In MRI most scans involve some amplitude modulated RF pulse to selectively excite spins over a certan bandwidth.
The flip angle of the pulses is set by the RF exciter amplifier level and this must always be calibrated to an get optimum MR signal.
The flip angle varies in a rughly sinusiodal manner with the excitation RF power.
Adiabatic pulses are fundamentally different from othr RF pulses used in MRI: Once the RF power exceeds a certain threshold the flip angle remains constant.
Why this happens is not easy to explain, but in the movies on this page you can get a glimpes on how it happens.
If you really want to know more about these pusles please read any of the following :
1. DeGraaf RA, Nicolay K. Adiabatic rf pulses: Applications to in vivo NMR. Concept Magnetic Res 1997;9(4):247-268.
2. Norris DG. Adiabatic radiofrequency pulse forms in biomedical nuclear magnetic resonance. Concept Magnetic Res 2002;14(2):89-101.
3. Garwood M, DelaBarre L. The return of the frequency sweep: Designing adiabatic pulses for contemporary NMR. Journal of Magnetic Resonance 2001;153(2):155-177.
On this web page you get a chnage to see what happens with spins during and adiabatic pulse/
A Tan/Tanh Adiabatic Inversion Pulse
The hyperbolic Secant Pulse is a frequency selective adiabatic inversion pulse
The two movies below follow the path of magnetization vectors during a hyperbolic secant pulse
Pulse : 14.5 ms sech/tanh mu = 7, BW 1500 Hz
trunc = 0.01 gamma B1/2pi = 1000 Hz = (23 microT for 1H = about 2* threshold for adiabatic response).
Here red and blue are +/- 1 kHz and outside the excited band and green is on res
Note how the spins outside the inversion band are moved, but return to equlibrium.
Now red and blue are +/- 750 Hz and green is still on res
The red and the blue are right on the edge of the inversion band.
Spins at these frequencies will get a 90 degree flip.
A Tan/Tanh Adiabatic Inversion Pulse is a non-selective adiabatic inversion pulse.
It inverts spins over a wide frequency offset range that gets wider with increasing RF amplitude.
The useful bandwidth is roughly equal to the RF amplitude gamma *B1 /2pi in Hz and has no sharp boundaries.
The movie below follow the path of magnetization vectors during a tan/tanh inversion pulse
Pulse : 1 ms tanh/tan at gamma B1/2pi = 5kHz. Red and blue are +/- 1 kHz off resonance, green is on resonance.
A BIR 4 pulse is a composite pulse made of four segments. Each segment is half of an adiabatic inversion pulse.
This BIR4 is made up of four tanh/tan segmets and it is also a non-selective adiabatic pulse. The unique feature of a BIR4 pulse is that the flip angle can be set to values other than 90 or 180 degrees (at RF amplitudes above the adiabatic threshold). The flip angle is determined not by the RF amplitude, but by a phase jump between segments 2 and 3 and segments 1 and 4.
It rotates spins over a wide frequency offset range that gets wider with increasing RF amplitude.
The useful bandwidth is agian roughly equal to the RF amplitude gamma *B1 /2pi in Hz and has no sharp boundaries.
The movie below shows the path of magnetization vectors during a tan/tanh inversion pulse
Pulse : 4 ms (tanh.tan based) BIR4 at gamma B1/2pi = 5kHz. Red and blue are +/- 1 kHz off resonance, green is on resonance.
Note how after everything the three spin vectors converge to yield a 60 dgree flip for all.
Well,.... you can clearly see how that works! Right?