Time-domain feature extraction for target specificity in photoacoustic remote sensing microscopy.

Photoacoustic remote sensing (PARS) microscopy is an emerging label-free optical absorption imaging modality. PARS operates by capturing nanosecond-scale optical fluctuations produced by photoacoustic pressures. These time-domain (TD) variations are usually projected by amplitude to determine optical absorption magnitude. However, valuable details on a target's material properties (e.g., density, speed of sound) are contained within the TD signals. This work uses a novel, to the best of our knowledge, clustering method to learn TD features, based on signal shape, which relate to underlying material traits. A modified K-means method is used to cluster TD data, capturing representative signal features. These features are then used to form virtual colorizations which may highlight tissues based on their underlying material properties. Applied in fresh resected murine brain tissue, colorized visualizations highlight distinct regions of tissue. This may potentially facilitate differentiation of tissue constituents (e.g., myelinated and unmyelinated axons, cell nuclei) in a single acquisition.

Hyperspectral absorption microscopy using photoacoustic remote sensing.

An improved method of remote optical absorption spectroscopy and hyperspectral optical absorption imaging is described which takes advantage of the photoacoustic remote sensing detection architecture. A wide collection of photoacoustic excitation wavelengths ranging from 210 nm to 1550 nm was provided by a nanosecond tunable source allowing access to various salient endogenous chromophores such as DNA, hemeproteins, and lipids. Sensitivity of the device was demonstrated by characterizing the infrared absorption spectrum of water. Meanwhile, the efficacy of the technique was explored by recovering cell nuclei and oxygen saturation from a live chicken embryo model and by recovering adipocytes from freshly resected murine adipose tissue. This represents a continued investigation into the characteristics of the hyperspectral photoacoustic remote sensing technique which may represent an effective means of non-destructive endogenous contrast characterization and visualization.

Non-contact reflection-mode optical absorption spectroscopy using photoacoustic remote sensing.

A method of remote optical absorption spectroscopy is described that utilizes the photoacoustic remote sensing detection technique. A nanosecond tunable excitation source is used to excite thermo-elastic pressure-induced elasto-optic modulations within targets across a wide wavelength range from 210 to 680 nm, providing optical absorption contrast. These modulations are read remotely as back-reflected intensity variations within a continuous-wave 1310 nm detection beam. The absorption spectra of several samples including dyes and biological macromolecules are captured with an 8 mm working distance in reflection-mode without the use of containment chambers or acoustic detection. This represents an initial investigation into the characteristics of this technique, which may facilitate optical absorption measurement within previously inaccessible sample types due to their size or opacity.

All-optical label-free human breast tissue block histology using photoacoustic remote sensing.

The direct imaging of tissue preserved in formalin-fixed paraffin-embedded (FFPE) blocks remains a challenge. There are presently millions of tissues preserved as FFPE blocks whose assessment via bright-field microscopes requires them to be sectioned and subsequently stained. These processes are laborious, resource-intensive, and time consuming. In this Letter, we utilize an ultraviolet laser with photoacoustic remote sensing to provide a novel method that enables direct label-free pathological assessment of FFPE blocks. We demonstrate the efficacy of this technique by imaging human breast tissue, highlighting salient features such as ducts, adipocytes, and ductal hyperplasia. This direct imaging of FFPE blocks facilitates pathological assessment much earlier in the histopathological workflow, saving valuable time in clinical and research settings. The presented non-contact label-free reflection-mode device enables augmentation of existing histopathological workflows and aims to expand the arsenal of imaging technologies available to clinicians.

Label-free, non-contact, in vivo ophthalmic imaging using photoacoustic remote sensing microscopy.

We present, to the best of our knowledge, the first label-free, non-contact, in vivo imaging of the ocular vasculature using photoacoustic remote sensing (PARS) microscopy. Both anterior and posterior segments of a mouse eye were imaged. Vasculature of the iris, sclera, and retina tissues were clearly resolved. To the best of our knowledge, this is the first study showing non-contact photoacoustic imaging conducted on in vivo ocular tissue. We believe that PARS microscopy has the potential to advance the diagnosis and treatment of ocular diseases.

Rapid High-Resolution Mosaic Acquisition for Photoacoustic Remote Sensing.

