Depth-Resolved Assessment of Atherosclerosis by Intravascular Photoacoustic-Ultrasound Imaging

Coronary heart disease is the leading cause of death in the United States and the incidence is projected to increase by 18% by 2030. Yet, there remains a pressing clinical need for tools to detect vulnerable atherosclerotic plaques that can rupture and lead to major adverse cardiac events. Plaques that are considered most vulnerable for rupture are thin-capped fibroatheromas, which are grossly defined by hallmarks of a thin fibrous cap, a large lipid-rich necrotic core, inflammatory infiltrate, and positive remodeling. These plaques are often structurally non-obstructive to moderately obstructive, thus asymptomatic and clinically unidentifiable with routine angiography and stress testing. Rather, their vulnerability is a product of their chemical composition. We have developed a dual-mode intravascular catheter which is capable of producing co-registered cross-sectional images of arterial wall morphology and lipid content, via ultrasound and photoacoustic modes, respectively. Validation of this capability will rely on interrogation of atherosclerotic coronary arteries from humans and peripheral arteries from swine, with comparison to gold-standard histopathology and competing technologies. Here, we present ex vivo validation of a novel intravascular photoacoustic-ultrasound (IVPA-US) imaging catheter and the first systematic in vivo IVPA-US imaging study in a preclinical swine model with native disease, necessary benchmarks before proceeding with translation to clinic. We aim to ultimately demonstrate predictive utility to detect plaques that are vulnerable to rupture and trigger adverse cardiac events. In addition, this will be instrumental in elucidating the mechanism of plaque rupture, the development of preventive and therapeutic interventions, and reducing coronary heart disease-related mortality

Lipid detection in pig arteries using intravascular photoacoustic imaging

Heart disease is the leading cause of death in the United States and worldwide. Each year over 370,000 people died from coronary artery disease in America. As the primary form of coronary artery disease, atherosclerosis behaves as lipid-rich plaque development inside an artery wall. Vulnerable plaques are those prone to rupture, which may result in thrombus or even death. Typical hallmarks of a vulnerable plaque include thin fibrous cap, a large lipid-rich necrotic core and inflammatory infiltrate. The identification and accurate detection of these lipid depositions in the arterial wall is crucial in the diagnosis of atherosclerosis. However, none of the current clinical imaging tools can provide accurate and reliable detection of the lipid-rich necrotic core in human arteries. Intravascular photoacoustic (IVPA) imaging is an emerging technique that can provide lipid-specific detection with depth resolution. Our research focuses on applying the catheter-based IVPA imaging technique for lipid-laden plaque detection within the artery of an Ossabaw swine model. A high sensitivity IVPA imaging system developed in our lab was performed to imaging the carotid arteries from the pig model ex vivo. The imaging results showed that the exact location and size of the lipid core can be identified, which agrees with the gold standard histology result. We also compared the results of our IVPA system with the commercial near infrared spectroscopy (NIRS) imaging system. They both successfully indicated the lipid appearance at the same location. However, our imaging modality provided more information of the lipid including lipid core size, depth and distribution. This is a significant improvement of plaque burden estimation and the diagnosis of atherosclerosis in the human artery

Highly Sensitive Intravascular Photoacoustic Imaging with a Collinear Catheter Probe

A collinear catheter for label-free intravascular photoacoustic imaging was developed with a diameter of 1.6 mm. The collinear overlap between optical and acoustic waves enabled photoacoustic imaging of a human coronary artery from lumen to perivascular fat

Comparative Quantification of Arterial Lipid by Intravascular Photoacoustic-Ultrasound Imaging and Near-Infrared Spectroscopy-Intravascular Ultrasound

Intravascular photoacoustic-ultrasound (IVPA-US) imaging and near-infrared spectroscopy-intravascular ultrasound (NIRS-IVUS) are two hybrid modalities that detect arterial lipid, with comparison necessary to understand the relative advantages of each. We performed in vivo and ex vivo IVPA-US imaging of the iliac arteries of Ossabaw swine with metabolic syndrome (MetS) and lean swine to investigate sensitivity for early-stage atherosclerosis. We repeated imaging ex vivo with NIRS-IVUS for comparison to IVPA-US and histology. Both modalities showed significantly greater lipid in MetS vs. lean swine, but only IVPA-US localized the lipid as perivascular. To investigate late-stage atherosclerosis, we performed ex vivo IVPA-US imaging of a human coronary artery with comparison to NIRS-IVUS and histology. Two advanced fibroatheromas were identified, with agreement between IVPA-measured lipid area and NIRS-derived lipid content. As confirmed histologically, IVPA-US has sensitivity to detect lipid content similar to NIRS-IVUS and provides additional depth resolution, enabling quantification and localization of lipid cores within plaques

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

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

Real-time intravascular photoacoustic-ultrasound imaging of lipid-laden plaque at speed of video-rate level

