MR safety: fast T₁ thermometry of the RF-induced heating of medical devices.

Determining the MR compatibility of medical implants and devices is becoming increasingly relevant. In most cases, the heating of conductive implants due to radiefrequency (RF) excitation pulses is measured by fluoroptic temperature sensors in relevant tests for approval. Another common method to determine these heating effects is MR thermometry using the proton resonance frequency. This method gives good results in homogeneous phantoms. However in many cases, technical shortcomings such as susceptibility artifacts prohibit exact proton resonance frequency thermometry near medical implants. Therefore, this work aimed at developing a fast T₁-based method which allows controlled MR-related heating of a medical implant while simultaneously quantifying the spatial and temporal temperature distribution. To this end, an inversion recovery snapshot Fast Low-Angle Shot (FLASH) sequence was modified with additional off-resonant heating pulses. With an accelerated imaging method and a sliding-window technique, every 7.6 s a new temperature map could be generated with a spatial in-plane resolution of 2 mm. The temperature deviation from calculated temperature values to reference fluoroptic probe was found to be smaller than 1 K.