The aim of this study is to examine the impact of dark signal on assessing the image quality in computerized mammography. A GE analogue mammography unit was calibrated to be used with an Agfa CR35-X system. Tests for quality control were performed and Image Plates (IPs) were erased by the CR system. All IPs were first exposed to known amounts of scatter radiation and then used to take mammographic images of phantoms with varying density (a PASMAM-1054 Phantom with PMMA and Al inserts). Since high Signal-to-noise ratio (SNR) and high Contrast-to-noise ratio (CNR) are of great importance in mammographic image quality, we measured SNR and CNR of the taken images. Our results show the effect of scatter radiation on SNR and CNR. Scatter radiation increases SNR and decreases CNR. The impact of background signal was found not to be negligible. Dark signal deteriorates image quality in CR mammography. Consequently, further investigation may be warranted and daily erasure of the IPs is recommended in order to limit dark signal effect on image quality.