ASTM E2007-00
(Guide)Standard Guide for Computed Radiology (Photostimulable Luminescence (PSL) Method)
Standard Guide for Computed Radiology (Photostimulable Luminescence (PSL) Method)
SCOPE
1.1 This guide covers practices and image quality measuring systems for the detection, display, and recording of CR data files. These data files, used in materials examination, are generated by penetrating radiation passing through the subject material and producing an image via a storage phosphor imaging plate. Although the described radiation sources are specifically X-ray and gamma-ray, the general concepts can be used for other radiation sources such as neutrons. The image detection and display techniques are nonfilm, but the use of a hard copy as a means for permanent recording of the image is not precluded.
1.2 This guide is for tutorial purposes only and to outline the general principles of computed radiology (CR) imaging.
1.3 The values stated in SI units are to be regarded as the standard. The inch-pound units given in parentheses are for information only.
1.4 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. For specific safety precautionary statements, see Section 7.
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Designation:E2007–00
Standard Guide for
Computed Radiology (Photostimulable Luminescence (PSL)
Method)
This standard is issued under the fixed designation E 2007; 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 cators (IQI) Used for Radiology
E 1817 Practice for Controlling Quality of Radiological
1.1 This guide covers practices and image quality measur-
Examination by Using Representative Quality Indicators
ingsystemsforthedetection,display,andrecordingofCRdata
(RQIs)
files. These data files, used in materials examination, are
2.2 Federal Standard:
generated by penetrating radiation passing through the subject
Fed. Std. No. 21-CFR 1020.40 Safety Requirements for
material and producing an image via a storage phosphor
Cabinet X-Ray Machines
imaging plate. Although the described radiation sources are
specifically X-ray and gamma-ray, the general concepts can be
3. Summary of Guide
used for other radiation sources such as neutrons. The image
3.1 This guide outlines the practices for the use of CR
detection and display techniques are nonfilm, but the use of a
methods and techniques for materials examination. It is in-
hard copy as a means for permanent recording of the image is
tended to provide a basic understanding of the method and the
not precluded.
techniques involved. The selection of a storage phosphor
1.2 This guide is for tutorial purposes only. It outlines the
imaging plate, radiation source, and radiological techniques,
general principles of computed radiology (CR) imaging in
whicharenecessaryinachievinguserCRperformancerequire-
which luminescence is emitted by a storage phosphor imaging
ments, is described.
plate,bymeansofphotostimulationafterthedetectorhasbeen
penetrated by x-rays or gamma radiation.
4. Significance and Use
1.3 The values stated in SI units are to be regarded as the
4.1 This guide establishes an introduction to the theory and
standard. The inch-pound units given in parentheses are for
use of CR.The X-, gamma-ray detector discussed in this guide
information only.
is a storage phosphor imaging plate, hereafter referred to as
1.4 This standard does not purport to address all of the
SPIP.TheSPIP,whichisthekeycomponentintheCRprocess,
safety concerns, if any, associated with its use. It is the
differentiates CR from other radiologic methods. This guide is
responsibility of the user of this standard to establish appro-
a tutorial standard, and therefore it does not present specified
priate safety and health practices and determine the applica-
image quality levels as would be used to address the accep-
bility of regulatory limitations prior to use. For specific safety
tance or rejection criteria established between two contracting
precautionary statements, see Section 7.
parties, for example, NDT facility or consumer of NDT
2. Referenced Documents services, or both. It is not a detailed how-to procedure to be
used by the NDTfacility or consumer of NDTservice, or both.
2.1 ASTM Standards:
4.2 Table 1 lists the general performance, complexity, and
E 142 Method for Controlling Quality of Radiographic
2 relative cost of CR systems.
Testing
E 747 Practice for Design, Manufacture and Material
5. Background
Grouping Classification of Wire Image Quality Indicators
2 5.1 Inspired by the success of computed tomography (CT),
(IQI) Used for Radiology
new methods of radiologic imaging have been developed that
E 1025 Practice for Design, Manufacture, and Material
utilize recent advances in electronics and computer technolo-
Grouping Classification of Hole-Type Image Quality Indi-
gies to realize better image quality, and to evolve new imaging
modalities. These are generally in a category in which the
X-raysensorismainlyeithertheconventionalimageintensifier
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-
andtelevision-cameracombinationorthelineararraysensoras
tive Testing and is the direct responsibility of Subcommittee E07.01 on Radiology
(X and Gamma) Method.
