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.
Peters
R D
RD
Department of Medical Biophysics, University of Toronto, Sunnybrook and Women's College Health Science Center, Ontario, Canada. peters@sten.sunnybrook.utoronto.ca
Henkelman
R M
RM
D000818:Animals; D016477:Artifacts; D002417:Cattle; D046508:Culture Techniques; D004553:Electric Conductivity; D004574:Electromagnetic Fields; D006358:Hot Temperature; D006979:Hyperthermia, Induced; D008099:Liver / Q000473:pathology / Q000502:physiology*; D008279:Magnetic Resonance Imaging; D008954:Models, Biological; D019047:Phantoms, Imaging; D013821:Thermometers