Optical & Ultrasonic Imaging Laboratory

Trans-illumination intestine projection imaging of intestinal motility in mice

Functional intestinal imaging holds importance for the diagnosis and evaluation of treatment of gastrointestinal diseases. Currently, preclinical imaging of intestinal motility in animal models is performed either invasively with excised intestines or noninvasively under anesthesia, and cannot reveal intestinal dynamics in the awake condition. Capitalizing on near-infrared optics and a high-absorbing contrast agent, we report the Trans-illumination Intestine Projection (TIP) imaging system for free-moving mice. After a complete system evaluation, we performed in vivo studies, and obtained peristalsis and segmentation motor patterns of free-moving mice. We show the in vivo typical segmentation motor pattern, that was previously shown in ex vivo studies to be controlled by intestinal pacemaker cells. We also show the effects of anesthesia on motor patterns, highlighting the possibility to study the role of the extrinsic nervous system in controlling motor patterns, which requires unanesthetized live animals. Combining with light-field technologies, we further demonstrated 3D imaging of intestine in vivo (3D-TIP). Importantly, the added depth information allows us to extract intestines located away from the abdominal wall, and to quantify intestinal motor patterns along different directions. The TIP system should open up avenues for functional imaging of the GI tract in conscious animals in natural physiological states.

D. Wang, H. Zhang, T. Vu, Y. Zhan, A. Malhotra, P. Wang, U. Chitgupi, A. Rai, S. Zhang, L. Wang, J. D. Huizinga, J. F. Lovell & J. Xia*, Trans-illumination intestine projection imaging of intestinal motility in mice, Nature Communications, 12, 1682, 2021

Dual Scan Mammoscope (DSM) — A New Portable Photoacoustic Breast Imaging System with Scanning in Craniocaudal Plane

Objective: We present a new photoacoustic tomography system that provides visualization of angiographic features in a human breast with mammogram-like images. Methods: The system images a mildly compressed breast, from both top and bottom, using two 128-element, 2.25 MHz linear transducer arrays and line optical fiber bundles. The mild compression is achieved using plastic films, which is a more comfortable experience for the patient compared to rigid metal plates used in a traditional mammogram. Results: We could image a D cup-sized breast of 7 cm thickness within 1 minute and achieved a spatial resolution of around 1 mm in all directions. Conclusion: Our system possesses the benefits of portability, speedy scanning, and patient comfort. The craniocaudal-view images can be easily correlated with existing imaging modalities for data interpretation. Significance: Early cancer detection plays a critical role in overall cancer survival rate. Our system may address the limitations of mammogram and offer a radiation-free screening technique for patients with dense breasts.

N. Nyayapathi, R. Lim, H. Zhang, W. Zheng, Y. Wang, M. Tiao, K.W. Oh, X.C. Fan, E. Bonaccio, K. Takabe, J. Xia. Dual Scan Mammoscope (DSM) — A New Portable Photoacoustic Breast Imaging System with Scanning in Craniocaudal Plane. IEEE Transactions on Biomedical Engineering 2019.

A portable three-dimensional photoacoustic tomography system for imaging of chronic foot ulcers

Background: chronic leg ulcers affect approximately 6.5 million americans and the disorder is associated with a range of serious complications. since many chronic ulcers have underlying vascular insufficiency, accurate assessment of tissue perfusion is critical to treatment planning and post-surgical monitoring. however, existing clinical tests fail to meet this need in practice due to their low sensitivity or accuracy. Methods: in this paper, we introduce a portable photoacoustic tomography (pat) system for wound assessment. since hemoglobin serves as the major endogenous contrast at near-infrared wavelengths, pat provides label-free, three-dimensional imaging of hemoglobin distribution, which is closely related to blood perfusion. the proposed system consists of a 128-element linear transducer array, a data acquisition (daq) system, and a pulsed nd:yag laser source, all mounted on a portable cart for easy clinical testing. Results: we validated our system through both phantom and human imaging studies. the phantom imaging results indicate that the system’s spatial resolution ranges from 0.5 mm along the axial direction to 1.3 mm along the elevational direction. the healthy volunteer result shows clear foot vasculature, indicating good perfusion. the preliminary patient imaging results agree very well with the clinical test, demonstrating that pat has a high potential for assessing the circulation around the wound. Conclusions: we believe that our technique will be a valuable tool for assessing tissue perfusion and guiding wound treatment in vascular clinics.

Wang Yuehang, Ye Zhan, Linda M. Harris, Sikandar Khan, and Jun Xia. A portable three-dimensional photoacoustic tomography system for imaging of chronic foot ulcers. Quantitative Imaging in Medicine and Surgery, 2019.

