ASTM E1570-19

Standard Practice for Fan Beam Computed Tomographic (CT) Examination

SIGNIFICANCE AND USE
5.1 This practice establishes the basic parameters for the application and control of fan-beam CT examinations. This practice is written so it can be specified on the engineering drawing, specification, or contract. It will require a detailed procedure delineating the technique or procedure requirements and shall be approved by the Cognizant Engineering Organization (CEO).  
5.2 The requirements in this practice shall be used when placing a CT system into NDT service and establishing a baseline of system performance measures. Monitoring the system performance over time shall be performed, including calibration procedures, performance measurements, and system maintenance in accordance with Section 9.
SCOPE
1.1 This practice establishes the minimum requirements for computed tomography (CT) examination of test objects using fan beam systems (systems that generate one or a few CT cross sectional slices at a time). The examination may be used to nondestructively disclose physical features or anomalies within a test object by providing radiological density and geometric measurements. This practice implicitly assumes the use of penetrating radiation, specifically X-ray and γ-ray.  
1.2 CT is broadly applicable to any material or test object through which a beam of penetrating radiation passes. The principal advantage of CT is that it provides densitometric (that is, radiological density and geometry) images of thin cross sections through an object without the structural superposition in projection radiography.  
1.3 There are areas in this practice that may require agreement between the purchaser and the supplier, or specific direction from the cognizant engineering organization. These items should be addressed in the purchase order or the contract. Generally, the items are application specific or performance related, or both.  
1.4 Techniques and applications employed with CT are diverse. This practice is not intended to be limiting or restrictive. Refer to Guides E1441 and E1672 that provide additional information and guidance on CT fundamentals and tradeoffs in designing or purchasing a CT system, or both.  
1.5 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.6 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.  
1.7 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: 14-Jun-2019
• ICS: 11.040.50 - Radiographic equipment
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.01 - Radiography (X and Gamma) Method

Relations

• Replaces: ASTM E1570-11 - Standard Practice for Computed Tomographic (CT) Examination
• Effective Date: 15-Jun-2019
• Refers: ASTM E1316-24 - Standard Terminology for <?Pub Dtl?>Nondestructive Examinations
• Effective Date: 01-Feb-2024
• Refers: ASTM E2767-24 - Standard Practice for Digital Imaging and Communication in Nondestructive Evaluation (DICONDE) for X-ray Computed Tomography (CT) Test Methods
• 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 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 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 E2767-13 - Standard Practice for Digital Imaging and Communication in Nondestructive Evaluation &#40;DICONDE&#41; for X-ray Computed Tomography &#40;CT&#41; Test Methods
• Effective Date: 15-Dec-2013

Overview

ASTM E1570-19: Standard Practice for Fan Beam Computed Tomographic (CT) Examination establishes the minimum requirements for the use of fan-beam computed tomography (CT) systems in nondestructive testing (NDT). Issued by ASTM International, this standard is designed to promote reliability, consistency, and quality in fan-beam CT examinations used to evaluate the internal structure of a variety of materials and components. The standard is applicable to CT systems using penetrating X-ray or gamma-ray radiation to generate cross-sectional images of test objects, helping to identify internal features or detect anomalies without damaging the object.

Key Topics

• Scope of Practice: ASTM E1570-19 outlines procedures for the application and control of fan-beam CT, including minimum system and operational requirements. It is intended for reference in engineering drawings, specifications, and contracts, ensuring standardized examinations across industries.
• Quality Control and System Performance: The practice requires that a comprehensive quality control program be established, including calibration, monitoring, and maintenance of CT equipment to ensure ongoing reliability and baseline performance.
• System Components: The standard describes essential subsystems of fan-beam CT, such as:
  • Penetrating radiation sources (X-ray or gamma-ray)
  • Detector arrays (single, linear, or area detectors)
  • Mechanical scanning/manipulation assemblies
  • Computer systems for image acquisition, reconstruction, visualization, and storage
• Examination Procedure: Detailed documentation is required, covering equipment qualifications, scan parameters, reconstruction methods, image display criteria, archiving, and analysis techniques.
• Performance and Safety: Procedures must also address safety, health, and regulatory compliance, with specific requirements highlighted for radiation safety and personnel qualification.

Applications

Fan-beam CT examinations are highly valued across a broad range of industries for nondestructive evaluation (NDE) and quality assurance:

• Aerospace and Defense: Inspection of complex components for defects, inclusions, porosity, or assembly verification without disassembly.
• Automotive: Evaluation of castings, welds, and composite materials to ensure structural integrity.
• Manufacturing: Assessment of critical parts, additive manufactured components, and materials for quality control.
• Research and Development: Detailed internal imaging for prototype validation, failure analysis, and material characterization.
• Regulatory and Contractual Compliance: Use of standardized CT practices enhances traceability and confidence among suppliers, purchasers, and regulatory bodies.

The use of ASTM E1570-19 allows organizations to:

• Establish a consistent baseline for their CT system performance.
• Monitor system functionality over time with defined calibration and performance checks.
• Achieve reproducibility in CT-based inspections and measurements.

Related Standards

ASTM E1570-19 references and is complemented by several other international and ASTM standards, ensuring a comprehensive framework for CT-based NDT:

• ASTM E1316: Terminology for Nondestructive Examinations
• ASTM E1441: Guide for Computed Tomography (CT) Imaging
• ASTM E1672: Guide for Computed Tomography (CT) System Selection
• ASTM E1695: Test Method for CT System Performance Measurement
• ASTM E2339 and E2767: Practices for Digital Imaging and Communication in Nondestructive Evaluation (DICONDE)
• ISO 15708 Series: International standards for radiation-based computed tomography in NDT
• ANSI/ASNT CP-189, SNT-TC-1A, NAS-410, ISO 9712: Personnel qualification and certification standards
• ANSI N43.3; 21 CFR 1020.40; 29 CFR 1910.96: Radiation safety standards

Conclusion

ASTM E1570-19 provides a robust framework for organizations seeking reliable, repeatable, and standardized procedures in fan-beam CT examination. By following its guidelines-from equipment setup through documentation and quality control-industries can enhance nondestructive testing outcomes, support regulatory compliance, and improve product safety and reliability. Using this standard helps ensure that fan-beam CT applications deliver high-quality, actionable results for internal inspections across diverse materials and sectors.

