PULSE SEQUENCES

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PULSE SEQUENCES

• Emphasizing the differences among spin density,
  T1, and T2 relaxation time constants of the
  tissues is the key to the exquisite contrast
  sensitivity of MR images.
• Tailoring the pulse sequencesthat is, the
  timing, order, polarity, and repetition frequency
  of the RF pulses and applied magnetic field
  gradientsmakes the emitted signals dependent on
  T1, T2 or spin density relaxation
  characteristics.

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• MR relies on three major pulse sequences
• spin echo,
• inversion recovery, and
• gradient recalled echo.

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• When these used in conjunction with localization
  methods (i.e., the ability to spatially encode
  the signal to produce an image,
  contrast-weighted images are obtained.

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• SPIN ECHO

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• Spin echo describes the excitation of the
  magnetized protons in a sample with an RF pulse
  and production of the FID, followed by a second
  RF pulse to produce an echo.
• Timing between the RU pulses allows separation of
  the initial FID and the echo and the ability to
  adjust tissue contrast.

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Time of Echo

• An initial 90-degree pulse produces the maximal
  transverse magnetization, Mxy, and places the
  spins in phase coherence.
• The signal exponentially decays with T2
  relaxation caused by intrinsic and extrinsic
  magnetic field variations.

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• After a time delay of TE/2, where TE is the time
  of echo, a 180-degree RF pulse is applied, which
  inverts the spin system and induces a rephasing
  of the transverse magnetization.
• The spins are rephased and produce a measurable
  signal at a time equal to the time of echo (TE).

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• This sequence is depicted in the rotating frame.

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• The echo reforms in the opposite direction from
  the initial transverse magnetization vector, so
  the spins experience the opposite external
  magnetic field inhomogeneities and this strategy
  cancels their effect.
• The b0 inhomogeneiry-canceling effect that the
  spin echo pulse sequence produces has been
  likened to a foot race on a track.

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• The racers start running at the 90-degree pulse,
  but quickly their tight grouping at the starting
  line spreads out (dephases) as they run at
  different speeds.

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• After a short period, the runners are spread out
  along the track, with the fastest runners in
  front and the slower ones in the rear.
• At this time (TE/2), a 180-degree pulse is
  applied and the runners all instantly reverse
  their direction, but they keep running at the
  same speed as before.

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• Immediately after the 180-degree rephasing RF
  pulse, the fastest runners are the farthest
  behind and the slowest runners are in front of
  the pack.
• Under these conditions, the fast runners at the
  end of the pack will catch the slow runners at
  the front of the pack as they all run past the
  starring line together (i.e., at time TE).

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• Even in a field of runners in which each runs at
  a markedly different speed from the others, they
  all will recross the starting line at exactly TE.
  
• The MR signal is at a maximum (i.e., the peak of
  the FID envelope) as the runners are all in phase
  when they cross the starting line.

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• They can rim off in the other direction, and
  after another time interval of TE/2 reverse their
  direction and run back to the starting line.
• Again, after a second TE period, they will all
  cross the starting line (and the FID signal will
  be at its third peak), then head off in the other
  direction.

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• This process can be repeated.

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• The maximal echo amplitude depends on the T2
  constant and not on T2, which is the decay
  constant that includes magnetic field
  inhomogeneities.
• Of course all MR signals depend on the proton
  density of the tissue sample, as well.

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• Just before and after the peak amplitude of the
  echo (centered at time TE) digital sampling and
  acquisition of the signal occurs.

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• Spin echo formation separates the RF excitation
  and signal acquisition events by finite periods
  of time, which emphasizes the fact that
  relaxation phenomena are being observed and
  encoded into the images.
• Contrast in the image is produced because
  different tissue types relax differently (based
  on their T1 and T2 characteristics).

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• Multiple echoes generated by 180-degree pulses
  after the initial excitation allow the
  determination of the true T2 of the sample.

