Acoustic noise during functional magnetic resonance imaging

Acoustic noise generated by TMS in typical environment and inside an MRI scanner

BACKGROUND

The operation of a transcranial magnetic stimulation (TMS) coil produces high-intensity impulse sounds. In TMS, a magnetic field is generated by a short-duration pulse in the range of thousands of amperes in the TMS coil. When placed in a strong magnetic field, such as inside a magnetic resonance imaging (MRI) bore, the interaction of the magnetic field and the current in the TMS coil can cause strong forces on the coil casing. The strengths of these forces depend on the coil orientation in the main magnetic field (B0). Part of the energy in this process is dissipated in the form of acoustic noise.

OBJECTIVE

Our objective was to measure the sound pressure levels (SPL) of TMS "click" sounds created by commercial TMS stimulators and coils in a typical environment and inside a 3-T MRI scanner and advance the knowledge of the acoustic behaviour of TMS to safely conduct TMS alone as well as concurrently with functional MRI (fMRI).

METHODS

We report SPL measurements of two commercial MRI-compatible TMS systems in the 3-T B0 field of an MRI scanner and in the earth's magnetic field. Also, we present the acoustic noise measurements of four commercial TMS stimulators and three different TMS coils in a typical operational environment without the B0 field.

RESULTS

The maximum peak SPL measured was 158 dB(C) inside the 3-T MRI scanner. Outside the scanner, the maximum peak SPL was 117 dB(C). Inside the scanner, the peak SPL increased by 21-45 dB(C) depending on the stimulator and the orientation of the electric field relative to the B field.

CONCLUSIONS

Hearing protection is obligatory during concurrent TMS-fMRI experiments and highly recommended during any TMS experiment. The manufacturing of quieter TMS systems is encouraged to reduce the risk of hearing damage and other unwanted effects.

Strategies to improve the magnetic resonance imaging experience for autistic individuals: a cross-sectional study exploring parents and carers' experiences

BACKGROUND

Autistic individuals encounter numerous barriers in accessing healthcare, including communication difficulties, sensory sensitivities, and a lack of appropriate adjustments. These issues are particularly acute during MRI scans, which involve confined spaces, loud noises, and the necessity to remain still. There remains no unified approach to preparing autistic individuals for MRI procedures.

METHODS

A cross-sectional online survey was conducted with parents and carers of autistic individuals in the UK to explore their experiences, barriers, and recommendations concerning MRI scans. The survey collected demographic information and experiential accounts of previous MRI procedures. Quantitative data were analysed descriptively, while key themes were identified within the qualitative data through inductive thematic analysis.

RESULTS

Sixteen parents/carers participated. The majority reported difficulties with communication, inadequate pre-scan preparation, and insufficient adjustments during MRI scans for their autistic children. Key barriers included an overwhelming sensory environment, radiographers' limited understanding of autism, and anxiety stemming from uncertainties about the procedure. Recommended improvements encompassed accessible communication, pre-visit familiarisation, noise-reduction and sensory adaptations, staff training on autism, and greater flexibility to meet individual needs.

CONCLUSIONS

There is an urgent need to enhance MRI experiences for autistic individuals. This can be achieved through improved staff knowledge, effective communication strategies, thorough pre-scan preparation, and tailored reasonable adjustments. Co-producing clear MRI guidelines with the autism community could standardise sensitive practices. An individualised approach is crucial for reducing anxiety and facilitating participation. Empowering radiographers through autism-specific education and incorporating insights from autistic individuals and their families could transform MRI experiences and outcomes.

Fetal MRI at 3 T: Principles to Optimize Success

Fetal MRI has emerged as a cornerstone of prenatal imaging, helping to establish the correct diagnosis in pregnancies affected by congenital anomalies. In the past decade, 3 T imaging was introduced as an alternative to increase the signal-to-noise ratio (SNR) of the pulse sequences and improve anatomic detail. However, imaging at a higher field strength is not without challenges. Many artifacts that are barely appreciable at 1.5 T are amplified at 3 T. A systematic approach to imaging at 3 T that incorporates appropriate patient positioning, a thoughtful protocol design, and sequence optimization minimizes the impact of these artifacts and allows radiologists to reap the benefits of the increased SNR. The sequences used are the same at both field strengths and include single-shot T2-weighted, balanced steady-state free-precession, three-dimensional T1-weighted spoiled gradient-echo, and echo-planar imaging. Synergistic use of these acquisitions to sample various tissue contrasts and in various planes provides valuable information about fetal anatomy and pathologic conditions. In the authors' experience, fetal imaging at 3 T outperforms imaging at 1.5 T for most indications when performed under optimal circumstances. The authors condense the cumulative experience of fetal imaging specialists and MRI technologists who practice at a large referral center into a guideline covering all major aspects of fetal MRI at 3 T, from patient preparation to image interpretation. © RSNA, 2023 Quiz questions for this article are available in the supplemental material.

Direct comparison of gradient Fidelity and acoustic noise of the same MRI system at 3 T and 0.75 T

PURPOSE

To analyze the difference between gradient fidelity and acoustic noise of the same MRI scanner operated at product field strength (3 T) and lower field strength (0.75 T).

METHODS

Gradient modulation transfer functions (GMTFs) were measured using a four-slice 2D phase-encoded chirp-based sequence on the same scanner operated at 3 T and, following ramp-down, at 0.75 T with identical gradient specifications (40 mT/m, 200 T/m/s). Calibrated audio measurements were performed at both field strengths to correlate audio spectra with GMTFs.

RESULTS

While eddy currents were independent of field strength, mechanical resonances were substantially decreased at lower field, resulting in a reduction of GMTF distortions by up to 95% (88% on average) at the mechanical resonances of the gradient system. Audio spectra amplitudes were reduced by up to 87% when comparing 0.75 T versus 3 T.

CONCLUSION

Lower static fields lead to reduced Lorentz forces on the gradient coil and, in turn, to reduced mechanical resonances, thereby improving gradient fidelity. Simultaneously, the reduction of acoustic noise may help to improve patient comfort.

