Photoacoustic Computed Tomography System Research Articles

Development of Low-Cost Fast Photoacoustic Computed Tomography: System Characterization and Phantom Study

A low-cost Photoacoustic Computed Tomography (PACT) system consisting of 16 single-element transducers has been developed. Our design proposes a fast rotating mechanism of 360° rotation around the imaging target, generating comparable images to those produced by large-number-element (e.g., 512, 1024, etc.) ring-array PACT systems. The 2D images with a temporal resolution of 1.5 s and a spatial resolution of 240 µm were achieved. The performance of the proposed system was evaluated by imaging complex phantom. The purpose of the proposed development is to provide researchers a low-cost alternative 2D photoacoustic computed tomography system with comparable resolution to the current high performance expensive ring-array PACT systems.

A Novel Dictionary-Based Image Reconstruction for Photoacoustic Computed Tomography

One of the major concerns in photoacoustic computed tomography (PACT) is obtaining a high-quality image using the minimum number of ultrasound transducers/view angles. This issue is of importance when a cost-effective PACT system is needed. On the other hand, analytical reconstruction algorithms such as back projection (BP) and time reversal, when a limited number of view angles is used, cause artifacts in the reconstructed image. Iterative algorithms provide a higher image quality, compared to BP, due to a model used for image reconstruction. The performance of the model can be further improved using the sparsity concept. In this paper, we propose using a novel sparse dictionary to capture important features of the photoacoustic signal and eliminate the artifacts while few transducers is used. Our dictionary is an optimum combination of Wavelet Transform (WT), Discrete Cosine Transform (DCT), and Total Variation (TV). We utilize two quality assessment metrics including peak signal-to-noise ratio and edge preservation index to quantitatively evaluate the reconstructed images. The results show that the proposed method can generate high-quality images having fewer artifacts and preserved edges, when fewer view angles are used for reconstruction in PACT.

Calibrating reconstruction radius in a multi single-element ultrasound-transducer-based photoacoustic computed tomography system.

In a circular scanning photoacoustic computed tomography (PAT/PACT) system, a single-element ultrasound transducer (SUT) (rotates in full 360° around the sample) or a full-ring array transducer is used to acquire the photoacoustic (PA) data from the target object. SUT takes several minutes to acquire the PA data, whereas the full-ring array transducer takes only few seconds. Hence, for real-time imaging, full-ring circular array transducers are preferred. However, these are custom built, very expensive, and not available readily on the market, whereas SUTs are cheap and easily available. Thus, PACT systems can be made cost effective by using SUTs. To improve the data acquisition speed, multiple SUTs can be employed at the same time. This will reduce the acquisition time by N-fold if N numbers of SUTs are used, each rotating 360/N degrees. Experimentally, all SUTs cannot be placed exactly at the same distance from the scanning center. Hence, the acquired PA data from each transducer need to be reconstructed with their corresponding radii in a delay-and-sum reconstruction algorithm. This requires the exact location of each SUT from the scanning center. Here, we propose a calibration method to find out the distance from the scanning center at which each SUT acquires the PA data. Three numerical phantoms were used to show the efficacy of the proposed method, and later it was validated with experimental data (point source phantom).

Internal-illumination photoacoustic computed tomography.

We report a photoacoustic computed tomography (PACT) system using a customized optical fiber with a cylindrical diffuser to internally illuminate deep targets. The traditional external light illumination in PACT usually limits the penetration depth to a few centimeters from the tissue surface, mainly due to strong optical attenuation along the light propagation path from the outside in. By contrast, internal light illumination, with external ultrasound detection, can potentially detect much deeper targets. Different from previous internal illumination PACT implementations using forward-looking optical fibers, our internal-illumination PACT system uses a customized optical fiber with a 3-cm-long conoid needle diffuser attached to the fiber tip, which can homogeneously illuminate the surrounding space and substantially enlarge the field of view. We characterized the internal illumination distribution and PACT system performance. We performed tissue phantom and in vivo animal studies to further demonstrate the superior imaging depth using internal illumination over external illumination. We imaged a 7.5-cm-deep leaf target embedded in optically scattering medium and the beating heart of a mouse overlaid with 3.7-cm-thick chicken tissue. Our results have collectively demonstrated that the internal light illumination combined with external ultrasound detection might be a useful strategy to improve the penetration depth of PACT in imaging deep organs of large animals and humans.

