Comparison of four magnetic resonance methods for mapping small temperature changes

Non-invasive detection of small temperature changes (<1 degrees C) is pivot al to the further advance of regional hyperthermia as a treatment modality for deep-seated tumours. Magnetic resonance (MR) thermography methods are c onsidered to be a promising approach. Four methods exploiting temperature-d ependent parameters were evaluated in phantom experiments. The investigated temperature indicators were spin-lattice relaxation time T-1, diffusion co efficient D, shift of water proton resonance frequency (water PRF) and reso nance frequency shift of the methoxy group of the praseodymium complex (Pr probe). The respective pulse sequences employed to detect temperature-depen dent signal changes were the multiple readout single inversion recovery (T One by Multiple Read Out Pulses; TOMROP), the pulsed gradient spin echo (PG SE), the fast low-angle shot (FLASH) with phase difference reconstruction, and the classical chemical shift imaging (CSI). Applying these sequences, e xperiments were performed in two separate and consecutive steps. In the fir st step, calibration curves were recorded for all four methods. In the seco nd step, applying these calibration data, maps of temperature changes were generated and verified. With the equal total acquisition time of approximat ely 4 min for all four methods, the uncertainties of temperature changes de rived from the calibration curves were less than 1 degrees C (Pr probe 0.11 degrees C, water PRF 0.22 degrees C, D 0.48 degrees C and T-1 0.93 degrees C). The corresponding maps of temperature changes exhibited slightly highe r errors but still in the range or less than 1 degrees C (0.97 degrees C, 0 .41 degrees C, 0.70 degrees C, 1.06 degrees C respectively). The calibratio n results indicate the Pr probe method to be most sensitive and accurate. H owever, this advantage could only be partially transferred to the thermogra phic maps because of the coarse 16 x 16 matrix of the classical CSI sequenc e. Therefore, at present the water PRF method appears to be most suitable f or MR monitoring of small temperature changes during hyperthermia treatment .