Purpose: To introduce a method of designing single and parallel transmit (pTx) 3D adiabatic π pulses for inverting and refocusing spins that are insensitive to transmit B1 ((Formula presented.)) inhomogeneity. Theory and Methods: A 3D adiabatic pulse is created by replacing each piece-wise constant element (or sub-pulse) of an adiabatic full passage (AFP) by a 2D selective pulse. In this study, the parent AFP is an HS1 and each sub-pulse is a 2D pulse derived from a jinc function designed using a spiral k-trajectory. Spatial selectivity in the third direction is achieved by blipping the slab-selective gradient between sub-pulses, yielding a rectangular slab profile identical to that of the parent AFP. The slew-rate limited sub-pulse can be undersampled utilizing pTx, thus shortening the overall pulse width. Simulations and experiments demonstrate the quality of spatial selectivity and adiabaticity achievable. Results: The 3D adiabatic pulse inverts and refocus spins in a sharply demarcated cylindrical volume. When stepping RF amplitude, an adiabatic threshold is observed above which the flip angle remains π. Experimental results demonstrate that pTx is an effective means to significantly improve pulse performance. Conclusion: A method of designing 3D adiabatic pulses insensitive to B1 inhomogeneity has been developed. pTx can shorten these pulses while retaining their adiabatic character. Magn Reson Med 79:701–710, 2018.
Bibliographical notePublisher Copyright:
© 2017 International Society for Magnetic Resonance in Medicine
- adiabatic pulse
- hyperbolic secant
- magnetic field inhomogeneity
- multidimensional pulse
- parallel transmission