Photoacoustic Tomography (PAT) systems based on Fabry-Perot (FP) sensors provide high-resolution images limited by the system’s sensitivity. The sensitivity is limited by the optical Q-factor of the FP cavity (i.e., the optical confinement of the interrogation laser beam in the FP cavity). In existing systems, a focused Gaussian beam is used to interrogate the sensor. While providing a small acoustic element required for high-resolution imaging, this interrogation beam naturally diverges inside the FP cavity, leading to the current sensitivity limit. To break this limit, a new approach of interrogating the FP sensor using a Bessel beam is investigated. The Noise Equivalent Pressure (NEP) and both axial and lateral PAT resolutions using Bessel beam interrogation were quantified. Bessel beam interrogation provided lower NEP, similar axial resolution, but lower lateral resolution. Thus, Bessel beam might be an alternative interrogation scheme for deep PAT imaging as high sensitivity is needed and the lateral resolution is limited by the aperture of the PAT system.
Miniaturising ultrasonic field mapping systems could lead to novel endoscopes capable of photoacoustic tomography and other techniques. However, developing high-resolution arrays of sensitive, sub-millimetre scale ultrasound sensors presents a challenge for traditional piezoelectric transducers. To address this challenge, we conceived an ultrasonic detection concept in which an optical ultrasonic sensor array is read out using a laser beam scanned through a 0.24 mm diameter multimode optical fibre using optical wavefront shaping. We demonstrate this system enables ultrasonic field mapping with ⪆2500 measurement points, paving the way to developing miniaturized photoacoustic endoscopes and other ultrasonic systems based on the presented concept.
Fabry-Perot (FP) ultrasound sensors are widely used for Photoacoustic Tomography (PAT), affording high resolution (<100 μm) images, with a penetration depth of about 1 cm, limited by system's sensitivity. The sensitivity is, in turn, limited by the shape of the Gaussian beam typically used to interrogate the FP sensor, which is not well "confined" inside the FP cavity. To overcome this limitation, a novel PAT system employing Bessel beam to interrogate the FP sensor was prototyped. Unlike Gaussian beams, Bessel beams are well confined in the FP cavity, increasing the system's sensitivity by multiple orders of magnitude, paving the way to multi-centimetre clinical PAT imaging with high resolution
The impact of optical absorption in the spacer layer of Fabry-Pérot (FP) ultrasound sensors is discussed. It is shown that absorption significantly limits the sensitivity of planoconcave microresonators (PCMRs; FP type sensors with a planoconcave geometry). Using materials of lower absorption or selecting sensor interrogation wavelengths to avoid absorption peaks in existing spacer materials could provide at least an order of magnitude higher sensitivity, paving the way to multi-cm deep-tissue PA imaging applications.
There has been considerable interest in extending photoacoustic imaging techniques to endoscopic devices, which would enable a diverse range of applications, e.g. assessment of coronary artery disease or surgical guidance.
However, the difficulty of miniaturising traditional piezoelectric sensors has mostly prevented tomography-mode endoscopic imaging, where an array of sensors is used to reconstruct the full ultrasound field to centimeter-scale depths.
In this work we demonstrate how wavefront shaping through multimode fibres onto a Fabry-Perot optical ultrasound sensor can overcome this limitation, producing an endoscopic imaging system with a footprint an order of magnitude smaller than the state of the art.
Fabry-Pérot (FP) polymer film sensors are used as ultrasound sensors for Photoacoustic (PA) imaging. Optical models predict that FP sensors should have higher sensitivity than observed experimentally. The models assume FP sensors to be optically flat whereas in practice the polymer film spacer exhibits a degree of surface roughness. To understand the impact of the roughness, an optical model of rough FP sensors was developed. Theoretical results show that roughness can reduce the optical sensitivity by a factor of two. The model will help to guide the design of FP sensors to optimize their sensitivity and, therefore, the imaging depth.
Deep tissue applications (>1 cm) for photoacoustic imaging are currently limited for traditional Fabry-Pérot ultrasound transducers interrogated by tightly focused Gaussian beams due to beam walk-off. We investigate the optical confinement of the beam using plano-concave microresonators with a model based on the ABCD formalism and the use of high-sensitive and high-density multi-element arrays. The results show an improvement in the sensitivity enabling an increase of the penetration depth in tomographic photoacoustic imaging.
We present Jolab: an open source package for performing full-wave simulation of light propagation in optical systems. Jolab enables a very broad range or researchers, engineers and practitioners to simulate light propagation through complex optical systems. Jolab takes a relatively simple script as its input in which the optical system is defined and light is propagated by each optical components sequentially using built-in functions. Jolab scripts are simple and readable and their structure is designed to mimic the design of optical systems making it easy to learn. We will present a range of examples including time-domain simulations and wavefront-shaping experiments.
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.
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.
The Fabry-Perot interferometer (FPI) is widely used in photoacoustic imaging (PAI) as an ultrasound (US) sensor due to its high sensitivity to weak US waves. Such high sensitivity is important as it allows for increasing the depth in tissue at which PAI can access, thus strongly influencing its clinical applicability. FPI sensitivity is impacted by many factors including the FPI mirror reflectivity, focussed beam spot size, FPI cavity thickness and aberrations introduced by the optical readout system. Improving FPI sensitivity requires a mathematical model of its optical response which takes all of these factors into account. Previous attempts to construct such a model have been critically limited by unrealistic assumptions. In this work we have developed a general model of FPI optical readout which based upon electromagnetic theory. By making very few assumptions, the model is able to replicate experimental results and allows insight to be gained into the operating principles of the sensor.
Polymer film Fabry-Perot (FP) sensors are commonly used to detect ultrasound for Photoacoustic (PA) imaging providing high resolution 3D images. Such high image quality is possible due to their low Noise Equivalent Pressure (NEP) because of their broadband response and small acoustic element size. The acoustic element size is small (<100 μm) as defined, to first approximation, by the spot size of the focused interrogation beam. However, it has been difficult until now to gain an accurate intuitive understanding of the working principle of FP sensors interrogated with a focused beam. To overcome this limitation a highly realistic rigorous model of the FP sensor’s optical response has used to establish a new intuitive understanding. The origin of fringe depth reduction and asymmetry associated with the FP sensors optical response is explained using the model developed.
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