MR-guided Focused Ultrasound (MRgFUS) is used transcranially to ablate brain tissue for the treatment of neurological diseases. Temperature monitoring of the skull is desired for increasing treatment safety and efficacy. Proton resonance frequency shift MR fails to detect heating in the cortical bone of the skull. T1-based MR thermometry uses T1 mapping to observe a linear increase in T1 with temperature but requires long acquisitions. We demonstrate a new thermometry method dependent on the linear relationship between signal magnitude from a T1-weighted 3D Spiral Ultra-short Echo Time sequence and test it across three scanners in multiple trials.