Quiet MRI with novel acoustic noise reduction

Eddy currents analysis methods for an MRI longitudinal gradient coil

PURPOSE

The rapid switching of the gradient fields induces eddy currents in neighboring metallic structures, causing undesirable effects. Numerical computations are thus required to understand eddy-currents effects for designing/implementing mitigation (involving passive shielding) and compensating techniques (using pre-emphasis). Previously, the network-analysis (NA) method was introduced to compute z-gradient eddy currents, although limited to a circularly symmetric and unconnected coil. Multi-layer integral method (MIM) method was recently introduced, modifying the circuit equation involving stream functions. We tailor MIM (TMIM) for a more general eddy-currents analysis in thin structures. Z-gradient eddy currents are analyzed and then compared using three methods (NA, TMIM, and Ansys). The analysis helps to evaluate the efficiency of passive shielding and to compensate eddy currents.

METHODS

NA and TMIM computational frameworks for harmonic and transient eddy-currents analysis were implemented and cross-validated against Ansys Maxwell. A pre-emphasis pulse was modeled for compensating eddy currents.

RESULTS

Eddy-currents analysis of an unconnected z-gradient coil in both the passive shield and cryostat were computed, and results were comparable to the least computationally efficient Ansys simulations. Although NA computations are fast, TMIM is implemented with reasonable efficiency and applied to circularly unsymmetric geometries. TMIM computations were further validated against Ansys using a connected z-gradient. Our computations allowed the effective evaluation of the performance of three various passive-shielding configurations, non-capped, capped, and slitted (for the first time), and an effective pre-emphasis compensation model was computed.

CONCLUSION

Three eddy-currents analysis methods were studied and compared. Computationally efficient TMIM allows both harmonic and transient eddy-currents analysis involving different/complex gradient configurations/situations as well as involved shielding structures. Eddy-currents pre-emphasis compensation was demonstrated.

Effect of acoustic noise reduction technology on image quality: a multivendor study

The purpose of this study was to clarify the appropriate use of a combination of pulse sequences and acoustic noise reduction technology in general-purpose brain magnetic resonance imaging. Five pulse sequences commonly used in brain magnetic resonance imaging examinations-turbo spin-echo T2-weighted imaging, T1-weighted fluid-attenuated inversion recovery, T2-weighted fluid-attenuated inversion recovery, diffusion-weighted imaging, and magnetic resonance angiography-were performed on healthy participants at three vendors where acoustic noise reduction technology was available. The results showed that acoustic noise reduction technology reduced sound pressure levels and altered image quality in all pulse sequences across all vendors' magnetic resonance imaging scanners. Although T2-weighted imaging and T1-weighted fluid-attenuated inversion recovery resulted in little image quality degradation, T2-weighted fluid-attenuated inversion recovery, diffusion-weighted imaging, and magnetic resonance angiography had significant image degradation. Therefore, acoustic noise reduction technology should be used with caution.

Improving Patient Comfort in MRI with Predictive Acoustic Noise Cancelling

With sound pressure levels reaching up to 130 dB, acoustic noise in Magnetic Resonance Imaging (MRI) is one of the main sources of patient discomfort in otherwise one of the safest medical imaging modalities. In this work, a noise prediction-based approach, termed predictive noise cancelling (PNC), is applied, for the first time, to suppress noise in MRI. In PN C the noise from the scanner gradient coils is predicted based on linear time-invariant models, which relate the individual gradient coil (X, Y and Z) input to the acoustic noise output. A model setup was constructed of a custom speaker box and MRI -compatible microphone to demonstrate live noise reduction. Additional tuning steps, including output channel equalization and clock mismatch correction, were performed to maximize noise reduction. A calibration sequence was designed to determine the model and tuning parameters. Analysis of actual scanner noise shows an upper limit of 21 dB noise reduction with the proposed linear model. For the components of a clinical example sequence, the setup demonstrated in-bore live noise reduction of up to 10 dB (7.01 ± 0.31 dB, 6.42 ± 2.04 dB and 9.28 ± 0.26 dB for X, Y and Z gradient coils respectively) in the presence of system imperfections. Clinical relevance - The results indicate promising noise attenuation without the need to modify scanner hardware or compromises in acquisition speed or quality. This has potential to substantially and cost effectively improve patient comfort in clinical MRI.

