Validating processing algorithms for photoacoustic images is complex due to a gap between simulated and experimental data. To address this challenge, we present a multi-device dataset of well-characterised phantoms and investigate the simulation gap using a supervised calibration of the forward model. We use N=15 phantoms for calibration and systematically compare simulated and experimental data from the remaining N=15 phantoms. Our results highlight the importance of the device geometry, impulse response, and noise for accurate simulation. By reducing the simulation gap and providing an open dataset, our work will contribute to advancing data-driven photoacoustic image processing techniques.
Optoacoustic tomography is typically implemented with bulky solid-state lasers delivering per-pulse energies in the millijoule range. Light emitting diodes (LEDs) represent a cost-effective and portable alternative for signal excitation further offering excellent pulse-to-pulse stability. Herein, we describe a full-view LED-based optoacoustic tomography (FLOAT) system for deep-tissue in vivo imaging. A custom-made electronic unit driving a stacked array of LEDs attains stable light pulses with total per-pulse energy of 0.48 mJ and 100 ns pulse width. The LED array was arranged on a circular configuration and integrated in a full-ring ultrasound array enabling full-view tomographic imaging performance in cross-sectional (2D) geometry. As a proof of concept, we scanned the medial phalanx of the index finger without extrinsic administration of a contrast agent. We anticipate that this compact, affordable, and versatile illumination technology will facilitate dissemination of the optoacoustic technology in resource-limited settings.
KEYWORDS: Animals, Optoacoustics, Head, Spherical lenses, Tomography, Light sources and illumination, Imaging systems, Data acquisition, Whole body imaging, In vivo imaging
Small animal preclinical research is indispensable to study human disease progression and to monitor potential treatment therapies. Optoacoustic tomography has been recognized as a powerful imaging modality for preclinical whole-body imaging of rodents. In particular, spiral volumetric optoacoustic tomography (SVOT) capitalizes on the large angular coverage of a spherical transducer array to provide otherwise-unattainable optoacoustic images of mice. However, only thoracic and/or abdominal regions of the animal could be imaged with this approach. Efficient whole-body coverage indeed demands continuous acoustic coupling between the animal and the detector surface. In this work, we implement panoramic (3600) head-to-tail imaging of mice with SVOT combined with multi-beam illumination. For this, a dedicated animal holder enables uninterrupted acoustic coupling for whole-body scans. Proper coverage of cranial regions in addition to thoracic and abdominal regions is then feasible in a single set up. Self-gated motion rejection and dual speed-of-sound correction algorithms were employed to optimize the image fidelity. The developed system is highly suitable for label-free imaging of hemodynamics across individual organs, total body accumulation and clearance dynamics of molecular agents and drugs, as well as for monitoring responses to stimuli with unparalleled contrast and spatio-temporal
resolution.
The feasibility of real-time tracking of microparticles intravenously injected into living organisms can significantly facilitate the development of new biomedical applications, including blood flow characterization, drug delivery, and many others. However, existing imaging modalities generally lack the sensitivity to detect the weak signals generated by individual particles flowing through vascular networks deep within biological tissues. Also, the temporal resolution is usually insufficient to track the particles in an entire three-dimensional region. Herein, we capitalize on the unique advantages of a state-of-the-art high-frame-rate optoacoustic tomographic imaging system to visualize and track monodisperse core-shell microparticles with a diameter of ~4 μm in the mouse brain vasculature. The feasibility of localizing individual solid particles smaller than red blood cells opens new opportunities for mapping the blood flow velocity, enhancing the resolution and visibility of optoacoustic images, and developing new biosensing assays.
Accurate visualization of biological events at scales in the order of seconds requires high frame rate acquisition of image data from living tissues. Yet, fast imaging performance commonly comes at the cost of limited field-of-view (FOV) and reduced image quality. Here we present single-sweep volumetric optoacoustic tomography (sSVOT) for whole-body imaging of mice from head to tail within 1.8 s. sSVOT capitalizes on a custom-made spherical matrix array transducer together with a multi-beam illumination approach, the latter playing a critical role in maximizing the effective FOV. We compare the performance metrics to the previously reported whole-body mouse imaging implementations.
Significance: The proposed binary tomography approach was able to recover the vasculature structures accurately, which could potentially enable the utilization of binary tomography algorithm in scenarios such as therapy monitoring and hemorrhage detection in different organs.
Aim: Photoacoustic tomography (PAT) involves reconstruction of vascular networks having direct implications in cancer research, cardiovascular studies, and neuroimaging. Various methods have been proposed for recovering vascular networks in photoacoustic imaging; however, most methods are two-step (image reconstruction and image segmentation) in nature. We propose a binary PAT approach wherein direct reconstruction of vascular network from the acquired photoacoustic sinogram data is plausible.
