ASTM E1931-16(2022)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E1931 − 16 (Reapproved 2022)
Standard Guide for
Non-computed X-Ray Compton Scatter Tomography
This standard is issued under the fixed designation E1931; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope provided concerning the acceptance or rejection of examina-
tion objects examined with CST.
1.1 Purpose—This guide covers a tutorial introduction to
familiarize the reader with the operational capabilities and 1.4 Limitations—As with all nondestructive examination
limitations inherent in a single non-computed X-ray Compton
methods, CST has limitations and is complementary to other
ScatterTomography(CST).Alsoincludedisabriefdescription NDE methods. Chief among the limitations is the difficulty in
ofthephysicsandtypicalhardwareconfigurationforCST.This
performing CST on thick sections of high-Z materials. CST is
single technique is still used for a small number of inspections. best applied to thinner sections of lower Z materials. The
This is not meant as comprehensive guide covering the variety
following provides a general idea of the range of CST
of Compton scattering techniques that are now used for applicability when using a 160 keV constant potential X-ray
non-destructive testing and security screen screening.
source:
Material Practical Thickness Range
1.2 Advantages—X-ray Compton Scatter Tomography
(CST) is a radiologic nondestructive examination method with
Steel Up to about 3 mm ( ⁄8 in.)
several advantages that include:
Aluminum Up to about 25 mm (1 in.)
Aerospace composites Up to about 50 mm (2 in.)
1.2.1 The ability to perform X-ray examination without
Polyurethane Foam Up to about 300 mm (12 in.)
access to the opposite side of the examination object;
The limitations of the technique must also consider the
1.2.2 The X-ray beam need not completely penetrate the
required X, Y, and Z axis resolutions, the speed of image
examination object allowing thick objects to be partially
formation, image quality and the difference in the X-ray
examined. Thick examination objects become part of the
scattering characteristics of the parent material and the internal
radiation shielding thereby reducing the radiation hazard;
features that are to be imaged.
1.2.3 The ability to examine and image object subsurface
features with minimal influence from surface features;
1.5 The values stated in both inch-pound and SI units are to
1.2.4 The ability to obtain high-contrast images from low
be regarded separately as the standard. The values given in
subject contrast materials that normally produce low-contrast
parentheses are for information only.
imageswhenusingtraditionaltransmittedbeamX-rayimaging
1.6 This standard does not purport to address all of the
methods; and
safety concerns, if any, associated with its use. It is the
1.2.5 The ability to obtain depth information of object
responsibility of the user of this standard to establish appro-
features thereby providing a three-dimensional examination.
priate safety, health, and environmental practices and deter-
The ability to obtain depth information presupposes the use of
mine the applicability of regulatory limitations prior to use.
a highly collimated detector system having a narrow angle of
1.7 This international standard was developed in accor-
acceptance.
dance with internationally recognized principles on standard-
1.3 Applications—Thisguidedoesnotspecifywhichexami-
ization established in the Decision on Principles for the
nation objects are suitable, or unsuitable, for CST. As with
Development of International Standards, Guides and Recom-
most nondestructive examination techniques, CST is highly
mendations issued by the World Trade Organization Technical
application specific thereby requiring the suitability of the
Barriers to Trade (TBT) Committee.
method to be first demonstrated in the application laboratory.
This guide does not provide guidance in the standardized
2. Referenced Documents
practice or application of CST techniques. No guidance is
2.1 ASTM Standards:
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-
tiveTesting and is the direct responsibility of E07.01 on Radiology (X and Gamma)
Method. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedJune1,2022.PublishedJuly2022.Originallyapproved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 1997. Last previous edition approved in 2016 as E1931 – 16. DOI: 10.1520/ Standards volume information, refer to the standard’s Document Summary page on
E1931-16R22. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1931 − 16 (2022)
E747 Practice for Design, Manufacture and Material Group- respect to the examination object thickness (slice images) and
ing Classification of Wire Image Quality Indicators (IQI) which are at right angles to the X-ray beam. Each two-
Used for Radiology dimensional slice image (X–Y axes) is produced at an incre-
E1025 Practice for Design, Manufacture, and Material mental distance along and orthogonal to the X-ray beam
Grouping Classification of Hole-Type Image Quality In-
(Z–axis). A stack of CST images therefore represents a solid
dicators (IQI) Used for Radiography volume within the examination object. Each slice image
E1255 Practice for Radioscopy
contains examination object information which lies predomi-
E1316 Terminology for Nondestructive Examinations nantly within the desired slice. To make an analogy as to how
E1441 Guide for Computed Tomography (CT)
CST works, consider a book. The examination object may be
E1453 Guide for Storage of Magnetic Tape Media that larger or smaller (in length, width and depth) then the analo-
Contains Analog or Digital Radioscopic Data
gous book. The CST slice images are the pages in the book.
