Intravascular confocal photoacoustic endoscope with dual-element ultrasonic transducer

Recent Advances in Photoacoustic Imaging: Current Status and Future Perspectives

Photoacoustic imaging (PAI) is an emerging hybrid imaging modality that combines high-contrast optical imaging with high-spatial-resolution ultrasound imaging. PAI can provide a high spatial resolution and significant imaging depth by utilizing the distinctive spectroscopic characteristics of tissue, which gives it a wide variety of applications in biomedicine and preclinical research. In addition, it is non-ionizing and non-invasive, and photoacoustic (PA) signals are generated by a short-pulse laser under thermal expansion. In this study, we describe the basic principles of PAI, recent advances in research in human and animal tissues, and future perspectives.

Perspectives on endoscopic functional photoacoustic microscopy

Endoscopy, enabling high-resolution imaging of deep tissues and internal organs, plays an important role in basic research and clinical practice. Recent advances in photoacoustic microscopy (PAM), demonstrating excellent capabilities in high-resolution functional imaging, have sparked significant interest in its integration into the field of endoscopy. However, there are challenges in achieving functional PAM in the endoscopic setting. This Perspective article discusses current progress in the development of endoscopic PAM and the challenges related to functional measurements. Then, it points out potential directions to advance endoscopic PAM for functional imaging by leveraging fiber optics, microfabrication, optical engineering, and computational approaches. Finally, it highlights emerging opportunities for functional endoscopic PAM in basic and translational biomedicine.

Optical-resolution functional gastrointestinal photoacoustic endoscopy based on optical heterodyne detection of ultrasound

Photoacoustic endoscopy shows promise in the detection of gastrointestinal cancer, inflammation, and other lesions. High-resolution endoscopic imaging of the hemodynamic response necessitates a small-sized, high-sensitivity ultrasound sensor. Here, we utilize a laser ultrasound sensor to develop a miniaturized, optical-resolution photoacoustic endoscope. The sensor can boost the acoustic response by a gain factor of ω_o/Ω (the frequency ratio of the signal light and measured ultrasound) by measuring the acoustically induced optical phase change. As a result, we achieve a noise-equivalent pressure density (NEPD) below 1.5 mPa·Hz^-1/2 over the measured range of 5 to 25 MHz. The heterodyne phase detection using dual-frequency laser beams of the sensor can offer resistance to thermal drift and vibrational perturbations. The endoscope is used to in vivo image a rat rectum and visualize the oxygen saturation changes during acute inflammation, which can hardly be observed with other imaging modalities.

Intravascular polarization-sensitive optical coherence tomography based on polarization mode delay

Intravascular polarization-sensitive optical coherence tomography (IV-PSOCT) provides depth-resolved tissue birefringence which can be used to evaluate the mechanical stability of a plaque. In our previous study, we reported a new strategy to construct polarization-sensitive optical coherence tomography in a microscope platform. Here, we demonstrated that this technology can be implemented in an endoscope platform, which has many clinical applications. A conventional intravascular OCT system can be modified for IV-PSOCT by introducing a 12-m polarization-maintaining fiber-based imaging probe. Its two polarization modes separately produce OCT images of polarization detection channels spatially distinguished by an image separation of 2.7 mm. We experimentally validated our IV-PSOCT with chicken tendon, chicken breast, and coronary artery as the image samples. We found that the birefringent properties can be successfully visualized by our IV-PSOCT.

Advances in Endoscopic Photoacoustic Imaging

Photoacoustic (PA) imaging is able to provide extremely high molecular contrast while maintaining the superior imaging depth of ultrasound (US) imaging. Conventional microscopic PA imaging has limited access to deeper tissue due to strong light scattering and attenuation. Endoscopic PA technology enables direct delivery of excitation light into the interior of a hollow organ or cavity of the body for functional and molecular PA imaging of target tissue. Various endoscopic PA probes have been developed for different applications, including the intravascular imaging of lipids in atherosclerotic plaque and endoscopic imaging of colon cancer. In this paper, the authors review representative probe configurations and corresponding preclinical applications. In addition, the potential challenges and future directions of endoscopic PA imaging are discussed.

In vivo safety study using radiation at wavelengths and dosages relevant to intravascular imaging

SIGNIFICANCE

Intravascular photoacoustic (IVPA) imaging can identify native lipid in atherosclerotic plaques in vivo. However, the large number of laser pulses required to produce 3D images is a safety concern that has not been fully addressed.

AIM

We aim to evaluate if irradiation at wavelengths and dosages relevant to IVPA imaging causes target vessel damage.

APPROACH

We irradiate the carotid artery of swine at one of several energy dosages using radiation at 1064 or 1720 nm and use histological evaluation by a pathologist to identify dose-dependent damage.

RESULTS

Media necrosis was the only dose-dependent form of injury. Damage was present at a cumulative fluence of 50  J  /  cm2 when using 1720 nm light. Damage was more equivocally identified at 700  J  /  cm2 using 1064 nm.

