1、Designation: E 1570 00 (Reapproved 2005)e1Standard Practice forComputed Tomographic (CT) Examination1This standard is issued under the fixed designation E 1570; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revis
2、ion. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice is for computed tomography (CT), whichmay be used to nondestructively disclose physical features oranomalies with
3、in a test object by providing radiological densityand geometric measurements. This practice implicitly assumesthe use of penetrating radiation, specifically X-ray and g-ray.1.2 CT systems utilize a set of transmission measurementsmade along paths through the test object from many differentdirections
4、. Each of the transmission measurements is digitizedand stored in a computer, where they are subsequently recon-structed by one of a variety of techniques. A treatment of CTprinciples is given in Guide E 1441.1.3 CT is broadly applicable to any material or test objectthrough which a beam of penetrat
5、ing radiation passes. Theprincipal advantage of CT is that it provides densitometric (thatis, radiological density and geometry) images of thin crosssections through an object without the structural superpositionin projection radiography.1.4 This practice describes procedures for performing CTexamin
6、ations. This practice is to address the general use of CTtechnology and thereby facilitate its use.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 healt
7、h practices and determine the applica-bility of regulatory limitations prior to use. For specific safetystatements, see Section 8, NBS Handbook 114, and FederalStandards 21 CFR 1020.40 and 29 CFR 1910.96.2. Referenced Documents2.1 ASTM Standards:2E 1316 Terminology for Nondestructive TestingE 1441 G
8、uide for Computed Tomography (CT) ImagingE 1695 Test Method for Measurement of Computed Tomog-raphy (CT) System Performance2.2 NIST Standard:NBS Handbook 114 General Safety Standard for Installa-tions. Using Non-Medical X-Ray and Sealed Gamma-RaySources, Energies Up to 10 MeV32.3 Federal Standards:2
9、1 CFR 1020.40 Safety Requirements of Cabinet X RaySystems429 CFR 1910.96 Ionizing Radiation42.4 ASNT Documents:5SNT-TC-1A Recommended Practice for Personnel Qualifi-cation and Certification in Nondestructive TestingANSI/ASNT-CP-189 Qualification and Certification ofNondestructive Testing Personnel2.
10、5 Military Standard:MIL-STD-410 Nondestructive Testing Personnel Qualifica-tion and Certification42.6 AIA Standard:NAS-410 Certification and Qualification of NondestructiveTesting Personnel63. Terminology3.1 DefinitionsFor definitions of terms used in this guide,refer to Terminology E 1316 and Annex
11、 A1 in Guide E 1441.4. Summary of Practice4.1 Requirements in this practice are intended to control thereliability and quality of the CT images.4.2 CT systems are made up of a number of subsystems; thefunction served by each subsystem is common in almost all CTscanners. Section 7 describes the follo
12、wing subsystems:4.2.1 Source of penetrating radiation,4.2.2 Radiation detector or an array of detectors,4.2.3 Mechanical scanning assembly, and4.2.4 Computer system including:4.2.4.1 Image reconstruction software/hardware,4.2.4.2 Image display/analysis system,1This practice is under the jurisdiction
13、 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 Dec. 1, 2005. Published February 2006. Originallyapproved in 1993. Last previous edition approved in 2000 as E 1570 - 00.2For referenced
14、 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 National Institute of Standards and Technology (NIST), 100Bu
15、reau Dr., Stop 3460, Gaithersburg, MD 20899-3460.4Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098.5Available from American Society for Nondestructive Testing, 1711 ArlingatePlaza, P.O. Box 28518, Columbus, OH 43228-0518.6A
16、vailable from Aerospace Industries Association of America, Inc. (AIA), 1250Eye St., NW, Washington, DC 20005.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4.2.4.3 Data storage system, and4.2.4.4 Operator interface.4.3 Section 8 des
17、cribes and defines the procedures forestablishing and maintaining quality control of CT services.4.4 The extent to which a CT image reproduces an object ora feature within an object is influenced by spatial resolution,statistical noise, slice plane thickness, and artifacts of theimaging system. Oper
18、ating parameters should strike an overallbalance between image quality, inspection time, and cost.These parameters should be considered for CT system configu-rations, components, and procedures. The setting and optimi-zation of CT system parameters is discussed in Section 9.4.5 Methods for the measu
