Physics, Techniques and ProceduresCoherent steady-state technique
type of
gradient echo pulse sequence in which the
transverse magnetization Mxy remaining at the end of each sequence repetition is allowed to contribute to the signal in subsequent repetitions. If the
repetition time TR is longer than the
T2 relaxation time T2, the
transverse magnetization generated in any pulse sequence repetition will essentially disappear before the next repetition. However, if
TR <
T2, the impact of the
transverse magnetization remaining from the previous repetition must be considered. As a simple example, consider the case in which the system is exactly on resonance,
TR <
T2, and the
flip angle is low. The
transverse magnetization created by each
excitation pulse will add constructively to that remaining from the previous cycle, and the equilibrium (or steady state)
transverse magnetization just after the pulse will be higher than that which would have been observed if
T2 had been very short (which would have caused the
transverse magnetization from the previous cycle to have disappeared). This steady-state
transverse magnetization depends on
T2 even when
TE is zero.
In addition to its dependence on TR, flip angle, T1 and T2, the steady-state transverse magnetization also depends on the phase angle between the transverse magnetization at the end of one cycle and that created by the next excitation pulse. This phase angle f is in turn affected by resonance offsets due to magnetic field inhomogeneity. If f varies throughout the image, the result will be inhomogenous signal intensity (shading) which can be very severe. To avoid this problem, most coherent steady-state techniques employ dephasing gradients in at least in one direction (frequency encoding or phase encoding) to guarantee that each voxel contains all possible values of f. The resulting signal intensity reflects an averaged transverse steady state. A fully refocused sequence with a properly selected and uniform f would yield higher signal, especially for tissues with long T2 relaxation times. As mentioned above, this method can be very sensitive to magnetic field inhomogeneity, but may be feasible at lower magnetic field H strength and very short TR.
Coherent steady-state sequences can measure the free induction decay FID generated just after each excitation pulse or the echo formed just prior to the next pulse. The former types are far more common; examples include GRASS, FISP, FAST, and FFE. The latter type have stronger T2 dependence but lower signal to noise ratio SNR ; commercially available examples go by the names of SSFP, CE-FAST, PSIF, and CE-FFE-T2. An example of a fully refocussed FID sequence is true-FISP (see gradient echo pulse sequence).
NP