From Virtual Pixel Grids to Overlapped PSF for PET Systems with Monolithic Crystals

In this work we take advantage of the monolithic continuous crystals in PET photon detection. The called Impact Bin Method (IBM) proposes to use the smaller bin size given by the electronics to position the photon impact and then, build a Point Spread Function (PSF) around this point. We use a rectangular PSF to facilitate the estimation of the voxels emission probabilities. These probabilities are calculated as the volumes of intersection between the tube of response generated by the PSF and the voxels. To perform a proof of concept of IBM, we use a dedicated breast PET consisting of two rings with 12 modules each. Each module contains a single LYSO monolithic crystal (no pixelated), coupled to a PSPMT. The dimensions of the crystal are 40 mm × 40 mm × 12 mm and the maximum pixel size allowed by electronics is 300 × 300 bins per block. Then, if we consider this pixel size, the minimum area for each individual bin will be 0.13 mm × 0.13 mm. We compare images reconstructed with the maximum virtual binning of 300 × 300, with the regular virtual binning of 20 × 20 and finally with IBM. Using IBM we avoid the regular virtual grid, so the PSF from different hits can overlap between them. The results reconstructing with IBM show an improvement of 10% in spatial resolution and a 7% in the recovery coefficients without compromise the image uniformity.