Conventional photoacoustic (PA) imaging suffers from visibility artefacts due to limitations in ultrasound transducer bandwidth, viewing angles, and the use of sparse arrays. PA fluctuation imaging (PAFI), exploiting the signal changes due to blood flow, compensates for these artefacts, at the cost of temporal resolution.
Our study addresses this limitation employing a deep learning approach in which PAFI images serve as ground truths for training a 3D neural network to obtain real-time single-shot artefact-free images.
Following a pre-training with simulated examples, a 3D-ResUnet network was trained with 90 PA chicken embryo vasculature volumes as input and corresponding PAFI as ground truths. Notably, inclusion of experimental data significantly improves predictions over simulation-only training, even accounting for transducer angular filtering.
Furthermore, applying the same network exclusively trained in-ovo to predict the femoral artery in mice demonstrates the potential of this method for real-time, full-visibility multispectral PA imaging in vivo using sparse arrays.
Spatial light distribution prediction is highly useful but challenging; no imaging method is currently capable of measuring it at different depths. This study introduces a novel technique for fluence quantification within blood vessels through the ratio of photoacoustic fluctuation imaging (PAFI) and ultrasound power Doppler (USPD). However, their direct coupling fails in accurately estimating the fluence due to differences in the Point Spread Functions (PSFs), leading to varying image resolution and amplitude dependence over vessel sizes. To address this, we propose a model-based matrix approach to apply a non-stationary PSF filter to USPD. Validation through 3D simulations and experiments with tissue-mimicking phantoms demonstrates accurate fluence recovery. Results indicate a robust correlation with the Monte Carlo-simulated ground truth, even in unresolved vessels. This direct imaging technique uniquely offers precise measurement of light distribution in ubiquitous blood vessels, showing great potential for clinical applications and quantitative photoacoustic inverse problems.
KEYWORDS: Monte Carlo methods, Simulation of CCA and DLA aggregates, Photography, Data modeling, Visibility, Image quality, Computer simulations, Photoacoustic imaging, Veins, Transducers
In conventional photoacoustics (PA) imaging, the finite size and limited-bandwidth of ultrasound transducers often lead to visibility artifacts resulting in a degraded image quality. We propose a reconstruction algorithm based on deep learning to address theses issues. An in vitro vasculature mimicking model has been used in order to show the capability of a conventional neural network to remove these artefacts in an experimental configuration. The deep learning algorithm is trained using couples of PA images and ground truth photographs. The uncertainty of the model prediction is estimated through the Monte Carlo dropout method allowing the display of a pixel-wise degree of confidence. Finally, the interest of using simulation data through transfer learning in order to reduce the size of the experimental dataset is investigated.
Photoacoustic imaging (PAI) provides optical contrast at depth beyond the optical transport mean free path. From the generation of ultrasound by light absorption, images can be reconstructed at the acoustic resolution ( 100 μm) with a penetration of a few cm. The design of imaging systems often leads to limited view artifacts, where a part of the information needed for a complete reconstruction of the objects is missing. We theoretically show that a dynamic approach based on the analysis of fluctuations induced by blood flow can suppress visibility artefacts. We demonstrate the performance of 3D Photoacoustic Fluctuation imaging (PAFI) using a spherical array with limited number of channels (256
elements, 8 MHz) in the chicken embryo model. Due to the low number of channels, standard PAI reconstructions additionally suffer from a poor contrast, which is enhanced by 2 to 3-fold using PAFI. We present an implementation of simultaneous PAFI and Ultrasound Power Doppler and present some results with coupled flow direction evaluations and optical contrast. Photoacoustic fluctuation imaging overcomes many limitations of conventional imaging and will be further evaluated for in-vivo imaging.
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.
One limitation of microendoscopy is the device footprint that should be minimal for many applications. Here we present a minimally-invasive endoscope based on a multimode fiber that combines photoacoustic and fluorescence sensing. With the use of a fast spatial-light modulator, it is possible to rapidly learn the transmission matrix during a prior calibration step. A focused spot can then be produced to raster-scan a sample at the distal tip of the fiber. Our setup provides both photoacoustic and fluorescence microscopic images of test samples in vitro (fluorescent beads and red blood cells) through a single multimode fiber.
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.
We present a ultra-thin system that combines optical resolution photoacoustic microscopy and fluorescence imaging based on a multimode fiber and a fiber optical hydrophone with only 250μm cross section.
