Artefacts In Clinical MRI by nikeborome

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									       Artefacts In Clinical MRI
                   Kris Armoogum MSc
             Department of Medical Physics,
                    Ninewells Hospital
                         Dundee
                        DD2 1QW
            Kris.Armoogum@tuht.scot.nhs.uk
            Stephen.Gandy@tuht.scot.nhs.uk


14th Scottish MRI Seminar, Wednesday 19th November 2003
                         Introduction
•    Artefacts are parts of reconstructed images that are not present in
     the true anatomy.
•    Artefacts are dependent on a variety of factors from patient
     movement to magnetic field inhomogeneities.
•    Artefacts can lead to misdiagnosis if they are not recognised
     and/or removed.
•    Ideally, we want all image artefacts to be below the level of user's
     perception.

Main classifications -

1.   Movement Artefacts
2.   Geometrical Artefacts
3.   Resolution/Sequence Artefacts
4.   Bo Artefacts
5.   RF Artefacts
6.   Noise
1. Movement Artefacts
• Motion Artefacts – patient
• Flow artefact – inflow/washout effect,
  diastole, systole, arterial flow
• Reducing flow artefacts – Gradient
  Moment Nulling
• Respiratory compensation, triggering,
  ROPE, navigator echoes
          Motion Artefact - Patient

•   Patient movement – as outer
    areas of k-space acquired
•   May mimic truncation artefact
•   Difference – truncation artefact
    diminishes with distance from
    the high contrast boundary
•   If related to pulsation of vessels,
    this can be reduced by applying
    an anterior sat band



                                          T2W SE Thoracic Spine
      Patient Movement Artefact
• Smearing of image
• Particularly in phase direction
Solutions
• Immobilise the patient more effectively
• Reduce the scan time – reduce NSA, breathold, shorter TR, less k-space
   lines
• Reduce the scan time (reduced k-space acquisition) e.g. HASTE, TSE with
   large turbo factor
       Flow Artefact – Inflow Effect
•   T1W GE sequence
•   RF pulse saturates the blood momentarily in the slice (yellow)
•   If blood is stationary, long T1 of blood means that no signal available
    for successive RF pulses to excite – hypointense signal
•   If velocity of inflowing blood > z/TR then full inflow occurs and the next
    RF pulse ‘sees’ unsaturated spins in the slice – ‘Bright Blood’ signal
•   Other tissues within the slice are saturated, and therefore suppressed

           NO FLOW                   Next TR
                                                                Image
                  90o pulse                 Dark signal


            z
           FLOW                      Next TR
                  90o pulse                Bright signal
Flow Artefact – Washout Effect
•   Spin-echo sequence - 90o pulse excites spins in the slice
•   In the absence of flow, bright signal is seen because the spins
    experience both the 90o and 180o pulses
•   In the presence of flow, blood flows out of the slice and does not
    experience the 180o pulse – no rephasing, ‘Black Blood’ signal
•   D.G. Nishimura "Time-of-Flight MR Angiography."
    Magn. Reson. Med. 14:194-201 (1990)
NO FLOW                     TE/2                 TE
                                                                Image
       90o pulse          180o pulse           Signal




FLOW
       90o pulse          180o pulse           No signal
               Flow Artefact I - Body
               +1
Grad       T
                         T
                    -1




                                 S

Phase                        D
Shift




    •   Diastole (filling phase), systole (emptying phase).
    •   Aortic ghosts in PE direction [because FE step (msec) takes much less
        time than a PE (sec) step]
    •   Physiological modulation
    Why only the PE Direction?

Why is motion artefact only seen in the PE direction?
• A FE step takes much less time (of the order of msec)
  than a PE step (of the order of seconds)
• Most motion that occurs during clinical MRI is much
  slower than the rapid sampling process along the FE axis
• However, each PE line is separated by the time interval
  TR which is long enough for blood to flow/move/dephase
  between successive phase encodings
              Phase Shift Effects
     S

              D
                         ωm

                              ωo   FT        ωo-ωm            ωo+ωm

                                                        ωo
          MR Signal


•   Systolic-diastolic switching of the flow velocity (at frequency ωm)
    modulates the MR signal (with frequency ωο)
•   Two frequency sidebands (upper and lower frequency sideband
    components) appear as ghosts either side of the primary image
            Flow Artefact II - Brain                                           A
        •    Velocity profile – laminar flow
             (Re < 2100)
        •    Velocity zero at wall, fastest at
             centre of lumen
        •    Continuous spread of velocities
            +1
Grad    T
                      T
                 -1




