Magnetic resonance imaging study of silicon microelectrodes.

Abstract

Current brain mapping technologies to study brain function or treat neurological disorders can be divided into two categories: those with high temporal resolution and those with high spatial resolution. Unfortunately, there is no single technique that includes both characteristics. Some attempts have been reported to join the high temporal resolution and high spatial specificity obtained with extracellular electrophysiological recordings, using wire microelectrodes, and the high volume coverage and good spatial resolution of the Blood Oxygenation Level Dependant (BOLD) phenomenon, obtained with functional magnetic resonance imaging (fMRI). Unfortunately, the magnetic susceptibility difference between tissue and metallic microelectrodes causes significant image distortions that, while making obvious the presence of the electrodes, also produces significant image artifacts that compromise the anatomical or functional information in regions surrounding the electrodes. This work addresses these problems and presents the first MR-compatible multichannel silicon microelectrode system, used for neural recordings and electrical stimulation of the central nervous system for animal models. A standard chronic assembly, developed and produced by the University of Michigan Center for Neural Communication Technology, was tested on a 2 Tesla magnet to detect forces, heating, and image distortions. It was then modified to minimize or eliminate susceptibility artifacts, tissue damage, and electrode displacement, while maintaining good image quality and safety to the animals. The final design is fully MR-compatible and has been successfully tested on Guinea pigs. In addition, the magnetic field perturbations produced by different wire and silicon microelectrodes were simulated using a Fourier-based method. The results showed that silicon microelectrodes, developed at the University of Michigan, are one of the most MR-compatible devices currently available. Finally, a method for <italic>in vivo</italic> MRI detection of implanted silicon microelectrodes was investigated. The method uses phase and susceptibility weighted imaging with a new phase-mask that enhances the visibility of sub-voxel size features. The mask is based on a sigmoidal function that intensifies small positive and negative phase variation, with respect to the background, of MR images. Although the method degrades the signal to noise ratio of the images, it also enhances small features not easily detected with current MRI methods.