Proceedings of the Second APSIPA Annual Summit and Conference, pages 543–548,Biopolis, Singapore, 14-17 December 2010.

Active Noise Control System for MR NoiseMasafumi Kumamoto∗, Yoshinobu Kajikawa∗, Toru Tani†, and Yoshimasa Kurumi†

∗ Kansai University, Yamate-cho, Suita-shi, Osaka, 564-8680, JapanE-mail: kaji@kansai-u.ac.jp Tel: +81-6-6368-1121

† Shiga University of Medical Science, Tsukinowa-cho, Seta, Otsu-shi, Shiga, 520-2192, JapanE-mail: [Tan.kurumi]@belle.shiga-med.ac.jp Tel: +81-77-548-2111

Abstract—We propose an active noise control (ANC) systemfor reducing periodic noise generated in a high field area suchas noise generated from magnetic resonance imaging (MRI)devices (MR noise). The proposed ANC system utilizes opticalmicrophones and piezoelectric loudspeakers, because specificacoustic equipment is required to overcome the high-field prob-lem, and consists of a head-mounted structure to control noisenear the user’s ears and to compensate for low output ofthe piezoelectric loudspeaker. Moreover, internal model control(IMC)-based feedback ANC is employed because the MR noiseincludes some periodic components and is predictable. Ourexperimental results demonstrate that the proposed ANC system(head-mounted structure) can significantly reduce MR noise byapproximately 20 dB in a high field in an actual MRI room evenif the imaging mode changes frequently.

I. INTRODUCTION

Recently, magnetic resonance imaging (MRI) devices,which are used to take images inside of the body, have beenintroduced in many medical institutions on the grounds ofsafety and convenience. In particular, an open-configurationMR system[1] has been introduced to conduct microwavecoagulation therapy with near-realtime MR images. However,taking images with an MRI device leads to intense noise(referred to as MR noise in this paper) because the gradientcoil vibrates in the MRI device owing to the Lorentz force.The exposure to the intense noise may cause operators andother medical staff to suffer extreme stress and preventscommunication between staff. This may lead to accidents[2].

So far, many various approaches have been applied toreduce MR noise. These approaches are classified into passivenoise control, the design of silent MRI pulse sequences,and active noise control. Passive noise control includes theuse of earplugs or ear protection, modifying the gradientcoil design, and minimizing the vibration resonances of thecoil assembly[3]. However, this approach is mainly effectivefor high-frequency noise but requires bulky size and is noteffective for low-frequency noise. Unfortunately, MR noisehas a high sound pressure level (SPL) at low frequencies.Moreover, the passive noise control devices prevent verbalcommunication between medical staff during surgery utilizingthe MRI system[2].

The design of silent MRI pulse sequences relies on selectingimaging parameters to reduce the related acoustic noise. Theuse of silent MRI pulse sequences results in approximately20 dB attenuation of the SPL. However, this technique limitsimaging sequences and reduces image resolution[2].

(a) (b)

Fig. 1. Pictures of acoustic equipment for use in the presence magnetic field.(a) An optical microphone. (b) A piezoelectric loudspeaker.

(a) (b)

Fig. 2. Pictures of a piezoelectric loudspeaker with various vibration trans-mitters. (a) A plastic plate. (b) A plastic cup.

On the other hand, active noise control (ANC)[4], [5] mayoffer an alternative technique for reducing MR noise. ANCis a technique based on the principle of superposition, i.e., anantinoise with equal amplitude and opposite phase is generatedand combined with an unwanted noise, thus resulting in itscancellation. The application of ANC to MR noise has beenpreviously reported [6], [7], [8], [9], [10], and approximately15 to 25 dB noise reduction has been reported. However, thesestudies had the following limitations. First, the noise reductionexperiments were conducted by simulations or in a laboratoryset up, not in actual MRI rooms[6], [7], [8], [10]. Second, in allstudies a headset-based system was utilized, which preventedverbal communication between medical staff and provided afeeling of pressure on the user’s ears and separation from theoutside acoustical environment. Third, in none of studies wasthe user’s movements considered. Thus, the results of thesestudies were of limited use for medical staff. Finally, most ofthe conventional studies on the use of ANC for reducing MRnoise utilized a feedforward ANC system based on the filtered-x (FX) algorithm. The feedforward ANC system requiresat least two microphones (reference and error microphones)and sufficient distance must be maintained between the two

543

10-0105430548©2010 APSIPA. All rights reserved.

microphones to ensure causality, thus increasing the size ofthe system. Moreover, the reference signal detected at thereference microphone must have a strong correlation with thenoise reaching the error microphone. Unfortunately, it is verydifficult to place the reference microphone at an appropriateposition because the MRI device is very large and the noisesource is unlocatable.

