ASTM E1672-95(2001)
(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) An American National Standard
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.
1. Scope easier to interpret than conventional radiological images. The
new purchaser can quickly learn to read CT data because
1.1 This guide covers guidelines for translating application
images correspond more closely to the way the human mind
requirements into computed tomography (CT) system
visualizes 3-D structures than conventional projection radiol-
requirements/specifications and establishes a common termi-
ogy. Further, because CT images are digital, the images may be
nology to guide both purchaser and supplier in the CT system
enhanced, analyzed, compressed, archived, input as data into
selection process. This guide is applicable to the purchaser of
performance calculations, compared with digital data from
both CT systems and scan services. Computed tomography
other nondestructive evaluation modalities, or transmitted to
systems are complex instruments, consisting of many compo-
other locations for remote viewing. While many of the details
nents that must correctly interact in order to yield images that
are generic in nature, this guide implicitly assumes the use of
repeatedly reproduce satisfactory examination results. Com-
penetrating radiation, specifically X rays and gamma rays.
puted tomography system purchasers are generally concerned
1.5 This standard does not purport to address all of the
with application requirements. Computed tomography system
safety concerns, if any, associated with its use. It is the
suppliers are generally concerned with the system component
responsibility of the user of this standard to establish appro-
selection to meet the purchaser’s performance requirements.
priate safety and health practices and determine the applica-
This guide is not intended to be limiting or restrictive, but
bility of regulatory limitations prior to use.
rather to address the relationships between application require-
ments and performance specifications that must be understood
2. Referenced Documents
and considered for proper CT system selection.
2.1 ASTM Standards:
1.2 Computed tomography (CT) may be used for new
E 1316 Terminology for Nondestructive Examinations
applications or in place of film radiography, provided that the
E 1441 Guide for Computed Tomography (CT) Imaging
capability to disclose physical features or indications that form
E 1570 Practice for Computed Tomographic (CT) Exami-
the acceptance/rejection criteria is fully documented and avail-
nation
able for review.
1.3 Computed tomography (CT) systems use a set of trans-
3. Terminology
mission measurements made along a set of paths projected
3.1 Definitions—For definitions of terms used in this guide,
through the examination object from many different directions.
refer to Terminology E 1316 and Guide E 1441, Appendix X1.
Each of the transmission measurements within these views is
3.2 Definitions of Terms Specific to This Standard:
digitized and stored in a computer, where they are subsequently
3.2.1 purchaser—purchaser or customer of CT system or
conditioned (for example, normalized and corrected) and
scan service.
reconstructed by one of a variety of techniques. An in-depth
3.2.2 scan service—use of a CT system, on a contract basis,
treatment of CT principles is given in Guide E 1441.
for a specific examination application. A scan service acquisi-
1.4 Computed tomography (CT), as with conventional radi-
tion requires the matching of a specific examination application
ography and radioscopic examinations, is broadly applicable to
to an existing CT machine, resulting in the procurement of CT
any material or examination object through which a beam of
system time to perform the examination. Results of scan
penetrating radiation may be passed and detected, including
service are contractually determined but typically include
metals, plastics, ceramics, metallic/nonmetallic composite ma-
some, all, or more than the following: meetings, reports,
terial and assemblies. The principal advantage of CT is that it
images, pictures, and data.
provides densitometric (that is, radiological density and geom-
3.2.3 subsystem—one or more system components inte-
etry) images of thin cross sections through an object. Because
grated together that make up a functional entity.
of the absence of structural superposition, images are much
3.2.4 supplier—suppliers/owners/builders of CT systems.
3.2.5 system component—generic term for a unit of equip-
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-
ment or hardware on the system.
tive Testing and is the direct responsibility of Subcommittee E07.01 on Radiology
(X and Gamma) Method.
Current edition approved Jan. 15, 1995. Published March 1995. Annual Book of ASTM Standards, Vol 03.03.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1672
TABLE 1 Computed Tomography (CT) System Examination
3.2.6 throughput—number of CT scans performed in a
Requirements and Their Major Ramifications
given time frame.
