ASTM E1441-11
(Guide)Standard Guide for Computed Tomography (CT) Imaging
Standard Guide for Computed Tomography (CT) Imaging
SIGNIFICANCE AND USE
This guide provides a tutorial introduction to the theory and use of computed tomography. This guide begins with a overview intended for the interested reader with a general technical background. Subsequent, more technical sections describe the physical and mathematical basis of CT technology, the hardware and software requirements of CT equipment, and the fundamental measures of CT performance. This guide includes an extensive glossary (with discussion) of CT terminology and an extensive list of references to more technical publications on the subject. Most importantly, this guide establishes consensus definitions for basic measures of CT performance, enabling purchasers and suppliers of CT systems and services to communicate unambiguously with reference to a recognized standard. This guide also provides a few carefully selected equations relating measures of CT performance to key system parameters.
General Description of Computed Tomography—CT is a radiographic inspection method that uses a computer to reconstruct an image of a cross-sectional plane (slice) through an object. The resulting cross-sectional image is a quantitative map of the linear X-ray attenuation coefficient, μ, at each point in the plane. The linear attenuation coefficient characterizes the local instantaneous rate at which X-rays are removed during the scan, by scatter or absorption, from the incident radiation as it propagates through the object (See 7.5). The attenuation of the X-rays as they interact with matter is a well-studied problem (1) and is the result of several different interaction mechanisms. For industrial CT systems with peak X-ray energy below a few MeV, all but a few minor effects can be accounted for in terms of the sum of just two interactions: photoelectric absorption and Compton scattering (1). The photoelectric interaction is strongly dependent on the atomic number and density of the absorbing medium; the Compton scattering is predominantly a function of the electron d...
SCOPE
1.1 Computed tomography (CT) is a radiographic method that provides an ideal examination technique whenever the primary goal is to locate and size planar and volumetric detail in three dimensions. Because of the relatively good penetrability of X-rays, as well as the sensitivity of absorption cross sections to atomic chemistry, CT permits the nondestructive physical and, to a limited extent, chemical characterization of the internal structure of materials. Also, since the method is X-ray based, it applies equally well to metallic and non-metallic specimens, solid and fibrous materials, and smooth and irregularly surfaced objects. When used in conjunction with other nondestructive evaluation (NDE) methods, such as ultrasound, CT data can provide evaluations of material integrity that cannot currently be provided nondestructively by any other means.
1.2 This guide is intended to satisfy two general needs for users of industrial CT equipment: (1) the need for a tutorial guide addressing the general principles of X-ray CT as they apply to industrial imaging; and (2) the need for a consistent set of CT performance parameter definitions, including how these performance parameters relate to CT system specifications. Potential users and buyers, as well as experienced CT inspectors, will find this guide a useful source of information for determining the suitability of CT for particular examination problems, for predicting CT system performance in new situations, and for developing and prescribing new scan procedures.
1.3 This guide does not specify test objects and test procedures for comparing the relative performance of different CT systems; nor does it treat CT inspection techniques, such as the best selection of scan parameters, the preferred implementation of scan procedures, the analysis of image data to extract densitometric information, or the establishment of accept/reject criteria for a new object.
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Designation: E1441 − 11
Standard Guide for
1
Computed Tomography (CT) Imaging
This standard is issued under the fixed designation E1441; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber 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 U.S. Department of Defense.
1. Scope* 1.4 Standard practices and methods are not within the
purview of this guide. The reader is advised, however, that
1.1 Computed tomography (CT) is a radiographic method
examination practices are generally part and application
that provides an ideal examination technique whenever the
specific, and industrial CT usage is new enough that in many
primary goal is to locate and size planar and volumetric detail
instances a consensus has not yet emerged. The situation is
in three dimensions. Because of the relatively good penetra-
complicated further by the fact that CT system hardware and
bility of X-rays, as well as the sensitivity of absorption cross
performance capabilities are still undergoing significant evo-
sections to atomic chemistry, CT permits the nondestructive
lution and improvement. Consequently, an attempt to address
physical and, to a limited extent, chemical characterization of
generic examination procedures is eschewed in favor of
the internal structure of materials. Also, since the method is
providing a thorough treatment of the principles by which
X-ray based, it applies equally well to metallic and non-
examination methods can be developed or existing ones
metallic specimens, solid and fibrous materials, and smooth
revised.
and irregularly surfaced objects. When used in conjunction
with other nondestructive evaluation (NDE) methods, such as
1.5 TheprincipaladvantageofCTisthatitnondestructively
ultrasound, CT data can provide evaluations of material integ-
provides quantitative densitometric (that is, density and geom-
rity that cannot currently be provided nondestructively by any
etry) images of thin cross sections through an object. Because
other means.
of the absence of structural noise from detail outside the thin
plane of inspection, images are much easier to interpret than
1.2 This guide is intended to satisfy two general needs for
conventionalradiographicdata.Thenewusercanlearnquickly
users of industrial CT equipment: (1) the need for a tutorial
(often upon first exposure to the technology) to read CT data
guide addressing the general principles of X-ray CT as they
because the images correspond more closely to the way the
applytoindustrialimaging;and(2)theneedforaconsistentset
human mind visualizes three-dimensional structures than con-
of CT performance parameter definitions, including how these
ventional projection radiography. Further, because CT images
performance parameters relate to CT system specifications.
