Daniel Güllmar - Academia.edu (original) (raw)

Papers by Daniel Güllmar

Research paper thumbnail of Unidentified bright objects in neurofibromatosis type 1: Conventional MRI in the follow-up and correlation of microstructural lesions on diffusion tensor images

European Journal of Paediatric Neurology, 2012

To evaluate the evolution of unidentified bright objects (UBOs) in individuals with neurofibromat... more To evaluate the evolution of unidentified bright objects (UBOs) in individuals with neurofibromatosis type 1 (NF1) by serial magnetic resonance imaging (MRI), and to relate this to regional fractional anisotropy (FA). Materials and Methods: The signal pattern of the T2-weighted sequences in the basal ganglia, thalamus, brain stem, and cerebellum for 27 NF1 individuals and a control group were analyzed by diffusion tensor imaging (DTI). The presence or absence of UBOs in 2 consecutive MRI examinations was related to FA. Results: We demonstrated significant differences in FA for the basal ganglia, cerebellum, and thalamus between NF1 patients and controls (P 0.05), even with a reduction or disappearance of UBOs. Conclusions: MRI allows for adequate monitoring of the temporal and spatial distribution of UBOs in patients with NF1. DTI confirmed changes in FA despite the disappearance or reduction of UBOs, thereby confirming the hypothesis that microstructural damage occurs in specific brain regions of NF1 patients.

Research paper thumbnail of Anisotropic EEG/MEG volume conductor modeling based on Diffusion Tensor Imaging

Research paper thumbnail of Understanding the concept of a novel tool requires interaction of the dorsal and ventral streams

Cerebral Cortex

The left hemisphere tool-use network consists of the dorso-dorsal, ventro-dorsal, and ventral str... more The left hemisphere tool-use network consists of the dorso-dorsal, ventro-dorsal, and ventral streams, each with distinct computational abilities. In the dual-loop model, the ventral pathway through the extreme capsule is associated with conceptual understanding. We performed a learning experiment with fMRI to investigate how these streams interact when confronted with novel tools. In session one, subjects observed pictures and video sequences in real world action of known and unknown tools and were asked whether they knew the tools and whether they understood their function. In session two, video sequences of unknown tools were presented again, followed again by the question of understanding their function. Different conditions were compared to each other and effective connectivity (EC) in the tool-use network was examined. During concept acquisition of an unknown tool, EC between dorsal and ventral streams was found posterior in fusiform gyrus and anterior in inferior frontal gyru...

Research paper thumbnail of Anisotropic EEG/MEG volume conductor modeling based on Diffusion Tensor Imaging

Research paper thumbnail of High-Resolution MR Imaging of the Human Brainstem In vivo at 7 Tesla

The human brainstem, which comprises a multitude of axonal nerve fibers and nuclei, plays an impo... more The human brainstem, which comprises a multitude of axonal nerve fibers and nuclei, plays an important functional role in the human brain. Depicting its anatomy non-invasively with high spatial resolution may thus in turn help to better relate normal and pathological anatomical variations to medical conditions as well as neurological and peripheral functions. We explored the potential of high-resolution magnetic resonance imaging (MRI) at 7 T for depicting the intricate anatomy of the human brainstem in vivo by acquiring and generating images with multiple contrasts: T 2-weighted images, quantitative maps of longitudinal relaxation rate (R 1 maps) and effective transverse relaxation rate (R 2 * maps), magnetic susceptibility maps, and direction-encoded track-density images. Images and quantitative maps were compared with histological stains and anatomical atlases to identify nerve nuclei and nerve fibers. Among the investigated contrasts, susceptibility maps displayed the largest number of brainstem structures. Contrary to R 1 maps andT 2-weighted images, which showed rather homogeneous contrast, R 2 * maps, magnetic susceptibility maps, and track-density images clearly displayed a multitude of smaller and larger fiber bundles. Several brainstem nuclei were identifiable in sections covering the pons and medulla oblongata, including the spinal trigeminal nucleus and the reticulotegmental nucleus on magnetic susceptibility maps as well as the inferior olive on R 1 , R 2 * , and susceptibility maps. The substantia nigra and red nuclei were visible in all contrasts. In conclusion, high-resolution, multi-contrast MR imaging at 7T is a versatile tool to non-invasively assess the individual anatomy and tissue composition of the human brainstem.

