Margret Hund-Georgiadis

An Investigation of Functional and Anatomical Connectivity Using Magnetic Resonance Imaging

This article examines functional and anatomical connectivity in healthy human subjects measured with magnetic resonance imaging methods. Anatomical connectivity in white matter is obtained from measurements of the diffusion tensor. A Monte-Carlo simulation determines the probability that a particle diffuses between two points, with the probability of a jump in a particular direction from a given voxel being based on the local value of the diffusion tensor components. Functional connectivity between grey matter pixels is assessed without recourse to a specific activation paradigm, by calculating the correlation coefficient between random fluctuations in the blood oxygenation level-dependent signal time course in different pixels. The methods are used to examine the anatomical and functional connectivities between crowns of adjacent gyri. A high functional connectivity was found between grey matter pixels, with white matter displaying only very low correlation. A comparison of the measurements of anatomical and functional connectivity found that there is no simple correlation between these measures, except that low values of functional connectivity were not found together with high values of anatomical connectivity. Furthermore pairs of regions situated around the central sulcus indicated a dependence of the two connectivity measures on each other. These results are in accordance with an interpretation that regions which are clearly directly linked by white matter fiber tracts should show high functional connectivity, but that the inverse need not be true as functional connectivity may also be indirectly mediated via more distant grey matter regions.

Motor-learning-related changes in piano players and non-musicians revealed by functional magnetic-resonance signals

Abstract In this study, we investigated blood-flow-related magnetic-resonance (MR) signal changes and the time course underlying short-term motor learning of the dominant right hand in ten piano players (PPs) and 23 non-musicians (NMs), using a complex finger-tapping task. The activation patterns were analyzed for selected regions of interest (ROIs) within the two examined groups and were related to the subjects’ performance. A functional learning profile, based on the regional blood-oxygenation-level-dependent (BOLD) signal changes, was assessed in both groups. All subjects achieved significant increases in tapping frequency during the training session of 35 min in the scanner. PPs, however, performed significantly better than NMs and showed increasing activation in the contralateral primary motor cortex throughout motor learning in the scanner. At the same time, involvement of secondary motor areas, such as bilateral supplementary motor area, premotor, and cerebellar areas, diminished relative to the NMs throughout the training session. Extended activation of primary and secondary motor areas in the initial training stage (7–14 min) and rapid attenuation were the main functional patterns underlying short-term learning in the NM group; attenuation was particularly marked in the primary motor cortices as compared with the PPs. When tapping of the rehearsed sequence was performed with the left hand, transfer effects of motor learning were evident in both groups. Involvement of all relevant motor components was smaller than after initial training with the right hand. Ipsilateral premotor and primary motor contributions, however, showed slight increases of activation, indicating that dominant cortices influence complex sequence learning of the non-dominant hand. In summary, the involvement of primary and secondary motor cortices in motor learning is dependent on experience. Interhemispheric transfer effects are present.

Non-invasive regime for language lateralization in right- and left-handers by means of functional MRI and dichotic listening.

Language lateralization was assessed by two independent functional techniques, fMRI and a dichotic listening test (DLT), in an attempt to establish a reliable and non-invasive protocol of dominance determination. This should particularly address the high intraindividual variability of language lateralization and allow decision-making in individual cases. Functional MRI of word classification tasks showed robust language lateralization in 17 right-handers and 17 left-handers in terms of activation in the inferior frontal gyrus. The DLT was introduced as a complementary tool to MR mapping for language dominance assessment, providing information on perceptual language processing located in superior temporal cortices. The overall agreement of lateralization assessment between the two techniques was 97.1%. Conflicting results were found in one subject, and diverging indices in ten further subjects. Increasing age, non-familial sinistrality, and a non-dominant writing hand were identified as the main factors explaining the observed mismatch between the two techniques. This finding stresses the concept of an intrahemispheric distribution of language function that is obviously associated with certain behavioral characteristics.

