1、Designation: E 1672 06Standard Guide forComputed Tomography (CT) System Selection1This standard is issued under the fixed designation E 1672; 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 pa
2、rentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers guidelines for translating applicationrequirements into computed tomography (CT) systemrequirements/specifications and establish
3、es 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 instruments, consisting of many compo-nents that must correctly interact in order to
4、yield images thatrepeatedly reproduce satisfactory examination results. Com-puted tomography system purchasers are generally concernedwith application requirements. Computed tomography systemsuppliers are generally concerned with the system componentselection to meet the purchasers performance requi
5、rements.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 system selection.1.2 Computed tomography (CT) may be used for newapplications
6、or in place of film radiography, provided that thecapability to disclose physical features or indications that formthe acceptance/rejection criteria is fully documented and avail-able for review.1.3 Computed tomography (CT) systems use a set of trans-mission measurements made along a set of paths pr
7、ojectedthrough 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, normalized and corrected) and reconstructed byone of a variety of techniques. An in-depth treat
8、ment of CTprinciples is given in Guide E 1441.1.4 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, met
9、allic/nonmetallic composite materialand assemblies. The principal advantage of CT is that itprovides densitometric (that is, radiological density and geom-etry) images of thin cross sections through an object. Becauseof the absence of structural superposition, images are mucheasier to interpret than
10、 conventional radiological images. Thenew purchaser can quickly learn to read CT data becauseimages correspond more closely to the way the human mindvisualizes 3-D structures than conventional projection radiol-ogy. Further, because CT images are digital, the images may beenhanced, analyzed, compres
11、sed, archived, input as data intoperformance calculations, compared with digital data fromother nondestructive evaluation modalities, or transmitted toother locations for remote viewing. While many of the detailsare generic in nature, this guide implicitly assumes the use ofpenetrating radiation, sp
12、ecifically X rays and gamma rays.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 of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitat
13、ions prior to use.2. Referenced Documents2.1 ASTM Standards:2E 1316 Terminology for Nondestructive ExaminationsE 1441 Guide for Computed Tomography (CT) ImagingE 1570 Practice for Computed Tomographic (CT) Exami-nation3. Terminology3.1 DefinitionsFor definitions of terms used in this guide,refer to
14、Terminology E 1316 and Guide E 1441, 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 service acquisi-tion requires
15、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, reports,images, pictures, a
16、nd data.1This guide is under the jurisdiction 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, 2006. Published January 2007. Originallyapproved in 1995. Last previous edition app
17、roved in 2001 as E 1672 - 95(2001)e1.2For 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.1Copyright ASTM Internat
18、ional, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.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-m
19、ent or hardware on the system.3.2.6 throughputnumber of 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
20、equipment suppliers perspective is critical to thesuccessful 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 pu
21、rchaser to prepare a CT system specification. Aspecification 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
22、the system design, since the system components thatare 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,o
23、r both, (spatial resolution and contrast discrimination); 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
24、 8 covers some management cost considerationsin CT system 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 requi
25、rements to system components that mustoccur for a useful 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
26、 to purchasers needing to pro-cure a CT system for a specific 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.
