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Flow Phenomena

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					Flow Phenomena
     Time of flight
 Entry slice phenomina
 Intra-voxel dephasing
                Introduction
   Explores artefacts Produced from nuclei
    that move during the acquisition of data
   Flowing nuclei exhibit different contrast
    characteristics from their neighbouring
    stationary nuclei.
   Originate primarily from nuclei in blood
    and CSF
   The motion causes mismapping of signals
    and results in artefacts known as flow
    motion artefacts or phase ghosting.
   The cuases of flow artefact are known as
    flow phenomena
     The principal phenomina

   Time of flight
   Entry slice phenomina
   Intra-voxel dephasing
         Mechanisms of flow
   Laminar flow – flow that is at different but
    consistent velocities. At the centre of the lumen
    of the vessel is faster than at the wall. The
    volocity difference across the vessel is constant.
   Turbulent flow – flow at different velocities
    that fluctuates randomly. Velocity difference
    across the vessel changes erratically.
   Vortex flow – flow that is initially laminar but
    then passes through a stricture or stenosis in
    the vessel. Flow at the centre of the lumen has
    a high velocity but near the walls, the flow
    spirals.
   Stagnant flow – the velocity of flow slows
    down to a point of stagnation. It behaves like
    stationary tissue.
           Flow mechanisms

 Laminar                      vortex   turbulance
 flow

                  stricture




Laminar flow (constant velocity)is termed first
order motion
    Time of flight phenomenon
   To produce a signal, a nucleus must receive an
    excitation pulse and a rephasing pulse.
   If a nucleus receive only an excitation pulse but
    not rephased it does not produce a signal.
   If a nucleus is rephased but not received an
    excitation pulse it does not produce a signal
   Stationary nuclei always receive both excitation
    pulse and rephasing pulse and therefore produce
    a signal.
   Flowing nuclei present in the slice for excitation
    may exit the slice before rephasing and therefore
    not produce signal.
   This is called the time of flight phenomenon
   The effect of the phenomenon depends on the
    types of pulse sequence used.
Time of flight in spin echo pulse sequences
    In spin echo a slice is selected and a 900
     excitation pulse and a 1800 rephasing pulse are
     applied.
    Every slice is selectively excited and rephased.
    Stationary nuclei wihin the slice receive both
     pulses and produce a signal
    Nuclei flowing perpendicular to the slice may be
     present for the excitation pulse, but may not for
     the rephasing pulse.
    They may not produce a signal
    Similarly new nuclei which have not received
     excitation pulse may be precsent for the
     rephasing pulse
    They also not produce a signal
    This will result in a signal void from the nuclei, so
     the vessel appear dark
Time of flight phenomenon in spin
                echo
                             1800
                  900




    Flowing
    nuclei


                excited rephased


                                                     No signal

                 excited              Not rephased
                                                     No signal
                           rephased
  Not excited
                    slice
    Factors affecting the time of flight
                  effect

   Velocity of flow
   TE
   Slice thickness
               Velocity of flow

   At high velocity only a smaller proportion of
    nuclei are present for both excitation and
    rephasing pulses.
   As the velocity increases the time of flight effect
    is increased.
   At slow velocity more nuclei will be present for
    both excitation and rephasing pulses.
   Therefore, as the velocity decreases the effect of
    time of flight effect decreases.
   This is called flow related enhancement
        Effect of TE on TOF

   As the TE increases, a higher
    proportion of flowing nuclei have
    exited the slice between the
    excitation pulse and the rephasing
    pulse. Therefore with increasing TE
    the signal void increases.
         Slice thickness & TOF
   For a given constant velocity a nucleus take
    longer time to travel through a thicker slice.
   Therefore the nuclei are more likely to receive
    both excitation and the rephasing pulse.
   So the TOF effect is less on thick slices than on
    thin slices.



                                Thin
            Thick slice
                                slice




           signal              Signal
                               void
    Time of flight in gradient echo
          pulse sequences
   In gradient echo a variable RF excitation pulse is
    followed by a gradient rephasing.
   Each slice is selectively excited by the RF pulse,
    but the rephasing gradient is applied to the whole
    body
   So the flowing nucleus which was excited by the
    RF pulse is rephased by the gradient even if it
    has exited the slice, and produces a signal.
   In addition the short TR tend to saturate the
    stationary nuclei and the flowing nuclei appear to
    have a higher signal.
   Therefore in GE pulse sequences the flow signal
    enhancement is increased.
   So the GE sequences are said to be flow sensitive
                   Summary
   Time of flight phenomina produce
    • flow related enhancement     or
    • high velocity signal loss
   Flow related enhancement increases as
    the:
    • Velocity of flow decreases
    • TE decreases
    • Slice thichness increases
   High velocity signal void increases as the:
    • Velocity of flow increases
    • TE increases
    • Slice thickness decreases
Entry slice phenomenon (in-flow
             effect)
   Entry slice phenomenon is related to the
    excitation history of the nuclei.
   Nuclei that receive repeated RF pulses during the
    acquisition are said to be saturated or ‘beaten
    down’.
   The NMV of these nuclei eventually reach an
    equilibrium position, and produce a signal
    according to the TE,TR, flip angle and contrast
    characteristics of the tissue.
   Nuclei that have not received repeated RF pulses
    are said to be ‘Fresh’ as their NMV has not been
    beaten down.
   The signal produced by the ‘fresh’ nuclei and the
    ‘beaten down’ nuclei are different.
Saturated and fresh spins
  Magnetic           Fresh
  moments beaten     spins
  down by repeated
  RF puses




