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(4) There should be no unexpected variation in apparent texture or magnitude of the noise across the uniformimage 21 . If the place to place variation changes over time, areas where the noise appears to be less mayindicate degradation in the detector causing a loss of sharpness. Increases in noise may indicate other detectorproblems. Document such problems by measuring the local SNR ((pixel value – offset)/standard deviation) inROIs of about 100 mm 2 or less located in an area of ‘normal’ noise level and compare these with the area(s)where noise levels are observed to have changed.(5) For CR, bear in mind that the heel effect is not compensated for by the CR reader’s processing algorithms, sosome non-uniformity due to the X ray beam is to be expected.8.5.5.5. Recommendations and corrective actions(1) If dead pixels or other unacceptable artefacts are noted, or if significant non-uniformity is present, the serviceperson should be contacted to investigate and correct the problem.(2) CR systems have no flat fielding and will require the service engineer to investigate unacceptable artefacts ina manner similar to that for screen film imaging.8.5.5.6. Time frame for corrective actionFor severe artefacts, corrective action should be taken immediately, before any further patients are imaged.For minor artefacts, corrective action should be taken at the next routine servicing of equipment.21Note that variations in signal level (especially in CR systems) and noise due to the heel effect are to be expected.104
8.6. EVALUATION OF SYSTEM RESOLUTION8.6.1. Modulation transfer functionIn digital mammography the major system related physical factor affecting spatial resolution is the signalblurring within the detector and the integration of the signal over the area of the del to form a single reading. Thiscan be assessed by calculating the MTF from the spread of the signal in the image of a sharp, high contrast edge. Ifa square object is imaged, the edges in both the horizontal and the vertical directions can be measured, and anevaluation can be performed for both signal rising and signal falling conditions. Slanting the edge slightly allowsdetermination of the ‘pre-sampled’ MTF, that is, the MTF that would exist before sampling to form the digitalimage [51–54]. The method presented here is a field measurement of ‘effective system resolution at the top surfaceof the breast’ rather than a rigorous laboratory test. This incorporates the effects due to the focal spot as well asthose due to detector characteristics.8.6.1.1. Scope— Objective: To determine the MTF associated with the detector and focal spot of the system.— Minimum frequency: Annually and after changes or service to the detector, tube or CR plate reader.8.6.1.2. Instrumentation(1) MTF test tool. A square of metal foil with very straight edges, 20–50 mm on a side. The test object may bemade of a variety of materials such as copper (70 μm thick), stainless steel [55], brass backing with tungstenor lead [56], or niobium (20–30 μm thick) [57]. Ideally, whatever the material used, the thickness of this foilwill provide measurable X ray transmission at mammographic energies so that a reliable signal is obtainedbeneath the foil.(2) A 45 mm thick slab of PMMA to support the MTF test tool (Fig. 33). Alternatively, the square can bepermanently mounted on a larger (100 mm × 100 mm) 1.5–2 mm thick aluminium plate, angled slightly withrespect to the edge of the sheet.(3) MTF software (see: http://humanhealth.iaea.org).8.6.1.3. Methodology(1) Place the MTF tool 45 mm above the breast support table such that the square is angled slightly (2–5°) withrespect to the chest wall edge of the breast support table. Use of a pre-mounted square achieves thisautomatically.(2) Make an exposure using manual factors similar to those used for a clinical image of the average breast.Choose factors such that there are no pixel values in the uniform regions of the phantom or MTF square thatreach the maximum pixel value for the system or that fall to the minimum value.(3) Download the ‘for processing’ image onto the computer on which the MTF software is available. Run thesoftware according to the provider’s directions to calculate the MTF from the ‘for processing’ image. It isdesirable to obtain MTFs on both the rising edge and the falling edge of the edge spread data.(4) Record the spatial frequencies at which the MTF has fallen to 50 and 20% in the x and y directions on theMTF–resolution chart (Chart 9 in Annex II). Also record the MTF value at 2.5, 5 and 7.5 cycles/mm on thatsheet.105
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X raysdel matrixCs scintillatorLine
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QUALITY ASSURANCE PROGRAMMEFOR DIGI
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IAEA HUMAN HEALTH SERIES No. 17QUAL
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FOREWORDThe application of radiatio
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An effective QA programme is necess
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The Integrating the Healthcare Ente
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technology. Systems containing othe
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X raysTop electrodea-Se layerCharge
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dose received by inhomogeneous real
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2.3.8.1. Soft copy displayFlat pane
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3. ELEMENTS OF HIGH QUALITY MAMMOGR
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— The ability to mask edges of ma
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Disposal orsaleNew facility(start h
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4.2.3. Radiographer (mammography te
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5.2.2. Electronic versus geometric
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To allow full utilization of the di
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5.7. ARTEFACTSWhile the incidence o
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FIG. 10. Detector crystallization.
