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1、 NEMA Standards Publication MS 10-2006 Determination of Local Specific Absorption Rate (SAR) in Diagnostic Magnetic Resonance Imaging Published by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 www.nema.org 2006 by the National Electrical Manufact
2、urers 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 International and Pan American Copyright Conventions. NOTICE AND DISCLAIMER The information in
3、 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 developed. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this docum
4、ent. The National Electrical Manufacturers Association (NEMA) standards 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
5、who have an interest in the topic covered by this publication. While 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 info
6、rmation or the soundness of any judgments contained in its standards 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
7、 publication, use of, application, or reliance on this document. NEMA 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
8、 your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or seller?s 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 serv
9、ices 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 document should rely on his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the ex
10、ercise of reasonable care in any given circumstances. Information and 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 u
11、ndertake to police or enforce compliance with the contents of this document. 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 s
12、hall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement. MS 10-2006 Page i Copyright 2006 by the National Electrical Manufacturers Association. CONTENTS Page Foreword.ii Section 1 GENERAL1 1.1Rationale1 1.2Scope.1 1.3References 1 1.4Definitions2 S
13、ection 2 DETERMINING LOCAL SAR3 2.1Properties of the Tissue Equivalent Phantom.3 2.2RF Coil Loading Characteristics of the Tissue Equivalent Phantom.6 2.3Scan Conditions 6 2.4Experimental Measurement.7 Section 3 REPORTING RESULTS9 3.1Parameters9 3.2Phantom Parameters.9 3.3Local SAR Results.9 3.4Addi
14、tional Data 9 3.5Repeatability Data.10 3.6Sources of Error 10 Appendix THEORETICAL ESTIMATE OF LOCAL SAR.11 ? MS 10-2006 Page ii Copyright 2006 by the National Electrical Manufacturers Association. Foreword This Standards Publication is classified as a NEMA Standard unless otherwise noted. It descri
15、bes a measurement method for local specific absorption rate (SAR). The measurement method requires construction of a radio frequency phantom for a given frequency and the use of radio frequency- transparent thermometry. The procedure is intended for local SAR only. The method specifically does not a
16、ddress whole-body SAR. This Standards Publication has been 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 development.
17、 At the time it was approved, the section was composed of the following members: Computer Imaging Reference Systems?Norfolk, VA Fonar Corporation?Melville, NY GE Healthcare, Inc.?Milwaukee, WI Hitachi Medical Systems America, Inc.?Twinsburg, OH Invivo Corp.?Gainesville, FL Philips Medical Systems No
18、rth America?Bothell, WA Siemens Medical Solutions, Inc.?Malvern, PA Toshiba America Medical Systems?Tustin, CA User needs have been considered throughout the development of this publication. Proposed or recommended revisions should be submitted to: Vice-President, Technical Services National Electri
19、cal Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 ? MS 10-2006 Page 1 Copyright 2006 by the National Electrical Manufacturers Association. Section 1 GENERAL 1.1 RATIONALE Local SAR is a parameter that relates to the safety of magnetic resonance (MR) scanners. The pri
20、mary safety concern with transmit surface coils involves local SAR. Local SAR may be highest near conductors. Methods for determining local SAR are needed for ensuring safe operation of transmit coils during MR exams. This standard does not attempt to establish relationships between SAR and body tem
21、perature. 1.2 SCOPE This document defines methods for determining the local specific absorption rate of diagnostic magnetic resonance imaging radio frequency coils under a specific set of conditions. This document does not address whole-body SAR. Heating other than radio frequency heating (such as t
22、hermal heating from surface coil blocking networks) is not addressed in this standard. 1.3 REFERENCES 1. Durney, C.H., Johnson, C.C., Barber, P.W., et al, 1978, Radiofrequency radiation dosimetry handbook. USAF School of Aerospace Medicine, Report SAM-TR- 78-22, Brooks Air Force Base, Texas, 2nd ed.
23、 2. Schenck, J.F., E.B. Boskamp, D.J. Schaefer, W.D. Barber and R.H. Vander Heiden, ?Estimating Local SAR Produced by RF Transmitter Coils: Examples Using the Birdcage Coil.? Abstracts of the International Society of Magnetic Resonance in Medicine, Sixth Meeting, Sydney, Australia, p. 649, 1998. 3.