                           Continuous
                                                        B
Phase
Shift                                   •   A = flow artefact from eye movement
                                        •   B = flow artefact from sagittal sinus
      Flow From Sagittal Sinus




•   Distinguishable from Truncation artefact – artefact propagates
    across the anatomy.
•   Truncation artefact diminishes with distance from the high
    contrast boundary.
              Further Flow Artefacts

A                           B                               C




    •   Flow from middle (A) cerebral arteries, (B) eye movement & sagittal
        sinus, and (C) salivary glands (swallowing)
          Flow – Popliteal Artery


                                                  Popliteal Artery




•   Popliteal artery flow generates artefacts across the femur.
•   More prominent when fat suppression removes the marrow signal.
•   May disturb interpretation of bone bruising or subchondral cysts.
              Phase Shift Effects
                       ωm

                          ωo   FT
                                     ωo-ωmax            ωo+ωmax
                                                  ωo

         MR Signal


•   Complex modulation frequency, made up of many component waves
•   FT results in a spread of upper and lower sidebands - artefact
•   Worse for GE images, due to bright blood inflow effect
•   SE dark blood inflow effect – less severe phase artefacts
Reducing Phase Flow Effects - I
Low bandwidth                               High bandwidth         +2G


                     +G

     Grad       2T                               Grad    T
                               2T                              T

                          -G

                                                             -2G




                                    Large
                                                                   Small
  Phase                                        Phase
  Shift                                        Shift



      •     Use sequence with a high bandwidth – shorter TE
      •     Amount of dephasing generally goes up with TE2
      •     Shorter TE minimises velocity induced phase effects
        Gradient Moment Nulling
•   Gradient Moment Nulling (GMN) – technique used for flow
    compensation

                                           •    Stationary tissue (0o) – unaffected
        +1                     +1          •    Constant velocity blood (1o motion) –
                    -2                          rephased by +1-2+1 gradients
                                           •    Constant acceleration blood (2o) –
                                                need –1+3-3+1 gradients to rephase
                                           •    Jerk motion (3o) – need +1-4+6-4+1
                                                gradients to rephase
Phase
shift
                                               2o flow comp     3o flow comp
                                                     +3                  +6
                                                          +1   +1                  +1
             Stationary tissue
             Constant velocity blood
                                                -1                  -4        -4
             Constant acceleration blood
                                                     -3
         Flow Compensation
                                             Thoracic spine
                         Vertebral foramen




•   Two cords appear to be present in first T2W TSE image.
•   The ‘extra cord’ is flow artefact from pulsatile CSF flow.
•   First order (+1-2+1 gradient) flow compensation
    (Gradient Moment Nulling) results in the RHS image.
                Swallowing Motion
                                   Cervical spine




                                                S
                  CIV                           A
                   CV                           T




           A                       P




•   Any patient motion during a scan can cause PE artefacts (A-P above).
•   Left image - artefact generated by patient swallowing during data
    acquisition - increased signal intensity in the spinal cord.
•   Eliminated by applying presaturation RF pulses to the anatomy that
    was generating the artefact.
•   Sat band visible on RHS image.
           Respiratory Artefact
Periodic respiratory motion – ghosting above and below
the body
•   Remove by breathold imaging (<18sec scan time).
•   Increase the NSA (anatomy SNR improved relative to ghosts).
•   NSA 4 to 6 ~ respiratory compensation.
    Respiratory Gating/Triggering

                                        • Reduces respiratory artefact
Bellows pressure = electrical trigger
                                        •   Bellows placed over abdomen.
                                        •   Sequence TR ‘gated’ via patient
                                            breathing rate.
                           Inhalation   •   Equivalent to TR ~ 4000ms,
                                            (breathing rate 15/min) so only
                           Exhalation       able to generate PDW and T2W
                                            scans (long TR reduces T1 effect).
                                        •   Signal acquired when chest wall is
                                            in same position - minimises ghost
                                            images.
       Respiratory Compensation
   • Reduces respiratory artefact
   •   ROPE – Respiratory Ordered Phase Encoding (also uses bellows).
   •   Typical TR for a T1W sequence = 500 msec.
   •   Typical breathing rate = 15 breaths per min, i.e. 4000 msec period.
   •   Can therefore fit 8 TR’s (8 PE steps) per breathing cycle.
   •   Outer k-lines (image boundary detail) acquired at peak inhalation.
   •   Central k-lines (signal, contrast) are acquired at peak exhalation.



                                 K-space

                         +64                     -64

Inhalation

Exhalation
Respiratory Gating or ROPE ?

