1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA MS 2-2008 (R2014)Determination of Two-DimensionalGeometric Distortion inDiagnostic MagneticResonance ImagesNEMA Standards Publication MS 2-2008 (R2014) Determination of Two-Dimensional Geometric Distortion in Diagnostic Magn
2、etic Resonance Images Published by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 900 Rosslyn, Virginia 22209 www.nema.org 2008 National Electrical Manufacturers Association. All rights, including translation into other languages, reserved under the Universal Copyright
3、Convention, the Berne Convention for the Protection of Literary and Artistic Works, and the International and Pan American copyright conventions. 2008 National Electrical Manufacturers Association NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the conse
4、nsus 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. The National Electrical Manufacturers Association (NEMA) s
5、tandards 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.
6、While NEMA 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 stan
7、dards and 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
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9、o guarantee 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 und
10、ertaking to 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. Informat
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12、this document. 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 responsibil
13、ity of the certifier or maker of the statement. MS 2-2008 (R2014) Page i 2008 National Electrical Manufacturers Association CONTENTS Page Preamble ii Foreword . iii Rationale . iv Scope v History . v Section 1 DEFINITIONS 1 1.1 Specification Volume . 1 1.2 Specification Area 1 Section 2 METHODS OF M
14、EASUREMENT . 2 2.1 Phantom Design 2 2.2 Scan Conditions 2 2.3 Measurement Procedure . 2 Example 1 3 Example 2 4 Example 3 5 Section 3 REPORTING OF RESULTS . 6 3.1 Reporting of Two-Dimensional Geometric Distortion 6 MS 2-2008 (R2014) Page ii 2008 National Electrical Manufacturers Association Preamble
15、 This is one of a series of test standards developed by the medical diagnostic imaging industry for the measurement of performance parameters governing image quality of magnetic resonance (MR) imaging (MRI) systems. These test standards are intended for the use of equipment manufacturers, prospectiv
16、e purchasers, and users alike. Manufacturers are permitted to use these standards for the determination of system performance specifications. This standardization of performance specifications is of benefit to the prospective equipment purchaser, and the parameters supplied with each NEMA measuremen
17、t serve as a guide to those factors that can influence the measurement. These standards can also serve as reference 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. S
18、ome test standards require instrumentation better suited to factory measurements, while others require the facilities 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 mo
19、de of the system. For example, standard calibration procedures, standard clinical sequences, and standard reconstruction 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 s
20、et of key magnetic resonance image quality parameters. This standards publication describes the measurement of one 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 qu
21、ality parameters shall have carried out the tests in accordance with the procedures specified in the published standards. 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 proce
22、dure must be included with any measurement claimed equivalent to the NEMA standard. The validity or equivalence of 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 publi
23、cation is to be reported, together with the value of the parameter. Justification for the claimed uncertainty limits shall also be provided by a listing and discussion of sources and magnitudes of error. MS 2-2008 (R2014) Page iii 2008 National Electrical Manufacturers Association Foreword This stan
24、dards publication is classified as a NEMA standard unless otherwise noted. It is intended for use by MRI system manufacturers, manufacturers of accessory equipment (including special purpose gradient coils), and MRI end users. The purpose of this procedure is to provide a standard means for measurin
25、g and reporting two-dimensional geometric distortion in an MRI system. Two-dimensional geometric distortion is defined here as the maximum percent difference between measured distances in an image and the actual corresponding phantom dimensions. Radial measurements, i.e., between points spanning the
26、 geometric center of the test object, are used to characterize the geometric distortion. Measurements will be evenly spaced with an angular separation less than or equal to 45 in order to sample sufficiently the angular variation of geometric distortion. This standards publication has been developed
27、 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: Vice President, Technical Services National Electrical Manufacturers Asso
28、ciation 1300 North 17th Street, Suite 900 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 was approved, the section was composed of the following members: Computer Imaging
29、 Reference SystemsNorfolk, VA Confirma, Inc.Bellevue, WA GE Healthcare, Inc.Milwaukee, WI Hitachi Medical Systems America, Inc.Twinsburg, OH Invivo Corporation, Gainesville, FL Medipattern CorporationToronto, ON, Canada Philips HealthcareAndover, MA Siemens Medical Solutions, Inc.Malvern, PA Toshiba
30、 America Medical SystemsTustin, CA MS 2-2008 (R2014) Page iv 2008 National Electrical Manufacturers Association Rationale One of the essential attributes of an MRI scanner is its ability to produce a two-dimensional map, or image, of the nuclear magnetic resonance (NMR) signal as a function of posit