Applications in Awake Animal Magnetic Resonance Imaging

There are numerous publications on methods and applications for awake functional MRI across different species, e.g., voles, rabbits, cats, dogs, and rhesus macaques. Each of these species, most obviously rhesus monkey, have general or unique attributes that provide a better understanding of the human condition. However, much of the work today is done on rodents. The growing number of small bore (≤30 cm) high field systems 7T- 11.7T favor the use of small animals. To that point, this review is primarily focused on rodents and their many applications in awake function MRI. Applications include, pharmacological MRI, drugs of abuse, sensory evoked stimuli, brain disorders, pain, social behavior, and fear.

Silent susceptibility-weighted angiography to detect hemorrhagic lesions in the brain: a clinical and phantom study

PURPOSE

To compare the effectiveness of silent susceptibility-weighted angiography (sSWAN), a new imaging technique with lower acoustic noise, with conventional susceptibility-weighted angiography (cSWAN) in the detection of intracranial hemorrhagic lesions.

METHODS

We measured the acoustic and background noise during sSWAN and cSWAN imaging and calculated the contrast-to-noise ratio (CNR) of the phantom consisting of eight chambers with different concentrations of superparamagnetic iron oxide. In the clinical study, we calculated the CNRs of hemorrhagic lesions in 15 patients and evaluated the images for conspicuity and artifact on each sequence and scored them on a 4-point scale. We also evaluated whether hypointense areas observed on sSWAN or cSWAN increased in size from those on T2*-weighted imaging (T2*-WI).

RESULTS

Acoustic noise for sSWAN (57.9 ± 0.32 dB [background noise 51.3 dB]) was significantly less than that for cSWAN (89.0 ± 0.22 dB [background noise 50.9 dB]). The CNRs of phantoms for sSWAN were slightly but not significantly lower than those for cSWAN (P = 0.18). The CNRs of hemorrhagic lesions did not show significant differences between sSWAN and cSWAN (P = 0.17). There were no significant differences between sSWAN and cSWAN with respect to the scores for conspicuity, artifact, and change in size of hypointense areas from T2*-WI.

CONCLUSION

sSWAN is equivalent to cSWAN with respect to the image quality for the detection of hemorrhagic lesions but has lower acoustic noise.

MRI acoustic noise-modulated computer animations for patient distraction and entertainment with application in pediatric psychiatric patients

PURPOSE

To reduce patient anxiety caused by the MRI scanner acoustic noise.

MATERIAL AND METHODS

We developed a simple and low-cost system for patient distraction using visual computer animations that were synchronized to the MRI scanner's acoustic noise during the MRI exam. The system was implemented on a 3T MRI system and tested in 28 pediatric patients with bipolar disorder. The patients were randomized to receive noise-synchronized animations in the form of abstract animations in addition to music (n = 13, F/M = 6/7, age = 10.9 ± 2.5 years) or, as a control, receive only music (n = 15, F/M = 7/8, age = 11.6 ± 2.3 years). After completion of the scans, all subjects answered a questionnaire about their scan experience and the perceived scan duration.

RESULTS

The scan duration with multisensory input (animations and music) was perceived to be ~15% shorter than in the control group (43 min vs. 50 min, P < 0.05). However, the overall scan experience was scored less favorably (3.9 vs. 4.6 in the control group, P < 0.04).

CONCLUSIONS

This simple system provided patient distraction and entertainment leading to perceived shorter scan times, but the provided visualization with abstract animations was not favored by this patient cohort.

Software-based noise reduction in cranial magnetic resonance imaging: Influence on image quality

Objectives

To investigate acoustic noise reduction, image quality and white matter lesion detection rates of cranial magnetic resonance imaging (MRI) scans acquired with and without sequence-based acoustic noise reduction software.

Material and methods

Thirty-one patients, including 18 men and 13 women, with a mean age of 58.3±14.5 years underwent cranial MRI. A fluid-attenuated inversion recovery (FLAIR) sequence was acquired with and without acoustic noise reduction using the Quiet Suite (QS) software (Siemens Healthcare). During data acquisition, peak sound pressure levels were measured with a sound level meter (Testo, Typ 815). In addition, two observers assessed subjective image quality for both sequences using a five-point scale (1 very good—5 inadequate). Signal-to-noise ratio (SNR) was measured for both sequences in the following regions: white matter, gray matter, and cerebrospinal fluid. Furthermore, lesion detection rates in white matter pathologies were evaluated by two observers for both sequences. Acoustic noise, image quality including SNR and white matter lesion detection rates were compared using the Mann-Whitney-U-test.