Intravascular photoacoustic-ultrasound (IVPA-US) imaging is an emerging hybrid modality for the detection of lipidladen plaques by providing simultaneous morphological and lipid-specific chemical information of an artery wall. The clinical utility of IVPA-US technology requires real-time imaging and display at speed of video-rate level. Here, we demonstrate a compact and 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, differentiated A-line strategy, and real-time image processing and display algorithms. By imaging pulsatile motion at different imaging speeds, 16 frames per second was deemed to be adequate to suppress motion artifacts from cardiac pulsation for in vivo applications. Our lateral resolution results further verified the number of A-lines used for a cross-sectional IVPA image reconstruction. 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

Epicardial Adipose Tissue Removal Potentiates Outward Remodeling and Arrests Coronary Atherogenesis

BACKGROUND: Pericoronary epicardial adipose tissue (cEAT) serves as a metabolic and paracrine organ that contributes to inflammation and is associated with macrovascular coronary artery disease (CAD) development. Although there is a strong correlation in humans between cEAT volume and CAD severity, there remains a paucity of experimental data demonstrating a causal link of cEAT to CAD. The current study tested the hypothesis that surgical resection of cEAT attenuates inflammation and CAD progression. METHODS: Female Ossabaw miniature swine (n = 12) were fed an atherogenic diet for 8 months and randomly allocated into sham (n = 5) or adipectomy (n = 7) groups. Both groups underwent a thoracotomy, opening of the pericardial sac, and placement of radioopaque clips to mark the proximal left anterior descending artery. Adipectomy swine underwent removal of 1 to 1.5 cm2 of cEAT from the proximal artery. After sham or adipectomy, CAD severity was assessed with intravascular ultrasonography. Swine recovered for an additional 3 months on an atherogenic diet, and CAD was assessed immediately before euthanasia. Artery sections were processed for histologic and immunohistochemical analysis. RESULTS: Severity of CAD as assessed by percent stenosis was reduced in the adipectomy cohort compared with shams; however, plaque size remained unaltered, whereas larger plaque sizes developed in sham-operated swine. Adipectomy resulted in an expanded arterial diameter, similar to the Glagov phenomenon of positive outward remodeling. No differences in inflammatory marker expression were observed. CONCLUSIONS: These data indicate that cEAT resection did not alter inflammatory marker expression, but arrested CAD progression through increased positive outward remodeling and arrest of atherogenesis

Alloxan-induced diabetes exacerbates coronary atherosclerosis and calcification in Ossabaw miniature swine with metabolic syndrome

Abstract Background There is a preponderance of evidence implicating diabetes with increased coronary artery disease (CAD) and calcification (CAC) in human patients with metabolic syndrome (MetS), but the effect of diabetes on CAD severity in animal models remains controversial. We investigated whether diabetes exacerbates CAD/CAC and intracellular free calcium ([Ca2+]i) dysregulation in the clinically relevant Ossabaw miniature swine model of MetS. Methods Sixteen swine, eight with alloxan-induced diabetes, were fed a hypercaloric, atherogenic diet for 6 months. Alloxan-induced pancreatic beta cell damage was examined by immunohistochemical staining of insulin. The metabolic profile was confirmed by body weight, complete blood panel, intravenous glucose tolerance test (IVGTT), and meal tolerance test. CAD severity was assessed with intravascular ultrasound and histology. [Ca2+]i handling in coronary smooth muscle (CSM) cells was assessed with fura-2 ratiometric imaging. Results Fasting and post-prandial blood glucose, total cholesterol, and serum triglycerides were elevated in MetS-diabetic swine. This group also exhibited hypoinsulinemia during IVGTT and less pancreatic beta cell mass when compared to lean and MetS-nondiabetic swine. IVUS analysis revealed that MetS-diabetic swine had greater percent wall coverage, percent plaque burden, and calcium index when compared to lean and MetS-nondiabetic swine. Fura-2 imaging of CSM [Ca2+]i revealed that MetS-nondiabetic swine exhibited increased sarcoplasmic reticulum Ca2+ store release and Ca2+ influx through voltage-gated Ca2+ channels compared to lean swine. MetS-diabetic swine exhibited impaired Ca2+ efflux. Conclusions Diabetes exacerbates coronary atherosclerosis and calcification in Ossabaw miniature swine with MetS, accompanied by progression of [Ca2+]i dysregulation in advanced CAD/CAC. These results recapitulate increased CAD in humans with diabetes and establish Ossabaw miniature swine as an animal model for future MetS/diabetes comorbidity studies

Spectral analysis assisted photoacoustic imaging for lipid composition differentiation

Recent advances in atherosclerotic plaque detection have shown that not only does lipid core size and depth play important roles in plaque rupture and thrombi formation, but lipid composition, especially cholesterol deposition, is equally important in determining lesion vulnerability. Here, we demonstrate a spectral analysis assisted photoacoustic imaging approach to differentiate and map lipid compositions within an artery wall. The approach is based on the classification of spectral curves obtained from the sliding windows along time-of-flight photoacoustic signals via a numerical k-means clustering method. The evaluation result on a vessel-mimicking phantom containing cholesterol and olive oil shows accuracy and efficiency of this method, suggesting the potential to apply this approach in assessment of atherosclerotic plaques