The Effect of Frequency on the Magnitude of Cyclic Variation of Backscatter in Dogs and Implications for Prompt Detection of Acute Myocardial Ischemia

Contracting myocardium exhibits a cyclic variation of the magnitude of ultrasonic backscatter, with maxima occurring at end-diastole and minima at end-systole. In order to determine whether this effect exhibits frequency dependence, the magnitude of cyclic variation of integrated backscatter was measured for five frequency bands: 3–4 MHz, 4–5 MHz, 5–6 MHz, 6–7 MHz, and 7–8 MHz. In ten normal dogs, the magnitude of cyclic variation (Af) was found to increase with ultrasonic frequency in an approximately linear fashion. The least squares linear fit to the data yielded M = 2.5 dB + 0.24f dB/MHz where f is the ultrasonic frequency (MHz). Subsequently, the potential frequency dependence of the ability to detect the immediate consequences of myocardial ischemia was investigated. Acute ischemic injury was induced in each of seven dogs by ligation of a coronary artery. The magnitude of cyclic variation of integrated backscatter was measured in regions of myocardium supplied by this artery both before and after ligation. Ischemic myocardium was clearly differentiable from normal myocardium in all five frequency bands. The magnitude of cyclic variation of integrated backscatter demonstrated substantial recovery upon reperfusion in all five frequency bands. These results offer promise for the detection of ischemia in humans using clinical imaging systems.