1、Designation: E 1931 97 (Reapproved 2003)Standard Guide forX-Ray Compton Scatter Tomography1This standard is issued under the fixed designation E 1931; 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 PurposeThis guide covers a tutorial introduction tofamiliarize the reader with the operational capabilities andlimitations inherent in X-
3、ray Compton Scatter Tomography(CST). Also included is a brief description of the physics andtypical hardware configuration for CST.1.2 AdvantagesX-ray Compton Scatter Tomography(CST) is a radiologic nondestructive examination method withseveral advantages that include:1.2.1 The ability to perform X-
4、ray examination withoutaccess to the opposite side of the examination object;1.2.2 The X-ray beam need not completely penetrate theexamination object allowing thick objects to be partiallyexamined. Thick examination objects become part of theradiation shielding thereby reducing the radiation hazard;
5、1.2.3 The ability to image examination object subsurfacefeatures with minimal influence from surface features;1.2.4 The ability to obtain high-contrast images from lowsubject contrast materials that normally produce low-contrastimages when using traditional transmitted beam X-ray imagingmethods; and
6、1.2.5 The ability to obtain depth information for examina-tion object features thereby providing three-dimensional ex-amination. The ability to obtain depth information presupposesthe use of a highly collimated detector system having a narrowangle of acceptance.1.3 ApplicationsThis guide does not sp
7、ecify which ex-amination objects are suitable, or unsuitable, for CST. As withmost nondestructive examination techniques, CST is highlyapplication specific thereby requiring the suitability of themethod to be first demonstrated in the application laboratory.This guide does not provide guidance in th
8、e standardizedpractice or application of CST techniques. No guidance isprovided concerning the acceptance or rejection of examina-tion objects examined with CST.1.4 LimitationsAs with all nondestructive examinationmethods, CST has limitations and is complementary to otherNDE methods. Chief among the
9、 limitations is the difficulty inperforming CST on thick sections of high-Z materials. CST isbest applied to thinner sections of lower Z materials. Thefollowing provides a general idea of the range of CSTapplicability when using a 160 keV constant potential X-raysource:Material Practical Thickness R
10、angeSteel Up to about 3 mm 18 in.Aluminum Up to about 25 mm 1 in.Aerospace composites Up to about 50 mm 2 in.The limitations of the technique must also consider therequired X, Y, and Z axis resolutions, the speed of imageformation, image quality and the difference in the X-rayscattering characterist
11、ics of the parent material and the internalfeatures that are to be imaged.1.5 The values stated in both inch-pound and SI units are tobe regarded separately as the standard. The values given inparentheses are for information only.1.6 This standard does not purport to address all of thesafety concern
12、s, 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 theapplicability of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:E 747 Practice for Design, Manufacture, an
13、d MaterialGrouping Classification of Wire Image Quality Indicators(IQI) Used for Radiology2E 1025 Practice for Hole-Type Image Quality IndicatorsUsed for Radiography2E 1255 Practice for Radioscopy2E 1316 Standard Terminology for Nondestructive Examina-tions2E 1441 Guide for Computed Tomography (CT)
14、Imaging2E 1453 Guide for the Storage of Media that ContainsRadioscopic Data2E 1475 Guide for Data Fields for Computerized Transfer ofDigital Radiological Examination Data2E 1647 Practice for Determining Contrast Sensitivity inRadioscopy22.2 ANSI/ASNT Standards:1This guide is under the jurisdiction o
15、f 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 March 10, 2003. Published May 2003. Originallyapproved in 1997. Last previous edition approved in 1997 as E 1931 97.2Annual Book of ASTM S
16、tandards, Vol 03.03.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.ASNT Recommended Practice No. SNT-TC-1A PersonnelQualification and Certification in Nondestructive Test-ing3ANSI/ASNT CP-1 89 Standard for Qualification and Certi-fi
17、cation in Nondestructive Testing Personnel32.3 Military Standard:MIL-STD-410 Nondestructive Testing Personnel Qualifica-tion and Certification43. Terminology3.1 Definitions:3.1.1 CST, being a radiologic examination method, usesmuch the same vocabulary as other X-ray examination meth-ods. A number of
18、 terms used in this standard are defined inTerminology E 1316. It may also be helpful to read GuideE 1441.4. Summary of Guide4.1 DescriptionCompton Scatter Tomography is auniquely different nondestructive test method utilizing pen-etrating X-ray or gamma-ray radiation. Unlike computedtomography (CT)
