Mark D. Herbst, M.D., Ph.D.

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Chapter 4

• Mark D. Herbst, M.D., Ph.D.

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Magnetization

• MRI depends on magnetization of tissues
• Temporary magnetization occurs when the hydrogen
  nuclei are laingned in one direction more than
  other directions
• The sum of all the individual spin vectors is the
  net magnetization vector.

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Maximum magnetization depends on 3 things

• Concentration of magnetic nuclei (PD)
• Magnetic sensitivity of the nuclide (1.0 for H-1)
• Magnetic field strength (B0)

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MR Image

• The MR image shows different tissues as different
  levels of gray because the magnetization of each
  tissue is different when the picture is
  snapped.
• The level of magnetization at any given time in a
  tissue depends on several factors, the most
  important being T1, T2, and PD.

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Tissue Magnetization

• Occurs initially a within several seconds of
  placing the patient in the magnet.

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Magnetization Direction

• Two main directions
• Longitudinal (along the z direction, same as the
  magnetic field of the magnet)
• Transverse (in the xy plane)

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Flipping or tipping

• When the net magnetization vector is subjected to
  RF at the Larmor frequency, it begins to tilt out
  of alignment with the magnetic field direction of
  the magnet (z) and toward the xy plane.
• This tilt is called the flip angle or tip angle
  and is usually 90 or 180 degrees, but can be any
  angle.

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Terms to know

• Excitation-any time the equilibrium condition is
  disturbed by an RF pulse
• Equilibrium-protons in the magnet are undisturbed
  by RF long enough to achieve the maximum
  magnetization in the z direction
• Relaxation- process of going back to equilibrium
  after excitation
• Saturation-when no signal can be obtained from
  tissue because it has absorbed all the RF it can
  (like a saturated sponge) and both energy levels
  are equally populated.

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T1 relaxation

• Describes the regrowth of the net magnetization
  vector in the z direction
• T1 is defined by the time it takes for it to
  regrow 63

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T2 Relaxation

• Describes the decay of magnetization in the xy
  plane (dephasing)
• Defined as the time it takes for magnetization in
  the xy plane to decay 63
• Dephasing is caused by spin-spin interactions or
  by diffusion through magnetic field gradients

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Effect of B0 on relaxation times

• No effect on T2
• Increased T1 with increasing B0

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Transverse magnetization

• Produces contrast between tissues
• Generates RF collected by receive coils (signal,
  echoes) to be converted to images

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Magnetic susceptibility

• Diamagnetic-negative or low magnetic
  susceptibility (air, cortical bone)
• Paramagnetic (have unpaired electrons like ions
  of gadolinium, manganese, iron, chromium
  nitroxide, molecular oxygen magnetite (a
  superparamagnetic iron oxide) cause marked
  decrease in T1, mild decrease in T2)
• Ferromagnetic-metallic iron, ferritin