  Human skull poses a significant barrier for the propagation of ultrasound
waves. Development of methods enabling more efficient ultrasound transmission
into and from the brain is therefore critical for the advancement of
ultrasound-mediated transcranial imaging or actuation techniques. We report on
the first observation of guided acoustic waves in the near-field of an ex vivo
human skull specimen in the frequency range between 0.2 and 1.5 MHz. In
contrast to what was previously observed for the guided wave propagation in
thin rodent skulls, the guided wave observed in a higher frequency regime
corresponds to a quasi-Rayleigh wave, mostly confined to the cortical bone
layer. The newly discovered near-field properties of the human skull are
expected to facilitate the development of more efficient diagnostic and
therapeutic techniques based on transcranial ultrasound.