Research paper thumbnail of CT Dataset of a Human Head

Cranial CT scan of a 51 years old male obtained in a follow-up check after drill hole trepanation... more Cranial CT scan of a 51 years old male obtained in a follow-up check after drill hole trepanation frontal left. The data originate from a GE Healthcare CT System Lightspeed VCT with the following study parameters: Slice thickness 0.625 mm, Reconstruction diameter 252 mm and Inplane pixel spacing 0.492 x 0.492 mm

Research paper thumbnail of Simulation von tangentialer und radialer elektrischer Gehirnaktivität: unterschiedliche Empfindlichkeit in EEG und MEG

Biomedizinische Technik, Oct 26, 2010

Basierend auf der Hauptrichtung der neuronaler Ströme bezüglich der lokalen Schädelform ist es üb... more Basierend auf der Hauptrichtung der neuronaler Ströme bezüglich der lokalen Schädelform ist es üblich zwischen tangentialer und radialer Aktivität zu unterscheiden. Tangentiale Aktivität hat ihren Ursprung hauptsächlich in den Wänden der Sulci, während radiale Aktivität hauptsächlich in den Böden der Sulci und den Gyri zu finden ist. Es ist etabliert, dass MEG sensitive für tangentiale Aktivität und EEG für tangentiale und radiale Aktivität ist. Daher ist es erstaunlich, dass Studien an epileptischen Patienten Fälle berichten in denen Spikes im MEG aber nicht im EEG gefunden werden. Eine niedrige Sensitivität des MEGs bezüglich der Hintergrundaktivität wurde als möglicher Grund für diesen Befund diskutiert. Daher wird in dieser Studie das Signal-zu-Rausch Verhältnis (SNR) von simulierten Spikes mit verschiedenen Orientierungen unter Berücksichtigung verschiedener Hintergrundaktivität in realistischen Kopfmodellen analysiert. Für eine feste, realistische Hintergrundaktivität zeigt sich ein höheres SNR im MEG solange die Orientierung des Spikes nicht mehr als 30 Grad von der tangentialen Richtung abweicht. Im EEG ist das SNR der Spikes dagegen höher solange die Orientierung der Aktivität nicht mehr als 45 Grad von der radialen Richtung abweicht. Die gezeigten Ergebnisse können zur Erklärung der experimentell gefundenen Unterschiede in EEG und MEG Signalen beitragen.

Research paper thumbnail of Investigation of the spatial spectrum of cortical contours on cross-sections of the brain

ABSTRACT The human brain possesses a highly structured surface. A quantitative examination of the... more ABSTRACT The human brain possesses a highly structured surface. A quantitative examination of the cortical surface is of particular interest for many questions, e.g. the investigation the relation between the spatial frequencies of cortical surfaces and spatial frequencies within Electroencephalogram and the analysis of changes of the structure of the human brain caused by diseases. In this paper a Fourier analysis approach to investigate the spatial spectrum of contour data of the cortical surface is introduced. The method was validated using artificial data with known spatial spectrum and employed to investigate the spatial fre­ quencies of the cortical surface of a human subject.

Research paper thumbnail of Association between white matter fiber structure and reward-related reactivity of the ventral striatum

Human Brain Mapping, Apr 24, 2013

Individual responsiveness to rewards or rewarding stimuli may affect various domains of normal as... more Individual responsiveness to rewards or rewarding stimuli may affect various domains of normal as well as pathological behavior. The ventral striatum/nucleus accumbens (NAcc) constitutes a key brain structure in the regulation of reward-appetitive behavior. It remains unclear, however, to which extent individual reward-related BOLD response in the NAcc is dependent on individual characteristics of connecting white matter fiber tracts. Using tract-based spatial statistics (TBSS) and statistical parametric mapping (SPM) this combined DTI-fMRI study investigated this question by correlating NAcc BOLD signal upon receipt of a monetary reward with different white matter characteristics (FA, axial diffusivity, radial diffusivity). The results show that increased integrity of white matter as assessed by FA in the cingulate and corpus callosum, the inferior fronto-occipital fasciculus, the anterior thalamic radiation and the anterior limb of the internal capsule was positively correlated with reward-related activation in the NAcc. There were no negative correlations as well as no significant results regarding axial and radial diffusivity. These findings indicate that microstructural properties of fiber tracts connecting, amongst others, the cortex with the striatum may influence intensity of reward-related responsiveness of the ventral striatum by constraining or increasing efficiency in information transfer within relevant circuitries involved in processing of reward.

