ASTM E1935-97

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 1935 – 97
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 3.1.1 The definitions of terms relating to CT, that appear in
Terminology E 1316 and Guide E 1441, shall apply to the
1.1 This test method covers instruction for determining the
terms used in this test method.
density calibration of X- and g-ray computed tomography (CT)
3.2 Definitions of Terms Specific to This Standard:
systems and for using this information to measure material
3.2.1 density calibration—calibration of a CT system for
densities from CT images. The calibration is based on an
accurate representation of material densities in test objects.
examination of the CT image of a disk of material with
3.2.2 effective energy—the equivalent monoenergetic en-
embedded specimens of known composition and density. The
ergy for a polyenergetic CT system. Thus, the actual, polyen-
measured mean CT values of the known standards are deter-
ergetic CT system yields the same measured attenuation
mined from an analysis of the image, and their linear attenu-
coefficient for a test object as a theoretical, monoenergetic CT
ation coefficients are determined by multiplying their measured
system at the effective energy.
physical density by their published mass attenuation coeffi-
3.2.3 phantom—a part or item being used to calibrate CT
cient. The density calibration is performed by applying a linear
density.
regression to the data. Once calibrated, the linear attenuation
3.2.4 test object—a part or specimen being subjected to CT
coefficient of an unknown feature in an image can be measured
examination.
from a determination of its mean CT value. Its density can then
be extracted from a knowledge of its mass attenuation coeffi-
4. Basis of Application
cient, or one representative of the feature.
4.1 The procedure is generic and requires mutual agreement
1.2 CT provides an excellent method of nondestructively
between purchaser and supplier on many points.
measuring density variations, which would be very difficult to
quantify otherwise. Density is inherently a volumetric property
5. Significance and Use
of matter. As the measurement volume shrinks, local material
5.1 This test method allows specification of the density
inhomogeneities become more important; and measured values
calibration procedures to be used to calibrate and perform
will begin to vary about the bulk density value of the material.
material density measurements using CT image data. Such
1.3 All values are stated in SI units.
measurements can be used to evaluate parts, characterize a
1.4 This standard does not purport to address the safety
particular system, or compare different systems, provided that
concerns, if any, associated with its use. It is the responsibility
observed variations are dominated by true changes in object
of the user of this standard to establish appropriate safety and
density rather than by image artifacts. The specified procedure
health practices and determine the applicability of regulatory
may also be used to determine the effective X-ray energy of a
limitations prior to use.
CT system.
2. Referenced Documents 5.2 The recommended test method is more accurate and less
susceptible to errors than alternative CT-based approaches,
2.1 ASTM Standards:
2 because it takes into account the effective energy of the CT
E 1316 Terminology for Nondestructive Examinations
2 system and the energy-dependent effects of the X-ray attenu-
E 1441 Guide for Computed Tomography (CT) Imaging
ation process.
E 1570 Practice for Computed Tomographic (CT) Exami-
2 5.3 This (or any) test method for measuring density is valid
nation
only to the extent that observed CT-number variations are
3. Terminology reflective of true changes in object density rather than image
artifacts. Artifacts are always present at some level and can
3.1 Definitions:
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
This test method is under the jurisdiction of ASTM Committee E-7 on
measurements; that is, they are performed in regions of the
Nondestructive Testing and is the direct responsibility of Subcommittee E07.01 on
Radiology (X and Gamma) Method. image which are not overly influenced by artifacts.
Current edition approved Dec. 10, 1997. Published June 1998.
