Temperature Monitoring Using Chemical Shift

Abstract

Methodologies in MRS provide noninvasive thermometry with a number of temperature-dependent MR parameters. Among these, the most representative is the proton chemical shift of the hydroxyl group. The electronic shielding effect of protons in this group changes linearly with temperature owing to intermolecular hydrogen bonding. Internally referenced measurements of the chemical shift difference between water and the protons in methyl or ethyl groups, whose frequencies are temperature insensitive, can provide precise measures of temperature. The proton chemical shift differences between water and N-acetylaspartate, creatine, choline, and the methylene chain in fat, or the relative change of the water proton chemical shift, may be used to monitor temperature in biological tissues. However, factors such as bulk susceptibility, electrolytes, pH, and macromolecules also affect chemical shifts in tissues. Although the internal reference technique can reduce the susceptibility effect, the bond-breaking effect of electrolytes can alter the absolute value, as well as the temperature coefficient of the chemical shift. Thus, absolute temperature monitoring in vivo has not been achieved to date. Nevertheless, monitoring relative temperature differences is useful. Alternative methodologies for MRS temperature monitoring include the use of proton chemical shifts in lanthanide complexes, chemical exchange saturation transfer agents, nonproton nuclei, spectroscopic T 1 measurements, and quantum coherence.

MR techniques for guiding high‐intensity focused ultrasound (HIFU) treatments

“…It should also be noted that a difference in the temperature‐induced susceptibility change between fat and aqueous tissues leads to significant temperature error . Moreover, any nonwater signal component, such as fat, needs to be suppressed in order to avoid erroneous phase calculation unless the fat signal is used as an internal reference for measuring water PRF in MR spectroscopy (MRS) . Despite these drawbacks, PRF shift thermometry using the phase mapping method is the only clinically available noninvasive temperature imaging modality for HIFU.…”