ASTM E2736-17(2022)

Standard Guide for Digital Detector Array Radiography

SIGNIFICANCE AND USE
4.1 This standard provides a guide for the other DDA standards (see Practices E2597, E2698, and E2737). It is not intended for use with computed radiography apparatus. Figure 1 describes how this standard is interrelated with the aforementioned standards.
FIG. 1 Flow Diagram Representing the Connection Between the Four DDA Standards  
4.2 This guide is intended to assist the user to understand the definitions and corresponding performance parameters used in related standards as stated in 4.1 in order to make an informed decision on how a given DDA can be used in the target application.  
4.3 This guide is also intended to assist cognizant engineering officers, prime manufacturers, and the general service and manufacturing customer base that may rely on DDAs to provide advanced radiological results so that these parties may set their own acceptance criteria for use of these DDAs by suppliers and shops to verify that their parts and structures are of sound integrity to enter into service.  
4.4 The manufacturer characterization standard for DDA (see Practice E2597) serves as a starting point for the end user to select a DDA for the specific application at hand. DDA manufacturers and system integrators will provide DDA performance data using standardized geometry, X-ray beam spectra, and phantoms as prescribed in Practice E2597. The end user will look at these performance results and compare DDA metrics from various manufacturers and will decide on a DDA that can meet the specification required for inspection by the end user. See Sections 5 and 8 for a discussion on the characterization tests and guidelines for selection of DDAs for specific applications.  
4.5 Practice E2698 is designed to assist the end user to set up the DDA with minimum requirements for radiological examinations. This standard will also help the user to get the required SNR, to set up the required magnification, and provides guidance for viewing and storage of radiographs. Discussion is also...
SCOPE
1.1 This standard is a user guide, which is intended to serve as a tutorial for selection and use of various digital detector array systems nominally composed of the detector array and an imaging system to perform digital radiography. This guide also serves as an in-detail reference for the following standards: Practices E2597, E2698, and E2737.  
1.2 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.  
1.3 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: 30-Nov-2022
• ICS: 11.040.50 - Radiographic equipment
• 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 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 E2698-18 - Standard Practice for Radiographic Examination Using Digital Detector Arrays
• Effective Date: 01-Feb-2018
• Refers: ASTM E1316-18 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jan-2018
• 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 E1000-16 - Standard Guide for Radioscopy
• Effective Date: 01-Dec-2016
• 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 E1316-14 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jun-2014
• Refers: ASTM E1316-14e1 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jun-2014
• Refers: ASTM E1316-13d - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Dec-2013

Overview

ASTM E2736-17(2022): Standard Guide for Digital Detector Array Radiography serves as a comprehensive tutorial and reference for the deployment and selection of digital detector array (DDA) systems in digital radiography. Published by ASTM International, this standard is structured to guide users through the advantages, technical considerations, and performance parameters associated with DDAs, referencing supporting standards such as Practices E2597, E2698, and E2737.

ASTM E2736-17(2022) is tailored for industry professionals-including engineering officers, manufacturers, and service providers-who utilize DDA radiography for non-destructive testing (NDT), quality assurance, and integrity verification of materials and structures. It specifically focuses on digital detector arrays, excluding computed radiography systems.

Key Topics

• Definitions and Performance Parameters: The guide clarifies terminology and key metrics associated with DDAs, assisting users in understanding parameters such as signal-to-noise ratio (SNR), achievable contrast sensitivity, basic spatial resolution, dynamic range, and material thickness range.
• Selection and Comparison: End users are provided with guidance to interpret manufacturer-supplied DDA performance data, evaluate options using standardized metrics, and select systems appropriate to their application requirements.
• System and Technology Overview: The standard reviews the architecture of DDA systems, detailing their main components-X-ray capture media (such as scintillators and photoconductors), pixelized readout structures (e.g., CCDs, CMOS, amorphous silicon panels), and digitization methods.
• Integration with Related Standards: ASTM E2736-17(2022) shows how it interrelates with Practices E2597 (manufacturing characterization), E2698 (radiographic examination procedures), and E2737 (performance evaluation and stability).
• Image Quality Insights: Detailed information is provided on image quality determinants, including resolution, SNR, contrast, and detector-specific effects such as lag, burn-in, and bad pixel response.
• Scope of Applicability: The guide points out that DDAs are effective over a wide range of X-ray and gamma ray energies, suitable for various inspection scenarios.

Applications

The practical value of ASTM E2736-17(2022) lies in its broad support for non-destructive testing applications across multiple industries:

• Aerospace and Automotive: Inline testing of critical castings, welds, and components for defects using high-resolution DDA imaging.
• Manufacturing Process Control: Diagnostic and quality assurance imaging in production lines, facilitating real-time defect recognition and process feedback.
• Infrastructure and Construction: Inspection of steel, aluminum, and titanium structures for internal flaws or damage without physical alteration.
• Pipeline and Energy Sectors: Portable and fixed DDA solutions for routine and specialized inspection of pipes, pressure vessels, and complex assemblies.
• Integration into Automation: Use in robotic scanning, computed tomography systems, and advanced image processing for automatic defect detection.
• Performance Verification: Establishment and verification of acceptance criteria for suppliers and in-service components by leveraging DDA radiographic techniques.

Related Standards

When implementing DDA radiography under ASTM E2736-17(2022), it is recommended to also consult:

• ASTM E2597: Practice for Manufacturing Characterization of Digital Detector Arrays
• ASTM E2698: Practice for Radiographic Examination Using Digital Detector Arrays
• ASTM E2737: Practice for Digital Detector Array Performance Evaluation and Long-Term Stability
• ASTM E94, E155, E2002, E1815, E2422, E2445, E2446, E2660, E2669: Additional guidelines related to radiographic examination, reference images, and detector testing
• ISO 17636-2: International requirements for radiographic testing of welds using digital techniques

These standards, when used alongside ASTM E2736-17(2022), enhance the reliability, repeatability, and comparability of digital radiography and ensure alignment with global best practices in non-destructive testing.