19、, CST produces radioscopic images which arenot computed images. Multiple slice images can be simulta-neously produced so that the time per slice image is in therange of a few seconds. CST produces images that are thin withrespect to the examination object thickness (slice images) andwhich are at rig
20、ht angles to the X-ray beam. Each two-dimensional slice image (XY axes) is produced at an incre-mental distance along and orthogonal to the X-ray beam(Zaxis). A stack of CST images therefore represents a solidvolume within the examination object. Each slice imagecontains examination object informati
21、on which lies predomi-nantly within the desired slice. To make an analogy as to howCST works, consider a book. The examination object may belarger or smaller (in length, width and depth) then the analo-gous book. The CST slice images are the pages in the book.Paging through the slice images provides
22、 information aboutexamination object features lying at different depths within theexamination object.4.2 Image FormationCST produces one or more digitalslice plane images per scan. Multiple slice images can beproduced in times ranging from a few seconds to a few minutesdepending upon the examined ar
23、ea, desired spatial resolutionand signal-to-noise ratio. The image is digital and is typicallyassembled by microcomputer. CST images are free fromreconstruction artifacts as the CST image is produced directlyand is not a calculated image. Because CST images are digital,they may be enhanced, analyzed
24、, archived and in generalhandled as any other digital information.4.3 Calibration StandardsAs with all nondestructive ex-aminations, known standards are required for the calibrationand performance monitoring of the CST method. PracticeE 1255 calibration block standards that are representative ofthe
25、actual examination object are the best means for CSTperformance monitoring. Conventional radiologic performancemeasuring devices, such as Test Method E 747 and PracticeE 1025 image quality indicators or Practice E 1647 contrastsensitivity gages are designed for transmitted X-ray beamimaging and are
26、of little use for CST. With appropriatecalibration, CST can be utilized to make three-dimensionalmeasurements of internal examination object features.5. Significance and Use5.1 Principal Advantage of Compton Scatter TomographyThe principal advantage of CST is the ability to performthree-dimensional
27、X-ray examination without the requirementfor access to the back side of the examination object. CSToffers the possibility to perform X-ray examination that is notpossible by any other method. The CST sub-surface sliceimage is minimally affected by examination object featuresoutside the plane of exam
28、ination. The result is a radioscopicimage that contains information primarily from the slice plane.Scattered radiation limits image quality in normal radiographicand radioscopic imaging. Scatter radiation does not have thesame detrimental effect upon CST because scatter radiation isused to form the
29、image. In fact, the more radiation theexamination object scatters, the better the CST result. Lowsubject contrast materials that cannot be imaged well byconventional radiographic and radioscopic means are oftenexcellent candidates for CST. Very high contrast sensitivitiesand excellent spatial resolu
30、tion are possible with CST tomog-raphy.5.2 LimitationsAs with any nondestructive testingmethod, CST has its limitations. The technique is useful onreasonably thick sections of low-density materials. While a 25mm 1 in. depth in aluminum or 50 mm 2 in. in plastic isachievable, the examination depth is
31、 decreased dramatically asthe material density increases. Proper image interpretationrequires the use of standards and examination objects withknown internal conditions or representative quality indicators(RQIs). The examination volume is typically small, on theorder of a few cubic inches and may re
32、quire a few minutes toimage. Therefore, completely examining large structures withCST requires intensive re-positioning of the examinationvolume that can be time-consuming. As with other penetratingradiation methods, the radiation hazard must be properlyaddressed.6. Technical Description6.1 General
33、Description of Compton Scatter TomographyTransmitted beam radiologic techniques used in radiography,radioscopy and computed tomography have dominated the useof penetrating radiation for industrial nondestructive examina-tion. The transmitted beam technique depends upon the pen-etrating radiation att
34、enuation mechanisms of photoelectricabsorption and Compton scattering. For low-Z materials atenergies up to about 50 keV, the photoelectric effect is thedominant attenuation mechanism. As X-ray energy increases,Compton scattering becomes the dominant attenuation mecha-nism for large scattering angle
