Gradient Echo Sequences -Approach to steady state

 
A steady state (SSFP: steady state free precession) will be established after a sufficient number (>5T1/TR) of excitation and dephasing periods. The approach to the steady state is shown below for a gradient echo sequence with T1=T2=10TR, and flip angle α of 50°.

 
Again, the distribution of the magnetization gets more and more complicated. A more appropriate and comprehensive way to display the flow of magnetization from pulse to pulse is to transform the spatially distributed magnetization into the Fourier space.

Magnetization in Fourier space: the phase graph description

The phase graph corresponds to the Fourier series of the spatially distributed magnetization. In this case the evolution of the transverse (F) and longitudinal (Z) Fourier components is analyzed. An excitation pulse leads to a mixture of Z and F components, gradient pulses increase the index of the Fourier component F.

The images below depict the flow of transverse F components as a function of the number of excitation pulses. The magnitude of the F components (spin echoes left, gradient echoes right) are shown in yellow. The two images on the left have been calculated with T1=40TR, T2=30TR, images on the right without relaxation. Flip angle was α=50°.








RF-spoiling using quadratically increased phases of the excitation pulses produces a totally different distribution of F and Z states. While F states with high amplitude are concentrated at the center of the phase graph for non-spoiled sequences, RF-spoiling gives a more flat and brought F distribution around the center. As a consequence the FID signal (F0) is reduced (T1-weighted) as compared to the unspoiled case.

The images below depict the flow of transverse F components as a function of the number of excitation pulses for an RF-spoiled gradient echo sequence. The magnitude o the F components (spin echoes left, gradient echoes right) are shown in yellow. The two images on the left have been calculated with a quadratic phase increment of 50°, images on the with 117°. Flip angle was α=50°, and T1=40TR, T2=30TR.








Burst, the Stopped Pulse experiment

A nearly constant distribution of spin echo F components can also be achieved with (somewhat different) quadratically increasing phases of the excitation pulses. This is used in the BURST or Stopped Pulse experiment on order to have constant echo amplitudes after the BURST excitation train. Left is the phase graph of a BURST sequence with 60 excitation pulses followed by a single 180° refocusing pulse. Flip angle of the BURST pulses was 90°/ √60 and quadratic pulse phase, no relaxation. The 180° refocusing pulse was not applied for the second simulation shown right.


 
 


Last updated: Montag, 18.04.2011