1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA MS 1-2008 (R2014)Determination of Signal-to-Noise Ratio(SNR) in DiagnosticMagnetic ResonanceImagingNEMA Standards Publication MS 1-2008 (R2014) Determination of Signal-to-Noise Ratio (SNR) in Diagnostic Magnetic Resonance Im
2、aging 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 Convention, the
3、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 consensus of persons
4、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 and guideline publications, of which the document contained
5、 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 NEMA administers the process and establishes rules to promote
6、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 guideline publications. NEMA disclaims liability for any p
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8、ed, 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 the performance of any individual manufacturer or sellers
9、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 perform any duty owed by any person or entity to someone e
10、lse. 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 other standards on the topic covered by this publication may b
11、e 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 document. NEMA does not certify, test, or inspect products, desig
12、ns, 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 certifier or maker of the statement. NEMA MS 1-2008 (R2014)
13、 Page i 2008 National Electrical Manufacturers Association CONTENTS Page Preamble ii Foreword . iii Rationale . iv Scope iv Section 1 DEFINITIONS 1 1.1 Specification Volume . 1 1.2 Specification Area 1 1.3 Measurement Region of Interest (MROI) 1 1.4 Image Signal 1 1.5 Image Noise. 1 1.6 Image Signal
14、-to-Noise Ratio . 1 1.7 Image Artifact. 1 1.8 Voxel Dimensions 1 1.9 Real Image 2 1.10 Magnitude Image . 2 Section 2 METHODS OF MEASUREMENT . 3 2.1 Test Hardware . 3 2.1.1 Size of the Signal Producing Volume 3 2.1.2 RF Coil Loading Characteristics 3 2.1.3 MR Characteristics of the Signal Producing V
15、olume 3 2.2 Scan Conditions 3 2.3 Measurement Procedure . 4 2.3.1 Image Signal Determination 4 2.3.2 Procedures to Evaluate Image Noise . 4 2.3.3 Procedure to Determine SNR . 9 Section 3 RESULTS 10 3.1 Reporting of Results 10 3.1.1 Geometric and Phantom Information 10 3.1.2 Data Acquisition Paramete
16、rs . 10 3.1.3 SNR Results 11 3.1.4 Reconstruction Parameters . 11 3.1.5 Additional Data 11 3.1.6 Repeatability Data . 11 3.1.7 Sources of Error 11 Appendices A An Alternative Measure of Noise Standard Deviation . 12 B Changes to Standard . 13 Table 31 Data Acquisition Parameters 10 Figures 11 A Sche
17、matic Showing the Relative Location of Defined Terms 2 21 A Schematic Showing Example Background Noise Measurement Region Locations for Method 4. . 9 NEMA MS 1-2008 (R2014) Page ii 2008 National Electrical Manufacturers Association Preamble This is one of a series of test standards developed by the
18、medical diagnostic imaging industry for the 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
19、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 measurement serve as a guide to those factors that can influence the measure
20、ment. 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 require instrumentation better suited to factor
21、y 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 mode of the system. For example, standard calibration procedures, st
22、andard 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 resonance image quality parameters. This standa
23、rds 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 shall have carried out the tests in accordance wi
24、th 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 included with any measurement claimed equivalent to t
25、he 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 reported, together with the value of the parameter
26、. Justification for the claimed uncertainty limits shall also be provided by a listing and discussion of sources and magnitudes of error. NEMA MS 1-2008 (R2014) Page iii 2008 National Electrical Manufacturers Association Foreword This standards publication is classified as a NEMA standard unless oth
27、erwise noted. It describes four methods to measure image signal-to-noise ratio (SNR). It is intended for use by MRI system manufacturers, manufacturers of accessory equipment (including radiofrequency coils), and by MRI end users. The major feature of the first method is that the SNR performance of
28、the system is evaluated using a standard clinical scan procedure. However, it should be noted that since this method involves the subtraction of two images, it can be very sensitive to system instabilities that may occur during the data acquisition process. If results are highly variable, it is advi
29、sable to perform the alternative calculation of standard deviation, described in method #1, or use methods #2, #3, or #4. These alternative methods have been designed to be less susceptible to system instabilities and can be used to determine if any variability in the SNR is due to system instabilit
30、y or genuinely poor SNR. Both methods are intended to measure thermal and other broadband, non-structured noise, and specifically do not address low frequency variations in an image or artifacts as defined herein. This standards publication has been developed by the Magnetic Resonance Section of the
31、 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, Engineering Department National Electrical Manufacturers Association 1300 North 17th Street, Suite
