DOES MAGNETIC RESONANCE IMAGING NOISE AFFECT THE COCHLEA? THE PROTECTIVE ROLE OF EARPLUGS
2Karabük Training and Research Hospital, Department of Radiology, Karabük, Türkiye
3Karabuk University, Department of Audiometry, Karabuk, Türkiye
Summary
Background: Magnetic resonance imaging (MRI) produces high-intensity acoustic noise that can negatively influence auditory function. This study aimed to examine the effects of noise generated during MRI scans of different body regions on cochlear function and to evaluate the protective effectiveness of hearing protection devices (HPD).Materials and Methods: A total of 60 individuals were included: 20 who underwent brain MRI without HPD (brain group), 20 who underwent lower extremity MRI without HPD (lower extremity group), and 20 who underwent brain MRI while using HPD (brain+HPD group). Distortion Product Otoacoustic Emissions (DPOAEs) were measured for all participants immediately before and after the MRI procedure.
Results: Following MRI, significant decreases in the signal-to-noise ratio (SNR) at 2000 Hz and 4000 Hz were observed in the brain group (p<0.05). In the lower extremity group, SNR reduction was detected at 4000 Hz (p<0.05), indicating that even MRI scans of non-cranial regions generate noise intense enough to influence cochlear responses. In contrast, the brain+HPD group showed no significant change in SNR values at any tested frequency after MRI (p>0.05).
Conclusions: These findings highlight that MRI-related noise can transiently affect auditory function, and appropriate HPD use effectively minimizes these adverse effects and should therefore be recommended during MRI procedures, especially for high-noise sequences.
Introduction
The auditory organ is sensitive to mechanical sound waves and transmits them to the brain by converting them into neural energy. However, high-intensity sounds can damage this organ and lead to noise-induced hearing loss (NIHL) in individuals.[1] NIHL is the second most prevalent form of hearing loss after age-related hearing loss, despite noise being a controllable risk factor.[1] Noise can affect the sensory cells in the inner ear through mechanical or metabolic pathways.[2] In this process, the more vulnerable outer hair cells (OHCs) are among the first structures to be damaged. Mechanical effects usually occur directly due to excessive noise exposure and are referred to as acoustic trauma. Metabolic effects, on the other hand, develop over time due to an increase in free radicals and the induction of oxidative stress in the inner ear following noise exposure. Some of the damage that occurs in the inner ear after noise exposure may be reversible, which is known as a temporary threshold shift.[3] However, long-term exposure or severe acoustic trauma can result in permanent sensorineural hearing loss. Although gene therapies hold promise for the treatment of permanent NIHL, currently there is no effective medical or surgical treatment available. A time-weighted average noise exposure limit of 85 A-weighted decibels (dBA) over eight hours is advised by the National Institute for Occupational Safety and Health (NIOSH). For noise levels of 100 dBA, the recommended exposure time is only 15 minutes.[4]Magnetic Resonance Imaging (MRI) is a technology that provides detailed cross-sectional imaging of many regions of the human body with high soft-tissue resolution. It has a wide range of applications, including the analysis of pathologies in various areas such as brain parenchyma, mapping of white matter tracts, and observation of brain activity. Although scanners are generally perceived as quiet, changes in the direction of current in the gradient coils within the device lead to vibration and sound production. The gradient coils are subject to Lorentz forces, which produce this noise.[5] Studies have shown that MRI noise can reach approximately 86 dBA, with sound pressure levels rising up to 120 dBA.[6] Such high noise levels produced by MRI can lead to NIHL. Therefore, in countries like Türkiye, where MRI is commonly used in diagnostic and follow-up processes, it is crucial to take appropriate protective measures.[7]
Hearing level is determined by pure-tone audiometry, which is considered the gold standard test[8]. However, as a behavioral test, pure-tone audiometry is subjective and may be insufficient in detecting minor cochlear damage. Therefore, otoacoustic emissions, which provide a more sensitive and objective assessment of OHC function, are used to detect such subtle cochlear damage. One type of otoacoustic emission, distortion product otoacoustic emissions (DPOAEs), is employed to evaluate cochlear function in a frequency-specific manner.[8]
The aim of this study was to investigate the effects of noise generated during MRI scans of different body regions on cochlear function and to evaluate the effectiveness of hearing protection devices (HPD) in reducing these effects.
