Simulation-corrected focusing to the vertebral canal

Abstract

Phased arrays have long been used to deliver focused ultrasound to the brain, but applications in the spinal cord remain comparatively unexplored. This is largely due to the aberrating effect of vertebral bone on the incoming wavefront, where variable density and complex geometry distort the pressure field in the canal. For controlled focusing, phase and amplitude corrections must therefore be calculated in advance. Here, we validate a previously-developed ray acoustics model for transvertebral focusing with a bilateral spine-specific array. Benchtop trials were conducted with ex vivo human vertebrae and ray acoustics beamforming was compared to a geometric baseline and hydrophone-corrected gold standard. Planar shift in the 90% contour was evaluated via raster scans of the sagittal and coronal planes. Ray acoustics correction reduced mean sagittal shift from 1.76 ± 0.79 mm to 1.63 ± 0.86 mm and reduced mean coronal shift from 0.99 ± 0.54 mm to 0.76 ± 0.63 mm, while hydrophone correction produced mean sagittal and coronal shifts of 1.40 ± 0.83 mm and 0.53 ± 0.47 mm, respectively. Large variance in simulation-corrected results is hypothesized to stem from nonuniform attenuation and intervertebral acoustic windows, a possibility that will be explored in future in silico and benchtop work.