iaea human health series publications - SEDIM

control limit. The upper and lower values indicating that the process is ‘out of control’ and requiring that correctiveaction be taken. It is prudent to immediately repeat the measurement to verify that the system is ‘out ofcontrol’ before taking corrective action. If the repeated measurement is ‘out of control’, then corrective actionis required immediately (or in some cases within 30 days). Synonym for action limit.D max , D min . The maximum and minimum optical density on a film. D max is the darkest area of the film, where thehighest exposure of X rays occurs. D min is the base plus fog optical density of the film.del. A physical detector element that determines the sampling aperture of the detector.densitometer. An instrument for measuring the optical density or degree of blackening of film.density difference (DD). The difference in optical density between the high density and the speed index. Thisprovides an index of contrast of the mammography film.detective quantum efficiency (DQE). DQE is a measure of the efficiency of an imaging system in transferringinformation from the X ray pattern incident on the detector to the output image. Specifically it is evaluated bydividing the square of the signal-to-noise ratio (SNR) of the output image by the square of the SNR in theincident X ray pattern. DQE ranges between 0 and 1. The only source of noise in an ideal detector results fromthe incident X ray quantum statistics. DQE is reduced if not all of the incident X rays are absorbed by thedetector and/or the noise is increased from its theoretical value due to sources of noise added by the detector.dry processing. Laser film printers that use thermal processes to produce and fix the blackness on the transparencyfilm. They use a heat sensitive development process rather than chemicals and normally have frequent selfrecalibrationto ensure stability of images.dynamic range. The range of exposures over which a detector can acquire image data in a single image. Typicaldigital systems will respond to exposures as low as 1 μGy and as high as 50 mGy outside the breast.effective resolution. The limiting spatial resolution measured with a line pair test pattern located at the level of thetop surface of the average breast. It is affected by both focal spot size and detector resolution. The effectiveresolution is typically measured in the orthogonal directions parallel to and perpendicular to theanode–cathode axis.exposure. The act of initiating and producing X radiation from an X ray unit.exposure index (EI). An index that gives an indicative relative value of the amount of radiation absorbed by a CRplate. Currently different manufacturers have different proprietary exposure indices.exposure time. The duration of primary X rays striking the breast and image receptor.fog. The unwanted signal added to an image by the exposure of the image receptor to light, radiation or heatbetween patient exposures.heel effect. Non-uniformity of the radiation field striking the image receptor caused by the geometry of the X raytarget. X ray intensity is higher toward the chest wall than toward the nipple, due to the increased path lengththrough the target and filter of X rays striking the nipple side of the image receptor.illuminance. A photometric quantity describing the light intensity per unit area falling on a surface. The SI unit forilluminance is the lux (candela-steradians per square meter).image noise. See radiographic noise.174