Future Directions for Chemosensory Connectomes: Best Practices and Specific Challenges

Ecological chemosensory stimuli almost always evoke responses in more than one sensory system. Moreover, any sensory processing takes place along a hierarchy of brain regions. So far, the field of chemosensory neuroimaging is dominated by studies that examine the role of brain regions in isolation. However, to completely understand neural processing of chemosensation, we must also examine interactions between regions. In general, the use of connectivity methods has increased in the neuroimaging field, providing important insights to physical sensory processing, such as vision, audition, and touch. A similar trend has been observed in chemosensory neuroimaging, however, these established techniques have largely not been rigorously applied to imaging studies on the chemical senses, leaving network insights overlooked. In this article, we first highlight some recent work in chemosensory connectomics and we summarize different connectomics techniques. Then, we outline specific challenges for chemosensory connectome neuroimaging studies. Finally, we review best practices from the general connectomics and neuroimaging fields. We recommend future studies to develop or use the following methods we perceive as key to improve chemosensory connectomics: (1) optimized study designs, (2) reporting guidelines, (3) consensus on brain parcellations, (4) consortium research, and (5) data sharing.

Optimized lung tumor diagnosis system using enhanced version of crow search algorithm, Zernike moments, and support vector machine

Early detection of lung tumors is so important to heal this disease in the initial steps. Automatic computer-aided detection of this disease is a good method for reducing human mistakes and improving detection precision. The major concept here is to propose the best CAD system for lung tumor detection. In the presented technique, after pre-processing and segmentation of the lung area, its features including different orders of Zernike moments have been extracted. After features extraction, they have been injected into an optimized version of Support Vector Machine (SVM) for final diagnosis. The optimization of the SVM is based on an enhanced design of the Crow Search Algorithm (ECSA). For validating the proposed method, it was applied to three datasets including Lung CT-Diagnosis, TCIA, and RIDER Lung CT collection, and the results are validated by comparing with three state-of-the-art methods including Walwalker method, Mon method, and Naik method to indicate the system superiority toward the compared methods. The system is also analyzed based on different orders of Zernike moment to select the best order. The final results indicate that the suggested method has a suitable accuracy for diagnosing lung cancer.

The effects of a simulated fMRI environment on voice intensity in individuals with Parkinson's disease hypophonia and older healthy adults

PURPOSE

Functional magnetic resonance imaging (fMRI) has promise for understanding neural mechanisms of neurogenic speech and voice disorders. However, performing vocal tasks within the fMRI environment may not always be analogous to performance outside of the scanner. Using a mock MRI scanner, this study examines the effects of a simulated scanning environment on vowel intensity in individuals with Parkinson's disease (PD) and hypophonia and older healthy control (OHC) participants.

METHOD

Thirty participants (15 PD, 15 OHC) performed a sustained /ɑ/ vowel production task in three conditions: 1) Upright, 2) Mock Scanner + No Noise, and 3) Mock Scanner + MRI noise. We used a linear mixed-effects (multi-level) model to evaluate the contributions of group and recording environment to vowel intensity. A second linear mixed-effects model was also used to evaluate the contributions of PD medication state (On vs. Off) to voice intensity.

RESULTS

Vowel intensity was significantly lower for PD compared to the OHC group. The intensity of vowels produced in the Upright condition was significantly lower compared to the Mock Scanner + No Noise condition, while vowel intensity in the Mock Scanner + MRI Noise condition was significantly higher compared to the Mock Scanner + No Noise condition. A group by condition interaction also indicated that the addition of scanner noise had a greater impact on the PD group. A second analysis conducted within the PD group showed no effects of medication state on vowel intensity.

CONCLUSION

Our findings demonstrate that performance on voice production tasks is altered for PD and OHC groups when translated into the fMRI environment, even in the absence of acoustic scanner noise. For fMRI studies of voice in PD hypophonia, careful thought should be given to how the presence of acoustic noise may differentially affect PD and OHC, for both group and task comparisons.

A Miniature, Fiber-Optic Vibrometer for Measuring Unintended Acoustic Output of Active Hearing Implants during Magnetic Resonance Imaging

Making use of magnetic resonance imaging (MRI) for diagnostics on patients with implanted medical devices requires caution due to mutual interactions between the device and the electromagnetic fields used by the scanner that can cause a number of adverse events. The presented study offers a novel test method to quantify the risk of unintended output of acoustically stimulating hearing implants. The design and operating principle of an all-optical, MRI safe vibrometer is outlined, followed by an experimental verification of a prototype. Results obtained in an MRI environment indicate that the system can detect peak displacements down to 8 pm for audible frequencies. Feasibility testing was performed with an active middle ear implant that was exposed to several pulse sequences in a 1.5 Tesla MRI environment. Magnetic field induced actuator vibrations, measured during scanning, turned out to be equivalent to estimated sound pressure levels between 25 and 85 dB SPL, depending on the signal frequency. These sound pressure levels are situated well below ambient sound pressure levels generated by the MRI scanning process. The presented case study therefore indicates a limited risk of audible unintended output for the examined hearing implant during MRI.

Comparison of continuous sampling with active noise cancelation and sparse sampling for cortical and subcortical auditory functional MRI

PURPOSE

Detecting sound-related activity using functional MRI requires the auditory stimulus to be more salient than the intense background scanner acoustic noise. Various strategies can reduce the impact of scanner acoustic noise, including "sparse" temporal sampling with single/clustered acquisitions providing intervals without any background scanner acoustic noise, or active noise cancelation (ANC) during "continuous" temporal sampling, which generates an acoustic signal that adds destructively to the scanner acoustic noise, substantially reducing the acoustic energy at the participant's eardrum. Furthermore, multiband functional MRI allows multiple slices to be collected simultaneously, thereby reducing scanner acoustic noise in a given sampling period.

METHODS

Isotropic multiband functional MRI (1.5 mm) with sparse sampling (effective TR = 9000 ms, acquisition duration = 1962 ms) and continuous sampling (TR = 2000 ms) with ANC were compared in 15 normally hearing participants. A sustained broadband noise stimulus was presented to drive activation of both sustained and transient auditory responses within subcortical and cortical auditory regions.

RESULTS

Robust broadband noise-related activity was detected throughout the auditory pathways. Continuous sampling with ANC was found to give a statistically significant advantage over sparse sampling for the detection of the transient (onset) stimulus responses, particularly in the auditory cortex (P < .001) and inferior colliculus (P < .001), whereas gains provided by sparse over continuous ANC for detecting offset and sustained responses were marginal (p ~ 0.05 in superior olivary complex, inferior colliculus, medial geniculate body, and auditory cortex).

CONCLUSIONS

Sparse and continuous ANC multiband functional MRI protocols provide differing advantages for observing the transient (onset and offset) and sustained stimulus responses.

