Comprehensive approach for correction of motion and distortion in diffusion-weighted MRI

Uncinate fasciculus connectivity in patients with psychogenic nonepileptic seizures: A preliminary diffusion tensor tractography study

The amygdala, hippocampus, and medial prefrontal cortex are limbic brain regions connected by the uncinate fasciculus (UF) and implicated in emotion regulation. The aim of this study was to assess the connectivity characteristics of the UF in patients with psychogenic nonepileptic seizures (PNES) and matched healthy controls. We hypothesized that white matter connectivity of the UF in patients with PNES would differ from that in healthy controls. Eight patients with PNES and eight age- and sex-matched healthy controls underwent 3T MRI and 32-direction diffusion tensor imaging (DTI). Computation of DTI indices including fractional anisotropy (FA) and diffusion tensor tractography was performed. Two regions of interest were defined to manually trace the UF in each hemisphere for each subject. Fractional anisotropy and the number of reconstructed streamlines for the left and right hemispheres of the UF and the degree of asymmetry for each measure were compared between groups. Correlations between UF measures and clinical variables were also performed. Patients with PNES exhibited a significantly greater number of UF streamlines in the right hemisphere tract than in the left hemisphere (p=0.031), with such difference not observed in controls (p=0.81). This was reflected in a significant group difference in the asymmetry index (AI) for the number of streamlines, with more rightward asymmetry in patients with PNES (p=0.021). Average FA of the UF was similar between groups and between hemispheres for each group (all p>0.05). Age at illness onset was correlated with the AI for FA (r=-0.87; p=0.0045). Previously observed differences in emotion processing between controls and patients with PNES may be related to the differences in the rightward asymmetry in the number of UF streamlines in patients with PNES. Age at PNES onset appears to also have a role in the FA asymmetry of the UF. This is the first study to investigate the structural connectivity in these regions involved in emotional regulation in patients with PNES; further research is necessary to clarify the complex relationships between clinical measures and DTI characteristics.

Multimodal neuroimaging based classification of autism spectrum disorder using anatomical, neurochemical, and white matter correlates

Neuroimaging techniques, such as fMRI, structural MRI, diffusion tensor imaging (DTI), and proton magnetic resonance spectroscopy (1H-MRS) have uncovered evidence for widespread functional and anatomical brain abnormalities in autism spectrum disorder (ASD) suggesting it to be a system-wide neural systems disorder. Nevertheless, most previous studies have focused on examining one index of neuropathology through a single neuroimaging modality, and seldom using multiple modalities to examine the same cohort of individuals. The current study aims to bring together multiple brain imaging modalities (structural MRI, DTI, and 1H-MRS) to investigate the neural architecture in the same set of individuals (19 high-functioning adults with ASD and 18 typically developing (TD) peers). Morphometry analysis revealed increased cortical thickness in ASD participants, relative to typical controls, across the left cingulate, left pars opercularis of the inferior frontal gyrus, left inferior temporal cortex, and right precuneus, and reduced cortical thickness in right cuneus and right precentral gyrus. ASD adults also had reduced fractional anisotropy (FA) and increased radial diffusivity (RD) for two clusters on the forceps minor of the corpus callosum, revealed by DTI analyses. 1H-MRS results showed a reduction in the N-acetylaspartate/Creatine ratio in dorsal anterior cingulate cortex (dACC) in ASD participants. A decision tree classification analysis across the three modalities resulted in classification accuracy of 91.9% with FA, RD, and cortical thickness as key predictors. Examining the same cohort of adults with ASD and their TD peers, this study found alterations in cortical thickness, white matter (WM) connectivity, and neurochemical concentration in ASD. These findings underscore the potential for multimodal imaging to better inform on the neural characteristics most relevant to the disorder.

Diffusion-weighted imaging of the liver in patients with chronic liver disease: comparison of monopolar and bipolar diffusion gradients for image quality and lesion detection

OBJECTIVE

The objective of our study was to compare diffusion-weighted imaging (DWI) sequences using a bipolar versus a monopolar single-shot echo-planar imaging (EPI) gradient design for image quality and for lesion detection and characterization in patients with liver disease.

MATERIALS AND METHODS

In this retrospective study, 77 patients with chronic liver disease who underwent MRI including bipolar and monopolar DWI at 1.5 T were assessed. Two independent observers reviewed the DWI studies for image quality and the detection and characterization of liver lesions. The reference standard for diagnosis was established by consensus review of two different observers using imaging characteristics on conventional MRI sequences, lesion stability over time, pathologic correlation, or a combination of these findings. The estimated signal-to-noise ratio (SNR) of liver parenchyma and apparent diffusion coefficients (ADCs) of the liver and lesions were calculated for both sequences. ROC analysis was conducted to evaluate the performance of ADC for the diagnosis of hepatocellular carcinoma (HCC).

RESULTS

Eighty-five lesions, 50 HCCs and 35 benign lesions, were detected with the reference standard in 38 patients. There was equivalent image quality for the bipolar and monopolar sequences (p = 0.24-0.42). The HCC detection rate for observers 1 and 2 was slightly better with bipolar DWI (50.0% and 52.0%, respectively) compared with monopolar DWI (44.0% and 46.0%); however, this difference did not reach statistical significance. The estimated SNR was higher with the monopolar sequence than with the bipolar sequence (p ≤ 0.001). The AUC for the ROC curve was 0.691 for bipolar DWI and 0.649 for monopolar DWI when ADC was used for the characterization of HCC, which is not a statistically significant difference (p = 0.59).

CONCLUSION

The higher estimated SNR yielded by the monopolar DWI sequence did not translate into better HCC detection compared with the bipolar DWI sequence. ADC has a limited role for HCC characterization in patients with liver disease.

Statistical Instability of TBSS Analysis Based on DTI Fitting Algorithm

Voxel-based DTI analysis is an important approach in the comparison of subject groups by detecting and localizing gray and white matter changes in the brain. One of the principal problems for intersubject comparison is the absence of a "gold standard" processing pipeline. As a result, contradictory results may be obtained from identical data using different data processing pipelines, for example, in the data normalization or smoothing procedures. Tract-based spatial statistics (TBSS) shows potential to overcome this problem by automatic detection of white matter changes and decreasing variation in the performed analysis. However, skeleton projection approaches, such as TBSS, critically depend on the accuracy of the diffusion scalar metric estimations. In this work, we demonstrate that the agreement and reliability of TBSS results depend on the applied DTI data processing algorithm. Statistical tests have been performed using two in vivo measured datasets and compared with different implementations of the least squares algorithm. As a result, we recommend repeating TBSS analysis using different fitting algorithms, in particular, using on iteratively-assessed robust estimators, as accurate and more reliable approach in voxel-based analysis, particularly, for TBSS. Repeating TBSS analysis allows one to detect and localize suspicious regions in white matter which were estimated as the regions with significant difference. Finally, we did not find a favorite fitting algorithm (or class of them) which can be marked as more reliable for group comparison.

