Gradient echo pulse sequences

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Gradient echo pulse sequences

• Conventional gradient echo
• Steady state Coherent gradient echo
• Steady state Incoherent gradient echo
• Steady state free precession
• Ultra fast sequences
• Echo planer imaging (EPI)

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Gradient echo pulse sequences

• Conventional gradient echo
• Uses variable flip angles so that, TR and
  therefore the scan time, can be reduced without
  producing saturation.
• A gradient instead of 180 rephasing RF pulse is
  used to rephase the FID.
• The frequency encoding gradient is used for this
  purpose.
• A gradient is quicker to apply than a 180 pulse
• Therefore the minimum TE can be reduced.

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• Frequency encoding gradient is initially applied
  negatively to speed up the dephasing of the FID.
• Then its polarity is reversed producing rephasing
  of the gradient echo.
• Gradient does not compensate for magnetic field
  inhomogenities
• So the resultant echo displays great deal of T2
  information
• Used to acquire T2, T1, and proton density
  weighting
• Allow for reduction in scan time as the TR is
  greatly reduced

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Conventional gradient echo
TR
RF
RF
Echo
FID
FID
Frequency encode
rephase
dephase
TE
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Uses of gradient echo

• Used for single slice breath-hold acquisitions in
  the abdomen
• Used for dynamic contrast enhancement
• Used to produce angiographic type images, because
  the flowing nuclei which have been previously
  excited, always give a signal as gradient
  rephasing is not slice selective.

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Manipulating Parameters

• Flip angle and TR determines the degree of
  saturation and, therefore the T1 weighting.
• For saturation flip angle should be large and TR
  short so that full recovery cannot occur.
• To prevent saturation, flip angle should be small
  and the TR long enough to permit full recovery.
• TE controls the amount of T2 dephasing
• To minimize T2 Te should be short
• To maximize T2 TE should be long.

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Typical values

• T1 weighting
• Large flip angle 70 -110 degrees
• Short TE 5-10 ms
• Short TR less than 50 ms
• Average scan time several seconds to minutes
• T2 weighting
• Small flip angle 5 -20 degrees
• Long TE 15 -25 ms
• Short TR enough for full recovery as flip angle
  is small
• Scan time several seconds to minutes
• Proton density weighting
• Small flip angle 5 -20 degrees
• Short TE 5 -10 ms
• Short TR for full recovery as flip angle is small
• Scan time several seconds to minutes

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The steady state

• This is a stage where the TR is shorter than the
  T1 and T2 times of the tissues.
• There is no time for the transverse magnetization
  to decay, before the pulse sequence is repeated
• There is coexistence of both longitudinal and
  transverse magnetization
• The flip angle and TR maintain the steady state
  which holds the longitudinal and transverse
  components and the NMV steady during the data
  acquisition
• Flip angles of 300 to 450 and TR of 20 to 50 ms
  achieves a steady state.

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B0
Longitudinal component held steady
NMV held steady
Transverse component held steady

• If steady state is maintained, the transverse
  component does not have time to decay during
  pulse sequence.
• This transverse magnetization, produced as a
  result of previous excitations , is called the
  residual transverse magnetization(RTM)

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• The residual transverse magnetization (RTM)
  affects the contrast as it results in tissues
  with long T2 times , appearing bright on the
  image
• Most gradient echo sequences use the steady
  state, as the shortest TR and scan times are
  achieved.
• Gradient echo sequences are classified according
  to whether the residual transverse magnetization
  is in phase (coherent) or out of phase
  (incoherent).

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Learning point

• The steady state involves repeatedly applying RF
  pulses at TR less than the T1 and T2 of all
  tissues
• This train of RF pulses generates two signals
• A FID signal which occurs as a result of the
  withdrawal of the RF pulse and contains T2
  information
• A spin echo whose peak occurs at the same time as
  an RF pulse

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• This happens because every RF pulse contains
  individual radio waves that have sufficient
  energy to rephase a previous FID
• These radiowaves rephase the RTM left over from
  previous RF excitation pulses to form a spin
  echo.
• This ocurs at exactly the same time as the next
  RF pulse as the RTM takes the same time to
  rephase as it took to dephase. Therfore when
  utilizing steady state , the TR TAU of the spin
  echo.

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A FID and a spin echo occur at each RF pulse
RF pulse 2
Produces own FID and rephases FID of pulse 1
RF pulse 3
RF pulse 1
Spin echo
FID
FID
FID
dephasing
rephasing
TR
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Summary

• Any two RF pulses produce a spin echo.
• The first RF pulse excites the nuclei regardless
  of its net amplitude
• The second RF pulse rephases the FID resulting
  from the first.
• The spin echoes produced are sometimes called
  Hahn or stimulated echoes.
• This concept applies to all pulse sequences that
  use steady state.