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: E2736 − 17 (Reapproved 2022)
Standard Guide for
Digital Detector Array Radiography
This standard is issued under the fixed designation E2736; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope amination by Using Representative Quality Indicators
(RQIs)
1.1 This standard is a user guide, which is intended to serve
E2002Practice for Determining Image Unsharpness and
as a tutorial for selection and use of various digital detector
Basic Spatial Resolution in Radiography and Radioscopy
arraysystemsnominallycomposedofthedetectorarrayandan
E2422Digital Reference Images for Inspection of Alumi-
imagingsystemtoperformdigitalradiography.Thisguidealso
num Castings
serves as an in-detail reference for the following standards:
E2445Practice for Performance Evaluation and Long-Term
Practices E2597, E2698, and E2737.
Stability of Computed Radiography Systems
1.2 This standard does not purport to address all of the
E2446Practice for Manufacturing Characterization of Com-
safety concerns, if any, associated with its use. It is the
puted Radiography Systems
responsibility of the user of this standard to establish appro-
E2597Practice for Manufacturing Characterization of Digi-
priate safety, health, and environmental practices and deter-
tal Detector Arrays
mine the applicability of regulatory limitations prior to use.
E2660Digital Reference Images for Investment Steel Cast-
1.3 This international standard was developed in accor-
ings for Aerospace Applications
dance with internationally recognized principles on standard-
E2669Digital Reference Images for Titanium Castings
ization established in the Decision on Principles for the
E2698Practice for Radiographic Examination Using Digital
Development of International Standards, Guides and Recom-
Detector Arrays
mendations issued by the World Trade Organization Technical
E2737Practice for Digital Detector Array Performance
Barriers to Trade (TBT) Committee.
Evaluation and Long-Term Stability
2.2 ISO Document:
2. Referenced Documents
ISO 17636-2 Non-Destructive Testing of Welds—
2.1 ASTM Standards:
Radiographic Testing - Part 2: X- and Gamma-Ray Tech-
E94Guide for Radiographic Examination Using Industrial
niques with Digital Detector
Radiographic Film
E155Reference Radiographs for Inspection of Aluminum 3. Terminology
and Magnesium Castings
3.1 Definitions of Terms Specific to This Standard:
E192Reference Radiographs of Investment Steel Castings
3.1.1 achievable contrast sensitivity (CSa)—best contrast
for Aerospace Applications
sensitivity (see Terminology E1316 for a definition of contrast
E1000Guide for Radioscopy
sensitivity)obtainableusingastandardphantomwithanX-ray
E1316Terminology for Nondestructive Examinations
technique that has little contribution from scatter.
E1320Reference Radiographs for Titanium Castings
3.1.2 bad pixel—a bad pixel is a pixel identified with a
E1815Test Method for Classification of Film Systems for
performance outside of the specification for a pixel of a DDA
Industrial Radiography
as defined in Practice E2597.
E1817Practice for Controlling Quality of Radiological Ex-
3.1.3 burn-in—change in gain of the scintillator or photo-
conductor that persists well beyond the exposure.
1 3.1.4 effıciency—SNR (see 3.1.6 of Practice E2597) di-
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc- n
tive Testing and is the direct responsibility of Subcommittee E07.01 on Radiology vided by the square root of the dose (in mGy) and is used to
(X and Gamma) Method.
measuretheresponseofthedetectoratdifferentbeamenergies
Current edition approved Dec. 1, 2022. Published December 2022. Originally
and qualities.
approved in 2010. Last previous edition approved in 2017 as E2736–17. DOI:
10.1520/E2736-17R22.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Available from International Organization for Standardization (ISO), ISO
Standards volume information, refer to the standard’s Document Summary page on Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier,
the ASTM website. Geneva, Switzerland, http://www.iso.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2736 − 17 (2022)
3.1.5 grooved wedge—awedgewithonegroove,thatis5% 3.1.10 specific material thickness range (SMTR)—material
of the base material thickness and that is used for achievable thickness range within which a given image quality is
contrast sensitivity measurement in Practice E2597.
achieved.
3.1.6 internal scatter radiation (ISR)—scattered radiation
4. Significance and Use
within the detector (from scintillator, photodiodes, electronics,
shielding, or other detector hardware).
4.1 This standard provides a guide for the other DDA
3.1.7 lag—residual signal in the DDA that occurs shortly
standards (see Practices E2597, E2698, and E2737). It is not
after the exposure is completed.
intended for use with computed radiography apparatus. Figure
3.1.8 phantom—a part or item being used to quantify DDA 1 describes how this standard is interrelated with the afore-
mentioned standards.
characterization metrics.
3.1.9 SNR —SNR, normalized by the basic spatial resolu-
N
4.2 This guide is intended to assist the user to understand
tion SR as measured directly in the digital image and/or
b
thedefinitionsandcorrespondingperformanceparametersused
calculated from measured SNR by:
measured
in related standards as stated in 4.1 in order to make an
88.6 µm informed decision on how a given DDA can be used in the
SNR 5SNR 3 (1)
S D
N measured
SR target application.
b
FIG. 1 Flow Diagram Representing the Connection Between the Four DDA Standards
E2736 − 17 (2022)
4.3 This guide is also intended to assist cognizant engineer- 5.1.3 Area—DDAs are area imaging devices and as such
ing officers, prime manufacturers, and the general service and can capture much more information in a single exposure than
manufacturing customer base that may rely on DDAs to a Linear Detector Array (LDA), or a linear-DDA. For the
provide advanced radiological results so that these parties may balance of this standard, the term LDA shall be considered a
set their own acceptance criteria for use of these DDAs by linear detector array, while a DDAshall be considered an area
