ASTM E1931-97(2003)
(Guide)Standard Guide for X-Ray Compton Scatter Tomography
Standard Guide for X-Ray Compton Scatter Tomography
SIGNIFICANCE AND USE
Principal Advantage of Compton Scatter Tomography—The principal advantage of CST is the ability to perform three-dimensional X-ray examination without the requirement for access to the back side of the examination object. CST offers the possibility to perform X-ray examination that is not possible by any other method. The CST sub-surface slice image is minimally affected by examination object features outside the plane of examination. The result is a radioscopic image that contains information primarily from the slice plane. Scattered radiation limits image quality in normal radiographic and radioscopic imaging. Scatter radiation does not have the same detrimental effect upon CST because scatter radiation is used to form the image. In fact, the more radiation the examination object scatters, the better the CST result. Low subject contrast materials that cannot be imaged well by conventional radiographic and radioscopic means are often excellent candidates for CST. Very high contrast sensitivities and excellent spatial resolution are possible with CST tomography.
Limitations—As with any nondestructive testing method, CST has its limitations. The technique is useful on reasonably thick sections of low-density materials. While a 25 mm [1 in.] depth in aluminum or 50 mm [2 in.] in plastic is achievable, the examination depth is decreased dramatically as the material density increases. Proper image interpretation requires the use of standards and examination objects with known internal conditions or representative quality indicators (RQIs). The examination volume is typically small, on the order of a few cubic inches and may require a few minutes to image. Therefore, completely examining large structures with CST requires intensive re-positioning of the examination volume that can be time-consuming. As with other penetrating radiation methods, the radiation hazard must be properly addressed.
SCOPE
1.1 Purpose—This guide covers a tutorial introduction to familiarize the reader with the operational capabilities and limitations inherent in X-ray Compton Scatter Tomography (CST). Also included is a brief description of the physics and typical hardware configuration for CST.
1.2 Advantages—X-ray Compton Scatter Tomography (CST) is a radiologic nondestructive examination method with several advantages that include:
1.2.1 The ability to perform X-ray examination without access to the opposite side of the examination object;
1.2.2 The X-ray beam need not completely penetrate the examination object allowing thick objects to be partially examined. Thick examination objects become part of the radiation shielding thereby reducing the radiation hazard;
1.2.3 The ability to image examination object subsurface features with minimal influence from surface features;
1.2.4 The ability to obtain high-contrast images from low subject contrast materials that normally produce low-contrast images when using traditional transmitted beam X-ray imaging methods; and
1.2.5 The ability to obtain depth information for examination object features thereby providing three-dimensional examination. The ability to obtain depth information presupposes the use of a highly collimated detector system having a narrow angle of acceptance.
1.3 Applications—This guide does not specify which examination objects are suitable, or unsuitable, for CST. As with most nondestructive examination techniques, CST is highly application specific thereby requiring the suitability of the method to be first demonstrated in the application laboratory. This guide does not provide guidance in the standardized practice or application of CST techniques. No guidance is provided concerning the acceptance or rejection of examination objects examined with CST.
1.4 Limitations—As with all nondestructive examination methods, CST has limitations and is complementary to other NDE methods. Chief among the limitations is the difficulty in performing CST on th...
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Designation:E1931–97 (Reapproved 2003)
Standard Guide for
X-Ray Compton Scatter Tomography
This standard is issued under the fixed designation E 1931; 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 performing CST on thick sections of high-Z materials. CST is
best applied to thinner sections of lower Z materials. The
1.1 Purpose—This guide covers a tutorial introduction to
following provides a general idea of the range of CST
familiarize the reader with the operational capabilities and
applicability when using a 160 keV constant potential X-ray
limitations inherent in X-ray Compton Scatter Tomography
source:
(CST). Also included is a brief description of the physics and
Material Practical Thickness Range
typical hardware configuration for CST.
1.2 Advantages—X-ray Compton Scatter Tomography
Steel Up to about 3 mm [ ⁄8 in.]
