This Conference Presentation, “Functional photoacoustic tomography of the human brain,” was recorded at SPIE Photonics West held in San Francisco, California, United States

Currently, there is an inability to obtain fast realistic label-free virtual histopathological images of tissues. We previously introduced ultraviolet photoacoustic remote sensing microscopy as a method to obtain virtual hematoxylin contrast albeit without the ability to obtain virtual eosin contrast. By utilizing UV scattering as a high-resolution eosin channel we are able to produce complete H&E-like virtual histology of unstained human breast lumpectomy specimen sections. By further leveraging a novel colormap matching algorithm with this UV scattering, we generate H&E-like output that is shown to have strong concordance with true H&E-stained adjacent sections, showing promising diagnostic utility.

A combined synthetic aperture optical coherence microscopy and ultraviolet photoacoustic remote sensing system is presented, capable of fast scanning of tissues for tumor margin inspection. It provides a fast 3D OCT mode for imaging tissues to depths of ~1mm, and a superficial virtual histology mode provided by absorption contrast UV-PARS for virtual hematoxylin contrast and coherence-gated scattering microscopy for virtual eosin contrast. Breast lumpectomy specimens are scanned in each mode to evaluate the extent of features in depth and generate en-face images with histological detail and realism, providing results accurately interpretable by pathologists.

This study examined the performance of a clinical translatable needle photoacoustic sensing probe in intact prostates removed through prostatectomy procedures. Compared to the prototype probe in our previous studies, the latest version probe possesses fiber optic hydrophone with a dome-shaped tip that provides better sensitivity and sampling volume. The optical components were integrated into a customized clinical standard steel needle with a side opening and protected by medical grade polyurethane. The needle probe was examined in a setup mimicking a transrectal ultrasound guided transperineal prostate biopsy procedure. Preliminary tests showed promising results in differentiating between benign and aggressive cancer tissues in prostate.

The feasibility of using photoacoustic imaging (PAI) to measure electrically-evoked hemodynamic responses in a squirrel monkey brain in vivo was examined. A linear-array photoacoustic computed tomography (PACT) system and a high-resolution photoacoustic microscopy (PAM) system were built for imaging subcortical and cortical brain regions, respectively. The hemodynamic responses at multiple cortices, including premotor, primary motor, and primary somatosensory cortices, were monitored. The variations could be observed in all cortices and their underlying cortical and subcortical brain regions. The results from this study validated the potential of PAI technique for multiscale and multi-resolution functional brain mapping for non-human primates.

In this work, a synchronized dual-modal imaging system is used for in-vivo, non-contact ophthalmic imaging. The apparatus is comprised of both Photoacoustic Remote Sensing (PARS) and Swept-Source Optical Coherence Tomography (SS-OCT) subsystems. The PARS utilizes a multi-wavelength excitation source to target hemoglobin absorption and an 830 nm interrogation source to detect photoacoustic signals. PARS provides the measurements for computing blood oxygen saturation (sO2) mapping in the mouse and rat eyes. Meanwhile, a 1060 nm SS-OCT is employed to obtain volumetric tissue structure. To our knowledge, this is the first report of non-contact functional photoacoustic imaging in ophthalmic applications.

This Conference Presentation, “3D ultrasound guided-photoacoustic vascular imaging of pancreatic tumors to predict response to tyrosine kinase inhibitor therapy,” was recorded at SPIE Photonics West held in San Francisco, California, United States

This Conference Presentation, “Photoacoustic image guided oxygen enhanced photodynamic therapy of hypoxic tumors,” was recorded at SPIE Photonics West held in San Francisco, California, United States

Excessive bleaching can cause changes in the amino acid composition as well as the surface structure of the hair. Thus, proper hair bleaching and quantification is important to achieve cosmetic purposes while maintaining the healthy properties of the hair. Here, we propose a novel method to quantify the degree of hair bleaching at the nanoscale resolution using a photoactivated atomic force microscopy (pAFM). We demonstrated that acquiring and quantifying pAFM images of hair according to bleaching time can help determine the appropriate bleaching time. We believe that this result will help to prevent unwanted hair damage due to bleaching.

In molecular photoacoustic imaging, the administration of an external photoacoustic contrast agent increases the photoacoustic yield and clarifies pathophysiological observation over the lesion site. Recognizing the necessity of a cyanine-based dye that complements the optical properties of ready-approved indocyanine green, we proceeded with molecular photoacoustic imaging in vitro and in vivo using cypate with a high absorption rate and photothermal conversion yield in the near-infrared I region. In addition, the particles are functionalized with tumor-avid peptide to facilitate receptor-based active delivery.

