iaea human health series publications - SEDIM

(10) Place the thinner test object (25 mm PMMA or 1.27 mm aluminium) on the magnification stand.(11) Select the manual or AEC mode used clinically for magnification imaging of an average breast.(12) Expose the image. For CR systems, use the settings noted above.(13) Enter the imaging parameters into Chart 8 in Annex II (and into Chart 3, if required).(14) Repeat steps 9–13 for other magnification stand positions (magnification factors) and clinically relevanttarget–filter combinations. It is not necessary to exhaustively image all possible permutations ofmagnification stand positions and target–filter combinations, but each magnification stand position should beused at least once, and the most commonly used target–filter combinations should be tested at least once withthe magnification stand.(15) Examine the unprocessed images on a workstation.(16) Select a window width and window level that allow artefact severity assessment without accentuating thenoise excessively.(17) Record the window width and level settings used on Chart 8 in Annex II.(18) Carefully examine the images of the uniform phantom for artefacts. Record any visible artefacts on Chart 8 inAnnex II. If possible, save images illustrating the artefacts.(19) For CR systems, examine the images for evidence of defects in the plates, dirt on the plates (reduced densityspecks) and dirt in the reader (reduced density lines across the images).(20) If any artefacts or non-uniformities are observed, rotate the phantom 90° and repeat the exposure andacquisition procedure. For CR systems, the cassette may be placed on the table top, rotated at a 45° angle, todetermine if the artefact is caused by the filter or X ray tube versus the imaging plate or the reader. Anyartefacts or non-uniformities that keep a fixed orientation relative to the image receptor in both images andthat are deemed significant probably require a recalibration of the gain file specific to the target–filtercombination in question.(21) The image should be examined for flat field uniformity to ensure that the image does not have nonuniformitiesacross the field of view.(22) Review the images of all clinically used target–filter combinations.(23) Display the uniformity image with a zoom factor that displays full resolution (1:1 pixel mapping). Reduce thewindow width until the noise pattern becomes apparent. Pan over the entire image, examining it for variationsin the amount of noise and in the texture of the noise from place to place in the image.(24) Ensure that some of the thickness tracking images are also examined, to verify that the flat fielding algorithmworks well across the thickness range of 20–70 mm, and not just for the thickness of the facility’s uniformphantom.(25) Record all recommended technique factors and viewing window and level settings on Chart 3 in Annex II.8.5.5.4. Interpretation of results and conclusionsArtefacts may include: dust or dirt, ‘ghost’ images left over from repeated test or clinical exposures,blotchiness due to thickness variations of the filter, dirt or corrosion on the filter, stitching artefacts,clipping/electronic noise, spatial non-uniformities, artefacts, plus or minus signal variations, flat fielding artefacts,grid lines, streaking in the horizontal or vertical directions, bad pixels and other equipment induced artefacts.Artefacts are also caused by dirt or debris in the tube port or on the underside of the breast support plate or grid.Some examples of artefacts associated with digital mammography systems are illustrated in Section 5.7 and inAppendix III.(1) There should be no visible dead pixels, missing lines or missing columns of data at a level that could interferewith the detection of anatomical structures or could mimic structures that do not actually exist in the breast.(2) There should be no visually distracting structured noise patterns in a uniform phantom image.(3) There should be no regions of discernibly different density on an unprocessed image of a uniform phantom.103