Symmetry Enforced, a priori RF Shimming in MRI for the Human Head at High Field Strengths.
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.
Jörg Felder, N.Jon Shah
To read the full article Download Full Article