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: E1570 − 19
Standard Practice for
Fan Beam Computed Tomographic (CT) Examination
This standard is issued under the fixed designation E1570; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope* 1.7 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1.1 This practice establishes the minimum requirements for
ization established in the Decision on Principles for the
computed tomography (CT) examination of test objects using
Development of International Standards, Guides and Recom-
fan beam systems (systems that generate one or a few CTcross
mendations issued by the World Trade Organization Technical
sectional slices at a time). The examination may be used to
Barriers to Trade (TBT) Committee.
nondestructivelydisclosephysicalfeaturesoranomalieswithin
a test object by providing radiological density and geometric
2. Referenced Documents
measurements. This practice implicitly assumes the use of
2.1 ASTM Standards:
penetrating radiation, specifically X-ray and γ-ray.
E1316 Terminology for Nondestructive Examinations
1.2 CT is broadly applicable to any material or test object
E1441 Guide for Computed Tomography (CT) Imaging
through which a beam of penetrating radiation passes. The
E1672 Guide for Computed Tomography (CT) System Se-
principaladvantageofCTisthatitprovidesdensitometric(that
lection
is, radiological density and geometry) images of thin cross
E1695 Test Method for Measurement of Computed Tomog-
sections through an object without the structural superposition
raphy (CT) System Performance
in projection radiography.
E2339 Practice for Digital Imaging and Communication in
1.3 There are areas in this practice that may require agree-
Nondestructive Evaluation (DICONDE)
ment between the purchaser and the supplier, or specific
E2767 Practice for Digital Imaging and Communication in
direction from the cognizant engineering organization. These
Nondestructive Evaluation (DICONDE) for X-ray Com-
itemsshouldbeaddressedinthepurchaseorderorthecontract.
puted Tomography (CT) Test Methods
Generally, the items are application specific or performance
2.2 ASNT Documents:
related, or both.
SNT-TC-1A Recommended Practice for Personnel Qualifi-
1.4 Techniques and applications employed with CT are
cation and Certification in Nondestructive Testing
diverse. This practice is not intended to be limiting or restric-
ANSI/ASNT-CP-189 QualificationandCertificationofNon-
tive. Refer to Guides E1441 and E1672 that provide additional
destructive Testing Personnel
information and guidance on CTfundamentals and tradeoffs in
2.3 AIA Standard:
designing or purchasing a CT system, or both.
NAS-410 Certification and Qualification of Nondestructive
1.5 Units—The values stated in SI units are to be regarded
Testing Personnel
as standard. No other units of measurement are included in this
2.4 ISO Standards:
standard.
ISO 9712 International Standard for NondestructiveTesting
1.6 This standard does not purport to address all of the Personnel Qualification and Certification
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter- 2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
mine the applicability of regulatory limitations prior to use.
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
AvailablefromAmericanSocietyforNondestructiveTesting(ASNT),P.O.Box
This practice is under the jurisdiction of ASTM Committee E07 on Nonde- 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org.
structive Testing and is the direct responsibility of Subcommittee E07.01 on Available fromAerospace IndustriesAssociation ofAmerica, Inc. (AIA), 1000
Radiology (X and Gamma) Method. WilsonBlvd.,Suite1700,Arlington,VA22209-3928,http://www.aia-aerospace.org.
Current edition approved June 15, 2019. Published August 2019. Originally Available from International Organization for Standardization (ISO), ISO
approved in 1993. Last previous edition approved in 2011 as E1570 – 11. DOI: Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier,
10.1520/E1570-19. Geneva, Switzerland, http://www.iso.org.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1570 − 19
ISO 15708-1:2017-02 International Standard for Non- 4.1.3 Section 10 describes the documentation for an archive
destructive Testing - Radiation Methods for Computed of the CT examination.
Tomography - Part 1: Terminology
5. Significance and Use
ISO 15708-2:2017-02 International Standard for Non-
destructive Testing - Radiation Methods for Computed
5.1 This practice establishes the basic parameters for the
Tomography - Part 2: Principles, Equipment and Samples application and control of fan-beam CT examinations. This
ISO 15708-3:2017-02 International Standard for Non-
practice is written so it can be specified on the engineering
destructive Testing - Radiation Methods for Computed drawing, specification, or contract. It will require a detailed
Tomography - Part 3: Operation and Interpretation
procedure delineating the technique or procedure requirements
ISO 15708-4:2017-02 International Standard for Non- and shall be approved by the Cognizant Engineering Organi-
destructive Testing - Radiation Methods for Computed
zation (CEO).
Tomography - Part 4: Qualification
5.2 The requirements in this practice shall be used when
2.5 SMPTE Standard:
placing a CT system into NDT service and establishing a
SMPTE RP 133 Specifications for Medical Diagnostic Im-
baseline of system performance measures. Monitoring the
agingTest Pattern forTelevision Monitors and Hard-Copy
system performance over time shall be performed, including
Recording Cameras
calibration procedures, performance measurements, and sys-
tem maintenance in accordance with Section 9.
3. Terminology
6. Basis of Application
3.1 Definitions—ThedefinitionsoftermsrelatingtoCT,that
appear in Terminology E1316, shall apply to the terms used in
6.1 The following items are subject to contractual agree-
this practice.
ment between the parties using or referencing this standard.
3.2 Definitions of Terms Specific to This Standard:
6.2 Personnel Qualification
3.2.1 digital driving level (DDL), n—for computer graphics
6.2.1 If specified in the contractual agreement, personnel
display boards,thedigitalvaluethatcorrespondstoaparticular
performing examinations to this standard shall be qualified in
monochrome grayscale level.
accordance with a nationally or internationally recognized
3.2.1.1 Discussion—A particular DDL “drives out” a par-
NDT personnel qualification practice or standard such as
ticular visible shade of gray. For example, in an 8-bit display,
ANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, ISO 9712, or a
a DDL assumes 256 values from 0 to 255.
similar document and certified by the employer or certifying
3.2.2 effective energy, n—the equivalent monoenergetic en- agency, as applicable. The practice or standard used and its
ergy for a polyenergetic CT system; thus, the actual, polyen- applicable revision shall be identified in the contractual agree-
ment between the using parties.
ergetic CT system yields the same measured attenuation
coefficient for an examination object as a theoretical, monoen-
6.3 Procedures and Techniques—The procedures and tech-
ergetic CT system at the effective energy.
niques to be utilized shall be as specified in the contractual
3.2.3 fan beam, n—a beam of radiation that is restricted to
agreement.
one dimension in linear extent with a thickness limited to the
6.4 Reporting Criteria/Acceptance Criteria—Reporting cri-
detector height.
teria for the examination results shall be in accordance with
3.2.4 purchaser, n—as used within this document, the pur-
Section10unlessotherwisespecified.Sinceacceptancecriteria
chaser of computed tomographic services refers to the entity
are not specified in this standard, they shall be specified in the
thatrequiresthecomputedtomographicservices;thepurchaser
contractual agreement.
