Nuclear Magnetic Resonance II

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Nuclear Magnetic Resonance II

• 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 contrast sensitivity in MR
  images
• Tailoring the pulse sequences timing, order,
  polarity, and repetition frequency of the RF
  pulses and applied magnetic field gradients
  makes the emitted signals dependent on T1, T2 or
  spin density relaxation characteristics

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Pulse sequences (cont.)

• MR relies on three major pulse sequences
• Spin echo
• Inversion recovery
• Gradient recalled echo
• 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

• 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 RF pulses allows separation of
  the initial FID and the echo and the ability to
  adjust tissue contrast

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Time of repetition

• The standard spin echo pulse sequence uses a
  series of 90-degree pulses separation 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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Partial saturation

• After the TR interval, the next 90-degree pulse
  is applied, but usually before the complete
  longitudinal magnetization recovery of the
  tissues
• 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
• Tissues become partially saturated, with the
  amount of saturation depending on the T1
  relaxation time

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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
• The signal, S, produced by an NMR system is
  proportional to other factors as follows
• ?H is the spin (proton) density
• f(v) is the signal arising from fluid flow

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Spin echo contrast weighting

• 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 spin density
  or flow) will change the signal S
• The signal in adjacent voxels will be different
  when T1 or T2 changes between these voxels
• 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 T1-weighted spin echo sequence is designed to
  produce contrast chiefly based on the T1
  characteristics of the tissues by de-emphasizing
  T2 contributions
• This is done using 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
• Tissues with shorter T1 have higher image
  intensity

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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
• To minimize T1 differences of the tissues, a
  relatively long TR is used
• Signal amplitude differences in the FID are
  preserved with a short TE, so the influences of
  T2 are minimized
• Tissues with higher spin density (e.g., fat, CSF)
  have higher image intensity

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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
• Compared with a T1-weighted image, inversion of
  tissue contrast occurs, because short-T1 tissues
  usually have a short T2, and long-T1 tissues
  usually have a long T2

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Spin echo parameters

• T1 contrast TR 400-600 msec, TE 5-30 msec
• Spin density contrast TR 1,500-3,000 msec, TE
  5-30 msec
• T2 contrast TR 1,500-3,000 msec, TE 60-150
  msec
• 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

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Inversion recovery

• Inversion recovery, also known as inversion
  recovery spin echo, emphasizes T1 relaxation
  times of the tissues by extending the amplitude
  of the longitudinal recovery by a factor of two
• An initial 180-degree RT pulse inverts the Mz
  longitudinal magnetization to Mz
• After a delay time known as the time of inversion
  (TI), a 90-degree RF pulse rotates the recovered
  fraction of Mz spins into the transverse plane to
  generate the FID

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Inversion recovery (cont.)

• A second 180-degree pulse at time TE/2 produces
  an echo signal at TE
• The echo amplitude of a given tissue depends on
  TI, TE, TR, and the magnitude of Mz
• The signal intensity is approximated as follows

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Inversion recovery (cont.)

• Inversion recovery sequence produces negative
  longitudinal magnetization that results in
  negative (in phase) or positive (out of phase)
  transverse magnetization when a short TI is used
• Actual signals encoded in one of two ways
• A magnitude (absolute value) signal flips
  negative Mz values to positive values
• A phase signal preserves the full range of the
  longitudinal magnetization from Mz to Mz

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Short tau inversion recovery

• Short tau inversion recovery, or STIR, is a pulse
  sequence that uses a very short TI and magnitude
  signal processing
• Materials with short T1 have a lower signal
  intensity (the reverse of a standard T1-weighted
  image)
• All tissues pass through zero amplitude (Mz 0),
  known as the bounce point or tissue null
• Judicious TI selection can suppress a given
  tissue signal (e.g., fat)

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Fluid attenuated inversion recovery

• The use of longer TI reduces the signal levels of
  CSF and other tissues with long T1 relaxation
  constants
• Fluid attenuated inversion recovery, or FLAIR,
  reduces CSF signal and other water-bound anatomy
  in the MR image by using a TI selected at or near
  the bounce point of CSF

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Gradient recalled echo

• The gradient recalled echo (GRE) technique uses a
  magnetic field gradient to induce the formation
  of an echo, instead of the 180-degree pulses
  discussed earlier
• The GRE is not a true spin echo technique but a
  purposeful dephasing and rephasing of the FID
• Magnetic field inhomogeneities and tissue
  susceptibilities are emphasized in GRE

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GRE and flip angles

• Depending on the desired contrast, flip angles of
  a few degrees to more than 90 degrees are applied
• With short TR, smaller flip angles are used
  because there is less time to excite the spin
  system, and more transverse magnetization is
  actually generated with smaller flip angles
• This occurs because magnetization quickly builds
  up in the tissue sample
• Tissue contrast in GRE pulse sequences depends on
  TR, TE, and the flip angle

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GRE with long TR (gt200 msec)

• For the GRE sequence with long TR and flip
  angles greater than 45 degrees, contrast
  behaviour is similar to that of spin echo
  sequences
• Major difference is the generation of signals
  based on T2 contrast rather than T2 contrast
• In terms of clinical interpretation, the
  mechanisms of T2 contrast are different from
  those of T2, particularly for MRI contrast agents
• A long TE tends to show the differences between
  T2 and T2

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Steady-state precession with short TR (lt50 msec)

• Flip angle has the major impact on the contrast
  weighting of the resultant images
• Small flip angles spin density weighting is the
  single most important factor
• Moderate flip angles contrast depends on the
  difference in T2/T1 ratio between the tissues
• Large flip angles contrast depends on T2 and
  T1 weighting an increase in TE contributes to
  the T2 contrast in the image

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