Do quiescent arachnoid cysts alter CNS functional organization? A fMRI and morphometric study

Objective: To investigate whether congenital and clinically quiescent arachnoid cysts (AC) in the left temporal fossa alter the functional organization of adjacent cortices. Methods: fMRI mapping was applied in five right-handed asymptomatic patients to determine the functional organization of language. Moreover, morphometry was performed in each patient to gain the size of cortical surface areas and cortical thickness values in the neighboring brain adjacent to the AC and explicitly in the left opercular region. Results: Four patients showed a clear left hemisphere language dominance regardless of the cyst size; a mixed laterality of language organization was found in the remaining patient. An interesting dissociation of morphometric data was assessed when comparing strongly language-related cortices in the inferior frontal gyrus with the entire neighboring cortices. Morphometry in the neighboring brain regions of the AC showed 1) overall reduced cortical surface areas and 2) a decrease in cortical thickness compared to the homologous right side. However, the surface area of the fronto-opercular region in the left inferior frontal gyrus—i.e., the pars triangularis and the pars opercularis—was larger on the left as compared to the right side. Both structures have earlier been identified to represent the morphologic substrate of language dominance in the left hemisphere. Conclusion: Arachnoid cysts do not disturb the normal asymmetry of hemisphere language organization despite delicate locations adjacent to the left inferior frontal gyrus.

Morphology-based cortical thickness estimation

We describe a new approach to estimating the cortical thickness of human brains using magnetic resonance imaging data. Our algorithm is part of a processing chain consisting of a brain segmentation (skull stripping), as well as white and grey matter segmentation procedures. In this paper, only the grey matter segmentation together with the cortical thickness estimation is described. In contrast to many existing methods, our estimation method is voxel-based and does not use any surface meshes. While this fact poses a principal limit on the accuracy that can be achieved by our method, it offers tremendous advantages with respect to practical applicability. In particular, it is applicable to data sets showing severe cortical atrophies that involve areas of high curvature and extremely thin gyral stalks. In contrast to many other methods, it is entirely automatic and very fast with computation times of a few minutes. Our method has been used in two clinical studies involving a total of 27 patients and 23 healthy subjects.

Morphometry demonstrates loss of cortical thickness in cerebral microangiopathy

ObjectiveTo evaluate the role of MR morphometry in the characterization of cerebral microangiopathy (CMA) in relation to clinical and neuropsychological impairment.Subjects and Methods3D MR images of 27 patients and 27 age–matched controls were morphometrically analysed for regional thickness. The normalized values were related to the patients’ clinical and neuropsychological scores. The patients were categorised according to the amount of structural MR signal changes. A ventricle index reflecting internal atrophy was related to MR morphology and cortical thickness as an indicator for external atrophy.ResultsCortical thickness was significantly reduced in the patients group (3.03mm ± 0.26 vs. 3.22mm ± 0.13 in controls, p = 0.001). The severest loss of cortical thickness occurred in severe CMA. Internal and external atrophy evolved in parallel and both showed a significant relationship with structural MR–abnormalities (p < 0.05; r = –0.7; r = 0.67; r = –0.74, respectively). Neuropsychological performance correlated strongly with the loss of cortical thickness.ConclusionsCortical thickness was identified as the most sensitive parameter to characterize CMA. A strong correlation was found of morphometric parameters to the severity of CMA based on a score derived from T2–weighted MRI. The degree of cortical atrophy was directly related to the degree of neuropsychological impairment. Our findings suggest that the cortical thickness is a valid marker in the structural and clinical characterization of CMA.

