Non-invasive transcranial ultrasound therapy guided by CT-scans.

Brain therapy using focused ultrasound remains very limited due to the strong aberrations induced by the skull. A technique using time-reversal was validated recently in-vivo on 20 sheep. The principal handicap of this technique is the need of an hydrophone at the focal point for the first step of the time-reversal procedure, which is minimally invasive but lightly traumatizing. A completely noninvasive therapy requires a reliable model of the acoustical properties of the skull in order to simulate this first step. 3-D simulations based on high-resolution CT images of a skull have been successfully performed with a finite differences code developed in our Laboratory. Thanks to 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 and this technic was validated in-vitro in the laboratory. A stereotactic frame has been designed and built in order to perform non invasive transcranial focusing. Here we will describe all the steps of our new protocol, from the CT-scans to the therapy treatment and the first in vivo results on monkeys will be presented. This protocol is based on protocols already existing in radiotherapy.