Test-retest reliability of diffusion measures in cerebral white matter: A multiband diffusion MRI study

BACKGROUND

To investigate the test-retest (TRT) reliability of the diffusion measures in cerebral white matter obtained from the diffusion MRI dataset acquired with multiband acceleration.

METHODS

With the multiband diffusion MRI dataset with two repeated scanning sessions, the TRT reliability of diffusion measures (fractional anisotropy [FA], mean diffusivity [MD], primary diffusivity [PD] and transverse diffusivity [TD]) was investigated through several fully automated analysis methods, including two voxel-level analyses (voxel-based analysis [VBA] and tract-based spatial statistics [TBSS]) and an atlas ROI-based analysis. The reproducibility was assessed by the intra-class correlation coefficient (ICC).

RESULTS

Our results demonstrated moderate to high reproducibility (ICC > 0.4) of diffusion measures from the multiband EPI sequence with different analysis approaches. Across different measures, FA exhibited the highest reproducibility (mean ICC = 0.70), while MD showed the lowest reliability (mean ICC = 0.55) (P = 0.006). Additionally, ICCs varied across different tract ROIs: Commissural tracts showed higher reproducibility than other categories of tracts (projection, association and brainstem), while the brainstem tracts exhibited the poorest reliability (P = 0.004).

CONCLUSION

Our findings suggest a potential utility of the multiband EPI sequence for exploring individual differences of cerebral white matter and provide reference for future white matter studies.

DR-BUDDI (Diffeomorphic Registration for Blip-Up blip-Down Diffusion Imaging) method for correcting echo planar imaging distortions

We propose an echo planar imaging (EPI) distortion correction method (DR-BUDDI), specialized for diffusion MRI, which uses data acquired twice with reversed phase encoding directions, often referred to as blip-up blip-down acquisitions. DR-BUDDI can incorporate information from an undistorted structural MRI and also use diffusion-weighted images (DWI) to guide the registration, improving the quality of the registration in the presence of large deformations and in white matter regions. DR-BUDDI does not require the transformations for correcting blip-up and blip-down images to be the exact inverse of each other. Imposing the theoretical "blip-up blip-down distortion symmetry" may not be appropriate in the presence of common clinical scanning artifacts such as motion, ghosting, Gibbs ringing, vibrations, and low signal-to-noise. The performance of DR-BUDDI is evaluated with several data sets and compared to other existing blip-up blip-down correction approaches. The proposed method is robust and generally outperforms existing approaches. The inclusion of the DWIs in the correction process proves to be important to obtain a reliable correction of distortions in the brain stem. Methods that do not use DWIs may produce a visually appealing correction of the non-diffusion weighted images, but the directionally encoded color maps computed from the tensor reveal an abnormal anatomy of the white matter pathways.

Analysis of the contribution of experimental bias, experimental noise, and inter-subject biological variability on the assessment of developmental trajectories in diffusion MRI studies of the brain

Metrics derived from the diffusion tensor, such as fractional anisotropy (FA) and mean diffusivity (MD) have been used in many studies of postnatal brain development. A common finding of previous studies is that these tensor-derived measures vary widely even in healthy populations. This variability can be due to inherent inter-individual biological differences as well as experimental noise. Moreover, when comparing different studies, additional variability can be introduced by different acquisition protocols. In this study we examined scans of 61 individuals (aged 4-22 years) from the NIH MRI study of normal brain development. Two scans were collected with different protocols (low and high resolution). Our goal was to separate the contributions of biological variability and experimental noise to the overall measured variance, as well as to assess potential systematic effects related to the use of different protocols. We analyzed FA and MD in seventeen regions of interest. We found that biological variability for both FA and MD varies widely across brain regions; biological variability is highest for FA in the lateral part of the splenium and body of the corpus callosum along with the cingulum and the superior longitudinal fasciculus, and for MD in the optic radiations and the lateral part of the splenium. These regions with high inter-individual biological variability are the most likely candidates for assessing genetic and environmental effects in the developing brain. With respect to protocol-related effects, the lower resolution acquisition resulted in higher MD and lower FA values for the majority of regions compared with the higher resolution protocol. However, the majority of the regions did not show any age-protocol interaction, indicating similar trajectories were obtained irrespective of the protocol used.

Subject-Motion Correction in HARDI Acquisitions: Choices and Consequences

Diffusion-weighted imaging (DWI) is known to be prone to artifacts related to motion originating from subject movement, cardiac pulsation, and breathing, but also to mechanical issues such as table vibrations. Given the necessity for rigorous quality control and motion correction, users are often left to use simple heuristics to select correction schemes, which involves simple qualitative viewing of the set of DWI data, or the selection of transformation parameter thresholds for detection of motion outliers. The scientific community offers strong theoretical and experimental work on noise reduction and orientation distribution function (ODF) reconstruction techniques for HARDI data, where post-acquisition motion correction is widely performed, e.g., using the open-source DTIprep software (1), FSL (the FMRIB Software Library) (2), or TORTOISE (3). Nonetheless, effects and consequences of the selection of motion correction schemes on the final analysis, and the eventual risk of introducing confounding factors when comparing populations, are much less known and far beyond simple intuitive guessing. Hence, standard users lack clear guidelines and recommendations in practical settings. This paper reports a comprehensive evaluation framework to systematically assess the outcome of different motion correction choices commonly used by the scientific community on different DWI-derived measures. We make use of human brain HARDI data from a well-controlled motion experiment to simulate various degrees of motion corruption and noise contamination. Choices for correction include exclusion/scrubbing or registration of motion corrupted directions with different choices of interpolation, as well as the option of interpolation of all directions. The comparative evaluation is based on a study of the impact of motion correction using four metrics that quantify (1) similarity of fiber orientation distribution functions (fODFs), (2) deviation of local fiber orientations, (3) global brain connectivity via graph diffusion distance (GDD), and (4) the reproducibility of prominent and anatomically defined fiber tracts. Effects of various motion correction choices are systematically explored and illustrated, leading to a general conclusion of discouraging users from setting ad hoc thresholds on the estimated motion parameters beyond which volumes are claimed to be corrupted.

Efficacy of distortion correction on diffusion imaging: comparison of FSL eddy and eddy_correct using 30 and 60 directions diffusion encoding

Diffusion imaging is a unique noninvasive tool to detect brain white matter trajectory and integrity in vivo. However, this technique suffers from spatial distortion and signal pileup or dropout originating from local susceptibility gradients and eddy currents. Although there are several methods to mitigate these problems, most techniques can be applicable either to susceptibility or eddy-current induced distortion alone with a few exceptions. The present study compared the correction efficiency of FSL tools, "eddy_correct" and the combination of "eddy" and "topup" in terms of diffusion-derived fractional anisotropy (FA). The brain diffusion images were acquired from 10 healthy subjects using 30 and 60 directions encoding schemes based on the electrostatic repulsive forces. For the 30 directions encoding, 2 sets of diffusion images were acquired with the same parameters, except for the phase-encode blips which had opposing polarities along the anteroposterior direction. For the 60 directions encoding, non-diffusion-weighted and diffusion-weighted images were obtained with forward phase-encoding blips and non-diffusion-weighted images with the same parameter, except for the phase-encode blips, which had opposing polarities. FA images without and with distortion correction were compared in a voxel-wise manner with tract-based spatial statistics. We showed that images corrected with eddy and topup possessed higher FA values than images uncorrected and corrected with eddy_correct with trilinear (FSL default setting) or spline interpolation in most white matter skeletons, using both encoding schemes. Furthermore, the 60 directions encoding scheme was superior as measured by increased FA values to the 30 directions encoding scheme, despite comparable acquisition time. This study supports the combination of eddy and topup as a superior correction tool in diffusion imaging rather than the eddy_correct tool, especially with trilinear interpolation, using 60 directions encoding scheme.

