Half‐fourier‐acquisition single‐shot turbo spin‐echo (HASTE) MRI of the lung at 3 Tesla using parallel imaging with 32‐receiver channel technology

Purpose

To assess the feasibility of half‐Fourier‐acquisition single‐shot turbo spin‐echo (HASTE) of the lung at 3 Tesla (T) using parallel imaging with a prototype of a 32‐channel torso array coil, and to determine the optimum acceleration factor for the delineation of intrapulmonary anatomy.

Materials and Methods

Nine volunteers were examined on a 32‐channel 3T MRI system using a prototype 32‐channel‐torso‐array‐coil. HASTE‐MRI of the lung was acquired at both, end‐inspiratory and end‐expiratory breathhold with parallel imaging (Generalized autocalibrating partially parallel acquisitions = GRAPPA) using acceleration factors ranging between R = 1 (TE = 42 ms) and R = 6 (TE = 16 ms). The image quality of intrapulmonary anatomy and subjectively perceived noise level was analyzed by two radiologists in consensus. In addition quantitative measurements of the signal‐to‐noise ratio (SNR) of HASTE with different acceleration factors were assessed in phantom measurements.

Results

Using an acceleration factor of R = 4 image blurring was substantially reduced compared with lower acceleration factors resulting in sharp delineation of intrapulmonary structures in expiratory scans. For inspiratory scans an acceleration factor of 2 provided the best image quality. Expiratory scans had a higher subjectively perceived SNR than inspiratory scans.

Conclusion

Using optimized multi‐element coil geometry HASTE‐MRI of the lung is feasible at 3T with acceleration factors up to 4. Compared with nonaccelerated acquisitions, shorter echo times and reduced image blurring are achieved. Expiratory scanning may be favorable to compensate for susceptibility associated signal loss at 3T. J. Magn. Reson. Imaging 2009;30:541–546. © 2009 Wiley‐Liss, Inc.     

Denoising sparse images from GRAPPA using the nullspace method

To accelerate magnetic resonance imaging using uniformly undersampled (nonrandom) parallel imaging beyond what is achievable with generalized autocalibrating partially parallel acquisitions (GRAPPA) alone, the DEnoising of Sparse Images from GRAPPA using the Nullspace method is developed. The trade‐off between denoising and smoothing the GRAPPA solution is studied for different levels of acceleration. Several brain images reconstructed from uniformly undersampled k‐space data using DEnoising of Sparse Images from GRAPPA using the Nullspace method are compared against reconstructions using existing methods in terms of difference images (a qualitative measure), peak‐signal‐to‐noise ratio, and noise amplification (g‐factors) as measured using the pseudo‐multiple replica method. Effects of smoothing, including contrast loss, are studied in synthetic phantom data. In the experiments presented, the contrast loss and spatial resolution are competitive with existing methods. Results for several brain images demonstrate significant improvements over GRAPPA at high acceleration factors in denoising performance with limited blurring or smoothing artifacts. In addition, the measured g‐factors suggest that DEnoising of Sparse Images from GRAPPA using the Nullspace method mitigates noise amplification better than both GRAPPA and L₁ iterative self‐consistent parallel imaging reconstruction (the latter limited here by uniform undersampling). Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

k-t-Space accelerated myocardial perfusion

Purpose

To investigate the performance of the recently introduced spatiotemporal parallel imaging technique called parallel MRI with extended and averaged generalized autocalibrating partially parallel acquisitions (GRAPPA) kernels (PEAK‐GRAPPA) for myocardial perfusion measurements.

Materials and Methods

A study with 11 patients with myocardial infarction was performed to compare nonaccelerated perfusion imaging, i.e., fully acquired k‐space data, with the results of conventional GRAPPA and PEAK‐GRAPPA with a net acceleration factor of 2.4 to 3.4. Signal time courses reflecting the passage of the contrast agent bolus in different regions of the heart were evaluated for these different reconstruction methods.

Results

Reconstruction with PEAK‐GRAPPA demonstrated considerably improved image quality compared to conventional GRAPPA. In addition, signal time courses for PEAK‐GRAPPA demonstrated an excellent agreement compared to full k‐space data, which is necessary for an accurate qualitative and quantitative assessment of myocardial perfusion.

