This PDF file contains the front matter associated with SPIE Proceedings Volume 11240, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

A 3D high resolution scanner has been developed specifically for clinical use. The novel scanner architecture employing multiple interrogation beams can acquire a 3D image in less than 1 second. An initial technical validation study has been undertaken in human volunteers to determine repeatability, reproducibility and patient acceptability. Thereafter, a first-in-man clinical study aimed at assessing diagnostic accuracy in patients with inflammatory diseases has been completed.

Thyroid cancer is one of the most commonly diagnosed cancers in the world. Ultrasonography and fine-needle aspiration biopsy are the typical standard-of-care method for diagnosing thyroid nodules. However, about 20% of fine-needle aspiration biopsies generate undeterminable results, which can lead to overdiagnosis and overtreatment. In this study, we propose photoacoustic imaging as an additional triaging tool for identifying cancerous nodules in vivo. We enrolled and photoacoustically imaged 28 patients (19 malignant and 9 benign) who have thyroid nodules. To perform multispectral analysis, we used a series of 5 different wavelengths (i.e., 700, 756, 796, 866, and 900 nm), which were selected based on the optical absorption property of oxy- and deoxy-hemoglobin. All the raw data were automatically stored for further off-line processing, while the corresponding images were visualized on the clinical ultrasound machine in real-time. By using the multispectral photoacoustic data, we calculated the oxygen saturation values of the nodule areas. The result showed that the oxygen saturation level of malignant nodules was lower than that of benign nodules (p < 0.005), which matched with the well-known property of cancerous nodules. Based on the oxygen saturation value, malignant and benign nodules were differentiable with a sensitivity of 80% and specificity of 89%. The result showed the great potential of multispectral photoacoustic analysis as a novel method to identify malignancy of thyroid nodules in vivo. We also verified the robustness of the result by testing reproducibility and comparing inter-physician interpretation.

Rupture of carotid plaques triggers stroke. Current diagnosis of stroke is based on lumen stenosis, resulting in sever overtreatment. Photoacoustic (PA) imaging can provide comprehensive and patient-specific assessment of plaque vulnerability, and prevent overtreatment. However, no in vivo PA imaging of carotid plaque is available due to low SNR. Here, we present a fast PA/US imaging system and motion corrected averaging algorithm to increase PA SNR. The imaging system and algorithm are verified ex vivo, and in vivo on patients during carotid endarterectomy (intra-operatively). The results may accelerate the clinical translation of PA imaging of carotid plaques.

Cutaneous melanoma accounts for only 5% of skin cancer, but it is as dangerous as it is associated with 75% skincancer- related deaths. Clinical decision-making and prognosis is the thickness of melanoma into the tissue. Another feature is that the cancer that can occur anywhere on the body, including the face, chest, thigh, soles, and groin, and its size is also very diverse. Here, we developed a hand-held scanner and obtained 3D photoacoustic images of in vivo human melanoma by using multispectral real-time clinical photoacoustic and ultrasound imaging system with the scanner. The scanner allowed wide-field scanning of 3.8 cm (transducer aperture size) × 3 cm (scanning range). Four patients were recruited to obtain photoacoustic melanoma images of various sites (thigh, sole, etc.), types (in situ, invasive, etc.) and sizes (sub-mm to cm). Five wavelengths were used to perform spectral unmixing. The penetration depth of melanoma was successfully confirmed by the multispectral photoacoustic images. The melanoma depth measured by photoacoustic imaging was significantly similar to histopathologic results obtained after excision (mean absolute error = 0.6 mm). In this study, we acquired small-to-large size and various types of melanoma multispectral photoacoustic images in vivo. We hope that this study will be an additional criterion for histopathological results that may have a positive impact on the diagnosis, treatment, and prognosis of melanomas.

Identifying fibrosis against inflammation in the intestinal strictures is critical to the management of CD. Our pioneering study has shown that spectroscopic photoacoustic (PA) imaging is capable of differentiating inflammatory and fibrotic intestinal strictures in animals in vivo. We also validated the feasibility of acquiring PA signals from intestinal strictures transcutaneously. In this study, we further investigated the capability of transcutaneous PA imaging in characterizing intestinal inflammation and fibrosis in human subjects. The findings in PA imaging were validated by US Doppler images and histopathology.

Surgery remains the primary method of care for multiple types of solid cancer. The goal of surgical oncology is to remove all tumorous tissue from the body. Frozen sectioning is commonly used during surgery to assess margin status. However, this method can be unreliable as the slides can be difficult to interpret. Using a recently reported imaging modality, Photoacoustic Remote Sensing (PARS), we present the first in human non-contact histology-like imaging in reflection mode. Cellular morphology alongside blood vessels are imaged in the human breast, gastrointestinal, and skin tissues. These images then compared with conventional hematoxylin and eosin-stained samples.

Colorectal cancer is the second most common malignancy diagnosed globally and the 4th leading cause of cancer mortality. Critical gaps exist in diagnostic and surveillance imaging modalities for colorectal neoplasia. We have conducted a pilot study using a real-time co-registered photoacoustic (PAT) and ultrasound (US) tomography system. A total of 23 ex vivo human colorectal tissue samples (19 colon and 4 rectum) were imaged immediately after surgical resection. These results indicate potential of using PAT/US for future cancer screening and post-treatment surveillance of colon and rectum. The image resolution of the current system is low (~ 250 μm axial resolution) due to the commercial endo-cavity ultrasound transducer array (6 MHz central frequency, 80% bandwidth). To solve the problem of image resolution, we decoded the pin configuration of a high-frequency transducer array (15 MHz central frequency, 9-18 MHz bandwidth) and adapted it to our home-made 128 channels ultrasound pulsing and receiving system to perform high-frequency PAT/US imaging. We achieved a lateral resolution of ~ 150 μm and axial resolution of ~ 120 μm. We also imaged a post-treated human rectum sample to evaluate the system performance.

Skin aging caused by ultraviolet light exposure is one of the serious problems from the viewpoint of beauty and healthcare. This is because ultraviolet light can cause age spot, wrinkles, at the worst case, skin cancer and so on. To evaluate skin aging, various modalities are being used, such as histopathological diagnosis, optical coherence tomography, ultrasound examination (B-mode imaging). However, they have disadvantages in terms of invasiveness, penetration depth and tissue specificity, respectively. To overcome these defects, photoacoustic imaging (PAI), a novel modality was used in this work. This modality can portray differences of tissue characteristics non-invasively. In this experiment, human skin tissues in various generations (i.e. various degrees of photo-aging) were measured by using optical resolution photoacoustic microscopy (OR-PAM) and acoustic resolution photoacoustic microscopy (AR-PAM). To verify the feasibility of quantitative skin aging evaluation with PA technique, signals from sectioned human skin (cheek and buttock; female from 28 to 95 years old) were measured with PA microscopy. The effects of photo-aging progress on the signal intensity were investigated. The results demonstrated that the PA signal from the dermis increases with aging progress. These analyses demonstrate the feasibility of quantitative skin aging evaluation with a PA imaging system.

Using spectral photoacoustic imaging (sPAI) to estimate oxygen saturation of tissue at depth suffers from inaccuracies due to the unknown optical absorption and scattering properties of tissue. Because of the high scattering and absorption of light by tissue, the estimation of concentrations of Hb and HbO2 from the measured photoacoustic (PA) signal intensity can be erroneous. Simulation of wavelength-dependent light transport in tissue can help to estimate the local fluence distribution within the tissue. In this work, a Monte Carlo simulation has been implemented to simulate the fluence distribution in placental tissue. We obtained sPAI images of ex vivo human placental tissue and demonstrate improved estimations of hemoglobin oxygen saturation by using a fluence correction derived from the Monte Carlo simulation. The results show that with simulation correction, the oxygen saturation value is 12.61±3% which is closer to the value 6.8% directly measured from ex vivo human placenta using invasive oxygen probe.

In this work, we present the first time use of photoacoustic imaging for assessing the quality of donor kidneys pre-transplantation. There is a pressing clinical need to quantify the fibrotic (scarring) burden in a non-invasive manner to give clinicians crucial information before they decide whether to accept a donor kidney. Our results in human kidneys show that photoacoustic imaging can be a robust tool for assessing the degree of scarring, an important predictor of post-transplantation clinical outcome.

To assess cancer resection margins, post-operative histological diagnosis using hematoxylin and eosin (H&E) stained slides remains the gold standard due to the lack of effective intra-operative approaches. Wait times may be up to two weeks and subsequent treatments may be necessary. Therefore, we are motivated to introduce Chromophore Selective Multi-Wavelength Photoacoustic Remote Sensing, an all-optical, non-contact, reflection-mode, label-free approach to produce H&E-like images of human tissue. This work is a step towards in-situ imaging, rapid clinical assessment of tissue, and may permit future developments as a live intraoperative surgical microscope.

Beamforming algorithms are widely used for photoacoustic (PA) imaging to reconstruct the initial pressure map. In the reconstruction process, they typically assumed that the imaged biological tissue was a homogeneous medium. However, as biological tissue is generally heterogeneous, the misassumption causes suboptimal image reconstruction. Because it is difficult to predict the heterogeneity of a medium, it was still common to reconstruct images assuming a uniform medium. To solve this problem, we introduce a deep learning-based algorithm that can correct the speed of sound (SoS) aberration in the PA image. We trained a neural network with the multiple simulation datasets and successfully corrected SoS aberrations in a PA in vivo image of the human forearm. We observed that the proposed algorithm effectively suppressed side lobes and noise in the PA image and greatly improves image quality.

Clinical PA (photoacoustic) /US (ultrasound) imaging has been widely explored and in most cases is performed in 2D. However, 2D PA/US imaging technology has problems of low reproducibility and high operator dependency. For these reasons, we developed a clinical handheld 3D PA/US scanner using an 1D linear array US transducer and a compact mechanical scanner, which is managed by a Scotch yoke mechanism. The total weight, overall size and the field of view (FOV) of the scanner are 950 g, 100 × 80 × 100 mm³ and 38 × 40 mm², respectively. We have successfully evaluated the feasibility of clinical use by acquiring various images of the human body parts such as face, neck and calf. We believe that the hand-held scanner can be used for a variety of clinical applications.

We present the results on development of the 3D imaging platform combining photoacoustic tomography and fluorescence (PAFT) for preclinical and biological research. This combined multimodal imaging instrument addresses known deficiencies in sensitivity, spatial resolution, and anatomical registration of the individual imaging components. Multiangle photoacoustic projections, excited by an OPO operating in the near-infrared window, of a live anesthetized animal are used to reconstruct large volumes (30 cm³) that show deep anatomical vasculature and blood-rich tissues with resolutions exceeding 150 μm. A sCMOS camera is used for simultaneous co-registered multi-angle optical imaging. The images of a fluorescent dual-contrast agent are then reconstructed into a 3D volume using a tomographic algorithm. A separate 532-nm low-energy pulsed laser excitation is used for skin topography and imaging of superficial vasculature. All three imaging channels can be combined to produce spatially accurate in vivo volumes showing an animal’s skin, deep anatomical structures, and distribution of photosensitive molecular contrast agents. PAFT’s photoacoustic sensitivity was assessed using contrast agents in a phantom study. We demonstrate biomedical imaging application of PAFT’s combined imaging modalities by observing biodistribution of a dual-contrast agent injected intravenously to in vivo preclinical murine models.

