Photoacoustic imaging of human coronary atherosclerosis in two spectral bands

The application of frequency-domain photoacoustics to temperature-dependent measurements of the Grüneisen parameter in lipids

The Grüneisen parameter is an essential factor in biomedical photoacoustic (PA) diagnostics. In most PA imaging applications, the variation of the Grüneisen parameter with tissue type is insignificant. This is not the case for PA imaging and characterization of lipids, as they have a very distinct Grüneisen parameter compared with other tissue types. One example of PA applications involving lipids is the imaging and characterization of atherosclerotic plaques. Intravascular photoacoustic (IVPA) imaging is a promising diagnostic tool that can evaluate both plaque severity and composition. The literature for IVPA has mainly focused on using the difference in absorption coefficients between plaque components and healthy arterial tissues. However, the Grüneisen parameters for lipids and their behavior with temperature have not been well established in the literature. In this study we employ frequency-domain photoacoustic measurements to estimate the Grüneisen parameter by virtue of the ability of this modality to independently measure both the absorption coefficient and the Grüneisen parameter through the use of the phase channel. The values of the Grüneisen parameters of some lipids are calculated as functions of temperature in the range 25-45 °C.

Red blood cell membrane damage by light-induced thermal gradient under optical trap

Rapid membrane damage of optically trapped red blood cells (RBCs) was observed at trapping powers ≥280 mW. An excellent agreement between the estimated laser-induced thermal gradient across trapped cell's membrane and that typically required for membrane electropermeabilization suggests a mechanism involving temperature gradient-induced electropermeabilization of membrane. Also the rapid collapse of the trapped cell due to membrane rupture was seen to cause shock waves in the surroundings permeabilizing nearby untrapped cells. When the experiments were carried out with RBCs collected from type II diabetic patients, a noticeable change in the damage rate compared to normal RBCs was seen suggesting a novel optical diagnosis method for the disease.

A Dual-Modality Hybrid Imaging System Harnesses Radioluminescence and Sound to Reveal Molecular Pathology of Atherosclerotic Plaques

Atherosclerosis is a progressive inflammatory condition caused by an unstable lesion, called thin-cap fibro atheromata (TCFA) that underlies coronary artery disease (CAD)-one of the leading causes of death worldwide. Therefore, early clinical diagnosis and effective risk stratification is important for CAD management as well as preventing progression to catastrophic events. However, early detection could be difficult due to their small size, motion, obscuring 18F-FDG uptake by adjacent myocardium, and complex morphological/biological features. To overcome these limitations, we developed a catheter-based Circumferential-Intravascular-Radioluminescence-Photoacoustic-Imaging (CIRPI) system that can detect vulnerable plaques in coronary arteries and characterizes them with respect to pathology and biology. Our CIRPI system combined two imaging modalities: Circumferential Radioluminescence Imaging (CRI) and PhotoAcoustic Tomography (PAT) within a novel optical probe. The probe's CaF2:Eu based scintillating imaging window provides a 360° view of human (n = 7) and murine carotid (n = 10) arterial plaques by converting β-particles into visible photons during 18F-FDG decay. A 60× and 63× higher radioluminescent signals were detected from the human and murine plaque inflammations, respectively, compared to the control. The system's photoacoustic imaging provided a comprehensive analysis of the plaque compositions and its morphologic information. These results were further verified with IVIS-200, immunohistochemical analysis, and autoradiography.

Recent Development of Technology and Application of Photoacoustic Molecular Imaging Toward Clinical Translation

The deep imaging capability and optical absorption contrast offered by photoacoustic imaging promote the use of this technology in clinical applications. By exploiting the optical absorption properties of endogenous chromophores such as hemoglobin and lipid, molecular information at a depth of a few centimeters can be unveiled. This information shows promise to reveal lesions indicating early stage of various human diseases, such as cancer and atherosclerosis. In addition, the use of exogenous contrast agents can further extend the capability of photoacoustic imaging in clinical diagnosis and treatment. In this review, the current state of the art and applications of photoacoustic molecular probes will be critically reviewed, as well as some spearheading translational efforts that have taken place over the past 5 years. Some of the most critical barriers to clinical translation of this novel technology will be discussed, and some thoughts will be given on future endeavors and pathways.

