Improving the longitudinal spot width of an optimizingly designed point-like scatterer-arrayed transcranial ultrasonic lens

Abstract

As a non-invasive therapy to promote the effect of the thrombolytic agent in the acute phase of cerebral infarction, transcranial irradiation by ultrasonic waves at the site of the occlusion has been validated and is expected to achieve clinical application. Such irradiation may, however, cause damage to the brain by interference of reflected waves within the skull. In order to avoid such a phenomenon, we proposed a transcranial acoustic lens formed by optimizing the arrangement of point-like scatterers. The optimum arrangement of scatterers was determined so that the sound field vanishes within the skull except at the focus(es) and becomes higher at the focus(es) than in the surrounding regions. The ultrasonic waves are strongly scattered by air bubbles in the water-like medium, so we attempted to install a crossbar switch in the medium covering the head and constructed an acoustic lens by producing an instantaneously adaptive air bubble array. This lens can focus an incident plane wave to the diffraction limit in the direction perpendicular to the incident direction, while the peak width in the propagation direction is more than five times higher. In this work, in order to practically improve it, we propose to arrange a point-like scatterer so as to partially cover the head and further increase the number of layers. The shape of the skull is modeled based on CT data, and more realistic parameters are employed than in a previous paper [T. Ueta, J. Appl. Phys. 132, 144504 (2022)].