ASTM E2007-10(2023)

Standard Guide for Computed Radiography

SIGNIFICANCE AND USE
4.1 This guide is intended as a source of tutorial and reference information that can be used during establishment of computed radiography techniques and procedures by qualified CR personnel for specific applications. All materials presented within this guide may not be suited for all levels of computed radiographic personnel.  
4.2 This guide is intended to build upon an established basic knowledge of radiographic fundamentals (that is, film systems) as may be found in Guide E94. Similarly, materials presented within this guide are not intended as “all-inclusive” but are intended to address basic CR topics and issues that complement a general knowledge of computed radiography as described in 1.2 and 3.2.28.  
4.3 Materials presented within this guide may be useful in the development of end-user training programs designed by qualified CR personnel or activities that perform similar functions. Computed radiography is considered a rapidly advancing inspection technology that will require the user maintain knowledge of the latest CR apparatus and technique innovations. Section 11 of this guide contains technical reference materials that may be useful in further advancement of knowledge associated with computed radiography.
SCOPE
1.1 This guide provides general tutorial information regarding the fundamental and physical principles of computed radiography (CR), definitions and terminology required to understand the basic CR process. An introduction to some of the limitations that are typically encountered during the establishment of techniques and basic image processing methods are also provided. This guide does not provide specific techniques or acceptance criteria for specific end-user inspection applications. Information presented within this guide may be useful in conjunction with those standards of 1.2.  
1.2 CR techniques for general inspection applications may be found in Practice E2033. Technical qualification attributes for CR systems may be found in Practice E2445. Criteria for classification of CR system technical performance levels may be found in Practice E2446. Reference Images Standards E2422, E2660, and E2669 contain digital reference acceptance illustrations.  
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 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.

General Information

• Status: Published
• Publication Date: 31-May-2023
• ICS: 17.180.01 - Optics and optical measurements in general
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.01 - Radiography (X and Gamma) Method

Relations

• Refers: ASTM E1316-24 - Standard Terminology for <?Pub Dtl?>Nondestructive Examinations
• Effective Date: 01-Feb-2024
• Refers: ASTM E746-23 - Standard Practice for Determining Relative Image Quality Response of Industrial Radiographic Imaging Systems below 4 MeV
• Effective Date: 01-Dec-2023
• Refers: ASTM E1316-19b - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Dec-2019
• Refers: ASTM E1316-19 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Mar-2019
• Refers: ASTM E1025-18 - Standard Practice for Design, Manufacture, and Material Grouping Classification of Hole-Type Image Quality Indicators (IQI) Used for Radiography
• Effective Date: 01-Feb-2018
• Refers: ASTM E746-18 - Standard Practice for Determining Relative Image Quality Response of Industrial Radiographic Imaging Systems
• Effective Date: 01-Feb-2018
• Refers: ASTM E1316-18 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jan-2018
• Refers: ASTM E746-17 - Standard Practice for Determining Relative Image Quality Response of Industrial Radiographic Imaging Systems
• Effective Date: 01-Nov-2017
• Refers: ASTM E1316-17a - Standard Terminology for Nondestructive Examinations
• Effective Date: 15-Jun-2017
• Refers: ASTM E1316-17 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Feb-2017
• Refers: ASTM E1316-16a - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Aug-2016
• Refers: ASTM E1316-16 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Feb-2016
• Refers: ASTM E1316-15a - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Dec-2015
• Refers: ASTM E1316-15 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Sep-2015
• Refers: ASTM E746-07(2014) - Standard Practice for Determining Relative Image Quality Response of Industrial Radiographic Imaging Systems
• Effective Date: 01-Jul-2014

Overview

ASTM E2007-10(2023): Standard Guide for Computed Radiography provides comprehensive tutorial and reference information on the fundamental and physical principles underlying computed radiography (CR). Developed by ASTM International, this guide supports qualified personnel in establishing and refining computed radiography techniques and procedures for various non-destructive testing (NDT) applications.

This guide is designed to complement a basic understanding of radiographic examination using film, while introducing key computed radiography concepts, terminology, system components, and technical topics. It helps practitioners navigate limitations, highlight image processing fundamentals, and stay informed about developments in CR technology.

Key Topics

• Computed Radiography Fundamentals:
  Explains the principles of computed radiography using storage phosphor imaging plates (IPs), describing core steps from image acquisition to digital processing and display.

• System Components:
  Details the typical CR system, including imaging plates, plate readers, computers, electronic display devices, and supporting software.

• Essential Terminology:
  Defines important concepts such as spatial resolution, contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR), pixel value, gray scale, and digital image processing.

• Latent Image Formation and Processing:
  Discusses the phenomenon of photostimulable luminescence (PSL) in storage phosphors, the process of capturing and reading the digital latent image, and managing residual images.

• Image Quality Factors:
  Addresses key factors affecting CR image quality, such as exposure parameters, resolution, dynamic range, digital noise, and the importance of proper handling and processing.

• System Calibration and Reference Materials:
  Introduces the use of digital reference images and technical reference materials for system performance evaluation and classification.

Applications

Computed radiography is widely used across multiple industries that require reliable, high-resolution imaging for inspection and evaluation:

• Industrial Non-Destructive Testing (NDT):
  CR enables detailed inspection of welds, castings, and structural components in sectors such as aerospace, automotive, oil and gas, and power generation.

• Quality Assurance and Control:
  The ability to produce, store, and analyze high-quality digital images enhances process control and traceability in manufacturing environments.