Research paper thumbnail of Comparison of different T2 fitting models in renal tissue relaxometry

While relaxation rate estimation substantially varies on the chosen fitting method, a tissue-spec... more While relaxation rate estimation substantially varies on the chosen fitting method, a tissue-specific T2 fitting scheme for the kidney is lacking. The current study aims to compare multiple T2 fitting methods, including square signal and mono-exponential with constant offsets, in high SNR and low SNR scenarios from identical data points. We compare standard deviations for fitting robustness and RMSE for the goodness of fit. Our finding suggests a mono-exponential with constant offsets is the most suitable method, for it yielded the lowest RMSE both in high and low SNR.

Research paper thumbnail of Exploring the potential of StyleGAN projection for quantitative maps from diffusion-weighted MR images

Synthetic medical images can be generated with a StyleGAN and are indistinguishable from real dat... more Synthetic medical images can be generated with a StyleGAN and are indistinguishable from real data even by experts. However, the projection of real data via latent space onto a synthetic image shows clear deviations from the original (at least on the second image). This plays a major role especially when using GANs to perform tasks such as image correction (e.g. noise reduction), image interpolation or image interpretation by analyzing the latent space. Based on the results shown, it is highly recommended to perform an analysis of the projection accuracy before applying any of these applications.

Research paper thumbnail of Combined imaging of white matter and brain activation in schizophrenia with DTI and fMRI: Evidence for temporolimbic and prefrontal alterations

Schizophrenia Research, 2006

Research paper thumbnail of Influence of volume conductor modeling on source reconstruction in magnetoencephalography and electroencephalography

Summary form only given. The function and structure of the human brain is immensely complex and, ... more Summary form only given. The function and structure of the human brain is immensely complex and, at the same time, the key to understanding human behavior and many of today's prevailing diseases. In most cases, this system cannot be investigated directly, but only non-invasively from outside the head. Although several non-invasive measurement modalities are available, only magnetoencephalography (MEG) and electroencephalography (EEG) provide information with a high temporal resolution. In order to reconstruct the neuronal activity underlying measured EEG and MEG data both the forward problem (computing the electromagnetic field due to given sources) and the inverse problem (finding the best fitting sources to explain given data) have to be solved. The forward problem involves a source model and a model with the conductivities of the head. The conductivity model can be as simple as a homogeneously conducting sphere or as complex as a finite element model consisting of millions of elements, each with a different anisotropic conductivity tensor. The question is addressed how complex the employed forward model should be, and, more specifically, the influence of anisotropic volume conduction and the influence of conductivity inhomogeneities are evaluated. For this purpose high resolution finite element models of the rabbit and the human head are employed in combination with individual conductivity tensors to quantify the influence of white matter anisotropy on the solution of the forward and inverse problem in EEG and MEG. Although the current state of the art in the analysis of this influence of brain tissue anisotropy on source reconstruction does not yet allow a final conclusion, the results available indicate that the expected average source localization error due to anisotropic white matter conductivity might be within the principal accuracy limits of current inverse procedures. However, in some percent of the cases a considerably larger localization error might occur. In contrast, dipole orientation and dipole strength estimation are influenced significantly by anisotropy. Skull conductivity inhomogeneities such as the spongy bone structure embedded in the compact bone or surgical holes or fontanels in infants have a non-negligible effect on the EEG and MEG forward and inverse problem solution. Especially when source positions are expected to be in the vicinity of the conductivity inhomogeneity and when a large difference with respect to the skull conductivity is indicated, the modeling approach should take the inhomogeneities into account. In conclusion, models taking into account tissue anisotropy and conductivity inhomogeneities information are expected to improve source estimation procedures. Depending on the question addressed, the complexity of the forward and inverse solution approach has to be chosen.

