Effects of the acoustic noise of the gradient systems on fMRI: a study on auditory, motor, and visual cortices

Model-based reconstruction for looping-star MRI

PURPOSE

The aim of this study was to develop a reconstruction method that more fully models the signals and reconstructs gradient echo (GRE) images without sacrificing the signal to noise ratio and spatial resolution, compared to conventional gridding and model-based image reconstruction method.

METHODS

By modeling the trajectories for every spoke and simplifying the scenario to only echo-in and echo-out mixture, the approach explicitly models the overlapping echoes. After modeling the overlapping echoes with two system matrices, we use the conjugate gradient algorithm (CG-SENSE) with the nonuniform FFT (NUFFT) to optimize the image reconstruction cost function.

RESULTS

The proposed method is demonstrated in phantoms and in-vivo volunteer experiments for three-dimensional, high-resolution T2*-weighted imaging and functional MRI tasks. Compared to the gridding method, the high resolution protocol exhibits improved spatial resolution and reduced signal loss as a result of less intra-voxel dephasing. The fMRI task shows that the proposed model-based method produced images with reduced artifacts and blurring as well as more stable and prominent time courses.

CONCLUSION

The proposed model-based reconstruction results shows improved spatial resolution and reduced artifacts. The fMRI task shows improved time series and activation map due to the reduced overlapping echoes and under-sampling artifacts.

Silent zero TE MR neuroimaging: Current state-of-the-art and future directions

Magnetic Resonance Imaging (MRI) scanners produce loud acoustic noise originating from vibrational Lorentz forces induced by rapidly changing currents in the magnetic field gradient coils. Using zero echo time (ZTE) MRI pulse sequences, gradient switching can be reduced to a minimum, which enables near silent operation.Besides silent MRI, ZTE offers further interesting characteristics, including a nominal echo time of TE = 0 (thus capturing short-lived signals from MR tissues which are otherwise MR-invisible), 3D radial sampling (providing motion robustness), and ultra-short repetition times (providing fast and efficient scanning).In this work we describe the main concepts behind ZTE imaging with a focus on conceptual understanding of the imaging sequences, relevant acquisition parameters, commonly observed image artefacts, and image contrasts. We will further describe a range of methods for anatomical and functional neuroimaging, together with recommendations for successful implementation.

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.

BOLD and EEG signal variability at rest differently relate to aging in the human brain

Variability of neural activity is regarded as a crucial feature of healthy brain function, and several neuroimaging approaches have been employed to assess it noninvasively. Studies on the variability of both evoked brain response and spontaneous brain signals have shown remarkable changes with aging but it is unclear if the different measures of brain signal variability - identified with either hemodynamic or electrophysiological methods - reflect the same underlying physiology. In this study, we aimed to explore age differences of spontaneous brain signal variability with two different imaging modalities (EEG, fMRI) in healthy younger (25 ​± ​3 years, N ​= ​135) and older (67 ​± ​4 years, N ​= ​54) adults. Consistent with the previous studies, we found lower blood oxygenation level dependent (BOLD) variability in the older subjects as well as less signal variability in the amplitude of low-frequency oscillations (1-12 ​Hz), measured in source space. These age-related reductions were mostly observed in the areas that overlap with the default mode network. Moreover, age-related increases of variability in the amplitude of beta-band frequency EEG oscillations (15-25 ​Hz) were seen predominantly in temporal brain regions. There were significant sex differences in EEG signal variability in various brain regions while no significant sex differences were observed in BOLD signal variability. Bivariate and multivariate correlation analyses revealed no significant associations between EEG- and fMRI-based variability measures. In summary, we show that both BOLD and EEG signal variability reflect aging-related processes but are likely to be dominated by different physiological origins, which relate differentially to age and sex.

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.

EEG Mu (µ) rhythm spectra and oscillatory activity differentiate stuttering from non-stuttering adults

Stuttering is linked to sensorimotor deficits related to internal modeling mechanisms. This study compared spectral power and oscillatory activity of EEG mu (μ) rhythms between persons who stutter (PWS) and controls in listening and auditory discrimination tasks. EEG data were analyzed from passive listening in noise and accurate (same/different) discrimination of tones or syllables in quiet and noisy backgrounds. Independent component analysis identified left and/or right μ rhythms with characteristic alpha (α) and beta (β) peaks localized to premotor/motor regions in 23 of 27 people who stutter (PWS) and 24 of 27 controls. PWS produced μ spectra with reduced β amplitudes across conditions, suggesting reduced forward modeling capacity. Group time-frequency differences were associated with noisy conditions only. PWS showed increased μ-β desynchronization when listening to noise and early in discrimination events, suggesting evidence of heightened motor activity that might be related to forward modeling deficits. PWS also showed reduced μ-α synchronization in discrimination conditions, indicating reduced sensory gating. Together these findings indicate spectral and oscillatory analyses of μ rhythms are sensitive to stuttering. More specifically, they can reveal stuttering-related sensorimotor processing differences in listening and auditory discrimination that also may be influenced by basal ganglia deficits.

The impact of simulated MRI scanner background noise on visual attention processes as measured by the EEG

Environmental noise is known to affect personal well-being as well as cognitive processes. Besides daily life, environmental noise can also occur in experimental research settings, e.g. when being in a magnetic resonance scanner. Scanner background noise (SBN) might pose serious confounds for experimental findings, even when non-auditory settings are examined. In the current experiment we tested if SBN alters bottom-up and top-down related processes of selective visual attention mechanisms. Participants completed two blocks of a visual change detection task, one block in silence and one block under SBN exposure. SBN was found to decrease accuracy in measures of visual attention. This effect was modulated by the temporal occurrence of SBN. When SBN was encountered in the first block, it prevented a significant improvement of accuracy in the second block. When SBN appeared in the second block, it significantly decreased accuracy. Neurophysiological findings showed a strong frontal positivity shift only when SBN was present in the first block, suggesting an inhibitory process to counteract the interfering SBN. Common correlates of both top-down and bottom-up processes of selective visual attention were not specifically affected by SBN exposure. Further research appears necessary to entirely rule out confounds of SBN in assessing visual attention.

