Composite pulses for RF phase encoded MRI: A simulation study

Abstract

BackgroundIn B1 encoded MRI, a realistic non-linear phase RF encoding coil will generate an inhomogeneous B1 field that leads to spatially dependent flip angles. The non-linearity of the B1 phase gradient can be compensated for in the reconstruction, but B1 inhomogeneity remains a problem. The effect of B1 inhomogeneity on tip angles for conventional, B0 encoded MRI, may be minimized using composite pulses. The objective of this study was to explore the feasibility of using composite pulses with non-linear RF phase encoding coils and to identify the most appropriate composite pulse scheme. MethodsRF encoded signals were simulated via the Bloch equation for various symmetric, asymmetric and antisymmetric composite pulses. The simulated signals were reconstructed using a constrained least squares method. ResultsRoot mean square reconstruction errors varied from 6% (for an asymmetric composite pulse) to 9.7% (for an antisymmetric composite pulse). ConclusionAn asymmetric composite pulse scheme created images with fewer artifacts than other composite pulse schemes in inhomogeneous B0 and B1 fields making it the best choice for decreasing the effects of spatially varying flip angles. This is contrary to the conclusion that antisymmetric composite pulses are the best ones to use for spin echo sequences in conventional, B0 encoded, MRI.