High-pulse energy supercontinuum laser for high-resolution spectroscopic photoacoustic imaging of lipids in the 1650-1850 nm region

We propose a cost-effective high-pulse energy supercontinuum (SC) source based on a telecom range diode laser-based amplifier and a few meters of standard single-mode optical fiber, with a pulse energy density as high as ~25 nJ/nm in the 1650-1850 nm regime (factor >3 times higher than any SC source ever used in this wavelength range). We demonstrate how such an SC source combined with a tunable filter allows high-resolution spectroscopic photoacoustic imaging and the spectroscopy of lipids in the first overtone transition band of C-H bonds (1650-1850 nm). We show the successful discrimination of two different lipids (cholesterol and lipid in adipose tissue) and the photoacoustic cross-sectional scan of lipid-rich adipose tissue at three different locations. The proposed high-pulse energy SC laser paves a new direction towards compact, broadband and cost-effective source for spectroscopic photoacoustic imaging.

Investigation on the Effect of Spatial Compounding on Photoacoustic Images of Carotid Plaques in the In Vivo Available Rotational Range

Photoacoustic imaging (PAI) is a promising imaging modality due to its high optical specificity. However, the low signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of in vivo PA images are major challenges that prevent PAI from finding its place in clinics. This paper investigates the merit of spatial compounding of PA images in arterial phantoms and the achievable improvements of SNR, when in vivo conditions are mimicked. The analysis of the compounding technique was performed on a polyvinyl alcohol vessel phantom with black threads embedded in its wall. The in vivo conditions were mimicked by limiting the rotation range in ±30°, adding turbid surrounding medium, and filling the lumen with porcine blood. Finally, the performance of the technique was evaluated in ex vivo human carotid plaque samples. Results showed that spatial compounding elevates the SNR by 5-10 dB and CNR by 1-5 dB, depending on the location of the absorbers. This paper elucidates prospective in vivo PA characterization of carotid plaques by proposing a method to enhance PA image quality.

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.

Multispectral photoacoustic microscopy of lipids using a pulsed supercontinuum laser

We demonstrate optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue between 1050-1714 nm using a pulsed supercontinuum laser based on a large-mode-area photonic crystal fiber. OR-PAM experiments of lipid-rich samples show the expected optical absorption peaks near 1210 and 1720 nm. These results show that pulsed supercontinuum lasers are promising for OR-PAM applications such as label-free histology of lipid-rich tissue and imaging small animal models of disease.

Optimization of dual-wavelength intravascular photoacoustic imaging of atherosclerotic plaques using Monte Carlo optical modeling

Coronary heart disease (the presence of coronary atherosclerotic plaques) is a significant health problem in the industrialized world. A clinical method to accurately visualize and characterize atherosclerotic plaques is needed. Intravascular photoacoustic (IVPA) imaging is being developed to fill this role, but questions remain regarding optimal imaging wavelengths. We utilized a Monte Carlo optical model to simulate IVPA excitation in coronary tissues, identifying optimal wavelengths for plaque characterization. Near-infrared wavelengths (≤1800  nm) were simulated, and single- and dual-wavelength data were analyzed for accuracy of plaque characterization. Results indicate light penetration is best in the range of 1050 to 1370 nm, where 5% residual fluence can be achieved at clinically relevant depths of ≥2  mm in arteries. Across the arterial wall, fluence may vary by over 10-fold, confounding plaque characterization. For single-wavelength results, plaque segmentation accuracy peaked at 1210 and 1720 nm, though correlation was poor (<0.13). Dual-wavelength analysis proved promising, with 1210 nm as the most successful primary wavelength (≈1.0). Results suggest that, without flushing the luminal blood, a primary and secondary wavelength near 1210 and 1350 nm, respectively, may offer the best implementation of dual-wavelength IVPA imaging. These findings could guide the development of a cost-effective clinical system by highlighting optimal wavelengths and improving plaque characterization.