Development of low-cost photoacoustic imaging systems using very low-energy pulsed laser diodes.

With the growing application of photoacoustic imaging (PAI) in medical fields, there is a need to make them more compact, portable, and affordable. Therefore, we designed very low-cost PAI systems by replacing the expensive and sophisticated laser with a very low-energy laser diode. We implemented photoacoustic (PA) microscopy, both reflection and transmission modes, as well as PA computed tomography systems. The images obtained from tissue-mimicking phantoms and biological samples determine the feasibility of using a very low-energy laser diode in these configurations.

High-resolution deep functional imaging of the whole mouse brain by photoacoustic computed tomography in vivo.

Photoacoustic computed tomography (PACT) is a non-invasive imaging technique offering high contrast, high resolution, and deep penetration in biological tissues. We report a PACT system equipped with a high frequency linear transducer array for mapping the microvascular network of a whole mouse brain with the skull intact and studying its hemodynamic activities. The linear array was scanned in the coronal plane to collect data from different angles, and full-view images were synthesized from the limited-view images in which vessels were only partially revealed. We investigated spontaneous neural activities in the deep brain by monitoring the concentration of hemoglobin in the blood vessels and observed strong interhemispherical correlations between several chosen functional regions, both in the cortical layer and in the deep regions. We also studied neural activities during an epileptic seizure and observed the epileptic wave spreading around the injection site and the wave propagating in the opposite hemisphere.

Dry coupling for whole-body small-animal photoacoustic computed tomography.

We have enhanced photoacoustic computed tomography with dry acoustic coupling that eliminates water immersion anxiety and wrinkling of the animal and facilitates incorporating complementary modalities and procedures. The dry acoustic coupler is made of a tubular elastic membrane enclosed by a closed transparent water tank. The tubular membrane ensures water-free contact with the animal, and the closed water tank allows pressurization for animal stabilization. The dry coupler was tested using a whole-body small-animal ring-shaped photoacoustic computed tomography system. Dry coupling was found to provide image quality comparable to that of conventional water coupling.

Photoacoustic elastography.

Elastography can noninvasively map the elasticity distribution in biological tissue, which can potentially be used to reveal disease conditions. In this Letter, we have demonstrated photoacoustic elastography by using a linear-array photoacoustic computed tomography system. The feasibility of photoacoustic elastography was first demonstrated by imaging the strains of single-layer and bilayer gelatin phantoms with various stiffness values. The measured strains agreed well with theoretical values, with an average error of less than 5.2%. Next, in vivo photoacoustic elastography was demonstrated on a mouse leg, where the fat and muscle distribution was mapped based on the elasticity contrast. We confirmed the photoacoustic elastography results by ultrasound elastography performed simultaneously.

Photoacoustic tomography of vascular compliance in humans.

Characterization of blood vessel elastic properties can help in detecting thrombosis and preventing life-threatening conditions such as acute myocardial infarction or stroke. Vascular elastic photoacoustic tomography (VE-PAT) is proposed to measure blood vessel compliance in humans. Implemented on a linear-array-based photoacoustic computed tomography system, VE-PAT can quantify blood vessel compliance changes due to simulated thrombosis and occlusion. The feasibility of the VE-PAT system was first demonstrated by measuring the strains under uniaxial loading in perfused blood vessel phantoms and quantifying their compliance changes due to the simulated thrombosis. The VE-PAT system detected a decrease in the compliances of blood vessel phantoms with simulated thrombosis, which was validated by a standard compression test. The VE-PAT system was then applied to assess blood vessel compliance in a human subject. Experimental results showed a decrease in compliance when an occlusion occurred downstream from the measurement point in the blood vessels, demonstrating VE-PAT’s potential for clinical thrombosis detection.

In vivo deep brain imaging of rats using oral-cavity illuminated photoacoustic computed tomography.

Using internal illumination with an optical fiber in the oral cavity, we demonstrate, for the first time, photoacoustic computed tomography (PACT) of the deep brain of rats in vivo. The experiment was performed on a full-ring-array PACT system, with the capability of providing high-speed cross-sectional imaging of the brain. Compared with external illumination through the cranial skull, internal illumination delivers more light to the base of the brain. Consequently, in vivo photoacoustic images clearly reveal deep brain structures such as the hypothalamus, brain stem, and cerebral medulla.