(CST) is a radiologic nondestructive examination method with
Aluminum Up to about 25 mm [1 in.]
Aerospace composites Up to about 50 mm [2 in.]
several advantages that include:
1.2.1 The ability to perform X-ray examination without
The limitations of the technique must also consider the
access to the opposite side of the examination object;
required X, Y, and Z axis resolutions, the speed of image
1.2.2 The X-ray beam need not completely penetrate the
formation, image quality and the difference in the X-ray
examination object allowing thick objects to be partially
scattering characteristics of the parent material and the internal
examined. Thick examination objects become part of the
features that are to be imaged.
radiation shielding thereby reducing the radiation hazard;
1.5 The values stated in both inch-pound and SI units are to
1.2.3 The ability to image examination object subsurface
be regarded separately as the standard. The values given in
features with minimal influence from surface features;
parentheses are for information only.
1.2.4 The ability to obtain high-contrast images from low
1.6 This standard does not purport to address all of the
subject contrast materials that normally produce low-contrast
safety concerns, if any, associated with its use. It is the
imageswhenusingtraditionaltransmittedbeamX-rayimaging
responsibility of the user of this standard to establish appro-
methods; and
priate safety and health practices and to determine the
1.2.5 The ability to obtain depth information for examina-
applicability of regulatory limitations prior to use.
tion object features thereby providing three-dimensional ex-
2. Referenced Documents
amination.The ability to obtain depth information presupposes
the use of a highly collimated detector system having a narrow
2.1 ASTM Standards:
angle of acceptance.
E 747 Practice for Design, Manufacture, and Material
1.3 Applications—This guide does not specify which ex-
Grouping Classification of Wire Image Quality Indicators
amination objects are suitable, or unsuitable, for CST.As with
(IQI) Used for Radiology
most nondestructive examination techniques, CST is highly
E 1025 Practice for Hole-Type Image Quality Indicators
application specific thereby requiring the suitability of the
Used for Radiography
method to be first demonstrated in the application laboratory.
E 1255 Practice for Radioscopy
This guide does not provide guidance in the standardized
E 1316 StandardTerminology for Nondestructive Examina-
practice or application of CST techniques. No guidance is
tions
provided concerning the acceptance or rejection of examina-
E 1441 Guide for Computed Tomography (CT) Imaging
tion objects examined with CST.
E 1453 Guide for the Storage of Media that Contains
1.4 Limitations—As with all nondestructive examination
Radioscopic Data
methods, CST has limitations and is complementary to other
E 1475 Guide for Data Fields for Computerized Transfer of
NDE methods. Chief among the limitations is the difficulty in
Digital Radiological Examination Data
E 1647 Practice for Determining Contrast Sensitivity in
Radioscopy
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-
2.2 ANSI/ASNT Standards:
tive Testing and is the direct responsibility of Subcommittee E07.01 on Radiology
(X and Gamma) Method.
Current edition approved March 10, 2003. Published May 2003. Originally
approved in 1997. Last previous edition approved in 1997 as E 1931 – 97. 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.
E1931–97 (2003)
ASNT Recommended Practice No. SNT-TC-1A Personnel the actual examination object are the best means for CST
Qualification and Certification in Nondestructive Test- performancemonitoring.Conventionalradiologicperformance
ing measuring devices, such as Test Method E 747 and Practice
ANSI/ASNT CP-1 89 Standard for Qualification and Certi- E 1025 image quality indicators or Practice E 1647 contrast
fication in Nondestructive Testing Personnel sensitivity gages are designed for transmitted X-ray beam
2.3 Military Standard: imaging and are of little use for CST. With appropriate
MIL-STD-410 Nondestructive Testing Personnel Qualifica- calibration, CST can be utilized to make three-dimensional
tion and Certification measurements of internal examination object features.