31、ion within the object under study. It is natural, therefore, to seek to determine the extent to which the spatial relationships in the image correspond to the actual spatial relationships in the object. Geometric distortion in a two-dimensional MRI image can be caused by a number of factors, includi
32、ng, but not limited to, inhomogeneity of the main magnetic field (B0), gradient field nonlinearity, gradient amplitude miscalibration, eddy current effects, and magnetic susceptibility effects. A number of specialized techniques have been developed to identify particular sources of geometric distort
33、ion, to quantify and in some cases to correct for the distortions introduced. For example, magnetic field mapping and shimming techniques are essential tools employed by the MR system manufacturer to reduce B0 inhomogeneities to a minimum. Nevertheless, geometric distortion correction algorithms are
34、 sometimes employed in the reconstruction of an image in order to further reduce distortions. Such algorithms are permitted under this standard, so long as they are an integral step in the reconstruction of an image. Two-dimensional geometric distortion in an MR image can be characterized in a numbe
35、r of different ways. The choice of a measurement and reporting method was guided by a desire for computational simplicity and ease of implementation on all MR systems. It was also intended that the results could be obtained in a relatively short time. Consequently, a simple figure of merit was chose
36、n to characterize the two-dimensional geometric distortion likely to be encountered when using a typical clinical pulse sequence. Because readout gradient strength has a direct bearing on the degree of in-plane distortion due to static field inhomogeneity, the imaging bandwidth chosen for this test
37、must accompany the report of results. MS 2-2008 (R2014) Page v 2008 National Electrical Manufacturers Association Scope This standards publication describes a method for determining the maximum percent difference between measured distances in an image and actual corresponding phantom dimensions. The
38、 procedure described evaluates geometric distortion in three orthogonal planes passing through the center of the specification volume. This procedure does not address the question of the absolute positional accuracy of the image of the test object in the frame of reference of the magnet. Only the re
39、lative separation of selected points on the test object is considered. This procedure does not address the measurement of slice flatness or the accuracy of slice positioning. History MS 2-2003 differs from its predecessor MS 2-1989 in the following ways: 1. A Rationale has been added. 2. The Scope o
40、f the standard has been more precisely delineated. 3. The scan conditions explicitly stipulate that the spatial resolution be the same in the frequency- and phase-encoding directions. 4. Geometric distortion correction algorithms are permitted so long as they are an integral step in the reconstructi
41、on of an image. These were not considered in the predecessor standard. 5. Numerous editorial changes have been made to improve readability. MS 2-2008 differs from its predecessor MS 2-2003 in the following way: The definition for “Specification Volume” has been revised. MS 2-2008 (R2014) Page vi 200
42、8 National Electrical Manufacturers Association MS 2-2008 (R2014) Page 1 2008 National Electrical Manufacturers Association Section 1 DEFINITIONS 1.1 SPECIFICATION VOLUME The specification volume is the imaging volume within which a manufacturer guarantees image performance specifications. Images or
43、 portions of images outside this volume may not necessarily meet performance specifications, but may still be useful for diagnostic purposes. For head scans, the specification volume must enclose, as a minimum, a 10 centimeter diameter spherical volume (dsv) centered in the RF head coil. For body sc
44、ans, the specification volume must enclose, as a minimum, a 20-cm dsv centered in the RF body coil. 1.2 SPECIFICATION AREA The specification area is the intersection of the specification volume and the image plane. MS 2-2008 (R2014) Page 2 2008 National Electrical Manufacturers Association Section 2
45、 METHODS OF MEASUREMENT 2.1 PHANTOM DESIGN The phantom shall be designed so that geometric distortion can be measured over the perimeter of the entire specification area. The phantom shall consist of a series of rings, holes, pins, or other defined edges that follow the perimeter of the specificatio
46、n area. The phantom shall be at least twice as thick as the slice thickness used in the measurement to minimize the influence of slice curvature. RF coil loading need not be considered in the design of the phantom since RF coil loading will not affect the distortion measurement. 2.2 SCAN CONDITIONS
47、Image data shall be acquired using a typical clinical pulse sequence and reconstruction process. In this context, a typical clinical pulse sequence is one that might be relied upon for an accurate determination of the dimensions of an anatomical structure or feature. In addition, the following scan
48、conditions shall obtain: 1. Phantom centered in the RF receive coil; 1. 2. Spin echo pulse sequence (first echo); 3. TE (echo time) and TR (repetition time) within the clinically selectable range; 4. Single-slice acquisition centered at isocenter; 5. maximum dimension of the specification area in or
49、der to minimize errors due to large pixel size; 6. 7. Square pixel encoding (identical spatial resolution in frequency and phase). 2.3 MEASUREMENT PROCEDURE The phantom shall be positioned and the data acquired as specified in Section 2.2. Separate scans shall be performed in the three orthogonal planes (axial, sagittal, and coronal) passing through the center of the specification volume. The distance between diametrically opposed pairs of points shall be measured. The phantom shall have at least four pairs of evenly distributed reference points that lie on the pe
