1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA MS 4-2010Acoustic NoiseMeasurement Procedurefor Diagnostic MagneticResonance ImagingDevicesNEMA Standards Publication MS 4-2010 Acoustic Noise Measurement Procedure for Diagnostic Magnetic Resonance Imaging Devices Published
2、 by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 www.nema.org 2010 by National Electrical Manufacturers Association. All rights, including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention
3、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 time
4、it was developed. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this document. NEMA standards and guideline publications, of which the document contained herein is one, are developed through a voluntary consensus standard
5、s 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 development of consensus, it does not write the docu
6、ment 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, property, or other damages of any nature whatsoever,
7、 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 or completeness of any information published
8、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 by virtue of this standard or guide. In publish
9、ing 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 document should rely on his or her own indepe
10、ndent 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 sources, which the user may wish to consult f
11、or 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 for safety or health purposes. Any certificatio
12、n 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 4-2010 Page i Contents Preamble .ii Foreword.iii Introduction.iv Rationale.iv Section 1 R
13、EFERENCED STANDARDS AND DEFINITIONS 1 1.1 Scope. 1 1.2 Referenced Standards 1 1.3 Definitions 1 1.3.1 SPL (Sound Pressure Level) 1 1.3.2 A-weighting. 1 1.3.3 ISLM (Integrating Sound Level Meter) . 2 1.3.4 LAeq2 1.3.5 Lpeak. 2 Section 2 DATA ACQUISITION PARAMETERS 3 2.1 Maximum Gradient Acoustic Nois
14、e (MGAN) Method 3 2.2 Maximum Clinical Acoustic Noise (MCAN) Method 3 Section 3 METHODS OF MEASUREMENT5 3.1 Test Hardware . 5 3.2 Hardware Setup. 5 3.3 MGAN Measurement. 5 3.4 MCAN Measurement. 8 Section 4 RESULTS 10 4.1 Reporting Results 10 Annex . 11 Copyright 2010 by the National Electrical Manuf
15、acturers Association. MS 4-2010 Page ii Preamble This is one of a series of test standards developed by the medical diagnostic industry for the measurement of parameters governing the safety of Magnetic Resonance (MR) Imaging (MRI) systems. These test standards are intended for the use of equipment
16、manufacturers, testing houses, prospective purchasers, and users alike. Manufacturers are permitted to use these standards for the determination of system performance specifications. This standardization of performance specifications is of benefit to the prospective equipment purchaser. The paramete
17、rs 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 quality assurance. It must be recognized, however, that not all test standards lend themselves to me
18、asurement at the installation site. Some test standards require instrumentation better suited to factory measurements, while others require the facilities of an instrumentation laboratory to ensure stable test conditions necessary for reliable measurements. The NEMA test procedures shall be carried
19、out using the normal clinical operating mode of the system. For example, standard calibration procedures and standard reconstruction processes shall be used. No modifications to alter test results shall be used unless otherwise specified in these standards. The NEMA Technical Committee of the MR Sec
20、tion has identified a set of key magnetic resonance safety parameters. This standard describes the measurement of one of these parameters. Equivalence It is intended and expected that manufacturers or others who claim compliance with these NEMA standard test procedures for the determination of safet
21、y parameters shall have carried 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
22、 must be included with any measurement claimed to be equivalent to the NEMA standard. The validity or equivalence of the modified procedure will be determined by each reader. Uncertainty of the Measurements The measurement uncertainty of the safety parameter determined using this standard is to be r
23、eported, 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. Copyright 2010 by the National Electrical Manufacturers Association. MS 4-2010 Page iii Foreword This standards p
24、ublication is classified as a NEMA standard unless otherwise noted. It describes the test conditions and parameters that approximate the worst case acoustic noise levels that a particular magnet/gradient system combination produces when using pulsed gradient waveforms. This standard also describes h
25、ow the acoustic noise levels are to be measured. In the absence of specific guidelines for sound level exposure with MR imaging equipment, this procedure references the OSHA guidelines for acoustic noise exposure and the IEC standards for sound level meters. This standards publication has been devel
26、oped by the Magnetic Resonance Section 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: Executive Director, Medical Imaging A+max, A-max) millitesla/meter Ri
27、se and fall times (r1and r2; r1and r2) microseconds Gradient pulse repetition time (TR) milliseconds Gradient pulse on time (TP) milliseconds Meter response time*- 2.2 MAXIMUM CLINICAL ACOUSTIC NOISE (MCAN) METHOD The following parameters must be reported for the MCAN method per Section 4: Parmetr D
28、imension B0(static field strength) tesla Sequence type/name - Repetition time (TR) milliseconds Echo time(s) milliseconds Number of slices - Slice thickness millimeters Field-of-view millimeters Data acquisition matrix size - Acquisition bandwidth Hz/pixel Scan plane - Other (e.g., flow compensation
29、, on/off, thickness, - orientation, preparation and saturation pulses, spoilers, etc.) Meter response time* - *For short integration periods (10 seconds or less), the different responses of a FAST and SLOW network can produce slightly different levels of LAeq; the discrepancy will “average out” and
