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Currently, single photon emission computed tomography (SPECT) is a technique widely used in nuclear medicine imaging. The wide range of possible radiotracer energies and the relative low cost make SPECT a more commonly used cancer diagnostic tool with respect to other more precise imaging techniques, like positron emission tomography (PET). However, SPECT cameras use mechanical collimation for the localization of the photon source, which leads to intrinsic limitations in sensitivity and spatial resolution. Compton gamma cameras overcome these limitations by using the kinematics of Compton scattering to localize the radioactive source.
The advantages of a Compton camera over a SPECT camera are: higher sensitivity, larger field of view and larger energy range. The proposed Compton camera is developed in the framework of the Voxel Imaging PET (VIP) project that also investigates the improvement of PET and PEM detectors for nuclear medicine diagnosis. The Compton camera consists of two different detectors, a scatterer and an absorber, made of pixelated Si and CdTe sensors, respectively. Both detectors are built up by assembling a sufficient number of independent modules with different layouts for the positioning of the Si or CdTe active volumes. The scatterer detector has a parallelepiped shape with a 540 mm x 380 mm x 26 mm size. The absorber detector of the Compton camera has a parallelepiped shape of size 540 mm x 380 mm x 62 mm.
Figure 1. VIP Compton Camera
Each absorber module (Figure. 1) hosts a number of 10 mm x 20 mm x 2 mm pixelated CdTe detectors. Each of the resulting 1 mm x 1 mm x 2 mm voxels is coupled to a fully integrated signal processing microchip and constitutes a completely independent detector for the energy, position, and arrival time of the photons. The chosen voxel size represents a good compromise between spatial resolution and fabrication cost. The ROC with CdTe detectors is mounted on a 50 μm kapton printed circuit board (PCB) with a connector for the external bus on one side while the signal radiation is expected to enter from the opposite side. The absorber module is designed to host four CdTe sensors corresponding to 4 cm interaction depth for the best compromise between cost and detection efficiency. An arbitrary number of modules can be stacked to obtain the preferred detector shape and size.
The
scatterer is built following a similar modular design as for the
absorber. Voxels of 1 mm x 1 mm x 2 mm size are chosen for
consistency with CdTe sensors. The nominal depth of interaction for
Si is set to 2 cm with the same optimization procedure as for the
CdTe. The scatterer and absorber are aligned and placed in parallel
planes with 10 cm between them. The distance between scatterer and
absorber is optimized for the best compromise between spatial
resolution and detection efficiency. The advantages of the proposed
design are: easily portability, operational at room temperatures,
excellent energy resolution of about 1.7% at 511 keV, and excellent
spatial resolution with millimeter-size voxels and operational in
strong magnetic fields.
Using the Geant4-based Architecture for Medicine-Oriented Simulations (GAMOS) software and software developed by the VIP group we are studying the expected performance of the camera in realistic conditions. Given the flexibility of the VIP module detector we expect to build a proof of concept prototype in order to compare the simulation with real laboratory data.
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