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FIG. 13. A more subtle example of a
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FIG. 16. An example of poor collima
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7. RADIOGRAPHER’S QUALITY CONTROL
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7.1. DAILY TESTS7.1.1. Monitor insp
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7.1.3. Daily flat field phantom ima
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7.1.4. Visual inspection for artefa
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7.1.5. Laser printer sensitometry7.
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7.1.6. Image plate erasure (CR syst
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FIG. 19. Modified TG18-QC pattern w
Page 65 and 66: FIG. 21. Radiologist workstation wi
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Page 69 and 70: 7.2.3. Weekly quality control test
Page 71 and 72: Note: It is not necessary to view t
Page 73 and 74: TABLE 7. TOLERANCES FOR IMAGING WEE
Page 75 and 76: 7.2.4.4. Interpretation of results
Page 77 and 78: 7.3.1.5. Recommendations and correc
Page 79 and 80: artefacts are typically best seen w
Page 81 and 82: 7.3.3.5. Recommendations and correc
Page 83 and 84: 7.4.1.3. Methodology(1) Annotate th
Page 85 and 86: RECORD OF DIGITAL MAMMOGRAPHY REPEA
Page 87 and 88: (3) Including examinations of at le
Page 89 and 90: 7.5. SEMI-ANNUAL TESTS7.5.1. Comput
Page 91 and 92: 8. MEDICAL PHYSICIST’S QUALITY CO
Page 93 and 94: 8.1. NOTE ON IMAGE NAMING CONVENTIO
Page 95 and 96: (16) On any randomly selected patie
Page 97 and 98: FIG. 30. Positioning of bathroom sc
Page 99 and 100: (6) The target, filter, kV, grid, d
Page 101 and 102: (2) Stack the 20 and 25 mm PMMA sla
Page 103 and 104: TABLE 13. ACCEPTABLE AND ACHIEVABLE
Page 105 and 106: 8.5. DETECTOR PERFORMANCE8.5.1. Bas
Page 107 and 108: 8.5.2. Detector response and noise8
Page 109 and 110: the service engineer responsible fo
Page 111 and 112: (13) Using the annotation tool on t
Page 113 and 114: Breast supportPMMA slabBreast suppo
Page 115: (10) Place the thinner test object
Page 119 and 120: TABLE 17. ACCEPTABLE FREQUENCIES AT
Page 121 and 122: 8.7. X RAY EQUIPMENT CHARACTERISTIC
Page 123 and 124: 8.7.2. Incident air kerma at the en
Page 125 and 126: 8.8. DOSIMETRY8.8.1. Mean glandular
Page 127 and 128: TABLE 21. ACCEPTABLE AND ACHIEVABLE
Page 129 and 130: FIG. 34. Suggested layout for ruler
Page 131 and 132: 8.10. IMAGE DISPLAY QUALITYThe accu
Page 133 and 134: FIG. 35. Modified TG18-QC test patt
Page 135 and 136: 8.10.1.5. Interpretation of results
Page 137 and 138: 8.10.2. Monitor luminance response
Page 139 and 140: TABLE 22. MONITOR PERFORMANCE TOLER
Page 141 and 142: FIG. 38. Measurement of illuminatio
Page 143 and 144: 8.11. LASER PRINTER8.11.1. Laser pr
Page 145 and 146: 8.11.1.5. Recommendations and corre
Page 147: 8.12.1.4. Interpretation of results
Page 150 and 151: iopsies are performed, a sink is of
Page 152 and 153: Appendix IISPECIFICATIONS OF TEST E
Page 154 and 155: TABLE 24. SPECIFICATIONS OF TEST EQ
Page 156 and 157: FIG. 40. Image acquired with a unif
Page 158 and 159: FIG. 43. The left hand image is par
Page 160 and 161: FIG. 45. A TG18-QC test pattern dis
Page 162 and 163: [29] ALSAGER, A., YOUNG, K.C., ODUK
Page 165 and 166: Annex IRADIOGRAPHER DATA COLLECTION
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Chart 2 — DIGITAL MAMMOGRAPHY (DM
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Chart 4 LASER PRINTER SENSITOMETRY
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Chart 6 — WEEKLY DISPLAY MONITOR
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Chart 7(b) WEEKLY QC TEST OBJECT:
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Chart 9 — SAFETY AND FUNCTION CHE
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Chart 11 — LASER PRINTER ARTEFACT
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Chart 13(a) — RECORD OF DIGITAL M
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Chart 14 — CR PLATE SENSITIVITY M
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Annex IIITHE MAMMOGRAPHY EXTRACTThe
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GLOSSARYair kerma. The energy depos
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image quality. The overall merit of
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CONTRIBUTORS TO DRAFTING AND REVIEW
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