suppliers and shops to verify that their parts and structures are device.Whencapturinganareaofinterest,aDDAmaycapture
of sound integrity to enter into service. a full area in a single exposure, while an LDAcaptures only a
single line in an exposure. To capture an area of interest, the
4.4 The manufacturer characterization standard for DDA
LDAneeds a series of adjacent line scans and thereby a series
(see Practice E2597) serves as a starting point for the end user
of exposures.
to select a DDA for the specific application at hand. DDA
5.1.4 Despite the advantages in 5.1.3, there is a tradeoff in
manufacturers and system integrators will provide DDA per-
that the DDA devices are prone to higher levels of Compton
formance data using standardized geometry, X-ray beam
scatter. For example, the use of a fan beam over the use of a
spectra, and phantoms as prescribed in Practice E2597. The
cone beam will inherently produce less Compton scatter from
end user will look at these performance results and compare
the object, or from the side walls of an X-ray cabinet.
DDAmetrics from various manufacturers and will decide on a
Secondly, the LDA can more effectively collimate this scatter
DDAthat can meet the specification required for inspection by
using a narrow slit.That said, the DDAcan be configured with
the end user. See Sections 5 and 8 for a discussion on the
a set of adjustable jaws both about the detector and/or the
characterization tests and guidelines for selection of DDAs for
sourcetoreducethescatterfield,butthebenefitofitsprojected
specific applications.
coverage is diminished.
4.5 Practice E2698 is designed to assist the end user to set
5.1.5 When considering the SNR as a tradeoff, the LDAhas
up the DDA with minimum requirements for radiological
an advantage compared to DDA’s. LDA’s have significantly
examinations. This standard will also help the user to get the
thicker scintillators, and thereby require a lower radiation dose
required SNR, to set up the required magnification, and
toproduceasuccessfulimageperlinecomparedtoaDDA.For
provides guidance for viewing and storage of radiographs.
a given overall exposure time, because of the thicker scintil-
Discussion is also added to help the user with marking and
lator in an LDA, there could be situations where it produces a
identification of parts during radiological examinations.
higher SNR compared to the thinner scintillator in a DDA.
Also, for a given SNR, there could be situations where more
4.6 Practice E2737 is designed to help the end user with a
overall exposure time is needed for a DDA.
set of tests so that the stability of the performance of the DDA
5.1.6 DDAsalsoenableafacilecorrectionfordifferencesin
can be confirmed. Additional guidance is provided in this
pixel value response (gain) across the DDA, X-ray beam
document to support this standard.
shading, offset levels of the device, and bad pixels. These
4.7 Figure 1 provides a summary of the interconnectivity of
correctionsresultinahighlyuniformandlinearresponsetothe
these four DDA standards.
X-ray beam and provides the capability to provide very high
4.8 DDAs may be used with significant success under a
and uniform signal to noise response with respect to the X-ray
wide energy range, i.e. from 10 kV to 20 MeV if configured beam incident on the DDA. These corrections are not neces-
appropriately. However in this document some phantoms such
sarily simple to perform with film or computed radiography
as the duplex wire gauge (Practice E2002) may not give an systems where the SNR might be limited by a structure noise
accurate representation of the basic spatial resolution at ener-
inherent in the imaging medium, or the shading of the X-ray
gies above 600 kV. beam.
5.1.7 Unlike film or computed radiography (CR) systems,
5. DDA Technology Description
theabilitytoflexthesensor,forexamplearoundapipehasnot
yet been realized, and is certainly one of the advantages of
5.1 General Discussion:
these other imaging media over DDA devices.
5.1.1 DDAs are seeing increased use in industries to en-
5.1.8 Another difference between film/CR is that these
hanceproductivityandqualityofnondestructivetesting.DDAs
devices/media will enable very long exposure times. For
are being used for in-service nondestructive testing, as a
example unlike a DDA there is not any restriction on frame
diagnostic tool in the manufacturing process, and for inline
rate. That said, a DDAcan overcome this shortfall by averag-
testing on production lines. DDAs are also being used as hand
ing frames to achieve the desired image quality. All X-ray
held, or scanned devices for pipeline inspections, in industrial
imaging devices will suffer from poor SNR if the dynamic
computed tomography systems, and as part of large robotic
nature of the inspection is too fast to capture enough photons
scanning systems for imaging of large or complex structures.
as an object transits the beam.
Because of the digital nature of the data, a variety of new
applications and techniques have emerged recently, enabling 5.2 DDA architecture:
quantitative inspection and automatic defect recognition.
5.2.1 A common aspect of the different forms of this
5.1.2 DDAs can be used to detect various forms of electro- technology is the use of discrete sensors (position-sensitive)
magneticradiation,orparticles,includinggammarays,X-rays, where, the data from each discrete location is read out into a
neutrons,orotherformsofpenetratingradiation.Thisstandard file structure to form pixels of a digital image file. In all its
focuses on X-rays and gamma rays. simplicity, the device has an X-ray capture material as its
E2736 − 17 (2022)
primary means for detecting X-rays, which is then coupled to for each of these steps, and the reader is referred to (1, 2, 3)
a solid-state pixelized structure, where such a structure is for further discussion on this topic.
similar to the imaging chips used in visible-wavelength digital
5.3 Digitization Methods:
photography and videography devices. Figure 2 shows a block
5.3.1 Digitization techniques typically convert the analog
diagram of a typical digital X-ray imaging system.
signal to discrete pixel values. For DDAs the digitization is
5.2.2 An important difference between X-ray imaging and
typically, 8-bit (256 pixel values), 12-bits (4096 pixel values),
