Intracranial MR and implant safety

INTRODUCTION: Surgical treatment for temporal lobe epilepsy is highly effective when clinicians can correlate the location of structural and functional abnormalities in a patient [1]. Magnetic Resonance Imaging (MRI) is the favored neuroimaging method for identifying structural lesions in TLE [2], while intracranial electrodes reveal waveforms characteristic of seizure generating tissue. Unfortunately, existing MRI methods often lack the sensitivity required to identify subtle abnormalities, and existing intracranial electrodes create magnetic-susceptibility artifacts that prevent imaging of the tissue from which the electrodes record. We aim to transform the region normally obscured in MRI into the region of greatest spatial resolution by minimizing the electrode artifact and integrating an implantable MRI antenna. METHODS: We designed an implantable radio frequency (RF) coil to integrate with an intracranial electrode. We evaluated several RF-coil geometries and determined a 1-mm-diameter solenoid, known in the field of nuclear magnetic resonance (NMR) as the microcoil, as the optimal coil geometry for this application. We modify the NMR-microcoil design to allow implantation of the RF coil, by winding the microcoil on medical-grade silicone tubing and attaching circuit components on the tubing. RESULTS: The implantable RF coil provided MRI of neural tissue (butcher-grade Ovis aries). The images were obtained by using the microcoil as a transceiver for a 3-T Scanner (Siemens AG, Berlin) Fig. (a) TR/TE = 3000/22 ms, FOV = 26 mm, 256x256, slice thickness = 0.40 mm, number of slices 30, NA = 10, pixel size = 0.098 mm); Fig. (b) TR/TE = 123/48 ms, FOV = 22 mm, 1024x1024, slice thickness = 0.17 mm, number of slices 15, NA = 3, pixel size = 0.02 mm. CONCLUSSIONS: We achieved the milestone of imaging with the MRI microcoil. While the spatial resolutions represent a nearly ten-fold improvement in spatial resolution over existing techniques, we must improve performance of our device to reduce image times. DISCUSSION: The next generation of implantable microcoils is underway, including coil modifications and development of circuitry to allow use as a receive-only coil. Safety experiments to assess, identify, and reduce MR-related heating of depth electrodes at 3 Tesla have also begun with promising results. Future work includes quantification of coil performance, development of an artifact-free electrode, and safety experiments to quantify MR-related heating of the microcoils.