Study of different pinhole configurations for small animal tumor imaging

In this work, we design and study the properties of a novel geometric criteria-based multipinhole (MP) SPECT system, for imaging tumors and gene expression in small animals. The MP configuration was determined based on the following geometric criteria viz: 1) place the pinholes evenly in the vertical and horizontal directions of the collimator (so that a large number of axial planes in the animal are completely sampled), 2) select a pinhole arrangement that gives the maximum sensitivity, 3) select a configuration that maximizes the distance between adjacent pinholes (less overlap in projection data) and 4) arrange the pinholes such that the projections from the volume of interest (VOI) containing the tumor and surrounding tissues are not truncated. The reconstructed images obtained using the criteria-based MP collimators were evaluated with those obtained using other MP configurations. Image degradation factors such as pinhole diameter, pinhole penetration, intrinsic resolution of the crystal, depth dependent decrease in sensitivity and Poisson statistics were modeled using a Monte Carlo based projector. The criteria-based seven-pinhole configuration resulted in a faithful reconstruction of a larger volume, reducing the chance of systematic errors propagating into the quantitative values from the VOI. Further, the irregular pattern of the pinholes helped to eliminate the multiplexing artifacts often observed in MP reconstructions. A 100% improvement in contrast-to-noise ratio and a 40% reduction in the normalized standard deviation were observed for the criteria-based system, compared to the single pinhole collimator. When compared to SP the non-prewhitening filter analysis showed a significantly improved detectability of small lesions using the criteria-based collimator. Since it is relatively inexpensive to build an application-specific MP collimator we postulate that such geometric criteria-based pinhole collimators could be the modality of choice for small animal SPECT imaging.