 Satuurated
 spins
   Stationary nuclei within a slice become
    saturated after repeated RF pulses.
   Nuclei flowing perpendicular to the slice
    enter the slice fresh.
   They therefore produce a different signal
    from the stationary signal.
   This is called entry slice phenomenon or
    in-flow effect (as it is more prominent in
    the first slice of a ‘stack’ of slices.
   The slices in the middle of the stack
    exhibit less entry slice phenomenon ( as
    flowing nuclei have received more
    excitation pulses by the time they reach
    these slices)
Factors affecting the magnitude
of entry slice phenomenon (ESP)
   TR     - short TR reduces the
    magnitude of ESP
   Slice thickness – ESP increase with
    the slice thickness
   Velocity of flow – ESP increases as
    the velocity of flow increases
   Direction of flow – ( is the most
    important in determining the
    magnetude of ESP ) – next slide
            Direction of flow
   The direction may be co-current (same
    direction as the slice selection) or
    counter-current (opposite direction to
    the slice selection)
   Co-current flow – ESP decreases rapidly
    in the direction of slice selection
   Counter-current flow – ESP is more
    prominent and may still be present deep
    within the slice stack
  Co-current & counter current flow
                      vessel



Co-
current




                                   Counter
                                   -current




 Slices
          1   2       3        4
                  Summary
   ESP increases:
    • At the first slice in the stack
    • When using a long TR
    • In thin slices
    • With fast flow
    • In counter-current flow
         Intra-Voxel dephasing
   Gradients alter the magnetic field strength,
    precessional frequency and phase of nuclei.
   Nuclei flowing along a gradient rapidly accelerate
    or decelerate depending on the direction of flow
    and gradient application.
   Flowing nuclei therefore either gain phase or
    loose phase
   If a flowing nucleus is adjacent to a stationary
    nucleus in a voxel, there is a phase difference
    between the two nuclei.
   Therefore nuclei wihin the same voxel are out of
    phase with each other
   This will result in a reduction of total signal
    amplitude from the voxal.
   This is called intra-voxel dephasing.
        Type of flow and IVD

   The magnitude of intra-voxel
    dephasing depends on the degree of
    turbulence.
   In turbulent flow, IVD effects are
    irreversible.
   In laminar flow, the IVD can be
    compensated for as long the velocity
    is constant.
                Summary
   Flow affects image quality
   Time of flight effects give signal void
    or enhancement
   Entry slice phenomenon effects give
    a different signal intensity to flowing
    nuclei
   The signal intensity of the lumen is
    also affected by the mechanism of
    flow
Flow phenomena compensation
   Flowing nuclei produce a very
    confusing range of signal intensities.
   Ideally, these should be
    compensated for, inorder to minimise
    their adverse effects on image
    quality and interpretation.
   There are several methods to help
    minimise flow artefacts.
1. Gradient moment rephasing(nulling)
    Gradient moment rephasing compensates for the altered
     phase values of nuclei flowing along a gradient.
    It uses additional gradients to correct the altered phases
     back to their original values.
    In this way, flowing nuclei do not gain or loose phase due
     to the presence of the main gradient.
    This is performed by using slice select gradient and/or
     readout gradient.
    The gradient alters its polarity from positive to
     double negative and then back to positive again.
    A flowing nucleus traveling along these gradients,
     experiences different magnetic field strengths.
    In order to compensate altered phase values, the
     precessional frequency at the beginning of gradient
     moment rephasing must be the same as it is at the end.
    The net precessional frequency and phase change must
     therefore be zero.
      Gradient moment rephasing
                            +4000   +8000        +12000      0
   Frequency of a       0   Hz      Hz           Hz
   stationary nucleus
   along a normal
   gradient                                      +12000 Hz
                            +4000 Hz -16000 Hz                0