Annual Book of ASTM Standards, Vol 03.03.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 1935
FIG. 1 Density Calibration Phantom
5.4 Linear attenuation and mass attenuation may be mea-
where:
sured in various ways. For a discussion of attenuation and
w = the weight fraction of the ith elemental component.
i
attenuation measurement, see Guide E 1441 and Practice
6.1.5 For each density standard, the measured density, r,
E 1570.
shall be multiplied by its corresponding mass attenuation
coefficient, μ/r, as determined in 6.1.5. The linear attenuation
6. Apparatus
coefficient, μ, thus obtained shall be permanently recorded for
6.1 Unless otherwise agreed upon between the purchaser
each density calibration standard.
and supplier, the density calibration phantom shall be con-
6.1.6 A host disk to hold the density standards shall be
structed as follows (see Fig. 1):
fabricated. The opacity of the disk should approximate the
6.1.1 A selection of density standards bracketing the range
attenuation range of the test objects. If possible, the host disk
of densities of interest shall be chosen. For best results, the
should be of the same material as the test objects, but other
materials should have known composition and should be
requirements take precedence and may dictate the selection of
physically homogeneous on a scale comparable to the spatial
another material.
resolution of the CT system. It is a good idea to radiographi-
6.2 In general, it is very difficult to find acceptable materials
cally verify homogeneity and to independently verify chemical
for density standards. Published density data are generally not
composition. All materials should be manufactured to repro-
reliable enough for calibration purposes. Homogeneity often
ducible standards. Solids should be readily machinable and not
varies on a local scale and negatively influences the calibration
susceptible to surface damage.
procedure. Machine damage can increase the density at the
6.1.2 One or more cylinders of each density standard shall
surface of a sample, making it difficult to determine the density
be machined or prepared, or both. Selecting cylinders over
of the interior material crucial to the calibration process.
rectangles reduces the uncertainties and streaks that sharp
Lot-to-lot variations in composition or alloy fraction can make
corners have on volumetric determination and verification
it difficult to compute mass attenuation coefficients. For these
methods. The cylinders should be large enough that the mean
and other reasons, development of a good density calibration
CT number corresponding to each standard can be computed
phantom takes effort, resources and a willingness to iterate the
over a hundred or more uncorrupted (see 8.1.3) pixels but small
selection and production of standards until acceptable results
enough relative to the dimensions of the host disk that radial
are obtained.
effects are minimal.
6.2.1 Liquids make the best standards, because they can be
6.1.3 The physical density of each density standard shall be
precisely controlled and measured. However, liquids require
determined empirically by weighing and measuring the speci-
special handling considerations, are sensitive to temperature
mens as accurately as possible. It is a good idea to indepen-
variations, and often tend to precipitate, especially high-
dently verify the measured densities using volumetric displace-
concentration aqueous solutions. It is hard to find organic
ment methods.
liquids with densities above 1.5 g/cm or inorganic liquids
6.1.4 The mass attenuation coefficient, μ/r, at the effective
above 4.0 g/cm ; but for many purposes, they offer a suitable
energy of the system (see 8.3) shall be determined from a
choice.
reference table. For compounds, μ/r can be obtained by taking
6.2.2 Plastics are popular but in general make the worst
the weighted sum of its constituents, in accordance with the
standards. Most plastics have at best an approximately known
following equation:
polymerization and often contain unknown or proprietary
μ 5 μ/r5 w ~μ/r! (1)
m ( i i
i additives, making them poor choices for calibration standards.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 1935
They also tend to vary more than other materials from batch to surface damage caused by machining or compression. Ideally,
batch. Notable exceptions to these generalizations are brand- a circular region of interest should be used that includes a
name acrylics and brand-name fluorocarbons. hundred or more pixels but avoids the boundary region around
6.2.3 Metals are also popular, but they are generally avail- each density standard, especially if edge effects of any type are
able only in a limited number of discrete densities. They can clearly visible.
exhibit important lot-to-lot variations in alloy fractions; but
8.1.4 A table of linear attenuation coefficients versus mean
with careful selection or characterization, they can make good
CT numbers shall be prepared.
density calibration standards. Pure elements or very well
8.1.5 A least-squares fit to the equation N = a·μ + b shall
CT
known specimens offer an excellent option when they can be
be performed on the data stored in the table, where μ is the
obtained in the density range of interest.
linear attenuation coefficient and N is the CT number.
