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Chapter 6 LectureSpin Echo Imaging Methods
Mark D. Herbst, MD, PhD
• Two Main Types of MR methods– Spin Echo –uses RF pulse to get an echo– Gradient Echo –uses gradient pulse to get an
echo
The spin echo process
• Net magnetization vector starts at equilibrium, pointing up along the +z direction.
• 90 degree RF pulse M now in xy plane• Spins in xy plane begin dephasing, and when
totally dephased, M=0.• 180 degree refocusing RF pulse is applied• Spins in xy plane begin to rephase, and echo
forms. This echo is captured and stored in k-space.
The Spin Echo Process
Three basic spin echo methods
• PD-weighted (proton density) – long TR, short TE
• T2-weighted – long TR, long TE
• T1-weighted – short TR, short TE
Causes of Spin Dephasing
• Spin-spin interactions – energy exchange among the spinning nuclei causes relaxation (dephasing)
• Magnetic field inhomogeneity – cases spins to dephase, reversed by 180 degree RF refocusing pulse
Multiple Spin Echo
• Done by using more than one 180 degree RF refocusing pulse
• Commonly used to get both PDW and T2W images at the same time
• Used in FSE (fast spin echo, turbo spin echo) to get images faster by filling k-space faster than conventional spin echo (CSE)
Inversion Recovery
• Gives high level of T1 contrast• Used to suppress either water or fat (STIR
suppresses fat, FLAIR suppresses water)• Starts with a 180 degree RF inversion pulse,
placing M in the –z direction• Then, wait TI (inversion time) before applying
the 90 degree RF pulse that puts M in the xy plane
• Finally, apply the 180 degree RF refocusing pulse to form the echo for data collection, put it in k-space.
TI: inversion time
• The time of inversion
• The time that M is allowed to stay inverted (-z)
• The time between the initial 180 degree inversion pulse and the 90 degree pulse that puts M in the xy plane.
180 degree pulse
IR pulse diagram (STIR and FLAIR)
STIR – fat at null point
FLAIR -- water at null point
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