Diffusion-tensor MR imaging and tractography: exploring brain microstructure and connectivity.
ABSTRACT Diffusion magnetic resonance (MR) imaging is evolving into a potent tool in the examination of the central nervous system. Although it is often used for the detection of acute ischemia, evaluation of directionality in a diffusion measurement can be useful in white matter, which demonstrates strong diffusion anisotropy. Techniques such as diffusion-tensor imaging offer a glimpse into brain microstructure at a scale that is not easily accessible with other modalities, in some cases improving the detection and characterization of white matter abnormalities. Diffusion MR tractography offers an overall view of brain anatomy, including the degree of connectivity between different regions of the brain. However, optimal utilization of the wide range of data provided with directional diffusion MR measurements requires careful attention to acquisition and postprocessing. This article will review the principles of diffusion contrast and anisotropy, as well as clinical applications in psychiatric, developmental, neurodegenerative, neoplastic, demyelinating, and other types of disease.
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ABSTRACT: Ischemically damaged brain can be accompanied by secondary degeneration of associated axonal connections e.g. Wallerian degeneration. Diffusion tensor imaging (DTI) is widely used to investigate axonal injury but the cellular correlates of many of the degenerative changes remain speculative. We investigated the relationship of DTI of directly damaged cerebral cortex and secondary axonal degeneration in the cerebral peduncle with cellular alterations in pan-axonal neurofilament staining, myelination, reactive astrocytes, activation of microglia/macrophages and neuronal cell death. DTI measures (axial, radial and mean diffusivity, and fractional anisotropy (FA)) were acquired at hyperacute (3 h), acute (1 and 2 d) and chronic (1 and 4 week) times after transient cerebral hypoxia with unilateral ischemia in neonatal rats. The tissue pathology underlying ischemic and degenerative responses had a complex relationship with DTI parameters. DTI changes at hyperacute and subacute times were smaller in magnitude and tended to be transient and/or delayed in cerebral peduncle compared to cerebral cortex. In cerebral peduncle by 1 d post-insult, there were reductions in neurofilament staining corresponding with decreases in parallel diffusivity which were more sensitive than mean diffusivity in detecting axonal changes. Ipsilesional reductions in FA within cerebral peduncle were robust in detecting both early and chronic degenerative responses. At one or four weeks post-insult, radial diffusivity was increased ipsilaterally in the cerebral peduncle corresponding to pathological evidence of a lack of ontogenic myelination in this region. The detailed differences in progression and magnitude of DTI and histological changes reported provide a reference for identifying the potential contribution of various cellular responses to FA, and, parallel, radial, and mean diffusivity.NeuroImage: Clinical. 01/2014;
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ABSTRACT: Drug-resistant epilepsy is a chronic debilitating disorder, but many cases are potentially curable by surgery. The key to the successful epilepsy surgery with complete postoperative seizure-free status is precise localization of the epileptic zone that must be resected and the adjacent eloquent areas of gray and white matter that must be preserved to avoid neurological defects. This article reviews the current state of the art epilepsy imaging techniques facilitating successful epilepsy surgery. The overview of the imaging appearances of the most common epileptogenic etiologies that are amenable to surgical resection is also included in this review.Clinical nuclear medicine 06/2014; 39(6):511-26. · 3.92 Impact Factor
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ABSTRACT: The rhesus macaque exhibits age-related brain changes similar to humans, making an excellent model of normal aging. Calorie restriction is a dietary intervention that reduces age-related comorbidities in short-lived animals, and its effects are still under study in rhesus macaques. Here, using deterministic fiber tracking method, we examined the effects of age and calorie restriction on a diffusion tensor imaging measure of white matter integrity, fractional anisotropy (FA), within white matter tracks traversing the anterior (genu) and posterior (splenium) corpus callosum in rhesus monkeys. Our results show: (1) a significant inverse relationship between age and mean FA of tracks traversing the genu and splenium; (2) higher mean FA of the splenium tracks as compared to that of genu tracks across groups; and (3) no significant diet effect on mean track FA through either location. These results are congruent with the age-related decline in white matter integrity reported in humans and monkeys, and the anterior-to-posterior gradient in white matter vulnerability to normal aging in humans. Further studies are warranted to critically evaluate the effect of calorie restriction on brain aging in this unique cohort of elderly primates.Neuroscience Letters 03/2014; · 2.03 Impact Factor