Synovial angiogenesis and hypoxia in the joints are biomarkers of Rheumatoid Arthritis (RA). The ability to probe blood and accurately estimate the oxygen concentration make multiwavelength Photoacoustic (PA) imaging a potential tool for early detection of RA. In this work, a multiwavelength LED-based PA imaging system was characterized based on its imaging depth, resolution and accuracy of oxygen saturation estimation. A multicenter 3R (Replace, Refine and Reduce) focused small animal study was conducted. The 3R strategy was devised by reusing RA animal models, in vivo imaging of healthy animals and ex vivo studies with human blood. RA animal cadaver models with different levels of synovial angiogenesis (control, positive RA and treated) were imaged and compared against results from a previous study using the same samples. An ex vivo PA oxygen saturation imaging using human blood was validated against oximeter readings and further verified it with in vivo animal studies. An imaging depth of 8 mm with an SNR of 10 dB was achieved for RA samples. A difference in PA intensity was observed for RA models compared to control and treated group. The PA oxygen saturation estimation correlates with oximeter readings, which is confirmed with in vivo studies. The results show the efficacy of the LED-based PA imaging system in RA diagnosis based on synovial angiogenesis and hypoxia. The imaging depth, resolution and oxygen saturation estimate are sufficient to differentiate RA samples from control. Our future work will focus on validating the method using arthritis animal models and demonstrating the 3R potential.

We report on a comprehensive theoretical analysis of the optimal wavelengths for in vivo optoacoustic angiography in whole rodent brains. Governed by the competing processes of light attenuation through the brain tissues and optoacoustic signal generation due to absorption by blood, we identified three distinct spectral ranges centered around 580, 895 and 1100 nm optimally suited for imaging of vessels of different size and depth. The developed model was employed for numerical simulation of optoacoustic imaging of murine brain illustrating the effect of probing wavelength on visibility of the cerebral vasculature. In vivo imaging experiments further affirmed the capacity for whole-brain optoacoustic angiography in rodents at a wavelength of 1064nm.

Raster-scan optoacoustic angiography at 532 nm wavelength with 50 µm lateral resolution at 2 mm diagnostic depth was used for quantitative characterization of neoangiogenesis in human colon adenocarcinoma HT-29. Inhomogeneous distribution of areas with high and low vascularization was demonstrated in the tumors. During tumor development vessel growth from the periphery to the center of the tumor was shown. Increase of vascular density preceded the increase of tumor volume.

High power light emitting diodes (LEDs) can serve as fast, robust and affordable excitation sources for photoacoustic imaging. We test the in vivo imaging capabilities of a multi-modal imaging platform that comprises of LED-based photoacoustic (PA) and conventional ultrasound imaging. We characterized the dynamics of a fluorescent contrast agent and modulation of tissue oxygenation in vivo. Fast dynamic PA imaging shows an increase in PA signal with increasing fluorescent agent concentration (Tmax ~ 8 min). Oxygen challenge studies revealed changes in blood oxygenation saturation (SO2) levels in relation to the modulation of breathing air and oxygen.

Molecular Imaging techniques are a hotspot in oncology for their inherent ability to detect cancer early as well as characterize and stage them. The prognosis and treatment response can also be monitored with these imaging techniques. Specifically, photoacoustic imaging is a technique that recently had exponential growth in several biomedical applications, particularly for imaging tumors. Photoacoustic imaging involves excitation of the tissue with the nanosecond laser pulses and subsequent generation of acoustic waves with an ultrasound transducer. to reconstruct the image, has potential to detect and monitor cancer prognosis by using the optical contrast of hemoglobin. In this work, we evaluated the potential of LED-based photoacoustics in imaging heterogenous microvasculature in tumors.

Optical detection of ultrasound for photoacoustic imaging has received great interest. Recently, we have developed a new fiber-optic ultrasound sensor by exploiting dual-polarization fiber laser. It offers high sensitivity (40 Pa over 50 MHz) as well as good stability as a result of the self-heterodyning detection. In this work, the signal-to-noise ratio has been enhanced by suppressing the noise of the ultrasound sensing system via signal averaging. As a result of multiple measurements of a single photoacoustic signal, the total noise was reduced by 40%. With the enhanced detection capability, the sensors have been deployed as photoacoustic probes in different imaging modalities. We demonstrate fastscanning photoacoustic microscopy with a field-of-view 2×2 mm², a frame rate of 2 Hz to visualize the blood flow dynamics. By bending the flexible fiber optic sensor for geometrical focusing, PACT was realized to image a mouse brain with a spatial resolution of 70 μm. An all-fiber photoacoustic endoscope was built to in vivo image the vascular network of a rat rectum, with a lateral resolution of 10 μm, with a 2.3-mm probe diameter.

The detection of ultrasound via optical resonators is conventionally performed by tuning a continuous-wave (CW) laser to the linear slope of the resonance and monitoring the intensity modulation at the resonator output. In this work, we develop an alternative CW technique that can significantly reduce the measurement noise by monitoring variations in the phase, rather than intensity, at the resonator output. In our current implementation, which is based on a balanced Mach-Zehnder interferometer for phase detection, we demonstrate a 24-fold increase in the signal-to-noise ratio of the detected ultrasound signal over the conventional, intensity-monitoring approach.

Fabry-Perot (FP) based scanners are typically interrogated by scanning a single focused laser beam over its surface and measuring the reflected light. However, this approach can be relatively slow with acquisition times on the order of minutes. An alternative is to parallelise the read-out of the Fabry-Perot sensor by illuminating it with a large diameter collimated interrogation beam (e.g. 1 cm) and measuring the reflected light using an InGaAs camera. This approach allows acquiring 3D photoacoustic images in sub-second time frames. To demonstrate this speed advantage, a camera based FP scanner was developed and evaluated.

The manufacturing process of high sensitivity planar Fabry-Pérot (FP) sensors for Photoacoustic (PA) imaging is very challenging and typically results in non-uniformities of the cavity thickness. The non-uniformities leads to an angular tilt between the two mirrors forming the FP sensor. Based on a full wave model, we study the impact of this tilt which reveals a strong dependence between optical sensitivity and degree of tilt. As an example, an angular tilt as small as 0.1 mrad can reduce the sensitivity by 75%. To achieve high sensitivity FP sensors, high mirror reflectivities are required which in turn increases the impact of the non-uniformities in the cavity thickness. Therefore, the optimal design of the sensors is dependent on the manufacturing precision.

In optical detection of ultrasound, resonators with high Q-factors are frequently used to maximize sensitivity. However, in order to perform parallel interrogation, conventional interferometric techniques require an overlap between the resonator spectra, which is difficult to achieve with high Q-factor resonators. In this work, a new method is developed for simultaneous interrogation of optical resonators with non-overlapping spectra. The method is based on a phase modulation scheme for pulse interferometry (PM-PI) and requires only a single photodetector and sampling channel per ultrasound detector. Using PM-PI, parallel ultrasound detection is demonstrated with four high Q-factor resonators.

Silicon photonics represents an attractive platform for optical sensing of ultrasound owing to the high light confinement it can achieved, which can enable the development of detector with sub-micron sizes. However, the small elasto-optic coefficients of silicon and silica limit the sensitivity of conventional silicon-on-insulator (SOI) sensors, in which the silicon core is surrounded by a silica cladding. In this work, we demonstrate an order-of-magnitude increase in the response of a silicon-photonics waveguide to ultrasound by replacing the silica over-cladding with bisbenzocyclobutene (BCB) - a transparent polymer with a high elasto-optic coefficient.

Fiber optic Fabry-Perot interferometer is inherently suitable as the ultrasonic transducer for photoacoustic tomography due to its high sensitivity, broad bandwidth and small footprint. Interrogated by a narrow linewidth continuous wave laser, the sensor’s output power is modulated by the incident ultrasound. During the imaging process, the sensor’s sensitivity is maximized by locking the laser to a spectral point where the sensor’s reflectivity changes most rapidly with wavelength. Traditionally, one needs a fast tunable laser to scan the reflection spectrum of the sensor and subsequently lock the laser frequency to the proper spectral point using a feedback loop. The requirement of a wavelength-tunable, low-noise interrogation laser significantly raises system cost and inhibits parallel detection. In this paper, we present a fiber optic Fabry-Perot acoustic sensor whose reflection spectrum can be swiftly and robustly tuned using an economical visible diode laser. By controlling the power of the illumination laser, the temperature of the sensor cavity can be finely adjusted which leads to altered cavity length and shifted spectrum. With this technique, we are able to tune the spectrum by more than 10 nm with a precision less than 0.1 nm. The sensor was characterized to exhibit a flat frequency response up to 20 MHz and a noise-equivalent pressure below 200 Pa. This sensor can be batch-fabricated and its low cost and easy implementation make parallel detection feasible and affordable, potentially benefiting fast image acquisition. The performance of the sensor was demonstrated in multiple phantom and in vivo imaging experiments.

We present all-optical ultrasound transducers that is suitable for photoacoustic applications. We leverage the benefits of CMOS compatible on-chip photonic sensor that is realized by patterning the integrated photonic circuits overlaid on membrane cavity responding to incoming acoustic signals. The sensor is based on SiN based photonic ring resonator on top of 3-micron thick SiO2 membrane. Latched on the optical read out circuit, the sensor hardware architecture promises ultra-high sensitivity with the ability to detect as low as <1 mPa/sqrt(Hz) noise equivalent pressure. The hardware architecture can be further improved to leverage the benefits of existing photonic based signal conditioning schemes and adopt them to multiplex the ultrasound reception from multiple sensor elements.

Silicon photonics-based photonic crystal slab (PCS) ultrasound sensors have recently been demonstrated to have properties such as broadband (1-40 MHz) detection and sub-kPa sensitivities. Here, we study the effect of a thin PMMA overlayer (~ 300 nm) on the sensitivity of a PCS sensor. The presence of this overlayer can result in almost 50% of the electric field energy of the PCS guided resonance to reside outside of the slab. This allows for the PCS to be sensitized to multiple mechanisms, including photoelastic change and mechanical deformation of the overlayer, for ultrasonic detection. These mechanisms are investigated in this work.

There is a fundamental limitation in optical resolution photoacoustic microscopy (OR-PAM) systems, as optical resolution may only be maintained to a depth equal to the transport mean free path of the excitation laser. Here we present a non-contact OR-PAM that can break this barrier. Monte-Carlo simulations and experiments in tissue simulating phantoms validate that the proposed method is capable of imaging at over 2.5mm deep achieving micron resolution, and greater than 5mm at lower resolutions.

Fabry-Pérot etalon-based ultrasound detectors are typically interrogated with a focused Gaussian beam in order to achieve a desired acoustic element size. However, tightly focused Gaussian beams lead to beam ‘walk-off’ within the etalon cavity which reduces sensitivity. In previous work, the planar geometry of the Fabry-Pérot etalon has been replaced by a curved geometry matched to the interrogation beam geometry, thus preventing walk-off. In this work we instead propose using propagation invariant Bessel beams, thus matching the beam geometry to that of the planar etalon geometry, to reduce beam walk-off and increase sensitivity. Increased sensitivity may extend the imaging depth of Fabry-Pérot ultrasound detection systems and may thus enable photoacoustic imaging to access a range of deep tissue imaging applications.

Particles with sizes in the order of a few micrometers can significantly enhance the capabilities of optoacoustic imaging systems by improving visualization of arbitrarily oriented vascular structures and achieving resolution beyond the acoustic diffraction barrier. Particle tracking may also be used for mapping the blood flow in two and three dimensions. However, a trade-off exists between the particle absorption properties and size, whereas large sized microparticles also tend to arrest in the capillary network. We analyzed the flow of microparticles in an intracardiac perfusion mouse model in which blood is effectively substituted by artificial cerebrospinal fluid (ACSF). This enables mitigating the strong blood absorption background in the optoacoustic images thus facilitating the visualization of microparticles. A sequence of three-dimensional optoacoustic images of the mouse brain is then acquired at a high frame rate of 100 Hz after injection of the particles in the left heart ventricle. By visualizing the flow of particles of different sizes in microvascular structures it is possible to establish optimal trade-offs between the particle size, their optoacoustic signal and perfusion properties.

