The purpose of this study was to investigate the relationship between physical performance characteristics (such as signal-to-noise ratio and Detective Quantum Efficiency (DQE)) and psycho-physical performance (probability of detection), when aperiodic objects on a uniform background are imaged using two digital mammographic systems. The task simulated the detection of microcalcifications. A contrast detail study was performed using the Dutch CDMAM contrast-detail phantom. This phantom uses objects of different diameter and thickness. X-ray images of this phantom were generated by two digital x-ray imaging systems, one using a fiber optic taper to couple the light from a Min-R type phosphor to a CCD, the other one using a lens to couple the light from a Lanex phosphor to a CCD. Images were presented to human observers on the CRTs of the imaging systems in the context of a target detection task. Signal-to-noise ratio, MTF and DQE of both imaging systems were determined using standard image evaluation techniques. The lens coupled system had the highest DQE at low spatial frequencies, but a low MTF and DQE at high spatial frequencies. It yielded the highest detection probability overall in the observer performance study. The fiber optic system on the other hand had a significantly lower DQE at low spatial frequencies, but at high spatial frequencies it had significantly higher DQE and MTF than the lens coupled system. Its probability of detection throughout the performance studies was significantly lower than that of the lens coupled system. Furthermore, the probability of detection of the fiber optic system for small objects did not reflect its superior performance with respect to DQE and MTF at higher spatial frequencies. Presenting the DQE as function of object diameter rather than as function of spatial frequency permitted calculating the detection probability and fitting the Rose Model of Vision. The results serve as a reminder, that the detection of small aperiodic objects, even down to a diameter of about 100 micrometers is not only determined by DQE and MTF at high spatial frequencies but also by DQE and MTF at low spatial frequencies.