Radiography (2009) 15, 320e326

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Acoustic noise in magnetic resonance imaging: Anongoing issue

J.P. McNulty*, S. McNulty

UCD School of Medicine and Medical Science, Health Sciences Centre, UCD Belfield, Dublin 4, Ireland

Received 23 October 2008; revised 6 January 2009; accepted 19 January 2009Available online 15 February 2009

KEYWORDSAcoustic noise levels;Magnetic resonanceimaging;Noise reduction;Patient satisfaction;Hearing protection

* Corresponding author. DiagnosticMedicine and Medical Science, HeBelfield,Dublin4, Ireland.Tel.:þ3531

E-mail address: jonathan.mcnulty@

1078-8174/$ - see front matter ª 200doi:10.1016/j.radi.2009.01.001

Abstract Purpose: Acoustic noise creates a problem for both patients and staff within themagnetic resonance (MR) environment. This study qualitatively and quantitatively investigatesthe acoustic noise levels from two MR systems in one clinical department and demonstrates theadverse effects that the acoustic noise generated in magnetic resonance imaging (MRI) has ona patient’s experience of an MRI examination.Methods: A questionnaire was distributed to consenting patients undergoing one of twospecific MR examinations on two MR systems (System A and System B) of varying age and tech-nology in one clinical department. These evaluated the patient’s experience during the MRIexamination. Physical measurements of the maximum acoustic noise levels produced by eachsystem for various pulse sequences were also recorded using a sound level meter.Results: The results of the questionnaire survey demonstrated significantly greater toleranceof the acoustic noise levels of System B (mean noise level rating of 2.45 on LIKERT scale) incomparison to System A (mean noise level rating of 3.71 on LIKERT scale) (P Z 0.001). Signif-icantly lower noise level descriptions were also demonstrated (P Z 0.01). The maximum re-corded sound levels also confirmed that System B was quieter than the System A.Conclusion: It is has been demonstrated that the acoustic noise generated during an MRI exam-inations has an adverse effect on the patient experience during the examination. However,new technology has significantly reduced these effects and is improving patient comfort inMRI. It was shown quantitatively that the newer system’s advanced gradient technology wasquieter than the older system, in terms of the acoustic noise levels associated with a rangeof common pulse sequences.ª 2009 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.

Imaging, UCD School ofalth Sciences Centre, UCD7166545; fax:þ35317166547.

ucd.ie (J.P. McNulty).

9 The College of Radiographers.

Introduction

The gradient system and resulting gradient magnetic fieldare an essential part of magnetic resonance (MR) imageproduction.1e3 The rapidly altering magnetic fields producedduring scanning within an MR system result in the production

Published by Elsevier Ltd. All rights reserved.

Comparison of acoustic noise in magnetic resonance between two systems 321

of significant Lorenz forces which produce a vibration in thegradient coils.2 This is the primary source of acoustic noiseproduced in an MR system and may consist of a series of loudtapping, banging, or high pitched chirping noises as thegradient coils impact against their mountings.2,4e6 Thepresence of this acoustic noise is not completely avoidabledue to the fact that the gradient coils responsible for it arevital for image production.2,4 The acoustic noise produced bythe gradient coils is increased by: higher gradient duty cyclesand sharper pulse transition, decreasing the slice thickness,decreasing the field of view, decreasing the repetition time,decreasing the echo time or by using faster imaging tech-niques such as fast spin echo, echo planar imaging (EPI), andspiral k-space filling techniques.2,4e8 A secondary cause ofacoustic noise during an MR scan is from the RF pulsesintroduced to produce a signal from the patient. Howeverthis noise is negligible and completely masked by the loudernoise created by the gradient coils.9