A non-contact, dual-modal imaging apparatus is introduced which will be used to obtain the Metabolic Rate of Oxygen (mRO2) in the live murine eye, based on the blood flow rate and blood Oxygen Saturation (sO2) measurements. The apparatus is comprised of both Photoacoustic Remote Sensing (PARS) and Swept-Source Optical Coherence Tomography (SS-OCT) systems, operating synchronously. A phantom model will be imaged using the proposed system to validate the accuracy of the blood flow and sO2 measurements. To the best of our knowledge, this work would report for the first time, non-contact, in-vivo measurement of the mRO2 in the ophthalmic tissues.

We present a new functional photoacoustic microscopy system with the highest imaging speed and ultrawide field of view. The high imaging speed is enabled by a 12-facets polygon for fast scanning and a Raman-shifter system for fast dual-wavelength measurement of oxygen saturation in vivo. we imaged the dynamic functions in mouse brains in response to hypoxia challenge, sodium nitroprusside (SNP), and ischemic stroke. The experimental results have demonstrated that the high-speed photoacoustic microscopy system can be a powerful tool for studying the rapid hemodynamics in the mouse brains of a wide range of pathological and physiological models.

Following resection of cancerous tissues, specimens are excised from the surgical margins to be examined post-operatively for the presence of residual cancer cells. Hematoxylin and eosin (H&E) staining is the gold standard of histopathological assessment. Ultraviolet photoacoustic microscopy (UV-PARS), combined with scattering microscopy, provides virtual nuclei and cytoplasm contrast similar to H&E staining. A generative adversarial network (GAN) deep learning approach, specifically a CycleGAN, was used to perform style transfer to improve the histological realism of UV-PARS generated images. Post-CycleGAN images are easier for a pathologist to examine and can be input into existing machine learning pipelines for H&E-stained images.

Ironizing radiation acoustic imaging (iRAI) is a novel imaging concept with the potential to map the radiation dose delivery in real time during external beam radiation therapy. In this study, iRAI volumetric dose mapping was achieved with 2D matrix transducer array using a C-shape 3D conformal treatment plan with clinically relevant setting and a moving beam plan in both phantoms and rabbit model in vivo. With the unique ability to map the volumetric dose delivery in real time, iRAI 3D dose mapping can be developed into a new tool for quantifying the accuracy of dose delivery of radiation therapy.

The current functional brain mapping techniques such as fMRI and DOI suffer from limited spatial resolution. Photoacoustic (PA) imaging combines the sensitivity of optical imaging to hemodynamic variations, and spatial resolution of ultrasound detection. In this study, we built a label-free PA computed tomography (PACT) system with a ring-shaped ultrasound array to monitor the hemodynamic changes in the primary visual cortex (V1) of mice in response to retinal photostimulation. The responses of wild-type and retinal degenerate (rd1) mice were compared. A linear-array PACT system was also used to measure the visually-evoked subcortical responses. Therefore, PACT is potential tool to study the effect of retinal degeneration of mice on the visual pathway.

We present a new technique for improving the temporal stability of the Stimulated Raman Scattering (SRS)-based multispectral pulsed source by decreasing the temperature of the SRS medium. This technique reduces temporal fluctuations of the output SRS peaks, generates stable multiwavelength light, improves repeatability and accuracy of functional measurements. This stabilized temperature-regulated SRS-based source is combined with the wide field of view photoacoustic remote sensing microscope utilizing a telecentric scan lens as an imaging objective. In-vivo functional imaging experiments of the chorioallantois membrane of a chicken embryo (CAM) are performed for validation purposes.

This Conference Presentation, “Collagen detection in silk protein-based scaffolds through ultrasound and photoacoustic imaging,” was recorded at SPIE Photonics West held in San Francisco, California, United States

Ultraviolet photoacoustic remote sensing (UV-PARS) microscopy is a non-contact imaging modality capable of producing label-free absorption contrast images of cell nuclei. This virtual hematoxylin-like imaging combined with virtual eosin-like data from 1310 nm scattering microscopy can provide complete virtual H&E histologically in unstained tissues. Here, we develop contour scanning for applying UV-PARS and scattering-based virtual histology in fresh and formalin-fixed thick tissues. Our spectral-domain OCT-guided approach initially scans specimens in 3D, and a custom algorithm extracts the surface contour. A high-resolution UV-PARS scan is then performed using a z-axis stage for dynamic focusing to compensate for sample surface irregularities.