Phantom-based image quality test methods for photoacoustic imaging systems

As photoacoustic imaging (PAI) technologies advance and applications arise, there is increasing need for standardized approaches to provide objective, quantitative performance assessment at various stages of the product development and clinical translation process. We have developed a set of performance test methods for PAI systems based on breast-mimicking tissue phantoms containing embedded inclusions. Performance standards for mature imaging modalities [magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound] were used to guide selection of critical PAI image quality characteristics and experimental methods. Specifically, the tests were designed to address axial, lateral, and elevational spatial resolution, signal uniformity, penetration depth, sensitivity, spatial measurement accuracy, and PAI-ultrasound coregistration. As an initial demonstration of the utility of these test methods, we characterized the performance of a modular, bimodal PAI-ultrasound system using four clinical ultrasound transducers with varying design specifications. Results helped to inform optimization of acquisition and data processing procedures while providing quantitative elucidation of transducer-dependent differences in image quality. Comparison of solid, tissue-mimicking polymer phantoms with those based on Intralipid indicated the superiority of the former approach in simulating real-world conditions for PAI. This work provides a critical foundation for the establishment of well-validated test methods that will facilitate the maturation of PAI as a medical imaging technology.

Thermo-elastic optical coherence tomography

The absorption of nanosecond laser pulses induces rapid thermo-elastic deformation in tissue. A sub-micrometer scale displacement occurs within a few microseconds after the pulse arrival. In this Letter, we investigate the laser-induced thermo-elastic deformation using a 1.5 MHz phase-sensitive optical coherence tomography (OCT) system. A displacement image can be reconstructed, which enables a new modality of phase-sensitive OCT, called thermo-elastic OCT. An analysis of the results shows that the optical absorption is a dominating factor for the displacement. Thermo-elastic OCT is capable of visualizing inclusions that do not appear on the structural OCT image, providing additional tissue type information.

Advanced optoacoustic methods for multiscale imaging of in vivo dynamics

Visualization of dynamic functional and molecular events in an unperturbed in vivo environment is essential for understanding the complex biology of living organisms and of disease state and progression. To this end, optoacoustic (photoacoustic) sensing and imaging have demonstrated the exclusive capacity to maintain excellent optical contrast and high resolution in deep-tissue observations, far beyond the penetration limits of modern microscopy. Yet, the time domain is paramount for the observation and study of complex biological interactions that may be invisible in single snapshots of living systems. This review focuses on the recent advances in optoacoustic imaging assisted by smart molecular labeling and dynamic contrast enhancement approaches that enable new types of multiscale dynamic observations not attainable with other bio-imaging modalities. A wealth of investigated new research topics and clinical applications is further discussed, including imaging of large-scale brain activity patterns, volumetric visualization of moving organs and contrast agent kinetics, molecular imaging using targeted and genetically expressed labels, as well as three-dimensional handheld diagnostics of human subjects.

Tracking delivery of a drug surrogate in the porcine heart using photoacoustic imaging and spectroscopy

Although the drug-eluting stent (DES) has dramatically reduced the rate of coronary restenosis, it still occurs in up to 20% of patients with a DES. Monitoring drug delivery could be one way to decrease restenosis rates. We demonstrate real-time photoacoustic imaging and spectroscopy (PAIS) using a wavelength-tunable visible laser and clinical ultrasound scanner to track cardiac drug delivery. The photoacoustic signal was initially calibrated using porcine myocardial samples soaked with a known concentration of a drug surrogate (DiI). Next, an in situ coronary artery was perfused with DiI for 20 min and imaged to monitor dye transport in the tissue. Finally, a partially DiI-coated stent was inserted into the porcine brachiocephalic trunk for imaging. The photoacoustic signal was proportional to the DiI concentration between 2.4 and 120 ?? ? g / ml , and the dye was detected over 1.5 mm from the targeted coronary vessel. Photoacoustic imaging was also able to differentiate the DiI-coated portion of the stent from the uncoated region. These results suggest that PAIS can track drug delivery to cardiac tissue and detect drugs loaded onto a stent with sub-mm precision. Future work using PAIS may help improve DES design and reduce the probability of restenosis.