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.

Effects of scanner acoustic noise on intrinsic brain activity during auditory stimulation

INTRODUCTION

Although the effects of scanner background noise (SBN) during functional magnetic resonance imaging (fMRI) have been extensively investigated for the brain regions involved in auditory processing, its impact on other types of intrinsic brain activity has largely been neglected. The present study evaluated the influence of SBN on a number of intrinsic connectivity networks (ICNs) during auditory stimulation by comparing the results obtained using sparse temporal acquisition (STA) with those using continuous acquisition (CA).

METHODS

Fourteen healthy subjects were presented with classical music pieces in a block paradigm during two sessions of STA and CA. A volume-matched CA dataset (CAm) was generated by subsampling the CA dataset to temporally match it with the STA data. Independent component analysis was performed on the concatenated STA-CAm datasets, and voxel data, time courses, power spectra, and functional connectivity were compared.

RESULTS

The ICA revealed 19 ICNs; the auditory, default mode, salience, and frontoparietal networks showed greater activity in the STA. The spectral peaks in 17 networks corresponded to the stimulation cycles in the STA, while only five networks displayed this correspondence in the CA. The dorsal default mode and salience networks exhibited stronger correlations with the stimulus waveform in the STA.

CONCLUSIONS

SBN appeared to influence not only the areas of auditory response but also the majority of other ICNs, including attention and sensory networks. Therefore, SBN should be regarded as a serious nuisance factor during fMRI studies investigating intrinsic brain activity under external stimulation or task loads.

[Quantification and improvement of speech transmission performance using headphones in acoustic stimulated functional magnetic resonance imaging]

Functional magnetic resonance imaging (fMRI) has made a major contribution to the understanding of higher brain function, but fMRI with auditory stimulation, used in the planning of brain tumor surgery, is often inaccurate because there is a risk that the sounds used in the trial may not be correctly transmitted to the subjects due to acoustic noise. This prompted us to devise a method of digitizing sound transmission ability from the accuracy rate of 67 syllables, classified into three types. We evaluated this with and without acoustic noise during imaging. We also improved the structure of the headphones and compared their sound transmission ability with that of conventional headphones attached to an MRI device (a GE Signa HDxt 3.0 T). We calculated and compared the sound transmission ability of the conventional headphones with that of the improved model. The 95 percent upper confidence limit (UCL) was used as the threshold for accuracy rate of hearing for both headphone models. There was a statistically significant difference between the conventional model and the improved model during imaging (p < 0.01). The rate of accuracy of the improved model was 16 percent higher. 29 and 22 syllables were accurate at a 95% UCL in the improved model and the conventional model, respectively. This study revealed the evaluation system used in this study to be useful for correctly identifying syllables during fMRI.

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.

FMRI scanner noise interaction with affective neural processes

The purpose of the present study was the investigation of interaction effects between functional MRI scanner noise and affective neural processes. Stimuli comprised of psychoacoustically balanced musical pieces, expressing three different emotions (fear, neutral, joy). Participants (N=34, 19 female) were split into two groups, one subjected to continuous scanning and another subjected to sparse temporal scanning that features decreased scanner noise. Tests for interaction effects between scanning group (sparse/quieter vs continuous/noisier) and emotion (fear, neutral, joy) were performed. Results revealed interactions between the affective expression of stimuli and scanning group localized in bilateral auditory cortex, insula and visual cortex (calcarine sulcus). Post-hoc comparisons revealed that during sparse scanning, but not during continuous scanning, BOLD signals were significantly stronger for joy than for fear, as well as stronger for fear than for neutral in bilateral auditory cortex. During continuous scanning, but not during sparse scanning, BOLD signals were significantly stronger for joy than for neutral in the left auditory cortex and for joy than for fear in the calcarine sulcus. To the authors' knowledge, this is the first study to show a statistical interaction effect between scanner noise and affective processes and extends evidence suggesting scanner noise to be an important factor in functional MRI research that can affect and distort affective brain processes.

Effect of MRI acoustic noise on cerebral fludeoxyglucose uptake in simultaneous MR-PET imaging

Integrated scanners capable of simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) data acquisition are now available for human use. Although the scanners' manufacturers have made substantial efforts to understand and minimize the mutual electromagnetic interference between the 2 modalities, the potential physiological inference has not been evaluated. In this study, we have studied the influence of the acoustic noise produced by the magnetic resonance (MR) gradients on brain fludeoxyglucose (FDG) uptake in the Siemens MR-BrainPET prototype. Although particular attention was paid to the primary auditory cortex (PAC), a brain-wide analysis was also performed.

METHODS

The effects of the MR on the PET count rate and image quantification were first investigated in phantoms. Next, 10 healthy volunteers underwent 2 simultaneous FDG-PET/MR scans in the supine position with the FDG injection occurring inside the MR-BrainPET, alternating between a "quiet" (control) environment in which no MR sequences were run during the FDG uptake phase (the first 40 minutes after radiotracer administration) and a "noisy" (test) environment in which MR sequences were run for the entire time. Cortical and subcortical regions of interest were derived from the high-resolution morphological MR data using FreeSurfer. The changes in the FDG uptake in the FreeSurfer-derived regions of interest between the 2 conditions were analyzed from parametric and static PET images, and on a voxel-by-voxel basis using SPM8 and FreeSurfer.

RESULTS

Only minimal to no electromagnetic interference was observed for most of the MR sequences tested, with a maximum drop in count rate of 1.5% and a maximum change in the measured activity of 1.1% in the corresponding images. The region of interest-based analysis showed statistically significant increases in the right PAC in both the parametric (9.13% [4.73%]) and static (4.18% [2.87%]) images. The SPM8 analysis showed no statistically significant clusters in any images when a P < 0.05 (corrected) was used; however, a P < 0.001 (uncorrected) resolved bilateral statistically significant clusters of increased FDG uptake in the area of the PAC for the parametric image (left, 8.37% [1.55%]; right, 8.20% [1.17%]) but only unilateral increase in the static image (left, 8.68% [3.89%]).

