MRI Technology For Enhanced Radio Frequency Safety

Stanford University, Stanford CA

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Abstract

[unreadable] DESCRIPTION (provided by applicant): The overall goal of this work is to create an MRI imaging environment that eliminates the possibility of RF burns for recipients of cardiac pacemaker, deep brain stimulator, and other neuro-stimulator devices. Moreover, guaranteed RF safety is a necessary requirement for image guided interventions to be performed under MRI. Today, about 3 million Americans have implanted pacemakers that typically contraindicate any form of head, chest, or muskulo-skeletal MRI scan. Recent clinical safety studies for imaging device recipients at 1.5T have been performed without incident and no related fatalities for pacemakers have occurred since the 1980s. Guidelines for deep brain stimulator recipients typically require head transmit coils and only at 1.5T but at least two MR induced brain injuries have occurred at 1.0T. The general failure to identify adverse outcomes does not prove safety because these results cannot be extrapolated to other field strengths; guidelines are tied to scanner power which is reported inconsistently, and MRI systems lack robust methods of predicting and avoiding potential heating conditions based on physically existing preconditions. We believe the solutions for RF safe devices lie in improved engineering of the MR scanner itself. This will require an integration of electromagnetic safety sensors that can independently detect or search for dangerous resonances, MRI RF field mapping methods that can detect lead wire currents responsible for heating but at sensitivities well below the MR thermometry or physical heating thresholds, and distributed transmit array systems that deposit RF power only where needed. If we can detect and image if the physical conditions exist for heating, regardless of field strength, patient orientation, or device, we can create RF excitation systems that prevent heating. Specifically, our aims are to: 1) Develop electromagnetic safety devices external to the patient that prescreen for dangerous resonant conditions, detect wire currents in real time, or inhibit wire resonances. 2) Develop MRI pulse sequences that detect and quantify the presence of RF currents at levels below the sensitivity needed for detection by MR thermometry. 3) Develop transmit array excitation systems with optimized pulse sequences that maintain image quality but prevent electromagnetic coupling and RF heating near implanted devices. Our approach ultimately involves localizing the region of power deposition with transmit array elements, but even with standard transmit coils, independent RF sensors and/or specialized MRI field mapping methods will give a quantifiable and objective basis for determining if an RF hazard can exist. Achieving these goals will substantially increase access to MRI for a broad class of patients with cardiac or neurostimulator implants who are currently denied access out of fear of RF heating danger. [unreadable] [unreadable] [unreadable]