•   Gating: + simple technique
•   Gating: – effective TR very long (cannot do T1W)
•   ROPE: + shorter scan times
•   ROPE: – residual ghosts if patient breathes
    deeply
              Navigator Echoes
• Two slice selective directions and FE in the third direction
  (of motion).
• Small column of tissue excited across the diaphragm.
• Spin echo sequence – acquires series 1D images of the
  diaphragm boundary over time.
• Stack images side-by-side - intensity difference between
  diaphragm and lung indicates respiratory motion.
• Navigator echo is interleaved within main scan sequence.
• Data for main image can then be adjusted for respiratory
  motion by using data acquired during specific range of
  diaphragm motion.
2. Geometrical Artefacts
• Phase wrap
• Partial volume
• Cross talk
• Magic Angle artefact
                Phase Wrap - Aliasing
•   Regions outside FOV still produce a signal if in proximity to receiver coil.
•   Anatomy outside FOV is mapped inside FOV.
•   Corrected by - larger FOV or apply presat pulses to undesired tissue.
•   ‘No Phase Wrap’ – double the FOV; but because PE steps is doubled need to half
    number of averages to keep scan time constant.
•   Aliasing in FE direction can occur, but eliminated by filters (no time waste).


                     0
               -           +




           +200o
          = -160o   180o                      -180o        0      +180o

                                                  -160o   EQUIVALENT      +200o
                Partial Volume Effect
•   Partial volume occurs if slice thickness > thickness of tissue of interest
•   If small structure is entirely contained within the slice thickness along with
    other tissue of differing signal intensities then the resulting signal displayed on
    the image is a combination of these two intensities. This reduces contrast of
    the small structure.
•   If the slice is the same thickness or thinner than the small structure, only that
    structures signal intensity is displayed on the image.
•   Typically would use 3mm slices for cranial nerves and 5-10mm slices for liver.




                                                                     VII (Facial) and VIII
                                                                     (Acoustic) cranial nerves
                         Cross Talk
•   Perfect RF pulse is a sinc function (FT = ‘top hat’)
•   Real RF pulse is a truncated sinc (FT = ‘top hat with rounded edges’)
•   Inter-slice cross talk could cause increased T1 weighting and
    reduced SNR.
     How Does Cross Talk Occur?
                  TE = 20
                                     TR = 600
    SLICE 1    90o 180o                                    PE2


                          90o 180o
    SLICE 2                                                      PE2


                                                90o 180o
    SLICE N                                                               PE2

•   Typical TR for T1W scan = 600ms, typical TE = 20ms.
•   Theoretically possible to acquire 30 slices within the TR.
•   Cross talk region between slices 1 and 2 – experiences RF excitation from
    slice 1, then slice 2.
•   Effective TR is 20ms giving loss of signal due to lack of T1 recovery.
•   Solution - ‘interleave’ slices.
•   A 3D sequence avoids the problem altogether – contiguous slices.

                                                                       10-20% interslice gap
     Magic Angle Artefact (54.7o)
•   Collagen fibril orientation w.r.t. B0 field.
•   T2 lengthening at Magic Angle.
•   Result is that the T2W image becomes hyperintense at the
    magic angle.
•   Magic Angle is solution to: 3cos2θ-1=0 (from dipolar
    Hamiltonian mathematical theory)
•   Magic angle imaging of the median nerve (brain) which has a
    high collagen content.


                     Median Nerve           At Magic Angle
                     in brain
3. Resolution/Sequence Artefacts
• Truncation artefact
• Chemical Shift artefact (Types I and II)
            Truncation Artefact - Brain
              128                                                256
              x                                                  x
              256                                                256




•   Also known as Gibbs (‘ringing’) artefact.
•   Usually occurs in the PE direction at high contrast borders.
•   Due to undersampling of high spatial frequencies (sharp edged borders)
•   Remedied by taking more samples (e.g. 256 PE steps).
•   Truncation artefact causes ring-down effect because F.T. of truncated sinc
    function has ripples at the edges.