30、disappear after 20 seconds of integration. It is recommended that a FAST meter response time (125 milliseconds time constant per IEC 61672) be used for the measurement. However, a SLOW meter response time (1000 milliseconds time constant per IEC 61672) can be used for integration periods greater tha
31、n 20 seconds. (This note has been approved as Authorized Engineering Information.) Copyright 2010 by the National Electrical Manufacturers Association. MS 4-2010 Page 4 Figure 2-1 Maximum Amplitude Bipolar Gradient Amplifier Input Waveform (without Pre-Emphasis) Copyright 2010 by the National Electr
32、ical Manufacturers Association. MS 4-2010 Page 5 Section 3 METHODS OF MEASUREMENT 3.1 TEST HARDWARE The following equipment is required: ISLM: An integrating sound level meter that meets ANSI Standard S1.4 Type 0 or 1 or IEC Standard 61672 Type 0 or 1 Microphone: The microphone must be omnidirection
33、al and insensitive to magnetic fields or calibrated to account for the magnetic field A microphone extension cable For MGAN: A sine wave generator capable of frequency sweeps or a white noise source 3.2 HARDWARE SETUP The following setup shall be used: The ISLM with the extension cable shall be cali
34、brated using a sound level standard Using the microphone extension cable, the ISLM shall be placed sufficiently far from the magnet isocenter to be in a fringe field of less than 10 gauss. Care must be taken to ensure that environmental effects (e.g., pulsed gradient fields, EMI, etc.) are compensat
35、ed for or avoided All measurements shall be made with the microphone aimed in a direction orthogonal to the patient axis 3.3 MGAN MEASUREMENT To measure the MGAN, a trapezoidal bipolar test waveform (see Figure 2-1) shall be applied simultaneously to the X, Y, and Z gradient coils. Note that there i
36、s no dead time in the test waveform. The waveform shall be generated using the standard pulse sequence generation software and hardware supplied by the manufacturer. The generation of the test waveform is subject to the manufacturers pulse sequence design limitations (amplitude, rise-time, fall-time
37、, and duty cycle). Within these design limitations, the rise and fall times of the test waveform shall be the minimum allowed, the amplitude shall be the maximum allowed, and the duty cycle shall be the maximum allowed. The repetition time shall be chosen to correspond to the frequency of the loudes
38、t noise. Further, eddy current compensation, also known as pre-emphasis, must be included since this is the mode in which the system is operated. The acoustic noise will be greater with pre-emphasis. The MGAN measurement consists of three steps: Search for the frequency of the highest SPL to determi
39、ne the timing of the excitation waveform (sequence) Search for the position in the patient space that has the highest SPL Perform the noise measurement with the worst case frequency at the worst case position The fundamental frequency of the driving gradient waveform (i.e., f = 1/TR) that produces t
40、he highest SPL could be lower than the acoustic frequency of the highest SPL due to the presence of mechanical resonances in the MR system (see the Annex for further discussion). In addition, the fundamental frequency of the driving gradient waveform that produces the highest SPL may not be equal to
41、 the maximum frequency (fmax) the system could theoretically attain at maximum gradient amplitude. For these reasons, the system response must first be measured over a frequency range up to three times the frequency fmax(see the Annex for an explanation of the choice of upper limit). Copyright 2010
42、by the National Electrical Manufacturers Association. MS 4-2010 Page 6 The complete measurement procedure is as follows: 1. Set up the equipment as described in section 3.2. 2. Turn the RF amplifier off or disable its output. 3. Measure the ambient SPL LAeqat magnet isocenter using the following ISL
43、M settings: Detector: rms Time weighting: FAST or SLOW Frequency weighting: A-weighting Measurement period: 20 seconds The ambient noise level shall be at least 20 dB(A) smaller than the noise level generated by the MR system. 4. Compute fmax, the highest fundamental frequency of the driving gradien
44、t waveform that may theoretically be achieved on the MR system using a triangular pulse at maximum gradient strength and minimum rise time: fmax= 1/TR min, where TR minis the shortest possible repetition time TRof the waveform described in Figure 2-1 at maximum gradient strength A+maxand A-max, mini
45、mum transition times r1, r2, and with TP= 0. 5. Measure the system response function (noise spectrum) over the frequency range 0 fmax, there are two ways to maximize the acoustic noise produced by the continuously pulsed bipolar test waveform at one of those frequencies. The first way is to match th
46、e acoustic frequency of maximum SPL with the 3rdharmonic of the excitation frequency of the bipolar waveform. That is, if the excitation frequency of the bipolar pulse train is one-third the frequency of the highest SPL, then sequence 2 is a candidate for producing the highest possible SPL. This is
47、possible due to the definition of 3 fmax. This is the option chosen in this standard. The second way to put a high amount of energy into a frequency f fmaxis to directly excite it. However, this is only possible at reduced gradient strength, and it is unlikely to produce a significantly higher SPL.
48、Consequently, this option was not chosen, but it is worth mentioning. In this context, two very different spectra can be contemplated: a rather constant or slightly rising spectrum, and a spectrum dominated by one resonant frequency in the frequency range above fmax. A.2.3 Finding the Loudest Spot T
49、he standard stipulates finding the position of worst case noise along the patient axis because it may not occur at isocenter because of the presence of standing waves. Standing air waves may result in significant noise differences within a distance of one-half wavelength. At 1000 Hz, the wavelength in air is about 0.33 m. The distance between a noise maximum and a noise minimum in a standing wave at 1000 Hz is therefore in the range of 0.16 m. It is assumed that a search along the patient axis fulfills the requirement. Note