Survey of Acoustic Output in Neonatal Brain Protocols

BACKGROUND

Auditory and non-auditory safety concerns associated with the appreciable sound levels inherent to magnetic resonance imaging (MRI) procedures exist for neonates. However, current gaps in knowledge preclude making an adequate risk assessment.

PURPOSE

To measure acoustic exposure (duration, intensity, and frequency) during neonatal brain MRI and compare these values to existing hearing safety limits and data.

STUDY TYPE

Phantom.

PHANTOM

Cylindrical doped water phantom.

FIELD STRENGTH/SEQUENCE

Neonatal brain protocols acquired at 1-3 T. Scans in the model protocol included a diffusion tensor imaging scan, a gradient echo, a three-dimensional (3D) fast spin echo, 3D fast spin-echo single-shots, a spin echo, a turbo spin echo, a 3D arterial spin labeling scan, and a susceptibility-weighted fast spin-echo scan.

ASSESSMENT

The sound pressure levels (SPLs), frequency profile, and durations of five neonatal brain protocols on five MR scanners (scanner A [3 T, whole-body], scanner B [1.5 T, whole-body], scanner C [1 T, dedicated neonatal], scanner D [1.5 T, whole-body], and scanner E [3 T, whole-body]) located at three different sites were recorded. The SPLs were then compared to the International Electrotechnical Commission (IEC) hearing safety limit and existing data of infant non-auditory responses to loud sounds to assess risk.

STATISTICAL TESTS

Mann-Whitney U test to assess whether the dedicated neonatal scanner was quieter than the other machines.

RESULTS

The average level A-weighted equivalent value (LAEQ) across all five MR scanners and scans was 92.88 dBA and the range of LAEQs across all five MR scanners and scans was 80.8-105.31 dBA. The duration of the recorded neonatal protocols maintained by neonatal scanning facilities (from scanners A, B, and C) ranged from 27:33 to 37:06 minutes.

DATA CONCLUSION

Neonatal protocol sound levels straddled existing notions of risk, exceeding sound levels known to cause non-auditory responses in neonates but not exceeding the IEC MRI SPL safety limit.

LEVEL OF EVIDENCE

5 TECHNICAL EFFICACY: Stage 5.

Revealing the mechanisms behind novel auditory stimuli discrimination: An evaluation of silent functional MRI using looping star

Looping Star is a near‐silent, multi‐echo, 3D functional magnetic resonance imaging (fMRI) technique. It reduces acoustic noise by at least 25dBA, with respect to gradient‐recalled echo echo‐planar imaging (GRE‐EPI)‐based fMRI. Looping Star has successfully demonstrated sensitivity to the cerebral blood‐oxygen‐level‐dependent (BOLD) response during block design paradigms but has not been applied to event‐related auditory perception tasks. Demonstrating Looping Star's sensitivity to such tasks could (a) provide new insights into auditory processing studies, (b) minimise the need for invasive ear protection, and (c) facilitate the translation of numerous fMRI studies to investigations in sound‐averse patients. We aimed to demonstrate, for the first time, that multi‐echo Looping Star has sufficient sensitivity to the BOLD response, compared to that of GRE‐EPI, during a well‐established event‐related auditory discrimination paradigm: the “oddball” task. We also present the first quantitative evaluation of Looping Star's test–retest reliability using the intra‐class correlation coefficient. Twelve participants were scanned using single‐echo GRE‐EPI and multi‐echo Looping Star fMRI in two sessions. Random‐effects analyses were performed, evaluating the overall response to tones and differential tone recognition, and intermodality analyses were computed. We found that multi‐echo Looping Star exhibited consistent sensitivity to auditory stimulation relative to GRE‐EPI. However, Looping Star demonstrated lower test–retest reliability in comparison with GRE‐EPI. This could reflect differences in functional sensitivity between the techniques, though further study is necessary with additional cognitive paradigms as varying cognitive strategies between sessions may arise from elimination of acoustic scanner noise.

Acoustic Noise and Magnetic Resonance Imaging: A Narrative/Descriptive Review

Magnetic resonance imaging generates unwanted acoustic noise. This review describes the work characterizing the acoustic noise, and the various solutions to control and attenuate the acoustic noise. There are also discussions about the permissible limits, and guidance regarding acoustic noise exposure for staff, patients, and volunteers. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 1.

Neural representations of own-voice in the human auditory cortex

We have a keen sensitivity when it comes to the perception of our own voices. We can detect not only the differences between ourselves and others, but also slight modifications of our own voices. Here, we examined the neural correlates underlying such sensitive perception of one's own voice. In the experiments, we modified the subjects' own voices by using five types of filters. The subjects rated the similarity of the presented voices to their own. We compared BOLD (Blood Oxygen Level Dependent) signals between the voices that subjects rated as least similar to their own voice and those they rated as most similar. The contrast revealed that the bilateral superior temporal gyrus exhibited greater activities while listening to the voice least similar to their own voice and lesser activation while listening to the voice most similar to their own. Our results suggest that the superior temporal gyrus is involved in neural sharpening for the own-voice. The lesser degree of activations observed by the voices that were similar to the own-voice indicates that these areas not only respond to the differences between self and others, but also respond to the finer details of own-voices.

Left Inferior Frontal Gyrus Integrates Multisensory Information in Category Learning

Humans are able to categorize things they encounter in the world (e.g., a cat) by integrating multisensory information from the auditory and visual modalities with ease and speed. However, how the brain learns multisensory categories remains elusive. The present study used functional magnetic resonance imaging to investigate, for the first time, the neural mechanisms underpinning multisensory information-integration (II) category learning. A sensory-modality-general network, including the left insula, right inferior frontal gyrus (IFG), supplementary motor area, left precentral gyrus, bilateral parietal cortex, and right caudate and globus pallidus, was recruited for II categorization, regardless of whether the information came from a single modality or from multiple modalities. Putamen activity was higher in correct categorization than incorrect categorization. Critically, the left IFG and left body and tail of the caudate were activated in multisensory II categorization but not in unisensory II categorization, which suggests this network plays a specific role in integrating multisensory information during category learning. The present results extend our understanding of the role of the left IFG in multisensory processing from the linguistic domain to a broader role in audiovisual learning.