E1475 Guide for Data Fields for Computerized Transfer of
Paging through the slice images provides information about
Digital Radiological Examination Data
examination object features lying at different depths within the
E1647 Practice for Determining Contrast Sensitivity in Ra-
examination object.
diology
4.2 Image Formation—CST produces one or more digital
2.2 ANSI/ASNT Standards:
slice plane images per scan. Multiple slice images can be
SNT-TC-1A ASNT Recommended Practice for Personnel
producedintimesrangingfromafewsecondstoafewminutes
Qualification and Certification in Nondestructive Testing
depending upon the examined area, desired spatial resolution
ANSI/ASNT CP-189 Standard for Qualification and Certifi-
and signal-to-noise ratio. The image is digital and is typically
cation in Nondestructive Testing Personnel
assembled by computer. CST images are free from reconstruc-
2.3 Military Standard:
tion artifacts as the CST image is produced directly and is not
MIL-STD-410 Nondestructive Testing Personnel Qualifica-
a calculated image. Because CST images are digital, they may
tion and Certification
be enhanced, analyzed, archived and in general handled as any
2.4 Aerospace Standard:
other digital information.
AIA-NAS-410 Aerospace Industries Association, National
Aerospace Standard-4105 Certification and Qualification 4.3 Calibration Standards—As with all nondestructive
of Nondestructive Test Personnel examinations, known standards are required for the calibration
and performance monitoring of the CST method. Practice
2.5 ISO Standard:
E1255 calibration block standards that are representative of the
ISO 9712 Nondestructive Testing—Qualification and Certi-
actual examination object are the best means for CST perfor-
fication of NDT Personnel
mance monitoring. Conventional radiologic performance mea-
3. Terminology
suring devices, such as Test Method E747 and Practice E1025
image quality indicators or Practice E1647 contrast sensitivity
3.1 Definitions:
gagesaredesignedfortransmittedX-raybeamimagingandare
3.1.1 CST, being a radiologic examination method, uses
of little use for CST. With appropriate calibration, CST can be
much the same vocabulary as other X-ray examination meth-
utilized to make three-dimensional measurements of internal
ods. A number of terms used in this standard are defined in
examination object features.
Terminology E1316. It may also be helpful to read Guide
E1441.
5. Significance and Use
4. Summary of Guide
5.1 Principal Advantage of Compton Scatter Tomography—
4.1 Description—Compton Scatter Tomography is a
The principal advantage of CST is the ability to perform
uniquely different nondestructive test method utilizing pen-
three-dimensional X-ray examination without the requirement
etrating X-ray or gamma-ray radiation. Unlike computed
for access to the back side of the examination object. CST
tomography(CT),CSTproducesradioscopicimageswhichare
offers the possibility to perform X-ray examination that is not
not computed images. Multiple slice images can be simultane-
possible by any other method. The CST sub-surface slice
ously produced so that the time per slice image is in the range
image is minimally affected by examination object features
of a few seconds. CST can produce images that are thin with
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
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
and radioscopic imaging. Scatter radiation does not have the
4th Floor, New York, NY 10036, http://www.ansi.org.
Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,
same detrimental effect upon CST because scatter radiation is
Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http://
used to form the image. In fact, the more radiation the
www.dodssp.daps.mil.
5 examination object scatters, the better the CST result. Low
Available from Federal Aviation Administration (FAA), 800 Independence
subject contrast materials that cannot be imaged well by
Ave., SW, Washington, DC 20591, http://www.faa.gov.
This document has superseded MIL-STD-410; however, MIL-STD-410 is still
conventional radiographic and radioscopic means are often
acceptable to the FAA
excellent candidates for CST. Very high contrast sensitivities
Available from International Organization for Standardization (ISO), ISO
and excellent spatial resolution are possible with CST tomog-
Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier,
Geneva, Switzerland, http://www.iso.org. raphy.
E1931 − 16 (2022)
5.2 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.
FIG. 1 Relationship Between Photoelectric, Compton, and Pair
6. Basis of Application
Production Effects
5 7/2
6.1 Personnel Qualification is subject to contractual agree-
Photoelectric Effect: τ = Z / E
Compton Scattering: σ = Z / E
ment between the parties using or referencing this standard.
