Analytical study of performance in a 3D PET scanner.

The use of arrays of small discrete detectors and thin slices to achieve a high isotropic spatial resolution in positron emission tomography (PET) results in systems with a low ring sensitivity. In a multi-ring system, the overall sensitivity can be considerably improved by removing the interslice collimators to make full use of all cross slices coincidences, but this is achieved at the expense of increased scatter and accidental rates in the image. For imaging of small laboratory animals (diameter of 10 cm or less), the relieved burden of scatters, and to some extent accidentals, suggests that volumetric imaging may be of particular value. In order to evaluate the performance to be expected from a small animal PET scanner (10 cm diameter field) with and without the interplane collimators, the incident event rates for singles (unscattered and single scattered) and true, scatter and accidental coincidences were evaluated analytically. The performance was evaluated for various source sizes and activities in terms of five criteria: true/single ratio, scatter fraction, accidental fraction, image contrast and noise effective sensitivity. As a result of a 3-fold increase in scatter fraction and of a significant increase in accidental fraction for larger sources, the image contrast (true/(scatter+accidental)) is observed to always be inferior with the collimators removed. However, a significant improvement in noise effective sensitivity is obtained with the volumetric configuration, especially for small size sources placed at the centre of the field of view. It is concluded that the volumetric configuration is more advantageous than the multislice configuration to image small animals because the gain in sensitivity overcomes the loss of accuracy due to higher scatter and accidental rates.