Deep Learning of Static and Dynamic Brain Functional Networks for Early MCI Detection

While convolutional neural network (CNN) has been demonstrating powerful ability to learn hierarchical spatial features from medical images, it is still difficult to apply it directly to resting-state functional MRI (rs-fMRI) and the derived brain functional networks (BFNs). We propose a novel CNN framework to simultaneously learn embedded features from BFNs for brain disease diagnosis. Since BFNs can be built by considering both static and dynamic functional connectivity (FC), we first decompose rs-fMRI into multiple static BFNs with modified independent component analysis. Then, the voxel-wise variability in dynamic FC is used to quantify BFN dynamics. A set of paired 3D images representing static/dynamic BFNs can be fed into 3D CNNs, from which we can hierarchically and simultaneously learn static/dynamic BFN features. As a result, the dynamic BFN features can complement static BFN features and, at the meantime, different BFNs can help each other toward a joint and better classification. We validate our method with a publicly accessible, large cohort of rs-fMRI dataset in early-stage mild cognitive impairment (eMCI) diagnosis, which is one of the most challenging problems to the clinicians. By comparing with a conventional method, our method shows significant diagnostic performance improvement by almost 10%. This result demonstrates the effectiveness of deep learning in preclinical Alzheimer's disease diagnosis, based on the complex and high-dimensional voxel-wise spatiotemporal patterns of the resting-state brain functional connectomics. The framework provides a new but intuitive way to fully exploit deeply embedded diagnostic features from rs-fMRI for a better-individualized diagnosis of various neurological diseases.

Corticostriatal functional connectivity of bothersome tinnitus in single-sided deafness

Subjective tinnitus is an auditory phantom perceptual disorder without an objective biomarker. Bothersome tinnitus in single-sided deafness (SSD) is particularly challenging to treat because the deaf ear can no longer be stimulated by acoustic means. We contrasted an SSD cohort with bothersome tinnitus (TIN; N = 15) against an SSD cohort with no or non-bothersome tinnitus (NO TIN; N = 15) using resting-state functional magnetic resonance imaging (fMRI). All study participants had normal hearing in one ear and severe or profound hearing loss in the other. We evaluated corticostriatal functional connectivity differences by placing seeds in the caudate nucleus and Heschl’s Gyrus (HG) of both hemispheres. The TIN cohort showed increased functional connectivity between the left caudate and left HG, and left and right HG and the left caudate. Within the TIN cohort, functional connectivity between the right caudate and cuneus was correlated with the Tinnitus Functional Index (TFI) relaxation subscale. And, functional connectivity between the right caudate and superior lateral occipital cortex, and the right caudate and anterior supramarginal gyrus were correlated with the TFI control subscale. These findings support a striatal gating model of tinnitus and suggest tinnitus biomarkers to monitor treatment response and to target specific brain areas for innovative neuromodulation therapies.

Acoustic trauma from continuous noise: Minimum exposures, issues in clinical trial design, and comments on magnetic resonance imaging exposures

Acoustic trauma (AT) is permanent hearing loss after a single noise exposure. A few human cases resulting from continuous, i.e., nonimpulsive noise, have been reported as reviewed by Ward [(1991). "Hearing loss from noise and music," presented at Audio Engineering Society, New York, October 4-8]. This paper updates that review by examining 11 cases in nine reports, from 1950 to 2006, with the intention of determining minimum exposures that may cause AT, including the potential risk of exposure to noise from magnetic resonance imaging machines. Diffuse-field related levels above 120 dBA for 10 s or more, or above 130 dBA for 2-3 s (values well above OSHA's unprotected exposure limits), can lead to AT. These cases appear to represent a susceptible fraction of the population, because much more intense exposures (e.g., 130 dBA for 32 min) have been tolerated by groups of volunteers who suffered only temporary threshold shifts. AT from continuous noise is unlikely to occur in OSHA-compliant hearing conservation programs, and probably rare enough in the general civilian population that clinical trials of drugs aimed at treating it are unlikely to be practical. AT from impulse noise, such as gunfire, which is specifically not the topic of the current work, is more amenable to clinical trials, especially in military settings.

Spatiotemporal Empirical Mode Decomposition of Resting-State fMRI Signals: Application to Global Signal Regression

Resting-state functional connectivity MRI (rs-fcMRI) is a common method for mapping functional brain networks. However, estimation of these networks is affected by the presence of a common global systemic noise, or global signal (GS). Previous studies have shown that the common preprocessing steps of removing the GS may create spurious correlations between brain regions. In this paper, we decompose fMRI signals into 5 spatial and 3 temporal intrinsic mode functions (SIMF and TIMF, respectively) by means of the empirical mode decomposition (EMD), which is an adaptive data-driven method widely used to analyze non-linear and non-stationary phenomena. For each SIMF, functional connectivity matrices were computed by means of Pearson correlation between TIMFs of different brain areas. Thus, instead of a single connectivity matrix, we obtained 5 × 3 = 15 functional connectivity matrices. Given the high correlation and global efficiency values of the connectivity matrices related to the low spatial maps (SIMF3, SIMF4, and SIMF5), our results suggest that these maps can be considered as spatial global signal masks. Thus, by summing up the first two SIMFs extracted from the fMRI signals, we have automatically excluded the GS which is now voxel-specific. We compared the performance of our method with the conventional GS regression and to the results when the GS was not removed. While the correlation pattern identified by the other methods suffers from a low level of precision in identifying the correct brain network connectivity, our approach demonstrated expected connectivity patterns for the default mode network and task-positive network.

The Future of Contextual Fear Learning for PTSD Research: A Methodological Review of Neuroimaging Studies

There has been a great deal of recent interest in human models of contextual fear learning, particularly due to the use of such paradigms for investigating neural mechanisms related to the etiology of posttraumatic stress disorder. However, the construct of "context" in fear conditioning research is broad, and the operational definitions and methods used to investigate contextual fear learning in humans are wide ranging and lack specificity, making it difficult to interpret findings about neural activity. Here we will review neuroimaging studies of contextual fear acquisition in humans. We will discuss the methodology associated with four broad categories of how contextual fear learning is manipulated in imaging studies (colored backgrounds, static picture backgrounds, virtual reality, and configural stimuli) and highlight findings for the primary neural circuitry involved in each paradigm. Additionally, we will offer methodological recommendations for human studies of contextual fear acquisition, including using stimuli that distinguish configural learning from discrete cue associations and clarifying how context is experimentally operationalized.

