iaea human health series publications - SEDIM

7.3.2. Full field artefacts7.3.2.1. ScopeNote that this test is similar to the artefact evaluation component of the daily tests described in Section 7.1.3,and the methodology described there is largely reproduced in this section for completeness. However, on a monthlybasis, the test is extended to include all applicable focal spots, filters and magnification modes, but the tests ofMPV and SDNR are not required.— Objective: To ensure that there are no artefacts in the images that might mimic structures in the breast or thatmight increase the difficulty of interpretation.— Minimum frequency: Monthly, and whenever changes have been made to the digital mammography systemthat might affect the flat field performance.7.3.2.2. InstrumentationUniformly thick slab of PMMA. Typically this should be 45 mm thick; however, a slab of another thicknessthat has been supplied by the medical physics team or the manufacturer for flat field correction would also beacceptable. The slab should be free of scratches or other imperfections that would cause artefacts. It shouldpreferably cover the entire area of the image receptor (see Fig. 17). The same test object should be used insubsequent monthly tests.7.3.2.3. Methodology(1) Place the test object on the breast support, centred laterally and extending slightly beyond the chest wall edgeof the digital image receptor.(2) Apply a compression force typically used clinically (e.g. 80 N) 8 . Record the compression force used and thethickness indicated at the top of the full field artefacts chart (Chart 10 in Annex I).(3) If there is a separate AEC sensor, confirm that it is under the test object. Keep this in the same position everytime the test is performed.(4) Acquire an image of the test object using the same technique settings that would be used for a clinicalexposure of a standard breast. Normally this is achieved by using the automatic exposure mode. Otherwise,select the appropriate target, filter, kV, grid, density control position and operation mode (semiautomatic orautomatic). The DICOM ‘for processing’ version of the image should be used for this test (seeSection 2.3.7.1). For all tests of CR systems, the systems should be set to the modes indicated in Table 6.(5) Record the technique used to acquire the image on the full field artefacts chart (Chart 10 in Annex I). For CRsystems, the EI should be recorded.(6) If patient images are interpreted in soft copy, and if suitable analysis tools are available, view the ‘forprocessing’’ image on the acquisition workstation. Otherwise, view the image on the radiologist displayworkstation. Use the window width and window level recommended by the medical physicist. At thesesettings, the background of the phantom is displayed with a mid-grey. The window level should not beunreasonably narrow, as this can overstate noise or irrelevant artefacts. The same window width and windowlevel settings (±10) should be used each time an image is evaluated. An appropriate choice of window widthis critical for catching artefacts yet not ‘failing’ good images. Window width choice should be based on whatis appropriate for typical breast images or a phantom with breast-like features.(7) With the same window width and window level as used above, evaluate the entire image for overall appearanceand for artefacts. Examine the entire image for both broad area artefacts such as non-uniformities, blotches andstreaks, and for detailed artefacts such as black or white pixels, clusters of pixels, lines or specks. Broad area8The actual force should be similar to the typical value used clinically, but the same value should be used for all testing. Notethat in some systems and in some modes of operation, the compressed breast thickness is utilized in an automated algorithm todetermine the technique factors; this thickness is, in turn, dependent on the degree of compression applied.66