1、Designation: E1570 11Standard Practice forComputed Tomographic (CT) Examination1This standard is issued under the fixed designation E1570; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in paren
2、theses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope*1.1 This practice is for computed tomography (CT), whichmay be used to n
3、ondestructively disclose physical features oranomalies within 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
4、paths through the test object from many differentdirections. 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 E1441.1.3 CT is broadly applicable to a
5、ny material or test objectthrough which a beam of penetrating 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
6、This practice describes procedures for performing CTexaminations. 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 o
7、f this standard to establish appro-priate safety and health 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 Standa
8、rds:2E1316 Terminology for Nondestructive ExaminationsE1441 Guide for Computed Tomography (CT) ImagingE1695 Test Method for Measurement of Computed Tomog-raphy (CT) System Performance2.2 NIST Standard:ANSI N43.3 General Radiation Safety Installations UsingNon-Medical X-Ray and Sealed Gamma Sources u
9、p to 10MeV32.3 Federal Standards:421 CFR 1020.40 Safety Requirements of Cabinet X RaySystems29 CFR 1910.96 Ionizing Radiation2.4 ASNT Documents:5SNT-TC-1A Recommended Practice for Personnel Qualifi-cation and Certification in Nondestructive TestingANSI/ASNT-CP-189 Qualification and Certification ofN
10、ondestructive Testing Personnel2.5 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,refe
11、r to Terminology E1316 and Annex A1 in Guide E1441.4. 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
12、. Section 7 describes the following subsystems:4.2.1 Source of penetrating radiation,4.2.2 Radiation detector or an array of detectors,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
13、Gamma) Method.Current edition approved July 1, 2011. Published July 2011. Originally approvedin 1993. Last previous edition approved in 2005 as E1570 - 00(2005)1. DOI:10.1520/E1570-11.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm
14、.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), 100Bureau Dr., Stop 1070, Gaithersburg, MD 20899-1070, http:/www.nist.gov.4Available from Standardization
15、Documents Order Desk, DODSSP, Bldg. 4,Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http:/dodssp.daps.dla.mil.5Available fromAmerican Society for Nondestructive Testing (ASNT), P.O. Box28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http:/www.asnt.org.6Available from Aerospace Indust
16、ries Association of America, Inc. (AIA), 1000Wilson Blvd., Suite 1700,Arlington, VA22209-3928, http:/www.aia-aerospace.org.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United Sta
17、tes.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,4.2.4.3 Data storage system, and4.2.4.4 Operator interface.4.3 Section 8 describes and defines the procedures forestablishing and maintainin
18、g 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. Operating parameters should strike an overallbalance between image q
19、uality, 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 measurement of CT system perfor-mance are provided in Section 10 of t
20、his 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 prescribe operating procedures for new orroutine test objects.5.2 Th
21、is 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 provides the approach for performing CTexaminations. Supplemental inf
22、ormation covering specificitems where agreement between supplier7and purchaser8arenecessary is required. Generally the items are applicationspecific or performance related, or both. Examples include:system configuration, equipment qualification, performancemeasurement, and interpretation of results.
23、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 requirement, through a carefulanalysis of the benefits and limita
24、tions 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, X-ray generators(that is, X-ray tubes and linear accelerators)
25、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 polychromaticity of theenergy spectrum from an X-ray source causes a
26、rtifacts 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 few millimetres down toa few micrometres. Reducing the focal spot
27、 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 physical size, portability, low power requirements, sim-plicity
28、, 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 SR sources can employ essen-tially monochromatic radiation. With
29、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 employed, SR systems canin general image only small (about 1 mm
30、) objects.7.3 Radiation Detection SystemsThe detection system is atransducer that converts the transmitted radiation containinginformation about the test object into an electronic signalsuitable for processing. The detection system may consist of asingle sensing element, a linear array of sensing el
31、ements, or anarea array of sensing elements. The more detectors used, thefaster the required scan data can be collected; but there areimportant tradeoffs to be considered.7.3.1 Asingle detector provides the least efficient method ofcollecting data but entails minimal complexity, eliminatesdetector c
32、ross 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 typically accommodates good colli-mation a
33、nd 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 detectorefficiency, and creates seriou
34、s 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 motiongeometries are most common: tran
35、slate-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 test7As used within this document, the supplier of computed tomographic s
36、ervicerefers to the entity 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 com
37、puted tomographic services. The purchasermay be a part of the same organization as the supplier, or an outside organization.E1570 112object between translations by the fan angle of the beam andagain translating the object until a minimum of 180 degrees ofdata have been acquired. The advantage of thi
38、s design issimplicity, good view-to-view detector matching, flexibility inthe 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 longer scantime.7.4.2 With rotate-only motion, a co
39、mplete 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 scan speed is important.7.4.3 In volume CT, a c
40、omplete data set for the entire part isacquired in at least one rotation. This allows for much fasterdata acquisition, as the data required for multiple slices can beacquired in one rotation.7.5 Computer SystemCT requires substantial computa-tional resources, such as a large capacity for image stora
41、ge andarchival and the ability to efficiently perform numerous math-ematical computations, especially for the back-projection op-eration. Computational speed can be augmented by eithergeneralized array processors or specialized back-projectionhardware. The particular implementations will change asco
42、mputer hardware evolves, but high computational power willremain a fundamental requirement for efficient CT examina-tion. A separate workstation for image analysis and displayoften is appropriate.7.6 Image Reconstruction Software The aim of CT is toobtain information regarding the nature of material
43、 occupyingexact positions inside a test object. In current CT scanners, thisinformation is obtained by “reconstructing” individual cross-sections of the test object from the measured intensity of X-raybeams transmitted through that cross section. An exact math-ematical theory of image reconstruction
44、 exists for idealizeddata. This theory is applied although the physical measure-ments do not fully meet the requirements of the theory. Whenapplied to actual measurements, algorithms based on thistheory produce images with blurring and noise, the extent ofwhich depends on the quantity and quality of
45、 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, they are planes), (2) both the source focalspot and the detector elements are infinitely small (that is, theyare points), (3) the physic
46、al measurements correspond to totalattenuation along the line between the source and detector, and(4) the radiation is, or can be treated as, effectively monoen-ergetic. A reconstruction algorithm is a collection of step-by-step instructions that define how to convert the measurementsof total attenu
47、ation 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 evolved. They can be broadlygrouped into three classes of algorithms: matrix inversionmethods, finite series-expansion methods, and transf
48、orm meth-ods. See Guide E1441 for treatment of reconstruction algo-rithms.7.6.3 If the test object is larger than the prescribed field ofview (FOV), either by necessity or by accident, unexpected andunpredictable artifacts or a measurable degradation of imagequality can result.7.7 Image DisplayThe f
49、unction of the image display is toconvey derived information (that is, an image) of the test objectto the system operator. For manual evaluation systems, thedisplayed 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 colordisplays. These units can be switched between color andgray-scale presentation to suit the preference of t
