Fingerprints of Neuronal Activation
A major line of our research is a more specific understanding of the measured MR signal during brain activation, the variability of this contrast across different cortical and subcortical regions, its dependence on the underlying structure and shape of the microvasculature, and the correlation of this contrast to neuronal activation as a function of spatial and temporal resolution. An important step towards a better understanding of MR signal formation in neuronal tissue will be achieved with multimodal integrated micro devices composed of MR detectors, optical and electrical detectors with a size of only a few hundreds of micrometers. Along these lines, we also try to assess the spatial limit of BOLD fMRI at ultra-high fields, and whether it is possible at all, to reliably detect subunits of the primary cortex such as layers or columns.
current research
Though it is used in a large number of neuroscientific studies, the BOLD effect, arising due to a complex interplay of changes in blood volume, blood oxygenation and blood flow, is still not completely understood. Intrinsic optical imaging can help to disentangle the different contributions, being able to measure changes in oxygenation and blood volume separately and quantitatively.
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While the vessel size specificity of gradient and spin echoes has been extensively characterized in simulations and measurements, other sequence types that recently gain increasing attention in ultra-high field and high-resolution BOLD fMRI such as GRASE are yet not fully characterized. GRASE, that is based on a CPMG excitation scheme, samples a gradient echo train within consecutive (gradient and rf) refocusing pulses
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Magnetic resonance imaging and spectroscopy are versatile tools for probing brain physiology, but their intrinsically low sensitivity limits the achievable spatial and temporal resolution. Here, we introduce a monolithically integrated NMR-on-a-chip needle that co-integrates an ultra-sensitive 300-µm NMR coil with a complete NMR transceiver, enabling for the first time in-vivo measurements of blood oxygenation and flow in nanoliter volumes at a sampling rate of 200 Hz.
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Balanced steady-state free precession (bSSFP) is a BOLD-sensitive acquisition method that is highly sensitive to small vessels in the range of 5 to 20 µm. Thus, it shows a high potential to represent oxygenation changes within the microvasculature in contrast to gradient echoes with their huge and unspecific sensitivity to larger draining veins. A major drawback of bSSFP is its significantly reduced imaging speed compared to EPI.
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Magnetic Resonance Imaging at ultra high field strengths can help to increase the signal amplitude and thus to improve spatial or temporal resolution. But how much? And how does that affect the contrast?
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The primary visual cortex of humans contains patches of neurons responding preferentially to stimulation of one eye (the ocular dominance columns). Multiple previous studies attempted to detect their activity using fMRI.
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Although being applied in thousands of neuroscientific studies, the BOLD effect and the underlying neurovascular coupling are still a question of intense research. A major challenge is the complex generation of the BOLD effect as a combination of several interacting contributions, mainly the change in cerebral blood volume (CBV), cerebral blood flow (CBF) and blood oxygenation, which have different temporal and spatial distributions.
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The thalamus, a central relay for sensory and motor signals in the brain, is still under investigation for its specific role in human sensorimotor processing. Given the small, centrally located nuclei in the thalamus, high-resolution imaging is crucial. This study uses ultra-high field fMRI (≥ 7 Tesla) to improve the signal-to-noise ratio, allowing the measurement of functional responses in individual thalamic nuclei.
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The thalamus acts as a central communications hub for the brain, relaying information from the senses and cortico-cortical interactions. The cortex heavily relies on thalamic nuclei interactions for most of its functional processing. Despite this, human thalamic nuclei's detailed functional interactions and behavioral association with the cerebral cortex remain partially uncharted.
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