ASTM E1935-97(2019)
(Test Method)Standard Test Method for Calibrating and Measuring CT Density
Standard Test Method for Calibrating and Measuring CT Density
SIGNIFICANCE AND USE
5.1 This test method allows specification of the density calibration procedures to be used to calibrate and perform material density measurements using CT image data. Such measurements can be used to evaluate parts, characterize a particular system, or compare different systems, provided that observed variations are dominated by true changes in object density rather than by image artifacts. The specified procedure may also be used to determine the effective X-ray energy of a CT system.
5.2 The recommended test method is more accurate and less susceptible to errors than alternative CT-based approaches, because it takes into account the effective energy of the CT system and the energy-dependent effects of the X-ray attenuation process.
FIG. 1 Density Calibration Phantom
5.3 This (or any) test method for measuring density is valid only to the extent that observed CT-number variations are reflective of true changes in object density rather than image artifacts. Artifacts are always present at some level and can masquerade as density variations. Beam hardening artifacts are particularly detrimental. It is the responsibility of the user to determine or establish, or both, the validity of the density measurements; that is, they are performed in regions of the image which are not overly influenced by artifacts.
5.4 Linear attenuation and mass attenuation may be measured in various ways. For a discussion of attenuation and attenuation measurement, see Guide E1441 and Practice E1570.
SCOPE
1.1 This test method covers instruction for determining the density calibration of X- and γ-ray computed tomography (CT) systems and for using this information to measure material densities from CT images. The calibration is based on an examination of the CT image of a disk of material with embedded specimens of known composition and density. The measured mean CT values of the known standards are determined from an analysis of the image, and their linear attenuation coefficients are determined by multiplying their measured physical density by their published mass attenuation coefficient. The density calibration is performed by applying a linear regression to the data. Once calibrated, the linear attenuation coefficient of an unknown feature in an image can be measured from a determination of its mean CT value. Its density can then be extracted from a knowledge of its mass attenuation coefficient, or one representative of the feature.
1.2 CT provides an excellent method of nondestructively measuring density variations, which would be very difficult to quantify otherwise. Density is inherently a volumetric property of matter. As the measurement volume shrinks, local material inhomogeneities become more important; and measured values will begin to vary about the bulk density value of the material.
1.3 All values are stated in SI units.
1.4 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
General Information
Relations
Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E1935 − 97 (Reapproved 2019)
Standard Test Method for
Calibrating and Measuring CT Density
This standard is issued under the fixed designation E1935; 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.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 This test method covers instruction for determining the
E1316 Terminology for Nondestructive Examinations
density calibration of X- and γ-ray computed tomography (CT)
E1441 Guide for Computed Tomography (CT)
systems and for using this information to measure material
E1570 Practice for Fan Beam Computed Tomographic (CT)
densities from CT images. The calibration is based on an
Examination
examination of the CT image of a disk of material with
embedded specimens of known composition and density. The
3. Terminology
measured mean CT values of the known standards are deter-
mined from an analysis of the image, and their linear attenu-
3.1 Definitions:
ationcoefficientsaredeterminedbymultiplyingtheirmeasured
3.1.1 The definitions of terms relating to CT, that appear in
physical density by their published mass attenuation coeffi-
Terminology E1316 and Guide E1441, shall apply to the terms
cient.The density calibration is performed by applying a linear
used in this test method.
regression to the data. Once calibrated, the linear attenuation
3.2 Definitions of Terms Specific to This Standard:
coefficient of an unknown feature in an image can be measured
3.2.1 density calibration—calibration of a CT system for
from a determination of its mean CTvalue. Its density can then
accurate representation of material densities in examination
be extracted from a knowledge of its mass attenuation
objects.
coefficient, or one representative of the feature.
3.2.2 effective energy—theequivalentmonoenergeticenergy
1.2 CT provides an excellent method of nondestructively
for a polyenergetic CT system. Thus, the actual, polyenergetic
measuring density variations, which would be very difficult to
CTsystem yields the same measured attenuation coefficient for
quantify otherwise. Density is inherently a volumetric property
an examination object as a theoretical, monoenergetic CT
of matter. As the measurement volume shrinks, local material
system at the effective energy.
inhomogeneities become more important; and measured values
3.2.3 phantom—a part or item being used to calibrate CT
will begin to vary about the bulk density value of the material.
density.
1.3 All values are stated in SI units.
3.2.4 examination object—a part or specimen being sub-
jected to CT examination.
1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
4. Basis of Application
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
4.1 The procedure is generic and requires mutual agreement
mine the applicability of regulatory limitations prior to use. between purchaser and supplier on many points.
