Using Adiabatic Inversion Pulses for Frequency Selective Excitation

 Adiabatic RF pulses are well known for robust, B1-insensitive (i.e. changing the pulse amplitude above a minimum cut-off has little impact on its performance) and offset-independent inversion. But they can also be used for B1-insensitive narrow-band selective excitation. 


How?


Any inversion pulse, adiabatic or not, always produces transverse magnetization (Mxy) centered around the transition band of the frequency profile. Take for instance this 30-ms Hyperbolic Secant adiabatic inversion pulse (HS1), with a time-bandwidth factor (R) of 20, and its simulated frequency profile.


The transverse magnetization is excited centered around the transition bands of the inversion profile, and has an excitation bandwidth of ca. 155 Hz. This bandwidth depends on the pulse duration - longer the pulse, smaller the bandwidth - and also on the pulse shape. The offset of the excitation bands depends on the inversion bandwidth, and is ~±333 Hz.

More importantly, this excitation is completely adiabatic in nature as can be seen from the RF amplitude profile simulated at the -333 Hz offset and the magnetization trajectory.





Outside this excitation band, the magnetization is either fully inverted, or un-excited, also adiabatically as shown in the RF amplitude profile simulations at ±183 Hz from the excitation band offset frequency.





This type of selective adiabatic excitation was used by de Graaf & Nicolay [1] to design an adiabatic water-suppression pulse-sequence called SWAMP.


Figure from Reference 1

References

  1. de Graaf, R. A., & Nicolay, K. (1998). Adiabatic water suppression using frequency selective excitation. Magnetic Resonance in Medicine, 40(5), 690–696. https://doi.org/10.1002/mrm.1910400508

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