Proton-resonance frequency shift MR thermometry is affected by changes in the electrical conductivity of tissue

The proton‐resonance frequency (PRF) shift method of MR thermometry provides an easy and practical means of quantitatively monitoring in vivo temperatures for MR image‐guided thermal‐coagulation therapy. However, reported discrepancies in the numerical value of the PRF‐thermal coefficient persist, when measured in a variety of experimental conditions and in different tissue types, both ex vivo and in vivo. In this report, a potential source of variation in the PRF‐shift method of thermometry is identified that manifests as a constant incremental phase shift per unit change in temperature that is independent of the echo‐time setting, when constructing temperature‐sensitive phase images from a gradient‐echo pulse sequence. It is proposed that this confounding phase‐shift offset arises from thermally induced changes in the electrical conductivity of the material. To this end, it is demonstrated that the MR‐derived temperature changes could be in error by as much as 28%, as measured from a simple calibration experiment on freshly excised cow liver. A simple method of overcoming this phase‐shift offset is described. Magn Reson Med 43:62–71, 2000. © 2000 Wiley‐Liss, Inc.