CONCLUSIONS

Although the operation of the BrainPET prototype is virtually unaffected by the MR scanner, the acoustic noise produced by the MR gradients causes a focal increase in the FDG uptake in the PAC, which could affect the interpretation of pathological (or brain-activation-related) changes in the FDG uptake in this region if the expected effects are of comparable amplitude.

Standardizing the intrinsic brain: towards robust measurement of inter-individual variation in 1000 functional connectomes

As researchers increase their efforts to characterize variations in the functional connectome across studies and individuals, concerns about the many sources of nuisance variation present and their impact on resting state fMRI (R-fMRI) measures continue to grow. Although substantial within-site variation can exist, efforts to aggregate data across multiple sites such as the 1000 Functional Connectomes Project (FCP) and International Neuroimaging Data-sharing Initiative (INDI) datasets amplify these concerns. The present work draws upon standardization approaches commonly used in the microarray gene expression literature, and to a lesser extent recent imaging studies, and compares them with respect to their impact on relationships between common R-fMRI measures and nuisance variables (e.g., imaging site, motion), as well as phenotypic variables of interest (age, sex). Standardization approaches differed with regard to whether they were applied post-hoc vs. during pre-processing, and at the individual vs. group level; additionally they varied in whether they addressed additive effects vs. additive+multiplicative effects, and were parametric vs. non-parametric. While all standardization approaches were effective at reducing undesirable relationships with nuisance variables, post-hoc approaches were generally more effective than global signal regression (GSR). Across approaches, correction for additive effects (global mean) appeared to be more important than for multiplicative effects (global SD) for all R-fMRI measures, with the exception of amplitude of low frequency fluctuations (ALFF). Group-level post-hoc standardizations for mean-centering and variance-standardization were found to be advantageous in their ability to avoid the introduction of artifactual relationships with standardization parameters; though results between individual and group-level post-hoc approaches were highly similar overall. While post-hoc standardization procedures drastically increased test-retest (TRT) reliability for ALFF, modest reductions were observed for other measures after post-hoc standardizations-a phenomena likely attributable to the separation of voxel-wise from global differences among subjects (global mean and SD demonstrated moderate TRT reliability for these measures). Finally, the present work calls into question previous observations of increased anatomical specificity for GSR over mean centering, and draws attention to the near equivalence of global and gray matter signal regression.

Brain networks of novelty-driven involuntary and cued voluntary auditory attention shifting

In everyday life, we need a capacity to flexibly shift attention between alternative sound sources. However, relatively little work has been done to elucidate the mechanisms of attention shifting in the auditory domain. Here, we used a mixed event-related/sparse-sampling fMRI approach to investigate this essential cognitive function. In each 10-sec trial, subjects were instructed to wait for an auditory "cue" signaling the location where a subsequent "target" sound was likely to be presented. The target was occasionally replaced by an unexpected "novel" sound in the uncued ear, to trigger involuntary attention shifting. To maximize the attention effects, cues, targets, and novels were embedded within dichotic 800-Hz vs. 1500-Hz pure-tone "standard" trains. The sound of clustered fMRI acquisition (starting at t = 7.82 sec) served as a controlled trial-end signal. Our approach revealed notable activation differences between the conditions. Cued voluntary attention shifting activated the superior intra--parietal sulcus (IPS), whereas novelty-triggered involuntary orienting activated the inferior IPS and certain subareas of the precuneus. Clearly more widespread activations were observed during voluntary than involuntary orienting in the premotor cortex, including the frontal eye fields. Moreover, we found -evidence for a frontoinsular-cingular attentional control network, consisting of the anterior insula, inferior frontal cortex, and medial frontal cortices, which were activated during both target discrimination and voluntary attention shifting. Finally, novels and targets activated much wider areas of superior temporal auditory cortices than shifting cues.

How challenges in auditory fMRI led to general advancements for the field

In the early years of fMRI research, the auditory neuroscience community sought to expand its knowledge of the underlying physiology of hearing, while also seeking to come to grips with the inherent acoustic disadvantages of working in the fMRI environment. Early collaborative efforts between prominent auditory research laboratories and prominent fMRI centers led to development of a number of key technical advances that have subsequently been widely used to elucidate principles of auditory neurophysiology. Perhaps the key imaging advance was the simultaneous and parallel development of strategies to use pulse sequences in which the volume acquisitions were "clustered," providing gaps in which stimuli could be presented without direct masking. Such sequences have become widespread in fMRI studies using auditory stimuli and also in a range of translational research domains. This review presents the parallel stories of the people and the auditory neurophysiology research that led to these sequences.

Robustness of intrinsic connectivity networks in the human brain to the presence of acoustic scanner noise

Evoked responses in functional magnetic resonance imaging (fMRI) are affected by the presence of acoustic scanner noise (ASN). Particularly, stimulus-related activation of the auditory system and deactivation of the default mode network have repeatedly been shown to diminish. In contrast, little is known about the influence of ASN on the spontaneous fluctuations in brain activity that are crucial for network-related neuroimaging methods like independent component analysis (ICA) or functional and effective connectivity analysis (ECA). The present study assessed the robustness of intrinsic connectivity networks in the human brain to the presence of ASN by comparing 'silent' (sparse) and 'noisy' (continuous) acquisition schemes, both during task performance and during rest. In agreement with existing literature, ASN strongly diminished conventional evoked response levels. In contrast, ICA and ECA robustly identified similar functional networks regardless of the scanning method. ASN affected the strength of only few independent components, and effective connectivity was hardly sensitive to ASN overall. However, unexpectedly, ICA revealed notable differences in the underlying neurodynamics. In particular, low-frequency network oscillations dominated in the commonly used continuous scanning environment, but signal spectra were significantly flatter during the less noisy sparse scanning runs. We tentatively attribute these differences to the ubiquitous influence of ASN on alertness and arousal.