Methods
Ethical approval for this study was obtained from the Non-Interventional Research Ethics Committee of XXX University. Written and verbal informed consent was obtained from all participants included in the study (2024/1844).This study was conducted on patients who presented to the radiology clinic for MRI scans. These patients were divided into groups based on the type of MRI they were scheduled to undergo and whether or not they used a HPD. Participants were divided into three groups:
• Group 1 (brain): 20 patients undergoing brain MRI without HPD.
• Group 2 (lower extremity): 20 patients undergoing lower extremity MRI without HPD.
• Group 3 (brain + HPD): 20 patients undergoing brain MRI with foam earplug HPD (E.A.R 1100, Noise Reduction Rate [NRR] 37 dB, USA).
The medical histories of the participants were reviewed. All participants underwent acoustic immittance evaluation, pure-tone audiometry, and DPOAE testing before MRI. MRI scans were performed using a 1.5-T MRI scanner (Siemens, Germany). Brain MRI sequences averaged 5.5 minutes, whereas lower extremity MRI sequences averaged 8 minutes (knee, foot/ankle MRIs). After MRI, DPOAE testing was repeated. Exclusion criteria included systemic, psychiatric, cardiac or otologic disorders, any degree of hearing loss, or abnormal tympanometric findings (type B or C). Figure 1 shows the patient selection.
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Figure 1: Flowchart of the study design and patient allocation. All participants underwent pure-tone audiometry and tympanometry, followed by the application of exclusion criteria. Eligible participants (n = 60) completed DPOAE testing before MRI (pre-MRI). Patients were then allocated to three groups according to MRI type and HPD use. Post-MRI DPOAE testing was performed in all groups. |
Audiological evaluation
Tympanometric assessments were performed using the GSI TympStar Pro 2 acoustic immittance meter (Grason-Stadler, Minnesota, USA). An appropriate probe was inserted into the participant's ear, and tympanometry was conducted using a 226 Hz probe tone.
The Madsen ASTERA computerized audiometer (Natus, Taastrup, Denmark) was used to conduct the pure tone audiometry test. TDH 39 supra-aural headphones were used to assess bilateral air conduction thresholds at frequencies ranging from 0.25 to 8 kHz. The average air conduction thresholds at 0.5 to 4 kHz were used to calculate the pure tone average (PTA). A PTA greater than 25 dB was considered indicative of hearing loss.
DPOAE testing was performed using the Neuro-Audio system (Neurosoft, Russia). A snugly fitting probe was placed in the participant's ear. DPOAEs were recorded using primary tone levels of F1 = 65 dB and F2 = 55 dB, with a frequency ratio of F2/F1, and the distortion product calculated at 2F1-F2. Signal-to-noise ratios (SNRs) were assessed at 1000, 1429, 2000, 2857, 4000, and 5714 Hz.
Statistical analysis
The Statistical Package for the Social Sciences (SPSS, IBM Corp., Armonk, NY, USA) version 21.0 was used for all statistical analyses. Histograms and normal distribution curves were employed to evaluate the normality of the data distribution. When comparing groups, the one-way ANOVA test was utilized for normally distributed data and the Kruskal-Wallis test for non-normally distributed variables. The influence of MRI was evaluated using the paired t-test or the Wilcoxon signed-rank test. A threshold of p < 0.05 was adopted to determine statistical significance at the 95% confidence level.
Results
The brain MRI group included 16 (80%) females and 4 (20%) males, with a mean age of 39.75 ± 13.56 years. The lower extremity MRI group comprised 14 (70%) females and 6 (30%) males, with a mean age of 42.15 ± 13.59 years. The brain+HPD group consisted of 12 (60%) females and 8 (40%) males, with a mean age of 34.15 ± 14.31 years. Regarding age and sex, there were no statistically significant differences between the groups (p = 0.251 and p = 0.386, respectively). All MRI scans were performed without contrast agents. In the lower extremity group, 12 (60%) of the MRI scans were knee MRIs, 6 (30%) were ankle MRIs, and 2 (10%) were foot MRIs.
