Using Rose's metal alloy as a pinhole collimator material in preclinical small-animal imaging: a Monte Carlo evaluation.

Abstract

Pinhole collimation is the most common method of high-resolution preclinical single photon emission computed tomography imaging. The collimators are usually constructed from dense materials with high atomic numbers, such as gold and platinum, which are expensive and not always flexible in the fabrication step. In this work, the authors have investigated the properties of a fusible alloy called Rose's metal and its potential in pinhole preclinical imaging. When compared to current standard pinhole materials such as gold and platinum, Rose's metal has a lower density and a relatively low effective atomic number. However, it is inexpensive, has a low melting point, and does not contract when solidifying. Once cast, the piece can be machined with high precision. The aim of this study was to evaluate the imaging properties for Rose's metal and compare them with those of standard materials. After validating their Monte Carlo code by comparing its results with published data and the results from analytical calculations, they investigated different pinhole geometries by varying the collimator material, acceptance angle, aperture diameter, and photon incident angle. The penetration-to-scatter and penetration-to-total component ratios, sensitivity, and the spatial resolution were determined for gold, tungsten, and Rose's metal for two radionuclides, (99)Tc(m) and (125)I. The Rose's metal pinhole-imaging simulations show higher penetration/total and scatter/total ratios. For example, the penetration/total is 50% for gold and 75% for Rose's metal when simulating (99)Tc(m) with a 0.3 mm aperture diameter and a 60° acceptance angle. However, the degradation in spatial resolution remained below 10% relative to the spatial resolution for gold for acceptance angles below 40° and aperture diameters larger than 0.5 mm. Extra penetration and scatter associated with Rose's metal contribute to degradation in the spatial resolution, but this degradation is not always substantial. The most important factor besides the collimator material was the acceptance angle. This should be kept to a minimum to prevent unnecessary scatter and penetration. For (125)I, the difference in spatial resolution between gold and Rose's metal is very small, 2.2% in the worst-case scenario. Based on these results, the authors conclude that Rose's metal is an alternative to standard materials not only for low-energy photon imaging but also for medium-energy applications that require low-cost, flexible pinhole configurations and designs, and that can tolerate a degraded spatial resolution.