1、Designation: E 1441 00 (Reapproved 2005)Standard Guide forComputed Tomography (CT) Imaging1This standard is issued under the fixed designation E 1441; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A num
2、ber in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 Computed tomography (CT) is a radiographic methodthat provides an ideal examination technique whenever theprimary goal is to locate and s
3、ize planar and volumetric detailin three dimensions. Because of the relatively good penetra-bility of X-rays, as well as the sensitivity of absorption crosssections to atomic chemistry, CT permits the nondestructivephysical and, to a limited extent, chemical characterization ofthe internal structure
4、 of materials. Also, since the method isX-ray based, it applies equally well to metallic and non-metallic specimens, solid and fibrous materials, and smoothand irregularly surfaced objects. When used in conjunctionwith other nondestructive evaluation (NDE) methods, such asultrasound, CT data can pro
5、vide evaluations of material integ-rity that cannot currently be provided nondestructively by anyother means.1.2 This guide is intended to satisfy two general needs forusers of industrial CT equipment: (1) the need for a tutorialguide addressing the general principles of X-ray CT as theyapply to ind
6、ustrial imaging; and (2) the need for a consistent setof CT performance parameter definitions, including how theseperformance parameters relate to CT system specifications.Potential users and buyers, as well as experienced CT inspec-tors, will find this guide a useful source of information fordeterm
7、ining the suitability of CT for particular examinationproblems, for predicting CT system performance in newsituations, and for developing and prescribing new scan pro-cedures.1.3 This guide does not specify test objects and test proce-dures for comparing the relative performance of different CTsyste
8、ms; nor does it treat CT inspection techniques, such as thebest selection of scan parameters, the preferred implementationof scan procedures, the analysis of image data to extractdensitometric information, or the establishment of accept/rejectcriteria for a new object.1.4 Standard practices and meth
9、ods are not within thepurview of this guide. The reader is advised, however, thatexamination practices are generally part and application spe-cific, and industrial CT usage is new enough that in manyinstances a consensus has not yet emerged. The situation iscomplicated further by the fact that CT sy
10、stem hardware andperformance capabilities are still undergoing significant evo-lution and improvement. Consequently, an attempt to addressgeneric examination procedures is eschewed in favor ofproviding a thorough treatment of the principles by whichexamination methods can be developed or existing on
11、esrevised.1.5 The principal advantage of CT is that it nondestructivelyprovides quantitative densitometric (that is, density and geom-etry) images of thin cross sections through an object. Becauseof the absence of structural noise from detail outside the thinplane of inspection, images are much easi
12、er to interpret thanconventional radiographic data. The new user can learn quickly(often upon first exposure to the technology) to read CT databecause the images correspond more closely to the way thehuman mind visualizes three-dimensional structures than con-ventional projection radiography. Furthe
13、r, because CT imagesare digital, they may be enhanced, analyzed, compressed,archived, input as data into performance calculations, com-pared with digital data from other NDE modalities, or trans-mitted to other locations for remote viewing. Additionally, CTimages exhibit enhanced contrast discrimina
14、tion over compactareas larger than 20 to 25 pixels. This capability has noclassical analog. Contrast discrimination of better than 0.1 % atthree-sigma confidence levels over areas as small as one-fifthof one percent the size of the object of interest are common.1.6 With proper calibration, dimension
15、al inspections andabsolute density determinations can also be made very accu-rately. Dimensionally, virtually all CT systems provide a pixelresolution of roughly 1 part in 1000 (since, at present,1024 3 1024 images are the norm), and metrological algo-rithms can often measure dimensions to one-tenth
16、 of one pixelor so with three-sigma accuracies. For small objects (less than4 in. in diameter), this translates into accuracies of approxi-mately 0.1 mm 0.003 to 0.005 in. at three-sigma. For muchlarger objects, the corresponding figure will be proportionallygreater. Attenuation values can also be r
17、elated accurately tomaterial densities. If details in the image are known to be purehomogeneous elements, the density values may still be suffi-cient to identify materials in some cases. For the case in whichno a priori information is available, CT densities cannot be1This guide is under the jurisdi
