Photoacoustic monopole radiation in one, two, and three dimensions

Singular value decomposition analysis of a photoacoustic imaging system and 3D imaging at 0.7 FPS

Photoacoustic imaging is a non-ionizing imaging modality that provides contrast consistent with optical imaging techniques while the resolution and penetration depth is similar to ultrasound techniques. In a previous publication [Opt. Express 18, 11406 (2010)], a technique was introduced to experimentally acquire the imaging operator for a photoacoustic imaging system. While this was an important foundation for future work, we have recently improved the experimental procedure allowing for a more densely populated imaging operator to be acquired. Subsets of the imaging operator were produced by varying the transducer count as well as the measurement space temporal sampling rate. Examination of the matrix rank and the effect of contributing object space singular vectors to image reconstruction were performed. For a PAI system collecting only limited data projections, matrix rank increased linearly with transducer count and measurement space temporal sampling rate. Image reconstruction using a regularized pseudoinverse of the imaging operator was performed on photoacoustic signals from a point source, line source, and an array of point sources derived from the imaging operator. As expected, image quality increased for each object with increasing transducer count and measurement space temporal sampling rate. Using the same approach, but on experimentally sampled photoacoustic signals from a moving point-like source, acquisition, data transfer, reconstruction and image display took 1.4 s using one laser pulse per 3D frame. With relatively simple hardware improvements to data transfer and computation speed, our current imaging results imply that acquisition and display of 3D photoacoustic images at laser repetition rates of 10Hz is easily achieved.

Vaporization of perfluorocarbon droplets using optical irradiation

Micron-sized liquid perfluorocarbon (PFC) droplets are currently being investigated as activatable agents for medical imaging and cancer therapy. After injection into the bloodstream, superheated PFC droplets can be vaporized to a gas phase for ultrasound imaging, or for cancer therapy via targeted drug delivery and vessel occlusion. Droplet vaporization has been previously demonstrated using acoustic methods. We propose using laser irradiation as a means to induce PFC droplet vaporization using a method we term optical droplet vaporization (ODV). In order to facilitate ODV of PFC droplets which have negligible absorption in the infrared spectrum, optical absorbing nanoparticles were incorporated into the droplet. In this study, micron-sized PFC droplets loaded with silica-coated lead sulfide (PbS) nanoparticles were evaluated using a 1064 nm laser and ultra-high frequency photoacoustic ultrasound (at 200 and 375 MHz). The photoacoustic response was proportional to nanoparticle loading and successful optical droplet vaporization of individual PFC droplets was confirmed using photoacoustic, acoustic, and optical measurements. A minimum laser fluence of 1.4 J/cm(2) was required to vaporize the droplets. The vaporization of PFC droplets via laser irradiation can lead to the activation of PFC agents in tissues previously not accessible using standard ultrasound-based techniques.

Fast, limited-data photoacoustic imaging for multiplexed systems using a frequency-domain estimation technique

PURPOSE

A new frequency-domain estimation algorithm has been developed that uses a priori information to simultaneously improve imaging quality and time resolution in photoacoustic tomography with incomplete data sets.

METHODS

The method involves application of a single-stage Wiener optimal filter to augment data sets by interpolation between measurement locations using relationships determined in a reference scan. The filter can be applied in real-time using FFT methods using either fixed or dynamic references and used with any imaging algorithm. The performance of the method is compared to a modified version of constrained backprojection algorithms using simulations and experimental investigations.

RESULTS

Simulations demonstrate the effectiveness of the approach for tracking dynamic photoacoustic activity for data sets with limited views (90 degrees) or tomographic views with a reduced number of acquisition angles at any given time (< or = 32). Experimental data of contrast uptake and washout using a 512-element curved transducer with 8:1 electronic multiplexing with the algorithm demonstrate full two-dimensional tomographic imaging with a temporal resolution better than 130 ms.

CONCLUSIONS

The estimation algorithm enables high spatial resolution, real-time imaging of dynamic physiological events or volumetric regions for photoacoustic systems employing multiplexing or scanning.

