ASTM E1672-95
(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 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 test results. Computed to-
penetrating radiation, specifically X rays and gamma rays.
mography system purchasers are generally concerned with
1.5 This standard does not purport to address all of the
application requirements. Computed tomography system sup-
safety concerns, if any, associated with its use. It is the
pliers 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 test object from many different directions. Each of
refer to Terminology E 1316 and Guide E 1441, Appendix X1.
the transmission measurements within these views is digitized
3.2 Definitions of Terms Specific to This Standard:
and stored in a computer, where they are subsequently condi-
3.2.1 purchaser—purchaser or customer of CT system or
tioned (for example, normalized and corrected) and recon-
scan service.
structed 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 test object through which a beam of penetrating
system time to perform the examination. Results of scan
radiation may be passed and detected, including metals,
service are contractually determined but typically include
plastics, ceramics, metallic/nonmetallic composite material
some, all, or more than the following: meetings, reports,
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 E-7 on Nondestructive
ment or hardware on the system.
Testing and is the direct responsibility of Subcommittee E07.01 on Radiology (X
and Gamma) Method.
Annual Book of ASTM Standards, Vol 03.03.
Current edition approved Jan. 15, 1995. Published March 1995.
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
Test object, size and weight Mechanical handling equipment 7.27.2
4.1 This guide provides guidelines for the translation of
Test object radiation Dynamic range 7.37.3
examination requirements to system components and specifi-
penetrability
cations. Understanding the CT purchaser’s perspective as well Radiation source 7.3.17.3.1
Detectability 7.47.4
as the CT equipment supplier’s perspective is critical to the
Spatial resolution Detector size/aperture 7.4.1.17.4.1.1
successful acquisition of new CT hardware or implementation,
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
Contrast discrimination Strength/energy of radiation 7.4.27.4.2
understanding of the performance capabilities of the system
source
components making up the CT system is needed in order for a
Detector size/source spot size 7.4.2.17.4.2.1
Artifact level Mechanical handling equipment 7.4.37.4.3
CT system purchaser to prepare a CT system specification. A
Throughput/speed of CT process 7.57.5
specification is required for acquisition of either CT system
Scan time (Spatial resolution) 7.5.17.5.1
hardware or scan services for a specific examination applica-
(Contrast discrimination)
Image matrix size (number of Number/configuration of 7.5.27.5.2
tion.
pixels in image) detectors
4.2 Section 7 identifies typical purchaser’s examination
Amount of data acquired
requirements that must be met. These purchaser requirements
Computer/hardware resources
Slice thickness range Detector configuration/collimators 7.5.37.5.3
factor into the system design, since the system components that
System dynamic range
are selected for the CT system will have to meet the purchas-
Operator interface 7.67.6
er’s requirements. Some of the purchaser’s requirements are:
Operator console 7.6.17.6.1
Computer resources 7.6.27.6.2
the ability to support the object under test, that is, size and
Ease of use 7.6.37.6.3
weight; detection capability for size of defects and flaws, or
Trade-offs 7.6.47.6.4
both, (spatial resolution and contrast discrimination); dimen-
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
utilizing one system component over another as well as its role
the choice of system components and subsystems.
in the overall subsystem. Fig. 1 is a functional block diagram
4.3 Section 8 covers some management cost considerations
for a generic CT system.
in CT system procurements.
7.2 Test Object, Size and Weight—The most basic consid-
4.4 Section 9 provides some recommendations for the
eration for selecting a CT system is the test object’s physical
procurement of CT systems.
dimensions and characteristics, such as size, weight, and
material. The physical dimensions, weight, and attenuation of
5. Significance and Use
the test object dictate the size of the mechanical subsystem that
5.1 This guide will aid the purchaser in generating a CT
handles the test object and the type of radiation source and
system specification. This guide covers the conversion of
detectors, or both, needed. To select a system for scan services,
purchaser’s requirements to system components that must
the issues of CT system size, test object size and weight, and
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
radiation energy must be addressed first. Considerations like source. As in any radiological situation, penetrability is a
detectability and throughput cannot be addressed until these function of test object material, density and morphology (shape
have been satisfactorily resolved. Price-performance tradeoffs and features/geometry). The rules for selecting CT source
must be examined to guard against needless costs. energy are approximately the same as those for conventional
radiography, with the understanding that for CT, the incident
7.2.1 The maximum height and diameter of a test object that
radiation must be able to penetrate the maximum absorption
can be examined on a CT system defines the equipment
path length through the test object in the plane of the scan. The
examination envelope. The weight of the test object and any
lowest signal value should be larger than the root-mean-square
associated fixturing must be within the manipulation system
(RMS) of the electronic noise. The required flux is determined
capability. For example, a very different mechanical sub-
by how many photons are needed for statistical considerations.
system will be required to support and accurately move a large,
The spot size is determined by the spatial resolution and
heavy object than to move a small, light object. Similarly, the
specimen geometry requirements.
logistics and fixturing for handling a large number of similar
items will be a much different problem than for handling a 7.3.1 X-ray Sources—Electrical X-ray generators offer a
one-of-a-kind item.
wider selection in peak energy and intensity and have the
added safety feature of discontinued radiation production when
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 test object is trans-
the energy spectrum causes artifacts such as cupping (the
lated in a direction perpendicular to the direction and parallel
anomalous decreasing attenuation toward the center of a
to the plane of the X-ray beam. Full data sets are obtained by
homogeneous object) in the image if uncorrected. X-ray tubes
rotating the test article between translations by the fan angle of
and linear accelerators (linacs) are typically several orders of
the beam and again translating the test object until a minimum
magnitude more intense than isotope sources. However, X-ray
of 180° of data have been acquired. The advantage of this
generators have the disadvantage that they are inherently less
design is simplicity, good view-to-view detector matching,
stable than isotope sources. X rays produced from electrical
flexibility in the choice of scan parameters, and ability to
radiation generators have source spot sizes ranging from a few
accommodate a wide range of different object sizes, including
millimetres down to a few micrometres. Reducing the source
objects too big to be subtended by the X-ray fan. The
spot size reduces geometric unsharpness, thereby enhancing
disadvantage is longer scan time.
detail sensitivity. Smaller source spots permit higher spatial
7.2.2.2 Rotate-Only Motion—The test object remains sta-
resolution but at the expense of reduced X-ray beam intensity.
tionary and the source and detector system is rotated around it.
Reduced X-ray beam intensity implies that only smaller or less
A 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
applications where the object to be examined fits within the fan
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, rotate, and elevate func-
disadvantages are limited intensity per unit area, limited peak
tions incorporated in one integrally constructed piece of
energy, and increased regulatory concerns.
hardware is relatively cost invariant for vertical motions up to
7.3.3 Synchrotron Radiation (SR) Sources—Synchrotron ra-
some limit, but increases drastically above that point. The
diation (SR)
...
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