Shear wave elastography for lipid content detection in transverse arterial cross-sections

Rupture of plaques in atherosclerotic carotid arteries is a main cause of stroke. Plaques with a large soft lipid-rich core are more prone to rupture than predominantly fibrous, stiff plaques. Detecting and monitoring the presence of lipids in plaques noninvasively and fast remains challenging. Ultrasound shear wave elastography is fast and noninvasive. Experimental studies showed that detection of locally softer regions in vessel-mimicking phantoms is possible. However, these studies only report imaging in a longitudinal imaging plane. Since it is not possible to detect lipids over the full vessel circumference when imaging in a longitudinal plane, this study investigates the feasibility of shear wave elastography in transverse imaging planes. A SuperSonic Imagine Aixplorer equipped with a linear array probe was used to induce shear waves in vessel-mimicking polyvinylalcohol phantoms of varying dimensions and shear moduli using a single focused ultrasonic push (300 μs). The push was followed by 0° plane wave acquisitions (n = 100, PRF ≥ 10 kHz) to detect the axial motion induced in the vessel wall. To visualize the generated circumferentially propagating shear waves, axial motion maps were created showing the axial phase shift as a function of circumferential position for circular paths in the vessel wall. Approximately linearly propagating wave fronts were observed for all phantoms as a function of circumferential position. For homogeneous phantoms, one wave was observed, which made a roundtrip, whereas a slow and a fast wave were observed in two-layered `soft plaque´ phantoms suggesting that soft lipid-rich regions can be detected. Finally, preliminary in vivo results were obtained in a volunteer using a Verasonics ultrasound system which confirmed that creation of circumferentially propagating waves is also possible in vivo. Based on these observations we conclude that shear wave elastography in transverse imaging planes is feasible and promising for lipid-core detection in plaques of superficial arteries.