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GE-Coherent residual transverse magnetization

• This pulse sequence uses a variable flip angle
  excitation pulse followed by gradient rephasing,
  to produce a gradient echo.
• Steady state is maintained by selecting TR
  shorter than T1 and T2
• There is therefore RTM left over when the next
  excitation pulse is applied.
• The RTM is kept coherent by a process known as
  rewinding.

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• Rewinding is achieved by reversing the slope of
  the phase encoding gradient after readout.
• This results in RTM rephasing, so that it is in
  phase at the beginning of the next repetition
• This alows the RTM to build up so that tissues
  with a long T2 time produce a high signal.

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Coherent gradient echo pulse sequence
TR
Rewinder gradient
Phase encode
readout
FID
echo
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Sample image

• T2 weighted image using a coherent gradient
  echo. TE 15 ms, TR 40 ms, flip 350, breath
  holding single slice obtained in 11s

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Uses of coherent gradient echo

• This pulse sequences produce T2 weighted images.
• As fluid is bright they give an angiographic,
  myelographic or arthrographic effect.
• Can be used to determine whether a vessel is
  patent, or whether an area contains fluid.
• Can be acquired slice by slice, or in a 3D volume
  acquisition
• As the TR is short, a sliice can be acquired in a
  single breath hold.

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Parameters

• To maintain the steady state
• Flip angles 30 45
• TR 20-50 ms
• To maximize T2 long TE 15- 25 ms
• Use gradient moment rephasing to accentuate T2
• Average scan time
• Seconds for single slice
• 4-15 min for volumes
• (to minimize T2 to produce T1 or proton density
  weighting TE should be the shortest possible)

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Advantages Disadvantages

• Very fast scans, breath holding possible
• Very sensitive to flow so good for angiography
• Can be acquired in a volume acquisition

• Poor SNR in 2D acquisitions
• Magnetic susceptibility increases
• Loud gradient noise

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Incoherent(spoiled) RTM

• Pulse sequence that use incoherent RTM begin with
  a variable flip angle excitation pulse, and use
  gradient rephasing to produce agrdient echo.
• The steady state is maintained so that RTM is
  left over from the previuos RF.
• The RTM is spoiled so that its effect on image
  contrast is minimal.
• There are two ways to achieve spoiling
• Digitized RF spoiling
• Gradient spoiling

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RF spoiling

• RF spoiling is achived by controling the phase of
  the digitised RF pulses that are transmitted.
• The digitized RF is transmitted at a specific
  frequency phase
• The resultant NMV and transverse component are
  fillped to a certain position in the transvere
  plane.
• The receiver coil can lock onto the phase of the
  RF that has just being transmitted and receives
  only signal at that phase.
• Transverse magnetization at other phases or
  positions in the transverse plane are not recived
  by the coil

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• Each RF is delivered at a different phase and the
  receiver coil is locked to recieve signal only at
  that phase
• This process continues and the TRM which is at a
  different phase is ignored by the receiver coil.
• So the effect of RTM on the image is eleminated.
• T2 is therefore cannot predominate and T1 and
  proton density weighting prevails.

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Uses

• RF spoild GE pulse sequencs produce T1 or proton
  density weited images, although fluid may have a
  rather high signal due to gradient rephasing
• Can be used for 2D and volume acquisition and as
  the TR is short the 2D acquisition can be used to
  acquire T1 weighted breath-hold images.
• Demonstrate good T1 anatomy

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Example for RF spoiled GE 5.21

• T1/proton density weighted image using RF
  spoiling. TE 6 ms, TR 35 ms, flip 35, part of
  volume acqusition which took 7 minutes

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Parameters

• To maintain steady state
• Flip angle 30-45
• TR 20-50 ms
• To maximise T1 short TE 5-10ms
• Average scan time several seconds for single
  slice, 4-15 min for volumes
• Advantages
• Can be acquired in a volume or 2D
• Breath holding possible
• Good SNR and anatomical detail in volumes

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Gradient spoiling

• Gradient spoiling is the opposite of rewinding.
• The slice select, phase encoding, and frequency
  encoding gradients can be used to dephase the
  RTM, so that it is incoherent at the beginning of
  the next repetition.
• T2 effects are reduced
• Uses and parameters are similar to those in RF
  spoiling.
• Can be used to achieve T2 when the parameteres
  are similar to those in conventional GE.(because
  GS is less efficient than RF spoiling and moreT2
  information is present in the signal)

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Steady state free prcession (SSFP)

• Can be used to get shortest possible TR and scan
  time with steady state GE
• Used to produce more T2 weighted images than
  conventional gradient echo sequences.
• The pulse sequences used here help to obtain
  images that have a sufficiently long TE and less
  T2 when using steady state than other gradient
  echo pulse sequences.
• This is achieved in the manner described below.