35、s in low-Z materials. Pairproduction comes into play above 1.02 MeV and can becomethe dominant effect for higher X-ray energies. Photoelectric3Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036.4Available from Standardization Document Order Desk
36、, Bldg. 4 Section D, 700Robbins Ave. Philadelphia, PA 19111-5094, Ans:NPODS.E 1931 97 (2003)2absorption is strongly dependent upon the atomic number andalso the electron density of the absorbing material. Comptonscattering also depends upon the Z of the scattering material,but to a lesser degree tha
37、n is the photoelectric effect. Theserelationships may be seen in Fig. 1. The following relationshipsshow the approximate dependence of the photoelectric effectNOTE 1Hubbell, J.H. and Seltzer, S.M., Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients, 1 keV to 20 MeV
38、for ElementsZ=1to92and48Additional Substances of Dosimetric Interest, NISTIR 5632, 1996. Available from National Institute of Standards andTechnology (NIST), Gaithersburg, MD 20899.FIG. 1 Linear Absorption and Scatter Coefficients for Polyethylene, Aluminum and IronE 1931 97 (2003)3and Compton scatt
39、ering upon target material Z and incidentX-ray energy E:Photoelectric Effect Z5/ E7/2Compton Scattering Z / EPair Production: Z2(lnE- constant)6.1.1 CST is best suited for lower Z materials such asaluminum ( Z=13 ) using a commercially available 160 keVX-ray generating system. Somewhat higher Z mate
40、rials may beexamined by utilizing a higher energy X-ray generator rated at225, 320, or 450 keV. It is useful to envision the CST processas one where the X-rays that produce the CST image originatefrom many discrete points within the examined volume. EachCompton scatter event generates a lower energy
41、 X-ray thatemanates from the scattering site. Singly scattered X rays thatreach the detector carry information about the examinationobject material characteristics at the site where it was gener-ated. The scatter radiation is also affected by the materialthrough which it passes on the way to the det
42、ector. The externalsource of primary penetrating radiation, that may be either Xrays or gamma rays, interact by the Compton scatteringprocess. The primary radiation must have adequate energy andintensity to generate sufficient scattered radiation at the exami-nation site to allow detection. The exam
43、ination depth is limitedto that depth from which sufficient scattered radiation can reachthe detector to form a usable image. The examination object istherefore effectively imaged from the inside out. The CSTimage is formed voxel (volume element) by voxel in rasterfashion where the detectors field-o
44、f-view intersects with thecentral X-ray beam at the examination site. The primaryradiation beam source and scattered radiation detector arehighly collimated to assure collection of singly-scattered ra-diation from a known small volume of the examination object.Multiple scattered radiation causes a l
45、oss of spatial resolution.Moving the intersection of the radiation source and detectorlines of sight in a systematic fashion allows a tomogram, orslice image to be produced. Changing the distance at which theradiation source and detector lines of sight intersect allows thetomogram to be produced at
46、a selected depth below theexamination object surface.6.2 Significant Differences in the Transmitted Beam andCompton Scattered X-Ray Imaging TechniquesThe differ-ences between conventional transmitted beam and ComptonScatter Imaging are so significant that CST must be considereda separate examination
47、 technique. For transmitted beam tech-niques, the radiation source characteristics must be carefullycontrolled. The energy and intensity must be selected carefullyto fully penetrate the ation object and provide the requiredcontrast sensitivity. Thick sections of high-density materialsrequire a high-
48、energy radiation source while thin sections oflow-density materials require a low-energy radiation source.For CST applications, the energy and intensity of the primaryradiation beam is relatively less important. The primary radia-tion beam energy and intensity are not critical as long as theyremain
49、stable and are sufficient to generate adequate scatterradiation at the CST examination depth. Small focal spot sizeis critical to transmitted beam image sharpness. The primaryradiation beam focal spot size is of much less significance forCST techniques. What is important is high specific activity, orthe number of X rays or gamma rays generated per unit area (orvolume) of the primary radiation source resulting in a lowernoise CST image and faster examination speed. For this reasonan X-ray source is often a better choice than a radioisotope forCST. Radiation d