may be a part of the same organization as the supplier, or an
7. Equipment
outside organization.
7.1 ManydifferentCTsystemconfigurationsarepossible.It
3.2.5 supplier, n—as used within this document, the supplier
of computed tomographic service refers to the entity that is important that the user understands the advantages and
limitations of each (see Guide E1441). The provider and the
physically provides the computed tomographic services; the
user of the system should be fully aware of the capabilities and
supplier may be a part of the same organization as the
limitations of each system proposed.
purchaser, or an outside organization.
7.2 All CT systems have four major subsystems: radiation
4. Summary of Practice
source, radiation detectors, mechanical handling system, and
4.1 This practice is organized as follows:
computer system. The following represents the requirements
4.1.1 Section 7 describes CT Equipment;
on each subsystem for a CT examination.
4.1.2 Section 8 describes the minimum requirements of a
7.2.1 Source Setup—The radiation source shall be selected
CT examination; Section 9 describes the CT system perfor-
to provide the maximum signal-to-noise ratio (SNR), contrast
mance requirements; and
tonoiseratio(CNR),andcontrastsensitivitywhilemaintaining
the necessary spatial resolution. See Guide E1441 for a
detailed discussion.
Available from Society of Motion Picture and Television Engineers (SMPTE),
7.2.2 Radiation Detection Systems—The detection system
White Plains Plaza, 445 Hamilton Ave Ste 601, White Plains, NY 10601-1827,
https://www.smpte.org. may consist of a single sensing element, a linear array of
E1570 − 19
sensing elements, or a two-dimensional array in an area 7.3.1.1 The minimum brightness as measured off the image
detector. The more detectors used, the faster the required scan display screen at maximum Digital Driving Level (DDL) shall
data can be collected; but there are important tradeoffs to be be 250 cd/m2.
considered. 7.3.1.2 The minimum contrast as determined by the ratio of
the screen brightness at the maximum DDL compared to the
7.2.2.1 The detector cannot be operated without computing
hardware and software for image acquisition. screen brightness at the minimum DDL shall be 250:1.
7.3.1.3 The image display shall be capable of displaying
7.2.2.2 The acquisition software shall be capable of acquir-
linear patterns of alternating pixels at full contrast in both the
ing images projection by projection from the detector and
horizontal and vertical directions without aliasing.
integrating, or averaging frames, or both.
7.3.1.4 The display shall be free of discernable geometric
7.2.2.3 The acquisition software shall perform an image
distortion.
calibration to correct the inhomogeneities of the detector.
7.3.1.5 The display shall be free of screen flicker, charac-
7.2.2.4 Users shall comply with the detector manufacturers’
terized by high frequency fluctuation of high contrast image
requirements of temperatures and humidity conditions for both
details.
operation and shipping.
7.3.1.6 The image display shall be capable of displaying a
7.2.2.5 The detector shall be calibrated using the manufac-
5 % DDL block againsta0%DDL background and simulta-
turers’ recommendation both for frequency of calibration and
neously displaying a 95 % DDL block against a 100 %
the method used. Other calibration methods are allowed as
background in a manner clearly perceptible to the user. An
long as approved by the CEO.
image display test pattern, in accordance with the requirements
7.2.2.6 The user shall ensure that all exposures are within
of SMPTE RP 133, shall be configured for the system display
the linear operating range of the detector, using either infor-
resolution and aspect ratio.Alternate test patterns may be used
mation obtained from the manufacturer or data obtained by the
providedtheyincludethefeaturesdescribedinSMPTERP133
user/CEO.
required to perform the quality tests specified in this practice.
NOTE 1—Collimation (the restriction of the possible paths for radiation
7.3.1.7 The monitor shall be capable of discriminating the
by placement of absorbing material) near
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E1570 − 11 E1570 − 19
Standard Practice for
Fan Beam Computed Tomographic (CT) Examination
This standard is issued under the fixed designation E1570; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope*
1.1 This practice is establishes the minimum requirements for computed tomography (CT), which (CT) examination of test
objects using fan beam systems (systems that generate one or a few CT cross sectional slices at a time). The examination may be
used to nondestructively disclose physical features or anomalies within a test object by providing radiological density and
geometric measurements. This practice implicitly assumes the use of penetrating radiation, specifically X-ray and γ-ray.
1.2 CT systems utilize a set of transmission measurements made along paths through the test object from many different
directions. Each of the transmission measurements is digitized and stored in a computer, where they are subsequently reconstructed
by one of a variety of techniques. A treatment of CT principles is given in Guide E1441.
1.2 CT is broadly applicable to any material or test object through which a beam of penetrating radiation passes. The principal
advantage of CT is that it provides densitometric (that is, radiological density and geometry) images of thin cross sections through
an object without the structural superposition in projection radiography.
1.3 There are areas in this practice that may require agreement between the purchaser and the supplier, or specific direction from
the cognizant engineering organization. These items should be addressed in the purchase order or the contract. Generally, the items
are application specific or performance related, or both.
1.4 This practice describes procedures for performing CT examinations. Techniques and applications employed with CT are
diverse. This practice is to address the general use of not intended to be limiting or restrictive. Refer to Guides E1441 and E1672CT
technology and thereby facilitate its use. that provide additional information and guidance on CT fundamentals and tradeoffs in
designing or purchasing a CT system, or both.
1.5 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this
standard.
1.6 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.For specific safety statements, see Section 8, NBS Handbook 114, and Federal
Standards 21 CFR 1020.40 and 29 CFR 1910.96.
1.7 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.
2. Referenced Documents
2.1 ASTM Standards:
E1316 Terminology for Nondestructive Examinations
E1441 Guide for Computed Tomography (CT) Imaging
E1672 Guide for Computed Tomography (CT) System Selection
E1695 Test Method for Measurement of Computed Tomography (CT) System Performance
E2339 Practice for Digital Imaging and Communication in Nondestructive Evaluation (DICONDE)
This practice is under the jurisdiction of ASTM Committee E07 on Nondestructive Testing and is the direct responsibility of Subcommittee E07.01 on Radiology (X and
Gamma) Method.
Current edition approved July 1, 2011June 15, 2019. Published July 2011August 2019. Originally approved in 1993. Last previous edition approved in 20052011 as
ε1
E1570 - 00E1570 – 11.(2005) . DOI: 10.1520/E1570-11.10.1520/E1570-19.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1570 − 19
E2767 Practice for Digital Imaging and Communication in Nondestructive Evaluation (DICONDE) for X-ray Computed
Tomography (CT) Test Methods
2.2 NIST Standard:
ANSI N43.3 General Radiation Safety Installations Using Non-Medical X-Ray and Sealed Gamma Sources up to 10 MeV
2.3 Federal Standards:
21 CFR 1020.40 Safety Requirements of Cabinet X Ray Systems
29 CFR 1910.96 Ionizing Radiation