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• Signal amplitude is measured at several points in
  time, and an exponential curve is fit to this
  measured data

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• The T2 value is one of the curve-fitting
  coefficients.

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Time of Repetition and Partial Saturation

• The standard spin echo pulse sequence uses a
  series of 90-degree pulses separated by a period
  known as the time of repetition (TR), which
  typically ranges from about 300 to 3,000 msec.
• A time delay between excitation pulses allows
  recovery of the longitudinal magnetization.

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• During this period, the FID and the echo produce
  the MR signal.
• After the ER interval, the next 90-degree pulse
  is applied, but usually before the complete
  longitudinal magnetization recovery of the
  tissues.

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• In this instance, the FID generated is less than
  the first FID.
• After the second 90-degree pulse, a steady-state
  longitudinal magnetization produces the same FID
  amplitude from each subsequent 90-degree pulse
  (spins are rotated through 360 degrees and are
  reintroduced in the transverse plane).

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• Tissues become partially saturated (i.e., the
  full transverse magnetization is decreased from
  the equilibrium magnetization), with the amount
  of saturation dependent on the T1 relaxation
  time.

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• A short-T1 tissue has less saturation than a
  long-T1 tissue.

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• For spin echo sequences, partial saturation of
  the longitudinal magnetization depends on the TR
  and T1 of the tissues.
• Partial saturation has an impact on tissue
  contrast.

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Spin Echo Contrast Weighting

• Contrast in an image is proportional to the
  difference in signal intensity between adjacent
  pixels in the image, corresponding to two
  different voxels in the patient.

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• The signal, S, produced by an NMR system is
  proportional to other factors as follows
• where pH is the spin (proton) density, f(v) is
  the signal arising from fluid flow, T1 and T2 are
  physical properties of tissue, and TR and TE are
  pulse sequence controls on the MRI machine.

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• The equation shows that for the same values of TR
  and TE (i.e., for the same pulse sequence),
  different values of T1 or T2 (or of rH or f(v))
  will change the signal S.
• The signal in adjacent voxels will be different
  when T1 or T2 changes between those two voxels,
  and this is the essence of how contrast is formed
  in MRI.

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• Importantly, by changing the pulse sequence
  parameters TR and TE, the contrast dependence in
  the image can be weighted toward T1 or toward T2.

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T1 Weighting

• A T1weighted spin echo sequence is designed to
  produce contrast chiefly based on the T1
  characteristics of tissues by de-emphasizing T2
  contributions.
• This is achieved with the use of a relatively
  short TR to maximize the differences in
  longitudinal magnetization during the return to
  equilibrium, and a short TE to minimize T2
  dependency during signal acquisition.

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• In the longitudinal recovery and transverse decay
  diagram, note that the TR time on the abscissa of
  the figure on the left (longitudinal recovery)
  intersects the individual tissue curves and
  projects over to the figure on the right
  (transverse decay).

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• These values represent the amount of
  magnetization that is available to produce the
  transverse signal, and therefore the individual
  tissue curves on right-hand figure start at this
  point at time T 0.
• The horizontal projections (arrows) graphically
  demonstrate how the T1 values modulate the
  overall MRI signal.

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• When TR is chosen to be 400 to 600 msec, the
  difference in longitudinal magnetization
  relaxation times (T1) between tissues is
  emphasized.

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• Four common cerebral tissuesfat, white matter,
  gray matter, CSFare shown in the diagrams.
• The amount of transverse magnetization (which
  gives rise to a measurable signal) after the
  90-degree pulse depends on the amount of
  longitudinal recovery that has occurred in the
  tissue of the excited sample.

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• Fat, with a short T1, has a large signal, because
  the short T1 value allows rapid recovery of the
  Mz vector.
• The short T1 value means that the spins rapidly
  reassume their equilibrium conditions.