visible-light imaging is the size of the read-out device. The
14-bits (16,384 pixel values) or 16-bits (65,536 pixel values),
imagers found in cameras and for visible-light are typically on
and 20-bits (1,048,576). The higher the bit depth, the more
the order of 1 to 2 cm in area. Since X-rays are not easily
finely the signal intensity is sampled.
focused, as is the case for visible light, the imaging medium
5.3.2 The digitization does not necessarily define the pixel
must be the size of the object. Hence, the challenge lies in
value range of the DDA. The useful range of performance is
meeting the requirement of a large uniform imaging area
defined by the ability of the read device to capture signal in a
without loss of spatial information. This in turn requires high
linear relation to the signal generated by the primary conver-
pixel densities of the read-out device over the object under
sion device.Awide linear range warrants the use of a high bit
examination, as well a primary sensing medium that also
depthdigitizer.Itshouldbenotedthatifdigitizationisnothigh
retains the radiologic pattern in its structure. Therefore, each enough to cover the information content from the read device,
DDA consists of a primary X-ray or gamma ray capture digitization noise might result. This can be manifested as a
medium followed by a pixelized read structure, with various posterization effect, where discrete bands of pixel values are
meansoftransferringtheabovesaidcapturedpattern.Foreach observed in the image.
of these elements, there are numerous options that can be 5.3.3 Conversely, if digitization range is selected that is
selected in the creation of DDAs. For the primary X-ray significantly wider than the range of the read-out device then
conversion material, there are either luminescent materials the added sampling may not necessarily improve performance.
such as scintillators or phosphors, and photoconductive mate- Secondly, if the digitization limit is well beyond the linear
range of the read structure, these added pixel values would not
rials also known as direct converter semiconductors.
be useable. For example, 16-bits of digitization do not neces-
5.2.3 For read-out structures, the technology consists of
sarily indicate 65,536 values of linear responsivity.
charge coupled detectors (CCDs), complementary metal oxide
5.3.4 The useful range of a detector is frequently defined as
silicon (CMOS) based detectors, and amorphous silicon thin
the maximum usable level, without saturation in relation to the
film transistor diode read-out structures. Other materials and
noise floor of the DDA, where again no useful differentiation
structures are also possible, but in the end, a pixelized pattern
canbeextractedfromthedata.Thisissometimesreferredtoas
is captured and transferred to a computer for review.
the detector dynamic range.
5.2.4 Eachprimaryconversionmaterialcanbecoupledwith
5.3.5 The dynamic range is different from the specific
the various read structures mentioned through a wide range of
material thickness range (SMTR) as defined in this standard
coupling media, devices, or circuitry. With these possible
and Practice E2597. That range is a true practical range of the
combinations, there are many different types of DDAs that
DDA at hand, a range significantly tighter than the DDA
have been produced. But all result in a digital X-ray or gamma
dynamic range.
ray image that can be used for different NDT applications.
5.2.5 Following the capture of the X-rays and conversion
into an analog signal on the read-out device, this signal is 4
The boldface numbers in parentheses refer to a list of references at the end of
typically amplified and digitized.There are numerous schemes this standard.
FIG. 2 Block Diagram of a Typical Digital X-Ray Imaging System
E2736 − 17 (2022)
5.3.6 TheSMTRisoneofthepropertiestoconsiderinDDA propertiesallowforhighefficiency,stableandrobustoperation
selection, as it impacts the thickness range that can be yielding ideal imaging performance:
interpreted in a single view. This is dependent on the charac-
(1)High stopping power for X-rays obtained by high
teristicsofthereaddevice,thedigitizationlevel,andtheX-ray
atomicnumber,addedthickness,and,ortheuseofhighdensity
techniqueemployed.Thistestprovidesameansofdetermining
materials without loss of spatial information due to scattering
an effective range without understanding the subtle nuances of
processes within the scintillator.
the detector readout, and avoids erroneous parallels between
(2)High X-ray to light conversion efficiency
bit depth and its relation to thickness range, and maximum
(3)Matched emission spectrum of the scintillator to the
possible signal from a device.
spectral sensitivity of the light collection device
5.4 Specific DDA components—Therearenumerousoptions (4)Low afterglow during and after termination of the
ineachcomponentoftheimagingchaintoproduceaDDA.To
X-ray illumination.
understand the options and limitations of each category, and to
(5)Stable output during long or intense exposure to radia-
bestassesswhichtechnologytopursueforagivenapplication,
tion. No gain changes for example based on dose levels.
theunderlyingtechnologywillbediscussedbeginningwiththe
(6)Temperature independence of light output.
image capture medium. This is followed by the image read
(7)Stable mechanical and chemical properties.
structureandthentheimagetransferdeviceisdiscussedforthe
5.4.1.1 The scintillator based on CsI:Tl (thallium doped
various configurations of the read-out devices. For a more
cesiumiodide)hasshownconsiderablesuccessasascintillator
detailed description of the architectures of these devices, the
because of the following reasons:
reader is referred to Ref. (2).
(1)Cesium iodide can be formed into needles (see Fig. 3)
5.4.1 X-ray Capture—Scintillators (phosphors)—
and coupled directly to a diode read structure or a fiber optic
Scintillators are materials that convert X-ray or gamma ray
component to direct the light to the photodiodes without
photons into visible-light photons, which are then converted to
significantlightlossoropticalscatter.Thisisthemostefficient
a digital signal using technologies such as amorphous silicon
