1、 American National Standard for Evaluating the Image Quality of X-ray Computed Tomography (CT) Security-Screening Systems Sponsored by the National Committee on Radiation Instrumentation, N42 IEEE 3 Park Avenue New York, NY 10016-5997 USA 23 May 2011 Accredited by the American National Standards Ins
2、titute ANSI N42.45-2011ANSI N42.45- 2011 American National Standard for Evaluating the Image Quality of X-ray Computed Tomography (CT) Security-Screening Systems Sponsor National Committee on Radiation Instrumentation, N42 Accredited by the American National Standards Institute Secretariat Institute
3、 of Electrical and Electronics Engineers, Inc. Approved 26 January 2011 American National Standards Institute Abstract: Test methods and test articles for the evaluation of the image quality of CT security-screening systems are provided. The quality of data for automated analysis is the primary conc
4、ern. This standard does not address the systems ability to use its image data to automatically detect explosives or other threat materials, which is typically verified by an appropriate regulatory body. Keywords: ANSI N42.25, computed tomography, contraband, detection, explosives, image quality, lug
5、gage, screening, security The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright 2011 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 23 May 2011. Printed in the United States of America. IEEE is
6、 a registered trademark in the U.S. Patent +1-978-750-8400. Permission to photocopy portions of any individual standard for educational classroom use can also be obtained through the Copyright Clearance Center. v Copyright 2011 IEEE. All rights reserved. Introduction This introduction is not part of
7、 ANSI 42.45-2011, American National Standard Evaluating the Image Quality of X-ray Computed Tomography (CT) Security-Screening Systems. This standard is the responsibility of the Accredited American Standards Committee on Radiation Instrumentation, N42. The standard was approved on N42 letter ballot
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15、rent interpretations can be accessed at the following URL: http:/standards.ieee.org/reading/ieee/interp/ index.html. vi Copyright 2011 IEEE. All rights reserved. Patents Attention is called to the possibility that implementation of this standard may require use of subject matter covered by patent ri
16、ghts. By publication of this standard, no position is taken with respect to the existence or validity of any patent rights in connection therewith. The IEEE is not responsible for identifying Essential Patent Claims for which a license may be required, for conducting inquiries into the legal validit
17、y or scope of Patents Claims or determining whether any licensing terms or conditions provided in connection with submission of a Letter of Assurance, if any, or in any licensing agreements are reasonable or non-discriminatory. Users of this standard are expressly advised that determination of the v
18、alidity of any patent rights, and the risk of infringement of such rights, is entirely their own responsibility. Further information may be obtained from the IEEE Standards Association. vii Copyright 2011 IEEE. All rights reserved. Participants At the time it approved this standard, the Accredited S
19、tandards Committee on Radiation Instrumentation, N42, had the following members: Michael P. Unterweger, Chair Michael Kipness, Administrative secretary Organization represented Name of representative CanberraBob Huckins Chew, M.HJack M. Selby Commerce Dept, U.S. NIST .Michael P. Unterweger Computer
20、Dependability Associates, LLC.Gary Johnson Consultant Frank X. Masse Department of Homeland Security. Peter Shebell Entergy-ANO Ron Schwartz Health Physics Soc. Sandy Perle Harvard University Joseph Ring Institute of Electrical any deviations shall be noted by the evaluators in the manually recorded
21、 data. If the system is approved to operate under more than one configuration, the user may want to request the test be carried out at all appropriate settings. Evaluation is to be based on images or other data normally used in standard mode of operation. The exposure time and level shall be chosen
22、as that used when the CT system is operated for the intended use in the security screening application. A calibration of the CT system shall be carried out prior to any testing. Any recalibration of the CT system shall be allowed according to the standard operation of the system. In order to assure
23、that the system is using a configuration approved by the appropriate regulatory agency, the user may wish to request that the vendor provide the specific settings used during the evaluation such as: tube voltage(s), amperage, voxel size, belt speed, etc. The test articles (see Clause 5) shall be pre
24、sented to the system in a controlled position and orientation. The main axis of each test article should be parallel to the conveyor belt motion direction and the front of the article shall enter the system first (front designated via labeling). The test method described in 6.3 determines the angle
25、of rotation and side to side offset of the test articles relative to the center line of the system conveyor. Results for rotations more than 2 degrees off parallel from conveyor center line shall be rejected. This standard requires that the test articles be measured at the center of the belt, direct
26、ly on the belt, within 2 cm of the conveyor center line. If the user decides to also run the test articles off center line, the parallel requirement shall still be met, and the data shall be segregated and treated separately. For reference, Figure 1 shows the coordinate system that shall be used for
27、 all procedure descriptions. The z axis is aligned along the direction of the conveyor motion. The y axis is in the vertical direction and the x axis is across the belt. The positive/negative direction of the axis system is immaterial as used in this standard. Figure 1 Reference axes for procedures
28、Not all the methods stated are applicable to all CT systems. Each method shall identify whether it is applicable to all CT types or only a subset. Each of the test methods specified in this standard can include required procedures and examples of optional techniques for achieving the required result
