Magnetic resonance temperature imaging for guidance of thermotherapy

Continuous thermometry during a hyperthermic procedure may help to correct for local differences in heat conduction and energy absorption, and thus al low optimization of the thermal therapy. Noninvasive, three-dimensional map ping of temperature changes is feasible with magnetic resonance (MR) and ma y be based on the relaxation time T-1, the diffusion coefficient (D), or pr oton resonance frequency (PRF) of tissue water. The use of temperature-sens itive contrast agents and proton spectroscopic imaging can provide absolute temperature measurements. The principles and performance of these methods are reviewed in this paper. The excellent linearity and near-independence w ith respect to tissue type, together with good temperature sensitivity, mak e PRP-based temperature AM the preferred choice for many applications at mi d to high field strength (greater than or equal to1 T). The PRF methods emp loy radiofrequency spoiled gradient-echo imaging methods. A standard deviat ion of less than VC, for a temporal resolution below 1 second and a spatial resolution of about 2 mm, is feasible for a single slice for immobile tiss ues. Corrections should be made for temperature-induced susceptibility effe cts in the PRF method. If spin-echo methods are preferred, for example when field homogeneity is poor due to small ferromagnetic parts in the needle, the D- and T-1-based methods may give better results. The sensitivity of th e D method is higher that that of the T-1 methods provided that motion arti facts are avoided and the trace of D is evaluated. Pat suppression is neces sary for most tissues when T1, D, or PRF methods are employed. The latter t hree methods require excellent registration to correct for displacements be tween scans. J. Magn. Reson. Imaging 2000; 12:525-533. (C) 2000 Wiley-Liss, Inc.