24、Chou CK, Chen GW, Guy AW, Luk KH (1984): Formulas for preparing phantom muscle tissue at various radiofrequencies. Bioelectromagnetics 5(4):435-441. 4. Gandhi, OP and Chien, JY, 1992, “Absorption and Distribution Patterns of RF Fields,“ Annals of the New York Academy of Sciences, 649:132. 5. Grandol
25、fo, M, Polichetti, A, Vecchia, P, and Gandhi, OP, 1992, “Spatial Distribution of RF Power in Critical Organs during Magnetic Resonance Imaging,“ Annals of the New York Academy of Sciences, 649:178. 6. Schaefer, D.J. and E.C. Burdette, 1981, “Complex Permittivity of Chicken Brain from 13.56 MHz to 2.
26、45 GHz,“ Abstracts of the Third Annual Meeting of the Bioelectromagnetics Society, Washington, D.C., August, 1981, abstract published, p. 59. 7. M. A. Stuchly, S. S. Stuchly, “Coaxial line reflection method for measuring dielectric properties of biological substances at radio and microwave frequenci
27、es - A review,“ IEEE Trans. Instrum. Meas., vol. 29, n?3, September 1980, pp 176 - 183. 8. IEC 60601-2-33, 2nd edition, Medical Electrical Equipment - Part 2: Particular Requirements for The Safety of Magnetic Resonance Equipment for Medical Diagnosis, International Electrotechnical Commission (IEC)
28、*, 3, rue de Varemb, P.O. Box 131, CH - 1211 Geneva 20, Switzerland (In the United States, copies of this standard can be obtained from the American National Standards Institute (ANSI), 11 West 42nd Street, New York, NY 10036), (2002). ? MS 10-2006 Page 2 Copyright 2006 by the National Electrical Ma
29、nufacturers Association. 1.4 DEFINITIONS local SAR: Local SAR is the local rate of energy deposition per unit mass. It is expressed in W/kg. It serves as an approximate measure of heating potential. electrical conductivity (! !): Electrical conductivity is the ratio of current density to electric fi
30、eld. electrical permitivity (!): Electrical permittivity is the ratio of electric flux density to electric field. tissue density (“): Tissue density is the mass per unit volume of the tissue. radian frequency (#): Radian frequency is 2 “ times the frequency. For this standard the frequency of intere
31、st is the resonant frequency of the transmit coil. tissue equivalent phantom: A tissue equivalent phantom is a phantom whose radio frequency electrical and thermal properties approximate those of human muscle at the desired frequency. ? MS 10-2006 Page 3 Copyright 2006 by the National Electrical Man
32、ufacturers Association. Section 2 DETERMINING LOCAL SAR 2.1 PROPERTIES OF THE TISSUE EQUIVALENT PHANTOM Construct or obtain an appropriate tissue equivalent phantom for the coil under test. Phantom dimensions shall mimic the position and extent of patient anatomy in clinical use. However, phantom di
33、mensions need not extend more than half a coil width beyond coil conductors. When patients are smaller than such a dimension, the phantom dimension need be no larger than that of typical adult patients. It is imperative that phantom construction permits coil conductors to be placed as close to the p
34、hantom tissue equivalent material as is the case for patients. Sample phantom materials include: gelling agent such as TX151 (Oil Center Research, P.O. Box 71871, Lafayette, LA 70501), polyethylene powder - not needed below 100 MHz (Wadco CA Inc., 2102 Curry Street, Long Beach, CA 90805), aluminum p
35、owder - needed below 100 MHz (Bakers USP, reagent grade, J.T. Baker Chemical Company, Phillipsburg, NJ), sodium chloride (reagent grade), and deionized water. Table 2-1 may be used for determining phantom composition (see Bioelectromagnetics 5:435-441 (1984). The phantom electrical properties are mo
36、st accurate at 22oC. It may be desirable to add a small quantity of paramagnetic ion to the local SAR phantom to reduce relaxation times T1, T2, and the B1 calibration time. For example, 2.5 grams of copper sulfate (reagent grade) per liter will produce T1=125 ms and T2=102 ms at 1.5 T. More stable
