Biomedical subjects

Thomas Bauermann

Publications and source records attributed to Thomas Bauermann.

5 recordsLinked to original sources

PubMed2007

White matter fiber tracking computation based on diffusion tensor imaging for clinical applications.

Fiber tracking allows the in vivo reconstruction of human brain white matter fiber trajectories based on magnetic resonance diffusion tensor imaging (MR-DTI), but its application in the clinical routine is still in its infancy. In this study, we present a new software for fiber tracking, developed on top of a general-purpose DICOM (digital imaging and communications in medicine) framework, which can be easily integrated into existing picture archiving and communication system (PACS) of radiological institutions. Images combining anatomical information and the localization of different fiber tract trajectories can be encoded and exported in DICOM and Analyze formats, which are valuable resources in the clinical applications of this method. Fiber tracking was implemented based on existing line propagation algorithms, but it includes a heuristic for fiber crossings in the case of disk-shaped diffusion tensors. We successfully performed fiber tracking on MR-DTI data sets from 26 patients with different types of brain lesions affecting the corticospinal tracts. In all cases, the trajectories of the central spinal tract (pyramidal tract) were reconstructed and could be applied at the planning phase of the surgery as well as in intraoperative neuronavigation.

Adolescent↗

PubMed2006

Brainstem and cerebellar fMRI-activation during horizontal and vertical optokinetic stimulation.

Animal studies have shown that not only cortical, but also brainstem and cerebellar areas are involved in the initiation and generation of optokinetic nystagmus (OKN), e.g., cortico-(pretecto)pontine-olivo-cerebellar pathways. The aim of this fMRI study was to identify and differentiate brainstem and cerebellar areas involved in horizontal and vertical OKN (h/vOKN) in humans. In a group of nine healthy volunteers, hOKN and vOKN were statistically compared with a stationary control condition. There were common activated regions for hOKN and vOKN directions located in the transition zone between the posterior thalamus and the mesencephalon bilaterally covering the pretectal nucleus complex, which is known to be a major structure within the afferent branch of the optokinetic system. Furthermore, during hOKN, activation occurred bilaterally in the mediodorsal and dorsolateral ponto-medullary brainstem, which could be best attributed to the reticular formation, especially the paramedian pontine reticular formation (PPRF). For vOKN, additional activated areas in the dorsal mesencephalic brainstem could be best localized to the ocular motor nuclei and the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF). For both OKN directions, the cerebellar activation was localized in the oculomotor vermis (declive VI, folium and tuber VIIA/B, in part pyramis VIIIA), and the flocculus bilaterally as well as widespread in the cerebellar hemispheres. In conclusion, fMRI allowed first attributions of neuronal substrates in the cerebellum and brainstem to hOKN and vOKN in humans. Consistent with the animal data, the dorsal ponto-medullary routes were involved bilaterally for hOKN, whereas the rostral mesencephalic routes were involved for vOKN.

Adult↗

PubMed2006

Direction-dependent visual cortex activation during horizontal optokinetic stimulation (fMRI study).

Looking at a moving pattern induces optokinetic nystagmus (OKN) and activates an assembly of cortical areas in the visual cortex, including lateral occipitotemporal (motion-sensitive area MT/V5) and adjacent occipitoparietal areas as well as ocular motor areas such as the prefrontal cortex, frontal, supplementary, and parietal eye fields. The aim of this functional MRI (fMRI) study was to investigate (1) whether stimulus direction-dependent effects can be found, especially in the cortical eye fields, and (2) whether there is a hemispheric dominance of ocular motor areas. In a group of 15 healthy subjects, OKN in rightward and leftward directions was visually elicited and statistically compared with the control condition (stationary target) and with each other. Direction-dependent differences were not found in the cortical eye fields, but an asymmetry of activation occurred in paramedian visual cortex areas, and there were stronger activations in the hemisphere contralateral to the slow OKN phase (pursuit). This can be explained by a shift of the mean eye position of gaze (beating field) in the direction of the fast nystagmus phases of approximately 2.6 degrees, causing asymmetrical visual cortex stimulation. The absence of a significant difference in the activation pattern of the cortical eye fields supports the view that the processing of eye movements in both horizontal directions is mediated in the same cortical ocular motor areas. Furthermore, no hemispheric dominance for OKN processing was found in right-handed volunteers.

Adult↗

PubMed2004

New partially parallel acquisition technique in cerebral imaging: preliminary findings.

In MRI applications where short acquisition time is necessary, the increase of acquisition speed is often at the expense of image resolution and SNR. In such cases, the newly developed parallel acquisition techniques could provide images without mentioned limitations and in reasonably shortened measurement time. A newly designed eight-channel head coil array (i-PAT coil) allowing for parallel acquisition of independently reconstructed images (GRAPPA mode) has been tested for its applicability in neuroradiology. Image homogeneity was tested in standard phantom and healthy volunteers. BOLD signal changes were studied in a group of six volunteers using finger tapping stimulation. Phantom studies revealed an important drop of signal even after the use of a normalization filter in the center of the image and an important increase of artifact power with reduction of measurement time strongly depending on the combination of acceleration parameters. The additional application of a parallel acquisition technique such as GRAPPA decreases measurement time in the range of about 30%, but further reduction is often possible only at the expense of SNR. This technique performs best in conditions in which imaging speed is important, such as CE MRA, but time resolution still does not allow the acquisition of angiograms separating the arterial and venous phase. Significantly larger areas of BOLD activation were found using the i-PAT coil compared to the standard head coil. Being an eight-channel surface coil array, peripheral cortical structures profit from high SNR as high-resolution imaging of small cortical dysplasias and functional activation of cortical areas imaged by BOLD contrast. In BOLD contrast imaging, susceptibility artifacts are reduced, but only if an appropriate combination of acceleration parameters is used.

Artifacts↗

PubMed2003

Asymmetry in the human primary somatosensory cortex and handedness.

Brain asymmetry is a phenomenon well known for handedness and language specialization and has also been studied in motor cortex. Less is known about hemispheric asymmetries in the somatosensory cortex. In the present study, we systematically investigated the representation of somatosensory function analyzing early subcortical and cortical somatosensory-evoked potentials (SEP) after electrical stimulation of the right and left median nerve. In 16 subjects, we compared thresholds, the peripheral neurogram at Erb point, and, using MRI-based EEG source analysis, the P14 brainstem component as well as N20 and P22, the earliest cortical responses from the primary sensorimotor cortex. Handedness was documented using the Edinburgh Inventory and a dichotic listening test was performed as a measure for language dominance. Whereas thresholds, Erb potential, and P14 were symmetrical, amplitudes of the cortical N20 showed significant hemispheric asymmetry. In the left hemisphere, the N20 amplitude was higher, its generator was located further medial, and it had a stronger dipole moment. There was no difference in dipole orientation. As a possible morphological correlate, the size of the left postcentral gyrus exceeded that of the right. The cortical P22 component showed a lower amplitude and a trend toward weaker dipole strength in the left hemisphere. Across subjects, there were no significant correlations between laterality indices of N20, the size of the postcentral gyrus, handedness, or ear advantage. These data show that asymmetry of median nerve SEP occurs at the cortical level, only. However, both functional and morphological cortical asymmetry of somatosensory representation appears to vary independently of motor and language functions.

Adult↗