ASTM E1695-95
(Test Method)Standard Test Method for Measurement of Computed Tomography (CT) System Performance
Standard Test Method for Measurement of Computed Tomography (CT) System Performance
SCOPE
1.1 This test method provides instruction for determining the spatial resolution and contrast sensitivity in X-ray and -ray computed tomography (CT) images. The determination is based on examination of the CT image of a uniform disk of material. The spatial resolution measurement is derived from an image analysis of the sharpness at the edge of the disk. The contrast sensitivity measurement is derived from an image analysis of the statistical noise at the center of the disk.
1.2 This test method is more quantitative and less susceptible to interpretation than alternative approaches because the required disk is easy to fabricate and the analysis is immune to cupping artifacts. This test method may not yield meaningful results if the disk image occupies less than a significant fraction of the field of view.
1.3 This test method may also be used to evaluate other performance parameters. Among those characteristics of a CT system that are detectable with this test method are: the mid-frequency enhancement of the reconstruction kernel, the presence (or absence) of detector crosstalk, the undersampling of views, and the clipping of unphysical (that is, negative) CT numbers (see Air Force Technical Report WL-TR-94-4021). It is highly likely that other characteristics as well can be detected with this test method.
1.4 The values stated in SI units are to be regarded as the standard. Inch-pound units are provided for information only.
1.5 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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E 1695 – 95 An American National Standard
Standard Test Method for
Measurement of Computed Tomography (CT) System
Performance
This standard is issued under the fixed designation E 1695; 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 E 1570 Practice for Computed Tomographic (CT) Exami-
nation
1.1 This test method provides instruction for determining
the spatial resolution and contrast sensitivity in X-ray and
3. Terminology
g-ray computed tomography (CT) images. The determination
3.1 Definitions—The definitions of terms relating to
is based on examination of the CT image of a uniform disk of
Gamma- and X-Radiology, which appear in Terminology
material. The spatial resolution measurement is derived from
E 1316 and Guide E 1441, shall apply to the terms used in this
an image analysis of the sharpness at the edge of the disk. The
test method.
contrast sensitivity measurement is derived from an image
3.2 Definitions of Terms Specific to This Standard:
analysis of the statistical noise at the center of the disk.
3.2.1 phantom—a part or item being used to quantify CT
1.2 This test method is more quantitative and less suscep-
system performance.
tible to interpretation than alternative approaches because the
3.2.2 test object—a part or specimen being subjected to CT
required disk is easy to fabricate and the analysis is immune to
examination.
cupping artifacts. This test method may not yield meaningful
3.3 Acronyms:
results if the disk image occupies less than a significant fraction
3.3.1 ERF—edge response function.
of the field of view.
3.3.2 PSF—point spread function.
1.3 This test method may also be used to evaluate other
3.3.3 MTF—modulation transfer function.
performance parameters. Among those characteristics of a CT
3.3.4 CDF—contrast discrimination function.
system that are detectable with this test method are: the
mid-frequency enhancement of the reconstruction kernel, the
4. Significance and Use
presence (or absence) of detector crosstalk, the undersampling
4.1 Two factors affecting the quality of a CT image are
of views, and the clipping of unphysical (that is, negative) CT
2 geometrical unsharpness and random noise. Geometrical un-
numbers (see Air Force Technical Report WL-TR-94-4021 ). It
sharpness limits the spatial resolution of a CT system, that is,
is highly likely that other characteristics as well can be detected
its ability to image fine structural detail in an object. Random
with this test method.
noise limits the contrast sensitivity of a CT system, that is, its
1.4 The values stated in SI units are to be regarded as the
ability to detect the presence or absence of features in an
standard. Inch-pound units are provided for information only.
object. Spatial resolution and contrast sensitivity may be
1.5 This standard does not purport to address all of the
measured in various ways. ASTM specifies spatial resolution
safety concerns, if any, associated with its use. It is the
be quantified in terms of the modulation transfer function
responsibility of the user of this standard to establish appro-
(MTF) and contrast sensitivity be quantified in terms of the
priate safety and health practices and determine the applica-
contrast discrimination function (CDF) (see Guide E 1441 and
bility of regulatory limitations prior to use.
Practice E 1570). This test method allows the purchaser or the
provider of CT systems or services, or both, to measure and
2. Referenced Documents
specify spatial resolution and contrast sensitivity.
