1、 NEMA MS 8 CHARACTERIZATION OF THE SPECIFIC ABSORPTION RATE FOR MAGNETIC RESONANCE IMAGING SYSTEMS NEMA Standards Publication MS 8-2008 Characterization of the Specific Absorption Rate for Magnetic Resonance Imaging Systems Published by National Electrical Manufacturers Association 1300 North 17thSt
2、reet, Suite 1752 Rosslyn, VA 22209 www.nema.org 2008 by the National Electrical Manufacturers Association. 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 Intern
3、ational and Pan American Copyright Conventions. 2008 by the National Electrical Manufacturers Association. NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the document at the time it was
4、 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 publications, of which the document contained herein is one, are de
5、veloped 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 fairness in the develo
6、pment 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 personal injury, proper
7、ty, 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 no guaranty or warranty, express or implied, as to the accuracy
8、 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 products or services b
9、y 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 else. Anyone using this
10、 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 be available from other
11、 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, designs, or installations f
12、or 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. MS 8-2008 Page i 2008 by the National Electri
13、cal Manufacturers Association. CONTENTS Page Preamble ii Foreword iii Introductioniv Scopev Equivalence v Uncertainty of the Measurements v Section 1 REFERENCES AND DEFINITIONS 1 1.1 References .1 1.2 Definitions.1 1.2.1 Specific Absorption Rate (SAR).1 1.2.2 Tip Angle 1 1.2.3 Landmark .1 1.2.4 Phan
14、tom 1: Unloaded Tip Angle Calibration Phantom for Pulse-Energy Coil Loss Determination.1 1.2.5 Phantom 2: Pulse-Energy Device under Test (Human or Phantom to be Tested)2 1.2.6 Phantom 3: Calorimeteric Test Phantom .2 1.2.7 Pforward.2 1.2.8 Preflected2 1.2.9 Pother2 1.2.10 Pobject.2 1.2.11 Pcoil.2 1.
15、2.12 Pforward2 1.2.13 Preflected.3 1.2.14 Pother.3 1.2.15 B1 .3 Section 2 PULSE-ENERGY METHOD.5 2.1 Test Hardware 5 2.2 Hardware Setup5 2.3 Pulse Energy SAR Measurement Procedure.5 Section 3 CALORIMETRY METHOD 11 3.1 Test Hardware . 11 3.2 Hardware Setup. 11 3.3 SAR Measurement Procedure. 11 Section
16、 4 RESULTS 13 4.1 Reporting SAR Results 13 Appendix DOCUMENT CHANGES. 15 Figures 1-1 Examples of test phantoms 4 2-2 Possible arrangements for measuring radiofrequency power absorption in linear transmit coils.9 2-3 Method to find average power per TR using coupler forward power port and an oscillos
17、cope capable of finding peak and rms levels of the waveform. . 10 MS 8-2008 Page ii 2008 by the National Electrical Manufacturers Association. Preamble This is one of a series of test standards developed by the medical diagnostic industry for the measurement of performance parameters governing the i
18、mage quality of Magnetic Resonance 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 specifications. This standardization o
19、f 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 acceptance testing and periodic q
20、uality 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 instrumentation laboratory to assure the
21、 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 shall be used. No modificatio
22、ns to alter test results shall be used unless otherwise specified in these standards. The NEMA Magnetic Resonance subdivision has identified a set of key magnetic resonance parameters. This Standard describes the measurement of one of these parameters. MS 8-2008 Page iii 2008 by the National Electri
23、cal Manufacturers Association. Foreword Unless otherwise noted, this Publication has been approved as a NEMA Standard. It describes the test conditions and parameters that ensure accurate measurement of the Specific Absorption Rate (SAR). This Standard does not attempt to establish relationships bet
24、ween SAR and body temperature. This Standards Publication was developed by the Magnetic Resonance Section of the National Electrical Manufacturers Association. Section approval of the standard does not necessarily imply that all section members voted for its approval or participated in its developme
25、nt. At the time it was approved, the section was composed of the following members: Computer Imaging Reference Systems Norfolk, Va. GE Healthcare, Inc. Milwaukee, Wisc. Hitachi Medical Systems America, Inc. Twinsburg, Oh. Invivo Gainesville, Fla. Medipattern Corporation Toronto, Ontario Medtronic Na
26、vigation Yokneam, Israel Philips Healthcare Bothell, Wash. Siemens Medical Solutions, Inc. Malvern, Penn. Toshiba America Medical Systems Tustin, Calif. User needs have been considered throughout the development of this publication. Proposed or recommended revisions should be submitted to: Vice-Pres
27、ident, Technical Services National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 MS 8-2008 Page iv 2008 by the National Electrical Manufacturers Association. Introduction In magnetic resonance (MR) imaging, radiofrequency (RF) magnetic fields are used to i
28、nterrogate a region of interest. These RF fields induce currents in the body, which may lead to heating. It is not considered prudent to raise the core temperature in a patient above 39.2C (roughly a 2.2 degree rise from thermoneutral) 1,2. If patient exposure to radiofrequency magnetic fields durin
29、g MR scanning is in-sufficient to produce a core temperature rise in excess of 1C and localized heating greater than 38C in the head, 39C in the trunk, and 40C in the extremities, RF heating is considered to be within safe levels 3,4,5. Parameters such as bore temperature, ambient temperature, relat
