ASTM E1570-00(2005)e1
(Practice)Standard Practice for Computed Tomographic (CT) Examination
Standard Practice for Computed Tomographic (CT) Examination
SIGNIFICANCE AND USE
This practice is applicable for the systematic assessment of the internal structure of a material or assembly using CT technology. This practice may be used for review by system operators, or to prescribe operating procedures for new or routine test objects.
This practice provides the basis for the formation of a program for quality control and its continuation through calibration, standardization, reference samples, inspection plans, and procedures.
SCOPE
1.1 This practice is for computed tomography (CT), which may be used to nondestructively disclose physical features or anomalies within a test object by providing radiological density and geometric measurements. This practice implicitly assumes the use of penetrating radiation, specifically X-ray and -ray.
1.2 CT systems utilize a set of transmission measurements made along paths through the test object from many different directions. Each of the transmission measurements is digitized and stored in a computer, where they are subsequently reconstructed by one of a variety of techniques. A treatment of CT principles is given in Guide E 1441.
1.3 CT is broadly applicable to any material or test object through which a beam of penetrating radiation passes. The principal advantage of CT is that it provides densitometric (that is, radiological density and geometry) images of thin cross sections through an object without the structural superposition in projection radiography.
1.4 This practice describes procedures for performing CT examinations. This practice is to address the general use of CT technology and thereby facilitate its use.
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. For specific safety statements, see Section , NBS Handbook 114, and Federal Standards 21 CFR 1020.40 and 29 CFR 1910.96.
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Designation:E1570–00 (Reapproved 2005)
Standard Practice for
Computed Tomographic (CT) Examination
This standard is issued under the fixed designation E1570; 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 (´) 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 E1695 Test Method for Measurement of Computed Tomog-
raphy (CT) System Performance
1.1 This practice is for computed tomography (CT), which
2.2 NIST Standard:
may be used to nondestructively disclose physical features or
NBS Handbook 114 General Safety Standard for Installa-
anomalieswithinatestobjectbyprovidingradiologicaldensity
tions. Using Non-Medical X-Ray and Sealed Gamma-Ray
and geometric measurements. This practice implicitly assumes
Sources, Energies Up to 10 MeV
the use of penetrating radiation, specifically X-ray and g-ray.
2.3 Federal Standards:
1.2 CT systems utilize a set of transmission measurements
21 CFR 1020.40 Safety Requirements of Cabinet X Ray
made along paths through the test object from many different
Systems
directions. Each of the transmission measurements is digitized
29 CFR 1910.96 Ionizing Radiation
and stored in a computer, where they are subsequently recon-
2.4 ASNT Documents:
structed by one of a variety of techniques. A treatment of CT
SNT-TC-1A Recommended Practice for Personnel Qualifi-
principles is given in Guide E1441.
cation and Certification in Nondestructive Testing
1.3 CT is broadly applicable to any material or test object
ANSI/ASNT-CP-189 Qualification and Certification of
through which a beam of penetrating radiation passes. The
Nondestructive Testing Personnel
principaladvantageofCTisthatitprovidesdensitometric(that
2.5 Military Standard:
is, radiological density and geometry) images of thin cross
MIL-STD-410 Nondestructive Testing Personnel Qualifica-
sections through an object without the structural superposition
tion and Certification
in projection radiography.
2.6 AIA Standard:
1.4 This practice describes procedures for performing CT
NAS-410 Certification and Qualification of Nondestructive
examinations. This practice is to address the general use of CT
Testing Personnel
technology and thereby facilitate its use.
1.5 This standard does not purport to address all of the
3. Terminology
safety concerns, if any, associated with its use. It is the
3.1 Definitions—For definitions of terms used in this guide,
responsibility of the user of this standard to establish appro-
refer to Terminology E1316 and Annex A1 in Guide E1441.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. For specific safety
4. Summary of Practice
statements, see Section 8, NBS Handbook 114, and Federal
4.1 Requirements in this practice are intended to control the
Standards 21 CFR 1020.40 and 29 CFR 1910.96.
reliability and quality of the CT images.