High-resolution diffusion tensor imaging of the human kidneys using a free-breathing, multi-slice, targeted field of view approach

Fractional anisotropy (FA) obtained by diffusion tensor imaging (DTI) can be used to image the kidneys without any contrast media. FA of the medulla has been shown to correlate with kidney function. It is expected that higher spatial resolution would improve the depiction of small structures within the kidney. However, the achievement of high spatial resolution in renal DTI remains challenging as a result of respiratory motion and susceptibility to diffusion imaging artefacts. In this study, a targeted field of view (TFOV) method was used to obtain high-resolution FA maps and colour-coded diffusion tensor orientations, together with measures of the medullary and cortical FA, in 12 healthy subjects. Subjects were scanned with two implementations (dual and single kidney) of a TFOV DTI method. DTI scans were performed during free breathing with a navigator-triggered sequence. Results showed high consistency in the greyscale FA, colour-coded FA and diffusion tensors across subjects and between dual- and single-kidney scans, which have in-plane voxel sizes of 2 × 2 mm(2) and 1.2 × 1.2 mm(2) , respectively. The ability to acquire multiple contiguous slices allowed the medulla and cortical FA to be quantified over the entire kidney volume. The mean medulla and cortical FA values were 0.38 ± 0.017 and 0.21 ± 0.019, respectively, for the dual-kidney scan, and 0.35 ± 0.032 and 0.20 ± 0.014, respectively, for the single-kidney scan. The mean FA between the medulla and cortex was significantly different (p < 0.001) for both dual- and single-kidney implementations. High-spatial-resolution DTI shows promise for improving the characterization and non-invasive assessment of kidney function.

White matter consequences of retinal receptor and ganglion cell damage

PURPOSE

Patients with Leber hereditary optic neuropathy (LHON) and cone-rod dystrophy (CRD) have central vision loss; but CRD damages the retinal photoreceptor layer, and LHON damages the retinal ganglion cell (RGC) layer. Using diffusion MRI, we measured how these two types of retinal damage affect the optic tract (ganglion cell axons) and optic radiation (geniculo-striate axons).

METHODS

Adult onset CRD (n = 5), LHON (n = 6), and healthy controls (n = 14) participated in the study. We used probabilistic fiber tractography to identify the optic tract and the optic radiation. We compared axial and radial diffusivity at many positions along the optic tract and the optic radiation.

RESULTS

In both types of patients, diffusion measures within the optic tract and the optic radiation differ from controls. The optic tract change is principally a decrease in axial diffusivity; the optic radiation change is principally an increase in radial diffusivity.

CONCLUSIONS

Both photoreceptor layer (CRD) and retinal ganglion cell (LHON) retinal disease causes substantial change in the visual white matter. These changes can be measured using diffusion MRI. The diffusion changes measured in the optic tract and the optic radiation differ, suggesting that they are caused by different biological mechanisms.

A DTI-based tractography study of effects on brain structure associated with prenatal alcohol exposure in newborns

Prenatal alcohol exposure (PAE) is known to have severe, long-term consequences for brain and behavioral development already detectable in infancy and childhood. Resulting features of fetal alcohol spectrum disorders include cognitive and behavioral effects, as well as facial anomalies and growth deficits. Diffusion tensor imaging (DTI) and tractography were used to analyze white matter (WM) development in 11 newborns (age since conception <45 weeks) whose mothers were recruited during pregnancy. Comparisons were made with nine age-matched controls born to abstainers or light drinkers from the same Cape Coloured (mixed ancestry) community near Cape Town, South Africa. DTI parameters, T1 relaxation time, proton density and volumes were used to quantify and investigate group differences in WM in the newborn brains. Probabilistic tractography was used to estimate and to delineate similar tract locations among the subjects for transcallosal pathways, cortico-spinal projection fibers, and cortico-cortical association fibers. In each of these WM networks, the axial diffusivity was the parameter that showed the strongest association with maternal drinking. The strongest relations were observed in medial and inferior WM, regions in which the myelination process typically begins. In contrast to studies of older individuals with PAE, fractional anisotropy did not exhibit a consistent and significant relation with alcohol exposure. To our knowledge, this is the first DTI-tractography study of prenatally alcohol exposed newborns.

Evaluating contextual processing in diffusion MRI: application to optic radiation reconstruction for epilepsy surgery

Diffusion MRI and tractography allow for investigation of the architectural configuration of white matter in vivo, offering new avenues for applications like presurgical planning. Despite the promising outlook, there are many pitfalls that complicate its use for (clinical) application. Amongst these are inaccuracies in the geometry of the diffusion profiles on which tractography is based, and poor alignment with neighboring profiles. Recently developed contextual processing techniques, including enhancement and well-posed geometric sharpening, have shown to result in sharper and better aligned diffusion profiles. However, the research that has been conducted up to now is mainly of theoretical nature, and so far these techniques have only been evaluated by visual inspection of the diffusion profiles. In this work, the method is evaluated in a clinically relevant application: the reconstruction of the optic radiation for epilepsy surgery. For this evaluation we have developed a framework in which we incorporate a novel scoring procedure for individual pathways. We demonstrate that, using enhancement and sharpening, the extraction of an anatomically plausible reconstruction of the optic radiation from a large amount of probabilistic pathways is greatly improved in three healthy controls, where currently used methods fail to do so. Furthermore, challenging reconstructions of the optic radiation in three epilepsy surgery candidates with extensive brain lesions demonstrate that it is beneficial to integrate these methods in surgical planning.

Diffusion tensor tractography-based analysis of the cingulum: clinical utility and findings in traumatic brain injury with chronic sequels

INTRODUCTION

To evaluate the clinical utility of quantitative diffusion tensor tractography (DTT) and tractography-based core analysis (TBCA) of the cingulum by defining the reproducibility, normal values, and findings in traumatic brain injury (TBI).

METHODS

Eighty patients with TBI and normal routine MRI and 78 controls underwent MRI at 3T. To determine reproducibility, 12 subjects were scanned twice. Superior (SC) and inferior (IC) cingulum were analyzed separately by DTT (fractional anisotropy (FA) thresholds 0.15 and 0.30). TBCA was performed from volumes defined by tractography with gradually changed FA thresholds. FA values were correlated with clinical and neuropsychological data.