Shaping and timing gradient pulses to reduce MRI acoustic noise

A method to reduce the acoustic noise generated by gradient systems in MRI has been recently proposed; such a method is based on the linear response theory. Since the physical cause of MRI acoustic noise is the time derivative of the gradient current, a common trapezoid current shape produces an acoustic gradient coil response mainly during the rising and falling edge. In the falling edge, the coil acoustic response presents a 180 degrees phase difference compared to the rising edge. Therefore, by varying the width of the trapezoid and keeping the ramps constant, it is possible to suppress one selected frequency and its higher harmonics. This value is matched to one of the prominent resonance frequencies of the gradient coil system. The idea of cancelling a single frequency is extended to a second frequency, using two successive trapezoid-shaped pulses presented at a selected interval. Overall sound pressure level reduction of 6 and 10 dB is found for the two trapezoid shapes and a single pulse shape, respectively. The acoustically optimized pulse shape proposed is additionally tested in a simulated echo planar imaging readout train, obtaining a sound pressure level reduction of 12 dB for the best case.

An efficient feedback active noise control algorithm based on reduced-order linear predictive modeling of FMRI acoustic noise

Functional magnetic resonance imaging (fMRI) acoustic noise exhibits an almost periodic nature (quasi-periodicity) due to the repetitive nature of currents in the gradient coils. Small changes occur in the waveform in consecutive periods due to the background noise and slow drifts in the electroacoustic transfer functions that map the gradient coil waveforms to the measured acoustic waveforms. The period depends on the number of slices per second, when echo planar imaging (EPI) sequencing is used. Linear predictability of fMRI acoustic noise has a direct effect on the performance of active noise control (ANC) systems targeted to cancel the acoustic noise. It is shown that by incorporating some samples from the previous period, very high linear prediction accuracy can be reached with a very low order predictor. This has direct implications on feedback ANC systems since their performance is governed by the predictability of the acoustic noise to be cancelled. The low complexity linear prediction of fMRI acoustic noise developed in this paper is used to derive an effective and low-cost feedback ANC system.

A comprehensive experimental study of micro-perforated panel acoustic absorbers in MRI scanners

OBJECT

A comprehensive experimental study has been conducted to investigate the possibilities of using micro-perforated panel (MPP) acoustic absorbers in magnetic resonance imaging (MRI) scanners.

MATERIALS AND METHOD

The experimental acoustic measurements include measurements in an impedance tube, measurements in an MRI scanner bore mock-up, and in situ measurements in an actual MRI scanner.

RESULTS

The experimental results are in good agreement with theoretical calculations. This study confirms that MPP acoustic absorbers have multiple absorption frequency bands and wider frequency bands at higher frequency ranges when they are used in cylindrically shaped ducts such as MRI scanner bores. It has also been found that the acoustic noise level in the scanner bore is significantly increased when the air gap depth behind the MPP is too large.

CONCLUSION

This study shows that an MPP absorber, when properly designed, is effective in reducing the acoustic noise in MRI scanners. And, when designing an MPP absorber for MRI scanners, the air gap depth should be carefully considered.

Background MR gradient noise and non-auditory BOLD activations: a data-driven perspective

The effect of echoplanar imaging (EPI) acoustic background noise on blood oxygenation level dependent (BOLD) activations was investigated. Two EPI pulse sequences were compared: (i) conventional EPI with a pulsating sound component of typically 8-10 Hz, which is a potent physiological stimulus, and (ii) the more recently developed continuous-sound EPI, which is perceived as less distractive despite equivalent peak sound pressure levels. Sixteen healthy subjects performed an established demanding visual n-back working memory task. Using an exploratory data analysis technique (tensorial probabilistic independent component analysis; tensor-PICA), we studied the inter-session/within-subject response variability introduced by continuous-sound versus conventional EPI acoustic background noise in addition to temporal and spatial signal characteristics. The analysis revealed a task-related component associated with the established higher-level working memory and motor feedback response network, which exhibited a significant 19% increase in its average effect size for the continuous-sound as opposed to conventional EPI. Stimulus-related lower-level activations, such as primary visual areas, were not modified. EPI acoustic background noise influences much more than the auditory system per se. This analysis provides additional evidence for an enhancement of task-related, extra-auditory BOLD activations by continuous-sound EPI due to less distractive acoustic background gradient noise.

Assessing the influence of scanner background noise on auditory processing. II. An fMRI study comparing auditory processing in the absence and presence of recorded scanner noise using a sparse design

Several studies reported decreased signal intensities within auditory areas for experimental designs employing continuous scanner background noise (SBN) in comparison to designs with less or no SBN. This study examined the source for this SBN-induced masking effect of the blood oxygenation level-dependent (BOLD) response by directly comparing two experimental sessions with the same auditory stimulation, which was presented either with or without recorded scanner background noise (RecSBN). Ten subjects listened to a series of four one-syllable words and had to decide whether two of the words were identical. The words were either presented with a silent background or with added RecSBN. This was then contrasted with either silence or RecSBN. A sparse temporal sampling method was used in both sessions, which enabled us to directly assess the influence of RecSBN without varying scanning parameters, acquisition quantities, or auditory stimulations. Our results suggest that previously reported SBN-induced masking of the BOLD response in experimental designs with SBN might be caused by an interaction between increased baseline levels and nonlinearity effects within auditory cortices. Adding SBN to an experimental condition does not enhance signal intensities to the same degree that SBN does when presented with a silent background, and therefore contrasting an experimental and baseline condition that both have SBN may lead to signal decreases. In addition, our study shows this effect is greatest in Heschl's gyrus, but can also be observed in higher-order auditory areas.

