1、Designation: E2446 16Standard Practice forManufacturing Characterization of Computed RadiographySystems1This standard is issued under the fixed designation E2446; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last rev
2、ision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice describes the manufacturing characteriza-tion of computed radiography (CR) systems, consisting of aparticular ph
3、osphor imaging plate (IP), scanner, software, andan image display monitor, in combination with specified metalscreens for industrial radiography.1.2 The practice defines system tests to be used to charac-terize the systems of different suppliers and make themcomparable for users.1.3 This practice is
4、 intended for use by manufacturers of CRsystems or certification agencies to provide quantitative resultsof CR system characteristics for nondestructive testing (NDT)user or purchaser consumption. Some of these tests requirespecialized test phantoms to ensure consistency of resultsamong suppliers or
5、 manufacturers. These tests are not intendedfor users to complete, nor are they intended for long termstability tracking and lifetime measurements. However, theymay be used for this purpose, if so desired.1.4 The CR system performance is described by the basicspatial resolution, contrast, signal and
6、 noise parameters, andthe equivalent penetrameter sensitivity (EPS). Some of theseparameters are used to compare with DDAcharacterization andfilm characterization data (see Practice E2597 and Test MethodE1815).NOTE 1For film system characterization, the signal is represented bythe optical density of
7、 2 (above fog and base) and the noise as granularity.The signal-to-noise ratio is normalized by the aperture (similar to the basicspatial resolution) of the system and is part of characterization. Thisnormalization is given by the scanning circular aperture of 100 m of themicro-photometer, which is
8、defined in Test Method E1815 for film systemcharacterization.1.5 The measurement of CR systems in this practice isrestricted to a selected radiation quality to simplify the proce-dure. The properties of CR systems will change with radiationenergy but not the ranking of CR system performance. Users o
9、fthis practice may carry out the tests at different or additionalradiation qualities (X-ray or gamma ray) if required.1.6 The values stated in SI are to be regarded as thestandard.1.7 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is therespo
10、nsibility of the user of this standard to establish appro-priate safety and health practices and to determine theapplicability of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E746 Practice for Determining Relative Image Quality Re-sponse of Industrial Radiographic I
11、maging SystemsE1165 Test Method for Measurement of Focal Spots ofIndustrial X-Ray Tubes by Pinhole ImagingE1316 Terminology for Nondestructive ExaminationsE1815 Test Method for Classification of Film Systems forIndustrial RadiographyE2002 Practice for Determining Total Image Unsharpnessand Basic Spa
12、tial Resolution in Radiography and Radios-copyE2007 Guide for Computed RadiographyE2033 Practice for Computed Radiology (PhotostimulableLuminescence Method)E2445 Practice for Performance Evaluation and Long-TermStability of Computed Radiography SystemsE2597 Practice for Manufacturing Characterizatio
13、n of Digi-tal Detector ArraysE2903 Test Method for Measurement of the Effective FocalSpot Size of Mini and Micro Focus X-ray Tubes2.2 ISO Standard:3ISO 17636-2 Non-Destructive Testing of WeldsRadiographic TestingPart 2: X- and Gamma Ray Tech-nologies with Digital Detectors1This practice is under the
14、 jurisdiction of ASTM Committee E07 on Nonde-structive Testing and is the direct responsibility of Subcommittee E07.01 onRadiology (X and Gamma) Method.Current edition approved June 1, 2016. Published June 2016. Originallyapproved in 2005. Last previous edition approved in 2015 as E2446 15. DOI:10.1
15、520/E2446-16.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from International Organization for S
16、tandardization (ISO), 1, ch. dela Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http:/www.iso.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13. Terminology3.1 DefinitionsThe definition of terms relating to gamma-and X-radio
17、graphy, which appear in Terminology E1316,Guide E2007, and Practice E2033, shall apply to the terms usedin this practice.3.2 Definitions of Terms Specific to This Standard:3.2.1 computed radiography system (CR system)A com-plete system of a storage phosphor imaging plate (IP), acorresponding read ou
