1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA MS 6-2008 (R2014)Determination of Signal-to-Noise Ratio andImage Uniformity forSingle-ChannelNon-Volume Coils inDiagnostic MR ImagingNEMA Standards Publication MS 6-2008 (R2014) Determination of Signal-to-Noise Ratio and Ima
2、ge Uniformity for Single-Channel Non-Volume Coils in Diagnostic MR Imaging 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 oth
3、er 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. 2008 National Electrical Manufacturers Association NOTICE AND DISCLAIMER The information in this publ
4、ication 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 agreement among every person participating in the development of this document. The N
5、ational Electrical Manufacturers Association (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 views of persons who have a
6、n 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 completeness of any information or
7、 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 resulting from the publicati
8、on, use of, application, or reliance on this document. NEMA disclaims and makes no guaranty or warranty, expressed 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 pa
9、rticular 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 professional or other services for
10、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 determining the exercise of
11、 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, nor does it undertake
12、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 safety-related information in this document shall not
13、be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement. MS 6-2008 (R2014) Page i 2008 National Electrical Manufacturers Association CONTENTS Page Preamble . iii Foreword . iv Rationale . v Scope vi Section 1 DEFINITIONS 1 1.1 Coil-Related Definitions . 1
14、1.1.1 Sensitive Volume 1 1.1.2 Sensitive Area . 1 1.1.3 Imaging Region of Interest (IROI) . 1 1.1.4 Reference Position 1 1.2 Analysis-Related Definitions 1 1.2.1 Characterization Volume . 1 1.2.2 Characterization Area . 1 1.2.3 Measurement Region of Interest (MROI) 1 1.2.4 Measurement Subregion of I
15、nterest (SROI) . 1 1.3 Phantom-Related Definitions . 1 1.3.1 Signal-Producing Volume (Phantom) 1 1.4 Image-Related Definitions . 2 1.4.1 Image Artifact 2 1.4.2 Image Uniformity/Non-uniformity 2 1.4.3 Baseline Pixel Offset Value . 2 1.4.4 Image Signal . 2 1.4.5 Image Noise 2 1.4.6 Image Signal-to-Noi
16、se Ratio . 2 Section 2 METHODS OF MEASUREMENT 3 2.1 Test Hardware . 3 2.1.1 MR Characteristics of the Signal Producing Volume (Phantom) 3 2.1.2 RF Coil Loading Characteristics . 3 2.1.3 RF Coil and Positioning Device 3 2.2 Selection of Measurement Geometry 3 2.2.1 Selection of the Reference Position
17、 , Characterization Volume and Area . 3 2.2.2 Measurement Region-of-Interest (MROI) . 4 2.2.3 Noise Evaluation Area. 4 2.2.4 Slice Positions . 4 2.3 Scan Conditions 5 2.4 Primary Measurement Procedure for SNR 6 2.5 Alternate Single-Image Measurement Procedure For SNR 7 2.6 Primary Measurement Proced
18、ure for Image Uniformity 7 2.7 Alternate Measurement Procedure for Image Uniformity 8 Section 3 REPORTING OF RESULTS 11 3.1 Reporting of SNR 11 3.1.1 Geometric Information. 11 3.1.2 Data Acquisition Parameters 11 3.1.3 SNR Results 12 MS 6-2008 (R2014) Page ii 2008 National Electrical Manufacturers A
19、ssociation 3.2 Reporting of Uniformity 12 3.2.1 Geometric and Phantom Information 12 3.2.2 Data Acquisition Parameters 12 3.2.3 Uniformity Results . 12 3.3 Uncertainty of Measurements 12 Annex A Changes To Standard . 13 Figures 2-1 Simplest Geometry of Noise Evaluation Area 4 2-2 Complex Geometry of
20、 Noise Evaluation Area 4 2-3 Slice Positions for a Surface Coil . 5 2-4 ROI for Image Uniformity Measurement . 9 MS 6-2008 (R2014) Page iii 2008 National Electrical Manufacturers Association Preamble This is one of a series of test standards developed by the medical diagnostic imaging industry for t
21、he measurement of performance parameters governing image quality of magnetic resonance (MR) imaging systems. These test standards are intended for the use of equipment manufacturers, prospective purchasers, and users alike. Manufacturers are permitted to use these standards for the determination of
22、system performance specifications. This standardization of performance specifications is of benefit to the prospective equipment purchaser, and 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 r
23、eference 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
24、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 mode of the system. For example, standard calibration procedures, standard clinical sequences, and standard r
25、econstruction 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
26、 of two parameters for special purpose single-channel non-volume coils; signal-to-noise ratio (SNR) and uniformity. 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 sha
27、ll 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 include
28、d 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 each image quality parameter determined using this standards publication is to be rep
29、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. MS 6-2008 (R2014) Page iv 2008 National Electrical Manufacturers Association Foreword This standards publication is
30、 classified as a NEMA standard unless otherwise noted. It is intended for use by manufacturers of MRI systems and accessory equipment and by MRI end users. It describes a method for evaluating single-channel non-volume special purpose radio-frequency (RF) coils for use with magnetic resonance (MR) i