Hence, we propose a novel ANC structure that can realizehigh noise reduction, allow clear verbal communication be-tween medical staff, be comfortably mounted without a feelingof pressure on the ears, and achieve noise reduction regardlessof the user’s movements. The proposed ANC structure is com-posed of two microphones arranged near the user’s ears andtwo loudspeakers closely arranged against the user’s ears. Wecall this structure a head-mounted ANC system to distinguishit from the headset-based ANC system. The head-mountedANC system utilizes optical microphones and piezoelectricloudspeakers to realize noise reduction under a high-field areaand can compensate the inadequate output SPL of the piezo-electric loudspeakers because the loudspeakers are close tothe user’s ears. Moreover, the proposed ANC system employsan internal model control (IMC)-based feedback system[11],[12], [13] which can reduce predictable noises, is independentof direction of arrival of noise, and is small in comparisonwith a feedforward ANC system. The proposed ANC systemcan effectively reduce the MR noise between 500 Hz and2500 Hz because MR noise consists of many strong periodiccomponents. Our experimental results demonstrate that theproposed ANC system can reduce MR noise by approximately20 dB in a high field in an actual MRI room.

II. SUITABLE ACOUSTIC EQUIPMENT IN MRI ROOM

MRI devices generate intense magnetic fields of approxi-mately 0.5 T. Therefore, it is necessary for acoustic equipmentin the ANC system to satisfy the following conditions. Thefirst condition is that equipment must work normally in anintense magnetic field. The second condition is that equipmentmust not affect the MR image. Hence, acoustic equipmentcontaining magnetic materials cannot be used for the ANCsystem. An optical microphone and a piezoelectric loudspeakerare acoustic devices satisfying these conditions. The opticalmicrophone detects the light of a light-emitting diode that hasbeen reflected by a photodiode and detects sound by measuringthe displacement of a diaphragm, and thus satisfies the aboveconditions because no magnetic materials are used. Also, thepiezoelectric loudspeaker transforms an electrical signal intomechanical vibration by using the piezoelectric effect. Figure 1shows the optical microphone and piezoelectric loudspeakerused in this study.

The optical microphone has a flatness within ± 3 dB from10 to 15000 Hz, linearity for an input signal of 141 dB, anda sensitivity of 98 mV/Pa. The piezoelectric loudspeaker usedin the experiments M-PZT-02 (Eishin Denki Co., Ltd.) has afrequency band of approximately 150 to 30000 Hz.

20

40

60

80

100

120

100 1000 3000

SPL

[dB

]

Frequency [Hz]

Original

Plastic cupPlastic plate

(a)

0

10

30

40

50

60

0 500 1000 1500 2000 2500 3000TH

D [%

]Frequency [Hz]

20

Original

Plastic cupPlastic plate

(b)Fig. 3. Characteristics curves for a piezoelectric loudspeaker with variousvibration transmitters. (a) Sound pressure level in dB. (b) Total harmonicdistortion (%).

NLMS

Σ

ΣSecondary path model

Secondary pathNoise control filter

Delay operatorSecondary path model

dn^

rn

yn

en

dn

Fig. 4. Block diagram of IMC-based feedback ANC system.

LPF

poweramp

preamp

D/Ach0

LPF

DSPTMS320C6713

A/Dch1

C6713DSK+DSK6713IFA

MSPAMP800

Mr

Opticalmicrophone

Piezo-electricloudspeaker

Rr

LPF

poweramp

preamp

D/Ach0

LPF

DSPTMS320C6713

A/Dch1

C6713DSK+DSK6713IFA

MSPAMP800

Ml

Opticalmicrophone

Piezo-electricloudspeaker

Rl5cm 5cm

Fig. 5. System composition of the head-mounted ANC system.

