Measuring and Reducing Acoustic Noise in MRI Studies of Infants: A Review of Existing Guidelines and Development of New Methods

Background : Magnetic Resonance Imaging (MRI) is an important tool for charting brain development. This research initiative may provide critical insight into the neurodevelopmental processes associated with the emergence of ASD. Despite the promising potential of MRI, one major disadvantage is the intensity of acoustic noise that scanners generate while in operation. With noise levels reaching up to 120db, hearing protection is critical to ensure the comfort and safety of infant participants. Unfortunately, effectively reducing noise exposure in infant populations poses unique challenges that are not readily met by commercially-available hearing protection devices (HPDs). For instance, common HPDs, including earplugs, earmuffs, and headphones, do not have a mechanism to gauge if attenuation has been reduced during a session due to movement of the HPD. Instead, these HPDs rely on proper initial placement to facilitate noise reduction. In addition, few guidelines exist for what constitutes an acceptable level of noise in an MRI study of infants or how to reliably measure infants’ acoustic environment. In the current study, we aim to attenuate the intensity of noise levels during MRI in order to accommodate infant participants.

Objectives: The aims of this project are to: (1) review existing guidelines for sound measurement and attenuation in MRI studies of infants; (2) record accurate measurements of acoustic emissions from a Siemens 3T Tim Trio to determine the amount of sound attenuation necessary for a safe infant scan; and (3) create a HPD, specifically designed for infants, that provides sufficient sound attenuation and allows researchers to monitor the level of sound attenuation in real time, thereby ensuring that effectiveness is maintained throughout the scan session.

Methods: Studies on acoustic emissions from MRIs, ANSI standards for measuring HPD effectiveness, and sound conduction pathways in the human body were reviewed.  Acoustic emissions generated by the MRI and the effectiveness of a HPD designed for infants will be measured and tested in 1-to 9-month-olds (n=20).

Results: Findings from the literature review guided the development of our framework for creating and measuring the effectiveness of a customized HPD for infants. The attenuation properties of our HPD will be tested outside the scanner by playing recordings of scanner sounds (MRI, DTI, and fMRI sequences to be used in infant studies) at reduced amplitudes (~60db).  Attenuation will be measured with built-in MR-safe microphones that record sound pressure levels (SPL) outside each ear. The noise reduction (NR) scale will be used in recording these measurements with the transfer function of open ear (TFOE) correction to account for natural amplification by the pinna and ear canal. The microphones integrated into the HPD design will allow for continuous monitoring of sound attenuation during a MRI scan.

Conclusions: This research demonstrates a first step towards the development of an HPD, custom-made for infants, that reduces scanner noise to a safe level and allows real-time monitoring of the effectiveness of the HPD throughout a scan session. Using the principles learned from the existing literature, our immediate next steps include completing the design and testing of this customized HPD.