31、maging (MRI) systems. These coils are used to receive signal from a limited region of interest. These include linear or quadrature combined surface coils, flexible coils, pairs of coils such as Helmholtz coils, or coils that partially surround a specific tissue such as the calf or other extremity. B
32、oth receive-only and transmit-receive coils are included. The system head and body coils, and single-channel volume specialty coils, are excluded (see MS 1 and MS 3). Also excluded are coils that require multiple receiver channels for operation (array coils, see MS 9). However, if analyzing one chan
33、nel of an array coil, this standard may be used. Independent of intended use, for the purpose of this standard, the coils being analyzed will be called “surface coils.” These coils achieve good signal-to-noise performance because of their increased filling factor; in most cases, however, these coils
34、 have a non-uniform signal distribution. The purpose of this procedure is to provide a standard means for measuring and reporting the signal-to-noise ratio (SNR) and uniformity of signal intensity in images acquired with surface coils. These quantities are helpful in evaluating coil performance and
35、effectiveness. Evaluations are performed on phantom images generated using standard clinical scan protocols. This standards publication has been developed by the Magnetic Resonance Section of the Medical Imaging Technology Alliance, a division of the National Electrical Manufacturers Association. Us
36、er needs have been considered throughout the development of this publication. Proposed or recommended revisions should be submitted to: Vice President, Technical Services Department Medical Imaging Technology Alliance/National Electrical Manufacturers Association 1300 North 17th Street, Suite 900 Ro
37、sslyn, 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 Reference SystemsNorfolk, VA GE Healthcare,
38、 Inc.Milwaukee, WI Hitachi Medical Systems America, Inc.Twinsburg, OH InvivoGainesville, FL Medipattern Corporation Toronto, ON, Canada Philips HealthcareAndover, MA Siemens Medical Solutions, Inc.Malvern, PA Toshiba America Medical SystemsTustin, C A MS 6-2008 (R2014) Page v 2008 National Electrica
39、l Manufacturers Association Rationale Typically, coils are constructed to optimize the signal-to-noise ratio (SNR) of images from a restricted imaging region of interest (IROI). A simple example would be a single loop surface coil placed close to the IROI as compared to a cylindrical volume coil tha
40、t may surround the IROI, thereby encompassing a volume. This measurement procedure defines the volume near the surface coil, which produces a useful signal, and estimates the SNR about a reference point within that volume. Methods to characterize image uniformity for such coils are also provided. Th
41、e increased signal-to-noise performance of some of these coils is accompanied by a loss of image uniformity. While a high level of image uniformity is generally a desirable goal for volume coils, the reduction of signal from areas outside the IROI can be exploited using surface coils to reduce motio
42、n artifacts or to reduce image wrap-around artifacts caused by under-sampling when the field of view is small. Therefore, it is appropriate to map the sensitive volume of a surface coil, since a simple figure of merit for uniformity can be misleading. The SNR is a sensitive, but rather non-specific,
43、 measure of MR system performance. It can be used to assess the effect of alterations in the MR system (excluding the coil), or it can be used to compare the performance of two coils. Given that the sensitivity of many surface coils is spatially dependent, the assessment of the effect of alterations
44、 in the MR system can be achieved by measuring the SNR about a fixed reference point relative to the coil position. Since different coils are designed for different coil-to-tissue distances, it is not possible to fix a single reference position that is appropriate for all coil designs. The reference
45、 position selected shall approximate the position of the IROI for which the coil is used or intended. This standard allows flexibility in the choice of reference position; however, this flexibility may prevent direct comparison of different coil designs. The loading of surface coils may vary substan
46、tially from application to application and even from exam to exam depending on coil placement. Because of these variations, a generic coil loading scheme is not included in this standard. Since both loaded and unloaded SNRs are sensitive to changes in the remainder of the MR system (although the loa
47、ded SNR may be more representative of typical conditions), one condition shall be selected for this measurement procedure. Phantoms are objects that contain MR signal producing material and are generally used for SNR and uniformity testing of RF coils. It is recognized that as field strength (freque
48、ncy of operation) increases, wavelength effects become more significant, particularly above 64MHz. Therefore, this standard allows for the use of water-based or non-aqueous (e.g. oil-based) phantom fluids, without regard to field strength or frequency of operation, and emphasizes instead that the ph
49、antom fluid that is actually used be adequately specified for purposes of reproducibility. The use of geometric distortion correction algorithms and image uniformity correction algorithms is becoming increasingly common, and in some situations necessary. Both types of corrections will alter image uniformity results reported in this standard. While it was the original intent of this standard to characterize the coil without these corrections, it is also the intent of the standard to test the coil under typical clinical conditions. . Multiple measurement p
