Johanna Vannesjö from University of Zürich, Balgrist research center will give a talk at NeuroSpin on October 21th. 

Short abstract:

Signal encoding in MRI is crucially dependent on a stable and homogenous background magnetic field, upon which time-varying linear gradient fields are added for spatial encoding. Several factors, however, disturb the encoding fields, giving rise to artefacts that can obscure relevant image features and bias quantitative measures, especially at ultra-high field. In this talk, I will discuss common factors compromising the encoding magnetic fields, and show examples of their relevance for neuroimaging (brain and spinal cord). First, the spatially encoding gradient time-courses generally deviate from ideal, which especially impacts fast readouts, such as EPI and spiral imaging, that are commonly used for fMRI. The deviations can largely be captured using a linear time-invariant model of the gradient system, which can serve as basis for image reconstruction. Based on the gradient system model we show high-resolution fMRI of the visual cortex using single-shot spiral readouts at 7T. Second, field homogeneity is perturbed by magnetic susceptibility differences between air and different tissue types. This is especially a challenge in spinal cord imaging, and advanced shim solutions such as slice-wise shimming are called for to improve homogeneity. Third, the encoding process is also perturbed by time-varying magnetic fields related to the breathing cycle. These commonly give rise to ghosting in structural imaging and apparent motion in EPI acquisitions. The field fluctuations are particularly severe in the spinal cord due to the proximity to the lungs and the thorax. We have shown that correcting for the field fluctuations can improve image quality as well as quantitative measures. Taken together, an awareness of magnetic field imperfections as a source of artefacts, and the use of various correction methods, is crucial to take full advantage of the potentials of ultra-high field imaging of the brain and spinal cord.