Low-noise small-size microring ultrasonic detectors for high-resolution photoacoustic imaging

Small size polymer microring resonators have been exploited for photoacoustic (PA) imaging. To demonstrate the advantages of the wide acceptance angle of ultrasound detection of small size microrings, photoacoustic tomography (PAT), and delay-and-sum beamforming PA imaging was conducted. In PAT, we compared the imaging quality using different sizes of detectors with similar noise-equivalent pressures and the same wideband response: 500 μm hydrophone and 100, 60, and 40 μm microrings. The results show significantly improved imaging contrast and high resolution over the whole imaging region using smaller size detectors. The uniform high resolution in PAT imaging using 40 μm microrings indicates the potential to resolve microvasculature over a large imaging region. The improved lateral resolution of two-dimensional and three-dimensional delay-and-sum beamforming PA imaging using a synthetic array demonstrate another advantageous application of small microrings. The small microrings can also be applied to other ultrasound-related imaging applications.

Target detection and quantification using a hybrid hand-held diffuse optical tomography and photoacoustic tomography system

We present a photoacoustic tomography-guided diffuse optical tomography approach using a hand-held probe for detection and characterization of deeply-seated targets embedded in a turbid medium. Diffuse optical tomography guided by coregistered ultrasound, MRI, and x ray has demonstrated a great clinical potential to overcome lesion location uncertainty and to improve light quantification accuracy. However, due to the different contrast mechanisms, some lesions may not be detectable by a nonoptical modality but yet have high optical contrast. Photoacoustic tomography utilizes a short-pulsed laser beam to diffusively penetrate into tissue. Upon absorption of the light by the target, photoacoustic waves are generated and used to reconstruct, at ultrasound resolution, the optical absorption distribution that reveals optical contrast. However, the robustness of optical property quantification of targets by photoacoustic tomography is complicated because of the wide range of ultrasound transducer sensitivity, the orientation and shape of the targets relative to the ultrasound array, and the uniformity of the laser beam. We show in this paper that the relative optical absorption map provided by photoacoustic tomography can potentially guide the diffuse optical tomography to accurately reconstruct target absorption maps.

Target detection and quantification using a hybrid hand-held diffuse optical tomography and photoacoustic tomography system

We present a photoacoustic tomography-guided diffuse optical tomography approach using a hand-held probe for detection and characterization of deeply-seated targets embedded in a turbid medium. Diffuse optical tomography guided by coregistered ultrasound, MRI, and x ray has demonstrated a great clinical potential to overcome lesion location uncertainty and to improve light quantification accuracy. However, due to the different contrast mechanisms, some lesions may not be detectable by a nonoptical modality but yet have high optical contrast. Photoacoustic tomography utilizes a short-pulsed laser beam to diffusively penetrate into tissue. Upon absorption of the light by the target, photoacoustic waves are generated and used to reconstruct, at ultrasound resolution, the optical absorption distribution that reveals optical contrast. However, the robustness of optical property quantification of targets by photoacoustic tomography is complicated because of the wide range of ultrasound transducer sensitivity, the orientation and shape of the targets relative to the ultrasound array, and the uniformity of the laser beam. We show in this paper that the relative optical absorption map provided by photoacoustic tomography can potentially guide the diffuse optical tomography to accurately reconstruct target absorption maps.

An imaging model incorporating ultrasonic transducer properties for three-dimensional optoacoustic tomography

Optoacoustic tomography (OAT) is a hybrid imaging modality that combines the advantages of optical and ultrasound imaging. Most existing reconstruction algorithms for OAT assume that the ultrasound transducers employed to record the measurement data are point-like. When transducers with large detecting areas and/or compact measurement geometries are utilized, this assumption can result in conspicuous image blurring and distortions in the reconstructed images. In this work, a new OAT imaging model that incorporates the spatial and temporal responses of an ultrasound transducer is introduced. A discrete form of the imaging model is implemented and its numerical properties are investigated. We demonstrate that use of the imaging model in an iterative reconstruction method can improve the spatial resolution of the optoacoustic images as compared to those reconstructed assuming point-like ultrasound transducers.

Coded excitation for photoacoustic imaging using a high-speed diode laser

A Q-switched Nd:YAG laser providing nanosecond pulse durations and millijoule pulse energies is suitable for typical biomedical PA applications. However, such lasers are both bulky and expensive. An alternative method is to use a diode laser, which can achieve a higher pulse repetition frequency. Although the energy from a diode laser is generally too low for effective PA generation, this can be remedied by using high-speed coded laser pulses, with the signal intensity of the received signal being enhanced by pulse compression. In this study we tested a version of this method that employs coded excitation. A 20-MHz PA transducer was used for backward-mode PA detection. A frequency-coded PA signal was generated by tuning the interval between two adjacent laser pulses. The experimental results showed that this methodology improved the signal-to-noise ratio of the decoded PA signal by up to 19.3 dB, although high range side lobes were also present. These side lobes could be reduced by optimizing the compression filter. In contrast to the Golay codes proposed in the literature, the proposed coded excitation requires only a single stimulus.