It has been shown that sub-diffraction structures can be resolved in acoustic resolution photoacoustic imaging thanks to norm-based iterative reconstruction algorithms exploiting prior knowledge of the point spread function (PSF) of the imaging system. Here, we demonstrate that super-resolution is still achievable when the receiving ultrasonic probe has much fewer elements than used conventionally (8 against 128). To this end, a proof-of-concept experiment was conducted. A microfluidic circuit containing five parallel microchannels (channel’s width 40μm, center-to center distance 180μm) filled with dye was exposed to 5ns laser pulses (=532nm, fluence=3.0mJ/cm2, PRF=100Hz). Photoacoustic signals generated by the sample were captured by a linear ultrasonic array (128 elements, pitch=0.1mm, fc=15MHz) connected to an acquisition device. The forward problem is modelled in a matrix form Y=AX, where Y are the measured photoacoustic signals and X is the object to reconstruct. The matrix A contained the PSFs at all points of the reconstruction grid, and was derived from a single PSF acquired experimentally for a 10-μm wide microchannel. For the reconstruction, we used a sparsity-based minimization algorithm. While the conventional image obtained by beamforming the signals measured with all the 128 elements of the probe cannot resolve the individual microchannels, our sparsity-based reconstruction leads to super-resolved images with only 8 elements of the probe (regularly spaced over the full probe aperture), with an image quality comparable to that obtained with all the 128 elements. These results pave the way towards super-resolution in 3D photoacoustic imaging with sparse transducers arrays.
Limited-view and bandwidth-limited acquisition in acoustic–resolution photoacoustic imaging lead to known imaging artifacts. To eliminate these artifacts, it has first been proposed to use fluctuations induced by multiple speckle illuminations to recover otherwise invisible features. However, a very small size of the optical speckle grain at depth in tissue against the acoustic resolution makes this approach unrealistic in practice. Here, we demonstrate experimentally in vitro that fluctuations induced by blood flow at physiological concentration may be exploited to improve visibility in photoacoustic imaging. We first illustrate how our method reveals features otherwise invisible due to the source directivity: a bended capillary tube (inner diameter 100µm) filled with blood flowing at 1.7cm/s was illuminated by 5ns laser pulses (=532nm, fluence=3.0mJ/cm2, PRF=100Hz) and imaged with a linear ultrasound array (128 elements, pitch=0.1mm, fc=15MHz) connected to an acquisition device. Being partially invisible in the conventional image, the whole capillary is reconstructed by means of a second-order analysis of photoacoustic images. Second, we illustrate how our approach allows for visualization of the inside of large objects otherwise invisible due to highpass filtering: we performed a second-order analysis on photoacoustic data resulting from illumination (=800nm, fluence=9.0mJ/cm2, PRF=10Hz/100Hz) of a glass tube (inner diameter 1mm) with blood flowing at 1cm/s. Whether the tube is perpendicular to the imaging plane or is lying inside the imaging plane parallel to the probe, the whole blood volume is visible on the fluctuation-based image, whereas conventional imaging only reveals the blood stream boundaries.
We present an ultra-thin endoscope that combines a multimode optical fiber (MMF) attached to
an optical hydrophone for simultaneous optical-resolution photoacoustic microscopy and fluorescence
imaging. The MMF is used for light delivery and fluorescence collection and the hydrophone
for acoustic detection; a digital micro-mirror device (DMD) modulates the amplitude of the optical
wavefront of a pulsed laser coupled into the MMF, controlling the illumination at the distal tip.
The DMD allows for fast calibration approaches to reach calibration and measurement times of a
few seconds.
We obtain optical-diffraction-limited images with full field illumination recording the intensity
of a series of various calibrated speckle patterns produced by different configurations of the DMD
at the input, with no wavefront shaping. The intensity fluctuations from speckle pattern to speckle
pattern encodes for the position at which the signal is emitted. The fluorescence signal from the sample is
collected with the MMF and detected with a PMT at the proximal side. For the acoustic detection,
embedding the ultrasound detection within the device avoids the absorption of high-frequency ultrasound
by the tissue and therefore removes any limitation on the insertion depth. The footprint of
the probe is 250 um x 125 um making it thinner than common GRIN lenses used for endoscopy.
To best of our knowledge, our approach provides the thinnest endoscope head capable
of obtaining optical-resolution photoacoustic and fluorescence images simultaneously.