Tripling the detection view of high-frequency linear-array-based photoacoustic computed tomography by using two planar acoustic reflectors.

Linear-array-based photoacoustic computed tomography (PACT) suffers from a limited view. Circular scanning does increase the detection view angle but is time-consuming. Therefore, it is desirable to increase the detection view angle of linear-array-based PACT without sacrificing imaging speed. Two planar acoustic reflectors placed at 120 degrees to each other were added to a linear-array-based PACT system. Each reflector redirects originally undetectable photoacoustic waves back to the transducer array elements, and together they triple the original detection view angle of the PACT system. Adding two reflectors increased the detection view angle from 80 to 240 degrees. As a comparison, a single-reflector PACT has a detection view angle of only 160 degrees. A leaf skeleton phantom with a rich vascular network was imaged with the double-reflector PACT, and most of its features were recovered. The two acoustic reflectors triple the detection view angle of a linear-array-based PACT without compromising the original imaging speed. This nearly full-view detection capability produces higher-quality images than single-reflector PACT or conventional PACT without reflectors.

Retrospective respiration-gated whole-body photoacoustic computed tomography of mice.

Photoacoustic tomography (PAT) is an emerging technique that has a great potential for preclinical whole-body imaging. To date, most whole-body PAT systems require multiple laser shots to generate one cross-sectional image, yielding a frame rate of <1 Hz. Because a mouse breathes at up to 3 Hz, without proper gating mechanisms, acquired images are susceptible to motion artifacts. Here, we introduce, for the first time to our knowledge, retrospective respiratory gating for whole-body photoacoustic computed tomography. This new method involves simultaneous capturing of the animal's respiratory waveform during photoacoustic data acquisition. The recorded photoacoustic signals are sorted and clustered according to the respiratory phase, and an image of the animal at each respiratory phase is reconstructed subsequently from the corresponding cluster. The new method was tested in a ring-shaped confocal photoacoustic computed tomography system with a hardware-limited frame rate of 0.625 Hz. After respiratory gating, we observed sharper vascular and anatomical images at different positions of the animal body. The entire breathing cycle can also be visualized at 20 frames/cycle.

Enhancement of photoacoustic tomography by ultrasonic computed tomography based on optical excitation of elements of a full-ring transducer array

Photoacoustic computed tomography (PACT) is a hybrid technique that combines optical excitation and ultrasonic detection to provide high-resolution images in deep tissues. In the image reconstruction, a constant speed of sound (SOS) is normally assumed. This assumption, however, is often not strictly satisfied in deep tissue imaging, due to acoustic heterogeneities within the object and between the object and the coupling medium. If these heterogeneities are not accounted for, they will cause distortions and artifacts in the reconstructed images. In this Letter, we incorporated ultrasonic computed tomography (USCT), which measures the SOS distribution within the object, into our full-ring array PACT system. Without the need for ultrasonic transmitting electronics, USCT was performed using the same laser beam as for PACT measurement. By scanning the laser beam on the array surface, we can sequentially fire different elements. As a first demonstration of the system, we studied the effect of acoustic heterogeneities on photoacoustic vascular imaging. We verified that constant SOS is a reasonable approximation when the SOS variation is small. When the variation is large, distortion will be observed in the periphery of the object, especially in the tangential direction.

Tumor glucose metabolism imagedin vivoin small animals with whole-body photoacoustic computed tomography

With the increasing use of small animals for human disease studies, small-animal whole-body molecular imaging plays an important role in biomedical research. Currently, none of the existing imaging modalities can provide both anatomical and glucose molecular information, leading to higher costs of building dual-modality systems. Even with image co-registration, the spatial resolution of the molecular imaging modality is not improved. Utilizing a ring-shaped confocal photoacoustic computed tomography system, we demonstrate, for the first time, that both anatomy and glucose uptake can be imaged in a single modality. Anatomy was imaged with the endogenous hemoglobin contrast, and glucose metabolism was imaged with a near-infrared dye-labeled 2-deoxyglucose.