A quantitative study to design an experimental setup for photoacoustic imaging

During the last decade, a new modality called photoacoustic imaging has emerged. The increasing interest for this new modality is due to the fact that it combines advantages of ultrasound and optical imaging, i.e. the high contrast due to optical absorption and the low acoustic attenuation in biological tissues. It is thus possible to study vascularization because blood has high optical absorption coefficient. Papers in the literature often focus on applications and rarely discuss quantitative parameters. The goal of this paper is to provide quantitative elements to design an acquisition setup. By defining the targeted resolution and penetration depth, it is then possible to evaluate which kind of excitation and reception systems have to be used. First, we recall theoretical background related to photoacoustic effect before to describe the experiments based on a nanosecond laser at 1064 nm and 2.25-5 MHz transducers. Second, we present results about the relation linking fluence laser to signal amplitude and axial and lateral resolutions of our acquisition setup. We verify the linear relation between fluence and amplitude before to estimate axial resolution at 550 μm for a 2.25 MHz ultrasonic transducer. Concerning lateral resolution, we show that a reconstruction technique based on curvilinear acquisition of 30 lines improves it by a factor of 3 compared to a lateral displacement. Future works will include improvement of lateral resolution using probes, like in ultrasound imaging, instead of single-element transducers.

Carbon nanotube composite optoacoustic transmitters for strong and high frequency ultrasound generation

We demonstrate carbon nanotube (CNT) composite-based optoacoustic transmitters that generate strong and high frequency ultrasound. The composite consists of CNTs grown on a substrate, which are embedded in elastomeric polymer used as an acoustic transfer medium. Under pulsed laser excitation, the composite generates very strong optoacoustic pressure: 18 times stronger than a Cr film reference and five times stronger than a gold nanoparticle composite with the same polymer. This enhancement persists over a broadband frequency range of up to 120 MHz and is confirmed by calculation. We suggest the CNT-polymer composites as highly efficient optoacoustic transmitters for high resolution ultrasound imaging.

Analysis of a photoacoustic imaging system by the crosstalk matrix and singular value decomposition

Photoacoustic imaging is a hybrid imaging modality capable of producing contrast similar to optical imaging techniques but with increased penetration depth and resolution in turbid media by encoding the information as acoustic waves. In general, it is important to characterize the performance of a photoacoustic imaging system by parameters such as sensitivity, resolution, and contrast. However, system characterization can extend beyond these metrics by implementing advanced analysis via the crosstalk matrix and singular value decomposition. A method was developed to experimentally measure a matrix that represented the imaging operator for a photoacoustic imaging system. Computations to produce the crosstalk matrix were completed to provide insight into the spatially dependent sensitivity and aliasing for the photoacoustic imaging system. Further analysis of the imaging operator was done via singular value decomposition to estimate the capability of the imaging system to reconstruct objects and the inherent sensitivity to those objects. The results provided by singular value decomposition were compared to SVD results from a de-noised imaging operator to estimate the number of measurable singular vectors for the system. These characterization techniques can be broadly applied to any photoacoustic system and, with regards to the studied system, could be used as a basis for improvements to future iterations.

Photoacoustic tomography of water in phantoms and tissue

Photoacoustic tomography (PAT) has been widely used to image optically absorptive objects in both human and animal tissues. For the first time, we present imaging of water with laser-based PAT. We photoacoustically measure the absorption spectra of water-ethanol mixtures at various water concentrations, and then image water-ethanol and pure-water inclusions in gel and a water inclusion in fat tissue. The significant difference in photoacoustic signals between water and fat tissue indicates that the laser-based PAT has the potential to detect water content in tissue.

Reconstruction of high quality photoacoustic tomography with a limited-view scanning

The goal of this work is to resolve the limited-view problem of photoacoustic tomography (PAT). We report a two-loop iteration method to inverse the photoacoustic sources from the measured photoacoustic signals. PAT reconstruction with this method does not depend on the detection path. Therefore, the proposed method can provide recognizable image even when the detector only scans a small angle (about 20 degrees approximately 30 degrees). The comparison with the delay-and-sum method shows the advantage of the proposed method in reconstructing image from incomplete data.

