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The parallel hole (PH) collimator is the most commonly used collimator in single photon emission computed tomography (SPECT) imaging. A well-known limitation of the PH collimator is its low geometric efficiency. For better geometric efficiency without compromising the resolution, a rotating slat (RS) collimator is a potential alternative. In our previous work, a linear relation between the image resolution and the optimal collimator aperture was derived for both the PH and the RS collimator systems, and the two collimator systems were compared with the optimized geometries using different uniform phantoms. In this study, the PH and the RS collimator system were further compared using digital contrast phantoms. Three figures of merit (FOMs) were calculated for comparison, i.e., the contrast-to-noise ratio (CNR) in a pixel of interest, the CNR in a region of interest, and the signal-to-noise ratio (SNR) of a channelized Hotelling observer (CHO). All the FOMs were calculated analytically using an efficient approximation method. The comparison was done in both planar imaging and volume imaging. The size of the digitized phantom was varied, resulting in a detector area coverage from 2.5% to 40%. The main results are: (1) With the optimal collimator apertures, the gains (RS over PH) in all FOMs show the same trends in the system comparison. (2) The performance of the two collimators is similar for cold spot imaging with the largest phantom given in this study, while RS outperforms PH for all the other cases. (3) For system optimization, the optimal collimator apertures depend on the FOM to be optimized, however the ratio of the apertures (PH/RS) is always around √2, a value that we derived analytically in our previous work.