ASTM E1672-95(2001)e1
(Guide)Standard Guide for Computed Tomography (CT) System Selection
Standard Guide for Computed Tomography (CT) System Selection
SCOPE
1.1 This guide covers guidelines for translating application requirements into computed tomography (CT) system requirements/specifications and establishes a common terminology to guide both purchaser and supplier in the CT system selection process. This guide is applicable to the purchaser of both CT systems and scan services. Computed tomography systems are complex instruments, consisting of many components that must correctly interact in order to yield images that repeatedly reproduce satisfactory examination results. Computed tomography system purchasers are generally concerned with application requirements. Computed tomography system suppliers are generally concerned with the system component selection to meet the purchaser's performance requirements. This guide is not intended to be limiting or restrictive, but rather to address the relationships between application requirements and performance specifications that must be understood and considered for proper CT system selection.
1.2 Computed tomography (CT) may be used for new applications or in place of film radiography, provided that the capability to disclose physical features or indications that form the acceptance/rejection criteria is fully documented and available for review.
1.3 Computed tomography (CT) systems use a set of transmission measurements made along a set of paths projected through the examination object from many different directions. Each of the transmission measurements within these views is digitized and stored in a computer, where they are subsequently conditioned (for example, normalized and corrected) and reconstructed by one of a variety of techniques. An in-depth treatment of CT principles is given in Guide E1441.
1.4 Computed tomography (CT), as with conventional radiography and radioscopic examinations, is broadly applicable to any material or examination object through which a beam of penetrating radiation may be passed and detected, including metals, plastics, ceramics, metallic/nonmetallic composite material and assemblies. The principal advantage of CT is that it provides densitometric (that is, radiological density and geometry) images of thin cross sections through an object. Because of the absence of structural superposition, images are much easier to interpret than conventional radiological images. The new purchaser can quickly learn to read CT data because images correspond more closely to the way the human mind visualizes 3-D structures than conventional projection radiology. Further, because CT images are digital, the images may be enhanced, analyzed, compressed, archived, input as data into performance calculations, compared with digital data from other nondestructive evaluation modalities, or transmitted to other locations for remote viewing. While many of the details are generic in nature, this guide implicitly assumes the use of penetrating radiation, specifically X rays and gamma rays.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E 1672 – 95 (Reapproved 2001)
Standard Guide for
Computed Tomography (CT) System Selection
This standard is issued under the fixed designation E 1672; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—The term “test” was changed or removed editorially. Footnote 5 was also changed editorially in July 2001.
1. Scope and assemblies. The principal advantage of CT is that it
provides densitometric (that is, radiological density and geom-
1.1 This guide covers guidelines for translating application
etry) images of thin cross sections through an object. Because
requirements into computed tomography (CT) system
of the absence of structural superposition, images are much
requirements/specifications and establishes a common termi-
easier to interpret than conventional radiological images. The
nology to guide both purchaser and supplier in the CT system
new purchaser can quickly learn to read CT data because
selection process. This guide is applicable to the purchaser of
images correspond more closely to the way the human mind
both CT systems and scan services. Computed tomography
visualizes 3-D structures than conventional projection radiol-
systems are complex instruments, consisting of many compo-
ogy.Further,becauseCTimagesaredigital,theimagesmaybe
nents that must correctly interact in order to yield images that
enhanced, analyzed, compressed, archived, input as data into
repeatedly reproduce satisfactory examination results. Com-
performance calculations, compared with digital data from
puted tomography system purchasers are generally concerned
other nondestructive evaluation modalities, or transmitted to
with application requirements. Computed tomography system
other locations for remote viewing. While many of the details
suppliers are generally concerned with the system component
are generic in nature, this guide implicitly assumes the use of
selection to meet the purchaser’s performance requirements.
penetrating radiation, specifically X rays and gamma rays.
This guide is not intended to be limiting or restrictive, but
1.5 This standard does not purport to address all of the
rather to address the relationships between application require-
safety concerns, if any, associated with its use. It is the
ments and performance specifications that must be understood
responsibility of the user of this standard to establish appro-
and considered for proper CT system selection.
priate safety and health practices and determine the applica-
1.2 Computed tomography (CT) may be used for new
bility of regulatory limitations prior to use.
applications or in place of film radiography, provided that the
capability to disclose physical features or indications that form
2. Referenced Documents
the acceptance/rejection criteria is fully documented and avail-
2.1 ASTM Standards:
able for review.
E 1316 Terminology for Nondestructive Examinations
1.3 Computed tomography (CT) systems use a set of trans-
E 1441 Guide for Computed Tomography (CT) Imaging
mission measurements made along a set of paths projected
E 1570 Practice for Computed Tomographic (CT) Exami-
through the object from many different directions. Each of the
nation
transmission measurements within these views is digitized and
stored in a computer, where they are subsequently conditioned
3. Terminology
(for example, normalized and corrected) and reconstructed by
3.1 Definitions—For definitions of terms used in this guide,
one of a variety of techniques. An in-depth treatment of CT
refer to Terminology E 1316 and Guide E 1441,Appendix X1.
principles is given in Guide E 1441.
