1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA MS 10-2010Determination of Local Specific Absorption Rate (SAR) in Diagnostic Magnetic Resonance ImagingNEMA Standards Publication MS 10-2010 Determination of Local Specific Absorption Rate (SAR) in Diagnostic Magnetic Reson
2、ance Imaging Published by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 www.nema.org 2010 by the National Electrical Manufacturers Association. All rights, including translation into other languages, reserved under the Universal Copyright Convent
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10、 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 the topic covered by this publica
11、tion 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 not certify, test, or inspect produ
12、cts, designs, or installations for safety or health purposes. Any certification or other statement of compliance with any health or safetyrelated information in this document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement. MS 10-2010 Pag
13、e i Copyright 2010 by the National Electrical Manufacturers Association. CONTENTS Page Foreword . ii Section 1 GENERAL 1 1.1 Rationale 1 1.2 Scope . 1 1.3 References 1 1.4 Definitions 2 Section 2 DETERMINING LOCAL SAR 3 2.1 Properties of the Tissue Equivalent Phantom . 3 2.2 RF Coil Loading Characte
14、ristics of the Tissue Equivalent Phantom . 6 2.3 Scan Conditions 6 2.4 Experimental Measurement . 7 Section 3 REPORTING RESULTS 9 3.1 Parameters 9 3.2 Phantom Parameters . 9 3.3 Local SAR Results . 9 3.4 Additional Data 9 3.5 Repeatability Data . 10 3.6 Sources of Error 10 Appendix THEORETICAL ESTIM
15、ATE OF LOCAL SAR . 11 MS 10-2010 Page ii Copyright 2010 by the National Electrical Manufacturers Association. Foreword This Standards Publication is classified as a NEMA Standard unless otherwise noted. It describes a measurement method for local specific absorption rate (SAR). The measurement metho
16、d 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 address whole-body SAR. This Standards Publication has been developed by the Magnetic Reso
17、nance 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. At the time it was approved, the section was composed of the following members: Computer
18、 Imaging Reference SystemsNorfolk, VA GE Healthcare, Inc.Milwaukee, WI Hitachi Medical Systems America, Inc.Twinsburg, OH Medipattern Toronto, Ontario Philips Medical Systems North AmericaBothell, WA Siemens Medical Solutions, Inc.Malvern, PA Time Medical Toronto, Ontario Toshiba America Medical Sys
19、tems 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 Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 MS 10-2010 P
20、age 1 Copyright 2010 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 primary safety concern with both transmit surface coils and receive-only surface coils involves loca
21、l SAR Local SAR may be highest near conductors. Methods for determining local SAR are needed for ensuring safe operation of coils during MR exams. This standard does not attempt to establish relationships between SAR and body temperature. 1.2 SCOPE This document defines methods for determining the l
22、ocal 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. Local SAR may be determined near coil conductors, transmission lines, or any other desired region. It may be useful in some
23、cases to first do the pulse energy method described in NEMA MS-8 so that both whole-body and local SAR are measured. Heating other than radio frequency heating (such as thermal heating from surface coil blocking networks) is not addressed in this standard. 1.3 REFERENCES 1. Durney, C.H., Johnson, C.
24、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. 2. Schenck, J.F., E.B. Boskamp, D.J. Schaefer, W.D. Barber and R.H. Vander Heiden, “Estimating Local SAR Produced by RF Transmitt
25、er Coils: Examples Using the Birdcage Coil.” Abstracts of the International Society of Magnetic Resonance in Medicine, Sixth Meeting, Sydney, Australia, p. 649, 1998. 3. Chou CK, Chen GW, Guy AW, Luk KH (1984): Formulas for preparing phantom muscle tissue at various radiofrequencies. Bioelectromagne
26、tics 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. Grandolfo, M, Polichetti, A, Vecchia, P, and Gandhi, OP, 1992, “Spatial Distribution of RF Power in Critical Organs during Magnetic Reson
27、ance 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 2.45 GHz,“ Abstracts of the Third Annual Meeting of the Bioelectromagnetics Society, Washington, D.C., August, 1981, abstract publis
28、hed, p. 59. 7. M. A. Stuchly, S. S. Stuchly, “Coaxial line reflection method for measuring dielectric properties of biological substances at radio and microwave frequencies - A review,“ IEEE Trans. Instrum. Meas., vol. 29, n3, September 1980, pp 176 - 183. 8. IEC 60601-2-33, 2nd edition, Medical Ele
29、ctrical Equipment - Part 2: Particular Requirements for The Safety of Magnetic Resonance Equipment for Medical Diagnosis, International Electrotechnical Commission (IEC)*, 3, rue de Varemb, P.O. Box 131, CH - 1211 Geneva 20, Switzerland (In the United MS 10-2010 Page 2 Copyright 2010 by the National
30、 Electrical Manufacturers Association. States, copies of this standard can be obtained from the American National Standards Institute (ANSI), 11 West 42nd Street, New York, NY 10036), (2002). 9. Park, SM, Nyenhuis, JA, Smith, CD, et al, “Gelled Versus Nongelled Phantom Material for Measurement of MR
31、I-Induced Temperature Increases With Bioimplants“, IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 2003, pp 3367- 3371. 1.4 DEFINITIONS Average SAR: the rate of energy deposition per unit mass. B1rms: The root mean square value of B1 over the pulse sequence, assuming B1 is properly calibra
32、ted. 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. Safety standards usually average over the worst-case 10 g. electrical conductivity (): Electrical conductivity is the ratio of current de
33、nsity to electric field. 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 freq
34、uency of interest 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 10-2010 Page 3 Copyright 2010 by the National E
35、lectrical Manufacturers 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. Howeve
36、r, phantom dimensions 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 c
37、lose to the phantom 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
38、), aluminum powder - 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 prop
39、erties are most 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 B1calibration 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.
40、More stable 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),
41、the dielectric 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
42、 convection for example by including a gelling agent. Electrical properties of muscle tissue provided by the US Air Force (1) The following table summarizes field strength, muscle dielectric constant and conductivity as a function of frequency or magnetic field strength for protons. MS 10-2010 Page
43、4 Copyright 2010 by the National Electrical Manufacturers Association. Table 2-1 PHANTOM CONDUCTIVITY AND 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.
44、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 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
45、 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 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
46、 62.09 0.94 8 340.59 61.08 0.96 MS 10-2010 Page 5 Copyright 2010 by the National Electrical Manufacturers 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.
47、2 (loading is proportional to conductivity and SAR). A more sophisticated measurement of both conductivity 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 co
48、mplex scattering parameter S11) should be referenced to the end of the open coaxial cable. The capacitance 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
49、 to resistance. Let 0represent the permittivity of free space. Note that C0= 0f(g) = f(g) = 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, Cpand resistance, Rp. The relative dielectric constant, rof the phantom material is: r = Cp/C0. The conductivity, of 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 comp