2.2 ASNT Documents:
SNT-TC-1A Recommended Practice for Personnel Qualification and Certification in Nondestructive Testing
ANSI/ASNT-CP-189 Qualification and Certification of Nondestructive Testing Personnel
2.5 Military Standard:
MIL-STD-410 Nondestructive Testing Personnel Qualification and Certification
2.3 AIA Standard:
NAS-410 Certification and Qualification of Nondestructive Testing Personnel
2.4 ISO Standards:
ISO 9712 International Standard for Nondestructive Testing Personnel Qualification and Certification
ISO 15708-1:2017-02 International Standard for Non-destructive Testing - Radiation Methods for Computed Tomography - Part
1: Terminology
ISO 15708-2:2017-02 International Standard for Non-destructive Testing - Radiation Methods for Computed Tomography - Part
2: Principles, Equipment and Samples
ISO 15708-3:2017-02 International Standard for Non-destructive Testing - Radiation Methods for Computed Tomography - Part
3: Operation and Interpretation
ISO 15708-4:2017-02 International Standard for Non-destructive Testing - Radiation Methods for Computed Tomography - Part
4: Qualification
2.5 SMPTE Standard:
SMPTE RP 133 Specifications for Medical Diagnostic Imaging Test Pattern for Television Monitors and Hard-Copy Recording
Cameras
3. Terminology
3.1 Definitions—ForThe definitions of terms used in this guide, refer torelating to CT, that appear in Terminology E1316 and
Annex A1 in Guide, shall apply to the terms used in this E1441.practice.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 digital driving level (DDL), n—for computer graphics display boards, the digital value that corresponds to a particular
monochrome grayscale level.
3.2.1.1 Discussion—
A particular DDL “drives out” a particular visible shade of gray. For example, in an 8-bit display, a DDL assumes 256 values from
0 to 255.
3.2.2 effective energy, n—the equivalent monoenergetic energy for a polyenergetic CT system; thus, the actual, polyenergetic
CT system yields the same measured attenuation coefficient for an examination object as a theoretical, monoenergetic CT system
at the effective energy.
3.2.3 fan beam, n—a beam of radiation that is restricted to one dimension in linear extent with a thickness limited to the detector
height.
3.2.4 purchaser, n—as used within this document, the purchaser of computed tomographic services refers to the entity that
requires the computed tomographic services; the purchaser may be a part of the same organization as the supplier, or an outside
organization.
Available from National Institute of Standards and Technology (NIST), 100 Bureau Dr., Stop 1070, Gaithersburg, MD 20899-1070, http://www.nist.gov.
Available from Standardization Documents Order Desk, DODSSP, Bldg. 4, Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http://dodssp.daps.dla.mil.
Available from American Society for Nondestructive Testing (ASNT), P.O. Box 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org.
Available from Aerospace Industries Association of America, Inc. (AIA), 1000 Wilson Blvd., Suite 1700, Arlington, VA 22209-3928, http://www.aia-aerospace.org.
As used within this document, the supplier of computed tomographic service refers to the entity that physically provides the computed tomographic services. The supplier
may be a part of the same organization as the purchaser, or an outside organization.Available from International Organization for Standardization (ISO), ISO Central
Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva, Switzerland, http://www.iso.org.
As used within this document, the purchaser of computed tomographic services refer to the entity that requires the computed tomographic services. The purchaser may
be a part of the same organization as the supplier, or an outside organization.Available from Society of Motion Picture and Television Engineers (SMPTE), White Plains Plaza,
445 Hamilton Ave Ste 601, White Plains, NY 10601-1827, https://www.smpte.org.
E1570 − 19
3.2.5 supplier, n—as used within this document, the supplier of computed tomographic service refers to the entity that physically
provides the computed tomographic services; the supplier may be a part of the same organization as the purchaser, or an outside
organization.
4. Summary of Practice
4.1 Requirements in this practice are intended to control the reliability and quality of the CT images.
4.2 CT systems are made up of a number of subsystems; the function served by each subsystem is common in almost all CT
scanners. Section 7 describes the following subsystems:
4.2.1 Source of penetrating radiation,
4.2.2 Radiation detector or an array of detectors,
4.2.3 Mechanical scanning assembly, and
4.2.4 Computer system including:
4.2.4.1 Image reconstruction software/hardware,
4.2.4.2 Image display/analysis system,
4.2.4.3 Data storage system, and
4.2.4.4 Operator interface.
4.3 Section 8 describes and defines the procedures for establishing and maintaining quality control of CT services.
4.4 The extent to which a CT image reproduces an object or a feature within an object is influenced by spatial resolution,
statistical noise, slice plane thickness, and artifacts of the imaging system. Operating parameters should strike an overall balance
between image quality, inspection time, and cost. These parameters should be considered for CT system configurations,
components, and procedures. The setting and optimization of CT system parameters is discussed in Section 9.
4.1 Methods for the measurement of CT system performance are provided in Section This practice is organized as follows:10
of this practice.
4.1.1 Section 7 describes CT Equipment;
4.1.2 Section 8 describes the minimum requirements of a CT examination; Section 9 describes the CT system performance
requirements; and
4.1.3 Section 10 describes the documentation for an archive of the CT examination.
5. Significance and Use
5.1 This practice is applicable establishes the basic parameters for the systematic assessment of the internal structure of a
material or assembly using CT technology. This practice may be used for review by system operators, or to prescribe operating
procedures for new or routine test objects.application and control of fan-beam CT examinations. This practice is written so it can
be specified on the engineering drawing, specification, or contract. It will require a detailed procedure delineating the technique
or procedure requirements and shall be approved by the Cognizant Engineering Organization (CEO).
5.2 This practice provides the basis for the formation of a program for quality control and its continuation through calibration,
standardization, referenceThe requirements in this practice shall be used when placing a CT system into NDT service and
establishing a baseline of system performance measures. Monitoring the system performance over time shall be performed,
including calibration procedures, performance measurements, and system maintenance in accordance with Section 9samples,
inspection plans, and procedures.
6. Basis of Application
6.1 The following items are subject to contractual agreement between the parties using or referencing this standard.
6.2 This practice provides the approach for performing CT examinations. Supplemental information covering specific items
7 8
where agreement between supplier and purchaser are necessary is required. Generally the items are application specific or
performance related, or both. Examples include: system configuration, equipment qualification, performance measurement, and
interpretation of results.Personnel Qualification
6.2.1 If specified in the contractual agreement, personnel performing examinations to this standard shall be qualified in
accordance with a nationally or internationally recognized NDT personnel qualification practice or standard such as ANSI/ASNT-
CP-189, SNT-TC-1A, NAS-410, ISO 9712, or a similar document and certified by the employer or certifying agency, as applicable.
