Self-navigated prospective motion correction for 3D-EPI acquisition.

PURPOSE: Although 3D EPI is more susceptible to motion artifacts than 2D EPI, it presents some benefits for functional MRI, including the absence of spin-history artifacts, greater potential for parallel imaging acceleration, and better functional sensitivity in high-resolution imaging. Here we present a self-navigated 3D-EPI sequence suitable for prospective motion-corrected functional MRI without additional hardware or pulses. METHODS: For each volume acquisition, the first 24 of the 52 partitions being acquired are accumulated to a new feedback block that was added to the image reconstruction pipeline. After zero-filling the remaining partitions, the feedback block constructs a volumetric self-navigator (vSNav), co-registers it to the reference vSNav acquired during the first volume acquisition, and sends motion estimates to the sequence. The sequence then updates its FOV and acquires subsequent partitions with the adjusted FOV, until the next update is received. The sequence was validated without and with intentional motion in phantom and in vivo on a 3T Skyra. RESULTS: For phantom scans, the FOV was updated 0.704 s after acquisition of the vSNav partitions, and for in vivo scans after 0.768 s. Both phantom and in vivo data demonstrated stable motion estimates in the absence of motion. For in vivo acquisitions, prospective head-pose estimates using the vSNav's and retrospective estimates with FLIRT (FMRIB's Linear Image Registration Tool) agreed to within 0.23 mm (< 10% of the slice thickness) and 0.14° in all directions. CONCLUSION: Depending when motion occurs during a volume acquisition, the proposed method fully corrects the FOV and recovers image quality within one volume acquisition.

Real-time measurement and correction of both B0 changes and subject motion in diffusion tensor imaging using a double volumetric navigated (DvNav) sequence.

Diffusion tensor imaging (DTI) requires a set of diffusion weighted measurements in order to acquire enough information to characterize local structure. The MRI scanner automatically performs a shimming process by acquiring a field map before the start of a DTI scan. Changes in B0, which can occur throughout the DTI acquisition due to several factors (including heating of the iron shim coils or subject motion), cause significant signal distortions that result in warped diffusion tensor (DT) parameter estimates. In this work we introduce a novel technique to simultaneously measure, report and correct in real time subject motion and changes in B0 field homogeneity, both in and through the imaging plane. This is achieved using double volumetric navigators (DvNav), i.e. a pair of 3D EPI acquisitions, interleaved with the DTI pulse sequence. Changes in the B0 field are evaluated in terms of zero-order (frequency) and first-order (linear gradients) shim. The ability of the DvNav to accurately estimate the shim parameters was first validated in a water phantom. Two healthy subjects were scanned both in the presence and absence of motion using standard, motion corrected (single navigator, vNav), and DvNav DTI sequences. The difference in performance between the proposed 3D EPI field maps and the standard 3D gradient echo field maps of the MRI scanner was also evaluated in a phantom and two healthy subjects. The DvNav sequence was shown to accurately measure and correct changes in B0 following manual adjustments of the scanner's central frequency and the linear shim gradients. Compared to other methods, the DvNav produced DTI results that showed greater spatial overlap with anatomical references, particularly in scans with subject motion. This is largely due to the ability of the DvNav system to correct shim changes and subject motion between each volume acquisition, thus reducing shear distortion.

Assessing the performance of different DTI motion correction strategies in the presence of EPI distortion correction.

Diffusion tensor imaging (DTI) is susceptible to several artifacts due to eddy currents, echo planar imaging (EPI) distortion and subject motion. While several techniques correct for individual distortion effects, no optimal combination of DTI acquisition and processing has been determined. Here, the effects of several motion correction techniques are investigated while also correcting for EPI distortion: prospective correction, using navigation; retrospective correction, using two different popular packages (FSL and TORTOISE); and the combination of both methods. Data from a pediatric group that exhibited incidental motion in varying degrees are analyzed. Comparisons are carried while implementing eddy current and EPI distortion correction. DTI parameter distributions, white matter (WM) maps and probabilistic tractography are examined. The importance of prospective correction during data acquisition is demonstrated. In contrast to some previous studies, results also show that the inclusion of retrospective processing also improved ellipsoid fits and both the sensitivity and specificity of group tractographic results, even for navigated data. Matches with anatomical WM maps are highest throughout the brain for data that have been both navigated and processed using TORTOISE. The inclusion of both prospective and retrospective motion correction with EPI distortion correction is important for DTI analysis, particularly when studying subject populations that are prone to motion. Hum Brain Mapp 37:4405-4424, 2016. © 2016 Wiley Periodicals, Inc.

Volumetric navigated MEGA-SPECIAL for real-time motion and shim corrected GABA editing.