2. Referenced Documents 4.2 CTsystems are made up of a number of subsystems; the
2 functionservedbyeachsubsystemiscommoninalmostallCT
2.1 ASTM Standards:
scanners. Section 7 describes the following subsystems:
E1316 Terminology for Nondestructive Examinations
4.2.1 Source of penetrating radiation,
E1441 Guide for Computed Tomography (CT) Imaging
4.2.2 Radiation detector or an array of detectors,
4.2.3 Mechanical scanning assembly, and
This practice is under the jurisdiction of ASTM Committee E07 on Nonde-
structive Testing and is the direct responsibility of Subcommittee E07.01 on
Radiology (X and Gamma) Method. Available from National Institute of Standards and Technology (NIST), 100
Current edition approved Dec. 1, 2005. Published February 2006. Originally Bureau Dr., Stop 3460, Gaithersburg, MD 20899-3460.
approved in 1993. Last previous edition approved in 2000 as E1570 - 00. DOI: Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,
10.1520/E1570-00R05E01. Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098.
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For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available from American Society for Nondestructive Testing, 1711 Arlingate
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Plaza, P.O. Box 28518, Columbus, OH 43228-0518.
Standards volume information, refer to the standard’s Document Summary page on Available fromAerospace IndustriesAssociation ofAmerica, Inc. (AIA), 1250
the ASTM website. Eye St., NW, Washington, DC 20005.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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E1570–00 (2005)
4.2.4 Computer system including: the added safety feature of discontinued radiation production
4.2.4.1 Image reconstruction software/hardware, when switched off; however, the polychromaticity of the
4.2.4.2 Image display/analysis system, energy spectrum from an X-ray source causes artifacts such as
4.2.4.3 Data storage system, and beam hardening (the anomalous decreasing attenuation toward
4.2.4.4 Operator interface. the center of a homogeneous object) in the image if uncor-
4.3 Section 8 describes and defines the procedures for rected.
establishing and maintaining quality control of CT services.
7.2.1 X-rays produced from electrical radiation generators
4.4 The extent to which a CTimage reproduces an object or
have focal spot sizes ranging from a few millimetres down to
a feature within an object is influenced by spatial resolution,
a few micrometres. Reducing the focal spot size reduces
statistical noise, slice plane thickness, and artifacts of the
geometric unsharpness, thereby enhancing detail sensitivity.
imaging system. Operating parameters should strike an overall
Smaller focal spots permit higher spatial resolution, but at the
balance between image quality, inspection time, and cost.
expense of reduced X-ray beam intensity.
These parameters should be considered for CTsystem configu-
7.2.2 A radioisotope source can have the advantages of
rations, components, and procedures. The setting and optimi-
small physical size, portability, low power requirements, sim-
zation of CT system parameters is discussed in Section 9.
plicity, and stability of output. The disadvantages are limited
4.5 Methods for the measurement of CT system perfor-
intensity and limited peak energy.
mance are provided in Section 10 of this practice.
7.2.3 Synchrotron Radiation (SR) sources produce very
intense, naturally collimated, narrow bandwidth, tunable radia-
5. Significance and Use
tion. Thus, CT systems using SR sources can employ essen-
5.1 This practice is applicable for the systematic assessment
tially monochromatic radiation. With present technology, how-
of the internal structure of a material or assembly using CT
ever, practical SR energies are restricted to less than
technology. This practice may be used for review by system
approximately 20 to 30 keV. Since any CT system is limited to
operators, or to prescribe operating procedures for new or
the inspection of samples with radio-opacities consistent with
routine test objects.
the penetrating power of the X-ray employed, SR systems can
5.2 This practice provides the basis for the formation of a
in general image only small (about 1 mm) objects.
program for quality control and its continuation through
7.3 The detection system is a transducer that converts the
calibration, standardization, reference samples, inspection
transmitted radiation containing information about the test
plans, and procedures.
object into an electronic signal suitable for processing. The
detection system may consist of a single sensing element, a
6. Basis of Application
linear array of sensing elements, or an area array of sensing
6.1 This practice provides the approach for performing CT
elements. The more detectors used, the faster the required scan
examinations. Supplemental information covering specific
data can be collected; but there are important tradeoffs to be
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items where agreement between supplier and purchaser are
considered.
necessary is required. Generally the items are application
7.3.1 Asingle detector provides the least efficient method of
specific or performance related, or both. Examples include:
collecting data but entails minimal complexity, eliminates
system configuration, equipment qualification, performance
detector cross talk and detector matching, and allows an
measurement, and interpretation of results.
arbitrary degree of collimation and shielding to be imple-
mented.