The practice or standard used and its applicable revision shall be identified in the contractual agreement between the using parties.
6.3 Procedures and Techniques—The procedures and techniques to be utilized shall be as specified in the contractual agreement.
6.4 Reporting Criteria/Acceptance Criteria—Reporting criteria for the examination results shall be in accordance with Section
10 unless otherwise specified. Since acceptance criteria are not specified in this standard, they shall be specified in the contractual
agreement.
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7. System Configuration Equipment
7.1 Many different CT system configurations are possible and it is important to understand the advantages and limitations of
each. possible. It is important that the optimum system parameters be selected for each examination requirement, through a careful
analysis of the benefitsuser understands the advantages and limitations of the available systemeach (see Guide E1441components
). The provider and the chosen system configuration.user of the system should be fully aware of the capabilities and limitations
of each system proposed.
7.2 Radiation Sources—While the CT systems may utilize either gamma-ray or X-ray generators, the latter is used for most
applications. For a given focal spot size, X-ray generators (that is, X-ray tubes and linear accelerators) are several orders of
magnitude more intense than isotope sources. Most X-ray generators are adjustable in peak energy and intensity and have the added
safety feature of discontinued radiation production when switched off; however, the polychromaticity of the energy spectrum from
an X-ray source causes artifacts such as beam hardening (the anomalous decreasing attenuation toward the center of a
homogeneous object) in the image if uncorrected.
7.2.1 X-rays produced from electrical radiation generators have focal spot sizes ranging from a few millimetres down to a few
micrometres. Reducing the focal spot size reduces geometric unsharpness, thereby enhancing detail sensitivity. Smaller focal spots
permit higher spatial resolution, but at the expense of reduced X-ray beam intensity.
7.2.2 A radioisotope source can have the advantages of small physical size, portability, low power requirements, simplicity, and
stability of output. The disadvantages are limited intensity and limited peak energy.
7.2.3 Synchrotron Radiation (SR) sources produce very intense, naturally collimated, narrow bandwidth, tunable radiation.
Thus, CT systems using SR sources can employ essentially monochromatic radiation. With present technology, however, practical
SR energies are restricted to less than approximately 20 to 30 keV. Since any CT system is limited to the inspection of samples
with radio-opacities consistent with the penetrating power of the X-ray employed, SR systems can in general image only small
(about 1 mm) objects.
7.2 Radiation Detection Systems—The detection system is a transducer that converts the transmitted radiation containing
information about the test object into an electronic signal suitable for processing. The detection system may consist of a single
sensing element, a linear array of sensing elements, or an area array of sensing elements. The more detectors used, the faster the
required scan data can be collected; but there are important tradeoffs to be considered.All CT systems have four major subsystems:
radiation source, radiation detectors, mechanical handling system, and computer system. The following represents the requirements
on each subsystem for a CT examination.
7.2.1 Source Setup—A single detector provides the least efficient method of collecting data but entails minimal complexity,
eliminates detector cross talk and detector matching, and allows an arbitrary degree of collimation and shieldingThe radiation
source shall be selected to provide the maximum signal-to-noise ratio (SNR), contrast to noise ratio (CNR), and contrast sensitivity
while maintaining the necessary spatial resolution. See Guide E1441 to be implemented.for a detailed discussion.
7.2.2 Radiation Detection Systems—Linear arrays have reasonable scan times at moderate complexity, acceptable cross talk and
detector matching, and a flexible architecture that typically accommodates good collimation and shielding. Most commercially
available CT systems employ a linear array of detectors.The detection system may consist of a single sensing element, a linear
array of sensing elements, or a two-dimensional array in an area detector. The more detectors used, the faster the required scan
data can be collected; but there are important tradeoffs to be considered.
7.2.2.1 The detector cannot be operated without computing hardware and software for image acquisition.
7.2.2.2 The acquisition software shall be capable of acquiring images projection by projection from the detector and integrating,
or averaging frames, or both.
7.2.2.3 The acquisition software shall perform an image calibration to correct the inhomogeneities of the detector.
7.2.2.4 Users shall comply with the detector manufacturers’ requirements of temperatures and humidity conditions for both
operation and shipping.
7.2.2.5 The detector shall be calibrated using the manufacturers’ recommendation both for frequency of calibration and the
method used. Other calibration methods are allowed as long as approved by the CEO.
7.2.2.6 The user shall ensure that all exposures are within the linear operating range of the detector, using either information
obtained from the manufacturer or data obtained by the user/CEO.
NOTE 1—Collimation (the restriction of the possible paths for radiation by placement of absorbing material) near the radiation source is used to limit
the radiation beam to correspond to the general shape of the detection apparatus. In some cases, further collimation near the detector bank or for each
detector is used to determine the thickness of the slice and to reduce or eliminate scattered radiation from that which will ultimately be measured.
7.2.3 Manipulation/Handling System—An area detector provides a fast method of collecting data but entails the transfer and
storage of large amounts of information, forces tradeoffs between cross talk and detector efficiency, and creates serious collimation
and shielding challenges.The manipulation system has the function of holding the test object and providing the necessary range
of motions to position the test object between the radiation source and detector. Two types of scan motion geometries are most
common: translate-rotate motion and rotate-only motion.
7.2.3.1 Users shall comply with the handling system manufacturers’ requirements of weight, size, temperatures, and humidity
conditions for both operation and shipping.
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7.2.3.2 The handling system shall be calibrated using the manufacturers’ recommendation both for frequency of calibration and
the method used. Other calibration methods are allowed as long as approved by the CEO.
7.2.3.3 The user shall ensure that all exposures are within the fan-beam envelope of the source-handling system, using either
information obtained from the manufacturer or data obtained by the user/CEO.
7.2.4 Computer System(s)—The computer system provides the Operator Interface and performs the tasks of acquisition,
reconstruction, visualization, and storage. The system shall have the necessary storage and device interfaces to work with the
system provided.
7.4 Manipulation System—The manipulation system has the function of holding the test object and providing the necessary
range of motions to position the test object between the radiation source and detector. Two types of scan motion geometries are
most common: translate-rotate motion and rotate-only motion.
