Influence of detector thickness on resolution in three-headed gamma camera PET

In PET imaging with large area NaI scintillators, an uncertainty in the measured position of a photon hitting the detector at an oblique angle exists due to the thickness of the crystal in the detector. To evaluate the influence of the crystal thickness and the angle of incidence on the spatial resolution, simulations and measurements of a dual and triple headed PET system in different configurations were performed. In the simulations, the resolution was calculated as a function of: head radii, crystal thickness, head orientation and the maximum angle of incidence. The influence of these parameters (except crystal thickness) was also evaluated by measurements on a dual-headed AXIS and triple-headed IRIX PET systems. It was observed that reducing the average angle-of-incidence (and hence reducing the depth-of-interaction uncertainty) tended to improve the spatial resolution. This could be accomplished by restricting the maximum angle-of-incidence, increasing the separation of the heads, or changing the orientation of the heads (a 0-120-240 configuration has a greater average angle-of-incidence than a 0-90-180 or 0-180 configuration). However, when taking the effect of non-collinearity of the annihilation into account, increasing the separation of the heads results in a degrading resolution. The resolution improved by decreasing the crystal thickness (since depth-of-interaction uncertainty was reduced). The system sensitivity tended to decrease with the increase in resolution.