Simulation and Treatment Planning
Introduction
Transcranial focused ultrasound (FUS) is a therapeutic modality that can be used to induce transient or permanent changes in brain tissue. The most common applications include ablation, blood brain barrier opening, and neuromodulation.
As ultrasound propagates through the skull, the bone heterogeneously reflects, refracts, and attenuates the ultrasound waves. This alters the position, intensity, and shape of the beam’s focus. To account for the aberrations caused by the skull, phase corrections are applied to each transducer element such that the beams constructively interfere at the target of interest.
3D numeric simulations
One approach for estimating phase corrections is to use 3D numeric simulations. In general, numeric simulation frameworks use patient-specific skull models to account for heterogeneities of the skull. These models are derived from computed tomography (CT) images of individual patients. For every voxel in a skull model, acoustic properties can be assigned according to its CT Hounsfield unit (HU) value. To simulate the ultrasound, many algorithms solve second order partial differential wave and/or elastic equations using a finite difference time domain (FDTD) method, a pseudo spectral time domain (PSTD) method, or a hybrid angular spectrum (HAS) method.
In our lab, we have a 3D numeric simulation framework that uses the HAS method to estimate phase corrections. The HAS method is two orders of magnitude faster than FDTD and PSTD methods, reducing computation time from tens of hours to minutes. With its advantage in speed, HAS opens the possibility for performing real-time simulation of treatment and for rapidly adjusting to unforeseen treatment modifications.