Effects of Crystal Pixel Size and Collimator Geometry on the Performance of a Pixelated Crystal  -Camera Using Monte Carlo Simulation

Abstract

Dedicated g-cameras based on pixelated scintillators have long been used for breast tumor imaging. Intercrystal scattering (ICS) increases the background counting rate and degrades the image quality when small crystal pixels are used. Because of the small size of applied collimators, scattered radiation and septal penetration are high, and therefore collimator characteristics must be carefully considered. In our study, we investigated the influence of ICS events on position-detection accuracy (PDA) for pixelated crystals and the effects of different geometries of hexagonal-hole collimators on the performance of these cameras, using Monte Carlo simulation to optimize camera design. The arrays of thallium-doped cesium iodide detectors with different pixel dimensions that had been exposed to 140-keV photons of isotropic point source, 50 mm from the collimator surface, were simulated. Hexagonal-hole collimators were 10.5, 15, and 21 mm long. The septal thickness varied from 0.1 to 0.5 mm, with 3 different hole diameters. The results confirmed that by increasing the crystal pixel size, ICS was decreased and change of detection efficiency was negligible, but PDA, contrast-to-noise ratio, and spatial resolution (full width at half maximum) were increased. Our experiences confirmed that 2 · 2 mm was an optimum crystal pixel size, especially for a lower ICS fraction and an appropriate full width at half maximum. Because collimators are the limiting factor for spatial resolution and sensitivity, careful collimator design is of great importance.Key Words: pixelated g-camera; PDA; DE; ICS; CNR Nucl Med Technol 2010; 38:199-204 DOI: 10.2967/jnmt.109.073072 St andard g-cameras traditionally have been used to diagnose a major health problem for women-that is, breast carcinoma. System limits are essentially related to the spatial resolution of these standard g-cameras, prompting a need for researchers to build dedicated high-resolution g-cameras. The pixelated crystal, in comparison to continuous crystals, limits the degree to which the scintillation light spreads laterally and thereby can improve spatial resolution. Of course, such a design improves spatial resolution at the expense of a deterioration of the energy resolution resulting from light losses in the pixelated crystal, compared with that of a single, continuous crystal. However, the arrays of solid-state detectors, such as cadmium-zinc-telluride, provide a promising alternative technology as a compact high-resolution g-camera, with the advantage of improved energy resolution, compared with the resolution of pixilated scintillators. Many workers up to now have performed simulations and measurements to improve the performance of these cameras for detecting T1a tumors (size, ,5 mm) (1-7). Monte Carlo (MC) simulation techniques are useful when experimental measurements are not practical or experiments are too expensive. In an ideal environment, the parameters affecting the performance of an imaging system, such as collimator configuration and detector pixel size, are...

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