  Non-invasive measurement of absolute temperature is important for proper
characterization of various pathologies and for evaluation of thermal dose
during interventional procedures. The proton magnetic resonance (MR) frequency
shift method can be used to map relative temperature changes in tissues;
however, spatiotemporal variations in the main magnetic field and the lack of
an internal frequency reference at each location challenge the determination of
absolute temperature. Here, we introduce a novel multinuclear method for
absolute MR thermometry, based on the fact that the proton and sodium nuclei
exhibit a unique and distinct characteristic frequency dependence with
temperature. A one-to-one mapping between the precession frequency difference
of the two nuclei and absolute temperature is demonstrated. Proof-of-concept
experiments were conducted in aqueous solutions with different NaCl
concentrations, in agarose gel samples, and in freshly excised ex vivo mouse
tissues. One-dimensional chemical shift imaging was also conducted at two
steady-state temperatures, demonstrating excellent agreement with infrared
measurements.
