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1、NEMA Standards Publication MS 12-2006 Quantification and Mapping of Geometric Distortion for Special Applications Published by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 www.nema.org 2006 by the National Electrical Manufacturers Association. A
2、ll rights, including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Protection of Literary and Artistic Works, and the International and Pan American Copyright Conventions. NOTICE AND DISCLAIMER The information in this publication wa
3、s considered technically sound by the consensus of persons engaged in the development and approval of the document at the time it was developed. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this document. NEMA standards
4、and guideline publications, of which the document contained herein is one, are developed through a voluntary consensus standards development process. This process brings together volunteers and/or seeks out the views of persons who have an interest in the topic covered by this publication. While NEM
5、A administers the process and establishes rules to promote fairness in the development of consensus, it does not write the document and it does not independently test, evaluate, or verify the accuracy or completeness of any information or the soundness of any judgments contained in its standards and
6、 guideline publications. NEMA disclaims liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on this document. NEMA di
7、sclaims and makes no guaranty or warranty, express or implied, as to the accuracy or completeness of any information published herein, and disclaims and makes no warranty that the information in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee
8、 the performance of any individual manufacturer or sellers products or services by virtue of this standard or guide. In publishing and making this document available, NEMA is not undertaking to render professional or other services for or on behalf of any person or entity, nor is NEMA undertaking to
9、 perform any duty owed by any person or entity to someone else. Anyone using this document should rely on his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Information and oth
10、er standards on the topic covered by this publication may be available from other sources, which the user may wish to consult for additional views or information not covered by this publication. NEMA has no power, nor does it undertake to police or enforce compliance with the contents of this docume
11、nt. NEMA does not certify, test, or inspect products, designs, or installations for safety or health purposes. Any certification or other statement of compliance with any health or safety related information in this document shall not be attributable to NEMA and is solely the responsibility of the c
12、ertifier or maker of the statement. MS 12-2006 Page i Copyright 2006 by the National Electrical Manufacturers Association. CONTENTS Page Preambleii Foreword.iii Rationale.iv Scope v Section 1 DEFINITIONS1 1.1SPECIFICATION VOLUME.1 1.2REFERENCE POSITION1 1.3CHARACTERIZATION VOLUME1 1.4CHARACTERIZATIO
13、N AREA.1 1.5IMAGE ARTIFACT 1 1.6IMAGE DISTORTION1 1.7PHANTOM.1 Section 2 METHODS OF MEASUREMENT2 2.1TEST HARDWARE .2 2.1.1MR Characteristics of the Signal Producing Volume2 2.1.2Signal Producing Volume2 2.1.3Construction of the Signal Producing Volume 2 2.2SCAN CONDITIONS.4 2.2.1Reference Position a
14、nd Acquisition Orientation5 2.3MEASUREMENT PROCEDURE.5 2.3.1Measurement Procedure Hints and Tips 6 Section 3 REPORTING OF RESULTS7 3.1DATA REDUCTION.7 3.1.1Spatial Mapping 7 3.1.2Scatter Plots9 3.1.3Error Table 9 3.1.4Data Acquisition Parameters.10 3.2SOURCES OF ERROR.11 3.3REFERENCES11 ANNEX A Poly
15、nomial Analysis of the Displacement Errors in Magnetic Resonance Imaging12 ? MS 12-2006 Page ii Copyright 2006 by the National Electrical Manufacturers Association. Preamble This is one of a series of test standards developed by the medical diagnostic imaging industry for the measurement of performa
16、nce parameters governing image quality of magnetic resonance (MR) imaging (MRI) systems. These test standards are intended for the use of equipment manufacturers, testing houses, prospective purchasers, and users alike. Manufacturers are permitted to use these standards for the determination of syst
17、em performance specifications. This standardization of performance specifications is of benefit to the prospective equipment purchaser. The parameters supplied with each NEMA measurement serve as a guide to those factors that can influence the measurement. These standards can also serve as reference
18、 procedures for acceptance testing and periodic quality assurance. It must be recognized, however, that not all test standards lend themselves to measurement at the installation site. Some test standards require instrumentation better suited to factory measurements, while others require the faciliti
19、es of an instrumentation laboratory to assure stable test conditions necessary for reliable measurements. The NEMA test procedures are carried out using the normal clinical operating mode of the system. For example, standard calibration procedures, standard clinical sequences, and standard reconstru
20、ction processes shall be used. No modifications to alter test results shall be used unless otherwise specified in these standards. The NEMA Magnetic Resonance Section has identified a set of key magnetic resonance image quality parameters. This standards publication describes the measurement of one
21、of these parameters. Equivalence It is intended and expected that manufacturers or others who claim compliance with these NEMA standard test procedures for the determination of image quality parameters shall have carried out the tests in accordance with the procedures specified in the published stan
22、dards. In those cases where it is impossible or impractical to follow the literal prescription of a NEMA test procedure, a complete description of any deviation from the published procedure must be included with any measurement claimed equivalent to the NEMA standard. The validity or equivalence of
23、the modified procedure will be determined by the reader. Uncertainty of the Measurements The measurement uncertainty of the image quality parameter determined using this standards publication is to be reported, together with the value of the parameter. Justification for the claimed uncertainty limit
24、s shall also be provided by a listing and discussion of sources and magnitudes of error. ? MS 12-2006 Page iii Copyright 2006 by the National Electrical Manufacturers Association. Foreword This standards publication is classified as a NEMA standard unless otherwise noted. It describes a method for e
25、valuating the geometric distortion characteristics throughout a specified imaging volume of a Magnetic Resonance Imaging (MRI) system. The equipment contribution to geometric distortion in MRI systems is largely due to imperfections of the main magnetic field and the spatially encoding gradient subs
26、ystem. In addition, the object to be imaged by the MRI system may also induce magnetic field distortions that geometrically distort the image representation of the object to a lesser or greater extent than the MRI system imperfections, depending upon the object and scanning parameters. Since geometr
