David M. Morris

Brain Hemispheric Structural Efficiency and Interconnectivity Rightward Asymmetry in Human and Nonhuman Primates

Evidence for interregional structural asymmetries has been previously reported for brain anatomic regions supporting well-described functional lateralization. Here, we aimed to investigate whether the two brain hemispheres demonstrate dissimilar general structural attributes implying different principles on information flow management. Common left hemisphere/right hemisphere structural network properties are estimated and compared for right-handed healthy human subjects and a nonhuman primate, by means of 3 different diffusion-weighted magnetic resonance imaging fiber tractography algorithms and a graph theory framework. In both the human and the nonhuman primate, the data support the conclusion that, in terms of the graph framework, the right hemisphere is significantly more efficient and interconnected than the left hemisphere, whereas the left hemisphere presents more central or indispensable regions for the whole-brain structural network than the right hemisphere. From our point of view, in terms of functional principles, this pattern could be related with the fact that the left hemisphere has a leading role for highly demanding specific process, such as language and motor actions, which may require dedicated specialized networks, whereas the right hemisphere has a leading role for more general process, such as integration tasks, which may require a more general level of interconnection.

Distortion Correction for Diffusion-Weighted MRI Tractography and fMRI in the Temporal Lobes

Single shot echo‐planar imaging (EPI) sequences are currently the most commonly used sequences for diffusion‐weighted imaging (DWI) and functional magnetic resonance imaging (fMRI) as they allow relatively high signal to noise with rapid acquisition time. A major drawback of EPI is the substantial geometric distortion and signal loss that can occur due to magnetic field inhomogeneities close to air‐tissue boundaries. If DWI‐based tractography and fMRI are to be applied to these regions, then the distortions must be accurately corrected to achieve meaningful results. We describe robust acquisition and processing methods for correcting such distortions in spin echo (SE) EPI using a variant of the reversed direction k space traversal method with a number of novel additions. We demonstrate that dual direction k space traversal with maintained diffusion‐encoding gradient strength and direction results in correction of the great majority of eddy current‐associated distortions in DWI, in addition to those created by variations in magnetic susceptibility. We also provide examples to demonstrate that the presence of severe distortions cannot be ignored if meaningful tractography results are desired. The distortion correction routine was applied to SE‐EPI fMRI acquisitions and allowed detection of activation in the temporal lobe that had been previously found using PET but not conventional fMRI. Hum Brain Mapp, 2010.

Probabilistic fibre tracking: differentiation of connections from chance events.

Probabilistic tractography methods that use Monte Carlo sampling of voxelwise fibre orientation probability density functions suffer from distance-related artefacts due to the propagation of uncertainty along the tract path. These are manifested as a preferential weighting of regions close to the tracking start point at the expense of more distant regions--an effect that can mask genuine anatomical connections. We propose a methodology based on comparison of the conventional connection probability map with a null connection map that defines the distribution of connections expected by a random tracking process and that is dominated by the same distance effects. When the connection probability is significantly greater than the result of the null tracking result this identifies voxels where the diffusion information is providing more evidence of connection than that expected from random tracking. We show that the null connection probability map used is governed by Poisson statistics within each voxel, allowing analytical estimation of connection values that are significantly different to the null connection values. The resultant significant connection maps can be combined with the conventional probabilistic tractography output to produce maps of significant connections which reduce distance-related artefacts by removing areas where the observed frequency of connection is dominated simply by distance effects and not the diffusion information. This is achieved by applying an objective statistical interpretation of observed patterns of connection which cannot be achieved by simple thresholding of conventional probabilistic tractography maps due to the distance effect.

Using the Model-Based Residual Bootstrap to Quantify Uncertainty in Fiber Orientations From

Bootstrapping of repeated diffusion-weighted image datasets enables nonparametric quantification of the uncertainty in the inferred fiber orientation. The wild bootstrap and the residual bootstrap are model-based residual resampling methods which use a single dataset. Previously, the wild bootstrap method has been presented as an alternative to conventional bootstrapping for diffusion tensor imaging. Here we present a study of an implementation of model-based residual bootstrapping using q -ball analysis and compare the outputs with conventional bootstrapping. We show that model-based residual bootstrap q-ball generates results that closely match the output of the conventional bootstrap. Both the residual and conventional bootstrap of multifiber methods can be used to estimate the probability of different numbers of fiber populations existing in different brain tissues. Also, we have shown that these methods can be used to provide input for probabilistic tractography, avoiding existing limitations associated with data calibration and model selection.

