RESUMEN
Increased static field inhomogeneities are a burden for human brain MRI at Ultra-High-Field. In particular they cause enhanced Echo-Planar image distortions and signal losses due to magnetic susceptibility gradients at air-tissue interfaces in the subject's head. In the past decade, Multi-Coil Arrays (MCA) have been proposed to shim the field in the brain better than the 2nd or 3rd order Spherical Harmonic (SH) coils usually offered by MRI manufacturers. Here we present a novel MCA, named SCOTCH, optimized for whole brain shimming. Based on a cylindrical structure, it features several layers of small coils whose shape, size and location are found from a principal component analysis of ideal stream functions computed from an internal 100-brain fieldmap database. From an Open-Access external database of 126 brains, our SCOTCH implementation is shown to be equivalent to a partial 7th-order SH system with unlimited power, outperforming all known existing MCA prototypes. This result is further confirmed by a low-cost 30-cm diameter SCOTCH prototype built with 48 coils on 3 layers, and tested on 7 volunteers at 7T with a parallel-transmit RF coil made to be inserted in SCOTCH. Echo-Planar images of the subject brains before and after SCOTCH shimming show large signal recoveries, especially in the prefrontal cortex.
Asunto(s)
Procesamiento de Imagen Asistido por Computador , Imagen por Resonancia Magnética , Encéfalo/diagnóstico por imagen , Humanos , Procesamiento de Imagen Asistido por Computador/métodos , Imagen por Resonancia Magnética/métodos , Magnetismo , Ondas de RadioRESUMEN
OBJECTIVE: As the MRI main magnetic field rises for improved signal-to-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-mm-resolved brain fieldmaps acquired at 3T . 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 DISCUSSION: We 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 [Formula: see text]Hz inhomogeneity hard shim limit at 7T 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 region-specific shimming is shown to greatly improve homogeneity in critical zones such as the prefrontal cortex and around ear canals.