Christopher D. Kroenke

Modeling dendrite density from magnetic resonance diffusion measurements.

Diffusion-weighted imaging (DWI) provides a noninvasive tool to probe tissue microstructure. We propose a simplified model of neural cytoarchitecture intended to capture the essential features important for water diffusion as measured by NMR. Two components contribute to the NMR signal in this model: (i) the dendrites and axons, which are modeled as long cylinders with two diffusion coefficients, parallel (D(L)) and perpendicular (D(T)) to the cylindrical axis, and (ii) an isotropic monoexponential diffusion component describing water diffusion within and across all other structures, i.e., in extracellular space and glia cells. The model parameters are estimated from 153 diffusion-weighted images acquired from a formalin-fixed baboon brain. A close correspondence between the data and the signal model is found, with the model parameters consistent with literature values. The model provides an estimate of dendrite density from noninvasive MR diffusion measurements, a parameter likely to be of value for understanding normal as well as abnormal brain development and function.

A Population-Average MRI-Based Atlas Collection of the Rhesus Macaque

Magnetic resonance imaging (MRI) studies of non-human primates are becoming increasingly common; however, the well-developed voxel-based methodologies used in human studies are not readily applied to non-human primates. In the present study, we create a population-average MRI-based atlas collection for the rhesus macaque (Macaca mulatta) that can be used with common brain mapping packages such as SPM or FSL. In addition to creating a publicly available T1-weighted atlas (http://www.brainmap.wisc.edu/monkey.html), probabilistic tissue classification maps and T2-weighted atlases were also created. Theses atlases are aligned to the MRI volume from the Saleem, K.S. and Logothetis, N.K. (2006) atlas providing an explicit link to histological sections. Additionally, we have created a transform to integrate these atlases with the F99 surface-based atlas in CARET. It is anticipated that these tools will help facilitate voxel-based imaging methodologies in non-human primate species, which in turn may increase our understanding of brain function, development, and evolution.

Axons Pull on the Brain, But Tension Does Not Drive Cortical Folding

During human brain development, the cerebral cortex undergoes substantial folding, leading to its characteristic highly convoluted form. Folding is necessary to accommodate the expansion of the cerebral cortex; abnormal cortical folding is linked to various neurological disorders, including schizophrenia, epilepsy, autism, and mental retardation. Although this process requires mechanical forces, the specific force-generating mechanisms that drive folding remain unclear. The two most widely accepted hypotheses are as follows: (1) Folding is caused by differential growth of the cortex and (2) folding is caused by mechanical tension generated in axons. Direct evidence supporting either theory, however, is lacking. Here we show that axons are indeed under considerable tension in the developing ferret brain, but the patterns of tissue stress are not consistent with a causal role for axonal tension. In particular, microdissection assays reveal that significant tension exists along axons aligned circumferentially in subcortical white matter tracts, as well as those aligned radially inside developing gyri (outward folds). Contrary to previous speculation, however, axonal tension is not directed across developing gyri, suggesting that axon tension does not drive folding. On the other hand, using computational (finite element) models, we show that differential cortical growth accompanied by remodeling of the subplate leads to outward folds and stress fields that are consistent with our microdissection experiments, supporting a mechanism involving differential growth. Local perturbations, such as temporal differences in the initiation of cortical growth, can ensure consistent folding patterns. This study shows that a combination of experimental and computational mechanics can be used to evaluate competing hypotheses of morphogenesis, and illuminate the biomechanics of cortical folding.

