Implantable CMOS pixel sensor for positron imaging in rat brain

Abstract IMIC is a Monolithic Active Pixel Sensor prototype designed for the MAPSSIC project, which aims at developing wireless intracerebral probes dedicated to image positron-emitting source activity in the brain of awake and freely moving rats. Former experiments with the PIXSIC positron probe based on a passive sensor have validated the proof of concept, but have also shown limitations with regards to the probe robustness and to its transparency to annihilation photons. The IMIC circuit features a matrix of 16  ×  128 active pixels of 30  × 50  µm 2 size and targets to overcome the PIXSIC probe drawbacks by exploiting a thin sensitive layer of 18 µm, still featuring an overall thickness close to 300 µm. Additionally, by using a low power (55 nW/pixel) in-pixel front-end architecture providing binary output, IMIC solves the challenge of implanting an active sensor in tissues where overheating is forbidden. The needle-shaped sensor 610 µm  × 12000  µm was fabricated and tested in laboratory. The whole sensor dissipates 160 µW and its imaging capabilities were asserted with various sources: 55 Fe, 90 Sr and 18 F. These tests also demonstrated robust count-rate measurement with IMIC in the range 10–1000 counts/matrix/s. Finally, a dedicated setup qualitatively confirmed excellent in-sensitivity to 511 keV γ -rays. In this paper, we present the sensor requirements and its detailed design. We also discuss the first characterisation results and the outlook for the integration of IMIC into an implantable probe.