ASTM E1935-97(2003)

Standard Test Method for Calibrating and Meausring CT Density

SIGNIFICANCE AND USE
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.
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.
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.
Linear attenuation and mass attenuation may be measured in various ways. For a discussion of attenuation and attenuation measurement, see Guide E 1441 and Practice E 1570.
FIG. 1 Density Calibration Phantom
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 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.

General Information

• Status: Historical
• Publication Date: 09-Mar-2003
• ICS: 35.240.80 - IT applications in health care technology
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.01 - Radiology (X and Gamma) Method

Relations

• Replaces: ASTM E1935-97 - Standard Test Method for Calibrating and Meausring CT Density
• Effective Date: 10-Mar-2003
• Replaced By: ASTM E1935-97(2008) - Standard Test Method for Calibrating and Meausring CT Density
• Effective Date: 15-Dec-2008
• Referred By: ASTM E3166-20e1 - Standard Guide for Nondestructive Examination of Metal Additively Manufactured Aerospace Parts After Build
• Effective Date: 10-Mar-2003
• Referred By: ASTM E2533-21 - Standard Guide for Nondestructive Examination of Polymer Matrix Composites Used in Aerospace Applications
• Effective Date: 10-Mar-2003
• Referred By: ASTM E1441-19 - Standard Guide for Computed Tomography (CT)
• Effective Date: 10-Mar-2003
• Referred By: ASTM E1814-14(2022) - Standard Practice for Computed Tomographic (CT) Examination of Castings
• Effective Date: 10-Mar-2003
• Referred By: ASTM F3637-23 - Standard Guide for Additive Manufacturing of Metal — Finished Part Properties — Methods for Relative Density Measurement
• Effective Date: 10-Mar-2003