1.5 This international standard was developed in accor-
5. Significance and Use
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
5.1 This test method allows specification of the density
Development of International Standards, Guides and Recom-
calibration procedures to be used to calibrate and perform
mendations issued by the World Trade Organization Technical
material density measurements using CT image data. Such
Barriers to Trade (TBT) Committee.
measurements can be used to evaluate parts, characterize a
particular system, or compare different systems, provided that
observed variations are dominated by true changes in object
This test method is under the jurisdiction of ASTM Committee E07 on
Nondestructive Testing and is the direct responsibility of Subcommittee E07.01 on
Radiology (X and Gamma) Method. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Dec. 1, 2019. Published January 2020. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1997. Last previous edition approved in 2013 as E1935 – 97(2013). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/E1935-97R19. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1935 − 97 (2019)
density rather than by image artifacts. The specified procedure 6.1.2 One or more cylinders of each density standard shall
may also be used to determine the effective X-ray energy of a be machined or prepared, or both. Selecting cylinders over
CT system.
rectangles reduces the uncertainties and streaks that sharp
corners have on volumetric determination and verification
5.2 The recommended test method is more accurate and less
methods. The cylinders should be large enough that the mean
susceptible to errors than alternative CT-based approaches,
CT number corresponding to each standard can be computed
because it takes into account the effective energy of the CT
overahundredormoreuncorrupted(see8.1.3)pixelsbutsmall
system and the energy-dependent effects of the X-ray attenu-
enough relative to the dimensions of the host disk that radial
ation process.
effects are minimal.
5.3 This (or any) test method for measuring density is valid
6.1.3 The physical density of each density standard shall be
only to the extent that observed CT-number variations are
determined empirically by weighing and measuring the speci-
reflective of true changes in object density rather than image
mens as accurately as possible. It is a good idea to indepen-
artifacts. Artifacts are always present at some level and can
dently verify the measured densities using volumetric displace-
masquerade as density variations. Beam hardening artifacts are
ment methods.
particularly detrimental. It is the responsibility of the user to
determine or establish, or both, the validity of the density 6.1.4 The mass attenuation coefficient, µ/ρ, at the effective
measurements; that is, they are performed in regions of the
energy of the system (see 8.3) shall be determined from a
image which are not overly influenced by artifacts.
reference table. For compounds, µ/ρ can be obtained by taking
the weighted sum of its constituents, in accordance with the
5.4 Linear attenuation and mass attenuation may be mea-
following equation:
sured in various ways. For a discussion of attenuation and
attenuation measurement, see Guide E1441 and Practice
µ 5 µ/ρ 5 w µ/ρ (1)
~ !
m ( i
i
i
E1570.
where:
6. Apparatus
w = the weight fraction of the ith elemental component.
i
6.1 Unless otherwise agreed upon between the purchaser
6.1.5 For each density standard, the measured density, ρ,
and supplier, the density calibration phantom shall be con-
shall be multiplied by its corresponding mass attenuation
structed as follows (see Fig. 1):
coefficient, µ/ρ, as determined in 6.1.4. The linear attenuation
6.1.1 A selection of density standards bracketing the range
coefficient, µ, thus obtained shall be permanently recorded for
of densities of interest shall be chosen. For best results, the
each density calibration standard.
materials should have known composition and should be
6.1.6 A host disk to hold the density standards shall be
physically homogeneous on a scale comparable to the spatial
fabricated. The opacity of the disk should approximate the
resolution of the CT system. It is a good idea to radiographi-
attenuation range of the examination objects. If possible, the
cally verify homogeneity and to independently verify chemical
host disk should be of the same material as the examination
composition. All materials should be manufactured to repro-
ducible standards. Solids should be readily machinable and not objects, but other requirements take precedence and may
dictate the selection of another material.
susceptible to surface damage.
FIG. 1 Density Calibration Phantom
E1935 − 97 (2019)
6.2 In general, it is very difficult to find acceptable materials 7.1.3 The slice plane shall be adjusted to intercept the
for density standards. Published density data are generally not phantom approximately midway between the flat faces of the
reliable enough for calibration purposes. Homogeneity often disk.
varies on a local scale and negatively influences the calibration
7.1.4 The phantom shall be scanned using the same data
procedure. Machine damage can increase the density at the
acquisition parameters, and the data shall be processed using
surface of a sample, making it difficult to determine the density
the same steps (for example, beam-hardening corrections)
of the interior material crucial to the calibration process.
applied to examination objects.
