Mammography equipment: principles, features, selection.

Abstract

1. Three-phase and constant potential generators may offer advantages over single-phase generators. These include shorter exposure times with less chance for motion unsharpenss, more adequate exposure and possible lower dose, and longer tube life. 2. Control of tube voltage in increments of 1 kVp are desirable for screen-film mammography and 2 kVp for xeromammography. 3. Tube loading limitations should be capable of allowing an adequate mA and mAs output. A mammography unit with a lower mA output may require longer exposure times with consequent motion unsharpness and/or higher dose. The maximum mA output is limited by the power rating of the tube and not by the power rating of the generator, which is usually much higher. Thus, tube rating charts rather than generator power rating should be used to compare the mA output of different mammography units. 4. Phototimer capability can reduce exam time, facilitate proper exposure, and reduce radiation dose. Units with a greater number of density steps and three or more photocell locations, one of which is near the edge of the film holder, provide better phototimer results. 5. A molybdenum target, beryllium window tube with 0.03 mm molybdenum added filtration is presently preferred for screen-film mammography. A tungsten target tube with 1 to 2 mm added aluminum filtration is advised for xeromammography. 6. Effective local spot size for contact mammography should be in the range of 0.2 to 0.5 mm. However, high resolution also requires sufficiently long source-to-image-receptor distances. Using measured focal spot size and source-to-image-receptor distance, it has been shown that some mammography units do not fully utilize the resolution capabilities of some available screen-film systems. On the other hand, a focal spot that is too small may limit the field size and lead to uneven energy distribution in the x-ray field. 7. In order to perform standard and supplementary mammographic projections, the C arm assembly must be capable of being located at multiple positions within a 180 degrees rotation. Greater degrees of rotation are not required. 8. The heat dissipation capability should be adequate to accommodate the anticipated work-load in terms of number of patients and patient scheduling. A mammographic unit with an anode heat capacity of 90,000 heat units or more should be adequate for most practices. A mammography unit with a heat capacity of 20,000 heat units or less may require waiting time between exposures or studies in a high-volume mammography practice. 9. The compression plate should be made of plastic, usually 1 to 4 mm thick.(ABSTRACT TRUNCATED AT 400 WORDS)