Absolute thermometry using hyperpolarized 129Xe free-induction decay and spin-echo chemical-shift imaging in rats

Purpose: To implement and test variants of chemical shift imaging (CSI) acquiring both free induction decays (FIDs) showing all dissolved-phase compartments and spin echoes for specifically assessing 129Xe in lipids in order to perform precise lipid-dissolved 129Xe MR thermometry in a rat model ofgeneral hypothermia. Methods: Imaging was performed at 2.89 T. T2 of 129Xe in lipids was determined in one rat by fitting exponentials to decaying signals of global spin-echo spectra. Four rats (conventional CSI) and six rats (turbo spectroscopic imaging) were scanned at three time points with core body temperature 37/34/37◦C. Lorentzian functions were fit to spectra from regions of interest to determine the water-referenced chemical shift of lipid-dissolved 129Xe in the abdomen. Absolute 129Xe-derived temperaturewas compared to values froma rectal probe. Results: Global T2 of 129Xe in lipids was determined as 251.3ms ± 81.4ms. Friedman tests showed significant changes of chemical shift with time for both sequence variants and both FID and spin-echo acquisitions. Mean and SD of 129Xe and rectal probe temperature differences were found to be −0.15◦C ± 0.93◦C (FID) and −0.38◦C ± 0.64◦C (spin echo) for conventional CSI as well as 0.03◦C ± 0.77◦C (FID) and−0.06◦C ± 0.76◦C (spin echo) for turbo spectroscopic imaging. Conclusion: 129Xe MRI using conventional CSI and turbo spectroscopic imaging of lipid-dissolved 129Xe enables precise temperature measurements in the rat’s abdomen using both FID and spin-echo acquisitions with acquisition of spin echoes enabling most precise temperature measurements.