Acoustic Noise and MRI Procedures

Acoustic Noise and MRI Procedures*

Various types of acoustic noise are produced during the operation of an MR system. Problems associated with acoustic noise for patients and healthcare professionals include annoyance, verbal communication difficulties, heightened anxiety, temporary hearing loss and, in extreme cases, the potential for permanent hearing impairment.

Acoustic noise may pose a particular problem to specific patient groups. For example, patients with psychiatric disorders may become confused or suffer from increased anxiety as the result of exposure to loud noise. Sedated patients may experience discomfort in association with high noise levels. In addition, neonates may have adverse reactions to acoustic noise.

HEARING AND THE IMPACT OF ACOUSTIC NOISE
The human ear is a highly sensitive wide-band receiver, with the typical frequency range for normal hearing being between 20-Hz to 20,000-Hz. The ear does not tend to judge sound powers in absolute terms, but assesses how much greater one power is than another. The logarithmic decibel scale, dB, is used when referring to sound power.

Noise is defined in terms of frequency spectrum (in Hz), intensity (in dB), and time duration. Noise can be steady-state, intermittent, impulsive, or explosive. Transient hearing loss may occur following exposure to loud noise, resulting in a temporary threshold shift (i.e., a shift in audible threshold). With regard to acoustic noise associated with MR imaging, Brummett et al. reported temporary shifts in hearing thresholds in 43% of the patients scanned without ear protection and patients with improperly fitted earplugs. Recovery from the effects of noise occurs in a relatively short period of time. However, if the noise insult is particularly severe, full recovery can take up to several weeks. If the noise is sufficiently injurious, a permanent threshold shift at specific frequencies may occur.

MRI-RELATED ACOUSTIC NOISE
The gradient magnetic field is the main source of acoustic noise associated with an MR procedure. This noise occurs during the rapid alterations of currents within the gradient coils. These currents, in the presence of the strong static magnetic field of the MR system, produce significant (Lorentz) forces that act upon the gradient coils. Acoustic noise, manifested as loud tapping, knocking, chirping, or squeaking sounds, is produced when the forces cause motion or vibration of the gradient coils as they impact against their mountings which, in turn, flex and vibrate.

Alteration of the gradient output (rise time or amplitude) by modifying MR imaging parameters causes the acoustic noise to vary. Noise tends to be enhanced by decreases in section thickness, field of view, repetition time, and echo time. In addition to dependence on imaging parameters, acoustic noise is dependent on the MR system hardware, construction, and the surrounding environment. Furthermore, noise characteristics have a spatial dependence. For example, noise levels have been found to vary by as much as 10 dB as a function of patient position along the bore of the MR system. The presence and size of the patient may also affect the level of acoustic noise.

CHARACTERISTICS OF MR SYSTEM-RELATED ACOUSTIC NOISE

Gradient magnetic field-induced noise levels have been measured during a variety of pulse sequences for MR systems with static magnetic field strengths ranging from 0.35 to 4-Tesla. For example, Hurwitz et al. reported that the MR imaging-related sound levels varied from 82 to 93 dB on the A-weighted scale and from 84 to 103 dB on the linear scale.

Later studies performed using a variety of MR parameters including “worst-case” pulse sequences that applied multiple gradients simultaneously (e.g., three-dimensional, fast gradient echo techniques) reported that these are among the loudest sequences, with acoustic noise levels that ranged from 103 to 113 dB (peak) on the A-weighted scale.

Additional studies measured acoustic noise generated by echo planar imaging (EPI) and fast spin echo sequences. Echo planar sequences tend to have extremely fast gradient switching times and high gradient amplitudes. Shellock et al. reported high levels of noise ranging from 114 to 115 dBA on two different high field strength (1.5-Tesla) MR systems tested when running EPI sequences with parameters chosen to represent “worst-case” protocols. At 3-Tesla, Hattori et al. recorded sound levels that ranged from 126 to 131 dB on a linear scale, recommending the use of both earplugs and headphones for ear protection relative to the use of 3-Tesla MR systems when certain pulse sequences are used.

ACOUSTIC NOISE AND PERMISSIBLE LIMITS
In general, acoustic noise levels recorded in the MR environment have been below the maximum limits permitted by the Occupational Safety and Health Administration of the United States. This is particularly the case when one considers that the duration of exposure is an important physical factor that determines the effect of noise on hearing.

The U.S. Food and Drug Administration released guidelines for acoustic noise levels that should not be exceeded in association with the operation of MR systems, as follows:

Sound Pressure Level - Peak unweighted sound pressure level greater than 140 dB. A-weighted root mean square (rms) sound pressure level greater than 99 dBA with hearing protection in place.

