1、Designation: E1672 12Standard Guide forComputed Tomography (CT) System Selection1This standard is issued under the fixed designation E1672; 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 pare
2、ntheses 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 guide covers guidelines for translating applicationrequirement
3、s into computed tomography (CT) systemrequirements/specifications and establishes a common termi-nology to guide both purchaser and supplier in the CT systemselection process. This guide is applicable to the purchaser ofboth CT systems and scan services. Computed tomographysystems are complex instru
4、ments, consisting of many compo-nents that must correctly interact in order to yield images thatrepeatedly reproduce satisfactory examination results. Com-puted tomography system purchasers are generally concernedwith application requirements. Computed tomography systemsuppliers are generally concer
5、ned with the system componentselection to meet the purchasers performance requirements.This guide is not intended to be limiting or restrictive, butrather to address the relationships between application require-ments and performance specifications that must be understoodand considered for proper CT
6、 system selection.1.2 Computed tomography (CT) may be used for newapplications or in place of radiography or radioscopy, providedthat the capability to disclose physical features or indicationsthat form the acceptance/rejection criteria is fully documentedand available for review. In general, CT has
7、 lower spatialresolution than film radiography and is of comparable spatialresolution with digital radiography or radioscopy unless mag-nification is used. Magnification can be used in CT orradiography/radioscopy to increase spatial resolution but con-currently with loss of field of view.1.3 Compute
8、d tomography (CT) systems use a set of trans-mission measurements made along a set of paths projectedthrough the object from many different directions. Each of thetransmission measurements within these views is digitized andstored in a computer, where they are subsequently conditioned(for example, n
9、ormalized and corrected) and reconstructed,typically into slices of the object normal to the set of projectionpaths by one of a variety of techniques. If many slices arereconstructed, a three dimensional representation of the objectis obtained. An in-depth treatment of CT principles is given inGuide
10、 E1441.1.4 Computed tomography (CT), as with conventional radi-ography and radioscopic examinations, is broadly applicable toany material or object through which a beam of penetratingradiation may be passed and detected, including metals,plastics, ceramics, metallic/nonmetallic composite materialand
11、 assemblies. The principal advantage of CT is that it has thepotential to provide densitometric (that is, radiological densityand geometry) images of thin cross sections through an object.In many newer systems the cross-sections are now combinedinto 3D data volumes for additional interpretation. Bec
12、ause ofthe absence of structural superposition, images may be mucheasier to interpret than conventional radiological images. Thenew purchaser can quickly learn to read CT data becauseimages correspond more closely to the way the human mindvisualizes 3D structures than conventional projection radiol-
13、ogy. Further, because CT images are digital, the images may beenhanced, analyzed, compressed, archived, input as data intoperformance calculations, compared with digital data fromother nondestructive evaluation modalities, or transmitted toother locations for remote viewing. 3D data sets can berende
14、red by computer graphics into solid models. The solidmodels can be sliced or segmented to reveal 3D internalinformation or output as CAD files. While many of the detailsare generic in nature, this guide implicitly assumes the use ofpenetrating radiation, specifically X rays and gamma rays.1.5 UnitsT
15、he 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.1This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-tive Testing a
16、nd is the direct responsibility of Subcommittee E07.01 on Radiology(X and Gamma) Method.Current edition approved June 15, 2012. Published September 2012. Originallyapproved in 1995. Last previous edition approved in 2006 as E1672 - 06. DOI:10.1520/E1672-12.1*A Summary of Changes section appears at t
17、he end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.1.6 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
18、establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E1316 Terminology for Nondestructive ExaminationsE1441 Guide for Computed Tomography (CT) ImagingE1570 Practice for Computed Tomograph