Results

Peak sound pressure levels were slightly but significantly reduced using QS, P≤0.017. Effective sound pressure, measured in Pascal, was decreased by 19.7%. There was no significant difference in subjective image quality between FLAIR sequences acquired without/with QS: observer 1: 2.03/2.07, P = 0.730; observer 2: 1.98/2.10, P = 0.362. In addition, SNR was significantly increased in white matter, P≤0.001, and gray matter, P = 0.006, using QS. The lesion detection rates did not decline utilizing QS: observer 1: P = 0.944 observer 2: P = 0.952.

Conclusions

Sequence-based noise reduction software such as QS can significantly reduce peak sound pressure levels, without a loss of subjective image quality and increase SNR at constant lesion detection rates.

Image quality assessment of silent T2 PROPELLER sequence for brain imaging in infants

OBJECTIVE

Infants are vulnerable to high acoustic noise. Acoustic noise generated by MR scanning can be reduced by a silent sequence. The purpose of this study is to compare the image quality of the conventional and silent T2 PROPELLER sequences for brain imaging in infants.

METHODS

A total of 36 scans were acquired from 24 infants using a 3 T MR scanner. Each patient underwent both conventional and silent T2 PROPELLER sequences. Acoustic noise level was measured. Quantitative and qualitative assessments were performed with the images taken with each sequence.

RESULTS

The sound pressure level of the conventional T2 PROPELLER imaging sequence was 92.1 dB and that of the silent T2 PROPELLER imaging sequence was 73.3 dB (reduction of 20%). On quantitative assessment, the two sequences (conventional vs silent T2 PROPELLER) did not show significant difference in relative contrast (0.069 vs 0.068, p value = 0.536) and signal-to-noise ratio (75.4 vs 114.8, p value = 0.098). Qualitative assessment of overall image quality (p value = 0.572), grey-white differentiation (p value = 0.986), shunt-related artefact (p value > 0.999), motion artefact (p value = 0.801) and myelination degree in different brain regions (p values ≥ 0.092) did not show significant difference between the two sequences.

CONCLUSION

The silent T2 PROPELLER sequence reduces acoustic noise and generated comparable image quality to that of the conventional sequence. Advances in knowledge: This is the first report to compare silent T2 PROPELLER images with that of conventional T2 PROPELLER images in children.

Comparison of image quality characteristics on Silent MR versus conventional MR imaging of brain lesions at 3 Tesla

OBJECTIVE:

To compare signal- and contrast-to-noise-ratio (SNR, CNR), conspicuity values and subjective image quality characteristics of Silent MRI and conventional MRI in brain disorders at 3 T.

METHODS:

26 patients were prospectively examined with a 3 T MRI. Silent Scan was added to standardized MR protocol. Silenz T₁ weighted (Tlw) and Silent T₂ weighted (T2w) sequences were compared to standard Tlw and T2w. Analysis was performed quantitatively (SNR, CNR, conspicuity values) and by visual assessment on a 4-point scale with regard to lesion visibility, lesion delineation, grey-white differentiation and diagnostic usefulness. Data were analyzed using Wilcoxon signed-rank and Sign test. p ≤ 0.05 was considered significant.

RESULTS:

Silenz Tlw vs Tlw provided decreased SNR, but increased CNR (SNRparenchyma, SNRlesion: p = 0.000, CNRlesion: p = 0.003). Silent T2w vs T2w showed better SNR and CNR values (SNR_parenchyma, p = 0.014; SNR_lesion, p = 0.005; CNR_lesion, p = 0.005). Conspicuity values were not significantly different on Silenz Tlw vs Tlw and Silent T2w vs T2w. The visual assessment revealed Silenz Tlw to be significantly superior to Tlw in terms of grey- white differentiation (p = 0.000), lesion visibility (p = 0.003) and overall diagnostic usefulness (p = 0.001). In terms of Silent T2w vs T2w, there was a significant difference in grey-white differentiation in favour of Silent T2w (p = 0.016).

CONCLUSION:

Silent Scan is suitable for 3 T with image quality characteristics comparable to conventional MRI.

