The physical characteristics of a charge-coupled device (CCD) image detector were evaluated, as well as its potential as a digital imaging device for small field mammographic applications such as preoperative needle localization. The detection system is based on a 2048 X 2048 pixel CCD operated in 1024 X 1024 mode. The CCD was optically coupled to an intensifying screen via a lens, without intermediate intensification. The thermal noise was suppressed to 0.15 electrons pixel-1s-1 by cooling the CCD with liquid nitrogen. The dominant source of noise was attributed to the on-chip amplifier during the readout process that was performed at 50 000 pixels s-1. The measured readout noise level was 15 electrons per pixel. The low-noise characteristics of this CCD prototype detector produced encouraging results under conditions simulating mammography, with a signal level close to one electron per pixel for each detected x ray. The mean glandular dose to the breast, based on the entrance exposure measured from a standard mammographic phantom would be 1.52 mGy (152 mrad). The ultimate spatial resolution of the system was approximately 8 cycles/mm but it was limited to about 5 cycles/mm when operated in the 1024 X 1024 imaging mode. Other physical characteristics of the system such as optical coupling efficiency, exposure response, and signal-to-noise ratio were evaluated. The results of this study suggest that the use of a scientific-grade CCD allows for very good low-contrast discrimination and moderate spatial resolution under conditions simulating mammography, but the current prototype is limited to a 9 X 9-cm2 field of view. The results of this study suggest that with realistic improvements in the optical coupling via a faster lens or fiberoptic coupling, additional improvements in image quality and dose are feasible.
