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1、 NEMA Standards Publication MS 1-2008 Determination of Signal-to-Noise Ratio (SNR) in Diagnostic Magnetic Resonance Imaging Published by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 www.nema.org 2008 by National Electrical Manufacturers Associat
2、ion. 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 American Copyright Conventions. NOTICE AND DISCLAIMER The information in this publicati
3、on 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. NEMA stand
4、ards 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. Whil
5、e 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 standard
6、s 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. NE
7、MA 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 will fulfill any of your particular purposes or needs. NEMA does not undertake to guar
8、antee 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 undertaki
9、ng 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. Information an
10、d 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 to police or enforce compliance with the contents of this d
11、ocument. 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 responsibility of
12、the certifier or maker of the statement. NEMA MS 1-2008 Page i CONTENTS Page Preamble ii Forewordiii Rationaleiv Scope.iv Section 1DEFINITIONS1 1.1Specification Volume.1 1.2Specification Area1 1.3Measurement Region of Interest (MROI)1 1.4Image Signal1 1.5Image Noise.1 1.6Image Signal-to-Noise Ratio.
13、1 1.7Image Artifact.1 1.8Voxel Dimensions1 1.9Real Image 2 1.10Magnitude Image.2 Section 2METHODS OF MEASUREMENT .3 2.1Test Hardware.3 2.1.1Size of the Signal Producing Volume3 2.1.2RF Coil Loading Characteristics3 2.1.3MR Characteristics of the Signal Producing Volume3 2.2Scan Conditions 3 2.3Measu
14、rement Procedure.4 2.3.1Image Signal Determination4 2.3.2Procedures to Evaluate Image Noise .4 2.3.3Procedure to Determine SNR .9 Section 3RESULTS10 3.1 Reporting of Results10 3.1.1Geometric and Phantom Information10 3.1.2Data Acquisition Parameters.10 3.1.3SNR Results11 3.1.4Reconstruction Paramete
15、rs.11 3.1.5Additional Data11 3.1.6Repeatability Data.11 3.1.7Sources of Error11 Appendices A An Alternative Measure of Noise Standard Deviation.12 B Changes to Standard.13 Table 31 Data Acquisition Parameters 10 Figures 11 A Schematic Showing the Relative Location of Defined Terms2 21 A Schematic Sh
16、owing Example Background Noise Measurement Region Locations for Method 49 Copyright 2008 by the National Electrical Manufacturers Association ? NEMA MS 1-2008 Page ii Preamble This is one of a series of test standards developed by the medical diagnostic imaging industry for the measurement of perfor
17、mance 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 system performance speci
18、fications. 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 reference procedures for
19、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 facilities of an instrum
20、entation 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 reconstruction processes
21、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 of one of these paramet
22、ers. 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 with the procedures specified in the published standards. In those
23、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 the NEMA standard. The validity or equivalence of the modified pro
24、cedure 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. Justification for the claimed uncertainty limits shall also be
25、provided by a listing and discussion of sources and magnitudes of error. Copyright 2008 by the National Electrical Manufacturers Association ? NEMA MS 1-2008 Page iii Foreword This standards publication is classified as a NEMA Standard unless otherwise noted. It describes four methods to measure ima
26、ge 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 the system is evaluated using a standard clinical scan
27、 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 advisable to perform the alternative calculation of standa
28、rd 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 instability or genuinely poor SNR. Both methods are intended to
29、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 National Electrical Manufacturers Association. User n
30、eeds 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 1752 Rosslyn, VA 22209 This standards publication was
31、 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 Section was composed of the following members: Computer Im
32、aging Reference SystemsNorfolk, VA GE Healthcare, Inc.Milwaukee, WI Hitachi Medical Systems America, Inc.Twinsburg, OH InvivoGainesville, FL Philips Medical Systems North AmericaBothell, WA Siemens Medical Solutions, Inc.Malvern, PA Toshiba America Medical SystemsTustin, CA Copyright 2008 by the Nat
33、ional Electrical Manufacturers Association ? NEMA MS 1-2008 Page iv 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 calibration, gain, coil tuni
34、ng, 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 using single-channel vo
35、lume 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). Copyright 2008 by the National Electrical Manufacturers Association ? NEMA MS 1-2008 Page 1 Section 1 DEFINITIONS 1.1 SPECIFICATIO
36、N 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 scans, the
37、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 the intersect
38、ion 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 SIGNAL The
39、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 is a single
40、 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 chemistry of
41、 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 by the numbe
42、r of data sampling points in the horizontal axis. This corresponds to the horizontal dimension of the data acquisition matrix. Copyright 2008 by the National Electrical Manufacturers Association ? NEMA MS 1-2008 Page 2 c. Pixel vertical dimension the field of view in the vertical direction divided b
43、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
44、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 Copyright 2008 by
45、the National Electrical Manufacturers Association ? NEMA MS 1-2008 Page 3 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 cr
46、oss-sectional 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
47、specification 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 per
48、formance that 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
49、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 The following are the desired MR characteristics of the signal producing volume: T1 50 milliseconds (at operating field strength) Spin density = density of H2O 20% (for water-based phantoms) The phantom materi