Acoustic Noise in MRI

Acoustic Noise in MRI

-Parajuli K, MSc Medical Imaging, IOM, Nepal

Why noise is a problem in MRI¹

·       It causes discomfort, anxiety and distraction to the patient.

·       It causes problem in verbal communication.

·       It can cause discomfort in sedated patients.

·       Some medications can increase the sensitivity of our ear to sound.

·       Temporary shift in the threshold of hearing has been reported.

·       Neonates have increased sensitivity to acoustic noise.

·       Unwanted activation of auditory cortex during functional MRI causes spurious patterns and is a serious problem.²

How is noise produced in MRI

There are many sources of noise in mri.

They can be categorized into four groups

a. Due to gradient current

b. Due to eddy current

c. Due to radio frequency waves and slice selection pulse

d. Due to ambient noise

Noise due to gradient current³

Gradient, spatial variation in magnetic field is produced by the electric currents flowing through coil. Large pulsed electrical current ranging from 200 A to 600A with rise time in the order of sub millisecond or millisecond is applied to copper windings held together by epoxy resin and fiber glass. Gradient coil is itself situated in the magnet bore assembly and thus experience the main magnetic field, a force called Lorentz force develops in between the gradient current and static magnetic field, which causes bending and buckling the coil producing compressional force. Force of 2000 N per meter of gradient coil is produced resulting in vibrations with acceleration levels in the order of 100 m/s². Lorentz force is given by the relation:

where dl is the element inside the magnetic field.

Fig 1: grey arrow showing the direction of gradient current and white arrow shows direction of Lorentz force.

Figure (2) shows two mode of vibration: cone shaped vibration in Z gradient (A) and banana shaped vibration in X-Y gradient (B).

Studies have shown similarity between spectrum of gradient current and that of noise. Also, amplitude of vibration is higher when frequency of gradient current matches the resonant frequency of gradient coil which depends on properties of length and elastic properties of material in coil assembly. It is thought to be due to better acoustic transfer function of the gradient system at resonant frequency. The time course of the gradient current depends on various pulse sequence parameters (TR, TE, FOV). Thus pulse sequence can be modified such that frequency of gradient current (also the frequency of vibration) does not match the resonant frequency of the coil. In EPI the frequency of readout gradient can be adjusted such that it is not same as resonant frequency of the gradient coil.

In addition, the wave for gradient can be changed from trapezoidal to sinusoidal.

Noise due to eddy current⁴

Gradient magnetic field induces eddy current, according to faradays law of electromagnetic induction, in the metallic structures located in the vicinity of the gradient coil for eg. inner cryostat in superconducting systems and Rf body coil, Rf shield etc. These structures conducting the eddy current are situated in a magnetic field because of which Lorenz force develops and induces vibration. Recent study by Eldestein et al ⁴have shown that main source of noise in MRI is due to eddy current induced vibration of the conducting structures located near the gradient coil.

RF Hearing¹

It is due to pressure waves generated because of expansion of tissues as a result of rise in temperature (1x10^-6 degrees C) caused by the RF induced heating. It is very nominal compared to gradient field induced acoustic noise and there is no proof regarding harmful effects due to RF hearing.

Ambient noise³

Inside the scanner room there are various other sources of noise for example:

1.    Noise due to air blower of air handling system that can produce noise upto 20 dbA.

2.    Pounding sound of low frequency due to liquid helium pump that can reach up to 80 dB.

How much is the noise produced in MRI^{5, 6}

Noise level in MRI depends upon the hardware construction and surrounding environment i.e the presence and absence of various structures that reflect sound. Thus sound level is measured in specific pressure level or sound pressure level which can be expressed in linear or A-weighted scale. The sensitivity of human ear depends upon the frequency of sound. A-weighted sound takes into account the different sensitivity of human ear to various frequencies of sound and is expressed as dBA or dB(A).dB(A) is better correlated with the risk of noise induced hearing loss.

Noise levels of 70 to 130 dB have been reported. Price et al found noise level to be 82.5+-0.1 dB (A) in 0.23 to 0.5T systems and 118.4 +-1.3 dB (A) in 3T systems. Hattori et al⁶ found that noise in 3T MR systems exceeded that of regulatory limit of 99 dB given by IEC. The peak sound pressure level was 125.7 dB for MR angiography, 130.7dB for SS-EPI (single shot spin echo EPI) on the linear scale.

What factor affects level of noise produced?

It depends upon the scan parameters like section thickness, FOV, TR and TE. It is because change in these parameters bring about change in gradient output (rise time and amplitude).Smaller the section thickness and FOV and shorter the TR, higher is the level of noise produced. It increases logarithmically with increase in magnetic field.

Studies have shown presence and absence of patient can also create difference in level of acoustic noise produced. The value was found to be larger in the former case.

What are the regulatory limits?

According to US FDA guidelines peak un-weighted sound pressure level should not be greater than 140 dB. Also, A-weighted root mean square sound pressure level should be greater than 99dBA with hearing protection in place.

UK has provided guidelines for operators as well, according to which hearing protection should be provided to the operators who can be exposed at the average of 85 dB over 8 hour a day.

