1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA MS 9-2008 (R2014)Characterization of Phased Array Coils forDiagnostic MagneticResonance ImagesNEMA Standards Publication MS 9-2008 (R2014) Characterization of Phased Array Coils for Diagnostic Magnetic Resonance Images Publi
2、shed 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 Berne Conve
3、ntion 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 engaged in
4、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 National Electrical Manufacturers Association (NEMA) standards and guideline publ
5、ications, 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 administers the
6、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 guideline publica
7、tions. 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 disclaims and makes
8、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 particular purposes or needs. NEMA does not undertake to guarantee the performance
9、 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 perform any dut
10、y 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 standards on
11、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. NEMA does no
12、t 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 certifier or mak
13、er of the statement. MS 9-2008 (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 1.1.1 Sensitive Volume 1 1.1.2 Sensitive Area 1 1.1.3 Imaging Region of Interest (
14、IROI) 1 1.1.4 Reference Position . 1 1.1.5 Phased Array Coils . 1 1.1.6 Sub-Coils 1 1.1.7 Sub-Coil Sets (or Modes) . 1 1.1.8 Phased Array Surface Sub-Coils . 1 1.1.9 Phased Array Volume Sub-Coils . 1 1.2 Analysis-Related Definitions . 2 1.2.1 Characterization Volume 2 1.2.2 Characterization Area . 2
15、 1.2.3 Measurement Region of Interest (MROI) . 2 1.2.4 Measurement Subregion of Interest (SROI) . 2 1.2.5 Specification Volume 2 1.3 Phantom-Related Definitions 2 1.3.1 Signal-Producing Volume (Phantom) . 2 1.4 Image-Related Definitions . 2 1.4.1 Image Artifact . 2 1.4.2 Image Uniformity/Nonuniformi
16、ty . 2 1.4.3 Baseline Pixel Offset Value 3 1.4.4 Image Signal . 3 1.4.5 Image Noise . 3 1.4.6 Image Signal-to-Noise Ratio. 3 Section 2 METHODS OF MEASUREMENT . 4 2.1 Test Hardware . 4 2.1.1 MR Characteristics of the Signal Producing Volume (Phantom) 4 2.1.2 RF Coil Loading Characteristics . 4 2.1.3
17、RF Coil and Positioning Device 4 2.2 Selection of Measurement Geometry . 4 2.2.1 Selection of the Reference Position , Characterization Volume and Area . 4 2.2.2 Measurement Region-of-Interest (MROI) . 5 2.2.3 Noise Evaluation Area 5 2.2.4 Slice Positions 5 2.3 Scan Conditions 5 2.4 Measurement Proc
18、edure 5 Section 3 REPORTING OF RESULTS . 7 3.1 Reporting of SNR 7 3.1.1 Geometric Information 7 3.1.2 Data-Acquisition Parameters 7 3.1.3 SNR Results . 8 MS 9-2008 (R2014) Page ii 2008 National Electrical Manufacturers Association 3.2 Reporting of Uniformity . 8 3.2.1 Geometric and Phantom Informati
19、on . 8 3.2.2 Data-Acquisition Parameters 8 3.2.3 Uniformity Results 8 3.3 Uncertainty of Measurements . 8 Annex A Changes to Standard 16 MS 9-2008 (R2014) Page iii 2008 National Electrical Manufacturers Association Preamble This is one of a series of test standards developed by the medical diagnosti
20、c 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 purchasers, and users alike. Manufacturers are permitted to use these stand
21、ards 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 measurement. These
22、 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 factory measureme
23、nts, 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, standard clin
24、ical 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 standards publica
25、tion describes the measurement of two of these parameters (SNR and Uniformity) for phased array coils. 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 carri
26、ed 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 included with any me
27、asurement 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 publication is to be reported, togethe
28、r 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 9-2008 (R2014) Page iv 2008 National Electrical Manufacturers Association Foreword This standards publication is classified as
29、 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 phased array radio-frequency (RF) coils used with MRI systems. Phased array coils consist of multiple receive-only coil e
30、lements that are used to detect signals from a limited portion of the anatomy. The output of each coil element, or combined set of elements, is connected to the input of an independent receiver chain that is part of a set of multiple receiver chains. Phased array coils may be composed of surface coi
31、ls, flexible coils, pairs of coils such as Helmholtz coils, or coils that surround a specific anatomical region as well as combinations of these coils. Phased array coils achieve good signal-to-noise performance because of their increased filling factor and the simultaneous use of smaller, higher si