Conclusion

Qualitative and quantitative results of patient measurements illustrate that the temporal fidelity of nonperiodic processes such as myocardial perfusion are preserved with PEAK‐GRAPPA up to net acceleration factors of more than 3 while showing a superior image quality compared to conventional GRAPPA and a sliding‐window reconstruction. J. Magn. Reson. Imaging 2008;28:1080–1085. © 2008 Wiley‐Liss, Inc.     

Multiband accelerated spin‐echo echo planar imaging with reduced peak RF power using time‐shifted RF pulses

Purpose:

To evaluate an alternative method for generating multibanded radiofrequency (RF) pulses for use in multiband slice‐accelerated imaging with slice‐GRAPPA unaliasing, substantially reducing the required peak power without bandwidth compromises. This allows much higher accelerations for spin‐echo methods such as SE‐fMRI and diffusion‐weighted MRI where multibanded slice acceleration has been limited by available peak power.

Theory and Methods:

Multibanded “time‐shifted” RF pulses were generated by inserting temporal shifts between the applications of RF energy for individual bands, avoiding worst‐case constructive interferences. Slice profiles and images in phantoms and human subjects were acquired at 3 T.

Results:

For typical sinc pulses, time‐shifted multibanded RF pulses were generated with little increase in required peak power compared to single‐banded pulses. Slice profile quality was improved by allowing for higher pulse bandwidths, and image quality was improved by allowing for optimum flip angles to be achieved.

Conclusion:

A simple approach has been demonstrated that significantly alleviates the restrictions imposed on achievable slice acceleration factors in multiband spin‐echo imaging due to the power requirements of multibanded RF pulses. This solution will allow for increased accelerations in diffusion‐weighted MRI applications where data acquisition times are normally very long and the ability to accelerate is extremely valuable. Magn Reson Med 69:1261–1267, 2013 Wiley Periodicals, Inc.     

Practical signal-to-noise ratio quantification for sensitivity encoding: application to coronary MR angiography

Purpose:

To develop and evaluate a practical method for the quantification of signal‐to‐noise ratio (SNR) on coronary MR angiograms (MRA) acquired with parallel imaging.

Materials and Methods:

To quantify the spatially varying noise due to parallel imaging reconstruction, a new method has been implemented incorporating image data acquisition followed by a fast noise scan during which radiofrequency pulses, cardiac triggering and navigator gating are disabled. The performance of this method was evaluated in a phantom study where SNR measurements were compared with those of a reference standard (multiple repetitions). Subsequently, SNR of myocardium and posterior skeletal muscle was determined on in vivo human coronary MRA.

Results:

In a phantom, the SNR measured using the proposed method deviated less than 10.1% from the reference method for small geometry factors (≤2). In vivo, the noise scan for a 10 min coronary MRA acquisition was acquired in 30 s. Higher signal and lower SNR, due to spatially varying noise, were found in myocardium compared with posterior skeletal muscle.

Conclusion:

SNR quantification based on a fast noise scan is a validated and easy‐to‐use method when applied to three‐dimensional coronary MRA obtained with parallel imaging as long as the geometry factor remains low. J. Magn. Reson. Imaging 2011;33:1330–1340. © 2011 Wiley‐Liss, Inc.     

Parallel MRI with extended and averaged GRAPPA kernels (PEAK-GRAPPA): optimized spatiotemporal dynamic imaging

Purpose

To evaluate an optimized k‐t‐space related reconstruction method for dynamic magnetic resonance imaging (MRI), a method called PEAK‐GRAPPA (Parallel MRI with Extended and Averaged GRAPPA Kernels) is presented which is based on an extended spatiotemporal GRAPPA kernel in combination with temporal averaging of coil weights.

Materials and Methods

The PEAK‐GRAPPA kernel consists of a uniform geometry with several spatial and temporal source points from acquired k‐space lines and several target points from missing k‐space lines. In order to improve the quality of coil weight estimation sets of coil weights are averaged over the temporal dimension.