19、ic (CT) Examina-tionE2339 Practice for Digital Imaging and Communication inNondestructive Evaluation (DICONDE)E2767 Practice for Digital Imaging and Communication inNondestructive Evaluation (DICONDE) for X-ray Com-puted Tomography (CT) Test Methods3. Terminology3.1 DefinitionsFor definitions of ter
20、ms used in this guide,refer to Terminology E1316 and Guide E1441, Appendix X1.3.2 Definitions of Terms Specific to This Standard:3.2.1 purchaserpurchaser or customer of CT system orscan service.3.2.2 scan serviceuse of a CT system, on a contract basis,for a specific examination application. A scan s
21、ervice acquisi-tion requires the matching of a specific examination applicationto an existing CT machine, resulting in the procurement of CTsystem time to perform the examination. Results of scanservice are contractually determined but typically includesome, all, or more than the following: meetings
22、, reports,images, pictures, and data.3.2.3 subsystemone or more system components inte-grated together that make up a functional entity.3.2.4 suppliersuppliers/owners/builders of CT systems.3.2.5 system componentgeneric term for a unit of equip-ment or hardware on the system.3.2.6 throughputnumber o
23、f CT scans performed in agiven time frame.4. Summary of Guide4.1 This guide provides guidelines for the translation ofexamination requirements to system components and specifi-cations. Understanding the CT purchasers perspective as wellas the CT equipment suppliers perspective is critical to thesucc
24、essful acquisition of new CT hardware or implementation,or both, of a specific application on existing equipment. Anunderstanding of the performance capabilities of the systemcomponents making up the CT system is needed in order for aCT system purchaser to prepare a CT system specification. Aspecifi
25、cation is required for acquisition of either CT systemhardware or scan services for a specific examination applica-tion.4.2 Section 7 identifies typical purchasers examinationrequirements that must be met. These purchaser requirementsfactor into the system design, since the system components thatare
26、 selected for the CT system will have to meet the purchas-ers requirements. Some of the purchasers requirements are:the ability to support the object under examination, that is, sizeand weight; detection capability for size of defects and flaws,or both, (spatial resolution and contrast discriminatio
27、n); dimen-sioning precision; artifact level; throughput; ease of use;archival procedures. Section 7 also describes the trade-offsbetween the CT performance as required by the purchaser andthe choice of system components and subsystems.4.3 Section 8 covers some management cost considerationsin CT sys
28、tem procurements.4.4 Section 9 provides some recommendations for theprocurement of CT systems.5. Significance and Use5.1 This guide will aid the purchaser in generating a CTsystem specification. This guide covers the conversion ofpurchasers requirements to system components that mustoccur for a usef
29、ul CT system specification to be prepared.5.2 Additional information can be gained in discussionswith potential suppliers or with independent consultants.5.3 This guide is applicable to purchasers seeking scanservices.5.4 This guide is applicable to purchasers needing to pro-cure a CT system for a s
30、pecific examination application.6. Basis of Application6.1 The following items should be agreed upon by thepurchaser and supplier.6.1.1 RequirementsGeneral system requirements are cov-ered in Section 7.7. Subsystems Capabilities and Limitations7.1 This section describes how various examination requi
31、re-ments affect the CT system components and subsystems.Trade-offs between requirements and hardware are cited. Table1 is a summary of these issues. Many different CT systemconfigurations are possible due to the wide range of systemcomponents available for integration into a single system. It isimpo
32、rtant to understand the capability and limitations ofutilizing one system component over another as well as its rolein the overall subsystem. Fig. 1 is a functional block diagramfor a generic CT system.7.1.1 Pencil-Beam, Fan-Beam and Cone-Beam Type Sys-tems:7.1.1.1 Pencil Beam SystemsThe x-ray beam
33、is colli-mated to a pencil and the effective pixel size becomes the sizeof the beam on the detector area. The beam is translated overthe object and the object rotated after each pass of the beamover the object or the beam and detector are translated androtated around the object to build up linear sl
34、ice profiles. If athree dimensional data set is desired the object or beam/detector must elevate so that multiple slices are generated. Theadvantage of this method is detector simplicity and scatterrejection with the primary disadvantage being long scan times.2For referenced ASTM standards, visit th
