In vivo analysis of the post-natal development of normal mouse brain by DTI

Diffusion kurtosis imaging: An efficient tool for evaluating age-related changes in rat brains

PURPOSE

To evaluate and determine age-related changes in rat brains by studying the diffusion kurtosis imaging results among different age groups of rats.

METHODS

Sprague-Dawley (SD) rats underwent conventional magnetic resonance imaging (MRI) and diffusion Kurtosis Imaging (DKI). Two diffusion values of mean kurtosis (MK) and kurtosis (K⊥ ) were measured and analyzed based on laterality, brain regions and age groups. The MK and K⊥ data were plotted against different age groups.

RESULTS

No laterality was found for the MK or K⊥ values in the cerebral cortex (CT), external capsule (EC), or caudate putamen (CPu) regions. In contrast, significant changes in these values were observed among different age groups. Changes of the MK and K⊥ values were significant in both hemispheres in the EC, the CT, and the CPu brain regions. The changes in the MK and K⊥ values showed a parabolic relationship with ages in all the brain regions.

CONCLUSION

No laterality in the MK and K⊥ values was observed for the EC, CT, or CPu regions of the rat brain. Significant changes in MK and K⊥ values were both observed among different age groups, thus suggesting diffusion kurtosis imaging as an efficient tool for studying brain aging in rats.

High-Resolution 3D in vivo Brain Diffusion Tensor Imaging at Ultrahigh Fields: Following Maturation on Juvenile and Adult Mice

Diffusion tensor imaging (DTI) is a well-established technique for mapping brain microstructure and white matter tracts in vivo. High resolution DTI, however, is usually associated with low intrinsic sensitivity and therefore long acquisition times. By increasing sensitivity, high magnetic fields can alleviate these demands, yet high fields are also typically associated with significant susceptibility-induced image distortions. This study explores the potential arising from employing new pulse sequences and emerging hardware at ultrahigh fields, to overcome these limitations. To this end, a 15.2 T MRI instrument equipped with a cryocooled surface transceiver coil was employed, and DTI experiments were compared between SPatiotemporal ENcoding (SPEN), a technique that tolerates well susceptibility-induced image distortions, and double-sampled Spin-Echo Echo-Planar Imaging (SE-EPI) methods. Following optimization, SE-EPI afforded whole brain DTI maps at 135 μm isotropic resolution that possessed higher signal-to-noise ratios (SNRs) than SPEN counterparts. SPEN, however, was a better alternative to SE-EPI when focusing on challenging regions of the mouse brain -including the olfactory bulb and the cerebellum. In these instances, the higher robustness of fully refocused SPEN acquisitions coupled to its built-in zooming abilities, provided in vivo DTI maps with 75 μm nominal isotropic spatial resolution. These DTI maps, and in particular the mean diffusion direction (MDD) details, exhibited variations that matched very well the anatomical features known from histological brain Atlases. Using these capabilities, the development of the olfactory bulb (OB) in live mice was followed from week 1 post-partum, until adulthood. The diffusivity of this organ showed a systematic decrease in its overall isotropic value and increase in its fractional anisotropy with age; this maturation was observed for all regions used in the OB's segmentation but was most evident for the lobules' centers, in particular for the granular cell layer. The complexity of the OB neuronal connections also increased during maturation, as evidenced by the growth in directionalities arising in the mean diffusivity direction maps.

Demyelination-Remyelination of the Rat Caudal Cerebellar Peduncle Evaluated with Magnetic Resonance Imaging

Remyelination is common under physiological conditions and usually occurs as a response to a pathological demyelinating event. Its potentiation is an important goal for the development of therapies against pathologies such as multiple sclerosis and white matter injury. Visualization and quantification in vivo of demyelination and remyelination processes are essential for longitudinal studies that will allow the testing and development of pro-myelinating strategies. In this study, ethidium bromide (EB) was stereotaxically injected into the caudal cerebellar peduncle (c.c.p.) in rats to produce demyelination; the resulting lesion was characterized (i) transversally through histology using Black-Gold II (BGII) staining, and (ii) longitudinally through diffusion-weighted magnetic resonance imaging (dMRI), by computing fractional anisotropy (FA) and diffusivity parameters to detect microstructural changes. Using this characterization, we evaluated, in the lesioned c.c.p., the effect of N-butyl-β-carboline-3-carboxylate (β-CCB), a potentiator of GABAergic signaling in oligodendrocytes. The dMRI analysis revealed significant changes in the anisotropic and diffusivity properties of the c.c.p. A decreased FA and increased radial diffusivity (λ_⊥) were evident following c.c.p. lesioning. These changes correlated strongly with an apparent decrease in myelin content as evidenced by BGII. Daily systemic β-CCB administration for 2 weeks in lesioned animals increased FA and decreased λ_⊥, suggesting an improvement in myelination, which was supported by histological analysis. This study shows that structural changes in the demyelination-remyelination of the caudal cerebellar peduncle (DRCCP) model can be monitored longitudinally by MRI, and it suggests that remyelination is enhanced by β-CCB treatment. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Association of LHPP genetic variation (rs35936514) with structural and functional connectivity of hippocampal-corticolimbic neural circuitry

A single nucleotide polymorphism at the LHPP gene (rs35936514) has been reported to be associated with major depressive disorder (MDD) in genome-wide association studies. We conducted a neuroimaging analysis to explore whether and which brain neural systems are affected by LHPP variation. Since LHPP variants seem to be associated with the hippocampus, we assessed the relationship between rs35936514 variation and structural-functional connectivity within a hippocampal-corticolimbic neural system implicated in MDD. A total of 122 Chinese subjects were divided into a CC homozygous group (CC genotype, n = 60) and a T allele-carrier group (CT/TT genotypes, n = 62). All subjects participated in resting-state functional magnetic resonance imaging (rs-fMRI) and diffusion tensor imaging (DTI) scans. Structural and functional connectivity data analyses were then performed. Compared to the CC group, the T allele-carrier group showed significantly higher fractional anisotropy (FA) values in the fornix as well as increased functional connectivity from the hippocampus to the rostral part of the anterior cingulate cortex (rACC). Moreover, a significant negative correlation between fornix FA value and hippocampus-rACC functional connectivity was identified (P < 0.05). These findings suggest that there is a relationship between rs35936514 variation and both structural and functional hippocampal-corticolimbic neural system involvement in MDD. LHPP may play an important role in the neuropathophysiology of MDD.

