The development of magnetic resonance imaging (MRI) scanners has profoundly enhanced our ability to non-invasively visualize the brain, spinal cord, and other parts of the body. MRI images allow physicians to diagnose medical conditions such as multiple sclerosis, acoustic neuromas, strokes, traumatic brain injury, and more. However, a frequent concern has revolved around the high-intensity noise produced by modern 3-Tesla (3T) MRI scanners that can reach peak sound pressure levels of 125.7 to 130.7 dB and have an average equivalent intensity of 110 to 115 dB. This high-intensity noise could easily cause hearing loss or induce tinnitus or hyperacusis (Nagoya J Med Sci. 2007 Jan;69(1-2):23). The intensity of noise produced by MRI scanners generally has a positive correlation with the magnetic field strength (i.e., 3T scanners are louder than 1.5T scanners). The noise produced is also predominantly of a low frequency (below 1 kHz), which is typically less damaging to the ear than noise in the higher 4 kHz range (J Magn Reson Imaging. 2005 Jul;22(1):163). Nevertheless, it seems reasonable that exposure to such high-intensity noise could result in noise-induced hearing loss (Radiology. 2018 Feb;286(2):609).
INTENSITIES OF MRI SCANNER NOISE
The risk of hearing loss from MRI scanner noise can be put into context by referencing the U.S. Occupational Safety and Health Administration (OSHA) regulations, which consider the intensity, spectral content, and duration of noise exposures. The maximum permissible exposure limit (PEL) is 90 dBA for an exposure duration of eight hours. The PEL increases by 5 dB with every halving of exposure duration (i.e., 95 dB for 4 h, 100 dB for 2 h, etc.). If a typical MRI scan lasts one hour, the PEL would increase to 105 dBA. Thus, modern 3T MRI scanners are capable of exceeding the PEL and could conceivably result in hearing loss. Fortunately, it is common practice for patients to wear protective earplugs during the scanning procedure, allowing for a passive sound attenuation of as much as 30 dB, which is sufficient to prevent permanent damage. Despite this theoretical presumption, a number of case reports have found evidence of hearing loss following MRI scans, leaving the matter somewhat unsettled (Case Rep Radiol. 2013;2013:510258).
CAN SCANNER NOISE CAUSE HEARING LOSS?
To determine if MRI scanner noise poses a significant risk for acquired hearing loss despite the use of hearing protection, several studies have evaluated hearing ability before and after scanning (Radiology. 2018 Feb;286(2):602; Lancet. 2002 Apr 27;359(9316):1485). Jin, et al., recently tested if the noise produced by a 3T MRI scanner could cause temporary or permanent noise-induced hearing loss by using an auditory brainstem response (ABR) threshold screening device (Radiology. 2018 Feb;286(2):602). The researchers obtained the thresholds of 26 normal hearing participants and compared these to thresholds obtained after the subjects underwent 3T MRI scanning. Participants wore protective earplugs rated for 21 dBA attenuation during the procedure. The average equivalent noise intensity during the nearly one-hour scan was 103.5-111.3 dBA, which equates to 82.5 to 90.3 dB after factoring in the attenuating properties of the protective earplug. Thus, the noise reaching the ear was below the PEL for one hour (105 dBA) and would conceivably be considered safe. The investigators found a slight 5 dB threshold increase immediately after the scan. However, thresholds returned to pre-scan levels four weeks later, suggesting no permanent damage.
Another study completed by Radomskij, et al., produced similar results with a 1.5T scanner (Lancet. 2002). The researchers assessed cochlear function using otoacoustic emissions (OAEs) before and after a 20- to 40-minute MRI scan with a time-averaged equivalent intensity ranging from 100 to 110 dBA, equivalent to a PEL of 91.3 dBA. All participants wore protective earplugs during the measurements, but the noise attenuation rating was not indicated. OAEs obtained immediately after MRI scanning decreased by 1.84 dB, compared with a 0.43 dB decrease for controls.
Taken together, these two studies indicate that patients who wear earplugs during an MRI scan could experience very slight, temporary hearing loss following exposure to scanner noise but are unlikely to experience any permanent damage. However, patients scanned without wearing earplugs would be at substantially increased risk for developing hearing loss if they were to undergo frequent MRI evaluations or are more susceptible to developing noise-induced hearing loss.
OTHER CONCERNS WITH SCANNER NOISE
Overall, it seems that MRI scanner noise does not pose a significant risk for permanent hearing loss as long as patients are appropriately fitted with hearing protection during the procedure. However, the radiology community should not become complacent on this matter. If patients do not wear protective earplugs appropriately, which is very common, sound attenuation would be greatly diminished, increasing the risk of hearing loss, particularly among individuals who require frequent MRI scans.
Furthermore, with the deployment of more powerful 9T MRI scanners, the risk of hearing loss could increase considerably if the noise levels were to exceed 150 dB. At these intensity levels, protective earplugs would not be sufficient in preventing hearing loss since even after the maximum possible passive sound attenuation, noise would remain at an unsafe loudness intensity and be able to be transmitted through bone conduction (J Magn Reson Imaging. 2005). Thus, scanner manufacturers should continue to dedicate resources to quieting modern machines. Adopting some already developed noise reduction techniques commonly used in the automobile and aircraft industries may prove useful for this task.
In conclusion, special attention should be given to the effective fitting of hearing protection during MRI scanning to reduce the risk of temporary and permanent hearing loss. Equally important, manufacturers of MRI scanners should focus on reducing scanner noise to prepare for future high magnetic strength MRI scanners.