CONCLUSIONS

In prior work, IVPA imaging of native lipid in swine has been successfully conducted below the damage thresholds identified. This indicates that it will be possible to use IVPA imaging in a clinical setting without damaging vessel tissue. Future work should determine if irradiation causes an increase in blood thrombogenicity and confirm whether damaged tissue will heal over longer time points.

Reliability assessment on intravascular photoacoustic imaging of lipid: ex vivo animal and human sample validation

Although the lipid-detecting IVPA imaging system has been developed in resolution, speed, and catheter size, there is no parameterization study of the reliability on the IVPA imaging for lipid diagnosis. Here, the sensitivity, specificity, and accuracy were calculated to assess the reliability of the IVPA imaging of lipid. Abdominal aortas from six rabbits with atherosclerosis, were subjected to the IVPA imaging and Oil Red O staining, and 75 groups of IVPA as well as corresponding histological images were obtained. Similarly, 125 groups of IVPA and histological results were obtained from five human carotid plaque samples. The sensitivity, specificity, and accuracy, calculated from the statistical data, were 96.8%, 83.3%, 94.6% and 97.3%, 72.7%, 95.2%, respectively. The numerical values of sensitivity, specificity, and accuracy demonstrated the reliability of IVPA imaging on distinguishing the lesions vessel with lipid-rich plaque, which provided the foundation for IVPA translation to clinical application.

Evaluation of Reconstruction Methodology for Helical Scan Guided Photoacoustic Endoscopy

Photoacoustic endoscopy (PAE), combining both advantages of optical contrast and acoustic resolution, can visualize the chemical-specific optical information of tissues inside human-body. Recently, its corresponding reconstruction methods have been extensively researched. However, most of them are limited on cylindrical scan trajectories, rather than a helical scan which is more clinically practical. On this note, this article proposes a methodology of imaging reconstruction and evaluation for helical scan guided PAE. Different from traditional reconstruction method, synthetic aperture focusing technique (SAFT), our method reconstructs image using wavefield extrapolation which significantly improves computational efficiency and even takes only 0.25 seconds for 3-D reconstructions. In addition, the proposed evaluation methodology can estimate the resolutions and deviations of reconstructed images in advance, and then can be used to optimize the PAE scan parameters. Groups of simulations as well as ex-vivo experiments with different scan parameters are provided to fully demonstrate the performance of the proposed techniques. The quantitatively measured angular resolutions and deviations agree well with our theoretical derivation results D√{r_s² +h²} / [1.25(r_s r_d +h²)] (rad) and -h l / (r_s r_d +h²) (rad), respectively D,r_d, r_s,h and l represent transducer diameter, radius of scan trajectory, radius of source position, unit helical pitch and the distance from targets to helical scan plane, respectively). This theoretical result also suits for circular and cylindrical scan in case of h = 0 .

Photoacoustic endoscopy: A progress review

Endoscopy has been widely used in biomedical imaging and integrated with various optical and acoustic imaging modalities. Photoacoustic imaging (PAI), one of the fastest growing biomedical imaging modalities, is a noninvasive and nonionizing method that owns rich optical contrast, deep acoustic penetration depth, multiscale and multiparametric imaging capability. Hence, it is preferred to miniaturize the volume of PAI and develop an emerged endoscopic imaging modality referred to as photoacoustic endoscopy (PAE). It has been developed for more than one decade since the first report of PAE. Unfortunately, until now, there is no mature photoacoustic endoscopic technique recognized in clinic due to various technical limitations. To address this concern, recent development of new scanning mechanisms, adoption of novel optical/acoustic devices, utilization of superior computation methods and exploration of multimodality strategies have significantly promoted the progress of PAE toward clinic. In this review, we comprehensively reviewed recent progresses in single- and multimodality PAE with new physics, mechanisms and strategies to achieve practical devices for potential applicable scenarios including esophageal, gastrointestinal, urogenital and intravascular imaging. We ended this review with challenges and prospects for future development of PAE.

A PMN-PT Composite-Based Circular Array for Endoscopic Ultrasonic Imaging

Endoscopic ultrasound (EUS), an interventional imaging technology, utilizes a circular array to delineate the cross-sectional morphology of internal organs through the gastrointestinal (GI) track. However, the performance of conventional EUS transducers has scope for improvement because of the ordinary piezoelectric parameters of Pb(Zr, Ti) [Formula: see text] (PZT) bulk ceramic as well as its inferior mechanical flexibility which can cause material cracks during the circular shaping process. To achieve both prominent imaging capabilities and high device reliability, a 128-element 6.8-MHz circular array transducer is developed using a Pb(Mg [Formula: see text]Nb [Formula: see text]) [Formula: see text]-PbTiO₃ (PMN-PT) 1-3 composite with a coefficient of high electromechanical coupling ( [Formula: see text]) and good mechanical flexibility. The characterization results exhibit a large average bandwidth of 58%, a high average sensitivity of 100 mV_pp, and a crosstalk of less than -37 dB near the center frequency. Imaging performance of the PMN-PT composite-based array transducer is evaluated by a wire phantom, an anechoic cyst phantom, and an ex-vivo swine intestine. This work demonstrates the superior performance of the crucial ultrasonic device based on an advanced PMN-PT composite material and may lead to the development of next-generation biomedical ultrasonic devices for clinical diagnosis and treatment.

