Large magnetic field inhomogeneity can be a significant cause of spatial flip-angle variation when using ordinary, limited-bandwidth RF pulses. Multidimensional RF pulses are particularly sensitive to inhomogeneity due to their extended pulse length, which decreases their bandwidth. Previously, it was shown that, by breaking a 2D pulse into multiple undersampled k-space segments, the excitation bandwidth can be increased at the expense of increased imaging time. The present study shows how this increased imaging time can be offset by undersampling acquisition k-space in a phase-encoded dimension that is in the direction of excitation segmentation. Data from each segment are viewed as originating from “virtual receive coils” rather than multiple physical coils. The undersampled data are reconstructed using parallel imaging techniques (e.g. as in GRAPPA). The method was tested in vivo with brain imaging at both 3 T and 4 T, and used in conjunction with a 32-channel head coil and conventional GRAPPA on the 3 T data. Relationships with existing techniques and future applications are discussed.
Bibliographical noteFunding Information:
This work was supported by the National Institutes of Health grants U01 EB025153 , P41 EB015894 , and T32 EB008389 . The authors would like to thank Drs. Steen Moeller and Gregor Adriany for useful discussions and for technical assistance and the reviewers for valuable recommendations to improve the clarity of the work.
- 2D pulse
- B inhomogeneity
- Parallel imaging
- Segmented pulse
- Virtual coil