1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA MS 12-2016 Quantification and Mapping of Geometric Distortion for Special ApplicationsNEMA Standards Publication MS 12-2016 Quantification and Mapping of Geometric Distortion for Special Applications Published by: National E
2、lectrical Manufacturers Association 1300 North 17thStreet, Suite 900 Rosslyn, Virginia 22209 www.nema.org 2016 National Electrical Manufacturers Association. All rights including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Protect
3、ion of Literary and Artistic Works, and the International and Pan American Copyright Conventions. NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the document at the time it was develope
4、d. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this document. NEMA standards and guideline publications, of which the document contained herein is one, are developed through a voluntary consensus standards development p
5、rocess. 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 NEMA administers the process and establishes rules to promote fairness in the development of consensus, it does not write the document and it doe
6、s not independently test, evaluate, or verify the accuracy or completeness of any information or the soundness of any judgments contained in its standards and guideline publications. NEMA disclaims liability for any personal injury, property, or other damages of any nature whatsoever, whether specia
7、l, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on this document. NEMA disclaims and makes no guaranty or warranty, express or implied, as to the accuracy or completeness of any information published herein, and dis
8、claims 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 the performance of any individual manufacturer or sellers products or services by virtue of this standard or guide. In publishing and making
9、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 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
10、or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Information and other standards on the topic covered by this publication may be available from other sources, which the user may wish to consult for additional v
11、iews 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 document. NEMA does not certify, test, or inspect products, designs, or installations for safety or health purposes. Any certification or other stat
12、ement 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 certifier or maker of the statement. NEMA MS 12-2016 Page i 2016 National Electrical Manufacturers Association CONTENTS Preamble ii Foreword .
13、iii Rationale . iv Scope v Section 1 Definitions . 1 1.1 Specification Volume . 1 1.2 Reference Position 1 1.3 Characterization Volume . 1 1.4 Characterization Area 1 1.5 Image Artifact 1 1.6 Image Distortion 1 1.7 Phantom 1 Section 2 Methods of Measurement . 3 2.1 Test Hardware . 3 2.1.1 MR Charact
14、eristics of the Signal Producing Volume 3 2.1.2 Signal Producing Volume 3 2.1.3 Construction of the Signal Producing Volume 3 2.2 Scan Conditions 5 2.2.1 Reference Position and Acquisition Orientation 6 2.3 Measurement Procedure . 7 2.3.1 Measurement Procedure Hints and Tips 7 Section 3 Reporting of
15、 Results . 9 3.1 Data Reduction 9 3.1.1 Spatial Mapping 9 3.1.2 Scatter Plots 11 3.1.3 Error Table 11 3.1.4 Data Acquisition Parameters . 12 3.2 Sources of Error 13 3.3 References 13 Annex A Polynomial Analysis of the Displacement Errors in Magnetic Resonance Imaging 14 Annex B Changes to Standard .