Acoustic noise creates a problem for both patients andstaff within the MR environment.4,10 It poses a particularproblem for patients with head injuries, the elderly, youngchildren and neonates, patients with psychiatric disorders,those with sensitive hearing, anaesthetised patients andalso animals in veterinary practices 6,11,12 and it affectspatients in many negative ways ranging from minor,reversible effects such as annoyance and communicationproblems to more serious effects such as transient andpermanent hearing loss.4,6,10 Brummet et al. reporteda temporary shift of hearing in 43% of patients scanned,regardless of ear protection.4 A simulation study oncommunication difficulties reported that at the noisiestpulse sequences the operator needed to shout via anintercom system for the patient to understand 50% of whatwas said using an earplug model.13 While Tseng et al. statedthat the noise levels associated with fMRI, often in excessof 130 dB(A), seriously limit the effectiveness of auditorystimuli.10 Many people undergoing MRI scans will experi-ence some sort of anxiety reaction. These reactions varyfrom mild apprehension, which occurs in approximately 35%of patients, to severe panic and/or claustrophobia whichoccurs in an estimated 5e10% of patients.14 It has beendemonstrated that there is a direct correlation betweenthe acoustic noise level and claustrophobia.15 A recentstudy by Tischler et al. stated that anxiety surrounding MRexaminations is an ongoing problem and continues to causedisruption.16 Therefore the effects of acoustic noise onpatients undergoing examinations require evaluation andthe attenuation methods employed to reduce this noisecould be further improved.

The purpose of this study was to qualitatively andquantitatively investigate the acoustic noise levels fromtwo MR systems in one clinical department. To our knowl-edge, the patient’s own opinions on the effects of acousticnoise have not been evaluated previously.

Methods

Study design

The entire study was undertaken in one of the five Dublinarea teaching hospitals. This institution was chosen as the

radiology department incorporated two MR systems ofvarying manufacturer and technologies. System A wasa 1.5 T Signa LX (GE Healthcare, Chalfont St. Giles, UK)while System B was a 1.5 T Magnetom Avanto (SiemensHealthcare, Erlangen, Germany). The maximum gradientfield strength and slew rates for System A were 22 mT/mand 77 mT/m/ms respectively, and 40 mT/m and 200 mT/m/ms for System B. Both MR systems were located adjacentto each other and utilised shared staffing. Prior tocommencing the study proper full ethical approval wasgiven by both the university and hospital research ethicscommittees. Two different methodological approacheswere taken as described below.

Questionnaire survey

A questionnaire was devised to investigate the opinions ofpatients with regard to the perceived acoustic noisegenerated while undergoing MRI examinations. A ques-tionnaire-based study was preferred to an interview-basedstudy as it would reduce the risk of interviewer bias.17

A ten item questionnaire was compiled consisting of twosections. The first section, containing seven questions, wasaimed directly at the patient. Five of these were closedquestions which were designed to produce instantlycomparable answers, allowing for more straightforwardanalysis. The participants were also required to completetwo 5-point LIKERT scales which were chosen asa measuring tool to gain a better understanding of patientattitudes about the acceptability and perceived level of theacoustic noise during their examination. Open-endedquestions were also included to elicit further informationfrom the participants.17e19 The remaining three questionswere designed for completion by the radiographer andprovided technical information relating to the pulsesequences and MR system employed for the examination.

It was decided that an equal number of patients wouldbe surveyed on each MR system over the course of a three-week period. Throughout this period adult patients under-going a departmental standard brain (three plane fastspoiled gradient recalled echo localiser, axial dual-echo T2and proton density weighted sequence, sagittal T1weighted fast spin echo sequence, and sagittal T2 weightedhigh resolution fast spin echo sequence) and a depart-mental standard lumbar spine (three plane fast spoiledgradient recalled echo localiser, sagittal T2 weighted fastspin echo sequence, sagittal T1 weighted fast spin echosequence, and axial T2 weighted fast spin echo sequence)examinations were invited to participate. Every effort wasmade to standardise the pulse sequences employed byadjusting repetition times, echo times, echo train lengths,bandwidths, slice thickness and slice spacing. Patients withhead injuries, young children and neonates, patients withpsychiatric disorders and those with hearing problems werenot invited to participate. A patient information leaflet wasprovided and all consenting patients then completed thequestionnaire following their MR examination. Patientscould withdraw their participation at any stage throughoutthe process. A total of 42 patients participated in thesurvey with 21 undergoing an examination on System A andan equal number on System B.

