Atherosclerotic plaque characterization by acoustic impedance analysis of intravascular ultrasound data

Abstract

Most acute coronary syndromes are the result of rupture of a vulnerable atherosclerotic plaque. Accurate in vivo identification of the plaque composition using intravascular ultrasound (IVUS) imaging has the potential to be a useful diagnostic tool helpful in detection of the vulnerable atheroma before rupture. Intensity at any point in an IVUS image of a vessel cross-section depends on the acoustic impedance of the tissue at any point, which in turn depends on the tissue density. Different plaque compositions have different densities. Taking advantage of this relationship between acoustic impedance and tissue density, we have investigated the possibility of classification of plaque on the basis of relative acoustic impedance value. Data was collected in vitro from 29 sites from 18 specimens of left anterior descending (LAD) coronary artery, excised under physiologic conditions, using a 40 MHz unfocused catheter. In this study, for a given arterial cross-section, we maintain one-to-one spatial correlation between histology and the corresponding IVUS image. One hundred seventeen regions of interest (ROIs) were grouped as fibrous (n = 45), fibrolipidic (n = 38), calcified (n = 6), calcified necrotic (n = 10) and lipid core (n = 18) based on histology. The radio frequency (RF) data was analyzed using plane wave born approximation (PWBA) deconvolved inverse scattering method which estimates relative acoustic impedance from the knowledge of reflected and incident pulses. We conducted statistical analysis based on the one-way analysis of variance (ANOVA) method. Based on relative acoustic impedance values, it was possible to distinguish between fibrolipidic and lipid core regions with a p value of 0.0029. Results with clearer distinction between relative impedance values of different compositions may be expected with a larger sample size. This study showed that relative acoustic impedance values calculated from PWBA deconvolved inverse scattering method can be employed for plaque characterization.