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2.3.3.2. Type 2: Flat plate CsI with photodiode arrayIn these systems, a CsI(Tl) phosphor layer is deposited directly onto a large area matrix of photodiodesformed on a flat plate amorphous silicon (a-Si, or α-Si) substrate (Fig. 2). Each light sensitive diode element isconnected by a thin film transistor (TFT) switch to a <strong>series</strong> of control lines and data lines such that the chargeproduced on the diode in response to light emission from the phosphor is read out and can be digitized. In suchsystems, the initial signal forming the DICOM ‘for processing’ image is linear with the amount of energy absorbedby the phosphor (subsequent non-linear transformations are usually performed on the images; see Section 2.3.7).2.3.3.3. Type 3: Flat plate amorphous selenium with electrode arrayThis system does not employ a phosphor. Instead, X rays are absorbed in a layer of amorphous selenium(a-Se, or α-Se), which is deposited on an array of electrodes formed on a large area a-Si substrate (Fig. 3). Anelectric field is imposed across the plate to collect the electron–hole pairs liberated upon X ray absorption. Thecharges drift to the electrode pads and are collected there. During the readout procedure, TFT switches on eachdetector element (del) are sequentially activated, one row at a time via control lines, and the charge is collectedalong data lines (running between columns of dels) connecting each del to readout electronics similar to those in aType 2 system [28]. An extension of this a-Se detector technology is a recent development using optical switchingtechnology rather than TFT to read the image signal.2.3.3.4. Type 4: Slot scanning photon counting detectorIn this system, the energy of absorbed X rays is converted to charge in a set of many single-line detectorsbased on depleted crystalline silicon or on high pressure gas ionization strips. The charge arising from theabsorption of an individual X ray photon is collected to form a pulse, which is counted to register that X ray.Individual linear detector arrays are arranged adjacently or spaced apart, and the assembly is scanned in a directionorthogonal to the detector lines to acquire the image (Fig. 4).X raysdel matrixCsI scintillatorLine driverPhotodiodeAmplifier, multiplexerand ADCContactsExploded viewof del elementshowing switchFIG. 2. Indirect flat plate detector based on a CsI scintillator with amorphous silicon (a-Si) switching diodes or thin film transistorreadout. The X rays absorbed in the CsI layer are first converted to light, which is then converted to a charge signal by the photodiodesand ultimately digitized. ADC — analogue to digital converter.10
X raysTop electrodea-Se layerChargecollectionelectrodeSignal outGatepulseCharge storagecapacitorTFTFIG. 3. Direct flat plate detector utilizing amorphous selenium (a-Se) as the X ray absorber. When a voltage is applied across the a-Selayer, the charges produced are collected by the electrodes and digitized.Front viewX ray tubeSide viewPre-collimatorX ray fanbeamCompressionplateGantryMechanical linkPre-collimatorBreastPost-collimatorSi strip detectorsPost-collimatorSi strip detectorsFIG. 4. Sectra MicroDose multi-slit scanning unit. Narrow slit collimators define fan beams that image part of the breast.Post-collimators further reduce the impact of scatter. The multi-slit device moves across the breast, ensuring that all breast tissue isimaged. The crystal-Si detector elements are also unique in that they collect and record the energy from individual X ray quanta.2.3.3.5. General comments about the equipment typesOne important consideration when purchasing equipment is the rate at which images can be acquired. Somesystems may require an extensive detector preparation cycle between images. The required interval is likely tochange as technology evolves, so it is important, especially in high volume facilities, to establish from the vendorwhat the inter-image time is.System performance, in terms of spatial resolution, starts with the effective size of the del and the spacingbetween dels, or ‘pitch’. The effective size or aperture can be smaller than the pitch if part of the del is insensitive11
Page 1 and 2: X raysdel matrixCs scintillatorLine
Page 3 and 4: QUALITY ASSURANCE PROGRAMMEFOR DIGI
Page 5 and 6: IAEA HUMAN HEALTH SERIES No. 17QUAL
Page 7 and 8: FOREWORDThe application of radiatio
Page 14: An effective QA programme is necess
Page 17 and 18: The Integrating the Healthcare Ente
Page 21: technology. Systems containing othe
Page 25 and 26: dose received by inhomogeneous real
Page 27 and 28: 2.3.8.1. Soft copy displayFlat pane
Page 29 and 30: 3. ELEMENTS OF HIGH QUALITY MAMMOGR
Page 31: — The ability to mask edges of ma
Page 34 and 35: Disposal orsaleNew facility(start h
Page 36 and 37: 4.2.3. Radiographer (mammography te
Page 38 and 39: 5.2.2. Electronic versus geometric
Page 40 and 41: To allow full utilization of the di
Page 42 and 43: 5.7. ARTEFACTSWhile the incidence o
Page 44 and 45: FIG. 10. Detector crystallization.