Imaging of tumor vasculature using Twente photoacoustic systems

Photoacoustic imaging is a hybrid imaging modality based on the detection of acoustic waves generated by the absorption of short laser pulses in biological tissue. It combines the advantages of excellent contrast achieved in optical techniques with the high resolution of ultrasound imaging. In this article we present a review of the work done at the University of Twente to image tumor angiogenesis in vivo using this technique. We start with a description and the technical details of the different photoacoustic systems developed in our laboratory, with their validation on phantoms. We then discuss small-animal studies with results of serial imaging of angiogenesis over a 10-day period at the site of tumor induction in a rat. Further, we present clinical results using a photoacoustic mammoscope of breast cancer imaging based on angiogenesis-driven optical absorption contrast.

On the speckle-free nature of photoacoustic tomography

PURPOSE

A long-standing conundrum is why photoacoustic tomography (PAT) possesses the unique ability to produce images devoid of speckle artifacts while all other coherent imaging technologies do not.

METHODS

In this paper, we explain the inherent mechanism that suppresses speckle in PAT, and the analysis was validated by simulations based on an experimental PAT system.

RESULTS

We found that the speckle-free feature of PAT results directly from the optical absorption contrast.

CONCLUSIONS

All optical absorbers expand on laser excitation, and therefore all initial photoacoustic pressure rises are positive, which engenders strong correlations among the photoacoustic waves from the absorbers. As a result, prominent boundaries always build up in photoacoustic images and suppress the interior speckle.

Polymer microring resonators for high-sensitivity and wideband photoacoustic imaging

Polymer microring resonators have been exploited for high-sensitivity and wideband photoacoustic imaging. To demonstrate high-sensitivity ultrasound detection, highfrequency photoacoustic imaging of a 49-microm-diameter black bead at an imaging depth of 5 mm was imaged photoacoustically using a synthetic 2-D array with 249 elements and a low laser fluence of 0.35 mJ/cm(2). A bandpass filter with a center frequency of 28 MHz and a bandwidth of 16 MHz was applied to all element data but without signal averaging, and a signal-to-noise ratio of 16.4 dB was obtained. A wideband detector response is essential for imaging reconstruction of multiscale objects, e.g., various sizes of tissues, by using a range of characteristic acoustic wavelengths. A simulation of photoacoustic tomography of beads shows that objects with their boundaries characteristic of high spatial frequencies and the inner structure primarily of low spatial frequency components can be faithfully reconstructed using such a detector. Photoacoustic tomography experiments of 49- and 301-microm-diameter beads were presented. A high resolution of 12.5 microm was obtained. The boundary of a 301-microm bead was imaged clearly. The results demonstrated that the high sensitivity and broadband response of polymer microring resonators have potential for high resolution and high-fidelity photoacoustic imaging.

Photoacoustic tomography and sensing in biomedicine

Photoacoustics has been broadly studied in biomedicine, for both human and small animal tissues. Photoacoustics uniquely combines the absorption contrast of light or radio frequency waves with ultrasound resolution. Moreover, it is non-ionizing and non-invasive, and is the fastest growing new biomedical method, with clinical applications on the way. This review provides a brief recap of recent developments in photoacoustics in biomedicine, from basic principles to applications. The emphasized areas include the new imaging modalities, hybrid detection methods, photoacoustic contrast agents and the photoacoustic Doppler effect, as well as translational research topics.

Development and characterization of an omnidirectional photoacoustic point source for calibration of a staring 3D photoacoustic imaging system

Photoacoustic imaging is a modality which makes use of the contrast provided by optical imaging techniques and the spatial resolution and penetration depth similar to acoustic imaging modalities. We have developed a method for fast 3D photoacoustic imaging using a sparse hemispherical array of transducers. Such a system requires characterization of the transducer's response to an ideal point source in order to accurately reconstruct objects in the imaging volume. First, an attempt was made to design an ideal photoacoustic point source via a combination of liquids which would appropriately scatter and absorb the light such that a spherical distribution was achieved. Methylene blue (MB(+)) was used as the primary optical absorber while Intralipid (IL) was used as the liquid responsible for the optical scatter. A multitude of combinations were tested and the signal uniformity was characterized. The combination of 200 microM MB(+) and 0.09% IL was found to produce the most uniform signal over the range of transducers in the hemispherical array. The liquid source was then characterized over a broader range of azimuthal and zenith angles where it was shown the azimuthal consistency was much greater than the stability seen in different zenith elevations. The source was then used in a calibration scan for an imaging volume of 40 x 40 x 40 mm(3). At 216 points evenly spaced in the imaging volume, parameters were recorded for signal amplitude, width, and time-of-flight. These calibration parameters could then be applied to an iterative reconstruction algorithm in an attempt to more accurately produce images.

