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1、 NEMA Standards Publication MS 3-2008 Determination of Image Uniformity in Diagnostic Magnetic Resonance Images Published by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 www.nema.org 2008 by the National Electrical Manufacturers Association. All
2、 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 publication was
3、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 standards an
4、d 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. While NEMA
5、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 standards and g
6、uideline 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. NEMA disc
7、laims 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 guarantee t
8、he 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 undertaking to p
9、erform 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 and other
10、 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 document
11、. 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 the cer
12、tifier or maker of the statement. NEMA MS 3-2008 Page i CONTENTS Page Preamble ii Forewordiii Rationaleiv Scope.iv Section 1DEFINITIONS1 1.1Measurement Region of Interest (MROI) 1 1.2Measurement Subregion of Interest (SROI)1 1.3Image Non-uniformity1 1.4Normalized Absolute Average Deviation.1 1.5Peak
13、 Deviation Non-uniformity1 1.6Gray-Scale Uniformity Map .1 1.7Maximum Selectable Volume1 1.8Specification Volume.2 1.9Specification Area2 Section 2METHODS OF MEASUREMENT .3 2.1Test Hardware.3 2.1.1Size of the Signal Producing Volume3 2.1.2MR Characteristics of the Signal Producing Volume3 2.2Scan Co
14、nditions 3 2.3Measurement Procedure.4 2.3.1Pre-processing of the Image.4 2.3.2Peak Deviation Non-uniformity5 2.3.3Gray-Scale Uniformity Map.5 2.3.4ACR MR Accreditation Program (ACR-MRAP) Uniformity .7 2.3.5Normalized Absolute Average Deviation Uniformity.7 Section 3SOURCES OF ERROR AND REPORTING OF
15、RESULTS .9 3.1Sources of Error 9 3.2Reporting of Results9 3.2.1Scan Parameters 9 Annex A CHANGES TO STANDARD .10 Copyright 2008 by the National Electrical Manufacturers Association. NEMA MS 3-2008 Page ii Preamble This is one of a series of test standards developed by the medical diagnostic imaging
16、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, prospective purchasers, and users alike. Manufacturers are permitted to use these standards for t
17、he 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 measurement. These standards
18、 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 factory measurements, while
19、 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, standard clinical seque
20、nces 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 standards publication descri
21、bes 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 with the procedures spec
22、ified 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 the NEMA standard. The
23、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. Justification for th
24、e claimed uncertainty limits shall also be provided by a listing and discussion of sources and magnitudes of error. Copyright 2008 by the National Electrical Manufacturers Association. NEMA MS 3-2008 Page iii Foreword This standards publication is classified as a NEMA standard unless otherwise noted
25、. It is intended for use by MRI system manufacturers, manufacturers of accessory equipment (including radio-frequency coils), and by MRI end users. The purpose of this standards publication is to provide a standard procedure for measuring and reporting image-intensity uniformity in an MRI system. Im
26、age uniformity refers to the ability of an MR imaging system to produce an identical signal response throughout the scanned volume when the object being imaged is homogenous. Image-intensity non- uniformity in a two-dimensional MR image of a uniform test object can be caused by a number of factors,
27、including RF coil geometry and penetration, non-uniformity of the transmitted RF field (B1 non-uniformity), inhomogeneity of the static magnetic field (B0 non-uniformity), inadequacies in gradient pulse calibration or eddy current corrections, and spatial positioning of the phantom. Image uniformity
28、 is quantified here in terms of the deviation of the image pixel intensities from the midrange value. The analysis of uniformity shall be performed over the region of interest that is typically occupied by clinical samples and not over the full available volume of the RF coil. This standards publica
29、tion has been developed by the Magnetic Resonance 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: Vice President, Engineering Department National El
30、ectrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, 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 followi
31、ng members: Computer Imaging 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 Co
32、pyright 2008 by the National Electrical Manufacturers Association. NEMA MS 3-2008 Page iv Rationale This Standard measures image non-uniformity (see 1.3) to characterize the low spatial frequency non- uniformities typical of MR images. It is important to quantify image non-uniformity because it alte