Changes in extracellular calcium concentrations ([Ca²⁺]e) can mediate a variety of biological responses in both excitable and nonexcitable cells. These changes can be seen in both physiological and pathological conditions; however, little is still known about their effects to neuronal excitability. Fluorescent calcium probes are essential tools for studying the fluctuation of calcium ions both in and out of cells. Unfortunately, current techniques utilizing these calcium probes have many limitations that have yet to be addressed, including lack of penetration depth and concurrent multiple site analysis in the whole brain. For example, fluorescence imaging suffers from light diffusion, a fundamental constraint that limits the imaging depth in tissue (< 1 mm). Photoacoustic tomography (PAT) has emerged as a promising imaging modality that overcomes this challenge. In this paper, we utilized a near infrared (NIR) ratiometric calcium fluorescent probe (Ca-NIR) as a unique photoacoustic calcium probe. Ca-NIR is based on fusing a selective calcium ligand BAPTA (1,2-bis-(o-aminophenoxy)ethane-N,N,N0,N0-tetraacetic acid) moiety to a dihydroxanthenehemicyanine fluorophore. We report the use of Ca-NIR as an efficient PA generating agent in various artificial cerebral-spinal fluid (aCSF) solutions with varying Ca²⁺ concentrations. Our result indicates high sensitivity of Ca-NIR to [Ca²⁺]_e fluctuations in aCSF and great potential of utilizing Ca-NIR in PAT as a method for noninvasive whole brain [Ca²⁺]_e imaging.

Photoacoustic signal can be significantly enhanced by improving the optical absorption, photothermal conversion efficiency, and thermoelastic expansion of an absorbing nanoprobe. Gold nanorods have low photothermal stability leading to spherical deformation and blue-shifting under high fluence lasers. Our work shows enhanced PA signal and high photothermal stability of metal chalcogenide-coated gold nanorods treated with ferricyanide. Preliminary data suggests up to 28 – 48X increase in PA amplitude and over 10X increase in photothermal stability compared to uncoated nanorods.

Bacterial infections lead to high oxidative stress due to production of reactive oxygen species like H2O2 and ONOO- inducing cell damage and death. ROS are a short-lived species making direct, precise and real time measurements difficult. Ag+ is a known bactericidal, disrupting metabolism and disulfide bond formation. In this work we synthesized a thernostic nanoparticle that can measure ROS concentrations and release Ag+ in the presence of ROS. The iodide-doped gold/silver hybrid nanoparticle is 10X more sensitive to H2O2 as compared to silver coated gold nanorods.

To accelerate the clinical translation of photoacoustic imaging (PAI), IPASC aims to define widely accepted test objects (‘phantoms’) for use with preclinical and clinical PAI systems and use these phantoms to enable quantitative comparison of PAI data. We propose the use of a polymer-mineral oil phantom composition, where all constituent materials have defined CAS numbers and are available from commercial chemistry suppliers. A pilot study involving 13 partner labs was designed and implemented. A detailed characterisation of the resulting acoustic and optical properties was used to evaluate the precision and accuracy of phantoms fabricated at multiple sites.

Test-objects for use in hybrid photoacoustic-ultrasound systems with a 3D geometry are introduced. They are designed to assess the system’s spatial resolution, imaging depth and image reconstruction algorithms. Considering the latter, one test-object is designed to test the robustness of algorithms when encountering heterogeneity in sound speeds. A further test-object is designed to test the accuracy of blood oxygenation estimation in image reconstruction. Finally, a novel semi-anthropomorphic photoacoustic-ultrasound breast phantom containing tumors and blood vessels is introduced. This phantom is equipped with adaptable blood oxygenation levels and is a valuable tool for the optimization and characterization of the imaging parameters.

High intensity focused ultrasound (HIFU) is a non-invasive thermal therapy during which a focused ultrasound beam is used to destroy cells within a confined volume of tissue. Due to its increased use and advancements in treatment delivery, various numerical models are being developed for use in treatment planning software. In order to validate these models, as well as to perform routine quality checks and transducer characterisation, a temperature monitoring technique capable of accurately mapping the temperature rise induced is necessary. Photoacoustic thermometry is a rapidly emerging technique for non-invasive temperature monitoring, where the temperature dependence of the Gruneisen parameter leads to changes in the recorded photoacoustic signal amplitude with temperature. In order to use this technique to assess heating induced by HIFU in a metrology setting, a suitable test material must first be selected that exhibits an increase in the generated photoacoustic signal with temperature. In this study, the temperature dependence of the photoacoustic conversion efficiency (μaΓ) of several tissue-mimicking materials was measured for temperatures between 22 °C and 50 °C. Materials included were agar-based phantoms, copolymer-in-oil, gel wax, PVA cryogels, PVCP and silicone. This information provided a basis for the development of a volumetric phantom, which was sonicated in a proof-of-concept integrated photoacoustic thermometry system for monitoring of HIFU-induced heating. The results show the suitability of agar-based phantoms and photoacoustic thermometry to image the 3D heat distribution generated by a HIFU transducer.

To accelerate the clinical translation of photoacoustic (PA) imaging, IPASC aims to provide open and publicly available reference datasets for testing of data reconstruction and spectral processing algorithms in a widely accepted data format. The International Photoacoustic Standardisation Consortium (IPASC) has identified and agreed on a list of essential metadata parameters to describe raw time series PA data and used it to develop an initial prototype of a standardized PA data format. We aim to apply the proposed format in an open database that provides reference datasets for testing of processing algorithms, thereby facilitating and advancing PA research and translation.

Due to a demonstrated capability to assess tumor angiogenesis and hypoxia in mammalian systems, there is great interest in applying optoacoustic tomography (OAT) to the study and screening of breast cancer. In order to translate OAT to clinical applications, in silico studies are crucial for studying imaging system parameters that might be impossible to assess via direct experimentation. Previous numerical phantoms have proven to be too unrealistic for rigorous testing of modern image reconstruction methods and clinically relevant signal detection tasks. Recently, the U.S. Food and Drug Administration has released software to generate realistic three-dimensional numerical realizations of the human female breast as part of the Virtual Imaging Clinical Trials for Regulatory Evaluation (VICTRE) project. By careful selection of physical attributes and material coefficients, the VICTRE breast phantom can be customized for particular imaging tasks, but no such customization has been given for OAT. We propose a general framework of in silico studies for OAT breast imaging using the VICTRE breast phantom. We will create an ensemble of OAT breast phantoms, using appropriate optical and acoustic parameters, that have typical sizes and tissue densities. Various lesions will be created and embedded based on clinical scenarios. We will define and perform several signal detection tasks by which the system performance may be compared. Generation of such an ensemble requires substantial computation but once produced, it can be utilized in other numerical simulation studies of the configuration of OAT imaging systems customized for diverse tasks. We will make this ensemble of phantoms publicly available online. The proposed framework will permit standardization of the assessment of 3D OAT data-acquisition parameters and image reconstruction methods.

Rectal adenocarcinoma is a common cancer in the United States. Current standard of care techniques (colonoscopy and MRI) have notable drawbacks and surgeons have aggressively put most patients into surgical intervention. Developing an efficient and sensitive method to evaluate rectal cancer is urgently needed. Here we have developed a new handheld co-registered ultrasound and acoustic-resolution photoacoustic endoscope (AR-PAE) to evaluate human rectal cancer in vivo. Normal rectal ultrasound images revealed typical layered structure, while photoacoustic images resolved rich vascular supply of submucosa. Our pilot patient data suggest that AR-PAE is effective to distinguish rectal cancer from normal rectum.

Minimally invasive procedures are increasingly being preferred over the conventional open procedures as it offers range of benefits to the patients such as quick recovery time, reduced infection rates, minimal tissue damage and scarring. However, in situ assessment of malignancies and identification of tumour margins remains a challenge with existing intraoperative imaging techniques. We present a novel laparoscopic probe that provides co-registered photoacoustic (PA) and white light endoscopy images. With the help of PA contrast, the probe can visualise depth-resolved microvasculature and thus offers the prospect of more sensitive detection of tumours based on abnormal vascular anatomy and function.

Photoacoustic (PA) imaging has shown its capability of characterizing intestinal inflammation and fibrosis endoscopically. With the purpose of clinical translation, we developed an endoscopic probe integrating an intracardiac ultrasound array and an 800 µm side-firing fiber optic inside a medical balloon catheter. The catheter probe, when collapsed, fits to the instrument channel of a colonoscope and can inflate for acoustic coupling when positioned at the disease location inside intestine. The performance of the probe in assessing the disease conditions including inflammation, fibrosis and muscle hypertrophy is under investigation in rabbits in vivo. The imaging results are validated by histopathology.

We present the all-fiber approach towards building a miniatured, optical-resolution photoacoustic endoscope. The catheter encapsulates two optical fibers for optical excitation and ultrasound detection, respectively. The ultrasound waves are detected with the laser-based fiber optic sensor, with a diameter of 125 μm, instead of a focused piezoelectric transducer. Photoacoustic endoscopic images from a rat rectum have been acquired in vivo with a range of 6.3 mm, a lateral resolution of 10 μm, and a 285° angular field-of-view. The catheter has a diameter of 2.3 mm and can be further reduced by replacing the bulk prism reflector.

Ultrasound transducers, one of the most widely used sensors in the era of the fourth Industrial Revolution, have been recognized and used in a variety of industries including medical, automotive, and robotics. In particular, recent research has focused on the development of multi-mode imaging systems that combine ultrasound and optical imaging to improve the accuracy of information acquisition. Unfortunately, its efficient combination has been severely limited due to the inherent opacity of conventional ultrasound transducers. These limitations cause off-axes between the ultrasound (US) and optical signal paths, resulting in low signal-to-ratio and bulky system. This is especially a critical problem for a photoacoustic (PA) imaging system that requires the ultrasonic transducer to detect the photoacoustic signal. Here, we introduce a newly developed optically transparent ultrasound transducer (TUT) to overcome the limitation. We combined the developed TUT with an optical resolution photoacoustic microscopy (OR-PAM). Using a mouse, we successfully acquired in vivo PA and US images and confirmed the feasibility of the TUT and TUT integrated OR-PAM system.

Non-alcoholic fatty liver disease (NAFLD) refers to the accumulation of excess fat in the liver without excessive alcohol consumption. Failure to detect NAFLD in advance can lead to fatal and irreversible liver diseases such as liver fibrosis, cirrhosis, and liver cancer, and thus the clinical need for rapid diagnosis of NAFLD is being raised. However, conventional NAFLD diagnostic methods, including biopsy and imaging, have inherent limitations due to technical or cost issues. For example, liver biopsy is invasive, MRI imaging is very expensive, CT is harmful for routine clinical use, and ultrasound is not very specific in diagnosing NAFLD. Here, we present photothermal strain imaging (pTSI), which uses the difference in thermal strain between fat and water during temperature change, as a new method for NAFLD diagnosis. The pTSI is a non-invasive, convenient, and cost-effective method of using a laser that matches the optical characteristics of the target. We developed a liver pTSI system to find fat accumulated in the liver using a clinical ultrasound imaging system and a lipid-sensitive continuous-wave laser. To demonstrate the feasibility of the pTSI system, in vitro experiments were performed using fat and chicken breast. The results showed that fat in the chicken breast was clearly distinguished. Finally, we performed in vivo experiments using NAFLD and normal rat. Through the difference in the strains that occurs when the laser heats the target, the progression of NAFLD could be determined.

We report on a new small animal imaging platform for concomitant noninvasive mapping of the absorbed optical energy, acoustic reflectivity, speed of sound and acoustic attenuation in whole mice with submillimeter resolution. In vivo mouse imaging experiments revealed fine details on the organ parenchyma, vascularization, tissue reflectivity, density and stiffness. The newly developed synergistic multimodal combination offers unmatched capabilities for imaging diverse tissue properties and biomarkers with high resolution, penetration and contrast.