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Figure 1 Responses to question relating to the acceptabilityof the acoustic noise experienced from 1 Z completelyacceptable to 5 Z completely unacceptable.

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322 J.P. McNulty, S. McNulty

Acoustic noise measurement

All measurements were made using a CEL-254 DigitalImpulse Sound Level Meter and matching CEL-112 calibrator(Casella CEL Ltd, Bedford, UK). This unit offered accuracyof �1 decibels (dB) with negligible magnetic field effects onmeasurement accuracy which was essential due to itsposition within the static and time-varying magneticfields.20 Following calibration the device was positioned ata distance of exactly 3 m from the isocentre of eachsystem. The ambient noise levels in each scan room wererecorded followed by the noise levels for a range ofcommon pulse sequences employed in brain and lumbarspine protocols on each system. All measures were upperacoustic noise levels recorded using the A-weighted decibelscale (dB(A)) as the peak sensitivity of the human ear is inthe region of 4 kHz, corresponding to the A-weighted scale,and this region is important in terms of hearing loss, as it isthe region where maximum hearing loss potentiallyoccurs.6,13,21

Statistical analysis

Data from each MR system pertaining to patient attitudesabout the acceptability and perceived level of the acousticnoise during their examination were compared and testedfor statistically significant differences with the non-para-metric Mann-Whitney U test by using a statistical softwarepackage (SPSS version 11.0 for Windows; SPSS, Chicago,USA). A P value of 0.01 (99% Confidence Level) wasconsidered to denote a statistically significantdifference.22

Results

A satisfactory distribution of brain and lumbar spineexaminations was achieved over the course of the three-week study period (Table 1). All patients participating inthe study were provided with hearing protection, with thetype of protection employed depending on which systemwas used. Those examined on System A and System B wereprovided with earplugs and headphones respectively.

All subjects were asked to grade the acceptability of theacoustic noise they experienced using a 5-point LIKERTscale with 1 being completely acceptable and 5 beingcompletely unacceptable. As can be seen from Fig. 1,System B achieved better overall acceptability ratings thanSystem A with a statistically significant difference betweenthe two (P Z 0.001).

Subjects were next asked to describe the perceivednoise levels using another 5-point LIKERT scale with options

Table 1 Breakdown of completed questionnaires.

MR system Examination Number of subjects

A Brain 14Lumbar spine 7

B Brain 13Lumbar spine 8

ranging from ‘Quiet’ to ‘Very Loud’. It is evident from Fig. 2that the perceived noise levels for System B were moreacceptable than for System A. A statistically significantdifference between the two systems was again demon-strated (P Z 0.01). The majority of the perceived noiselevels for brain examinations ranged from ‘Loud’ to ‘VeryLoud’ for System A and from ‘Moderate’ to ‘Very Loud’ forSystem B while for the lumbar spine examinations rangedfrom ‘Moderate’ to ‘Loud’ for System A and from ‘FairlyQuiet’ to ‘Moderate’ for System B (Fig. 3).

Acoustic noise measurement

The ambient noise levels in the scan rooms of System A andSystem B were 61.8 dB(A) and 57.6 dB(A) respectively. Forall pulse sequences employed System B was found to beconsiderably quieter than System A (Figs. 4 and 5). Overthe course of a standard brain examination, System Bachieved an upper acoustic noise level of 81.9 dB(A)compared to 87.2 dB(A) for System A. For the standardlumbar spine examination similar upper acoustic noiselevels were recorded at 81.6 dB(A) and 86.3 dB(A)respectively.

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Figure 2 Responses to question relating to the noise levels asperceived by the subject.

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Figure 3 Breakdown of perceived noise levels by system and examination.

