  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.
