Compensating for magnetic field inhomogeneity in multigradient-echo-based MR thermometry

Abstract

PURPOSE: MR thermometry (MRT) is a noninvasive method for measuring temperature that can potentially be used for radio frequency (RF) safety monitoring. This application requires measuring absolute temperature. In this study, a multigradient-echo (mGE) MRT sequence was used for that purpose. A drawback of this sequence, however, is that its accuracy is affected by background gradients. In this article, we present a method to minimize this effect and to improve absolute temperature measurements using MRI. THEORY: By determining background gradients using a B0 map or by combining data acquired with two opposing readout directions, the error can be removed in a homogenous phantom, thus improving temperature maps. METHODS: All scans were performed on a 3T system using ethylene glycol-filled phantoms. Background gradients were varied, and one phantom was uniformly heated to validate both compensation approaches. Independent temperature recordings were made with optical probes. RESULTS: Errors correlated closely to the background gradients in all experiments. Temperature distributions showed a much smaller standard deviation when the corrections were applied (0.21°C vs. 0.45°C) and correlated well with thermo-optical probes. CONCLUSION: The corrections offer the possibility to measure RF heating in phantoms more precisely. This allows mGE MRT to become a valuable tool in RF safety assessment.