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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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:E1695–95 (Reapproved 2001)
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 examination object—a part or specimen being sub-
required disk is easy to fabricate and the analysis is immune to
jected to CT examination.
cupping artifacts. This test method may not yield meaningful
3.3 Acronyms:Acronyms:
resultsifthediskimageoccupieslessthanasignificantfraction
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(seeAirForceTechnicalReportWL-TR-94-4021 ).It
sharpness limits the spatial resolution of a CT system, that is,
ishighlylikelythatothercharacteristicsaswellcanbedetected
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 E07 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.
task (that is, the examination object and the specified CT
Current edition approved March 15, 1995. Published May 1995.
parameters), the application requirements must be fixed before
X-Ray Computed Tomography Standards (WL-TR-94-4021). Bossi, R. H. and
Nelson, J. M. Air Force Contract No. F33615-88-C-5404.
Annual Book of ASTM Standards, Vol 03.03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1695–95 (2001)
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 examination object. The material
should preferably be the same as that of the examination
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
examination object.
Thickness The thickness of the disk shall be greater than the slice
thickness used to inspect the examination 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.
approximates the attenuation range of the examination object.
If possible, the phantom should be of the same material as the
examinationobject,buttheotherrequirementstakeprecedence
andmaydictatetheselectionofanothermaterial.Thedesignof
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
agreeduponbetweenpurchaserandsupplier,thephantomshall
be placed at the center of the field of view used for the
examination object.
6.2 Unless otherwise agreed upon between purchaser and
supplier, the data acquisition parameters shall be identical to
those used for examination 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 examination object reconstructions.
6.4 Unless otherwise agreed upon between purchaser and
FIG. 1 Disk Phantom
supplier, the display parameters shall be identical to those used
for examination object display. It shall be verified by exami-
the phantom can be designed. In general, each examination
nationthatthediskimageoccupiesanimageatleasttwo-thirds
task will require a separate phantom. The diameter of the disk
of the image matrix. Recommended guidelines are given in
relative to the field of view shall be such that the reconstructed
Table 2.
image of the disk occupies a significant fraction of the image
matrix. Recommended sizes are given inTable 2.The diameter
7. Interpretation of Results
and opacity of the disk shall be such that the phantom
7.1 Spatial Resolution—From the CT image data, generate
the composite profile of the edge of the disk to obtain the edge
TABLE 2 Suggested Measurement Parameters
response function (ERF). Calculate the derivative of the ERF
to obtain the point spread function (PSF). Calculate the
Image Matrix Disk Image Maximum Tile ERF Bin Number
amplitude of the Fourier Transform of the PSF and normalize
Size Diameter Size Size of Fit
the results to unity at zero frequency to obtain the modulation
(Pixels) (Pixels) (Pixels) (Pixels) Points
transfer function (MTF).
256 235 12 0.100 11
512 470 24 0.050 21
1024 940 48 0.025 41 The Fourier Transform and Its Applications, Ronald M. Bracewell, McGraw-
Hill, NY, ISBN 0-07-007013-X.
E1695–95 (2001)
7.1.1 Unless otherwise agreed upon between the purchaser should also be graphically presented, with the full width at half
and supplier, the ERF shall be generated as follows: maximum of the latter quantitatively indicated.
7.1.1.1 Calculate the center of mass of the disk.
7.2 Contrast Sensitivity—From the CTimage data, generate
7.1.1.2 Select the inner and outer radii with respect to the
a sequence of tile patterns which fit within the central region of
center of mass that comfortably bracket the edge.
the disk. For each pattern, calculate the mean CT value within
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
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