Comparison of acoustic noise in magnetic resonance between two systems 323

Discussion

Questionnaire survey

Due to the questionnaire-based nature of this study, it isassumed that all respondents are unbiased in theirresponses.17 The aim of this part of the study was to eval-uate patient responses to the acoustic noise levels associ-ated with two of the most common MR examinations. Itshould be noted that the results of this questionnairesurvey are subjective and that for ethical reasons it was notpossible to scan each participant on each MR system whichwould have allowed them to directly compare the twosystems. The participants’ heads and ears were positionedat the isocentre of the magnet for the standard brainexamination and away from the isocentre for the lumbarspine examination. Therefore greater perceived noiselevels were expected for those undergoing brain

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Figure 4 Maximum acoustic noise levels recorded for the three lo

examinations.23,24 This qualitative study should thereforebe viewed with this in mind.

Analysis of the questionnaire responses indicated anoverall preference for System B with a mean acceptabilityrating of 2.45 compared to a mean acceptability rating of3.71 for System A (Fig. 1). Perceived acoustic noise levels forSystem B were also found to be significantly quieter thanSystem A (Fig. 2). The breakdown of perceived noise levels bysystem and examination was as expected (Fig. 3). One open-ended question allowed participants to elaborate on how theacoustic noise made them feel. Responses to this varied fromfeelings of mild annoyance to feelings of fear with a highernumber scanned on System A responding to this question.

Sound level survey

The quantitative study also yielded some interesting, ifsomewhat unsurprising, results. The newer System B,

Axial T2 Sagittal HighResolution

equence

System ASystem B

udest sequences employed as part of a standard brain protocol.

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Figure 5 Maximum acoustic noise levels recorded for the three loudest sequences employed as part of a standard lumbar spineprotocol.

324 J.P. McNulty, S. McNulty

incorporating improved gradient technology performedbetter than System A. Upper acoustic noise levels for thebrain examinations were 81.9 dB(A) and 87.2 dB(A)respectively while upper levels for the lumbar spineexamination were 81.6 dB(A) and 86.3 dB(A) respectively. Itmust be noted that the three loudest pulse sequences werechosen for each examination on each system and that otherpulse sequences would likely have been significantlyquieter on System B. The reduced ambient noise levels inthe scan room for System B (57.6 dB(A)) compared toSystem A (61.8 dB(A)) should also be noted. The primarysource of this ambient noise is the repetitive sound made bythe cryogen reclamation system.5 The newer gradient andgradient software technology of System B, with measuresfor acoustic noise reduction and highly compact, solid-stategradient technology, are claimed to significantly reduceacoustic noise by up to 30 dB(A) when compared toconventional and older systems.25 Most modern systemsseal the gradient coils in vacuum chambers and utilise somedegree of gradient acoustic shielding while one study byHaywood et al. looked at active noise control which isintegrated with the switched read gradient.11

Irish safety, health and welfare at work (control of noiseat work) regulations recommend that employees should notbe exposed to average daily noise levels exceeding87 dB(A).26 This is in keeping with the UK legislationregarding the control of noise at work.27 In both instanceshearing protection must be available to employees exposedto noise over 80 dB(A) and must be worn if levels exceed85 dB(A). This would include staff present in the MR scanroom during imaging. For patients and volunteers hearingprotection should always be provided unless it can bedemonstrated that acoustic noise levels will not exceed80 dB(A).27,28 Thus it is evident from this study that forSystem A, with mean acoustic noise levels of 86 dB(A) forthe three noisiest brain sequences and 85.6 dB(A) for thethree noisiest lumbar spine sequences at a distance of 3 mfrom the isocentre, hearing protection should always beworn by healthcare personnel remaining in the scan room.It should also be noted that many authors have measuredupper acoustic levels of 103e132 dB(A) at the isocentre

during an MRI scan5,11,23,29,30 and it is for this reason thatnoise reduction techniques must be employed to reduce thedecibel levels to the patient and staff, who ideally shouldremain outside the scan room during acquisition. Fosteret al. calculated that without suitable hearing protectionthe maximum permitted daily exposure would be exceededat the centre of the head coil in less than 5 s of exposureduring an EPI sequence.23

