Investigation of transcranial ultrasound delivery via Lamb wave mode conversion leveraging a transducer pair

Abstract

Cranial Lamb waves have been recently proposed for the non-invasive identification of the mechanical properties of the cranial bone, the evaluation of its acoustic radiation characteristics, and the estimation of the elastic and transmission properties of cranial sutures. Few works have focused on the so-called Lamb mode conversion phenomenon for transcranial ultrasound delivery, in which an attempt was made to control the focal point within the brain for incident acoustic beams beyond the critical angle. In our study, we further investigate this problem by overcoming some of the limitations of previous works, which are mainly related to the use of simplified geometries and material properties. To this end, we experimentally and numerically investigate leaky Lamb waves in immersed cranial bones by adopting a two-step strategy. In the first step, the dispersion properties of leaky Lamb waves are retrieved in the 0–500 kHz range along the mechanical properties of the cranial bone layers. The material data are then used to determine, via numerical simulations, the range of frequencies in which Lamb mode conversion can be effectively exploited for transcranial focusing purposes. Finally, we experimentally validate the numerical findings by means of an immersed setup that utilizes a pair of ultrasonic transducers.