• Training and Certification:
  Materials in this standard support the development of end-user training programs for both new and experienced CR practitioners, facilitating consistent and up-to-date knowledge transfer.

• Archiving and Digital Workflow Integration:
  Digital images can be efficiently stored, retrieved, and shared, supporting remote diagnostics, regulatory compliance, and robust data management practices.

Related Standards

To effectively establish or enhance computed radiography practices, ASTM E2007-10(2023) should be used alongside these closely associated standards:

• ASTM E94: Guide for Radiographic Examination Using Industrial Radiographic Film
• ASTM E2033: Practice for Radiographic Examination Using Computed Radiography (Photostimulable Luminescence Method)
• ASTM E2445: Practice for Performance Evaluation and Long-Term Stability of Computed Radiography Systems
• ASTM E2446: Practice for Manufacturing Characterization of Computed Radiography Systems
• ASTM E2422, E2660, E2669: Standards containing digital reference images for various materials and applications
• ASTM E1316: Terminology for Nondestructive Examinations

Practical Value

By following the guidance in ASTM E2007-10(2023), organizations and professionals can:

• Ensure accurate, repeatable computed radiography results
• Improve detection sensitivity and inspection reliability
• Develop effective training and quality assurance programs
• Facilitate compliance with international best practices in CR and NDT

Regular reference to this standard supports the implementation and continual improvement of computed radiography systems, helping industries maintain leading-edge, efficient, and safe inspection practices.

Standards Content (Sample)