Pre-MRI comparison of DPOAE findings between groups
Pre-MRI DPOAE results by group are presented in Table 1. No statistically significant differences were found between the groups in DPOAE SNRs at 1000 Hz, 1429 Hz, 2000 Hz, 2857 Hz, 4000 Hz, or 5714 Hz (p > 0.05).
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Table 1: Pre-MRI DPOAE findings according to groups.
Effect of MRI on DPOAE findings
The effect of MRI on DPOAE SNR values is illustrated in Figure 2. In the brain group, post-MRI SNRs at 2000 Hz and 4000 Hz were significantly reduced (p < 0.05). In the lower extremity group, a significant decrease in SNR values was observed at 4000 Hz after MRI (p < 0.05). In contrast, the brain+HPD group showed no significant changes in SNR values following MRI (p > 0.05).
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Figure 2: Effect of MRI on DPOAE findings |
Post-MRI comparison of DPOAE findings between groups
None of the participants reported tinnitus or auditory hypersensitivity after MRI. Post-MRI DPOAE findings by group are shown in Table 2. There were no statistically significant differences between the groups in DPOAE SNR values at 1000 Hz, 1429 Hz, 2000 Hz, 2857 Hz, 4000 Hz, or 5714 Hz (p > 0.05).
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Discussion
MRI is frequently preferred in routine clinical practice due to its non-invasive nature and lack of ionizing radiation, allowing detailed evaluation of various body systems. Although 7 tesla (T) MRI devices exist today, the most commonly used scanners operate within the 0.5-3 T range. MRI primarily targets hydrogen atoms, which are abundant in water and fat tissues.[9] When a patient is placed within a strong magnetic field, the protons in the body align either parallel or anti-parallel to the magnetic field and begin to precess. During this process, radiofrequency pulses cause the hydrogen nuclei, each containing a single proton, to deviate from their alignment. When the radiofrequency pulses cease, the protons return to their original positions, releasing the absorbed energy. This energy is converted into signals by receiver coils, which are then used to generate images. However, MRI scanners can produce high levels of acoustic noise during imaging. Studies have shown that 1.5-T and 3-T MRI machines generate similar noise levels, which can exceed 120 dBA.[6,7] According to the equal energy principle, which determines the risk of NIHL, both the intensity and duration of noise exposure are critical factors. MRI scan durations vary depending on the body region examined and the use of contrast agents, typically ranging from 10 to 35 minutes.[7] In this study, we investigated the effects of noise generated during MRI scans of different anatomical regions on cochlear function and evaluated the effectiveness of HPDs, specifically earplugs, in mitigating these effects. Our findings demonstrated that, in the absence of earplug use, both brain and lower extremity MRI scans adversely affected cochlear function.Several studies have investigated the impact of noise generated during MRI scans on auditory function. Bahaloo et al.[10] examined pure-tone audiometry results in individuals exposed to head and neck MRI scans (1.5-T). They reported temporary hearing threshold shifts at 4 kHz, 6 kHz, and 14 kHz, which fully recovered after 24 hours. Radomskij et al.[11] divided patients exposed to 1.5-T MRI into two groups-those who used HPDs and those who did not-and assessed their hearing using transient otoacoustic emissions (TEOAEs). The authors found decreased TEOAE scores in individuals who did not use HPDs and emphasized the importance of proper HPD usage. Turay et al.[7] performed pre- and post-scan pure-tone audiometry, TEOAE, and DPOAE test on patients undergoing brain, head, neck, or cervical MRI scans. Although all patients used HPDs, the authors reported a significant decline in 4 kHz DPOAE results after MRI compared to pre-scan values. Bulğurcu et al.[12] evaluated auditory function with pure-tone audiometry and DPOAE before and after cranial MRI scans and reported no significant effects of MRI noise on these tests. In our study, participants were divided into two groups based on the MRI scan region, the brain and the lower extremity, and their auditory function was evaluated using DPOAE. Previous studies[7,10-12] focused primarily on the effects of brain, head, or neck MRI noise on auditory function. Our study is significant because it investigates the impact of MRI noise from different anatomical regions (brain and lower extremity) on auditory function. None of the participants in our study reported tinnitus or any auditory symptoms following MRI scans. However, in the brain MRI group, post-scan SNR values at 2 kHz and 4 kHz deteriorated compared to pre-scan measurements. Similarly, in the lower extremity group, 4 kHz SNR values decreased after the scan relative to baseline. These findings indicate that, although the impact of lower extremity MRI noise on auditory function was less pronounced than that of brain MRI noise, it still negatively affected cochlear function.
