ASTM E1441-00(2005)
(Guide)Standard Guide for Computed Tomography (CT) Imaging
Standard Guide for Computed Tomography (CT) Imaging
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.
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 eschewed in favor of providing a thorough treatment of the principles by which examination methods can be developed or existing ones revised.
1.5 The principal advantage of CT is that it nondestructively provides quantitative densitometric (that is, density and geometry) 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 first exposure to the technology) to read CT data because the images correspond more closely to the way the human mind visualizes three-dimensional structures than conventional projection radiography. Further, because CT images are digital, they may be enhanced, analyzed, compressed, archived, input as data into performance calculations, compared with digital data from other NDE modalities, or transmitted to other locations for remote viewing. Additionally, CT images exhibit enhanced contrast discrimination over compact areas larger than 20 to 25 pixels. This capability has no classical analog. Contrast discrimination of better than 0.1 % 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, virtually all CT systems provide a pixel resolution of roughly 1 part in 1000 (since, at presen...
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Designation: E1441 – 00 (Reapproved 2005)
Standard Guide for
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 examination practices are generally part and application spe-
cific, and industrial CT usage is new enough that in many
1.1 Computed tomography (CT) is a radiographic method
instances a consensus has not yet emerged. The situation is
that provides an ideal examination technique whenever the
complicated further by the fact that CT system hardware and
primary goal is to locate and size planar and volumetric detail
performance capabilities are still undergoing significant evo-
in three dimensions. Because of the relatively good penetra-
lution and improvement. Consequently, an attempt to address
bility of X-rays, as well as the sensitivity of absorption cross
generic examination procedures is eschewed in favor of
sections to atomic chemistry, CT permits the nondestructive
providing a thorough treatment of the principles by which
physical and, to a limited extent, chemical characterization of
examination methods can be developed or existing ones
the internal structure of materials. Also, since the method is
revised.
X-ray based, it applies equally well to metallic and non-
1.5 TheprincipaladvantageofCTisthatitnondestructively
metallic specimens, solid and fibrous materials, and smooth
provides quantitative densitometric (that is, density and geom-
and irregularly surfaced objects. When used in conjunction
etry) images of thin cross sections through an object. Because
with other nondestructive evaluation (NDE) methods, such as
of the absence of structural noise from detail outside the thin
ultrasound, CT data can provide evaluations of material integ-
plane of inspection, images are much easier to interpret than
rity that cannot currently be provided nondestructively by any
conventionalradiographicdata.Thenewusercanlearnquickly
other means.
(often upon first exposure to the technology) to read CT data
1.2 This guide is intended to satisfy two general needs for
because the images correspond more closely to the way the
users of industrial CT equipment: (1) the need for a tutorial
human mind visualizes three-dimensional structures than con-
guide addressing the general principles of X-ray CT as they
ventional projection radiography. Further, because CT images
applytoindustrialimaging;and(2)theneedforaconsistentset
are digital, they may be enhanced, analyzed, compressed,
of CT performance parameter definitions, including how these
archived, input as data into performance calculations, com-
performance parameters relate to CT system specifications.
pared with digital data from other NDE modalities, or trans-
Potential users and buyers, as well as experienced CT inspec-
mitted to other locations for remote viewing.Additionally, CT
tors, will find this guide a useful source of information for
images exhibit enhanced contrast discrimination over compact
determining the suitability of CT for particular examination
areas larger than 20 to 25 pixels. This capability has no
problems, for predicting CT system performance in new
classicalanalog.Contrastdiscriminationofbetterthan0.1%at
situations, and for developing and prescribing new scan pro-
three-sigma confidence levels over areas as small as one-fifth
cedures.
of one percent the size of the object of interest are common.