means for depositing light into photodiodes. All other scintil-
(a-Si) arrays, CCDs or CMOS devices together with an
lators lose light at the interface because of reduced optical
analog-to-digital converter. This will facilitate real time acqui-
couplingbetweenthescintillatorandthediodestructure.CsIis
sition of images without the need for offline processing. Since
very well matched in index of refraction to that of the entry
therearevariousstagesofconversioninvolvedinrecordingthe
layer of the amorphous silicon diode structure.The needle-like
digital image, it is very important to ensure that minimal
structure enables thick phosphor layers, which improves X-ray
information is lost during conversion in the scintillator. The
propertiesdesirableofidealscintillatorsarelistedbelow.These absorption without significant loss in spatial resolution.
FIG. 3 Architecture of CsI:Tl needle structure demonstrating light guiding nature following X-ray conversion to light, and the amor-
phous silicon architecture illustrating direct contact of the scintillator with the diode thin film transistor readout matrix.
E2736 − 17 (2022)
(2)The cesium iodide has a high effective atomic number scintillator material absorb this radiation and get excited.They
(Z)whichalsocontributestogoodX-rayabsorptionefficiency. de-excite by emitting the energy in the form of visible light.
The emitted energy is ‘luminescence,’ which falls broadly
The drawbacks of CsI are:
under two categories namely, fluorescence and phosphores-
(1)CsI:Tl has been prone to severe hysteresis effects, an
cence. These manifest as a two-component exponential
effect that leads to an unstable signal under constant flow of
X-rays, and this instability is non-linear with the dose rate decay—fast (prompt) for fluorescence and slow (delayed) for
phosphorescence. An ideal scintillator should essentially have
used. This can cause residual images (ghost images) to be
retained in the detector from prior scans referred to as burn-in. only a fast decay component with a linear conversion, that is,
In some circumstances, recent preparations have significantly light yield should be proportional to the deposited energy.Any
overcome this effect. phosphorescence might introduce residual latent artifacts into
(2)CsI is hygroscopic and sensitive to moisture, and must subsequent imagery and make interpretation difficult. Scintil-
be encapsulated to avoid loss in crystallinity.
lator phosphorescence can lead to image lag as defined herein,
(3)CsI:Tlhasaprimarydecaytimeof1microsecondat1/e
where features from prior images contaminate new scenes.
(to~37%ofpeaksignal),buthasalongdecaycomponentinto
5.4.2 Semiconductors (Photoconductors)—A photoconduc-
themillisecondrangethatisnon-zerobutwellbelow1%.This
tive material converts X-rays to electron-hole pairs that then
needs to be taken into consideration where a very opaque
getseparatedbytheinternalbiasofthedeviceasdefinedbythe
objectfollowseitheranopenairexposureoraverylowopacity
material properties, such as the manufactured charge imbal-
exposure, as the afterglow from the bright exposure may
ance into the semiconductor material. As with scintillating
encroach on the signal level of the dim exposure.
materials, another electronic element is needed to capture the
(4)In some cases if the CsI:Tl is not manufactured
signal produced, such as an electrode structure with
properly, the light out response can fade with deposited X-ray
pixelization, possibly with additional added electron bias on
dose. Upon introduction of a new detector, the light output
one electrode to separate the electron-hole pairs. But unlike a
should be monitored under a consistent dose rate and X-ray
scintillating material, there is a lower likelihood that the
technique over a period of several thousand Gray to determine
charges produced will have as much lateral spread as experi-
if the scintillator behaves with a stable response. If no changes
enced optically in luminescent materials. Also since the pho-
or only subtle changes are noted, then it is likely that the CsI
toconductive material converts the X-ray signal directly into
is manufactured in an appropriate manner. The light level
electron-hole pairs, there is greater conversion efficiency than
shouldmaintaina 610%variationunderaconsistentX-rayset
with the production of light, that first generates electron-hole
of conditions.
pairs prior to producing the light. For X-ray applications,
5.4.1.2 Other scintillators (phosphors) such as polycrystal-
photoconductivematerialssuchasamorphousselenium(a-Se),
line Gd O S:Tb have been successfully used, but have limita-
2 2
CdTe, and HgI2 have been used because of their high atomic
tions on how thick they can be made given that the powder
numbers, and the ability to manufacture these materials into a
architecture scatters the light produced from the deposited
monolithic structure. Other photoconductive materials are
X-rays. Nevertheless, these are simple phosphors to purchase
available, or may become available in the future. It should be
and implement, and like the CsI needles, can be optically
noted that although light is not generated from these materials,
coupled through a lens, or directly coupled to a read structure.
lag and burn-in effects can occur due to subtle effects of
For the latter, and as with CsI:Tl, this can be achieved via a
sweeping the charge out of the semiconductor. Direct convert-
fiber optic lens, an optical lens, or by direct coupling to the
ing devices are normally available in small modules that are
read-structure itself.
manufactured to one bigger detector for a suitable large DDA.
5.4.1.3 Certain scintillators such as Gd O S:Tb can be
2 2
Todaythereisstilltheproblemofthe "blind"areasbetweenthe
sinteredtoceramicimagingplateswithdiscretecellboundaries
modules.
yielding the same advantage of the CsI needle structures, but
typically without the temporal drawbacks of the CsI:Tl chem- 5.5 Capture of the converted image:
istry. However, they are difficult to grow directly onto diode
5.5.1 Charge-coupled devices (CCDs) are light imaging
structures, typically require an optical couplant to improve
devices that are typically small in size, and have high pixel
transfer efficiency due to index mismatch, and typically are
