Ultrasonic transmittance of rat skull as a function of frequency

The applications of focused ultrasound in the brain have the common obstacle of penetrating the skull. The purpose of this study is to characterize the ultrasonic transmittance of the rat skull, which is the common animal model used in clinical studies. We modeled the skull as an acoustic cavity and derived an expression for the ultrasonic transmittance that takes into account the multiple reflections in the skull. We built an experimental setup with which we measured the ultrasonic transmittance. By using XYZ scanner, focused transducers and a hydrophone, we obtained ultrasonic transmission images of ex-vivo rat skulls. We found these images to be consistent with the anatomical structure. We also measured the transmittance frequency response of the rat skulls. We observed interference patterns in frequency responses which is consistent with the acoustical cavity model we proposed. We observed relatively high transmittance at frequencies where the multiple reflections add up constructively, whereas we observed relatively low transmittance otherwise.