Research paper thumbnail of In vivo assessment of anisotropy of apparent magnetic susceptibility in white matter from a single orientation acquisition

NeuroImage, Nov 1, 2021

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Research paper thumbnail of High levels of neuroticism are associated with decreased cortical folding of the dorsolateral prefrontal cortex

European Archives of Psychiatry and Clinical Neuroscience, Apr 6, 2017

association was found with cortical thickness. The present study is the first to describe a linka... more association was found with cortical thickness. The present study is the first to describe a linkage between the extent of local cortical folding and the individual degree of neuroticism in healthy subjects. Because neuroticism is a vulnerability factor for common psychiatric diseases such as depression our finding indicates that alterations of DLPFC might constitute a neurobiological marker elevating risk for psychiatric burden.

Research paper thumbnail of Influence of tissue conductivity changes on the EEG signal in the human brain – A simulation study

Zeitschrift Fur Medizinische Physik, May 1, 2011

Tissue disorders due to brain pathologies, like tumors, ischemia, or vasogenic edema, are known t... more Tissue disorders due to brain pathologies, like tumors, ischemia, or vasogenic edema, are known to impact the propagation of electrical fields. By using the finite element method the EEG forward problem was solved within an adapted subspace of a simplified human head model. Simulated electric potentials on the scalp revealed strong influences on the magnitude of the signal in almost all cases, even for ischemic tissue in which conductivity is lower than in healthy tissue. Remarkably, due to the "shunting effect" and the diminishing anisotropy of tissue conductivity, the signal amplitude of a radial dipole located in a sulcus was found to be higher than the signal of a dipolar source on a gyrus if the ischemic area was located underneath. The results demonstrate that pathological tissue changes have to be taken into account when evaluating EEG signals, especially when performing source localization.

Research paper thumbnail of Structure-function relationships in the context of reinforcement-related learning: a combined diffusion tensor imaging–functional magnetic resonance imaging study

Neuroscience, Jun 1, 2010

In the context of probabilistic learning, previous functional magnetic resonance imaging studies ... more In the context of probabilistic learning, previous functional magnetic resonance imaging studies have shown decreasing uncertainty accompanying decreasing neuronal activation in task-relevant networks. Moreover, initial evidence points to a relationship between white matter structure and cognitive performance. Little is known, however, about the structural correlates underlying individual differences in activation and performance in the context of probabilistic learning. This combined functional magnetic resonance imaging-diffusion tensor imaging study aimed at investigating the individual ability to reduce processing resources with decreasing uncertainty in direct relation to individual characteristics in white matter brain structure. Results showed that more successful learners, as compared with less successful learners, exhibited stronger activation decreases with decreasing uncertainty. An increased mean and axial diffusivity in, among others, the inferior and superior longitudinal fasciculus, the posterior part of the cingulum bundle, and the corpus callosum were detectable in less successful learners compared with more successful learners. Most importantly, there was a negative correlation between uncertainty-related activation and diffusivity in a fronto-parieto-striatal network in less successful learners only, indicating a direct relation between diffusivity and the ability to reduce processing resources with decreasing uncertainty. These findings indicate that interindividual variations in white matter characteristics within the normal population might be linked to neuronal activation and critically influence individual learning performance.

Research paper thumbnail of Deterministic tracking of contrast enhanced MR-Angiography (CE-MRA) data by using established methods of Diffusion-Tensor-Imaging (DTI)

IFMBE proceedings, 2009

ABSTRACT

Research paper thumbnail of Development of an Experimental Setup to Investigate the Effect of Skull Discontinuities on the EEG

IFMBE proceedings, 2009

ABSTRACT

Research paper thumbnail of Time-Frequency-Space Components of Somatosenory Evoked Potentials in Rats

Springer eBooks, 2011

This paper presents a description of components extracted from somatosensory evoked potentials re... more This paper presents a description of components extracted from somatosensory evoked potentials recorded from rats after electrical stimulation of the forelimb. For the decomposition of SEPs Topographic Matching Pursuit method was used. This method is able to decompose multichannel data into spatio-temporal atoms defined by only a few parameters. Such parameterization allows convenient processing and analysis of data components. In the present study four distinct components could be identified in the SEP data.