Photoacoustic imaging is expected to be a next-generation diagnostic modality. However, systems using a solid state laser (SSL) are expensive, large in size and poor in operability of probes. In addition, protective goggles are required because of laser light. Therefore, we have adopted the LED technology and improved the signal-to-noise-ratio (SNR) of the LED-based system, which had been 1 / 2.3 million of the SSL, to the same level with four innovative technologies. These innovative technologies include a) High power and high density LED array technology: Use of high power LED chips with luminous efficiency comparable to laser diode, high density mounting of LED chips on Aluminum base and compact design, b) Giant and ultra-short-pulse drive circuit technology: High speed on-and-off by low-resistance Metal Oxide Semiconductor Field Effect Transistor (MOSFET), and position separation of high-voltage drive circuit from ultrasonic probe (USP) by series connection of LEDs, c) Optical pulse generation technology optimum for frequency response characteristic of USP, d) Noise reduction technology for faint signals using ultra-amplification: Minimization of quantization noise of Analog-digital-converter (ADC) by wide band ultra-amplification of 86 dB, and noise reduction by averaging of <100 times. Using these technologies, we have developed an LED-based photoacoustic imaging system. To use the system, we have discovered the mechanism of the absorption of pulsed light converted into photoacoustic signal detection is a "linear system" by frequency response characteristic analysis using an ideal point source phantom, and clarified the ultra-amplification over 80 dB and the SNR over 4 are required for real-time imaging using a biological phantom. Furthermore human in-vivo real-time functional imaging using dual-wavelength of both 820nm and 940nm has showed that the LED-based system can be used clinically.

Combined Photoacoustic (PA) and Ultrasound (US) imaging systems are finding more preclinical and clinical applications. However, majority of the commercial systems use expensive pulsed lasers. In most small animal studies and clinical applications like arthritis screening of finger joints, there is a need for tomographic imaging. In this work, we present PA and US tomographic imaging using a commercial imaging system with LED arrays as illumination source. We employ multiangle spatial compounding of PA and US images using a probe with a linear array transducer and four LED arrays, to form dual-mode tomographic images. Using phantom experiments, the proposed approach is validated and thoroughly tested. Further, the potential of the system is demonstrated by imaging knee joint and abdominal region of a mouse. This proposed approach has several advantages. First, the resolution and signal to noise ratio (SNR) are improved with the compounding of images from multiple angles. The resolution improvement owes to better axial resolution compared to lateral and high SNR with averaging. Secondly, the limited view artifacts and loss of information from the use of a linear array US probe is tackled. The US tomographic images of the mouse-knee RA model show structural details of the joint and blood vessels were visible in the tomographic PA images. The whole animal images enabled improved functional and structural information. An affordable PA/US tomographic imaging system with potential in clinical arthritis-screening and small animal imaging is presented.

We introduce a combined non-interferometric photoacoustic remote sensing (PARS) microscopy and spectral-domain optical coherence tomography (SD-OCT) system. This can provide complementary information for structural and functional imaging in biomedical applications, with intrinsic 2D registration of PARS absorption images and OCT scattering tomograms. The system includes a free-space Michelson interferometer, 147 kHz line scan spectrometer, and a 1060 nm superluminescent diode interrogation source offering up to 65 mW of power, along with a bandwidth of 65.6 nm. System validation involved imaging layered phantoms containing carbon fibers, with a PARS lateral resolution of 7 µm and OCT axial resolution of 8 µm.

Optical-resolution photoacoustic microscopy (OR-PAM) has become a popular tool in small-animal studies. However, previous OR-PAM techniques variously lacked a high imaging speed, a high spatial resolution, and/or a large field of view. Here we report a high-speed OR-PAM system using an innovative water-immersible polygon-mirror scanner, which has achieved a cross-sectional frame rate of as high as 1200 Hz over a 12-mm scanning range. Using this polygon-scanner-based OR-PAM system, we have performed various studies on mouse models with stroke and cardiac arrests. We expect that the new OR-PAM system will become a powerful tool for imaging hemodynamics and neuronal functions.

Ultraviolet-based photoacoustic microscopy (UV-PAM) has recently been demonstrated as a promising tool to overcome the time-consuming sample preparation procedure in traditional pathological analysis. In order to achieve high-speed UVPAM for clinical usage, we implemented UV-PAM with a single-axis galvo mirror scanner. With our UV laser operating at a repetition rate of 55 kHz, our system produced images ~5.5 times faster than the previously reported point-by-point raster scanning based UV-PAM, with a lateral resolution of ~1.0 μm. Histology-like images of a mouse brain slice were acquired by our system, showing its potential as an intraoperative imaging tool for surgical margin assessment.

We report a new multi-parametric photoacoustic microscopy (PAM) system, which enables high-resolution imaging of blood perfusion, oxygenation and flow at 0.2-Hz frame rate over an area of 4.5×4.5 mm2. Extending the laser scanning range by using a cylindrically focused transducer (focus: 50 µm by 4.5 mm), it increases the speed of our previous hybrid-scan system with a weakly focused transducer (focal diameter: 250 µm) by 18-fold without compromising the sensitivity. We have demonstrated the feasibility of this technique in the transmission mode in vivo. Further development of a reflection-mode system will enable real-time cortex-wide imaging of cerebral hemodynamics and metabolism.

Surgical removal of the head cuticle of Drosophila is necessary for optical brain imaging, leading to the Drosophila death in a short period of time and thus hindering long term monitoring. Targeting to the unmet need of surgery free procedure for Drosophila brain researches, in this study, laser scanning optical resolution photoacoustic microscopy (LSOR-PAM) of in vivo cuticle intact RCaMP labeled Drosophila mushroom body (MB) is presented. The MB is a higher-order olfactory center in Drosophila brain. This study paves the way toward exploring LSOR-PAM functional imaging capability of cuticle intact Drosophila brain under olfactory stimulus.

Due to limited ultrasound detection angle, photoacoustic microscopy may own a relatively low sensitivity. To break this limit, we develop an ultra-sensitive optical resolution photoacoustic microscopy based on a customized acoustic lens with high numerical aperture (0.74) (HNA-OR-PAM). The sensitivity of HNA-OR-PAM is improved to around 160% as the state-of-the-art OR-PAM. It has the capability to measure oxygen saturation of mice’s ear in vivo with ~10nJ pulse energy, reducing the nonlinear effect induced by high pulse energy. In addition, photoacoustic signal of tilted objects could be enhanced due to augmented ultrasound detection angle, which has been validated in our phantom study and the brain imaging experiment in vivo.

Two-photon photoacoustic microscopy (TP-PAM) can visualize deep structures in living tissues with high spatial resolution determined by the volume of nonlinear absorption. Generally, the out-of-focus background fluorescence limits the imaging depth in nonlinear optical microscopies. In this study, to overcome this drawback that is also expected to exist in TP-PAM, we propose TP-PAM with spatial overlap modulation using femtosecond optical pulse train. Because the modulation depth of the spatial overlap in the focal region is much greater than those in out-of-focus regions, the out-of-focus background is effectively rejected by extracting the modulated photoacoustic signals.

A multi-wavelength imaging technique based on the principle of stimulated Raman scattering (SRS) can be applied to photoacoustic (PA) techniques to produce label-free image contrast in wavelength-dependent targets in biological tissue. Current studies had limited approaches to optimizing the pulse energy of generated peaks. A comprehensive study of various parameters that affect the pulse energy of generated SRS peaks from a 532nm pulsed laser is presented; including fiber polarization, cut-off wavelength, length of fiber, pulse width, pulse repetition rate, and input power. Optimal conditions for designing a multi-wavelength laser source to image wavelength-dependent biological tissues with possible biomedical diagnostics and experimental applications are presented.

Fast switch between pulses of sufficient energy but different wavelengths plays an important role in fast functional photoacoustic imaging. Commonly used Stimulated Raman scattering through a long optical fiber can produce multiple wavelengths, but longer fiber means lower energy, which would decrease image quality. Shorter optical fiber is thus more desired for fast imaging. However, it should be noted that too short pulse separation may generate photoacoustic signals that are temporally overlapped. Here, we propose an approach to solve this overlapping problem in ultrafast PA imaging using a Fourier-domain based method. The validity of this method is confirmed through simulation firstly, and then it is applied to separate overlapped PA signals. Lastly, in vivo OR-PAM mapping of mouse ear’s sO2 photoacoustic imaging is achieved.

Photoacoustic endoscopy holds great potential for guiding minimally invasive procedures including fetal surgery, and tumour biopsy, as it can provide functional and molecular information of tissue with high spatial resolution. Multimode fibre has shown promise for the development of an ultrathin, forward-viewing photoacoustic probes. In this work, we report the development of a photoacoustic endomicroscopy system based on a multimode fibre and a new method for calibration. With a simple setup, this method exhibited high-speed calibration of MMFs, and it could be useful for the development and clinical translations of ultra-thin photoacoustic endomicroscopy probes.

Many applications of microendoscopy, including brain imaging, requires minimally invasive devices to minimize damage during insertion in the tissue. Here we present a minimally-invasive endoscope based on a multimode fiber that combines photoacoustic and fluorescence sensing. By learning the transmission matrix during a prior calibration step, a focused spot can be produced and raster-scanned over a sample at the distal tip of the fiber by use of a spatial light modulator. We demonstrate that our setup provides both photoacoustic and fluorescence microscopic images of test samples in vitro (fluorescent beads and red blood cells) through the same fiber.

Understanding the mechanisms of cardiac disorders largely depends on availability of multi-dimensional and multiparametric imaging methods capable of quantitative assessment of cardiac morphology and function. The imaging modalities commonly employed in cardiac research, such as ultrasonography and magnetic resonance imaging, are lacking sufficient contrast and/or spatio-temporal resolution in 3D in order to reveal the multi-scale nature of rapid electromechanical activity in a beating heart. Our recently developed volumetric optoacoustic tomography (VOT) platform offers versatile observations of the heart function with rich optical contrast at otherwise unattainable temporal and spatial resolutions. Herein, we further advance the imaging performance by developing compressed acquisition scheme to boost the temporal resolution of VOT into the kilohertz range, thus enabling 3D mapping of electromechanical wave propagation in the heart. Experiments in isolated mouse hearts were performed by exciting the entire imaged tissue volume with nanosecond-duration laser pulses at 1 kHz repetition rate pulse operating at 532 nm and sparse tomographic signal sampling using a custom-made 512-element spherical matrix ultrasound array. By analyzing the strain maps obtained from the rapid VOT image sequence, it was possible to quantify the phase velocity of the electromechanical cardiac waves, in good agreement with previously reported values.

In this work, we examine the potential of photoacoustic imaging for understanding the biophysical mechanism of nanobubble and microbubble-based vascular disrupting therapies. We present for the first time, a direct in-vivo comparison between sub-micron nanobubbles and the commercially available microbubbles. Our results show that PA imaging of tumor oxygenation is capable of measuring the nanobubble-induced almost 40% cell death as a result of vascular disruption.

Using spectroscopic photoacoustic imaging to quantitatively measure blood oxygenation saturation (sO2) is a difficult problem which requires prior tissue knowledge and costly computational methods. We have developed a convolutional neural network with a U-Net architecture to estimate the sO2 from spectroscopic photoacoustic data. The network was trained on Monte Carlo simulated spectroscopic PA data and predicted sO2 with only 4.49% error, an accuracy much higher than that of a linear spectral unmixing baseline. These results suggest that precise quantitative measurements of sO2 deep in tissue is attainable using machine learning approaches.

Recently, Eil et al. showed that tumors with necrotic cores had elevated K+ concentrations due to the release of K+ by necrosis, which acts to suppress T-cell function by high extracellular K+ concentration. These findings demonstrate the importance of developing a tool for imaging K+ distributions. Here, we demonstrate K+ nanosensor-enabled photoacoustic imaging for measuring K+ levels in vivo. The nanosensor is an in-house synthesized optical contrast agent that is sensitive to K+ levels within biological ranges. The use of this K+ nanosensor, combined with multi-spectral photoacoustic imaging, allowed measurement of K+ levels in vivo.

We investigate the use of a 256-channel spherically focused sparse array for volumic real-time ultrasound (US) and photoacoustic (PA) imaging of chicken embryo in vivo. Reconstructions were performed offline and signal processing techniques exploiting spatial and temporal dynamics of the blood flow were applied to visualize the vasculature. The resulting reduction of the clutter enhances the contrast by up to a factor of 2, providing an enhanced visualization of vascular networks. This methodology has a potential for in vivo 3D real time visualization of the vasculature and other features using complementary information provided by US and PA imaging.