Assessing the influence of scanner background noise on auditory processing. I. An fMRI study comparing three experimental designs with varying degrees of scanner noise

We compared two experimental designs aimed at minimizing the influence of scanner background noise (SBN) on functional MRI (fMRI) of auditory processes with one conventional fMRI design. Ten subjects listened to a series of four one-syllable words and had to decide whether two of the words were identical. This was contrasted with a no-stimulus control condition. All three experimental designs had a duration of approximately 17 min: 1) a behavior interleaved gradients (BIG; Eden et al. [1999] J Magn Reson Imaging 41:13-20) design (repetition time, TR, = 6 s), where stimuli were presented during the SBN-free periods between clustered volume acquisitions (CVA); 2) a sparse temporal sampling technique (STsamp; e.g., Gaab et al., [2003] Neuroimage 19:1417-1426) acquiring only one set of slices following each of the stimulations with a 16-s TR and jittered delay times between stimulus offset and image acquisition; and 3) an event-related design with continuous scanning (ERcont) using the stimulation design of STsamp but with a 2-s TR. The results demonstrated increased signal within Heschl's gyrus for the STsamp and BIG-CVA design in comparison to ERcont as well as differences in the overall functional anatomy among the designs. The possibility to obtain a time course of activation as well as the full recovery of the stimulus- and SBN-induced hemodynamic response function signal and lack of signal suppression from SBN during the STsamp design makes this technique a powerful approach for conducting auditory experiments using fMRI. Practical strengths and limitations of the three auditory acquisition paradigms are discussed.

Lateralization of functional magnetic resonance imaging (fMRI) activation in the auditory pathway of patients with lateralized tinnitus

INTRODUCTION

Tinnitus is hypothesized to be an auditory phantom phenomenon resulting from spontaneous neuronal activity somewhere along the auditory pathway. We performed fMRI of the entire auditory pathway, including the inferior colliculus (IC), the medial geniculate body (MGB) and the auditory cortex (AC), in 42 patients with tinnitus and 10 healthy volunteers to assess lateralization of fMRI activation.

METHODS

Subjects were scanned on a 3T MRI scanner. A T2*-weighted EPI silent gap sequence was used during the stimulation paradigm, which consisted of a blocked design of 12 epochs in which music presented binaurally through headphones, which was switched on and off for periods of 50 s. Using SPM2 software, single subject and group statistical parametric maps were calculated. Lateralization of activation was assessed qualitatively and quantitatively.

RESULTS

Tinnitus was lateralized in 35 patients (83%, 13 right-sided and 22 left-sided). Significant signal change (P(corrected) < 0.05) was found bilaterally in the primary and secondary AC, the IC and the MGB. Signal change was symmetrical in patients with bilateral tinnitus. In patients with lateralized tinnitus, fMRI activation was lateralized towards the side of perceived tinnitus in the primary AC and IC in patients with right-sided tinnitus, and in the MGB in patients with left-sided tinnitus. In healthy volunteers, activation in the primary AC was left-lateralized.

CONCLUSION

Our paradigm adequately visualized the auditory pathways in tinnitus patients. In lateralized tinnitus fMRI activation was also lateralized, supporting the hypothesis that tinnitus is an auditory phantom phenomenon.

Functional MR imaging of language processing: an overview of easy-to-implement paradigms for patient care and clinical research

Functional magnetic resonance (MR) imaging is one of the most commonly used functional neuroimaging techniques for studying the cerebral representation of language processing and is increasingly being used for both patient care and clinical research. In patient care, functional MR imaging is primarily used in the preoperative evaluation of (a) the relationship of a lesion to critical language areas and (b) hemispheric dominance. In clinical research, this modality is used to study language disorders due to neurologic disease and is generally aimed at language function recovery. A variety of language paradigms (verbal fluency, passive listening, comprehension) have been developed for the study of language processing and its separate components. All of the tasks are easy to implement, analyze, and perform. Silent gap acquisition is preferable for the imaging of specific language processing components because auditory stimuli are not degraded by imager noise. On the other hand, continuous acquisition allows more data to be acquired in less time, thereby increasing statistical power and decreasing the effects of motion artifacts. Although functional MR imaging cannot yet replace intraoperative electrocortical stimulation in patients undergoing neurosurgery, it may be useful for guiding surgical planning and mapping, thereby reducing the extent and duration of craniotomy.

Experimental design and analysis in functional MRI

As the experimental diversity of functional magnetic resonance imaging (fMRI) has grown since introduction of the technique in 1991, the appropriate application of statistical evaluation has become a significant research need. Advances in our understanding of both the dynamics of the underlying neurophysiology and the measurement process make fMRI a fertile ground for novel signal and image processing research. Recent analysis procedures seek to incorporate knowledge of the temporal and spatial characteristics of the blood oxygenation level dependent (BOLD) response measured by fMRI. Efforts that incorporate the nonstationary aspects of the BOLD response will become more important as future applications of fMRI are likely to be in conjunction with additional imaging modalities (e.g., fMRI combined with EEG) to acquire more complete physiologic data and permit greater refinement of our characterization of the central nervous system responses arising due to an applied stimulus.

Effect of fMRI acoustic noise on sensorimotor activation examined using optical topography

Functional magnetic resonance imaging (fMRI) is an important tool for noninvasively imaging the hemodynamic responses accompanying brain activity, but fMRI measurements are accompanied by loud acoustic noises resulting from Lorentz forces that cannot be completely excluded when the present technology is used. We used recorded fMRI acoustic noise and examined its effect on sensorimotor activation in optical topography measurement when subjects were instructed to tap the fingers of the right hand under a 23-dB non-noise condition and 46-, 56-, and 65-dB noise conditions. The results showed that the amplitude of the activation signal (relative change in concentration) for oxygenated hemoglobin in the sensorimotor cortex decreased with increasing noise. The activation signal for deoxygenated hemoglobin did not depend significantly on the noise level but did tend to decrease with increasing noise. These results suggest that fMRI acoustic noise affects the hemodynamics of cortical areas associated with the processing of information other than auditory information.