We present a ultra-thin endoscopy system for optical resolution photoacoustic microscopy. The system is based on a silica capillary waveguide of two hundred microns of diameter. The silica tube acts as a multi-mode optical waveguide for the illumination, while the hollow core of the capillary carries a fiber-based optical hydrophone to detect the photoacoustic waves. Embedding the ultrasound detection within the device avoids the absorption of high-frequency ultrasound by the tissue and therefore removes any limitation on the insertion depth.
To control the illumination at the distal tip of the capillary, a digital micromirror device modulates the amplitude of the optical wavefront which is coupled into the capillary. The DMD allows for fast calibration approaches to reach calibration and measurement times of a few seconds, as compared with current approaches limited to hours. We obtain optical-diffraction-limited images with full field illumination recording the intensity of a series of various speckle patterns produced by different configurations of the DMD at the input, with no wavefront shaping. The intensity fluctuations from shot to shot codes for the position at which it is measured. Computational methods based on correlation, pseudo-inverse and compressed sensing approaches are investigated and compared with raster-scanning an optimized focus for image reconstruction. To best of our knowledge, our approach provides the thinest endoscope head capable of obtaining optical resolution photoacoustic images.
The STORM and PALM techniques developed in the past decade in optics allow resolving sub-wavelength structures based on localization. Here, we demonstrate that localization can be used to go beyond the diffraction limit in acoustic resolution photoacoustic imaging. To this end, a proof-of-concept photoacoustic localization experiment was conducted. A silicone sample containing five parallel microchannels (channel’s width is 40μm; center-to center distance equals 180μm), fed with a water suspension of 10μm red coated microbeads at a constant flow rate, was exposed to 5ns laser pulses (wavelength=532nm, fluence=3.0 mJ/cm2). At each laser pulse the microbeads produced a photoacoustic response that was then detected by a linear US array (128 elements, L22-8v, Verasonics, USA) connected to an acquisition device (High Frequency Vantage 256, Verasonics, USA). The design of the microfluidic circuit and the concentration of microbeads ensured sparse but random distribution of microbeads at any moment. The photoacoustic data was processed by a delay-and-sum algorithm, whose output was correlated with the system’s PSF function to obtain the position of microbeads at each laser shot. These positions were accumulated onto the localization grid providing a super-resolved image of the micro-fluidic circuit. Although being indistinguishable in a conventional US image, the microchannels dimensions and position were accurately reconstructed on the localization grid with 34.8±1.3μm for the channel’s width and 179±2.5μm for the center-to-center distance. As the first demonstration of super-localization in photoacoustics, these results constitute the first step towards imaging of red blood cells at depth beyond the acoustic diffraction limit.
Optical-resolution photoacoustic microscopy offers a specific contrast to optical absorption. The limiting penetration depth of current techniques due to scattering produced by tissues makes endoscopic approaches attractive for photoacoustic imaging deep inside biological structures. Conventional approaches generally involves mechanically raster scanning a focused spot over the sample and acquiring an acoustic signal for each spot. Here, we demonstrate that a full-field illumination approach with multiple known speckle patterns generated by a multimode fiber can also provide diffraction-limited optical-resolution photoacoustic images. As a proof of principle we experimentally image micro-structured test samples illuminated with reference speckle patterns measured during a calibration step. A digital micromirror device modulating the incident light coupled into a multimode fiber provides the different speckle patterns at the distal tip of the fiber where the sample is placed.
We study and compare the performance in simulations and experiments of three different approaches; the first method is based on cross-correlation between the photoacoustic signal under multiple speckle illumination with the calibrated known speckle patterns, following approaches from ghost imaging. The second method is based on computing the pseudo-inverse of the reference matrix obtained from the calibration step. A third method based on compressed sensing exploits the sparsity of the sample achieving reconstructed images with a number of speckle realizations smaller than the number of speckle grains. Additionally, speckle-illumination-based photoacoustic microscopy provides a powerful framework for the development of novel reconstruction approaches, that can demand less computation time in case of compressed sensing approaches.
The resolution of photoacoustic imaging of blood vasculature is limited at depth by the acoustic diffraction limit. In this work, we propose to exploit the fluctuations caused by flowing absorbers (such as red blood cells in blood vessels) to perform photoacoustic imaging beyond the acoustic diffraction limit: following the super-resolution optical fluctuation imaging (SOFI) method, we analyze the n-th order statistics from the temporal photoacoustic fluctuations induced by flowing particles.
