Contrast Mechanisms

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					             Contrast
             Mechanisms
An Introduction to MRI Physics and Analysis

                    Michael Jay Schillaci, PhD
                      Monday, February 18th, 2008
Contrast Mechanisms
   Static Contrasts
       Sensitive to type, number and relaxation of spins
       E.g., T1, T2
   Motion Contrasts 
       Sensitive to movement of spins in space
       E.g., Dephasing, Diffusion, Perfusion
   Endogenous Contrast 
       Depends upon intrinsic properties of tissue
       E.g., BOLD fMRI
   Exogenous Contrast
       Uses injection of to track changes
       E.g., Nuclear Medicine (NMR)
Static Contrast
The Concept of Contrast
 Contrast = difference in signals emitted by water
   protons between different tissues
 For example, gray-white contrast is possible
   because T1 is different between these two types
   of tissue
  Static Contrast Imaging Methods

MR                       MR
Signal                   Signal


           T2 Decay
           transverse
                                  T1 Recovery
                                  longitudinal




         50 ms          time      1s             time
Most Common Static Contrasts


1. Weighted by the Proton Density

2. Weighted by the Transverse
   Relaxation Times (T2 and T2*)

3. Weighted by the Longitudinal
   Relaxation Time (T1)
                            The Effect of TR and TE on
                             Proton Density Contrast
                                                                     TR                        TE
            2.5                                                                               2.5




             2                                                                                 2
MR Signal




                                                                                  MR Signal
            1.5                                                                               1.5


                                        T1                                                                              T2 Decay
             1
                                        Recovery                                               1




            0.5                                                                               0.5




             0                                                                                 0
                  0   0.2   0.4   0.6   0.8   1   1.2   1.4   1.6   1.8     2                       0   10   20   30   40   50   60   70   80   90   100
                                                                          t (s)                                                                 t (ms)
Inversion Recovery to Boost T1
Contrast
               S = So * (1 – 2 e –t/T1)
     So

                      S = So * (1 – 2 e –t/T1’)




     -So
IR-Prepped T1 Contrast
T1 and T2 Values
   Equilibrium magnetization
       Depends on field
       Depends on H20 content

                                      N z 2
                                 M0         B0  pH 2O B0
                                      V k BT

                    T1 and T2 Values for Various Tissues and Fields1
        Material      % H2O2,3             T1 ( ms )4                               T2 ( ms )4

                                   B0 = 0.5 T      B0 = 1.5 T            B0 = 0.5 T           B0 = 1.5 T

     White matter       84.3             500                     600                74                     80
     Grey Matter        70.6             650                     900                87                    100
     CSF                99.0            1800                 4000                  600                  2000


                                                        1Table Adapted from: http://members.lycos.nl/mri/Nieuw/T1eng.htm
                                          2White/GreyMatter: http://www.fmrib.ox.ac.uk/~stuart/lectures/lecture4/sld004.htm
                                        3CSF Value: http://www.ivis.org/special_books/Braund/tipold/chapter_frm.asp?LA=1
                                         4Values From: Huettel Chapter 5 and http://members.lycos.nl/mri/Nieuw/T1eng.htm
Image Formation - General
   Integrate magnetization to get MRI signal
       Select Z “slice” and form image of XY plane variations
                                                    t
                                              i    xGX   yGY  dt
           S (t )     M x, y, t e
                      Area
                              XY
                                                    0
                                                                               dxdy

       Contrast comes from difference in magnetization values
       Measurement at different times gives different contrast




                             MRI Picture Adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm
Static Contrast – T1 and T2
                                                     T1 Contrast Weighting
   T1 Contrast
                                               TR                  TE
     Echoat T2 min
     Repeat at T1 max


   T2 Contrast
                                          Max T1 Contrast            Min T2 Contrast
     Echoat T2 max
                                                    T2 Contrast Weighting
     Repeat at T1 min
                                                             TR      TE