Q

Pregnancy complications such as preeclampsia and fetal growth restriction are sometimes thought to be caused by placental abnormalities associated with reduced oxygenation. Oxygen‐enhanced MRI (R1 contrast) and BOLD MRI (R2* contrast) have the potential to noninvasively investigate this oxygen environment at a range of gestational ages.

-Ball Analysis

UNLABELLED Placental insufficiency is a major cause of fetal growth restriction (FGR) and accumulating evidence indicates several aspects of placental morphology are altered in this condition. MRI provides quantitative indices that may be used in non-invasive assessment of the human placenta, such as relaxation time measurements, T1 and T2. We hypothesised that placental relaxation times relate to alterations in placental tissue morphology and hence may be useful in identifying the changes associated with FGR. We report on the first phase of testing this hypothesis, in a study of women in normal pregnancy. AIMS To assess relaxation time measurements in the placenta in normal pregnancy and correlate these with gestational age and stereological analyses of placental morphology following delivery. METHODS 30 women underwent MRI examination (1.5 T) between 20 and 41 weeks gestation. Placental T1 and T2 measurements were acquired from a mid-depth placental region, co-localised to a structural scan. Fixed, wax-embedded sections of these placentas collected at delivery were stained with hematoxylin/eosin and subjected to stereological analysis. RESULTS Placental T1 and T2 show a significant negative correlation with gestation, (Pearson correlation p=0.01, 0.03 respectively). 17 placentas were analysed stereologically. In the group as a whole there was no significant correlation between T1 and T2 and morphological features. However, in a subset of 7 pregnancies scanned within a week of delivery, a significant positive correlation was observed between the fibrin volume density and the ratio of fibrin: villous volume densities and T2 (Spearman correlation p=0.02, 0.03 respectively). DISCUSSION The correlations between placental T1 and T2 and gestation show that these variables are clearly influenced by changes in placental structure. Fibrin might be a key component but further work is needed to fully elucidate the major structural influences on placental T1 and T2.

R1 and R2* changes in the human placenta in response to maternal oxygen challenge

Magnetic resonance imaging allows the noninvasive observation of Po2 changes between air breathing and oxygen breathing through quantification of the magnetic longitudinal relaxation time T1. Changes in Po2 are proportional to changes in the longitudinal relaxation rate ΔR1 (where ΔR1 = 1/T1oxygen − 1/T1air). Knowledge of this response could inform clinical interventions using maternal oxygen administration antenatally to treat fetal growth restriction. We present in vivo measurements of the response of the fetal–placental unit to maternal hyperoxia.

Magnetic Resonance Imaging Relaxation Time Measurements of the Placenta at 1.5 T

Highlights • The dorsal stream has been postulated to constitute multiple pathways.• Tractography was used to map the connectivity of regions within the left SMG.• The arcuate fasciculus was subdivided into dorso-dorsal/ventro-dorsal pathways.• The parallel pathways appear to underlie functional heterogeneity within the SMG.

Absence of PO2 change in fetal brain despite PO2 increase in placenta in response to maternal oxygen challenge.

To evaluate between‐site agreement of apparent diffusion coefficient (ADC) measurements in preclinical magnetic resonance imaging (MRI) systems.

Using in vivo probabilistic tractography to reveal two segregated dorsal 'language-cognitive' pathways in the human brain.

Diffusion tensor imaging (DTI) studies have identified changes in white matter tracts in schizophrenia patients and those at high risk of transition. Schizotypal samples represent a group on the schizophrenia continuum that share some aetiological risk factors but without the confounds of illness. The aim of the current study was to compare tract microstructural coherence as measured by fractional anisotropy (FA) between 12 psychometrically defined schizotypes and controls. We investigated bilaterally the uncinate and arcuate fasciculi (UF and AF) via a probabilistic tractography algorithm (PICo), with FA values compared between groups. Partial correlations were also examined between measures of subclinical hallucinatory/delusional experiences and FA values. Participants with schizotypal features were found to have increased FA values in the left hemisphere UF only. In the whole sample there was a positive correlation between FA values and measures of hallucinatory experience in the right AF. These findings suggest subtle changes in microstructural coherence are found in individuals with schizotypal features, but are not similar to changes predominantly observed in clinical samples. Correlations between mild hallucinatory experience and FA values could indicate increasing tract coherence could be associated with symptom formation.