Our study demonstrated a common decline in 4 kHz SNR values in both the brain and lower extremity MRI groups. As reported in the literature, the cochlear region around 4 kHz is the most vulnerable to noise exposure, which is related to the acoustic properties of the external auditory canal and the characteristics of the noise itself.[13,14] While the resonance frequency of the external auditory canal ranges between 3000 and 3500 Hz, the average center frequency of MRI noise is roughly 3200 Hz. Around 4 kHz, the noise intensity, which is magnified at the resonance frequency of the external auditory canal, causes a half-octave frequency shift on the basilar membrane, leading to peak stimulation and eventual damage to the cochlear region.
There are different types of HPDs, with NRR typically ranging between 10 and 30 dB depending on the type.[15,16] Fortier et al.[4] evaluated the auditory function of participants who used HPDs and were repeatedly exposed to MRI noise (twice weekly for 1.5 years) using DPOAE measurements. The authors reported that MRI noise did not affect the auditory function of individuals who used HPDs. On the other hand, Turay et al.[7] demonstrated that even when participants used HPDs (headphones), MRI noise could still affect auditory function, particularly the 4 kHz DPOAE responses. Unlike Turay et al.'s study, our findings showed that HPDs (foam earplugs) effectively preserved auditory function. This discrepancy may be due to differences in the types, characteristics, or incorrect use of HPDs.[4] Earplugs and earmuff-type HPDs generally perform better than ear canal-type HPDs.[15] The HPDs used in our study were foam earplugs that conform to the shape of the external auditory canal, providing complete protection, and when used correctly, their NRR was 37 dB. Therefore, foam earplug-type HPDs may be more effective in attenuating MRI noise.
Noise initially affects the OHCs in the cochlea. The first damage to OHCs is often asymptomatic but can be detected by otoacoustic emissions and is typically repaired within 1-2 days by scar tissue formation, resulting in a temporary threshold shift.[14,17] Therefore, the absence of auditory symptoms in our participants after MRI exposure, despite the reduction in DPOAE SNR values, may indicate the presence of a temporary threshold shift. However, we were unable to perform long-term DPOAE follow-up in these individuals, which represents the main limitation of our study.
Considering the noise intensity during MRI scanning, it is thought that metabolic damage rather than acoustic trauma is more likely to develop in the cochlea after MRI. This is because acoustic trauma typically occurs at sound levels of 140 dB or higher.[18] However, several studies have reported that MRI noise can also cause acoustic trauma in patients. Revadi et al.[19] reported that a 3-T lumbosacral MRI generated noise at 118.4 dB, resulting in unilateral sudden hearing loss in a patient. The authors noted that the hearing loss improved within three days, but the tinnitus persisted. Mollasadeghi et al.[20] reported tinnitus and hearing loss (including an acoustic notch at 4 kHz) developing two days after a 1.5-T brain MRI. They stated that neither the hearing thresholds nor the tinnitus improved even after three months.
In addition to noise exposure, factors such as aging and exposure to chemicals can also negatively affect auditory function.[21] Therefore, these adverse factors may act synergistically with noise, potentially accelerating the development of acute auditory pathologies.[22] Consequently, MRI noise may cause acute hearing loss in individuals with such risk factors. This damage can worsen over time (2 to 30 days) as endolymph leaks through holes in the reticular lamina and mixes with cortilymph. This mechanism may lead to progressive hearing loss following the initial trauma.[23]
Conclusion
Our findings indicate that exposure to noise generated by both 1.5-T brain MRI and 1.5-T lower extremity MRI adversely affects auditory function when HPDs are not used. However, the appropriate use of HPDs appears to effectively protect the auditory system from the harmful effects of MRI noise.
FUNDING
This study was supported by the Karabük University Scientific Research Projects Coordination Unit (Project Number: KBÜBAP-24-DS-104).
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