ADVANCES IN KNOWLEDGE:

Silent Scan has a diagnostic value comparable to conventional MRI, with the advantage of a quiet MR exam improving patient MR experience.

Quiet T1-Weighted Pointwise Encoding Time Reduction with Radial Acquisition for Assessing Myelination in the Pediatric Brain

BACKGROUND AND PURPOSE

T1-weighted pointwise encoding time reduction with radial acquisition (PETRA) sequences require limited gradient activity and allow quiet scanning. We aimed to assess the usefulness of PETRA in pediatric brain imaging.

MATERIALS AND METHODS

We included consecutive pediatric patients who underwent both MPRAGE and PETRA. The contrast-to-noise and contrast ratios between WM and GM were compared in the cerebellar WM, internal capsule, and corpus callosum. The degree of myelination was rated by using 4-point scales at each of these locations plus the subcortical WM in the anterior frontal, anterior temporal, and posterior occipital lobes. Two radiologists made all assessments, and the intra- and interrater agreement was calculated by using intraclass correlation coefficients. Acoustic noise on MPRAGE and PETRA was measured.

RESULTS

We included 56 patients 5 days to 14 years of age (mean age, 36.6 months) who underwent both MPRAGE and PETRA. The contrast-to-noise and contrast ratios for PETRA were significantly higher than those for MPRAGE (P < .05), excluding the signal ratio for cerebellar WM. Excellent intra- and interrater agreement were obtained for myelination at all locations except the cerebellar WM. The acoustic noise on PETRA (58.2 dB[A]) was much lower than that on MPRAGE (87.4 dB[A]).

CONCLUSIONS

PETRA generally showed better objective imaging quality without a difference in subjective image-quality evaluation and produced much less acoustic noise compared with MPRAGE. We conclude that PETRA can substitute for MPRAGE in pediatric brain imaging.

MRI evaluation and safety in the developing brain

Magnetic resonance imaging (MRI) evaluation of the developing brain has dramatically increased over the last decade. Faster acquisitions and the development of advanced MRI sequences, such as magnetic resonance spectroscopy (MRS), diffusion tensor imaging (DTI), perfusion imaging, functional MR imaging (fMRI), and susceptibility-weighted imaging (SWI), as well as the use of higher magnetic field strengths has made MRI an invaluable tool for detailed evaluation of the developing brain. This article will provide an overview of the use and challenges associated with 1.5-T and 3-T static magnetic fields for evaluation of the developing brain. This review will also summarize the advantages, clinical challenges, and safety concerns specifically related to MRI in the fetus and newborn, including the implications of increased magnetic field strength, logistics related to transporting and monitoring of neonates during scanning, and sedation considerations, and a discussion of current technologies such as MRI conditional neonatal incubators and dedicated small-foot print neonatal intensive care unit (NICU) scanners.

Lateralization of music processing with noises in the auditory cortex: an fNIRS study

The present study is to determine the effects of background noise on the hemispheric lateralization in music processing by exposing 14 subjects to four different auditory environments: music segments only, noise segments only, music + noise segments, and the entire music interfered by noise segments. The hemodynamic responses in both hemispheres caused by the perception of music in 10 different conditions were measured using functional near-infrared spectroscopy. As a feature to distinguish stimulus-evoked hemodynamics, the difference between the mean and the minimum value of the hemodynamic response for a given stimulus was used. The right-hemispheric lateralization in music processing was about 75% (instead of continuous music, only music segments were heard). If the stimuli were only noises, the lateralization was about 65%. But, if the music was mixed with noises, the right-hemispheric lateralization has increased. Particularly, if the noise was a little bit lower than the music (i.e., music level 10~15%, noise level 10%), the entire subjects showed the right-hemispheric lateralization: This is due to the subjects' effort to hear the music in the presence of noises. However, too much noise has reduced the subjects' discerning efforts.

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.

