1、Designation: E2446 05 (Reapproved 2010)Standard Practice forClassification of Computed Radiology Systems1This 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 re
2、vision. 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 evaluation and classificationof a computed radiography (CR) system, a particular phosphorimaging
3、plate (IP), system scanner and software, in combina-tion with specified metal screens for industrial radiography. Itis intended to ensure that the evaluation of image quality, as faras this is influenced by the scanner/IP system, meets the needsof users.1.2 The practice defines system tests to be us
4、ed to classifythe systems of different suppliers and make them comparablefor users.1.3 The CR system performance is described by signal andnoise parameters. For film systems, the signal is represented bygradient and the noise by granularity. The signal-to-noise ratiois normalized by the basic spatia
5、l resolution of the system andis part of classification. The normalization is given by thescanning aperture of 100 m diameter for the micro-photometer, which is defined in Test Method E1815 for filmsystem classification. This practice describes how the param-eters shall be measured for CR systems.1.
6、4 The values stated in SI are to be regarded as thestandard.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and to determine theapplica
7、bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E1316 Terminology for Nondestructive ExaminationsE1815 Test Method for Classification of Film Systems forIndustrial RadiographyE2002 Practice for Determining Total Image Unsharpnessin RadiologyE2007 Guide for Co
8、mputed RadiographyE2033 Practice for Computed Radiology (PhotostimulableLuminescence Method)E2445 Practice for Qualification and Long-Term Stabilityof Computed Radiology Systems3. Terminology3.1 DefinitionsThe definition of terms relating to gamma-and X-radiology, which appear in Terminology E1316,
9、GuideE2007, and Practice E2033, shall apply to the terms used in thispractice.3.2 Definitions of Terms Specific to This Standard:3.2.1 computed radiology system (CR system)A completesystem of a storage phosphor imaging plate (IP), a correspond-ing read out unit (scanner or reader) and software, whic
10、hconverts the information of the IP into a digital image (see alsoGuide E2007).3.2.2 computed radiology system classA particular groupof storage phosphor imaging plate systems, which is charac-terized by a SNR (signal-to-noise ratio) range shown in Table1 and by a certain unsharpness range in a spec
11、ified exposurerange.3.2.3 ISO speed SIPxDefines the speed of a CR system andis calculated from the reciprocal dose value, measured in gray,which is necessary to obtain a specified minimum SNR of a CRsystem.3.2.4 signal-to-noise ratio (SNR)Quotient of mean valueof the linearized signal intensity and
12、standard deviation of thenoise (intensity distribution) at this signal intensity. The SNRdepends on the radiation dose and the CR system properties.3.2.5 modulation transfer function (MTF)The normalizedmagnitude of the Fourier-transform (FT) of the differentiatededge spread function (ESF) of the lin
13、earized PSL (photostimulated luminescence) intensity, measured perpendicular toa sharp edge. MTF describes the contrast transmission as afunction of the object size. In this practice, the MTF charac-terizes the unsharpness of the CR system. This depends on thescanning system itself and IP-type and c
14、assette employed.3.2.6 gain/amplificationOpto-electrical gain setting of thescanning system.1This practice is under the 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
15、 1, 2010. Published August 2010 Originallyapproved in 2005. Last previous edition approved in 2005 as E2446 05. DOI:10.1520/E2446-05R10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume in
16、formation, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.7 linearized signal intensitya numerical signal valueof a picture element (pixel) of the digital image, wh
17、ich isproportional to the radiation dose. The linearized signal inten-sity is zero, if the radiation dose is zero.3.2.8 basic spatial resolutionthe read-out value of un-sharpness measured with duplex wire IQI in accordance withPractice E2002 divided by 2 as effective pixel size of the CRsystem.4. Si
18、gnificance and Use4.1 There are several factors affecting the quality of a CRimage including the spatial resolution of the IP system,geometrical unsharpness, scatter and contrast sensitivity(signal-to-noise ratio), as well as software. There are severaladditional factors (for example, scanning param