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: E2007 − 10 (Reapproved 2023)
Standard Guide for
Computed Radiography
This standard is issued under the fixed designation E2007; 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 mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
1.1 This guide provides general tutorial information regard-
ing the fundamental and physical principles of computed
2. Referenced Documents
radiography (CR), definitions and terminology required to
2.1 ASTM Standards:
understand the basic CR process. An introduction to some of
E94 Guide for Radiographic Examination Using Industrial
the limitations that are typically encountered during the estab-
Radiographic Film
lishment of techniques and basic image processing methods are
E746 Practice for Determining Relative Image Quality Re-
also provided. This guide does not provide specific techniques
sponse of Industrial Radiographic Imaging Systems
or acceptance criteria for specific end-user inspection applica-
E747 Practice for Design, Manufacture and Material Group-
tions. Information presented within this guide may be useful in
ing Classification of Wire Image Quality Indicators (IQI)
conjunction with those standards of 1.2.
Used for Radiology
1.2 CR techniques for general inspection applications may
E1025 Practice for Design, Manufacture, and Material
be found in Practice E2033. Technical qualification attributes
Grouping Classification of Hole-Type Image Quality In-
for CR systems may be found in Practice E2445. Criteria for
dicators (IQI) Used for Radiography
classification of CR system technical performance levels may
E1316 Terminology for Nondestructive Examinations
be found in Practice E2446. Reference Images Standards
E1453 Guide for Storage of Magnetic Tape Media that
E2422, E2660, and E2669 contain digital reference acceptance
Contains Analog or Digital Radioscopic Data
illustrations.
E2002 Practice for Determining Image Unsharpness and
1.3 The values stated in SI units are to be regarded as the
Basic Spatial Resolution in Radiography and Radioscopy
standard. The inch-pound units given in parentheses are for
E2033 Practice for Radiographic Examination Using Com-
information only.
puted Radiography (Photostimulable Luminescence
Method)
1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the E2339 Practice for Digital Imaging and Communication in
Nondestructive Evaluation (DICONDE)
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter- E2422 Digital Reference Images for Inspection of Alumi-
num Castings
mine the applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accor- E2445 Practice for Performance Evaluation and Long-Term
Stability of Computed Radiography Systems
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the E2446 Practice for Manufacturing Characterization of Com-
puted Radiography Systems
Development of International Standards, Guides and Recom-
E2660 Digital Reference Images for Investment Steel Cast-
ings for Aerospace Applications
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-
tive Testing and is the direct responsibility of Subcommittee E07.01 on Radiology
(X and Gamma) Method. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved June 1, 2023. Published June 2023. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 1999. Last previous edition approved in 2016 as E2007 – 10 (2016). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/E2007-10R23. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2007 − 10 (2023)
E2669 Digital Reference Images for Titanium Castings image display, image storage and retrieval system and interac-
tive support software.
2.2 SMPTE Standard:
RP-133 Specifications for Medical Diagnostic Imaging Test
3.2.7 computed radiographic system class—a group of com-
Pattern for Television Monitors and Hard-Copy Recording
puted radiographic systems characterized with a standard
Cameras
image quality rating. Practice E2446, Table 1, provides such a
classification system.
3. Terminology
3.2.8 computed radiography—a radiological nondestructive
3.1 Unless otherwise provided within this guide, terminol-
testing method that uses storage phosphor imaging plates
ogy is in accordance with Terminology E1316.
(IP’s), a PSL stimulating light source, PSL capturing optics,
3.2 Definitions: optical-to-electrical conversion devices, analogue-to-digital
3.2.1 aliasing—artifacts that appear in an image when the data conversion electronics, a computer and software capable
spatial frequency of the input is higher than the output is of processing original digital image data and a means for
capable of reproducing. This will often appear as jagged or electronically displaying or printing resultant image data.
stepped sections in a line or as moiré patterns.
3.2.9 contrast and brightness—an application of digital
3.2.2 basic spatial resolution (SR )—terminology used to
image processing used to “re-map” displayed gray scale levels
b
describe the smallest degree of visible detail within a digital
of an original gray scale data matrix using different reference
image that is considered the effective pixel size.
lookup tables.
3.2.2.1 Discussion—The concept of basic spatial resolution
3.2.9.1 Discussion—This mode of image processing is also
involves the ability to separate two distinctly different image
known as “windowing” (contrast adjustment) and “leveling”
features from being perceived as a single image feature. When
(brightness adjustment) or simply “win-level” image process-
two identical image features are determined minimally distinct,
ing.
the single image feature is considered the effective pixel size.
3.2.10 contrast-to-noise ratio (CNR)—quotient of the digi-
If the physical sizes of the two distinct features are known, for
tal image contrast (see 3.2.13) and the averaged standard
example, widths of two parallel lines or bars with an included
deviation of the linear pixel values.
space equal to one line or bar, then the effective pixel size is
1 3.2.10.1 Discussion—CNR is a measure of image quality
considered ⁄2 of their sums. Example: A digital image is
that is dependent upon both digital image contrast and signal-
determined to resolve five line pairs per mm or a width of line
to-noise ratio (SNR) components. In addition to CNR, a digital
equivalent to five distinct lines within a millimetre. The basic
radiograph must also possess adequate sharpness or basic
spatial resolution is determined as 1/ [2 × 5 LP/ mm] or
spatial resolution to adequately detect desired features.
0.100 mm.
3.2.11 digital driving level (DDL)—terminology used to
3.2.3 binary/digital pixel data—a matrix of binary (0’s, 1’s)
describe displayed pixel brightness of a digital image on a
values resultant from conversion of PSL from each latent pixel
monitor resultant from digital mapping of various gray scale
(on the IP) to proportional (within the bit depth scanned)
levels within specific look-up-table(s).
electrical values. Binary digital data value is proportional to the
radiation dose received by each pixel. 3.2.11.1 Discussion— DDL is also known as monitor pixel
intensity value; thus, may not be the PV of the original digital
3.2.4 bit depth—the number “2” increased by the exponen-
image.
tial power of the analogue-to-digital (A/D) converter resolu-
