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.
[more]
Widefield optical imaging is an important tool for neuroscientific research, either for directly observing changes in the oxygenation and blood volume during activation with intrinsic optical imaging, or for using dyes or genetically encoded indicators to look at calcium inflow or electrical cellular events.
[more]
Arterial spin labeling (ASL) can benefit twofold from ultrahigh-field MRI. First, the higher field strength leads to an increase in the intrinsic signal-to-noise ratio (SNR), and second, a higher and longer-lasting perfusion-related signal change can be measured due to the longer longitudinal relaxation times.
[more]
An increasing number of studies aim to perform functional MRI measurements acquired at ultra-high field (UHF) with voxel dimensions in the sub-millimeter scale. Such studies are possible primarily due to the stronger BOLD effect at higher fields, resulting in an increased contrast-to-noise ratio (CNR) in fMRI, as well as the superlinear increase of image signal-to-noise ratio with field strength.
[more]
To fully understand the neurovascular bSSFP fingerprint observed in BOLD experiments, extravascular and intravascular contributions have to be identified separately. The extravascular BOLD component has been analyzed extensively dependent on vessel size, static magnetic field strength, and sequence parameters such as repetition time or flip angle by means of Monte Carlo methods.
[more]
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
[more]
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.
[more]
The BOLD sensitivity of different sequences to the underlying neurovascular vessel size is a key feature to characterize the measured BOLD signal changes.
[more]
The superior colliculus (SC) is a layered structure located in the midbrain. We exploited the improved spatial resolution and BOLD signal strength available at 9.4 T to investigate the depth profile of visual BOLD responses in the human SC based on distortion-corrected EPI data with a 1 mm isotropic resolution.
[more]
We investigate the superior colliculi to reveal how human sensory processing is managed by this small mid-brain structure. Ultra-high fields are central to enable detailed detection of layer-specific fMRI signals.
[more]
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.
[more]
This work focuses on the analysis and description of the neurovascular fingerprint of pass-band bSSFP, an imaging modality that has been introduced for functional BOLD imaging in 2001 (at the stop-band), and that was further advanced by several groups.
[more]
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?
[more]
In 2001, a novel method to detect BOLD response was proposed, which is based on the frequency sensitivity of the stop band of bSSFP. In this work, we demonstrate the feasibility of brain activation mapping based on rapid and high-resolution pass band bSSFP at 9.4T.
[more]
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.
[more]