19、rement of CT system perfor-mance are provided in Section 10 of this practice.5. Significance and Use5.1 This practice is applicable for the systematic assessmentof the internal structure of a material or assembly using CTtechnology. This practice may be used for review by systemoperators, or to pres
20、cribe operating procedures for new orroutine test objects.5.2 This practice provides the basis for the formation of aprogram for quality control and its continuation throughcalibration, standardization, reference samples, inspectionplans, and procedures.6. Basis of Application6.1 This practice provi
21、des the approach for performing CTexaminations. Supplemental information covering specificitems where agreement between supplier7and purchaser8arenecessary is required. Generally the items are applicationspecific or performance related, or both. Examples include:system configuration, equipment quali
22、fication, performancemeasurement, and interpretation of results.7. System Configuration7.1 Many different CT system configurations are possibleand it is important to understand the advantages and limitationsof each. It is important that the optimum system parameters beselected for each examination r
23、equirement, through a carefulanalysis of the benefits and limitations of the available systemcomponents and the chosen system configuration.7.2 Radiation SourcesWhile the CT systems may utilizeeither gamma-ray or X-ray generators, the latter is used formost applications. For a given focal spot size,
24、 X-ray generators(that is, X-ray tubes and linear accelerators) are several ordersof magnitude more intense than isotope sources. Most X-raygenerators are adjustable in peak energy and intensity and havethe added safety feature of discontinued radiation productionwhen switched off; however, the poly
25、chromaticity of theenergy spectrum from an X-ray source causes artifacts such asbeam hardening (the anomalous decreasing attenuation towardthe center of a homogeneous object) in the image if uncor-rected.7.2.1 X-rays produced from electrical radiation generatorshave focal spot sizes ranging from a f
26、ew millimetres down toa few micrometres. Reducing the focal spot size reducesgeometric unsharpness, thereby enhancing detail sensitivity.Smaller focal spots permit higher spatial resolution, but at theexpense of reduced X-ray beam intensity.7.2.2 A radioisotope source can have the advantages ofsmall
27、 physical size, portability, low power requirements, sim-plicity, and stability of output. The disadvantages are limitedintensity and limited peak energy.7.2.3 Synchrotron Radiation (SR) sources produce veryintense, naturally collimated, narrow bandwidth, tunable radia-tion. Thus, CT systems using S
28、R sources can employ essen-tially monochromatic radiation. With present technology, how-ever, practical SR energies are restricted to less thanapproximately 20 to 30 keV. Since any CT system is limited tothe inspection of samples with radio-opacities consistent withthe penetrating power of the X-ray
29、 employed, SR systems canin general image only small (about 1 mm) objects.7.3 The detection system is a transducer that converts thetransmitted radiation containing information about the testobject into an electronic signal suitable for processing. Thedetection system may consist of a single sensing
30、 element, alinear array of sensing elements, or an area array of sensingelements. The more detectors used, the faster the required scandata can be collected; but there are important tradeoffs to beconsidered.7.3.1 Asingle detector provides the least efficient method ofcollecting data but entails min
31、imal complexity, eliminatesdetector cross talk and detector matching, and allows anarbitrary degree of collimation and shielding to be imple-mented.7.3.2 Linear arrays have reasonable scan times at moderatecomplexity, acceptable cross talk and detector matching, and aflexible architecture that typic
32、ally accommodates good colli-mation and shielding. Most commercially available CT sys-tems employ a linear array of detectors.7.3.3 An area detector provides a fast method of collectingdata but entails the transfer and storage of large amounts ofinformation, forces tradeoffs between cross talk and d
33、etectorefficiency, and creates serious collimation and shielding chal-lenges.7.4 Manipulation SystemThe manipulation system has thefunction of holding the test object and providing the necessaryrange of motions to position the test object between theradiation source and detector. Two types of scan m
34、otiongeometries are most common: translate-rotate motion androtate-only motion.7.4.1 With translate-rotate motion, the object is translated ina direction perpendicular to the direction and in the plane of theX-ray beam. Full data sets are obtained by rotating the testobject between translations by t