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Composition of RF pulse

• RF pulse contains radio waves of differing
  amplitudes. The magnetude of RF pulse is an
  average of these amlitudes. E.g.
• 10 waves of amplitude of 100
• 2 waves of amplitude of 300
• 15 waves of amplitude of 600
• 5 waves of amplitude of 1800
• The average amplitude 19600/32 61.250

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• Therfore every RF pulse contains waves that on
  their own have sufficient magnitude to move
  magnetic moments within the NMV through 1800.
• These radio waves are therefore able to rephase a
  FID.
• In SSFP, the steady state can be maintained by
  using a flip angle between 300 and 450 with a TR
  of 20-50ms.
• Every TR an excitation pulse is applied.
• When the RF is switched off a FID is produced.

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• After the TR another excitation pulse is applied
  which also produces its own FID.
• The radiowaves within it that have an amplitude
  of 180 rephase the FID from the previous pulse,
  and a spin echo is produced.
• Each RF pulse therefore not only produces its own
  FID, but also rephases the FID produced from the
  previous excitation.
• As nuclei take as long to rephase as they took to
  dephase, the echo from the first excitation pulse
  occurs at the same time as the third excitation
  pulse.
• However this cannot be sampled, as RF cannot be
  transmitted and received at the same time.

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• To receive the spin echo, a rewinder gradient is
  used to speed up the rephasing process after the
  RF rephasing has begun.
• The rewinding moves the echo so that it occurs
  before the next excitation pulse, rather than
  during it.
• This way the resultant ehco can be received.
• It demonstrate more true T2 weighting than
  conventional gradient echo sequences.
• Because
• The effective TE is now longer than the TR.
• The rephasing is initiated by an RF pulse rather
  than a gradient so that more T2 and less T2
  information is present.

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1
3
2
FID 1
Echo of FID 1
FID 3
FID 2
TE
TR
1
3
2
Rewinder gradient
FID 1
Echo of FID 1
Effective TE
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Effective TE actual TE

• Actual TE is the time between the echo and the
  next excitation pulse
• Effective TE is the time from the echo to the
  excitation pulse that created its FID
• Effective TE (2xTR) actual TE
• If TR 50 ms, actual TE 10 ms
• Then effective TE 90 ms

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Uses of SSFP

• Used to acquire images that demonstrate true T2
  weighting.
• Especially useful in the brain and joints and on
  most systems can be used with both 2D and 3D
  volume acquisitions.

Effective TE 71 ms, TE 9ms, TR 40 ms, flip angle
350, volume scan time 9 minutes.
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Parameters

• To maintain steady state flip angle 30-45, TR
  20-50 ms
• Actual TE affects the effective TE unless the
  system uses a fixed TE.
• Average scan time 4-15 min volume acquisition
• Some manufacturers suggest decreasing the
  effective TE to reduce magnetic susceptibility,
  and increasing the flip angle to create more
  transverse magnetisation which results in higher
  SNR

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Comparison between Coherent, incoherent SSFP
RF pulse
RF pulse
RF pulse
FID spin echo
FID
FID
Spin echo
Gradient echo
Gradient echo
TE
TE
FID
TE
Spin echo moved away from RF pulse by rewinding
gradient
Coherent
Incoherent
SSFP
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Ultra-fast sequences

• Advances have been made in developing very fast
  pulse sequences.
• Usually employ the coherent or incoherent
  gradient echo sequences.
• The TE is significantly reduced by
• Aplying only a portion of the RF exciation pulse.
• Reading only a portion of the echo
• TE kept to a minimum (2.5 3.0 ms)
• TR and therefore the scan time is reduced.
• TR as low as 10 ms is achieved and about 16
  slices can be achieved in a single breath hold.

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• Many ultra-fast sequences use extra pulses
  applied before the pulse sequences begins, to
  pre-magnetise the tissue.
• This way certain contrast can be obtained.
• Pre-magnetisation is achieved in the following
  manner.
• Applying a 1800 pulse before the pulse sequence
  and a specified delay time similar to inversion
  recovery.
• Applying a combination of 900/1800/900 pulses
  before the pulse sequence begins.

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• (first 90 pulse produces transverse
  magnetisation. 180 pulse rephases this, and at a
  specific time later second 90 pulse is applied.
  This drives the coherent transverse
  magnetisation into the longitudinal plane. It is
  available to be fliped when the pulse sequence
  begins. This is used to produce T2 contrast and
  is sometimes known as driven equilibrium)

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Echo planer imaging(EPI)

• Fills all the lines of k-space during one TR
• Uses a single echo train
• Multiple Echos are generated and each is phase
  encoded by a different slope of gradient to fill
  all the required lines of k space.
• Echoes are generated either by 180 rephasing
  pulses or by gradients.
• Gradients are much faster