We performed a proof-of-concept experiment in a 5-channel microfluidic silicon-based circuit flown with a suspension of RBC-mimicking 10 µm red-tainted polymer spheres (Microparticles, GmbH, Berlin, Germany). The sample was illuminated with a 5 ns pulsed ND-YAG laser (532 nm, Innolas, Krailling, Germany) with a fluence of 3 mJ/cm^2 and imaged at a 20 Hz rate using a L22-8v probe (128 elements, Verasonics, Redmond, WA, USA) coupled to a Verasonics Vantage 256 ultrasound scanner. Whereas the resolution of conventional photoacoustic imaging was too low to resolve individual channels, the nth order statistical analysis of the photoacoustic fluctuations provided images with a resolution enhancement scaling as n^{1/2}, in agreement with the SOFI theory and with numerical simulations. As opposed to our previous work which exploited speckle-based photoacoustic fluctuations to increase the resolution, the approach proposed here based on sample fluctuations do not require coherent light and can be readily applied to conventional photoacoustic imaging setup. Furthermore, in order to discard the oscillatory behavior of the photoacoustic point-spread-function, we extended in this work the SOFI theory to complex-valued photoacoustic images.
Optical-resolution photoacoustic microscopy offers exquisite and specific contrast to optical absorption. Conventional approaches generally involves raster scanning a focused spot over the sample. Here, we demonstrate that a full-field illumination approach with multiple speckle illumination can also provide diffraction-limited optical-resolution photoacoustic images. Two different proof-of-concepts are demonstrated with micro-structured test samples. The first approach follows the principle of correlation/ghost imaging,1, 2 and is based on cross-correlating photoacoustic signals under multiple speckle illumination with known speckle patterns measured during a calibration step. The second approach is a speckle scanning microscopy technique, which adapts the technique proposed in fluorescence microscopy by Bertolotti and al.:3 in our work, spatially unresolved photoacoustic measurements are performed for various translations of unknown speckle patterns. A phase-retrieval algorithm is used to reconstruct the object from the knowledge of the modulus of its Fourier Transform yielded by the measurements. Because speckle patterns naturally appear in many various situations, including propagation through biological tissue or multi-mode fibers (for which focusing light is either very demanding if not impossible), speckle-illumination-based photoacoustic microscopy provides a powerful framework for the development of novel reconstruction approaches, well-suited to compressed sensing approaches.2
Optical-resolution photoacoustic endomicroscopy (OR-PAE) allows going beyond the limited penetration depth of conventional optical-resolution photoacoustic systems. Recently, it has been shown that OR-PAE may be performed through minimally invasive multimode fibers, by raster scanning a focus spot with optical wavefront shaping [1]. Here we introduce for the first time an approach to perform OR-PAE through a multimode fiber with a full-field illumination approach. By using multiple known speckle patterns, we show that it is possible to obtain optical-diffraction limited photoacoustic images, with the same resolution as that obtained by raster scanning a focus spot, i.e that of the speckle grain size. The fluctuations patterns of the photoacoustic amplitude at each pixel in the sample plane with the series of multiple speckle illumination were used to encode each pixel.
This approach with known speckle illumination requires an initial calibration stage, that consists in learn a set of fluctuation patterns pixel per pixel, which will encode patterns each pixel of the scanned area. A point-like absorber was scanned across the filed-of-view during the calibration stage to acquire the reference patterns. Image reconstruction may be carried out by cross-correlating the series of photoacoustic amplitude measured with the sample to the reference patterns obtained during the calibration stage.
In this work, the approach above was carried out both theoretically with Monte-carlo simulations and experimentally through a multi-mode fiber with samples made of absorbing spheres.
[1] Papadopoulos et al., " Optical-resolution photoacoustic microscopy by use of a multimode fiber", Appl. Phys. Lett., 102(21), 2013
In deep photoacoustic imaging, resolution is inherently limited by acoustic diffraction, and ultrasonic frequencies cannot be arbitrarily increased because of attenuation in tissue. Here we report on the use of multiple speckle illumination to perform super resolution photoacoustic imaging. We show that the analysis of speckle-induced second-order fluctuations of the photoacoustic signal combined with deconvolution enables to resolve optically absorbing structures below the acoustic diffraction limit.