Components/Subsystems
Requirement Reference
Affected
4. Summary of Guide
Examination object, size and Mechanical handling equipment 7.27.2
4.1 This guide provides guidelines for the translation of
weight
examination requirements to system components and specifi-
Examination object radiation Dynamic range 7.37.3
cations. Understanding the CT purchaser’s perspective as well penetrability
Radiation source 7.3.17.3.1
as the CT equipment supplier’s perspective is critical to the
Detectability 7.47.4
successful acquisition of new CT hardware or implementation,
Spatial resolution Detector size/aperture 7.4.1.17.4.1.1
Source size/source spot size 7.4.1.27.4.1.2
or both, of a specific application on existing equipment. An
Mechanical handling equipment 7.4.1.57.4.1.5
understanding of the performance capabilities of the system
Contrast discrimination Strength/energy of radiation 7.4.27.4.2
components making up the CT system is needed in order for a
source
Detector size/source spot size 7.4.2.17.4.2.1
CT system purchaser to prepare a CT system specification. A
Artifact level Mechanical handling equipment 7.4.37.4.3
specification is required for acquisition of either CT system
Throughput/speed of CT process 7.57.5
hardware or scan services for a specific examination applica-
Scan time (Spatial resolution) 7.5.17.5.1
(Contrast discrimination)
tion.
Image matrix size (number of Number/configuration of 7.5.27.5.2
4.2 Section 7 identifies typical purchaser’s examination
pixels in image) detectors
requirements that must be met. These purchaser requirements
Amount of data acquired
Computer/hardware resources
factor into the system design, since the system components that
Slice thickness range Detector configuration/collimators 7.5.37.5.3
are selected for the CT system will have to meet the purchas-
System dynamic range
er’s requirements. Some of the purchaser’s requirements are:
Operator interface 7.67.6
Operator console 7.6.17.6.1
the ability to support the object under examination, that is, size
Computer resources 7.6.27.6.2
and weight; detection capability for size of defects and flaws,
Ease of use 7.6.37.6.3
or both, (spatial resolution and contrast discrimination); dimen-
Trade-offs 7.6.47.6.4
sioning precision; artifact level; throughput; ease of use;
archival procedures. Section 7 also describes the trade-offs
between the CT performance as required by the purchaser and
the choice of system components and subsystems.
utilizing one system component over another as well as its role
4.3 Section 8 covers some management cost considerations
in the overall subsystem. Fig. 1 is a functional block diagram
in CT system procurements.
for a generic CT system.
4.4 Section 9 provides some recommendations for the
7.2 Examination Object, Size and Weight—The most basic
procurement of CT systems.
consideration for selecting a CT system is the examination
object’s physical dimensions and characteristics, such as size,
5. Significance and Use
weight, and material. The physical dimensions, weight, and
5.1 This guide will aid the purchaser in generating a CT
attenuation of the examination object dictate the size of the
system specification. This guide covers the conversion of
mechanical subsystem that handles the examination object and
purchaser’s requirements to system components that must
the type of radiation source and detectors, or both, needed. To
occur for a useful CT system specification to be prepared.
5.2 Additional information can be gained in discussions
with potential suppliers or with independent consultants.
5.3 This guide is applicable to purchasers seeking scan
services.
5.4 This guide is applicable to purchasers needing to pro-
cure a CT system for a specific examination application.
6. Basis of Application
6.1 The following items should be agreed upon by the
purchaser and supplier.
6.1.1 Requirements—General system requirements are cov-
ered in Section 7.
7. Subsystems Capabilities and Limitations
7.1 This section describes how various examination require-
ments affect the CT system components and subsystems.