RESULTS

The lowest coefficient of variation was obtained at DTT threshold 0.30 (2.0 and 2.4 % for SC and IC, respectively), but in proportion to standard deviations of normal controls, the reproducibility of TBCA was better in SC and similar to that of DTT in IC. In patients with TBI, volume reduction with loss of peripheral fibers was relatively common; mean FA was mostly normal in these tractograms. The frequency of FA reductions (>2 SD) was in DTT smaller than in TBCA, in which FA decrease was present in 42 (13.1 %) of the 320 measurements. Central FA values in SC predicted visuoperceptual ability, and those in left IC predicted cognitive speed, language, and communication ability (p < 0.05).

CONCLUSION

Tractography-based measurements have sufficient reproducibility for demonstration of severe abnormalities of the cingulum. TBCA is preferential for clinical FA analysis, because it measures corresponding areas in patients and controls without inaccuracies due to trauma-induced structural changes.

Altered corticomotor-cerebellar integrity in young ataxia telangiectasia patients

Magnetic resonance imaging (MRI) research in identifying altered brain structure and function in ataxia-telangiectasia, an autosomal recessive neurodegenerative disorder, is limited. Diffusion-weighted MRI were obtained from 11 ataxia telangiectasia patients (age range, 7-22 years; mean, 12 years) and 11 typically developing age-matched participants (age range, 8-23 years; mean, 13 years). Gray matter volume alterations in patients were compared with those of healthy controls using voxel-based morphometry, whereas tract-based spatial statistics was employed to elucidate white matter microstructure differences between groups. White matter microstructure was probed using quantitative fractional anisotropy and mean diffusivity measures. Reduced gray matter volume in both cerebellar hemispheres and in the precentral-postcentral gyrus in the left cerebral hemisphere was observed in ataxia telangiectasia patients compared with controls (P < 0.05, corrected for multiple comparisons). A significant reduction in fractional anisotropy in the cerebellar hemispheres, anterior/posterior horns of the medulla, cerebral peduncles, and internal capsule white matter, particularly in the left posterior limb of the internal capsule and corona radiata in the left cerebral hemisphere, was observed in patients compared with controls (P < 0.05). Mean diffusivity differences were observed within the left cerebellar hemisphere and the white matter of the superior lobule of the right cerebellar hemisphere (P < 0.05). Cerebellum-localized gray matter changes are seen in young ataxia telangiectasia patients along with white matter tract degeneration projecting from the cerebellum into corticomotor regions. The lack of cortical involvement may reflect early-stage white matter motor pathway degeneration within young patients.

Characterization and correction of eddy-current artifacts in unipolar and bipolar diffusion sequences using magnetic field monitoring

Diffusion tensor imaging (DTI) of moving organs is gaining increasing attention but robust performance requires sequence modifications and dedicated correction methods to account for system imperfections. In this study, eddy currents in the "unipolar" Stejskal-Tanner and the velocity-compensated "bipolar" spin-echo diffusion sequences were investigated and corrected for using a magnetic field monitoring approach in combination with higher-order image reconstruction. From the field-camera measurements, increased levels of second-order eddy currents were quantified in the unipolar sequence relative to the bipolar diffusion sequence while zeroth and linear orders were found to be similar between both sequences. Second-order image reconstruction based on field-monitoring data resulted in reduced spatial misalignment artifacts and residual displacements of less than 0.43 mm and 0.29 mm (in the unipolar and bipolar sequences, respectively) after second-order eddy-current correction. Results demonstrate the need for second-order correction in unipolar encoding schemes but also show that bipolar sequences benefit from second-order reconstruction to correct for incomplete intrinsic cancellation of eddy-currents.

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.

Cortico-striatal-thalamic network functional connectivity in hemiparkinsonism

Cortico-striatal-thalamic network functional connectivity (FC) and its relationship with levodopa (L-dopa) were investigated in 69 patients with hemiparkinsonism (25 drug-naïve [n-PD] and 44 under stable/optimized dopaminergic treatment [t-PD]) and 27 controls. Relative to controls, n-PD patients showed an increased FC between the left and the right basal ganglia, and a decreased connectivity of the affected caudate nucleus and thalamus with the ipsilateral frontal and insular cortices. Compared with both controls and n-PD patients, t-PD patients showed a decreased FC among the striatal and thalamic regions, and an increased FC between the striatum and temporal cortex, and between the thalamus and several sensorimotor, parietal, temporal, and occipital regions. In both n-PD and t-PD, patients with more severe motor disability had an increased striatal and/or thalamic FC with temporal, parietal, occipital, and cerebellar regions. Cortico-striatal-thalamic functional abnormalities occur in patients with hemiparkinsonism, antecede the onset of the motor symptoms on the opposite body side and are modulated by L-dopa. In patients with hemiparkinsonism, L-dopa is likely to facilitate a compensation of functional abnormalities possibly through an increased thalamic FC.

Diffusion tensor histogram analysis of pediatric diffuse intrinsic pontine glioma

Purpose. To evaluate tumor structure in children with diffuse intrinsic pontine glioma (DIPG) using histogram analyses of mean diffusivity (MD), determine potential treatment and corticosteroid-related effects on MD, and monitor changes in MD distributions over time. Materials and Methods. DTI was performed on a 1.5T GE scanner. Regions of interest included the entire FLAIR-defined tumor. MD data were used to calculate histograms. Patterns in MD distributions were evaluated and fitted using a two-normal mixture model. Treatment-related effects were evaluated using the R² statistic for linear mixed models and Cox proportional hazards models. Results. 12 patients with DIPG underwent one or more DTI exams. MD histogram distributions varied among patients. Over time, histogram peaks became shorter and broader (P = 0.0443). Two-normal mixture fitting revealed large lower curve proportions that were not associated with treatment response or outcome. Corticosteroid use affected MD histograms and was strongly associated with larger, sharper peaks (R² = 0.51, P = 0.0028). Conclusions. MD histograms of pediatric DIPG show significant interpatient and intratumoral differences and quantifiable changes in tumor structure over time. Corticosteroids greatly affected MD and must be considered a confounding factor when interpreting MD results in the context of treatment response.

Assessment of the structural brain network reveals altered connectivity in children with unilateral cerebral palsy due to periventricular white matter lesions

Background

Cerebral palsy (CP) is a term to describe the spectrum of disorders of impaired motor and sensory function caused by a brain lesion occurring early during development. Diffusion MRI and tractography have been shown to be useful in the study of white matter (WM) microstructure in tracts likely to be impacted by the static brain lesion.

Aim

The purpose of this study was to identify WM pathways with altered connectivity in children with unilateral CP caused by periventricular white matter lesions using a whole-brain connectivity approach.

