Anisotropy of ultrasonic parameters 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 hours 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 focussed transducer give frequency dependent information in the 4-14 MHz range. The attenuation coefficient in each muscle is consistently 60±25% lower for propagation perpendicular to the fibers than at 45°, whereas speed of sound shows a much milder anisotropy (slightly faster for the 90° orientation) which is inconclusive within experimental error. 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 close to 2.