MRI-guided High Intensity Focused Ultrasound (MRIgHIFU): Application to Cancer treatment – MRgHIFU

Cancer is one of the leading causes of death in the world. Its treatment relies on three well established pillars (surgery, radiation therapy, and chemotherapy) and several highly promising approaches such as immunotherapy. In the last decade, local therapies, based on thermal ablation or cryoablation have gained significant interest thanks to their mini-invasive, non-ionizing nature. Among those thermal ablation methods, High Intensity Focused Ultrasound (HIFU) stands out prominently because it is completely non-invasive, very flexible (in terms of tumor sizes and locations) and, combined with MR (Magnetic Resonance) imaging, very accurate and reliable since MRI provides detailed anatomical images and enables fast volumetric temperature imaging in real time during the treatment. HIFU is based on the concentration of high intensity ultrasound generated from an extra-corporeal source (the transducer) in the pathological tissues in order to raise the temperature above a lethal threshold. However there are technological challenges to solve to effectively integrate HIFU in the very constrained MR environment. The main constraints are the very restricted space due to the limited bore diameter, the extreme sensitivity to radio-frequency, and the presence of a strong magnetic field. The limited space in the bore and the sensitivity to magnetic susceptibility variations have led to the development of HIFU device with multiple small transducers driven as a phased array in order to perform a complete ablation of the tumor by steering the ultrasound beam and shifting the focal spot position without any transducer motion. However, with the current technology, independent signals are generated for each of the transducer elements by usually very bulky phased array generators that exhibit narrow bandwidth. The high power signals are then brought to the transducer through individual coaxial cables, via a large matching box. As the number of elements in the transducer increases, cable bundles become awfully bulky and lossy due to the use of micro-coaxial cables. In addition risks of RF interferences between the multiple high power RF signals driving the HIFU and the MR image acquisition increase dramatically. Ideally, the optimal HIFU system should have a phased array transducer with: i) a large number of small elements for a wide steering range of the focal spot; ii) unobtrusive cables for easy positioning of the transducer; iii) broadband usage to adjust the ultrasound frequency for the target; iv) a reliable determination of the position and orientation of the transducer for easy targeting; v) and a perfect MR compatibility with no RF inferences with the MR, while in operation.
The objective of the MRIgHIFU project is to develop a driving system, a phased array transducer and a localization device for the transducer that would address the issues of the current technologies and come closer to the ideal HIFU system. The prototype would provide the performance of massively multi-channels systems (>1000 elements) that integrate well in the MR environment by: i) integrating the power electronic system as well as the matching system in high-voltage CMOS technology in order to locate the electronics directly in the housing of the HIFU transducer, and ii) providing the HIFU with a pre-positioning system that will help in positioning the HIFU transducer during the intervention. This will lead to a compact HIFU system, much easier to use, and safer. In a medium term, the scaling of the system will make it possible to have much more channels and thus to increase significantly the ultrasonic beam deflection range, offering many new opportunities such as the possibility for the beam to follow mobile organs. Validation of the prototype will be performed in the IRIS platform of ICube (http://plateforme.icube.unistra.fr/iris) through pre-clinical experiments first in phantoms, and second in animal subjects.