Research paper thumbnail of Unidentified bright objects in neurofibromatosis type 1: Conventional MRI in the follow-up and correlation of microstructural lesions on diffusion tensor images

European Journal of Paediatric Neurology, 2012

To evaluate the evolution of unidentified bright objects (UBOs) in individuals with neurofibromat... more To evaluate the evolution of unidentified bright objects (UBOs) in individuals with neurofibromatosis type 1 (NF1) by serial magnetic resonance imaging (MRI), and to relate this to regional fractional anisotropy (FA). Materials and Methods: The signal pattern of the T2-weighted sequences in the basal ganglia, thalamus, brain stem, and cerebellum for 27 NF1 individuals and a control group were analyzed by diffusion tensor imaging (DTI). The presence or absence of UBOs in 2 consecutive MRI examinations was related to FA. Results: We demonstrated significant differences in FA for the basal ganglia, cerebellum, and thalamus between NF1 patients and controls (P 0.05), even with a reduction or disappearance of UBOs. Conclusions: MRI allows for adequate monitoring of the temporal and spatial distribution of UBOs in patients with NF1. DTI confirmed changes in FA despite the disappearance or reduction of UBOs, thereby confirming the hypothesis that microstructural damage occurs in specific brain regions of NF1 patients.

Research paper thumbnail of Anisotropic EEG/MEG volume conductor modeling based on Diffusion Tensor Imaging

Research paper thumbnail of Understanding the concept of a novel tool requires interaction of the dorsal and ventral streams

Cerebral Cortex

The left hemisphere tool-use network consists of the dorso-dorsal, ventro-dorsal, and ventral str... more The left hemisphere tool-use network consists of the dorso-dorsal, ventro-dorsal, and ventral streams, each with distinct computational abilities. In the dual-loop model, the ventral pathway through the extreme capsule is associated with conceptual understanding. We performed a learning experiment with fMRI to investigate how these streams interact when confronted with novel tools. In session one, subjects observed pictures and video sequences in real world action of known and unknown tools and were asked whether they knew the tools and whether they understood their function. In session two, video sequences of unknown tools were presented again, followed again by the question of understanding their function. Different conditions were compared to each other and effective connectivity (EC) in the tool-use network was examined. During concept acquisition of an unknown tool, EC between dorsal and ventral streams was found posterior in fusiform gyrus and anterior in inferior frontal gyru...

Research paper thumbnail of Anisotropic EEG/MEG volume conductor modeling based on Diffusion Tensor Imaging

Research paper thumbnail of High-Resolution MR Imaging of the Human Brainstem In vivo at 7 Tesla

The human brainstem, which comprises a multitude of axonal nerve fibers and nuclei, plays an impo... more The human brainstem, which comprises a multitude of axonal nerve fibers and nuclei, plays an important functional role in the human brain. Depicting its anatomy non-invasively with high spatial resolution may thus in turn help to better relate normal and pathological anatomical variations to medical conditions as well as neurological and peripheral functions. We explored the potential of high-resolution magnetic resonance imaging (MRI) at 7 T for depicting the intricate anatomy of the human brainstem in vivo by acquiring and generating images with multiple contrasts: T 2-weighted images, quantitative maps of longitudinal relaxation rate (R 1 maps) and effective transverse relaxation rate (R 2 * maps), magnetic susceptibility maps, and direction-encoded track-density images. Images and quantitative maps were compared with histological stains and anatomical atlases to identify nerve nuclei and nerve fibers. Among the investigated contrasts, susceptibility maps displayed the largest number of brainstem structures. Contrary to R 1 maps andT 2-weighted images, which showed rather homogeneous contrast, R 2 * maps, magnetic susceptibility maps, and track-density images clearly displayed a multitude of smaller and larger fiber bundles. Several brainstem nuclei were identifiable in sections covering the pons and medulla oblongata, including the spinal trigeminal nucleus and the reticulotegmental nucleus on magnetic susceptibility maps as well as the inferior olive on R 1 , R 2 * , and susceptibility maps. The substantia nigra and red nuclei were visible in all contrasts. In conclusion, high-resolution, multi-contrast MR imaging at 7T is a versatile tool to non-invasively assess the individual anatomy and tissue composition of the human brainstem.

