Except for the light emanating from the viewbox, the room should be dark. The viewboxes should ideally display two different levels of light, one average, the other, bright, for displaying studies of greater exposure. A rheostatically-controlled bright light should be available for viewing such overexposed areas as the skin line and for inspecting overexposed films. Some companies make view-boxes specifically designed for viewing mammograms. Some of these viewboxes are equipped with masking devices, built-in bright lights, and magnifiers.
The ACR's recommendations for mammography quality control to be performed by medical physicists (ACR, 1999) suggests performance criteria for mammographic viewbox measurements to be used as interim guidelines until further data are collected:
Although ACR (1999) recommends 50 lux or less, it is desirable to keep the room ambient illumination level as close to zero as possible.
The "Guide to Good Practice" in the section, "Viewboxes and Viewing Conditions," of ACR (1999), outlines the essential requirements for viewboxes and viewing conditions. For instance, masking the area around the mammograms is a necessity. This obliterates extrinsic light, which reduces contrast and limits the visibility of densities that have not been "bright-lighted." Direct or reflected light from windows, other viewboxes, and any other sources should not impinge in any way on the mammographic viewbox (KimmeSmith et al, 1997).
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