Focal volume reduction in transcranial focused ultrasound using spherical wave expansions

Transcranial focused ultrasound (tFUS) has been gaining increased attention as a non-invasive modality for treating brain diseases. However, accurately focusing on brain structures remains a challenge as the ultrasound is severely distorted by the presence of the skull. In this article, we propose a promising distortion correction method based on spherical wave expansions. It is demonstrated that the focal gain is directly related to the zero-order spherical harmonic coefficient, and suppressing higher-order coefficients significantly reduces the focal volume. Simulation results show that this method can correct distortion and effectively balance focal gain and volume, achieving a smaller focal spot with lower grating lobes compared to the commonly used time reversal technique. We also verified the capability of shifting the focal position in real time without additional simulations. This work provides an effective approach for tFUS treatments, offering enhanced precision and reduced focal volume.