1、Designation: E1441 11Standard Guide forComputed Tomography (CT) Imaging1This standard is issued under the fixed designation E1441; 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 parentheses i
2、ndicates 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 Computed tomography (CT) is a radiographic methodthat provides an ideal exam
3、ination technique whenever theprimary goal is to locate and size 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 limit
4、ed extent, chemical characterization ofthe internal structure 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
5、 evaluation (NDE) methods, such asultrasound, CT data can provide 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 addr
6、essing the general principles of X-ray CT as theyapply to industrial 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,
7、will find this guide a useful source of information fordetermining 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-dure
8、s for comparing the relative performance of different CTsystems; 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/re
9、jectcriteria for a new object.1.4 Standard practices and methods 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 emerge
10、d. The situation iscomplicated further by the fact that CT system 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 principle
11、s by whichexamination methods can be developed or existing onesrevised.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 det
12、ail outside the thinplane of inspection, images are much easier 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
13、 structures than con-ventional projection radiography. Further, 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
14、. Additionally, CTimages exhibit enhanced contrast discrimination 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
15、of interest are common.1.6 With proper calibration, dimensional 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 , and metrologicalalgorithms can often measure dimensions to o
16、ne-tenth of onepixel or so with three-sigma accuracies. For small objects (lessthan 100 mm (4 in.) in diameter), this translates into accuraciesof approximately 0.1 mm (0.003 to 0.005 in.) at three-sigma.For much larger objects, the corresponding figure will beproportionally greater. Attenuation val
17、ues can also be relatedaccurately to material densities. If details in the image areknown to be pure homogeneous elements, the density valuesmay still be sufficient to identify materials in some cases. Forthe case in which no a priori information is available, CT1This guide is under the jurisdiction
18、 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 July 1, 2011. Published July 2011. Originally approvedin 1991. Last previous edition approved in 2005 as E1441 - 00(2005). DOI:10.1520/E
19、1441-11.1*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 States.densities cannot be used to identify unknown materials unam-biguously, since an infinite spectrum of compounds
20、can beenvisioned that will yield any given observed attenuation. Inthis instance, the exceptional density sensitivity of CT can stillbe used to determine part morphology and highlight structuralirregularities.1.7 In some cases, dual energy (DE) CT scans can helpidentify unknown components. DE scans
21、provide accurateelectron density and atomic number images, providing bettercharacterizations of the materials. 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 additiona
22、l informationoften allows educated guesses on the probable composition ofan object to be made.1.8 As with 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
23、the user and radiometricallytranslucent at the X-ray energies employed by that particularsystem. Further, 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 a
24、re methods to compensate for incom-plete data which produce diagnostically useful images, theresultant images 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 ac
25、commodate attenuation ranges(peak-to-lowest-signal ratio) of approximately four orders ofmagnitude. This 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 feasib
26、ility of scanning a part that has notbeen examined previously.1.9 Another potential drawback with CT imaging 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
27、real,artifacts limit the users ability to quantitatively extract density,dimensional, or other data from 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 impro
28、ved engineeringpractice; others are inherent in the methodology. Examples ofthe former include scattered 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 m
29、uch to radiation scatter(which can in principle be largely eliminated) as to thepolychromaticity of the X-ray flux (which is inherent in the useof bremsstrahlung sources).1.10 Depending on the technology of the CT system,complete three-dimensional CT examinations can be timeconsuming. Thus, less tha
30、n 100 % CT examinations are oftennecessary or must be accommodated by complementing theinspection process with digital radiographic screening. Onepartial response to this problem is to use large slice thicknesses.This leads to reduced axial resolution and can introduce partialvolume artifacts in som
31、e cases; however, this is an acceptabletradeoff in many instances. In principle, this drawback can beeliminated by resorting to full volumetric scans using planardetectors instead of linear detectors (see (1) under 6.5.1.5).1.11 Complete part examinations demand large storagecapabilities or advanced
32、 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/reject software is objectdependent and to date has be
33、en developed and employed inonly a limited number of cases.1.12 UnitsThe values stated in SI units are to be regardedas standard. The values given in parentheses are mathematicalconversions to inch-pound units that are provided for informa-tion only and are not considered standard.1.13 This standard
34、 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 determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM
35、Standards:2E1316 Terminology for Nondestructive ExaminationsE1570 Practice for Computed Tomographic (CT) Examina-tion3. 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 withoutd
36、iscussion, in Terminology E1316. Because 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 gamm
37、a rays.3.2 Acronyms:3.2.1 BWbeam 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
38、.2.10 PDFprobability distribution 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
39、 ASTM website.E1441 1124. 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 fundame
40、ntal CT performance parameters, 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 over
41、view of CT: defining the 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 a
42、s their characterization 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 form
43、ulae.5. Significance and 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 b
44、asis of CTtechnology,the 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
45、, this guideestablishes 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 perfo
46、rmance to keysystem parameters.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
47、 coefficient, , at each 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
48、 interact with matter is 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
49、Compton scattering (1). 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