Research paper thumbnail of CT Dataset of a Human Head

Cranial CT scan of a 51 years old male obtained in a follow-up check after drill hole trepanation... more Cranial CT scan of a 51 years old male obtained in a follow-up check after drill hole trepanation frontal left. The data originate from a GE Healthcare CT System Lightspeed VCT with the following study parameters: Slice thickness 0.625 mm, Reconstruction diameter 252 mm and Inplane pixel spacing 0.492 x 0.492 mm

Research paper thumbnail of Simulation von tangentialer und radialer elektrischer Gehirnaktivität: unterschiedliche Empfindlichkeit in EEG und MEG

Biomedizinische Technik, Oct 26, 2010

Basierend auf der Hauptrichtung der neuronaler Ströme bezüglich der lokalen Schädelform ist es üb... more Basierend auf der Hauptrichtung der neuronaler Ströme bezüglich der lokalen Schädelform ist es üblich zwischen tangentialer und radialer Aktivität zu unterscheiden. Tangentiale Aktivität hat ihren Ursprung hauptsächlich in den Wänden der Sulci, während radiale Aktivität hauptsächlich in den Böden der Sulci und den Gyri zu finden ist. Es ist etabliert, dass MEG sensitive für tangentiale Aktivität und EEG für tangentiale und radiale Aktivität ist. Daher ist es erstaunlich, dass Studien an epileptischen Patienten Fälle berichten in denen Spikes im MEG aber nicht im EEG gefunden werden. Eine niedrige Sensitivität des MEGs bezüglich der Hintergrundaktivität wurde als möglicher Grund für diesen Befund diskutiert. Daher wird in dieser Studie das Signal-zu-Rausch Verhältnis (SNR) von simulierten Spikes mit verschiedenen Orientierungen unter Berücksichtigung verschiedener Hintergrundaktivität in realistischen Kopfmodellen analysiert. Für eine feste, realistische Hintergrundaktivität zeigt sich ein höheres SNR im MEG solange die Orientierung des Spikes nicht mehr als 30 Grad von der tangentialen Richtung abweicht. Im EEG ist das SNR der Spikes dagegen höher solange die Orientierung der Aktivität nicht mehr als 45 Grad von der radialen Richtung abweicht. Die gezeigten Ergebnisse können zur Erklärung der experimentell gefundenen Unterschiede in EEG und MEG Signalen beitragen.

Research paper thumbnail of Investigation of the spatial spectrum of cortical contours on cross-sections of the brain

ABSTRACT The human brain possesses a highly structured surface. A quantitative examination of the... more ABSTRACT The human brain possesses a highly structured surface. A quantitative examination of the cortical surface is of particular interest for many questions, e.g. the investigation the relation between the spatial frequencies of cortical surfaces and spatial frequencies within Electroencephalogram and the analysis of changes of the structure of the human brain caused by diseases. In this paper a Fourier analysis approach to investigate the spatial spectrum of contour data of the cortical surface is introduced. The method was validated using artificial data with known spatial spectrum and employed to investigate the spatial fre­ quencies of the cortical surface of a human subject.

Research paper thumbnail of Association between white matter fiber structure and reward-related reactivity of the ventral striatum

Human Brain Mapping, Apr 24, 2013

Individual responsiveness to rewards or rewarding stimuli may affect various domains of normal as... more Individual responsiveness to rewards or rewarding stimuli may affect various domains of normal as well as pathological behavior. The ventral striatum/nucleus accumbens (NAcc) constitutes a key brain structure in the regulation of reward-appetitive behavior. It remains unclear, however, to which extent individual reward-related BOLD response in the NAcc is dependent on individual characteristics of connecting white matter fiber tracts. Using tract-based spatial statistics (TBSS) and statistical parametric mapping (SPM) this combined DTI-fMRI study investigated this question by correlating NAcc BOLD signal upon receipt of a monetary reward with different white matter characteristics (FA, axial diffusivity, radial diffusivity). The results show that increased integrity of white matter as assessed by FA in the cingulate and corpus callosum, the inferior fronto-occipital fasciculus, the anterior thalamic radiation and the anterior limb of the internal capsule was positively correlated with reward-related activation in the NAcc. There were no negative correlations as well as no significant results regarding axial and radial diffusivity. These findings indicate that microstructural properties of fiber tracts connecting, amongst others, the cortex with the striatum may influence intensity of reward-related responsiveness of the ventral striatum by constraining or increasing efficiency in information transfer within relevant circuitries involved in processing of reward.