Current edition approved May 10, 2000. Published July 2000. Originally
published as E 2007–99. Last previous edition E 2007–99a. Available from Standardization Documents Order Desk, Bldg. 4, Section D,
Annual Book of ASTM Standards, Vol 03.03. 700 Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E2007–00
TABLE 1 Computed Radiology (PSL Method)
Availability Good
Equipment needed Phosphor storage imaging plate, plate reader and work station
Usual readout Electronics/visual
methods
Other readout Film
methods
Practical Dependent on application and equipment
resolution
Contrast sensitivity in % Dependent on energy and material
Useful kVcp range
Min 10kV
Max 32MeV
Optimum kVcp As low as practical
Maximum field of 14[in] 3 17[in]
view
Relative
Extremely high
sensitivity to
X rays
Relative cost Low to moderate
Approximate useful
Five years or determined by handling
life
Special remarks Digital and versatile
used in CT.The basic quality of the digital image is not limited 5.4 CR can utilize various software algorithms for image
by digital processing but in large measure by the performance enhancement and optical disks for digital file storage. This
of the sensor itself in regard to spacial resolution and signal to advanced imaging system greatly expands the versatility of
noise ratio. radiology. Potential industrial applications include production
5.2 The earliest written reference to fluorescence, the phe- examination of aircraft components, welds in rocket-motor
nomenon which causes materials to emit light in response to housings, castings, transistors, microcircuits, circuit-boards,
external stimuli, dates back to 1500 B.C. in China. This valve positions, erosion and corrosion of pipes, integrity of
phenomenon did not attract scientific interest until 1603, when pipe welds, solenoid valves, fuses, relays, tires, reinforced
the discovery of the Bolognese stone in Italy led to investiga- plastics and automotive parts.
tion by a large number of researchers. One of these was 5.5 Limitation:
Becquerel, who, in his 1869 book La Lumiere, revealed that he 5.5.1 Handling Characteristics—Potentially, a CR imaging
had discovered the phenomenon of stimulated luminescence in plate may be erased and reused thousands of times. The
the course of his work with phosphors. primary limiting factor, as is the case with lead intensifying
5.2.1 Photo stimulated luminescence (PSL) is a phenom- screens, is physical handling. Frequency of handling, bending
enon in which a phosphor that has ceased emitting light and cleaning determines the plate’s useful lifespan.
because of the removal of the stimulus once again emits light
6. Interpretation and Reference Standards
when excited by light with a longer wavelength. The phenom-
enon is quite common since photostimulable phosphors cover 6.1 Acceptance Standards—As written by other organiza-
a broad range of materials—compounds of elements from tions for film radiography may be employed for CR inspection
GroupsIIBandVI(forexample,ZnS),compoundsofelements provided appropriate adjustments are made to accommodate
from Groups 1A and VIIA, VIIB and V VIB (for example, the differences represented by the CR data files.
alkali halides), diamond, Groups 2A and VIIA, VIIB and V 6.2 ASTM Reference Standards—Reference digital image
2+
VIB (for example, barium fluorohalides—Ba FX-EU X=Br, standards, complementing existing ASTM reference film ra-
I, etc.), oxides (for example, Zn2Si04:Mn and LaOBr:Ce, Tb), diographic standards must be developed. Subcommittee
and even certain organic compounds. The materials therefore E07.01 work aimed at developing such standards is underway.
lend themselves to data storage because the stimulus or
7. Safety Precautions
primary excitation could be used to write data to the material,
the light or secondary excitation to read the data back. Storage 7.1 The safety procedures for the handling and use of
phosphor imaging plate (IP) is a name given to a two- ionizing radiation sources must be followed. Mandatory rules
dimensional flexible sensor that can store a latent image and regulations are published by governmental licensing agen-
obtained from X rays, electron beams or other types of cies, and guidelines for control of radiation are available in
radiation, using photostimulable phosphors (P.P.), and then publications such as the Fed. Std. No. 21-CFR 1020.40.
sequentially reproduces them as a digital file by releasing the Careful radiation surveys should be made in accordance with
PSL with a laser beam, piping the PSL to a photomultiplier regulations and codes and should be conducted in the exami-
tube (PMT) and then digitizing the resulting electrical signal. nation area as well as adjacent areas under all possible
5.3 With the introduction of photostimulable luminescence operating conditions.
imaging systems in the early 1980’s, CR was born by the
8. Radiation Sources
combination of this highly advanced photographic technology
with recent advances in computer technologies. 8.1 General:
E2007–00
8.1.1 The sources of radiation for CR imaging systems many different energies and the object may be composed of
described in this guide are X-ray machines and radioactive atomsofmanydifferentkinds.Exactpredictionofthefluxfield
isotopes. The energy range available extends from a few kV to
falling upon the SPIP is therefore, difficult. Approximations
32 MeV. Since examination systems in general require high
can be made, since the mathematics and data are available to
dose rates, X-ray machines are the primary radiation source.
