[Ultrasonic tissue characterization of diseased myocardium by scanning acoustic microscopy]

Ultrasonic tissue characterization of the myocardium has become very important because the requirement for evaluating local cardiac function has increased. In the present study, the acoustic properties of the tissue elements in myocardial infarction were measured by a specially developed scanning acoustic microscope (SAM) system, and the echo intensity of the infarcted myocardium was assessed. Ten samples of infarcted myocardium obtained at autopsy were formalin-fixed, paraffin-embedded and cut into 10 microns thick slices. The SAM system was operated in the frequency range of 100 to 200 MHz. The values of the attenuation constant and sound speed were measured in five tissue elements, normal myocardium, degenerated myocardium, fibrosis, granulation, and hemorrhage by comparing the optical microscopic image and acoustic images. The density of each tissue element was measured by the CuSO4 method, and the specific acoustic impedance was calculated by multiplying the sound speed and the density of each tissue element. The specific acoustic impedance was 1.75 x 10(6) Ns/m3 in normal myocardium, 1.69 x 10(6) Ns/m3 in degenerated myocardium, and 1.85 x 10(6) Ns/m3 in fibrosis. The intensity of the reflected ultrasound at the interface between two different tissue elements was calculated by the difference of specific acoustic impedance. Using the sensitivity changing method in clinical echocardiography, the intensity of myocardial scar was -15 dB compared to the pericardial echo. The corrected ideal ultrasonic reflection between degenerated myocardium and fibrosis was calculated as -15.4 dB. The results suggest that measurement of acoustic properties is very important to provide basic data for ultrasonic characterization of myocardium tissue.