Pair Production : κ = Z (lnE - constant)
6.1.1 If specified in the contractual agreement, personnel
performing examinations to this standard shall be qualified in
accordance with a nationally or internationally recognized
moving left to right in the graph, the green lines represent the
NDT personnel qualification practice or standard such as
boundaries of the regions where the photoelectric, Compton
ANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, MIL-STD-
and pair-production events are dominant, respectively. The
410E, ISO 9712, or a similar document and certified by the
graph is useful for understanding the X-ray properties of a
employer or certifying agency, as applicable. The practice or
material, by knowing its predominant mass number Z. For
standard used and its applicable revision shall be identified in
example, it could be used to estimate its interaction regime
the contractual agreement between the using parties.
based on the Z of the material and energy of interrogation
photons.
7. Technical Description
7.1.1 CST is best suited for lower Z materials such as
7.1 General Description of Compton Scatter Tomography—
aluminum ( Z=13 ) using a commercially available 160 keV
Transmitted beam radiologic techniques used in radiography,
X-ray generating system. Somewhat higher Z materials may be
radioscopy and computed tomography have dominated the use
examined by utilizing a higher energy X-ray generator rated at
of penetrating radiation for industrial nondestructive examina-
225, 320, or 450 keV.
tion. The transmitted beam technique depends upon the pen-
7.1.2 It is useful to envision the CST process as one where
etrating radiation attenuation mechanisms of photoelectric
the X-rays that produce the CST image originate from many
absorption and Compton scattering. For low- Z materials at
discrete points within the examined volume. Each Compton
energies up to about 50 keV, the photoelectric effect is the
scatter event generates a lower energy X ray that emanates
dominant attenuation mechanism. As X-ray energy increases,
from the scattering site. Singly scattered X rays that reach the
Compton scattering becomes the dominant attenuation mecha-
detector carry information about the examination object mate-
nism.Pairproductioncomesintoplayabove1.02MeVandcan
rial characteristics at the site where it was generated. The
become the dominant effect for higher X-ray energies. The
scatter radiation is also affected by the material through which
following relationships are plotted in Fig. 1 and show the
it passes on the way to the detector. The external source of
approximate dependence of the photoelectric effect and Comp-
primary penetrating radiation, that may be either X rays or
ton scattering upon target material atomic number (Z) and
gamma rays, interact by the Compton scattering process. The
incident X-ray energy (E):
primary radiation must have adequate energy and intensity to
The terms τ, σ, and κ represent the cross sections of the
generate sufficient scattered radiation at the examination site to
photoelectric, Compton scatter and pair-production interaction
allow detection. The examination depth is limited to that depth
of a given X-ray photon that has energy level E. There is no
from which sufficient scattered radiation can reach the detector
analytic expression for the probability of photoelectric absorp-
to form a usable image. The examination object is therefore
tion as it varies as a function of E and Z. For the gamma and
effectively imaged from the inside out. The CST image is
X-ray energies of interest, the exponent of Z can vary between
formed voxel (volume element) by voxel in raster fashion
4 and 5. The green lines are contours where the indicated
where the detector’s field-of-view intersects with the central
cross sections are equal. Since the cross sections are propor-
X-ray beam at the examination site. The primary radiation
tional to the probability of the particular type of interaction,
beam source and scattered radiation detector are highly colli-
mated to assure collection of single-scattered radiation from a
known small volume of the examination object. Multiple
Knoll, G. F., Radiation Detection and Measurement, 3rd Ed. John Wiley and
Sons, New York, 2000. scattered radiation causes a loss of spatial resolution, but can
E1931 − 16 (2022)
enhance contrast of features. Moving the intersection of the to have a higher flux than a standard X-ray tube (5.5 mm FOC
radiation source and detector lines of sight in a systematic at 3000W) while maintaining the same image quality. For this
fashion allows a tomogram, or slice image to be produced. reason an X-ray source is often a better choice than a
Changing the distance at which the radiation source and radioisotope for CST. Radiation detection and other image
detector lines of sight intersect allows the tomogram to be formingconsiderationsmayalsodiffersubstantiallyfromother
produced at a selected depth below the examination object radiologic imaging methods.
surface.