Miniature pig model of human adolescent brain white matter development

BACKGROUND

Neuroscience research in brain development and disorders can benefit from an in vivo animal model that portrays normal white matter (WM) development trajectories and has a sufficiently large cerebrum for imaging with human MRI scanners and protocols.

NEW METHOD

Twelve three-month-old Sinclair™ miniature pigs (Sus scrofa domestica) were longitudinally evaluated during adolescent development using advanced diffusion weighted imaging (DWI) focused on cerebral WM. Animals had three MRI scans every 23.95 ± 3.73 days using a 3-T scanner. The DWI imaging protocol closely modeled advanced human structural protocols and consisted of fifteen b-shells (b = 0-3500 s/mm2) with 32-directions/shell. DWI data were analyzed using diffusion kurtosis and bi-exponential modeling that provided measurements that included fractional anisotropy (FA), radial kurtosis, kurtosis anisotropy (KA), axial kurtosis, tortuosity, and permeability-diffusivity index (PDI).

RESULTS

Significant longitudinal effects of brain development were observed for whole-brain average FA, KA, and PDI (all p < 0.001). There were expected regional differences in trends, with corpus callosum fibers showing the highest rate of change.

COMPARISON WITH EXISTING METHOD(S)

Pigs have a large, gyrencephalic brain that can be studied using clinical MRI scanners/protocols. Pigs are less complex than non-human primates thus satisfying the "replacement" principle of animal research.

CONCLUSIONS

Longitudinal effects were observed for whole-brain and regional diffusion measurements. The changes in diffusion measurements were interepreted as evidence for ongoing myelination and maturation of cerebral WM. Corpus callosum and superficial cortical WM showed the expected higher rates of change, mirroring results in humans.

Effects of ANK3 variation on gray and white matter in bipolar disorder

The single-nucleotide polymorphism rs9804190 in the Ankyrin G (ANK3) gene has been reported in genome-wide association studies to be associated with bipolar disorder (BD). However, the neural system effects of rs9804190 in BD are not known. We investigated associations between rs9804190 and gray and white matter (GM and WM, respectively) structure within a frontotemporal neural system implicated in BD. A total of 187 adolescent and adult European Americans were studied: a group homozygous for the C allele (52 individuals with BD and 56 controls) and a T-carrier group, carrying the high-risk T allele (38 BD and 41 controls). Subjects participated in high-resolution structural magnetic resonance imaging and diffusion tensor imaging (DTI) scanning. Frontotemporal region of interest (ROI) and whole-brain exploratory analyses were conducted. DTI ROI-based analysis revealed a significant diagnosis by genotype interaction within the uncinate fasciculus (P⩽0.05), with BD subjects carrying the T (risk) allele showing decreased fractional anisotropy compared with other subgroups, independent of age. Genotype effects were not observed in frontotemporal GM volume. These findings support effects of rs9804190 on frontotemporal WM in adolescents and adults with BD and suggest a mechanism contributing to WM pathology in BD.

Determining mean fractional anisotropy using DDCOSY: preliminary results in biological tissues

Complex materials are ubiquitous in science, engineering and nature. One important parameter for characterising their morphology is the degree of anisotropy. Magnetic resonance imaging offers non-invasive methods for quantitative measurements of the materials anisotropy, most commonly via diffusion tensor imaging and the subsequent extraction of the spatially resolved fractional anisotropy (FA) value. Here, we propose an alternative way of determining the FA as a sample average for cases where spatially resolved methods are not needed or not applicable. It is based on a particular diffusion-diffusion correlation spectroscopy protocol, allowing for the extraction of the mean (i.e. sample averaged) FA value. We demonstrate that mean FA values obtained from three anisotropic biological tissues are consistent with those extracted using diffusion tensor imaging. Moreover, we show that differences of mean FA values in healthy and tumour-bearing mouse brains allow to distinguish these tissue types. We anticipate that the proposed method will be beneficial in the wider context of medical and material science. Copyright © 2016 John Wiley & Sons, Ltd.

Spatio-Temporal Patterns of Demyelination and Remyelination in the Cuprizone Mouse Model

Cuprizone administration in mice provides a reproducible model of demyelination and spontaneous remyelination, and has been useful in understanding important aspects of human disease, including multiple sclerosis. In this study, we apply high spatial resolution quantitative MRI techniques to establish the spatio-temporal patterns of acute demyelination in C57BL/6 mice after 6 weeks of cuprizone administration, and subsequent remyelination after 6 weeks of post-cuprizone recovery. MRI measurements were complemented with Black Gold II stain for myelin and immunohistochemical stains for associated tissue changes. Gene expression was evaluated using the Allen Gene Expression Atlas. Twenty-five C57BL/6 male mice were split into control and cuprizone groups; MRI data were obtained at baseline, after 6 weeks of cuprizone, and 6 weeks post-cuprizone. High-resolution (100 μm isotropic) whole-brain coverage magnetization transfer ratio (MTR) parametric maps demonstrated concurrent caudal-to-rostral and medial-to-lateral gradients of MTR decrease within corpus callosum (CC) that correlated well with demyelination assessed histologically. Our results show that demyelination was not limited to the midsagittal line of the corpus callosum, and also that opposing gradients of demyelination occur in the lateral and medial CC. T2-weighted MRI gray/white matter contrast was strong at baseline, weak after 6 weeks of cuprizone treatment, and returned to a limited extent after recovery. MTR decreases during demyelination were observed throughout the brain, most clearly in callosal white matter. Myelin damage and repair appear to be influenced by proximity to oligodendrocyte progenitor cell populations and exhibit an inverse correlation with myelin basic protein gene expression. These findings suggest that susceptibility to injury and ability to repair vary across the brain, and whole-brain analysis is necessary to accurately characterize this model. Whole-brain parametric mapping across time is essential for gaining a real understanding of disease processes in-vivo. MTR increases in healthy mice throughout adolescence and adulthood were observed, illustrating the need for appropriate age-matched controls. Elucidating the unique and site-specific demyelination in the cuprizone model may offer new insights into in mechanisms of both damage and repair in human demyelinating diseases.

