Anisotropy of ultrasonic propagation and scattering properties in fresh rat skeletal muscle in vitro

The anisotropy of frequency-dependent backscatter coefficient, attenuation, and speed of sound is assessed in fresh rat skeletal muscle within 5 h post-mortem. Excised rat semimembranosus and soleus muscles are measured in 37 °C Tyrode solution, with the muscle fibers at 90°and 45°orientations to the incident sound beam. Reflected and through transmission signals from either a 6- or 10-MHz focused transducer give frequency dependent information in the 4-14 MHz range. The attenuation coefficient in each muscle is consistently a factor of 2.0±0.4 lower for propagation perpendicular to the fibers than at 45°, whereas speed of sound shows a much milder anisotropy, and is slightly faster for the 90°orientation. The largest anisotropy is seen in the backscatter coefficient, most notably in the semimembranosus where the magnitude at 90°is over an order of magnitude greater than at 45°, with the frequency dependence in both cases giving a power law between 1.5 and 2.0. (C) 2000 Acoustical Society of America.