7.3 Theory of Compton Scatter Tomography—In the energy
7.2 Significant Differences in the Transmitted Beam and range appropriate for CST (roughly 50 keV to 1 MeV), the
Compton Scattered X-Ray Imaging Techniques—The differ- primary interaction mechanisms between electromagnetic ra-
ences between conventional transmitted beam and Compton diation and matter are photoelectric absorption and inelastic
Scatter Imaging are so significant that CSTmust be considered (Compton) scatter. Fig. 2 illustrates the principles of photo-
a separate examination technique. For transmitted beam electricabsorptionandComptonscattering.AsanXrayhaving
techniques, the radiation source characteristics must be care- an energy E collides with an electron, the electron absorbs
fully controlled. The energy and intensity must be selected energy from the incoming X-ray photon and is ejected from its
carefully to fully penetrate the object and provide the required shell. In the case of photoelectric absorption, the incoming
contrast sensitivity. Thick sections of high-density materials photon’s energy is totally absorbed. As the energy E of the
require a high-energy radiation source while thin sections of incoming photon increases, the probability of photoelectric
low-density materials require a low-energy radiation source. absorption decreases while the probability of Compton scatter-
For CST applications, the energy and intensity of the primary ing increases. The Compton scattering creates a new X ray
'
radiation beam is relatively less important. The primary radia- having and energy E, and travelling at an angle θ with respect
tion beam energy and intensity are not critical as long as they to the direction of the original primary X ray.
remain stable and are sufficient to generate adequate scatter 7.3.1 Material linear attenuation coefficients due to photo-
radiation at the CST examination depth. Small focal spot size electric absorption and Compton scattering vary with energy.
is critical to transmitted beam image sharpness. The primary The linear absorption coefficient µ falls sharply with increas-
τ
radiation focal spot size is also of significant importance for ing energy, while the scatter coefficient µ remains nearly
σ
CST. For CST the beam must be collimated. A smaller focal constant. For low-Z materials, scatter begins to dominate
spot can have a shorter collimator to achieve the same level of photoelectricabsorptionatprimaryradiationenergiesabove50
collimation.The 1/R (where R is the distance from the object) keV allowing the use of scatter radiation instead of the
effect for X-ray source allows a high power, small focal spot attenuated primary beam radiation for imaging purposes. It
X-ray tube (1.0 mm X-ray focal spot size (FOC) at 1800 W), should also be noted that unlike the linear attenuation
FIG. 2 Principles of Photoelectric Absorption and Compton Scattering
E1931 − 16 (2022)
coefficient, the scatter coefficient is relatively independent of detector and the primary radiation source can therefore be
the primary penetrating radiation energy E . Many of the positioned on the same side of the examination object in this
restrictions on energy selection associated with transmitted
energy range.
beam techniques are not a consideration with CST. For
7.3.4 The intensity of radiation I scattered from a volume
SC
example, low-density aerospace composite materials can be
element (voxel) V inside the examination object can be
Voxel
imaged at higher energies of 100 keV or more producing high
approximated as follows:
contrast using CST techniques.
2µτ 2µ W 2µ’t’
c
I 5Κ·n ·V ·I e ~1 2 e !~e !1M (3)
SC e Voxel 0
7.3.2 The energy of the scattered X ray is given by:
where:
E
'
E 5 (1)
K = constant of proportionality representing the dif-
E
11 1 2 cos θ
S D ~ !
ferential scatter cross-section, detector efficien-
m c
e
cies and all other object-dependent effects,
where:
n = number of electrons per unit volume acting as
e
'
E = energy of the scattered X ray,
scatter centers,
E = energy of the primary radiation photon,
0 I = incident flux,
-µt
c = speed of light,
e = the attenuation along the primary beam path,
-µσW
m c = rest energy of the electron,
e 1–e = the fraction of photons scattered from the primary
m = electron mass, and
e beam in a voxel of length W.The scattering voxel
θ = scattering angle.
is the intersection of the incident pencil beam
with the solid angle subtended by the detector,
It can be seen from Eq 1 that the energy of the scatter
'
dV = the volume of the scattering voxel,
radiation E decreases with increasing scattering angle θ. The
W = lengthofthescatteringvoxelalongthepathofthe
amount of Compton scattering in any material is proportional
pencil beam,
to its electron density.
µσ = the Compton linear attenuation coefficient, as
7.3.3 Disregarding the effects of pair production that come
defined earlier,
into play above 1.02 MeV, the total attenuation is the sum of
-µ’t'
e = the attenuation along the scattered beam path t'. µ'
attenuation due to photoelectric absorption and Compton
is the total linear attenuation coefficient of the
scattering:
lower energy scattered beam,
µ 5 µ 1µ (2)
τ σ µ' = the total linear attenuation coefficient of the lower
energy scattered beam, and
where µ, µ , and µ are the total, photoelectric-absorption
τ σ
M = multiple scatter component originating from
and Compton scattering linear attenuation coefficients, respec-
quanta scattered more than once outside the
tively.
voxel.
