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Artifacts in Cardiac MRI

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					Artifacts in Cardiac MRI
 Andreas Sigfridsson, IMV/IMT/CMIV
        <sigge@imv.liu.se>
            2007-04-27
                                          sigge@imv.liu.se


           Dynamic imaging
Images are acquired line-by-line in k-
space
   Motion during acquisition causes artifacts, not
    blurring
   Special care must be taken
      Triggering
      Acquisition ordering
                                              sigge@imv.liu.se


                      Example
Simulation example
   256x256, TR=20 ms, total scan time ~5 s




       Image domain                k-space
                                                                   sigge@imv.liu.se


                    Signal saturation
           Signal recovery - relaxation

                                                                              t
                                                                         
                                                      M z  1 e             T1   t 0




                TR  T1                                 TR  T1
  1                                    1
                               Mz                                                   Mz

0.5                                  0.5



  0                                    0


                                     -0.5
-0.5                                        0   0.2    0.4   0.6   0.8        1   1.2
       0        5         10    15

                                       1
  1
                                                     sigge@imv.liu.se


                 Signal saturation
Progress to steady-state


                 TR  T1             90
        1
                                                Mz

       0.5


        0


      -0.5
             0    0.2   0.4   0.6   0.8   1   1.2


        1
                                                Mx

       0.5


        0
                                                 sigge@imv.liu.se


         Varying signal during inflow
    The MR signal is increased during inflow
    Signal increase related to flow velocity
        More magnetization because of T1
         decay/recovery
                                   Flowing spins, fresh signal




Static spins, saturated signal
                                                           sigge@imv.liu.se


          Varying signal intensity
Simulated six sinusoidal heart beats
0.9


0.8


0.7


0.6


0.5


0.4


0.3


0.2


0.1
      0   50    100    150   200   250   300




               Intensity                       Image (animation)
                                 sigge@imv.liu.se


Varying signal intensity, results
Reconstructed image




    Normal contrast   High contrast
                                            sigge@imv.liu.se


Varying signal intensity, k-space
 Image artifacts because of k-space
 sampling
    Different lines are acquired with different
     signal intensity
                                                          sigge@imv.liu.se


     Varying signal intensity, analysis
     Analyze signal

    Image(t )  Imagestatic  Intensity (t ) * Imagedynamic
       K (k y )  K static(k y )  Intensity (k y ) * K dynamic(k y )
       FT( K )  FT( K static)  FT( Intensity * K dynamic)
       FT( K )  FT( K static)  FT( Intensity)  FT( K dynamic)

Imagereconstructed  Imagestatic  FT( Intensity (t ))  Imagedynamic
                                                                         sigge@imv.liu.se


                     Slow sine variation
0.9                                          140


0.8                                          120

0.7
                                             100

0.6
                                             80
0.5
                                             60
0.4

                                             40
0.3


0.2                                          20


0.1                                           0
      0   50   100   150   200   250   300         0   50    100   150      200   250   300




               Intensity                                    FT(Intensity)
                                 sigge@imv.liu.se


Varying signal intensity, results
Reconstructed image




    Normal contrast   High contrast
                                                                          sigge@imv.liu.se


                     Fast sine variation
0.9                                          140


0.8                                          120

0.7
                                             100

0.6
                                             80
0.5
                                             60
0.4

                                             40
0.3


0.2                                          20


0.1                                           0
      0   50   100   150   200   250   300         0   50     100   150     200   250   300




               Intensity                                    FT(Intensity)
                                                                          sigge@imv.liu.se


                     Fast sine variation
0.9                                          140


0.8                                          120

0.7
                                             100

0.6
                                             80
0.5
                                             60
0.4

                                             40
0.3


0.2                                          20


0.1                                           0
      0   50   100   150   200   250   300         0   50     100   150     200   250   300




0.9                                          140


0.8                                          120

0.7
                                             100

0.6
                                             80
0.5
                                             60
0.4

                                             40
0.3


0.2                                          20


0.1                                           0
      0   50   100   150   200   250   300         0   50     100   150     200   250   300




               Intensity                                    FT(Intensity)
                                                                       sigge@imv.liu.se


                “Physiological” variation
          Aortic flow (handcrafted, not actual data)
 1

0.9

0.8

0.7

0.6

0.5

0.4

0.3
                                                       Intensity (whole scan)
0.2
                                             1


0.1                                         0.9

                                            0.8


 0                                          0.7

      0     5      10      15     20   25   0.6

                                            0.5

                                            0.4



           Intensity (one beat)
                                            0.3

                                            0.2

                                            0.1

                                             0
                                                  0   50   100   150    200     250   300
                                         sigge@imv.liu.se


“Physiological” variation




       Intensity variation (animation)
                                                                                 sigge@imv.liu.se


          “Physiological” variation

 1
                                                                         FT(Intensity)
0.9

0.8

0.7                                      90
0.6

0.5
                                         80
0.4

0.3

0.2                                      70
0.1

 0
      0   50   100     150   200   250   60       300




                                         50
           Intensity
                                         40

                                         30

                                         20

                                         10

                                         0
                                              0         50   100   150     200      250   300
                                       sigge@imv.liu.se


   “Physiological” variation
Reconstructed images




  Normal contrast      High contrast
                                               sigge@imv.liu.se


                   Remedies
Triggering
   Wait for ECG R-tag
   Wait additional trigger delay time
   Acquire one (or several) k-space lines
Acquisition reordering
   Measure current cardiac (or respiratory phase)
   Choose k-space line to acquire
   Creates a slow varying intensity curve along k-space
                                                                           sigge@imv.liu.se


            Triggering - random phase
          If triggering is not accurate, random
          variations will still be left
 1                                               90

0.9                                              80

0.8
                                                 70
0.7
                                                 60
0.6
                                                 50
0.5
                                                 40
0.4
                                                 30
0.3

0.2                                              20

0.1                                              10

 0                                               0
      0     50   100   150     200   250   300        0   50   100   150    200   250   300




                   Intensity                                   FT(Intensity)
                                     sigge@imv.liu.se


Random phase, reconstructed




   Normal contrast   High contrast
                                                                               sigge@imv.liu.se


                Acquisition reordering
 1                                                 90

0.9                                                80

0.8
                                                   70

0.7
                                                   60
0.6
                                                   50
0.5
                                                   40
0.4
                                                   30
0.3
                                                   20
0.2

                                                   10
0.1

 0                                                 0
      0   50   100      150      200   250   300        0   50     100   150     200   250   300




                     Intensity                                   FT(Intensity)
                                                                      sigge@imv.liu.se


               Acquisition reordering
90                                          90

80                                          80

70                                          70

60                                          60

50                                          50

40                                          40

30                                          30

20                                          20

10                                          10

0                                           0
     0   50   100   150   200   250   300        0   50   100   150    200   250   300




                Random                                      Ordered
                                   sigge@imv.liu.se


     Acquisition reordering
Reconstructed images




    Normal contrast    High contrast
                           sigge@imv.liu.se


         Actual examples
ECG triggering failed
                                          sigge@imv.liu.se


           Dynamic imaging
Images are acquired line-by-line in k-
space
   Motion during acquisition causes artifacts, not
    blurring
   Special care must be taken
      Triggering
      Acquisition ordering
   Artifacts might be predictable through use of
    Fourier transform

				
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