Use of fast spin echo for phase shift magnetic resonance thermometry

Purpose: To propose a modified fast spin echo (FSE) magnetic resonance imaging sequence for MR thermometry, employing the proton resonance frequency (PRF) shift by means of MR phase maps. Despite their obvious advantages of speed and high signal-to-noise ratio (SNR), FSE sequences have not until now been used for this purpose due to the restraints imposed by the Carr-Purcell-Meiboom-Gill (CPMG) conditions. Materials and Methods: The new FSE combines a new phase modulation scheme that maintains magnetization that ordinarily is destroyed under CPMG conditions, while employing conventional FSE gradient waveforms. The echoes are read in a single shot using 128 readouts in 650 msec, with a phase sensitive preparation using an optional time shift T before the start of the refocusing gradient waveforms. This feature allows the quantification of temperature dependent phase shifts. We tested the sequence by imaging a heated agar gel phantom while cooling, using different values for T. Results: There was good correlation between FSE and fiberoptic-based temperature measurements in the phantom(r(2) greater than or equal to 0.95). Temperature sensitivity could be adjusted by varying the T value. Conclusion: With the proposed non-CPMG FSE sequence it is feasible to quantify temperature changes by means of the PRF shift.