Hyperthermia MRI temperature measurement: Evaluation of measurement stabilisation strategies for extremity and breast tumours

Purpose: MR thermometry using the proton resonance frequency shift (PRFS) method has been used to measure temperature changes during clinical hyperthermia treatment. However, frequency drift of the MRI system can add large errors to the measured temperature change. These drifts can be measured and corrected using oil references placed around the treatment region. In this study, the number and position of four or more oil references were investigated to obtain a practical approach to correct frequency drift during PRFS thermometry in phantoms and in vivo. Materials and methods: Experiments were performed in a 140 MHz four antenna mini-annular phased array (MAPA) heat applicator (for treatment of extremity tumours) and an applicator for heating of the breast, with symmetric and asymmetric positioning of the oil references, respectively. Temperature change PRFS images were obtained during an hour or more of measurement with no application of heat. Afterwards, errors in calculating temperature change due to system drift were quantified with and without various oil reference correction arrangements. Results: Results showed good temperature correction in phantoms and in a human leg, with average errors of 0.28°C and 0.94°C respectively. There was further improvement in the leg when using eight or more oil references, reducing the average error to 0.44°C, while the phantoms showed no significant improvement. Conclusions: These results indicate that oil reference correction performs well in vivo, and that eight references can improve the correction by up to 0.5°C compared to four references.