1、 NEMA MS 12 QUANTIFICATION AND MAPPING OF GEOMETRIC DISTORTION FOR SPECIAL APPLICATIONS NEMA Standards Publication MS 12-2010 Quantification and Mapping of Geometric Distortion for Special Applications Published by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Ros
2、slyn, VA 22209 www.nema.org 2010 by the National Electrical Manufacturers Association. All 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 Amer
3、ican 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 developed. Consensus does not necessarily mean that there is unanimous agreeme
4、nt 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 process. This process brings together volunteers and/or seeks out the v
5、iews 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 does not independently test, evaluate, or verify the accuracy or complete
6、ness 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 special, indirect, consequential, or compensatory, directly or indirectly re
7、sulting 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 disclaims and makes no warranty that the information in this document wil
8、l 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 this document available, NEMA is not undertaking to render professiona
9、l 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 or, as appropriate, seek the advice of a competent professional in det
10、ermining 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 views or information not covered by this publication. NEMA has no power
11、, 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 statement of compliance with any health or safetyrelated information in th
12、is document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement. MS 12-2010 Page i CONTENTS Page Preambleii Foreword .iii Rationale .iv Scope v Section 1 DEFINITIONS . 1 1.1 Specification Volume . 1 1.2 Reference Position 1 1.3 Characterizati
13、on 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 Characteristics of the Signal Producing Volume 3 2.1.2 Signal Producing Volume 3 2.1.3 Construction of the Signal Producing Volume 3 2
14、.2 Scan Conditions 5 2.2.1 Reference Position and Acquisition Orientation 6 2.3 Measurement Procedure. 6 2.3.1 Measurement Procedure Hints and Tips 7 Section 3 REPORTING OF RESULTS. 8 3.1 Data Reduction 8 3.1.1 Spatial Mapping 8 3.1.2 Scatter Plots 10 3.1.3 Error Table 10 3.1.4 Data Acquisition Para
15、meters. 11 3.2 Sources of Error 12 3.3 References 12 ANNEX A POLYNOMIAL ANALYSIS OF THE DISPLACEMENT ERRORS IN MAGNETIC RESONANCE IMAGING 13 ANNEX B CHANGES TO STANDARD. 14 Copyright 2010 by the National Electrical Manufacturers Association. MS 12-2010 Page ii Preamble This is one of a series of tes
16、t 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, testing houses, prospective purchasers, a
17、nd 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. The parameters supplied with each NEMA measurement serve as a guide
18、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. Some test standards
19、require instrumentation better suited to factory measurements, while others require the facilities of an instrumentation laboratory to ensure stable test conditions necessary for reliable measurements. The NEMA test procedures are carried out using the normal clinical operating mode of the system. F
20、or 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 set of key magnetic
21、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 quality parameters sh
22、all 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 procedure must be includ
23、ed 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 publication is to be rep
24、orted, 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. Copyright 2010 by the National Electrical Manufacturers Association. MS 12-2010 Page iii Foreword This standards pu
25、blication 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 Magnetic Resonance Imaging (MRI) system. The equipment contribution to geometric distortion in MRI systems is la
26、rgely 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 distortions that geometrically distort the image representation of the object to a lesser or greater extent than
27、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 when MR images are used quantitatively. The purpose of this procedure is to provide a standard means for
28、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 geometric accuracy is crucial (e.g., image guided interventions.) This information is also helpful in evalu
29、ating 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 measurement methods have not been designed for compatibility with existing NEMA methods but some of the me
30、thods 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. Evaluations are performed on images generated using standard clinical scan protocols. This standards publicatio
31、n 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 Resonance Section of the National Electrical Manufacturers Association. User needs have been considered throughout
32、 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 exclusively with absolute error measurements because it is assumed the end user will need geometric distortion er
33、ror 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 introduce some geometric errors. It is not possible to remove the phantom induced errors within the scope of
34、 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 geometric errors due to the MR imaging system and errors that arise from measuring geometric distortion wi
35、th 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 performed with three dimensional acquisitions and large volume phantoms, but the cost, weight, and size of t
36、he 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 image acquisitions in different orientations. It is recognized that the use of a two dimensional phantom will
37、 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 applicati
38、ons. However, this standard does not supercede MS 2, Determination of Two-Dimensional Geometric Distortion in Diagnostic Magnetic Resonance Images. Standard MS 2 is designed to produce simple figures of merit that describe basic geometric distortions, or image field of view errors, that could arise
39、from imaging gradient amplitude scaling errors. Copyright 2010 by the National Electrical Manufacturers Association. MS 12-2010 Page vi Copyright 2010 by the National Electrical Manufacturers Association. MS 12-2010 Page 1 Section 1 DEFINITIONS 1.1 SPECIFICATION VOLUME The specification volume is th
40、e 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 diagnostic or image guided purposes. For head scans, the specification volume m
41、ust 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 reference position is a well defined point within the nearest measurable phantom
42、 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 the phantom. The user may define the reference position as the middle of a d
43、iscrete 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 characterization volume is the intersection of the image volume and the spe
44、cification 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 is the intersection of the image plane and the specification volume and it
45、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 third dimension to fill no more than the thickness of the imaging plane. 1.5 IMA
46、GE ARTIFACT An image artifact is an image anomaly (excluding random noise) that is not representative 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 exp
47、ected. 1.6 IMAGE DISTORTION Image distortion is the spatial deviation of an arbitrary point 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 pro
48、ducing materials, used for MR image testing purposes. Two dimensional phantoms have large extent in two dimensions (much larger than one pixel in plane) and have limited extent in the third dimension, approximately equal to the slice thickness. Copyright 2010 by the National Electrical Manufacturers
49、 Association. MS 12-2010 Page 2 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). Copyright 2010 by the National Electrical Manufacturers Association. MS 12-2010 Page 3 Section 2 METHODS OF MEASUREMENT 2.1 TEST HARDWARE 2.1.1 MR Characteristics of the Signal Producing Volume The following are the MR characteristics of the signal producing volume: T150 milliseconds (at operating field strength) Spin density = density of H2O 20%. Single peak NMR spectrum.
