Compared to clinical whole-body PET systems, small animal PET systems have a much smaller detector diameter and the individual detector modules typically have smaller cross-sectional area comprising significantly smaller crystal elements. Thus, arranging rectangular blocks into a circle yields a significant number of relatively large wedge-shaped gaps between adjacent detector sides that provide a path for crystal-scattered photons to escape. For an 8 cm diameter, 8 cm long cylinder, these escape paths limit the system photon sensitivity to < 6% for a center point source () (350–650 keV energy window). We saw in and when all detector photon interactions (scatter + photoelectric) are simulated, the photon sensitivity decreases as the number of detector sides increase, whereas when only the photoelectric interactions are simulated the sensitivity curve essentially remains constant independent of the number of detector sides. This implies that the decrease of photon sensitivity with increasing number of detector sides (and hence increasing number of gaps) seen in the simulation curves of is mainly due to crystal Compton scatter photons escaping into the gaps.
As the number of system polygon sides increases, the number of inter-detector gaps also increases, which allows more scattered photons to escape. This result has been verified from analytical estimates, which only consider the geometric efficiency (solid angle coverage) and intrinsic coincidence detection efficiency (including photon attenuation in the crystals and coincidence time and energy window settings) and do not account for escaped crystal-scattered photons. Unlike the simulation results, which took into account loss of photons that scatter in the crystal, the analytical sensitivity curves in remain flat as the number of detector sides (and gaps) in the system polygon increases.
Maximum photon sensitivity (~8.5% for LSO-PSAPD and ~15% for CZT; see , ‘All interactions’ curves) is obtained for a system comprising four detector ‘sides’ in a box-shaped geometry (see , middle). This is due to the relatively high geometric efficiency and relatively few gaps between detector sides of the system polygon, which improves the probability of absorbing multiple interactions in the detectors. When the four corner gaps are filled in a box geometry >25% improvement in photon sensitivity is obtained providing >10% absolute photon sensitivity for LSO-PSAPD (4 ns, 350–650 keV) and >19% for CZT (16 ns, 350–650 keV) detectors, respectively.
In a different context, a four-panel box system was proposed for a breast-dedicated PET system (Jinyi et al 2002
). However, that four-sided geometry was proposed as a way to increase the number of LORs and improve tomographic image reconstruction compared to the dual-panel breast-dedicated PET approach (Murthy et al 2000
) rather than having a goal to optimize photon sensitivity for a given FOV size. Huber and Moses (1999)
also proposed a conceptual full box system with six detector panels to achieve 4π
solid angle coverage that provides high photon sensitivity for a small animal PET system, but without analyzing factors that limit photon sensitivity including the distribution of gaps created between two adjacent rectangular-shaped detector modules that we have studied here.
Detector modules used in clinical whole-body PET systems (e.g. cylinder configuration with 83 cm system diameter, 16 cm axial FOV) are built from relatively large detectors (>5 × 5 cm2 cross-sectional area, 2 cm thick), and large crystals (>4 × 4 × 20 mm3), and the large diameter means that the inter-module gaps are relatively narrow. Thus, although the geometric efficiency is much lower than for the small animal PET systems studied, the whole-body PET system design provides higher intrinsic detection efficiency due to a higher probability of absorbing multiple interactions with a relatively lower probability of crystal scatter photons escaping through the inter-module gaps. Hence, the number of detector sides (and inter-detector gaps) did not show a significant effect on photon sensitivity for the fixed 83 cm transaxial FOV width clinical whole-body PET system configuration ().