Results

The kernel geometry leads to strongly decreased reconstruction times compared to the recently introduced k‐t‐GRAPPA using different kernel geometries with only one target point per kernel to fit. Improved results were obtained in terms of the root mean square error and the signal‐to‐noise ratio as demonstrated by in vivo cardiac imaging.

Conclusion

Using a uniform kernel geometry for weight estimation with the properties of uncorrelated noise of different acquired timeframes, optimized results were achieved in terms of error level, signal‐to‐noise ratio, and reconstruction time. J. Magn. Reson. Imaging 2008;28:1226–1232. © 2008 Wiley‐Liss, Inc.     

Improving quality of arterial spin labeling MR imaging at 3 Tesla with a 32-channel coil and parallel imaging

Purpose:

To compare 12‐channel and 32‐channel phased‐array coils and to determine the optimal parallel imaging (PI) technique and factor for brain perfusion imaging using Pulsed Arterial Spin labeling (PASL) at 3 Tesla (T).

Materials and Methods:

Twenty‐seven healthy volunteers underwent 10 different PASL perfusion PICORE Q2TIPS scans at 3T using 12‐channel and 32‐channel coils without PI and with GRAPPA or mSENSE using factor 2. PI with factor 3 and 4 were used only with the 32‐channel coil. Visual quality was assessed using four parameters. Quantitative analyses were performed using temporal noise, contrast‐to‐noise and signal‐to‐noise ratios (CNR, SNR).

Results:

Compared with 12‐channel acquisition, the scores for 32‐channel acquisition were significantly higher for overall visual quality, lower for noise and higher for SNR and CNR. With the 32‐channel coil, artifact compromise achieved the best score with PI factor 2. Noise increased, SNR and CNR decreased with PI factor. However mSENSE 2 scores were not always significantly different from acquisition without PI.

Conclusion:

For PASL at 3T, the 32‐channel coil at 3T provided better quality than the 12‐channel coil. With the 32‐channel coil, mSENSE 2 seemed to offer the best compromise for decreasing artifacts without significantly reducing SNR, CNR. J. Magn. Reson. Imaging 2012;35:1233‐1239. © 2012 Wiley Periodicals, Inc.     

Breathheld autocalibrated phase‐contrast imaging

Purpose:

To compare generalized autocalibrating partially parallel acquisitions (GRAPPA), modified sensitivity encoding (mSENSE), and SENSE in phase‐contrast magnetic resonance imaging (PC‐MRI) applications.

Materials and Methods:

Aliasing of the torso can occur in PC‐MRI applications. If the data are further undersampled for parallel imaging, SENSE can be problematic in correctly unaliasing signals due to coil sensitivity maps that do not match that of the aliased volume. Here, a method for estimating coil sensitivities in flow applications is described. Normal volunteers (n = 5) were scanned on a 1.5 T MRI scanner and underwent PC‐MRI scans using GRAPPA, mSENSE, SENSE, and conventional PC‐MRI acquisitions. Peak velocity and flow through the aorta and pulmonary artery were evaluated.

Results:

Bland–Altman statistics for flow in the aorta and pulmonary artery acquired with mSENSE and GRAPPA methods (R = 2 and R = 3 cases) have comparable mean differences to flow acquired with conventional PC‐MRI. GRAPPA and mSENSE PC‐MRI have more robust measurements than SENSE when there is aliasing artifact caused by insufficient coil sensitivity maps. For peak velocity, there are no considerable differences among the mSENSE, GRAPPA, and SENSE reconstructions and are comparable to conventional PC‐MRI.

Conclusion:

Flow measurements of images reconstructed with autocalibration techniques have comparable agreement with conventional PC‐MRI and provide robust measurements in the presence of wraparound. J. Magn. Reson. Imaging 2010;31:1004–1014. ©2010 Wiley‐Liss, Inc.     

fMRI of human olfaction at the individual level: Interindividual variability

Purpose:

To determine whether the range of normal variation of human olfactory functional magnetic resonance imaging (fMRI) activations in healthy single subjects is compatible with the detection of atypical patterns.