Approach: Binary tomography approach relies on solving a dual-optimization problem to reconstruct images with every pixel resulting in a binary outcome (i.e., either background or the absorber). Further, the binary tomography approach was compared against backprojection, Tikhonov regularization, and sparse recovery-based schemes.
Results: Numerical simulations, physical phantom experiment, and in-vivo rat brain vasculature data were used to compare the performance of different algorithms. The results indicate that the binary tomography approach improved the vasculature recovery by 10% using in-silico data with respect to the Dice similarity coefficient against the other reconstruction methods.
Conclusion: The proposed algorithm demonstrates superior vasculature recovery with limited data both visually and based on quantitative image metrics.
KEYWORDS: Image restoration, Acquisition tracking and pointing, Transducers, Photoacoustic tomography, Numerical simulations, In vivo imaging, Data acquisition, Acoustics, Photoacoustic spectroscopy
The recovery of the initial pressure rise distribution tends to be an ill-posed problem in the presence of noise and when limited independent data is available, necessitating regularization. The standard regularization schemes include Tikhonov, l1 -norm, and total-variation. These regularization schemes weigh the singular values equally irrespective of the noise level present in the data. This work introduces a fractional framework to weigh the singular values with respect to a fractional power. This fractional framework was implemented for Tikhonov, l1-norm, and total-variation regularization schemes. The fractional framework outperformed the standard regularization schemes by 54% in terms of observed contrast/signal-to-noise-ratio.
Visualizing whole-body dynamics across entire living organisms is crucial for understanding complex biology, disease progression as well as evaluating efficacy of new drugs and therapies. Existing small animal functional and molecular imaging modalities either suffer from low spatial and temporal resolution, limited penetration depth or poor contrast. In this work, we present flash scanning volumetric optoacoustic tomography (fSVOT) imaging system that enables the acquisition speeds required for visualizing fast kinetics and biodistribution of optical contrast agents across whole mice. fSVOT can render images of intricate vascular and organ anatomy with rich contrast by capitalizing on the large angular coverage of a spherical matrix array transducer rapidly scanned around the mouse. Volumetric (three-dimensional) images with 200 µm resolution can be acquired within 45 seconds, which corresponds to an imaging speed gain of an order of magnitude with respect to existing state-of-the-art modalities offering comparable resolution performance. We demonstrate volumetric tracking and quantification of gold nanorod kinetics and their differential uptake across the spleen, liver and kidneys. Overall, fSVOT offers unprecedented capabilities for multi-scale imaging of pharmacokinetics and bio-distribution of agents with high contrast, resolution and image acquisition speed.
Conventional photoacoustic tomography (PAT) systems use bulky, expensive Q-switched Nd:YAG lasers which limits its usage in clinics. The low repetition rate of these lasers makes them not suitable for real-time imaging as well. Compact pulsed laser diodes (PLD) are being currently used for PAT systems due to their high repetition rates and compact in size. These lasers can be mounted inside the PAT circular scanning geometry making the PAT system more portable. Using acoustic reflector based single-element ultrasound transducer (SUTR), the scanning radius can be reduced thereby making the PAT system more compact. In this work, we present a portable, high frame rate, more compact second generation PLD-PAT system using eight SUTRs in circular scanning geometry. This system provides high frame rate of ~1 Hz. We demonstrate the performance of this system using phantom imaging studies.
KEYWORDS: Signal to noise ratio, Data modeling, Sensors, Image resolution, Photoacoustic tomography, In vivo imaging, Acoustics, Model-based design, Tissues, Tomography
Photoacoustic tomography tends to be an ill-conditioned problem with noisy limited data requiring imposition of regularization constraints, such as standard Tikhonov (ST) or total variation (TV), to reconstruct meaningful initial pressure rise distribution from the tomographic acoustic measurements acquired at the boundary of the tissue. However, these regularization schemes do not account for nonuniform sensitivity arising due to limited detector placement at the boundary of tissue as well as other system parameters. For the first time, two regularization schemes were developed within the Tikhonov framework to address these issues in photoacoustic imaging. The model resolution, based on spatially varying regularization, and fidelity-embedded regularization, based on orthogonality between the columns of system matrix, were introduced. These were systematically evaluated with the help of numerical and in-vivo mice data. It was shown that the performance of the proposed spatially varying regularization schemes were superior (with at least 2 dB or 1.58 times improvement in the signal-to-noise ratio) compared to ST-/TV-based regularization schemes.