Toward the detection of intraplaque hemorrhage in carotid artery lesions using photoacoustic imaging

Photoacoustic imaging (PAI) may have the ability to reveal the composition and the anatomical structure of carotid plaques, which determines its mechanical properties and vulnerability. We used PAI and plane wave ultrasound (PUS) imaging to obtain three-dimensional (3-D) images of endarterectomy samples ex vivo and compared the results with histology to investigate the potential of PAI-based identification of intraplaque hemorrhage. Seven carotid plaque samples were obtained from patients undergoing carotid endarterectomy and imaged with a fully integrated hand-held photoacoustic (PA) probe, consisting of a pulsed diode laser ( t pulse = 130 ?? ns , E pulse = 1 ?? mJ , ? = 808 ?? nm ) and a linear array transducer ( f c = 7.5 ?? MHz ). The samples were rotated 360 deg with 10 deg steps, and data were spatially compounded to obtain complete 3-D images of the plaques. Areas of high absorption in the 3-D datasets were identified and compared to histological data of the plaques. Data in six out of seven endarterectomy samples revealed the presence of intraplaque hemorrhages that were not visible in the PUS images. Due to the noninvasive nature of PAI, this ex vivo study may elucidate preclinical studies toward the in vivo, noninvasive, vulnerability assessment of the atherosclerotic carotid plaque.

Real-time volumetric lipid imaging in vivo by intravascular photoacoustics at 20 frames per second

Lipid deposition can be assessed with combined intravascular photoacoustic/ultrasound (IVPA/US) imaging. To date, the clinical translation of IVPA/US imaging has been stalled by a low imaging speed and catheter complexity. In this paper, we demonstrate imaging of lipid targets in swine coronary arteries in vivo, at a clinically useful frame rate of 20 s-1. We confirmed image contrast for atherosclerotic plaque in human samples ex vivo. The system is on a mobile platform and provides real-time data visualization during acquisition. We achieved an IVPA signal-to-noise ratio of 20 dB. These data show that clinical translation of IVPA is possible in principle.

A computer-based simulator for intravascular photoacoustic images

Intravascular photoacoustic (IVPA) is a newly developed catheter-based imaging technique for the diagnosis of arterial atherosclerosis. A framework of simulating IVPA transversal images from a cross-sectional vessel model with given optical and acoustic parameters was presented. The light illumination and transportation in multi-layered wall and atherosclerotic plaque tissues were modeled through Monte Carlo (MC) simulation. The generation and transmission of photoacoustic (PA) waves in the acoustically homogeneous medium were modeled through the PA wave equation, which is solved explicitly with a finite difference time domain (FDTD) algorithm in polar coordinates. Finally, a series of cross-sectional gray-scale images displaying the distribution of the deposited optical energy were reconstructed from the time-dependent acoustic pressure series with a time-reversal based algorithm. Experimental results demonstrate a good correlation between the simulated IVPA images and the optical absorption distribution profiles. The simulator provides a powerful tool for generating IVPA image data sets, which are used to improve the imaging catheter and to test the performance of image post-processing algorithms.

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.

Real-Time Intravascular Ultrasound and Photoacoustic Imaging

Combined intravascular ultrasound and intravascular photoacoustic (IVUS/IVPA) imaging is an emerging hybrid modality being explored as a means of improving the characterization of atherosclerotic plaque anatomical and compositional features. While initial demonstrations of the technique have been encouraging, they have been limited by catheter rotation and data acquisition, displaying, and processing rates on the order of several seconds per frame as well as the use of off-line image processing. Herein, we present a complete IVUS/IVPA imaging system and method capable of real-time IVUS/IVPA imaging, with online data acquisition, image processing, and display of both IVUS and IVPA images. The integrated IVUS/IVPA catheter is fully contained within a 1-mm outer diameter torque cable coupled on the proximal end to a custom-designed spindle enabling optical and electrical coupling to system hardware, including a nanosecond-pulsed laser with a controllable pulse repetition frequency capable of greater than 10 kHz, motor and servo drive, a US pulser/receiver, and a 200-MHz digitizer. The system performance is characterized and demonstrated on a vessel-mimicking phantom with an embedded coronary stent intended to provide IVPA contrast within content of an IVUS image.