Materials and Methods:

In an event‐related olfactory experiment, the variability of fMRI activation in six bilateral olfactory areas known to be affected in neurodegenerative diseases was measured in a region of interest (ROI) analysis in terms of intensity, localization, and overlap on 51 subjects. fMRI measurements were compared against measurements from a visual experiment performed on 25 subjects.

Results:

Olfaction induced activations with low intensity, high variability, and a 4‐fold lower contrast‐to‐noise ratio (CNR) than vision. Even in the best case (piriform cortex), mean pairwise activation overlap was still less than 40%. None of the olfactory ROIs showed significant activation for all subjects at the permissive threshold of P < 0.001. A gender‐dependent significantly stronger activation was found in the bilateral piriform cortex of male subjects.

Conclusion:

Linking t‐statistics and CNR showed that for all olfactory ROIs, CNR is either near or below the estimated threshold of 0.73 found to be necessary to obtain significant activations. In our experimental conditions the low reliability of olfactory activations should prompt major reservations over using fMRI of human olfaction as a diagnostic tool in single subjects. J. Magn. Reson. Imaging 2013;37:92–100. © 2012 Wiley Periodicals, Inc.     

Accelerating time‐resolved MRA with multiecho acquisition

A new four‐dimensional magnetic resonance angiograpy (MRA) technique called contrast‐enhanced angiography with multiecho and radial k‐space is introduced, which accelerates the acquisition using multiecho while maintaining a high spatial resolution and increasing the signal‐to‐noise ratio (SNR). An acceleration factor of approximately 2 is achieved without parallel imaging or undersampling by multiecho (i.e., echo‐planar imaging) acquisition. SNR is gained from (1) longer pulse repetition times, which allow more time for T₁ regrowth; (2) decreased specific absorption rate, which allows use of flip angles that maximize contrast at high field; and (3) minimized effects of a transient contrast bolus signal with a shorter temporal footprint. Simulations, phantom studies, and in vivo scans were performed. Contrast‐enhanced angiography with multiecho and radial k‐space can be combined with parallel imaging techniques such as Generalized Autocalibrating Partially Parallel Acquisitions (GRAPPA) to provide additional 2‐fold acceleration in addition to higher SNR to trade off for parallel imaging. This technique can be useful in diagnosing vascular lesions where accurate dynamic information is necessary. Magn Reson Med 63:1520–1528, 2010. © 2010 Wiley‐Liss, Inc.     

Evaluation of multicoil breast arrays for parallel imaging

Purpose:

To evaluate three multicoil breast arrays for both conventional and SENSE‐accelerated imaging.

Materials and Methods:

Two commercially available 8‐element coils and a prototype 16‐element coil were compared. One 8‐element array had adjustable coils located next to the breast tissue and the other had a fixed coil arrangement; both were designed to allow parallel imaging in the left–right direction. The 16‐element coil was designed to have coil sensitivity variation in both the left–right and superior–inferior directions, and also had adjustable coils. Their performance was assessed in terms of signal‐to‐noise ratio (SNR), g‐factor, and uniformity with a custom‐built phantom.

Results:

The 16‐element array with adjustable coils provided the highest SNR, while the 8‐element coil with a fixed coil arrangement had the best uniformity. All coils performed well for SENSE acceleration in the left–right direction. The 8‐element coils did not have the capability for acceleration in the superior–inferior direction across the whole volume. The 16‐element coil enabled acceleration in the superior–inferior direction in addition to the left–right direction.

Conclusion:

Smaller, adjustable coil elements located next to breast tissue can provide greater SNR than larger, fixed coil elements. A multicoil breast array with high intrinsic SNR and low g‐factors enables high‐quality parallel imaging. J. Magn. Reson. Imaging 2010; 31: 328–338. © 2010 Wiley‐Liss, Inc.     

Signal fluctuations induced by non‐T1‐related confounds in variable TR fMRI experiments

Purpose

To assess and model signal fluctuations induced by non‐T₁‐related confounds in variable repetition time (TR) functional magnetic resonance imaging (fMRI) and to develop a compensation procedure to correct for the non‐T₁‐related artifacts.