Mechanical stages are routinely used to scan large expanses of biological specimens in photoacoustic imaging. This is primarily due to the limited field of view (FOV) provided by optical scanning. However, stage scanning becomes impractical at higher scanning speeds, or potentially unfeasible with heavier samples. Also, the slow scan-rate of the stages makes high resolution scanning a time-consuming process. Some clinical applications such as microsurgery require submicron resolution in a reflection-mode configuration necessitating a method that can acquire large field of views with a small raster scanning step size. In this study, we describe a method that combines mechanical stages with optical scanning for the rapid acquisition of high-resolution large FOVs. Optical scanning is used to acquire small frames in a two-dimensional grid formed by the mechanical stages. These frames are captured with specific overlap for effective image registration. Using a step size of 200 nm, we demonstrate mosaics of carbon fiber networks with FOVs of 0.8 × 0.8 mm2 captured in under 70 s with 1.2 µm image resolution. Larger mosaics yielding an imaging area of 3 × 3 mm2 are also shown. The method is validated by imaging a 1 × 1 mm2 section of unstained histopathological human tissue.

Real-time functional photoacoustic remote sensing microscopy.

A fiber-tetherable non-contact photoacoustic remote sensing microscopy system capable of multiplex functional imaging is reported. By utilizing stimulated Raman scattering within an over-pumped polarization-maintaining single-mode optical fiber, rapid pulse-to-pulse switching (500 kHz) of excitation spectral content is demonstrated and utilized as a photoacoustic excitation source. These rapid acquisitions aim to reduce motion artifacts and facilitate high frame rates appropriate for real-time feedback to users. The system is characterized by estimating blood oxygen saturation in blood-flow phantoms and within a mouse ear in vivo.

Multi-channel feature extraction for virtual histological staining of photon absorption remote sensing images.

Accurate and fast histological staining is crucial in histopathology, impacting diagnostic precision and reliability. Traditional staining methods are time-consuming and subjective, causing delays in diagnosis. Digital pathology plays a vital role in advancing and optimizing histology processes to improve efficiency and reduce turnaround times. This study introduces a novel deep learning-based framework for virtual histological staining using photon absorption remote sensing (PARS) images. By extracting features from PARS time-resolved signals using a variant of the K-means method, valuable multi-modal information is captured. The proposed multi-channel cycleGAN model expands on the traditional cycleGAN framework, allowing the inclusion of additional features. Experimental results reveal that specific combinations of features outperform the conventional channels by improving the labeling of tissue structures prior to model training. Applied to human skin and mouse brain tissue, the results underscore the significance of choosing the optimal combination of features, as it reveals a substantial visual and quantitative concurrence between the virtually stained and the gold standard chemically stained hematoxylin and eosin images, surpassing the performance of other feature combinations. Accurate virtual staining is valuable for reliable diagnostic information, aiding pathologists in disease classification, grading, and treatment planning. This study aims to advance label-free histological imaging and opens doors for intraoperative microscopy applications.

Label-free complete absorption microscopy using second generation photoacoustic remote sensing.

In the past decades, absorption modalities have emerged as powerful tools for label-free functional and structural imaging of cells and tissues. Many biomolecules present unique absorption spectra providing chromophore-specific information on properties such as chemical bonding, and sample composition. As chromophores absorb photons the absorbed energy is emitted as photons (radiative relaxation) or converted to heat and under specific conditions pressure (non-radiative relaxation). Modalities like fluorescence microscopy may capture radiative relaxation to provide contrast, while modalities like photoacoustic microscopy may leverage non-radiative heat and pressures. Here we show an all-optical non-contact total-absorption photoacoustic remote sensing (TA-PARS) microscope, which can capture both radiative and non-radiative absorption effects in a single acquisition. The TA-PARS yields an absorption metric proposed as the quantum efficiency ratio (QER), which visualizes a biomolecule's proportional radiative and non-radiative absorption response. The TA-PARS provides label-free visualization of a range of biomolecules enabling convincing analogues to traditional histochemical staining of tissues, effectively providing label-free Hematoxylin and Eosin (H&E)-like visualizations. These findings establish an effective all-optical non-contact total-absorption microscope for label-free inspection of biological materials.

Deformable mirror-based photoacoustic remote sensing (PARS) microscopy for depth scanning.

Optically shifting the focal plane to allow depth scanning of delicate biological structures and processes in their natural environment offers an appealing alternative to conventional mechanical scanning. Our technique uses a deformable mirror-based photoacoustic remote sensing microscopy (PARS) with a focus shifting of Δz ∼ 240 µm. We achieve this by integrating a deformable mirror that functions as a varifocal mirror for axial scanning. First, the system's focal shift capability was demonstrated with USAF resolution targets and carbon fiber phantoms, followed by in-vivo visualizations of blood vessels in chicken embryo chorioallantoic membrane (CAM). This work represents an initial step toward developing a non-contact, label-free, and aberration-free PARS imaging system with axial scanning capability.

