Simulating extravascular artifacts surrounding cortical draining veins to increase neuronal specificity of ultra-high-field fMRI

• Applicant: Dr. Jörg Pfannmöller
• Subject Area: Clinical Neurology; Neurosurgery and Neuroradiology ; Cognitive, Systems and Behavioural Neurobiology ; Medical Physics, Biomedical Technology ; Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: from 2017 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 388285513

fMRI is the most commonly used imaging method for non-invasively mapping brain function in humans. Modern MRI technology has provided imaging methods with unprecedentedly high spatio-temporal resolution, enabling fMRI investigations which are dramatically enhancing our knowledge about the working human brain. All fMRI techniques seek to infer patterns of neuronal activity by measuring the accompanying changes in blood supply. The most commonly used fMRI signal is the blood oxygen level dependent or BOLD contrast, which is typically acquired with gradient echo MRI sequences. This technique provides the most sensitive and robust fMRI measurement and is therefore well suited to exploit the available spatio-temporal imaging resolution. However, a major problem that limits the ability of gradient-echo BOLD to infer underlying neuronal activity is the degradation of the spatial specificity due to artifacts given by signal displacement from draining veins at the cortical surface and from intra cortical veins. Those artifacts can be discounted using a priori knowledge of the functional architecture in combination with careful experimental design, which provided sufficient specificity to detect neuronal activation across the cerebral cortical layers. The prospect of “laminar fMRI” has spurred recent interest in methods designed to account for these artifacts that limit BOLD fMRI by combining knowledge about the cortical vascularization, which is strikingly regular and consistent across the cortex, with models of how the fMRI signal is influenced by draining veins. Here we propose to adapt the Ogawa/Boxerman model to simulate the intravascular and extravascular gradient-echo BOLD signals for a broadly applicable removal of draining vein artifacts to improve resolution. Our new implementation will first be benchmarked against previous simulations and estimations of the artifact. In a second step the model for artifact removal will be constructed after the calibration of the simulation. Subsequently, we validate our artifact range and voxel size based model for artifact removal in an independent sample. Calibration and validation will be carried out using fMRI data of a well-known topographic map in the primary visual cortex acquired at 3T and 7T. This serves as a proof for our hypothesis that accounting for the extravascular signal can improve the neuronal specificity of fMRI. Currently, several endeavors are underway to construct next generation MRI scanners with field strengths up to 20T, paving the way for future decreases in fMRI voxel sizes. However, the artifact ranges and magnitudes increase together with MR field strength. Thus, careful modeling of the relationship between neuronal activity and the observed hemodynamic signals underlying fMRI measures will be even more important to exploit these future technologies. Therefore, we will use our simulations to predict the spatial resolution and the maximum achievable specificity from 3 to 20 Tesla.

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Professor Jonathan Polimeni, Ph.D.