Embodiments relate to a sensor system for a brain computer interface (BCI) that enable detection and decoding of brain activity by optical tomography. The sensor system includes an array of pixels arranged as grouped pixel units to provide increased dynamic range. One or more of the grouped pixel units can operate in a saturated mode while providing information useful for decoding brain activity. Furthermore, the grouped pixel units are arranged to enable fast readout by a pixel scanner, thereby increasing detection and decoding ability by systems implementing the sensor design. The grouped pixel units of the sensor system are aligned with optical fibers of an interface to a body region of a user, where the optical fibers can be retained in position relative to the grouped pixel units by an optically transparent substrate that provides mechanical support while minimizing factors associated with divergence of light transmitted through optical fibers.