32、gnal-to-noise receive coil elements. 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 phased array coils. These quantities are helpful in evaluating the impact of system
33、 changes on performance or in demonstrating effectiveness for FDA applications. The measurement methods have been designed for compatibility with existing NEMA methods for determining the SNR and signal intensity uniformity of head, body, and special purpose coil images (see MS 1, MS 3, and MS 6). E
34、valuations are performed on images generated using standard clinical scan protocols. This standards publication is intended for use by MRI system manufacturers, manufacturers of accessory equipment (including RF coils), and MRI end users. This standards publication has been developed by the Magnetic
35、 Resonance Section of the Medical Imaging Technology Alliance, a division 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, Technical Services
36、 Department Medical Imaging Technology Alliance/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 development. At the tim
37、e 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 Medipattern CorporationToronto, ON, Canada Medtronic NavigationYokneam, Israel
38、 Philips HealthcareAndover, MA Siemens Medical Solutions, Inc.Malvern, PA Toshiba America Medical SystemsTustin, CA MS 9-2008 (R2014) Page v 2008 National Electrical Manufacturers Association Rationale This standard makes extensive references to three earlier standards in its treatment of Phased Arr
39、ay coils. To aid the reader, it is strongly recommended that copies of these earlier standards be on hand while using this standard. The earlier standards referenced herein are: MS 1 Determination of Signal-to-Noise Ratio (SNR) in Diagnostic Magnetic Resonance Imaging MS 3 Determination of Image Uni
40、formity in Diagnostic Magnetic Resonance Images MS 6 Determination of Signal-to-Noise Ratio and Image Uniformity for Single-Channel Non-Volume Coils in Diagnostic MR Imaging Phased array coils are constructed to optimize the signal-to-noise ratio (SNR) of images from a restricted volume of interest
41、within the patient. This measurement procedure seeks to estimate the SNR and uniformity inside the imaging volume of the phased array coil. Phased array coils that consist of volume coil elements (sub-coils) may provide a uniform signal response over the sensitive volume. Therefore in this standard
42、SNR and uniformity for volume sub-coils may be measured and characterized using methods similar to those used for volume coils such as a head or body coil (MS 1 and MS 3). For some sub-coils used in the construction of a phased array coil, the increased signal-to-noise performance may be accompanied
43、 by a loss of image uniformity. While image uniformity is generally a desirable goal, the reduction of signal from areas outside the region of interest can be exploited to reduce motion artifacts or to reduce wrap-around artifacts caused by undersampling when the field of view is small. In this stan
44、dard, SNR and uniformity of surface coil elements of a phased array is measured using the methods prescribed in MS 6. The SNR is a sensitive, but rather non-specific, 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
45、 be used to compare the performance of two coils. Given that the sensitivity of some phased array coils is spatially dependent, the assessment of the effect of alterations in the MR system can be achieved by measuring the SNR about a fixed reference point relative to the coil position. Since differe
46、nt phased array sub-coils are designed for different coil-to-tissue distances, it is not possible to fix a single reference position that is appropriate for all sub-coils, complicating direct comparison of different coils. The reference position selected shall approximate the position of the anatomi
47、cal feature for which the selected sub-coil is used or intended. The loading of phased array RF coils varies substantially from application to application and even from exam to exam depending on coil placement. Because of these variations, a generic loading scheme is not included in this standard. S
48、ince both loaded and unloaded SNRs are sensitive to changes in the remainder of the MR system (although the loaded SNR may be more representative of clinical conditions), either shall be permitted in this measurement procedure. Phantoms are objects that contain MR signal producing material and are g
49、enerally used for SNR and uniformity testing of RF coils. It is recognized that as field strength (frequency 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 phantom fluid that is actually used be adequately specified for purposes of reproducibility. The use of geometric distortion correction algorithms and image uniformity
