RELATION OF ULTRASONIC BACKSCATTER AND ACOUSTIC PROPAGATION PROPERTIES TO MYOFIBRILLAR LENGTH AND MYOCARDIAL THICKNESS

Background Ultrasonic backscatter demonstrates a cardiac cycle-depende nt modulation. The exact mechanism of the modulation is under debate. The objective of the present study was to test the hypothesis that a c hange in size and configuration of myofilaments from systole to diasto le alters acoustic propagation properties and backscatter. Methods and Results In vivo measurements were made of integrated backscatter at 5 MHz (IBR5), followed by in vitro measurements of ultrasonic attenuati on, speed, and heterogeneity index using a scanning laser acoustic mic roscope at 100 MHz. Studies were performed in canine hearts (16) arres ted in systole (8) with calcium chloride or arrested in diastole (8) w ith potassium chloride. Sarcomere length was measured with a calibrate d eyepiece on a Ziess microscope. Wall thickness was measured with cal ipers. The attenuation coefficient of 220+/-34 dB/cm during systole wa s significantly higher than the coefficient of 189+/-24 dB/cm during d iastole (P<.01); the IBR5 of -44.7+/-1.2 dB during systole was signifi cantly greater than the IBR5 of -47.0+/-1.0 dB during diastole (P<.01) ; the ultrasonic speed of 1591+/-11 m/s during systole was higher than the speed of 1575+/-4.2 m/s during diastole (P<.01); and the heteroge neity index of 7.4+/-1.8 m/s during systole was significantly lower th an the index of 9.0+/-2.0 m/s during diastole (P<.02). The sarcomere l ength of 1.804+/-0.142 mu m during diastole was significantly higher t han the length of 1.075+/-0.177 mu m during systole (P<.01). Wall thic kness was significantly greater during systole than during diastole (2 0+/-3 versus 9+/-3 mm, P<.01). Conclusions Ultrasonic backscatter and propagation properties are directly related to sarcomere length and my ocardial thickness and may be responsible for cardiac cycle-dependent variation in backscatter.