The Impact of Acoustic fMRI-Noise on Olfactory Sensitivity and Perception

Sensory perception is neither static nor simple. The senses influence each other during multisensory stimulation and can be both suppressive and super-additive. As most knowledge of human olfactory perception is derived from functional neuroimaging studies, in particular fMRI, our current understanding of olfactory perception has systematically been investigated in an environment with concurrent loud sounds. To date, the confounding effects of acoustic fMRI-noise during scanning on olfactory perception have not yet been investigated. In this study we investigate how acoustic noise derived from the rapid switching of MR gradient coils, affects olfactory perception. For this, 50 subjects were tested in both a silent setting and an fMRI-noise setting, in a randomised order. We found that fMRI-related acoustic noise had a significant negative effect on the olfactory detection threshold score. No significant effects were identified on olfactory discrimination, identification, identification certainty, hedonic rating, or intensity rating.

Does 3-T fetal MRI induce adverse acoustic effects in the neonate? A preliminary study comparing postnatal auditory test performance of fetuses scanned at 1.5 and 3 T

BACKGROUND

Fetal MRI at 3 T is associated with increased acoustic noise relative to 1.5 T.

OBJECTIVE

The goal of this study is to determine if there is an increased prevalence of congenital hearing loss in neonates who had a 3-T prenatal MR vs. those who had it at 1.5 T.

MATERIALS AND METHODS

We retrospectively identified all subjects who had 3-T fetal MRI between 2012 and 2016 and also underwent universal neonatal hearing screening within 60 days of birth. Fetuses with incomplete hearing screening, magnetic resonance imaging (MRI) studies at both field strengths or fetuses affected by conditions associated with hearing loss were excluded. A random group of controls scanned at 1.5 T was identified. Five subjects had repeat same-strength MRIs (one at 3 T and four at 1.5 T). The pass/fail rate of the transient otoacoustic emissions test and auditory brainstem response test were compared using the Fisher exact test. A logistic regression was performed to assess the effects of other known risk factors for congenital hearing loss.

RESULTS

Three hundred forty fetal MRI examinations were performed at 3 T, of which 62 met inclusion criteria. A control population of 1.5-T fetal MRI patients was created using the same exclusion criteria, with 62 patients randomly selected from the eligible population. The fail rates of transient otoacoustic emissions test for the 1.5-T and 3-T groups were 9.7% and 6.5%, respectively, and for the auditory brainstem response test were 3.2% and 1.6%, respectively. There was no significant difference in the fail rate of either test between groups (P=0.74 for transient otoacoustic emissions test, and P=0.8 for auditory brainstem response test). The median gestational age of the 3-T group was 30 weeks, 1 day, significantly higher (P<0.001) than the 1.5-T group (median gestational age: 20 weeks, 2 days).

CONCLUSION

Our findings suggest that the increase in noise associated with 3 T does not increase the rate of clinically detectable hearing abnormalities.

Accelerated silent echo-planar imaging

PURPOSE

The standard approach to Echo-Planar Imaging (EPI) is to use trapezoidal readout (RO) gradients with blipped phase-encoding (PE) gradients. Sinusoidal RO gradients with constant PE gradients can reduce acoustic noise. However, this sequence, originally introduced by Mansfield et al., constitutes major challenges for Cartesian parallel imaging techniques. In this study two alternatives to reconstruct a non-blipped EPI are proposed and evaluated.

THEORY AND METHODS

The first method separates the acquired k-space data into odd and even echoes and applies Cartesian GRAPPA separately to each partial data set. Afterwards, the resulting reconstructed data sets for each echo are summed in image space. In the second method, an iterative parallel-imaging algorithm is used to reconstruct images from the highly non-Cartesian data samples.

RESULTS

Compared to blipped-EPI, the first method reduces image SNR depending on the acceleration factor between 11% and 60%. For an acceleration factor of 3 folding artefacts appear. The second method produces slight fold-over artefacts although image SNR is on the same level as the blipped approach.

CONCLUSION

In this study, we have introduced two new approaches to EPI that allow the use of Cartesian parallel imaging in conjunction with continuous data sampling. In addition to providing a reduction in acoustic noise compared to the standard blipped PE EPI sequence, the proposed techniques improve sampling efficiency, resulting in a reduction of the echo-spacing. Of the two methods, the second approach, based on an iterative image reconstruction, provides higher SNR, but requires a longer reconstruction time.

Sensory prediction errors in the continuum of psychosis

Sensory prediction errors are fundamental brain responses that signal a violation of expectation in either the internal or external sensory environment, and are therefore crucial for survival and adaptive behaviour. Patients with schizophrenia show deficits in these internal and external sensory prediction errors, which can be measured using electroencephalography (EEG) components such as N1 and mismatch negativity (MMN), respectively. New evidence suggests that these deficits in sensory prediction errors are more widely distributed on a continuum of psychosis, whereas psychotic experiences exist to varying degrees throughout the general population. In this paper, we review recent findings in sensory prediction errors in the auditory domain across the continuum of psychosis, and discuss these in light of the predictive coding hypothesis.

Pavlovian conditioning-induced hallucinations result from overweighting of perceptual priors

Some people hear voices that others do not, but only some of those people seek treatment. Using a Pavlovian learning task, we induced conditioned hallucinations in four groups of people who differed orthogonally in their voice-hearing and treatment-seeking statuses. People who hear voices were significantly more susceptible to the effect. Using functional neuroimaging and computational modeling of perception, we identified processes that differentiated voice-hearers from non-voice-hearers and treatment-seekers from non-treatment-seekers and characterized a brain circuit that mediated the conditioned hallucinations. These data demonstrate the profound and sometimes pathological impact of top-down cognitive processes on perception and may represent an objective means to discern people with a need for treatment from those without.

fMRI Mapping of Brain Activity Associated with the Vocal Production of Consonant and Dissonant Intervals

The neural correlates of consonance and dissonance perception have been widely studied, but not the neural correlates of consonance and dissonance production. The most straightforward manner of musical production is singing, but, from an imaging perspective, it still presents more challenges than listening because it involves motor activity. The accurate singing of musical intervals requires integration between auditory feedback processing and vocal motor control in order to correctly produce each note. This protocol presents a method that permits the monitoring of neural activations associated with the vocal production of consonant and dissonant intervals. Four musical intervals, two consonant and two dissonant, are used as stimuli, both for an auditory discrimination test and a task that involves first listening to and then reproducing given intervals. Participants, all female vocal students at the conservatory level, were studied using functional Magnetic Resonance Imaging (fMRI) during the performance of the singing task, with the listening task serving as a control condition. In this manner, the activity of both the motor and auditory systems was observed, and a measure of vocal accuracy during the singing task was also obtained. Thus, the protocol can also be used to track activations associated with singing different types of intervals or with singing the required notes more accurately. The results indicate that singing dissonant intervals requires greater participation of the neural mechanisms responsible for the integration of external feedback from the auditory and sensorimotor systems than does singing consonant intervals.