tion. Example 1) In a 2-bit image, there are four (2 ) possible
3.2.12 digital dynamic range—maximum material thickness
combinations for a pixel: 00, 01, 10 and 11. If “00” represents
latitude that renders acceptable levels of specified image
black and “11” represents white, then “01” equals dark gray
quality performance within a specified pixel intensity value
and “10” equals light gray. The bit depth is two, but the number
range.
of gray scales shades that can be represented is 2 or 4.
3.2.12.1 Discussion—Digital dynamic range should not be
Example 2): A 12-bit A/D converter would have 4096 (2 )
confused with computer file bit depth.
gray scales shades that can be represented.
3.2.13 digital image contrast—pixel value difference be-
3.2.5 blooming or flare—an undesirable condition exhibited
tween any two areas of interest within a computed radiograph.
by some image conversion devices brought about by exceeding
3.2.13.1 Discussion—Digital contrast = PV2 – PV1 where
the allowable input brightness for the device, causing the
PV2 is the pixel value of area of interest “2” and PV1 is the
image to go into saturation, producing an image of degraded
pixel value of area of interest “1” on a computed radiograph.
spatial resolution and gray scale rendition.
Visually displayed image contrast can be altered via digital
3.2.6 computed radiographic system—all hardware and
re-mapping (see 3.2.11) or re-assignment of specific gray scale
software components necessary to produce a computed radio-
shades to image pixels.
graph. Essential components of a CR system consisting of: an
3.2.14 digital image noise—imaging information within a
imaging plate, an imaging plate readout scanner, electronic
computed radiograph that is not directly correlated with the
degree of radiation attenuation by the object or feature being
examined and/or insufficient radiation quanta absorbed within
Available from Society of Motion Picture and Television Engineers (SMPTE),
3 Barker Ave, 5th Floor, White Plains, NY 10601. the detector IP.
E2007 − 10 (2023)
FIG. 1 Basic Computed Radiography Process
3.2.14.1 Discussion—Digital image noise results from ran- 3.2.18 image morphing—a potentially degraded CR image
dom spatial distribution of photons absorbed within the IP and resultant from over processing (that is, over driving) an
interferes with the visibility of small or faint detail due to
original CR image.
statistical variations of pixel intensity value.
3.2.18.1 Discussion—“Morphing” can occur following sev-
3.2.15 digital image processing—the use of algorithms to eral increments of image processing where each preceding
image was “overwritten” resulting in an image that is notice-
change original digital image data for the purpose of enhance-
ment of some aspect of the image. ably altered from the original.
3.2.15.1 Discussion—Examples include: contrast,
3.2.19 look up table (LUT)—one or more fields of binary
brightness, pixel density change (digital enlargement), digital
digital values arbitrarily assigned to a range of reference gray
filters, gamma correction, and pseudo colors. Some digital
scale levels (viewed on an electronic display as shades of
processing operations such as sharpening filters, once saved,
“gray”).
permanently change the original binary data matrix (Fig. 1,
3.2.19.1 Discussion—A LUT is used (applied) to convert
Step 5).
binary digital pixel data to proportional shades of “gray” that
3.2.16 equivalent penetrameter sensitivity (EPS)—that
define the CR image. LUT’s are key reference files that allow
thickness of penetrameter, expressed as a percentage of the
binary digital pixel data to be viewed with many combinations
section thickness radiographed, in which a 2T hole would be
of pixel gray scales over the entire range of a digital image (see
visible under the same radiographic conditions. EPS is calcu-
Fig. 5-A).
lated by: EPS% = 100/ X (√ Th/2), where: h = hole diameter,
3.2.20 original digital image—a digital gray scale (see
T = step thickness and X = thickness of test object (see
3.2.17) image resultant from application of original binary
Terminology E1316 and Practices E1025, E747, and E746).
digital pixel data to a linear look-up table (see 3.2.24 and
3.2.17 gray scale—a term used to describe an image con-
3.2.19 prior to any image processing.
taining shades of gray rather than color. Gray scale is the range
3.2.20.1 Discussion—This original gray scale image is usu-
of gray shades assigned to image pixels that result in visually
ally considered the beginning of the “computed radiograph”,
perceived pixel display brightness.
since without this basic conversion (to gray scales) there would
3.2.17.1 Discussion—The number of shades is usually posi-
be no discernable radiographic image (see Fig. 5-B).
tive integer values taken from the bit depth. For example: an
8-bit gray scale image has up to 256 total shades of gray from 3.2.21 photostimulable luminescence (PSL)—photostimula-
0 to 255, with 0 representing white image areas and 255 ble luminescence (PSL) is a physical phenomenon in which a
representing black image areas with 254 shades of gray in halogenated phosphor compound emits bluish light when
between. excited by a source of red spectrum light.
E2007 − 10 (2023)
3.2.22 pixel brightness—the luminous (monitor) display 3.2.28 spatial resolution—terminology used to define a
intensity of pixel(s) that can be controlled by means of component of optical image quality associated with distinction
of closely spaced adjacent multiple features.
electronic monitor brightness level settings or changes of
digital driving level (see 3.2.11).
3.2.28.1 Discussion—The concept of optical resolution in-
volves the ability to separate multiple closely spaced
3.2.23 pixel density—the number of pixels within a digital
components, for example, optical line pairs, into two or more
image of fixed dimensions (that is, length and width).
distinctly different components within a defined unit of space.
3.2.23.1 Discussion—for digital raster images, the conven-
Example: an optical imaging system that is said to resolve two
tion is to describe pixel density in terms of the number of
line pairs within one mm of linear space (that is, 2 Lp/mm)
pixel-columns (width) and number of pixel-rows (height). An
contains five individual components: two closely spaced adja-
alternate convention is to describe the total number of pixels in
cent line components, an intervening space between the lines
the image area (typically given as the number of mega pixels),
and space on the outside boundaries of the two lines.
which can be calculated by multiplying pixel-columns by
3.2.29 storage phosphor imaging plate (IP)—a photostimu-
pixel-rows. Another convention includes describing pixel den-
lable luminescent material that is capable of storing a latent
sity per area-unit or per length-unit such as pixels per in./mm.
radiographic image of a material being examined and, upon
Resolution (see 7.1.5) of a digital image is related to pixel
stimulation by a source of red spectrum light, will generate
density.
luminescence (PSL) proportional to radiation absorbed.
3.2.24 pixel value (PV)—a positive integer numerical value
3.2.29.1 Discussion—When performing computed
