The undesirable and strong noise from MRI devices often cause adverse effects on human hearing function, such as decreasing hearing sensitivity, increasing threshold, and even to deafness¹. A follow-up case study found the potential permanent effect of 1.5T MRI noise on hearing function by comparing the hearing tests before, 25 min and 3 months after MR examinations². However, another 3T MRI case study obeserved the temporary effects, i.e. immediate unilateral hearing loss³. Neverthess, such noise-induced temporary or permanent effects on hearing functions, especially for different age populations remain unclear. Thus, a longitudinal study targeting 3T MRI was conducted to disclose such noise-induced effects on hearing functions of young adults and neonates.The Internal Review Board approved this study and all written informed consents were obtained. Patients 29 healthy adults (male/female, 12/7; age-range, 18~25yr) and 20 neonates (male/female, 14/6; postnatal-days range, 4~28days) were recruited and performed by 3T MRI head examinations. Hearing protection For neonates, chloral hydrate (50 mg/kg) was administered orally for sedation and the neonate worn with micro-earplugs and earmuffs. For adults, micro-earplugs were used to protect their hearing functions. MR Protocols All MRI examinations were performed using a 3T scanner (GE, Signa HDxt) with an 8-channel head coil. The protocols were listed in Table 1. Noise level measurements During the examinations, the acoustic noise levels at the magnet center (scanning room) and operator site (operator room) were synchronously measured by a sound analyzer and a microphone. Hearing function tests Dynamic auditory brainstem response (ABR) tests were used to estimate participants’ hearing function within 24 hours before (test1; all participants), within 20 minutes after (test2; all participants) and 25 days after (test3; 26 adults were re-examined, others were excluded due to no significant changes between test1 and test2) the MRI examinations, respectively. Statistical analysis The paired t-tests were employed to compare the ABR results of hearing function between test1 and test2; between test1 and test3. Moreover, in each test, differences between left and right hearing functions; between male and female in terms of unilateral hearing function (i.e. left or right ear) were further compared by nonparametric test.

For the scanned MR protocols, the measured peak of sound pressure levels (SPL) in the magnet center ranged from 118.2dBA to 125.9dBA that were slightly larger than 115dBA; while equivalent SPLs varied from 103.5dBA to 116.4dBA (Table 2). And the strongest SPLs (L_eq=111.3dBA, L_peak=123.2dBA) resulted from the ESWAN protocols. Besides, the SPLs without subject were obviously lower than those with subjects.

As for young adults, the ABR test2 showed statistically significant decreases compared with those of test1 (left, p=0.013; right, p=0.001); while no significant differences were found in neonates (left, p=0.171; right, p=0.748) (Figure 1). The re-examined test3 for young adults showed no significant differences with test1 (left, p=0.185; right, p=0.791) (Figure 1). Beyond, for all participants in each test, no significant differences were observed between left and right hearing functions (p_min=0.124); between male and female in terms of unilateral hearing function (p_min=0.334) (Table 3).

Our study indicated that the 3T MR noise indeed led to a temporary threshold shift of hearing function in young adults, whereas no effects were observed in neonates. Previous 1.5T MR case reported the permanent effect on patient’s hearing function that may be closely linked with the absence of hearing proction²; while 3T MR case observed the the temporary hearing loss of patient worn with 3M-foamear plugs³. Being consistent with latter case, our young adult group with hearing protection showed temporary threshold shifts.

Besides, the ABR results of neonates were higher than those of young adults (mean, 21.25dB v.s. 13.84dB). The underlying reason may lie in the developmental differences between neonates and young adults. Specifically, during the neonatal period, cochlea has developed into a very adult-like configuration; and brainstem also reaches a mature state⁴. Such developmental progress enables the reflection of brainstem activity in behavioral responses to sound, e.g. phonetic discrimination. However, the myelination of auditory nerve initiates later⁴. It is the conductive mechanism and neural immaturity that lead to the higher ABR thresholds of neonates than young adults⁵. Due to the above facts, the perception ability of neonates to acoustic noise appears to be weaker than young adults, and thus rarely transient effects of MR noise were found in neonates.

Due to immaturity of hearing nerve system, neonates show weaker sensitivity to acoustic noise than adults. The 3T MRI noise resulted in transient decrease of young adults’ hearing function (i.e. temporary threshold shift), whereas showed rarely effects on neonates.