Imaging Sequences part II by oUsDWTj

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									Imaging Sequences
      part II
•   Gradient Echo
•   Spin Echo
•   Fast Spin Echo
•   Inversion Recovery
   Spin Echo Refresher
• 900 RF pulse followed by 1800
  RF pulse
• least artifact prone sequence
• moderately high SAR
                   Spin Echo

gradient
           frequency encode   readout

  RF pulse              RF pulse



signal

                FID            spin
                               echo
                Spin Echo
                pulse timing
                       
    RF


  slice


 phase

readout
                                 echo
 signal

                     TE
  Spin Echo Contrast




T1 weighted    T2 weighted
  Multi Echo Spin Echo
         rationale
• conventional imaging uses a multi-
  slice 2D technique
  – at a given TR time, number of slices
    depends on the TE time
  T2 weighted imaging:
     long TR
     long TE
  PD weighted imaging:
     long TR
     short TE
     Multi Echo Spin Echo
• designed to obtain simultaneously
  multiple echos
• generally used for PD and T2 weighted
  imaging
• no time penalty for first echo
   – inserted before second echo
• can do multiple echos (usually 4) to
  calculate T2 relaxation values
           Multi Echo Spin Echo

gradient

                           
    RF pulse          RF pulses




  signal

                  TE 1
                                     TE 2
          Multi Echo Spin Echo
                pulse timing
                        
    RF


  slice


 phase

readout

 signal

                   echo 1             echo 2
 Spin Echo Contrast




PD weighted    T2 weighted
  Multi Echo Spin Echo

• Summary
 – simultaneously generates PD
   and T2 weighted images
 – no time penalty for acquisition of
   PD weighted image
 – no mis-registration between
   echos
       Fast Spin Echo
• Rationale
  – importance of T2 weighted
    images
    • most clinically useful
    • longest to acquire
    • lowest S/N
  – need for higher spatial resolution
       Fast Spin Echo
    historical perspective
• faster T2 weighted imaging
  – gradient echo (T2*)
  – reduced data acquisition
    • “half-NEX”, “half-Fourier” imaging
    • rectangular FOV
    • S/N or spatial resolution penalty
  – altered flip angle SE imaging
    • “prise”, “thrift”
       Fast Spin Echo
• single most important time
  limiting factor is the acquisition
  of enough data to reconstruct
  an image
• at a given image resolution, the
  number of phase encodings
  determines the imaging time
       Fast Spin Echo
• each phase encoding is
  obtained as a unique echo
  following a single excitation
  with a 90 degree RF pulse
                       Spin Echo
                      pulse timing
                 TR            …..
                                       

                               …..
                        echo                               echo




phase encode n                       phase encode n+1

                      echo n                            echo n+1

                                     TR         …..
              Spin Echo


Spin Echo Imaging Time =

            time-between-90-degrees times

            total-number-of-unique-echos times

            number-of-signal-averages
                    Spin Echo
                    scan time
    • time-between-90-degrees = TR
    • total-number-of-unique-echos = phase encodings
    • number-of-averages = NEX, NSA



                        R
                       T m sec * p ha se enc od ing s* NEX
sc a n time min utes =
                                       60,000
        Fast Spin Echo
        implementation
• collect multiple echos per TR
  – similar to multi-echo SE
  – number of echos per TR referred
    to as the “echo train”
• re-sort the data collection order
  to achieve the desired image
  contrast (effective TE time)
          Multi Echo Spin Echo
                pulse timing
                            
    RF


                            only 1 phase
  slice                     encode
                            per TR
 phase

readout

 signal

                   echo 1                  echo 2
                 Fast Spin Echo
                       pulse timing
                                  
    RF


  slice


 phase

readout
                                             multiple
 signal                                      phase
                                             encodes
                   echo train                per TR
                 Fast Spin Echo
                   scan time
    •   time-between-90-degrees = TR
    •   total-number-of-unique-echos = phase encodings
    •   number-of-averages = NEX, NSA
    •   echo-train-length = ETL



                        R
                       T m sec * p ha se enc od ing s* NEX
sc a n time min utes =
                                    60,000* ETL
        Fast Spin Echo
         advantages
• acquisition time reduced
  proportional to echo train
  length (ETL)
• can trade-off some of the time
  savings to improve images
  – increased NEX
  – increased resolution
        Fast Spin Echo
         advantages
• image contrast similar to SE
• scan parameters
  – TR
  – TE
  – echo train length
        Fast Spin Echo
        disadvantages
• new hardware required
• ear protection may be
  necessary
• higher SAR
  – many 1800 flips closely spaced
• motion sensitive
         Fast Spin Echo
         disadvantages
• reduced number of slices for
  equivalent TR SE scan
• MT effects alter image contrast
• TE time imprecise
• image blurring may occur
• fat remains relatively bright on long
  TR/long TE scans
   • “J-coupling”
            Fast Spin Echo
TE 20       disadvantages
           Want:                      Do:
           TR 3000, TE 80             TR 3000, ET 4
TE 40                                 20 msec IES




TE 60
                                        computer


TE 80   Get:                TE 70ef
        TR 3000, TE 70ef
        Fast Spin Echo
        disadvantages
• each echo “belongs” to a
  different TE image
• combining the echos to form a
  single image creates artifacts
  – worse with shorter effective TE
    times
      Fast Spin Echo
         blurring




