Study of Sensitivity and Resolution for Full Ring PET Prototypes based on Continuous Crystals and analytical modeling of the light distribution.

Sensitivity and spatial resolution are the main parameters to
 maximize in the performance of a PET scanner. For this purpose, detectors
 consisting of a combination of continuous crystals optically coupled to segmented
 photodetectors have been employed. With the use of continuous crystals the
 sensitivity is increased with respect to the pixelated crystals. In addition, spatial
 resolution is no longer limited to the crystal size. The main drawback is the
 difficulty in determining the interaction position.
 In this work, we present the characterization of the performance of a full
 ring based on cuboid continuous crystals coupled to SiPMs. To this end, we
 have employed the simulations developed in a previous work for our experimental
 detector head. Sensitivity could be further enhanced by using tapered crystals.
 This enhancement is obtained by increasing the solid angle coverage, reducing
 the wedge-shaped gaps between contiguous detectors. The performance of the
 scanners based on both crystal geometries was characterized following NEMA NU
 4-2008 standardized protocol in order to compare them. An average sensitivity
 gain over the entire axial field of view of 13.63% has been obtained with tapered
 geometry while similar performance of the spatial resolution has been proven with
 both scanners. The activity at which NECR and True peak occur is smaller and
 the peak value is greater for tapered crystals than for cuboid crystals. Moreover,
 a higher degree of homogeneity was obtained in the sensitivity map due to the
 tighter packing of the crystals, which reduces the gaps and results in a better
 recovery of homogeneous regions than for the cuboid configuration. Some of the
 results obtained, such as spatial resolution, depend on the interaction position
 estimation and may vary if other method is employed.