1G-1 Treatment of Near-Skull Brain Tissue Using Shear Mode Conversion

Shear mode transmission through the skull has been previously proposed as a new trans-skull propagation technique for noninvasive therapeutic ultrasound [Clement, J. Acoust. Soc. Am. 115(3), 1356-1364 (2004)]. The main advantage of choosing shear over longitudinal mode resides on the fact that there is less phase shift with the first one. In the present study, the regions of the brain suitable for shear mode transmission were established for a focusing therapeutic device. The therapeutic device consists of a spherically-curved transducer that has 30 active annular-elements, a focal length of 10 cm, an aperture between 60 and 30deg and operates at 0.74 MHz. The regions suitable for shear mode transmission were determined by the shear-wave acoustic windows that matched the shape of the device acoustic field. The acoustic windows were calculated using segmentation and triangulation of the scalp from a 3D-MRI brain dataset. Nine heads of healthy adults were analyzed. The surface considered for the calculations consisted on the scalp region found above the supra-orbital margin. For every inspected point in the brain volume, the axis of the therapeutic device was determined by the vector between this inspection point and a point located in the center of the brain. By selecting specific rings in the therapeutic device that matched the acoustic window, numerical predictions of the acoustic field, where shear mode conversion through the skull was considered, were obtained and compared to the case of only-water conditions. The brain tissue that is close to the skull showed suitable acoustic windows for shear waves. The central region of the brain seems to be unreachable using shear mode with this setup. Usable acoustic window for shear waves were detected when the acoustic window for shear waves matched more than 50% of the acoustic window of the therapeutic device. The average and standard deviation of the depth from the scalp showing a matching value equal or larger than 50- % were, respectively, 2.39 and 0.17 cm. This matching had a value close to 100% in the tissue near to the internal face of the skull, and its value decreased as the inspection point was closer to the center of the brain. The average of the largest depth in tissue from the scalp showing a usable acoustic window was 3.69 cm and was found in the tissue close to the parietal lobes region. The average of the smallest depth showing usable acoustic windows was 0.96 cm (right below the skull) and was found in the frontal lobe region