Simultaneous Skull and Brain Monitoring in Transcranial MRgFUS Surgeries Using a 3D Stack-of-Spirals Sequence: A Report

Purpose: To develop an MRI approach for simultaneous skull and brain monitoring in clinical transcranial MR-guided focused ultrasound (MRgFUS) surgeries.Methods: A 3D stack-of-spirals dual-echo UTE sequence was implemented, incorporating spiral-out ultrashort-TE (UTE) acquisition, spiral-retraced-in-out (RIO) normal-TE acquisitions, and fat suppression pulses. The variable TE technique enabled achieving a UTE as short as 0.05ms, allowing T1 mapping of the skull using the variable flip angle (VFA) method. For normal-TE imaging, a fixed TE was selected based on Bloch simulation to enhance phase measurement accuracy using the proton resonance frequency (PRF) shift method in the brain. The proposed method integrates spiral deblurring methods that preserve both magnitude and phase information, in-plane trajectory measurement to improve trajectory fidelity, and B1 maps to improve T1 mapping. This approach was evaluated in both laboratory and clinical setups.Results: During laboratory cooling, estimated temperatures closely matched with the ground truth, with standard deviations of 0.60°C and 0.57°C per gel ROI, and demonstrated a linear relationship with cortical bone T1 (2.51ms/°C). A similar trend was observed during laboratory heating, with a coefficient of 3.86 ms/°C. In phantom heating in a clinical environment, a concurrent increase in gel temperature and bone T1 was observed at the boundaries, peaking at 12°C and 7.5 ms, respectively. In patient scan results, B1 correction reduced variability in the T1 map of the skull’s top region, while the estimated brain temperature remained stable, with a standard deviation of 0.95°C.Conclusions:The proposed method, integrating the 3D stack-of-spirals dual-echo UTE sequence and corrections for both blurring artifacts and the B1 field, enabled simultaneous skull-temperature-change detection and brain thermometry with high accuracy and precision under transcranial MRgFUS conditions.