Connectome and Maturation Profiles of the Developing Mouse Brain Using Diffusion Tensor Imaging

This paper presents a comprehensive effort to establish a structural mouse connectome using diffusion tensor magnetic resonance imaging coupled with connectivity analysis tools. This work lays the foundation for imaging-based structural connectomics of the mouse brain, potentially facilitating a whole-brain network analysis to quantify brain changes in connectivity during development, as well as deviations from it related to genetic effects. A connectomic trajectory of maturation during postnatal ages 2-80 days is presented in the C57BL/6J mouse strain, using a whole-brain connectivity analysis, followed by investigations based on local and global network features. The global network measures of density, global efficiency, and modularity demonstrated a nonlinear relationship with age. The regional network metrics, namely degree and local efficiency, displayed a differential change in the major subcortical structures such as the thalamus and hippocampus, and cortical regions such as visual and motor cortex. Finally, the connectomes were used to derive an index of "brain connectivity index," which demonstrated a high correlation (r = 0.95) with the chronological age, indicating that brain connectivity is a good marker of normal age progression, hence valuable in detecting subtle deviations from normality caused by genetic, environmental, or pharmacological manipulations.

Progressive volume loss and white matter degeneration in cstb-deficient mice: a diffusion tensor and longitudinal volumetry MRI study

Unverricht-Lundborg type progressive myoclonus epilepsy (EPM1, OMIM 254800) is an autosomal recessive disorder characterized by onset at the age of 6 to 16 years, incapacitating stimulus-sensitive myoclonus and tonic-clonic epileptic seizures. It is caused by mutations in the gene encoding cystatin B. Previously, widespread white matter changes and atrophy has been detected both in adult EPM1 patients and in 6-month-old cystatin B-deficient mice, a mouse model for the EPM1 disease. In order to elucidate the spatiotemporal dynamics of the brain atrophy and white matter changes in EPM1, we conducted longitudinal in vivo magnetic resonance imaging and ex vivo diffusion tensor imaging accompanied with tract-based spatial statistics analysis to compare volumetric changes and fractional anisotropy in the brains of 1 to 6 months of age cystatin B-deficient and control mice. The results reveal progressive but non-uniform volume loss of the cystatin B-deficient mouse brains, indicating that different neuronal populations possess distinct sensitivity to the damage caused by cystatin B deficiency. The diffusion tensor imaging data reveal early and progressive white matter alterations in cystatin B-deficient mice affecting all major tracts. The results also indicate that the white matter damage in the cystatin B-deficient brain is most likely secondary to glial activation and neurodegenerative events rather than a primary result of CSTB deficiency. The data also show that diffusion tensor imaging combined with TBSS analysis provides a feasible approach not only to follow white matter damage in neurodegenerative mouse models but also to detect fractional anisotropy changes related to normal white matter maturation and reorganisation.

Quantitative magnetic susceptibility of the developing mouse brain reveals microstructural changes in the white matter

Cerebral development involves a complex cascade of events which are difficult to visualize and quantify in vivo. In this study we combine information from Diffusion Tensor Imaging (DTI) and Quantitative Susceptibility Mapping (QSM) to analyze developing mouse brains at five stages up to 56days postnatal. Susceptibility maps were calculated using frequency shifts in gradient echo MR images acquired at 9.4T. The mean apparent magnetic susceptibility and magnetic susceptibility anisotropy of major white matter tracts were evaluated as a function of age. During the first two weeks, susceptibility of white matter appeared paramagnetic relative to surrounding gray matter; it then gradually became more diamagnetic. While diffusion anisotropy was already apparent and high at postnatal day 2, susceptibility anisotropy only became significant during the third week. This mismatch indicated different microstructural underpinnings for diffusion anisotropy and susceptibility anisotropy. Histological exams were also performed to evaluate myelin and iron content. It is confirmed that the main source of susceptibility contrast in WM is the myelin content. The ability to quantify the magnetic properties of white matter will provide valuable information on the architecture of the brain during development and potentially a more specific indicator for myelin degenerative diseases.

Effects of repeated adolescent stress and serotonin transporter gene partial knockout in mice on behaviors and brain structures relevant to major depression

In humans, exposure to stress during development is associated with structural and functional alterations of the prefrontal cortex (PFC), amygdala (AMY), and hippocampus (HC) and their circuits of connectivity, and with an increased risk for developing major depressive disorder particularly in carriers of the short (s) variant of the serotonin transporter (5-HTT) gene-linked polymorphic region (5-HTTLPR). Although changes in these regions are found in carriers of the s allele and/or in depressed patients, evidence for a specific genotype × developmental stress effect on brain structure and function is limited. Here, we investigated the effect of repeated stress exposure during adolescence in mice with partial knockout of the 5-HTT gene (HET) vs. wildtype (WT) on early-adulthood behavioral measures and brain structure [using magnetic resonance imaging (MRI)] relevant to human major depression. Behaviorally, adolescent stress (AS) increased anxiety and decreased activity and did so to a similar degree in HET and WT. In a probabilistic reversal learning task, HET-AS mice achieved fewer reversals than did HET-No-AS mice. 5-HTT genotype and AS were without effect on corticosterone stress response. In terms of structural brain differences, AS reduced the volume of two long-range white matter tracts, the optic tract (OT) and the cerebral peduncle (CP), in WT mice specifically. In a region-of-interest analysis, AS was associated with increased HC volume and HET genotype with a decreased frontal lobe volume. In conclusion, we found that 5-HTT and AS genotype exerted long-term effects on behavior and development of brain regions relevant to human depression.

Diffusion MRI of the developing cerebral cortical gray matter can be used to detect abnormalities in tissue microstructure associated with fetal ethanol exposure

Fetal alcohol spectrum disorders (FASDs) comprise a wide range of neurological deficits that result from fetal exposure to ethanol (EtOH), and are the leading cause of environmentally related birth defects and mental retardation in the western world. One aspect of diagnostic and therapeutic intervention strategies that could substantially improve our ability to combat this significant problem would be to facilitate earlier detection of the disorders within individuals. Light microscopy-based investigations performed by several laboratories have previously shown that morphological development of neurons within the early-developing cerebral cortex is abnormal within the brains of animals exposed to EtOH during fetal development. We and others have recently demonstrated that diffusion MRI can be of utility for detecting abnormal cellular morphological development in the developing cerebral cortex. We therefore assessed whether diffusion tensor imaging (DTI) could be used to distinguish the developing cerebral cortices of ex vivo rat pup brains born from dams treated with EtOH (EtOH; 4.5 g/kg, 25%) or calorie-matched quantities of maltose/dextrin (M/D) throughout gestation. Water diffusion and tissue microstructure were investigated using DTI (fractional anisotropy, FA) and histology (anisotropy index, AI), respectively. Both FA and AI decreased with age, and were higher in the EtOH than the M/D group at postnatal ages (P)0, P3, and P6. Additionally, there was a significant correlation between FA and AI measurements. These findings provide evidence that disruptions in cerebral cortical development induced by EtOH exposure can be revealed by water diffusion anisotropy patterns, and that these disruptions are directly related to cerebral cortical differentiation.