Methods

Data of 50 children with unilateral CP caused by periventricular white matter lesions (5–17 years; manual ability classification system [MACS] I = 25/II = 25) and 17 children with typical development (CTD; 7–16 years) were analysed. Structural and High Angular Resolution Diffusion weighted Images (HARDI; 64 directions, b = 3000 s/mm²) were acquired at 3 T. Connectomes were calculated using whole-brain probabilistic tractography in combination with structural parcellation of the cortex and subcortical structures. Connections with altered fractional anisotropy (FA) in children with unilateral CP compared to CTD were identified using network-based statistics (NBS). The relationship between FA and performance of the impaired hand in bimanual tasks (Assisting Hand Assessment—AHA) was assessed in connections that showed significant differences in FA compared to CTD.

Results

FA was reduced in children with unilateral CP compared to CTD. Seven pathways, including the corticospinal, thalamocortical, and fronto-parietal association pathways were identified simultaneously in children with left and right unilateral CP. There was a positive relationship between performance of the impaired hand in bimanual tasks and FA within the cortico-spinal and thalamo-cortical pathways (r² = 0.16–0.44; p < 0.05).

Conclusion

This study shows that network-based analysis of structural connectivity can identify alterations in FA in unilateral CP, and that these alterations in FA are related to clinical function. Application of this connectome-based analysis to investigate alterations in connectivity following treatment may elucidate the neurological correlates of improved functioning due to intervention.

•

Alterations in FA in children with CP were assessed using the connectome approach.

•

FA is reduced in corticospinal, thalamocortical, and association tracts in CP.

•

Higher FA is associated with better performance in bimanual tasks.

Prevention of motion-induced signal loss in diffusion-weighted echo-planar imaging by dynamic restoration of gradient moments

PURPOSE

Head motion is a significant problem in diffusion-weighted imaging as it may cause signal attenuation due to residual dephasing during strong diffusion encoding gradients even in single-shot acquisitions. Here, we present a new real-time method to prevent motion-induced signal loss in DWI of the brain.

METHODS

The method requires a fast motion tracking system (optical in the current implementation). Two alterations were made to a standard diffusion-weighted echo-planar imaging sequence: first, real-time motion correction ensures that slices are correctly aligned relative to the moving brain. Second, the tracking data are used to calculate the motion-induced gradient moment imbalance which occurs during the diffusion encoding periods, and a brief gradient blip is inserted immediately prior to the signal readout to restore the gradient moment balance.

RESULTS

Phantom experiments show that the direction as well as magnitude of the gradient moment imbalance affects the characteristics of unwanted signal attenuation. In human subjects, the addition of a moment-restoring blip prevented signal loss and improved the reproducibility and reliability of diffusion tensor measures even in the presence of substantial head movements.

CONCLUSION

The method presented can improve robustness for clinical routine scanning in populations that are prone to head movements, such as children and uncooperative adult patients.

Joint fractional segmentation and multi-tensor estimation in diffusion MRI

In this paper we present a novel Bayesian approach for fractional segmentation of white matter tracts and simultaneous estimation of a multi-tensor diffusion model. Our model consists of several white matter tracts, each with a corresponding weight and tensor compartment in each voxel. By incorporating a prior that assumes the tensor fields inside each tract are spatially correlated, we are able to reliably estimate multiple tensor compartments in fiber crossing regions, even with low angular diffusion-weighted imaging (DWI). Our model distinguishes the diffusion compartment associated with each tract, which reduces the effects of partial voluming and achieves more reliable statistics of diffusion measurements. We test our method on synthetic data with known ground truth and show that we can recover the correct volume fractions and tensor compartments. We also demonstrate that the proposed method results in improved segmentation and diffusion measurement statistics on real data in the presence of crossing tracts and partial voluming.

Neurobiological basis of head motion in brain imaging

Individual differences in brain metrics, especially connectivity measured with functional MRI, can correlate with differences in motion during data collection. The assumption has been that motion causes artifactual differences in brain connectivity that must and can be corrected. Here we propose that differences in brain connectivity can also represent a neurobiological trait that predisposes to differences in motion. We support this possibility with an analysis of intra- versus intersubject differences in connectivity comparing high- to low-motion subgroups. Intersubject analysis identified a correlate of head motion consisting of reduced distant functional connectivity primarily in the default network in individuals with high head motion. Similar connectivity differences were not found in analysis of intrasubject data. Instead, this correlate of head motion was a stable property in individuals across time. These findings suggest that motion-associated differences in brain connectivity cannot fully be attributed to motion artifacts but rather also reflect individual variability in functional organization.

A PRELIMINARY STUDY ON THE EFFECT OF MOTION CORRECTION ON HARDI RECONSTRUCTION

Post-acquisition motion correction is widely performed in diffusion-weighted imaging (DWI) to guarantee voxel-wise correspondence between DWIs. Whereas this is primarily motivated to save as many scans as possible if corrupted by motion, users do not fully understand the consequences of different types of interpolation schemes on the final analysis. Nonetheless, interpolation might increase the partial volume effect while not preserving the volume of the diffusion profile, whereas excluding poor DWIs may affect the ability to resolve crossing fibers especially with small separation angles. In this paper, we investigate the effect of interpolating diffusion measurements as well as the elimination of bad directions on the reconstructed fiber orientation diffusion functions and on the estimated fiber orientations. We demonstrate such an effect on synthetic and real HARDI datasets. Our experiments demonstrate that the effect of interpolation is more significant with small fibers separation angles where the exclusion of motion-corrupted directions decreases the ability to resolve such crossing fibers.

Evaluation of hepatic fibrosis using intravoxel incoherent motion in diffusion-weighted liver MRI

OBJECTIVES

To determine whether intravoxel incoherent motion (IVIM)-diffusion-weighted image (DWI)-derived parameters showed better diagnostic performance than the apparent diffusion coefficient (ADC(total)) for the evaluation of hepatic fibrosis (HF).

METHODS

This retrospective study was approved by institutional review board, and informed consent was waived. Fifty-five patients with chronic liver disease who had undergone IVIM-DWI using 8 b-values at 3 T were included. True diffusion coefficient (Dt), pseudo-diffusion coefficient (Dp), perfusion fraction (f), and ADC(total) were calculated. Receiver operating characteristic analysis was performed for all parameters for the HF staging.

RESULTS

All parameters showed a significant correlation with the HF stages (-0.31 to -0.72, P < 0.05). All parameters were significantly higher in F0 to F1 than in F4 (P < 0.05). The Dp showed better performance than the ADC(total) in differentiating significant HF (≥F2) from F0 to F1.

CONCLUSIONS

The IVIM-derived parameters and ADC(total) showed significant correlation with HF. The D p showed better diagnostic performance for differentiating significant HF than did ADC(total).

