Nuclear Spin Energy

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Nuclear Spin Energy
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The Larmor Equation
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Net Magnetization
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Alignment in an Applied Magnetic Field
Bo
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NMR/MRI Sensitivity

• E h ? B
• N-/N e-E/kT 7 x10-6 at 1T for protons

The higher the field, the larger the net
magnetization and the bigger the MR signal !!!!
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1 Joule 1 watt sec -1
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Larmor Revisited
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Nuclear Precession and the Larmor Relationship
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The MRI Signal the FID

• The initial magnitude of the signal is
  proportional to the total number of spins and
  spin polarization
• The signal decays with a time constant of 1/T2

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Spin Dephasing
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Spin Echo Formation
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Spin Refocusing
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Diagrammatic T2

• Spin echoes are formed by refocusing the
  magnetic vectors
• They can be formed by application of 180 degree
  pulses (see right) or by a reversal of the
  gradient field (GRE or GE)

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The Consequences of T1, TR on CNR

• The net magnetic vector at any time will
  determine the signal strength

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Measuring T2
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T2-weighted (W) Contrast

• MRI almost always acquires images from echoes
• For a T2W sequence, the longer the TE, the more
  the loss of signal from those areas with short T2s

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