Endoscopy can be used to obtain high-resolution images at large depths in biological tissues. Usually endoscopic devices have a diameter ranging from 1 to few millimeters. Using digital phase conjugation, it is possible to adapt ultrathin multimode fibers to endoscopic purposes. Recently, we demonstrated that a 330 μm diameter, water-filled silica capillary waveguide can guide high frequency ultrasound waves through a 3 cm thick fat layer, allowing optical resolution photoacoustic imaging. Here we demonstrate that using digital phase conjugation, the same water-filled capillary waveguide (3 cm long) can be used as an endoscopic probe to obtain both fluorescence and optical resolution photoacoustic imaging, with no optical or acoustic elements at the tip of the waveguide. We study the consequences of using digital phase conjugation combined with a capillary waveguide and we conclude with possible future improvements of our endoscopic approach.
Photoacoustic imaging can achieve high-resolution three-dimensional (3-D) visualization of optical absorbers at penetration depths of ∼1 cm in biological tissues by detecting optically induced high ultrasound frequencies. Tomographic acquisition with ultrasound linear arrays offers an easy implementation of single-side access, parallelized, and high-frequency detection, but usually comes with an image quality impaired by the directionality of the detectors. Indeed, a simple translation of the array perpendicular to its median imaging plane is often used, but results both in a poor resolution in the translation direction and strong limited-view artifacts. To improve the spatial resolution and the visibility of complex structures while retaining a planar detection geometry, we introduce, in this paper, a rotate-translate scanning scheme and investigate the performance of a scanner implemented at 15 MHz center frequency. The developed system achieved a quasi-isotropic uniform 3-D resolution of ∼170 μm over a cubic volume of side length 8.5 mm, i.e., an improvement in the resolution in the translation direction by almost one order of magnitude. Dual-wavelength imaging was also demonstrated with ultrafast wavelength shifting. The validity of our approach was shown in vitro. We discuss the ability to enable in vivo imaging for preclinical and clinical studies.
Photoacoustics has recently been proposed as a potential method to guide and/or monitor therapy based on high-intensity focused ultrasound (HIFU). We experimentally demonstrate the creation of a HIFU lesion at the location of an optical absorber, by use of photoacoustic signals emitted by the absorber detected on a dual mode transducer array. To do so, a dedicated ultrasound array intended to both detect photoacoustic waves and emit HIFU with the same elements was used. Such a dual-mode array provides automatically coregistered reference frames for photoacoustic detection and HIFU emission, a highly desired feature for methods involving guidance or monitoring of HIFU by use of photoacoustics. The prototype is first characterized in terms of both photoacoustic and HIFU performances. The probe is then used to perform an idealized scenario of photoacoustic-guided therapy, where photoacoustic signals generated by an absorbing thread embedded in a piece of chicken breast are used to automatically refocus a HIFU beam with a time-reversal mirror and necrose the tissue at the location of the absorber.
Both mechanical and optical imaging of biological tissue can provide relevant contrasts in terms of biomedical tissue
characterization. While ultrasound imaging can easily be performed at depth thanks to the weak scattering of ultrasound
in soft tissue, the optical spatial resolution is limited for thick tissue by the strong scattering of light. In this paper, we
present a technique involving the optical detection of a transient displacement caused by the acoustic radiation force
created at cm depth by a focused intense short ultrasound burst (typically ~millisecond). This localized displacement
disturbs the optical paths and allows localizing the information with a resolution dictated by the ultrasound spatial
distribution. Using a high-speed camera, our objective was to detect and time-resolve displacements in the focal region
and the associated propagation shear waves. Experiments were carried out in attenuating tissue-like media illuminated by
a continuous laser source. In this work, we refined the optical detection scheme in order to work with low photons flux,
based on two-phase heterodyne interferometry setup. We used tissue-mimicking phantoms with different optical and
shear mechanical contrast. We demonstrate that it is possible to detect both types of contrast, and moreover to
discriminate between these two types of contrast.
We present a camera-based optical detection scheme designed to detect the transient motion created by the acoustic radiation force in elastic media. An optically diffusive tissue mimicking phantom was illuminated with coherent laser light, and a high speed camera (2 kHz frame rate) was used to acquire and cross-correlate consecutive speckle patterns. Time-resolved transient decorrelations of the optical speckle were measured as the results of localised motion induced in the medium by the radiation force and subsequent propagating shear waves. As opposed to classical acousto-optic techniques which are sensitive to vibrations induced by compressional waves at ultrasonic frequencies, the proposed technique is sensitive only to the low frequency transient motion induced in the medium by the radiation force. It therefore provides a way to assess both optical and shear mechanical properties.