Functional and structural network impairment in childhood frontal lobe epilepsy

In childhood frontal lobe epilepsy (FLE), cognitive impairment and educational underachievement are serious, well-known co-morbidities. The broad scale of affected cognitive domains suggests wide-spread network disturbances that not only involves, but also extends beyond the frontal lobe. In this study we have investigated whole brain connectional properties of children with FLE in relation to their cognitive impairment and compared them with healthy controls. Functional connectivity (FC) of the networks was derived from dynamic fluctuations of resting state fMRI and structural connectivity (SC) was obtained from fiber tractograms of diffusion weighted MRI. The whole brain network was characterized with graph theoretical metrics and decomposed into modules. Subsequently, the graph metrics and the connectivity within and between modules were related to cognitive performance. Functional network disturbances in FLE were related to increased clustering, increased path length, and stronger modularity compared to healthy controls, which was accompanied by stronger within- and weaker between-module functional connectivity. Although structural path length and clustering appeared normal in children with FLE, structural modularity increased with stronger cognitive impairment. It is concluded that decreased coupling between large-scale functional network modules is a hallmark for impaired cognition in childhood FLE.

Reduced integrity of sensorimotor projections traversing the posterior limb of the internal capsule in children with congenital hemiparesis

There is reduced integrity of corticospinal projections that traverse the posterior limb of the internal capsule (PLIC) in children with unilateral cerebral palsy (CP). It remains unclear whether there are changes in integrity of other projections traversing the PLIC. Forty children with congenital hemiparesis and 15 typically developing children underwent structural and diffusion-weighted MRI. All children with congenital hemiparesis showed lesions to the periventricular white matter. Structural images were parcellated into 34 cortical regions per hemisphere and posterior limb of the internal capsule was identified. PLIC connections to each cortical region were extracted using probabilistic tractography. Differences between hemispheres for each cortical projection (asymmetry index (AI)) and tract microstructure (fractional anisotropy (FA), mean diffusivity (MD)) were assessed. The results showed that 17 children (42.5%) with congenital hemiparesis showed bilateral lesions on structural MRI. Projections to the primary motor cortex (precentral gyrus and paracentral lobule) showed greater asymmetry in unilateral CP group compared to typically developing children and indicate reduced projections on the hemisphere contralateral to the impaired limb (i.e., contralateral hemisphere). Reduced FA and increased MD were also observed for connections with the primary motor cortex, primary sensory cortex (postcentral gyrus) and precuneus on the contralateral hemisphere in children with congenital hemiparesis. Similar changes were observed between children with unilateral and bilateral lesions on structural MRI. Notably, microstructural changes were associated with deficits in both sensory and motor function. The findings further unravel the underlying neuroanatomical correlates of sensorimotor deficits in children with congenital hemiparesis.

A unified approach to diffusion direction sensitive slice registration and 3-D DTI reconstruction from moving fetal brain anatomy

This paper presents an approach to 3-D diffusion tensor image (DTI) reconstruction from multi-slice diffusion weighted (DW) magnetic resonance imaging acquisitions of the moving fetal brain. Motion scatters the slice measurements in the spatial and spherical diffusion domain with respect to the underlying anatomy. Previous image registration techniques have been described to estimate the between slice fetal head motion, allowing the reconstruction of 3D a diffusion estimate on a regular grid using interpolation. We propose Approach to Unified Diffusion Sensitive Slice Alignment and Reconstruction (AUDiSSAR) that explicitly formulates a process for diffusion direction sensitive DW-slice-to-DTI-volume alignment. This also incorporates image resolution modeling to iteratively deconvolve the effects of the imaging point spread function using the multiple views provided by thick slices acquired in different anatomical planes. The algorithm is implemented using a multi-resolution iterative scheme and multiple real and synthetic data are used to evaluate the performance of the technique. An accuracy experiment using synthetically created motion data of an adult head and an experiment using synthetic motion added to sedated fetal monkey dataset show a significant improvement in motion-trajectory estimation compared to current state-of-the-art approaches. The performance of the method is then evaluated on challenging but clinically typical in utero fetal scans of four different human cases, showing improved rendition of cortical anatomy and extraction of white matter tracts. While the experimental work focuses on DTI reconstruction (second-order tensor model), the proposed reconstruction framework can employ any 5-D diffusion volume model that can be represented by the spatial parameterizations of an orientation distribution function.

Automated assessment of the quality of diffusion tensor imaging data using color cast of color-encoded fractional anisotropy images

Diffusion tensor imaging (DTI) data often suffer from artifacts caused by motion. These artifacts are especially severe in DTI data from infants, and implementing tight quality controls is therefore imperative for DTI studies of infants. Currently, routine procedures for quality assurance of DTI data involve the slice-wise visual inspection of color-encoded, fractional anisotropy (CFA) images. Such procedures often yield inconsistent results across different data sets, across different operators who are examining those data sets, and sometimes even across time when the same operator inspects the same data set on two different occasions. We propose a more consistent, reliable, and effective method to evaluate the quality of CFA images automatically using their color cast, which is calculated on the distribution statistics of the 2D histogram in the color space as defined by the International Commission on Illumination (CIE) on lightness and a and b (LAB) for the color-opponent dimensions (also known as the CIELAB color space) of the images. Experimental results using DTI data acquired from neonates verified that this proposed method is rapid and accurate. The method thus provides a new tool for real-time quality assurance for DTI data.

Comparison of the accuracy of PET/CT and PET/MRI spatial registration of multiple metastatic lesions

OBJECTIVE

The purpose of this study was to compare the accuracy of the spatial registration of conventional PET/CT with that of hybrid PET/MRI of patients with FDG-avid metastatic lesions.

SUBJECTS AND METHODS

Thirteen patients with known metastatic lesions underwent FDG PET/CT followed by PET/MRI with a hybrid whole-body system. The inclusion criterion for tumor analysis was spherical or oval FDG-avid tumor clearly identified with both CT and MRI. The spatial coordinates (x, y, z) of the visually estimated centers of the lesions were determined for PET/CT (PET and CT independently) and PET/MRI (PET, T1-weighted gradient-echo sequence with radial stack-of-stars trajectory, T2-weighted sequence), and the b0 images of an echo-planar imaging (EPI) diffusion-weighted imaging (DWI) acquisition. All MRI sequences were performed in the axial plane with free breathing. The spatial coordinates of the estimated centers of the lesions were determined for PET and CT and PET and MRI sequences. Distance between the isocenter of the lesion on PET images and on the images obtained with the anatomic modalities was measured, and misregistration (in millimeters) was calculated. The degree of misregistration was compared between PET/CT and PET/MRI with a paired Student t test.

RESULTS

Nineteen lesions were evaluated. On PET/CT images, the average of the total misregistration in all planes of CT compared with PET was 4.13 ± 4.24 mm. On PET/MR images, lesion misregistration between PET and T1-weighted gradient-echo images had a shift of 2.41 ± 1.38 mm and between PET and b0 DW images was 5.97 ± 2.83 mm. Similar results were calculated for 11 lesions on T2-weighted images. The shift on T2-weighted images compared with PET images was 2.24 ± 1.12 mm. Paired Student t test calculations for PET/CT compared with PET/MRI T1-weighted gradient-echo images with a radial stack-of-stars trajectory, b0 DW images, and T2-weighted images showed significant differences (p < 0.05). Similar results were seen in the analysis of six lung lesions.