19、eters), whichaffect the accurate reading of images on exposed IPs using anoptical scanner.4.2 This practice is to be used to establish a classification ofCR system classes on the basis of a normalized SNR. Due tothe difference between the methods, it is required to specify theCR system classes with
20、spatial resolution values. The CRsystem classes in this document do not refer to any particularmanufacturers imaging plates. A CR system class results fromthe use of a particular imaging plate together with the exposureconditions, particularly total exposure, the scanner type andsoftware and the sca
21、nning parameters. This classificationsystem provides a means to compare differing CR technolo-gies, as is common practice with film systems, which guidesthe user to the appropriate configuration, IP and technique forthe application at hand. The class selected may not match theimaging performance of
22、a corresponding film class due to thedifference in the spatial resolution and scatter sensitivity.Therefore, the practice should always use IQIs for proof ofcontrast sensitivity and spatial resolution.4.3 The quality factors can be determined most accuratelyby the tests described in this practice. S
23、ome of the system testsrequire special tools, which may not be available in userlaboratories. Simpler tests are described for quality assurancein Practice E2445, which are designed for a fast test of thequality of CR systems and long-term stability and are recom-mended as practical user tests, shoul
24、d the user not have thespecial tools available as needed for the tests in this practice.4.4 Manufacturers of industrial CR systems will use thispractice. Users of industrial CR systems may also perform thetests and measurements outlined in this practice, provided thatthe required test equipment is u
25、sed and the methodology isstrictly followed. Any alternative methods may be applied ifequivalence to the methods of this practice is proven to theappropriate Cognizant Engineering Organization.4.5 The publication of CR system classes will enablespecifying bodies and contracting parties to agree to p
26、articularsystem class, as a first step in arriving at the appropriatesettings of a system, or the selection of a system. Confirmationof necessary image quality shall be achieved by using PracticeE2033.5. Apparatus5.1 CR system evaluation depends on the combined prop-erties of the phosphor imaging pl
27、ate (IP) type, the scanner andsoftware used, and the selected scan parameters. Therefore,documentation for each test shall include the IP type, scanner,software and scan parameters, and the results shall be calcu-lated and tabulated prior to arriving at a class assignment. Theapplied test equipment
28、for SNR measurement (Fig. 1) andTABLE 1 CR System ClassificationCR SystemClassificationMinimumSignal-Noise RatioASTM IP Special/Y 130ASTM IP I/Y 65ASTM IP II/Y 52ASTM IP III/Y 43FIG. 1 Scheme of Experimental Arrangement for the Step Exposure MethodE2446 05 (2010)2algorithm 6.1.1 correspond to Test M
29、ethod E1815. The recom-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.5.2 The step wedge method (
30、Fig. 2) describes a simplerprocedure for SNR measurement than described in TestMethod E1815, which permits obtaining similar results withless expense, and less accuracy.6. Procedure for quantitative measurement of imagequality parameters6.1 Measurement of the Normalized Signal-to-Noise Ratio(SNR)6.1
31、.1 Step Exposure MethodFor measurement of theSNR, the following steps are taken (see also Test MethodE1815):6.1.1.1 The IP, with front and back lead screens of 0.1 mm(0.004 in.) thickness in the typical exposure cassette, shall bepositioned in front of an X-ray tube with tungsten anode. Makethe expo
32、sures with an 8 mm (0.32 in.) copper filter at the X-raytube and the kilovoltage set such that the half value layer incopper is 3.5 mm (0.14 in.). The kilovoltage setting will beapproximately 220 kV.6.1.1.2 Determine the required exact kilovoltage setting bymaking an exposure (or an exposure rate) m
33、easurement withthe detector placed at a distance of at least 750 mm (29.5 in.)from the tube target and an 8 mm (0.32 in.) copper filter at thetube. Then make a second measurement with a total of 11.5mm (0.45 in.) of copper at the tube. These filters should bemade of 99.9 % pure copper.6.1.1.3 Calcul
34、ate the ratio of the first and second readings. Ifthis ratio is not 2, 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 sensitive layer of the IP shall face the X-ra