While the acoustic noise levels recommended for patients undergoing MR procedures on an infrequent and short-term basis may appear to be somewhat conservative, they are deemed appropriate when one considers that individuals with underlying health conditions may have problems with noise at certain levels or frequencies. Acoustic noise produced during MR procedures represents a potential risk to such patients. As previously mentioned, the possibility exists that substantial gradient magnetic field-induced noise may produce hearing problems in patients who are susceptible to the damaging effects of loud noises.

The exposure of staff and other healthcare workers in the MR environment is also a concern (e.g., those involved in interventional MR procedures or who remain in the room for patient management reasons). Accordingly, if loud noises exist in the MR environment, staff members should routinely wear ear protection if they remain in the room during the operation of the scanner. In the United Kingdom, guidelines issued by the Department of Health recommend hearing protection be worn by staff exposed to an average of 85 dB over an eight hour day.

ACOUSTIC NOISE CONTROL TECHNIQUES

Passive noise control. The simplest and least expensive means of preventing problems associated with acoustic noise during MR procedures is to encourage the routine use of disposable earplugs or headphones. Earplugs, when properly used, can abate noise by 10 to 30 dB, which is usually an adequate amount of sound attenuation for the MR environment. The use of earplugs typically provides a sufficient decrease in acoustic noise that, in turn, is capable of preventing hearing problems. Therefore, for MR systems that generate substantial acoustic noise, patients should be required to wear these protective devices.

Unfortunately, passive noise control methods suffer from a number of limitations. For example, these devices hamper verbal communication with patients during the operation of the MR system. Additionally, standard earplugs are often too large for the ear canal of adolescents and infants. Importantly, passive noise control devices offer non-uniform noise attenuation over the hearing range. While high frequencies may be well attenuated, attenuation is often poor at low frequencies. This is problematic because, for certain pulse sequences, the low frequency range is where the peak MR imaging-related acoustic noise is generated.

Active Noise Control (ANC). A significant reduction in the level of acoustic noise caused by MR procedures has been accomplished using the active noise cancellation technique. Controlling noise from a particular source by introducing “anti-phase noise” to interfere destructively with the noise source is not a new idea. For example, Goldman et al. combined passive noise control and an active noise control system (i.e. an active system built into a headphone) to achieve an average noise reduction of approximately 14 dB.

Advances in digital signal processing technology allow efficient active noise control systems to be realized at a moderate cost. The anti-noise system involves a continuous feedback loop with continuous sampling of the sounds in the noise environment so that the gradient magnetic field-induced noise is attenuated. It is possible to attenuate the pseudo-periodic scanner noise while allowing the transmission of vocal communication or music to be maintained.

OTHER SOURCES OF MR SYSTEM-RELATED ACOUSTIC NOISE

RF hearing. When the human head is subjected to pulsed radiofrequency (RF) radiation at certain frequencies, an audible sound perceived as a click, buzz, chirp, or knocking noise may be heard. This acoustic phenomenon is referred to as “RF hearing”, “RF sound” or “microwave hearing”.

Thermoelastic expansion is believed responsible for the production of RF hearing, whereby there is absorption of RF energy that produces a minute temperature elevation over a brief time period in the tissue of the head. Subsequently, a pressure wave is induced that is sensed by the hair cells of the cochlea via bone conduction. In this manner, the pulse of RF energy is transferred into an acoustic wave within the human head and sensed by the hearing organs.

With specific reference to the operation of MR scanners, RF hearing has been found to be associated with frequencies ranging from 2.4- to 170-MHz. The gradient magnetic field-induced acoustic noise that occurs during an MR procedure is significantly louder than the sounds associated with RF hearing. Therefore, noises produced by the RF auditory phenomenon are effectively masked and not perceived by patients. Currently, there is no evidence of any detrimental health effect related to the presence of RF hearing.

Noise From Subsidiary Systems. Patient comfort fans and cryogen reclamation systems associated with superconducting magnets of MR systems are the main sources of ambient acoustic noise found in the MR environment. Acoustic noise produced by these subsidiary systems is considerably less than that caused by gradient magnetic fields.

[*Portions of this content were excerpted with permission from McJury M. Acoustic Noise and Magnetic Resonance Procedures, In: Magnetic Resonance Procedures: Health Effects and Safety, FG Shellock, Editor, CRC Press, Boca Raton, FL, 2001 and McJury M, Shellock FG. Acoustic noise and MR procedures: A review. Journal of Magnetic Resonance Imaging 12: 37-45, 2000.]

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