16、 15 NEMA MS 12-2016 Page ii 2016 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 performance parameters governing image quality of magnetic resonance (MR) imaging (MRI) system
17、s. 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 system performance specifications. This standardization of performance specifications is o
18、f 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 procedures for acceptance testing and periodic quality assurance. It must be recogniz
19、ed, 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 facilities of an instrumentation laboratory to ensure stable test conditions necessary for rel
20、iable 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 reconstruction processes shall be used. No modifications to alter test results shall be used un
21、less 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 of these parameters. Equivalence It is intended and expected that manufacturers or oth
22、ers 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 standards. In those cases where it is impossible or impractical to follow the literal pres
23、cription 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 the modified procedure will be determined by the reader. Uncertainty of the Measuremen
24、ts 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 limits shall also be provided by a listing and discussion of sources and magnitudes of erro
25、r. NEMA MS 12-2016 Page iii 2016 National Electrical Manufacturers Association Foreword This standards publication is classified as a NEMA standard unless otherwise noted. It describes a method for evaluating the geometric distortion characteristics throughout a specified imaging volume of a Magneti
26、c 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 subsystem. In addition, the object to be imaged by the MRI system may also induce magnetic field distorti
27、ons 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 geometric distortion is spatially variable, it is important to understand the spatial distribution of error
28、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, this information is helpful in matching MR scanner characteristics to clinical requirements, when ge
29、ometric 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 continually reaffirm system performance, or in demonstrating effectiveness for FDA applications. The m
30、easurement 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, Determination of Two-Dimensional Geometric Distortion in Diagnostic Magnetic Resonance Images. Evalu
31、ations 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 equipment, and MRI end users. This standards publication has been developed by the Magnetic Resonanc
32、e 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: Executive Director, Medical Imaging the results are spatial in nature. This standard deals exclusi
33、vely 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 phantom used for these measurements will also in
34、troduce 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 standard to be able to differentiate between
35、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 recognizes that these measurements are ideally pe
36、rformed 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 conjunction with a set of two-dimensional ima
37、ge 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
38、 that seek to continually reaffirm system performance, or in demonstrating effectiveness for FDA applications. However, this standard does not supersede NEMA MS 2 Determination of Two-Dimensional Geometric Distortion in Diagnostic Magnetic Resonance Images. MS 2 is designed to produce simple figures
39、 of merit that describe basic geometric distortions, or image field of view errors, that could arise from imaging gradient amplitude scaling errors. NEMA MS 12-2016 Page vi 2016 National Electrical Manufacturers Association NEMA MS 12-2016 Page 1 2016 National Electrical Manufacturers Association Se
40、ction 1 Definitions 1.1 Specification Volume The specification volume is the imaging volume within which a manufacturer guarantees image performance specifications. Images or portions of images outside this volume will not necessarily meet performance specifications, but may still be useful for diag
41、nostic or image guided purposes. For head scans, the specification volume must enclose, as a minimum, a 10 cm diameter spherical volume (dsv) centered in the RF head coil. For body scans, the specification volume must enclose a 20 cm dsv centered in the RF body coil. 1.2 Reference Position The refer
42、ence position is a well-defined point within the nearest measurable phantom element to magnet isocenter. The reference position must not be more than 1.5 cm from magnet isocenter (half of the element spacing). The reference position within the reference element will vary depending on the design of t
43、he phantom. The user may define the reference position as the middle of a discrete element, or the middle of the intersection of continuous elements, or any of the four corners created by intersecting continuous elements, or some other consistent, well-defined point. 1.3 Characterization Volume The
44、characterization volume is the intersection of the image volume and the specification volume and shall include the reference position. This implies that the phantom used to acquire the spatial distortion measurements is a three dimensional object. 1.4 Characterization Area The characterization area
45、is the intersection of the image plane and the specification volume, and it shall include the reference position, unless otherwise specifically noted. This implies that the phantom used to acquire the spatial distortion measurements is a planar two-dimensional object with sufficient size in the thir
46、d dimension to fill no more than the thickness of the imaging plane. In the case of a three-dimensional object and threedimensional image acquisitions, an arbitrarily extracted slice can also be used. 1.5 Image Artifact An image artifact is an image anomaly (excluding random noise) that is not repre
47、sentative of the structure or chemistry of the object being scanned, or, that is derived from the structure or chemistry of the object being scanned but which appears in the image at a location other than expected. 1.6 Image Distortion Image distortion is the spatial deviation of an arbitrary point
48、in the imaging volume from its expected true location. Image distortion will be specified as absolute (magnitude or signed magnitude) quantity. 1.7 Phantom A combination of signal producing and non-signal producing materials, used for MR image testing purposes. Two-dimensional phantoms have large ex
49、tent in two dimensions (much larger than one pixel in plane) and have limited extent in the third dimension, approximately equal to the slice thickness. NEMA MS 12-2016 Page 2 2016 National Electrical Manufacturers Association Three-dimensional phantoms have large extent in all three dimensions (much larger than one pixel in plane and much larger than nominal thickness in the orthogonal direction). NEMA MS 12-2016 Page 3 2016 National Electrical Manufacturers Association Section 2 Methods of Measurement 2.1 Test Hardware 2.1.1 MR Characteristics of the Signal Producing Volume