CONCLUSION

PET/MRI T1-weighted gradient-echo images with a radial stack-of-stars trajectory and T2-weighted images had more accurate spatial registration than PET/CT images. This may be because that the whole-body PET/MRI system used can perform simultaneous acquisition, whereas the PET/CT system acquires data sequentially. However, the EPI-based b0 DWI datasets were significantly misregistered compared with the PET/CT datasets, especially in the thorax. Radiologists reading PET/MR images should be aware of the potential for misregistration on images obtained with EPI-based DWI sequences because of inherent spatial distortion associated with this type of MRI acquisition.

Improved segmentation of white matter tracts with adaptive Riemannian metrics

We present a novel geodesic approach to segmentation of white matter tracts from diffusion tensor imaging (DTI). Compared to deterministic and stochastic tractography, geodesic approaches treat the geometry of the brain white matter as a manifold, often using the inverse tensor field as a Riemannian metric. The white matter pathways are then inferred from the resulting geodesics, which have the desirable property that they tend to follow the main eigenvectors of the tensors, yet still have the flexibility to deviate from these directions when it results in lower costs. While this makes such methods more robust to noise, the choice of Riemannian metric in these methods is ad hoc. A serious drawback of current geodesic methods is that geodesics tend to deviate from the major eigenvectors in high-curvature areas in order to achieve the shortest path. In this paper we propose a method for learning an adaptive Riemannian metric from the DTI data, where the resulting geodesics more closely follow the principal eigenvector of the diffusion tensors even in high-curvature regions. We also develop a way to automatically segment the white matter tracts based on the computed geodesics. We show the robustness of our method on simulated data with different noise levels. We also compare our method with tractography methods and geodesic approaches using other Riemannian metrics and demonstrate that the proposed method results in improved geodesics and segmentations using both synthetic and real DTI data.

Spurious group differences due to head motion in a diffusion MRI study

Diffusion-weighted MRI (DW-MRI) has become a popular imaging modality for probing the microstructural properties of white matter and comparing them between populations in vivo. However, the contrast in DW-MRI arises from the microscopic random motion of water molecules in brain tissues, which makes it particularly sensitive to macroscopic head motion. Although this has been known since the introduction of DW-MRI, most studies that use this modality for group comparisons do not report measures of head motion for each group and rely on registration-based correction methods that cannot eliminate the full effects of head motion on the DW-MRI contrast. In this work we use data from children with autism and typically developing children to investigate the effects of head motion on differences in anisotropy and diffusivity measures between groups. We show that group differences in head motion can induce group differences in DW-MRI measures, and that this is the case even when comparing groups that include control subjects only, where no anisotropy or diffusivity differences are expected. We also show that such effects can be more prominent in some white-matter pathways than others, and that they can be ameliorated by including motion as a nuisance regressor in the analyses. Our results demonstrate the importance of taking head motion into account in any population study where one group might exhibit more head motion than the other.

T2*-correction in dynamic contrast-enhanced MRI from double-echo acquisitions

PURPOSE

To evaluate the importance of T2*-effects on the arterial input function (AIF) and on the resulting dynamic parameter estimation in dynamic contrast-enhanced (DCE) MRI of high-grade gliomas.

MATERIALS AND METHODS

Seven patients with high-grade gliomas were imaged in total 50 times using a double-echo DCE sequence. Kinetic analysis using the extended Tofts model was performed using AIFs with and without correction for T2*-effects, and the resulting estimates of the transfer constant (K(trans) ), blood plasma volume (vp ), and the rate constant (kep ) were compared. Numerical simulations were done for comparison with clinical results as well as to further investigate the dependency of parameter values on the magnitude of T2*-induced errors.

RESULTS

All kinetic parameters were found to be overestimated if T2*-effects in the AIF were not accounted for; with vp being most severely affected. The relative error in each parameter was dependent on the absolute parameter magnitude, resulting in incorrect parametric tumor distributions in the presence of uncorrected AIF T2*-effects.

CONCLUSION

In DCE, a sufficiently short echo time should be used or corrections for T2*-effects based on double-echo acquisition should be made for correct quantification of kinetic parameters.

SPatial REgression Analysis of Diffusion tensor imaging (SPREAD) for longitudinal progression of neurodegenerative disease in individual subjects

OBJECTIVES

To develop a novel statistical method for analysis of longitudinal DTI data in individual subjects.

MATERIALS AND METHODS

The proposed SPatial REgression Analysis of Diffusion tensor imaging (SPREAD) method incorporates a spatial regression fitting of DTI data among neighboring voxels and a resampling method among data at different times. Both numerical simulations and real DTI data from healthy volunteers and multiple sclerosis (MS) patients were used in the study to evaluate this method.

RESULTS

Statistical inference based on SPREAD was shown to perform well through both group comparisons among simulated DTI data of individuals (especially when the group size is smaller than 5) and longitudinal comparisons of human DTI data within the same individual.

CONCLUSIONS

When pathological changes of neurodegenerative diseases are heterogeneous in a population, SPREAD provides a unique way to assess abnormality during disease progression at the individual level. Consequently, it has the potential to shed light on how the brain has changed as a result of disease or injury.

Diffusion tensor imaging study of the response to constraint-induced movement therapy of children with hemiparetic cerebral palsy and adults with chronic stroke

OBJECTIVE

To investigate the relationship of white matter integrity and path of the corticospinal tract (CST) on arm function before and after constraint-induced (CI) movement therapy in children with hemiparetic cerebral palsy (CP) and adults with chronic stroke.

DESIGN

Study 1 used a multiple-baseline pre-post design. Study 2 was a randomized controlled trial.

SETTING

Outpatient rehabilitation laboratory.

PARTICIPANTS

Study 1 included children with hemiparetic CP (n=10; mean age ± SD, 3.2±1.7y). Study 2 included adults with chronic stroke (n=26; mean age ± SD, 65.4±13.6y) who received either CI therapy or a comparison therapy.

INTERVENTIONS

Children in study 1 received CI therapy for 3.5h/d for 15 consecutive weekdays. Adults in study 2 received either CI therapy or a comparison therapy for 3.5h/d for 10 consecutive weekdays.

MAIN OUTCOME MEASURES

Diffusion tensor imaging was performed to quantify white matter integrity. Motor ability was assessed in children using the Pediatric Motor Activity Log-Revised and Pediatric Arm Function Test, and in adults with the Motor Activity Log and Wolf Motor Function Test.

RESULTS

Participants in both studies improved in real-world arm function and motor capacity. Children and adults with disrupted/displaced CSTs and children with reduced fractional anisotropy values were worse on pretreatment tests of motor function than participants with unaltered CSTs. However, neither integrity (fractional anisotropy) nor distorted or disrupted path of the CST affected motor improvement after treatment.

CONCLUSIONS

Participants who had reduced integrity, displacement, or interruption of their CST performed worse on pretreatment motor testing. However, this had no effect on their ability to benefit from CI therapy. The results for children and adults are consistent with one another.