18、ction of ASTM Committee E07 on Nondestruc-tive Testing and is the direct responsibility of Subcommittee E07.01 on Radiology(X and Gamma) Method.Current edition approved Dec. 1, 2005. Published February 2006. Originallyapproved in 1991. Last previous edition approved in 2000 as E 1441 - 00.1Copyright
19、 ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.used to identify unknown materials unambiguously, since aninfinite spectrum of compounds can be envisioned that willyield any given observed attenuation. In this instance, theexceptional density
20、sensitivity of CT can still be used todetermine part morphology and highlight structural irregulari-ties.1.7 In some cases, dual energy (DE) CT scans can helpidentify unknown components. DE scans provide accurateelectron density and atomic number images, providing bettercharacterizations of the mate
21、rials. In the case of knownmaterials, the additional information can be traded for im-proved conspicuity, faster scans, or improved characterization.In the case of unknown materials, the additional informationoften allows educated guesses on the probable composition ofan object to be made.1.8 As wit
22、h any modality, CT has its limitations. The mostfundamental is that candidate objects for examination must besmall enough to be accommodated by the handling system ofthe CT equipment available to the user and radiometricallytranslucent at the X-ray energies employed by that particularsystem. Further
23、, CT reconstruction algorithms require that afull 180 degrees of data be collected by the scanner. Object sizeor opacity limits the amount of data that can be taken in someinstances. While there are methods to compensate for incom-plete data which produce diagnostically useful images, theresultant i
24、mages are necessarily inferior to images from com-plete data sets. For this reason, complete data sets andradiometric transparency should be thought of as requirements.Current CT technology can accommodate attenuation ranges(peak-to-lowest-signal ratio) of approximately four orders ofmagnitude. This
25、 information, in conjunction with an estimateof the worst-case chord through a new object and a knowledgeof the average energy of the X-ray flux, can be used to make aneducated guess on the feasibility of scanning a part that has notbeen examined previously.1.9 Another potential drawback with CT ima
26、ging is thepossibility of artifacts in the data. As used here, an artifact isanything in the image that does not accurately reflect truestructure in the part being inspected. Because they are not real,artifacts limit the users ability to quantitatively extract density,dimensional, or other data from
27、 an image. Therefore, as withany technique, the user must learn to recognize and be able todiscount common artifacts subjectively. Some image artifactscan be reduced or eliminated with CT by improved engineeringpractice; others are inherent in the methodology. Examples ofthe former include scattered
28、 radiation and electronic noise.Examples of the latter include edge streaks and partial volumeeffects. Some artifacts are a little of both. A good example isthe cupping artifact, which is due as much to radiation scatter(which can in principle be largely eliminated) as to thepolychromaticity of the
29、X-ray flux (which is inherent in the useof bremsstrahlung sources).1.10 Because CT scan times are typically on the order ofminutes per image, complete three-dimensional CT examina-tions can be time consuming. Thus, less than 100 % CTexaminations are often necessary or must be accommodated bycompleme
30、nting the inspection process with digital radio-graphic screening. One partial response to this problem is touse large slice thicknesses. This leads to reduced axial resolu-tion and can introduce partial volume artifacts in some cases;however, this is an acceptable tradeoff in many instances. Inprin
31、ciple, this drawback can be eliminated by resorting to fullvolumetric scans. However, since CT is to a large extenttechnology driven, volumetric CT systems are currently lim-ited in the size of object that can be examined and the contrastof features that can be discriminated.1.11 Complete part exami
32、nations demand large storagecapabilities or advanced display techniques, or both, andequipment to help the operator review the huge volume of datagenerated. This can be compensated for by state-of-the-artgraphics hardware and automatic examination software to aidthe user. However, automated accept/r
33、eject software is objectdependent and to date has been developed and employed inonly a limited number of cases.1.12 The values stated in SI units are to be regarded as thestandard. The values given in brackets are provided forinformation only.1.13 This standard does not purport to address all of the
