Temperature mapping of frozen tissue using eddy current compensated half excitation RF pulses

Cryosurgery has been shown to be an effective therapy for prostate cancer. Temperature monitoring throughout the cryosurgical iceball could dramatical ly improve efficacy, since end temperatures of at least -40 degreesC are re quired. The results of this study indicate that MR thermometry based on tis sue R-2(*) has the potential to provide this information. Frozen tissue app ears as a complete signal void on conventional MR1. Ultrashort echo times ( TEs), achievable with half pulse excitation and a short spiral readout, all ow frozen tissue to be imaged and MR characteristics to be measured. Howeve r, half pulse excitation is highly sensitive to eddy current distortions of the slice-select gradient. In this work, the effects of eddy currents on t he half pulse technique are characterized and methods to overcome these eff ects are developed. The methods include: 1) eddy current compensated slice- select gradients, and 2) a correction for the phase shift between the first and second half excitations at the center of the slice. The effectiveness of these methods is demonstrated in R-2(*) maps calculated within the froze n region during cryoablation. (C) 2001 Wiley-Liss, Inc.