Fast accurate MR thermometry using phase referenced asymmetric spin-echo EPI at high field

Purpose: A novel highly accurate method for MR thermometry, effective at high field, is introduced and validated, which corrects for slow and fast field fluctuations by means of reference images. Methods: An asymmetric spin-echo (ASE) EPI sequence was made frequency-selective to water or a reference substance by controlling the slice-select gradient polarity, or the durations of the excitation and refocusing RF pulses. Images were acquired pairwise, and the temperature-sensitive water images were corrected for field fluctuations using the reference images. In a phantom RF heating experiment, dissolved dimethyl sulfoxide was used as a reference substance. Temperature stability was tested in-vivo on the human brain, referenced using subcutaneous scalp fat. Water and fat phase images were acquired only 50 ms apart. Bloch simulations validated the frequency selection accuracy. Results: ASE imaging using simple frequency selection methods provides highly accurate referenced MR thermometry in phantoms and in-vivo at 7 Tesla. Effects of field fluctuations caused by field drift, breathing and heart beat were corrected. The technique is highly robust against B1 inhomogeneities. Conclusion: Frequency selection using gradient-reversal can enable fast accurate referenced in-vivo MR thermometry, assisting thermal characterization of RF coils and possibly in-vivo SAR monitoring