Reduced Flip Angle FSE
Why would you want to use reduced flip angles in FSE? Wouldn't smaller flip angles kill the MR signal?
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A major limitation of fast (turbo) spin echo sequences is excessive heating of tissues. This is a direct consequence of the high concentration of 180°-pulses intrinsic to their design. Energy deposition from an RF-pulse is proportional to the square of the flip angle (α²). Thus, a 180°-pulse deposits 4x the energy of a 90°-pulse.
By reducing the RF-flip angles from 180º to the 50º−150º range, thermal energy deposition can be significantly reduced. But wouldn't these smaller flip angles also reduce the MR signal? The answer is yes, but not as much as you might imagine.
When RF-flip angles are reduced below 180°, the magnetization at each echo is no longer refocused purely into the transverse plane. A considerable portion of the magnetization is "stored" along the longitudinal direction and regrows via T1-relaxation mechanisms. Each successive non-180°-pulse in the train continues to split the magnetization into more longitudinal and transverse components. Stimulated echoes from these stored longitudinal components make progressively larger contributions to the MR signal, which now depends on both T1 and T2. It is these stimulated echoes, acting along multiple extra coherence pathways, that allow a higher than expected signal to be obtained when flip angle is reduced.
For a set of refocusing pulses all with the same flip angle α <180°, the system initially oscillates but reaches a pseudo-steady state (PSS) of transverse magnetization that reduced in amplitude by a factor of approximately sin(α/2) compared to an all 180°-sequence. So, for example, if 100° instead of 180°-pulses were used, the FSE signal would be reduced to sin(50º) ≈ 77% of its maximum value. Once established, the level of the PSS can be increased by varying flip angle along the course of the FSE echo train. This technique is known by the acronym TRAPS (TRansition between Pseudo-steady States) and is illustrated below.
Three variations of reduced flip angle FSE imaging. In the first case, constant 100º RF-pulses are used resulting in a pseudo-steady stateafter a few oscillations. In the second variation a transition between pseudo-steady states is achieved by increasing the flip angle to 120º halfway through the sequence. The third variation begins with preparatory pulses followed by ramped flip angles. The resultant theoretical signal intensities are shown below (ignoring T1 and T2 effects).
Several additional modifications to the reduced flip angle approach allow for even higher average signals and reduced energy deposition. The initial level of the PSS can be significantly increased by beginning the sequence with a set of 5-10 preparatory or "start-up" pulses to catalyze the process. Another modification is to linearly or sinusoidally ramp the flip angles from low to high values along the echo train. When the lowest flip angles are used early in the train, a very high fraction (~80%) of magnetization remains stored along the longitudinal axis. This stored magnetization can then be "drawn upon" to permit the generation of stronger signals later in the echo train. The largest flip angles are applied at the time of the lowest order phase encode steps, since these determine over MR signal and basic image contrast. A third modification is to use symmetric hyperechoes, the subject of the next Q&A.
References
Alsop DC. The sensitivity of low flip angle RARE imaging. Magn Reson Med 1997;37:176–184.
Busse RF, Brau ACS, Vu A, et al. Effects of refocusing flip angle modulation and view ordering in 3D fast spin echo. Magn Reson Med 2008;60:640-649.
Hennig J, Weigel M, Scheffler K. Multiecho sequences with variable refocusing flip angles: optimization of signal behavior using smooth transitions between pseudo steady states (TRAPS). Magn Reson Med 2003;49:527–535.
Mugler JP III. Optimized three-dimensional fast spin echo MRI. J Magn Reson Imaging 2014;39:745-767. (Excellent up-to-date review).
Alsop DC. The sensitivity of low flip angle RARE imaging. Magn Reson Med 1997;37:176–184.
Busse RF, Brau ACS, Vu A, et al. Effects of refocusing flip angle modulation and view ordering in 3D fast spin echo. Magn Reson Med 2008;60:640-649.
Hennig J, Weigel M, Scheffler K. Multiecho sequences with variable refocusing flip angles: optimization of signal behavior using smooth transitions between pseudo steady states (TRAPS). Magn Reson Med 2003;49:527–535.
Mugler JP III. Optimized three-dimensional fast spin echo MRI. J Magn Reson Imaging 2014;39:745-767. (Excellent up-to-date review).