Dimensioning A Versatile CdZnTe Small Field Of View Gamma-Camera With SINDBAD, a Mixed Analytical-Monte Carlo Simulation Tool

Small field of view (FOV) SPECT imaging can benefit from the improving performances of CdZnTe pixelated detectors, especially their excellent spatial resolution. An additional feature, specific to semiconductor detectors, is the measurement of the depth of interaction (DOI). LETI is developing a MICRO GAMma-ray Imager. From technological, electronics and physical constraints, the CdZnTe detector size is 48 mm times 48 mm times 4.6 mm with a 0.75 mm pixel pitch. We have chosen a versatile collimation architecture. The Anger analytical formulae for collimators help us to dimension two parallel square hole collimators with different sensitivity-spatial resolution trade-offs. To get high resolution (0.6 mm) in at least 1cm
²
FOV, we change to a pinhole collimator with a focal length of 50 mm or so. Simulations of selected setups are then made with SINDBAD, a LETI code developed to simulate radiographic chains. The collimator and attenuation phantom are described through a CAD tool. Either direct analytical simulation or Monte Carlo simulation or both can be done. A voxelized emission phantom can be simulated through an external loop. As scattering and fluorescence are not dominant in small FOV exams, the ray-tracing approach is an attractive way to help the architecture design. A variety of parallel hole or pinhole collimators are simulated. For parallel collimators, simulation shows that, compared to a typical NaI device, a High Sensitivity collimator can yield a gain in sensitivity of 4.7 with the same spatial resolution, whereas a High Resolution collimator can improve the spatial resolution by a factor of 1.9 without degrading sensitivity. For single pinhole collimators, the benefit of small detector pitch is counterbalanced by the reduced detector size. SINDBAD simulation helps to emphasize the effects of pinhole profile and of parallax errors.