Feasibility of shear-mode transcranial ultrasound imaging

Abstract

Despite ultrasound’s potential to provide a low cost method for imaging blood flow and diagnosing certain brain disorders, distortion and low signal to noise ratios caused by the skull have severely limited the use of existing clinical devices, such as trancranial Doppler sonography. Presently we investigate the potential to propagate ultrasound through the skull with reduced distortion and higher signal amplitudes by using high incident angles. In such cases the ultrasound angle of entry is set beyond Snell’s critical angle for the longitudinal pressure wave, so that propagation in the bone is purely due to a shear wave. This wave then converts back to a longitudinal acoustic wave in the brain. This conversion from a longitudinal wave (skin) to a shear wave (skull) and again to a longitudinal wave (brain) does not necessarily produce a highly distorted or small-amplitude wave. Basic images and measurements of shear speed-of-sound and attenuation values for ex vivo human skull bone will be presented for frequencies between 0.25 MHz and 2 MHz. Similar measurements with porcine samples will also be shown, indicating the large discrepancy between shear characteristics of the two species; the porcine samples showing no detectable shear mode.