ANISOTROPY OF THE APPARENT FREQUENCY-DEPENDENCE OF BACKSCATTER IN FORMALIN-FIXED HUMAN MYOCARDIUM

Measurements of the frequency dependence of ultrasonic backscatter are presented for specific angles of insonification for regions of infarc ted and noninfarcted human myocardium. A 5-MHz transducer was used to insonify cylindrical cores taken from 7 noninfarcted regions and 12 in farcted regions of the left ventricular free wall of 6 formalin-fixed human hearts explanted because of ischemic cardiomyopathy. The depende nce of apparent (uncompensated for diffraction effects and attenuation ) backscatter on frequency was approximated by a power-law dependence, \B(f)\(2) = af(n). Under ideal conditions in a lossless medium, the e ffect of not compensating for the effects of diffraction and attenuati on leads to the value of n to be 2.0 for Rayleigh scatterers while the frequency dependence of the fully compensated backscatter coefficient would be f(4). The value of n was determined over the frequency range , 3-7 MHz. Both noninfarcted and infarcted myocardium exhibited anisot ropy of the frequency dependence of backscatter, with maxima occurring at angles that were perpendicular to the predominant myofiber directi on and minima when parallel to the fibers. Perpendicular insonificatio n yielded results for it of 1.8+/-0.1 for noninfarcted myocardium and 1.2+/-0.1 for infarcted myocardium while parallel insonification yield ed results of 0.4+/-0.1 for noninfarcted and 0.0+/-0.1 for infarcted m yocardium. The functional form of the angle-dependent backscatter is s imilar for both noninfarcted and infarcted myocardium, although the fr equency dependence is clearly different for both tissue states for all angles of insonification. The results of this study indicate that the anisotropy of the frequency dependence of backscatter may play a sign ificant role in ultrasonic imaging and is an important consideration f or ultrasonic tissue characterization in myocardium. (C) 1997 Acoustic al Society of America.