Next-generation brain–computer interfaces (BCIs) for healthy individuals are expected to largely rely on noninvasive functional imaging methods to record cortex-wide neural activity because of the risk associated with surgically implanted devices. In this work, we present a fully integrated 1.8 × 1.8 mm single chip that can be arrayed on a wearable patch to perform noninvasive, functional brain imaging over large cortical areas. This chip node contains two bonded vertical-cavity surfaceemitting lasers (VCSELs), an 8 × 8 single-photon avalanche diode (SPAD) array with event-driven time-to-digital converters (TDCs) per row, and a digital back-end for on-chip histogramming and time-gating. We achieved 70-ps resolution for time-of-flight (ToF) imaging at the record-high 100-MHz laser repetition rate with 80-mW total power. We showed that time-gating improves the imaging contrast by as much as 36% using a brain-skull phantom. The fully integrated and compact node presented here is the key enabler for future high-spatiotemporal-resolution timedomain diffuse optical tomography (TD-DOT) imaging arrays.
Index Terms—Diffuse optical tomography (DOT), noninvasive brain imaging, time-of-flight (ToF) imager, wearable patch.