Single-shot hybrid photoacoustic-fluorescent microendoscopy through a multimode fiber with wavefront shaping

We present a minimally-invasive endoscope based on a multimode fiber that combines photoacoustic and fluorescence sensing. From the measurement of a transmission matrix during a prior calibration step, a focused spot is produced and raster-scanned over a sample at the distal tip of the fiber by use of a fast spatial light modulator. An ultra-sensitive fiber-optic ultrasound sensor for photoacoustic detection placed next to the fiber is combined with a photodetector to obtain both fluorescence and photoacoustic images with a distal imaging tip no larger than 250 µm. The high signal-to-noise ratio provided by wavefront shaping based focusing and the ultra-sensitive ultrasound sensor enables imaging with a single laser shot per pixel, demonstrating fast two-dimensional hybrid in vitro imaging of red blood cells and fluorescent beads.

Photoacoustic and ultrasound (PAUS) dermoscope with high sensitivity and penetration depth by using a bimorph transducer

A bimorph transducer was proposed to improve the detection sensitivity and imaging depth of photoacoustic and ultrasound (PAUS) dermoscope. By applying the bimorph transducer, the imaging depth and sensitivity of PAUS dermoscope were enhanced by simultaneously improving excitation efficiency and reception bandwidth. The integrated design of the imaging head of the dermoscope makes it highly convenient for detecting human skin. The PAUS imaging performance was demonstrated via visualizing subcutaneous tumor and depicting full structures of different skin layers from epidermis to subcutaneous tissue. The results confirm that the dermoscope with the bimorph transducer is well suited for PA and US dual-modality imaging, which can provide multi-information for skin disease.

High-robustness intravascular photoacoustic endoscope with a hermetically sealed opto-sono capsule

The prevailing open-structure intravascular photoacoustic (IVPA) endoscope emits a gradually deformed laser beam with exposed optical or acoustical components bearing pollution and damage in arterial lumen. Deformed laser beam scanning, which causes a low excitation efficiency and serious deterioration of the transverse resolution, is a current big obstacle to the application of photoacoustic endoscopy in intravascular imaging. Hence, the stable and reliable IVPA endoscope is indispensable. In this letter, we designed a high-robustness intravascular photoacoustic (HR-IVPA) endoscope with a hermetically sealed opto-sono capsule. The distal end of the opto-sono capsule was integrated with miniaturized optics, including a customized C-Lens and a customized total-reflection prism (TRP). The TRP was first applied to a side-viewing IVPA endoscope, featuring a high-throughput energy coupling efficiency of 90% and a cut-off free damage threshold. The optical path structure of the endoscope, optimized using optical simulation tools, overcame the ambiguous focus shift caused by chromatic dispersion and achieved a waist size of 20 µm as well as a focus depth of 4 mm in water at the wavelength of 1200 nm. The mass phantom experiments demonstrated that the HR-IVPA endoscope afforded repeatable IVPA images with a relatively constant signal-to-noise ratio (SNR) of about ∼41.8 dB and a transverse resolution of about ∼23 µm. The imaging experiments of the stent and lipid further demonstrated the robustness and validated the imaging ability of the HR-IVPA endoscope, which opens a new avenue for improving the endoscopic imaging capability, strengthening the credible detection of atherosclerotic cardiovascular disease.

Fan-shaped scanning approach for miniaturized photoacoustic tomography

We describe a novel scanning approach for miniaturized photoacoustic tomography (PAT), based on fan-shaped scanning of a single transducer at one or two discrete positions. This approach is tested and evaluated using several phantom and animal experiments. The results obtained show that this new scanning approach provides high image quality in the configuration of miniaturized handheld or endoscopic PAT with improved effective field of view and penetration depth.

Sensitivity Enhanced Photoacoustic Imaging Using a High-Frequency PZT Transducer with an Integrated Front-End Amplifier

Photoacoustic (PA) imaging is a hybrid imaging technique that can provide both structural and functional information of biological tissues. Due to limited permissible laser energy deposited on tissues, highly sensitive PA imaging is required. Here, we developed a 20 MHz lead zirconium titanate (PZT) transducer (1.5 mm × 3 mm) with front-end amplifier circuits for local signal processing to achieve sensitivity enhanced PA imaging. The electrical and acoustic performance was characterized. Experiments on phantoms and chicken breast tissue were conducted to validate the imaging performance. The fabricated prototype shows a bandwidth of 63% and achieves a noise equivalent pressure (NEP) of 0.24 mPa/√Hz and a receiving sensitivity of 62.1 μV/Pa at 20 MHz without degradation of the bandwidth. PA imaging of wire phantoms demonstrates that the prototype is capable of improving the detection sensitivity by 10 dB compared with the traditional transducer without integrated amplifier. In addition, in vitro experiments on chicken breast tissue show that structures could be imaged with enhanced contrast using the prototype and the imaging depth range was improved by 1 mm. These results demonstrate that the transducer with an integrated front-end amplifier enables highly sensitive PA imaging with improved penetration depth. The proposed method holds the potential for visualization of deep tissue structures and enhanced detection of weak physiological changes.