Mescher-Garwood (MEGA) editing with spin echo full intensity acquired localization (MEGA-SPECIAL, MSpc) is a technique to acquire γ-aminobutyric acid (GABA) without macromolecule (MM) contamination at a TE of 68 ms. However, due to the requirement of multiple shot-localization, it is often susceptible to subject motion and B0 inhomogeneity. A method is presented for real-time shim and motion correction (ShMoCo) using volumetric navigators to correct for motion and motion-related B0 inhomogeneity during MSpc acquisition. A phantom experiment demonstrates that ShMoCo restores the GABA peak and improves spectral quality in the presence of motion and zero- and first-order shim changes. The ShMoCo scans were validated in three subjects who performed up-down and left-right head rotations. Qualitative assessment of these scans indicates effective reduction of subtraction artefacts and well edited GABA peaks, while quantitative analysis indicates superior fitting and spectral quality relative to scans with no correction. Copyright © 2015 John Wiley & Sons, Ltd.

Motion artifact reduction in pediatric diffusion tensor imaging using fast prospective correction.

PURPOSE: To evaluate the patterns of head motion in scans of young children and to examine the influence of corrective techniques, both qualitatively and quantitatively. We investigate changes that both retrospective (with and without diffusion table reorientation) and prospective (implemented with a short navigator sequence) motion correction induce in the resulting diffusion tensor measures. MATERIALS AND METHODS: Eighteen pediatric subjects (aged 5-6 years) were scanned using 1) a twice-refocused, 2D diffusion pulse sequence, 2) a prospectively motion-corrected, navigated diffusion sequence with reacquisition of a maximum of five corrupted diffusion volumes, and 3) a T1 -weighted structural image. Mean fractional anisotropy (FA) values in white and gray matter regions, as well as tractography in the brainstem and projection fibers, were evaluated to assess differences arising from retrospective (via FLIRT in FSL) and prospective motion correction. In addition to human scans, a stationary phantom was also used for further evaluation. RESULTS: In several white and gray matter regions retrospective correction led to significantly (P < 0.05) reduced FA means and altered distributions compared to the navigated sequence. Spurious tractographic changes in the retrospectively corrected data were also observed in subject data, as well as in phantom and simulated data. CONCLUSION: Due to the heterogeneity of brain structures and the comparatively low resolution (∼2 mm) of diffusion data using 2D single shot sequencing, retrospective motion correction is susceptible to distortion from partial voluming. These changes often negatively bias diffusion tensor imaging parameters. Prospective motion correction was shown to produce smaller changes.

Volumetric navigators for real-time motion correction in diffusion tensor imaging.

Prospective motion correction methods using an optical system, diffusion-weighted prospective acquisition correction, or a free induction decay navigator have recently been applied to correct for motion in diffusion tensor imaging. These methods have some limitations and drawbacks. This article describes a novel technique using a three-dimensional-echo planar imaging navigator, of which the contrast is independent of the b-value, to perform prospective motion correction in diffusion weighted images, without having to reacquire volumes during which motion occurred, unless motion exceeded some preset thresholds. Water phantom and human brain data were acquired using the standard and navigated diffusion sequences, and the mean and whole brain histogram of the fractional anisotropy and mean diffusivity were analyzed. Our results show that adding the navigator does not influence the diffusion sequence. With head motion, the whole brain histogram-fractional anisotropy shows a shift toward lower anisotropy with a significant decrease in both the mean fractional anisotropy and the fractional anisotropy histogram peak location (P<0.01), whereas the whole brain histogram-mean diffusivity shows a shift toward higher diffusivity with a significant increase in the mean diffusivity (P<0.01), even after retrospective motion correction. These changes in the mean and the shape of the histograms are recovered substantially in the prospective motion corrected data acquired using the navigated sequence.

Early Antiretroviral Therapy in HIV-Infected Children Is Associated with Diffuse White Matter Structural Abnormality and Corpus Callosum Sparing.

BACKGROUND AND PURPOSE: Fractional anisotropy in the frontal white matter, corpus callosum, and internal capsule is abnormal in human immunodeficiency virus-positive (HIV+) adults. We describe the distribution and nature of white matter abnormalities in a cohort of children who started antiretroviral therapy within the first year of life and the benefit of early treatment by using DTI measures (fractional anisotropy and mean, axial, and radial diffusion). MATERIALS AND METHODS: DTI was performed on children in a neurodevelopmental substudy from the Children with HIV Early Antiretroviral trial. Voxel-based group comparisons were obtained to determine regions where fractional anisotropy and mean diffusion differed between HIV+ and uninfected children. Associations of DTI parameters with the timing of antiretroviral therapy initiation were examined. RESULTS: Thirty-nine HIV+ children (15 boys; mean age, 5.4 years) and 13 controls (5 boys; mean age, 5.7 years) were scanned. Two clusters with lower fractional anisotropy and 7 clusters with increased mean diffusion were identified in the HIV+ group, with symmetric distribution predominantly due to increased radial diffusion, suggestive of decreased myelination. Corticospinal tracts rather than the corpus callosum were predominantly involved. Children on early-interrupted antiretroviral therapy had lower fractional anisotropy compared with those receiving continuous treatment. CONCLUSIONS: HIV+ children at 5 years of age have white matter abnormalities measured by fractional anisotropy, despite early antiretroviral therapy, suggesting that early antiretroviral therapy does not fully protect the white matter from either peripartum or in utero infection. In contrast to adults, the corticospinal tracts are predominantly involved rather than the corpus callosum, possibly due to early antiretroviral therapy. Continuous early antiretroviral therapy can limit white matter damage.