Photoacoustic (PA) tomography (PAT) is a promising technology for noninvasive temperature sensing. However, traditional PA thermometry can measure only the temperature changes relative to a baseline. Here we report a new thermal-energy-memory-based PA thermometry (TEMPT) to quantify the Grüneisen parameter and recover the absolute temperature distribution in deep tissues. We have validated the feasibility of TEMPT on tissue-mimicking phantoms and achieved a measurement accuracy of ~0.5 °C at 1.5 cm depth. As proof-of-concept, we applied TEMPT for temperature mapping during focused ultrasound treatment in mice in vivo. TEMPT is expected to find applications in thermotherapy on small animal models.

Recently, radio-frequency (RF) delay-multiply-and-sum (RF-DMAS) algorithm has been proposed for photoacoustic (PA) imaging, featuring improved signal-to-noise ratio, contrast and lateral resolution. However, it requires oversampling to avoid aliasing and additional band-pass filtering (BPF) to keep the harmonic components for imaging. Here we propose baseband DMAS (BB-DMAS) algorithm for PA array imaging, offering similar results to the RF-DAMS ones but with simplified signal processing and additional flexibility. No oversampling and BPF is required. Experimental results show that the BB-DMAS algorithm provides similar image quality to the RF-DAMS one with lower complexity. The image quality can also be flexibly tuned accordingly.

Photoacoustic tomography (PAT) is an imaging modality developed during the past few decades. In the inverse problem of PAT, the aim is to estimate the spatial distribution of an initial pressure p₀ generated by the photoacoustic effect, when photoacoustic time-series pt measured on the boundary of the imaged target are given. To produce accurate photoacoustic images, the forward model linking p₀ to p_t has to model the measurement setup and the underlying physics to a sufficient accuracy. Use of an inaccurate model can lead to significant errors in the solution of the inverse problem. In this work, we study the effect and compensation of modelling errors due to uncertainties in ultrasound sensor locations in PAT using Bayesian approximation error modelling. The approach is evaluated with simulated and experimental data using various levels of measurement noise, uncertainties in sensor locations and varying sensor geometries. The results indicate that even small errors in the modelling of ultrasound sensor locations can lead to large errors in the solution of the inverse problem. Furthermore, the magnitude of these errors is affected by the amount of measurement noise and the measurement The modelling errors can, however, be well compensated by the approximation error modelling.

Multispectral optoacoustic tomography (MSOT) offers the unique capability to map the distribution of spectrally distinctive endogenous and exogenous substances in heterogeneous biological tissues by exciting the sample at various wavelengths and detecting the optoacoustically-induced ultrasound waves. This powerful functional and molecular imaging capability can greatly benefit from hybridization with pulse-echo ultrasound (US), which provides additional information on tissue anatomy and blood flow. However, speed of sound variations and acoustic mismatches in the imaged object generally lead to errors in the coregistration of compounded images and loss of spatial resolution in both imaging modalities. The spatially- and wavelength-dependent light fluence attenuation further limits the quantitative capabilities of MSOT. Proper segmentation of different regions and assignment of corresponding acoustic and optical properties turns then essential for maximizing the performance of hybrid optoacoustic and ultrasound (OPUS) imaging. Particularly, accurate segmentation of the boundary of the sample can significantly improve the images rendered. Herein, we propose an automatic segmentation method based on a convolutional neural network (CNN) for segmenting the mouse boundary in a pre-clinical OPUS system. The experimental performance of the method, as characterized with the Dice coefficient metric between the network output and the ground truth (manually segmented) images, is shown to be superior than that of a state-of-the-art active contour segmentation method in a series of two-dimensional (cross-sectional) OPUS images of the mouse brain, liver and kidney regions.

Utilization of the acousto-optic effect by detection of light scattered within tissues and modulated by focused ultrasound pulses could provide diagnostic information impossible to obtain by purely acoustic or optical imaging modalities. It could also support photoacoustic imaging by mapping fluence rate distribution. However, practical implementation of this technique encounters numerous difficulties preventing it from rapid adoption in clinical use. One of the important limitations that has not yet been adequately addressed is that in many practical medical applications the region of interest may be accessed only from one side. In the present study we introduce the results of investigations on acousto-optic detection and localization of optically distinct inclusions inside acoustically homogeneous phantoms using a linear ultrasound array with electronically scanned focus and optical fibers arranged in reflectance geometry. Speckle contrast differences between speckle patterns captured in absence and presence of ultrasound pulses with different focal point coordinates are determined for various samples. The results allow clear distinction between phantoms with and without optically absorbing inclusions, although these are neither visible from the surface nor distinguishable on ultrasound images. It is also shown, that data analysis allows to obtain further cues on localization of absorbing regions. Conditions and limitations in this regard are discussed.

One of the major applications of multispectral photoacoustic imaging is the recovery of functional tissue properties with the goal of distinguishing different tissue classes. In this work, we tackle this challenge by employing a deep learning-based algorithm called learned spectral decoloring for quantitative photoacoustic imaging. With the combination of tissue classification, sO2 estimation, and uncertainty quantification, powerful analyses and visualizations of multispectral photoacoustic images can be created. Consequently, these could be valuable tools for the clinical translation of photoacoustic imaging.

Tremendous progress in synthetic micro/nanomotors has been made for potential biomedical applications. However, existing micro/nanomotor platforms are inefficient for deep tissue imaging and motion control in vivo. Here, we present a photoacoustic computed tomography (PACT) guided investigation of micromotors in intestines in vivo. The micromotors enveloped in microcapsules exhibit efficient propulsion in various biofluids once released. PACT has visualized the migration of micromotor capsules toward the targeted regions in real time in vivo. The integration of the developed microrobotic system and PACT enables deep imaging and precise control of the micromotors in vivo.

Diffuse correlation spectroscopy (DCS) is an established diffuse optical technique that uses the analysis of temporal speckle intensity fluctuations to measure blood flow in tissue. As a non-invasive technique, DCS has been used to monitor patient cerebral blood flow at the bedside. Though an effective measurement tool, extra-cerebral contamination of the DCS signal limits the sensitivity to changes in brain blood flow. In order to overcome this depth sensitivity challenge, we present a method, overlapping volumes, acousto-optic modulated DCS (AOM-DCS), to improve sensitivity to deeper tissue structures.

The spatial resolution of photoacoustic (PA) computed tomography (PACT) is limited by acoustic diffraction. Here, we report in vivo superresolution PACT, which breaks the acoustic diffraction limit by localizing the centers of single dyed droplets. The dyed droplets generate much stronger PA signals than blood and can flow smoothly in blood vessels; thus, they are excellent tracers for localization-based superresolution imaging. The flowing droplets were first localized, and then their center positions were used to construct a superresolution image that exhibits sharper features and more finely resolved vascular details. A 6-fold improvement in spatial resolution has been realized in vivo.

Assessment of morphological changes in cerebral venous sinus of small animal models is important to gain insights of various disease conditions such as intracranial hypotension, Idiopathic intracranial hypertension (IIH), Cerebral venous sinus thrombosis, subdural hematoma etc. Photoacoustic Tomography (PAT), a fast-growing non-invasive hybrid imaging modality which combines high optical contrast and resolution in deep tissue imaging offers a novel, rapid and cost-effective way to analyze the morphological changes of venous sinus in comparison with the conventional imaging modalities. In this study, we examined the morphological changes of sagittal sinus in the rat brain due to intracranial pressure changes induced by Cerebrospinal fluid (CSF) extraction using low cost pulsed laser diode (PLD) based desktop (PAT) system. Our results indicate that the desktop PLD-PAT system can be employed to evaluate the changes in the cerebral venous sinus in preclinical models. We observed a ~30% average increase in the area of sagittal venous sinus from the baseline, when the CSF is extracted.

Chicken embryo remains a commonly used model for biomedical research due to its ease of availability, accessibility for surgical manipulations, and biological similarities with humans. Although, it is being extensively used for cancer, cardiovascular, and developmental studies, non-destructive imaging of live embryo vasculature with high optical contrast remains a challenge. In this work, photoacoustic tomography of chicken embryos developing in bioengineered eggshell was performed. Chicken embryos at different developmental stages were irradiated with laser pulses at wavelengths 532 nm and 740 nm to acquire cross sectional images at various depths. We have shown a label free method for imaging of live chicken embryo while maintaining its integrity. We demonstrated that our method has the potential to be a powerful non-invasive imaging method for studying vasculature growth and development in chicken embryos.

Photoacoustic imaging holds promise in wide range of clinical and preclinical applications. Since photoacoustic imaging can be implemented in a conventional ultrasound scanner by adding light illumination, it is straight forward to realize dual-mode imaging offering complementary contrast. We recently developed an LED-based photoacoustic and ultrasound imaging system (AcousticX) with unprecedented 2D and 3D functional and structural imaging capabilities. Pulse energy offered by our LED arrays is orders of magnitude lower than conventional lasers and we perform frame averaging to keep up with the SNR, reducing the display frame rate. Even though the pulse repetition frequency of our LED arrays is 4 KHz, image frame rate we can achieve is limited by the large number of frame averages used to improve SNR. In this work, we present a deep learning-based approach to reduce the frame averaging in LED-based photoacoustic imaging without compromising the SNR. We have used convolutional neural network (U-Net) model in deep learning for improving the images with less averaging. When compared with traditional denoising methods, deep learning enables us to optimize parameters through network training. We used images from various other photoacoustic imaging systems with higher laser energy and broadband ultrasound transducers, which can generate PA images with high resolution and SNR with minimal or no averaging as training data. We validate our algorithm using LED-based photoacoustic images of phantoms utilizing Indocyanine green and methylene blue as contrast agents. In all cases, we achieved improvement in the SNR by denoising the images with lesser averaging, thereby increasing the framerate. Results demonstrate the potential of deep learning algorithms in improving temporal resolution and SNR in LED-based photoacoustic imaging.

Photoacoustic microscopy (PAM) is a biological visualization technique that can provide high spatial resolution and high contrast images of deep structures in living tissues. However, because of the spherical aberration of the objective lens and the wavefront distortion due to the surface shape and light scattering of the specimen, obtained photoacoustic images in deep tissues are sometimes blurred or distorted. In order to solve this problem, we have developed a PAM using a transmissive liquid-crystal adaptive optics (AO) element. The transmissive and thin structure of the AO element can be easily installed in the PAM system. Using photoacoustic images of a USAF 1951 resolution test target measured through the glass substrate (thickness; 1.5-mm), the lateral resolutions in PAM were estimated with and without the AO element, when a flashlamp-pumped nanosecond pulse laser (pulse width, 5-ns; wavelength, 500-nm) and water-immersion objective lens (NA = 0.8) were employed. The lateral resolution of PAM at the depth of 1.5-mm was improved from 1.04 ± 0.04 μm to 0.53 ± 0.10 μm by optimizing AO corrections. We have also visualized small blood vessels in mouse ear in vivo by PAM with AO correction. Thus, by optimizing the AO correction according to the imaging depth, our proposed PAM improves the spatial resolution in biological tissues.

In photoacoustic imaging, accurate spectral unmixing is required for revealing functional and molecular information of the tissue using multispectral photoacoustic imaging data. A significant challenge in deep-tissue photoacoustic imaging is the nonlinear dependence of the received photoacoustic signals on the local optical fluence and molecular distribution. To overcome this, we have deployed an end-to-end unsupervised neural network based on autoencoders. The proposed method employs the physical properties as the constraints to the neural network which effectively performs the unmixing and outputs the individual molecular concentration maps without a-priori knowledge of their absorption spectra. The algorithm is tested on a set of simulated multispectral photoacoustic images comprising of oxyhemoglobin, deoxy-hemoglobin and indocyanine green targets embedded inside a tissue mimicking medium. These in silico experiments demonstrated promising photoacoustic spectral unmixing results using a completely unsupervised deep learning approach.