   Magnetization is given
    by
                       
                          TR
                               T 2 
                                  TE
     M XY    M 0 1  e T 1  e
                            
                                      
                                          Min T1 Contrast           Max T2 Contrast
                  
                
                             
                   re covery    decay
Static Contrast Images
   Examples from the Siemens 3T
     T1   and T2 Weighted Images
       T1 Weighted Image (T1WI)           T2 Weighted Image (T2WI)
           (Gray Matter – White Matter)     (Gray Matter – CSF Contrast)
Flip Angle
   RF Pulse Determines Flip Angle
        Duration and magnitude are important
        Rotation determines amount of magnetization measured
            +z

                       M
            B0

                 
                            MZ             
                                  +y
            BC       MXY
       +x
    M Z  M cos     M XY  M sin  
                                           Adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm
Field Strength
   Increased field strength
       Net magnetization in material is greater
       Increased contrast means signal is increased
       Image1 resolution is better

          Muscle



                           Tissue




                                      1MRI   adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm
Static Contrast - T2* Relaxation
   T2* accounts for magnetic
    defects and effects
         1    1    1     1
                    
        T 2* T 2 T 2 M T 2 MS

     T2 is relaxation due to spin-spin
      interaction of nuclei
     T2M is relaxation induced by      M    B0
      inhomogeneities of main
      magnet
     T2MS is relaxation induced by
      magnetic susceptibility of
                                        M    m
      material
Motion Contrast
Motion Contrast - Dephasing
   Dephasing of H2O and Fat
       MRI signal is a composite of Fat and H2O signals
       H2O and Fat resonate at different frequencies
            T1F = 210 ms, T1W = 2000 ms ( b/c T1F > T1W → fat is brighter … )
       Relative phase gives TE dependence


                                       MF


                                     ΦFW
                                              MW



     Parallel ( ΦFW = 0o )                         Anti-Parallel (ΦFW = 180o )
        @ TE = 13.42 ms                                @ TE = 15.66 ms
Motion Contrast Imaging Methods

Prepare magnetization to make signal
  sensitive to different motion properties

   Flow  weighting (bulk movement of blood)
   Diffusion weighting (water - random motion)
   Perfusion weighting (blood flow into capillaries)
Flow Weighting: MR Angiogram

        • Time-of-Flight Contrast

        • Phase Contrast
Time-of-Flight Contrast

       Saturation   Excitation             Acquisition
                                 No Flow
                                                      No
                                                      Signal
                                 Medium
                                  Flow                Mediu
                                                      m
                                  High                Signal
                                  Flow

                                                      High
                                                      Signal
         Vessel      Vessel                  Vessel
Pulse Sequence: Time-of-Flight Contrast

                          Time to allow fresh
                          flow enter the slice
                  90o                            90o
        RF
             Excitation

        Gx

                              Saturation                 Image
                                                       Acquisition
        Gy


        Gz
Phase Contrast (Velocity Encoding)
                T                2T
              G( x  vt)dt   G( x  vt)dt
                0                T

            GvT 2
                      Blood Flow v




     Externally Applied               Externally Applied
     Spatial Gradient G               Spatial Gradient -G
                            T
           0                                       2T
                                                        Time
Pulse Sequence: Phase Contrast
              90o
   RF
        Excitation   G

   Gx
                                Phase
                         -G
                                Image
   Gy
                              Acquisition


   Gz
MR Angiogram
Random Motion: Water Diffusion
    Diffusion Weighting

                                                      l  2 Dt


           2
           D 2G 2T 3
S  Soe    3



                  Externally Applied       Externally Applied
                  Spatial Gradient G       Spatial Gradient -G
                                       T
                          0                           2T
                                                           Time
Pulse Sequence: Gradient-Echo
      Diffusion Weighting
         Excitation
    RF        90o

                      G
                          G
                          -
    Gx

                                Image
    Gy
                              Acquisition


    Gz


                Large Lobes
Pulse Sequence: Spin-Echo
    Diffusion Weighting
                    180o
          90o
RF
     Excitation G          G

