Geant4 evaluation of the impact of spatial resolution improvement on the contrast recovery coefficient in a small-animal PET system with collimation

Abstract

Two limitations on the resolution of a reconstructed PET image are sampling and detector pixel size. Using collimation that partially blocks each crystal reduces the effective crystal size. Using different collimation positions increases sampling. In this study we determine the Contrast Recovery Coefficient (CRC) for a small-animal PET scanner with and without collimation in the transverse direction. We performed simulations of a single-slice small-animal PET system (205 mm diameter and 2 ? 2 ? 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> LYSO crystals). The septa forming the collimation were 1 ? 2 ? 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> tungsten pieces covering half of each crystal transaxially. Phantoms (25 mm diameter) with one cold and three hot lesions with diameter D (D = 0.5, 1, 2, 3 mm) were simulated with two S:B ratios (4:1, 6:1). CRC = (S/B-1)/(T-1) where S and B are mean lesion (hot or cold) and background count densities, and T is true uptake ratio. CRC was measured from reconstructions to quantify the impact of the resolution improvement. Results show collimation improves mean CRC compared to non-collimated PET. For 1 mm hot lesions (4:1), scanned for the same duration, the collimated mean hot lesion CRC values (STD) were 0.44 (0.03) (center) and 0.24 (0.02) (off-center), where STD is the standard deviation of measured CRCs of an ensemble of images. Non-collimated results were 0.31 (0.01) and 0.18 (0.01), respectively. Although the total number of coincidences for the same scan time is fewer by about a factor of 4 in the collimated system, the measured mean CRC is higher. The efficiency loss in collimated PET manifests itself as worse STD in measured CRC and noisier images. When the collimated PET scan time is increased, the STD of measured CRCs improves and reaches that of non-collimated PET. In certain imaging scenarios, it may be possible to scan longer with a collimated PET system to make up for the efficiency loss. In conclusion, our study shows the use of collimation has the potential to improve the quantification and detection capabilities of a small-animal PET system.