Photoacoustic imaging is a hybrid imaging technique with broad preclinical and clinical applications. Most of the optical-resolution photoacoustic microscopy (PAM) configurations are based on 3D scanning motor to scan the transducer together with the imaged sample or utilizing galvanometer scanning mirrors. In this paper, a novel low-cost linear laser scanning (LLS) system for photoacoustic microscopy is presented. The linear laser scanning system performs the two-dimensional scanning of the beam across a focusing lens. The prototype built for LLS system features a precision machined aluminum base with mounting platforms for optical components to insure proper alignment through the imaging process. The LLS relies on two stepper motors (1.8-degree, 200 steps per revolution) separately actuated, to move two 45-degree mirror mounts set onto high precision linear slides (44.5 mm of travel) which allow the laser beam to be moved across a tissue sample in a 2D axis. The threaded rod which is used to connect the linear slides and mirror assemblies has a travel distance per turn of 1/20.8” meaning for every inch of travel the rod must turn 20.8 times. The mechanical resolution of the system was estimated to be 6 μm. In this system, a pulsed laser diode with a wavelength of peak radiant intensity of 905 nm and output peak power of 220 W with 50 ns pulse width and repetition rate of 10 kHz was utilized as a light source.

Photoacoustic (PA) imaging is a hybrid imaging technology which combines the best of optical (contrast) and ultrasound (resolution) imaging. PA imaging uses intrinsic contrast agents in the body like blood (hemoglobin), melanin, etc. But the contrast from the intrinsic contrast agents might not be sufficient for different applications. So, external contrast agents are needed for improving the contrast of PA images. Organic dyes, inorganic dyes, and nanomaterials can be used as photoacoustic contrast agents. The major issue with using external contrast agent is that they often need FDA approval to be used for in-vivo studies. Availability of FDA approved contrast agents for PA imaging is very limited. In this work, we present the feasibility of using food and food-based dyes as photoacoustic contrast agents. We use commonly used foods like coffee, tea, chocolate and food colorants as contrast agents. We use alpinion’s E-CUBE dual mode ultrasound and photoacoustic imaging system with mobile Nd:YAG laser pumped by OPO laser to demonstrate the efficiency of the contrast agents. The contrast agents were compared with methylene blue. Out of the many different agents we tested, coffee, chocolate and few other dyes proved as efficient photoacoustic contrast agents. We performed photoacoustic spectroscopy to identify at which wavelength the dye performed best. We also tested the contrast agents for imaging sentinel lymph nodes in rats and the results are very similar to methylene blue. This will enable the transition of photoacoustic imaging to the clinics more easily.

Photoacoustic (PA) tomography can be useful to measure the hemoglobin concentration in blood vessels and tissues, and provide important information to determine the course of treatment for hypoxic malignant cancer which possesses the resistance to treatment. To image the hemoglobin concentration and oxygen saturation, the reconstruction of absorption coefficients at multiple wavelengths is required to correct the penetration depths of the multiple wavelengths of light. In this study, the authors tried the image reconstruction of the absorption coefficient for various concentration of photon absorber. Additionally, the effects of the regularizations minimizing 1-norm and p-norm (0<p<1) were compared to examine the improvement of the spatial resolution limited by the characteristics and configurations of the ultrasound transducer and illumination. The PA pressure waves were measured by the probe consisted of the optical fiber and the focused ultrasound transducer. The absorption coefficients of the tubes containing black ink in the phantom made of aqueous solution of Intralipid were from 0.2 to 1.0 mm^-1 at a wavelength of 755 nm, corresponding to the range from normoxic to hypoxic condition of the whole blood. Although the tubes with various absorption coefficients were reconstructed with 1-norm regularization, the tube was reconstructed broadly. On the other hand, the regularization technique minimizing p-norm (0<p<1) localized the tube better than 1-norm regularization. The regularization minimizing p-norm (0<p<1) will improve the spatial resolution of the PA tomography.

We present a dual-wavelength fast mechanical scanning optical-resolution photoacoustic microscopy system (OR-PAM) in this paper. Conventional mechanical scanning OR-PAM system takes ~ 30 min for one frame of single wavelength imaging. The acquisition of two wavelengths will double the total imaging time. By using a fast-linear stage, a high signal-to-noise (SNR) optical and acoustic combiner and 2 lasers at different wavelengths, we achieved a B-scan rate of 12 Hz and reduced the acquisition time to ~1.5 min for dual-wavelength imaging. This system can be used in large field optical resolution imaging. Contrast-free vascular and functional imaging can be achieved using this system. Both ex vivo and in vivo imaging results are demonstrated.

A limited number of patients with ovarian cancer are diagnosed at its early stages. The reason is that current medical imaging techniques still present low sensitivity in the detection of this disease. Molecular imaging, such as photoacoustic (PA), can play an important role to improve ovarian cancer detection. For gynecologic applications, a transvaginal (endo-cavity) ultrasound probe is needed. In this study, the time reversal (TR) and delay and sum (DAS) image reconstruction methods were evaluated for PA signals acquired using a transvaginal transducer. PA signals were recorded from two different phantoms. Phantom A consisted of six hair strands immersed in a water tank positioned at different depths. Image reconstruction using TR after interpolating the acquired scan-lines from 128 to 256 (TR256) and to 512 lines (TR512) were also evaluated. Furthermore, simulations using the same configuration of phantom A were conducted and a similarity index (SSIM) was calculated for each reconstruction method. Phantom B was manufactured using the copolymer styrene-ethylene/butylene-styrene in mineral oil. The phantom had a cubic shape with a cavity to place the transducer and three irregular inclusions were added to the phantom. These inclusions were filled with black pigment to improve light absorption. The SSIM values obtained from simulations for DAS, TR, TR256 and TR512 were 0.52, 0.60, 0.68 and 0.72, respectively. Signal to noise ratio (SNR) and contrast to noise ratio (CNR), for phantom B images, were calculated to compare both methods. The SNR values for DAS, TR, TR256 and TR512 were 19.5 dB, 21.1 dB, 22.0 dB and 22.5 dB and the CNR values were 13.4 dB, 14.5 dB, 15.5 dB and 16.0 dB, respectively. Results showed better performance when the TR method was used, including SNR, CNR, and lateral and axial resolution. However, DAS was less time consuming compared to TR, maintaining a reasonable image quality.

Quantitative photoacoustic imaging (QPAI) is a hybrid imaging technique aimed at reconstructing optical parameters from photoacoustic signals detected around the biological tissues. The recovery of optical parameters is a nonlinear, ill-posed inverse problem which is usually solved by iterative optimization methods based on the error minimization strategy. Most of the iterative algorithms are empirical and computationally expensive, leading to inadequate performance in practical application. In this work, we propose a deep learning-based QPAI approach to efficiently recover the optical absorption coefficient of biological tissues from the reconstructed result of initial pressure. The method involves a U-Net architecture based on the fully convolutional neural network. The Monte Carlo simulation with the wide-field illumination has been used to generate simulation data for the network training. The feasibility of the proposed method was demonstrated through numerical simulations, and its applicability to quantitatively reconstruct the distribution of optical absorption in the practical situation is further verified in phantom experiments. High image performance of this method in accuracy, efficiency and fidelity from both simulated and experimental results, suggests the enormous potential in biomedical applications in the future.

In the near-infrared optical window (NIR, 650 to 1000 nm), biological tissue generates a low intrinsic photoacoustic (PA) signal. This window can be utilized to reduce the background noise, and thus enhance visualization of contrast agents. Exogenous contrast agents have been investigated and co-applied with photoacoustic imaging such as organic materials (indocyanine green, astaxanthin, Prussian blue) and inorganic materials (gold nanorods, gold nanostars). However, these available contrast agents are usually associated with low thermal stability or large size, leading to unreliable photoacoustic signal. Conventional gold nanoparticles have an absorption peak of 520 nm, which corresponds to the absorption spectrum of hemoglobin. This study investigated the synthesis of novel ultrapure, biocompatible, and photostable chain-like gold nanoparticles (CGNPs), which shift the peak absorbance of GNPs from visible window (520 nm) to NIR window, while keeping the GNPs at a smaller size. These GNPs were fabricated by a femtosecond laser and were combined together with two organic polymers. The surface was then modified with PEG and conjugated with RGD ligands. The capacity of CGNPs for photoacoustic microscopy (PAM) and OCT were examined on 4 white New Zealand rabbits using a choroidal neovascularization (CNV) model. CNV was created by laser-induced retinal vein occlusion. Then, all animals were administered GNPs at concentration of 5 mg/mL. PAM and OCT were obtained before and after the injection at various time points, including 2 h, 4 h, 8 h, 24 h, days 2, 3, 5, 7, 9. 11, and 14. In vivo PAM and OCT imaging demonstrated that CNV was observed after the injection of CGNPs. In comparison with the signal before the injection, CGNPs produces 18- fold greater photoacoustic contrast and exhibits a 176 % increase in OCT signal, given the reduced background signal in the NIR window. The newly fabricated CGNPs have the capacity to improve visualization in living animals, while minimizing signal from hemoglobin and other endogenous contrast agents.

The spatial resolution in photoacoustic imaging is essentially limited by acoustic attenuation, which can be numerically compensated only up to a theoretical limit. The physical background for this “ill-posedness” is the second law of thermodynamics: the loss of information is equal to the entropy production, which is the energy decay of the attenuated wave divided by the temperature. As acoustic attenuation increases with higher frequencies, a cut-off frequency can be determined, where the information content for that frequency gets so low that it cannot be distinguished from equilibrium distribution within a certain statistical significance. This cut-off frequency can be determined also by setting the amplitude of the attenuated signal in frequency domain equal to the noise-level. Compensating for acoustic attenuation requires to solve an ill-posed inverse problem, where an adequate regularization parameter is the cut-off frequency, when the acoustic wave amplitude is damped just below the noise level. If additional information, such as positivity or sparsity is used, this theoretical resolution limit can be overcome. This is experimentally demonstrated for the propagation of planar acoustic waves in fat tissue, which are induced by short laser pulses and measured by piezoelectric transducers. For fatty porcine tissue the frequency dependent acoustic attenuation was measured. This was used to invert the problem and by using additional information, in the form of positivity and sparsity (Douglas-Rachford splitting algorithm) the resolution could be enhanced significantly compared to the limit given by the cut-off frequency from attenuation through 20 mm of porcine fat tissue.

Delay-and-sum (DAS) beamforming is the most commonly used algorithm to form photoacoustic (PA) and ultrasound (US) images because of its simple implementation. However, it has several drawbacks such as low image resolution and contrast. To deal with this problem, delay-multiply-and-sum (DMAS) beamforming algorithm was developed a few years ago. It is known that DMAS can improve the image quality by providing higher contrast and narrower main lobe compared to DAS, but its calculation speed is too slow to be implemented for clinical applications. Herein, we introduce an improved DMAS in terms of both imaging speed and quality, and we demonstrated real-time clinical PA imaging. The proposed DMAS provided better lateral resolution and signal-to-noise ratio (SNR) than the original DMAS through a modified coherence factor. Then we accelerated its computation speed by optimizing the algorithm and parallelizing the process using a graphics processing unit (GPU). We quantitatively compared the processing time and the image quality of the proposed algorithm with the conventional algorithms. As the result, it was observed that our proposed algorithm showed better spatial resolution and SNR while achieving real-time imaging framerate. Due to the improvement, the proposed algorithm was successfully implemented on a programmable clinical PA/US imaging system and showed clearer real-time PA images than the conventional DAS images.

