Excitation and detection

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Excitation and detection
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Excitation and detection
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Excitation and detection
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Hydrogen nucleus in water (H20) is a tiny
spinning magnet

• 0.5 T spins at 21 MHz (22 million times per
  sec.)
• 1 T 42 MHz

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Hydrogen nucleus in water (H20) is a tiny
spinning magnet
RADIOFREQUENCY

• 0.5 T spins at 21 MHz (22 million times per
  sec.)
• 1 T 42 MHz

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RF excitation

• Net magnetization can be excited by a pulse of
  radio waves at the Larmor frequency
• depends on field strength

RF
Excitation
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Excitation- RF coils
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RF coils

• A special coil (antenna) is used to produce the
  RF field
• coil is tuned to the appropriate resonant
  frequency.

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Surface coil Helmholtz
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Coils are made from tuned circuits
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Effect of a 90 degree pulse
Lab frame
Rotating frame
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What is the fraction of low energy spins to high
energy spins after a 90o pulse?
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Detection
Chen and Hoult

• After a 90o pulse bulk magnetisation is tipped
  into the x-y plane- Mxy
• all spins in phase with no net population
  difference between the up and down states
• Mxy will precess about Bo (once B1 is off)
• rotates at the Larmor frequency.
• Rotating Mxy induces oscillating e.m.f. in
  pick-up coil that is tuned to RF frequencies

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Quadrature PSD

• By comparing these two signals you can detect the
  circular motion of the magnetisation
• The resulting signal is usually considered to be
  complex, hence the labels re and im.

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• Typical signal from slice off resonance by 50Hz
  from the transmitter would look this
• (As a rule of thumb, the signal decays to nothing
  in a time equal to 5T2).

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RF Safety
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RF heating
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RF heating with field strength

• More power required to image at high field
• RF deposition
• Beware of hotspots
• Coil modelling

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Skin burns
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Henderson, Neurosurgery, 57, 1063, 2005
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Gradients
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Inducing E fields- movement and gradients

• Can induce current densities and E fields of from
  gradient switching and moving through field

E
J
Crozier
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RF heating with field strength

• More power required to image at high field
• RF deposition
• Beware of hotspots
• Coil modelling

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Neurostimulator
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Henderson, Neurosurgery, 57, 1063, 2005
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