densities.They use a transparent poly-silicon gate structure for
more expensive to produce and couple to large diode struc-
reading out the device, and because of their high pixel fill
tures.
factor are very efficient in collecting the light produced from
5.4.1.4 Certain glass scintillators based on terbium activa-
the phosphor material. Unlike amorphous silicon pixel
tion can be formed into fiber optic scintillating plates yielding
structures, current limitations in crystal growth methods have
the same advantage of the CsI needle structures. These plates
restricted the fabrication of these devices into larger arrays.A
tend to also have some temporal drawbacks, and are not as
larger field of view can be accomplished with CCDs through a
efficient in converting X-rays to light as any of the other
lens or a fiber optic transfer device to view a phosphor or
scintillators already mentioned.
scintillator screen.The downside of the lens approach is that it
5.4.1.5 Other materials are under development, and the
has very poor light collection efficiency, while fiber optic
above sections are not intended to cover all possible options.
image plates have significantly improved light collection
efficiency, but are expensive and are not amenable to large
5.4.1.6 Temporal Properties of scintillators—When radia-
tion impinges upon a scintillator, the atoms/molecules in the fields of view. For small field of view applications, the directly
E2736 − 17 (2022)
coupled charge coupled device approach will provide high 6. DDA Properties
spatial resolution and high light collection efficiency.
6.1 An important prerequisite for a good digital X-ray
5.5.2 CMOS read structures are based on Complementary
detector system is the capability of the system to control the
Metal-Oxide Semiconductors, which is a dominant semicon-
interplay of all its components (the entire imaging chain) and
ductor circuit for microprocessors, memories and application
reflect the capability of the system in the final image. The
specific integrated circuits (ASICs). CMOS technology, lever-
technology of image capture, the representation of images as
aging the multi-billion dollar semiconductor industry enables
digital data, their processing, enhancing of data for a specific
low cost production of pixelized devices. Like CCDs, they are imagedisplay,andthenatureofthedisplaytechnology,forma
formed with crystalline silicon, but the read structure is significant part of this capability. From an image interpretation
standpoint, the quality of images from the detector is an
individually addressed. Unlike CCDs, where charge is trans-
important metric for the choice of the detector and system
ferred across active pixel regions, CMOS technology has
specifications. This section introduces the image quality
individually addressed pixels. CMOS image sensors draw less
parameters/metrics that form the basis for selection, and
power than CCDs. However, they are known to produce more
monitoring performance as delineated in Practices E2597,
electronic noise than CCDs. Like CCDs, they can couple to
E2698, and E2737.
various scintillators either directly, or by lens or fiber optics.
5.5.3 Amorphous silicon read structures—Larger amor- 6.2 The dominant contributions to a digital radiographic
image, and hence the final image quality, come from two
phous silicon based thin film transistor pixelized read struc-
sources: (a) the inherent property of a detector and (b) the
tures have been made commercially available as large flat
radiographic technique itself. Some of the inherent properties
panel devices. Figure 3 provides a schematic of an amorphous
ofthedetectorwhichinfluencetheimagequalityare,(1)signal
silicon DDA architecture. Amorphous silicon, through large
and noise performance for a given dose, (2) basic spatial
area silicon deposition and processing/etching techniques of-
resolution, (3) normalized signal-to-noise ratio—SNR-
fers a solution to the size constraints of CCDs and CMOS
normalized for spatial resolution, (4) detection efficiency, (5)
devices. Since the phosphor or photoconductor layer is typi-
detector lag (residual images, ghosting), (6) internal scatter
cally deposited or coupled directly onto the silicon, efficient
radiation and (7) bad pixels. The other metrics such as (8)
optical or electron transfer is easily obtained. However, the
achievable contrast sensitivity, and (9) specific material thick-
readoutcircuitryinthesedevicesrequiresalargepixelspaceto
ness range are dependent on both, the DDAused as well as the
accommodate the thin film transistor (TFT) and data lines and
object under test.Another strong factor is the radiation quality
scan (gate) lines required for operation, thus limiting how
of the X-ray beam used for imaging.
small a pixel this device can permit. The amorphous silicon
6.2.1 A standardized methodology has been established for
readstructureiscomposedofoveramillionpixelsthatinclude
evaluatingtheinherentdetectorpropertiesofDDAsaslistedin
photodiodes. The diode has a sensitivity that peaks in the
6.2andmaybefoundinPracticeE2597.Thispracticeprovides
middle of the visible spectrum where a number of good
procedures for evaluating and recording DDA properties by
phosphors emit. The electric charges generated within every
manufacturers or providers so that a potential purchaser may
pixel of the photodiode are read by the active matrix of TFTs
comparedevicesunderstandardizedconditionsandtechniques
in place. The TFT matrix, which is essentially a matrix of
in order to make an informed decision on the purchase. The
switches,isscannedprogressively.Attheendofeachdata-line
ASTMstandardsuggeststhatprovidersofDDAsofferaspider
is a charge- integrating amplifier, which converts the charge
diagramthatsummarizestheperformanceofadetectorusinga
packet to a voltage, followed by a programmable gain stage
numerical grading scheme listed in the standard that highlights
andanAnalog-to-DigitalConverter(ADC),whichconvertsthe
the strengths or weaknesses of a DDA. The purchaser can
voltage to a digital number that is transferred to a computer,
easily review those diagrams and decide what is most impor-
where the data is formed into an N×M (N=number of
tant for the application at hand.
columns and M=number of rows) pixel image.