1.3 This guide does not specify test objects and test proce-
1.6 With proper calibration, dimensional inspections and
dures for comparing the relative performance of different CT
absolute density determinations can also be made very accu-
systems;nordoesittreatCTinspectiontechniques,suchasthe
rately. Dimensionally, virtually all CT systems provide a pixel
bestselectionofscanparameters,thepreferredimplementation
resolution of roughly 1 part in 1000 (since, at present,
of scan procedures, the analysis of image data to extract
1024 31024 images are the norm), and metrological algo-
densitometricinformation,ortheestablishmentofaccept/reject
rithms can often measure dimensions to one-tenth of one pixel
criteria for a new object.
or so with three-sigma accuracies. For small objects (less than
1.4 Standard practices and methods are not within the
4 in. in diameter), this translates into accuracies of approxi-
purview of this guide. The reader is advised, however, that
mately 0.1 mm (0.003 to 0.005 in.) at three-sigma. For much
larger objects, the corresponding figure will be proportionally
This guide is under the jurisdiction ofASTM Committee E07 on Nondestruc-
greater. Attenuation values can also be related accurately to
tive Testing and is the direct responsibility of Subcommittee E07.01 on Radiology
material densities. If details in the image are known to be pure
(X and Gamma) Method.
homogeneous elements, the density values may still be suffi-
Current edition approved Dec. 1, 2005. Published February 2006. Originally
approved in 1991. Last previous edition approved in 2000 as E1441-00. DOI:
cient to identify materials in some cases. For the case in which
10.1520/E1441-00R05.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1441 – 00 (2005)
no a priori information is available, CT densities cannot be graphic screening. One partial response to this problem is to
used to identify unknown materials unambiguously, since an use large slice thicknesses. This leads to reduced axial resolu-
infinite spectrum of compounds can be envisioned that will tion and can introduce partial volume artifacts in some cases;
yield any given observed attenuation. In this instance, the however, this is an acceptable tradeoff in many instances. In
exceptional density sensitivity of CT can still be used to principle, this drawback can be eliminated by resorting to full
determine part morphology and highlight structural irregulari- volumetric scans. However, since CT is to a large extent
ties. technology driven, volumetric CT systems are currently lim-
ited in the size of object that can be examined and the contrast
1.7 In some cases, dual energy (DE) CT scans can help
of features that can be discriminated.
identify unknown components. DE scans provide accurate
1.11 Complete part examinations demand large storage
electron density and atomic number images, providing better
capabilities or advanced display techniques, or both, and
characterizations of the materials. In the case of known
equipmenttohelptheoperatorreviewthehugevolumeofdata
materials, the additional information can be traded for im-
generated. This can be compensated for by state-of-the-art
proved conspicuity, faster scans, or improved characterization.
graphics hardware and automatic examination software to aid
In the case of unknown materials, the additional information
the user. However, automated accept/reject software is object
often allows educated guesses on the probable composition of
dependent and to date has been developed and employed in
an object to be made.
only a limited number of cases.
1.8 As with any modality, CT has its limitations. The most
1.12 The values stated in SI units are to be regarded as the
fundamental is that candidate objects for examination must be
standard. The values given in brackets are provided for
small enough to be accommodated by the handling system of
information only.
the CT equipment available to the user and radiometrically
1.13 This standard does not purport to address all of the
translucent at the X-ray energies employed by that particular
safety concerns, if any, associated with its use. It is the
system. Further, CT reconstruction algorithms require that a
responsibility of the user of this standard to establish appro-
full180degreesofdatabecollectedbythescanner.Objectsize
priate safety and health practices and determine the applica-
or opacity limits the amount of data that can be taken in some
bility of regulatory limitations prior to use.
instances. While there are methods to compensate for incom-
plete data which produce diagnostically useful images, the
2. Referenced Documents
resultant images are necessarily inferior to images from com-
2.1 ASTM Standards:
plete data sets. For this reason, complete data sets and
E1316 Terminology for Nondestructive Examinations
radiometrictransparencyshouldbethoughtofasrequirements.
E1570 Practice for Computed Tomographic (CT) Examina-
Current CT technology can accommodate attenuation ranges
tion
(peak-to-lowest-signal ratio) of approximately four orders of
magnitude. This information, in conjunction with an estimate
3. Terminology
of the worst-case chord through a new object and a knowledge
3.1 Definitions—CT, being a radiographic modality, uses
oftheaverageenergyoftheX-rayflux,canbeusedtomakean
much the same vocabulary as other X-ray techniques. A
educatedguessonthefeasibilityofscanningapartthathasnot
number of terms are not referenced, or are referenced without
been examined previously.