Gx
                                 Image

Gy                             Acquisition



Gz
Diffusion Anisotropy
Determination of fMRI Using
the Directionality of Diffusion
            Tensor
  Advantages of DWI
1. The absolute magnitude of the diffusion
   coefficient (ADC) can help determine proton pools
   with different mobility
2. The diffusion direction can indicate fiber tracks


  ADC                 Anisotropy
Fiber Tractography
DTI and fMRI

               D
                   A
         B


               C
                    Perfusion

The injection of fluid into a blood vessel in order
to reach an organ or tissue, usually to supply
nutrients and oxygen.

In practice, we often mean capillary perfusion as
most delivery/exchanges happen in the capillary
beds.
    Perfusion Weighting:
Arterial Spin Labeling (ASL)
                             Imaging Plane




                       Labeling Coil
              Transmission
          Arterial Spin Labeling Can Also
        Be Achieved Without Additional Coils
                                         Pulsed Labeling
                                                                                Imaging Plane




Alternating
Inversion
                                              Alternating
                                              Inversion




                        FAIR                                    EPISTAR
              Flow-sensitive Alternating IR   EPI Signal Targeting with Alternating Radiofrequency
Pulse Sequence: Perfusion Imaging
                 180o                       90o   180o
  RF

  Gx
                                                         Image
  Gy
       Odd
       Scan          Alternating opposite
                       Distal Inversion
  Gz
                        Even
                        Scan
                 180                              180
                 o                          90o   o

  RF

  Gx
                                                         Image
  Gy
          Alternating
       Proximal Inversion    Odd Scan
                             Even Scan
  Gz
Advantages of ASL Perfusion Imaging

   1. It is non-invasive
   2. Combined with proper diffusion weighting
      to eliminate flow signal first, it can be used
      to assess capillary perfusion
Perfusion Contrast
       Perfusion Map
Diffusion         Perfusion
Summary of Time Characteristics
   Spin-Lattice Relaxation (T1)
       nuclei realign with the magnetic field
   Spin-Spin Relaxation (T2)
       nuclei quickly become incoherent
   Magnetic Effects and Defects (T2*)
       gradients increase/decrease coherence
   Echo Time (TE)
       when DAQ occurs
   Repeat Time (TR)
       time between RF pulses




                Image adapted from: http://www.med.nagasaki-u.ac.jp/radiolgy/MRI%20of%20the%20FOOT/MRI-CDNUH/Fig9.html
Endogenous
Contrast
Hemoglobin and Magnetism
   The Hemoglobin (Hb) Molecule
     An organic molecule containing four
    heme groups (with iron in each) and
    globular protein (globin).
   Oxygen Characteristics
     Oxygen bound - oxyhemoglobin (Hb)
     No oxygen bound - deoxyhemoglobin (dHb)
   Magnetic Properties
     Hbis diamagnetic - no dipole
     dHb is paramagnetic - slight dipole
Oxygen and Field Strength
 Apply magnetic field to brain
 Blood oxygen level differs




   dHb is paramagnetic
     field   increased
   Hb diamagnetic
     Field   decreased
Endogenous Contrast - fMRI
   Depends on internal biological compound
     Blood   deoxygenation affects T2 Recovery




                       Decreasing Relaxation Time
                                                                         T2



                                                       T1




                                                    Increasing Blood Oxygenation Level
BOLD - Endogenous Contrast
   Blood Oxygenation Level Dependent Contrast
     dHb is paramagnetic, Hb is less
     Susceptibility of blood increases linearly with oxygenation
     BOLD subject to T2* criteria


   Oxygen is extracted from capillaries
     Arteriesare fully oxygenated
     Venous blood has increased proportion of dHb
     Difference between Hb and dHb states is greater for veins
     Therefore BOLD is result of venous blood changes
BOLD - T2* Contrast


MR                   MR
Signa                Signal
l
          T2 Decay


                              T1 Recovery



        50 ms                 1s

				
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