Long-term functional outcomes and correlation with regional brain connectivity by MRI diffusion tractography metrics in a near-term rabbit model of intrauterine growth restriction

BACKGROUND

Intrauterine growth restriction (IUGR) affects 5-10% of all newborns and is associated with increased risk of memory, attention and anxiety problems in late childhood and adolescence. The neurostructural correlates of long-term abnormal neurodevelopment associated with IUGR are unknown. Thus, the aim of this study was to provide a comprehensive description of the long-term functional and neurostructural correlates of abnormal neurodevelopment associated with IUGR in a near-term rabbit model (delivered at 30 days of gestation) and evaluate the development of quantitative imaging biomarkers of abnormal neurodevelopment based on diffusion magnetic resonance imaging (MRI) parameters and connectivity.

METHODOLOGY

At +70 postnatal days, 10 cases and 11 controls were functionally evaluated with the Open Field Behavioral Test which evaluates anxiety and attention and the Object Recognition Task that evaluates short-term memory and attention. Subsequently, brains were collected, fixed and a high resolution MRI was performed. Differences in diffusion parameters were analyzed by means of voxel-based and connectivity analysis measuring the number of fibers reconstructed within anxiety, attention and short-term memory networks over the total fibers.

PRINCIPAL FINDINGS

The results of the neurobehavioral and cognitive assessment showed a significant higher degree of anxiety, attention and memory problems in cases compared to controls in most of the variables explored. Voxel-based analysis (VBA) revealed significant differences between groups in multiple brain regions mainly in grey matter structures, whereas connectivity analysis demonstrated lower ratios of fibers within the networks in cases, reaching the statistical significance only in the left hemisphere for both networks. Finally, VBA and connectivity results were also correlated with functional outcome.

CONCLUSIONS

The rabbit model used reproduced long-term functional impairments and their neurostructural correlates of abnormal neurodevelopment associated with IUGR. The description of the pattern of microstructural changes underlying functional defects may help to develop biomarkers based in diffusion MRI and connectivity analysis.

Diffusion tensor magnetic resonance histology reveals microstructural changes in the developing rat brain

The postnatal period is a remarkably dynamic phase of brain growth and development characterized by large-scale macrostructural changes, as well as dramatic microstructural changes, including myelination and cortical layering. This crucial period of neurodevelopment is uniquely susceptible to a wide variety of insults that may lead to neurologic disease. MRI is an important tool for studying both normal and abnormal neurodevelopmental changes, and quantitative imaging strategies like diffusion tensor imaging (DTI) allow visualization of many of the complex microstructural changes that occur during postnatal life. Diffusion tensor magnetic resonance histology (DT-MRH) provides particularly unique insight into cytoarchitectural changes in the developing brain. In this study, we used DT-MRH to track microstructural changes in the rat brain throughout normal postnatal neurodevelopment. We provide examples of diffusion tensor parameter changes in both white matter and gray matter structures, and correlate these changes with changes in cytoarchitecture. Finally, we provide a comprehensive database of image sets as a foundation for future studies using DT-MRH to characterize abnormal neurodevelopment in rodent models of neurodevelopmental disease.

Multimodal magnetic resonance imaging assessment of white matter aging trajectories over the lifespan of healthy individuals

BACKGROUND

Postmortem and volumetric imaging data suggest that brain myelination is a dynamic lifelong process that, in vulnerable late-myelinating regions, peaks in middle age. We examined whether known regional differences in axon size and age at myelination influence the timing and rates of development and degeneration/repair trajectories of white matter (WM) microstructure biomarkers.

METHODS

Healthy subjects (n = 171) 14-93 years of age were examined with transverse relaxation rate (R(2)) and four diffusion tensor imaging measures (fractional anisotropy [FA] and radial, axial, and mean diffusivity [RD, AxD, MD, respectively]) of frontal lobe, genu, and splenium of the corpus callosum WM (FWM, GWM, and SWM, respectively).

RESULTS

Only R(2) reflected known levels of myelin content with high values in late-myelinating FWM and GWM regions and low ones in early-myelinating SWM. In FWM and GWM, all metrics except FA had significant quadratic components that peaked at different ages (R(2) < RD < MD < AxD), with FWM peaking later than GWM. Factor analysis revealed that, although they defined different factors, R(2) and RD were the metrics most closely associated with each other and differed from AxD, which entered into a third factor.

CONCLUSIONS

The R(2) and RD trajectories were most dynamic in late-myelinating regions and reflect age-related differences in myelination, whereas AxD reflects axonal size and extra-axonal space. The FA and MD had limited specificity. The data suggest that the healthy adult brain undergoes continual change driven by development and repair processes devoted to creating and maintaining synchronous function among neural networks on which optimal cognition and behavior depend.

Deafferentation-induced plasticity of visual callosal connections: predicting critical periods and analyzing cortical abnormalities using diffusion tensor imaging

Callosal connections form elaborate patterns that bear close association with striate and extrastriate visual areas. Although it is known that retinal input is required for normal callosal development, there is little information regarding the period during which the retina is critically needed and whether this period correlates with the same developmental stage across species. Here we review the timing of this critical period, identified in rodents and ferrets by the effects that timed enucleations have on mature callosal connections, and compare it to other developmental milestones in these species. Subsequently, we compare these events to diffusion tensor imaging (DTI) measurements of water diffusion anisotropy within developing cerebral cortex. We observed that the relationship between the timing of the critical period and the DTI-characterized developmental trajectory is strikingly similar in rodents and ferrets, which opens the possibility of using cortical DTI trajectories for predicting the critical period in species, such as humans, in which this period likely occurs prenatally. Last, we discuss the potential of utilizing DTI to distinguish normal from abnormal cerebral cortical development, both within the context of aberrant connectivity induced by early retinal deafferentation, and more generally as a potential tool for detecting abnormalities associated with neurodevelopmental disorders.

The diffusion tensor imaging toolbox

During the past few years, The Journal of Neuroscience has published more than 30 articles that describe investigations that used Diffusion Tensor Imaging (DTI) and related techniques as a primary observation method. This illustrates a growing interest in DTI within the basic and clinical neuroscience communities. This article summarizes DTI methodology in terms that can be immediately understood by the neuroscientist who has little previous exposure to DTI. It describes the fundamentals of water molecular diffusion coefficient measurement in brain tissue and illustrates how these fundamentals can be used to form vivid and useful depictions of white matter macroscopic and microscopic anatomy. It also describes current research applications and the technique's attributes and limitations. It is hoped that this article will help the readers of this Journal to more effectively evaluate neuroscience studies that use DTI.