34、safety 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 determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E 1316 Terminology for Nonde
35、structive TestingE 1570 Practice for Computed Tomographic (CT) Exami-nation3. Terminology3.1 DefinitionsCT, being a radiographic modality, usesmuch the same vocabulary as other X-ray techniques. Anumber of terms are not referenced, or are referenced withoutdiscussion, in Terminology E 1316. Because
36、they have mean-ings or carry implications unique to CT, they appear withexplanation in Appendix X1. Throughout this guide, the term“X-ray” is used to denote penetrating electromagnetic radia-tion; however, electromagnetic radiation may be either X-raysor gamma rays.3.2 Acronyms:Acronyms:3.2.1 BWbeam
37、 width.3.2.2 CDDcontrast-detail-dose.3.2.3 CTcomputed tomography.3.2.4 CATcomputerized axial tomography.3.2.5 DRdigital radiography.3.2.6 ERFedge response function.3.2.7 LSFline spread function.3.2.8 MTFmodulation transfer function.3.2.9 NDEnondestructive evaluation.3.2.10 PDFprobability distributio
38、n function.3.2.11 PSFpoint spread function.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.E 1441 00 (2005)24
39、. Summary of Guide4.1 This guide provides a tutorial introduction to the tech-nology and terminology of CT. It deals extensively with thephysical and mathematical basis of CT, discusses the basichardware configuration of all CT systems, defines a compre-hensive set of fundamental CT performance para
40、meters, andpresents a useful method of characterizing and predictingsystem performance. Also, extensive descriptions of terms andreferences to publications relevant to the subject are provided.4.2 This guide is divided into three main sections. Sections5 and 6 provide an overview of CT: defining the
41、 process,discussing the performance characteristics of CT systems, anddescribing the basic elements of all CT systems. Section 8addresses the physical and mathematical basis of CT imaging.Section 8 addresses in more detail a number of importantperformance parameters as well as their characterization
42、 andverification. This section is more technical than the othersections, but it is probably the most important of all. Itestablishes a single, unified set of performance definitions andrelates them to more basic system parameters with a fewcarefully selected mathematical formulae.5. Significance and
43、 Use5.1 This guide provides a tutorial introduction to the theoryand use of computed tomography. This guide begins with aoverview intended for the interested reader with a generaltechnical background. Subsequent, more technical sectionsdescribe the physical and mathematical basis of CTtechnology,the
44、 hardware and software requirements of CT equipment, andthe fundamental measures of CT performance. This guideincludes an extensive glossary (with discussion) of CT termi-nology and an extensive list of references to more technicalpublications on the subject. Most importantly, this guideestablishes
45、consensus definitions for basic measures of CTperformance, enabling purchasers and suppliers of CT systemsand services to communicate unambiguously with reference toa recognized standard. This guide also provides a few carefullyselected equations relating measures of CT performance to keysystem para
46、meters.5.2 General Description of Computed TomographyCT isa radiographic inspection method that uses a computer toreconstruct an image of a cross-sectional plane (slice) throughan object. The resulting cross-sectional image is a quantitativemap of the linear X-ray attenuation coefficient, , at each
47、pointin the plane. The linear attenuation coefficient characterizes thelocal instantaneous rate at which X-rays are removed duringthe scan, by scatter or absorption, from the incident radiation asit propagates through the object (See 7.5). The attenuation ofthe X-rays as they interact with matter is
48、 a well-studiedproblem (1)3and is the result of several different interactionmechanisms. For industrial CT systems with peak X-rayenergy below a few MeV, all but a few minor effects can beaccounted for in terms of the sum of just two interactions:photoelectric absorption and Compton scattering (1).
49、Thephotoelectric interaction is strongly dependent on the atomicnumber and density of the absorbing medium; the Comptonscattering is predominantly a function of the electron density ofthe material. Photoelectric attenuation dominates at lowerenergies and becomes more important with higher atomicnumber, while Compton scattering dominates at higher ener-gies and becomes more important at lower atomic number. Inspecial situations, these dependencies can be used to advantage(see 7.6.2 and references therein).5.2.1 One particularly important property of the total linearatte