                                      F.T.
                                                                        ω
                                 t
Truncation Artefact or Syrinx?
 • Problematic down centre of spinal cord –
   could be misinterpreted as a syrinx


                          CV




                         Syrinx
          (fluid filled cavity in spinal cord)
    Chemical Shift Artefact – Type I
                                        T1W Lumbar spine



•   T1W image of lumbar spine.
•   Low BW sequence used.
•   Frequency shift of a few pixels
    is visible at the base of each               L III

    vertebra (black line).
•   Vertebra-disc boundary detail is
                                                L IV
    lost at the top of each vertebra.
•   Observation of small disc
    herniations in L spine difficult.
     Chemical Shift (I) - Explained
      Displacement of
      yolk (fat) on LHS
      image




                          Egg (low BW)     Egg (high BW)

•   Protons from different molecules (eg: fat & water) precess at different
    frequencies.
•   Protons in H2O precess slightly faster than those in fat, (diff. is 3.5 ppm)
•   Chemical shift = 3.5ppm = 224Hz at 1.5T [ ω0 = γ.B0 :: (42.6MHz/T)(1.5T) ::
    64MHz :: 3.5ppm x 64MHz = 224Hz ]
•   LHS = 12.5kHz (low BW), 256 resolution.
•   Chemical shift is 4.6 pixels [ 224 / (12.5kHz/256) ]
•   Chemical shift also occurs between silicone & fat/water (Breast MRI)
•   Modify CS by using fat suppression, increase the bandwidth, swap freq and
    phase directions, or lower the Bo field (impractical)!
    Chemical Shift – Type II Artefact
       Out of
                                             In
       Phase                                 Phase
                 Liver       Thoracic
                             aorta




Worked example
•   Applies to Gradient Echo techniques, (not in SE because of 180º refocusing
    pulse).
•   Fat and water proton resonant frequencies differ by 3.5ppm.
•   For an imaging field strength of 1.5T, ω=λ= 64 MHz (from ω0 = γ.B0 ).
•   Difference between fat and water proton resonant frequencies is therefore
    about 224 Hz, ( ω diff ).
•   The phase of the fat and water spin vectors will thus coincide at 1/ωdiff, which is
    4.6 ms.
•   If a TE of 4.6 ms is used, then the fat and water components of the signal will
    be in phase. If a TE of 6.9 ms (4.6 + 2.3) is used then the fat and water
    components of the signal will be out of phase.
     Chemical Shift Type II Artefact
         F   3.5 PPM   W
                       ωo                       WF                   W

                                                        F
                                        4.6ms                2.3ms


•   Phase cancellation artefact – gradient echo sequences
•   Water precesses slightly faster than fat (phase difference between them)
•   Phase differences accumulate between water and fat signal
•   Vary the TE, f+W (in phase), f-w (out of phase – black boundary artefact)
•   At 1.5T, f-w occurs in 4.6ms multiples, starting at about 2.3ms (then 6.9,
    11.5, 16.1 ms) - artefact
•   At 1.5T, f+w occurs at 4.6ms (then 9.2, 13.8, 18.4 ms) – no artefact
•   Dixon technique – ip+op images = water image, ip-op = fat image
•   The artefact can occur in both encoding directions
•   Not a problem in SE images since 180o pulse refocuses chemical shift
4. Bo Artefacts
• Susceptibility artefacts
• Metallic artefacts
• Bo Inhomogeneity
           Susceptibility Artefacts
•   Occur when two materials with different magnetic susceptibility (χ)
    lie together, (tissue-air & tissue-fat).
•   Local Bo changes cause spin dephasing at the boundary causing
    signal loss.
•   Haemosiderin (end stage of haemorrhage) deposits (high χ) – local
    susceptibility changes in tissue.
•   Susceptibility artefacts can be useful - bony trabeculae (low χ).
•   Use a FSE and keep TE short to minimise susceptibility artefacts.
                Metallic Artefacts
•   Similar to susceptibility artefacts.
•   Metals have much higher susceptibility than tissue.
•   Large Bo inhomogeneities around object causing signal loss and
    distortion.
•   Implants absorb RF energy, so local field varies.
•   RF problems affect SE sequences as well as GE.
    Metallic Artefact




Small metal flake in lumbar spinal canal
    Bo Inhomogeneity and FatSat




•   Unsuccessful Fat suppression in T2W breast images.
•   Result of poor Bo field homogeneity.
•   Artefacts arise because of inability to distinguish fat and water
    frequencies locally.
•   Usually more prominent in images with a large FOV or off-axis.
•   Solution – improve the magnet shimming.
•   Modern magnets – auto shimming for very reliable fatsat.
5. RF Artefacts
• Ghosting
• RF interference
• Stimulated Echoes
• RF Coil artefacts
• Steady State artefacts
                                       Ghosting
•       Arises from any structure that moves during
        acquisition of data eg: chest wall, pulsatile
        movement of vessels, swallowing etc.)
•       Ghosts displaced along PE axis due to inherent
        time delay between phase encoding and
        readout.
•       Number and intensity depends upon period of
        modulation and the TR.