Determination of Cerebrovascular Reactivity by Means of fMRI Signal Changes in Cerebral Microangiopathy: A Correlation with Morphological Abnormalities

Background and Purpose: A reduced cerebrovascular reactivity (CR) is a risk factor of cerebrovascular disease. In this study, we implemented a protocol to assess CR by means of functional MRI (fMRI) using hyperventilation. Subjects and Methods: In 5 patients with cerebral microangiopathy (CM/lacunar infarction and white matter degeneration), 6 healthy elderly subjects (age-matched control), and 6 young healthy subjects, the CR in response to hyperventilation was evaluated by fMRI using gradient echo-planar Imaging. The percentage signal change normalized by end-tidal CO2 value was measured in various brain regions. Results: All subjects performed hyperventilation well without adverse reaction and significant gross motion. Patients with CM showed significant qualitative and quantitative differences (p < 0.05) as compared to controls. The volume of gray matter showing significant CR was significantly reduced in patients: by 40% in comparison to the age-matched elderly control group and by 60% when compared with the young controls. The CR impairment was most pronounced in the frontal cortices with a drastically reduced magnitude of the magnetic resonance (MR) signal change in the patients (–0.62 ± 0.2% in patients versus –2.0 ± 0.36% in age-matched controls, p < 0.0001). A strong relation was evident between the fMRI-based CR reduction in patients with CM and the individual severity of structural MR abnormalities (p = 0.002). Conclusion: This study demonstrates that fMRI-based signal changes in response to hyperventilation reliably reflect cerebral vasoreactivity. The protocol is feasible in healthy young and elderly controls and patients with CM. Quantitative and qualitative assessment of the signal decrease in the T2-weighted MR sequence and coregistration with individual anatomical data allow the generation of an individual cerebral vasoreactivity map. Future research will address the effect of CR reduction on neuropsychological parameters in patients with CM.

Intracerebral haemorrhage as a manifestation of Lyme Neuroborreliosis

Lyme Neuroborreliosis (LNB) has repeatedly been reported to cause cerebral vasculitis. However, there is no reliable information about the incidence of cerebral vessel affection. The majority of reports deal with ischaemic consequences, and there are a few reports of subarachnoid haemorrhage (SAH). We report a case of otherwise unexplained intracerebral haemorrhage (ICH) where clinical and laboratory tests have shown LNB. Late stage LNB might not only cause ischaemic, but haemorrhagic stroke as well.

Impaired hemodynamics and neural activation? A fMRI study of major cerebral artery stenosis

Functional MRI motor mapping was performed in two women with unilateral high-grade stenosis of the middle cerebral artery (MCA) to determine the influence of impaired hemodynamics on the blood oxygenation level dependent (BOLD) response. In both patients no structural lesions were present in primary motor pathways. A redistribution of the motor network to the healthy hemisphere was the main indicator of chronic hemodynamic compromise.

The effect of esmolol on cerebral blood flow, cerebral vasoreactivity, and cognitive performance: a functional magnetic resonance imaging study.

Background: Esmolol is often applied perioperatively to maintain stable hemodynamic conditions in neurosurgical patients. Little is known, however, about its effects on cerebral circulation. The authors employed functional magnetic resonance imaging based on blood oxygenation level-dependent contrast to explore the effect of esmolol on the human brain. The purpose of the study was to investigate the effect of esmolol on cerebral blood flow, cerebral vasoreactivity, and cognitive performance. Methods: Ten healthy volunteers were investigated in two separate experimental sessions using functional magnetic resonance imaging. During the first experimental session, a hyperventilation task and a cognitive task, subjects had to perform both tasks twice, once after administration of an esmolol bolus of 1 mg/kg followed by a continuous infusion of 150 μg · kg−1 · min−1 and once without β-blockade, in a random order. During the second experimental session subjects were scanned at resting state after administration of esmolol. Furthermore, the effect of the esmolol dose on hemodynamic changes caused by β-adrenergic stimulation with orciprenaline was investigated. Results: Esmolol decreased heart rate and blood pressure during the various experimental conditions and blunted the increase in heart rate and blood pressure caused by orciprenaline. Infusion of esmolol affects neither the blood oxygenation level-dependent contrast during the functional challenges nor the reaction times during the cognitive task. However, the esmolol bolus caused a brief blood oxygenation level-dependent contrast increase. Conclusion: The results indicate that effective β-blockade with esmolol does not affect cerebral blood flow, cerebrovascular reactivity, or cognitive performance.