Several algorithms exist to solve the photoacoustic image reconstruction problem depending on the expected reconstructed image features. These reconstruction algorithms promote typically one feature, such as being smooth or sharp, in the output image. Combining these features using a guided filtering approach was attempted in this work, which requires an input and guiding image. This approach act as a postprocessing step to improve commonly used Tikhonov or total variational regularization method. The result obtained from linear backprojection was used as a guiding image to improve these results. Using both numerical and experimental phantom cases, it was shown that the proposed guided filtering approach was able to improve (as high as 11.23 dB) the signal-to-noise ratio of the reconstructed images with the added advantage being computationally efficient. This approach was compared with state-of-the-art basis pursuit deconvolution as well as standard denoising methods and shown to outperform them.
Photoacoustic tomography is an emerging imaging modality which has paved its way in preclinical and clinical trials owing to the multiple advantages it offers. A typical PAT system consists of a laser beam which homogeneously illuminates the sample giving rise to photoacoustic (PA) waves, which are collected using an ultrasound transducer (UST) rotating around the sample. Low cost, high sensitivity and easy availability have made single-element transducers (SETs) a preferred choice for acquiring these A-lines PA signal. Two methods have been reported for collection of these A-lines by SETs- (1) Stop-and-go scan and (2) Continuous scan. In stop-and-go scan, the stepper motor moves the SET to a predefined position where the SET collects multiple A-lines. Once the desired number of A-lines at that point have been collected and saved, the stepper motor moves to the next position and the process continues. A continuous scan is one in which the stepper motor rotates the SET continuously at a predefined speed. The A-lines are thus collected by a moving SET and are saved once the motor has stopped. In this work, we have compared the two types of scanning methods in terms of image quality, signal-to-noise ratio and time of scan by performing experiments on phantoms.
Light absorption by the chromophores (hemoglobin, melanin, water etc.) present in any biological tissue results in local temperature rise. This rise in temperature results in generation of pressure waves due to the thermoelastic expansion of the tissue. In a circular scanning photoacoustic computed tomography (PACT) system, these pressure waves can be detected using a single-element ultrasound transducer (SUST) (while rotating in full 360° around the sample) or using a circular array transducer. SUST takes several minutes to acquire the PA data around the sample whereas the circular array transducer takes only a fraction of seconds. Hence, for real time imaging circular array transducers are preferred. However, these circular array transducers are custom made, expensive and not easily available in the market whereas SUSTs are cheap and readily available in the market. Using SUST for PACT systems is still cost effective. In order to reduce the scanning time to few seconds instead of using single SUST (rotating 360° ), multiple SUSTs can be used at the same time to acquire the PA data. This will reduce the scanning time by two-fold in case of two SUSTs (rotating 180° ) or by four-fold and eight-fold in case of four SUSTs (rotating 90° ) and eight SUSTs (rotating 45° ) respectively. Here we show that with multiple SUSTs, similar PA images (numerical and experimental phantom data) can be obtained as that of PA images obtained using single SUST.
As limited data photoacoustic tomographic image reconstruction problem is known to be ill-posed, the iterative reconstruction methods were proven to be effective in terms of providing good quality initial pressure distribution. Often, these iterative methods require a large number of iterations to converge to a solution, in turn making the image reconstruction procedure computationally inefficient. In this work, two variants of vector polynomial extrapolation techniques were deployed to accelerate two standard iterative photoacoustic image reconstruction algorithms, including regularized steepest descent and total variation regularization methods. It is shown using numerical and experimental phantom cases that these extrapolation methods that are proposed in this work can provide significant acceleration (as high as 4.7 times) along with added advantage of improving reconstructed image quality.
Photoacoustic (PA) signals collected at the boundary of tissue are always band-limited. A deep neural network was proposed to enhance the bandwidth (BW) of the detected PA signal, thereby improving the quantitative accuracy of the reconstructed PA images. A least square-based deconvolution method that utilizes the Tikhonov regularization framework was used for comparison with the proposed network. The proposed method was evaluated using both numerical and experimental data. The results indicate that the proposed method was capable of enhancing the BW of the detected PA signal, which inturn improves the contrast recovery and quality of reconstructed PA images without adding any significant computational burden.
