PET and MRI-guided focused ultrasound surgery for neurological applications

Magnetic Resonance (MR) guided Focused Ultrasound Surgery (MRgFUS) technology, combined with High Intensity Focused Ultrasound (HIFU) beams, has opened to new therapeutic protocols for various pathological conditions. The success of this therapy relies on the accuracy of the guidance for therapy provided by thermal mapping of sonication, which is obtained with MR imaging. In addition, in recent years, multimodality Positron Emission Tomography and Magnetic Resonance Imaging (PET/MRI) imaging has been developed. This technique is able to provide simultaneous functional and soft tissue morphological imaging and, for this reason, it is particularly engaging for brain imaging. The key concept behind this work is to assess the feasibility of a brain-dedicated PET- and MRI-guided FUS device providing real-time evaluation of the outcome of the HIFU therapy. At first, a method to improve imaging capabilities of small-ring PET scanners will be presented. It will be showed that thanks to this method it is possible to obtain high tomographic spatial resolution with an affordable, brain-dedicated, PET scanner based on monolithic scintillation crystals and able to work as an insert in a MRI system. Moreover, simulations of an anthropomorphic phantom will be made in order to evaluate the effect of different HIFU protocols and, in addition, to investigate the capability of thermal maps provided by MRI for assessing the effect of the HIFU treatment. Finally, from the comparison of the spatial resolutions provided by each imaging technique (PET, MRI and thermal MRI) and the HIFU therapy, it will be possible to assess the feasibility of the proposed multimodality device. The system suggested in this work should be composed of a MRI scanner with a PET insert and a customized MRI-compatible focused ultrasound applicator. It could be very useful for the diagnosis and therapy of brain tumors thanks to the possibility of a real-time evaluation of the effect of the HIFU treatment. The protocol for the therapy could be executed in three main steps: the diagnosis of the pathological condition by means of fused anatomical imaging from MRI and functional imaging from PET (with different radiotracers) that allow to identify the region where apply the therapy, the ablation of the tumor by means of HIFU beams and the simultaneous evaluation of the thermal response of the tissues by means MRI thermal maps and, finally, the assessment of the outcome of the therapy by means, again, of PET/MRI hybrid imaging. The results suggest that PET-and MRI-guided focused ultrasound surgery (PET/MRgFUS) could be an innovative, feasible and engaging technique for tumor ablation in the brain.