27、ic distortion is spatially variant, it is important to understand the spatial distribution of error when MR images are used quantitatively. The purpose of this procedure is to provide a standard means for measuring and reporting the geometric distortion characteristics of an MRI system. Clinically,
28、this information is helpful in matching MR scanner characteristics to clinical requirement, when geometric accuracy is crucial (e.g., image guided interventions.) This information is also helpful in evaluating the impact of system changes on performance, for quality control programs that seek to con
29、tinually reaffirm system performance, or in demonstrating effectiveness for FDA applications. The measurement methods have not been designed for compatibility with existing NEMA methods but some of the methods for reporting described in this standard may be compatible with data acquired for MS 2, “D
30、etermination of Two-Dimensional Geometric Distortion in Diagnostic Magnetic Resonance Images.” Evaluations are performed on images generated using standard clinical scan protocols. This standards publication is intended for use by MRI system manufacturers, testing houses, manufacturers of accessory
31、equipment, and MRI end users. This standards publication has been developed by the Magnetic Resonance Section of the National Electrical Manufacturers Association. User needs have been considered throughout the development of this publication. Proposed or recommended revisions should be submitted to
32、: Vice President, Engineering Department National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 Section approval of the standard does not necessarily imply that all section members voted for its approval or participated in its development. At the time it w
33、as approved, the section was composed of the following members: Computer Imaging Reference SystemsNorfolk, VA Fonar CorporationMelville, NY GE Healthcare, Inc.Milwaukee, WI Hitachi Medical Systems America, Inc.Twinsburg, OH Invivo Corp.Gainesville, FL Philips Medical Systems North AmericaBothell, WA
34、 Siemens Medical Solutions, Inc.Malvern, PA Toshiba America Medical SystemsTustin, CA ? MS 12-2006 Page iv Copyright 2006 by the National Electrical Manufacturers Association. Rationale Magnetic Resonance (MR) image formation is based, in part, on the ability to impose varying magnetic field strengt
35、hs at different locations within the imaging volume. These magnetic field variations change the precession frequency of the nuclei being imaged and are the basis of the MR image spatial localization process. The linear relationship of precession frequency with magnetic field strength permits the det
36、ermination of signal source location. Any mechanism that distorts the magnetic field will therefore introduce a spatial location error in the final image. The dominant equipment error sources are the inhomogeneity of the main magnetic field and the nonlinear characteristics of the spatially encoding
37、 gradient magnetic fields. In addition, the object to be imaged may also alter the magnetic field, thus creating spatial errors that may exceed the hardware induced errors in certain situations. As the accuracy of spatial information in MR images becomes more important, e.g. for image guided procedu
38、res, quantification of tumour position and volume, co-registration of images from different modalities etc., it becomes necessary to quantify these errors. For example, the geometric accuracy of spatial information is important for image-guided procedures when the intervention is not based on real-t
39、ime MR image guidance. Spatial geometric accuracy is also important if the MR images are being used to guide external beam radiation treatment planning because it is important to size the radiation beams appropriately and direct the radiation accurately. Additionally, if treatment progression is qua
40、ntified by volume measurements, it is important to understand how geometric distortion changes the perceived volume. Lastly, co-registration of images from other modalities with MR images improves with decreased geometric distortion in the MR image. This standard also has secondary benefits, such as
41、 quantifying the degree of gradient non-linearity and its impact on various quantitative measures, such as Apparent Diffusion Coefficient (ADC) measurements, where gradient non-linearity may introduce undesirable spatial non-uniformities in ADC images and phase contrast MRI where gradient non-linear
42、ities introduce flow velocity errors. Another secondary benefit of this standard is the ability to visualize the homogeneity of the main field by imaging the test phantom at extremely low imaging bandwidths when gradient non-linearity errors are dominated by main field inhomogeneity errors. ? MS 12-
43、2006 Page v Copyright 2006 by the National Electrical Manufacturers Association. Scope This standards publication defines test methods for measuring the absolute spatial variation of geometric accuracy within MR images. This standard presents the absolute geometric accuracy as a map, graph, or table
44、 throughout the imaging region rather than as simple figures of merit such as average or worst case error. Specifying both the acquisition and data presentation methods is the key function of this standard because the results are not easily reduced to a few simple figures of merit; the results are s
45、patial in nature. This standard deals exclusively with absolute error measurements because it is assumed the end user will need geometric distortion error measurements in absolute versus relative terms. While the intent of this standard is to quantify equipment induced geometric errors only, the pha
46、ntom used for these measurements will also introduce some geometric errors. It is not possible to remove the phantom induced errors within the scope of this standard, and this standard assumes that the measured errors are exclusively equipment errors. Therefore it is necessary for the user of this s
47、tandard to be able to differentiate between geometric errors due to the MR imaging system and errors that arise from measuring geometric distortion with a test object. The user should attempt to estimate the error the phantom introduces for the specific test conditions used. This standard also recog
48、nizes that these measurements are ideally performed with three dimensional acquisitions and large volume phantoms, but the cost, weight, and size of the required phantom may be prohibitive in certain situations. Therefore, this standard permits the use of a substantially two dimensional phantom in c
49、onjunction with a set of two dimensional image acquisitions in different orientations. It is recognized that the use of a two dimensional phantom will fundamentally undersample the three dimensional spatial error map. These procedures could also be helpful in evaluating the impact of system changes on performance, for quality control programs that seek to continually reaffirm system performance, or in demonstrating effectiveness for FDA applications. However, this standard does not supercede MS 2, “Determination of Two-D