29、s. This is necessary because of the range of implementations used in security CT equipment. Each test method identifies where latitude for deviation from the analysis techniques exists. Any deviation from provided example techniques shall be documented including rationale for deviating from the sugg
30、ested standard method of analysis. Such documentation shall be provided to the end user of the image quality evaluation. 5. Description of the test articles Execution of this standard requires two test articles. They are designated “test article A” and “test article B,” and are represented in Figure
31、 2 and Figure 3. The articles consist of several test objects supported in a machined frame within a commercial carrying case. The placement of the test objects has been selected to minimize artifacts from one test object interfering with the image of another test object. Additionally, the commercia
32、l cases have been modified to eliminate unnecessary artifacts (steel hinge pins replaced with plastic, some reinforcing structures have been machined off, etc.) When fabricating test articles for use in this standard, the test objects and the supporting structural frame shall be built in accordance
33、with the detailed drawing package included in Annex B. The outer case may be ANSI N42.45-2011 American National Standard for Evaluating the Image Quality of X-ray Computed Tomography (CT) Security-Screening Systems 5 Copyright 2011 IEEE. All rights reserved. selected for durability. It shall be larg
34、e enough to contain the specified support structure. The case shall be modified to remove any metal structures (hinges, handles, fasteners, etc.) from the sides, top and bottom. (Removing structures on the front and back surfaces is optional.) Also, the case shall be modified to remove any significa
35、nt plastic structure along the sides, top, and bottom that might interfere with the imaging of the test object. The outer case shall be outfitted with appropriate low-density foam inserts to hold the test article support structure firmly in place, parallel to the bottom plane of the case and aligned
36、 with its centerline. Figure 2 Test article A Test article A contains the test objects for the methods covered in the following subclauses: 6.3 Object length accuracy 6.4 Path length CT value and Zeff 6.7 Zeffand CT value uniformity 6.8 Streak artifacts 6.10 Image ANSI N42.45-2011 American National
37、Standard for Evaluating the Image Quality of X-ray Computed Tomography (CT) Security-Screening Systems 6 Copyright 2011 IEEE. All rights reserved. Figure 3 Test article B Test article B contains the test objects for the methods covered in the following subclauses: 6.3 Object length accuracy 6.5 Nois
38、e equivalent quanta (NEQ) 6.6 CT value consistency 6.9 Slice sensitivity profile (SSP) 6. Test procedures for image quality 6.1 General This standard specifies procedures for measuring a wide range of image quality indicators. Table 1 provides a list of the test procedures, the image quality indicat
39、ors they measure, and the test object they use. ANSI N42.45-2011 American National Standard for Evaluating the Image Quality of X-ray Computed Tomography (CT) Security-Screening Systems 7 Copyright 2011 IEEE. All rights reserved. Table 1 List of test methods and indicators measured Test procedure Im
40、age quality indicator Test object Test article Object length accuracy Length of objects relative to their expected values Acetalaplate with machined hole in each end A & B Path length CT value and Zeff Consistency of density and Zeffmeasured along a variable x-ray path length Acetal triangle A Noise
41、 equivalent quanta (NEQ) In-plane spatial resolution of the system normalized by the systems noise Acetal cylinder B CT value consistency (a) Measurement of the average CT value for a reference object (b) Variance of CT values within reference object Acetal cylinder B Zeff and CT value uniformity Me
42、asurement of Zeffand CT value through different thicknesses of attenuating metals Acetal cylinder wrapped with layers of aluminum, copper, tin, and lead A Streak artifacts Level of variation in a nominal Zeffmaterial induced by the presence of a high Zeffmaterial in close proximity Acetal cylinder w
43、ith imbedded tungsten alloy pins A Slice sensitivity profile (SSP) Resolution of the image along the direction of belt movement Acetal rectangular bar presented at 5 B Image registration Physical alignment between imaging subsystem frames of reference Acetal and aluminum plates arranged to form a bo
44、x with a diagonal acetal plate A aFULL SPECIFICATION: Use acetal copolymer (also called acetal, polyoxymethylene or POM) designated as ASTM D6100-05 S-POM0211LP. This is a general-purpose grade thermoplastic, with a density near 1.4 g/cm3, and that does not include fillers, impact modifiers or other
45、 additives including colorant, fiber, chemical lubricant, or heavy metals. Before construction of the NEQ test object, the core of the delivered acetal rod should be verified for low centerline porosity using the procedure specified in ASTM D6100-05. In addition to carrying out the cited image quali
46、ty methods, it is important to manually record certain data about the system being tested and the environment of the test. The manual data to be collected is specified in 6.2. The following subclauses discuss each method, detailing how the measurements are to be made. 6.2 Manually recorded data 6.2.
47、1 Purpose Manufacturers may produce a number of models under the same general product name, and they may perform differently under different environmental conditions. When looking at results from this evaluation method, it is important to be sure you are comparing similar machines under similar cond
48、itions. This section specifies the minimum data that shall be manually recorded about the system and the evaluation conditions. 6.2.2 System data The following data are available from the manufacturer of the system, or in some cases, provided by the system itself. It is not intended that the evaluat
49、ion team actually measure any of the data to record for this section. 6.2.2.1 General information a) Test method: State that the evaluation was carried out in accordance with this standard. b) Manufacturer: Record the name of the company producing the system. c) Model: Record the full model name/number for the system. ANSI N42.45-2011 American National Standard for Evaluating the Image Quality of X-ray Computed Tomography (CT) Security-Screening Systems 8 Copyright 2011 IEEE. All rights reserved. d) Serial number: Record the unique serial nu