                  Inhalation
         Exhalation
                                                    P2
          B

              A
                                                    P1




                               Chest wall


    •   Moving anatomy is mismapped into the FOV.
                Quadrature Ghost

•   Occurs due to differences in
    the gain of real and imaginary
    receiver channels
•   Phase errors between the two
    quadrature RF receive
    channels can also cause this
•   Ghost is displaced diagonally
    across the centre in both PE
    and FE directions
•   Solution - ? phase alternating
                      RF Interference
•   Zipper artefact appears as bright and dark zipper lines along PE.
•   External RF picked up by coils (e.g RF breakthrough waveguide filters).
•   Pulse oximeters (monitors the percentage of haemoglobin saturated with
    oxygen) use RF – can be picked up by MR coils.
•   RF from within the MR system may be coherent – bright spot on image.
•   Mains RF – modulated by 50Hz – regularly spaced faint zipper artefacts
    across image.




              RF breakthrough                   Zipper artefact
          Herring-Bone Artefact
•   Occurs due to the presence of a spike of noise (or an ‘arc’ from
    a static discharge) in the raw data.
•   FT (series of spikes) which is convolved with the image data.
•   Probably due to breakdown of RF system (poor RF decoupling).
•   Best solution – rescan the image.
            Halo Artefact
•   Results from signal clipping caused by overflow on the
    ADC’s.
•   Occurs if receiver gain is incorrectly set.
•   Signal becomes too large for the ADC range and
    information in the centre of k-space is lost.
•   Unusual - unless receiver gain is manually set.
         Stimulated Echoes (STE)
    1st 90o pulse    Dephasing    2nd 90o pulse       Hahn Echo
                                        z                 x   Lag

                                            x
                                                                     y
                                                  y
                                                              Lead


    3rd 90o pulse                                       Stim Echo




•   1st pulse forms transverse magnetisation
•   2nd pulse – remaining transverse components form Hahn echo
•   3rd pulse converts longitudinal magnetisation to transverse
    magnetisation, and components re-phase to form stimulated echo
STE – Coherence Pathways
    Phase   90o   90o             90o
                                                H=Hahn Echo
                                                S=Stimulated Echo
                   (Longitudinal)




                              H             STE

SHOULDER
                              •     STE has different spatial encoding
                                    and contrast
                              •     Avoid STE by using ‘spoiler’
                                    gradients to destroy residual
                                    transverse magnetisation, or use
                                    ‘rewinder’ gradients to prevent the
                                    STE occurring in the sampling
                                    window
                              •     Can also widen the bandwidth, or
                                    alter the TE to avoid STE
               RF Coil Artefacts
• One of the arrays of a
  phased array coil is out of
  phase with the other coils.
• Bands of signal addition
  and cancellation.
• Solution – call engineer!




                                Sagittal Pelvis
                  Surface Coil Flare
                                              Axial abdomen

•   The result of signal saturation at
    edge of surface coil.
•   Optimal signal is further in from edge.
•   Solution – Surface Coil Intensity
    Correction (SCIC) – algorithm that
    reduces the high intensity fat signal
    nearest the coil for improved
    visualisation.
•   SCIC is very useful for correcting
    sagittal and axial spine images.
      Steady State Imaging - Artefacts
                             Coronal abdomen




       Moire Fringes




•   Common on True FISP, balanced FFE, FIESTA (fully balanced gradients).
•   Related to variation of steady state condition due to Bo inhomogeneities.
•   Aliasing of one side of the body to the other results in superimposition of signals
    of different phases that alternatively add and cancel.
•   Equivalent to introducing a systematic error to the flip angle.
•   Require a short TE and good shimming – otherwise bands ~ 1/B0
•   Solution – phase alternation of RF pulse
Noise
                      Random Noise
•   Noise can be considered an artefact
    since it is unwanted.
•   Grainy, snowy, no recognisable
    pattern.
•   Solution – improve the SNR
•   Increase slice thickness, increase TR,
    reduce TE, decrease bandwidth,
    decrease pixel resolution, increase the
    FOV, increase phase steps, increase
    the number of averages
•   Remember ‘trade-offs’ (scan time [2D]
    = TR x NY x NEX).
And finally…
               Observer Artefact
• Self explanatory
• Otherwise known as
 “Upside-down Error”
• Solution – apply for time
  off !
                         References
•   MRI from Picture to Proton: Donald W. McRobbie, Elizabeth A.Moore,
    Martin J.Graves and Martin R.Prince Cambs Uni Press

•   All you need to know about MRI Physics: Moriel NessAiver

                          For further information

                    Kris.Armoogum@tuht.scot.nhs.uk
                    Stephen.Gandy@tuht.scot.nhs.uk

								
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