Materials and Methods

Radiofrequency disabled volume gradient sequences were effected at variable offsets between actual image acquisitions, enabling perturbation of the measurement system without perturbing longitudinal magnetization, allowing the study of non‐T₁‐related confounds that may arise in variable TR experiments. Three imaging sessions utilizing a daily quality assurance (DQA) phantom were conducted to assess the signal fluctuations, which were then modeled as a second‐order system. A modified projection procedure was implemented to correct for signal fluctuations arising from non‐T₁‐related confounds, and statistical analysis was performed to assess the significance of the artifacts with and without compensation.

Results

Assessment using phantom data reveals that the signal fluctuations induced by non‐T₁‐related confounds was consistent in shape across the phantom and well‐modeled by a second‐order system. The phantom exhibited significant spurious detections (at P < 0.01) almost uniformly across the central slices of the phantom.

Conclusion

Second‐order system modeling and compensation of non‐T₁‐related confounds achieves significant reduction of spurious detection of fMRI activity in a phantom. J. Magn. Reson. Imaging 2009;29:1234–1239. © 2009 Wiley‐Liss, Inc.     

Phase and amplitude correction for multi‐echo water–fat separation with bipolar acquisitions

Purpose:

To address phase and amplitude errors for multi‐point water–fat separation with “bipolar” acquisitions, which efficiently collect all echoes with alternating read‐out gradient polarities in one repetition.

Materials and Methods:

With the bipolar acquisitions, eddy currents and other system nonidealities can induce inconsistent phase errors between echoes, disrupting water–fat separation. Previous studies have addressed phase correction in the read‐out direction. However, the bipolar acquisitions may be subject to spatially high order phase errors as well as an amplitude modulation in the read‐out direction. A method to correct for the 2D phase and amplitude errors is introduced. Low resolution reference data with reversed gradient polarities are collected. From the pair of low‐resolution data collected with opposite gradient polarities, the two‐dimensional phase errors are estimated and corrected. The pair of data are then combined for water–fat separation.

Results:

We demonstrate that the proposed method can effectively remove the high order errors with phantom and in vivo experiments, including obliquely oriented scans.

Conclusion:

For bipolar multi‐echo acquisitions, uniform water–fat separation can be achieved by removing high order phase errors with the proposed method. J. Magn. Reson. Imaging 2010;31:1264–1271. © 2010 Wiley‐Liss, Inc.     

Comparison of intravascular and extracellular contrast media for absolute quantification of myocardial rest-perfusion using high-resolution MRI

Purpose:

To use the contrast agent gadofosveset for absolute quantification of myocardial perfusion and compare it with gadobenate dimeglumine (Gd‐BOPTA) using a high‐resolution generalized autocalibrating partially parallel acquisition (GRAPPA) sequence.

Materials and Methods:

Ten healthy volunteers were examined twice at two different dates with a first‐pass perfusion examination at rest using prebolus technique. We used a 1.5 T scanner and a 32 channel heart‐array coil with a steady‐state free precession (SSFP) true fast imaging with steady state precession (trueFISP) GRAPPA sequence (acceleration‐factor 3). Manual delineation of the myocardial contours was performed and absolute quantification was performed after baseline and contamination correction. At the first appointment, 1cc/4cc of the extracellular contrast agent Gd‐BOPTA were administered, on the second date, 1cc/4cc of the blood pool contrast agent (CA) gadofosveset. At each date the examination was repeated after a 15‐minute time interval.

Results:

Using gadofosveset perfusion the value (in cc/g/min) at rest was 0.66 ± 0.25 (mean ± standard deviation) for the first, and 0.55 ± 0.24 for the second CA application; for Gd‐BOPTA it was 0.62 ± 0.25 and 0.45 ± 0.23. No significant difference was found between the acquired perfusion values. The apparent mean residence time in the myocardium was 23 seconds for gadofosveset and 19.5 seconds for Gd‐BOPTA. Neither signal‐to‐noise ratio (SNR) nor subjectively rated image contrast showed a significant difference.