EEG reveals the effect of fMRI scanner noise on noise-sensitive subjects

One drawback of fMRI is that subjects must endure intense noise during testing. This may be annoying to some people and acceptable to others. The aim of this study was to examine, by means of event-related potentials (ERPs), the possible influence of this noise on brain activity while performing a mental reasoning task. Subjects carrying out tasks in a silent environment were compared with two groups executing the same tasks in an "fMRI-like" noisy environment, one of which consisted of subjects who were annoyed by the noise and the other of subjects who tolerated it easily. Subjects who were annoyed performed less well (i.e., produced more errors compared to the "no noise" group) and "not annoyed" subjects showed a speed-accuracy trade-off (i.e., reacted faster but made more errors compared to "no noise" subjects). Noise led to more pronounced N1 and P2 peaks but attenuated N2. As early ERP components are influenced by attention, this observation most likely reflects different attentional requirements. The slow cortical negative shift during task processing was significantly attenuated with "annoyed" subjects compared to "not annoyed" subjects. Emotion-related subcortical structures may be responsible for the observed difference. These findings suggest that individual reactions to fMRI scanner noise should be taken into account when designing fMRI studies and interpreting results.

Artifact reduction for simultaneous EEG/fMRI recording: Adaptive FIR reduction of imaging artifacts

OBJECTIVE

We present a new method of effectively removing imaging artifacts of electroencephalography (EEG) and extensively conserving the time-frequency features of EEG signals during simultaneous functional magnetic resonance imaging (fMRI) scanning under conventional conditions.

METHODS

Under the conventional conditions of a 5000 Hz EEG sampling rate, but in the absence of the MRI slice-timing signals, the imaging artifact during each slice scanning is theoretically inferred to be a linear combination of the average artifact waveform and its derivatives, deduced by band-limited Taylor's expansion. Technically, the imaging artifact reduction algorithm is equivalent to an adaptive finite impulse response (FIR) filter.

RESULTS

The capability of this novel method removing the imaging artifacts of EEG recording during fMRI scanning has been demonstrated by a phantom experiment. Moreover, the effectiveness of this method in conserving the time-frequency features of EEG activity has been evaluated by both visually evoked experiments and alpha waves.

CONCLUSIONS

The adaptive FIR method is an effective method of removing the imaging artifacts under conventional conditions, and also conserving the time-frequency EEG signals.

SIGNIFICANCE

The proposed adaptive FIR method, removing the imaging artifacts, combined with the wavelet-based non-linear noise reduction (WNNR) method [Wan X, Iwata K, Riera J, Ozaki T, Kitamura M, Kawashima R. Artifact reduction for EEG/fMRI recording: Nonlinear reduction of ballistocardiogram artifacts. Clin Neurophysiol 2006;117:668-80], reducing the ballistocardiogram artifacts (BAs), makes it feasible to obtain accurate EEG signals from the simultaneous EEG recordings during fMRI scanning.

Activation of Cortical and Subcortical Auditory Structures at 3 T by Means of a Functional Magnetic Resonance Imaging Paradigm Suitable for Clinical Use

OBJECTIVES

Visualization of functional magnetic resonance imaging (fMRI) activation of subcortical auditory structures remains challenging because of the cardiac-related pulsatile movement of both the brainstem and the cerebrospinal fluid and involved, until now, special scanning, pre- and postprocessing techniques, which are not convenient in clinical settings. The aim of this study is to examine the activation in both cortical and subcortical auditory structures by means of an fMRI paradigm, which is suitable for clinical use.

MATERIALS AND METHODS

Twenty subjects (13 volunteers and 7 patients) were examined on a 3 T imaging system with binaural musical stimulation.

RESULTS

Both cortical and subcortical auditory structures are successfully visualized in volunteers and patients.

CONCLUSIONS

Activation of both the cortical and subcortical auditory structures can be visualized by means of an appropriate fMRI setup at 3 T. This paradigm can easily be used in patients with tumors and/or hearing disorders.

Effect of fMRI acoustic noise on non-auditory working memory task: comparison between continuous and pulsed sound emitting EPI

Conventional blood oxygenation level-dependent (BOLD) based functional magnetic resonance imaging (fMRI) is accompanied by substantial acoustic gradient noise. This noise can influence the performance as well as neuronal activations. Conventional fMRI typically has a pulsed noise component, which is a particularly efficient auditory stimulus. We investigated whether the elimination of this pulsed noise component in a recent modification of continuous-sound fMRI modifies neuronal activations in a cognitively demanding non-auditory working memory task. Sixteen normal subjects performed a letter variant n-back task. Brain activity and psychomotor performance was examined during fMRI with continuous-sound fMRI and conventional fMRI. We found greater BOLD responses in bilateral medial frontal gyrus, left middle frontal gyrus, left middle temporal gyrus, left hippocampus, right superior frontal gyrus, right precuneus and right cingulate gyrus with continuous-sound compared to conventional fMRI. Conversely, BOLD responses were greater in bilateral cingulate gyrus, left middle and superior frontal gyrus and right lingual gyrus with conventional compared to continuous-sound fMRI. There were no differences in psychomotor performance between both scanning protocols. Although behavioral performance was not affected, acoustic gradient noise interferes with neuronal activations in non-auditory cognitive tasks and represents a putative systematic confound.

fMRI-acoustic noise alters brain activation during working memory tasks

Scanner noise during functional magnetic resonance imaging (fMRI) may interfere with brain function and change blood oxygenation level dependent (BOLD) signals, a problem that generally worsens at the higher field strengths. Therefore, we studied the effect of increased acoustic noise on fMRI during verbal working memory (WM) processing. The sound pressure level of scanner noise was increased by 12 dBA from "Quiet" to "Loud" echo planar imaging (EPI) scans by utilizing resonant vibration modes of the gradient coil. A WM paradigm with graded levels of task difficulty was used to further access WM load. Increased scanner noise produced increased BOLD responses (percent signal change) bilaterally in the cerebellum, inferior (IFG), medial (medFG), and superior (SFG) frontal, fusiform (FusG), and the lingual (LG) gyri, and decreased BOLD responses bilaterally in the anterior cingulate gyrus (ACG) and the putamen. This finding suggests greater recruitment of attention resources in these brain regions, probably to compensate for interference due to louder scanner noise. Increased working memory load increased the BOLD signals in IFG and the cerebellum, but decreased the BOLD signals in the putamen and the LG. These findings also support the idea that brain function requires additional attention resources under noisier conditions. Load- and acoustic-noise-related changes in BOLD responses correlated negatively in the WM network. This study demonstrates that MR noise affects brain activation pattern. Future comparisons between studies performed under different acoustic conditions (due to differing magnetic field strengths, pulse sequences, or scanner manufacturers) might require knowledge of the sound pressure level of acoustic noise during fMRI.