directly associated with each binary picture data element
radiography, an IP is used in lieu of a film. When establishing
(pixel) of an original digital image where gray scale shades
techniques related to source focal geometries, the IP is referred
(see 3.2.17) are assigned in linear proportion to radiation
to as a detector (that is, source-to detector-distance or SDD).
exposure dose received by that area.
3.2.30 unsharpness—terminology used to describe an attri-
3.2.24.1 Discussion—Computed radiography uses gray
bute of image quality associated with blurring or loss of
scale shades to render visual perceptions of image contrast;
distinction within a radiographic image.
thus, linear pixel value (PV) is used to measure a specific shade
3.2.30.1 Discussion—Measured total unsharpness is de-
of gray that corresponds to the quantity of radiation exposure
scribed with a numerical value corresponding with a measure
absorbed within a particular area of a part. With this
of definition (that is, distinction) associated with the geometry
relationship, a PV of “0” can correspond with “0” radiation
of exposure and inherent unsharpness of the CR system (that is,
dose (white image area of a negative image view) whereas a
inherent or total unsharpness). Guide E94 provides fundamen-
PV of “4095” can correspond with a saturated detector (black
tal guidance related to geometrical unsharpness and Practice
image area of a negative image view) for a 12 bit CR system.
E2002 provides a standard practice for measurement of total
PV is directly related to original binary pixel data via a
unsharpness.
common linear look-up-table (Fig. 5 A and B illustrate). The
number of available pixel value integers within an image is
4. Significance and Use
associated with the number of available gray scale shades for
the bit depth of the image.
4.1 This guide is intended as a source of tutorial and
reference information that can be used during establishment of
3.2.25 PSL afterglow—continued luminescence from a stor-
computed radiography techniques and procedures by qualified
age phosphor immediately following removal of an external
CR personnel for specific applications. All materials presented
photostimulating source.
within this guide may not be suited for all levels of computed
3.2.25.1 Discussion—A bluish luminescence continues for a
radiographic personnel.
short period of time after termination of the photostimulating
4.2 This guide is intended to build upon an established basic
source as illustrated in Fig. 12.
knowledge of radiographic fundamentals (that is, film systems)
3.2.26 relative image quality response (RIQR)—a means for
as may be found in Guide E94. Similarly, materials presented
determining the image quality performance response of a given
within this guide are not intended as “all-inclusive” but are
radiological imaging system in relative comparison to the
intended to address basic CR topics and issues that comple-
image quality response of another radiological imaging system.
ment a general knowledge of computed radiography as de-
3.2.26.1 Discussion—RIQR methods are not intended as a
scribed in 1.2 and 3.2.28.
direct measure of image quality for a specific radiographic
4.3 Materials presented within this guide may be useful in
technique application. Practice E746 provides a standard RIQR
the development of end-user training programs designed by
method.
qualified CR personnel or activities that perform similar
3.2.27 signal-to-noise ratio (SNR)—quotient of mean linear
functions. Computed radiography is considered a rapidly
pixel value and standard deviation of mean linear pixel values
advancing inspection technology that will require the user
(noise) for a defined detector area-of-interest in a digital image.
maintain knowledge of the latest CR apparatus and technique
3.2.27.1 Discussion—Notwithstanding extraneous sources innovations. Section 11 of this guide contains technical refer-
of digital image noise, SNR will normally increase as exposure ence materials that may be useful in further advancement of
dose is increased. knowledge associated with computed radiography.
E2007 − 10 (2023)
Illustration courtesy of Fujifilm NDT Systems
FIG. 2 Cross Section of a Typical Storage Phosphor Imaging Plate
5. Computed Radiography Fundamentals within a binary data matrix (Fig. 4 illustrates assignment of
binary data to a pixel matrix).
5.1 This section introduces and describes primary core
The actual size of the binary pixel element (length and
components and processes of a basic computed radiography
width) is determined by the scanning speed of the transport
process. The user of this standard guide is advised that
mechanism in one direction and the clock speed of the
computed radiography is a rapidly evolving technology where
sampling along each scan line (how fast the laser spot moves
innovations involving core steps and processes are continually
divided by the sampling rate). Although resolution is limited by
under refinement. Tutorial information presented in this section
pixel size, the size of individual phosphor crystals, the phos-
is intended to illustrate the fundamental computed radiography
phor layer thickness of the image plate, laser spot size and
process and not necessarily any specific commercial CR
optics also contribute to the overall quality (resolution) of the
system.
image. Each of these components thus becomes a very essen-
5.2 Acquiring the CR Image—Computed radiography (CR)
tial contributor to the overall binary matrix that represents the
is one of several different modes of digital radiography that
digital image. These individual elements represent the smallest
employs re-usable photostimulable luminescence (PSL) stor-
unit of storage of a binary digital image that can be discretely
age phosphor imaging plates (commonly called IP’s) for
controlled by the CR data acquisition and display system
acquisition of radiographic images. Fig. 1 illustrates an ex-
components and are commonly called “pixels.” The term
ample of the fundamental steps of a basic CR process arrange-
“pixel” is thus derived from two word components of the
ment.
digital matrix, that is, picture (or pix) and elements (els) or
In this illustration, a conventional (that is, Guide E94)
“pixels.” Picture elements or pixels become the basis for all
radiographic exposure geometry/arrangement is used to expose
technical imaging attributes that comprise quality and compo-
a part positioned between the radiation source and IP.
sition of the resultant image. An organized matrix of picture
Step 1 involves exposure of the IP (Fig. 2 illustrates typical
elements (pixels) containing binary data is called a binary pixel
cross section details of an IP) and creation of a residual latent
data matrix since proportional gray levels have not yet been
image with delayed luminescence properties (Section 6 details
assigned. (see 11.1.2) contains basic tutorial information on
physics).
binary numbering system and its usefulness for digital appli-
Step 2 involves index scanning the exposed IP with a
cations).
stimulus source of red light from a laser beam (Fig. 3 illustrates
Step 6: Computer algorithms (a string of mathematical
Steps 2 through 8).
instructions) are applied that match binary pixel data with
