SiliPET: design of an ultra high resolution small animal PET scanner based on stacks of semiconductor detectors

We have studied with Monte Carlo simulations, using the EGSnrc code, a new scanner for small-animal positron emission tomography (PET) based on stacks of double sided semiconductor detectors. In small animal PET imaging (mice) Compton scattering within the animal itself is not an issue and therefore registration of the energy information is not necessary. Low Z materials can therefore be studied for designing a high performance scanner. Each stack is composed of planar detectors with dimension 60times60times1 mm³ and 128 orthogonal strips on both sides to read the two coordinates of the interaction, the third coordinate being the detector number in the stack. Coincidence events are recorded only if two plane detectors in two different stacks registered an energy deposition over threshold. In this way we achieve a precise determination of the interaction point of the two 511 keV photons, overcoming three of the crucial scanner limits: depth of interaction measurement thus eliminating parallax error; inter-detector multiple interactions by requiring a single hit; reduced distance between the detectors reduces significantly effects due to the non colinearity of the emitted gamma rays. The reduced dimensions of the scanner also improve the sold angle coverage resulting in a high sensitivity. This suggests that this is a promising new approach for small animal PET imaging. The sinogram profile for a pointlike source of ¹⁸F in a 3 cm diameter water phantom is 0.52 mm FWHM. Due to the large solid angle coverage and relatively high Compton efficiency, the simulated sensitivity at the center of the FOV is 5.1% with 4 stacks each 4 cm thick placed in a box-like configuration. Preliminary results of a proof of principle measurement done with the MEGA advanced Compton imager using a sime1 mm diameter ²²Na source showed a focal plane ray tracing FWHM of 1 mm