treat any single photon energy and atomic type, but in practice
The types of X-ray sources available are conventional X-ray
great reliance must be placed on the experience of the user. In
generators that extend in energy up to 420 kV. Energy sources
spite of these difficulties, successful CR SPIP’s have been
from 1 MeV and above are generally represented by linear
developed, and perform well.The criteria for choice depend on
accelerators.
many factors, which, depending on the application, may, or
8.1.2 Useable isotope sources have energy levels from 84
may not be critical. Obviously, these criteria will include the
170 60
keV (Thulium-170, Tm ) up to 1.25 MeV (Cobalt-60, Co ).
following factors.
With high specific activities, these sources should be consid-
9.4.1 Field of View—The field of view of the SPIP and its
ered for special application where their field mobility and
resolution are interrelated. The resolution of the SPIP is fixed
operational simplicity can be of significant advantage.
by its physical characteristics, so if the X-ray image is
8.1.3 The factors to be considered in determining the
projected upon it full-size (the object and image planes in
desiredradiationsourceareenergy,focalgeometry,waveform,
contact),theresultantresolutionwillbeapproximatelyequalto
half life, and radiation output.
that of the SPIP. When SPIP resolution becomes the limiting
8.2 Selection of Sources:
factor, the object may be moved away from the SPIP, and
8.2.1 Low-Energy Sources—The radiation source selected
for a specific examination system depends upon the material towards the source to enlarge the projected image and thus
being examined, its mass,itsthickness,andtherequiredrateof allow smaller details to be resolved by the same SPIP. As the
examination. In the energy range up to 420 kV, the X-ray units
image is magnified, however, the detail contrast is reduced and
have an adjustable energy range so that they are applicable to
its outlines are less distinct. (See 10.3.) It is apparent, also, that
a wide range of materials. Specifically, 50-kV units operate
when geometric magnification is used, the area of the object
down to a few kV; 160-kVequipment operates down to 20 kV;
that is imaged on the SPIP is proportionally reduced. As a
and 420-kV equipment operates down to about 85 kV.
generalrule,X-raymagnificationsshouldnotexceed5xexcept
8.2.2 High-Energy Sources—The increased efficiency of
when using X-ray sources with very small (microfocus)
X-ray production at higher accelerating potentials makes
anodes. In such cases, magnifications in the order of 10 to 20x
available a large radiation flux, and this makes possible the
are useful. When using conventional focal-spot X-ray sources,
examination of greater thicknesses of material. High radiation
magnifications from 1.2 to 1.5 provide a good compromise
energies in general produce lower image contrast, so that as a
between contrast and resolution in the magnified image.
guide the minimum thickness of material examined should not
9.4.2 Inherent Sensitivity—The basic sensitivity of the SPIP
be less than 2.5 half-value layers of material. The maximum
may be defined as its ability to respond to small, local
thickness of material can extend up to 10 half-value layers.
variationsinradiantenergytodisplaythefeaturesofinterestin
8.3 Source Geometry:
the object being examined. It would seem that an SPIPthat can
8.3.1 Althoughthephysicalsizeofthesourceofradiationis
displayintensitychangesontheorderof1to2 %atresolutions
a parameter that may vary considerably, the radiation sensor is
approaching that of film radiography would satisfy all of the
generally the principal source of unsharpness.
requirements for successful CR imaging. It is not nearly that
9. CR Storage Phosphor Imaging Plate
simple. Often good technique is more important than the
9.1 A CR storage phosphor imaging plate (SPIP) is de-
details of the imaging system itself. The geometry of the
scribed as a reusable detector (flexible or rigid) that stores
system with respect to field of view, resolution and contrast is
penetrating radiation energy as a latent image.
a very important consideration as is the control of scattered
9.2 When X-ray photons pass through an object, they are
radiation. Scattered X rays entering the imaging system pro-
attenuated. At low-to-medium energies this attenuation is
duce background similar to fogging in a radiograph. This
caused primarily by photoelectric absorption, or Compton
scatter not only introduces radiant energy containing no useful
scattering. At high energies, scattering is by pair production
information into the imaging system but also impairs system
(over 1 MeV) and photon photonuclear processes (at about
sensitivity and resolution. Careful filtering and collimation of
11.5 MeV).As a result of attenuation, the character of
...
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