During the scan, the IP is stimulated to release deposited
arbitrary files (called look-up-tables) to assign individual pixel
energy of the latent image in the form of bluish photostimu-
gray scale levels. Example: for 4096 possible shades or levels
lated visible light.
Step 3: The bluish photostimulated light (PSL) is then of gray for a 12-bit image, gray scale levels are thus derived
when a computer assigns equal divisions between white (“0”)
collected by an optical system containing a chromatic filter
(that prevents the red stimulus light from being collected) and and black (“4095”) with each incremental division a derivative
(shade) of black or white (that is, gray) for a negative view
channeled to a photo-multiplier tube (PMT).
Step 4: PSL light is converted by the PMT to analogue image. An example is to assign gray scale levels in linear
proportion to the magnitude of the binary numbers (that is, a
electrical signals in proportion to quantity of PSL collected.
Step 5: Analog electrical signals are amplified, filtered, higher binary number associated with a greater amount of
passed through an analog-to-digital (A/D) converter and photo stimulated light for that pixel registration can be as-
“clock” synchronized to a spatially correct pixel location signed a corresponding darker gray value) to create an original
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Illustration courtesy of Carestream Health
FIG. 3 Fundamental CR Image Acquisition and Display Process
FIG. 4 Assignment of Binary Data to a Pixel Matrix (3-bit depth illustrated)
gray scale data matrix with a standard format (DICONDE, the resultant CR digital image can have a similar gray tonal
TIFF, BITMAP, etc.) ready for software transformation. Fig. appearance as its film counterpart (as illustrated with the LUT
5-A illustrates a simple linear look-up-table for an original gray shown in Fig. 5-A in that as gray values become larger,
scale data matrix where binary numbers are also represented by displayed luminance becomes smaller. With the digital image
their corresponding numerical integers (called pixel value display, inspected features can be characterized and disposi-
integers). In this example for a 12-bit image, there are 4096 tioned similar to a radiographic film. Both image modalities
gray scale divisions that precisely correspond with 4096 require evaluations within environments of subdued back-
numerical pixel value integers. Fig. 5-B illustrates a graphical ground lighting. Aside from these basic similarities, however,
version of the application as might be applied by an algorithm the CR digital image is an entirely different imaging modality
to produce an image with a gray tonal appearance (visually that requires some basic knowledge of digital imaging funda-
similar to a radiographic film). Most algorithms employed for mentals in order to understand and effectively apply the
original CR images assign gray scale values in linear propor- technology; c) Once the original digital image is visualized,
tion to the magnitude of each binary pixel (value). The range additional image processing techniques (see Section 8) may be
(number) of selectable gray values is defined within the image performed to further enhance inspection feature details and
viewing software as “bit depth.” complete the inspection evaluation process. This entire process
Step 7: a) Viewing software is used to transform the original is called computed radiography because of the extreme depen-
gray scale data matrix into an original image; b) The original dence on complex computational processes in order to render
image can be output to an electronic display monitor or printer; a meaningful radiographic image. Finally (Step 8), original
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FIG. 5 (A) Original 12-bit Linear Look-Up Table / (B) Graph Version of Applied Linear LUT
and/or processed digital images and related electronic records phosphors. Photo stimulated luminescence (PSL) is a phenom-
may be saved to optical, magnetic or print media for future use. enon which is quite common since photostimulable phosphors
Some applications may benefit from a high quality digital print cover a broad range of materials—compounds of elements
of the saved image. Typical CR system commercial hardware from Groups IIB and VI (for example, ZnS), compounds of
components are illustrated in Fig. 6. Computed radiographic elements from Groups 1A and VIIB, diamond, oxides (for
technology is complex in nature; therefore, subsequent sections example, Zn2Si04:Mn and LaOBr;Ce,Tb), and even certain
of this standard are intended to provide some additional levels organic compounds. The materials, therefore, lend themselves
of detail associated with the basic computed radiography to data storage because radiation could be used to write data to
process. Additional levels of information may be found within the material, the light or secondary excitation to read the data
the bibliography, Section 11. back. Storage phosphor imaging plate (IP) is a name given to
a two-dimensional sensor (see Fig. 2) that can store a latent
6. Brief History and Physics of Computed Radiography
image obtained from X-rays, electron beams, or other types of
radiation, using photostimulable phosphors.
6.1 Photo-Stimulated Luminescence (PSL) is a physical
phenomenon in which a halogenated phosphor compound
6.3 Recent History of Computed Radiography—With the
emits bluish light when excited by a source of red spectrum
introduction of photostimulable luminescence imaging systems
light. In other words, phosphors capable of “PSL” exhibit a
in the early 1980’s in combination with continued advance-
unique physical property of delayed release of visible light
ments in computer technologies, CR was “born.” In the early
subsequent to radiation exposure; thus, the reason this type of
1990’s, further advancements in computer technologies in
phosphor is sometimes referred to as a “storage phosphor.”
conjunction with refined phosphor imaging plate developments
illustrates the photo excitation process when this phosphor is
initiated limited applications, mostly driven by the medical
exposed (following exposure of the phosphor to radiation) to a
industry. The medical industry became interested in CR for two
source of red light (He-Ne or semiconductor laser). The
reasons: 1) The desire for electronic transport of digital images
“bluish-purple” light emitted during this stimulation is referred
for remote diagnostics and 2) The increased latitude of diag-
to as “photostimulated luminescence” or “PSL” for short.
nostic capability with a single patient exposure. Throughout
During collection of PSL light for computed radiography, the
the 90’s, technology advancements in CR were driven primar-
red light source is separated from PSL using a chromatic filter
ily by the medical industry for similar reasons. In the late 90’s,
(see Fig. 3). The “PSL” process is the very heart of CR
as image quality attributes continued to improve, industrial
technology and is thus important for understanding how
radiographers became more interested in CR for its ability to
computed radiography works.
detect small features within heavier materials with reliabilities