SE TE 20         FSE TE 20
          Fast Spin Echo
            limitations
• solutions:
  – use mainly for T2 weighted imaging
  – limit the ET length (~ 8)
  – many phase encodes (192 +)
             Fast Spin Echo
               limitations
• solutions:
  – choose long TE times (> 100 msec)
  – choose long TR times (> 4000 msec)
     • increases fat-fluid contrast
  – for PD imaging,
     –use shorter echo trains (4) and wider
       receive bandwidths (32 kHz)
     –alternatively, use fatsat
        Fast Spin Echo
      interecho spacing
• interecho spacing is the time
  between echos, ~ 16 msec
  minimum on current equipment
• echo trains vary from 2 on up
  on current equipment
• little signal is available with
  long echo train imaging
       Fast Spin Echo
 interecho spacing, example
• 16 ETL, 16 msec IES results in
  echos at the following:
  – 16, 32, 48, 64, 80, 96, 112, 128,
    144, 160, 176, 192, 208, 224,
    240, 256 msec
  – last 5 or 6 echos have so little
    signal that there is little
    contribution to the final image
       Fast Spin Echo
 interecho spacing, example
• time of last echo determines
  the number of slices per TR
• long echo trains greatly reduce
  the number of slices per TR,
  even if the effective TE is short
         Fast Spin Echo
       interecho spacing
• hardware upgrade (echo-
  planar capable) will decrease
  interecho spacing (6-8 msec)
  – better image quality for same
    echo train lengths
  – more slices per TR for identical
    echo train lengths
        Fast Spin Echo
         conclusions
• should be called “faster” spin echo
• produces superior T2 weighted
  images in a shorter time than
  conventional SE
• great innovation
• artifact prone
      Inversion Recovery
• initially used to generate heavily
  T1 weighted images
• popular in U.K. for brain imaging
• 1800 inversion pulse followed by
  a spin echo or fast spin echo
  sequence
    Inversion Recovery
• three image parameters
  – TI
  – TR
  – TE
            Inversion Recovery
                                             TR
                             

                     




            TI

                                  TE

inversion recovery
                           conventional SE or FSE sequence
                        Inversion Recovery

         1

        0.8

        0.6

        0.4

        0.2
                                                               long T1
MZ/M0    0
                                                               short T1




                                          1000


                                                 1250


                                                        1500
                  250


                        500


                                 750
              0




    -0.2

    -0.4

    -0.6

    -0.8

         -1
                              TI (msec)
             Initial 1800 Flip
                 inversion
              z                   z

                                      0
Before                                       After
 ML=M                                         ML=-M
 MXY=0                  0 RF               MXY=0
         y          x        y          x

             t=t0                 t=t0+
             T1 Relaxation
               recovery
             z            z


After
 ML=-M
 MXY=0
         y       x    y          x

             t=t0+        t=TI
                     900 Flip

              z                   z

                                      0
Before                                      After
 ML=Msin()                                  MXY= ML
                         0 RF
         y           x        y         x

              t=t0                t=t0+
                   Second 1800 Flip
       z                                               z
                   dephased
                     z                    z
   y       x                                       y       x

                                                  rephased


               y         x            y       x


900 RF               1800 RF

  t=0                        t=TE/2                t=TE
               STIR
• Short time-to-inversion
  inversion recovery imaging
• “fat nulling”
• exploits the zero crossing
  effect of IR imaging
  – all signal is in XY plane after TI
    time and subsequent 900 pulse
    produces no signal
                        Inversion Recovery

         1

        0.8

        0.6

        0.4

        0.2
                                                               long T1
MZ/M0    0
                                                               short T1




                                          1000


                                                 1250


                                                        1500
                  250


                        500


                                 750
              0




    -0.2

    -0.4

    -0.6

    -0.8

         -1
                              TI (msec)
                  STIR
• optimal inversion time for fat
  nulling dependent on T1
  relaxation time
       Field Strength   TI time
           (Tesla)      (msec)
             0.3          80
            0.5          110
            1.0          130
            1.5          150
              STIR
           advantages
• robust technique
  – works better than fat saturation
    over a large FOV (>30 cms)
  – better at lower field strengths
• high visibility for fluid
  – long T1 bright on STIR
  – long T2 bright on STIR, given
    long enough TE
            STIR
        disadvantages
• poor S/N
  – improved with multiple averages
    • FSE
  – improved with shorter TE times
• incompatible with gadolinium
  – shorter T1 relaxation post-contrast
             STIR
         disadvantages
• red marrow signal can obscure subtle
  edema
  – use TE=48 to knock signal down from
    marrow
• modified IR
  – TE=70-100
  – TI=110 @ 1.5T
  – excellent fluid sensitivity in soft tissues
                   Summary
• Spin echo
   – 90 degree pulse, dephase, 180 degree pulse, rephase-
     echo
• Multi-echo spin echo
   – 90 degree pulse, dephase, 180 degree pulse, rephase-1st-
     echo, 180 degree pulse, rephase-2nd-echo
• Fast spin echo
   – obtain multiple phase encoded echos with a single 90
     degree pulse
   – echo train length determines “turbo” factor
• Inversion recovery
   – 180 degree pulse, inversion time, then SE or FSE
     sequence
   – STIR enables fat suppression over large FOVs or for open
     magnets

								
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