Photothermoacoustic imaging of biological tissues: maximum depth characterization comparison of time and frequency-domain measurements

The photothermoacoustic (PTA) or photoacoustic (PA) effect induced in light-absorbing materials can be observed either as a transient signal in time domain or as a periodic response to modulated optical excitation. Both techniques can be utilized for creating an image of subsurface light-absorbing structures (chromophores). In biological materials, the optical contrast information can be related to physiological activity and chemical composition of a test specimen. The present study compares experimentally the two PA imaging modalities with respect to the maximum imaging depth achieved in scattering media with optical properties similar to biological tissues. Depth profilometric measurements were carried out using a dual-mode laser system and a set of aqueous light-scattering solutions mimicking photon propagation in tissue. Various detection schemes and signal processing methods were tested to characterize the depth sensitivity of PA measurements. The obtained results demonstrate the capabilities of both techniques and can be used in specific PTA imaging applications for development of image reconstruction algorithms aimed at maximizing system performance. Our results demonstrate that submillimeter-resolution depth-selective PA imaging can be achieved without nanosecond-pulsed laser systems by appropriate modulation of a continuous laser source and a signal processing algorithm adapted to specific parameters of the PA response.

Weight factors for limited angle photoacoustic tomography

Photoacoustic tomography (PAT) is based on the generation of ultrasound waves by heating an object with short light pulses. A three-dimensional image of the distribution of absorbed energy within the object is reconstructed from signals measured around the object with either point-like or extended, linear sensors. Limited angle artefacts arise when the curve or surface connecting neighbouring detectors is not closed around the object. For this case, there exists a 'detection region' in which all boundaries of an object are visible in the reconstruction. All straight lines passing through each point in this region intersect the detection curve or surface at least once. Although for these points an accurate reconstruction is possible, direct back projection leads to artefacts when some of the straight lines intersect the detection surface twice and others just once. In this work, special weight functions for direct, non-iterative back projection are presented that reduce these kinds of artefacts. A clear improvement in image quality is shown in simulations for three-dimensional (3D) imaging with point detectors and for two-dimensional (2D) imaging using line detectors compared to reconstruction without weight factors. For the 2D case also an experiment is shown. The presented weight factors make commonly used back projection formulae suitable for a more accurate reconstruction of the initial pressure distribution in cases where the detection aperture only covers a limited angle, and the region of interest lies within the detection region.

The use of pulse synthesis for optimization of photoacoustic measurements

In this paper the use of pulse shaping in photoacoustic (PA) measurements is presented. The benefits of this approach are demonstrated by utilizing it for optimization of either the responsivity or the sensitivity of PA measurements. The optimization is based on the observation that the temporal properties of the PA effect can be represented as a linear system which can be fully characterized by its impulse response. Accordingly, the response of the PA system to an input optical pulse, whose instantaneous power is arbitrarily shaped, can be analytically predicted via a convolution between the pulse envelope and the PA impulse response. Additionally, the same formalism can be used to show that the response of the PA system to a pulse whose instantaneous power is a reversed version of the impulse response, i.e. a matched pulse, would exhibit optimal peak amplitude when compared with all other pulses with the same energy. Pulses can also be designed to optimize the sensitivity of the measurement to a variation in a specific system parameter. The use of the matched pulses can improve SNR and enable a reduction in the total optical energy required for obtaining a detectable signal. This may be important for applications where the optical power is restricted or for dynamical measurements where long integration times are prohibited. To implement this new approach, a novel PA optical setup which enabled synthesis of excitation waveforms with arbitrary temporal envelopes was constructed. The setup was based on a tunable laser source, operating in the near-IR range, and an external electro-optic modulator. Using this setup, our approach for system characterization and response prediction was tested and the superiority of the matched pulses over other common types of pulses of equal energy was demonstrated.