A numerical study of the acoustic radiation due to eddy current-cryostat interactions

PURPOSE

To investigate the acoustic radiation due to eddy current-cryostat interactions and perform a qualitative analysis on noise reduction methods.

METHODS

In order to evaluate the sound pressure level (SPL) of the eddy current induced warm bore wall vibration, a Finite Element (FE) model was created to simulate the noises from both the warm bore wall vibration and the gradient coil assembly. For the SPL reduction of the warm bore wall vibration, we first improved the active shielding of the gradient coil, thus reducing the eddy current on the warm bore wall. A damping treatment was then applied to the warm bore wall to control the acoustic radiation.

RESULTS

Initial simulations show that the SPL of the warm bore wall is higher than that of the gradient assembly with typical design shielding ratios at many frequencies. Subsequent simulation results of eddy current control and damping treatment application show that the average SPL reduction of the warm bore wall can be as high as 9.6 dB, and even higher in some frequency bands.

CONCLUSIONS

Combining eddy current control and suggested damping scheme, the noise level in a MRI system can be effectively reduced.

Neural activity related to discrimination and vocal production of consonant and dissonant musical intervals

BACKGROUND

Relationships between musical pitches are described as either consonant, when associated with a pleasant and harmonious sensation, or dissonant, when associated with an inharmonious feeling. The accurate singing of musical intervals requires communication between auditory feedback processing and vocal motor control (i.e. audio-vocal integration) to ensure that each note is produced correctly. The objective of this study is to investigate the neural mechanisms through which trained musicians produce consonant and dissonant intervals.

METHODOLOGY

We utilized 4 musical intervals (specifically, an octave, a major seventh, a fifth, and a tritone) as the main stimuli for auditory discrimination testing, and we used the same interval tasks to assess vocal accuracy in a group of musicians (11 subjects, all female vocal students at conservatory level). The intervals were chosen so as to test for differences in recognition and production of consonant and dissonant intervals, as well as narrow and wide intervals. The subjects were studied using fMRI during performance of the interval tasks; the control condition consisted of passive listening.

RESULTS

Singing dissonant intervals as opposed to singing consonant intervals led to an increase in activation in several regions, most notably the primary auditory cortex, the primary somatosensory cortex, the amygdala, the left putamen, and the right insula. Singing wide intervals as opposed to singing narrow intervals resulted in the activation of the right anterior insula. Moreover, we also observed a correlation between singing in tune and brain activity in the premotor cortex, and a positive correlation between training and activation of primary somatosensory cortex, primary motor cortex, and premotor cortex during singing. When singing dissonant intervals, a higher degree of training correlated with the right thalamus and the left putamen.

CONCLUSIONS/SIGNIFICANCE

Our results indicate that singing dissonant intervals requires greater involvement of neural mechanisms associated with integrating external feedback from auditory and sensorimotor systems than singing consonant intervals, and it would then seem likely that dissonant intervals are intoned by adjusting the neural mechanisms used for the production of consonant intervals. Singing wide intervals requires a greater degree of control than singing narrow intervals, as it involves neural mechanisms which again involve the integration of internal and external feedback.

Temporal pattern of acoustic imaging noise asymmetrically modulates activation in the auditory cortex

This study investigated the hemisphere-specific effects of the temporal pattern of imaging related acoustic noise on auditory cortex activation. Hemodynamic responses (HDRs) to five temporal patterns of imaging noise corresponding to noise generated by unique combinations of imaging volume and effective repetition time (TR), were obtained using a stroboscopic event-related paradigm with extra-long (≥27.5 s) TR to minimize inter-acquisition effects. In addition to confirmation that fMRI responses in auditory cortex do not behave in a linear manner, temporal patterns of imaging noise were found to modulate both the shape and spatial extent of hemodynamic responses, with classically non-auditory areas exhibiting responses to longer duration noise conditions. Hemispheric analysis revealed the right primary auditory cortex to be more sensitive than the left to the presence of imaging related acoustic noise. Right primary auditory cortex responses were significantly larger during all the conditions. This asymmetry of response to imaging related acoustic noise could lead to different baseline activation levels during acquisition schemes using short TR, inducing an observed asymmetry in the responses to an intended acoustic stimulus through limitations of dynamic range, rather than due to differences in neuronal processing of the stimulus. These results emphasize the importance of accounting for the temporal pattern of the acoustic noise when comparing findings across different fMRI studies, especially those involving acoustic stimulation.

Multisensory Competition Is Modulated by Sensory Pathway Interactions with Fronto-Sensorimotor and Default-Mode Network Regions

Multisensory information competes for preferential access to consciousness. It remains unknown what neural processes cause one particular modality to win multisensory competition and eventually dominate behavior. Thus, in a paradigm in which human participants sought to make simultaneous auditory and visual detection responses, we sought to identify prestimulus and poststimulus neural signals that were associated with auditory and visual dominance on each trial. Behaviorally, visual detection responses preceded auditory responses more frequently than vice versa. Even when visual responses were preceded by auditory responses, they recovered more quickly from previous responses, indicating the dominance of vision over audition. Neurally, visual precedence was associated with increased prestimulus activity in the prefrontal cortex and reduced prestimulus activity in the default-mode network, and increased poststimulus connectivity between the prefrontal cortex and the visual system. Moreover, the dorsal visual stream showed not only increased activity in post-perceptual phases but also enhanced connectivity with the sensorimotor cortex, indicating the functional role of the dorsal visual stream in prioritizing the flow of visual information into the motor system. In contrast, auditory precedence was associated with increased prestimulus activity in the auditory cortex and increased poststimulus neural coupling between the auditory and the sensorimotor cortex. Finally, whenever one modality lost multisensory competition, the corresponding sensory cortex showed enhanced connectivity with the default-mode network. Overall, the outcome of audiovisual competition depended on dynamic interactions between sensory systems and both the fronto-sensorimotor and the default-mode network. Together, these results revealed both the neural causes and the neural consequences of visual and auditory dominance during multisensory competition.