32、 900 Rosslyn, VA 22209 This standards publication was developed by the Magnetic Resonance Section. 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 Magnetic Resonance Secti
33、on was composed of the following members: Computer Imaging Reference SystemsNorfolk, VA GE Healthcare, Inc.Milwaukee, WI Hitachi Medical Systems America, Inc.Twinsburg, OH InvivoGainesville, FL Philips HealthcareAndover MA Siemens Medical Solutions, Inc.Malvern, PA Toshiba America Medical SystemsTus
34、tin, CA NEMA MS 1-2008 (R2014) Page iv 2008 National Electrical Manufacturers Association Rationale Image SNR is a parameter that relates to clinical usefulness of magnetic resonance images and also is a sensitive measure of hardware performance. Experience has shown that variations in system calibr
35、ation, gain, coil tuning, radiofrequency shielding, or other similar parameters are usually demonstrated by a corresponding change in image SNR. Scope This document defines methods for measuring the signal-to-noise ratio of magnetic resonance images obtained under a specific set of conditions, and u
36、sing single-channel volume receiver coils. This document does not address the use of special purpose coils (see MS 6) or coils that employ multiple receiver channels for operation (see MS 9). NEMA MS 1-2008 (R2014) Page 1 2008 National Electrical Manufacturers Association Section 1 DEFINITIONS 1.1 S
37、PECIFICATION VOLUME The specification volume is the imaging volume within which a manufacturer guarantees image performance specifications. Images or portions of images outside this volume may not necessarily meet performance specifications, but may still be useful for diagnostic purposes. For head
38、scans, the specification volume must enclose, as a minimum, a 10 centimeter diameter spherical volume (dsv) centered in the RF head coil. For body scans, 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 th
39、e intersection of the specification volume and the image plane. 1.3 MEASUREMENT REGION OF INTEREST (MROI) The measurement region of interest (MROI) is a centered, regularly shaped geometric area enclosing at least 75% of the area of the image of the signal-producing volume of the phantom. 1.4 IMAGE
40、SIGNAL The image signal is the mean pixel value within the MROI (minus the baseline pixel offset, if any) in the original, unsubtracted image. 1.5 IMAGE NOISE The random variations in pixel intensity in the MROI are called image noise. 1.6 IMAGE SIGNAL-TO-NOISE RATIO The image signal-to-noise ratio
41、is a single number obtained by dividing the image signal by the image noise. 1.7 IMAGE 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, if it is an anomaly derived from the structure or c
42、hemistry of the object being scanned, appears in the image at a location other than expected. 1.8 VOXEL DIMENSIONS This parameter is composed of three elements: a. Slice thickness operator selected slice thickness. b. Pixel horizontal dimension the field of view in the horizontal direction divided b
43、y the number of data sampling points in the horizontal axis. This corresponds to the horizontal dimension of the data acquisition matrix. NEMA MS 1-2008 (R2014) Page 2 2008 National Electrical Manufacturers Association c. Pixel vertical dimension the field of view in the vertical direction divided b
44、y the number of data sampling points in the vertical axis. This corresponds to the vertical dimension of the data acquisition matrix. 1.9 REAL IMAGE After reconstruction, the image pixels are vectors with real and imaginary parts. A phase correction is applied to the image in an attempt to rotate th
45、e noise-free signal to the real axis. The positive and negative real components of each pixel form the real image. 1.10 MAGNITUDE IMAGE The magnitude image is formed by taking the magnitude of each complex pixel. Figure 11 A Schematic Showing the Relative Location of Defined Terms NEMA MS 1-2008 (R2
46、014) Page 3 2008 National Electrical Manufacturers Association Section 2 METHODS OF MEASUREMENT 2.1 TEST HARDWARE 2.1.1 Size of the Signal Producing Volume The size of the signal producing volume is determined by the thickness of the slice being imaged (per the protocol in 2.2) and the cross-section
47、al area resulting from the intersection of the image plane and the phantom. The size of this cross-sectional area must meet the following requirements: a. Head specification volume: in the image plane, the phantom shall enclose, as a minimum, a 10 centimeter diameter circle or 85% of the specificati
48、on area, whichever is larger. b. Body specification volume: in the image plane, the phantom shall enclose, as a minimum, a 20 centimeter diameter circle or 85% of the specification area, whichever is larger. 2.1.2 RF Coil Loading Characteristics In order to approximate the image noise performance th
49、at would be typically encountered in a clinical situation, the RF receive coil of interest (i.e., either head or body coil), must be electrically loaded. Loading may be accomplished by phantoms or other reproducible means. When loaded, the following electrical parameters must be the same as when a 50 to 90 kg human is positioned for a scan (within the stated tolerances): a. Coil 3 dB bandwidth: 15% b. Coil impedance: 20% magnitude, 20 phase c. Coil center frequency shift: 1% of center frequency 2.1.3 MR Characteristics of the Signal Producing Volume Th