Virtual histological staining of label-free total absorption photoacoustic remote sensing (TA-PARS).

Histopathological visualizations are a pillar of modern medicine and biological research. Surgical oncology relies exclusively on post-operative histology to determine definitive surgical success and guide adjuvant treatments. The current histology workflow is based on bright-field microscopic assessment of histochemical stained tissues and has some major limitations. For example, the preparation of stained specimens for brightfield assessment requires lengthy sample processing, delaying interventions for days or even weeks. Therefore, there is a pressing need for improved histopathology methods. In this paper, we present a deep-learning-based approach for virtual label-free histochemical staining of total-absorption photoacoustic remote sensing (TA-PARS) images of unstained tissue. TA-PARS provides an array of directly measured label-free contrasts such as scattering and total absorption (radiative and non-radiative), ideal for developing H&E colorizations without the need to infer arbitrary tissue structures. We use a Pix2Pix generative adversarial network to develop visualizations analogous to H&E staining from label-free TA-PARS images. Thin sections of human skin tissue were first virtually stained with the TA-PARS, then were chemically stained with H&E producing a one-to-one comparison between the virtual and chemical staining. The one-to-one matched virtually- and chemically- stained images exhibit high concordance validating the digital colorization of the TA-PARS images against the gold standard H&E. TA-PARS images were reviewed by four dermatologic pathologists who confirmed they are of diagnostic quality, and that resolution, contrast, and color permitted interpretation as if they were H&E. The presented approach paves the way for the development of TA-PARS slide-free histological imaging, which promises to dramatically reduce the time from specimen resection to histological imaging.

In-vivo functional and structural retinal imaging using multiwavelength photoacoustic remote sensing microscopy.

Many important eye diseases as well as systemic disorders manifest themselves in the retina. Retinal imaging technologies are rapidly growing and can provide ever-increasing amounts of information about the structure, function, and molecular composition of retinal tissue in-vivo. Photoacoustic remote sensing (PARS) is a novel imaging modality based on all-optical detection of photoacoustic signals, which makes it suitable for a wide range of medical applications. In this study, PARS is applied for in-vivo imaging of the retina and estimating oxygen saturation in the retinal vasculature. To our knowledge, this is the first time that a non-contact photoacoustic imaging technique is applied for in-vivo imaging of the retina. Here, optical coherence tomography is also used as a well-established retinal imaging technique to navigate the PARS imaging beams and demonstrate the capabilities of the optical imaging setup. The system is applied for in-vivo imaging of both microanatomy and the microvasculature of the retina. The developed system has the potential to advance the understanding of the ocular environment and to help in monitoring of ophthalmic diseases.

Single acquisition label-free histology-like imaging with dual-contrast photoacoustic remote sensing microscopy.

SIGNIFICANCE: Histopathological analysis of tissues is an essential tool for grading, staging, diagnosing, and resecting cancers and other malignancies. Current histopathological imaging techniques require substantial sample processing, prior to staining with hematoxylin and eosin (H&E) dyes, to highlight nuclear and cellular morphology. Sample preparation and staining is resource intensive and introduces potential for variability during sample preparation. AIM: We present a method for direct label-free histopathological assessment of formalin-fixed paraffin-embedded tissue blocks and thin tissue sections using a dual-contrast photoacoustic remote sensing (PARS) microscopy system. APPROACH: To emulate the nuclear and cellular contrast of H&E staining, we leverage unique properties of the PARS system. Here, the ultraviolet excitation PARS microscope takes advantage of DNA's unique optical absorption to provide nuclear contrast analogous to hematoxylin staining of cell nuclei. Concurrently, the optical scattering contrast of the PARS detection system is leveraged to provide bulk tissue contrast reminiscent of eosin staining of cell membranes. RESULTS: We demonstrate the efficacy of this technique by imaging human breast tissue and human skin tissues in formalin-fixed paraffin-embedded tissue blocks and frozen sections, respectively. Salient nuclear and extranuclear features such as cancerous cells, glands and ducts, adipocytes, and stromal structures such as collagen are captured. CONCLUSIONS: The presented dual-contrast PARS microscope enables label-free visualization of tissues with contrast and quality comparable to the current gold standard for histopathological analysis. Thus, the proposed system is well positioned to augment existing histopathological workflows, providing histological imaging directly on unstained tissue blocks and sections.

Single acquisition label-free histology-like imaging with dual-contrast photoacoustic remote sensing microscopy (Errata).