Prenatal Cerebral Ischemia Disrupts MRI-Defined Cortical Microstructure Through Disturbances in Neuronal Arborization

Preterm ischemia disrupts MRI-defined maturation of the cerebral cortex by impairing the differentiation of cortical neurons in fetal lambs. Early Start for Better Brains Despite all of the recent advances in medical care for premature newborns, these infants still often experience complications. In particular, cognitive problems and developmental delays are common in this patient population and can be difficult to predict. Now, two sets of authors have obtained new data that approach this problem from different angles using diffusion tensor magnetic resonance imaging (MRI) in human infants and newborn lambs. Vinall and coauthors examined 95 premature newborn babies who were born at 24 to 32 weeks of gestation. The authors performed two sets of MRI scans on these infants: one scan was done about 2 months before their due dates and the other scan when they reached full term. The authors also tracked the infants’ growth parameters—weight, length, and head size—as well as data on other factors that could affect brain growth, including the presence of infections or other serious illnesses. A detailed analysis of the MRI scans showed that the development of normal brain structure correlated with postnatal growth (and presumably nutrition) even after accounting for any other illnesses the infants may have experienced early in life. Dean et al. took a different approach to studying premature brain development: they analyzed the brain structures of fetal lambs that had experienced ischemia in utero at a time that corresponded to about two-thirds of full gestation time. The lambs were analyzed both by MRI and by histological analysis of the brain at 1, 2, or 4 weeks after an in utero ischemic event, and these data were compared to those of age-matched animals that did not undergo ischemic episodes. Here, the authors also saw abnormalities in brain development by MRI and correlated them with histological and structural aberrations. The growth impairment seen in the animals’ brains by MRI corresponded to disturbances in the branching of neuronal dendrites and abnormal formation of synapse connections with other neurons. More studies are needed to understand how postnatal growth, nutrition, illness, and prenatal ischemia affect the developing brain to develop methods for preventing any resulting injury. In addition, long-term studies should help to determine how differences in brain anatomy and MRI data translate into developmental and cognitive outcomes. Children who survive preterm birth exhibit persistent unexplained disturbances in cerebral cortical growth with associated cognitive and learning disabilities. The mechanisms underlying these deficits remain elusive. We used ex vivo diffusion magnetic resonance imaging to demonstrate in a preterm large-animal model that cerebral ischemia impairs cortical growth and the normal maturational decline in cortical fractional anisotropy (FA). Analysis of pyramidal neurons revealed that cortical deficits were associated with impaired expansion of the dendritic arbor and reduced synaptic density. Together, these findings suggest a link between abnormal cortical FA and disturbances of neuronal morphological development. To experimentally investigate this possibility, we measured the orientation distribution of dendritic branches and observed that it corresponds with the theoretically predicted pattern of increased anisotropy within cases that exhibited elevated cortical FA after ischemia. We conclude that cortical growth impairments are associated with diffuse disturbances in the dendritic arbor and synapse formation of cortical neurons, which may underlie the cognitive and learning disabilities in survivors of preterm birth. Further, measurement of cortical FA may be useful for noninvasively detecting neurological disorders affecting cortical development.

Determination of Axonal and Dendritic Orientation Distributions Within the Developing Cerebral Cortex by Diffusion Tensor Imaging

As neurons of the developing brain form functional circuits, they undergo morphological differentiation. In immature cerebral cortex, radially-oriented cellular processes of undifferentiated neurons impede water diffusion parallel, but not perpendicular, to the pial surface, as measured via diffusion-weighted magnetic resonance imaging, and give rise to water diffusion anisotropy. As the cerebral cortex matures, the loss of water diffusion anisotropy accompanies cellular morphological differentiation. A quantitative relationship is proposed here to relate water diffusion anisotropy measurements directly to characteristics of neuronal morphology. This expression incorporates the effects of local diffusion anisotropy within cellular processes, as well as the effects of anisotropy in the orientations of cellular processes. To obtain experimental support for the proposed relationship, tissue from 13 and 31 day-old ferrets was stained using the rapid Golgi technique, and the 3-D orientation distribution of neuronal proceses was characterized using confocal microscopic examination of reflected visible light images. Coregistration of the MRI and Golgi data enables a quantitative evaluation of the proposed theory, and excellent agreement with the theoretical results, as well as agreement with previously published values for locally-induced water diffusion anisotropy and volume fraction of the neuropil, is observed.