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:E1935–97 (Reapproved 2003)
Standard Test Method for
Calibrating and Measuring CT Density
This standard is issued under the fixed designation E 1935; 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 3. Terminology
1.1 This test method covers instruction for determining the 3.1 Definitions:
densitycalibrationofX-and g-raycomputedtomography(CT) 3.1.1 The definitions of terms relating to CT, that appear in
systems and for using this information to measure material Terminology E 1316 and Guide E 1441, shall apply to the
densities from CT images. The calibration is based on an terms used in this test method.
examination of the CT image of a disk of material with 3.2 Definitions of Terms Specific to This Standard:
embedded specimens of known composition and density. The 3.2.1 density calibration—calibration of a CT system for
measured mean CT values of the known standards are deter- accurate representation of material densities in examination
mined from an analysis of the image, and their linear attenu- objects.
ationcoefficientsaredeterminedbymultiplyingtheirmeasured 3.2.2 effective energy—the equivalent monoenergetic en-
physical density by their published mass attenuation coeffi- ergy for a polyenergetic CT system. Thus, the actual, polyen-
cient.The density calibration is performed by applying a linear ergetic CT system yields the same measured attenuation
regression to the data. Once calibrated, the linear attenuation coefficient for an examination object as a theoretical, monoen-
coefficient of an unknown feature in an image can be measured ergetic CT system at the effective energy.
from a determination of its mean CTvalue. Its density can then 3.2.3 phantom—a part or item being used to calibrate CT
be extracted from a knowledge of its mass attenuation coeffi- density.
cient, or one representative of the feature. 3.2.4 examination object—a part or specimen being sub-
1.2 CT provides an excellent method of nondestructively jected to CT examination.
measuring density variations, which would be very difficult to
4. Basis of Application
quantify otherwise. Density is inherently a volumetric property
4.1 The procedure is generic and requires mutual agreement
of matter. As the measurement volume shrinks, local material
between purchaser and supplier on many points.
inhomogeneities become more important; and measured values
will begin to vary about the bulk density value of the material.
5. Significance and Use
1.3 All values are stated in SI units.
5.1 This test method allows specification of the density
1.4 This standard does not purport to address the safety
calibration procedures to be used to calibrate and perform
concerns, if any, associated with its use. It is the responsibility
material density measurements using CT image data. Such
of the user of this standard to establish appropriate safety and
measurements can be used to evaluate parts, characterize a
health practices and determine the applicability of regulatory
particular system, or compare different systems, provided that
limitations prior to use.
observed variations are dominated by true changes in object
2. Referenced Documents
density rather than by image artifacts. The specified procedure
2.1 ASTM Standards: may also be used to determine the effective X-ray energy of a
CT system.
E 1316 Terminology for Nondestructive Examinations
E 1441 Guide for Computed Tomography (CT) Imaging 5.2 The recommended test method is more accurate and less
susceptible to errors than alternative CT-based approaches,
E 1570 Practice for Computed Tomographic (CT) Exami-
nation because it takes into account the effective energy of the CT
system and the energy-dependent effects of the X-ray attenu-
ation process.
5.3 This (or any) test method for measuring density is valid
This test method is under the jurisdiction of ASTM Committee E07 on
Nondestructive Testing and is the direct responsibility of Subcommittee E07.01 on
only to the extent that observed CT-number variations are
Radiology (X and Gamma) Method.
reflective of true changes in object density rather than image
Current edition approved March 10, 2003. Published May 2003. Originally
artifacts. Artifacts are always present at some level and can
approved in 1997. Last previous edition approved in 1997 as E 1935 - 97.
Annual Book of ASTM Standards, Vol 03.03. masquerade as density variations. Beam hardening artifacts are
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1935–97 (2003)
FIG. 1 Density Calibration Phantom
particularly detrimental. It is the responsibility of the user to reference table. For compounds, µ/r can be obtained by taking
determine or establish, or both, the validity of the density
the weighted sum of its constituents, in accordance with the
measurements; that is, they are performed in regions of the
following equation:
image which are not overly influenced by artifacts.
µ 5 µ/r5 w ~µ/r! (1)
(
m i i
5.4 Linear attenuation and mass attenuation may be mea- i
sured in various ways. For a discussion of attenuation and
where:
attenuation measurement, see Guide E 1441 and Practice
w = the weight fraction of the ith elemental component.
i
E 1570.
6.1.5 For each density standard, the measured density, r,
6. Apparatus shall be multiplied by its corresponding mass attenuation
coefficient, µ/r, as determined in 6.1.4. The linear attenuation
6.1 Unless otherwise agreed upon between the purchaser
coefficient, µ, thus obtained shall be permanently recorded for
and supplier, the density calibration phantom shall be con-
each density calibration standard.
structed as follows (see Fig. 1):
6.1.1 A selection of density standards bracketing the range 6.1.6 A host disk to hold the density standards shall be
of densities of interest shall be chosen. For best results, the
fabricated. The opacity of the disk should approximate the
materials should have known composition and should be
attenuation range of the examination objects. If possible, the
physically homogeneous on a scale comparable to the spatial
host disk should be of the same material as the examination
resolution of the CT system. It is a good idea to radiographi-
objects, but other requirements take precedence and may
cally verify homogeneity and to independently verify chemical
dictate the selection of another material.
composition. All materials should be manufactured to repro-
6.2 In general, it is very difficult to find acceptable materials
ducible standards. Solids should be readily machinable and not
for density standards. Published density data are generally not
susceptible to surface damage.
reliable enough for calibration purposes. Homogeneity often
6.1.2 One or more cylinders of each density standard shall
varies on a local scale and negatively influences the calibration
be machined or prepared, or both. Selecting cylinders over
procedure. Machine damage can increase the density at the
rectangles reduces the uncertainties and streaks that sharp
surface of a sample, making it difficult to determine the density
corners have on volumetric determination and verification
of the interior material crucial to the calibration process.
methods. The cylinders should be large enough that the mean
