BRAIN Project: Development and Validation of a Magnetocorticography array using optimized optically pumped magnetometers

National Institute Of Mental Health

Investigators

Abstract

Due to substantial delays in procurement of the dual-axis OPM sensor system, the upgraded system was not installed at the NIH until late June. Thus, initial tests of prototype arrays using our original single-axis sensors and new dual axis sensors have been delayed. Currently, we are in the process of fabricating a temporary fixture that can hold both the single-axis and new dual-axis sensors, to facilitate exact comparison of the two systems using our previously developed calibrator array. In order to simulate comparisons between the original single axis sensors and the new dual axis sensors, we developed a new method for realistic simulation using an MRI image. In Monte Carlo simulations, low magnitude "noise" sources are placed on vertices on the pial surface to serve as interfering activity, along with additional sources of interest, and beamformer reconstructions are used to determine how well different sensor types could resolve the sources of interest. We determined that a dual-axis OPM sensor significantly outperforms the single axis sensors which we have used in our prior work. A tri-axial sensor offered only modest improvement in performance. In addition, we also determined that the performance of a high resolution MCoG array could be dramatically improved by designing a fixture that can conform to the head rather than a fixed array, although this introduces additional complexity in sensor localization. Thus, we will be working on developing both fixed arrays and arrays that allow some flexibility in positioning the sensors on the head, in collaboration with Dr. Knappe, our co-investigator on the grant. Another component of the work is the development of a gradiometer optimized for MCoG. We have tested two different types of lasers as possible candidates for the multi-cell MCoG Optimized gradiometer. The first one is a DFB laser with 5 mW of output power. While this one would give us enough pumping power, the laser would be much more difficult to integrate than the current VCSEL lasers used. The noise suppression scheme with this laser was tested and the results showed residual laser noise beyond the photon shot noise that was not suppressed. We therefore also tested a second type of laser. The output power was around 500 mW. We tested the optical spectrum, LIV characteristics, beam divergence, and polarization behavior. We demonstrated the differential intensity-noise-rejection scheme and characterized is for several laser parameters. We demonstrated that we can reach the photon shot noise limit with this scheme. We tested four of these lasers to ensure reproducibility of the results. We believe that we have successfully identified a laser candidate and demonstrated the viability of the proposed scheme.