Lot-to-lot variations in composition or alloy fraction can make
it difficult to compute mass attenuation coefficients. For these
8. Interpretation of Results
and other reasons, development of a good density calibration
8.1 Unless otherwise agreed upon between the purchaser
phantom takes effort, resources and a willingness to iterate the
andsupplier,theimageofthedensitycalibrationphantomshall
selection and production of standards until acceptable results
be analyzed as follows:
are obtained.
8.1.1 The phantom scan data shall be reconstructed using
6.2.1 Liquids make the best standards, because they can be
the same reconstruction parameters and post-processing steps,
precisely controlled and measured. However, liquids require
special handling considerations, are sensitive to temperature if any, used for examination object data.
variations, and often tend to precipitate, especially high-
8.1.2 The phantom image shall be displayed using the same
concentration aqueous solutions. It is hard to find organic
display parameters used for viewing examination object im-
liquids with densities above 1.5 g/cm or inorganic liquids
ages.
above 4.0 g/cm ; but for many purposes, they offer a suitable
8.1.3 The mean CT numbers of the density standards in the
choice.
CTimage shall be measured. Special attention needs to be paid
6.2.2 Plastics are popular but in general make the worst
to this part of the measurement process.As much of the area of
standards. Most plastics have at best an approximately known
each specimen as practical should be used, but care must be
polymerization and often contain unknown or proprietary
takentoinsurethatonlyvalidpixelsareincluded.Forexample,
additives, making them poor choices for calibration standards.
a square region of interest in a round sample could yield biased
They also tend to vary more than other materials from batch to
results if there are significant radial effects, such as from beam
batch. Notable exceptions to these generalizations are brand-
hardening or a higher density around the perimeter due to
name acrylics and brand-name fluorocarbons.
surface damage caused by machining or compression. Ideally,
6.2.3 Metals are also popular, but they are generally avail-
a circular region of interest should be used that includes a
able only in a limited number of discrete densities. They can
hundred or more pixels but avoids the boundary region around
exhibit important lot-to-lot variations in alloy fractions; but
each density standard, especially if edge effects of any type are
with careful selection or characterization, they can make good
clearly visible.
density calibration standards. Pure elements or very well
8.1.4 A table of linear attenuation coefficients versus mean
known specimens offer an excellent option when they can be
CT numbers shall be prepared.
obtained in the density range of interest.
8.1.5 A least-squares fit to the equation N = a·µ + b shall
CT
6.2.4 Each material must be treated on a case-by-case basis.
be performed on the data stored in the table, where µ is the
Reactor-grade graphite provides a good case study. Reactor-
linear attenuation coefficient and N is the CT number.
CT
grade graphite is available in a variety of shapes, in very pure
8.1.6 The resulting linear curve shall be used as the density
form, and in a number of densities.At first glance, it appears to
calibration. Using the inferred linear relationship between CT
offer an attractive choice in a density range without many
number and linear attenuation coefficient, the measured CT
viable alternatives. However, upon closer examination, the
value, N , of any material can be used to calculate a best
material is found to be susceptible to surface damage during CT
estimate of its associated linear attenuation coefficient, µ.
machining and to exhibit important inhomogeneities in density
on linear scales of about 1 mm. Surface damage makes it
8.2 Unless otherwise agreed upon between the purchaser
nearly impossible to determine the core density of the sample
and supplier, the density of a region of interest in an exami-
gravimetrically, because the total weight is biased by a denser
nation object shall be determined as follows:
outer shell. Inhomogeneities make it difficult to extract accu-
8.2.1 The mean CT number in the region of interest shall be
rate mean CT numbers from an image of a sample that is not
measured.
large in diameter compared to 1 mm.
8.2.2 From the known calibration parameters, the linear
7. Procedure
attenuation coefficient of the region of interest shall be ob-
tained using the equation N = a·µ + b.
7.1 Unless otherwise agreed upon between the purchaser CT
and supplier, the density calibration phantom shall be scanned 8.2.3 Thedensityoftheregionofinterestshallbecalculated
as follows: by dividing the obtained linear attenuation by the appropriate
7.1.1 The phantom shall be mounted on the CT system with tabulated value of µ/ρ at the effective energy of the system (see
the orientation of its axis of revolution normal to the scan 8.3). If µ/ρ is not known for the feature of interest, a nominal
plane. value for µ/ρ may be used. Variations in µ/ρ are minor, and
7.1.2 The phantom shall be placed at the same location used basically independent of material in the energy range of about
for examination object scans. 200 keV to about 2 MeV. Outside this range, the selection of a
E1935 − 97 (2019)
TABLE 2 Density Calibration Data at an Effective Energy of 3800
nominal val
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