5. Significance and Use
3. Terminology
5.1 PrincipalAdvantage of Compton Scatter Tomography—
3.1 Definitions:
The principal advantage of CST is the ability to perform
3.1.1 CST, being a radiologic examination method, uses
three-dimensional X-ray examination without the requirement
much the same vocabulary as other X-ray examination meth-
for access to the back side of the examination object. CST
ods. A number of terms used in this standard are defined in
offers the possibility to perform X-ray examination that is not
Terminology E 1316. It may also be helpful to read Guide
possible by any other method. The CST sub-surface slice
E 1441.
image is minimally affected by examination object features
4. Summary of Guide outside the plane of examination. The result is a radioscopic
image that contains information primarily from the slice plane.
4.1 Description—Compton Scatter Tomography is a
Scattered radiation limits image quality in normal radiographic
uniquely different nondestructive test method utilizing pen-
and radioscopic imaging. Scatter radiation does not have the
etrating X-ray or gamma-ray radiation. Unlike computed
same detrimental effect upon CST because scatter radiation is
tomography(CT),CSTproducesradioscopicimageswhichare
used to form the image. In fact, the more radiation the
not computed images. Multiple slice images can be simulta-
examination object scatters, the better the CST result. Low
neously produced so that the time per slice image is in the
subject contrast materials that cannot be imaged well by
rangeofafewseconds.CSTproducesimagesthatarethinwith
conventional radiographic and radioscopic means are often
respect to the examination object thickness (slice images) and
excellent candidates for CST. Very high contrast sensitivities
which are at right angles to the X-ray beam. Each two-
and excellent spatial resolution are possible with CST tomog-
dimensional slice image (X–Y axes) is produced at an incre-
raphy.
mental distance along and orthogonal to the X-ray beam
5.2 Limitations—As with any nondestructive testing
(Z–axis). A stack of CST images therefore represents a solid
method, CST has its limitations. The technique is useful on
volume within the examination object. Each slice image
reasonably thick sections of low-density materials. While a 25
contains examination object information which lies predomi-
mm [1 in.] depth in aluminum or 50 mm [2 in.] in plastic is
nantly within the desired slice. To make an analogy as to how
achievable, the examination depth is decreased dramatically as
CST works, consider a book. The examination object may be
the material density increases. Proper image interpretation
larger or smaller (in length, width and depth) then the analo-
requires the use of standards and examination objects with
gous book. The CST slice images are the pages in the book.
known internal conditions or representative quality indicators
Paging through the slice images provides information about
(RQIs). The examination volume is typically small, on the
examination object features lying at different depths within the
order of a few cubic inches and may require a few minutes to
examination object.
image. Therefore, completely examining large structures with
4.2 Image Formation—CST produces one or more digital
CST requires intensive re-positioning of the examination
slice plane images per scan. Multiple slice images can be
volume that can be time-consuming.As with other penetrating
producedintimesrangingfromafewsecondstoafewminutes
radiation methods, the radiation hazard must be properly
depending upon the examined area, desired spatial resolution
addressed.
and signal-to-noise ratio. The image is digital and is typically
assembled by microcomputer. CST images are free from
6. Technical Description
reconstruction artifacts as the CST image is produced directly
6.1 General Description of Compton Scatter Tomography—
and is not a calculated image. Because CST images are digital,
Transmitted beam radiologic techniques used in radiography,
they may be enhanced, analyzed, archived and in general
radioscopy and computed tomography have dominated the use
handled as any other digital information.
of penetrating radiation for industrial nondestructive examina-
4.3 Calibration Standards—As with all nondestructive ex-
tion. The transmitted beam technique depends upon the pen-
aminations, known standards are required for the calibration
etrating radiation attenuation mechanisms of photoelectric
and performance monitoring of the CST method. Practice
absorption and Compton scattering. For low-Z materials at
E 1255 calibration block standards that are representative of
energies up to about 50 keV, the photoelectric effect is the
dominant attenuation mechanism. As X-ray energy increases,
Compton scattering becomes the dominant attenuation mecha-
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
nism for large scattering angles in low-Z materials. Pair
4th Floor, New York, NY 10036.