Influence of gradient acoustic noise on fMRI response in the human visual cortex

A paired-stimuli paradigm combined with fMRI was utilized to study the effect of gradient acoustic noise on fMRI response in the human primary visual cortex (V1) in terms of the auditory-visual cross-modal neural interaction. The gradient noise generated during the fMRI acquisition was used as the primary stimulus, and a single flashing light was used as the secondary stimulus. An interstimulus interval (ISI) separated the two. Six tasks were designed with different ISIs ranging from 50 to 700 ms. Both BOLD signal intensity and the number of activated pixels in V1 were analyzed and examined, and they showed a significant reduction when the gradient noise preceded the flashing light by approximately 300 ms. These results indicate that the gradient acoustic noise generated during fMRI acquisitions does interfere with neural behavior and the BOLD signal in the human visual cortex. This interference is modulated by the delay between the gradient noise and visual stimulation, and it can be studied quantitatively when the stimulation paradigm is designed appropriately. This study provides evidence of the auditory-visual interaction during fMRI studies, and the results should have an impact on fMRI applications.

Verbal communication in MR environments: effect of MR system acoustic noise on speech understanding

PURPOSE

To assess the masking effect of magnetic resonance (MR)-related acoustic noise and the effect of passive hearing protection on speech understanding.

MATERIALS AND METHODS

Acoustic recordings were made at 1.5 T at patient and operator (interventionalist in the MR suite) locations for relevant pulse sequences. In an audiologic laboratory, speech-to-noise ratios (STNRs) were determined, defined as the difference between the absolute sound pressure levels of MR noise and speech. The recorded noise of the MR sequences was played simultaneously with the recorded sentences at various intensities, and 15 healthy volunteers (seven women, eight men; median age, 27 years) repeated these sentences as accurately as possible. The STNR that corresponded with a 50% correct repetition was used as the measure for speech intelligibility. In addition, the effect of passive hearing protection on speech intelligibility was tested by using an earplug model.

RESULTS

Overall, speech understanding was reduced more at operator than at patient location. Most problematic were fast gradient-recalled-echo train and spiral k-space sequences. As the absolute sound pressure level of these sequences was approximately 100 dB at patient location, the vocal effort needed to attain 50% intelligibility was shouting (>77 dB). At operator location, less effort was required because of the lower sound pressure levels of the MR noise. Fast spoiled gradient-recalled-echo and echo-planar imaging sequences showed relatively favorable results with raised voice at operator location and loud speaking at patient location. The use of hearing protection slightly improved STNR.

CONCLUSION

At 1.5 T, the level of MR noise requires that large vocal effort is used, at the operator and especially at the patient location. Depending on the specific MR sequence used, loud speaking or shouting is needed to achieve adequate bidirectional communication with the patient. The wearing of earplugs improves speech intelligibility.

Acoustic noise concerns in functional magnetic resonance imaging

Magnetic resonance (MR) acoustic scanner noise may negatively affect the performance of functional magnetic resonance imaging (fMRI), a problem that worsens at the higher field strengths proposed to enhance fMRI. We present an overview of the current knowledge on the effects of confounding acoustic MR noise in fMRI experiments. The principles and effectiveness of various methods to reduce acoustic noise in fMRI are discussed, practical considerations are addressed and recommendations are made.

Effects of noise from functional magnetic resonance imaging on auditory event-related potentials in working memory task

The effects of functional magnetic resonance imaging (fMRI) acoustic noise were investigated on the parameters of event-related responses (ERPs) elicited during auditory matching-to-sample location and pitch working memory tasks. Stimuli were tones with varying location (left or right) and frequency (high or low). Subjects were instructed to memorize and compare either the locations or frequencies of the stimuli with each other. Tape-recorded fMRI acoustic noise was presented in half of the experimental blocks. The fMRI noise considerably enhanced the P1 component, reduced the amplitude and increased the latency of the N1, shortened the latency of the N2, and enhanced the amplitude of the P3 in both tasks. The N1 amplitude was higher in the location than pitch task in both noise and no-noise blocks, whereas the task-related N1 latency difference was present in the no-noise blocks only. Although the task-related differences between spatial and nonspatial auditory responses were partially preserved in noise, the finding that the acoustic gradient noise accompanying functional MR imaging modulated the auditory ERPs implies that the noise may confound the results of auditory fMRI experiments especially when studying higher cognitive processing.

Pain dynamics observed by functional magnetic resonance imaging: differential regression analysis technique

PURPOSE

To observe the dynamic responses of the cortical areas related to the pain processing by using the differential regression analysis (DRA) technique in functional magnetic resonance imaging (fMRI) and investigation of pain mechanisms.

MATERIALS AND METHODS

For pain studies, thermal stimulation was applied by immersing the index finger into a hot bath of water with a temperature of 50-52 degrees C. Motor (finger tapping) and visual (flickering light) stimulation experiments were conducted to elucidate the physiological differences between the simple sensory tasks and pain tasks. To obtain dynamic responses, T values (regression analysis) were sequentially estimated by using a series of shifted differential window functions (narrow width).

RESULTS

By using the DRA technique, well-defined prompt responses were observed for both motor and visual stimuli. On the other hand, in the pain experiment, a set of sequentially varying responses was observed for the thalamus (Thal), the dorsal anterior cingulate cortex (dACC), the caudal ACC (cACC), and the rostral ACC (rACC). This time-dependent response suggests the dynamics of pain signal processing in cortical areas.

CONCLUSION

The results support the hypothesis that the activated areas are similar to the previously reported pain processing areas; however, new sequential responses were observed, suggesting that the technique may reveal dynamics of pain perception and their pathway, important elements in understanding the mechanism of pain. The DRA technique can provide a new opportunity for many spatiotemporal analyses, for example, the physiologically complex and little-studied physiological phenomena, such as pain dynamics.