7.4.1 With translate-rotate motion, the object is translated in a direction perpendicular to the direction and in the plane of the
X-ray beam. Full data sets are obtained by rotating the test object between translations by the fan angle of the beam and again
translating the object until a minimum of 180 degrees of data have been acquired. The advantage of this design is simplicity, good
view-to-view detector matching, flexibility in the choice of scan parameters, and ability to accommodate a wide range of different
object sizes including objects too big to be subtended by the X-ray fan. The disadvantage is longer scan time.
7.4.2 With rotate-only motion, a complete view is collected by the detector array during each sampling interval. A rotate-only
scan has lower motion penalty than a translate-rotate scan and is attractive for industrial applications where the part to be examined
fits within the fan beam and scan speed is important.
7.4.3 In volume CT, a complete data set for the entire part is acquired in at least one rotation. This allows for much faster data
acquisition, as the data required for multiple slices can be acquired in one rotation.
7.5 Computer System—CT requires substantial computational resources, such as a large capacity for image storage and archival
and the ability to efficiently perform numerous mathematical computations, especially for the back-projection operation.
Computational speed can be augmented by either generalized array processors or specialized back-projection hardware. The
particular implementations will change as computer hardware evolves, but high computational power will remain a fundamental
requirement for efficient CT examination. A separate workstation for image analysis and display often is appropriate.
7.6 Image Reconstruction Software— The aim of CT is to obtain information regarding the nature of material occupying exact
positions inside a test object. In current CT scanners, this information is obtained by “reconstructing” individual cross-sections of
the test object from the measured intensity of X-ray beams transmitted through that cross section. An exact mathematical theory
of image reconstruction exists for idealized data. This theory is applied although the physical measurements do not fully meet the
requirements of the theory. When applied to actual measurements, algorithms based on this theory produce images with blurring
and noise, the extent of which depends on the quantity and quality of the measurements.
7.6.1 The simplifying assumptions made in setting up the theory of reconstruction algorithms are: (1) cross sections are
infinitely thin (that is, they are planes), (2) both the source focal spot and the detector elements are infinitely small (that is, they
are points), (3) the physical measurements correspond to total attenuation along the line between the source and detector, and (4)
the radiation is, or can be treated as, effectively monoenergetic. A reconstruction algorithm is a collection of step-by-step
instructions that define how to convert the measurements of total attenuation to a map of linear attenuation coefficients over the
field of view.
7.6.2 A number of methods for recovering an estimate of the cross section of an object have evolved. They can be broadly
grouped into three classes of algorithms: matrix inversion methods, finite series-expansion methods, and transform methods. See
Guide E1441 for treatment of reconstruction algorithms.
7.6.3 If the test object is larger than the prescribed field of view (FOV), either by necessity or by accident, unexpected and
unpredictable artifacts or a measurable degradation of image quality can result.
7.3 Image Display—The An additional key element of the CT system is the function of the image display isdisplay, specifically
to convey derived information (that is, an image) of the test object to the system operator. For manual evaluation systems, the
displayed image is used as the basis for accepting or rejecting the test object, subject to the operator’s interpretation of the CT data.
7.3.1 Generally, CT image display requires a special graphics monitor; television image presentation is of lower quality but may
be acceptable. Most industrial systems utilize color displays. These units can be switched between color and gray-scale
presentation to suit the preference of the viewer, but it should be noted that gray-scale images presented on a color monitor are
not as sharp as those on a gray-scale monitor. The use of color permits the viewer to distinguish a greater range of variations in
an image than gray-scale does. Depending on the application, this may be an advantage or a disadvantage. Sharply contrasting
colors may introduce false, distinct definition between boundaries. While at times advantageous, unwanted instances can be
corrected through the choice of color (or monochrome) scale.The image display shall meet the following requirements as a
minimum. Alternate image displays or requirements may be used with CEO approval.
7.3.1.1 The minimum brightness as measured off the image display screen at maximum Digital Driving Level (DDL) shall be
250 cd/m2.
7.3.1.2 The minimum contrast as determined by the ratio of the screen brightness at the maximum DDL compared to the screen
brightness at the minimum DDL shall be 250:1.
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7.3.1.3 The image display shall be capable of displaying linear patterns of alternating pixels at full contrast in both the horizontal
and vertical directions without aliasing.
7.3.1.4 The display shall be free of discernable geometric distortion.
7.3.1.5 The display shall be free of screen flicker, characterized by high frequency fluctuation of high contrast image details.
7.3.1.6 The image display shall be capable of displaying a 5 % DDL block against a 0 % DDL background and simultaneously
displaying a 95 % DDL block against a 100 % background in a manner clearly perceptible to the user. An image display test
pattern, in accordance with the requirements of SMPTE RP 133, shall be configured for the system display resolution and aspect
ratio. Alternate test patterns may be used provided they include the features described in SMPTE RP 133 required to perform the
quality tests specified in this practice.
7.3.1.7 The monitor shall be capable of discriminating the horizontal and vertical low contrast (1 %) modulation patterns at the
display center and each of the four corner locations.
7.4 Data Storage Medium—Storage—Many CT applications require an archival-quality record of the CT examination. This
could be in the form of raw data or reconstructed data. Therefore, formats and headers of digital data need to be specified so
information can be retrieved at a later date. Each archiving system has its own specifics as to image quality, archival storage
properties, equipment, and media cost. Computer systems are designed to interface to a wide variety of peripherals. As technology
advances or needs change, or both, equipment can be easily and affordably upgraded. The examination record archiving system
should be chosen on the basis of these and specific requirements as to archival storage properties, equipment, and media cost and
other pertinent parameters, as agreed upon by the supplier and purchaser of CT services. The reproduction quality of the archival
method should be sufficient to demonstrate the same image quality as was used to qualify the CT system. Storage of images to
the DICONDE standard Practices E2339 and E2767 is recommended.
7.9 Operator Interface—The operator interface determines much of the function of the rest of the CT system. The control panel
and image display system are the two significant subsystems affected. The control software, hardware mechanisms, and interface
to a remote data workstation if applicable, are among those controlled by this interface. Override logic, emergency shutdown, and