35、ysource. For gamma radiography with Ir-192, the measurementsshall be carried out with 0.3 mm lead screens in front andbehind the IP.Also 8 mm Cu shall be used for pre-filtering (seeFig. 1).6.1.1.5 The scanner shall read with a dynamic range of$12bit and operate at its highest spatial resolution or a
36、 spatialresolution for which the classification shall be carried out.Background and anti-shading correction may be used beforethe analysis of data, if it relates to the standard measurementprocedure for all measurements. The procedure shall be carriedout and documented for all sensitivity and latitu
37、de ranges andall read-out pixel sizes if any of these parameters change theSNR-analysis.6.1.1.6 IPs are exposed in a similar way to film radiographyand under the conditions described: signal and noise (sPSL)orSNR over dose curve shall be measured. It is especiallyimportant that the exposure of the I
38、P for the SNR measure-ments be spatially uniform. Any nonuniformities in X-raytransmission of the cassette front, or defects in the Pb foil or inthe phosphor itself could influence the SNR measurement. Nomajor scratches or dust shall be visible in the measurementarea. Therefore, exercise considerabl
39、e care in selection andplacement of the aperture, and selection and maintenance ofthe cassette, the lead screens and the phosphor screen. Toachieve a uniform region of interest on to the IP, the followingstandard protocol is recommended. Other approaches may beused as long as a uniform exposure is c
40、reated. 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 all possible pixel sizes, if the resultschange. The digital read-out i
41、ntensity values (gray values) shallbe calibrated in such a way, that they are linear in relation to theradiation dose, which corresponds to the photo stimulatedluminescence (PSL) intensity of the exposed IPs. These cali-brated gray values shall be used for the calculation of the SNR.In order to get
42、a reliable result at least six measurements shallbe made on different samples, and the results are to beaveraged for each of the twelve or more dose levels measured.FIG. 2 Scheme for the Measurement of the SNR by the Step Wedge MethodE2446 05 (2010)36.1.1.7 The signal (intensity Imeas) and noise (st
43、andarddeviation sPSL) shall be computed from a region withoutshading or artifacts. Sample SNR values shall be taken indifferent regions of the image area under test to ensure thatSNR values are within 10 % stable. The size of the ROI usedto measure the mean signal and noise shall be at least 20 by 5
44、5pixels and it should be an area ROI. An example technique forassuring reliable signal-to-noise measurements is describedbelow. This can be achieved using a commonly available imageprocessing tool. The signal and noise shall be calculated froma data set of 1100 values or more per exposed area. Theun
45、filtered data set is subdivided into 55 groups or more with 20values per group. For each group with index i, the value Imeas_iis calculated as the mean of the unfiltered group values and thevalue sPSLiis calculated from the same group values. Anincreased number of groups yields a better (lower) unce
46、rtaintyof the result. Due to the filtering effect of this groupingprocedure, the sPSLi-values shall be corrected by the followingequation:sPSLi_corr5 1.0179 sPSLi(1)NOTE 1The values sPSLiare multiplied with 1.0179 to correct for thefollowing median unbiased estimation. Assume k is the number ofconse
47、cutive observations within a group and C is the critical value of thechi-square distribution for a = 0.5 with k-1 degrees of freedom. In case of20 observations the values sPSLishall be multiplied with 1.0179 forstatistical correction (see also ISO/DIS 10505). The factor 1.0179 corre-sponds to the co
48、rrection sqrt (k-1)/c) of ISO/WD 10505 for grouping witha group size of 20 elements (k = 20) for application of a median procedure(c = 18.33765).6.1.1.8 The final value Imeasis obtained by the median of allImeas_ivalues. The final sPSLvalue is obtained by the medianof all sPSLi_corrvalues. sPSLshall
49、 be calculated as referencevalue to a resolution of 100 m, measured with a circularaperture, or 88.6 m measured with a squared aperture. Thefinal value sPSL100is calculated bysPSL1005sPSL SRmax/88.6! (2)SRmax= Maximum value of basic spatial resolution in mas measured in 6.3.NOTE 2Test Method E1815 requires the use of a micro-photodensitometer with circular aperture of 100 m* diameter for the measure-ment of granularity sD. Because the pixels in digital images are organizedin squares, the corresponding pixel size is calculated bysqrt (100 m*)2p / 4) = 88.6