Multimodal optoacoustic and multiphoton microscopy of human carotid atheroma

Carotid artery atherosclerosis is a main cause of stroke. Understanding atherosclerosis biology is critical in the development of targeted prevention and treatment strategies. Consequently, there is demand for advanced tools investigating atheroma pathology. We consider hybrid optoacoustic and multiphoton microscopy for the integrated and complementary interrogation of plaque tissue constituents and their mutual interactions. Herein, we visualize human carotid plaque using a hybrid multimodal imaging system that combines optical resolution optoacoustic (photoacoustic) microscopy, second and third harmonic generation microscopy, and two-photon excitation fluorescence microscopy. Our data suggest more comprehensive insights in the pathophysiology of atheroma formation and destabilization, by enabling congruent visualization of structural and biological features critical for the atherosclerotic process and its acute complications, such as red blood cells and collagen.

Near-infrared multispectral photoacoustic microscopy using a graded-index fiber amplifier

We demonstrate optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue using a multi-wavelength pulsed laser based on nonlinear fiber optics. 1047 nm laser pulses are converted to 1098, 1153, 1215, and 1270 nm pulses via stimulated Raman scattering in a graded-index multimode fiber. Multispectral PAM of a lipid phantom is demonstrated with our low-cost and simple technique.

A Review of Intravascular Ultrasound-based Multimodal Intravascular Imaging: The Synergistic Approach to Characterizing Vulnerable Plaques

Catheter-based intravascular imaging modalities are being developed to visualize pathologies in coronary arteries, such as high-risk vulnerable atherosclerotic plaques known as thin-cap fibroatheroma, to guide therapeutic strategy at preventing heart attacks. Mounting evidences have shown three distinctive histopathological features-the presence of a thin fibrous cap, a lipid-rich necrotic core, and numerous infiltrating macrophages-are key markers of increased vulnerability in atherosclerotic plaques. To visualize these changes, the majority of catheter-based imaging modalities used intravascular ultrasound (IVUS) as the technical foundation and integrated emerging intravascular imaging techniques to enhance the characterization of vulnerable plaques. However, no current imaging technology is the unequivocal "gold standard" for the diagnosis of vulnerable atherosclerotic plaques. Each intravascular imaging technology possesses its own unique features that yield valuable information although encumbered by inherent limitations not seen in other modalities. In this context, the aim of this review is to discuss current scientific innovations, technical challenges, and prospective strategies in the development of IVUS-based multi-modality intravascular imaging systems aimed at assessing atherosclerotic plaque vulnerability.

Characterization of lipid-rich plaques using spectroscopic optical coherence tomography

Intravascular optical coherence tomography (IV-OCT) is a high-resolution imaging method used to visualize the internal structures of walls of coronary arteries in vivo. However, accurate characterization of atherosclerotic plaques with gray-scale IV-OCT images is often limited by various intrinsic artifacts. In this study, we present an algorithm for characterizing lipid-rich plaques with a spectroscopic OCT technique based on a Gaussian center of mass (GCOM) metric. The GCOM metric, which reflects the absorbance properties of lipids, was validated using a lipid phantom. In addition, the proposed characterization method was successfully demonstrated in vivo using an atherosclerotic rabbit model and was found to have a sensitivity and specificity of 94.3% and 76.7% for lipid classification, respectively.