Multimodal intravascular imaging technology for characterization of atherosclerosis

Early detection of vulnerable plaques is the critical step in the prevention of acute coronary events. Morphology, composition, and mechanical property of a coronary artery have been demonstrated to be the key characteristics for the identification of vulnerable plaques. Several intravascular multimodal imaging technologies providing co-registered simultaneous images have been developed and applied in clinical studies to improve the characterization of atherosclerosis. In this paper, the authors review the present system and probe designs of representative intravascular multimodal techniques. In addition, the scientific innovations, potential limitations, and future directions of these technologies are also discussed.

Contact-free endoscopic photoacoustic sensing using speckle analysis

Photoacoustic endoscopy (PAE) is an emerging imaging modality, which offers a high imaging penetration and a high optical contrast in soft tissue. Most of the developed endoscopic photoacoustic sensing systems use miniaturized contact ultrasound transducers or complex optical approaches. In this work, a new fiber-based detection technique using speckle analysis for contact-free signal detection is presented. Phantom and ex vivo experiments are performed in transmission and reflection mode for proof of concept. In summary, the potential of the technique for endoscopic photoacoustic signal detection is demonstrated. The new technique might help in future to broaden the applications of PAE in imaging or guiding minimally invasive laser procedures.

Minimally invasive photoacoustic imaging: Current status and future perspectives

Photoacoustic imaging (PAI) is an emerging biomedical imaging modality that is based on optical absorption contrast, capable of revealing distinct spectroscopic signatures of tissue at high spatial resolution and large imaging depths. However, clinical applications of conventional non-invasive PAI systems have been restricted to examinations of tissues at depths less than a few cm due to strong light attenuation. Minimally invasive photoacoustic imaging (miPAI) has greatly extended the landscape of PAI by delivering excitation light within tissue through miniature fibre-optic probes. In the past decade, various miPAI systems have been developed with demonstrated applicability in several clinical fields. In this article, we present an overview of the current status of miPAI and our thoughts on future perspectives.

Ultrafine intravascular photoacoustic endoscope with a 0.7 mm diameter probe

Intravascular photoacoustic (IVPA) imaging, benefiting from high optical contrast, large imaging depth and absorption specificity, is of great potential for lipid-rich plaque detection. However, the diameters of reported IVPA endoscopes are too big to intervene into the coronary artery branches. Here, by designing an ultracompact house embedded with a side-fire fiber and a miniature single-element ultrasound transducer, we developed an ultrafine IVPA endoscope with a diameter of 0.7 mm aiming at coronary artery branches atherosclerotic plaque detection. The reliability and feasibility of the ultrafine IVPA endoscope was demonstrated by imaging a stent with a 1.6 mm inner diameter. Furthermore, the photoacoustic imaging and ultrasound imaging of a mouse thoracic aorta with an inner diameter of 1.15 mm was conducted to verify the clinical potentiality of the endoscope, and the PA images have good consistency with histological staining results. To the best of our knowledge, this is the first time we have achieved the IVPA imaging in fine vessel by the 0.7 mm diameter ultrafine photoacoustic endoscope, which paved a way for the translation of the IVPA endoscope to clinical application.

Optoacoustic image formation approaches-a clinical perspective

Clinical translation of optoacoustic imaging is fostered by the rapid technical advances in imaging performance as well as the growing number of clinicians recognizing the immense diagnostic potential of this technology. Clinical optoacoustic systems are available in multiple configurations, including hand-held and endoscopic probes as well as raster-scan approaches. The hardware design must be adapted to the accessible portion of the imaged region and other application-specific requirements pertaining the achievable depth, field of view or spatio-temporal resolution. Equally important is the adequate choice of the signal and image processing approach, which is largely responsible for the resulting imaging performance. Thus, new image reconstruction algorithms are constantly evolving in parallel to the newly-developed set-ups. This review focuses on recent progress on optoacoustic image formation algorithms and processing methods in the clinical setting. Major reconstruction challenges include real-time image rendering in two and three dimensions, efficient hybridization with other imaging modalitites as well as accurate interpretation and quantification of bio-markers, herein discussed in the context of ongoing progress in clinical translation.

Photoacoustic imaging for guidance of interventions in cardiovascular medicine

Imaging guidance is paramount to procedural success in minimally invasive interventions. Catheter-based therapies are the standard of care in the treatment of many cardiac disorders, including coronary artery disease, structural heart disease and electrophysiological conditions. Many of these diseases are caused by, or effect, a change in vasculature or cardiac tissue composition, which can potentially be detected by photoacoustic imaging. This review summarizes the state of the art in photoacoustic imaging approaches that have been proposed for intervention guidance in cardiovascular care. All of these techniques are currently in the preclinical phase. We will conclude with an outlook towards clinical applications.

