Noninvasive MRI thermometry with the proton resonance frequency method: study of susceptibility effects.

The temperature dependence of proton resonance frequency (PRF) is related to the temperature dependence of the screening constant and of the volume susceptibility constant. To evaluate the relative importance, an experimental setup was designed allowing quantification of both effects in different tissues, notably pure water in a gel structure, and porcine muscle and fat tissue. The temperature varied from 28 to 44 degrees C, a range significant for hyperthermia applications. Good agreement with results from the literature was obtained for water. Porcine muscle tissue behaves like water. Its screening constant varies linearly at a rate of 0.97 10(-8) (degree C)-1 and the effects of temperature-induced changes of the susceptibility constant are negligible for muscle thermometry applications. The PRF-temperature relation in fat tissue, however, is almost completely determined by susceptibility effects.