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Continuing developments in diagnostic imaging technology have resulted in the gradual replacement of film screen imaging systems by digital devices. Small-field mammographic imaging is one area in which this new technology has been successfully implemented. These systems are designed primarily for stereotactic localisation, although a number can be additionally used for spot imaging. In the past year, nearly all mammography X-ray units purchased with a stereotactic attachment have been supplied with a digital imaging option. As part of its continuing evaluation programme, KCARE has evaluated the four systems currently offered in the UK. All these are based on charge couple device (CCD) technology. Measurements of breast dose and image quality were made. The use of conventional mammography test objects, although providing relevant image quality data, is made difficult by the limited field size. Thus, additionally, one commercially available and two in-house test objects were employed, all based on a contrast detail format.

The results showed that, when compared to film screen imaging, low contrast sensitivity is improved and limiting resolution is poorer. Dose levels were generally slightly higher for the digital imaging systems.

In addition to subjective measurements, an objective evaluation was also carried out. Several parameters including detective quantum efficiency (DQE) and modulation transfer function (MTF) were derived and the results are presented.