33、rs image contrast. Various factors influence MR image non-uniformity such as B1 non-uniformity (both in transmit and receive), quality of B0 shim, geometric distortion, etc. Various techniques exist to correct for some forms of image nonunifomity such as RF receive coil correction algorithms. The us
34、e 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
35、characterize the coil without these corrections, it is also the intent of the standard to test the coil under typical clinical conditions. Therefore the results will be reported both ways where possible. This standard characterizes image non-uniformity qualitatively with a gray-scale uniformity map
36、and quantitatively with simple figures of merit that analyze the range of pixel intensities relative to some measure of the average image intensity. Scope This document defines a method for measuring image-uniformity performance of diagnostic magnetic resonance imaging systems using single channel v
37、olume coils and performing proton imaging. This document does not address the use of surface coils, chemical shift imaging, or spectroscopy. Image uniformity can be characterized in a number of different ways. The choice of a measurement and reporting method was guided by a desire for simplicity, ac
38、curacy, and ease of implementation on all MR systems. The peak deviation method represents a single image test that can quickly determine and report uniformity with a single number. It works best with very high SNR images. The gray-scale uniformity map image represents a single image test that can v
39、isually describe image uniformity. Both the ACR MR Accreditation Procedure method and the Normalized Absolute Average Deviation method resolve some of the image SNR issues of the peak deviation method. Measurements are made over a volume that is representative of the region used for typical clinical
40、 studies. Due to the difficulty in handling large phantoms, it is permitted to use a test phantom that only covers 85% of the specification area. This document does not address the use of special purpose coils (see MS 6) or coils that require multiple receiver channels for operation (see MS 9). Copy
41、right 2008 by the National Electrical Manufacturers Association. NEMA MS 3-2008 Page 1 Section 1 DEFINITIONS 1.1 MEASUREMENT REGION OF INTEREST (MROI) A centered, regular geometric area enclosing at least 75 percent (area) of the image of the signal producing volume of the phantom. 1.2 MEASUREMENT S
42、UBREGION OF INTEREST (SROI) A regular geometric area wholly enclosed within the MROI that covers approximately 0.15% of the number of image pixels (e.g. 100 pixels for a 256x256 matrix image). 1.3 IMAGE NON-UNIFORMITY The signal pixel intensity variations within an image that are repeatable from sca
43、n to scan. An absence of image non-uniformity (N) is defined as N=0% and perfect image uniformity (U) is defined as U=100%. The relationship is: U = 100 N. 1.4 NORMALIZED ABSOLUTE AVERAGE DEVIATION A measure of uniformity defined by the average absolute deviation from the mean within the MROI, norma
44、lized with respect to the mean within the MROI. 1.5 PEAK DEVIATION NON-UNIFORMITY The maximum absolute deviation from the midrange pixel value (2.3.2, Equation 3) expressed as a percentage. 1.6 GRAY-SCALE UNIFORMITY MAP The percentage deviation from a midrange pixel value expressed in a gray-scale i
45、mage format. 1.7 MAXIMUM SELECTABLE VOLUME For a given scan orientation (i.e., transverse, sagittal, coronal), the volume enclosed by the maximum selectable fields of view and the planes of maximum selectable slice offset from isocenter. Copyright 2008 by the National Electrical Manufacturers Associ
46、ation. NEMA MS 3-2008 Page 2 1.8 SPECIFICATION VOLUME The imaging volume over which a manufacturer guarantees image performance specifications will hold. Images or portions of images outside this volume will not necessarily meet performance specifications, but may still be useful for diagnostic purp
47、oses and require subjective judgment on the part of the diagnosing physician. For head scans, the specification volume must enclose, as a minimum, a 10 cm diameter spherical volume (dsv) centered in the RF head coil, and similarly for body scans, it must enclose, as a minimum, a 20 cm dsv centered i
48、n the RF body coil. 1.9 SPECIFICATION AREA The intersection of the specification volume and the image plane. Copyright 2008 by the National Electrical Manufacturers Association. NEMA MS 3-2008 Page 3 Section 2 METHODS OF MEASUREMENT 2.1 TEST HARDWARE 2.1.1 Size of the Signal Producing Volume The siz
49、e of the signal producing volume is determined by the thickness of the slice being imaged (per the protocol in 2.2) and the cross-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 cm diameter circle or 85% of the specification area, whichever is larger. b. Body specification volume: in the image plane, the phantom shall enclose, as a minimum, a 20 cm diameter cir