High Sensitivity Collimators for Optimising Lesion Detection in SPECT Images

Parallel hole collimators which are currently routinely used in SPECT imaging were designed a few decades ago, when filtered backprojection (FBP) was used for tomographic reconstruction. Statistical reconstruction methods with precise modeling of the projection measurement offer a different standard, and the question of optimizing resolution - sensitivity tradeoff by choosing appropriate collimator aperture needs to be revised. In this paper we search for a parallel hole collimator which offers best performance in detection of hot lesions on a uniform background. To evaluate the image quality we use standard quantitative measures like the contrast recovery coefficient, coefficient of variation, and contrast to noise ratio. We also performed signal detection by human observers followed by Receiver Operating Characteristics (ROC) analysis for a few lesions close to the limit of visibility. Our results consistently indicate that optimal performance is achieved with collimators of sensitivity 6 - 9 times that of the conventional high resolution (HR) collimator. When the optimized collimator is applied, the image quality of the conventional image obtained with the HR can be achieved at scanning time or activity dose reduced by factor of 3.