Altered White Matter Microstructure in Children With Attention- Deficit/Hyperactivity Disorder

OBJECTIVE Identification of biomarkers is a priority for attention-deficit/hyperactivity disorder (ADHD). Studies have documented macrostructural brain alterations in ADHD, but few have examined white matter microstructure, particularly in preadolescent children. Given dramatic white matter maturation across childhood, microstructural differences seen in adolescents and adults with ADHD may reflect compensatory restructuring, rather than early neurophenotypic markers of the disorder. METHOD Using tract-based spatial statistics, mean fractional anisotropy (FA) maps were created using diffusion tensor imaging. FA, mean diffusivity (MD), and associated axial and radial diffusivities were compared between 16 children with ADHD and 20 healthy children (age 7-9 years). RESULTS Youth with ADHD showed decreased FA in frontoparietal, frontolimbic, cerebellar, corona radiata, and temporo-occipital white matter compared with controls. In addition, ADHD was associated with lower MD in the posterior limb of the internal capsule and frontoparietal white matter and greater MD in frontolimbic white matter. Lower axial diffusion and/or higher radial diffusion were differentially observed for youth with ADHD in earlier versus later maturing areas of group FA/MD difference. CONCLUSIONS This study suggests that, even prior to adolescence, ADHD represents a disorder of altered structural connectivity of the brain, characterized by distributed atypical white matter microstructure. In addition, later maturing frontolimbic pathways were abnormal in children with ADHD, likely due to delayed or decreased myelination, a finding not previously demonstrated in the adolescent or adult stages of the disorder. These results suggest that disruptions in white matter microstructure may play a key role in the early pathophysiology of ADHD.

Histopathological correlates of magnetic resonance imaging–defined chronic perinatal white matter injury

Although magnetic resonance imaging (MRI) is the optimal imaging modality to define cerebral white‐matter injury (WMI) in preterm survivors, the histopathological features of MRI‐defined chronic lesions are poorly defined. We hypothesized that chronic WMI is related to a combination of delayed oligodendrocyte (OL) lineage cell death and arrested maturation of preoligodendrocytes (preOLs). We determined whether ex vivo MRI can distinguish distinct microglial and astroglial responses related to WMI progression and arrested preOL differentiation.

Measurement of the Dynamic Shear Modulus of Mouse Brain Tissue In Vivo by Magnetic Resonance Elastography

In this study, the magnetic resonance (MR) elastography technique was used to estimate the dynamic shear modulus of mouse brain tissue in vivo. The technique allows visualization and measurement of mechanical shear waves excited by lateral vibration of the skull. Quantitative measurements of displacement in three dimensions during vibration at 1200 Hz were obtained by applying oscillatory magnetic field gradients at the same frequency during a MR imaging sequence. Contrast in the resulting phase images of the mouse brain is proportional to displacement. To obtain estimates of shear modulus, measured displacement fields were fitted to the shear wave equation. Validation of the procedure was performed on gel characterized by independent rheometry tests and on data from finite element simulations. Brain tissue is, in reality, viscoelastic and nonlinear. The current estimates of dynamic shear modulus are strictly relevant only to small oscillations at a specific frequency, but these estimates may be obtained at high frequencies (and thus high deformation rates), noninvasively throughout the brain. These data complement measurements of nonlinear viscoelastic properties obtained by others at slower rates, either ex vivo or invasively.