6.2 Early History of Photo Stimulated Luminensce—The approaching some classes of film systems. In 1999, continued
earliest written reference to fluorescence, the phenomenon that industrial user interests led to the development and publication
causes materials to emit light in response to external stimuli, of ASTM’s first computed radiography standard, Practice
dates back to 1500 B.C. in China. This phenomenon did not E2033. ASME adopted its first article for ASME Code com-
attract scientific interest until 1603, when the discovery of the pliant computed radiography in 2004. In 2005, further interests
Bolognese stone in Italy led to investigation by a large number from industrial users led to the development and publishing of
of researchers. One of these was Becquerel, who, in his 1869 Practices E2445 and E2446. ASTM published its first ever set
book La Lumiere, revealed that he had discovered the phenom- of all-digital reference images (Digital Reference Images
enon of stimulated luminescence in the course of his work with E2422) for the inspection of aluminum castings in 2005.
E2007 − 10 (2023)
Illustrations courtesy of Carestream Health & Fujifilm NDT Systems
FIG. 6 Typical CR Scanner, Workstation, and Image Plate
Illustration courtesy of Fujifilm NDT Systems
FIG. 7 Spectra of Photostimulated Luminescence and Excitation
6.4 PSL Crystal Structure—Fig. 8 illustrates the basic physi-
cal structure of a typical Barium Fluorohalide phosphor crystal.
Fig. 9 illustrates a photo-micrograph of these type crystal
grains as seen through a scanning electron microscope at
approximately 5 microns. These crystal structures are the basis
of the phosphor layer shown in Fig. 2 and constitute the heart
of the physical “PSL” process described in the following text.
6.5 Latent Image Formation—A widely-accepted mecha-
nism for PSL in europium-activated halides was proposed by
Takahashi et al (see 11.1.10). In the phosphor-making process,
+
halogen ion vacancies, or “F ” centers, are created. Upon
exposure of the phosphor particles to ionizing radiation (Fig.
10 provides an energy level diagram that illustrates this
process), electrons are excited to a higher energy level (con-
+
duction band) and leave behind a hole at the Eu2 ion (valance
band). While some of these electrons immediately recombine
+
and excite the Eu2 to promptly emit, others are trapped at the
Illustration courtesy of Fujifilm NDT Systems
+
F centers to form metastable F centers, also known as color
FIG. 8 BaFBr Crystal Structure
centers, from the German word “Farbe,” which means color.
The energy stored in these electron-hole pairs is the basis of the
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6.8 CR Latent Image Issues—Now that some of the funda-
mental physics of CR are established, we need to understand
how this knowledge relates to everyday use and production of
quality CR images. Most radiographers have a good under-
standing of the importance in the use of lead intensifying
screens during film applications. It is known, for example, that
lead foil placed in intimate contact with film during exposure
to radiation will intensify the formation of the film latent image
and the physical mechanism (see 11.1.11) responsible for this
is electrons liberated during radiation absorption within the
lead screens. In this case, production of secondary electrons is
desirable and actually contributes to the productive formation
of the radiographic latent image. With CR, however, electrons
generated within lead screens do not result in any appreciable
gain or accelerated formation of latent image sites. CR latent
image formation is thus primarily dependent upon radiation
absorption within the phosphor layer of the image plate. For
Illustration courtesy of Carestream Health
this reason, unfiltered CR image plates are usually more
FIG. 9 Conventional BaFX: Eu Grains (5 microns)
sensitive to direct exposure of ionizing radiation than film. At
higher levels of radiation energy (in the approximate range of
750 keV or higher), radiation absorption within lead screens
CR latent image and remains quite stable for hours. This
(as well as the part under examination) will be more propor-
mechanism has been disputed by some and supported by
tionately influenced by the Compton process (see 11.1.14). The
others; however, the end result is photostimulable lumines-
greater proportion of Compton absorption within lead screens
cence.
results in an increased proportion of secondary (non-
6.6 Processing the Latent Image—When this phosphor
directional) radiation photons that can be re-distributed to the
(bearing the latent image) is subsequently exposed (that is,
image plate during part exposure reducing overall image
scanned with a laser as shown in Fig. 3) to a source of red light,
quality results. It is therefore, important to control unwanted
most of the trapped electrons are “liberated” and return to the
secondary radiation from lead screens as well as other sources
lower energy level (valence band) of the phosphor molecule
during the acquisition of quality CR images with higher energy
causing PSL to be emitted. Fig. 11 provides a simplified
applications. A relatively thin layer of copper or steel filter
graphic illustration of this process that may be helpful in better
screen positioned between the image plate and lead screen is
understanding the fundamentals of this unique process.
often sufficient to control unwanted secondary scattering from
6.7 Residual Latent Image Removal—Following a normal
lead screens.
latent image process scan (see Fig. 3), all phosphors on the
7. Basic Computed Radiography Techniques
imaging plate must be further exposed to a high intensity
source of white light in order to remove any remaining 7.1 Many exposure and technique arrangements for CR are
“residual” trapped electrons in the F centers. This process is often very similar to conventional film radiographic methods as
referred to as an IP “erasure” and is usually performed described in Guide E94, dependent upon the application. There
subsequent to the IP scan and prior to any subsequent re- are, however, numerous technical and physical issues that
exposures of the IP. If an erasure cycle is not performed, an differentiate CR exposure techniques from film that require
unwanted residual latent image may be superimposed on the careful consideration during development of specific CR tech-
next CR exposure if the IP is re-exposed soon after the first niques. Successful CR techniques are usually dependent upon
exposure. In the event no subsequent re-exposure of the IP is exposure technique (Step 1, Fig. 1) in conjunction with
performed, any residual latent image (trapped electrons) will
adequate image processing techniques (see Section 8) to
eventually fade as natural sources of red light energy (heat, achieve required image quality/dynamic range objectives.
etc.) cause remaining electrons to be liberated via the same Similar to film systems, CR techniques are dependent upon
physical process described above. Similarly, if erased IP’s are control of contrast, noise and resolution imaging properties.
stored near sources of radiation (background or other sources 7.1.1 Exposure Level and Image Quality—In general, CR
