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library:time_to_repeat [2025/05/05 16:12] – scottlibrary:time_to_repeat [2025/05/05 16:28] (current) – scott
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 This technique is common in spine imaging, and is an option available on almost all scanner generations. At the end of the echo train, an additional -90 degree RF pulse is tacked on to force any remaining magnetization back toward equilibrium. This in effect replaces the the waiting period (the 3000+ TR) needed for T1 recovery, allowing much shorter TR's (Low 2000's) to be used with heavily T2 weighted images. With typical parameters, fluids will also be brighter than they normally would with comparable TE's. This technique is common in spine imaging, and is an option available on almost all scanner generations. At the end of the echo train, an additional -90 degree RF pulse is tacked on to force any remaining magnetization back toward equilibrium. This in effect replaces the the waiting period (the 3000+ TR) needed for T1 recovery, allowing much shorter TR's (Low 2000's) to be used with heavily T2 weighted images. With typical parameters, fluids will also be brighter than they normally would with comparable TE's.
  
-===TR in Gradient Echo===+===TR in Fast Gradient Echo===
  
-When TR's become very short (<T1 and T2), a steady state can be achieved with the longitudinal magnetization.+When TR's become very short (<T1 and T2), a steady state can be achieved with the longitudinal magnetization. This means that after each TR, the same amount of longitudinal magnetization is recovered as is lost to RF excitation. To prevent total saturation of the signal, lower flip angles (5-15 degrees) are used. When utilizing these sequences, the TR may not be selectable and is no longer the primary parameter controlling image contrast, instead flip angle and sequence type will be most important. 
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