Mapping cortical responses to speech using high-density diffuse optical tomography

The functional neuroanatomy of speech processing has been investigated using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) for more than 20years. However, these approaches have relatively poor temporal resolution and/or challenges of acoustic contamination due to the constraints of echoplanar fMRI. Furthermore, these methods are contraindicated because of safety concerns in longitudinal studies and research with children (PET) or in studies of patients with metal implants (fMRI). High-density diffuse optical tomography (HD-DOT) permits presenting speech in a quiet acoustic environment, has excellent temporal resolution relative to the hemodynamic response, and provides noninvasive and metal-compatible imaging. However, the performance of HD-DOT in imaging the brain regions involved in speech processing is not fully established. In the current study, we use an auditory sentence comprehension task to evaluate the ability of HD-DOT to map the cortical networks supporting speech processing. Using sentences with two levels of linguistic complexity, along with a control condition consisting of unintelligible noise-vocoded speech, we recovered a hierarchically organized speech network that matches the results of previous fMRI studies. Specifically, hearing intelligible speech resulted in increased activity in bilateral temporal cortex and left frontal cortex, with syntactically complex speech leading to additional activity in left posterior temporal cortex and left inferior frontal gyrus. These results demonstrate the feasibility of using HD-DOT to map spatially distributed brain networks supporting higher-order cognitive faculties such as spoken language.

Adaptive speech enhancement using directional microphone in a 4-T MRI scanner

OBJECTIVE

To evaluate the effectiveness of the proposed adaptive speech enhancement (ASE) system for the magnetic resonance imaging (MRI) environment to reduce the loud scanning noise without disrupting the communication between patients and MRI operators.

MATERIALS AND METHODS

The developed system employed the idea of differential directional microphones for measuring and distinguishing the speech signals and MRI acoustic noises simultaneously. Two-stage adaptive filters with normalized least mean square algorithms were adopted. Two common MRI scanning sequences, echo planar imaging (EPI) and gradient echo multi-slice (GEMS), were tested using a 4T MRI scanner.

RESULTS

A total of 1.4 and 3.3 dB speech enhancements quantified by the cepstral distance assessment were achieved for the speech signal contaminated with the EPI and GEMS noises, respectively. The speech signal was noticeably recovered, and a clear speech waveform was observed after treated with the ASE system. Furthermore, a non-adaptive post-processing approach [i.e. simply using spectral subtraction (SS) technique] was also adopted to process the abovementioned results. Additional reductions were achieved for the non-coherent MRI acoustic noises.

CONCLUSION

The results showed that combining the proposed ASE system along with the SS approach has a great potential for treating MRI acoustic noise to guarantee an effective communication from patient to MRI operators.

Methodological challenges and solutions in auditory functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) studies involve substantial acoustic noise. This review covers the difficulties posed by such noise for auditory neuroscience, as well as a number of possible solutions that have emerged. Acoustic noise can affect the processing of auditory stimuli by making them inaudible or unintelligible, and can result in reduced sensitivity to auditory activation in auditory cortex. Equally importantly, acoustic noise may also lead to increased listening effort, meaning that even when auditory stimuli are perceived, neural processing may differ from when the same stimuli are presented in quiet. These and other challenges have motivated a number of approaches for collecting auditory fMRI data. Although using a continuous echoplanar imaging (EPI) sequence provides high quality imaging data, these data may also be contaminated by background acoustic noise. Traditional sparse imaging has the advantage of avoiding acoustic noise during stimulus presentation, but at a cost of reduced temporal resolution. Recently, three classes of techniques have been developed to circumvent these limitations. The first is Interleaved Silent Steady State (ISSS) imaging, a variation of sparse imaging that involves collecting multiple volumes following a silent period while maintaining steady-state longitudinal magnetization. The second involves active noise control to limit the impact of acoustic scanner noise. Finally, novel MRI sequences that reduce the amount of acoustic noise produced during fMRI make the use of continuous scanning a more practical option. Together these advances provide unprecedented opportunities for researchers to collect high-quality data of hemodynamic responses to auditory stimuli using fMRI.

Manganese enhanced magnetic resonance imaging (MEMRI): a powerful new imaging method to study tinnitus

Manganese enhanced magnetic resonance imaging (MEMRI) is a method used primarily in basic science experiments to advance the understanding of information processing in central nervous system pathways. With this mechanistic approach, manganese (Mn(2+)) acts as a calcium surrogate, whereby voltage-gated calcium channels allow for activity driven entry of Mn(2+) into neurons. The detection and quantification of neuronal activity via Mn(2+) accumulation is facilitated by "hemodynamic-independent contrast" using high resolution MRI scans. This review emphasizes initial efforts to-date in the development and application of MEMRI for evaluating tinnitus (the perception of sound in the absence of overt acoustic stimulation). Perspectives from leaders in the field highlight MEMRI related studies by comparing and contrasting this technique when tinnitus is induced by high-level noise exposure and salicylate administration. Together, these studies underscore the considerable potential of MEMRI for advancing the field of auditory neuroscience in general and tinnitus research in particular. Because of the technical and functional gaps that are filled by this method and the prospect that human studies are on the near horizon, MEMRI should be of considerable interest to the auditory research community. This article is part of a Special Issue entitled <Annual Reviews 2014>.

Auditory neuroimaging with fMRI and PET

For much of the past 30 years, investigations of auditory perception and language have been enhanced or even driven by the use of functional neuroimaging techniques that specialize in localization of central responses. Beginning with investigations using positron emission tomography (PET) and gradually shifting primarily to usage of functional magnetic resonance imaging (fMRI), auditory neuroimaging has greatly advanced our understanding of the organization and response properties of brain regions critical to the perception of and communication with the acoustic world in which we live. As the complexity of the questions being addressed has increased, the techniques, experiments and analyses applied have also become more nuanced and specialized. A brief review of the history of these investigations sets the stage for an overview and analysis of how these neuroimaging modalities are becoming ever more effective tools for understanding the auditory brain. We conclude with a brief discussion of open methodological issues as well as potential clinical applications for auditory neuroimaging. This article is part of a Special Issue entitled Human Auditory Neuroimaging.

