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In this study, the parallel hole (PH) and the rotating slat (RS) collimators were compared based on a planar contrast phantom. Three figures of merit (FOMs) were applied for the comparison: 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). For the comparison purpose, all the FOMs were calculated analytically using an efficient approximation method. The comparison was mainly performed for planar imaging. The main results are: (1) By doing iterative reconstructions, it was verified that the analytical method can give accurate predictions for the proposed FOMs. (2) The gains (RS over PH) in all FOMs show the same tendencies in the system comparison. (3) PH only outperforms RS for cold spot imaging with a large phantom, while RS is better for all the other cases. (4) 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 Â¿2, a value that we derived analytically in our previous work.