are digital, they may be enhanced, analyzed, compressed,
Potential users and buyers, as well as experienced CT
archived, input as data into performance calculations, com-
inspectors, will find this guide a useful source of information
pared with digital data from other NDE modalities, or trans-
fordeterminingthesuitabilityofCTforparticularexamination
mitted to other locations for remote viewing.Additionally, CT
problems, for predicting CT system performance in new
images exhibit enhanced contrast discrimination over compact
situations, and for developing and prescribing new scan pro-
areas larger than 20 to 25 pixels. This capability has no
cedures.
classicalanalog.Contrastdiscriminationofbetterthan0.1%at
1.3 This guide does not specify test objects and test proce-
three-sigma confidence levels over areas as small as one-fifth
dures for comparing the relative performance of different CT
of one percent the size of the object of interest are common.
systems;nordoesittreatCTinspectiontechniques,suchasthe
1.6 With proper calibration, dimensional inspections and
bestselectionofscanparameters,thepreferredimplementation
of scan procedures, the analysis of image data to extract absolute density determinations can also be made very accu-
rately. Dimensionally, virtually all CT systems provide a pixel
densitometricinformation,ortheestablishmentofaccept/reject
criteria for a new object. resolution of roughly 1 part in 1000 , and metrological
algorithms can often measure dimensions to one-tenth of one
pixel or so with three-sigma accuracies. For small objects (less
1
than100mm(4in.)indiameter),thistranslatesintoaccuracies
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
of approximately 0.1 mm (0.003 to 0.005 i
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation:E1441–00 (Reapproved 2005) Designation: E1441 – 11
Standard Guide for
1
Computed Tomography (CT) Imaging
This standard is issued under the fixed designation E1441; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber 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*
1.1 Computed tomography (CT) is a radiographic method that provides an ideal examination technique whenever the primary
goal is to locate and size planar and volumetric detail in three dimensions. Because of the relatively good penetrability of X-rays,
aswellasthesensitivityofabsorptioncrosssectionstoatomicchemistry,CTpermitsthenondestructivephysicaland,toalimited
extent, chemical characterization of the internal structure of materials. Also, since the method is X-ray based, it applies equally
well to metallic and non-metallic specimens, solid and fibrous materials, and smooth and irregularly surfaced objects. When used
in conjunction with other nondestructive evaluation (NDE) methods, such as ultrasound, CT data can provide evaluations of
material integrity that cannot currently be provided nondestructively by any other means.
1.2 This guide is intended to satisfy two general needs for users of industrial CT equipment: (1) the need for a tutorial guide
addressing the general principles of X-ray CT as they apply to industrial imaging; and (2) the need for a consistent set of CT
performanceparameterdefinitions,includinghowtheseperformanceparametersrelatetoCTsystemspecifications.Potentialusers
andbuyers,aswellasexperiencedCTinspectors,willfindthisguideausefulsourceofinformationfordeterminingthesuitability
of CT for particular examination problems, for predicting CT system performance in new situations, and for developing and
prescribing new scan procedures.
1.3 ThisguidedoesnotspecifytestobjectsandtestproceduresforcomparingtherelativeperformanceofdifferentCTsystems;
nor does it treat CT inspection techniques, such as the best selection of scan parameters, the preferred implementation of scan
procedures, the analysis of image data to extract densitometric information, or the establishment of accept/reject criteria for a new
object.
1.4 Standard practices and methods are not within the purview of this guide. The reader is advised, however, that examination
practices are generally part and application specific, and industrial CT usage is new enough that in many instances a consensus
has not yet emerged. The situation is complicated further by the fact that CT system hardware and performance capabilities are
still undergoing significant evolution and improvement. Consequently, an attempt to address generic examination procedures is
eschewedinfavorofprovidingathoroughtreatmentoftheprinciplesbywhichexaminationmethodscanbedevelopedorexisting
ones revised.
1.5 TheprincipaladvantageofCTisthatitnondestructivelyprovidesquantitativedensitometric(thatis,densityandgeometry)
images of thin cross sections through an object. Because of the absence of structural noise from detail outside the thin plane of
inspection, images are much easier to interpret than conventional radiographic data. The new user can learn quickly (often upon
firstexposuretothetechnology)toreadCTdatabecausetheimagescorrespondmorecloselytothewaythehumanmindvisualizes
three-dimensional structures than conventional projection radiography. Further, because CT images are digital, they may be
enhanced,analyzed,compressed,archived,inputasdataintoperformancecalculations,comparedwithdigitaldatafromotherNDE
modalities,ortransmittedtootherlocationsforremoteviewing.Additionally,CTimagesexhibitenhancedcontrastdiscrimination
overcompactareaslargerthan20to25pixels.Thiscapabilityhasnoclassicalanalog.Contrastdiscriminationofbetterthan0.1%
at three-sigma confidence levels over areas as small as one-fifth of one percent the size of the object of interest are common.
1.6 With proper calibration, dimensional inspections and absolute density determinations can also be made very accurately.
Dimensionally,virtuallyallCTsystemsprovideapixelresolutionofroughly1partin1000(since,atpresent,102431024images
are the norm), , and metrological algorithms can often measure dimensions to one-tenth of one pixel or so with three-sigma
accuracies.Forsmallobjects(lessthan100mm(4in.)indiameter),thistranslatesintoaccu
...
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