High-resolution and extended-depth-of-field photoacoustic endomicroscopy by scanning-domain synthesis of optical beams

Photoacoustic endomicroscopy (PAEM) is capable of imaging fine structures in digestive tract. However, conventional PAEM employs a tightly focused laser beam to irradiate the object, which results in a limited depth-of-field (DOF). Here, we propose a scanning-domain synthesis of optical beams (SDSOB) to optimize both transverse resolution and the DOF by synthetic effective focused beams in scanning domain for the PAEM. By utilizing the SDSOB technique, multiple defocused and scattered beams are refocused to synthesize virtual focuses covering a large range of depth. A transverse point spread function that is 5.7-time sharper, and a transverse spatial bandwidth that is 8.5-time broader than those of the conventional PAEM were simulatively obtained through SDSOB-PAEM at the defocus distance of 2.4 mm. We validated the transverse resolution improvement at both in- and out-focus positions via phantom experiments of carbon fibers. In addition, in vivo rabbit experiments were conducted to acquire vascular images over radial depth range of 900 µm. And further morphological analysis revealed that the SDSOB images were acquired with abundant vascular branches and nodes, large total-length and small average-length of blood vessels, which indicated that the SDSOB-PAEM achieved high-resolution imaging in distinct rectal layers. All these results suggest that the SDSOB-PAEM possesses high transverse resolution and extended DOF, which demonstrates the SDSOB-PAEM can provide more accurate information for clinical assessment.

Tapered fiber-based intravascular photoacoustic endoscopy for high-resolution and deep-penetration imaging of lipid-rich plaque

High-resolution intravascular photoacoustic (IVPA) imaging can potentially improve the identification of atherosclerosis plaque. However, the absorption of water and the low coupled laser energy resulted in insufficient excitation energy provided by the single-mode fiber-based IVPA endoscope to achieve high-resolution and deep-penetration plaque imaging. In this paper, we developed a 1 mm diameter IVPA endoscope assembled with a Ø25-Ø9 μm tapered fiber. Owing to high coupling efficiency and the small output facula of tapered fiber, the IVPA endoscope has an optimal lateral resolution of 18 μm and a large imaging-depth covering from the intima to the peri-adventitial adipose, as confirmed by imaging results respectively. Furthermore, IVPA imaging in the blood has confirmed that the tapered fiber-based endoscope can display the distribution and the relative concentration of lipid in ex vivo plaque precisely. By the obtained histology-like images, IVPA imaging shows great potential for accurately imaging atherosclerosis plaque.

Optoacoustic mesoscopy for biomedicine

Fuelled by innovation, optical microscopy plays a critical role in the life sciences and medicine, from basic discovery to clinical diagnostics. However, optical microscopy is limited by typical penetration depths of a few hundred micrometres for in vivo interrogations in the visible spectrum. Optoacoustic microscopy complements optical microscopy by imaging the absorption of light, but it is similarly limited by penetration depth. In this Review, we summarize progress in the development and applicability of optoacoustic mesoscopy (OPAM); that is, optoacoustic imaging with acoustic resolution and wide-bandwidth ultrasound detection. OPAM extends the capabilities of optical imaging beyond the depths accessible to optical and optoacoustic microscopy, and thus enables new applications. We explain the operational principles of OPAM, its placement as a bridge between optoacoustic microscopy and optoacoustic macroscopy, and its performance in the label-free visualization of tissue pathophysiology, such as inflammation, oxygenation, vascularization and angiogenesis. We also review emerging applications of OPAM in clinical and biological imaging.

In vivo photoacoustic/ultrasonic dual-modality endoscopy with a miniaturized full field-of-view catheter

Endoscopy is an essential clinical tool for the diagnosis of gastrointestinal (GI) tract cancer. A photoacoustic system that elegantly combines optical and ultrasound endoscopy advantages by providing high-sensitivity functional information and large imaging depth is a potentially powerful tool for GI tract imaging. Recently, several photoacoustic endoscopic imaging systems have been proposed and developed. However, the relatively large size and rigid length of the catheter make it difficult to translate them into wide clinical applications; while the existing system of a relatively small catheter, capable of in vivo animal imaging, is unable to acquire full (360°) field-of-view cross-section images. In this study, we developed a photoacoustic/ultrasonic dual-modality endoscopic system and a corresponding miniaturized, encapsulated imaging catheter, which provides a full 360° field-of-view. The diameter of the catheter is 2.5 mm, which is compatible with the 2.8-mm instrumental channel of a conventional clinical optical endoscope. Using this system, we demonstrate in vivo 3-dimensional endoscopic photoacoustic/ultrasonic imaging of the colorectum of a healthy Sprague Dawley rat, by depicting vasculature and morphology of the GI tract. The significantly improved imaging field of view, reduced catheter size, high-quality imaging results suggest that the developed photoacoustic/ultrasonic dual-modality endoscopy has a great potential to be translated into a broad range of clinical applications in gastroenterology.