37、solutions might include manganese chloride (T1= 360 ms and T2 = 44 ms for a 0.0003 M aqueous solution) or nickel chloride (T1 =77 ms and T2 = 67 ms for a 0.02 M aqueous solution) at 1.5 T. Note that heating depends directly on tissue conductivity. When standing waves matter (for B0 1.5 T), the diele
38、ctric constant will also play a significant role. Therefore, it is important that phantom conductivity and dielectric constant simulate the electrical properties of muscle tissue. The specific heat and density of the phantom should approximate muscle. In addition the phantom should minimize convecti
39、on for example by including a gelling agent. Electrical properties of muscle tissue provided by the US Air Force may be found at the following URL: ftp:/medical.nema.org/MEDICAL/NEMAPublic/Mag_Res/Dielectric_Properties_of_Tissue-files/ ftp:/medical.nema.org/MEDICAL/NEMAPublic/Mag_Res/Dielectric_Prop
40、erties_of_Tissue.htm The following table summarizes field strength, muscle dielectric constant and conductivity as a function of frequency or magnetic field strength for protons. ? MS 10-2006 Page 4 Copyright 2006 by the National Electrical Manufacturers Association. Table 2-1 PHANTOM CONDUCTIVITY A
41、ND DIELECTRIC PROPERTIES (EXTRAPOLATED FROM REFERENCE 1) B0 (T) Frequency (MHz) Muscle Dielectric Constant (# #r) Muscle Conductivity (S/m) 0.1 4.26 631.33 0.58 0.2 8.51 249.35 0.63 0.3 12.77 156.95 0.65 0.4 17.03 122.44 0.65 0.5 21.29 106.03 0.66 0.6 25.54 96.98 0.66 0.7 29.80 91.45 0.66 0.8 34.06
42、87.82 0.66 0.9 38.32 85.29 0.66 1 42.57 83.44 0.67 1.1 46.83 82.04 0.67 1.2 51.09 80.95 0.67 1.3 55.35 80.06 0.67 1.4 59.60 79.32 0.67 1.5 63.86 78.70 0.68 1.6 68.12 78.15 0.68 1.7 72.37 77.67 0.68 1.8 76.63 77.24 0.69 1.9 80.89 76.84 0.69 2 85.15 76.47 0.69 2.5 106.43 74.86 0.71 3 127.72 73.43 0.73
43、 3.1 131.98 73.15 0.74 3.5 149.01 72.05 0.75 4 170.29 70.69 0.78 4.5 191.58 69.35 0.80 5 212.87 68.03 0.83 5.5 234.15 66.74 0.85 6 255.44 65.49 0.87 6.5 276.73 64.30 0.90 7 298.01 63.17 0.92 7.5 319.30 62.09 0.94 8 340.59 61.08 0.96 ? MS 10-2006 Page 5 Copyright 2006 by the National Electrical Manuf
44、acturers Association. An adequate check on phantom conductivity is to use the phantom to load the coil and compare to loading the coil with tissue. The results shall agree within tolerances given in 2.1.2 (loading is proportional to conductivity and SAR). A more sophisticated measurement of both con
45、ductivity and dielectric constant at the frequency of interest can be made with a network analyzer and a section of rigid (50 !) coaxial cable open and flat on one end. Impedance measurements (from the complex scattering parameter S11) should be referenced to the end of the open coaxial cable. The c
46、apacitance seen when the open coaxial cable is in air (and several cable diameters away from any other material), C0, is recorded. Let f(g) represent a function of geometry related to both capacitance and to resistance. Let “0 represent the permittivity of free space. Note that C0 = “0 f(g) = f(g) =
47、 C0 / “0. Next, ensuring there are no air gaps (and averaging at least six measurements), place the open-ended coaxial probe on the phantom material and record the capacitance, Cp and resistance, Rp. The relative dielectric constant, “r of the phantom material is: “r = Cp/C0. The conductivity, # of
48、the phantom material is: # = Rp / f(g). Figure 2-1 MEASUREMENT OF CONDUCTIVITY AND DIELECTRIC CONSTANT Note that references 3, 4, and 5 contain recipes for phantom compositions that approximate muscle electrical properties at various frequencies. ? MS 10-2006 Page 6 Copyright 2006 by the National El
49、ectrical Manufacturers Association. Table 2-2 COMPOSITION OF PHANTOM MUSCLE TISSUES FOR VARIOUS RADIOFREQUENCIES (NOTE?Percentages are by weight.) Reference 3 Frequency (MHz) TX-151 (%) Polyethylene Powder (%) Aluminum Powder (%) H20 (%) NaCl (%) 2450 8.46 15.01 75.48 1.051 915 8.42 15.44 75.15 0.996 750 8.42 15.44 75.15 0.996 433 8.42 15.44 75.15 0.996 300 8.42 15.44 75.15 0.996 200 8.39 15.79 74.92 0.894 100 9.81 2.12 87.59 0.482 77.0 10.36 2.72 86.5 0.424 40.68 9.68 9.2 80.