of ionizing radiation) an unwanted residual latent image image quality is directly proportional to the quantity of
(background) may develop within affected phosphors of the IP. meaningful radiation exposure received by the IP, just as it is
Fig. 12 illustrates a typical life cycle for the eventual genera- with film. Exposure level is most effectively determined in CR
tion of PSL with bluish X-ray luminescence during radiation via measuring the linear pixel value within the image area of
exposure, bluish after-glow luminescence subsequent to radia- interest, similar to measuring a film system’s optical density
tion exposure, a bluish luminescence (PSL) during exposure to with a densitometer device. With a digital “negative” image, a
a high intensity source of “red” light stimulus (scanning) darker pixel value means more radiation reached that pixel (on
followed by a bluish luminescence after-glow (see 3.2.25) the scanned IP) than a lighter pixel value. A good fundamental
subsequent to scanning. Since this process is primarily passive, place to begin adapting to CR techniques is with the CR
the actual phosphor is often referred to as a “storage phosphor.” exposure curve. A good practice is to create an exposure
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Illustration courtesy of Fujifilm NDT Systems
+2
FIG. 10 Energy Level Diagram Illustrating Mechanism for Generating PSL in BaFBr: Eu Crystal
Illustration courtesy of Fujifilm NDT Systems
FIG. 11 Illustration of PSL Generation
relationship (exposure dose/quanta versus pixel value) for each words, as pixel value increases, CR system signal-to-noise
major material (including thickness ranges inspected) and type (SNR) performance and Practice E746 equivalent penetrameter
of radiation used. Fig. 13-A illustrates a typical CR exposure sensitivity (EPS), as illustrated in Fig. 13-B usually improves
relationship for a specific material, specific thickness, type of as well. (Note, SNR usually does not increase linearly with
radiation source and exposure arrangement. Exposure is mea- increasing exposure dose and will eventually achieve a maxi-
sured in units of time at a specified intensity and SDD, that is, mum value beyond which additional exposure dose will not
180 seconds @ 10 milliamps; 90 seconds @ 60 curies generate further improved SNR performance). Each user
(minimum), etc. An alternate means of controlling exposure should qualify a specific pixel value range using exposure data
could be expressed as 1800 mA-s at a specified SDD, not to that demonstrates satisfactory levels of image quality perfor-
exceed 180 seconds, or 5400 Curie seconds at a specified SDD, mance for the inspection application. Although dependent upon
not to exceed 90 seconds. The concept is to achieve a specified the particular CR system used, most all CR systems will reach
exposure level within a specified time “window,” thus control- a point of exposure saturation at some point on the higher end
ling quanta and dose. CR exposure data can be linear (within a of the exposure range where image quality can become
specified linearity tolerance) or logarithmic (depending upon significantly diminished. A CR system is considered “satu-
LUT’s and equipment used) over a fairly wide range of rated” when a sufficiently large amount of phosphor crystals
exposure levels resulting in predictable contrast (PV 2-PV 1) are overexposed (or the PMT can no longer differentiate,
level for the same material thickness difference (illustrated in depending on the scanner settings) to the extent that no
Fig. 13-A). Additionally, as exposure level is increased with meaningful contrast is obtained between an inspected feature
CR, image quality performance will normally improve to a and its surrounding background. For example: the overall
point due to increase of contrast-to-noise ratio (CNR). In other image quality of a 12 bit high-resolution CR system (as
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uniformities within the phosphor materials of the IP detector
(that is, irregular size, non-uniformly spaced or simply an
insufficient mass of crystals); 2) the IP detector receives an
insufficient quanta of radiation photons to affect an adequate
signal-to-noise ratio (SNR); 3) primary radiation scattering
(absorption) within the test part material under examination; 4)
secondary radiation scattering from the exposure environment.
Computed radiography image plate detectors that employ
(PSL) materials are especially prone to higher noise levels
since these materials are generally more sensitive to ionizing
radiation than silver-based film, especially to lower energy
photons. Noise levels in computed radiographs can usually be
controlled or minimized by: 1) use of a phosphor detector with
fine, uniformly distributed and dense crystal materials; 2) use
of a radiation source and exposure arrangement for the specific
mass (of the examined material) that results in higher quanta of
radiation absorbed within the detector for a given exposure
interval; 3) careful attention to control of all sources of
secondary radiation exposure (adequate use of filters,
diaphragms, collimators and other scatter reducing materials).
Although all three of these sources are important, relatively
low absorbed radiation quanta in conjunction with a “noisy”
Illustration courtesy of Fujifilm NDT Systems
image plate or CR system detector is often the predominantly
FIG. 12 Typical PSL Life Cycle
objectionable source of image noise with computed radiogra-
phy (Fig. 15 illustrates). Radiation quanta (absorbed within the
image plate detector) are affected by: 1) material composition
determined by EPS or SNR) can become significantly dimin-
and thickness of the examined part; 2) penetrating energy level
ished as pixel values exceed approximately ⁄4 bit depth or
of radiation being used; 3) the intensity of radiation or activity
≈3000 pixel value at a particular scanner setting. Again, the
levels of the primary exposure source. Dosage of radiation
exact determination will depend upon the specific CR system
received by the detector is also an important consideration in
used to measure image quality values. In order for a user to
control of image noise provided that all other CR exposure
change contrast from that shown in Fig. 13-A, the slope of the
attributes are “balanced” to minimize noise or maximize
curve must be increased or decreased. This can be potentially
contrast-to-noise ratio (CNR).
accomplished with a change of IP/scanner system or via image
7.1.4 Image Plate Effıciency—The efficiency (noise and
processing (covered in Section 8).
resolution) of the IP detector will be determined, in large
7.1.2 Dynamic (Pixel Value) Range—CR has the unique
measure, by the meaningful PSL that is directly returned to the
property (when compared to single film systems) of displaying
CR optics for each spatially correct pixel area. As the phosphor
a wide range of visible gray scale levels for a defined range of
imaging layer becomes thicker, for example, there is greater
material thickness, especially when image processing is used;
likelihood that a “stray” PSL photon will be captured outside of
however, CR image quality is very dependent upon achieving
the spatially correct pixel area (see Fig. 15). When
...