Lot-to-lot variations in composition or alloy fraction can make
CT number corresponding to each standard can be computed
it difficult to compute mass attenuation coefficients. For these
overahundredormoreuncorrupted(see8.1.3)pixelsbutsmall
and other reasons, development of a good density calibration
enough relative to the dimensions of the host disk that radial
phantom takes effort, resources and a willingness to iterate the
effects are minimal.
selection and production of standards until acceptable results
6.1.3 The physical density of each density standard shall be
are obtained.
determined empirically by weighing and measuring the speci-
6.2.1 Liquids make the best standards, because they can be
mens as accurately as possible. It is a good idea to indepen-
precisely controlled and measured. However, liquids require
dently verify the measured densities using volumetric displace-
ment methods. special handling considerations, are sensitive to temperature
variations, and often tend to precipitate, especially high-
6.1.4 The mass attenuation coefficient, µ/r, at the effective
energy of the system (see 8.3) shall be determined from a concentration aqueous solutions. It is hard to find organic
E1935–97 (2003)
liquids with densities above 1.5 g/cm or inorganic liquids 8.1.3 The mean CT numbers of the density standards in the
above 4.0 g/cm ; but for many purposes, they offer a suitable CTimage shall be measured. Special attention needs to be paid
choice. to this part of the measurement process.As much of the area of
6.2.2 Plastics are popular but in general make the worst each specimen as practical should be used, but care must be
standards. Most plastics have at best an approximately known takentoinsurethatonlyvalidpixelsareincluded.Forexample,
polymerization and often contain unknown or proprietary a square region of interest in a round sample could yield biased
additives, making them poor choices for calibration standards. results if there are significant radial effects, such as from beam
They also tend to vary more than other materials from batch to hardening or a higher density around the perimeter due to
batch. Notable exceptions to these generalizations are brand- surface damage caused by machining or compression. Ideally,
name acrylics and brand-name fluorocarbons. a circular region of interest should be used that includes a
6.2.3 Metals are also popular, but they are generally avail- hundred or more pixels but avoids the boundary region around
able only in a limited number of discrete densities. They can each density standard, especially if edge effects of any type are
exhibit important lot-to-lot variations in alloy fractions; but clearly visible.
with careful selection or characterization, they can make good 8.1.4 A table of linear attenuation coefficients versus mean
density calibration standards. Pure elements or very well
CT numbers shall be prepared.
known specimens offer an excellent option when they can be
8.1.5 A least-squares fit to the equation N = a·µ + b shall
CT
obtained in the density range of interest.
be performed on the data stored in the table, where µ is the
6.2.4 Each material must be treated on a case-by-case basis.
linear attenuation coefficient and N is the CT number.
CT
Reactor-grade graphite provides a good case study. Reactor-
8.1.6 The resulting linear curve shall be used as the density
grade graphite is available in a variety of shapes, in very pure
calibration. Using the inferred linear relationship between CT
form, and in a number of densities.At first glance, it appears to
number and linear attenuation coefficient, the measured CT
offer an attractive choice in a density range without many
value, N , of any material can be used to calculate a best
CT
viable alternatives. However, upon closer examination, the
estimate of its associated linear attenuation coefficient, µ.
material is found to be susceptible to surface damage during
8.2 Unless otherwise agreed upon between the purchaser
machining and to exhibit important inhomogeneities in density
and supplier, the density of a region of interest in an exami-
on linear scales of about 1 mm. Surface damage makes it
nation object shall be determined as follows:
nearly impossible to determine the core density of the sample
8.2.1 The mean CT number in the region of interest shall be
gravimetrically, because the total weight is biased by a denser
measured.
outer shell. Inhomogeneities make it difficult to extract accu-
8.2.2 From the known calibration parameters, the linear
rate mean CT numbers from an image of a sample that is not
attenuation coefficient of the region of interest shall be ob-
large in diameter compared to 1 mm.
tained using the equation N = a·µ + b.
CT
8.2.3 Thedensityoftheregionofinterestshallbecalculated
7. Procedure
by dividing the obtained linear attenuation by the appropriate
7.1 Unless otherwise agreed upon between the purchaser
tabulated value of µ/r at the effective energy of the system (see
and supplier, the density calibration phantom shall be scanned
8.3). If µ/r is not known for the feature of interest, a nominal
as follows:
value for µ/r may be used. Variations in µ/r are minor, and
7.1.1 The phantom shall be mounted on the CT system with
basically independent of material in the energy range of about
the orientation of its axis of revolution normal to the scan
200 keV to about 2 MeV. Outside this range, the selection of a
plane.
nominal value is more sensitive. Adoption of an appropriate
7.1.2 The phantom shall be placed at the same location used
nominal value is a matter of agreement between purchaser and
for examination object scans.
supplier.
7.1.3 The slice plane shall be adjusted to intercept the
8.3 Unless otherwise agreed upon between the purchaser
phantom approximately midway between the flat faces of the
and supplier, the effective energy of the CT system shall be
disk.
determined as follows:
7.1.4 The phantom shall be scanned using the same data
8.3.1 A table of linear attenuation coefficients versus mean
acquisition parameters, and the data shall be processed using
CT numbers shall be prepared for several X-ray energies
the same steps (for example, beam-hardening corrections)
bracketing the effective energy of the CT system, as shown in
applied to examination objects.
8.4.1.
8. Interpretation of Results 8.3.2 For each X-ray energy, a least-squares fit to the
equation N = a·µ + b shall be performed and the correlation
8.1 Unless otherwise agreed upon between the purchaser CT
coefficient recorded.
andsupplier,theimageofthedensitycalibrationphantomshall
8.3.3 The energy value in the table that yields the best fit
be analyzed as follows:
(that is, the largest value of the correlation coefficient) shall be
8.1.1 The phantom scan data shall be reconstructed using
selected as the effective energy of the CT system.
the same reconstruction parameters and post-processing steps,
8.3.4 If the effective energy has been determined previously
if any, used for examination object data.
under the same or similar conditions, this step may be skipped
8.1.2 The phantom image shall be displayed using the same
with the consent of the buyer.
display parameters used fo
...