KEYWORDS: Photoacoustic tomography, Transducers, Tomography, Ultrasonography, Biomedical optics, Brain imaging, Reflectors, Acoustics, Data acquisition, Imaging systems, Signal to noise ratio
Photoacoustic tomography (PAT) is a non-ionizing biomedical imaging modality which finds applications in brain imaging, tumor angiogenesis, monitoring of vascularization, breast cancer imaging, monitoring of oxygen saturation levels etc. Typical PAT systems uses Q-switched Nd:YAG laser light illumination, single element large ultrasound transducer (UST) as detector. By holding the UST in horizontal plane and moving it in a circular motion around the sample in full 2π radians photoacoustic data is collected and images are reconstructed. The horizontal positioning of the UST make the scanning radius large, leading to larger water tank and also increases the load on the motor that rotates the UST. To overcome this limitation, we present a compact photoacoustic tomographic (ComPAT) system. In this ComPAT system, instead of holding the UST in horizontal plane, it is held in vertical plane and the photoacoustic waves generated at the sample are detected by the UST after it is reflected at 45° by an acoustic reflector attached to the transducer body. With this we can reduce the water tank size and load on the motor, thus overall PAT system size can be reduced. Here we show that with the ComPAT system nearly similar PA images (phantom and in vivo data) can be obtained as that of the existing PAT systems using both flat and cylindrically focused transducers.
In photoacoustic/optoacoustic tomography (PAT/OAT) for a circular scanning geometry, the axial/radial resolution is not variant spatially and also do not depend on the ultrasound transducer (UST) aperture. But the tangential resolution is affected by the size of the detector aperture and is spatially variant. To counter this problem many techniques such as attaching a negative lens to the transducer surface, or using virtual detectors were proposed. However these techniques have difficulties. Therefore, a modified delay-and-sum reconstruction algorithm was proposed which can be used with the normal UST to improve the tangential resolution. In this work, we demonstrate the improvement of tangential resolution using the modified delay-and-sum reconstruction algorithm with experimental data. We have obtained more than twofold improvement of resolution in the tangential direction using non-focused and cylindrically focused USTs in a circular scanning geometry. We also observe that shape of the target object can also be preserved which is helpful for diagnosis and treatment purposes.
In recent years, high-repetition rate pulsed laser diode (PLD) was used as an alternative to the Nd:YAG lasers for photoacoustic tomography (PAT). The use of PLD makes the overall PAT system, a low-cost, portable, and high frame rate imaging tool for preclinical applications. In this work, we will present a portable in vivo pulsed laser diode based photoacoustic tomography (PLD–PAT) system. The PLD is integrated inside a circular scanning geometry. The PLD can provide near-infrared (∼803 nm) pulses with pulse duration ∼136 ns, and pulse energy ∼1.4 mJ / pulse at 7 kHz repetition rate. The system will be demonstrated for in vivo fast imaging of small animal brain. To enhance the contrast of brain imaging, experiments will be carried out using contrast agents which have strong absorption around laser excitation wavelength. This low-cost, portable small animal brain imaging system could be very useful for brain tumor imaging and therapy.
KEYWORDS: Acquisition tracking and pointing, Reflectors, Acoustics, Transducers, Signal to noise ratio, Photoacoustic tomography, Imaging systems, Data acquisition, Sensors, Ultrasonography
A typical photoacoustic tomography (PAT) system uses a Q-switched Nd:YAG laser for irradiating the sample and a single-element ultrasound transducer (UST) for acquiring the photoacoustic data. Conventionally, in PAT systems, the UST is held in a horizontal position and moved in a circular motion around the sample in full 2π radians. Horizontal positioning of the UST requires a large water tank to house, and load on the motor is also high. To overcome this limitation, we used the UST in the vertical plane instead of the horizontal plane. The photoacoustic (PA) waves generated from the sample are directed to the detector surface using an acoustic reflector placed at 45 deg to the transducer body. Hence, we can reduce the scanning radius, which, in turn, will reduce the size of the water tank and load on the motor, and the overall conventional PAT system size can be minimized. In this work, we demonstrate that with this system configuration, we acquire nearly similar images for phantom and in vivo data as that of the conventional PAT system using both flat and focused USTs.
For a circular scanning geometry in photoacoustic tomography, the axial/radial resolution is spatially invariant and is not affected by the ultrasound transducer (UST, detector) aperture. However, the tangential resolution is dependent on the detector aperture size and it varies spatially. Many techniques were proposed to improve the tangential resolution, such as attaching a concave lens in front of the nonfocused transducer or using a virtual point detector. Both of these methods have difficulties. Therefore, a modified delay-and-sum reconstruction algorithm has been proposed which can be used together with a standard ultrasound detector (nonfocused) to improve the tangential resolution. In this work, we validate the modified delay-and-sum algorithm experimentally for both flat and cylindrically focused USTs. More than threefold improvement in tangential resolution is observed. It is also shown that the object shape is recovered with this modified algorithm, which is very helpful for diagnosis and treatment purposes.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.