Slide 1 - Faculty Web Server by shuifanglj

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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 sequences—that is, the
      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.
   MR relies on three major pulse
    sequences:
    • spin echo,
    • inversion recovery, and
    • gradient recalled echo.
   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.
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 RU pulses allows
      separation of the initial FID and the echo and
      the ability to adjust tissue contrast.
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.
   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).
   This sequence is
    depicted in the
    rotating frame.
   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.
   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.
   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.
   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).
   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.
   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.
   This process can be repeated.
   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.
   Just before and after the peak amplitude
    of the echo (centered at time TE) digital
    sampling and acquisition of the signal
    occurs.
   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).
   Multiple echoes generated by 180-
    degree pulses after the initial excitation
    allow the determination of the ―true T2‖
    of the sample.
   Signal amplitude is measured at several
    points in time, and an exponential curve
    is fit to this measured data
   The T2 value is one of the curve-fitting
    coefficients.
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.
   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.
   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).
   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.
   A short-T1 tissue has less saturation
    than a long-T1 tissue.
   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.
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.
   The signal, S, produced by an NMR
    system is proportional to other factors as
    follows:
                          
          S   H f (v) 1  e   TR / T 1
                                            e   TE / T 2


    • 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.
   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 H 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.
   Importantly, by changing the pulse
    sequence parameters TR and TE, the
    contrast dependence in the image can
    be weighted toward T1 or toward T2.
T1 Weighting
   A ―T1–weighted‖ 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.
   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).
   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.
   When TR is chosen to be 400 to 600
    msec, the difference in longitudinal
    magnetization relaxation times (T1)
    between tissues is emphasized.
   Four common cerebral tissues—fat,
    white matter, gray matter, CSF—are
    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.
   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.
   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.
   T1-weighted images therefore require a
    short TR and a short TE for the spin
    echo pulse sequence.
   A typical T1-
    weighted axial image
    of the brain acquired
    with TR = 500 msec
    and TE = 8 msec is
    illustrated.
   Fat is the most intense signal (shortest
    T1);
    • White matter and gray matter have
        intermediate intensities; and
    •   CSF has the lowest intensity (longest T1).
   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.
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.
   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.
   The figure illustrates the longitudinal
    recovery and transverse decay diagram
    for spin density weighting.
   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 > fat > gray matter > white
      matter).
   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.
• This figure shows a spin density-weighted
 image with TR = 2,400 msec and TE = 30
 msec.
   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.
   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.
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.
   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).
   Generation of T2 contrast differences is
    shown in the figure.
   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.
   A T2-weighted
    image, demonstrates
    the contrast
    inversion and high
    tissue contrast
    features, compared
    with the T1-weighted
    image.
   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.
   The typical T2-weighted sequence uses
    a TR of approximately 2,000 to 4,000
    msec and a TE of 80 to 120 msec.
      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.

								
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