All-optical dual photoacoustic and optical coherence tomography intravascular probe

Intravascular imaging in percutaneous coronary interventions can be an invaluable tool in the treatment of coronary artery disease. It is of significant interest to provide molecular imaging contrast that is complementary to structural contrast provided by optical coherence tomography (OCT) and intravascular ultrasound imaging (IVUS). In this study, we developed a dual-modality intravascular imaging probe comprising a commercial OCT catheter and a high sensitivity fiber optic ultrasound sensor, to provide both photoacoustic (PA) and OCT imaging. With PA imaging, the lateral resolution varied from 18 μm to 40 μm; the axial resolution was consistently in the vicinity of 45 μm. We demonstrated the clinical potential of the probe with 2-D circumferential PA and OCT imaging, and with multispectral PA imaging.

Autofocusing optical-resolution photoacoustic endoscopy

Photoacoustic (PA) endoscopy has the potential to diagnose early diseases in the gastrointestinal tract. For the first time, to our knowledge, we developed an autofocusing PA endoscope (AF-PAE) for the usually irregular gastrointestinal tract imaging to solve the deterioration of transverse resolution caused by the defocus scanning of the probe. The 9-mm-diameter AF-PAE probe integrated a 6-mm aspheric lens and 6-mm liquid lens to automatically adjust the optical focal length, and an unfocused ultrasonic transducer with a center frequency of 15 MHz is coaxially set for detecting PA signals. With this probe, the AF-PAE achieved a focus-shifting range from approximately 2 to 10 mm with high transverse resolution and image contrast in a 360° field of view. Phantom experiment and vasculature distribution of a resected rabbit rectum have been performed to demonstrate the imaging ability of the AF-PAE for potential clinical applications in colorectal vessel imaging and subsequent diagnosis.

Multimodal Intravascular Photoacoustic and Ultrasound Imaging

The rupture of atherosclerotic plaques is the leading cause of death in developed countries. Early identification of vulnerable plaque is the essential step in preventing acute coronary events. Intravascular photoacoustic (IVPA) technology is able to visualize chemical composition of atherosclerotic plaque with high specificity and sensitivity. Integrated with intravascular ultrasound (IVUS) imaging, this multimodal intravascular IVPA/IVUS imaging technology is able to provide both structural and chemical compositions of arterial walls for detecting and characterizing atherosclerotic plaques. In this paper, we present representative multimodal IVPA/IVUS imaging systems and discuss current scientific innovations, potential limitations, and prospective improvements for characterization of coronary atherosclerosis.

Fast assessment of lipid content in arteries in vivo by intravascular photoacoustic tomography

Intravascular photoacoustic tomography is an emerging technology for mapping lipid deposition within an arterial wall for the investigation of the vulnerability of atherosclerotic plaques to rupture. By converting localized laser absorption in lipid-rich biological tissue into ultrasonic waves through thermoelastic expansion, intravascular photoacoustic tomography is uniquely capable of imaging the entire arterial wall with chemical selectivity and depth resolution. However, technical challenges, including an imaging catheter with sufficient sensitivity and depth and a functional sheath material without significant signal attenuation and artifact generation for both photoacoustics and ultrasound, have prevented in vivo application of intravascular photoacoustic imaging for clinical translation. Here, we present a highly sensitive quasi-collinear dual-mode photoacoustic/ultrasound catheter with elaborately selected sheath material, and demonstrated the performance of our intravascular photoacoustic tomography system by in vivo imaging of lipid distribution in rabbit aortas under clinically relevant conditions at imaging speeds up to 16 frames per second. Ex vivo evaluation of fresh human coronary arteries further confirmed the performance of our imaging system for accurate lipid localization and quantification of the entire arterial wall, indicating its clinical significance and translational capability.

Photoacoustic and hyperspectral dual-modality endoscope

We have developed a dual-modality endoscope composed of photoacoustic (PA) and hyperspectral imaging, capable of visualizing both structural and functional properties of bio-tissue. The endoscope's composition and scanning mechanism was described, and the feasibility of the dual-modality endoscope was verified by mimic phantom experiments. Lately, we demonstrated its endoscopic workability through in vivo experiments. The experimental results showed that the proposed herein hybrid endoscope can provide optical imaging of the surface and tomography imaging for the deeper features, and a functional oxygen saturation rate map of the same imaging area. We demonstrated optical-resolution PA imaging of vascular structures and an oxygen saturation rate map in a rabbit's rectum. It confirmed that this dual-modality endoscope can play an important role in comprehensive clinical applications.