On the nature of the NAA diffusion attenuated MR signal in the central nervous system

In the brain, on a macroscopic scale, diffusion of the intraneuronal constituent N‐acetyl‐L‐aspartate (NAA) appears to be isotropic. In contrast, on a microscopic scale, NAA diffusion is likely highly anisotropic, with displacements perpendicular to neuronal fibers being markedly hindered, and parallel displacements less so. In this report we first substantiate that local anisotropy influences NAA diffusion in vivo by observing differing diffusivities parallel and perpendicular to human corpus callosum axonal fibers. We then extend our measurements to large voxels within rat brains. As expected, the macroscopic apparent diffusion coefficient (ADC) of NAA is practically isotropic due to averaging of the numerous and diverse fiber orientations. We demonstrate that the substantially non‐monoexponential diffusion‐mediated MR signal decay vs. b value can be quantitatively explained by a theoretical model of NAA confined to an ensemble of differently oriented neuronal fibers. On the microscopic scale, NAA diffusion is found to be strongly anisotropic, with displacements occurring almost exclusively parallel to the local fiber axis. This parallel diffusivity, ADC‖, is 0.36 ± 0.01 μm2/ms, and ADC⟂ is essentially zero. From ADC‖ the apparent viscosity of the neuron cytoplasm is estimated to be twice as large as that of a temperature‐matched dilute aqueous solution. Magn Reson Med 52:1052–1059, 2004.

Human Neural Stem Cells Induce Functional Myelination in Mice with Severe Dysmyelination

Transplanted banked human neural stem cells produce functional myelin detected by MRI in juvenile mice with severe dysmyelination. Bringing Insulation Up to Code Faulty insulation around household wiring is an electric shock and fire hazard; likewise, defects in the insulation around nerve fibers—the myelin sheath—can have destructive effects. Because of myelin’s crucial roles in promoting the rapid transmission of nerve impulses and in axon integrity, mutations that affect myelin formation in the central nervous system cause severe neurological decline. Uchida et al. and Gupta et al. now investigate the use of neural stem cells—which can differentiate into myelin-producing oligodendrocytes—as a potential treatment for such disorders. Previous work showed that transplantation of human oligodendrocyte progenitors into newborn shiverer (Shi) mice, a hypomyelination model, could prolong survival. In the new work, Uchida et al. transplanted human neural stem cells, which had been expanded and banked, into the brains of newborn and juvenile Shi mice. Whereas the newborn mice were asymptomatic, the juvenile mice were already symptomatic and displayed advanced dysmyelination. These transplanted cells preferentially differentiated into oligodendrocytes that generated myelin, which ensheathed axons and improved nerve conduction in both categories of mice. In an open-label phase 1 study, Gupta et al. then tested the safety and efficacy of such cells in four young boys with a severe, fatal form of Pelizaeus-Merzbacher disease (PMD), a rare X-linked condition in which oligodendrocytes cannot myelinate axons. Human neural stem cells were transplanted directly into the brain; the procedure and transplantation were well tolerated. Magnetic resonance imaging techniques, performed before transplant and five times in the following year, were used to assess myelination. The imaging results were consistent with donor cell–derived myelination in the transplantation region in three of the four patients. These results support further study of potential clinical benefits of neural stem cell transplantation in PMD and other dysmyelination disorders. Shiverer-immunodeficient (Shi-id) mice demonstrate defective myelination in the central nervous system (CNS) and significant ataxia by 2 to 3 weeks of life. Expanded, banked human neural stem cells (HuCNS-SCs) were transplanted into three sites in the brains of neonatal or juvenile Shi-id mice, which were asymptomatic or showed advanced hypomyelination, respectively. In both groups of mice, HuCNS-SCs engrafted and underwent preferential differentiation into oligodendrocytes. These oligodendrocytes generated compact myelin with normalized nodal organization, ultrastructure, and axon conduction velocities. Myelination was equivalent in neonatal and juvenile mice by quantitative histopathology and high-field ex vivo magnetic resonance imaging, which, through fractional anisotropy, revealed CNS myelination 5 to 7 weeks after HuCNS-SC transplantation. Transplanted HuCNS-SCs generated functional myelin in the CNS, even in animals with severe symptomatic hypomyelination, suggesting that this strategy may be useful for treating dysmyelinating diseases.