18、t unit (scanner or reader), software andan image display monitor, which converts the information ofthe IP into a digital image (see also Guide E2007).3.2.2 computed radiography system performance levelAparticular group of CR performance levels, which is charac-terized by a SNRN(signal-to-noise ratio
19、) range, an interpolatedbasic spatial resolution range iSRbdetectorand equivalent pen-etrameter sensitivity (EPS) shown in Table 4 in a specifiedexposure range.3.2.3 gain/amplificationOpto-electrical gain setting of thescanning system.3.2.4 ISO speed SIPxDefines the speed of a CR system andis calcul
20、ated from the reciprocal dose value, measured in Gray(Gy), which is necessary to obtain a specified minimum SNRNof a CR system performance level.3.2.5 linearized signal intensitya numerical signal valueof a picture element (pixel) of the digital image, which isproportional to the radiation dose. The
21、 linearized signal inten-sity is zero, if the radiation dose is zero.4. Significance and Use4.1 There are several factors affecting the quality of a CRimage including the basic spatial resolution of the IP system,geometrical unsharpness, scatter and contrast sensitivity. Thereare several additional
22、factors (for example, software andscanning parameters) that affect the accurate reading of imageson exposed IPs using an optical scanner.4.2 This practice is to be used to establish a characterizationof CR system by performance levels on the basis of anormalized SNR, interpolated basic spatial detec
23、tor resolutionand EPS. The CR system performance levels in this practice donot refer to any particular manufacturersimaging plates.ACRsystem performance level results from the use of a particularimaging plate together with the exposure conditions, standard-ized phantom, the scanner type, and softwar
24、e and the scanningparameters. This characterization system provides a means tocompare differing CR technologies, as is common practicewith film systems, which guides the user to the appropriateconfiguration, IP, and technique for the application at hand.The performance level selected may not match t
25、he imagingperformance of a corresponding film class because of thedifference in the spatial resolution and scatter sensitivity.Therefore, the user should always use IQIs for proof of contrastsensitivity and basic spatial resolution.4.3 The measured performance parameters are presented ina characteri
26、zation chart. This enables users to select specificCR systems by the different characterization data to find thebest system for his specific application.4.4 The quality factors can be determined most accuratelyby the tests described in this practice. Some of the system testsrequire special tools, wh
27、ich may not be available in userlaboratories. Simpler tests are described for quality assuranceand long term stability tests in Practice E2445.4.5 Manufacturers of industrial CR systems or certificationagencies will use this practice. Users of industrial CR systemsmay use Practice E2445 or perform s
28、ome of the described testsand measurements outlined in this practice, provided that therequired test equipment is used and the methodology is strictlyfollowed. Any alternative methods or radiation qualities maybe applied if equivalence to the methods of this practice isproven to the appropriate cogn
29、izant engineering organization.4.6 The publication of CR system performance levels willenable specifying bodies and contracting parties to agree toparticular system performance level, as a first step in arrivingat the appropriate settings of a system, or the selection of asystem. Confirmation of nec
30、essary image quality shall beachieved by using Practice E2033.5. Apparatus5.1 CR system evaluation depends on the combined prop-erties of the phosphor imaging plate (IP) type, the scanner andsoftware used, and the selected scan parameters and imagedisplay monitor. Therefore, documentation for each t
31、est shallinclude the IP type, scanner, software, scan parameters, andimage display monitor, and the results shall be calculated andtabulated before arriving at a performance assignment. Theapplied test equipment for SNR measurement (Fig. 1) andalgorithm 6.1.1 correspond to Test Method E1815. The rec
32、om-mended thickness for aperture test object (diaphragm) is10.2 mm (0.4 in.) of Pb. The SDD shall be at least 1 m (39 in.).Do not use any material (for example, lead) behind the cassetteand leave a free space of at least 1 m (39 in.) behind thecassette or use a steel screen of about 0.5 mm (0.02 in.