Mesenchymal stem cells restore cortical rewiring after neonatal ischemia in mice

OBJECTIVE

A study was undertaken to investigate the effect of neonatal hypoxic-ischemic (HI) brain damage and mesenchymal stem cell (MSC) treatment on the structure and contralesional connectivity of motor function-related cerebral areas.

METHODS

Brain remodeling after HI±MSC treatment in neonatal mice was analyzed using diffusion tensor magnetic resonance imaging, immunohistochemistry, anterograde tracing with biotinylated dextran amine (BDA), and retrograde tracing with fluorescent pseudorabies virus (PRV).

RESULTS

MSC treatment after HI reduced contralesional rewiring taking place after HI. Following MSC treatment, fractional anisotropy values, which were increased in both ipsi- and contralesional cortices and decreased in the corpus callosum (CC) after HI, were normalized to the level observed in sham-operated mice. These results were corroborated by myelin basic protein intensity and staining pattern in these areas. Anterograde tracing of ipsilesional motor neurons showed that after MSC treatment, fewer BDA-positive fibers crossed the CC and extended into the contralesional motor cortex compared to HI mice. This remodeling was functional, because retrograde labeling showed increased connectivity between impaired (left) forepaw and the contralesional (left) motor cortex after HI, whereas MSC treatment reduced this connection and increased the connection between the impaired (left) forepaw and the ipsilesional (right) motor cortex. Finally, the extent of contralesional rewiring measured with BDA and PRV tracing was related to sensorimotor dysfunction.

INTERPRETATION

This is the first study to describe MSC treatment after neonatal HI markedly reducing contralesional axonal remodeling induced by HI brain injury.

The translational role of diffusion tensor image analysis in animal models of developmental pathologies

Diffusion tensor magnetic resonance imaging (DTI) has proven itself a powerful technique for clinical investigation of the neurobiological targets and mechanisms underlying developmental pathologies. The success of DTI in clinical studies has demonstrated its great potential for understanding translational animal models of clinical disorders, and preclinical animal researchers are beginning to embrace this new technology to study developmental pathologies. In animal models, genetics can be effectively controlled, drugs consistently administered, subject compliance ensured, and image acquisition times dramatically increased to reduce between-subject variability and improve image quality. When pairing these strengths with the many positive attributes of DTI, such as the ability to investigate microstructural brain organization and connectivity, it becomes possible to delve deeper into the study of both normal and abnormal development. The purpose of this review is to provide new preclinical investigators with an introductory source of information about the analysis of data resulting from small animal DTI studies to facilitate the translation of these studies to clinical data. In addition to an in-depth review of translational analysis techniques, we present a number of relevant clinical and animal studies using DTI to investigate developmental insults in order to further illustrate techniques and to highlight where small animal DTI could potentially provide a wealth of translational data to inform clinical researchers.

Atlas-based automatic mouse brain image segmentation revisited: model complexity vs. image registration

Although many atlas-based segmentation methods have been developed and validated for the human brain, limited work has been done for the mouse brain. This paper investigated roles of image registration and segmentation model complexity in the mouse brain segmentation. We employed four segmentation models [single atlas, multiatlas, simultaneous truth and performance level estimation (STAPLE) and Markov random field (MRF) via four different image registration algorithms (affine, B-spline free-form deformation (FFD), Demons and large deformation diffeomorphic metric mapping (LDDMM)] for delineating 19 structures from in vivo magnetic resonance microscopy images. We validated their accuracies against manual segmentation. Our results revealed that LDDMM outperformed Demons, FFD and affine in any of the segmentation models. Under the same registration, increasing segmentation model complexity from single atlas to multiatlas, STAPLE or MRF significantly improved the segmentation accuracy. Interestingly, the multiatlas-based segmentation using nonlinear registrations (FFD, Demons and LDDMM) had similar performance to their STAPLE counterparts, while they both outperformed their MRF counterparts. Furthermore, when the single-atlas affine segmentation was used as reference, the improvement due to nonlinear registrations (FFD, Demons and LDDMM) in the single-atlas segmentation model was greater than that due to increasing model complexity (multiatlas, STAPLE and MRF affine segmentation). Hence, we concluded that image registration plays a more crucial role in the atlas-based automatic mouse brain segmentation as compared to model complexity. Multiple atlases with LDDMM can best improve the segmentation accuracy in the mouse brain among all segmentation models tested in this study.

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 processes 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.

Murine diffusion imaging using snapshot interleaved EPI acquisition at 7T

Diffusion tensor imaging (DTI) is a powerful magnetic resonance imaging tool for quantitative assessment of white matter micro structure. The majority of DTI methods employ Echo Planar Imaging (EPI) because it is insensitive to motion. However, EPI suffers from distortions and signal losses induced by inhomogeneities in magnetic field susceptibility. This is particularly accentuated in murine imaging at very high magnetic fields. The purpose of this study is to demonstrate that a Snapshot Interleaved EPI acquisition block combined with a stimulated echo module for diffusion sensitization can be successfully used to obtain high quality DTI of a mouse brain at 7T. This technique preserves the EPI speed but reduces its susceptibility artifacts and signal losses. Signal to noise ratio is also reduced but remains higher than in the DTI acquisitions based on a fast low angle shot technique. In vivo results using this new approach are presented along with a full description of the methodology.

MR diffusion kurtosis imaging for neural tissue characterization

In conventional diffusion tensor imaging (DTI), water diffusion distribution is described as a 2nd-order three-dimensional (3D) diffusivity tensor. It assumes that diffusion occurs in a free and unrestricted environment with a Gaussian distribution of diffusion displacement, and consequently that diffusion weighted (DW) signal decays with diffusion factor (b-value) monoexponentially. In biological tissue, complex cellular microstructures make water diffusion a highly hindered or restricted process. Non-monoexponential decays are experimentally observed in both white matter and gray matter. As a result, DTI quantitation is b-value dependent and DTI fails to fully utilize the diffusion measurements that are inherent to tissue microstructure. Diffusion kurtosis imaging (DKI) characterizes restricted diffusion and can be readily implemented on most clinical scanners. It provides a higher-order description of water diffusion process by a 2nd-order 3D diffusivity tensor as in conventional DTI together with a 4th-order 3D kurtosis tensor. Because kurtosis is a measure of the deviation of the diffusion displacement profile from a Gaussian distribution, DKI analyses quantify the degree of diffusion restriction or tissue complexity without any biophysical assumption. In this work, the theory of diffusion kurtosis and DKI including the directional kurtosis analysis is revisited. Several recent rodent DKI studies from our group are summarized, and DKI and DTI compared for their efficacy in detecting neural tissue alterations. They demonstrate that DKI offers a more comprehensive approach than DTI in describing the complex water diffusion process in vivo. By estimating both diffusivity and kurtosis, it may provide improved sensitivity and specificity in MR diffusion characterization of neural tissues.