Conclusion:

The application of gadofosveset for an absolute quantification of myocardial perfusion is possible. Yet the acquired perfusion values show no significant differences to those determined with Gd‐BOPTA, maintained the same SNR and comparable perfusion values, and did not picture the expected concentration time‐course for an intravasal CA in the first pass. J. Magn. Reson. Imaging 2011;33:1047–1051. © 2011 Wiley‐Liss, Inc.     

Objective and subjective evaluation of adaptive speech enhancement methods for functional MRI

Purpose:

To recover speech corrupted by functional magnetic resonance imaging (fMRI) acoustic noise using two‐channel adaptive speech enhancement techniques.

Materials and Methods:

Speech corrupted by noise generated from a 3 T MRI scanner was recorded using diffuse‐field microphones and a data acquisition board. Multiband and subband adaptive speech enhancement methods are used to recover the speech signal from the recordings. Normalized least mean squares (NLMS) algorithm was used for updating the filter coefficients in each band.

Results:

The methods are successful in enhancing the speech quality. They are successful in improving the convergence rate of the adaptive filter. Multiband and subband methods have a similar performance in terms of noise reduction and in the subjective tests. The subband method introduces less speech distortion compared to the multiband method. The subband method requires a lower number of computations per sample.

Conclusion:

Adaptive speech enhancement techniques are effective in reducing fMRI background noise in the recordings. Based on the analysis, we conclude that subband‐based methods are more suited for enhancing speech corrupted by fMRI noise. J. Magn. Reson. Imaging 2010;31:46–55. © 2009 Wiley‐Liss, Inc.     

128-channel body MRI with a flexible high-density receiver-coil array

Purpose

To determine whether the promise of high‐density many‐coil MRI receiver arrays for enabling highly accelerated parallel imaging can be realized in practice.

Materials and Methods

A 128‐channel body receiver‐coil array and custom MRI system were developed. The array comprises two clamshells containing 64 coils each, with the posterior array built to maximize signal‐to‐noise ratio (SNR) and the anterior array design incorporating considerations of weight and flexibility as well. Phantom imaging and human body imaging were performed using a variety of reduction factors and 2D and 3D pulse sequences.

Results

The ratio of SNR relative to a 32‐element array of similar footprint was 1.03 in the center of an elliptical loading phantom and 1.7 on average in the outer regions. Maximum g‐factors dropped from 5.5 (for 32 channels) to 2.0 (for 128 channels) for 4 × 4 acceleration and from 25 to 3.3 for 5 × 5 acceleration. Residual aliasing artifacts for a right/left (R/L) reduction factor of 8 in human body imaging were significantly reduced relative to the 32‐channel array.

Conclusion

MRI with a large number of receiver channels enables significantly higher acceleration factors for parallel imaging and improved SNR, provided losses from the coils and electronics are kept negligible. J. Magn. Reson. Imaging 2008;28:1219–1225. © 2008 Wiley‐Liss, Inc.     

Diffusion tensor imaging (DTI) with retrospective motion correction for large-scale pediatric imaging

Purpose:

To develop and implement a clinical DTI technique suitable for the pediatric setting that retrospectively corrects for large motion without the need for rescanning and/or reacquisition strategies, and to deliver high‐quality DTI images (both in the presence and absence of large motion) using procedures that reduce image noise and artifacts.

Materials and Methods:

We implemented an in‐house built generalized autocalibrating partially parallel acquisitions (GRAPPA)‐accelerated diffusion tensor (DT) echo‐planar imaging (EPI) sequence at 1.5T and 3T on 1600 patients between 1 month and 18 years old. To reconstruct the data, we developed a fully automated tailored reconstruction software that selects the best GRAPPA and ghost calibration weights; does 3D rigid‐body realignment with importance weighting; and employs phase correction and complex averaging to lower Rician noise and reduce phase artifacts. For select cases we investigated the use of an additional volume rejection criterion and b‐matrix correction for large motion.

Results:

The DTI image reconstruction procedures developed here were extremely robust in correcting for motion, failing on only three subjects, while providing the radiologists high‐quality data for routine evaluation.

