The emergence of new animal models that mimic human disorders enables new fundamental and therapeutical approaches of these diseases. The increasing number of studies on animal models implies the development of new imaging tools adapted to the particular constraints of small animal studies. To that aim, the past few years have seen the development of numerous high resolution PET systems having a sensitivity and a spatial resolution well suited to the small animal imaging. Nevertheless, although these systems have already and extensively demonstrated their interest in the biomedical imaging field, they suffer from a few drawbacks (low temporal resolution, animal immobilization, high cost) that have stimulated the development of complementary approaches. In that particular context, our objective is to develop a beta radiosensitive probe surgically implantable in an animal brain that can measure autonomously the variation of injected radioactivity in a small tissue volume (typically a few mm3) on awake and freely moving animals. To reach that aim, the detector and its acquisition system must be entirely worn by the animal without wires except on the animal itself. This means to design a beta sensitive detector of very small size based on a technology that allows to produce a numerical signal compatible with the use of a telemetric system. This also means to develop a detector compact enough to be fixed on the rodent head without altering the animal movements. Moreover, the whole system must be allow the implantation of at least two probes in order to be able to measure on same animal, specific and non-specific binding rates of a radiotracer. Theoretical and experimental investigations carried out to demonstrate the feasibility of such a detector will be presented. On this basis, first architecture of the detector based on pixellated Si sensor technology will be proposed and detailed.