Laser optoacoustic imaging system for detection of breast cancer

We designed, fabricated and tested the laser optoacoustic imaging system for breast cancer detection (LOIS-64), which fuses optical and acoustic imaging techniques in one modality by utilizing pulsed optical illumination and ultrawide-band ultrasonic detection of resulting optoacoustic (OA) signals. The system was designed to image a single breast slice in craniocaudal or mediolateral projection with an arc-shaped array of 64 ultrawide-band acoustic transducers. The system resolution on breast phantoms was at least 0.5 mm. The single-channel sensitivity of 1.66 mVPa was estimated to be sufficient for single-pulse imaging of 6 to 11 mm tumors through the whole imaging slice of the breast. The implemented signal processing using the wavelet transform allowed significant reduction of the low-frequency (LF) acoustic noise, allowed localization of the optoacoustic signals from tumors, and enhanced the contrast and sharpened the boundaries of the optoacoustic images of the tumors. During the preliminary clinical studies on 27 patients, the LOIS-64 was able to visualize 18 out of 20 malignant lesions suspected from mammography and ultrasound images and confirmed by the biopsy performed after the optoacoustic tomography (OAT) procedure.

Toward contrast-enhanced microwave-induced thermoacoustic imaging of breast cancer: an experimental study of the effects of microbubbles on simple thermoacoustic targets

Microwave-induced thermoacoustic tomography (MI-TAT) is an imaging technique that exploits dielectric contrast at microwave frequencies while creating images with ultrasound resolution. We propose the use of microbubbles as a dielectric contrast agent for enhancing the sensitivity of MI-TAT for breast cancer detection. As an initial investigation of this concept, we experimentally studied the extent to which the microwave-induced thermoacoustic response of a dielectric target is modified by the presence of air-filled glass microbubbles. We created mixtures of ethylene glycol with varying weight percentages of microbubbles and characterized both their microwave properties (0.5-6 GHz) and thermoacoustic response when irradiated with microwave energy at 3 GHz. Our data show that the microbubbles considerably lowered the relative permittivity, electrical conductivity and thermoacoustic response of the ethylene glycol mixtures. We hypothesize that the interstitial infusion of microbubbles to a tumor site will similarly create a smaller thermoacoustic response compared to the pre-contrast-agent response, thereby enhancing sensitivity through the use of differential imaging techniques.

Microwave-induced thermoacoustic scanning CT for high-contrast and noninvasive breast cancer imaging

A fast thermoacoustic computed tomography system with a multielement linear transducer array was developed to image biological tissues with circular scanning. The spatial resolution of the imaging system and the spectra of the thermoacoustic signals were analyzed. A modified integration backprojection algorithm using velocity potential was employed to recover the direct energy deposition distribution, signal processing methods, and reconstruction algorithms were validated by imaging a phantom. The differences of the microwave-frequency dielectric properties between malignant and normal adipose-dominated tissues in the breast are considerable, and the absorption contrast can reach as large as 6:1 at 1.2 GHz. An experiment of human breast tissue with a tumor was performed with this system; the thermoacoustic images reconstructed by a limited-field-filtered backprojection algorithm and a modified integration backprojection algorithm were also compared with a mammogram. Our results show that the system can provide a rapid and noninvasive approach for high-contrast breast cancer imaging.

Low-noise wideband ultrasound detection using polymer microring resonators

Polymer microring resonators for low-noise, wideband ultrasound detection are presented. Using a nanoimprinting technique, we fabricated polymer microring resonators with a quality factor of 6000 resulting in high sensitivity to ultrasound. A noise-equivalent pressure of 0.23 kPa over 1-75 MHz and a detection bandwidth of over 90 MHz at -3 dB were measured. These results demonstrate the potential of polymer microring resonators for high-frequency ultrasound and photoacoustic imaging. For a typical photoacoustic imaging test case, the high sensitivity demonstrated in these devices would increase imaging depth by a factor of 3 compared to state-of-the-art polyvinylidene fluoride detectors.

Photoacoustic imaging of biological tissue with intensity-modulated continuous-wave laser

We build a photoacoustic imaging system using an intensity-modulated continuous-wave laser source, which is an inexpensive, compact, and durable 120-mW laser diode. The goal is to significantly reduce the costs and sizes of photoacoustic imaging systems. By using a bowl-shaped piezoelectric transducer, whose numerical aperture is 0.85 and resonance frequency is 2.45 MHz, we image biological tissues with a lateral resolution of 0.45 mm, an axial resolution of 1 mm, and an SNR as high as 43 dB.