544

III. COMPENSATION OF SECONDARY SOURCE

A. Selection of vibration transmitter

The piezoelectric loudspeaker has a sound pressure fre-quency response that depends on the configuration and themass of the attached object (vibration transmitter), that is, theoutput SPL can be improved by attaching an object. Therefore,it is necessary to select an appropriate vibration transmitterthat achieves a high output SPL in a wide frequency band.We compare the frequency responses in the cases of usinga plastic plate and a plastic cup as the vibration transmitter.Figure 2 shows piezoelectric loudspeakers with these vibrationtransmitters. Figure 3 shows the corresponding SPL charac-teristics and total harmonic distortion rate. From Fig. 3, wecan see that the output SPL of the piezoelectric loudspeakercan be improved in the frequency band from 200 to 2000 Hzby attaching either object, in particular, the output SPL ofthe plastic cup is highest in the frequency band from 500to 2000 Hz. In addition, it can be seen from Fig. 3 thatthe total harmonic distortion rate can be reduced effectivelyin the frequency band from 800 to 1500 Hz by attachingeither vibration transmitter. Hence, we use the piezoelectricloudspeaker with a plastic cup as the secondary source of theANC system to reduce MR noise.

B. Head-mounted composition

The output SPL of the secondary source can be improvedby the use of a vibration transmitter, but the effective range isgreatly limited for MR noise reduction. Hence, we utilize anANC structure composed of two microphones arranged nearthe user’s ears and two loudspeakers closely arranged againstthe user’s ears. We call this structure a head-mounted ANCsystem to distinguish it from the headset-based ANC system.The head-mounted ANC system utilizes optical microphonesand piezoelectric loudspeakers to realize noise reduction ina high field and can compensate the inadequate output SPLof the piezoelectric loudspeakers because the loudspeakersare closely-arranged to the user’s ear, that is, the distanceof the secondary path is very short. Therefore, the antinoiseoutput from the secondary source can arrive at the errormicrophone before the antinoise attenuates and this structurecan consequently compensate the low output SPL of thepiezoelectric loudspeakers. Also, clear verbal communicationbetween medical staff is possible because the user’s ears arenot covered. In addition, since the user’s head is locatedbetween each microphone and loudspeaker pair, crosstalk doesnot arise. Hence, we can independently control the left andright channels with a single-channel feedback ANC system.

C. IMC-based feedback ANC system

The IMC-based feedback ANC system can be easily imple-mented in a DSP (Digital Signal Processor) because a structureis equivalent to the feedforward system[5]. The IMC basedANC system does not require any reference microphones andcan reduce spread acoustic noise with a small system becauseit is based on linear prediction. Hence, this system can onlycontrol periodic acoustic noises. Figure 4 shows a block

Fig. 6. An image of the head-mounted ANC system used in this experiment.

7.4m

7.6mMRI

A

B

C

Fig. 7. Schematic diagram of control positions in an actual MRI room.

Fig. 8. Open-configuration MR system used at Shiga university of MedicalScience in Japan.

Axial

Sagittal

Coronal

Fig. 9. A cross-sectional plane of each imaging modes.

545

0

-20

-40

-60

-80

-10010000 2500500 2000

ANC OFF

1500

ANC ON

Frequency [Hz]

Am

plitu

de [d

B]

0

-20

-40

-60

-80

-10010000 2500500 2000

ANC OFF

1500

ANC ON

Frequency [Hz]

Am

plitu

de [d

B]

(a) (b)0

-20

-40

-60

-80

-10010000 2500500 2000

ANC OFF

1500

ANC ON

Frequency [Hz]

Am

plitu

de [d

B]

0

-20

-40

-60

-80

-10010000 2500500 2000

ANC OFF

1500

ANC ON

Frequency [Hz]

Am

plitu

de [d

B]

(c) (d)0

-20

-40

-60

-80

-10010000 2500500 2000

ANC OFF

1500

ANC ON

Frequency [Hz]

Am

plitu

de [d

B]

0

-20

-40

-60

-80

-10010000 2500500 2000

ANC OFF

1500

ANC ON

Frequency [Hz]

Am

plitu

de [d

B]

(e) (f)

Fig. 10. Curves for error spectra at right channel for various taking modes. (a) Axial. (b) Sagittal. (c) Coronal. (d) Axial & Sagittal. (e) Axial & Coronal. (f)Sagittal & Coronal.

diagram of the feedback ANC system using the FXNLMSalgorithm. The updating algorithm of the noise control filteris expressed as follows:

wn+1 = wn +μ

‖rn‖2 + βrnen (1)

‖rn‖2 = rTn rn (2)

rn = cT dn−1 (3)dn = en + cT yn (4)yn = wT

n dn−1 (5)

wn = [wn(1) wn(2) · · ·wn(i) · · ·wn(N)]T (6)

c = [c(1) c(2) · · · c(i) · · · c(M)]T (7)

dn = [dn dn−1 · · · dn−i+1 · · · dn−N+1]T (8)rn = [rn rn−1 · · · rn−i+1 · · ·rn−N+1]T (9)yn = [yn yn−1 · · ·yn−i+1 · · ·yn−M+1]T , (10)

where wn is the tap-weight vector of the noise control filter, dn

is the input signal of the noise control filter, yn is the outputsignal of the noise control filter, rn is the filtered referencesignal, en is the error signal measured at the error microphone,μ is the step size parameter, β is the regularization parameter,and N and M are the tap lengths of the noise control filterand the secondary path model, respectively.

