Changes in ultrasonic attenuation and backscatter of muscle with state of contraction

We have previously demonstrated that backscatter (uncompensated for attenuation) of canine myocardium varies systematically throughout the cardiac cycle and in relation to regional contractile performance. To elucidate these phenomena under conditions independent of blood flow and complex myofibrillar architecture, we measured attenuation with a phase-insensitive receiver and backscatter over a wide range of frequencies in an intermittently tetanized (10 stimulations), isolated frog gastrocnemius preparation (n = 12 muscles). Muscle contraction, as compared with relaxation, was associated with increased values of slope of attenuation (0.78 ± 0.04 vs 0.58 ± 0.03 B/(cm MHz); p < 0.001) and increased values of integrated backscatter (-29.5 ± 0.9 vs -35.5 ± 0.8 dB; p < 0.005). Differences in attenuation and backscatter diminished with the number of muscle stimulations (as the muscle fatigued). Thus, quantitative ultrasonic indices of skeletal muscle vary systematically with the contractile performance of the tissue. Extrapolation of these results to cardiac muscle suggests that the sensitivity of these indices to contractile function of muscle may provide an approach for noninvasive assessment of intrinsic properties of myocardium that determines its performance.