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During the last decades, small animal imaging has shown an increasing interest in Single Photon Computed Tomography since animal models can mimic human diseases enabling a translation of results and conclusions of research done on animals to humans. SPECT equipped with a pinhole type collimator provides high resolution for in vivo imaging leading to an accurate quantitative measurement especially in small volume of interest.

Recent researches in PET instrumentation tend to improve the reconstructed spatial resolution using the gamma ray Depth Of Interaction (DOI) information. This method, already introduced by Moses et al., requires reading both ends of a scintillating crystal to be efficient. The recorded charges obtained by the two photodetector such as Photomultiplier Tubes (PMT) or Avalanche PhotoDiodes (APD) are dependent on the depth gamma ray interaction. Braem et al. recently used the contrast calculated as the ratio between the difference of the recorded charges and their sum to improve the DOI spatial resolution. In this work, we investigate the optimization of the crystal length and coating to improve the depth of interaction spatial resolution in PET applications. For a 1.5 × 1.5 mm2 LYSO:Ce crystal section read at both ends by PMT, we find that a length of 25 mm and a coating made with PMMA and 30% of TiO 2 are appropriate to reach a DOI spatial resolution of 1 mm.

Some current researches based on small animal model are dedicated to functional cerebral imaging which represents a fundamental tool to understand the mechanisms involved in neurodegenerative diseases. In the radiopharmaceutical development approach, the main challenge is to measure the radioactivity distribution in the brain of a subject with good temporal and spatial resolutions in order to investigate the behaviors of new radiopharmaceuticals inside the injected subject. In the case where the isotope is a single photon emitter, we present a 1 D gamma counter to investigate the possibility to obtain both high detection efficiency and good spatial resolution to access the kinetic of radiopharmaceutical in the brain of a mouse. The system consists in a 64 channels Multi-Anode PhotoMultiplier Tube (H8804, Hamamatsu Corp.) coupled to a highly segmented YAP:Ce crystal consisting of 32 strips of 0.575 × 18.4 × 10 mm3. The collimation is made of 33 tungsten slats of 100 μm thick with a 40 mm height optimized using GEANT4 Monte Carlo simulations. For a field of view of 18.4 × 18.4 mm2, the intrinsic spatial resolution is equal to 575 μm with a total detection efficiency of 0.29% for an extended source. The geometry of this new gamma counter gives one kinetic curve per strip to finally get the radiopharmaceutical kinetic in the whole brain of the mouse. To obtain a 2D projection of the brain radioactivity distribution, a spin rotation procedure can be applied. The gamma counter can also be combined to a microCT system to obtain anatomical information.