Experimental validation of 3D finite differences simulations of ultrasonic wave propagation through the skull

High Intensity Focused Ultrasound could provide a non invasive way for burning tumor located deep inside the brain. However, the skull induces strong aberrations both in phase and amplitude resulting in a spreading of the focus. Thus a efficient noninvasive therapy would require an adaptive focusing taking into account the acoustical properties of the skull. 3-D simulations based on high-resolution CT images of a human skull have been successfully performed with a 3D finite differences code, developed in our laboratory. From the skull porosity, directly extracted from the CT images, we reconstructed acoustic speed, density and absorption maps and performed the computation. Computed wavefronts are in good agreement with experimental wavefronts acquired through the same part of the skull. It shows that it is possible to model the acoustical properties of a human skull from CT images. Moreover, experimental focusing can be achieved by applying time reversal combined with amplitude compensation. Again the measured focusing pattern obtained from either experimental or computed wavefront are very close. It demonstrates the feasibility of non invasive focusing through the skull by simply using CT images.