Correlation distance dependence of the resonance frequency of intermolecular zero quantum coherences and its implication for MR thermometry

Purpose Because the resonance frequency of water–fat intermolecular zero‐quantum coherences (iZQCs) reflects the water–fat frequency separation at the microscopic scale, these frequencies have been proposed and used as a mean to obtain more accurate temperature information. The purpose of this work was to investigate the dependence of the water–fat iZQC resonance frequency on sample microstructure and on the specific choice of the correlation distance. Methods The effect of water–fat susceptibility gradients on the water–methylene iZQC resonance frequency was first computed and then measured for different water–fat emulsions and for a mixture of porcine muscle and fat. Similar measurements were also performed for mixed heteronuclear spin systems. Results A strong dependence of the iZQC resonance frequency on the sample microstructure and on the specific choice of the correlation distance was found for spin systems like water and fat that do not mix, but not for spin systems that mix at the molecular level. Conclusions Because water and fat spins do not mix at the molecular level, the water–fat iZQC resonance frequency and its temperature coefficient are not only affected by sample microstructure but also by the specific choice of the correlation distance. Magn Reson Med 79:1429–1438, 2018. © 2017 International Society for Magnetic Resonance in Medicine.