PIXSIC: A pixellated beta-microprobe for kinetic measurements of radiotracers on awake and freely moving small animals

We present a design study of PIXSIC, a new β⁺ radiosensitive microprobe implantable in rodent brain dedicated to in vivo and autonomous measurements of local time activity curves of beta radiotracers in a small (a few mm³) volume of brain tissue. This project follows the initial β microprobe previously developed at IMNC, which has been validated in several neurobiological experiments. This first prototype has been extensively used on anesthetized animals, but presents some critical limits for utilization on awake and freely moving animals. Consequently, we propose to develop a wireless setup that can be worn by an animal without constraints upon its movements. To that aim, we have chosen a Silicon-based detector, highly β sensitive, which allows for the development of a compact pixellated probe (typically 600 ×200 ×1000 μm⁺), read out with miniaturized wireless electronics. Using Monte-Carlo simulations, we show that high resistive Silicon pixels are appropriate for this purpose, assuming that the pixel dimensions are adapted to our specific signals. More precisely, a tradeoff has to be found between the sensitivity to β⁺ particles and to the 511 keV γ background resulting from annihilations of β⁺ with electrons. We demonstrate that pixels with maximized surface and minimized thickness can lead to an optimization of their β⁺ sensitivity with a relative transparency to the annihilation background.