MR-based skull parametric modeling for blood-brain barrier opening in non-human primates

Focused ultrasound (FUS) transcranial procedures require of simulation planning using the computed tomography (CT) of the skull, leading to ionizing radiation. Zero echo time (ZTE) magnetic resonance imaging (MRI) is a safe alternative for blood-brain barrier opening (BBBO) planning in non-human primates (NHP), as it provides good contrast for bone imaging. In this study, we applied a classical coordinate-to-coordinate linear-regression method to obtain the ZTE-to-CT relationship for six NHP skulls. First, we compared ZTE-based (pseudo-CT/pCT) and CT-based simulations ex-vivo, with differences of 2.39±0.49%, 3.65±0.52%, 0.49±0.00 mm and 0.00±0.00 mm for the focal volume, attenuation, axial and lateral shifts, respectively. The differences between pCT simulations and experiments were 2.53±1.33%, 1.74±0.57 mm and 0.30±0.11 mm for the focal attenuation, axial and lateral shifts, respectively. Finally, pCT-based and CT-based simulations for in-vivo planning resulted in differences of 3.39±3.09%, 6.04±1.34%, 2.05±1.40% and 0.71±0.16 mm for the focal volume, attenuation, in-focus BBBO volume, and sonicated skull thickness, respectively, where pCT and CT predicted a 49.12±11.05% and 47.06±12.17% of BBBO volume, respectively. These results demonstrate for the first time the feasibility of a ZTE-to-CT skull modeling method for BBBOplanning in NHPs, defining a safe and non-ionizing technique for skull acquisition in transcranial FUS applications.