Curved array photoacoustic tomographic system for small animal imaging

We present systematic characterization of a photoacoustic imaging system optimized for rapid, high-resolution tomographic imaging of small animals. The system is based on a 128-element ultrasonic transducer array with a 5-MHz center frequency and 80% bandwidth shaped to a quarter circle of 25 mm radius. A 16-channel data-acquisition module and dedicated channel detection electronics enable capture of a 90-deg field-of-view image in less than 1 s and a complete 360-deg scan using sample rotation within 15 s. Measurements on cylindrical phantom targets demonstrate a resolution of better than 200 microm and high-sensitivity detection of 580-microm blood tubing to depths greater than 3 cm in a turbid medium with reduced scattering coefficient mu(s) (')=7.8 cm(-1). The system is used to systematically investigate the effects of target size, orientation, and geometry on tomographic imaging. As a demonstration of these effects and the system imaging capabilities, we present tomographic photoacoustic images of the brain vasculature of an ex vivo mouse with varying measurement aperture. For the first time, according to our knowledge, resolution of sub-200-microm vessels with an overlying turbid medium of greater than 2 cm depth is demonstrated using only intrinsic biological contrast.

Image distortion in thermoacoustic tomography caused by microwave diffraction

We report an intrinsic image distortion in microwave-induced thermoacoustic tomography. The distortion, due to microwave diffraction in the object to be imaged, leads to nonuniform excitation of acoustic pressure during microwave illumination. Both numerical simulations and phantom experiments demonstrate this phenomenon. A method of partial correction is also provided.

Photoacoustic Doppler effect from flowing small light-absorbing particles

From the flow of a suspension of micrometer-scale carbon particles, the photoacoustic Doppler shift is observed. As predicted theoretically, the observed Doppler shift equals half of that in Doppler ultrasound and does not depend on the direction of laser illumination. This new physical phenomenon provides a basis for developing photoacoustic Doppler flowmetry, which can potentially be used for detecting fluid flow in optically scattering media and especially low-speed blood flow of relatively deep microcirculation in biological tissue.

Photoacoustic effect for multiply scattered light

We consider the photoacoustic effect for multiply scattered light in a random medium. Within the accuracy of the diffusion approximation to the radiative transport equation, we present a general analysis of the sensitivity of a photoacoustic wave to the presence of one or more small absorbing objects. Applications to tumor detection by photoacoustic imaging are suggested.

Three-dimensional finite-element-based photoacoustic tomography: reconstruction algorithm and simulations

In this paper, a finite element reconstruction algorithm for three-dimensional photoacoustic tomography is described. The algorithm is based on rigorous iterative solution to the Helmholtz photoacoustic wave equation coupled with regularization techniques and is able to recover both the images of absorbed optical energy density and acoustic speed simultaneously. The algorithm is tested using various numerical examples that mimic cancer detection and joint imaging. The results show that the algorithm is able to reconstruct photoacoustic images quantitatively in terms of the location, size, optical and acoustic properties of the target, and background media for various examples examined.

Limitations of quantitative photoacoustic measurements of blood oxygenation in small vessels

We investigate the feasibility of obtaining accurate quantitative information, such as local blood oxygenation level (sO2), with a spatial resolution of about 50 microm from spectral photoacoustic (PA) measurements. The optical wavelength dependence of the peak values of the PA signals is utilized to obtain the local blood oxygenation level. In our in vitro experimental models, the PA signal amplitude is found to be linearly proportional to the blood optical absorption coefficient when using ultrasonic transducers with central frequencies high enough such that the ultrasonic wavelengths are shorter than the light penetration depth into the blood vessels. For an optical wavelength in the 578-596 nm region, with a transducer central frequency that is above 25 MHz, the sensitivity and accuracy of sO2 inversion is shown to be better than 4%. The effect of the transducer focal position on the accuracy of quantifying blood oxygenation is found to be negligible. In vivo oxygenation measurements of rat skin microvasculature yield results consistent with those from in vitro studies, although factors specific to in vivo measurements, such as the spectral dependence of tissue optical attenuation, dramatically affect the accuracy of sO2 quantification in vivo.