Photoacoustic endomicroscopy based on a MEMS scanning mirror

In this Letter, we present a high-resolution photoacoustic endomicroscopy probe based on a microelectromechanical systems (MEMS) scanning mirror. The built-in optical assembly consists of a 0.7 mm graded-index (GRIN) lens for light focusing and a ϕ1  mm MEMS mirror to reflect and scan the beam. A miniaturized unfocused ultrasound transducer with a center frequency of 10 MHz is used for photoacoustic detection. Sharp blades, carbon fibers, and black tapes were utilized to evaluate the performance of the system. In vivo mouse ears and resected rectums were imaged to further demonstrate the feasibility of this probe for potential biological and clinical applications.

Real-time intravascular photoacoustic-ultrasound imaging of lipid-laden plaque in human coronary artery at 16 frames per second

Intravascular photoacoustic-ultrasound (IVPA-US) imaging is an emerging hybrid modality for the detection of lipid-laden plaques, as it provides simultaneous morphological and lipid-specific chemical information of an artery wall. Real-time imaging and display at video-rate speed are critical for clinical utility of the IVPA-US imaging technology. Here, we demonstrate a portable IVPA-US system capable of imaging at up to 25 frames per second in real-time display mode. This unprecedented imaging speed was achieved by concurrent innovations in excitation laser source, rotary joint assembly, 1 mm IVPA-US catheter size, differentiated A-line strategy, and real-time image processing and display algorithms. Spatial resolution, chemical specificity, and capability for imaging highly dynamic objects were evaluated by phantoms to characterize system performance. An imaging speed of 16 frames per second was determined to be adequate to suppress motion artifacts from cardiac pulsation for in vivo applications. The translational capability of this system for the detection of lipid-laden plaques was validated by ex vivo imaging of an atherosclerotic human coronary artery at 16 frames per second, which showed strong correlation to gold-standard histopathology. Thus, this high-speed IVPA-US imaging system presents significant advances in the translational intravascular and other endoscopic applications.

Simultaneous imaging of atherosclerotic plaque composition and structure with dual-mode photoacoustic and optical coherence tomography

The composition of plaque is a major determinant of coronary-related clinical syndromes. By combining photoacoustic tomography (PAT) and optical coherence tomography (OCT), the optical absorption and scattering properties of vascular plaque can be revealed and subsequently used to distinguish the plaque composition and structure. The feasibility and capacity of the dual-mode PAT-OCT technique for resolving vascular plaque was first testified by plaque composition mimicking experiment. PAT obtained lipid information due to optical absorption differences, while owing to scattering differences, OCT achieved imaging of collagen. Furthermore, by combining PAT and OCT, the morphological characteristic and scattering difference of normal and lipid-rich plaque in the ex vivo rabbit aorta was distinguished simultaneously. The experiments demonstrated that the combined PAT and OCT technique is a potential feasible method for detecting the composition and structure of lipid core and fibrous cap in atherosclerosis.

Combined frequency domain photoacoustic and ultrasound imaging for intravascular applications

Intravascular photoacoustic (IVPA) imaging has the potential to characterize lipid-rich structures based on the optical absorption contrast of tissues. In this study, we explore frequency domain photoacoustics (FDPA) for intravascular applications. The system employed an intensity-modulated continuous wave (CW) laser diode, delivering 1W over an intensity modulated chirp frequency of 4-12MHz. We demonstrated the feasibility of this approach on an agar vessel phantom with graphite and lipid targets, imaged using a planar acoustic transducer co-aligned with an optical fibre, allowing for the co-registration of IVUS and FDPA images. A frequency domain correlation method was used for signal processing and image reconstruction. The graphite and lipid targets show an increase in FDPA signal as compared to the background of 21dB and 16dB, respectively. Use of compact CW laser diodes may provide a valuable alternative for the development of photoacoustic intravascular devices instead of pulsed laser systems.

Frequency Analysis of the Photoacoustic Signal Generated by Coronary Atherosclerotic Plaque

The identification of unstable atherosclerotic plaques in the coronary arteries is emerging as an important tool for guiding percutaneous coronary interventions and may enable preventive treatment of such plaques in the future. Assessment of plaque stability requires imaging of both structure and composition. Spectroscopic photoacoustic (sPA) imaging can visualize atherosclerotic plaque composition on the basis of the optical absorption contrast. It is an established fact that the frequency content of the photoacoustic (PA) signal is correlated with structural tissue properties. As PA signals can be weak, it is important to match the transducer bandwidth to the signal frequency content for in vivo imaging. In this ex vivo study on human coronary arteries, we combined sPA imaging and analysis of frequency content of the PA signals. Using a broadband transducer (-3-dB one-way bandwidth of 10-35 MHz) and a 1-mm needle hydrophone (calibrated for 1-20 MHz), we covered a large frequency range of 1-35 MHz for receiving the PA signals. Spectroscopic PA imaging was performed at wavelengths ranging from 1125 to 1275 nm with a step of 2 nm, allowing discrimination between plaque lipids and adventitial tissue. Under sPA imaging guidance, the frequency content of the PA signals from the plaque lipids was quantified. Our data indicate that more than 80% of the PA energy of the coronary plaque lipids lies in the frequency band below 8 MHz. This frequency information can guide the choice of the transducer element used for PA catheter fabrication.