35、he fan angle of the beam andagain translating the object until a minimum of 180 degrees ofdata have been acquired. The advantage of this design issimplicity, good view-to-view detector matching, flexibility in7As used within this document, the supplier of computed tomographic servicerefers to the en
36、tity that physically provides the computed tomographic services. Thesupplier may be a part of the same organization as the purchaser, or an outsideorganization.8As used within this document, the purchaser of computed tomographic servicesrefer to the entity that requires the computed tomographic serv
37、ices. The purchasermay be a part of the same organization as the supplier, or an outside organization.E 1570 00 (2005)e12the choice of scan parameters, and ability to accommodate awide range of different object sizes including objects too big tobe subtended by the X-ray fan. The disadvantage is long
38、er scantime.7.4.2 With rotate-only motion, a complete view is collectedby the detector array during each sampling interval. A rotate-only scan has lower motion penalty than a translate-rotate scanand is attractive for industrial applications where the part to beexamined fits within the fan beam and
39、scan speed is important.7.5 Computer SystemCT requires substantial computa-tional resources, such as a large capacity for image storage andarchival and the ability to efficiently perform numerous math-ematical computations, especially for the back-projection op-eration. Computational speed can be au
40、gmented by eithergeneralized array processors or specialized back-projectionhardware. The particular implementations will change ascomputer hardware evolves, but high computational power willremain a fundamental requirement for efficient CT examina-tion. A separate workstation for image analysis and
41、 displayoften is appropriate.7.6 Image Reconstruction Software The aim of CT is toobtain information regarding the nature of material occupyingexact positions inside a test object. In current CT scanners, thisinformation is obtained by “reconstructing” individual cross-sections of the test object fr
42、om the measured intensity of X-raybeams transmitted through that cross section. An exact math-ematical theory of image reconstruction exists for idealizeddata. This theory is applied although the physical measure-ments do not fully meet the requirements of the theory. Whenapplied to actual measureme
43、nts, algorithms based on thistheory produce images with blurring and noise, the extent ofwhich depends on the quantity and quality of the measure-ments.7.6.1 The simplifying assumptions made in setting up thetheory of reconstruction algorithms are: (1) cross sections areinfinitely thin (that is, the
44、y are planes), (2) both the source focalspot and the detector elements are infinitely small (that is, theyare points), (3) the physical measurements correspond to totalattenuation along the line between the source and detector, and(4) the radiation is, or can be treated as, effectively monoen-ergeti
45、c. A reconstruction algorithm is a collection of step-by-step instructions that define how to convert the measurementsof total attenuation to a map of linear attenuation coefficientsover the field of view.7.6.2 Anumber of methods for recovering an estimate of thecross section of an object have evolv
46、ed. They can be broadlygrouped into three classes of algorithms: matrix inversionmethods, finite series-expansion methods, and transform meth-ods. See Guide E 1441 for treatment of reconstruction algo-rithms.7.6.3 If the test object is larger than the prescribed field ofview (FOV), either by necessi
47、ty or by accident, unexpected andunpredictable artifacts or a measurable degradation of imagequality can result.7.7 Image DisplayThe function of the image display is toconvey derived information (that is, an image) of the test objectto the system operator. For manual evaluation systems, thedisplayed
48、 image is used as the basis for accepting or rejectingthe test object, subject to the operators interpretation of the CTdata.7.7.1 Generally, CT image display requires a special graph-ics monitor; television image presentation is of lower qualitybut may be acceptable. Most industrial systems utilize
49、 colordisplays. These units can be switched between color andgray-scale presentation to suit the preference of the viewer, butit should be noted that gray-scale images presented on a colormonitor are not as sharp as those on a gray-scale monitor. Theuse of color permits the viewer to distinguish a greater range ofvariations in an image than gray-scale does. Depending on theapplication, this may be an advantage or a disadvantage.Sharply contrasting colors may introduce false, distinct defini-tion between boundaries. While at times advantageous, un-wanted instances can be correcte
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