Evaluation of Atherosclerosis by Acoustic Microscopy

Abstract

Acoustic microscopy can provide not only the morphology, but also the biomechanical properties of biological soft tissues. The biomechanics of atherosclerosis is important because the pathophysiology of atherosclerosis is closely related to mechanical properties and to mechanical stress. The initial event in acute coronary syndrome such as acute myocardial infarction or unstable angina is the rupture of the fibrous cap of atheromatous plaque. In addition to extrinsic physical stresses to the plaque, the intrinsic biomechanical property of the plaque is important for assessing the mechanism of the rupture. Two Scanning Acoustic Microscopes (SAM), operating in 100 to 200 MHz and in 800 MHz to 1.3 GHz frequency regions were equipped to measure the acoustic properties of atherosclerosis of human and mouse arteries. The values of attenuation and sound speed in the tissue components of atherosclerosis were measured by analyzing the frequency dependent characteristics of the amplitude and phase of the ultrasonic signals. Both values were highest in calcification and lowest in lipid pool. Although attenuation and sound speed were relatively high in intimal fibrosis, inhomogeneity of the acoustic parameters was found within the fibrous cap. Polarized microscopy of collagen stained with Picrosirius red showed that the attenuation of ultrasound was significantly higher in type I collagen with orange polarized color compared to type III collagen with green color. The SAM has shown that it is possible to detect plaque vulnerability and that it might improve our understanding of sudden rupture from a micro-mechanical point of view.