Fig. 3 is a polar plot of the Klein Nishina Compton scatter
The two exponential terms describe the radiation attenuation
(free electron scattering) angular intensity at 30 and 300 keV.
Although the scatter radiation angular distribution becomes along the primary radiation beam path t as well as the scattered
moreintenseintheforwarddirectionasenergyincreases,there beam path t'. Due to the lower X-ray energy along the scatter
is sufficient intensity at all angles to permit the technique. The path, µ' is not equal to µ. The last term represents the
FIG. 3 Polar Plot of Relative Scatter Intensity as a Function of Scattering Angle at Primary Radiation Energies of 30 and 300 keV
E1931 − 16 (2022)
2~µ 2 µ!W
D
contribution of photons that are scattered from other voxels, C 5 e 21 (8)
scattered more than once, and scattered into the solid angle of
C 5 ~1 2 ~µ 2 µ!W! 2 1 5 ~µ 2 µ !W (9)
D D
thepixel.Theinfluenceofthistypeofmultiplescatterradiation
Thus, in a transmission imaging system, contrast is directly
needstobeminimizedbecauseitdegradesthefidelityofimage
proportional to the discontinuity size.
information corresponding to the voxel of interest, where the
7.4.2 Fig.5isageneralizedrepresentationofaCSTsystem.
primary scatter originated. This may be accomplished by
The examination object of thickness L contains a small
tightly collimating the detector to limit its field-of-view to the
discontinuity of length W and having a linear attenuation
desired examination voxel and by software.
coefficient of µ .
D
7.4 Contrast Sensitivity—One significant benefit of CST as
7.4.2.1 The CST system contrast may be determined by
compared with conventional transmission imaging is increased
comparing the scatter signals, I and I , from two similarly
SC SC’
contrast sensitivity. Fig. 4 is a generalized representation of
located voxels. For one sided radiographic inspection using
transmission beam imaging to determine the relationship 9
backscatter imaging, the scatter signal is:
between discontinuity size and subject contrast.
2µ x 2µ x
1 1 2 2
I 5 I e µ We (10)
SC 0 scat
7.4.1 To find an expression for the sensitivity of the trans-
mitted beam technique, consider a homogeneous material of
with µ and µ being the total attenuation coefficients for the
1 2
thicknessLwhoseattenuationcoefficientisµexceptforasmall
entering and the scattered beam, x and x being the path
1 2
discontinuity region of length W and whose attenuation coef-
lengths of entering and scattered beam in the base material,
ficient is µ .
D µ W the probability of scatter in the scatter voxel by a
scat
7.4.1.1 The intensities of the radiation beams passing
material with linear scatter coefficient µ and a thickness W.
scat
throughthehomogeneousexaminationobjectwithandwithout
If the material in the scatter voxel at the same location changes
the small discontinuity are I and I , respectively, given by:
to µ the scatter signal becomes:
Τ Τ’
D,scat
2µL
e 2 2µ x
µ x 2 2
I 5 I (4) I ' 5 I e 1 1µ We (11)
T 0 sc 0 D,scat
2 µ L 2 W 1µ W
~ ~ ! !
D
I 5 I e (5)
T’ 0 The total attenuation coefficient for the scattered beam does
notchangebecausetheenergyshiftdependsonlyonthescatter
From these equations the subject contrast C is expressed as
anglebutnotonthephotonenergy(seeEq1).Accordingly,the
follows:
contrast is:
I 2 I
~ !
T’ T
2µ x 2µ x 2µ x 2µ x
C 5 3100% (6) 1 1 2 2 1 1 2 2
I 2 I I e µ We 2 I e µ We
sc sc' 0 scat 0 D,scat
I
T
C 5 5
2µ x 2µ x
1 1 12 2
I I e µ We
sc 0 scat
2µ L 2 W L1µ W 2µL
~ ~ ! !
D
e 2 e
C 5 (7)
µ 2 µ
2µL
scat D,scat
e
5 (12)
µ
scat
Assuming a small discontinuity allows the exponent to be
replaced by its power expansion to the first order providing the 9
Bossi, R. H., Friddell, K. D., Nelson, J. M. “Backscatter X-Ray Imaging,” Mat.
following expression: Eval., 46, 1988, pp. 1462–67.
FIG. 4 Schematic Representation of the Transmitted Beam Imaging System Technique
E1931 − 16 (2022)
FIG. 5 Schematic Representation of the Compton Scatter Tomography Technique
Again, here µ and µ are the linear scatter coefficients of CS
...