Does wave mode conversion at large incidence angles improve transcranial ultrasound transmission? It depends on the porosity

Many previous studies have concluded that a large incidence angle should be avoided during transcranial focused ultrasound therapy to ensure transmission through the skull. Alternatively, some other studies have shown that longitudinal-to-shear wave mode conversion might improve the transmission when the incidence angle is increased above the critical angle (i.e.,
$25^{\circ}$
to
$30^{\circ})$
. Through numerical and experimental studies, we provided additional insight into the question of why transmission through the skull at large angles of incidence is decreased in some cases but improved in other cases. In a numerical simulation study using a microCT image of the human skull, transmission decreased monotonically with increasing incidence angle for skull porosities of both 26.9% and 13.45%, while at a porosity of 2.69%, the transmission increased at large incidence angles. In an experimental study using 3D printed bone phantoms, a pore-free plate exhibited increased transmission at large incidence angle relative to the transmission at the longitudinal critical angle. The experiment using ex vivo skull segments also showed that increased transmission at large incidence angle for the skull with lower porosity.