Echo planar imaging at 4 Tesla with minimum acoustic noise

PURPOSE

To minimize the acoustic sound pressure levels of single-shot echo planar imaging (EPI) acquisitions on high magnetic field MRI scanners.

MATERIALS AND METHODS

The resonance frequencies of gradient coil vibrations, which depend on the coil length and the elastic properties of the materials in the coil assembly, were measured using piezoelectric transducers. The frequency of the EPI-readout train was adjusted to avoid the frequency ranges of mechanical resonances.

RESULTS

Our MRI system exhibited two sharp mechanical resonances (at 720 and 1220 Hz) that can increase vibrational amplitudes up to six-fold. A small adjustment of the EPI-readout frequency made it possible to reduce the sound pressure level of EPI-based perfusion and functional MRI scans by 12 dB.

CONCLUSION

Normal vibrational modes of MRI gradient coils can dramatically increase the sound pressure levels during echo planar imaging (EPI) scans. To minimize acoustic noise, the frequency of EPI-readout trains and the resonance frequencies of gradient coil vibrations need to be different.

Acoustic noise and functional magnetic resonance imaging: current strategies and future prospects

Functional magnetic resonance imaging (fMRI) has become the method of choice for studying the neural correlates of cognitive tasks. Nevertheless, the scanner produces acoustic noise during the image acquisition process, which is a problem in the study of auditory pathway and language generally. The scanner acoustic noise not only produces activation in brain regions involved in auditory processing, but also interferes with the stimulus presentation. Several strategies can be used to address this problem, including modifications of hardware and software. Although reduction of the source of the acoustic noise would be ideal, substantial hardware modifications to the current base of installed MRI systems would be required. Therefore, the most common strategy employed to minimize the problem involves software modifications. In this work we consider three main types of acquisitions: compressed, partially silent, and silent. For each implementation, paradigms using block and event-related designs are assessed. We also provide new data, using a silent event-related (SER) design, which demonstrate higher blood oxygen level-dependent (BOLD) response to a simple auditory cue when compared to a conventional image acquisition.

Impact of fMRI acoustic noise on the functional anatomy of visual mental imagery

One drawback of functional magnetic resonance imaging (fMRI) is that the subject must endure intense noise during testing. We examined the possible role of such noise on the activation of early visual cortex during visual mental imagery. We postulated that noise may require subjects to work harder to pay attention to the task, which in turn could alter the activation pattern found in a silent environment. To test this hypothesis, we used positron emission tomography (PET) to monitor regional Cerebral Blood Flow (rCBF) of six subjects while they performed an imagery task either in a silent environment or in an "fMRI-like" noisy environment. Both noisy and silent imagery conditions, as compared to their respective baselines, resulted in activation of a bilateral fronto-parietal network (related to spatial processing), a bilateral inferior temporal area (related to shape processing), and deactivation of anterior calcarine cortex. Among the visual areas, rCBF increased in the most posterior part of the calcarine cortex, but at level just below the statistical threshold. However, blood flow values in the calcarine cortex during the silent imagery condition (but not the noisy imagery condition) were strongly negatively correlated with accuracy; the more challenging subjects found the task, the more strongly the calcarine cortex was activated. The subjects made more errors in the noisy condition than in the silent condition, and a direct comparison of the two conditions revealed that noise resulted in an increase in rCBF in the anterior cingulate cortex (involved in performance monitoring) and in the Wernicke's area (required to encode the verbal cues used in the task). These results thus demonstrate a nonadditive effect of fMRI gradient noise, resulting in a slight but significant effect on both performance and the neural activation pattern.

Cortical and subcortical activation with monaural monosyllabic stimulation by functional MRI

Few reports have characterized auditory processing in monaural stimulation, which is important to the understanding of auditory brain activity in subjects with hearing loss. We therefore measured regional brain activity in response to monaural stimulation of 95 dB SPL monosyllables using functional magnetic resonance imaging (fMRI) in subjects with normal hearing and five with unilateral deafness as controls for 'cross-hearing'. Images were analyzed by statistical parametric mapping software. In subjects without hearing loss, the stimuli elicited cortical activation in the primary auditory (BA 41) and auditory association regions (BA 42, 22), particularly contralaterally where extent of activation was approximately 2.5 times the ipsilateral extent. All patients with profound unilateral deafness showed no statistically apparent response in the primary auditory and auditory association regions, ruling out an important influence from cross-hearing. We found fMRI to be a useful technique for analysis of auditory processing that should be applicable to patients with various hearing abnormalities.

Silent BOLD imaging

Pulsed magnetic field gradients in magnetic resonance imaging produce high levels of acoustic noise. In functional magnetic resonance imaging, acoustic scanner noise causes unwanted masking effects. Recently, we proposed a method to perform magnetic resonance imaging experiments undisturbed by acoustic scanner noise by utilizing the property of standard gradient coils to poorly submit acoustic noise in the low frequency range. The silent gradient scheme is now incorporated into a standard T(2)*-weighted sequence. Additionally, simultaneous multi-slice excitation (SIMEX) pulses were implemented to improve the intrinsic low volume coverage of the silent sequence. The proposed silent SIMEX technique was tested and compared with a standard noisy technique using auditory and visual stimulation paradigms. The scanner noise during the silent experiments could be reduced below the range of the ambient noise of the magnet room. This feasibility study shows a trend of decreased activated areas in the noisy experiment for both, the visual and auditory paradigm.

Comparing BOLD fMRI signal changes in the awake and anesthetized rat during electrical forepaw stimulation

The difference between awake curarized and alpha-chloralose anesthetized animals was studied with respect to the BOLD signal response in an fMRI experiment. By studying the activation of the cortex upon electrical forepaw stimulation in the same rat, but following consecutively applied curarization and alpha-chloralose anesthesia protocols, it was possible to compare quantitatively the effect of both immobilization protocols on the fMRI data. The largest BOLD signal change as a result of forepaw stimulation was found in the awake condition, however the activated areas are less specific than those in the anesthetized state leaving it more difficult to interpret.