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Symmetry Enforced, a priori RF Shimming in MRI for the Human Head at High Field Strengths.

Jörg Felder, N.Jon Shah

To obtain an a priori RF shim set for in vivo measurements of the human head where derivation and implementation are feasible on standard MRI systems without the need for sophisticated hard- or software. The method is targeted for initial anatomical scout scans and adjustments on high field scanners. Tailoring a rotationally symmetric phantom to be electrically large will produce characteristic interference patterns. This pattern is employed to enforce rotational symmetry in MRI images acquired with a multi-element transmit coil by adjusting transmit phases and/or amplitudes. Using the a priori shim on a human head defocuses the transmit fields, resulting in images with reduced signal dropouts. Three different sized adult heads were scanned with the predetermined shim set and a coil array consisting of eight transceiver elements. The images were found to show better image homogeneity than CP mode excitation and performed only slightly worse compared to a per subject RF shim. Relatively homogeneous excitation can be achieved with a pre-determined RF shim set. The homogeneity is sufficient for static field shimming and scout imaging. Primary application is the development of double resonant head coils with a fixed RF shim set for the proton channel intended for high field imaging in systems that do not poses parallel transmission capabilities.