Collimator study of a γ-Camera system using GATE

A collimator study for a small field, high resolution γ-Camera system by means of GATE (GEANT4 Application for Tomographic Emission) simulations is presented in this paper. The aim of this study was the optimal definition and design of the geometrical characteristics of a parallel hole Pb-collimator, suitable for our γ-Camera system, which is based on the R2486 (HAMAMATSU) Position Sensitive Photomultiplier Tube, for different radio-tracers. The methodology followed two basic steps: (a) A validation phase with an existing parallel hole Pb-collimator of hexagonal structure, which preceded the main study. In this phase, experimentally obtained results for planar images are directly compared to simulated data. A simple phantom structure, consisting of four parallel capillaries filled with ^99mTc water solution, was imaged by the γ-Camera system for several phantom-collimator distances and the measured and Monte Carlo calculated spatial projections were compared. (b) A GATE simulation setup for the main collimator study geometry was constructed and the γ-Camera detector is repeated 36 times (in steps of 10⁰) around a ring. This construction allows the simultaneous detection of data for further SPECT reconstruction studies. Simulation data are accumulated for three ellipsoidal sources placed at the center of the ring with different tracer energies, different relative intensities and for several collimator geometries. The collimator sensitivity is tabulated for each tracer energy according to the ratio D/T, where D represents the hole diameter and T the collimator thickness. Finally, the spatial resolution is defined for some basic collimator hole patterns (triangular, square, cylindrical and hexagonal). SPECT images are also reconstructed and the detected resolution is discussed.