33、) and alead plate of2mm(0.08 in.) just behind the cassette (steelscreen is positioned between cassette and lead) and in contactwith the cassette.5.2 The step wedge method (Fig. 2) describes a simplerprocedure for SNR measurement than described in TestMethod E1815, which permits obtaining similar res
34、ults withless expense, and less accuracy.6. Procedure for Quantitative Measurement of ImageQuality Parameters6.1 Measurement of the Normalized Signal-to-Noise Ratio(SNRN)6.1.1 Step Exposure MethodFor measurement of the SNR,the following steps are taken (see also Test Method E1815):6.1.1.1 The IP sha
35、ll be positioned in front of an X-ray tubewith tungsten anode. Make the exposures with an 8 mm(0.32 in.) copper filter at the X-ray tube and the kilovoltage setsuch that the half value layer in copper is 3.5 mm (0.14 in.).The kilovoltage setting will be approximately 220 kV. Metalscreens can be used
36、 in the cassette if the manufacturerrecommends its application. The focal spot size is not relevantfor SNR measurements.E2446 1626.1.1.2 Determine the required exact kilovoltage setting bymaking an exposure (or an exposure rate) measurement withthe detector placed at a distance of at least 750 mm (2
37、9.5 in.)from the tube target and an 8 mm (0.32 in.) copper filter at thetube. Then make a second measurement with a total of11.5 mm (0.45 in.) of copper at the tube. These filters should bemade of 99.9 % pure copper.6.1.1.3 Calculate the ratio of the first and second readings. Ifthis ratio is not 2,
38、 adjust the kilovoltage up or down and repeatthe measurements until a ratio of 2 (within 5 %) is obtained.Record the setting of kilovoltage for use with the further IPtests.6.1.1.4 The scanner shall read with a dynamic range of 12 bit and operate at its highest spatial resolution or a basicspatial r
39、esolution for which the characterization shall be carriedout. Background and anti-shading correction may be usedbefore the analysis of data, if it relates to the standardmeasurement procedure for all measurements.6.1.1.5 The procedure shall be carried out and documentedfor one or more agreed sets of
40、 scanner parameters per imagingplate type. It is recommended to use the standard parameters ofthe CR scanner as given by the manufacturer and the parameterset for the highest resolution.6.1.1.6 IPs are exposed under the conditions describedabove: A signal (S) and noise () or the quotient, the signal
41、 tonoise ratio (SNR vs. dose and pixel value curve shall bemeasuredsee Fig. 3 and Fig. 4). It is important that theexposure of the IP for the SNR measurements be spatiallyuniform. Any non-uniformities in X-ray transmission of thecassette front, or defects in a front metal screen or in thephosphor la
42、yer itself could influence the SNR measurement.No major scratches or dust shall be visible in the measurementarea. Therefore, exercise considerable care in selection andplacement of the aperture, selection, and maintenance of thecassette, the metal screens (if any), and the imaging plate. Toachieve
43、a uniform area of interest on to the IP, the followingFIG. 1 Scheme of Experimental Arrangement for the Step Exposure MethodFIG. 2 Scheme for the Measurement of the SNR by the Step Wedge MethodE2446 163standard protocol is recommended. Other approaches may beused as long as a uniform exposure is cre
44、ated. At least twelveareas (test areas) of 400 mm2(0.62 in.2) are evenly exposedon the same IP over the full working range of dose. Due to thedifferent construction principles of scanners, the measurementshall be performed for different pixel sizes as recommended bythe manufacturer. A waiting time o
45、f 15 minutes is recom-mended between exposure and scan of the IPs to avoiddistortions by fading effects. Typically the characterization isperformed for selected parameter sets only if agreed by themanufacturer and the certifying laboratory. The digital read-outpixel values shall be calibrated in suc
46、h a way that they arelinear in relation to the radiation dose, which corresponds tothe photo stimulated luminescence (PSL) intensity of theexposed IPs. These calibrated pixel values shall be used for thecalculation of the SNR to obtain a reliable result. Measure-ments shall be made on at least six d
47、ifferent samples, and theresults are to be averaged for each of the twelve or more doselevels measured.6.1.1.7 The signal (S) and noise (standard deviation )ofthe measured pixel values shall be calculated from a region ofinterest (RoI) without shading or artifacts. Sample SNR valuesshall be taken in
48、 different regions of the image area under testto ensure that SNR values are within 10 %. The size of the RoIused to measure the mean signal and noise shall be at least 40by 200 pixels.6.1.1.8 An example technique for ensuring reliable signal-to-noise measurements is described in the following. This
49、 canbe achieved using a commonly available image processingtool. The signal and noise shall be calculated from a data set of8000 values or more per exposed area. The unfiltered data setis subdivided into 200 groups or more with 40 values per groupFIG. 3 Scheme for Measurement of SNR in the RoI with Pixel Values PVijNOTE 1The tested CR system qualifies for:Level I performance from PV 350 4095 (see Table 4)Level II performance from PV 140 4095 (see Table 4)PVmax= 4095, as determined with procedure of 6.4FIG. 4 Example Plot of Measured SNRNVersus PV (12 bit sys