2.1 ASTM Standards:
E 1316 Terminology for Nondestructive Examinations
5. Apparatus
E 1441 Guide for Computed Tomography (CT) Imaging
5.1 Disk Phantom—The disk phantom shall be a right
cylinder of uniform material conforming to the design and
This test method is under the jurisdiction of ASTM Committee E-7 on
material requirements in Table 1 and Fig. 1. Since spatial
Nondestructive Testing and is the direct responsibility of Subcommittee E07.01 on
resolution and contrast sensitivity depend on the examination
Radiology (X and Gamma) Method.
Current edition approved March 15, 1995. Published May 1995. task (that is, the test object and the specified CT parameters),
X-Ray Computed Tomography Standards (WL-TR-94-4021). Bossi, R. H. and
the application requirements must be fixed before the phantom
Nelson, J. M. Air Force Contract No. F33615-88-C-5404.
can be designed. In general, each examination task will require
Annual Book of ASTM Standards, Vol 03.03.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1695
TABLE 1 Disk Phantom Design Requirements
Material The material, in conjunction with the diameter of the disk,
shall be such that the phantom approximates the
attenuation range of the test object. The material should
preferably be the same as that of the test object.
Diameter The diameter shall be such that the reconstruction of the disk
occupies a significant fraction of the resulting image. In
conjunction with the material, the diameter shall be such
that the phantom approximates the attenuation range of the
test object.
Thickness The thickness of the disk shall be greater than the slice
thickness used to inspect the test object.
Shape The perpendicularity of the axis of revolution with respect to
the surface used to mount the phantom on the CT system
shall not compromise the measurement of geometrical
unsharpness.
Finish The surface texture roughness of the curved surface shall not
compromise the measurement of geometrical unsharpness.
requirements take precedence and may dictate the selection of
another material. The design of the disk phantom is a matter of
agreement between the purchaser and the supplier.
6. Procedure
6.1 The phantom shall be mounted on the CT system with
the orientation of the axis of revolution of the disk normal to
the scan plane. The alignment shall not compromise the
measurement of geometrical unsharpness. Unless otherwise
agreed upon between purchaser and supplier, the phantom shall
be placed at the center of the field of view used for the test
object.
6.2 Unless otherwise agreed upon between purchaser and
supplier, the data acquisition parameters shall be identical to
those used for test object scans. The slice plane shall intercept
the phantom approximately midway between the flat faces of
the disk.
6.3 Unless otherwise agreed upon between purchaser and
supplier, the reconstruction parameters shall be identical to
those used for test object reconstructions.
6.4 Unless otherwise agreed upon between purchaser and
supplier, the display parameters shall be identical to those used
for test object display. It shall be verified by examination that
the disk image occupies an image at least two-thirds of the
FIG. 1 Disk Phantom
image matrix. Recommended guidelines are given in Table 2.
a separate phantom. The diameter of the disk relative to the
7. Interpretation of Results
field of view shall be such that the reconstructed image of the
7.1 Spatial Resolution—From the CT image data, generate
disk occupies a significant fraction of the image matrix.
the composite profile of the edge of the disk to obtain the edge
Recommended sizes are given in Table 2. The diameter and
response function (ERF). Calculate the derivative of the ERF
opacity of the disk shall be such that the phantom approximates
to obtain the point spread function (PSF). Calculate the
the attenuation range of the test object. If possible, the phantom 4
amplitude of the Fourier Transform of the PSF and normalize
should be of the same material as the test object, but the other
the results to unity at zero frequency to obtain the modulation
transfer function (MTF).
TABLE 2 Suggested Measurement Parameters
7.1.1 Unless otherwise agreed upon between the purchaser
and supplier, the ERF shall be generated as follows:
Image Matrix Disk Image Maximum Tile ERF Bin Number
7.1.1.1 Calculate the center of mass of the disk.
Size Diameter Size Size of Fit
7.1.1.2 Select the inner and outer radii with respect to the
(Pixels) (Pixels) (Pixels) (Pixels) Points
center of mass that comfortably bracket the edge.
256 235 12 0.100 11
512 470 24 0.050 21
The Fourier Transform and Its Applications, Ronald M. Bracewell, McGraw-
1024 940 48 0.025 41
Hill, NY, ISBN 0-07-007013-X.
E 1695
7.1.1.3 Compute the distance to the center of mass for all each tile and store the result in a table specific to that pattern.
pixels between the inner and outer radii. For each table of results, calculate the standard deviation to
obtain the standard error in the mean and store the result in a
7.1.1.4 Generate a table of pixel values in order of their
pixel distance from the center of mass. separate table in order of ascending tile size. Express each
standard error in the mean as a percent of its respective
7.
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