Magnetic Resonance Imaging 4

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Magnetic Resonance Imaging 4

• Gradient echo

V.G.Wimalasena Principal School of Radiography
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The gradient echo pulse sequence

• This refers to the use of a variable RF
  excitation pulse which flips the NMV through any
  angle (usually less than 900).
• The transverse component of magnetization is less
  than that in spin echo, because only part of the
  longitudinal magnetization is converted to
  transverse magnetization.

300
900
Small transverse component
Full transverse component
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• After the RF pulse is withdrawn, the FID signal
  is immediately produced due to inhomogeneities in
  the magnetic field and T2 dephasing occurs.
• The magnetic moments within the transverse
  component of magnetization dephase.
• They are then rephased by a gradient.
• The gradient causes a change in the magnetic
  field strength within the magnet.
• The gradient rephases the magnetic moments so
  that a signal can be received by the coil.
• The signal contains T1 and T2 information.
• This signal is called a gradient echo.

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Gradients

• Magnetic field Gradients perform many tasks.
• They are generated by passing currents through
  coils of wire situated within the bore of the
  magnet.
• The gradient field interacts with the main static
  field, so that the magnetic field strength along
  the axis of the gradient coil is altered in a
  linear way.
• The middle of the axis of the gradient remains at
  the field strength of the main magnetic field.
• This is called the isocentre.

High
A gradient magnetic field
Low
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• The magnetic field strength increases relative to
  isocentre in one direction, and decreases in the
  other direction of the gradient axis.
• Gradients are used to either dephase or rephase
  the magnetic moments

Bore of magnet
Isocentre
Magnetic field strength decreases (B0 - b)
Magnetic field strength increases (B0 b)
Magnetic field strength remains constant (B0)
Nuclei slow down
Nuclei speed up
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How gradient dephase rephase

• Dephasing

• Rephasing

In phase
Out of phase
Out of phase
F
F
S
S
S
F
Rephasing gradient
Nuclei speed up
Nuclei speed up
Nuclei slow down
Dephasing gradient
Nuclei slow down
In phase
FS
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Timing parameters weighting in gradient echo
RF pulse
Rephasing gradient
Gradient echo
TAU
TAU
TE
TR

• The TR, TE and flip angle affect image weighting
  and contrast

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T1 weighting in gradient echo

• Flip angle is large, and
• TR is short
• to avoid full recovery of longitudinal
  magnetization and maintain saturation and to
  maximize T1 differences.
• TE is short to minimize T2 differences

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T2 weighting in gradient echo

• Small flip angle, to minimize T1 recovery, and
• Long TR
• to allow full recovery of fat and water vectors
• Long TE, so that fat water to decay
  sufficiently to show the differences.
• (in practice because of smaller flip angle, TR
  can be kept relatively short)

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Proton density weighting in gradient echo

• Small flip angle to minimize T1 recovery
• Long TR to full recovery of longitudinal
  magnetization
• Short TE to minimize T2 decay

Typical values Long TR 100
ms Short TR less than 50ms Short
TE 5 10 ms Low flip angles
5 -20 Large flip angles 70 - 110
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Advantages of gradient echo

• Since gradients can rephase faster than 180 RF
  pulses the minimum TE is much shorter
• As the flip angle is small TR also can be reduced
• So the scan time is reduced

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Disadvantages of gradient echo

• There is no compensation for magnetic field
  inhhomogeneities.
• Therefore very susceptible to magetic field
  inhomogeneities and produce artefacts.
• T2 weighting is termed T2 weighting because T2
  effects are not eliminated.

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Comparison
TR TE Flip angle
Spin echo Long 2000ms Long 60ms 90 (usually)
Spin echo Short 250-700 ms Short 10 25 ms 90 (usually)
Gradient echo Long 100ms Long 15 25 ms Small 5 20 large 70 110
Gradient echo Short Less than 50 ms Short 5 10 ms Small 5 20 large 70 - 110
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Questions

• Define the term weighting
• What is meant by
• A T1 weighted image?
• A T2 weighted image?
• A proton density weighted image?
• When saturation occurs
• The NMV is pushed beyond the transverse plane
• The magnetic moments dephase
• The MR signal is received
• What values of TR and TE are needed for T1
  weighting and why?
• Why do we use a 1800 RF pulse in spin echo?
• List three main factors that make gradient echo
  sequences different from spin echo.
• What parameters control T1 and proton density
  weighting in gradient echo?
• What type of contrast will the following produce?
• TR 400 ms, TE 5ms, flip 120
• TR 50 ms, TE 15 ms, flip 35