In vivo diffusion tensor magnetic resonance imaging and fiber tracking of the mouse brain

Until very recently, the study of neural architecture using fixed tissue has been a major scientific focus of neurologists and neuroanatomists. A non-invasive detailed insight into the brain's axonal connectivity in vivo has only become possible since the development of diffusion tensor magnetic resonance imaging (DT-MRI). This unique approach of analyzing axonal projections in the living brain was used in the present study to describe major white matter fiber tracts of the mouse brain and also to identify for the first time non-invasively the rich connectivity between the amygdala and different target regions. To overcome the difficulties associated with high spatially and temporally resolved DT-MRI measurements a 4-shot diffusion weighted spin echo (SE) echo planar imaging (EPI) protocol was adapted to mouse brain imaging at 9.4T. Diffusion tensor was calculated from data sets acquired by using 30 diffusion gradient directions while keeping the acquisition time at 91 min. Two fiber tracking algorithms were employed. A deterministic approach (fiber assignment by continuous tracking - FACT algorithm) allowed us to identify and generate the 3D representations of various neural pathways. A probabilistic approach was further used for the generation of probability maps of connectivity with which it was possible to investigate - in a statistical sense - all possible connecting pathways between selected seed points. We show here applications to determine the connection probability between regions belonging to the visual or limbic systems. This method does not require a priori knowledge about the projections' trajectories and is shown to be efficient even if the investigated pathway is long or three-dimensionally complex. Additionally, high resolution images of rotational invariant parameters of the diffusion tensor, such as fractional anisotropy, volume ratio or main eigenvalues allowed quantitative comparisons in-between regions of interest (ROIs) and showed significant differences between various white matter regions.

Normobaric hyperoximia increases hypoxia-induced cerebral injury: DTI study in rats

Perinatal hypoxia affects normal neurological development and can lead to motor, behavioral and cognitive deficits. A common acute treatment for perinatal hypoxia is oxygen resuscitation (hyperoximia), a controversial treatment. Magnetic resonance imaging (MRI), including diffusion tensor imaging (DTI), was performed in a P7 rat model of perinatal hypoxia to determine the effect of hyperoximia. These studies were performed on two groups of animals: 1) animals which were subjected to ischemia followed by hypoxia (HI), and 2) HI followed by hyperoximic treatment (HHI). Lesion volumes on high resolution MRI and DTI derived measures, fractional anisotropy (FA), mean diffusivity (MD), and axial and radial diffusivities (lambda(l) and lambda(t), respectively) were measured in vivo one day, one week, and three weeks after injury. Most significant differences in the MRI and DTI measures were found at three weeks after injury. Specifically, three weeks after HHI injury resulted in significantly larger hyperintense lesion volumes (95.26 +/- 50.42 mm(3)) compared to HI (22.25 +/- 17.62 mm(3)). The radial diffusivity lambda(t) of the genu of corpus callosum was significantly larger in HHI (681 +/- 330 x 10(-6) mm(2)/sec) than in HI (486 +/- 96 x 10(-6) mm(2)/sec). Over all, most significant differences in all the DTI metrics (FA, MD, lambda(t), lambda(l)) at all time points were found in the corpus callosum. Our results suggest that treatment of perinatal hypoxia with normobaric oxygen does not ameliorate, but exacerbates damage.

Diffusion features of white matter in tuberous sclerosis with tractography

Normal-appearing white matter has been shown via diffusion tensor imaging to be affected in tuberous sclerosis complex. Under the hypothesis that some systems might be differentially affected, including the visual pathways and systems of social cognition, diffusion properties of various regions of white matter were compared. For 10 patients and 6 age-matched control subjects, 3 T magnetic resonance imaging was assessed using diffusion tensor imaging obtained in 35 directions. Three-dimensional volumes corresponding to the geniculocalcarine tracts were extracted via tractography, and two-dimensional regions of interest were used to sample other regions. Regression analysis indicated lower fractional anisotropy in the splenium of corpus callosum and geniculocalcarine tracts in tuberous sclerosis complex group, as well as lower axial diffusivity in the internal capsule, superior temporal gyrus, and geniculocalcarine tracts. Mean and radial diffusivity of the splenium of corpus callosum were higher in the tuberous sclerosis complex group. The differences in diffusion properties of white matter between tuberous sclerosis complex patients and control subjects suggest disorganized and structurally compromised axons with poor myelination. The visual and social cognition systems appear to be differentially involved, which might in part explain the behavioral and cognitive characteristics of the tuberous sclerosis complex population.

Magnetic resonance imaging and micro-computed tomography combined atlas of developing and adult mouse brains for stereotaxic surgery

Stereotaxic atlases of the mouse brain are important in neuroscience research for targeting of specific internal brain structures during surgical operations. The effectiveness of stereotaxic surgery depends on accurate mapping of the brain structures relative to landmarks on the skull. During postnatal development in the mouse, rapid growth-related changes in the brain occur concurrently with growth of bony plates at the cranial sutures, therefore adult mouse brain atlases cannot be used to precisely guide stereotaxis in developing brains. In this study, three-dimensional stereotaxic atlases of C57BL/6J mouse brains at six postnatal developmental stages: postnatal day (P) 7, P14, P21, P28, P63 and in adults (P140-P160) were developed, using diffusion tensor imaging (DTI) and micro-computed tomography (CT). At present, most widely-used stereotaxic atlases of the mouse brain are based on histology, but the anatomical fidelity of ex vivo atlases to in vivo mouse brains has not been evaluated previously. To account for ex vivo tissue distortion due to fixation as well as individual variability in the brain, we developed a population-averaged in vivo magnetic resonance imaging adult mouse brain stereotaxic atlas, and a distortion-corrected DTI atlas was generated by nonlinearly warping ex vivo data to the population-averaged in vivo atlas. These atlas resources were developed and made available through a new software user-interface with the objective of improving the accuracy of targeting brain structures during stereotaxic surgery in developing and adult C57BL/6J mouse brains.