Research paper thumbnail of Comparison of different T2 fitting models in renal tissue relaxometry

While relaxation rate estimation substantially varies on the chosen fitting method, a tissue-spec... more While relaxation rate estimation substantially varies on the chosen fitting method, a tissue-specific T2 fitting scheme for the kidney is lacking. The current study aims to compare multiple T2 fitting methods, including square signal and mono-exponential with constant offsets, in high SNR and low SNR scenarios from identical data points. We compare standard deviations for fitting robustness and RMSE for the goodness of fit. Our finding suggests a mono-exponential with constant offsets is the most suitable method, for it yielded the lowest RMSE both in high and low SNR.

Research paper thumbnail of Exploring the potential of StyleGAN projection for quantitative maps from diffusion-weighted MR images

Synthetic medical images can be generated with a StyleGAN and are indistinguishable from real dat... more Synthetic medical images can be generated with a StyleGAN and are indistinguishable from real data even by experts. However, the projection of real data via latent space onto a synthetic image shows clear deviations from the original (at least on the second image). This plays a major role especially when using GANs to perform tasks such as image correction (e.g. noise reduction), image interpolation or image interpretation by analyzing the latent space. Based on the results shown, it is highly recommended to perform an analysis of the projection accuracy before applying any of these applications.

Research paper thumbnail of Combined imaging of white matter and brain activation in schizophrenia with DTI and fMRI: Evidence for temporolimbic and prefrontal alterations

Schizophrenia Research, 2006

Research paper thumbnail of Influence of volume conductor modeling on source reconstruction in magnetoencephalography and electroencephalography

Summary form only given. The function and structure of the human brain is immensely complex and, ... more Summary form only given. The function and structure of the human brain is immensely complex and, at the same time, the key to understanding human behavior and many of today's prevailing diseases. In most cases, this system cannot be investigated directly, but only non-invasively from outside the head. Although several non-invasive measurement modalities are available, only magnetoencephalography (MEG) and electroencephalography (EEG) provide information with a high temporal resolution. In order to reconstruct the neuronal activity underlying measured EEG and MEG data both the forward problem (computing the electromagnetic field due to given sources) and the inverse problem (finding the best fitting sources to explain given data) have to be solved. The forward problem involves a source model and a model with the conductivities of the head. The conductivity model can be as simple as a homogeneously conducting sphere or as complex as a finite element model consisting of millions of elements, each with a different anisotropic conductivity tensor. The question is addressed how complex the employed forward model should be, and, more specifically, the influence of anisotropic volume conduction and the influence of conductivity inhomogeneities are evaluated. For this purpose high resolution finite element models of the rabbit and the human head are employed in combination with individual conductivity tensors to quantify the influence of white matter anisotropy on the solution of the forward and inverse problem in EEG and MEG. Although the current state of the art in the analysis of this influence of brain tissue anisotropy on source reconstruction does not yet allow a final conclusion, the results available indicate that the expected average source localization error due to anisotropic white matter conductivity might be within the principal accuracy limits of current inverse procedures. However, in some percent of the cases a considerably larger localization error might occur. In contrast, dipole orientation and dipole strength estimation are influenced significantly by anisotropy. Skull conductivity inhomogeneities such as the spongy bone structure embedded in the compact bone or surgical holes or fontanels in infants have a non-negligible effect on the EEG and MEG forward and inverse problem solution. Especially when source positions are expected to be in the vicinity of the conductivity inhomogeneity and when a large difference with respect to the skull conductivity is indicated, the modeling approach should take the inhomogeneities into account. In conclusion, models taking into account tissue anisotropy and conductivity inhomogeneities information are expected to improve source estimation procedures. Depending on the question addressed, the complexity of the forward and inverse solution approach has to be chosen.