Understanding thrombosis formation is necessary for developing safe and effective treatments. We fabricated sophisticated in vitro models of blood vessels with internal microchannels by using digital light processing-based 3D printing method. Photoacoustic microscopy (PAM) offers a useful platform for imaging 3D-printed vascular structures with different patterns of microchannels. Our results show that PAM can provide clear microchannel structures at depths up to 3.6 mm. We further quantified the blood oxygenation in the 3D-printed vascular models, showing that thrombi had much lower oxygenation than the normal blood. Integration of PAM with 3D printing/bioprinting will enable numerous applications in tissue engineering.

Phantoms were designed to evaluate boundary buildup effect and object detectability. Tissue-mimicking materials simulating breast fat and parenchyma were prepared through emulsification of silicone oil, ethylene glycol and polyacrylamide hydrogel. Imaging targets were prepared by adding either India ink to the water phase or nigrosin to the oil phase. Phantom and inclusion molds were fabricated using an affordable 3D printer, yielding phantoms containing stepped-cylinder inclusions with 1-8 cm-1 optical absorption coefficients and 1-5 mm diameters. Maximum imaging depth depended on whether target boundary buildup or filled-in features were analyzed, with boundary buildup being more detectable.

Many applications of photoacoustic imaging (PAI) involve transdermal light delivery, and variable epidermal melanin content may be a potential confounding factor causing signal attenuation and imaging artifacts such as clutter. We developed polyvinyl chloride plastisol (PVCP) phantoms including epidermal and dermal layers. Skin phantoms were placed atop a breast-mimicking PVCP phantom to assess image quality. Nigrosin was added to epidermal layers to simulate Fitzpatrick Types I-VI and yielded a melanin-like spectral slope. Image quality testing indicated that higher pigmentation caused stronger clutter and reduced imaging depth. Phantom-based test methods may support evaluation of PAI device sensitivity to skin pigmentation variation.

Percutaneous radiofrequency ablation (RFA) is used to destroy small liver tumors by locally inducing heat. However, there is a high tumor recurrence rate due to insufficient real-time image guidance during the procedure. We studied multi-wavelength photoacoustic imaging for identifying ablated tissue by taking the ratio of the photoacoustic signals at two wavelengths. To realize this, we first simulated the optical penetration in the liver and its influence on the optimal wavelength pair. Finally, the photoacoustic signals of treated and untreated bovine liver tissue were measured between 680 nm to 1100 nm to find candidate wavelength pairs for successful ratio imaging.

Photoacoustic computed tomography (PACT) has great potential in mouse brain imaging. Conventional PACT either assumes homogenous optical fluence or uses simplified attenuation model for optical fluence estimation, resulting in inaccurate estimation of absorption coefficient of the chromophore. To optimize the quantitative performance of PACT, we used MCX 3D Monte Carlo simulation to study the optical fluence distribution in a complete mouse brain model, which contains complete anatomy and blood vasculature information. Our results suggest that optical fluence decays five times globally due to strong scattering tissue and fluctuates locally due to additional optical heterogeneity introduced by blood vessels.

Previous photoacoustic remote sensing virtual histology approaches were too slow to use intraoperatively. We present a new scanning methodology with simultaneous galvanometer mirror and constant velocity mechanical scanning to greatly increase image acquisition speed. Human breast and prostate samples are imaged over an area of 4mm x 4mm in 40s with a 0.5μm resolution resolving both cancerous and healthy tissue. Histological detail is clearly visible in our images where tissue organization and subcellular nuclei density can be observed to aid histologists in determining margin status and cancer grading.

Photoacoustic remote sensing (PARS) microscopy suffers from slow imaging speeds as a result of so far being an exclusively laser scanning microscopy-based technique. Here we introduce a camera-based PARS approach using a 10 million frames-per-second camera together with oblique 532nm excitation and white-light interrogation. 2mm x 1.2mm images of 20µm diameter gold bonding wires are obtained in fractions of a second albeit with lower resolution. Using these wide-field images, regions-of-interest can be established. Additionally, the observation of supersonic wavefronts suggest the generation of shockwaves. This observation is used to derive an empirical model for the time evolution of PARS signals.

A novel photoacoustic remote sensing (PARS) microscopy architecture is presented which replaces the conventional normal-incidence back-reflected operation with independent angled-incidence excitation and collection optical pathways. This new architecture has demonstrated significant detection sensitivity improvements over normal-incidence embodiments, providing order of magnitudes reduction in sample exposure to the detection beam. The efficacy of this new architecture is explored on phantoms, bulk erythrocytes, and in vivo microvasculature. The talk will feature discussions centered around architecture design considerations and explore applications for such a device.

