iaea human health series publications - SEDIM

TABLE 3. TYPICAL TARGET–FILTER COMBINATIONS FOR DIGITAL MAMMOGRAPHYTarget Filter Thickness, in mmMo Mo 0.03 (GE, Planmed, Fujifilm, Siemens, Giotto)Mo Rh 0.025 (GE, Planmed, Fujifilm, Siemens, Giotto)Rh Rh 0.025 (GE)W Al 0.3 (Sectra D40), 0.43 (Sectra L30)W Rh 0.06 (Hologic, Planmed)0.05 (Fujifilm, Siemens)W Ag 0.06 (Hologic)0.075 (Planmed)0.05 (Giotto)3.2.2. Viewing conditionsIn digital mammography, images are generally viewed on a display monitor. Unlike film, the electronic displaycan be adjusted; to facilitate the ability to detect subtle lesions or anatomic features, the display must be properlycalibrated to match the performance of the human visual system. Therefore, it is helpful to have some understandingof psychophysical factors of vision [30, 37, 38]. The ability of a human viewer to detect a change in brightness, ΔL, isdependent on the average brightness, L, of the area surrounding the image. The smallest visibly detectable change inbrightness, ΔL min , is referred to as the ‘just noticeable difference’, or jnd. The size of a jnd tends to be constant forluminance levels above 100 candelas per square metre (cd/m 2 ); at lower luminance levels, the jnd increases rapidly(i.e. greater changes in brightness are required to evoke a visual response that there is a difference).The approach to calibration of a display for digital imaging is to apply ‘perceptual linearization’ to the imagedata that are to be displayed, such that each step in the image signal sent to the display represents an equalincrement of perceived contrast; that is, a constant number of jnds regardless of L. In other words, to achieve alinear range of perceived contrast steps, an unequal spacing of luminance levels is applied, depending on theluminance.Perceptual linearization is based on the relationship between the jnd and L, and is the basis of the DICOMGSDF, which aims to enforce consistency of medical images across various display devices. While there is littleconcern for constant rendition of jnd with film display devices owing to the typical luminance levels involved, thisis not the case with soft copy displays. In this case, contrast enhancement at low luminance levels can be achievedby GSDF calibration.3.2.2.1. Soft copyViewing conditions are very important for image interpretation from a monitor, especially given the lowluminance of the monitors [24–26]. There should be no glare reflected from the screen, and the walls behind thescreen should be about the same colour and intensity as a mid-grey background on the screen, with the backgroundroom illumination at around 30 lux (lx). There should be no distracting icons on the image screens, and radiologistsshould be positioned such that they are at the distance where they have maximum acuity.3.2.2.2. Hard copyIf hard copy digital mammograms and mammography films from prior studies are to be viewed, the followingconditions should be met:— A viewbox designed for mammography with a luminance of greater than 3000 cd/m 2 ;— Lamps in the viewbox matched for brightness and colour;18