Absolute thermometry using hyperpolarized 129 Xe 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 ${}^{129}$ Xe in lipids in order to perform precise lipid-dissolved ${}^{129}$ Xe MR thermometry in a rat model of general hypothermia.

Methods: Imaging was performed at 2.89 T. $T_2$ of ${}^{129}$ 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 ${}^{\circ }$ C. Lorentzian functions were fit to spectra from regions of interest to determine the water-referenced chemical shift of lipid-dissolved ${}^{129}$ Xe in the abdomen. Absolute ${}^{129}$ Xe-derived temperature was compared to values from a rectal probe.

Results: Global $T_2$ of ${}^{129}$ Xe in lipids was determined as $251.3\text{ms}\pm 81.4\text{ms}$ . 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 ${}^{129}$ Xe and rectal probe temperature differences were found to be $-0.15^{\circ }C\pm 0.93^{\circ }C$ (FID) and $-0.38^{\circ }C\pm 0.64^{\circ }C$ (spin echo) for conventional CSI as well as $0.03^{\circ }C\pm 0.77^{\circ }C$ (FID) and $-0.06^{\circ }C\pm 0.76^{\circ }C$ (spin echo) for turbo spectroscopic imaging.

Conclusion: ${}^{129}$ Xe MRI using conventional CSI and turbo spectroscopic imaging of lipid-dissolved ${}^{129}$ 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.

Keywords: $$$ {}^{129} $$$ Xe; MRI thermometry; chemical-shift imaging; lipid; rat; spin echo.