Transurethral Photoacoustic Endoscopy for Prostate Cancer: A Simulation Study

The purpose of this study was to optimize the configuration of a photoacoustic endoscope (PAE) for prostate cancer detection and therapy monitoring. The placement of optical fiber bundles and ultrasound detectors was chosen to maximize the photoacoustic imaging penetration depth. We performed both theoretical calculations and simulations of this optimized PAE configuration on a prostate-sized phantom containing tumor and various photosensitizer concentrations. The optimized configuration of PAE with transurethral light delivery simultaneously increases the imaging penetration depth and improves image quality. Thermal safety, investigated via COMSOL Multiphysics, shows that there is only a 4 mK temperature rise in the urethra during photoacoustic imaging, which will cause no thermal damage. One application of this PAE has been demonstrated for quasi-quantifying photosensitizer concentrations during photodynamic therapy. The sensitivity of the photoacoustic detection for TOOKAD was 0.18 ng/mg at a 763 nm laser wavelength. Results of this study will greatly enhance the potential of prostate PAE for in vivo monitoring of drug delivery and guidance of the laser-induced therapy for future clinical use.

High-sensitivity intravascular photoacoustic imaging of lipid-laden plaque with a collinear catheter design

A highly sensitive catheter probe is critical to catheter-based intravascular photoacoustic imaging. Here, we present a photoacoustic catheter probe design on the basis of collinear alignment of the incident optical wave and the photoacoustically generated sound wave within a miniature catheter housing for the first time. Such collinear catheter design with an outer diameter of 1.6 mm provided highly efficient overlap between optical and acoustic waves over an imaging depth of >6 mm in D₂O medium. Intravascular photoacoustic imaging of lipid-laden atherosclerotic plaque and perivascular fat was demonstrated, where a lab-built 500 Hz optical parametric oscillator outputting nanosecond optical pulses at a wavelength of 1.7 μm was used for overtone excitation of C-H bonds. In addition to intravascular imaging, the presented catheter design will benefit other photoacoustic applications such as needle-based intramuscular imaging.

Toward in vivo biopsy of melanoma based on photoacoustic and ultrasound dual imaging with an integrated detector

Melanoma is the most dangerous type of skin cancer with high lethal rate. Tumor thickness and tumor-associated vasculature are two key parameters for staging melanoma. Previous techniques for diagnosing melanoma have insurmountable restrictions, such as invasive, low specificity, or inaccurate depth measurement. Here we develop an integrated photoacoustic (PA) and ultrasound (US) imaging system dedicated to overcome these limitations. An integrated detector with sound-light coaxial/confocal design and flexible coupling mode is employed for the combined PA/US imaging strategy. PA imaging results enable a clear characterization of tumor angiogenesis with high resolution and high contrast. Furthermore, accurate thickness measurements of melanoma in different stages can be resolved with the simultaneously obtained PA/US image. Phantom experiments and in vivo animal experimental results demonstrate the integrated PA/US system could provide potential for noninvasive biopsy of melanoma.

High-speed intravascular photoacoustic imaging at 1.7 μm with a KTP-based OPO

Lipid deposition inside the arterial wall is a hallmark of plaque vulnerability. Based on overtone absorption of C-H bonds, intravascular photoacoustic (IVPA) catheter is a promising technology for quantifying the amount of lipid and its spatial distribution inside the arterial wall. Thus far, the clinical translation of IVPA technology is limited by its slow imaging speed due to lack of a high-pulse-energy high-repetition-rate laser source for lipid-specific first overtone excitation at 1.7 μm. Here, we demonstrate a potassium titanyl phosphate (KTP)-based optical parametric oscillator with output pulse energy up to 2 mJ at a wavelength of 1724 nm and with a repetition rate of 500 Hz. Using this laser and a ring-shape transducer, IVPA imaging at speed of 1 frame per sec was demonstrated. Performance of the IVPA imaging system's resolution, sensitivity, and specificity were characterized by carbon fiber and a lipid-mimicking phantom. The clinical utility of this technology was further evaluated ex vivo in an excised atherosclerotic human femoral artery with comparison to histology.

High speed intravascular photoacoustic imaging with fast optical parametric oscillator laser at 1.7 μm

Intravascular photoacoustic imaging at 1.7 μm spectral band has shown promising capabilities for lipid-rich vulnerable atherosclerotic plaque detection. In this work, we report a high speed catheter-based integrated intravascular photoacoustic/intravascular ultrasound (IVPA/IVUS) imaging system with a 500 Hz optical parametric oscillator laser at 1725 nm. A lipid-mimicking phantom and atherosclerotic rabbit abdominal aorta were imaged at 1 frame per second, which is two orders of magnitude faster than previously reported in IVPA imaging with the same wavelength. Clear photoacoustic signals by the absorption of lipid rich deposition demonstrated the ability of the system for high speed vulnerable atherosclerotic plaques detection.