Physical limits to human brain B0 shimming with spherical harmonics, engineering implications thereof

Abstract

Objective As the MRI main magnetic field rises for improved Signalto-Noise Ratio, susceptibility-induced B0-inhomogeneity increases proportionally, aggravating related artifacts. Considering only susceptibility disparities between air and biological tissue, we explore the topological conditions for which perfect shimming could be performed in a Region of Interest (ROI) such as the human brain or part thereof. Materials and Methods After theoretical considerations for perfect shimming, spherical harmonic (SH) shimming simulations of very high degree are performed, based on a 100-subject database of 1.7-mmresolved brain fieldmaps acquired at 3 T . In addition to the whole brain, shimmed ROIs include slabs targeting the prefrontal cortex, both or single temporal lobes, or spheres in the frontal brain above the nasal sinus. Results and DiscussionWe show "perfect" SH shimming is possible only if the ROI can be contained in a sphere that does not enclose sources of magnetic field inhomogeneity, which are gathered at the air-tissue interface. We establish a 13Hz inhomogeneity hard shim limit at 7 T for whole brain SH shimming, that can only be attained at shimming degree higher than 90. On the other hand, under limited power and SH degree resources, 3D regionspecific shimming is shown to greatly improve homogeneity in critical zones such as the prefrontal cortex and around ear canals.

Deep learning‐assisted model‐based off‐resonance correction for non‐Cartesian SWI

“…More advanced shim coil designs have been proposed recently 19,20 but still face technical and theoretical limitations. 21 Post-processing methods can therefore be necessary as a complement for more demanding cases, such as Cartesian EPI 22,23 where alternating gradient direction at every time frame can be used to deduce and revert off-resonance induced geometric distortions. However, this technique is not applicable to non-Cartesian readouts (e.g., spirals, [24][25][26] rosette, 27 SPARKLING [9][10][11] ) due to the multiple spatially-encoding gradients played simultaneously.…”

“…The spherical harmonic shimming technique is the current standard for all systems 15,18 but is generally limited to second or third‐order harmonics, which already provide critical improvements. More advanced shim coil designs have been proposed recently 19,20 but still face technical and theoretical limitations 21 . Post‐processing methods can therefore be necessary as a complement for more demanding cases, such as Cartesian EPI 22,23 where alternating gradient direction at every time frame can be used to deduce and revert off‐resonance induced geometric distortions.…”

Iterative static field map estimation for off‐resonance correction in non‐Cartesian susceptibility weighted imaging

“…Those experimental results permit to achieve improved correction performances, but remain limited by the number of shimming coils to be deployed to reach the theoretical limits. 25 For that reason, alternative retrospective approaches are instrumental in correcting off-resonance effects once the k-space data has been collected.…”

“…Recent works have been proposed to improve this method using more efficient coil designs 23 that accurately fit the expected inhomogeneity profiles 24 using complementary data. Those experimental results permit to achieve improved correction performances, but remain limited by the number of shimming coils to be deployed to reach the theoretical limits 25 . For that reason, alternative retrospective approaches are instrumental in correcting off‐resonance effects once the k‐space data has been collected.…”