546

IV. EXPERIMENTAL RESULTS

We demonstrate that the head-mounted ANC system us-ing piezoelectric loudspeakers and optical microphones cancontrol MR noise through some experiments carried out inan actual MRI room. Table I shows the basic measurementconditions. Figures 5 and 6 respectively show the compositionand an image of the head-mounted ANC system used inthe experiments. We implemented the IMC-based feedbackANC system in the TMS320C6713 DSP Starter Kit (TexasInstruments Co.). Also, the peripheral is a DSK6713IFA (HEGCo.) and the microphone amplifier is an MSPAMP800 (HEGCo.). Figure 7 shows control positions in the actual MRI room.Figure 8 shows the open-configuration MR system used atShiga university of Medical Science in Japan. This MR systemhas three different modes for taking near-realtime anatomicalimages, named the axial, sagittal, and coronal modes. Figure 9shows the cross-sectional plane corresponding to each mode.These modes are used independently or in combination totake anatomical images according to the operating surgeon’swishes.

First, we demonstrate that the proposed system can reduceMR noise at position A in Fig. 7 when the imaging mode ischanged. The mode of the MRI device is axial for the first25 s, and then the imaging mode changes every 15 s in theorder of sagittal, coronal, axial & sagittal, sagittal & coronal,and axial & coronal.

Figures 10 and 11 show the error spectra before and afterthe control of the ANC system for each imaging mode at theright channel and the error signal, respectively. From theseresults, it can be seen that approximately 20 dB noise reductioncan be achieved for the frequency components from 500 to2500 Hz regardless of the imaging mode. On the other hand,the frequency components below 500 Hz cannot be reduced.This reason for this can be explained from Fig. 12, whichshows the frequency characteristic of the secondary path modelused in this experiment. From Fig. 12, we can see that the gainis low below 500 Hz and the performance of the piezo electricloudspeaker consequently limits the reduction of MR noise.However, the effect on the perceived noise is small becausethe frequency components below 500 Hz are small.

Next, we demonstrate how the noise reduction abilitychanges with the distance to the MRI device. Three controlpositions are set A, B and C as shown in Fig. 7, where themeasurement conditions for positions B and C are the sameas that for position A. In this case, the imaging mode of theMRI device is the axial mode.

Figures 13 and 14 respectively show the error spectra anderror signals measured at the error microphone at each posi-tion. From these figures, it can be seen that the noise reductionability does not depend on the position of measurement in thisexperiment. Therefore, the proposed ANC system can operateeffectively near the MRI device. We also confirmed the sameresults for the other modes. From the above experiment results,the effectiveness of the proposed head-mounted ANC systemhas been demonstrated.

TABLE IMEASUREMENT CONDITIONS.

Step-size parameter 5.0× 10−3

Regularization parameter 1.0× 10−7

Tap length of noise control filter 800Tap length of secondary path model 100Sampling frequency 12000 HzCut-off frequency of low-pass filter 2500 Hz

0 10 20 30 40 50 60 70 80 90 100-0.8

-0.4

0

0.4

0.8

Time [sec]

Am

plitu

de

Fig. 11. Curve for error signal at right channel when the imaging modechanges frequently.

10000 2500500 20001500Frequency [Hz]

Am

plitu

de [d

B]

0-10

-40-50-60

-80

-20-30

-70

Fig. 12. Frequency characteristic of the secondary path model used in thisexperiment.

V. CONCLUSION

In this paper, we proposed a head-mounted ANC systemthat utilizes an IMC-based feedback scheme to improve theinsufficient output SPL of the secondary source and wedemonstrated its effectiveness through some experiment resultsobtained in an actual MRI room. The head-mounted ANCsystem using optical microphones and piezoelectric loudspeak-ers with a plastic cup can effectively reduce MR noise byapproximately 20 dB in a high field in an actual MRI roomeven if the imaging mode changes frequently. In addition, thenoise reduction ability can be maintained regardless of thecontrol position in the MRI room.

