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

Abstract

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 (Formula presented.) Xe in lipids in order to perform precise lipid-dissolved (Formula presented.) Xe MR thermometry in a rat model of general hypothermia. Methods: Imaging was performed at 2.89 T. (Formula presented.) of (Formula presented.) Xe 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 (Formula presented.) C. Lorentzian functions were fit to spectra from regions of interest to determine the water-referenced chemical shift of lipid-dissolved (Formula presented.) Xe in the abdomen. Absolute (Formula presented.) Xe-derived temperature was compared to values from a rectal probe. Results: Global (Formula presented.) of (Formula presented.) Xe in lipids was determined as (Formula presented.). 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 (Formula presented.) Xe and rectal probe temperature differences were found to be (Formula presented.) (FID) and (Formula presented.) (spin echo) for conventional CSI as well as (Formula presented.) (FID) and (Formula presented.) (spin echo) for turbo spectroscopic imaging. Conclusion: (Formula presented.) Xe MRI using conventional CSI and turbo spectroscopic imaging of lipid-dissolved (Formula presented.) Xe 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.