Optimal setting of image bounding box can improve registration accuracy of diffusion tensor tractography

PURPOSE

When we register diffusion tensor tractography (DTT) to anatomical images such as fast imaging employing steady-state acquisition (FIESTA), we register the B0 image to FIESTA. Precise registration of the DTT B0 image to FIESTA is possible with non-rigid registration compared to rigid registration, although the non-rigid methods lack convenience. We report the effect of image data bounding box settings on registration accuracy using a normalized mutual information (NMI) method METHODS: MRI scans of 10 patients were used in this study. Registration was performed without modification of the bounding box in the control group, and the results were compared with groups re-registered using multiple bounding boxes limited to the region of interest (ROI). The distance of misalignment after registration at 3 anatomical characteristic points that are common to both FIESTA and B0 images was used as an index of accuracy.

RESULTS

Mean ([Formula: see text]SD) misalignment at the 3 anatomical points decreased significantly from [Formula: see text] to [Formula: see text] mm, [Formula: see text]), [Formula: see text] to [Formula: see text] mm, ([Formula: see text], and [Formula: see text] to [Formula: see text] mm, ([Formula: see text], each showing improvement compared to the control group CONCLUSION: Narrowing the image data bounding box to the ROI improves the accuracy of registering B0 images to FIESTA by NMI method. With our proposed methodology, accuracy can be improved in extremely easy steps, and this methodology may prove useful for DTT registration to anatomical image.

Assessment of structural connectivity in the preterm brain at term equivalent age using diffusion MRI and T2 relaxometry: a network-based analysis

Preterm birth is associated with a high prevalence of adverse neurodevelopmental outcome. Non-invasive techniques which can probe the neural correlates underpinning these deficits are required. This can be achieved by measuring the structural network of connections within the preterm infant's brain using diffusion MRI and tractography. We used diffusion MRI and T₂ relaxometry to identify connections with altered white matter properties in preterm infants compared to term infants. Diffusion and T₂ data were obtained from 9 term neonates and 18 preterm-born infants (born <32 weeks gestational age) at term equivalent age. Probabilistic tractography incorporating multiple fibre orientations was used in combination with the Johns Hopkins neonatal brain atlas to calculate the structural network of connections. Connections of altered diffusivity or T₂, as well as their relationship with gestational age at birth and postmenstrual age at the time of MRI, were identified using the network based statistic framework. A total of 433 connections were assessed. FA was significantly reduced in 17, and T₂ significantly increased in 18 connections in preterm infants, following correction for multiple comparisons. Cortical networks associated with affected connections mainly involved left frontal and temporal cortical areas: regions which are associated with working memory, verbal comprehension and higher cognitive function – deficits which are often observed later in children and adults born preterm. Gestational age at birth correlated with T₂, but not diffusion in several connections. We found no association between diffusion or T₂ and postmenstrual age at the time of MRI in preterm infants. This study demonstrates that alterations in the structural network of connections can be identified in preterm infants at term equivalent age, and that incorporation of non-diffusion measures such as T₂ in the connectome framework provides complementary information for the assessment of brain development.

Quantitative analysis of the efficacy of gradient table correction on improving the accuracy of fiber tractography

PURPOSE

Several groups suggested that the gradient table of a DTI data set should be reoriented to compensate for head motion. Although the effects of this correction were demonstrated qualitatively, its efficacy was not demonstrated quantitatively to date. The main goal of this study was to investigate the efficacy of gradient table correction on improving the accuracy of fiber tractography.

METHODS

First, the effects of gradient table correction on the estimation of fractional anisotropy (FA) maps and the primary diffusion direction were quantified and compared with the inherent uncertainty in the estimation process. Then, the effects of gradient table correction on tractography were quantified.

RESULTS

The corrections in FA values were only a fraction of the typical values seen in major fasciculi and inter-subject variance. The corrections to the primary diffusion direction were also much smaller than the uncertainty inherent in the estimation of its direction. However, the directional estimates were biased due to head motion and deviated fiber tracking.

CONCLUSIONS

Corrections to FA values were negligible and are not expected to affect group comparisons. However, a small but consistent bias was introduced to the estimates of primary diffusion direction, which might affect brain connectivity analyses based on fiber tracking.

Effect of disease and recovery on functional anatomy in brain tumor patients: insights from functional MRI and diffusion tensor imaging

Patients with brain tumors provide a unique opportunity to understand functional brain plasticity. Using advanced imaging techniques, such as functional MRI and diffusion tensor imaging, we have gained tremendous knowledge of brain tumor behavior, transformation, infiltration and destruction of nearby structures. Using these advanced techniques as an adjunct with more proven techniques, such as direct cortical stimulation, intraoperative navigation and advanced microsurgical techniques, we now are able to better formulate safer resection trajectories, perform larger resections at reduced risk and better counsel patients and their families about possible complications. Brain mapping in patients with brain tumors and other lesions has shown us that the old idea of fixed function of the adult cerebral cortex is not entirely true. Improving care for patients with brain lesions in the future will depend on better understanding of the functional organization and plasticity of the adult brain. Advanced noninvasive brain imaging will undoubtedly play a role in advancing this understanding.

Reproducibility of corticospinal diffusion tensor tractography in normal subjects and hemiparetic stroke patients

PURPOSE

The reproducibility of corticospinal diffusion tensor tractography (DTT) for a guideline is important before longitudinal monitoring of the therapy effects in stroke patients. This study aimed to establish the reproducibility of corticospinal DTT indices in healthy subjects and chronic hemiparetic stroke patients.

MATERIALS AND METHODS

Written informed consents were obtained from 10 healthy subjects (mean age 25.8 ± 6.8 years), who underwent two scans in one session plus the third scan one week later, and from 15 patients (mean age 47.5 ± 9.1 years, 6-60 months after the onset of stroke, NIHSS scores between 9 and 20) who were scanned thrice on separate days within one month. Diffusion-tensor imaging was performed at 3T with 25 diffusion directions. Corticospinal tracts were reconstructed using fiber assignment by continuous tracking without and with motion/eddy-current corrections. Intra- and inter-rater as well as intra- and inter-session variations of the DTT derived indices (fiber number, apparent diffusion coefficient (ADC), and fractional anisotropy (FA)) were assessed.

RESULTS

Intra-session and inter-session coefficients of variations (CVs) are small for FA (1.13-2.09%) and ADC (0.45-1.64%), but much larger for fiber number (8.05-22.4%). Inter-session CVs in the stroke side of patients (22.4%) are higher than those in the normal sides (18.0%) and in the normal subjects (14.7%). Motion/eddy-current correction improved inter-session reproducibility only for the fiber number of the infarcted corticospinal tract (CV reduced from 22.4% to 14.1%).

CONCLUSION

The fiber number derived from corticospinal DTT shows substantially lower precision than ADC and FA, with infarcted tracts showing lower reproducibility than the healthy tissues.