MRI-based evaluation of MR-HIFU induced thermal effects

High intensity focused ultrasound (HIFU) is novel technology for non-invasive thermal therapy and can be combined with magnetic resonance imaging (MRI). MRI-guided HIFU (MR-HIFU) allows for real-time acquisition of MRI scans during the HIFU treatment, for planning, monitoring and evaluation. The purpose of the research presented in this thesis was to develop and test an MR-HIFU technology platform. The clinical performance of the most commonly used MR thermometry method was evaluated retrospectively, using clinical scans which were used for the monitoring of palliative MR-HIFU treatments of patients with painful bone metastases. The accuracy of the temperature measurements was found to be limited by respiration induced artefacts and the image quality was variable between the different types of lesions. The treatment of tumors in abdominal organs is an emerging application of MR-HIFU, but may be limited by undesired heating of the abdominal wall. A novel MR thermometry method was researched, with which the slow heating processes in the abdominal wall could be monitored. This method potentially allows for the measurement of absolute temperatures, but these measurements are currently showing systematic errors. The effect of the magnetic susceptibility on this thermometry method was further investigated using computer simulations. MR-HIFU can also be used to induce mild hyperthermia, which induces physiological effects. These physiological effects can be measured indirectly using perfusion-related MRI scans. Using these scans, changes were detected in physiological parameters as a result of MR-HIFU induced mild hyperthermia, in a pre-clinical setting. The exact interpretation of the parameters remains a challenge.