Measurements from 22 to 105 MHz of the apparent anisotropy of ultrasonic backscatter from coronary arteries with atherosclerotic plaques identified by intravascular ultrasound

It is speculated that the anisotropic structure of coronary arteries may cause images of the coronary arteries produced by future oblique and forward-looking IVUS catheters to appear significantly different than the images produced by current side-looking IVUS catheters. In anticipation of these systems and to understand better the fundamental properties of the coronary arteries and associated plaques, we compare the measured anisotropy of ultrasonic backscatter in coronary arteries between side-looking and forward-looking directions over the bandwidth from 22 to 105 MHz. To do so, 44 segments from 19 human coronary arteries were each measured ultrasonically in 8 configurations. Each segment was imaged first with two clinical IVUS systems, and subsequently in two orthogonal orientations by an acoustic microscope operating with 3 interchangeable transducers of nominal center frequencies of 25, 50, and 100 MHz. The backscatter trend observed with side-looking IVUS was consistent with radial acoustic microscopy results, but axial acoustic microscopy exhibited a reversal of the typical pattern, with the media demonstrating greater apparent integrated backscatter than the intima/plaque. These results suggest that future IVUS catheters operating in new imaging planes may need to account for tissue anisotropy, and may be able to exploit this anisotropy for diagnostic advantage.