Development of T-1-Thermometry for Monitoring the Skull during MRgFUS using a 3D Spiral Ultra-Short Echo Time Sequence

In MR-guided focused ultrasound (MRgFUS), acoustic waves are focused through the skull to destroy target brain tissue as a treatment for movement disorders. Bone attenuates ultrasound energy 20 times more efficiently than soft tissue. Despite current clinical precautions such as circulating cold water around the scalp and predicting the cooling time needed for the skull between sonications from a model, a recent study has shown that MRgFUS led to unintended skull lesions in 7 out of 30 patients. Current precautions are also incapable of limiting skull heating with off-center targets, and this discourages the use of MRgFUS for a wider variety of potential therapeutic targets, such as tumors. Furthermore, the cooling time estimate is not patient specific and can thus prolong an expensive, uncomfortable treatment needlessly. Thus, there is a need for skull thermometry.

Skull thermometry is challenging due to the extremely short T2* decay of cortical bone, which precludes the use of standard proton resonance frequency (PRF) methods, and because of the need for rapid imaging over a large field of view. Other researchers have shown a linear temperature dependence of T1 relaxation in cortical bone. Our initial goal was to investigate the feasibility and repeatability of T1-weighted thermometry under various conditions, such as different magnetic fields, mechanisms of heating, and methods of analysis. Inconsistent results from T1-weighted thermometry led us to focus on investigating the repeatability of T1-mapping thermometry instead and to determine whether T1-mapping thermometry can be accelerated to meet clinical constraints.

Using a non-selective ultra-short-echo-time (UTE) 3D spiral sequence, we demonstrate that rapid T1 thermometry is feasible, and that it is more repeatable and quantitative than T1-weighted imaging.