safety interlocks are also controlled at this point. There are three types of operator interfaces.
7.9.1 A simple programming console interface, where the operator types in commands on a keyboard. While being less “user
friendly,” this type can offer the greatest range of flexibility and versatility.
7.9.2 The dedicated console with specific function buttons and relatively rigid data and processing features. These systems are
usually developed explicitly for standardized, nonvarying examination tasks. They are designed to be “functionally hardwired” for
efficient throughput for that program. Medical CT equipment is often of this type.
7.9.3 A graphical user interface employing a software display of the menu or windowing type with means such as a pointing
device for entering responses and interacting with the system. This approach has the advantage of being able to combine the best
features of the other two types of operator interfaces.
7.10 Automation—A variation among CT systems is the extent to which users can create, modify or elaborate image
enhancement or automated evaluation processes. The level of sophistication and versatility of a user command language or a
“learning mode” is an important consideration for purchasers and suppliers who expect to scan a variety of objects or to improve
their processes as they gain experience with CT.
8. Documentation
8.1 Documentation of the examination protocol shall cover the following:
8.1.1 Equipment Qualifications—The following system features shall be included:
8.1.2 Test Object Scan Plan—A listing of test object(s), scan parameters and performance measurements to be extracted from
the image(s).
8.1.2.1 Data Acquisition Parameters—A listing of radiation source and detector-related variables include the following:
(1) Source energy,
(2) Intensity, current, Rad output or equivalent,
(3) Integration time, number of pulses or equivalent,
(4) Source spot size or isotope source size,
(5) Source filtration,
(6) Source collimation,
(7) Detector filtration,
(8) Detector collimation,
(9) Source-to-axis distances,
(10) Source-to-detector distance,
(11) Detector gain factor, gain range, or equivalent,
(12) Sampling parameters (linear increment, angular increment or equivalent),
(13) Number of detectors or channels,
(14) Scan mode, that is, translate-rotate or rotate only,
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(15) Calibration of detector air counts (no test object) and dark counts (no source) and frequency of calibration, and
(16) Position of slice plane and orientation of sample.
8.1.2.2 Image Reconstruction Parameters— A listing of expected image reconstruction variables including:
(1) Type of reconstruction (that is, normal, zoom, annular, limited-angle, and so forth),
(2) Conditioning of X-ray absorption measurements; reconstruction algorithm, view pre-processing, beam-hardening
corrections, non-linearity corrections,
(3) Reconstruction diameter (field of view),
(4) Reconstruction pixel size, slice thickness or equivalent,
(5) Linear sampling intervals (if appropriate),
(6) Reconstruction matrix size,
(7) Pixel size and coordinates, and
(8) Position orientation/size (for zoom).
8.1.2.3 Image Display Parameters—A listing of the techniques and the intervals applied for standardizing the video image
display as to brightness, contrast, focus, and linearity, which includes the following:
(1) Provisions for displaying a quantized color bar or gray scale to assist in this operation.
(2) Method used for adjusting the monitor and ensuring that the full range of colors or shades of gray are properly displayed.
(3) Transformation from CT number to color or gray scale look-up table (LUT).
(4) Upper and lower limits on the range of CT numbers displayed (or the equivalent description in terms of a range about an
average value).
(5) If a nonlinear display technique, like histogram equalization or log transformation is used, describe the method.
8.1.2.4 Image Analysis—Digital image analysis techniques used to manipulate, alter, or quantify the image for the purpose of
CT examination must be documented. The documentation shall include the following:
8.1.2.5 Accept-Reject Criteria—A listing of accept/reject criteria.
8.1.2.6 Performance Evaluation—A listing of the qualification tests and the intervals at which they are applied (see 10.2).
8.1.3 Image Archiving Requirements—A listing of the requirements for preserving a historical record of the examination results.
The listing may include examination images along with written or electronically recorded alphanumeric or audio narrative
information, or both, sufficient to allow subsequent reevaluation or repetition of the CT examination. The listing shall specify data
types (that is, raw data, image data, 16-bit, 8-bit, specially processed images, and so forth) along with the format or medium used.
Data compression format, if applicable, shall be listed.
8.1.4 Examination Record Data—The examination record shall contain sufficient information to allow the CT examination to
be reevaluated or duplicated. Examination record data should be recorded simultaneously with the CT image and may be in writing
or a voice narrative, providing the following minimum data:
8.1.4.1 The CT system designation, examination date, operator identification, operating turn or shift, and other pertinent
examination and customer data,
8.1.4.2 Specific test object data as to part number, batch, serial number, and so forth (as applicable),
8.1.4.3 Test object orientation and site information (that is, scan height, slice thickness, and so forth) relative to system
coordinates or by reference to unique test object features. Slice planes can be annotated with respect to a preview radiogram, and
8.1.4.4 System performance monitoring by recording the results of the prescribed CT system performance monitoring tests, as
set forth in Section 10, at the beginning and end of a series of CT examinations, not to exceed the interval set forth in 8.1.2.6 for
system performance monitoring.
8. CT System Setup and Optimization Inspection Technique Requirements
9.1 CT Setup—In addition to the required flaw sensitivity, an examination setup should consider the expected distribution of
anomalies, an acceptable rate of false negatives (that is, passed defects) and an acceptable rate of false positives (normal data
mistaken for an anomaly). The following attributes should be considered when developing a CT setup for a group of test objects:
9.1.1 Specimen (size, weight, and composition factors that determine the source accelerating potential and the mechanical
handling equipment requirements),
9.1.2 Examination requirements (spatial resolution, contrast sensitivity, slice thickness, time),
9.1.3 System operation (system control, safety, calibration functions, scanning procedure),
9.1.4 Interaction with program flow (for example, concurrent data acquisition and review, automatic acquisition sequencing,
archiving, automatic anomaly recognition, data output for statistical process control), and
9.1.5 Part handling (logistics for loading and unloading the test object and the design and use of any associated fixturing.
9.2 Source Setup—Caution is advised against applying practices developed for projection radiography. Except at very high
energies, mass attenuation differences between materials (signal contrasts) tend to decrease as the mean X-ray energy is increased;
whereas, X-ray production and penetrability (signal levels) tend to increase under the same condition. Therefore, the optimum
source energy for a given part is not determined by the lowest possible X-ray energy that provides adequate penetration but rather
by the X-ray energy that produces the maximum signal-to-noise ratio (SNR). When a part consists of a single material or several