6.2.2 Subections 6.3 to 6.19 provide additional details into
5.5.4 Choice of Read Structure—For small field of view these important detector properties, and how these impact
applications, the directly coupled CCD or CMOS approach overall performance of an inspection. Section 9 provides
additional guidance into the selection of a DDA based on a
will provide high spatial resolution and high light collection
review of the performance metrics taken together.
efficiency.As mentioned, these devices have pixel pitch as fine
as 10 microns. For large field of view applications, the
6.3 Image Quality from a DDA—The SNR of the DDA,
amorphous silicon approach offers excellent collection effi-
using a specific radiation quality, and the relative contrast
ciency (no lenses), in a thin, compact, robust package.
sensed by the radiation beam in the object together constitute
However, pixel pitch is typically on the order of 100 microns
an element of the image quality that relates to the contrast
or larger, although smaller pixel pitch structures are likely to
sensitivity of the DDA. The higher the SNR, the better, or
appear soon. Note, phosphors or scintillators are typically
lowertheachievablecontrastsensitivity.Ahighsignaltonoise
coupled through fiber optic faceplates on CCD and CMOS
ratio improves contrast sensitivity as noise levels are sup-
chips.Thishasthedualbenefitofreducingradiationdamageto
pressed in relation to signal differences. The SNR of a DDA
the chip, and the flexibility of easy interchange of phosphors
system can be increased significantly by capturing multiple
for the application at hand. images with identical settings and integrating in a computer
E2736 − 17 (2022)
(frameaveraging).Theabilityoftheimagingchaintomaintain of quanta interacting with the scintillator represents the pri-
the spatial information that originally impinged onto the mary quantum sink of the detector. If we assume N represents
primary detection medium is another critical element of the a measure of the signal, then the variance σ is linearly
resultingimagequality.Thisistypicallyreferredtoasthebasic proportional to N . Hence, the signal-to-noise ratio (SNR) is
spatial resolution, SR .
defined as √N . SNR therefore increases as the square root of
b
the number of quanta interacting with the detector. Regardless
6.4 Signal and Noise—The signal recorded by a DDAis the
of the value of the X-ray quantum efficiency, the maximum
responseoftheDDAtoagivenradiationdose.Thenoiseisthe
signal-to-noise ratio of the system will occur at this point
variationofthesignalreadusingtheDDAforthesameamount
(SNR= √N ).Ifthesignal-to-noiseratiooftheimagingsystem
ofdose.SignalandnoisecharacteristicsoftheDDAdependon
is essentially determined there, the system is said to be X-ray
the radiation quality and the DDAstructure. Radiation quality
quantum limited in performance. For example, performance
which is defined as the beam spectrum used, is directly related
will only improve if more X-rays are captured. The phosphor
to the efficiency of the DDA that is related to the quantum
layer typically creates a large gain factor at this point.
efficiencyoftheconversionlayer,i.e.,scintillatororphotocon-
Following this, any subsequent inefficiency in emitting the
ductor layer. The higher the quantum efficiency of the conver-
lightandcapturingitbythephotodiodewillresultinlossesand
sionlayer,thehighertheSNRwillbe.TheDDAstructurehere
additionalsourcesofnoise.Ifthenumberofquantafallsbelow
refers to the type of conversion layer used, type of signal
the primary quantum sink, then a secondary quantum sink will
conversion chain employed, and the associated electronics
be formed and becomes an additional important noise source.
design. In an optimized DDA system where the DDA follows
Poissonstatistics,thenoiseisproportionaltothesquarerootof
6.6 For most detection systems discussed here, where the
the signal level captured and thus the higher the efficiency of
phosphor is in direct contact with the diode as in the flat panel
capturingandconvertingtheradiationtoavisible,orelectronic
detectors,thelimitingsourceofnoiseisthequantumefficiency
signalattheDDA,thehighertheperformanceoftheDDA.For
of the X-ray conversion material. Additional discussions on
example with higher signal levels, the SNR is increased, and
SNR of digital detectors are found elsewhere (3).
lower contrast, subtle features may be discerned in an image.
6.7 For direct conversion systems, the photoconductor is in
6.5 The transmitted X-ray beam signal propagates through
direct contact with the read device, and with efficient charge
various energy conversion stages of an imaging system, as
transfer through the photoconductor into the read device, the
discussedin5.2.InFig.4,N quantaareincidentonaspecified
limiting source of noise is the quantum efficiency of the X-ray
areaofthedetectorsurface(stage0).Afractionofthese,given
conversion material.
by the absorption efficiency (quantum efficiency) of the
material, interact (stage 1). Here it is important that the 6.8 Sincenoiseisrelatedtothesquarerootofthenumberof
absorption efficiency is high, or a larger X-ray dose would be X-ray quanta absorbed, it is crucial for efficient detection
neededtoarriveatadesiredsignallevel.ThemeannumberN systems to have a sufficient signal level to avoid quantum
FIG. 4 Quantum Statistics of X-Ray Imager.
E2736 − 17 (2022)
mottling. Quantum mottling here refers to the variation in the selectingadetectionsystem.Fromtheaspectofimagecontrast
signal level due to quantum noise. Quantum mottling makes sensitivity and spatial resolution, it is desirable to have the
detectionofsmallercontrastfeaturesmoredifficult.Inmedical largest pixel that will allow detection of the features of interest
imaging, regulations allow a certain maximum dose to the in the radiographic examination. For example, it is not neces-
patient and optimal signal levels may not be obtainable. In this sary to select a 10-µm pixel pitch if the application is for the
scenario, it is critical to absorb as many X-ray photons as detection of large foreign objects in an engine nacelle.
possible, and then to transfer that energy efficiently, and not Similarly, aircraft fatigue crack probability of detection will be