We have developed surface-crosslinked polymer nanodroplets having capabilities of therapeutic enhancement for high intensity focused ultrasound (HIFU) treatment and image guidance by photoacoustic (PA) and ultrasound (US) imaging. Six-arm-branched poly(ethylene glycol) (PEG) is crosslinked with Pluronic F127 mixed with naphthalocyanine (Nc), and sonicated with liquid perfluorohexane (PFH) to form nanoparticles encapsulating Nc and PFH (Nc/PFH@PCPN). The Nc dye provides high optical absorption at 850 nm wavelength for PA imaging, and improves the stability of the nanodroplets due to its hydrophobicity. PFH evaporates upon reception of HIFU waves to form microbubbles that enhances both visual contrast for US imaging and cavitation effect for HIFU treatment. We have shown from experiments that the nanodroplets significantly increase the signal contrast of the PA/US images, and at the same time they enhance the HIFU cavitation effect and induce necrosis and apoptosis of tumor cells. Thus, we believe Nc/PFH@PCPN can be a biocompatible and stable multi-functional agent for HIFU therapy guided by PA/US images.

Photoacoustic imaging (PAI) is a rapidly growing imaging modality which offers the advantages of good optical contrast and high ultrasound resolution. Although PAI provides imaging depth beyond the optical diffusion limit, penetration depth in biological samples is limited due to absorption and scattering of light in tissues. Improvement in imaging depth has been achieved by irradiating the sample with laser pulses of near infrared-I (NIR-I) region (700 nm–900 nm) due to decreased scattering of light in tissues within this optical window. Recently, further improvement in imaging depth has been reported by irradiating the sample in near infrared-II (NIR-II) region (900 nm-1700 nm). In this work, imaging depth in breast tissues when samples were irradiated by wavelengths in different optical windows has been compared. Initially, Monte Carlo simulation for light propagation in biological tissues was performed to compute imaging depth for excitation wavelengths of 532 nm, 800 nm, and 1064 nm. Further, photoacoustic tomography at 532 nm, 740 nm, and 1064 nm and acoustic resolution photoacoustic microscopy at 570 nm and 1064 nm were conducted to validate the results. We have shown that maximum imaging depth is achieved by NIR-I (740 nm/ 800 nm) when surface energy for all wavelengths is kept constant. However, when the energy density is proportional to maximum permissible exposure (MPE) at corresponding wavelength, maximum imaging depth is achieved by 1064 nm (NIR-II window). Therefore, we conclude that increased MPE in NIR-II window is responsible for the improved penetration depth in breast tissue in this region.

We present a new laboratory setup for photoacoustic, transmission ultrasound and reflection ultrasound tomography. The system is based on a pair of independently rotating hemisphere segments on which are mounted acoustic transmitters and detectors. The interchangeability of the elements, as well as the ability to fully customise the acquisition protocol, allows for a considerable amount of flexibility in testing out different imaging approaches in both 2D and 3D. The current focus is the development of laser-induced ultrasound (LIUS) transmitters for use in tomographic imaging. We propose a transmitter design tuned for speed-of-sound mapping with a 1MHz centred response.

Photoacoustic remote sensing (PARS) is a non-contact imaging modality that is based on the optical absorption contrast of endogenous molecules. PARS has shown promise in vascular imaging, blood oxygenation estimation, and virtual biopsy without the need for exogenous labels. Here we demonstrate simultaneous imaging of cell nuclei and blood using UV and visible excitation wavelengths. This is important for decoupling blood signals from cell nuclei signals in removed tissue and resection beds. A 532nm fiber laser is split with one light path frequency doubled using a CLBO crystal to 266nm. These two wavelength lasers are co-aligned and co-focused with a 1310nm interrogation beam and using a reflective objective to image microvasculature and cell nuclei with intrinsic optical absorptions at 532nm and 266nm, respectively. These images are taken serially and co-registered with lateral resolutions of 1.2μm and 0.44μm respectively. Co-alignment using multiple wavelengths is demonstrated using carbon fiber phantoms. We imaged both paraffin embedded tissue and in vivo mouse ear. Cell nuclei in sectioned tissues were clearly visualized with a SNR of 42dB while hemoglobin demonstrated an SNR of 39dB. In vivo cell nuclei and vasculature images produced an SNR up to 40dB and 35dB, respectively.

One of the challenges of quantitative Photoacoustic (PA) imaging is unmixing the optical absorption (μa) of the tissue from system response (C) and Grüneisen parameter (Γ). In this study, we have calculated the absorption coefficient and functional parameters, i.e. total hemoglobin (tHb) and oxygen saturation (sO2) of 5 blood tubes with sO2 values ranging from 24.9% to 97.6% at different depths in intralipid solution. Beer’s law is used to calculate the optical fluence in the target area. Initial values for μ_a and C×Γ are found by fitting a line to the log of PA beam data. These initial values are iteratively updated using a conjugate gradient method. This process is repeated for all 11 wavelengths. The absorption coefficient spectrum follows the molar extinction coefficient spectrum of deoxy hemoglobin for lower sO2 percentages, and it becomes closer to the spectrum of oxy hemoglobin when the sO2 percentage increases. The calculated absorption coefficients at 11 wavelengths are used to estimate the absolute value of the tHb and sO2 of each blood sample at different depths. The mean error of the estimated tHb values for blood tubes at all depths with respect to the real values are less than 13%. Moreover, the largest sO2 estimation error is 7.5% for the blood sample with sO2 of 24.9%. Our quantitative PA method performed well for the data collected from blood samples. We are investigating this method on our clinical data.

PhotoSound Technologies specializes in the development of electronics solutions for massive parallel data acquisition applicable to the fields of photoacoustics (PA), X-ray acoustics, including 3D dosimetry, and ultrasound. PhotoSound’s Legion ADC256 R1.1, released in 2018, is a 256-channel 12-bit ADC with a sampling rate of 40 MHz. The ADC256’s average data bandwidth is limited by its USB3 PC interface, which has a data rate up to 3 Gbps per board. Multiple ADC256 boards can operate fully in parallel. On software level configurations, multiple ADC256 boards are represented as a single ADC board with increased number of channels. The incoming ultrasound (US) upgrades and modifications of ADC256 will enable combination and alternation of US and PA modes using the same probe. PhotoSound MoleculUS is a medical-grade Telemed US system combined with a PA-optimized ADC. MoleculUS utilizes clinical US probes to produce US images which can be interleaved with PA imaging by enabling optical fiber illumination. The other ADC256 modification, advanced PAUS oriented for research, will have PCIe PC interface for raw PA and US data and arbitrary software control over beamformer profiles, limited by high-voltage power only. The data in ultrasound and photoacoustics modes is user accessible in raw format and can be delivered to CUDA GPU using MATLAB parallel computing (CUDA) toolbox or other tools. Multiple PAUS boards can work in parallel in both PA and US modes.

Extrinsic contrast agents with excellent light absorption properties in the second near-infrared (NIR-II, 1000-1350 nm) region can be a key to enhance the contrast of photoacoustic imaging (PAI) in deep tissues. Here, we demonstrated a photoacoustic (PA) contrast agent at 1064 nm optical wavelength for deep-tissue in vivo PAI. We successfully synthesized nickel(II) dithiolene-based polymeric nanoparticles (NNP) that have strong absorption at NIR-II light and generate improved PA signal with a 1064 nm pulse laser. To confirm the feasibility of the NNP, we have conducted both in vitro and in vivo PA experiments and acquired highly contrast-enhanced PA images. We successfully obtained contrast-enhanced PA images of a tube filled with NNP deeply located below several layers of chicken tissue. The maximum PAI penetration depth was about 5 cm. Next, we performed bladder, sentinel lymph node and gastrointestinal tract, which are clinically important, PAI in rats to confirm that NNP could be utilized as a PA agent in deep tissues in vivo. NNP was injected into each of the three cases, and we confirmed that PA contrast was significantly increased after the injections. These results demonstrate that the enhanced PA signals generated by irradiating 1064 nm laser to NNP in deep-tissue has sufficient contrast for PAI. Based on the excellent absorbability of NNP at 1064 nm and the translability of clinical PAI systems, this study is expected to provide a great opportunity for a variety of studies on non-invasive deep tissue in vivo.

In this paper, we numerically simulate the photoacoustic signal waveforms based on the Huygens-Fresnel principle. In this model, laser absorption medium which is the source of generating photoacoustic signal is divided into microspheres. A N-shaped carrier ultrasonic spherical wave is generated by each microsphere due to the absorption of short laser pulse and propagates outwards from the sphere center. The N-shaped waves reach the detection point through the direct propagation and the reflection from the medium interface. The photoacoustic signal generated by the overall absorption medium detected in the observation point is calculated as the summation of all these individual N-shaped photoacoustic waves including the original and reflected waves by considering the temporal delay and attenuation induced by the propagation distance. The envelope of the resulted summation is the transducer-detectable photoacoustic signal waveform. The photoacoustic signal profiles and spectra under different media interface boundary conditions and propagation distances are studied. The effect of optical absorption to photoacoustic signal bandwidth is studied as well. This numerical investigation demonstrates the formation of the detected photoacoustic signals and improves the understanding of the mechanism of the photoacoustic signal generation.

X-ray induced acoustic (XA) imaging is novel biomedical imaging technology that can potentially provide an X-ray absorption contrast with high resolution and relatively low dose. In this study, we improved the XA computed tomography (XACT) system using a pulsed X-ray source and a 96-element arc shaped transducer array to provide volumetric images in phantoms. The volumetric XACT images were reconstructed by custom developed software based on a back projection algorithm accelerated by a graphics processor unit (GPU). Using the developed system and software, we successfully acquired the volumetric XACT images of lead targets.

Photoacoustic microscopy (PAM), an emerging biomedical imaging technology, has demonstrated the label-free imaging capability to visualize biomolecules with the aid of superior optical contrast in them. Especially employing ultraviolet (UV) laser at the wavelength of 266 nm, we have developed an UV-PAM. Unlike conventional histology methods such as frozen and formalin-fixed paraffin-embedded (FFPE) sections, UV-PAM can illustrate cell nuclei by utilizing superior light absorption of DNA/RNA without time consuming procedures. In-vitro experiments were conducted to evaluate the spatial resolutions of the developed system. The measured lateral resolution was 1.3 μm, and axial resolution was 62.2 μm. Then we performed ex-vivo experiments using frozen sections of mouse brain to demonstrate the imaging capability of UVPAM as a rapid histology tool. Oxidative stress induced by kainic acid (KA) was monitored using UV-PAM, which is considered as a significant cause for epileptic neuronal brain damage. We have shown the apoptotic feature resulted from the KA-induced hippocampal cell death in a mouse brain section. In contrast to the brain section of the control mouse model, the substantial nuclear marginalization of hippocampal cell death was illustrated in the vulnerable neurons of the CA1 and CA3 regions on the KA-treated mouse with PA imaging. In addition, the PA histologic results were evidenced by the corresponding HE stained images on both the control and the KA-treated mouse, showing similar hippocampal cell death. The PA histologic results could also provide its potential application for use in the monitoring of the morphological changes observed in astrocytes including hypotrophy, hyperplasia, and neoplasia. Further, it might be a beneficial histologic tool for treatment monitoring of neurodegenerative diseases such as acute traumatic brain injury and neuroprotective effects of treatments on the diseases.

Arthritis, which is associated with local articular rigidity, swelling and pain as well as systemic development of fever and a sense of fatigue, is a disorder leading quickly to deterioration of quality of life. Approximately 300,000 rheumatoid arthritis patients reside in Japan alone1: 1% of the world’s population has the disorder worldwide. Although X-ray CT, MRI, and ultrasonic Doppler method are used for examination and diagnosis, various difficulties persist such as radiation exposure, administration of contrast agents, and difficulty in earlier diagnosis and quantitative evaluation. To resolve these difficulties, we developed a handheld photoacoustic imaging system. This study investigated the feasibility of evaluating the degree of inflammation using photoacoustic imaging with multiple wavelengths using in vivo measurements of model rats. Changes in signal intensity depending on the presence or absence of the disorder were examined. Results confirmed that the signal intensity can be intensified at diseased joints. Then, the changes with different time elapsed from drug administration were examined using rats. Results clarified that the degree of inflammation can be evaluated by photoacoustic spectral shapes, which change along with the progress of the inflammation. These analyses verified the usefulness of photoacoustic imaging for diagnosing and evaluating arthritis.

