Biomedical subjects

Martin A Koch

Publications and source records attributed to Martin A Koch.

6 recordsLinked to original sources

PubMed2006

Gradient and stimulated echo (GRASTE) imaging.

As a modification of single-shot stimulated echo acquisition mode (STEAM) MRI, a gradient and stimulated echo (GRASTE) sequence is presented that acquires multiple gradient echoes in addition to each stimulated echo. While "contiguous" GRASTE exploits all stimulated echoes for the central part of k-space and the gradient echoes for outer lines, "interleaved" GRASTE assigns all echoes of a particular readout interval to directly neighboring lines. Phase distortions may be corrected by the reference signals of a single readout interval without phase encoding. Experimental results obtained for the human brain demonstrate that contiguous GRASTE yields up to 30% better SNR per acquisition time than conventional single-shot STEAM due to a better efficiency and maintains most of its robustness. Interleaved GRASTE can improve the SNR by a factor of 2 because of the possibility of using larger flip angles in the readout interval. However, its more pronounced sensitivity to off-resonance effects requires short echo trains.

Brain Mapping↗

PubMed2004

Diffusion tensor imaging detects early Wallerian degeneration of the pyramidal tract after ischemic stroke.

We used diffusion tensor imaging (DTI) to assess Wallerian degeneration of the pyramidal tract within the first 2 weeks after ischemic stroke, and correlated the extent of Wallerian degeneration with the motor deficit. Nine patients with middle cerebral artery stroke were examined 2-16 days after stroke by DTI and T2-weighted MRI. We measured fractional anisotropy (FA), averaged diffusivity (Dav), eigenvalues of the diffusion tensor and T2-weighted signal in the cerebral peduncle and compared these values between the affected and the unaffected side and between patients and six controls. FA was significantly reduced on the affected side compared to the unaffected side and compared to the control group. The largest eigenvalue was reduced, whereas the smallest eigenvalue was elevated on the affected side. There was no significant difference in T2-weighted signal and Dav. The decrease of anisotropy correlated positively with the motor deficit at the time of DTI study and 90 days after stroke. The reduction of anisotropy mirrors the disintegration of axonal structures, as it occurs in the early phase of Wallerian degeneration. DTI detects changes of water diffusion related to beginning pyramidal tract degeneration within the first 2 weeks after stroke that are not yet visible in conventional T2-weighted or orientationally averaged diffusion weighted MRI. We demonstrated for the first time a correlation of early DTI findings of pyramidal tract damage with the motor deficit. DTI can help prognosing recovery of motor function after stroke within the early subacute phase.

Aged↗

PubMed2003

Diffusion tensor MRI of early upper motor neuron involvement in amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative system disorder affecting both upper and lower motor neurons. Despite supportive electrophysiological investigations, the involvement of the upper motor neuron is often difficult to assess at an early stage of disease. Diffusion tensor MRI provides an estimate of the orientation of fibre bundles in white matter on the basis of the diffusion characteristics of water. Diffusivity is generally higher in directions along fibre tracts than perpendicular to them. This degree of directionality of diffusion can be measured as fractional anisotropy. Changes in tissue structure due to degeneration of the corticospinal fibres can lead to a modification of the degree of directionality which can be detected by diffusion tensor MRI. We investigated 15 patients with ALS, six of whom had no clinical signs of upper motor neuron involvement at the time of MRI investigation, but developed pyramidal tract symptoms later in the course of their disease. These patients met the El Escorial criteria as their disease progressed. We found a decrease in fractional anisotropy in the corticospinal tract, corpus callosum and thalamus in all 15 ALS patients, including the patients without clinical signs of upper motor neuron lesion, compared with healthy controls. Regression analysis showed a negative correlation between fractional anisotropy and central motor conduction time obtained by transcranial magnetic stimulation, allowing spatial differentiation between the degenerated corticospinal tract fibres that supply the upper and lower extremities. Thus, diffusion tensor MRI can be used to assess upper motor neuron involvement in ALS patients before clinical symptoms of corticospinal tract lesion become apparent, and it may therefore contribute to earlier diagnosis of motor neuron disease.

Adult↗

PubMed2003

Functional magnetic resonance imaging: a review of methodological aspects and clinical applications.

This paper gives an overview of the recent literature on methodological developments of functional magnetic resonance imaging (fMRI) and recent trends in clinical applications. With the recent introduction of high-field systems and methodological developments leading to more robust signal behavior, fMRI is in a transition state from a research modality for use by experts to a standard procedure with useful applications in patient management. Compared to the use in neuroscientific research, which is often based on BOLD techniques alone, the application in patients is distinguished by a multiparametric characterization of the brain using a combination of several techniques. Neuronal fiber tracking based on diffusion anisotropy measurements, in particular, has already turned out to provide relevant supplementary information to the BOLD-based cortical activation maps.

Brain↗

PubMed2002

Disconnection of speech-relevant brain areas in persistent developmental stuttering.

BACKGROUND: The neuronal basis of persistent developmental stuttering is unknown. The disorder could be related to a reduced left hemisphere dominance, which functional neuroimaging data suggest might lead to right hemispheric motor and premotor overactivation. Alternatively, the core deficit underlying stuttering might be located in the speech-dominant left hemisphere. Furthermore, magnetoencephalography study results show profound timing disturbances between areas involved in language preparation and execution in the left hemisphere, suggesting that persistent developmental stuttering might be related to impaired neuronal communication, possibly caused by a disruption of white matter fibre tracts. We aimed to establish whether disconnection between speech-related cortical areas was the structural basis of persistent developmental stuttering. METHODS: We analysed the speech of 15 people with persistent developmental stuttering and 15 closely matched controls for the percentage of syllables stuttered. We used diffusion tensor imaging to assess participants' brain tissue structure, and used and two-sample t test to compare diffusion characteristics between groups. FINDINGS: Diffusion characteristics of the group with persistent developmental stuttering and controls differed significantly immediately below the laryngeal and tongue representation in the left sensorimotor cortex (mean difference in fractional anisotropy 0.04 [95% CI 0.03-0.05]). INTERPRETATION: Our findings show that persistent developmental stuttering results from disturbed timing of activation in speech-relevant brain areas, and suggest that right hemisphere overactivation merely reflects a compensatory mechanism, analogous to right hemisphere activation in aphasia.

Adult↗

PubMed2002

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.

Algorithms↗