A Robust and Secure Palm Vessel Biometric Sensing System Based on Photoacoustics

In this work, we propose a new biometric sensing system based on photoacoustic imaging (PAI), which is an emerging technique that allows high-resolution visualization of optical absorption in deep tissue. Our system consists of an ultrasound linear transducer array, an ultrasound data acquisition system, and an Nd:YAG laser emitting 1064 nm wavelength. By scanning the array, we could get a three-dimensional (3D) image of palm vasculature. The 3D image is further combined with our newly developed algorithm, Earth Mover’s Distance - Radiographic Testing (EMD-RT), to provide precise matching and robust recognition rate. Compared to conventional vein sensing techniques, our system demonstrates deeper imaging depth and better spatial resolution, offering securer biometric features to fight against spoofs. In this study, we imaged twenty different hands at various poses and quantified our system performance. We found that the usability and accuracy of our system are comparable to conventional biometric techniques, such as fingerprint imaging and face identification. Our technique can open up avenues for better liveness detection and biometric measurements.

Wang Yuehang, Zhengxiong Li, Tri Vu, Nikhila Nyayapathi, Kwang W Oh, Wenyao Xu, and Jun Xia. A Robust and Secure Palm Vessel Biometric Sensing System Based on Photoacoustics. IEEE Sensors Journal 18(14): p. 5993-6000, 2018.

Ingestible Roasted Barley for Contrast-enhanced Photoacoustic Imaging in Animal and Human Subjects

Photoacoustic computed tomography (PACT) is an emerging imaging modality. While many contrast agents have been developed for PACT, these typically cannot immediately be used in humans due to the lengthy regulatory process. We screened two hundred types of ingestible foodstuff samples for photoacoustic contrast with 1064 nm pulse laser excitation, and identified roasted barley as a promising candidate. Twenty brands of roasted barley were further screened to identify the one with the strongest contrast, presumably based on complex chemical modifications incurred during the roasting process. Individual roasted barley particles could be detected through 3.5 cm of chicken-breast tissue and through the whole hand of healthy human volunteers. With PACT, but not ultrasound imaging, a single grain of roasted barley was detected in a field of hundreds of non-roasted particles. Upon oral administration, roasted barley enabled imaging of the gut and peristalsis in mice. Prepared roasted barley tea could be detected through 2.5 cm chicken breast tissue. When barley tea was administered to humans, photoacoustic imaging visualized swallowing dynamics in healthy volunteers. Thus, roasted barley represents an edible foodstuff that should be considered for photoacoustic contrast imaging of swallowing and gut processes, with immediate potential for clinical translation.

Depeng Wang, Dong Hyeun Lee, Haoyuan Huang, Tri Vu, Rachel Su Ann Lim, Nikhila Nyayapathi, Upendra Chitgupi, Maggie Liu, Jumin Geng, Jun Xia, Jonathan F. Lovell, Ingestible Roasted Barley for Contrast-enhanced Photoacoustic Imaging in Animal and Human Subjects, Biomaterials 175, 72-81 (2018)

Deep Tissue Photoacoustic Computed Tomography with a Fast and Compact Laser System

Photoacoustic computed tomography (PACT) holds great promise for biomedical imaging, but wide-spread implementation is impeded by the bulkiness of flash-lamp-pumped laser systems, which typically weigh between 50 - 200 kg, require continuous water cooling, and operate at a low repetition rate. Here, we demonstrate that compact lasers based on emerging diode technologies are well-suited for preclinical and clinical PACT. The diode-pumped laser used in this study had a miniature footprint (13×14×7cm3), weighed only 1.6 kg, and outputted up to 80 mJ per pulse at 1064 nm. In vitro, the laser system readily provided over 4 cm PACT depth in chicken breast tissue. In vivo, in addition to high resolution, non-invasive brain imaging in living mice, the system can operate at 50 Hz, which enabled high-speed cross-sectional imaging of murine cardiac and respiratory function. The system also provided high quality, high-frame rate, and non-invasive three-dimensional mapping of arm, palm, and breast vasculature at multi centimeter depths in living human subjects, demonstrating the clinical viability of compact lasers for PACT.