Bond-selective photoacoustic imaging by converting molecular vibration into acoustic waves

The quantized vibration of chemical bonds provides a way of detecting specific molecules in a complex tissue environment. Unlike pure optical methods, for which imaging depth is limited to a few hundred micrometers by significant optical scattering, photoacoustic detection of vibrational absorption breaks through the optical diffusion limit by taking advantage of diffused photons and weak acoustic scattering. Key features of this method include both high scalability of imaging depth from a few millimeters to a few centimeters and chemical bond selectivity as a novel contrast mechanism for photoacoustic imaging. Its biomedical applications spans detection of white matter loss and regeneration, assessment of breast tumor margins, and diagnosis of vulnerable atherosclerotic plaques. This review provides an overview of the recent advances made in vibration-based photoacoustic imaging and various biomedical applications enabled by this new technology.

Emerging Technology Update Intravascular Photoacoustic Imaging of Vulnerable Atherosclerotic Plaque

The identification of vulnerable atherosclerotic plaques in the coronary arteries is emerging as an important tool for guiding atherosclerosis diagnosis and interventions. Assessment of plaque vulnerability requires knowledge of both the structure and composition of the plaque. Intravascular photoacoustic (IVPA) imaging is able to show the morphology and composition of atherosclerotic plaque. With imminent improvements in IVPA imaging, it is becoming possible to assess human coronary artery disease in vivo. Although some challenges remain, IVPA imaging is on its way to being a powerful tool for visualising coronary atherosclerotic features that have been specifically associated with plaque vulnerability and clinical syndromes, and thus such imaging might become valuable for clinical risk assessment in the catheterisation laboratory.

Vibrational spectroscopic imaging of living systems: An emerging platform for biology and medicine

Vibrational spectroscopy has been extensively applied to the study of molecules in gas phase, in condensed phase, and at interfaces. The transition from spectroscopy to spectroscopic imaging of living systems, which allows the spectrum of biomolecules to act as natural contrast, is opening new opportunities to reveal cellular machinery and to enable molecule-based diagnosis. Such a transition, however, involves more than a simple combination of spectrometry and microscopy. We review recent efforts that have pushed the boundary of the vibrational spectroscopic imaging field in terms of spectral acquisition speed, detection sensitivity, spatial resolution, and imaging depth. We further highlight recent applications in functional analysis of single cells and in label-free detection of diseases.

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.

Specific imaging of atherosclerotic plaque lipids with two-wavelength intravascular photoacoustics

The lipid content in plaques is an important marker for identifying atherosclerotic lesions and disease states. Intravascular photoacoustic (IVPA) imaging can be used to visualize lipids in the artery. In this study, we further investigated lipid detection in the 1.7-µm spectral range. By exploiting the relative difference between the IVPA signal strengths at 1718 and 1734 nm, we could successfully detect and differentiate between the plaque lipids and peri-adventitial fat in human coronary arteries ex vivo. Our study demonstrates that IVPA imaging can positively identify atherosclerotic plaques using only two wavelengths, which could enable rapid data acquisition in vivo.

Near-infrared spectroscopic photoacoustic microscopy using a multi-color fiber laser source

We demonstrate a simple multi-wavelength optical source suitable for spectroscopic optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue. 1064 nm laser pulses are converted to multiple wavelengths beyond 1300 nm via nonlinear optical propagation in a birefringent optical fiber. OR-PAM experiments with lipid phantoms clearly show the expected absorption peak near 1210 nm. We believe this simple multi-color technique is a promising cost-effective approach to spectroscopic OR-PAM of lipid-rich tissue.

High-speed intravascular spectroscopic photoacoustic imaging at 1000 A-lines per second with a 0.9-mm diameter catheter

Intravascular spectroscopic photoacoustic technology can image atherosclerotic plaque composition with high sensitivity and specificity, which is critical for identifying vulnerable plaques. Here, we designed and engineered a catheter of 0.9 mm in diameter for intravascular photoacoustic (IVPA) imaging, smaller than the critical size of 1 mm required for clinical translation. Further, a quasifocusing photoacoustic excitation scheme was developed for the catheter, producing well-detectable IVPA signals from stents and lipids with a laser energy as low as ~30 μJ/pulse. As a result, this design enabled the use of a low-energy, high-repetition rate, ns-pulsed optical parametric oscillator laser for high-speed spectroscopic IVPA imaging at both the 1.2-μm and 1.7-μm spectral bands for lipid detection. Specifically, for each wavelength, a 1-kHz IVPA A-line rate was achieved, ~100-fold faster than previously reported IVPA systems offering a similar wavelength tuning range. Using the system, spectroscopic IVPA imaging of peri-adventitial adipose tissue from a porcine aorta segment was demonstrated. The significantly improved imaging speed, together with the reduced catheter size and multiwavelength spectroscopic imaging ability, suggests that the developed high-speed IVPA technology is of great potential to be further translated for in vivo applications.