The most commonly used noise reduction technique isthe simple earplug. Earplugs have many advantages such astheir widespread availability and relatively low cost. Theyare also a very efficient method of reducing the amount ofnoise to the patient. However, Toivonen et al. concludedthat training in earplug insertion is important for goodattenuation with averaged attenuations for the untrainedand trained groups, 21 dB(A) and 31 dB(A) respectively.31

The use of earphones providing music is a helpful distrac-tion for the patient which may also put them at ease.Although it may be difficult to hear the music over theacoustic noise, use can noticeably decrease anxiety levelsin patients.28,32 Earplugs, when used in conjunction withearphones, can reduce acoustic noise levels to the patientby 39e41 dB(A) for a functional MRI exam29 and the MHRAalso state that earplugs and headphones should be used incombination for noisier sequences.28 Another method ofnoise reduction is active noise control (ANC), or anti-noise,which is a more expensive alternative but can result inperceived reductions of 50e70%. Following a real-timeFourier analysis of the noise produced a similar signal ofopposite phase is produced with the two sets of soundwaves effectively cancelling each other out. Haywood et al.reported noise reductions of up to 40 dB(A) using an ANCsystem built directly into the gradient system 2,7,11,30 whileTseng et al. discussed the use of predictive algorithms fromrecordings of specific sequences to improve ANC due to theunique noise of each sequence.10 In this study differentpassive noise reduction techniques were employed on eachsystem, earplugs alone on System A and headphones onSystem B. Due to the design of the radiofrequency receivercoils employed on System A it was not possible to useheadphones. The slightly better attenuating properties,

Comparison of acoustic noise in magnetic resonance between two systems 325

ability to play music and better communication provided byheadphones were a possible influencing factor on the par-ticipant’s responses.32 However, several authors havestated that both passive techniques demonstrate compa-rable noise reduction.24,29,31 A phantom study could havebeen employed to correct for the resulting small differ-ences due to the method of passive noise reduction as itwas deemed to be beyond the scope of the study due to thelimited time available.

In order to record a true measure of acoustic noise to thepatient, the sound level meter would have been positionedat the isocentre of each system. Thus reduced acoustic noiselevels were recorded in this study and it can be assumed thatupper acoustic noise levels exceeded occupational recom-mendations closer to the isocentre. Price et al. found that atfield strengths between 1.5 and 3 T there was a significantincrease in the noise with a patient, rather than a testobject, present through constructive interference of thereflected sound waves.28,33 Many previous studies employedtest objects during scanning. Another possible limitation wasthe fact that the acoustics and surfaces of each scan roomvaried slightly and the sound may have reverberated andbeen attenuated to different degrees in each room duringrecording. Moelker et al. demonstrated significant reduc-tions at both the patient’s and health worker’s positionswithin the scan room by applying acoustic insulation in andaround an MR system.24

Conclusion

It is has been demonstrated that the acoustic noise gener-ated during a magnetic resonance imaging (MRI) examina-tions has an adverse effect on the patient experienceduring the examination. Upon reviewing the qualitative andquantitative data, a definite, direct correlation is demon-strated in both aspects of the study, with System B per-forming better. Statistically significant differences weredemonstrated between the two systems when compared interms of acoustic noise level acceptability and perceivedacoustic noise levels. It was shown quantitatively that thenewer system’s advanced gradient technology was quieterthan the older system, in terms of both the ambientacoustic noise levels and the acoustic noise levels associ-ated with a range of common pulse sequences. This studyshows, both quantitatively and qualitatively, that newtechnologies may significantly reduced the adverse effectsthat acoustic noise has on the patient experience in MRI. Itcan therefore be deduced from this study, despite somemethodological limitations, that the newer technologies inMRI noticeably reduce the acoustic noise levels to thepatient.

Conflict of Interest

None declared.

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