Depeng Wang, Yuehang Wang, Weiran Wang, Dandan Luo, Upendra Chitgupi, Jumin Geng, Yang Zhou, Lidai Wang, Jonathan F Lovell, and Jun Xia, Deep tissue photoacoustic computed tomography with a fast and compact laser system, Biomed. Opt. Express 8, 112-123 (2017)

Coherent-Weighted 3D Image Reconstruction in Linear-array-based Photoacoustic Tomography

While the majority of photoacoustic imaging systems used custom-made transducer arrays, commercially-available linear transducer arrays hold the benefits of affordable price, handheld convenience and wide clinical recognition. They are not widely used in photoacoustic imaging primarily because of the poor elevation resolution. Here, without modifying the imaging geometry and system, we propose addressing this limitation purely through image reconstruction. Our approach is based on the integration of two advanced image reconstruction techniques: focal-line-based three-dimensional image reconstruction and coherent weighting. We first numerically validated our approach through simulation and then experimentally tested it in phantom and in vivo. Both simulation and experimental results proved that the method can significantly improve the elevation resolution (up to 4 times in our experiment) and enhance object contrast.

Depeng Wang, Yuehang Wang, Yang Zhou, Jonathan F. Lovell and Jun Xia, Coherent-weighted three-dimensional image reconstruction in linear-array-based photoacoustic tomography, Biomedical Optics Express, 7(5), 1957-1965 (2016)

A Phosphorus Phthalocyanine Formulation with Intense Absorbance at 1000nm for Deep Optical Imaging

Although photoacoustic computed tomography (PACT) operates with high spatial resolution in biological tissues deeper than other optical modalities, light scattering is a limiting factor. The use of longer near infrared wavelengths reduces scattering. Recently, the rational design of a stable phosphorus phthalocyanine (P-Pc) with a long wavelength absorption band beyond 1000 nm has been reported. Here, we show that when dissolved in biocompatible liquid surfactants, P-Pc can give rise to formulations with intense absorption values of greater than 1000 at wavelengths beyond 1000 nm. Using the broadly accessible Nd:YAG pulse laser emission output of 1064 nm, P-Pc could be imaged through 11.6 cm of chicken breast with PACT. P-Pc accumulated passively in tumors following intravenous injection in mice as observed by PACT. Following oral administration, P-Pc passed through the intestine harmlessly, and PACT could be used to non-invasively observe intestine function. When placed under the arm of a healthy adult human, a PACT transducer on the top of the arm could readily detect P-Pc across an entire 5 cm arm. Thus, the approach of using biocompatible media with extreme absorption at 1064 nm readily enables high quality optical imaging in vitro and in vivo in humans at exceptional depths.

Yang Zhou, Depeng Wang, Yumiao Zhang, Upendra Chitgupi, Jumin Geng, Yuehang Wang, Yuzhen Zhang, Timothy R. Cook, Jun Xia and Jonathan F. Lovell, A Phosphorus Phthalocyanine Formulation with Intense Absorbance at 1000 nm for Deep Optical Imaging, Theranostics, 6(5), 688 (2016)

Slit-enabled Linear-array PAT with Near Isotropic Spatial Resolution in Three Dimension

Due to its unique capability of visualizing optical absorption in deep tissues, photoacoustic tomography is increasingly used in biomedical imaging. Among various types of transducer arrays, the linear array is perhaps the most widely used in photoacoustic tomography because it is commercially available and readily allows ultrasound imaging. However, the three-dimensional imaging capability of a linear array is limited due to its poor elevational resolution. While various scanning schemes have been proposed to address this problem, they all suffer from long scanning time to the best of our knowledge. To address this issue, we introduce slit-enabled three-dimensional photoacoustic tomography. The metal slit, placed at the array focus, causes the incoming photoacoustic waves to diffract along the elevation direction and, hence, significantly improves the elevation detection aperture and resolution. We tested the new system in both phantoms and animals. The slit improves the elevation resolution by 10 times without compromising scanning time.

Wang, Y., Wang, D., Zhang, Y., Geng, J., Lovell, J. F. and Xia, J, Slit-enabled linear-array photoacoustic tomography with near isotropic spatial resolution in three dimensions, Optics Letters, 41(1), 127-130 (2016)

Anatomical and Vascular Imaging

We report a novel small-animal whole-body imaging system called ring-shaped confocal photoacoustic computed tomography (RC-PACT). RC-PACT is based on a confocal design of free-space ring-shaped light illumination and 512-element full-ring ultrasonic array signal detection. The free-space light illumination maximizes the light delivery efficiency, and the full-ring signal detection ensures a full two-dimensional view aperture for accurate image reconstruction. Using cylindrically focused array elements, RC-PACT can image a thin cross section with 0.10 to 0.25mm in-plane resolutions and 1.6 s/frame acquisition time. By translating the mouse along the elevational direction, RC-PACT provides a series of cross-sectional images of the brain, liver, kidneys, and bladder.

