The assessment of image quality in mammography often relies on the subjective evaluation of films produced with test-objects containing structures such as masses, micro-calcifications and filaments. If the methodology is adequate to control the stability of a mammography unit, its use in a context of system optimization (from the x-ray spectrum to the detector response) is limited. Thus, a test-object which allows measurements of the detectability index d' from the non-prewhitening matched filter (NPWE) observer, was developed and tested. The test-object is 45 mm thick and allows the assessment of d' in areas of different glandular/fat compositions (i.e image quality evaluation taking into account the dynamic range parameter). To simulate the absorption of the skin, a 100% fat equivalent tissue, with a thickness of 5 mm, is placed on each side of the test-object. On a conventional unit, it is possible to assess the image parameters at three optical density levels (i.e. 0.5 - 0.8 ; 1.5 - 1.6 and 2.3 - 2.6) in one exposure. The imaging of this test object on digital units has also been tested satisfactorily.
The display of low-contrast structures and fine microcalcifications is essential for the early diagnosis of breast cancer. In order to achieve a high image quality level with a minimum amount of radiation delivered to the patient, the use of different spectra (Mo or Rh anode and filters) was introduced. The European Synchrotron Radiation Facility is able to produce a monochromatic beam with a high photon flux. It is thus a powerful tool to study the effect of beam energy on image quality and dose in mammography. Our image quality assessment is based on the calculation of the size of the smallest microcalcification detectable on a radiograph, derived from the statistical decision theory. The mean glandular dose is simultaneously measured. Compared with conventional mammography units, the monochromaticity of synchrotron beams improves contrast and the use of a slit instead of an anti-scatter grid leads to a higher primary beam transmission. The relative contribution of these two effects on image quality and dose is discussed.