production comes into play above 1.02 MeV and can become
Available from Standardization Document Order Desk, Bldg. 4 Section D, 700
Robbins Ave. Philadelphia, PA 19111-5094, Ans:NPODS. the dominant effect for higher X-ray energies. Photoelectric
E1931–97 (2003)
absorption is strongly dependent upon the atomic number and
also the electron density of the absorbing material. Compton
scattering also depends upon the Z of the scattering material,
but to a lesser degree than is the photoelectric effect. These
relationshipsmaybeseeninFig.1.Thefollowingrelationships
show the approximate dependence of the photoelectric effect
NOTE 1—Hubbell, J.H. and Seltzer, S.M.,Tables of X-Ray MassAttenuation Coefficients and Mass Energy-Absorption Coefficients, 1 keVto 20 MeV
for ElementsZ=1to92and48 Additional Substances of Dosimetric Interest, NISTIR 5632, 1996. Available from National Institute of Standards and
Technology (NIST), Gaithersburg, MD 20899.
FIG. 1 Linear Absorption and Scatter Coefficients for Polyethylene, Aluminum and Iron
E1931–97 (2003)
and Compton scattering upon target material Z and incident volume) of the primary radiation source resulting in a lower
X-ray energy E: noise CSTimage and faster examination speed. For this reason
5 7/2
Photoelectric Effect Z / E an X-ray source is often a better choice than a radioisotope for
Compton Scattering Z / E
CST. Radiation detection and other image forming consider-
Pair Production: Z (lnE - constant)
ations may also differ substantially from other radiologic
6.1.1 CST is best suited for lower Z materials such as
imaging methods.
aluminum ( Z=13 ) using a commercially available 160 keV
6.3 Theory of Compton Scatter Tomography—In the energy
X-ray generating system. Somewhat higher Z materials may be
range appropriate for CST (roughly 50 keV to 1 MeV), the
examined by utilizing a higher energy X-ray generator rated at
primary interaction mechanisms between electromagnetic ra-
225, 320, or 450 keV. It is useful to envision the CST process
diation and matter are photoelectric absorption and inelastic
as one where the X-rays that produce the CST image originate
(Compton) scatter. Fig. 2 illustrates the principles of photo-
from many discrete points within the examined volume. Each
electric absorption and Compton scattering. As an X-ray
Compton scatter event generates a lower energy X-ray that
having an energy E collides with an electron, the electron
emanates from the scattering site. Singly scattered X rays that
absorbs energy from the incoming X-ray photon and is ejected
reach the detector carry information about the examination
from its shell. In the case of photoelectric absorption, the
object material characteristics at the site where it was gener-
incoming photon’s energy is totally absorbed.As the energy E
ated. The scatter radiation is also affected by the material
of the incoming photon increases, the probability of photoelec-
throughwhichitpassesonthewaytothedetector.Theexternal
tric absorption decreases while the probability of Compton
source of primary penetrating radiation, that may be either X
scattering increases. The Compton scattering creates a new
rays or gamma rays, interact by the Compton scattering
X-ray having and energy E , and travelling at an angle u with
process. The primary radiation must have adequate energy and
respect to the direction of the original primary X-ray.
intensity to generate sufficient scattered radiation at the exami-
nation site to allow detection.The examination depth is limited 6.3.1 Fig. 1 shows how material linear attenuation coeffi-
tothatdepthfromwhichsufficientscatteredradiationcanreach cients due to photoelectric absorption and Compton scattering
the detector to form a usable image. The examination object is
vary with energy for polyethylene, aluminum and iron. The
therefore effectively imaged from the inside out. The CST linear absorption coefficient µ for all three materials falls
a
image is formed voxel (volume element) by voxel in raster
sharply with increasing energy, while the scatter coefficient µ
s
fashion where the detector’s field-of-view intersect
...
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