Xia, J., Chatni, M., Maslov, K., Guo, Z., Wang, K., Anastasio, M., Wang and L. V. Whole-body ring-shaped confocal photoacoustic computed tomography of small animals in vivo. Journal of Biomedical Optics, 17(5), 050506 (2012)

Functional Imaging

Photoacoustic tomography (PAT) is a hybrid imaging technique that has broad preclinical and clinical applications. Based on the photoacoustic effect, PAT directly measures specific optical absorption, which is the product of the tissue-intrinsic optical absorption coefficient and the local optical fluence. Therefore, quantitative PAT, such as absolute oxygen saturation (sO2) quantification, requires knowledge of the local optical fluence, which can only be estimated through invasive measurements or sophisticated modeling of light transportation. In this Letter, we circumvent this requirement by taking advantage of the dynamics in sO2. The new method works when the sO2 transition can be simultaneously monitored with multiple wavelengths. For each wavelength, the ratio of photoacoustic amplitudes measured at different sO2 states is utilized. Using the ratio cancels the contribution from optical fluence and allows calibration-free quantification of absolute sO2. The new method was validated through both phantom and in vivo experiments.

Xia, J., Danielli, A., Liu, Y., Wang, L., Maslov, K., Wang and L. V., Calibration-free quantification of absolute oxygen saturation based on the dynamics of photoacoustic signals, Opt. Lett., 38(15), 2800-2803 (2013)

Molecular Imaging

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.

Chatni, M. R., Xia, J., Sohn, R., Maslov, K., Guo, Z., Zhang, Y., Wang, K., Xia, Y., Anastasio, M., Arbeit, J. and Wang, L. V. Tumor glucose metabolism imaged in vivo in small animals with whole-body photoacoustic computed tomography, Journal of Biomedical Optics, 17(7), 076012 (2012)

Neurological Imaging

The increasing use of mouse models for human brain disease studies presents an emerging need for a new functional imaging modality. Using optical excitation and acoustic detection, we developed a functional connectivity photoacoustic tomography system, which allows noninvasive imaging of resting-state functional connectivity in the mouse brain, with a large field of view and a high spatial resolution. Bilateral correlations were observed in eight functional regions, including the olfactory bulb, limbic, parietal, somatosensory, retrosplenial, visual, motor, and temporal regions, as well as in several subregions. The borders and locations of these regions agreed well with the Paxinos mouse brain atlas. By subjecting the mouse to alternating hyperoxic and hypoxic conditions, strong and weak functional connectivities were observed, respectively. In addition to connectivity images, vascular images were simultaneously acquired. These studies show that functional connectivity photoacoustic tomography is a promising, noninvasive technique for functional imaging of the mouse brain.

Nasiriavanaki, M., Xia, J., Wan, H., Bauer, A. Q., Culver, J. P., and Wang, L. V., High-resolution photoacoustic tomography of resting-state functional connectivity in the mouse brain, Proc Natl Acad Sci U S A, 111(1), 21-26. doi: 10.1073/pnas.1311868111 (2014)

Combined PAT and Microscopy Imaging

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.

Xia, J., Huang, C., Maslov, K., Anastasio, M. A., Wang, L. V., Enhancement of photoacoustic tomography by ultrasonic computed tomography based on optica excitation of elements of a full-ring transducer array, Opt. Lett., 38(16), 3140-3143 (2013)

Combined PAT and High Intensity Focused Ultrasound Imaging for Flow Measurement

Blood flow speed is an important functional parameter. Doppler ultrasound flowmetry lacks sufficient sensitivity to slow blood flow (several to tens of millimeters per second) in deep tissue. To address this challenge, we developed ultrasonically encoded photoacoustic flowgraphy combining ultrasonic thermal tagging with photoacoustic imaging. Focused ultrasound generates a confined heat source in acoustically absorptive fluid. Thermal waves propagate with the flow and are directly visualized in pseudo color using photoacoustic computed tomography. The Doppler shift is employed to calculate the flow speed. This method requires only acoustic and optical absorption, and thus is applicable to continuous fluid. A blood flow speed as low as 0.24mm⋅s−1 was successfully measured. Deep blood flow imaging was experimentally demonstrated under 5-mm-thick chicken breast tissue.

Wang, L., Xia, J., Yao, J., Maslov, K. I., and Wang, L. V., Ultrasonically Encoded Photoacoustic Flowgraphy in Biological Tissue, Physical Review Letters, 111(20), 204301 (2013)

Research Funding