Optimized design and analysis of sparse-sampling FMRI experiments

Sparse-sampling is an important methodological advance in functional magnetic resonance imaging (fMRI), in which silent delays are introduced between MR volume acquisitions, allowing for the presentation of auditory stimuli without contamination by acoustic scanner noise and for overt vocal responses without motion-induced artifacts in the functional time series. As such, the sparse-sampling technique has become a mainstay of principled fMRI research into the cognitive and systems neuroscience of speech, language, hearing, and music. Despite being in use for over a decade, there has been little systematic investigation of the acquisition parameters, experimental design considerations, and statistical analysis approaches that bear on the results and interpretation of sparse-sampling fMRI experiments. In this report, we examined how design and analysis choices related to the duration of repetition time (TR) delay (an acquisition parameter), stimulation rate (an experimental design parameter), and model basis function (an analysis parameter) act independently and interactively to affect the neural activation profiles observed in fMRI. First, we conducted a series of computational simulations to explore the parameter space of sparse design and analysis with respect to these variables; second, we validated the results of these simulations in a series of sparse-sampling fMRI experiments. Overall, these experiments suggest the employment of three methodological approaches that can, in many situations, substantially improve the detection of neurophysiological response in sparse fMRI: (1) Sparse analyses should utilize a physiologically informed model that incorporates hemodynamic response convolution to reduce model error. (2) The design of sparse fMRI experiments should maintain a high rate of stimulus presentation to maximize effect size. (3) TR delays of short to intermediate length can be used between acquisitions of sparse-sampled functional image volumes to increase the number of samples and improve statistical power.

A real-time data acquisition and control of gradient coil noise for fMRI identification of hearing disorder in children with history of ear infection

Early ear infection and trauma, from birth to age 12 are known to have a significant effect on sensory and cognitive development. This effect can be demonstrated through the fMRI study of children who have a history of ear infection compared to a control group. A second research question is the extent to which brain plasticity at an early age can reduce the impact of infection on hearing and cognitive development. Functional Magnetic Resonance Imaging (fMRI) provides a mapping of brain activity in cognitive and sensory regions by recording the oxygenation state of the local cerebral blood flow. The gradient coils of fMRI scanners generate intense acoustic noise (GCN) - to which the subject is in close proximity - in the range of 90 to 140 db SPL during the imaging process. Clearly this noise will impress its signature on low level brain response patterns. An Active Noise Canceller (ANC) system can suppress the effect of GCN on the subject's perception of a phonetic stimulus at the phoneme, word or phrase level. Due to a superimposition of the frequency and time domain components of the test signal and GCN for MR test, the ANC filtering system performs its function in real time - we must capture the brain's response to the test signal AFTER the noise has been removed. This goal is achieved through the application of field programmable gate array (FPGA) technology of NI LabVIEW. The presentation (in the noisy fMRI environment) of test words and phrases to hearing impaired children can identify sources of distortion to their perceptual processes associated with GCN. Once this distortion has been identified, learning strategies may be introduced to replace the hearing function distorted by early infection as well as the short term effect of GCN. The study of speech cognition without the confounding effect of GCN and with the varying level of GCN for a repeated test signal at later age can be allowed to a measure of recovery through brain plasticity.

Perspective of functional magnetic resonance imaging in middle ear research

Functional magnetic resonance imaging (MRI) studies have frequently been applied to study sensory system such as vision, language, and cognition, but have proceeded at a considerably slower speed in investigating middle ear and central auditory processing. This is due to several factors, including the intrinsic anatomy of the middle ear system and inherent acoustic noise during acquisition of MRI data. However, accumulating evidences have demonstrated that clarification of some fundamental neural underpinnings of audition associated with middle ear mechanics can be achieved using functional MRI methods. This mini review attempted to take a narrow snapshot of the currently available functional MRI procedures and gave examples of what may be learned about hearing from their application. It is hoped that with these technical advancements, many new high impact applications in audition would follow. In particular, because the fMRI can be used in humans and in animals, fMRI may represent a unique tool that should promote translational research by enabling parallel analyses of physiological and pathological processes in the human and animal auditory system. This article is part of a special issue entitled "MEMRO 2012".

A survey analysis of acoustic trauma related to MR scans

PURPOSE

The maximum limit of MR scanner noise and necessity of ear protection is defined in the IEC standard (IEC60601-2-33) of MR safety. With improvements in MR scanner performance, pulse sequences generating higher scanning noise have been used clinically. In this study, we investigated the factors significantly related to potential acoustic trauma cases (PATC) after MR examinations. To consider the future direction for MR safety and prevention of acoustic trauma, issues related to noise generation by MR scanners and acoustic trauma were systematically reviewed.

METHODS

A statistical analysis was performed using the data set from a survey (n=974) conducted in 2010 by the JSMRM safety committee. Hierarchical clustering analysis was used to extract the characteristics of the responders. With this classification as a reference, tests of independence and a residual analysis were employed to evaluate the factors related to PATC.

RESULTS

No significant relationship was observed between the ear protection policy and the incidence or the reported outcome of PATC. While the two main clusters out of the six clusters extracted were associated with who reported the PATC and the confirmation process of the acoustic noise level of MR scanners, no cluster was associated with the frequency of PATC. An absence of PATC was significantly less reported (p=0.03) and more PATC was reported (p=0.04) by facilities with 3T MR systems.

DISCUSSION

Although the total frequency was 4 cases, it should be noted that persistent hearing disturbances are a possible consequence of MR examinations. Neither the condition of the subjects nor the ear protection method was significantly related to the probability of PATC, suggesting the difficulty of predicting the potential risk of acoustic trauma. It is recommended to more systematically follow up PATC cases and clarify the risk factors.

MRI acoustic noise can harm experimental and companion animals

PURPOSE

To assess possible damage to the hearing of experimental and companion animal subjects of magnetic resonance imaging (MRI) scans.

MATERIALS AND METHODS

Using animal hearing threshold data and sound level measurements from typical MRI pulse sequences, we estimated "equivalent loudness" experienced by several experimental and companion animals commonly subjects of MRI scans. We compared the equivalent loudness and exam duration to safe noise standards set by the National Institute for Occupational Safety and Health (NIOSH).

RESULTS

Monkeys, dogs, cats, pigs, and rabbits are frequently exposed to equivalent loudness levels during MRI scans beyond what is considered safe for human exposure. The sensitive frequency ranges for rats and mice are shifted substantially upward and their equivalent loudness levels fall within the NIOSH safe zone.

CONCLUSION

MRI exposes many animals to levels of noise and duration that would exceed NIOSH human exposure limits. Researchers and veterinarians should use hearing protection for animals during MRI scans. Experimental research animals used in MRI studies are frequently kept and reimaged, and hearing loss could result in changed behavior. Damage to companion animals' hearing could make them less sensitive to commands and generally worsen interactions with family members. Much quieter MRI scanners would help decrease stress and potential harm to scanned animals, normalize physiology during MRI, and enable MRI of awake animals.