Characterization of In Vivo Atherosclerotic Plaques in the Carotid Artery with Acoustic Radiation Force Impulse Imaging

Acoustic Radiation Force Impulse (ARFI) imaging is a useful method for characterizing the local mechanical properties of tissue. ARFI imaging uses high energy, focused ultrasound pulses to generate small displacements (1-10 um) in tissue. The response of tissue to these localized forces is observed using a series of imaging pulses, which track displacement of the tissue. ARFI imaging is complementary to B-mode imaging, and can readily distinguish between stiff and soft regions of tissue with high contrast. Currently, physicians cannot easily determine whether atherosclerotic plaques in carotid arteries are vulnerable to rupture. We are investigating the use of ARFI imaging to characterize atherosclerotic plaque in carotid arteries and to guide the treatment of these plaques. We have created custom beam sequences on a Siemens Antares(TM) scanner to implement ARFI imaging. Five patients with atherosclerotic plaque and four volunteers without known atherosclerotic plaque underwent B-Mode and ARFI scanning of their carotid arteries. Measurements of the complete vessel wall (adventitial layer plus IMT) with ARFI imaging are shown to be consistent with measurements of the intima-media thickness (IMT) in normal B-mode scans. In plaque-filled vessels, the mean near-wall IMT was 0.92 mm and the mean far-wall IMT was 1.24 mm. In ARFI imaging, vessel walls have mean thicknesses of 2.17 mm and 2.17 mm for the near and far walls, respectively. For volunteers without plaques, the near and far wall IMT measurements were 0.52 and 0.54 mm, respectively and the ARFI vessel thicknesses were 1.39 and 1.26 mm. Both IMT measurements and ARFI measurements were significantly (p < 0.005) greater for patients with plaques. We demonstrate high-resolution ARFI images of carotid plaques depicting their complex mechanical structure, which often involve regions of soft and hard plaque. We discuss methods of improving the contrast and resolution of ARFI vascular images.