Reducing temperature errors in transcranial MR‐guided focused ultrasound using a reduced‐field‐of‐view sequence

PURPOSE: To reduce temperature errors due to water motion in transcranial MR-guided focused ultrasound (tcMRgFUS) ablation. THEORY AND METHODS: In tcMRgFUS, water is circulated in the transducer bowl around the patient's head for acoustic coupling and heat removal. The water moves during sonications that are monitored by MR thermometry, which causes it to alias into the brain and create temperature errors. To reduce these errors, a two-dimensional excitation pulse was implemented in a gradient-recalled echo thermometry sequence. The pulse suppresses water signal by selectively exciting the brain only, which reduces the imaging FOV. Improvements in temperature precision compared to the conventional full-FOV scan were evaluated in healthy subject scans outside the tcMRgFUS system, gel phantom scans in the system with heating, and in 2x-accelerated head phantom scans in the system without heating. RESULTS: In vivo temperature precision (standard deviation of temperature errors) outside the tcMRgFUS system was improved 43% on average, due to the longer TR and TE of the reduced-FOV sequence. In the phantom heating experiments, the hot spot was less distorted in the reduced-FOV scans, and background temperature precision was improved 59% on average. In the accelerated head phantom temperature reconstructions, temperature precision was improved 89% using the reduced-FOV sequence. CONCLUSIONS: Reduced-FOV temperature imaging alleviates temperature errors due to water bath motion in tcMRgFUS, and enables accelerated temperature mapping with greater precision.