Spoiling refers to the disruption of transverse coherences that may persist from cycle to cycle in a GRE sequence. Spoiling ensures that immediately before each RF-pulse, the steady-state magnetization has no transverse components. The longitudinal magnetization (Mz) does reach a steady-state (Mss) in the z-direction, but the transverse magnetization (Mxy) is obliterated at the end of each cycle and no transverse steady-state (M'ss) is established.
1. Long TR spoiling. When TR>>T2*, the transverse magnetization will naturally decay to zero by the end of the cycle. Thus any gradient echo sequence using TR values of several hundred milliseconds or longer will be "naturally" spoiled. Spoiling can be enhanced by operating the sequence in a 2D multi-slice mode. Here, off-resonant effects from RF-pulses for other slices and their associated imaging gradients will further disrupt transverse coherences. Sequences that take advantage of this method include GE's MPGR and MERGE, Siemens' MEDIC, and Philips' multi-FFE.
Simplified pulse timing diagrams for a "Basic" (unspoiled) GRE sequence and and one using both gradient and RF-spoiling are shown below. The diagrams have been simplified for clarity (e.g. rewinder gradients and other subtleties have been left out).
Advanced Discussion (show/hide)»
Many vendors have adopted an RF phase-cycling pattern based on the work of Zur et al (1991). Here the phase at step j is denoted by Φj and given by the recursive formula
Φj = ½ Φo (j2 + j + 2)
where Φo is the initial RF phase. This formula leads to the following series of phase angles: Φo, 2Φo, 4Φo, 7Φo, 11Φo, etc. The starting phase angle (Φo) is often selected to be either 117º or 123º. The reason for choosing such an apparently bizarre value for Φo is because no small integer multiple of either 117º or 123º is evenly divisible by 360º.
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Haase A, Frahm J, Matthaei KD. FLASH imaging: rapid NMR imaging using low flip angle pulses. J Magn Reson 1986: 67: 258-266.
Zur Y, Wood ML, Neuringer LJ. Spoiling of transverse magnetization in steady state sequences. Magn Reson Med 1991; 21:251-263.
Why would you want to use a spoiled-GRE technique? How do you pick the parameters?