CT
6.2.4 Each material must be treated on a case-by-case basis.
8.1.6 The resulting linear curve shall be used as the density
Reactor-grade graphite provides a good case study. Reactor-
calibration. Using the inferred linear relationship between CT
grade graphite is available in a variety of shapes, in very pure
number and linear attenuation coefficient, the measured CT
form, and in a number of densities. At first glance, it appears to
value, N , of any material can be used to calculate a best
CT
offer an attractive choice in a density range without many
estimate of its associated linear attenuation coefficient, μ.
viable alternatives. However, upon closer examination, the
8.2 Unless otherwise agreed upon between the purchaser
material is found to be susceptible to surface damage during
and supplier, the density of a region of interest in a test object
machining and to exhibit important inhomogeneities in density
shall be determined as follows:
on linear scales of about 1 mm. Surface damage makes it
8.2.1 The mean CT number in the region of interest shall be
nearly impossible to determine the core density of the sample
measured.
gravimetrically, because the total weight is biased by a denser
8.2.2 From the known calibration parameters, the linear
outer shell. Inhomogeneities make it difficult to extract accu-
attenuation coefficient of the region of interest shall be ob-
rate mean CT numbers from an image of a sample that is not
tained using the equation N = a·μ + b.
CT
large in diameter compared to 1 mm.
8.2.3 The density of the region of interest shall be calculated
by dividing the obtained linear attenuation by the appropriate
7. Procedure
tabulated value of μ/r at the effective energy of the system (see
7.1 Unless otherwise agreed upon between the purchaser
8.3). If μ/r is not known for the feature of interest, a nominal
and supplier, the density calibration phantom shall be scanned
value for μ/r may be used. Variations in μ/r are minor, and
as follows:
basically independent of material in the energy range of about
7.1.1 The phantom shall be mounted on the CT system with
200 keV to about 2 MeV. Outside this range, the selection of a
the orientation of its axis of revolution normal to the scan
nominal value is more sensitive (see 2.2). Adoption of an
plane.
appropriate nominal value is a matter of agreement between
7.1.2 The phantom shall be placed at the same location used
purchaser and supplier.
for test object scans.
8.3 Unless otherwise agreed upon between the purchaser
7.1.3 The slice plane shall be adjusted to intercept the
and supplier, the effective energy of the CT system shall be
phantom approximately midway between the flat faces of the
determined as follows:
disk.
8.3.1 A table of linear attenuation coefficients versus mean
7.1.4 The phantom shall be scanned using the same data
CT numbers shall be prepared for several X-ray energies
acquisition parameters, and the data shall be processed using
bracketing the effective energy of the CT system, as shown in
the same steps (for example, beam-hardening corrections)
8.4.1.
applied to test objects.
8.3.2 For each X-ray energy, a least-squares fit to the
8. Interpretation of Results equation N = a·μ + b shall be performed and the correlation
CT
coefficient recorded.
8.1 Unless otherwise agreed upon between the purchaser
8.3.3 The energy value in the table that yields the best fit
and supplier, the image of the density calibration phantom shall
(that is, the largest value of the correlation coefficient) shall be
be analyzed as follows:
selected as the effective energy of the CT system.
8.1.1 The phantom scan data shall be reconstructed using
8.3.4 If the effective energy has been determined previously
the same reconstruction parameters and post-processing steps,
under the same or similar conditions, this step may be skipped
if any, used for test object data.
with the consent of the buyer.
8.1.2 The phantom image shall be displayed using the same
8.4 Illustrative Examples:
display parameters used for viewing test object images.
8.1.3 The mean CT numbers of the density standards in the 8.4.1 Effective Energy Determination—The process of de-
CT image shall be measured. Special attention needs to be paid termining the effective X-ray energy of a CT system is
to this part of
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