35、e 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.E1672 1227.1.1.2 Fan-Beam SystemsThe x-ray beam is collimatedto a fan and detected by a linear detect
36、or array that usually hasa collimator aperture. The pixel size is defined by the width ofthe fan-beam on the detector height (vertically) and by thedetector element pitch (horizontally). Linear profiles are cap-tured as the object or beam/detector rotates. If three dimen-sional data is desired the o
37、bject or beam/detector must elevateto capture multiple slices. The advantage of this method isfaster scan times than pencil-beam systems and some scatterrejection with the primary disadvantage being long scan timesfor 3D data.7.1.1.3 Cone-Beam SystemsThe x-ray beam is usuallycollimated to the entire
38、 or a selected portion of the active areaof a two dimensional detector array and full 2D images arecaptured as the object or beam/detector rotates. In this mannermultiple slices are generated without needing to elevate. Theprimary advantage of this technique is speed or acquiring 3Ddata, with the pr
39、imary disadvantage being increased scatter dueto larger field of view.7.2 Object, Size and WeightThe most basic considerationfor selecting a CT system is the examination objects physicaldimensions and characteristics, such as size, weight, andmaterial. The physical dimensions, weight, and attenuatio
40、n ofthe object dictate the size of the mechanical subsystem thathandles the examination object and the type of radiation sourceand detectors, or both, needed. To select a system for scanservices, the issues of CT system size, object size and weight,and radiation energy must be addressed first. Consi
41、derationslike detectability and throughput cannot be addressed untilthese have been satisfactorily resolved. Price-performancetradeoffs must be examined to guard against needless costs.7.2.1 The maximum height and diameter of an object thatcan be examined on a CT system defines the equipmentexaminat
42、ion envelope. Data must be captured over the entirewidth of the object for each view. If the projected x-ray beamthrough the object does not provide complete coverage, theobject or beam/detector must translate. Some specializedalgorithms may allow the reduction of this requirement butdetectability a
43、nd scan time may be affected. The weight of theobject and any associated fixturing must be within the manipu-lation system capability. For example, a very different me-chanical sub-system will be required to support and accuratelymove a large, heavy object than to move a small, light object.Similarl
44、y, the logistics and fixturing for handling a largenumber of similar items will be a much different problem thanfor handling a one-of-a-kind item.7.2.2 Two Most Common Types of Scan MotionGeometriesBoth geometries are applicable to 2D fan beamor 3D cone beam systems.7.2.2.1 Translate-Rotate MotionTh
45、e object or detector istranslated in a direction perpendicular to the direction andparallel to the plane of the X-ray beam. Full data sets areobtained by rotating the article between translations by the fanangle of the beam and again translating the object until aminimum of 180 of data have been acq
46、uired. The advantageof this design is simplicity, good view-to-view detector match-ing, flexibility in the choice of scan parameters, and ability toaccommodate a wide range of different object sizes, includingobjects too big to be subtended by the X-ray fan. Thedisadvantage is longer scan time. Reco
47、nstruction softwaremust correctly account for fan/cone beam effects which can becomplicated by translation of the object.7.2.2.2 Rotate-Only MotionThe object remains stationaryand the source and detector system is rotated around it or theobject rotates and the source and detector remain stationary.
48、Acomplete view is generally collected by the detector arrayduring each sampling interval. A rotate-only scan has lowermotion overhead than a translate-rotate scan, and is attractivefor industrial applications where the object to be examined fitsTABLE 1 Computed Tomography (CT) System ExaminationRequ
49、irements and Their Major RamificationsRequirementComponents/SubsystemsAffectedReferenceObject, size and weight Mechanical handling equipment 7.2Object radiationpenetrabilityDynamic range 7.3Radiation source 7.3.1Detectability 7.4Spatial resolution Detector size/aperture 7.4.1.1Source size/source spot size 7.4.1.2Mechanical handling equipment 7.4.1.5Contrast discrimination Strength/energy of radiationsource7.4.2Detector size/source spot size 7.4.2.1Artifact level Mechanical handling equipment 7.4.3Throughput/speed of CT process 7.5Scan time (Spatial resolution)
