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

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 $$ Xe in lipids in order to perform precise lipid-dissolved $$ Xe MR thermometry in a rat model of general hypothermia.

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

Global $$ of $$ Xe in lipids was determined as $$ . Friedman tests showed significant changes of chemical shift with time for both sequence variants and both FID and spin-echo acquisitions. Mean and SD of $$ Xe and rectal probe temperature differences were found to be $$ (FID) and $$ (spin echo) for conventional CSI as well as $$ (FID) and $$ (spin echo) for turbo spectroscopic imaging.

$$ Xe MRI using conventional CSI and turbo spectroscopic imaging of lipid-dissolved $$ 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.