1.7-μm spectroscopic spectral-domain optical coherence tomography for imaging lipid distribution within blood vessel

We have developed a spectroscopic optical coherence tomography (OCT) for imaging lipid distribution within blood vessel in order to detect coronary artery plaque. A 1.7-μm spectral-domain OCT with A-scan rate of 47 kHz is fabricated using a broadband light source based on super-luminescent diodes and spectrometers based on extended InGaAs line sensors. We demonstrate imaging of lipid distribution in an in vitro artery model with lipid. The sensitivity and specificity in the differentiation between artery and lipid are 87% and 90% in the training, respectively. The validation test also shows detection of lipid with an accuracy over 90%.

Development of a photoacoustic, ultrasound and fluorescence imaging catheter for the study of atherosclerotic plaque

Atherosclerotic cardiovascular diseases are a major cause of death in industrialized countries. Molecular imaging modalities are increasingly recognized to be a promising avenue towards improved diagnosis and for the evaluation of new drug therapies. In this work, we present an acquisition system and associated catheter enabling simultaneous photoacoustic, ultrasound and fluorescence imaging of arteries designed for in vivo imaging. The catheter performance is evaluated in tissue-mimicking phantoms. Simultaneous imaging with three modalities is demonstrated at frame rates of 30 images per second for ultrasound and fluorescence and 1 image per 13 seconds for photoacoustic. Acquired radio-frequency ultrasound data could be processed to obtain radial strain elastograms. With motorized pullback, 3D imaging of phantoms was performed using the three modalities.

Real-time optoacoustic tomography of indocyanine green perfusion and oxygenation parameters in human finger vasculature

We interrogated whether optoacoustic tomography could be employed to study blood functional parameters and biodistribution of injected fluorescent agents in humans. Using a multichannel scanner at a frame rate of 10 images per second, we obtained cross-sectional images of the human finger in real time, before and after the administration of indocyanine green. We demonstrated that multispectral optoacoustic tomography can sense fast flow kinetics and resolve spatiotemporal characteristics of a common fluorochrome in human vasculature at clinically relevant concentrations. We further register ICG images with oxygen saturation maps and anatomical views of the proximal interphalangeal joint of a healthy volunteer.

Intravascular photoacoustic imaging: a new tool for vulnerable plaque identification

The vulnerable atherosclerotic plaque is believed to be at the root of the majority of acute coronary events. Even though the exact origins of plaque vulnerability remain elusive, the thin-cap fibroatheroma, characterized by a lipid-rich necrotic core covered by a thin fibrous cap, is considered to be the most prominent type of vulnerable plaque. No clinically available imaging technique can characterize atherosclerotic lesions to the extent needed to determine plaque vulnerability prognostically. Intravascular photoacoustic imaging (IVPA) has the potential to take a significant step in that direction by imaging both plaque structure and composition. IVPA is a natural extension of intravascular ultrasound that adds tissue type specificity to the images. IVPA utilizes the optical contrast provided by the differences in the absorption spectra of plaque components to image composition. Its capability to image lipids in human coronary atherosclerosis has been shown extensively ex vivo and has recently been translated to an in vivo animal model. Other disease markers that have been successfully targeted are calcium and inflammatory markers, such as macrophages and matrix metalloproteinase; the latter two through application of exogenous contrast agents. By simultaneously displaying plaque morphology and composition, IVPA can provide a powerful prognostic marker for disease progression, and as such has the potential to transform the current practice in percutaneous coronary intervention.