Chapter 6 Lecture Spin Echo Imaging Methods Mark D. Herbst, MD, PhD

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Chapter 6 LectureSpin Echo Imaging MethodsMark
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

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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, M0.
• 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.

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The Spin Echo Process
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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

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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

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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)

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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.

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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.

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180 degree pulse
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IR pulse diagram (STIR and FLAIR)
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STIR fat at null point
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FLAIR -- water at null point