- Aka
- Fiesta: Fast Imaging Employing Steady-state Acquisition by GE
- Balanced-FFE (Fast Field Echo): Philips
- TrueFISP: Simens
- CISS: constructive interference in steady-state
- Balanced gradients are used along all three axes.
- "Balanced" means that the net gradient-induced dephasing over a TR interval is zero.
- Conversely, some of the gradients in FISP and PSIF are unbalanced, allowing recording of separate FID and echo components of the steady-state free precession signal. In True FISP, however, the balanced gradients refocus both components at the exact center of the TR interval as a single echo.
- Generation of a balanced Steady state free precession
- Due to the low flip angles used in gradient echo it is possible to have such a short TR that the transverse magnetisation in the Mxy direction has not fully dephased when the next RF pulse is applied. In balanced SSFP this remnant magnetisation is not spoiled but maintained using additional rephasing and dephasing gradients.
- Dephasing due to every positive gradient is ‘undone’ by an equal negative rephasing gradient. This includes the addition of a rewinder gradient that reverses the effect of the phase encoding gradient.
- The signal is acquired at the exact midpoint of the sequence so that TE=TR/2, which means that the image contrast is related to the T2/T1 ratio.
Tissue | T2/T2 |
Muscle | 0.05 |
Liver | 0.08 |
Brain | 0.11 |
Fat | 0.30 |
CSF/fluids | 0.70 |
- The coherent signal carries over to subsequent repetitions and is superimposed onto the transverse magnetisation generated by subsequent RF pulses.
- After a number of repetitions this gives rise to a steady state where the signals from multiple repetitions combine to give a much larger signal (and therefore better SNR than spoiled gradient echo).
- Sequence diagram
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