NEMA MS 11-2010 Determination of Gradient-Induced Electric Fields In Diagnostic Magnetic Resonance Imaging Published by National Electrical.pdf

1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA MS 11-2010 Determination of Gradient-Induced Electric Fields In Diagnostic Magnetic Resonance ImagingNEMA Standards Publication MS 11-2010 Determination of Gradient-Induced Electric Fields In Diagnostic Magnetic Resonance Im

2、aging Published by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 2010 by the 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. 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

4、 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 publications, of which the document contained herein is

5、 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 process and establishes rules to promote fairness i

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10、e 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 publication may be availabl

11、e 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 products, designs, or ins

12、tallations 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 11-2010 Page i Copyright 2006 b

13、y the National Electrical Manufacturers Association. CONTENTS Page Foreword ii Section 1 GENERAL 1 1.1 Rationale 1 1.2 Scope 1 1.3 Definitions.1 Section 2 EXPERIMENTAL MEASUREMENT OF GRADIENT-INDUCED ELECTRIC FIELDS FROM GRADIENT COILS.3 2.1 Electrical and Mechanical Properties and Dimensions of Ele

14、ctric Field Dipoles and Spacers .3 2.1.1 Measurement Phantom 3 2.1.2 Electric Field Components to be Measured4 2.1.3 Electric Field Dipole Leads5 2.1.4 Search Methods for Locating Position of Maximum Electric Field Magnitude.5 2.2 Measurement Protocol6 Section 3 REPORTING OF RESULTS 8 3.1 Measuremen

15、t Protocol8 3.2 Phantom Parameters8 3.3 Electric Field Measurement Results 8 3.4 Additional Data 8 3.5 Repeatability Data8 3.6 Sources of Error 8 Appendix THEORETICAL ESTIMATE OF GRADIENT-INDUCED ELECTRIC FIELDS FROM GRADIENT COILS.9 MS 11-2010 Page ii Copyright 2006 by the National Electrical Manuf

16、acturers Association. Foreword This Standards Publication is classified as a NEMA Standard unless otherwise noted. It describes experimental measurement methods for determining gradient-induced electric fields for each gradient axis at a radius of 20 cm off the patient axis. The measurement method r

17、equires construction of electric field dipoles, spacers, a phantom and the use of a high impedance device for measuring voltages. The electric field measurements are done in a solution with a conductivity similar to that of the body. Numerical methods of estimating the electric field are also discus

18、sed in an appendix. 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. At

19、 the time it was approved, the section was composed of the following members: Computer Imaging Reference SystemsNorfolk, VA Fonar CorporationMelville, NY GE Healthcare, Inc.Milwaukee, WI Hitachi Medical Systems America, Inc.Twinsburg, OH Invivo Corp.Gainesville, FL Philips Medical Systems North Amer

20、icaBothell, WA Siemens Medical Solutions, Inc.Malvern, PA Toshiba America Medical SystemsTustin, 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 Manufac

21、turers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 MS 11-2010 Page 1 Copyright 2006 by the National Electrical Manufacturers Association. Section 1 GENERAL 1.1 RATIONALE Gradient-induced electric fields affect the safety and comfort of patients. Electric field magnitude ultimate

22、ly determines whether patients experience discomfort. Typically, stimulation requires a gradient-induced electric field of at least 2 V/m. Methods for determining gradient-induced electric fields are needed to ensure safe operation of gradient coils during MR exams. The method described makes electr

23、ic field measurements inside a phantom loaded with electrically conductive material to simulate the body. The signal to noise ratio of an electrically short (infinitesimal) dipole in air is not likely to be adequate. 1.2 SCOPE This document defines methods for determining the gradient-induced electr

24、ic fields of diagnostic magnetic resonance imaging gradient coils (head and body) under a specific set of conditions. This document does not address the effect of electrical inhomogeneities in the body on internal, gradient-induced, electric fields. 1.3 DEFINITIONS gradient-induced electric field: E

25、lectric field in volts/m induced by time-varying gradient magnetic fields. electric field dipole probe: Two electrical conductors, used to measure electric field, lying on the same line. The conductors are electrically separated (not constrained to be at the same electrical potential) and also physi

26、cally separated by some distance, d (see Figure 2-2). z coordinate: While this spatial coordinate is conventionally defined as the direction of the static magnetic field, for the purposes of this standard, it is defined as the direction of the long axis of the patient. magnetic vector potential, A:

27、The magnetic vector potential, A, is related to the magnetic induction vector, B, such that: .AB electric field, E: The electric field, E, is related to the time, t, rate of change of the gradient magnetic vector potential, A and to the electrostatic potential, : . tAE Electrostatic potential arises

28、 from conservation of electric charge at discontinuities in electrical conductivity (such as the air-patient interface). In a conductive cylinder with cylindrically-symmetric z MS 11-2010 Page 2 Copyright 2006 by the National Electrical Manufacturers Association. gradients along the axis of the cyli

29、nder, the gradient of electrostatic potential is zero. The electrostatic potential does not vanish for gradient axes other than z. measurement phantom: Radiation resistance of electrically small dipole antennas at low frequencies make measurement of gradient-induced electric fields in air problemati

30、c. However, it is possible to infer the electric field by measuring a potential difference across current flowing in a conductive measurement phantom. The Measurement Phantom contains material similar to the conductivity of muscle at gradient frequencies (below approximately 3 KHz). In addition, the