30、ive humidity, air flow rate, perspiration, and blood flow influence temperature rise in the patient. A key variable in determining patient heating potential in an MR scanner is the power absorbed per unit mass, which is the specific absorption rate (SAR). An insulated slab of tissue initially at the
31、rmal equilibrium with its environment increases in temperature at a rate of approximately 1C per hour when exposed to a SAR of 1 W/kg. The MR scanning process applies a train of RF pulses, which have specific, calibrated tip angles. Each pulse results in some power absorption in the patient. The hig
32、hest absorbed energy per pulse takes place in those patients whose cross-sectional area is greatest. The highest absorbed power (and SAR) takes place in such patients when they are exposed to the highest permitted RF duty cycle. The greater the number of images (slices/echoes) per unit time the grea
33、ter the SAR. Note that scan time implies the length of time the scanner gradient or RF hardware is employed to produce an image. For example, the period over which the SAR from an echo planar scan is averaged is the entire time required to pulse the RF and gradients, not merely the pulse duration of
34、 the initial RF pulse. Determination of SAR may be done either calorimetrically or by measurements of energy per pulse. The pulse energy method may be used to determine SAR either in a phantom or a patient. Both methods are described in this standard; either method may be chosen. The pulse energy me
35、thod permits the use of low duty cycle scans for the test. The results from either method may then be extrapolated to other scan parameters and even to other waveforms. There is a need for measuring SAR in patients for developing and verifying various predictive safety algorithms. In addition, measu
36、rements of SAR in phantoms with electrical conductivities similar to patients are important for implant heating tests for MR compatibility. This standard was developed to try to fill these needs. Local SAR measurements are important for assessing localized heating. The local average SAR is the total
37、 power divided by the exposed mass. The (spatial) peak SAR is the SAR in the highest SAR occurring in any gram of tissue. While peak and local SAR levels are important in localized heating, they are difficult to measure directly in living patients. For this reason, determinations of peak and local S
38、AR levels are beyond the scope of this document. MS 8-2008 Page v 2008 by the National Electrical Manufacturers Association. Scope This NEMA Standards Publication describes two measurement procedures for whole-body SAR measurements, the calorimetric method and the pulse-energy method. Extrapolation
39、of these data to pa-tient temperature rise is beyond the scope of this document. This document does not apply to gradient (low-frequency time-varying magnetic fields) safety where nerve and cardiac excitation are the primary safety issues. Neither is it intended to apply to spatial peak or local ave
40、rage SAR nor does it address other factors involved with patient heating. The tests specified are only for volume RF transmit coils which produce relatively homogeneous RF fields. Equivalence It is intended and expected that manufacturers or others who claim compliance with these NEMA standard test
41、procedures for the determination of image quality parameters shall have carried out the tests in accordance with the procedures in the published standards. In those cases where it is impossible or impractical to follow the literal proscription of a NEMA test procedure, a complete description of any
42、deviation from the published procedure must be included with any measurement claimed to be equivalent to the NEMA standard. The validity or equivalence of the modified procedures will be determined by each reader. Uncertainty of the Measurements The measurement uncertainty of the parameter determine
43、d using this standard 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. MS 8-2008 Page vi 2008 by the National Electrical Manufacturers Association. MS 8
44、-2008 Page 1 2008 by the National Electrical Manufacturers Association. Section 1 REFERENCES AND DEFINITIONS 1.1 REFERENCES 1. Goldman, R.F., E.B. Green, and P.F. Iampietro, 1965, “Tolerance of Hot Wet Environments by Resting Men,“ J. Appl. Physiol., 20(2):271-277. 2. Wyndham, C.H., N.B. Strydom, J.
45、F. Morrison, C.G. Williams, G.A.G. Bredell, J.S. Maritz, and A. Munro, 1965, “Criteria for Physiological Limits for Work in Heat,“ J. Appl. Physiol., 20(1): 27-45. 3. Department of Health and Human Services, Food and Drug Administration. Magnetic Resonance Diagnostic Device; Panel Recommendation and
46、 Report on Petitions for MR Reclassification, Docket nos. 87P-0214/CP through 87P-0214/CP0013. Fed Reg 1988; 53(46):7575-7579. 4. Department of Health and Human Services, Food and Drug Administration. Recommendation and Report on Petitions for Magnetic Resonance Reclassification and Codification, Fi
47、nal Rule, 21 CFR Part 892. Fed Reg 1989; 54(20):5077-5088. 5. Adair, E.R. and L.G. Berglund, “On the Thermoregulatory Consequences of NMR Imaging,“ Magnetic Resonance Imaging, 1986, 4, 321-333. 6. F 2182, Standard Test Method for Measurement of Radio Frequency Induced Heating Near Passive Implants D
48、uring Magnetic Resonance Imaging, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. 1.2 DEFINITIONS 1.2.1 Specific Absorption Rate (SAR) The SAR is the average RF power absorbed by the object in watts divided by the relevant mass of the object i
49、n kilograms. In this document SAR is measured for applications using volume transmit coils. 1.2.2 Tip Angle The tip angle is the angle through which the macroscopic magnetization vector is moved by an RF pulse. 1.2.3 Landmark The landmark is the region of the body to be placed in the center of the RF coil. 1.2.4 Phantom 1: Unloaded Tip Angle Calibration Phantom for Pulse-Energy Coil Loss Determination Phantom 1 is a physically small object used during the calibration of tip angles and for the determination of RF coil losses with the pulse-ene