Photoacoustic imaging is a hybrid modality with advantages of both the ultrasound and optical imaging. This technique with optical spectroscopic contrast and acoustic resolution holds promise in wide range of clinical and preclinical applications. Since photoacoustic imaging also involves ultrasound detection, it is straight forward to implement pointof- care dual mode systems with capability of structural and functional imaging. High signal generated from skin-melanin layer is an important problem in any handheld reflection-mode photoacoustic probes. This problem caused by high light fluence just beneath the ultrasound probe results in reduction of vascular contrast and also causes difficulty in image interpretation. In this work, using our LED-based photoacoustic and ultrasound imaging system (AcousticX), we demonstrate the potential of using ultrasound acquisitions dynamically to suppress high photoacoustic signal from skin surface and qualitatively improve the image contrast and quality. In AcousticX, photoacoustic and conventional pulseecho acquisitions are performed in an intermittent manner resulting in a dual-mode display frame rate of 30 Hz. We make use of the line-by-line/planewave ultrasound acquisitions to automatically delineate the skin surface in quasi realtime and suppress photoacoustic signal from the identified area, resulting in improved image interpretation and contrast without losing temporal resolution. Real-time 2D and 3D (linear scan) imaging experiments were performed on finger and foot dorsum of healthy human volunteers to validate the new skin-signal suppression technique. Results give a direct confirmation that our ultrasound-assisted skin-signal reduction feature holds strong potential in enhancing vascular contrast and visual quality in real-time 2D and 3D LED-based photoacoustic imaging.

Early detection of lipid-rich vulnerable plaque is important to prevent aortic atherosclerotic plaque rupture, which can cause cerebral and cardiac infarction. Nevertheless, evaluating such plaques using conventional modalities is difficult. Photoacoustic imaging can reveal and clarify tissue characteristics. Some evaluations of lipid-rich plaque have used intravascular photoacoustic imaging systems. To reduce invasiveness and to ease handling, we developed a handheld photoacoustic imaging system. The possibility of detecting lipid-rich plaques was evaluated in phantom experiments. A plaque was modeled using a mimic plaque (a mix of oleic acid cholesterol and linoleic acid cholesterol) into an ovine aortic wall and silicone tube. The ovine aorta and the silicone tube were then filled with ovine blood. These objectives were fixed in pellucid deaerated water or in deaerated water including intralipid suspension. Laser light was guided to the model phantom surface by an optical fiber bundle close to the linear ultrasound probe. The photoacoustic signal distribution was measured as photoacoustic images. The photoacoustic images, taken using wavelengths at which light absorbance of lipid is high, show strong photoacoustic signals from the fat boundary. At 1150–1300 nm wavelengths, similarity between photoacoustic spectra and the absorption spectrum of lipid were evaluated by calculating the correlation coefficient in photoacoustic images. Results show high correlation (more than 0.9) at the boundary between the fat and the vessel wall. These analyses demonstrate detection of lipid-rich plaque even if a highly absorbing object, e.g. blood, is in proximity to the lipid.

Vision-based robotic control (also known as visual servoing) is promising for surgical tool tip tracking and automated visualization of photoacoustic targets during interventional procedures. However, reliable segmenta- tion in photoacoustic-based visual servoing was previously achieved with a light source that exceeds laser safety limits, due to the limited availability of laser safety limits for only skin or eyes and the associated difficulty with visualizing signals at the low laser energies within these safety limits for millimeter-sized light sources. Short-lag spatial coherence (SLSC) imaging is an advanced beamforming method that has shown offline promise toward enhancing the visualization of signals acquired with low laser energies. This paper summarizes the first known GPU-based, real-time implementation of SLSC for photoacoustic imaging and displays example images showing the application of this real-time algorithm to improve signal visualization and segmentation for visual servoing tasks. Results with ex vivo bovine tissue demonstrate that real-time SLSC imaging recovers signals obtained with low laser energies (i.e., ≤ 268 μJ) with mean ± standard deviation signal-to-noise ratios (SNRs) of 11.2 ± 2.4 (compared to 3.5 ± 0.8 with conventional delay-and-sum beamforming). Therefore, real-time SLSC imaging enables low laser energies for visual servoing within existing safety limits, which is promising for multiple surgical interventions.

Infant brain imaging is highly challenging but necessary for diagnosing various prevalent disorders including vascular malformations, encephalitis, and abusive head trauma. Conventional brain imaging technologies such as MRI, CT, and PET are not suitable for repeated use on neonates due to the use of ionizing radiation (CT and PET), need for patient transport, uncomfortable environment, high cost, and bulky equipment. A wearable photoacoustic imaging (PAI) hat can be an ideal candidate for this application. However, its practical realization suffers from many system design problems such as complex assembly, unviability of full-hat rotation around the neonatal head, ultrasound coupling, and requirements of <3,000 ultrasound data acquisition channels to cover the whole brain. Here, we present a modular photoacoustic imaging (PAI) hat solution that uses an innovative modular design approach, making it realizable by assembling individual working units while minimizing the challenges of back-end electronics. The modular photoacoustic hat consists of multiple PAI disc modules of 2 inches in diameter that conform to the shape of the local head surface and assembled on a hat to cover the whole neonatal brain. Each PAI disc is integrated with optical fibers for light excitation of brain tissue. For photoacoustic detection, the discs are either densely packed with ultrasound elements to eliminate the need for rotation or can have fewer ultrasound elements (usually in trapezoidal shape) on the rotating disc to overcome large number of data acquisition channels. In this article, we have demonstrated the design, integration and initial results of the proposed wearable PAI-hat.

Photoacoustic computed tomography (PACT) has been widely explored for studying human diseases as well as response to therapies. Most PACT systems employ a large footprint, bulky, and high-cost lasers. Light emitting diodes (LEDs) based B-mode photoacoustic imaging systems have emerged as a low cost and compact alternative, offering a unique opportunity to expedite the widespread adoption of photoacoustic imaging in clinical and resource-poor settings. The high pulse repetition rate of LEDs facilitates signal-to-noise ratio improvements through averaging in spite of lower pulse energy. Here, we present the development of first low-cost LED-based PACT system that uses multiple LED arrays and a linear ultrasound transducer to generate three-dimensional structural, functional and molecular images of the object. Similar to OPO based lasers, our LED-PACT system allows for the multi-wavelength photoacoustic imaging vital for mapping functional and molecular information. Our experiments demonstrate that this study will enable clinical and pre-clinical applications such as imaging human arthritis and whole body mouse imaging.

Photoacoustic imaging (PAI) maps functional and molecular optical contrasts of tissue at ultrasonic spatial resolution and imaging depth. To generate detectable PA signals from deeper regions, expensive, bulky and high-energy class IV lasers are conventionally employed. Light emitting diodes (LED) have recently emerged as an alternative excitation source for PA imaging offering many advantages including portability, affordability, speed, multi-wavelength excitation, and eye/skin safety. Although the output energy of LED’s is far lower than lasers, high pulse repetition rate offers possibility to average more frames and thus improve the SNR. In this work, we performed controlled experiments on tissue-mimicking phantoms to compare the PAI performance of laser and LED light sources comprehensively. Our studies demonstrate that the LED based PA systems are ideal for low resource and point-of-care settings where the required depth of penetration is within 2-3 cms., whereas a high-energy laser is found to be more effective for higher penetration depths (<3 cm). In addition, it is clear from our results that LED-based PA imaging offers higher frame rate with similar spatial resolution and decent signal to noise ratio, which is comparable to conventional laser-based photoacoustic imaging.

Conventional photoacoustic imaging (PAI) systems use bulky and high-cost laser sources to derive functional and molecular information of the tissue. Recently, light emitting diodes (LED) have emerged as an affordable and compact alternative illumination source for PAI. Despite their low energies, LEDs have provided sufficient photoacoustic contrast for in vivo imaging of mice and for certain clinical applications. This is largely due to PA signal averaging allowed by higher repetition rates of the LEDs without compromising on video frame rate photoacoustic imaging. In this work, using multiple in vivo and phantom experiments, we demonstrate the potential of LED-based photoacoustic and ultrasound imaging (2-D and 3-D) for real-time functional, molecular and structural characterization of tissue. This includes photoacoustic derived functional oxygen saturation information and mapping molecules such as melanin, methylene blue and indocyanine green, and ultrasound derived anatomical information of tissue. These results demonstrate that LED-based PA and US imaging hold strong potential for accelerating several pre-clinical and clinical applications, especially in resource-poor settings.

Reflectance confocal microscopy (RCM) has become a standard method for skin diagnostics. It is based on tightly focusing a continuous laser beam with an objective into the skin and collecting the back scattered light through a pinhole for generating images of selected planes to a depth of several 100 micrometers. For enhancing its diagnostic capability, the RCM can be combined with optical resolution photoacoustic microscopy (OR-PAM), providing strong optical absorption contrast for melanocytic lesions. We have developed a compact add-on to common optical objectives that is able to detect the photoacoustically generated transients with high bandwidth, using four piezoelectric elements made of poly-vinylidene fluoride (PVDF). The elements are arranged on four quadrants of a conical surface around the objective, taking advantage of the focusing effect of the slightly curved surface. For taking an image, the pulsed and continuous excitation beams are simultaneously scanned over the sample using a pair of galvanometric mirrors. Photoacoustic images of the selected plane are then generated for each separate sensing element and are subsequently added in order to achieve an enhanced signal-to-noise ratio. Simultaneously recorded back scattered light provides the input for the RCM mode. We present a characterization of the sensors and provide experimental results on phantoms.

We have developed a photoacoustics-based imaging system that combines optical contrasts with acoustic detection, to obtain a snapshot of the angiographic features in human breast. The system uses near-infrared (NIR) light at 1064 nm wavelength for excitation and hemoglobin in blood as endogenous contrast agent. The light source is a 10 ns Nd:YAG laser with 10-Hz pulse repetition rate. Tumor-angiogenesis, the increase in neovasculature in rapidly growing tumors, is a known biomarker for malignancy. By mapping total hemoglobin levels, we are able to pinpoint the tumor location based on vessel density. For acoustic detection, two 128-element linear-array transducers with 2.25 MHz central frequency are employed. Photoacoustic data is acquired by scanning the breast mildly compressed in the craniocaudal plane, similar to a mammogram, with a scan time of less than 1 minute. The system simultaneously acquires ultrasound (US) data, which can be correlated easily with the photoacoustic data obtained as well as clinical ultrasound images. The photoacoustic images can also be correlated with maximum intensity projection (MIP) subtraction images of contrast MRI (magnetic resonance imaging) 6 minutes post-injection of Gadolinium, and the same vessels could be identified. With our dual transducer geometry, we are able to visualize through 7 cm of breast tissue, a first in this field. The resolution was measured to be 0.97 mm in lateral and 1.05 mm in elevational directions. Our system offers high spatial resolution, fast imaging capability, and convenient correlation with all existing imaging modalities, along with better sensitivity towards dense breast tissue.

Radiofrequency ablation (RFA) procedures for liver cancer treatment are hindered by high tumor recurrence. This is thought to be due to the intrinsic limitation of the heating mechanism and insufficient real-time feedback from imaging modalities. Most RFA procedures are performed under ultrasound (US) imaging and there are limitations in accurate device guidance and ablation monitoring. We propose photoacoustic (PA) imaging as a potential add-on to US imaging to address these limitations. Specifically, we present two interstitial PA imaging methods. Firstly, an annular fiber probe that can encapsulate an RFA device in its lumen. This device enables RFA device guidance, visualization of major blood vessels and targeting tumor tissue. Secondly, we used a cylindrical diffuser-based interstitial illumination to differentiate coagulated and native tissue. We present our results on RFA device guidance and ablation visualization using these approaches. The contrast provided by PA imaging for RFA needle and multiple electrodes is compared against that of US images. The difference between coagulated and native ex vivo liver tissue using PA imaging is studied. Finally, we propose a protocol to incorporate the minimally invasive PA imaging for the clinical RFA procedures. We would like to conclude with a note on how the proposed approach can potentially improve the outcome of RFA procedures.