Conclusion:

This work suggests that, apart from the rare instance of continuous motion throughout the scan, high‐quality DTI brain data can be acquired using our proposed integrated sequence and reconstruction that uses a retrospective approach to motion correction. In addition, we demonstrate a substantial improvement in overall image quality by combining phase correction with complex averaging, which reduces the Rician noise that biases noisy data. J. Magn. Reson. Imaging 2012;36:961–971. © 2012 Wiley Periodicals, Inc.     

Real‐time noise cancellation for speech acquired in interactive functional magnetic resonance imaging studies

Purpose:

To present online scanner noise cancellation for speech acquired in functional magnetic resonance imaging (fMRI) studies.

Materials and Methods:

An online active noise cancellation method for speech acquired in fMRI studies was developed. The approach consists of two automated steps: 1) creation of an MR noise template in a short “test” fMRI scan; 2) application of the template for automatic recognition and subtraction of the MR noise from the acquired microphone signal during an fMRI study. The method was applied in an experimental paradigm where a subject and an investigator communicated in an interactive verbal generation task during fMRI.

Results:

By applying online active noise cancellation, the quality of the subject's speech was substantially improved. The present approach was found to be flexible, reliable, and easy to implement, providing a method for fMRI studies that investigate the neural correlates of interactive speech communication.

Conclusion:

Using online noise cancellation it is possible to improve the quality of acquired speech in fMRI. This approach may be recommended for interactive fMRI studies. J. Magn. Reson. Imaging 2010;32:705–713. © 2010 Wiley‐Liss, Inc.     

Reproducibility of single-subject fMRI language mapping with AMPLE normalization

Purpose:

To evaluate the reproducibility of presurgical functional MRI (fMRI) language mapping based on test–retest scans, comparing traditional activation t‐maps to relative activation maps normalized by activation mapping as percentage of local excitation (AMPLE).

Materials and Methods:

Language fMRI scans were performed by 12 healthy volunteer subjects undergoing a standard clinical presurgical mapping protocol in multiple independent scan sessions. Objective relative AMPLE activation maps were generated automatically by normalizing statistical t‐value maps to the local peak activation amplitude within each functional brain region. The spatial distribution of activation was quantified and compared across mapping algorithms, subjects, scanners, and pulse sequences.

Results:

The spatial distribution of traditional blood oxygen level‐dependent (BOLD) t‐value statistical activation maps was highly variable in test–retest scans of single subjects, whereas AMPLE normalized maps were highly reproducible in terms of the location, hemispheric laterality, and spatial extent of relative activation. AMPLE map reproducibility was good regardless of scanner, field strength, or pulse sequence used, but reproducibility was best for scans acquired on the same scanner using the same pulse sequence.

Conclusion:

Reproducibility of the spatial pattern of BOLD activation makes relative amplitude fMRI mapping a useful normalization tool for clinical imaging of language function, where reproducibility and quantitative measurements are critical concerns. J. Magn. Reson. Imaging 2012;36:569–580. © 2012 Wiley Periodicals, Inc.     

Correction of eddy current distortions in high angular resolution diffusion imaging

Purpose:

To correct distortions caused by eddy currents induced by large diffusion gradients during high angular resolution diffusion imaging without any auxiliary reference scans.

Materials and Methods:

Image distortion parameters were obtained by image coregistration, performed only between diffusion‐weighted images with close diffusion gradient orientations. A linear model that describes distortion parameters (translation, scale, and shear) as a function of diffusion gradient directions was numerically computed to allow individualized distortion correction for every diffusion‐weighted image.

Results:

The assumptions of the algorithm were successfully verified in a series of experiments on phantom and human scans. Application of the proposed algorithm in high angular resolution diffusion images markedly reduced eddy current distortions when compared to results obtained with previously published methods.

Conclusion:

The method can correct eddy current artifacts in the high angular resolution diffusion images, and it avoids the problematic procedure of cross‐correlating images with significantly different contrasts resulting from very different gradient orientations or strengths. J. Magn. Reson. Imaging 2013;37:1460–1467. © 2012 Wiley Periodicals, Inc.