discussion, in Terminology E1316. Because they have mean-
1.9 Another potential drawback with CT imaging is the
ings or carry implications unique to CT, they appear with
possibility of artifacts in the data. As used here, an artifact is
explanation in Appendix X1. Throughout this guide, the term
anything in the image that does not accurately reflect true
“X-ray” is used to denote penetrating electromagnetic radia-
structure in the part being inspected. Because they are not real,
tion; however, electromagnetic radiation may be either X-rays
artifactslimittheuser’sabilitytoquantitativelyextractdensity,
or gamma rays.
dimensional, or other data from an image. Therefore, as with
3.2 Acronyms:Acronyms:
any technique, the user must learn to recognize and be able to
3.2.1 BW—beam width.
discount common artifacts subjectively. Some image artifacts
3.2.2 CDD—contrast-detail-dose.
canbereducedoreliminatedwithCTbyimprovedengineering
3.2.3 CT—computed tomography.
practice; others are inherent in the methodology. Examples of
3.2.4 CAT—computerized axial tomography.
the former include scattered radiation and electronic noise.
3.2.5 DR—digital radiography.
Examples of the latter include edge streaks and partial volume
3.2.6 ERF—edge response function.
effects. Some artifacts are a little of both. A good example is
3.2.7 LSF—line spread function.
the cupping artifact, which is due as much to radiation scatter
3.2.8 MTF—modulation transfer function.
(which can in principle be largely eliminated) as to the
3.2.9 NDE—nondestructive evaluation.
polychromaticityoftheX-rayflux(whichisinherentintheuse
3.2.10 PDF—probability distribution function.
of bremsstrahlung sources).
3.2.11 PSF—point spread function.
1.10 Because CT scan times are typically on the order of
minutes per image, complete three-dimensional CT examina-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
tions can be time consuming. Thus, less than 100% CT
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
examinationsareoftennecessaryormustbeaccommodatedby
Standards volume information, refer to the standard’s Document Summary page on
complementing the inspection process with digital radio- the ASTM website.
E1441 – 00 (2005)
4. Summary of Guide number and density of the absorbing medium; the Compton
scatteringispredominantlyafunctionoftheelectrondensityof
4.1 This guide provides a tutorial introduction to the tech-
the material. Photoelectric attenuation dominates at lower
nology and terminology of CT. It deals extensively with the
energies and becomes more important with higher atomic
physical and mathematical basis of CT, discusses the basic
number, while Compton scattering dominates at higher ener-
hardware configuration of all CT systems, defines a compre-
gies and becomes more important at lower atomic number. In
hensive set of fundamental CT performance parameters, and
specialsituations,thesedependenciescanbeusedtoadvantage
presents a useful method of characterizing and predicting
(see 7.6.2 and references therein).
system performance.Also, extensive descriptions of terms and
5.2.1 One particularly important property of the total linear
references to publications relevant to the subject are provided.
attenuation coefficient is that it is proportional to material
4.2 This guide is divided into three main sections. Sections
density, which is of course a fundamental physical property of
5 and 6 provide an overview of CT: defining the process,
all matter. The fact that CT images are proportional to density
discussing the performance characteristics of CT systems, and
isperhapstheprincipalvirtueofthetechnologyandthereason
describing the basic elements of all CT systems. Section 8
that image data are often thought of as representing the
addresses the physical and mathematical basis of CT imaging.
distribution of material density within the object being in-
Section 8 addresses in more detail a number of important
spected. This is a dangerous oversimplification, however. The
performance parameters as well as their characterization and
linearattenuationcoefficientalsocarriesanenergydependence
verification. This section is more technical than the other
that is a function of material composition. This feature of the
sections, but it is probably the most important of all. It
attenuation coefficient may or may not (depending on the
establishes a single, unified set of performance definitions and
materials and the energies of the X-rays involved) be more
relates them to more basic system parameters with a few
important than the basic density dependence. In some in-
carefully selected mathematical formulae.
stances, this effect can be detrimental, masking the density
5. Significance and Use differences in a CT image; in other instances, it can be used to
advantage, enhancing the contrast between different materials
5.1 This guide provides a tutorial introduction to the theory
of similar density.
and use of computed tomography. This guide begins with a
5.2.2 The fundamental difference between CT and conven-
overview intended for the interested reader with a general
tional radiography is shown in Fig. 1. In conventional radiog-
technical background. Subsequent, more technical sections
raphy, information on the slice plane “P” projects into a single
describethephysicalandmathematicalbasisofCTtechnology,
line, “A-A;”
...
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