Adiabatic RF-Pulses
How do adiabatic pulses differ from regular RF-pulses? What are they used for?
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Adiabatic pulses are a class of amplitude- and frequency-modulated RF-pulses that are relatively insensitive to B1 inhomogeneity and frequency offset effects. They utilize the adiabatic principle explained in the previous Q&A wherein magnetization (M) is manipulated by a slow passage of the B1 field through resonance. With adiabatic pulses spins having different resonant frequencies are inverted or manipulated at different times. This differs from an "ordinary" amplitude modulated rectangular RF-pulse where all spins are affected simultaneously.
Additional advantages of adiabatic pulses include: 1) Less sensitive to RF miscalibration and hence have a high tolerance to field inhomogeneity; 2) accurate spin manipulation over a large range of RF power levels; and 3) minimization of sample heating and specific absorption rate (SAR).
Adiabatic pulses can be categorized as excitation, refocusing, and inversion pulses. No matter what their use, adiabatic pulses behave differently than non-adiabatic pulses. For example, the flip angle produced is not simply proportional to B1 magnitude and pulse length, but depends on how the B1 field varies in amplitude and phase throughout the pulse. Likewise, adiabatic pulses cannot be simply scaled or stretched to change their effect; doubling a 90° adiabatic pulse does not produce a 180° inversion pulse.
The design of adiabatic pulses is rather complex, often segmentally constructed out of hyperbolic tangent or secant functions, with independent amplitude and frequency (or phase) modulation. In addition to the simpler adiabatic inversion pulses used for fat or water suppression (SPAIR), an more modern and interesting class of plane rotation pulses is now becoming popular, especially the BIR-4 (B1-Insensitive Rotation) pulse.
Additional advantages of adiabatic pulses include: 1) Less sensitive to RF miscalibration and hence have a high tolerance to field inhomogeneity; 2) accurate spin manipulation over a large range of RF power levels; and 3) minimization of sample heating and specific absorption rate (SAR).
Adiabatic pulses can be categorized as excitation, refocusing, and inversion pulses. No matter what their use, adiabatic pulses behave differently than non-adiabatic pulses. For example, the flip angle produced is not simply proportional to B1 magnitude and pulse length, but depends on how the B1 field varies in amplitude and phase throughout the pulse. Likewise, adiabatic pulses cannot be simply scaled or stretched to change their effect; doubling a 90° adiabatic pulse does not produce a 180° inversion pulse.
The design of adiabatic pulses is rather complex, often segmentally constructed out of hyperbolic tangent or secant functions, with independent amplitude and frequency (or phase) modulation. In addition to the simpler adiabatic inversion pulses used for fat or water suppression (SPAIR), an more modern and interesting class of plane rotation pulses is now becoming popular, especially the BIR-4 (B1-Insensitive Rotation) pulse.
References
Tannus A, Garwood M. Adiabatic pulses. NMR in Biomed 1997;10:423-434
DeGraaf RA, Nicolay K. Adiabatic rf pulses: Applications to in vivo NMR. Concept Magn Res 1997 ;9:247-268.
Norris DG. Adiabatic radiofrequency pulse forms in biomedical nuclear magnetic resonance. Concept Magn Res 2002; 14:89-101.
Garwood M, De la Barre L. The return of the frequency sweep: Designing adiabatic pulses for contemporary NMR. J Magn Reson 2001; 153:155-177.
Tannus A, Garwood M. Adiabatic pulses. NMR in Biomed 1997;10:423-434
DeGraaf RA, Nicolay K. Adiabatic rf pulses: Applications to in vivo NMR. Concept Magn Res 1997 ;9:247-268.
Norris DG. Adiabatic radiofrequency pulse forms in biomedical nuclear magnetic resonance. Concept Magn Res 2002; 14:89-101.
Garwood M, De la Barre L. The return of the frequency sweep: Designing adiabatic pulses for contemporary NMR. J Magn Reson 2001; 153:155-177.
Related Questions
What is adiabatic excitation?
What is the rotating frame of reference?
What happens if the B1 field is applied "off resonance" (i.e., not exactly at the Larmor frequency)?
What is adiabatic excitation?
What is the rotating frame of reference?
What happens if the B1 field is applied "off resonance" (i.e., not exactly at the Larmor frequency)?