Masking level differences--a diffusion tensor imaging and functional MRI study

In our previous study we investigated Masking Level Differences (MLD) using functional Magnetic Resonance Imaging (fMRI), but were unable to confirm neural correlations for the MLD within the auditory cortex and inferior colliculus. Here we have duplicated conditions from our previous study, but have included more participants and changed the study site to a new location with a newer scanner and presentation system. Additionally, Diffusion Tensor Imaging (DTI) is included to allow investigation of fiber tracts that may be involved with MLDs. Twenty participants were included and underwent audiometric testing and MRI scanning. The current study revealed regions of increased and decreased activity within the auditory cortex when comparing the combined noise and signal of the dichotic MLD stimuli (N0Sπ and NπS0) with N0S0. Furthermore, we found evidence of inferior colliculus involvement. Our DTI findings show strong correlations between DTI measures within the brainstem and signal detection threshold levels. Patterns of correlation when the signal was presented only to the right ear showed an extensive network in the left hemisphere; however, the opposite was not true for the signal presented only to the left ear. Our current study was able to confirm what we had previously hypothesized using fMRI, while extending our investigation of MLDs to include the characteristics of connecting neural pathways.

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.

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.

The challenges of neonatal magnetic resonance imaging

Improved neonatal survival rates and antenatal diagnostic imaging is generating a growing demand for postnatal MRI examinations. Neonatal brain MRI is now becoming standard clinical care in many settings, but with the exception of some research centres, the technique has not been optimised for imaging neonates and small children. Here, we review some of the challenges involved in neonatal MRI, including recent advances in overall MR practicality and nursing practice, to address some of the ways in which the MR experience could be made more neonate-friendly.

Sinusoidal echo-planar imaging with parallel acquisition technique for reduced acoustic noise in auditory fMRI

PURPOSE

To extend the parameter restrictions of a silent echo-planar imaging (sEPI) sequence using sinusoidal readout (RO) gradients, in particular with increased spatial resolution. The sound pressure level (SPL) of the most feasible configurations is compared to conventional EPI having trapezoidal RO gradients.

MATERIALS AND METHODS

We enhanced the sEPI sequence by integrating a parallel acquisition technique (PAT) on a 3 T magnetic resonance imaging (MRI) system. The SPL was measured for matrix sizes of 64 × 64 and 128 × 128 pixels, without and with PAT (R = 2). The signal-to-noise ratio (SNR) was examined for both sinusoidal and trapezoidal RO gradients.

RESULTS

Compared to EPI PAT, the SPL could be reduced by up to 11.1 dB and 5.1 dB for matrix sizes of 64 × 64 and 128 × 128 pixels, respectively. The SNR of sinusoidal RO gradients is lower by a factor of 0.96 on average compared to trapezoidal RO gradients.

CONCLUSION

The sEPI PAT sequence allows for 1) increased resolution, 2) expanded RO frequency range toward lower frequencies, which is in general beneficial for SPL, or 3) shortened TE, TR, and RO train length. At the same time, it generates lower SPL compared to conventional EPI for a wide range of RO frequencies while having the same imaging parameters.

Acoustic noise transfer function in clinical MRI a multicenter analysis

RATIONALE AND OBJECTIVES

Acoustic noise both in terms of its magnitude and frequency during magnetic resonance imaging (MRI) scan is influenced by imaging parameters and pulse sequences. It varies because of many different factors such as structure, materials, and magnetic field strength. The purpose of our study is to evaluate the characteristics of acoustic noise independent of MRI scan protocol by measuring a gradient-pulse-to-acoustic-noise transfer function (GPAN-TF) at various MRI scanners.

MATERIALS AND METHODS

We measured sound pressure levels in the frequency domain in a 0.4-T, seven 1.5-T, and three 3.0-T clinical MRI systems when applying a simple narrower trapezoidal gradient pulse. We calculated a GPAN-TF [μPa/(mT/m)] in each gradient coil (ie, X, Y, and Z-axis) by the deconvolution process.

RESULTS

GPAN-TF at a high-frequency range (1000-10,000 Hz) was larger than that at low frequency for all MRI (P<0.01) scanners except for a low static field machine. For high frequency (>1000 Hz), the 3.0-T MRI scanner had a larger GPAN-TF than that of 0.4-T and 1.5-T (P < .01). MR scanner with a vacuum chamber reduced GPAN-TF at a lower frequency (P < .01), but this effect decreased at higher frequency.

CONCLUSION

GPAN-TF analysis makes it possible to obtain more detailed information on acoustic noise properties among MRI scanners.

Silent echo-planar imaging for auditory FMRI

INTRODUCTION

The effect of the acoustic scanner noise produced by gradient coil switching on the auditory evoked BOLD signal represents a well-known problem in auditory functional MRI (FMRI). In this paper, a new low-noise echo-planar imaging (EPI) sequence is presented that is optimized for auditory FMRI measurements.