Exposure limits given by Occupational Safety and Health Administration (OSHA)¹

85dB(A)         16hrs a day

90dB               8 hrs a day

95db               4 hrs a day

100dB             2 hrs a day

105dB             1 hrs a day

110dB             0.5 hrs a day

115dB             0.25 hrs a day

What are the noise reduction strategies?

1. Modification in design of hardware

a)  Use of active-passive shielded, vibration isolated and vacuum enclosed gradient coils¹

Gradient assembly can be shielded so that eddy current is not induced in the surrounding metallic structures in the MRI assembly. Active shielding refers to the use of secondary coil other than primary gradient coil(X, Y, Z) that oppose and cancel out the fringe field. Like primary coils there are three shielding coils. Passive shielding refers to the use of conducting layers that are attached on the outer radius of the gradient coil.

Gradient coil are arranged such that structural or mechanical propagation of vibration into other component is reduced this is called vibration isolated gradient coil. Also they can be enclosed in vacuum to prevent air conduction of the vibration.

b)    Use of quiet gradient coil

Current in the gradient coil are passed such that net Lorentz force is reduced.

2. Passive noise control

Ear plugs and headphones can be used to lessen the intensity by 10 to 30 dB. Disadvantage of this method is that it interferes the verbal communication with patients.

3. Active noise control⁸

It utilizes the sound, also called anti phase sound that interferes destructively with the sound perceived by the subject undergoing MRI. There are two main topologies for active noise control. Feed forward and feed-back.

Feed-back topology:

·       A microphone is placed near to the area where attenuation is required.

·       It receives the sound generated by scanner which is perceived by the subject and sends reference signal to the loud speaker.

·       Loud speaker, on the basis of reference signal sends control sound that interferes with the noise and produces cancellation effect.

·       Disadvantage of this method is that because of the time delay between production of control sound and reception of the same by the subject, high frequency noise cannot be adequately suppressed, making this sort of topology not suitable for EPI sequences where noise of frequency 3 -4 kHz are generated.

Feed forward topology

·       Microphone is located near to the sound source which sends the reference signal in advance to the loudspeaker.

·       Another microphone called “error” microphone is located near the subject and provides electrical copy of residual noise.

·       Advantage of this topology is that higher frequency noise can be attenuated because the reference signal is available on advance and can compensate for the delay due to time required for the propagation of sound from loudspeaker to error microphone. Thus this sort of topology can be used to reduce high frequency noise from EPI sequences.

4. Use of quiet pulse sequences⁹

Pulse sequence with less gradient pulsing can be used. For example Gradient pulsing can be reduced by using single shot sequences based on stimulated echo (STEAM).Further reduction in gradient pulsing can be done by modifying the projection reconstruction method in which the two gradient pulses are replaced by a single mechanically rotating DC gradient coil.

Use of soft gradient pulse is another option. Hennel et.al¹⁰ used band limited pulses that used sinusoidal ramp with least number of slope (ramp) and maximum duration of slope (ramp duration). Noise level was brought to as low as 30dBA in GRE and SE and 60dBA in RARE sequence. Authors named such pulses as soft pulse which can be obtained by convolving the hard pulses i.e the pulses with rectangular waveform with the cosine window of length ᴦ (Figure 3)

Thus there is a growing concern regarding the detrimental effects of acoustic noise generated in the MR systems and the ongoing advancements in MRI hardware, especially in  the gradient coil technology, are dedicated towards development of a quiet scanner.

References

1.      MC.Jury M,Shellock F.G Auditory noise associated with MR procedure:A Review J Magn Reson Imaging.2000 12:37-45

2.      Measuring MRI noise

3.      Adriaan Moelker and Peter M.T. Pattynama Acoustic Noise Concerns in Functional Magnetic Resonance ImagingHuman Brain Mapping .2003;20:123–141

4.      Edelstein WA, Hedeen RA, Mallozzi RP, El-Hamamsy SA, Ackermann RA, Havens TJ (2002): Making MRI quieter. Magn Reson Imaging 20:155–163.

5.      Roozen N.B ,Koevoets A.H and den Hamer A.J Active Vibration Control of Gradient Coils to Reduce Acoustic Noise of MRI Systems.

6.      Hattori Y,Fukatsu H,Ishikagi T :Measurement and evaluation  of the acoustic noise of  a 3 Tesla MR scanner Nagoya J.Med.Sci,2009;69.23-28

7.      Tomasi DG¹, Ernst T Echo planar imaging at 4 Tesla with minimum acoustic noise. J Magn Reson Imaging. 2003;18(1):128-30.

8.      Chamber J,Bullock D,Kahana Y.Developments in active noise control sound systems for magnetic resonance imaging.Applied Acoustics 2007;68.281-295

9.      Alibek S,Vogel M,Sun W etal.Acoustic noise reduction in MRI using silent scan:an initial experience.Diagn Interv radiol 2014;20:360-363

10.  F. Hennel,* F. Girard, and T. Loenneker ‘‘Silent’’ MRI With Soft Gradient Pulses Magnetic Resonance in Medicine ,1999;42:6–10