The errata correct the errors in citation numbering that appeared in the originally published article.

Functional and structural ophthalmic imaging using noncontact multimodal photoacoustic remote sensing microscopy and optical coherence tomography.

Early diagnosis of ocular diseases improves the understanding of pathophysiology and aids in accurate monitoring and effective treatment. Advanced, multimodal ocular imaging platforms play a crucial role in visualization of ocular components and provide clinicians with a valuable tool for evaluating various eye diseases. Here, for the first time we present a non-contact, multiwavelength photoacoustic remote sensing (PARS) microscopy and swept-source optical coherence tomography (SS-OCT) for in-vivo functional and structural imaging of the eye. The system provides complementary imaging contrasts of optical absorption and optical scattering, and is used for simultaneous, non-contact, in-vivo imaging of murine eye. Results of vasculature and structural imaging as well as melanin content in the retinal pigment epithelium layer are presented. Multiwavelength PARS microscopy using Stimulated Raman scattering is applied to enable in-vivo, non-contact oxygen saturation estimation in the ocular tissue. The reported work may be a major step towards clinical translation of ophthalmic technologies and has the potential to advance the diagnosis and treatment of ocular diseases.

Live feedback and 3D photoacoustic remote sensing.

BACKGROUND: As photoacoustic (PA) techniques progress towards clinical adoption, providing a high-speed live feedback becomes a high priority. To keep up with the instantaneous optical feedback of conventional light microscopes, PA imaging would need to provide a high-resolution video-rate live feed to the user. However, conventional PA microscopy typically trades resolution, sensitivity and imaging speed when optically scanning due to the difficult opto-acoustic confocal geometry. Here, we employ photoacoustic remote sensing (PARS), an all-optical technique that relies on optical confocal geometry, to provide a high-resolution live display in a reflection-mode PA architecture. METHODS: Employing a conventional x-y galvanometer scanner and a 600 KHz pulse repetition rate laser we implement a system capable of acquiring 2.5 frames per second in 2D. To complement this fast scanning optical system, we implement a computationally inexpensive image reconstruction method that is able to render the frames with minimal overhead, providing a live display. RESULTS: Employing the proposed method, we demonstrate a live feedback with frame rates as high as 2.5 Hz in 2D and also report the first results of 3D imaging with a non-contact label-free reflection-mode technique. The method is validated with phantom studies and in-vivo imaging. Employing a repetition rate of 600 KHz, a live feed of carbon fibers is realized with a C-scan rate of 2.5 Hz. The imaging resolution was measured to be 1.2 µm, the highest reported for a real-time reflection-mode architecture. The mean and peak SNR were measured to be 44 and 62 dB respectively in-vivo. 3D visualizations of carbon fiber phantoms and mouse ear microvasculature structure are also demonstrated. CONCLUSIONS: In summary, we present a method that has a small computational overhead for image rendering, resulting in a live display capable of real-time frame rates. We also report the first 3D imaging with a non-contact label-free reflection-mode PA technique. The all-optical confocal geometry required by PARS is significantly easier to implement and maintain than the opto-acoustic geometry of conventional PA microscopy techniques. This results in a system capable of high resolution and sensitivity, imaging at real-time rates. The authors believe this work represents a vital step towards a clinical high-resolution reflection-mode video-rate PA imaging system.

Three-dimensional virtual histology in unprocessed resected tissues with photoacoustic remote sensing (PARS) microscopy and optical coherence tomography (OCT).

Histological images are critical in the diagnosis and treatment of cancers. Unfortunately, current methods for capturing these microscopy images require resource intensive tissue preparation that may delay diagnosis for days or weeks. To streamline this process, clinicians are limited to assessing small macroscopically representative subsets of tissues. Here, a combined photoacoustic remote sensing (PARS) microscope and swept source optical coherence tomography system designed to circumvent these diagnostic limitations is presented. The proposed multimodal microscope provides label-free three-dimensional depth resolved virtual histology visualizations, capturing nuclear and extranuclear tissue morphology directly on thick unprocessed specimens. The capabilities of the proposed method are demonstrated directly in unprocessed formalin fixed resected tissues. The first images of nuclear contrast in resected human tissues, and the first three-dimensional visualization of subsurface nuclear morphology in resected Rattus tissues, captured with a non-contact photoacoustic system are presented here. Moreover, the proposed system captures the first co-registered OCT and PARS images enabling direct histological assessment of unprocessed tissues. This work represents a vital step towards the development of a rapid histological imaging modality to circumvent the limitations of current histopathology techniques.