31、 phantom shall accommodate electric field dipole probes for electric field measurements. Note that the phantom may be a cylinder or an annulus for systems where static magnetic fields are aligned with the patient axis. For all other systems, the phantom shall be a cylinder. The phantom length shall

32、be sufficient to span the gradients under test. For compliance with IEC 60601-2-33, 2nd edition (2002), the electric fields shall be measured on a 20 cm radius for body gradients and 10 cm radius for head gradients. The phantom axis shall coincide with the long axis of the patient. MS 11-2010 Page 3

33、 Copyright 2006 by the National Electrical Manufacturers Association. Section 2 EXPERIMENTAL MEASUREMENT OF GRADIENT-INDUCED ELECTRIC FIELDS FROM GRADIENT COILS 2.1 ELECTRICAL AND MECHANICAL PROPERTIES AND DIMENSIONS OF ELECTRIC FIELD DIPOLES AND SPACERS 2.1.1 Measurement Phantom A measurement phant

34、om shall be constructed to hold a cylindrical or else an annular (annular phantoms may only be used for systems where the static magnetic field is aligned with the patients axis) volume of aqueous sodium chloride solution (0.1% = 1 gram/liter). The annular conductivity of the measurement phantom sha

35、ll be 0.2 0.02 siemens/m to mimic that of human muscle. The outer radius in the aqueous sodium chloride shall be at least 20 cm for body gradient phantoms and 10 cm for head gradient phantoms. The inner radius in the sodium chloride solution shall be no more than 18 cm for body gradient phantoms and

36、 8 cm for head gradient phantoms. The length of the phantom shall be the dimension of the gradient coils or 1.75 m, whichever is less. For body gradients, the electric fields of interest lie on a 20 cm radius from the long axis of the patient. For head gradients, the electric fields of interest lie

37、on a 10 cm radius from the long axis of the patient. Dipoles no longer than 4 cm and insulated everywhere except at the tips, shall be inserted in the conductive solution as shown in Figure 2-1. The following paragraph gives guidance for gradient coils in solenoidal magnets. In general, the maximum

38、electric field will be found near the maximum current density for a given gradient coil independent of the type of magnet used. For transverse gradient coils in solenoidal magnets, the maximum electric field will be in the direction at the z location corresponding to maximum current density (Figure

39、2-3) and y=20 cm (A/P) (y=10 cm for head gradients) or x=20 cm (L/R)(x=10 cm for heads). A dipole shall be inserted, aligned with the electric field (conductor pattern) along at z=0 or wherever is appropriate to find the maximum electric field. A second dipole may be similarly placed 90 degrees away

40、 or the phantom may be rotated for measuring other gradient axes. If desired a set of dipoles aligned with z may be placed at z=0, 90 degrees apart to measure z components. These locations are illustrated in Figure 2-1. The axial (z) gradient coil in solenoidal magnets will produce electric fields a

41、long the direction at the z location corresponding to maximum current density (Figure 2-3). Another dipole aligned along the direction shall be placed at approximately z=30 cm (z=15 cm for heads) or the position of peak current density. During actual measurements the phantom may be moved along the z

42、 and directions to find the maximum electric field. MS 11-2010 Page 4 Copyright 2006 by the National Electrical Manufacturers Association. Figure 2-1 MEASUREMENT PHANTOM 2.1.2 Electric Field Components to be Measured Only the E and Ez components of the electric field are to be measured centered at a

43、 point where the highest electric field magnitude (on the surface of a 20 cm radius cylinder concentric with the patient axis) is anticipated. The electric field dipoles shall not exceed 4 cm in length. The dipole probes shall be insulated everywhere except at the tips. The dipole conductors shall l

44、ie along the same axis and shall be rigid so the separation between the tips is unambiguous. It may be desirable to mount the dipoles to something like rubber stoppers which attach to the measurement phantom. The measurement phantom shall be constructed to properly position dipole alignments. Dipole

45、 centers may be placed at the proper positions (i.e., surface of a 20 cm radius cylinder concentric with the patient axis) by rotating the measurement phantom and possibly displacing the phantom along z. MS 11-2010 Page 5 Copyright 2006 by the National Electrical Manufacturers Association. Figure 2-

46、2 ELECTRIC FIELD DIPOLE PROBE 2.1.3 Electric field dipole leads Leads from electric field dipoles shall be twisted or otherwise arranged to ensure that electric field measurements are coming only from the dipoles and not from the leads. Note that the electric field dipoles described here are so elec

47、trically small that almost no radiation (radiation resistance should be on the order of 10-11 ohms) would be expected from the dipole or from the leads. Therefore, impedance matching is not a concern. 2.1.4 Search Methods for Locating Position of Maximum Electric Field Magnitude If available, maps (

48、Z versus , when r = 0.2 m) of the magnetic vector potential magnitude (or electric field magnitude) for the active gradient coil(s) will be very helpful in determining the position of the maximum electric field magnitude. Only one gradient axis shall be active at a time. The peak electric field magn

49、itude (on the surface of a 20 cm radius cylinder concentric with the patient axis) should appear near the region of highest current density on the gradient coil. MS 11-2010 Page 6 Copyright 2006 by the National Electrical Manufacturers Association. Figure 2-3 EXAMPLE OF HIGHEST CURRENT DENSITY REGIONS FOR SOLENOIDAL MAGNETS 2.2 MEASUREMENT PROTOCOL Note that these measurements can be made for a gradient coil placed outside the magnet provided the patient table is positioned as it is in the scanner and the interaction