Theoretical LOR model incorporating spatial uncertainty in continuous detector PET

In this paper, we will describe a theoretical model of the spatial uncertainty for a line of response, due to the imperfect localization of events on the detector heads of the Positron Emission Tomography (PET) camera. We assume a Gaussian distribution of the position of interaction on a detector head, centered at the measured position. The probability that an event originates from a certain point in the FOV is calculated by integrating all the possible LORs through this point, weighted with the Gaussian probability of detection at the LORs end points. We have calculated these probabilities both for perpendicular and oblique coincidences. For the oblique coincidence case it was necessary to incorporate the effect of the crystal thickness in the calculations. We found that the probability function can not be analytically expressed in a closed form, and it was thus calculated by means of numerical integration. A Gaussian was fitted to the probability profiles for a given distance to the detectors. From these fits, we can conclude that the profiles can be accurately approximated by a Gaussian, both for perpendicular as for oblique coincidences. The FWHM reaches a maximum at the detector heads, and decreases towards the center of the FOV, as was expected.