Towards virtual biopsies of gastrointestinal tissues using photoacoustic remote sensing microscopy.

Gastrointestinal (GI) tissue biopsies provide critical diagnostic information for a wide variety of conditions such as neoplastic diseases (colorectal, small bowel and stomach cancers) and non-neoplastic diseases (inflammatory disorders, infection, celiac disease). Endoscopic biopsies collect small tissue samples that require resource intensive processing to permit histopathological analysis. Unfortunately, the sparsely collected biopsy samples may fail to capture the pathologic condition because selection of biopsy sites relies on macroscopic superficial tissue features and clinician judgement. Here, we present the first all-optical non-contact label-free non-interferometric photoacoustic microscopy system capable of performing "virtual biopsies". A modular photoacoustic remote sensing (PARS™) architecture is used facilitating imaging of unprocessed tissues providing information similar to conventional histopathological staining techniques. Prospectively this would allow gastroenterologists to assess subcellular tissue morphology in situ when selecting biopsy location. Tested on preserved unstained human and freshly resected murine tissues, the presented PARS microscope rapidly retrieves images of similar area to current biopsies, while maintaining comparable quality to the current standard for histopathological analysis. Additionally, results show the first label free assessment of subsurface cellular morphology in FFPE GI tissue blocks. Clinically relevant features are recovered including cellular details such as lamina propria within colon tissue and cell nuclear structure in resected smooth muscle. Constructed with a modular architecture, this system facilitates the future development of compact imaging heads. The modular PARS system overcomes many of the challenges with imaging unstained thick tissue in situ, representing a significant milestone in the development of a clinical microscope providing virtual biopsy capabilities.

Histopathology for Mohs micrographic surgery with photoacoustic remote sensing microscopy.

Mohs micrographic surgery (MMS) is a precise oncological technique where layers of tissue are resected and examined with intraoperative histopathology to minimize the removal of normal tissue while completely excising the cancer. To achieve intraoperative pathology, the tissue is frozen, sectioned and stained over a 20- to 60-minute period, then analyzed by the MMS surgeon. Surgery is continued one layer at a time until no cancerous cells remain, meaning MMS can take several hours to complete. Ideally, it would be desirable to circumvent or augment frozen sectioning methods and directly visualize subcellular morphology on the unprocessed excised tissues. Employing photoacoustic remote sensing (PARS) microscopy, we present a non-contact label-free reflection-mode method of performing such visualizations in frozen sections of human skin. PARS leverages endogenous optical absorption contrast within cell nuclei to provide visualizations reminiscent of histochemical staining techniques. Presented here, is the first true one to one comparison between PARS microscopy and standard histopathological imaging in human tissues. We demonstrate the ability of PARS microscopy to provide large grossing scans (>1 cm2, sufficient to visualize entire MMS sections) and regional scans with subcellular lateral resolution (300 nm).

Reflection-mode virtual histology using photoacoustic remote sensing microscopy.

Histological visualizations are critical to clinical disease management and are fundamental to biological understanding. However, current approaches that rely on bright-field microscopy require extensive tissue preparation prior to imaging. These processes are both labor intensive and contribute to creating significant delays in clinical feedback for treatment decisions that can extend to 2-3 weeks for standard paraffin-embedded tissue preparation and interpretation, especially if ancillary testing is needed. Here, we present the first comprehensive study on the broad application of a novel label-free reflection-mode imaging modality known as photoacoustic remote sensing (PARS) for visualizing salient subcellular structures from various common histopathological tissue preparations and for use in unprocessed freshly resected tissues. The PARS modality permits non-contact visualizations of intrinsic endogenous optical absorption contrast to be extracted from thick and opaque biological targets with optical resolution. The technique was examined both as a rapid assessment tool that is capable of managing large samples (> 1 cm2) in under 10 min, and as a high contrast imaging modality capable of extracting specific biological contrast to simulate conventional histological stains such as hematoxylin and eosin (H&E). The capabilities of the proposed method are demonstrated in a variety of human tissue preparations including formalin-fixed paraffin-embedded tissue blocks and unstained slides sectioned from these blocks, including normal and neoplastic human brain, and breast epithelium involved with breast cancer. Similarly, PARS images of human skin prepared by frozen section clearly demonstrated basal cell carcinoma and normal human skin tissue. Finally, we imaged unprocessed murine kidney and achieved histologically relevant subcellular morphology in fresh tissue. This represents a vital step towards an effective real-time clinical microscope that overcomes the limitations of standard histopathologic tissue preparations and enables real-time pathology assessment.