Enhanced ultrasound transmission through the human skull using shear mode conversion

TLDR

The skull’s shear speed provides a better impedance match, less refraction, and less phase alteration than its longitudinal counterpart. The study investigates a new transskull propagation technique that deliberately induces a shear mode in the skull bone to focus ultrasound in the brain. The method relies on mode conversion of incident longitudinal waves into shear waves in bone layers beyond Snell’s critical angle, then back to longitudinal waves in the brain, demonstrated in plastic phantoms and an ex vivo human skull. At 0.74 MHz the transskull shear method yields amplitudes comparable to or exceeding longitudinal propagation, and its reduced phase shift suggests it could simplify and extend existing noninvasive transskull focusing methods to larger brain regions. Med.

Abstract

A new transskull propagation technique, which deliberately induces a shear mode in the skull bone, is investigated. Incident waves beyond Snell’s critical angle experience a mode conversion from an incident longitudinal wave into a shear wave in the bone layers and then back to a longitudinal wave in the brain. The skull’s shear speed provides a better impedance match, less refraction, and less phase alteration than its longitudinal counterpart. Therefore, the idea of utilizing a shear wave for focusing ultrasound in the brain is examined. Demonstrations of the phenomena, and numerical predictions are first studied with plastic phantoms and then using an ex vivo human skull. It is shown that at a frequency of 0.74 MHz the transskull shear method produces an amplitude on the order of—and sometimes higher than—longitudinal propagation. Furthermore, since the shear wave experiences a reduced overall phase shift, this indicates that it is plausible for an existing noninvasive transskull focusing method [Clement, Phys. Med. Biol. 47(8), 1219–1236 (2002)] to be simplified and extended to a larger region in the brain.