Mild hypoxic-ischemic injury in the neonatal rat brain: longitudinal evaluation of white matter using diffusion tensor MR imaging

BACKGROUND AND PURPOSE

Selective white matter (WM) damage is a known sequela of mild hypoxic-ischemic (HI) injury in the neonatal rat model. The aim of this study was to evaluate longitudinally mild HI-induced WM damage (represented by the external capsule [EC]) by diffusion tensor MR imaging (DTI) and to correlate the findings with histology.

MATERIALS AND METHODS

Seven-day-old Sprague-Dawley rats (n = 19) underwent unilateral ligation of the left common carotid artery followed by hypoxia for 50 minutes to create mild HI injury. DTI was performed longitudinally at 5 time points from day 1 to day 90 postinjury (n = 19, 16, 13, 11, 9, respectively), and fractional anisotropy (FA), trace, radial diffusivity (lambda( perpendicular)), and axial diffusivity (lambda(//)) of the injury and control contralateral ECs were quantified. Rats were randomly sacrificed (n = 15, in total), and the corresponding ECs were stained with hematoxylin-eosin, Luxol fast blue (LFB), and neurofilament (NF) to evaluate morphologic changes, amount of myelin, and axonal count at every time point. A paired t test was applied to evaluate statistical differences between both ECs, and the Pearson correlation test was used to evaluate the relationships between DTI indices and histologic evaluations. In addition, longitudinal changes in DTI indices and histologic evaluations were analyzed by a linear mixed model and an analysis of variance test, respectively.

RESULTS

We demonstrated significantly decreased FA, increased lambda( perpendicular), and similar lambda(//) in the injury compared with the control EC, which was persistent through all time points. Histologic evaluation by LFB and NF staining showed reduced myelin stain intensity in the injury EC and similar axonal counts in both ECs. Longitudinally, there was an increase in FA, a decrease in lambda( perpendicular) and trace, and stability in lambda(//) in both ECs. Also, there was progressive reduction in the differences in FA, trace, and lambda( perpendicular) between the injury and control EC, especially between day 1 and day 7 postinjury and in tandem with changes in myelin stain. FA was significantly correlated with myelin stain (r = 0.681, P < .01) and axonal count (r = 0.673, P < .01), whereas lambda( perpendicular) was significantly correlated with myelin stain only (r = -0.528, P < .01), and lambda(//), with axonal count only (r = 0.372, P = .043).

CONCLUSIONS

Diffusion indices can reflect dysmyelination in mild HI injury, continual myelination of both injury and control ECs with growth, and the partial recovery of myelin postinjury. We propose that diffusion indices may be used as biomarkers to monitor noninvasively the longitudinal changes of mild HI-induced WM damage.

Myelination of the corpus callosum in male and female rats following complex environment housing during adulthood

Myelination is an important process in brain development, and delays or abnormalities in this process have been associated with a number of conditions including autism, developmental delay, attention deficit disorder, and schizophrenia. Myelination can be sensitive to developmental experience; however, although the adult brain remains highly plastic, it is unknown whether myelination continues to be sensitive to experience during adulthood. Male and female rats were socially housed until four months of age, at which time they were moved into either a complex or "enriched" environment (EC) or an isolated condition (IC). Although the area of the splenium (posterior 20% of the callosum, which contains axons from visual cortical neurons) increased by about 10% following two months of EC housing, the area occupied by myelinated axons was not influenced by adult housing condition. Instead, it was the area occupied by glial cell processes and unmyelinated axons which significantly increased following EC housing. Neither the size nor the myelin content of the genu (anterior 15% of the callosum) was sensitive to manipulations of adult housing condition, but males had more area occupied by myelinated axons in both callosal regions. Finally, the inability of two months of complex environment housing during adulthood to impact the number of myelinated axons in the splenium was confirmed in a subset of animals using quantitative electron microscopy. We conclude that the sensitivity of myelination to experience is reduced in adulthood relative to development in both sexes.

The MT pool size ratio and the DTI radial diffusivity may reflect the myelination in shiverer and control mice

A quantitative magnetization transfer (qMT) technique was employed to quantify the ratio of the sizes of the bound and free water proton pools in ex vivo mouse brains. The goal was to determine the pool size ratio sensitivity to myelin. Fixed brains from both shiverer mice and control littermates were imaged. The pool size ratio in the corpus callosum of shiverer mice was substantially lower than that in the control mice, while there was no distinguishable difference in the pool size ratio in the gray matter. These results correlate with diffusion tensor imaging (DTI) derived radial diffusivity which previously was shown to reflect myelin integrity in this animal model. Histological study reveals the presence of myelin in control mice white matter and the absence of myelin in shiverer mice white matter, supporting the qMT and DTI results. Our findings support the view that qMT may be used for estimating myelin integrity.

Region-specific susceptibilities to cuprizone-induced lesions in the mouse forebrain: Implications for the pathophysiology of schizophrenia

Cuprizone (CPZ) is a neurotoxic agent acting as a copper chelator. In our recent study, C57BL/6 mice given dietary CPZ (0.2%) showed impairments in spatial working memory, social interaction, and prepulse inhibition. These abnormalities are reminiscent of certain schizophrenia symptoms and are not likely due to damage in the whole brain or in any single white matter tract/brain region. We hypothesized that white matter damage resulting from CPZ-treatment may be site-specific rather than universal. We examined the forebrains of C57BL/6 mice given the CPZ-containing diet and compared them with those of controls. We assessed CPZ-induced demyelination in main white matter tracts of the forebrain, evaluated myelin break down in the neuropil of the main olfactory bulb (MOB), cerebral cortex (CTX), caudate putamen (CP), hippocampus (HP), thalamus (TH), and hypothalamus (HY), and counted the number of myelin sheath forming oligodendrocytes (OLs) in CTX, CP, TH, and HY. Obvious demyelination was observed in the corpus callosum, external capsule, CP, and dorsal hippocampal commissure whereas other tracts seemed to be unaffected. The neuropil of CTX, HP and MOB showed myelin break down, which was mild in TH and HY. The number of OLs was decreased in all the above regions of CPZ-treated mice although the degree of OL loss was not consistent across regions. The data provide further support for white matter abnormalities contributing to schizophrenia-like behaviors in mice.