Trade-offs between requirements and hardware are cited. Table
1 is a summary of these issues. Many different CT system
configurations are possible due to the wide range of system
components available for integration into a single system. It is
important to understand the capability and limitations of FIG. 1 Functional Block Diagram for a Generic CT System
E 1672
select a system for scan services, the issues of CT system size, examination envelope and weight, the most basic consideration
examination object size and weight, and radiation energy must
is radiation penetrability. Examination object penetrability
be addressed first. Considerations like detectability and determines the minimum effective energy and intensity for the
throughput cannot be addressed until these have been satisfac-
radiation source. As in any radiological situation, penetrability
torily resolved. Price-performance tradeoffs must be examined is a function of examination object material, density and
to guard against needless costs.
morphology (shape and features/geometry). The rules for
selecting CT source energy are approximately the same as
7.2.1 The maximum height and diameter of a examination
those for conventional radiography, with the understanding that
object that can be examined on a CT system defines the
equipment examination envelope. The weight of the examina- for CT, the incident radiation must be able to penetrate the
maximum absorption path length through the examination
tion object and any associated fixturing must be within the
manipulation system capability. For example, a very different object in the plane of the scan. The lowest signal value should
be larger than the root-mean-square (RMS) of the electronic
mechanical sub-system will be required to support and accu-
rately move a large, heavy object than to move a small, light noise. The required flux is determined by how many photons
are needed for statistical considerations. The spot size is
object. Similarly, the logistics and fixturing for handling a large
number of similar items will be a much different problem than determined by the spatial resolution and specimen geometry
for handling a one-of-a-kind item.
requirements.
7.2.2 Two Most Common Types of Scan Motion Geometries:
7.3.1 X-ray Sources—Electrical X-ray generators offer a
7.2.2.1 Translate-Rotate Motion—The examination object wider selection in peak energy and intensity and have the
added safety feature of discontinued radiation production when
is translated in a direction perpendicular to the direction and
parallel to the plane of the X-ray beam. Full data sets are switched off. The disadvantage is that the polychromaticity of
the energy spectrum causes artifacts such as cupping (the
obtained by rotating the examination article between transla-
tions by the fan angle of the beam and again translating the anomalous decreasing attenuation toward the center of a
homogeneous object) in the image if uncorrected. X-ray tubes
examination object until a minimum of 180° of data have been
acquired. The advantage of this design is simplicity, good and linear accelerators (linacs) are typically several orders of
magnitude more intense than isotope sources. However, X-ray
view-to-view detector matching, flexibility in the choice of
scan parameters, and ability to accommodate a wide range of generators have the disadvantage that they are inherently less
different object sizes, including objects too big to be subtended stable than isotope sources. X rays produced from electrical
by the X-ray fan. The disadvantage is longer scan time. radiation generators have source spot sizes ranging from a few
millimetres down to a few micrometres. Reducing the source
7.2.2.2 Rotate-Only Motion—The examination object re-
spot size reduces geometric unsharpness, thereby enhancing
mains stationary and the source and detector system is rotated
detail sensitivity. Smaller source spots permit higher spatial
around it. A complete view is collected by the detector array
resolution but at the expense of reduced X-ray beam intensity.
during each sampling interval. A rotate-only scan has lower
Reduced X-ray beam intensity implies that only smaller or less
motion overhead than a translate-rotate scan, and is attractive
dense objects can be inspected. Also to keep in mind, unlike
for industrial applications where the object to be examined fits
radiography, CT can require extended, continuous usage of the
within the fan beam, and scan speed is important. Irrespective
X-ray generator. Therefore, an increased cooling capacity of
of whether the sample translates and rotates, or both, or the
the X-ray generator should be considered in the design and
source/detector system rotates, the principles of CT are the
purchase, or both, in anticipation of the extended usage
same.
requirements.
7.2.3 The purchaser of CT equipment should be aware that
7.3.2 Radioisotope Sources—A radioisotope source can
important cost trade-offs may exist. For instance, the cost of a
have the advantages of small physical size, portability, low
mechanical subsystem with translate, rotate, and elevate func-
power requirements, simplicity and stability of output. The
tions incorpora
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