Frequency of a
stationary nucleus
along the
compensatory
gradient



Phase of a flowing
nucleus

 Frequency of a         0   +4000    -12000
 flowing nucleus            Hz                    0       0
                                     Hz
     Result of gradient movement
               rehasing
   Gradient moment rephasing reduces intra-
    voxel dephasing.
   As flowing nuclei have the same phase as
    stationary nuclei in the same voxel, their
    signals add constructively and therefore a
    brighter signal results.
   Gradient momemt rephasing gives flowing
    nuclei a bright signal as spins are in
    phase.
                        Note
   Gradient moment rephasing assumes a constant
    velocity across the gradients.
   Most effective in slow laminar flow (1st order
    motion)
   More effective in venous rather than arterial flow.
   As it uses extra grdients minimum TE is increased
   Fewer slices may be obtained for a given TR
   As flowing nuclei are bright usually used for T2 or
    T2* weighted images where fluid (blood & CSF) is
    bright anyway.
                      Example




Without GMR – shows             With GMR
mismapping of the flowing
nuclei within the aorta
(arrow)
               Pre-saturation
   Pre-satuation pulses are used to nullify the signal
    from flowing nuclei so that the effects of entry
    slice and TOF phenomena are minimised.
   It delivers a 900 RF pulse to a volume of tissue
    outside the FOV.
   The flowing nuclei within the volume receives this
    pulse.
   When they enter the slice stack, they receive the
    excitation pulse and become saturated.
   If it is fully saturated to 1800, it has no
    transverse component of magnetization and
    produce a signal void.
                              Flowing nuclei
     Saturation                                    vessel
     volume


Pre-saturation
RF pulse
                                                      FOV

                  Stationay


   Excitation




                                       Stationay
                  nuclei



   pulse




                                       nuclei
                     Caution!
   To be effective, pre-saturation pulses should be
    placed between the flow and the imaging stack.
   In sagital and axial imaging it is usually placed
    above and below the FOV.
   Right and left sturation pulses are some times
    used in coronal imaging of chest to saturate flow
    from subclavion vessels.
   Pre-saturation pulses are only useful if they are
    applied to tissue.
   If they are applied to air they are not effective.
   They increase the amount of RF that is delivered
    to the patient, and increase heating effect.
                   Caution!
   Pre-saturation pulses are applied around
    each slice just before the excitation pulse.
   The TR, and the number of slices govern
    the interval between the delivery of each
    pr-saturation pulse.
   To optimise pre-saturation, use all the
    slices permitted for a given TR.
   As pre- saturation produces a signal void,
    it is usually used in T1 and proton density
    weighted images where fluid (blood &
    CSF), is dark anyway.
Example -pre-saturated images
Artefact from flowing
nuclei within aorta
(without saturation)                  With pre-saturation




                  T1 weighted imaes
    Other uses of pre-saturation

   Pre-saturation nullifies signal and can
    therefore be used to specifically
    eliminate certain signals.
   The main uses of this are:
    • fat and water saturation
    • to reduce aliasing
      1. Fat & water saturation

   In fat hydrogen is linked to carbon and in water it
    is linked to oxygen.
   Therfore the precessional frequency of hydrogen
    in water and fat are different.
   AT 1.5T field strength this difference is 220 Hz (
    less in fat).
   To saturate fat signal, a 900 pre-saturation pulse
    must be applied at the precessional frequency of
    fatt to the whole FOV.
   To saturate water signal, the pre-saturation pulse
    at the precessional frequency of water is applied
    to the whole FOV
  Fat & water saturation pulses
                                       Water saturation
    Fat saturation
                          Saturation
                          pulse
Water
peak
             Fat
             peak    Saturation
                     pulse




           220Hz                            220Hz
                Examples
           Sagital T1 weighted images
Without fat saturation        With fat saturation
   T1 weighted image      T1 weighted image
    without water           with water
    saturation              saturation
     2. pre-saturation to prevent
               aliasing
   Aliasing is produced when anatomy
    exists outside the FOV. It can be
    reduced by pre-saturation to prevent
    signals from tissue outside the FOV.
    However there are anti-aliasing
    methods to prevent aliasing
    (discused later).
              Summary
1. Gradient Moment Rephasing:
 • Uses additional gradients to correct
   altered phase values
 • Reduces artefact from intra-voxel
   dephasing
 • Gives flowing nuclei a bright signal
 • Is mainly used in T2 or T2* weighted
   images
 • Is most effective on slow, laminar flow
   within the slice
                 Summary
2. Pre-saturation:
  • Uses additional RF pulses to nullify signal from
    flowing nuclei
  • Reduces artefacts due to time of flight and
    entry slice phnomenon
  • Gives flowing nuclei a signal void
  • Is mainly used in T1 weighted images
  • Is effective on fast and slow flow
  • Increases the RF deposition to the patient
  • Can be used to nullify signal from fat or water
    and to reduce aliasing.
END

				
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posted:10/31/2011
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