27、1 This section describes how various examination require-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 ava
28、ilable for integration into a single system. It isimportant 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.7.2 Object, Size and WeightThe most basic co
29、nsiderationfor selecting a CT system is the examination objects physicaldimensions and characteristics, such as size, weight, andmaterial. The physical dimensions, weight, and attenuation ofthe object dictate the size of the mechanical subsystem thathandles the examination object and the type of rad
30、iation 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. Considerationslike detectability and throughput cannot be addressed untilthese have been satisfactorily resolved. Price
31、-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 equipmentexamination envelope. The weight of the object and anyassociated fixturing must be within the manipulation systemcapabilit
32、y. For example, a very different mechanical sub-system will be required to support and accurately move a large,heavy object than to move a small, light object. Similarly, thelogistics and fixturing for handling a large number of similaritems will be a much different problem than for handling aone-of
33、-a-kind item.7.2.2 Two Most Common Types of Scan Motion Geometries:7.2.2.1 Translate-Rotate MotionThe object is translatedin a direction perpendicular to the direction and parallel to theTABLE 1 Computed Tomography (CT) System ExaminationRequirements and Their Major RamificationsRequirementComponent
34、s/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 7.4.1.5Contras
35、t 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) 7.5.1(Contrast discrimination)Image matrix size (number ofpixels in image)Number/configurat
36、ion ofdetectors7.5.2Amount of data acquiredComputer/hardware resourcesSlice thickness range Detector configuration/collimators 7.5.3System dynamic rangeOperator interface 7.6Operator console 7.6.1Computer resources 7.6.2Ease of use 7.6.3Trade-offs 7.6.4E1672062plane of the X-ray beam. Full data sets
37、 are obtained by rotatingthe article between translations by the fan angle of the beamand again translating the object until a minimum of 180 ofdata have been acquired. The advantage of this design issimplicity, good view-to-view detector matching, flexibility inthe choice of scan parameters, and ab
38、ility to accommodate awide range of different object sizes, including objects too big tobe subtended by the X-ray fan. The disadvantage is longer scantime.7.2.2.2 Rotate-Only MotionThe object remains stationaryand the source and detector system is rotated around it. Acomplete view is collected by th
39、e detector array during eachsampling interval. A rotate-only scan has lower motion over-head than a translate-rotate scan, and is attractive for industrialapplications where the object to be examined fits within the fanbeam, and scan speed is important. Irrespective of whether thesample translates a
40、nd rotates, or both, or the source/detectorsystem rotates, the principles of CT are the same.7.2.3 The purchaser of CT equipment should be aware thatimportant cost trade-offs may exist. For instance, the cost of amechanical subsystem with translate, rotate, and elevate func-tions incorporated in one
41、 integrally constructed piece ofhardware is relatively cost invariant for vertical motions up tosome limit, but increases drastically above that point. Thecasual specification of an elevation could have severe costimplications; whereas the simple expediency of turning theobject over could effectivel
42、y extend the examination envelopewith no cost impact. Similarly, the specification of a large fieldof view could drive system size and cost soaring; whereas theapplication of prior information or limited angle reconstructiontechniques, or both, could enable the examination with a muchsmaller scanner
43、.7.2.4 Automatic material handling equipment is an optionthat can be acquired with a CT system for mounting andremoving objects. The advantages are lower overhead andgreater throughput. The main disadvantages are added costsand complexity to the system design.7.3 Object Radiation PenetrabilityNext t
44、o examinationenvelope and weight, the most basic consideration is radiationpenetrability. Object penetrability determines the minimumeffective energy and intensity for the radiation source. As inany radiological situation, penetrability is a function of objectmaterial, density and morphology (shape
45、and features/geometry). The rules for selecting CT source energy areapproximately the same as those for conventional radiography,with the understanding that for CT, the incident radiation mustbe able to penetrate the maximum absorption path lengththrough the object in the plane of the scan. The lowe
46、st signalvalue should be larger than the root-mean-square (RMS) of theelectronic noise. The required flux is determined by how manyphotons are needed for statistical considerations. The spot sizeis determined by the spatial resolution and specimen geometryrequirements.7.3.1 X-ray SourcesElectrical X
47、-ray generators offer awider selection in peak energy and intensity and have theadded safety feature of discontinued radiation production whenswitched off. The disadvantage is that the polychromaticity ofthe energy spectrum causes artifacts such as cupping (theanomalous decreasing attenuation toward
48、 the center of ahomogeneous object) in the image if uncorrected. X-ray tubesand linear accelerators (linacs) are typically several orders ofmagnitude more intense than isotope sources. However, X-raygenerators have the disadvantage that they are inherently lessstable than isotope sources. X rays pro
49、duced from electricalradiation generators have source spot sizes ranging from a fewmillimetres down to a few micrometres. Reducing the sourcespot size reduces geometric unsharpness, thereby enhancingdetail sensitivity. Smaller source spots permit higher spatialresolution but at the expense of reduced X-ray beam intensity.Reduced X-ray beam intensity implies that only smaller or lessdense objects can be inspected. Also to keep in mind, unlikeradiography, CT can require extended, continuous usage of theX-ray generator. Therefore, an increased cooling capacity ofthe X