3.2 Definitions of Terms Specific to This Standard:
1.4 Computed tomography (CT), as with conventional radi-
3.2.1 purchaser—purchaser or customer of CT system or
ography and radioscopic examinations, is broadly applicable to
scan service.
any material or object through which a beam of penetrating
3.2.2 scan service—use of a CTsystem, on a contract basis,
radiation may be passed and detected, including metals,
for a specific examination application. A scan service acquisi-
plastics, ceramics, metallic/nonmetallic composite material
tionrequiresthematchingofaspecificexaminationapplication
to an existing CT machine, resulting in the procurement of CT
system time to perform the examination. Results of scan
This 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 January 15, 1995. Published March 1995. Annual Book of ASTM Standards, Vol 03.03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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E 1672 – 95 (2001)
TABLE 1 Computed Tomography (CT) System Examination
service are contractually determined but typically include
Requirements and Their Major Ramifications
some, all, or more than the following: meetings, reports,
Components/Subsystems
images, pictures, and data.
Requirement Reference
Affected
3.2.3 subsystem—one or more system components inte-
Object, size and weight Mechanical handling equipment 7.27.2
grated together that make up a functional entity.
Object radiation Dynamic range 7.37.3
3.2.4 supplier—suppliers/owners/builders of CT systems.
penetrability
Radiation source 7.3.17.3.1
3.2.5 system component—generic term for a unit of equip-
Detectability 7.47.4
ment or hardware on the system.
Spatial resolution Detector size/aperture 7.4.1.17.4.1.1
3.2.6 throughput—number of CT scans performed in a
Source size/source spot size 7.4.1.27.4.1.2
Mechanical handling equipment 7.4.1.57.4.1.5
given time frame.
Contrast discrimination Strength/energy of radiation 7.4.27.4.2
source
4. Summary of Guide
Detector size/source spot size 7.4.2.17.4.2.1
Artifact level Mechanical handling equipment 7.4.37.4.3
4.1 This guide provides guidelines for the translation of
Throughput/speed of CT process 7.57.5
examination requirements to system components and specifi-
Scan time (Spatial resolution) 7.5.17.5.1
(Contrast discrimination)
cations. Understanding the CT purchaser’s perspective as well
Image matrix size (number of Number/configuration of 7.5.27.5.2
as the CT equipment supplier’s perspective is critical to the
pixels in image) detectors
successful acquisition of new CT hardware or implementation,
Amount of data acquired
Computer/hardware resources
or both, of a specific application on existing equipment. An
Slice thickness range Detector configuration/collimators 7.5.37.5.3
understanding of the performance capabilities of the system
System dynamic range
components making up the CT system is needed in order for a Operator interface 7.67.6
Operator console 7.6.17.6.1
CT system purchaser to prepare a CT system specification. A
Computer resources 7.6.27.6.2
specification is required for acquisition of either CT system
Ease of use 7.6.37.6.3
hardware or scan services for a specific examination applica-
Trade-offs 7.6.47.6.4
tion.
4.2 Section 7 identifies typical purchaser’s examination
requirements that must be met. These purchaser requirements
7. Subsystems Capabilities and Limitations
factorintothesystemdesign,sincethesystemcomponentsthat
7.1 Thissectiondescribeshowvariousexaminationrequire-
are selected for the CT system will have to meet the purchas-
ments affect the CT system components and subsystems.
er’s requirements. Some of the purchaser’s requirements are:
Trade-offs between requirements and hardware are cited.Table
the ability to support the object under examination, that is, size
1 is a summary of these issues. Many different CT system
and weight; detection capability for size of defects and flaws,
configurations are possible due to the wide range of system
orboth,(spatialresolutionandcontrastdiscrimination);dimen-
components available for integration into a single system. It is
sioning precision; artifact level; throughput; ease of use;
important to understand the capability and limitations of
archival procedures. Section 7 also describes the trade-offs
utilizing one system component over another as well as its role
between the CT performance as required by the purchaser and
in the overall subsystem. Fig. 1 is a functional block diagram
the choice of system components and subsystems.
for a generic CT system.