Research paper thumbnail of In vivo assessment of anisotropy of apparent magnetic susceptibility in white matter from a single orientation acquisition

NeuroImage, Nov 1, 2021

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Research paper thumbnail of High levels of neuroticism are associated with decreased cortical folding of the dorsolateral prefrontal cortex

European Archives of Psychiatry and Clinical Neuroscience, Apr 6, 2017

association was found with cortical thickness. The present study is the first to describe a linka... more association was found with cortical thickness. The present study is the first to describe a linkage between the extent of local cortical folding and the individual degree of neuroticism in healthy subjects. Because neuroticism is a vulnerability factor for common psychiatric diseases such as depression our finding indicates that alterations of DLPFC might constitute a neurobiological marker elevating risk for psychiatric burden.

Research paper thumbnail of Influence of tissue conductivity changes on the EEG signal in the human brain – A simulation study

Zeitschrift Fur Medizinische Physik, May 1, 2011

Tissue disorders due to brain pathologies, like tumors, ischemia, or vasogenic edema, are known t... more Tissue disorders due to brain pathologies, like tumors, ischemia, or vasogenic edema, are known to impact the propagation of electrical fields. By using the finite element method the EEG forward problem was solved within an adapted subspace of a simplified human head model. Simulated electric potentials on the scalp revealed strong influences on the magnitude of the signal in almost all cases, even for ischemic tissue in which conductivity is lower than in healthy tissue. Remarkably, due to the "shunting effect" and the diminishing anisotropy of tissue conductivity, the signal amplitude of a radial dipole located in a sulcus was found to be higher than the signal of a dipolar source on a gyrus if the ischemic area was located underneath. The results demonstrate that pathological tissue changes have to be taken into account when evaluating EEG signals, especially when performing source localization.

Research paper thumbnail of Structure-function relationships in the context of reinforcement-related learning: a combined diffusion tensor imaging–functional magnetic resonance imaging study

Neuroscience, Jun 1, 2010

In the context of probabilistic learning, previous functional magnetic resonance imaging studies ... more In the context of probabilistic learning, previous functional magnetic resonance imaging studies have shown decreasing uncertainty accompanying decreasing neuronal activation in task-relevant networks. Moreover, initial evidence points to a relationship between white matter structure and cognitive performance. Little is known, however, about the structural correlates underlying individual differences in activation and performance in the context of probabilistic learning. This combined functional magnetic resonance imaging-diffusion tensor imaging study aimed at investigating the individual ability to reduce processing resources with decreasing uncertainty in direct relation to individual characteristics in white matter brain structure. Results showed that more successful learners, as compared with less successful learners, exhibited stronger activation decreases with decreasing uncertainty. An increased mean and axial diffusivity in, among others, the inferior and superior longitudinal fasciculus, the posterior part of the cingulum bundle, and the corpus callosum were detectable in less successful learners compared with more successful learners. Most importantly, there was a negative correlation between uncertainty-related activation and diffusivity in a fronto-parieto-striatal network in less successful learners only, indicating a direct relation between diffusivity and the ability to reduce processing resources with decreasing uncertainty. These findings indicate that interindividual variations in white matter characteristics within the normal population might be linked to neuronal activation and critically influence individual learning performance.

Research paper thumbnail of Deterministic tracking of contrast enhanced MR-Angiography (CE-MRA) data by using established methods of Diffusion-Tensor-Imaging (DTI)

IFMBE proceedings, 2009

ABSTRACT

Research paper thumbnail of Development of an Experimental Setup to Investigate the Effect of Skull Discontinuities on the EEG

IFMBE proceedings, 2009

ABSTRACT

Research paper thumbnail of Time-Frequency-Space Components of Somatosenory Evoked Potentials in Rats

Springer eBooks, 2011

This paper presents a description of components extracted from somatosensory evoked potentials re... more This paper presents a description of components extracted from somatosensory evoked potentials recorded from rats after electrical stimulation of the forelimb. For the decomposition of SEPs Topographic Matching Pursuit method was used. This method is able to decompose multichannel data into spatio-temporal atoms defined by only a few parameters. Such parameterization allows convenient processing and analysis of data components. In the present study four distinct components could be identified in the SEP data.