Thermoacoustic tomography with correction for acoustic speed variations

Thermoacoustic tomography (TAT) is a technique that measures microwave-induced thermoacoustic waves at the boundary of biological tissue and generates images of internal microwave absorption distributions from the measurements. Existing reconstruction algorithms for TAT are based on the assumption that the acoustic properties in the tissue are homogeneous. Biological tissue, however, has heterogeneous acoustic properties, which lead to distortion and blurring of small buried objects in the reconstructed images. In this paper we develop a correction method based on ultrasonic transmission tomography (UTT) to improve the image quality of TAT. Numerical simulations and phantom experiments verify the effectiveness of this correction method.

Transient gratings generated by particulate suspensions: The uniformly irradiated sphere and the point source

Expressions for the time dependence of the state variables in a transient grating experiment carried out on suspensions of particles can be determined by integration over space of the solutions for the temperature and photoacoustic pressure for a single particle. The method relies on independent computation of the thermal and acoustic modes of wave motion which are combined to give the temperature, pressure, and density in the grating as a function of time. Calculations are given for the uniformly irradiated droplet and the point source, the latter including the effects of a temperature-dependent thermal expansion coefficient. Transient grating experiments are reported in colloidal Pt that show features described in the calculation.

Fast calculation of pulsed photoacoustic fields in fluids using k-space methods

Two related numerical models that calculate the time-dependent pressure field radiated by an arbitrary photoacoustic source in a fluid, such as that generated by the absorption of a short laser pulse, are presented. Frequency-wavenumber (k-space) implementations have been used to produce fast and accurate predictions. Model I calculates the field everywhere at any instant of time, and is useful for visualizing the three-dimensional evolution of the wave field. Model II calculates pressure time series for points on a straight line or plane and is therefore useful for simulating array measurements. By mapping the vertical wavenumber spectrum directly to frequency, this model can calculate time series up to 50 times faster than current numerical models of photoacoustic propagation. As the propagating and evanescent parts of the field are calculated separately, model II can be used to calculate far- and near-field radiation patterns. Also, it can readily be adapted to calculate the velocity potential and thus particle velocity and acoustic intensity vectors. Both models exploit the efficiency of the fast Fourier transform, and can include the frequency-dependent directional response of an acoustic detector straightforwardly. The models were verified by comparison with a known analytic solution and a slower, but well-understood, numerical model.

Universal back-projection algorithm for photoacoustic computed tomography

We report results of a reconstruction algorithm for three-dimensional photoacoustic computed tomography. A universal back-projection formula is presented for three types of imaging geometries: planar, spherical, and cylindrical surfaces. A solid-angle weighting factor is introduced in the back-projection formula to compensate for the variations of detection views. A method for implementing this algorithm is described. Numerical simulation is used to demonstrate the performance of the algorithm.

Time-domain reconstruction algorithms and numerical simulations for thermoacoustic tomography in various geometries

In this paper, we present time-domain reconstruction algorithms for the thermoacoustic imaging of biological tissues. The algorithm for a spherical measurement configuration has recently been reported in another paper. Here, we extend the reconstruction algorithms to planar and cylindrical measurement configurations. First, we generalize the rigorous reconstruction formulas by employing Green's function technique. Then, in order to detect small (compared with the measurement geometry) but deeply buried objects, we can simplify the formulas when two practical conditions exist: 1) that the high-frequency components of the thermoacoustic signals contribute more to the spatial resolution than the low-frequency ones, and 2) that the detecting distances between the thermoacoustic sources and the detecting transducers are much greater than the wavelengths of the high-frequency thermoacoustic signals (i.e., those that are useful for imaging). The simplified formulas are computed with temporal back projections and coherent summations over spherical surfaces using certain spatial weighting factors. We refer to these reconstruction formulas as modified back projections. Numerical results are given to illustrate the validity of these algorithms.

The photoacoustic effect generated by an incompressible sphere

An incompressible sphere with a vanishing thermal expansivity suspended in a fluid can generate a photoacoustic effect when the heat deposited in the sphere by a light beam diffuses into the surrounding liquid causing it to expand and launch a sound wave. The properties of the photoacoustic effect for the sphere are found using a Green's function solution to the wave equation for pressure with Neumann boundary conditions. The results of the calculation show that the acoustic wave for fast heat liberation is an outgoing compressive pulse followed by a reflected pulse whose time profile is modified as a result of frequency dependent reflection from the sphere. For slow heat release by the sphere, the photoacoustic effect is shown to be proportional to the first time derivative of the heat flux at the particle-fluid interface.