Page 46 and 47: FIG. 13. A more subtle example of a
Page 48 and 49: FIG. 16. An example of poor collima
Page 51 and 52: 7. RADIOGRAPHER’S QUALITY CONTROL
Page 53 and 54: 7.1. DAILY TESTS7.1.1. Monitor insp
Page 55 and 56: 7.1.3. Daily flat field phantom ima
Page 57 and 58: 7.1.4. Visual inspection for artefa
Page 59 and 60: 7.1.5. Laser printer sensitometry7.
Page 61 and 62: 7.1.6. Image plate erasure (CR syst
Page 63 and 64: FIG. 19. Modified TG18-QC pattern w
Page 65 and 66: FIG. 21. Radiologist workstation wi
Page 67 and 68: 7.2.1.6. Time frame for corrective
Page 69 and 70: 7.2.3. Weekly quality control test
Page 71 and 72: Note: It is not necessary to view t
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TABLE 7. TOLERANCES FOR IMAGING WEE
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7.2.4.4. Interpretation of results
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7.3.1.5. Recommendations and correc
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artefacts are typically best seen w
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7.3.3.5. Recommendations and correc
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7.4.1.3. Methodology(1) Annotate th
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RECORD OF DIGITAL MAMMOGRAPHY REPEA
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(3) Including examinations of at le
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7.5. SEMI-ANNUAL TESTS7.5.1. Comput
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8. MEDICAL PHYSICIST’S QUALITY CO
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8.1. NOTE ON IMAGE NAMING CONVENTIO
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(16) On any randomly selected patie
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FIG. 30. Positioning of bathroom sc
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(6) The target, filter, kV, grid, d
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(2) Stack the 20 and 25 mm PMMA sla
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TABLE 13. ACCEPTABLE AND ACHIEVABLE
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8.5. DETECTOR PERFORMANCE8.5.1. Bas
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8.5.2. Detector response and noise8
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the service engineer responsible fo
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(13) Using the annotation tool on t
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Breast supportPMMA slabBreast suppo
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(10) Place the thinner test object
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8.6. EVALUATION OF SYSTEM RESOLUTIO
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TABLE 17. ACCEPTABLE FREQUENCIES AT
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8.7. X RAY EQUIPMENT CHARACTERISTIC
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8.7.2. Incident air kerma at the en
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8.8. DOSIMETRY8.8.1. Mean glandular
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TABLE 21. ACCEPTABLE AND ACHIEVABLE
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FIG. 34. Suggested layout for ruler
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8.10. IMAGE DISPLAY QUALITYThe accu
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FIG. 35. Modified TG18-QC test patt
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8.10.2. Monitor luminance response
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TABLE 22. MONITOR PERFORMANCE TOLER
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FIG. 38. Measurement of illuminatio
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8.11. LASER PRINTER8.11.1. Laser pr
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8.11.1.5. Recommendations and corre
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iopsies are performed, a sink is of
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Appendix IISPECIFICATIONS OF TEST E
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TABLE 24. SPECIFICATIONS OF TEST EQ
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FIG. 40. Image acquired with a unif
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FIG. 43. The left hand image is par
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FIG. 45. A TG18-QC test pattern dis
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[29] ALSAGER, A., YOUNG, K.C., ODUK
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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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