1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA MS 3-2008 (R2014)Determination of ImageUniformity in DiagnosticMagnetic ResonanceImagesNEMA Standards Publication MS 3-2008 (R2014) Determination of Image Uniformity in Diagnostic Magnetic Resonance Images Published by: Nati
2、onal 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 Berne Convention for the
3、 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 engaged in the developme
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13、tement. NEMA MS 3-2008 (R2014) Page i 2008 National Electrical Manufacturers Association CONTENTS Page Preamble ii Foreword . iii Rationale . iv Scope iv Section 1 DEFINITIONS 1 1.1 Measurement Region of Interest (MROI) 1 1.2 Measurement Subregion of Interest (SROI) 1 1.3 Image Non-uniformity 1 1.4
14、Normalized Absolute Average Deviation . 1 1.5 Peak Deviation Non-uniformity 1 1.6 Gray-Scale Uniformity Map . 1 1.7 Maximum Selectable Volume 1 1.8 Specification Volume . 2 1.9 Specification Area 2 Section 2 METHODS OF MEASUREMENT . 3 2.1 Test Hardware . 3 2.1.1 Size of the Signal Producing Volume 3
15、 2.1.2 MR Characteristics of the Signal Producing Volume 3 2.2 Scan Conditions 3 2.3 Measurement Procedure . 4 2.3.1 Pre-processing of the Image . 4 2.3.2 Peak Deviation Non-uniformity 5 2.3.3 Gray-Scale Uniformity Map . 5 2.3.4 ACR MR Accreditation Program (ACR-MRAP) Uniformity . 7 2.3.5 Normalized
16、 Absolute Average Deviation Uniformity . 7 Section 3 SOURCES OF ERROR AND REPORTING OF RESULTS . 9 3.1 Sources of Error 9 3.2 Reporting of Results 9 3.2.1 Scan Parameters 9 Annex A CHANGES TO STANDARD . 10 NEMA MS 3-2008 (R2014) Page ii 2008 National Electrical Manufacturers Association Preamble Thi
17、s is one of a series of test 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, prospective pu
18、rchasers, and 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, and the parameters supplied with each NEMA measurement se
19、rve as a guide 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
20、test standards require instrumentation better suited to factory 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 o
21、f the system. For 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
22、 key magnetic 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
23、 parameters shall 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
24、must be included 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 publicatio
25、n is to be reported, 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. NEMA MS 3-2008 (R2014) Page iii 2008 National Electrical Manufacturers Association Foreword This stan
26、dards publication is classified as a NEMA standard unless otherwise noted. 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 fo
27、r measuring and reporting image-intensity uniformity in an MRI system. Image 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
28、MR image of a uniform test object can be caused by a number of factors, 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 curre
29、nt corrections, and spatial positioning of the phantom. Image uniformity 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 n
30、ot over the full available volume of the RF coil. This standards publication 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 sh
31、ould be submitted to: Vice President, Engineering Department National Electrical Manufacturers Association 1300 North 17th Street, Suite 900 Rosslyn, VA 22209 Section approval of the standard does not necessarily imply that all section members voted for its approval or participated in its developmen
32、t. At the time it was approved, the section 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.Mal
33、vern, PA Toshiba America Medical SystemsTustin, CA NEMA MS 3-2008 (R2014) Page iv 2008 National Electrical Manufacturers Association 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 qua
34、ntify image non-uniformity because it alters 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
35、receive coil correction algorithms. 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 wa
36、s 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. Therefore the results will be reported both ways where possible. This standard characterizes image non-uniformity quali
37、tatively with a gray-scale uniformity map 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 resona
38、nce imaging systems using single channel volume 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
39、 was guided by a desire for simplicity, accuracy, 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
40、 represents a single image test that can visually 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 representati
41、ve of the region used for typical clinical 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 receiv
42、er channels for operation (see MS 9). NEMA MS 3-2008 (R2014) Page 1 2008 National Electrical Manufacturers Association 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 o
43、f the phantom. 1.2 MEASUREMENT SUBREGION 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 im
44、age that are repeatable from scan 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
45、 the mean within the MROI, normalized 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 v
46、alue expressed in a gray-scale image 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. NEMA MS 3-2008 (R2014) Page 2
47、2008 National Electrical Manufacturers Association 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 usef
48、ul for diagnostic purposes 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,
49、a 20 cm dsv centered in the RF body coil. 1.9 SPECIFICATION AREA The intersection of the specification volume and the image plane. NEMA MS 3-2008 (R2014) 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-sectional area resulting from the intersection of the image plane and the phantom. The size of this cross-sectional area must mee