Simulation showed () that the photon sensitivity for the box geometry saturates when a coincidence time window greater than two times the coincidence time resolution of the detectors is used (2 ns for LSO-PSAPD, 8 ns FWHM for CZT). Narrow energy window settings (450–572 keV for LSO-PSAPD and 496–526 keV for CZT) equal to twice the energy resolutions (12% FWHM at 511 keV for LSO-PSAPD and 3% FWHM at 511 keV for CZT) are desired to limit the scatter and random events. A relatively wide 350–650 keV window is typically used in the existing small animal PET systems to increase the photon sensitivity, but increases the acceptance of scatter and random events. Random events increase with energy window since many single photons that contribute to randoms also undergo scatter. With excellent energy resolution, a narrow energy window can be used to limit the scatter and random events without compromising photon sensitivity (). Due to good energy resolution (3% FWHM at 511 keV) for CZT, using a very narrow energy window reduces the photon sensitivity to a lesser degree than for the LSO-PSAPD system (energy resolution 12% FWHM at 511 keV).
summarizes the recorded photon sensitivities that are achieved for the previously developed small animal PET systems. For a system that minimizes the inter-detector module gaps, as in the case of the X-PET system, a high sensitivity (~10%) is achieved for a cylindrical system geometry. The special pentagon-shaped detectors in the X-PET systems fill all potential inter-module gaps at module edges with trapezoid-shaped crystals, providing highest photon sensitivity compared to other previously developed small animal PET systems. Filling all inter-module gaps in any polygon system geometry will provide high probability of detecting Compton-scattered photons, significantly increasing the photon sensitivity. The ideal cylindrical geometry system would be a solid annulus, such as described by Karp et al (1994)
for a human brain system using NaI(Tl) scintillation crystal, since it provides optimum photon sensitivity per detector volume. However, such detector design is limited by lower intrinsic detection efficiency, poor spatial resolution and higher dead time compared to the standard pixellated crystal designs. In addition, the large solid annulus cannot be built using the more desirable PET scintillation crystals such as LSO, LYSO, GSO or BGO. Compared to existing small animal PET systems that use conventional rectangular block detectors, the proposed box-shaped small animal PET system designs, if successful, provide on average more than three and fivefold improvements in photon sensitivity using LSO-PSAPD and CZT detector configurations, respectively, while achieving 1 mm FWHM intrinsic detector resolution, 3D interaction positioning, and superior energy resolution.
The flexibility to shift the four sides of a box geometry PET system provides the ability to adjust the useful FOV to the actual size of the object, which yields significant increase in photon sensitivity. For a small animal PET box-shaped system, adjusting the transaxial useful FOV to less than 5 cm provided more than 50 and 30% additional gain in photon sensitivity over the fixed 8 cm transaxial width for LSO-PSAPD and CZT detectors, respectively. Similarly, greater than 60% improvement in photon sensitivity was obtained when the four sides of the rectangular-shaped clinical system simulated were shifted with respect to each other to adjust the transaxial FOV to less than 50 × 50 cm2 compared to the existing state-of-the-art fixed diameter clinical PET systems ().
As discussed we are investigating new PET detector technologies that, if successful, will have the capability to accurately estimate the 3D coordinates of all interactions per event. However, please note that the key results of this work, which studied the effect of detector arrangements and gaps on photon sensitivity, do not assume the availability of such a 3D detector PET system, which at present does not exist. The results of this paper also hold for the existing systems that use rectangular-shaped detectors, such as those small animal systems listed in , as well as all clinical PET systems. These systems use 2D positioning detector modules that are only capable of estimating the detector element closest to the weighted mean position of all interactions for each event, and the total energy deposited per event, but cannot separately position the individual photon interactions or even estimate the weighted mean interaction depth. However, in order to be able to develop new system designs that arrange the detectors closer to the subject for substantial photon sensitivity improvements, without adding excessive parallax positioning errors that degrade spatial resolution as a function of radial coordinate, the detector system must also be able to localize the weighted mean photon interaction depth, which has been a topic of great interest (e.g. Yamaya et al (2005)
, Burr et al (2004)
, Schmand et al (1999)
, Ziemons et al (2005)
, Levin et al (2004a)
The focus of the paper was to study the factors that affect photon sensitivity. This work established that, for rectangular-shaped detectors and a small system diameter, a box-shaped system is preferable. But how does the box shape affect tomographic image reconstruction performance? Previous studies demonstrated that reconstructed spatial resolution, contrast recovery, image SNR and variance are comparable for a box shape compared to a cylinder (assuming the LSO-PSAPD detector technology with a weighted mean position algorithm) (Chinn et al 2005