Pulse sequences

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Pulse sequences
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Categorization

• Spin echo
• Conventional spin echo
• Fast spin echo
• Inversion recovery
• Gradient echo
• Coherent
• Incoherent
• Steady state free precession
• Ultra-fast imaging

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Conventional spin echo

• Illustration
• The situation before the patient is placed inside
  the magnet

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• The patient is now inside the bore of the magnet.
  The magnetization of the patients protons M0 is
  aligned along the Z axis

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• The 900 pulse is now applied. M0 is now
  completely removed from the Z axis and lies along
  the Y axis

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• Relaxation is now taking place. Spin-lattice
  (T1)relaxation caused the magnetization to
  re-grow along the Z axis. Spin-spin relaxation
  causes the magnetization vectors to dephase (move
  apart) while still in the X-Y plane

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• 1800 pulse is now given. All the vectors now
  point in the opposite direction. The
  magnetization vectors rephase (come together) in
  the X-Y plane. As they come together the echo is
  being formed

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• The magnetization is now completely rephased in
  the X-Y plane and points along the Y axis. This
  causes the full height of the echo. The actual
  MRI signal is taken here.

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• Relaxation continues to take place. The magnetic
  vectors again dephase in the X-Y plane while the
  regrowth along the Z axis continues

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• Complete relaxation has taken place. There is no
  net vector in the X-Y plane, and the
  magnetization is full grown along the Z axis.
  This is identical to the situation before the 900
  pulse was applied

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• The coordinate system in relation to the magnet

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• Short TE and short TR gives T1 weighted image
• Use two RF rephasing pulses generating two spin
  echoes to produce T2 and proton density weighting
• First echo has short TE and long TR produce
  proton density weighting
• Second echo has a long TE and a long TR produce
  T2 weighting

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uses

• Gold standard for most imaging
• May be used for every examintion
• T1 images useful for demonstrating anatomy
  because high SNR
• With contrast enhancement T1 images show
  pathology
• T2 images also demonstrate pathology
• Diseased tissues are generally more oedematous
  and/or vascular. They have increased water
  content and, have a high signal on T2 images

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Parameters

• T1 weighting
• Short TE 10-20 ms
• Short TR 300 600 ms
• Typical scan time 4-6 min
• Proton density/T2
• Short TE 20 ms/long TE 80 ms
• Long TR 2000 ms
• Typical scan time 7-15 min

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• Advantages
• Good image quality
• Very versatile
• True T2 weighting sensitive to pathology
• Disadvantages
• Scan times relatively long

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Fast spin echo

• In contrast to conventional spin echo, fast spin
  echo applies a train of 1800 pulses per TR and
  different phase encoding steps are used.
• Each 1800 pulse produce an echo (proton density
  T2)
• This drastically reduce the scan time
• The number of 1800 pulses in the train called the
  turbo factor or train length

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• E.g. if in conventional SE 256 phase matrix and 1
  NEX is used, the scan time is 256TR
• In FSE if the turbo factor is 16, the scan time
  is 256TR/16 16TR

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Uses

• Useful in most clinical applications
• Central nervous system, pelvis, musculoskeletal
  regions
• Note
• Fat remains bright on T2
• Unless fat saturation techniques are used
• Muscles appear darker in FSE images
• Artefacts from metal implants is significantly
  reduced

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Parameters

• T1 weighting
• Short effective TE less than 20ms
• Short TR 300 600 ms
• Turbo factor 2-6
• Typical scan time 30s to 1 min
• T2 weighting
• Long effective TE 100 ms
• Long TR 4000 ms
• Turbo factor 8-20
• Typical scan time 2 min

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Advantages Disadvantages

• Reduced scan time
• High resolution matrices and multiple NEX can be
  used
• Image quality improved
• Increase T2 information

• Some flow and motion effects increased
• Incompatible with some imaging options
• Fat bright on T2
• Reduces magnetic susceptibility effect, so should
  not be used when haemorrhage is suspected

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Inversion Recovery

• Starts with a 1800 inversion pulse.
• This inverts NMV through 1800 into full
  saturation.
• When inverting pulse removed NMV begins to relax
  back to B0
• A 900 excitation pulse is then applied at a time
  TI (Time from Inversion) from the 1800 inversion
  pulse

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TI
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• TI determines the weighting contrast
• Short TI gives T1 contrast
• Long TI gives proto density contrast
• After the 900 excitation pulse 1800 rephasing
  pulse is applied at a time TE
• This produces the spin echo
• TR is the time between each 1800 inverting pulse

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Uses

• conventionally used to produce heavily T1
  weighted images to demonstrate anatomy in
  contrast enhanced imaging
• Now more widely used in conjunction with fast
  spin echo to produce T2 weighted images

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Parameters
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STIR (short TI inversion recovery)

• Uses TI that corresponds to the time it takes fat
  to recover from full inversion to the transverse
  plane so that there is no longitudinal
  magnetization corresponding to fat.
• As a result the signal from fat is nulled.
• Used to achieve suppression of fat in T1 weighted
  images.
• TI 150 175 ms

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FLAIR (Fluid attenuated inversion Recovery)

• The signal from CSF is nulled by selecting a TI
  corresponding to the time of recovery of CSF from
  180 to the transverse plane and there is no
  longitudinal magnetization present.
• Used to suppress signal from CSF in T2 weighted
  images
• TI - 1700 -2200 ms