METHODS

The sequence produces a narrow-band acoustic frequency spectrum by using a sinusoidal readout echo train and a constant phase encoding gradient. This narrow band is adapted to the frequency response function of the MR scanner by varying the switching frequency of the sinusoidal readout gradient.

RESULTS

Compared to a manufacturer-provided standard EPI sequence, the acoustic noise reduction amounts to up to 20 dBA. Using a simple block design paradigm contrasting presentation of a pure tone during ON blocks and "silence" (absence of the tone) during OFF blocks, the new low-noise sequence was evaluated and compared to the standard EPI sequence. Statistical parametric mapping (SPM) resulted in higher levels of significance of auditory activation for the low-noise sequence.

DISCUSSION

These findings strongly suggest that the low-noise sequence may generate enhanced BOLD contrasts compared to the standard EPI sequences commonly used in FMRI.

Silent and continuous fMRI scanning differentially modulate activation in an auditory language comprehension task

Sparse temporal acquisition schemes have been adopted to investigate the neural correlates of human audition using blood-oxygen-level dependent (BOLD) based functional magnetic resonance imaging (fMRI) devoid of ambient confounding acoustic scanner noise. These schemes have previously been extended to clustered-sparse temporal acquisition designs which record several subsequent BOLD contrast images in rapid succession in order to enhance temporal sampling efficiency. In the present study we demonstrate that an event-related task design can effectively be combined with a clustered temporal acquisition technique in an auditory language comprehension task. The same fifteen volunteers performed two separate auditory runs which either applied customary fMRI acquisition (CA) composed of continuous scanner noise or "silent" fMRI built on a clustered temporal acquisition (CTA) protocol. In accord with our hypothesis, the CTA scheme relative to the CA protocol is accompanied by significantly stronger functional responses along the entire superior temporal plane. By contrast, the bilateral insulae engage more strongly during continuous scanning. A post-hoc region-of-interest analysis reveals cortical activation in subportions of the supratemporal plane which varies as a function of acquisition protocol. The middle part of the supratemporal plane shows a rightward asymmetry only for the CTA scheme while the posterior supratemporal plane exposes a significantly stronger leftward asymmetry during the CTA. Our findings implicate that silent fMRI is advantageous when it comes to the exploration of auditory and speech functions residing in the supratemporal plane.

Model gradient coil employing active acoustic control for MRI

Results are presented for a model three-axis gradient coil incorporating active acoustic control which is applied to the switched read gradient during a single-shot rapid echo-planar imaging (EPI) sequence at a field strength of 3.0 T. The total imaging acquisition time was 10.6 ms. Substantial noise reduction is achieved both within the magnet bore and outside the magnet. Typical internal noise reduction over the specimen area is 40 dB_A whereas outside the acoustic chamber the noise level is reduced by 60–77 dB_A. However these results are relative to a control winding which is switched in phase, adding 6 dB_A in its non-optimized mode, which is included in the quoted figures.

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.

Quantification of mechanical vibration during diffusion tensor imaging at 3 T

Subjects sense clear mechanical vibrations during diffusion tensor imaging (DTI). These vibrations, likely resulting from diffusion-sensitizing gradients, have been assumed to be of the same strength and phase in all parts of the magnetic resonance imaging (MRI) scanner so that they could be ignored. However, our measurements, carried out from several parts of the MRI scanner and its surroundings using an optical laser-based interferometer, demonstrate an uneven distribution of mechanical vibrations within the scanner. The measurements were performed during DT scanning at 3 T, with various diffusion-weighting parameters, by positioning a phantom in the head coil and/or a human subject on the patient bed. The vibration-related movement was caused by the diffusion-sensitizing gradients and was maximally 0.5 mm with typical settings used in brain imaging. The compensation for eddy currents, done with gradients in our DTI sequence, increased the vibration level by a factor of 1.5 or more with diffusion-weighting parameter b = 1000 s/mm(2) and by a factor of 3 or more with b = 3000 s/mm(2). Mechanical vibrations stayed at an acceptable level with b < or = 1000 s/mm(2), resulting in additional signal losses of 5-17%. Vibration levels might be reduced by adjusting imaging parameters, by modifying the gradient waveforms in the DTI sequence, and by redesigning the mechanics of patient bed to effectively dampen the movements.

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.