Photoacoustic imaging (PAI) has a great potential to assess vulnerable plaques in the carotid artery. However, in vivo, PAI suffers from low angular coverage, limited field of view (FOV), and lateral resolution especially when imaging a few centimeters deep in the tissue. To improve these shortcomings, here, we propose to image with multiple capacitive micromachined ultrasound transducers on a flexible substrate with orientation sensors to improve the image quality independent of the patient anatomy. We tested the multi-perspective PAI on a phantom and the experimental results demonstrate improvement in FOV, angular coverage, and resolution, strongly increasing the diagnostic capability of the PAI system.

In this work, a flexible transparent CMUT array is fabricated for through-illumination photoacoustic applications in which shape conformality is required. The array is composed of glass pieces with transparent structural layers on them, which are connected to each other via PDMS, making the array flexible. Unlike the previous flexible devices in the literature, the array is very robust to bending. The photoacoustic image of a wire target was successfully reconstructed using the fabricated flexible transparent CMUT array. The fabricated device offers promise for future photoacoustic tomography systems with through-array illumination.

Stiffness is a biomarker to distinguish intestinal inflammation and fibrosis in Crohn’s disease. This study investigated the performance of an endoscopic photoacoustic (PA)-ultrasound (US) catheter probe in quantifying intestinal stiffness in rabbits in vivo. The probe integrated a miniaturized US array and a side-firing fiber optic inside a medical balloon catheter. During the balloon dilation, the ratios between the intestinal wall deformation and PA signal change were quantified. The strain-PA ratios measured in vivo demonstrated a correlation of 0.8 (n=55, p=0.01) with the Young’s moduli of the assessed intestinal segments determined by microelastometry ex vivo.

This work involves the development of a novel endoscope based on Photoacoustic Remote Sensing (PARS), for label-free in vivo imaging of microvasculature. The PARS endoscopy is realized by raster-scanning the proximal end of a coherent, image-guide fiber bundle containing 30,000 cores arranged within a 600-µm diameter image-circle. A graded-index (GRIN) distal-end objective lens is used to focus and collect the reflected probe beam. Based on preliminary results, the endoscope is measured to have a resolution of less than 3-µm. The developed PARS endoscope is characterized using phantoms and validated for in vivo application using a live chicken embryo model.

An estimated 20-40% of patients who undergo breast-conserving surgery require repeat operations due to the length of time required post-surgery to analyze the resected sample. Recently, a novel non-contact microscopy technique known as ultraviolet photoacoustic remote sensing (UV-PARS) has been developed, which can produce virtual hematoxylin and eosin (H&E) stained images. By using a voice-coil scanning stage in conjunction with an on-demand pulsed laser, we demonstrate 1cm x 1cm gross UV-PARS virtual H&E scans in approximately 50 seconds, along with full-resolution 1cm x 1cm scans in approximately 8 minutes. With widefield high-speed scanning, UV-PARS shows promise for future translation to clinical application.

Optical-resolution photoacoustic microscopy (OR-PAM) can image biological tissues at micrometer level resolution. However, the imaging speed of traditional OR-PAM is often too slow for capturing dynamic information. In this work, we demonstrate a high-speed OR-PAM system using a water-immersible two-axis torsion-bending scanner, in which the fast axis employs the torsion scanning while the slow axis works at the bending mode. The system has achieved a cross-sectional frame rate of 400 Hz, and a volumetric imaging speed of 1 Hz over a field of view of 1.5 × 2.5 mm2. We have demonstrated high-speed OR-PAM of fast hemodynamic changes in vivo.

Photoacoustic Remote Sensing (PARS®) is a non-contact, label-free imaging modality that provides optical absorption contrast in biological tissues. Images are formed by raster-scanning over a target. A time-domain signal is collected at each point, representing initial pressure-induced via the photoacoustic effect. Conventionally, only the amplitude of the time-domain signals is considered to estimate pixel values, disregarding the rich temporal information present in the signals. For instance, the signal shape carries information, which may be related to specific biological structures. In this work, clustering based on signal shape is explored, followed by feature extraction, enabling the virtual labeling of PARS images.

The assessment of tissue composition using photoacoustic imaging (PAI) is a promising approach. However, the signal-to-noise ratio in PAI systems are quite low in comparison to ultrasound imaging’s especially at few centimeters depth. Multi-perspective photoacoustic imaging (MP-PAI) using multiple capacitive micromachined ultrasound transducers (CMUTs) on a flexible substrate provide a cost-efficient way to improve field of view (FOV), resolution, and angular coverage. In this work, an encoding scheme based on Hadamard codes is proposed to improve the SNR in MP-PAI. The concept is validated in an experiment using a carotid plaque tissue sample demonstrating the increase in SNR imaging with three CMUTs.