Does diffusion kurtosis imaging lead to better neural tissue characterization? A rodent brain maturation study

Diffusion kurtosis imaging (DKI) can be used to estimate excess kurtosis, which is a dimensionless measure for the deviation of water diffusion profile from Gaussian distribution. Several recent studies have applied DKI to probe the restricted water diffusion in biological tissues. The directional analysis has also been developed to obtain the directionally specific kurtosis. However, these studies could not directly evaluate the sensitivity of DKI in detecting subtle neural tissue alterations. Brain maturation is known to involve various biological events that can affect water diffusion properties, thus providing a sensitive platform to evaluate the efficacy of DKI. In this study, in vivo DKI experiments were performed in normal Sprague-Dawley rats of 3 different ages: postnatal days 13, 31 and 120 (N=6 for each group). Regional analysis was then performed for 4 white matter (WM) and 3 gray matter (GM) structures. Diffusivity and kurtosis estimates derived from DKI were shown to be highly sensitive to the developmental changes in these chosen structures. Conventional diffusion tensor imaging (DTI) parameters were also computed using monoexponential model, yielding reduced sensitivity and directional specificity in monitoring the brain maturation changes. These results demonstrated that, by measuring directionally specific diffusivity and kurtosis, DKI offers a more comprehensive and sensitive detection of tissue microstructural changes. Such imaging advance can provide a better MR diffusion characterization of neural tissues, both WM and GM, in normal, developmental and pathological states.

Quantification of brain maturation and growth patterns in C57BL/6J mice via computational neuroanatomy of diffusion tensor images

Diffusion Tensor magnetic resonance imaging and computational neuroanatomy are used to quantify postnatal developmental patterns of C57BL/6J mouse brain. Changes in neuronal organization and myelination occurring as the brain matures into adulthood are examined, and a normative baseline is developed, against which transgenic mice may be compared in genotype-phenotype studies. In early postnatal days, gray matter-based cortical and hippocampal structures exhibit high water diffusion anisotropy, presumably reflecting the radial neuronal organization. Anisotropy drops rapidly within a week, indicating that the underlying brain tissue becomes more isotropic in orientation, possibly due to formation of a complex randomly intertwined web of dendrites. Gradual white matter anisotropy increase implies progressively more organized axonal pathways, likely reflecting the myelination of axons forming tightly packed fiber bundles. In contrast to the spatially complex pattern of tissue maturation, volumetric growth is somewhat uniform, with the cortex and the cerebellum exhibiting slightly more pronounced growth. Temporally, structural growth rates demonstrate an initial rapid volumetric increase in most structures, gradually tapering off to a steady state by about 20 days. Fiber maturation reaches steady state in about 10 days for the cortex, to 30-40 days for the corpus callosum, the hippocampus, and the internal and external capsules.

Residual stress in the adult mouse brain

This work provides direct evidence that sustained tensile stress exists in white matter of the mature mouse brain. This finding has important implications for the mechanisms of brain development, as tension in neural axons has been hypothesized to drive cortical folding in the human brain. In addition, knowledge of residual stress is required to fully understand the mechanisms behind traumatic brain injury and changes in mechanical properties due to aging and disease. To estimate residual stress in the brain, we performed serial dissection experiments on 500-mum thick coronal slices from fresh adult mouse brains and developed finite element models for these experiments. Radial cuts were made either into cortical gray matter, or through the cortex and the underlying white matter tract composed of parallel neural axons. Cuts into cortical gray matter did not open, but cuts through both layers consistently opened at the point where the cut crossed the white matter. We infer that the cerebral white matter is under considerable tension in the circumferential direction in the coronal cerebral plane, parallel to most of the neural fibers, while the cerebral cortical gray matter is in compression. The models show that the observed deformation after cutting can be caused by more growth in the gray matter than in the white matter, with the estimated tensile stress in the white matter being on the order of 100-1,000 Pa.

Early postnatal development of rat brain: in vivo diffusion tensor imaging

Perinatal hypoxia is a major cause of neurodevelopmental deficits. Neuronal migration patterns are particularly sensitive to perinatal hypoxia/ischemia and are associated with the clinical deficits. The rat model of hypoxia/ischemia at P7 mimics that of perinatal injury in humans. Before assessing the effects of postnatal injury on brain development, it is essential to determine the normal developmental trajectories of various brain structures in individual animals. In vivo longitudinal diffusion tensor imaging (DTI) was performed from postnatal day 0 (P0) to P56 on Wistar rats. The DTI metrics, mean diffusivity (MD), fractional anisotropy (FA), axial (lambdal) and radial (lambdat) diffusivities, were determined for four gray matter and eight white matter structures. The FA of the cortical plate and the body of corpus callosum decreased significantly during the first 3 weeks after birth. The decrease in the cortical plate's FA value was associated mainly with an increase in lambdat. The initial decrease in FA of corpus callosum was associated with a significant decrease in lambdal. The FA of corpus callosum increased during the rest of the observational period, which was mainly associated with a decrease in lambdat. The FA of gray matter structures, hippocampus, caudate putamen, and cortical mantle did not show significant changes between P0 and P56. In contrast, the majority of white matter structures showed significant changes between P0 and P56. These temporal changes in the DTI metrics were related to the neuronal and axonal pruning and myelination that are known to occur in the developing brain.

Imaging white matter diffusion changes with development and recovery from brain injury

PURPOSE

This study reviews the application of diffusion tensor imaging (DTI) to the study of developmental and pathological changes in brain white matter. The ability to measure and monitor such changes in vivo would provide important opportunities for charting disease progression and monitoring response to therapeutic intervention. This study first reviews the use of DTI in studying normal human brain development. It goes on to illustrate how DTI has been used to provide insights into recovery from damage in selected brain disorders.

CONCLUSIONS

It is concluded that potential clinical applications of DTI include: (i) monitoring pathological change, (ii) providing markers that predict recovery and allow for individual targeting of therapy, (iii) providing outcome measures, (iv) providing measures of potentially compensatory structural changes and (v) improving understanding of normal brain anatomy to aid in interpretation of the consequences of localized damage.

Increased dosage of DYRK1A and brain volumetric alterations in a YAC model of partial trisomy 21

A yeast artificial chromosome (YAC) transgenic murine model of partial trisomy 21 overexpressing five human genes -- including DYRK1A, which encodes a serine threonine kinase involved in cell cycle control -- has been shown to present an increase in brain weight. We analyzed this new phenotype by measuring total and regional brain volumes at different ages, using a 7 Tesla magnetic resonance imaging volumetric approach. Volumetric measurements showed a total volume increase of 13.6% in adult mice. Changes in brain morphogenesis were already visible at a very early postnatal stage (postnatal days 2-7). Region-specific changes were characterized from postnatal day 15 to 5 months. These results, made it possible to define region-specific effects of DYRK1A overexpression, with the strongest increase seen in the thalamus-hypothalamus area (24%).