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• White and gray matter have intermediate T1
  values, and CSF, with a long T1, has a small
  signal.
• For the transverse decay (T2) diagram, a
  180-degree RF pulse applied at time TEI2 produces
  an echo at time TE.
• A short TE preserves the TI signal differences
  with minimal transverse decay, which reduces the
  signal dependence on T2.
• A long TE is counterproductive in terms of
  emphasizing TI contrast, because the signal
  becomes corrupted with T2 decay.

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• T1-weighted images therefore require a short TR
  and a short TE for the spin echo pulse sequence.

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• A typical T1-weighted axial image of the brain
  acquired with TR 500 msec and TE 8 msec is
  illustrated.

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• Fat is the most intense signal (shortest T1)
• White matter and gray matter have intermediate
  intensities and
• CSF has the lowest intensity (longest T1).

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• A typical spin echo T1-weighted image is acquired
  with a TR of about 400 to 600 msec and a TE of 5
  to 20 msec.

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Spin (Proton) Density Weighting

• Image contrast with spin density weighting relies
  mainly on differences in the number of
  magnetizable protons per volume of tissue.
• At thermal equilibrium, those tissues with a
  greater spin density exhibit a larger
  longitudinal magnetization.

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• Very hydrogenous tissues such as lipids and fats
  have a high spin density compared with
  proteinaceous soft tissues aqueous tissues such
  as CSF also have a relatively high spin density.

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• The figure illustrates the longitudinal recovery
  and transverse decay diagram for spin density
  weighting.

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• To minimize the T1 differences of the tissues, a
  relatively long TR is used.
• This allows significant longitudinal recovery so
  that the transverse magnetization differences are
  chiefly those resulting from variations in spin
  density (CSF gt fat gt gray matter gt white matter).

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• Signal amplitude differences in the FID are
  preserved with a short TE, so the influences of
  T2 differences are minimized.
• Spin density-weighted images therefore require a
  long TR and a short TE for the spin echo pulse
  sequence.

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• This figure shows a spin density-weighted image
  with TR 2,400 msec and TE 30 msec.

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• Fat and CSF display as a relatively bright
  signal, and a slight contrast inversion between
  white and gray matter occurs.
• A typical spin density-weighted image has a TR
  between 2,000 and 3,500 msec and a TE between 8
  and 30 msec.

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• This sequence achieves the highest overall signal
  and the highest signal-to-noise ratio (SNR) for
  spin echo imaging however, the image contrast is
  relatively poor, and therefore the
  contrast-to-noise ratio is not necessarily higher
  than with a T1- or T2-weighted image.

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T2 Weighting

• T2 weighting follows directly from the spin
  density weighting sequence
• Reduce T1 effects with a long TR, and accentuate
  T2 differences with a longer TE.

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• The T2-weighted signal is usually the second echo
  (produced by a second 180-degree pulse) of a
  long-TR spin echo pulse sequence (the first echo
  is spin density weighted).

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• Generation of T2 contrast differences is shown in
  the figure.

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• Compared with a T1-weighted image, inversion of
  tissue contrast occurs (CSF is brighter than fat
  instead of darker), because short-T1 tissues
  usually have a short T2, and long-T1 tissues have
  a long T2.
• Tissues with a long T2 (e.g., CSF) maintain
  transverse magnetization longer than short-T2
  tissues, and thus result in higher signal
  intensity.

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• A T2-weighted image, demonstrates the contrast
  inversion and high tissue contrast features,
  compared with the T1-weighted image.

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• As TE is increased, moreT2 contrast is achieved,
  at the expense of a reduced transverse
  magnetization signal.
• Even with low signal, window width and window
  level adjustments remap the signals over the full
  range of the display, so that the overall
  perceived intensity is similar for all images.

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• The typical T2-weighted sequence uses a TR of
  approximately 2,000 to 4,000 msec and a TE of 80
  to 120 msec.

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Spin Echo Parameters

• For conventional spin echo sequences, both a spin
  density and a T2-weighted contrast signal are
  acquired during each TR by acquiring two echoes
  with a short TE and a long TE.