introduce secondary quantum sinks. On the other hand, in low with a pixel pitch of 200 µm or larger, unless low
nondestructive testing, it may be possible to increase signal unsharpness magnification techniques are used. See Fig. 5 for
levels by selecting any or all of the following: (a) a longer a discussion on selection of a DDA based on the size of the
exposure time, (b) a combination of frames, either by integra- anticipated smallest defect, subject contrast, SNR, and the
tionoraveraging,(c)ahigherbeamflux,(d)ahigherradiation DDA pixel size. Fig. 5 also includes an indication Detection
beam energy (assuming absorption is still high at those Probability guide.
energies), (e) a closer working distance between source and
6.12 Pixel Pitch—The predominant factor that governs the
detector, or (f) a different DDAwith a more absorbing primary
spatial resolution of a detector is the pixel pitch. Pixel pitch
detection medium (phosphor or photoconductor). These tech-
represents the physical dimension of the pixels. Most DDAs
niquesmayprovideimprovedcontrastsensitivityduetohigher
have square type pixels. As the pixel pitch is reduced for
SNR levels. Some of these techniques, however, may not meet
increasing the resolution, the total number of pixels in the
other goals, such as throughput or allowable space needed for
image increases for a constant field of view. The file sizes for
aspecimenbetweenthedetectorsandtheX-raytube.Certainly
typical images run from 2 to 24 megabytes or greater. Other
a thicker absorbing material (scintillator or photoconductor)
factors that impact the spatial resolution of the image are (1)
may also impact the spatial resolution (see 6.12) possible from
the geometric unsharpness of the inspection, (2) the thickness
theDDA.Therefore,tradeoffsneedtobemadeinselectingthe
and properties of the scintillator or photoconductor material
appropriate DDA and technique to use for any given applica-
used to absorb X-rays, and (3) various sources of scatter that
tion.
might degrade the modulation of features in an image. For a
6.9 Outside of the quantum chain discussed above, additive
thick scintillator or photoconductive material, X-rays can
noise from the device in the form of fixed patterns, or other
scatter a greater distance depending on the X-ray energy
noise sources, or from the digitization process, can degrade an
employedandthusimpactthespatialresolution.Opticalspread
image even from the most efficient image chain. For a full
can also occur in scintillation materials, especially thicker
discussiononnoisesources,see(3).Therefore,thenoiseofthe
layers. In thick photoconductive materials, the bias levels to
device,aswellasthecouplingschemeisimportantinselecting
drive the carriers to the readout electrodes must also be high
the DDA for the application at hand. Appropriate calibrations
enough to avoid electron spreading that will degrade resolu-
(see Section 7) to remove fixed patterns within the DDA will
tion. Magnification radiography is one means to compensate
result in drastically improved noise performance from the
for the limitation in pixel pitch if the appropriate X-ray focal
device.
spot is available and can be used for the application at hand.
6.10 In a DDA system the detectability of a feature is
6.13 Basic Spatial Resolution (SR )—The smallest geo-
b
definedintermsofcontrast-to-noiseratio(CNR).Contrastina
metrical detail, which can be resolved using the DDA. It is
radiographic image is mainly driven by subject contrast (see
similar to the effective pixel size, and is typically expressed in
Practice E2597). DDAcontrast sensitivity as mentioned above
µm.Ameans to measure the SR is to use a duplex wire gage
b
isdependentontheSNRofthedevice,andthiscontrastactsas
(see E2002), and measure the unsharpness, which in turn
a threshold limit for detection of subject contrast. When the
records the wire pair that can be seen in the image with 20%
subject contrast is below the DDA contrast, not enough
contrast modulation.Acontrast modulation of 20% is usually
information will be available to create a signal level in the
assumed as a standard to determine if the wire pair is visible.
resulting image for visual perception. Hence, contrast sensitiv-
One half of the unsharpness value corresponds to the effective
ity is related to subject contrast and noise in the imaging
pixel size or the basic spatial resolution, as two pixels are
system.
typically required to resolve a wire (d) and its adjacent space
6.10.1 Subject contrast refers to relative subject contrast
(wire + space = 2d, the unsharpness). Figure 6 shows an
that depends on the material properties of the object being
exampleimageofaduplexwirepair.Onemethodtorecordthe
imagedandenergyofradiationused.Toresolveasmallchange
20% modulation in the duplex wire gauge is to use an
in thickness of an object (low subject contrast) and to achieve
interpolation method (see E2002).The contrast modulation for
a high CNR, a high SNR of the imaging system is required.
the wire pair is the percentage dip in the signal. The iSR is
b
Additionally, improved detection of subject contrast can be
calculated as the interpolation of the wire pair distances of the
obtained by using an optimized X-ray energy beam spectrum
lastwirepairwithmorethan20%dipbetweenthewiresinthe
that best separates features in the object.
pair,andthefirstwirepairwithlessthan20%dipbetweenthe
6.11 Spatial Resolution—The spatial resolution of the de- wires (see Fig. 6). Where, D1 is the diameter of the smallest
tectordeterminesthedetectabilityoffeaturesintheimagefrom wire pair with >20% resolution of the gap. D2 is the diameter
a pixel sampling consideration. The selection of the spatial of the largest wire pair with <20% resolution of the gap. R1
resolution of the DDA is also important in designing or and R2 is the modulation of the corresponding wire pair (dip
E2736 − 17 (2022)
FIG. 5 Number of Effective Pixels to Cover an Indication Based on the Contrast of the Feature as Well as the SNR of the DDA.
Single pixel coverage of the longest dimension of an indication is not recommended from the perspective of detection. It also may be
confused for a bad pixel and missed.
%value)ofD1andD2respectively.Anothermethodtocallthe where:
...