Overview

Cardiovascular disease (CVD) is the leading cause of death in the United States and globally. Heart attacks and strokes represent the majority of CVD-related deaths and are predominantly caused by ischemia secondary to atherosclerotic disease of major arteries. Unsurprisingly, considerable research effort has gone into understanding the etiology of atherosclerosis and developing diagnostic tools that can identify the early onset of the disease prior to ischemic event.
Atherosclerosis has often been dubbed the “silent killer” because it generally develops asymptomatically for decades before presenting as a heart attack or stroke.

doppler
B-mode and color flow Doppler (CFD) images of a carotid plaque with 60-99% stenosis (asterisk).

While atherosclerosis itself can narrow arteries over time and eventually cause ischemia, an acute-onset event such as a heart attack often occurs when a plaque is disrupted or ruptured. In the 1990s and early 2000s, numerous histopathology studies determined that certain morphological features made plaques more prone to rupture, leading to the development of the “vulnerable plaque” hypothesis. Namely, plaques that were composed of large lipid-rich necrotic cores and active inflammation covered with thin fibrous caps were highly associated with rupture and thrombosis. Importantly, these and other studies revealed that ruptured plaques were not necessarily those with the highest degree of stenosis, suggesting that plaque composition may be a superior biomarker to stratify plaque risk. However, a cost-effective imaging modality that can accurately identify vulnerable plaque has remained elusive.

Our lab has been investigating using ARFI imaging to improve ultrasonic plaque characterization. Our primary hypothesis is that fibrosis and calcification will appear stiff in an ARFI image while necrotic core and intraplaque hemorrhage (i.e. vulnerable features) will appear soft in an ARFI image.

Validation of ARFI in humans

Our most recent work on this project has been to validate ARFI imaging in a cohort of human patients undergoing carotid endarterectomy (CEA). CEA is a surgery performed on the carotid arteries in the neck to extract plaque in an effort to prevent a future stroke. CEA patients serve as nice study subjects because we can image their plaques in vivo prior to surgery and then collect the specimen that is removed after surgery for histological processing and validation. To date we have imaged 25 patients undergoing CEA and have demonstrated that indeed ARFI is capable of separating stiff plaque features (collagen/calcium) from soft plaque features (necrotic core/hemorrhage). Two examples of ARFI images of carotid plaque are given below.

carotid1

Fig. 1: Carotid ARFI image with matched histology and ex vivo µCT imaging from a symptomatic patient. (a) B-mode with (b) ARFI peak displacement image overlaid. Significant stenosis is visible due to plaque in both the proximal and distal wall. In the proximal wall, an area of large dispalcement (top magenta arrow) is observed, covered by a lower displacing cap. Relatively larger displacements are also seen in the distal wall, but with smaller absolute magnitude (bottom magenta arrow). (c) µCT image showing minor calcification in the distal wall (yellow arrow), that matches with a bright spot on B-mode (yellow arrow). (d) H&E stain and (e) CME stain show a Type Va plaque with necrotic core in both the proximal and distal walls (magenta arrows). Doppler image of this plaque is shown in the previous image.

carotid2

Fig. 2: Plaque from the common carotid artery (CCA) of a 57 year-old symptomatic male. B-mode imaging shows an echolucent plaque with two echogenic foci located ~10 mm apart (white arrows) suggesting calcification. Hybrid B-mode/ARFI peak displacement images (b) show a plaque with generally low displacement except for an area of higher displacement located on the right (yellow outline) suggesting a lipid/necrotic core covered with a stiffer fibrous cap. H&E staining (c) confirms the presence of two calcifications (black arrows), while combined Masson’s staining (d) shows a necrotic region (yellow outline) located above the leftmost calcification underneath a fibrous cap. Inset panels show a magnification of fibrotic (d.1) and necrotic regions (d.2). The necrotic region is characterzied by cholestrol clefts and free erythrocytes suggesting intra-plaque hemorrhage (d.2).