novel conceptual design of PET scanner, known as a Voxel Imaging PET
(VIP) pathfinder, based on pixelated semiconductor detector, has been
proposed to overcome the intrinsic limitations of state-of-art PET
devices based on scintillating crystals. The VIP-PET scanner uses
pixelated room temperature solid-state cadmium telluride (CdTe)
detectors. The new design of the PET system allows to yield a true
and high 3-D spatial resolution detector with a density of ~450
channels per cm3, greatly improve the energy resolution due to the
use of room temperature CdTe detectors, achieves very high
signal-to-noise ratio by rejecting most of the scattered events and
achieves very high detection efficiency for 511 keV photons thanks to
the high CdTe stopping power.
In addition, the detectors are not affected by the presence of a strong magnetic ﬁeld and can be used for developing simultaneous MR-PET imaging systems. The VIP-PET represents a detector ring that consists of 66 VIP sectors (Figure 1). The ring has an inner diameter of 42 cm, outer diameter of 54 cm and an axial field of view (FOV) of 25.4 cm.
Figure 1. General view of the VIP-PET scanner and a VIP-PET ring section. The scanner consists of 66 such sections.
Each VIP section contains 4 module blocks that are connected to the same electronics bus, as shown in Figure 1 (black region). A module block consists of 30 detector modules (Figure 2) and includes 24000 voxels, each one has size of 1 mm x 1 mm x 2 mm and has its own independent readout channel. A single detector module is made of 4 CdTe pixelated detectors (Figure 2). Each one has trapezoidal shape geometry, so the module blocks form a perfect cylindrical shape without cracks in between.
Figure 2. VIP-PET module block and a detector module. Each module blocks consists of 30 detector modules. Each detector module is made of 4 CdTe pixelated detectors. Every pixelated detector contains 200 voxels with dimensions 1 mm x 1 mm x 2 mm.
In order to evaluate the VIP-PET system performance and assess its image quality, the whole VIP geometry is simulated using the Geant4-based Architecture for Medicine-Oriented Simulations (GAMOS). The energy window is 503 - 519 keV (according to the achieved 1.57% at FWHM energy resolution). The channel trigger threshold is set at 10 keV. The dead time and the measuring time per voxel are 20 μs and 10 μs, respectively, the coincidence time window is 20 ns.
Simulation results show that VIP-PET has a competitive sensitivity (14.37 cps/kBq for the NEMA NU 2-2001 phantom and 21 cps/kBq for the NEMA NU 4-2008 one) and a superior signal purity with respect to scintillating crystals PETs. The measured NEC curve has the peak value of 908 kcps at 1.6 MBq/mL for a NEMA NU 4-2008 mouse phantom and 122 kcps at 5.3 kBq/mL for a NEMA NU 2-2001 phantom. VIP-PET can achieve an image resolution lower than ~1 mm FWHM in all directions with a negligible scatter fraction of 0.73% according to the NEMA NU 4-2008 test and 3.95% in case of the NEMA NU 2-2001 measurements, thereby collecting virtually noise-free data and this will allow the user to acquire a full PET scan in 15 seconds, thus minimizing the image blurring from human movements.