In the future, we will examine whether it is easy to com-municate verbally when the proposed ANC system is used.

ACKNOWLEDGMENT

This research was financially supported by Adaptable Seam-less Technology Transfer Program Through Target-DrivenR&D (A-STEP) of Japan Science and Technology Agency.

547

0

-20

-40

-60

-80

-10010000 2500500 2000

ANC OFF

1500

ANC ON

Frequency [Hz]

Am

plitu

de [d

B]

(a)0

-20

-40

-60

-80

-10010000 2500500 2000

ANC OFF

1500

ANC ON

Frequency [Hz]

Am

plitu

de [d

B]

(b)0

-20

-40

-60

-80

-10010000 2500500 2000

ANC OFF

1500

ANC ON

Frequency [Hz]

Am

plitu

de [d

B]

(c)

Fig. 13. Curves for error spectra at right channel for each position. (a) PositionA. (b) Position B. (c) Position C.

REFERENCES

[1] Y. Kurumi, T. Tani, S. Naka, H. Shiomi, T. Shimizu, H. Abe, Y. Endo,S. Morikawa, “MR-Guided Microwave Ablation for Malignancies,”International Journal of Clinical Oncology, vol. 12, no. 2, pp. 85-93,Apr. 2007.

[2] M. Mcjury, and F. G. Shellock, “Auditory Noise Associated with MRProcedures,” Journal of Magnetic Resonance Imaging, vol. 12, issue 1,pp. 37–45, Jul. 2000.

[3] http://www.toshiba-medical.eu/en/Our-Product-Range/MRI/Technologies/Pianissimo/

[4] P. A. Nelson and S. J. Eliott, Active Control of Sound, Academic PressLtd., London, 1992.

[5] S. M. Kuo and D. R. Morgan, Active Noise Control Systems, John Wiley& Sons, New York, 1996.

[6] M. Mcjury, R. W. Stewart, D. Crawford, and E. Toma, “The use ofactive noise control (ANC) to reduce acoustic noise generated duringMRI scanning: some initial results,” Magnetic Resonance Imaging, vol.15, issue 3, pp. 319–322, Jul. 1997.

0 10 20 30 40

Time [sec]

-1

0

1

Am

plitu

de

0.5

-0.5

(a)

-1

0

1

Am

plitu

de

0 10 20 30 40

Time [sec]

0.5

-0.5

(b)

-1

0

1A

mpl

itude

0.5

-0.5

0 10 20 30 40

Time [sec]

(c)

Fig. 14. Curves for error signals at right channel for each position. (a) PositionA. (b) Position B. (c) Position C.

[7] C. K. Chen, T. T. Chiueh, and J. H. Chen, “Active canclellation system ofacoustic noise in MR imaging,” IEEE Trans. on Biomedical Engineering,vol. 46, no. 2, pp. 186–191, Feb. 1999.

[8] J. Chambers, M. A. Akeroyd, A. Q. Summerfield, and A. R. Palmer,“Active control of the volume acquisition noise in functional magneticresonance imaging: Method and osychoacoustical evaluation,” J. AcoustSoc. Amer., vol. 110, no. 6, pp. 3041–3054, Dec. 2001.

[9] J. Chambers, D. Bullock, Y. Kahana, A. Kots, and A. Palmer, “Devel-opments in active noise control sound systems for magnetic resonanceimaging,” Applied Acoustics, vol. 68, issue 3, pp. 281–295, Mar. 2007.

[10] M. Li, T. C. Lim, and J. H. Lee, “Simulation study on active noisecontrol for a 4-T MRI scanner,” Magnetic Resonance Imaging, vol. 26,issue 3, pp. 393–400, Apr. 2008.

[11] S. J. Elliott and T. J. Sutton, “Performance of feedforward and feedbacksystems for active control,” IEEE Trans. Speech & Audio Processing,vol. 4, no. 3, pp. 214-223, May 1996.

[12] W. S. Gan, S. Mitra and S. M. Kuo, “Adaptive feedback active noisecontrol headset: Implementation, evaluation and its extensions,” IEEETrans. Consumer Electronics, vol. 51, no. 3, pp. 975-982, Aug. 2005.

[13] S. M. Kuo, S. Mitra and W. S. Gan, “Active noise control system forheadphone applications,” IEEE Trans. Control Systems Technology, vol.14, no. 2, pp. 331-335, Mar. 2006.

548