Hyperthermia induces T1 relaxation and blood flow changes in tumors. A MRI thermometry study in vivo

Regional hyperthermia in combination with chemotherapy and/or radiotherapy has proven to be an effective treatment concept for locally advanced deep-seated tumors. Simultaneous MR-imaging could be a promising approach for therapy optimization. Purpose of this study was the in vivo investigation of temperature induced longitudinal relaxation time (T1) and blood flow changes in a tumor model. Using a 1.5 Tesla MR system, the T1 sensitivity on temperature and the onset of tissue changes at temperatures >44°C were investigated in muscle samples. Also, fourteen Syrian Golden Hamsters with amelanotic melanoma A-MEL-3 were examined during heating of the tumors. Temperature induced blood flow and T1 changes were determined continuously during hyperthermia. Changes of T1 correlated linearly with temperature over a wide range (27-44°C) in the tissue sample. Tissue changes became notable above 44°C. In the tumor model, relative changes of T1 (unlike blood flow) showed linear correlation with temperature over the entire range of hyperthermia relevant temperatures (32-44°C). For a low thermal dose, T1 allows the assessment of temperature changes in tumors in vivo. At higher thermal doses, in addition to temperature changes, T1 also shows tissue changes. Both temperature and tissue changes supply important information for hyperthermia.