Compensation for Aberrations When Focusing Ultrasound Through the Skull Based on CT and MRI Data

Abstract

The study compares the capabilities of using 3D acoustic models of the human head, constructed using magnetic resonance imaging (MRI) and computed tomography (CT) data, to simulate ultrasound beam focusing when passing through skull bones and to compensate for aberrations caused by them. A CT and MRI dataset from one patient was considered. The MRI data were used to reconstruct segments of the human head (skin, skull, and brain) that were homogeneous in their internal structure. The most realistic CT model took into account the internal inhomogeneities of the skull bones and soft tissues. Field simulations and compensation for aberrations were performed using the Rayleigh integral and pseudospectral method for solving the wave equation in an inhomogeneous medium, implemented in the k-Wave software package. The transducer was considered to be a fully populated 256-element phased array with a frequency of 1 MHz and radius of curvature and an aperture of 200 mm. It was shown that when aberrations were compensated using an inhomogeneous CT model and a homogeneous MRI model, the pressure amplitude at the focus and focusing efficiency were different by less than 10%. Thus, a homogeneous MRI model can be used for preoperative assessment of the feasibility of transcranial ultrasound therapy. During therapy, it is preferable to take into account the internal structure of the skull bones based on CT data.