7. System Configuration
7.3.2 Linear arrays have reasonable scan times at moderate
7.1 Many different CT system configurations are possible
complexity, acceptable cross talk and detector matching, and a
anditisimportanttounderstandtheadvantagesandlimitations
flexible architecture that typically accommodates good colli-
of each. It is important that the optimum system parameters be
mation and shielding. Most commercially available CT sys-
selected for each examination requirement, through a careful
tems employ a linear array of detectors.
analysis of the benefits and limitations of the available system
7.3.3 An area detector provides a fast method of collecting
components and the chosen system configuration.
data but entails the transfer and storage of large amounts of
7.2 Radiation Sources—While the CT systems may utilize
information, forces tradeoffs between cross talk and detector
either gamma-ray or X-ray generators, the latter is used for
efficiency, and creates serious collimation and shielding chal-
most applications. For a given focal spot size, X-ray generators
lenges.
(that is, X-ray tubes and linear accelerators) are several orders
7.4 ManipulationSystem—Themanipulationsystemhasthe
of magnitude more intense than isotope sources. Most X-ray
function of holding the test object and providing the necessary
generators are adjustable in peak energy and intensity and have
range of motions to position the test object between the
radiation source and detector. Two types of scan motion
geometries are most common: translate-rotate motion and
As used within this document, the supplier of computed tomographic service
rotate-only motion.
refers to the entity that physically provides the computed tomographic services.The
supplier may be a part of the same organization as the purchaser, or an outside
7.4.1 With translate-rotate motion, the object is translated in
organization.
adirectionperpendiculartothedirectionandintheplaneofthe
As used within this document, the purchaser of computed tomographic services
X-ray beam. Full data sets are obtained by rotating the test
refer to the entity that requires the computed tomographic services. The purchaser
may be a part of the same organization as the supplier, or an outside organization. object between translations by the fan angle of the beam and
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E1570–00 (2005)
again translating the object until a minimum of 180 degrees of 7.7 Image Display—The function of the image display is to
data have been acquired. The advantage of this design is conveyderivedinformation(thatis,animage)ofthetestobject
simplicity, good view-to-view detector matching, flexibility in to the system operator. For manual evaluation systems, the
the choice of scan parameters, and ability to accommodate a displayed image is used as the basis for accepting or rejecting
wide range of different object sizes including objects too big to thetestobject,subjecttotheoperator’sinterpretationoftheCT
besubtendedbytheX-rayfan.Thedisadvantageislongerscan data.
time.
7.7.1 Generally, CT image display requires a special graph-
7.4.2 With rotate-only motion, a complete view is collected ics monitor; television image presentation is of lower quality
by the detector array during each sampling interval. A rotate- but may be acceptable. Most industrial systems utilize color
only scan has lower motion penalty than a translate-rotate scan displays. These units can be switched between color and
and is attractive for industrial applications where the part to be gray-scale presentation to suit the preference of the viewer, but
examined fits within the fan beam and scan speed is important. it should be noted that gray-scale images presented on a color
monitor are not as sharp as those on a gray-scale monitor. The
7.5 Computer System—CT requires substantial computa-
useofcolorpermitstheviewertodistinguishagreaterrangeof
tional resources, such as a large capacity for image storage and
variations in an image than gray-scale does. Depending on the
archival and the ability to efficiently perform numerous math-
application, this may be an advantage or a disadvantage.
ematical computations, especially for the back-projection op-
Sharply contrasting colors may introduce false, distinct defini-
eration. Computational speed can be augmented by either
tion between boundaries. While at times advantageous, un-
generalized array processors or specialized back-projection
wanted instances can be corrected through the choice of color
hardware. The particular implementations will change as
(or monochrome) scale.
computerhardwareevolves,buthighcomputationalpowerwill
remain a fundamental requirement for efficient CT examina- 7.8 Data Storage Medium—Many CT applications require
tion. A separate workstation for image analysis and display an archival-quality record of the CT examination. This could
often is appropriate. be in the form of raw data or reconstructed data. Therefore,
formats and headers of digital data need to be specified so
7.6 Image Reconstruction Software— The aim of CT is to
information can be retrieved at a later date. Each archiving
obtain information regarding the nature of material occupying
system has its own specifics as to image quality, archival
exact positions inside a test object. In current CTscanners, this
storage properties, equipment, and media cost. Computer
information is obtained by “reconstructing” individual cross-
systems are designed to interface to a wide variety of periph-
sections of the test object from the measured intensity of X-ray
erals. As technology advances or needs change, or both,
beams transmitted through that cross section. An exact math-
equipment can be easily and affordably upgraded. The exami-
ematical theory of image reconstruction exists for idealized
nation record archiving system should be chosen on the basis
data. This theory is applied although the physical measure-
of these and other pertinent parameters, as agreed upon by the
ments do not fully meet the requirements of the theory. When
supplier and purchaser of CT servic
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