materials with distinct physical density differences, the best SNR may be obtained at a high source energy. In such cases, the
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decreased image noise at higher energies is more important than the increased contrast at lower energies. When chemically
different components have the same or similar physical densities, the best discrimination of materials may be obtained at a low
source energy. In such cases, the increased contrast at lower energies is more important than the decreased image noise at higher
energies.
9.2.1 Unless suitable measures are taken to reduce the effects of scattered radiation, it will reduce contrast over the whole image,
or parts of it, and produce beam hardening artifacts. Scattered radiation is most serious for materials and thicknesses that have high
X-ray absorption, because the scattering is more significant compared to the primary image-forming radiation that reaches the
detector through the specimen.
9.2.2 Source collimation can limit the cross section of an X-ray beam to cover only the area of the test object that is of interest
in the examination. This reduces the radiation dose to the object and the amount of scattered radiation produced.
9.2.3 A radiation source often contains X-rays of differing energies. The use of source filtration will preferentially remove the
low-energy content of the X-ray spectrum. However, filtration decreases the total number of photons, which reduces the amount
of available signal and may increase the noise in the image. A tradeoff is clearly required, and some filtration generally is found
to be useful. The amount of filtration depends on the source spectrum and the nature and size of the test object. Filtration can be
mounted near the source or the detector. Filters are generally used to combat beam hardening artifacts (see 9.5.1). The influence
of scattered radiation can be addressed with filtration by reducing the number of more readily scattered low-energy photons.
Filtration used to reduce scattered radiation is typically more effective if placed in front of the dectector as opposed to placement
at the source.
9.3 Spatial Resolution—The spatial resolution of a CT system is a function of the source focal spot size, the width of any
detector apertures (linear detector arrays), and the source-to-detector and source-to-center of rotation distances. Many CT systems
permit the spatial resolution to be adjusted by allowing the user some degree of control over some or all of these parameters. Refer
to Guide E1441 for a more thorough discussion of the interactions between these different variables. The mechanical accuracy of
the positioning subsystem also can limit spatial resolution but the supplier of CT services typically has no control over this aspect
of the system operation.
9.3.1 Test object positioning can affect spatial resolution. Because of the extended sizes of the source spot and the active
detection elements, the effective width of a measurement ray varies along its path from source to detector. This is reflected in a
variation with object position of spatial resolution in images computed from measurements with such rays. The simplest
approximation to the minimum effective ray width for a source spot size S and a detector active aperture size A separated by a
distance L is approximately AS/(A + S), and occurs at a location LS/(A + S) from the source.
NOTE 1—If source and aperture differ substantially in size, this minimum is located close to the smaller; this is the case for a microfocus source and
for high resolution detector systems. Optimal spatial resolution can usually be obtained by placing the object as near as possible to this position, but
different tasks and object sizes should be checked experimentally.
NOTE 2—The best placement for spatial resolution may not be optimal for efficient use of detectors or for such other considerations as scatter sensitivity.
8.1 Contrast Sensitivity—Contrast sensitivity is affected by the noise in an image and is a strong function of the total number
of photons detected. Most CT systems permit the contrast sensitivity to be adjusted by allowing some degree of control over
parameters affecting the number of detected photons. At a given energy, the most important factors are: (CT systems shall be
qualified by the CEO for a particular application and product acceptance. The parameters specified1) source intensity, ( during
qualification shall2) the integration/counting time allowed for each individual measurement, ( be used to develop the inspection
techniques and procedure for production inspection. It shall3) be the size of the detector resolution aperture (single detector or
linear detector array), (responsibility of the user NDT facility to develop a workable examination technique recorded4) the size
of the detector slice thickness aperture (linear detector array), ( as a written procedure that is capable of consistently producing
the desired results 5) the source-to-detector distance, and (and quality6) the amount of filtration employed. Refer to Guide level.
When required by contract or purchase order, E1441 for a more thorough discussion of the interactions between these different
variables.the procedure shall be submitted to the CEO for approval.
9.4.1 Contrast sensitivity is also a function of the energy of the photons comprising the X-ray beam. For a fixed number of X-ray
photons incident on a uniform composition object, the contrast sensitivity would generally be best if they have an energy which
typically gives 13 % transmission (that is, where the typical product of thickness and linear attenuation coefficient equals two). This
value is the result of the balance between less relative contrast at higher transmissions and more noise at lower transmissions. This
exact result depends on the restrictions stated (fixed number of photons, uniform object composition, modest dynamic range), and
should not be applied blindly to other situations.
9.4.1.1 The optimal acceleration voltage for CT contrast sensitivity, for CT images made with X-ray generators, is not a simple
calculation. Because a given current in a X-ray generator at a voltage produces more photons at all energies (up to the end-point
energy) than would the same current at a lower voltage, there is a potential for better results at the highest voltage possible.
Whether this potential is realized in a particular case depends on whether the advantages of greater photon production efficiency
will be overcome by the lower current typically required to meet wattage limits for a given spot size, or by saturation effects in
the detection system. Different results have been reported for different systems and examination tasks; users should rely on tests
if they wish to determine the optimal voltage for a particular examination. Because of substantial differences in detection
characteristics, experience with X-ray film radiography should not be used to predict optimal settings for CT examinations.
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9.5 Image Artifacts—Artifact content is one of the more difficult aspects of image quality to control or quantify. Artifacts can
be viewed as correlated noise because they form fixed patterns under given conditions and are often the limiting factor in image
quality. Mitigating their effects is best done by removing or reducing the cause that gave rise to them, a task that in many instances
may not be feasible or practical. In some cases, it may be possible to reduce artifacts through the application of specialized
software. Refer to Guide E1441 for a more thorough discussion (also see 10.7). The use of special procedures or software, or both,
to verify the existence (or absence) of artifacts or reduce the influence of artifacts on
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