4.3 Section 8 covers some management cost considerations
7.2 Object, Size and Weight—The most basic consideration
in CT system procurements.
for selecting a CT system is the examination object’s physical
4.4 Section 9 provides some recommendations for the
dimensions and characteristics, such as size, weight, and
procurement of CT systems.
material. The physical dimensions, weight, and attenuation of
the object dictate the size of the mechanical subsystem that
5. Significance and Use
handlestheexaminationobjectandthetypeofradiationsource
5.1 This guide will aid the purchaser in generating a CT
and detectors, or both, needed. To select a system for scan
system specification. This guide covers the conversion of
services, the issues of CT system size, object size and weight,
purchaser’s requirements to system components that must
and radiation energy must be addressed first. Considerations
occur for a useful CT system specification to be prepared.
like detectability and throughput cannot be addressed until
5.2 Additional information can be gained in discussions
these have been satisfactorily resolved. Price-performance
with potential suppliers or with independent consultants.
tradeoffs must be examined to guard against needless costs.
5.3 This guide is applicable to purchasers seeking scan
7.2.1 The maximum height and diameter of an object that
services.
can be examined on a CT system defines the equipment
5.4 This guide is applicable to purchasers needing to pro-
examination envelope. The weight of the object and any
cure a CT system for a specific examination application.
associated fixturing must be within the manipulation system
capability. For example, a very different mechanical sub-
6. Basis of Application
systemwillberequiredtosupportandaccuratelymovealarge,
6.1 The following items should be agreed upon by the heavy object than to move a small, light object. Similarly, the
purchaser and supplier. logistics and fixturing for handling a large number of similar
6.1.1 Requirements—General system requirements are cov- items will be a much different problem than for handling a
ered in Section 7. one-of-a-kind item.
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E 1672 – 95 (2001)
removing objects. The advantages are lower overhead and
greater throughput. The main disadvantages are added costs
and complexity to the system design.
7.3 Object Radiation Penetrability—Next to examination
envelope and weight, the most basic consideration is radiation
penetrability. Object penetrability determines the minimum
effective energy and intensity for the radiation source. As in
any radiological situation, penetrability is a function of object
material, density and morphology (shape and features/
geometry). The rules for selecting CT source energy are
approximately the same as those for conventional radiography,
with the understanding that for CT, the incident radiation must
be able to penetrate the maximum absorption path length
through the object in the plane of the scan. The lowest signal
value should be larger than the root-mean-square (RMS) of the
electronic noise. The required flux is determined by how many
photons are needed for statistical considerations. The spot size
is determined by the spatial resolution and specimen geometry
requirements.
FIG. 1 Functional Block Diagram for a Generic CT System
7.3.1 X-ray Sources—Electrical X-ray generators offer a
wider selection in peak energy and intensity and have the
addedsafetyfeatureofdiscontinuedradiationproductionwhen
7.2.2 Two Most Common Types of Scan Motion Geometries:
switched off. The disadvantage is that the polychromaticity of
7.2.2.1 Translate-Rotate Motion—The object is translated
the energy spectrum causes artifacts such as cupping (the
in a direction perpendicular to the direction and parallel to the
anomalous decreasing attenuation toward the center of a
plane of the X-ray beam. Full data sets are obtained by rotating
homogeneous object) in the image if uncorrected. X-ray tubes
the article between translations by the fan angle of the beam
and linear accelerators (linacs) are typically several orders of
and again translating the object until a minimum of 180° of
magnitude more intense than isotope sources. However, X-ray
data have been acquired. The advantage of this design is
generators have the disadvantage that they are inherently less
simplicity, good view-to-view detector matching, flexibility in
stable than isotope sources. X rays produced from electrical
the choice of scan parameters, and ability to accommodate a
radiation generators have source spot sizes ranging from a few
widerangeofdifferentobjectsizes,includingobjectstoobigto
millimetres down to a few micrometres. Reducing the source
besubtendedbytheX-rayfan.Thedisadvantageislongerscan
spot size reduces geometric unsharpness, thereby enhancing
time.
detail sensitivity. Smaller source spots permit higher spatial
7.2.2.2 Rotate-Only Motion—The object remains stationary
resolution but at the expense of reduced X-ray beam intensity.
and the source and detector system is rotated around it. A
Reduced X-ray beam intensity implies that only smaller or less
complete view is collected by the detector array during each
dense objects can be inspected. Also to keep in mind, unlike
sampling interval. A rotate-only scan has lower motion over-
radiography, CT can require extended, continuous usage of the
head than a translate-rotate scan, and is attractive for industrial
X-ray generator. Therefore, an increased cooling capacity of
applicationswheretheobjecttobeexaminedfitswithinthefan
the X-ray generator should be considered in the design and
beam, and scan speed is important. Irrespective of whether the
purchase, or both, in anticipation of the extended usage
sample translates and rotates, or both, or the source/detector
requirements.
system rotates, the principles of CT are the same.
7.3.2 Radioisotope Sources—A radioisotope source can
7.2.3 The purchaser of CT equipment should be aware that
have the advantages of small physical size, portability, low-
important cost trade-offs may exist. For instance, the cost of a
power requirements, simplicity and stability of output. The
mechanical subsystem with translate,
...
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