Interferometric measurements for in-vivo imaging of biological tissues are strongly sensitive to the related speckle decorrelation time tc, whose effect is to reduce the contrast of the speckle pattern at the exit of the sample and thus blur detection. Though
acousto-optic imaging is a well suited technique for the case of
thick tissues, it has been shown that an acquisition rate in the
1-10kHz range is required for a good efficiency. We have previously built for this purpose an holographic setup that combines a fast but large area single photodetector and a photorefractive crystal, in order to measure a real-time acousto-optic signal by the so-called self-adaptive wavefront holography technique. In such a configuration, one critical point is the time response tPR of the
photorefractive effect, which depends on the photorefractive
configuration of the setup as well as the light intensity within
the crystal. We have developed an original in situ method that determines this time in measuring the acousto-optic response through a combination of an amplitude modulation of the ultrasound and a frequency de-tuning of the reference beam. We can measure precisely this time but also monitor it according to a theoretical model that we have previously described. This offers the possibility to adapt the response of the setup to the decorrelation time of the medium under study, and also to have a measurement of τc.
KEYWORDS: Acousto-optics, In vivo imaging, Liquids, Ultrasonography, Speckle, Modulation, Spatial resolution, Acoustics, Signal to noise ratio, Beam propagation method
The aim of this paper is to show that we can perform acousto-optical
signal acquisition of one datapoint (or voxel of a 3D image)
in a very short time (2 - 4 ms), in order to overcome the speckle
decorrelation effect. To demonstrate this, we have performed
experiments in in dynamic scattering media such as liquids.
We will show that we can work with pulsed wave ultrasound, to reduce the sound irradiation duration in order to be compatible with safety
limits. These are significant steps towards in-vivo experiments.
Ultrasound focusing through complex media can be achieved using time-reversal techniques. These techniques make use of back-propagating ultrasonic waves generated by localized sources. Such sources generally consist of high acoustic contrasts echoing ultrasonic waves generated by an incident ultrasonic field, or directly by point-like transducers inserted at the desired focusing location. In this work, we experimentally investigate time-reversal of acoustic waves generated by photo-acoustic emission. A frequency-doubled Q-switched Nd:YAG laser was used to illuminate phantom with 5-ns laser pulses. A 128-element ultrasonic transducer array, with a center frequency of 1.5 MHz, was used to detect acoustic waves generated by optically absorbing targets suspended in water. A dedicated 32-channel electronics was used to time-reverse and re-emit the detected ultrasonic field. Gel spheres dyed with India ink (diameter approximately 1-2 mm)illuminated by the laser beam were used to generate the photo-acoustic waves. Time-reversal of the detected field was performed to focus ultrasound in the presence of highly defocusing media in front of the transducer array. We demonstrate how this allows correcting for the aberration in order to provide good quality images in the isoplanetic region surrounding the photo-acoustic source.
The acousto-optical sensing (AOS) of a turbid medium is based on the interaction of multiply-scattered coherent laser light with an ultrasonic field. A phase-modulated photon field emanates from the interaction region and carries with it information about the acousto-optical properties of the media. Using a novel technique based on a photorefractive crystal interferometer, it is possible to detect the ultrasound-modulated optical signals generated by short ultrasound pulses. As opposed to continuous-wave (CW) ultrasound, pulsed ultrasound directly provides resolution along the ultrasonic propagation axis. In this work, a commercial ultrasound scanner (Analogic AN2300) was used in pulse mode (5 MHz central frequency) to generate both conventional ultrasound and AO images. Gel-based highly diffusive (μs'=10 cm-1) tissue-mimicking phantoms were fabricated, with embedded targets possessing acoustical and/or optical contrast. AO images of 26-mm thick phantoms were generated from optical signals averaged in the time-domain, without further signal processing, and were superimposed on the top of the ultrasound images. Good quality AO images of optical absorbers, intrinsically co-registered with the ultrasound images, were obtained within minutes. The axial resolution of the AO images was given by the spatial length of the ultrasound pulse, typically on the order of one mm in the MHz range. These results show that AO signals can be excited in pulse mode using a commercial scanner, and combined to conventional ultrasound images to provide more information related to the optical properties of the medium.
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