Contrast%20Mechanism%20and%20Pulse%20Sequences

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Contrast Mechanism and Pulse Sequences

• Allen W. Song
• Brain Imaging and Analysis Center
• Duke University

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III.1 Image Contrasts
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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

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Static Contrast Imaging Methods
1 s
50 ms
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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)

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The Effect of TR and TE on Proton Density Contrast
TR
TE
MR Signal
MR Signal
T2 Decay
T1 Recovery
t (ms)
t (s)
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Optimal Proton Density Contrast

• Technique use very long time between RF shots
  (large TR) and very short delay between
  excitation and readout window (short TE)
• Useful for anatomical reference scans
• Several minutes to acquire 256?256?128 volume
• 1 mm resolution

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Proton Density Weighted Image
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Transverse Relaxation Times
T2
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Since the Magnetic Field Factor is always
present, how can we isolate it to achieve a
singular T2 Contrast?
Fast Spin
Fast Spin
TE/2
t0
180o turn t TE/2
Fast Spin
Fast Spin
TE/2
tTE
Slow Spin
Slow Spin
TE/2
t0
180o turn t TE/2
Slow Spin
TE/2
Slow Spin
tTE
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The Effect of TR and TE on T2 and T2 Contrast
TR
TE
T1 Recovery
MR Signal
MR Signal
T2 Decay
T1 Contrast
T2 Contrast
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Optimal T2 and T2 Contrast

• Technique use large TR and intermediate TE
• Useful for functional (T2 contrast) and
  anatomical (T2 contrast to enhance fluid
  contrast) studies
• Several minutes for 256 ? 256 ? 128 volumes, or
  second to acquire 64 ? 64 ? 20 volume
• 1mm resolution for anatomical scans or 4 mm
  resolution better is possible with better
  gradient system, and a little longer time per
  volume

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T2 Weighted Image
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T2 Weighted Image
T2 Images
PD Images
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The Effect of TR and TE on T1 Contrast
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Optimal T1 Contrast

• Technique use intermediate timing between RF
  shots (intermediate TR) and very short TE, also
  use large flip angles
• Useful for creating gray/white matter contrast
  for anatomical reference
• Several minutes to acquire 256?256?128 volume
• 1 mm resolution

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T1 Weighted Image
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Inversion Recovery to Boost T1 Contrast
S So (1 2 e t/T1)
So
S So (1 2 e t/T1)
-So
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IR-Prepped T1 Contrast
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In summary, TR controls T1 weighting and TE
controls T2 weighting. Short T2 tissues are dark
on T2 images, but short T1 tissues are bright on
T1 images.
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Motion Contrast Imaging Methods

• Prepare magnetization to make signal sensitive to
  different motion properties
• Flow weighting (bulk movement of blood)
• Diffusion weighting (scalar or tensor)
• Perfusion weighting (blood flow into capillaries)

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Flow Weighting MR Angiogram

• Time-of-Flight Contrast
• Phase Contrast

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Time-of-Flight Contrast
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Pulse Sequence Time-of-Flight Contrast
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Phase Contrast (Velocity Encoding)
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Pulse Sequence Phase Contrast
90o
RF
Excitation
G
Gx
Phase Image Acquisition
-G
Gy
Gz
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MR Angiogram
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Diffusion Weighting
Externally Applied Spatial Gradient -G
Externally Applied Spatial Gradient G
T
2T
0
Time
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Pulse Sequence Gradient-Echo Diffusion Weighting
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Pulse Sequence Spin-Echo Diffusion Weighting
180o
90o
RF
G
G
Excitation
Gx
Image Acquisition
Gy
Gz
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Diffusion Anisotropy
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Determination of fMRI Using the Directionality
of Diffusion Tensor
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Advantages of DWI

• The absolute magnitude of the diffusion
• coefficient can help determine proton pools
  
• with different mobility
• 2. The diffusion direction can indicate fiber
  tracks

ADC
Anisotropy
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Fiber Tractography
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DTI and fMRI
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Perfusion Weighting Arterial Spin Labeling
Imaging Plane
Labeling Coil
Transmission
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Arterial Spin Labeling Can Also Be Achieved
Without Additional Coils
Pulsed Labeling
Imaging Plane
Alternating Inversion
Alternating Inversion
FAIR Flow-sensitive Alternating IR
EPISTAR EPI Signal Targeting with Alternating
Radiofrequency
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Pulse Sequence Perfusion Imaging
EPISTAR
FAIR
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Advantages of ASL Perfusion Imaging

• It can non-invasively image and quantify
• blood delivery
• Combined with proper diffusion weighting,
• it can assess capillary perfusion

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Perfusion Contrast
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Diffusion and Perfusion Contrast
Perfusion
Diffusion
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III.2 Some of the fundamental acquisition
methods and their k-space view
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k-Space Recap
Equations that govern k-space trajectory
Kx g/2p ?0t Gx(t) dt
Ky g/2p ?0t Gx(t) dt
These equations mean that the k-space
coordinates are determined by the area under the
gradient waveform
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Gradient Echo Imaging

• Signal is generated by magnetic field refocusing
  mechanism only (the use of negative and positive
  gradient)
• It reflects the uniformity of the magnetic field
• Signal intensity is governed by
• S So e-TE/T2
• where TE is the echo time (time from
  excitation to
• the center of k-space)
• Can be used to measure T2 value of the tissue

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MRI Pulse Sequence for Gradient Echo Imaging
Excitation
Slice Selection
Frequency Encoding
Phase Encoding
digitizer on
Readout
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K-space view of the gradient echo imaging
Ky
1 2 3 . . . . . . . n
Kx
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Multi-slice acquisition
Total acquisition time Number of views
Number of excitations TR
Is this the best we can do?
Interleaved excitation method
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TR
Excitation

Slice Selection

Frequency Encoding

Phase Encoding
Readout
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Spin Echo Imaging

• Signal is generated by radiofrequency pulse
  refocusing mechanism (the use of 180o pulse )
• It doesnt reflect the uniformity of the magnetic
  field
• Signal intensity is governed by
• S So e-TE/T2
• where TE is the echo time (time from
  excitation to
• the center of k-space)
• Can be used to measure T2 value of the tissue

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MRI Pulse Sequence for Spin Echo Imaging
180
90
Excitation
Slice Selection
Frequency Encoding
Phase Encoding
digitizer on
Readout
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K-space view of the spin echo imaging
Ky
1 2 3 . . . . . . . n
Kx
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Fast Imaging Sequences
How fast is fast imaging? In principle, any
technique that can generate an entire image with
sub-second temporal resolution can be called fast
imaging. For fMRI, we need to have temporal
resolution on the order of a few tens of ms to
be considered fast. Echo-planar imaging,
spiral imaging can be both achieve such speed.
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Echo Planar Imaging (EPI)

• Methods shown earlier take multiple RF shots to
  readout enough data to reconstruct a single image
• Each RF shot gets data with one value of phase
  encoding
• If gradient system (power supplies and gradient
  coil) are good enough, can read out all data
  required for one image after one RF shot
• Total time signal is available is about 2?T2
  80 ms
• Must make gradients sweep back and forth, doing
  all frequency and phase encoding steps in quick
  succession
• Can acquire 10-20 low resolution 2D images per
  second

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Pulse Sequence
K-space View
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Why EPI?

• Allows highest speed for dynamic contrast
• Highly sensitive to the susceptibility-induced
  field
• changes --- important for fMRI
• Efficient and regular k-space coverage and good
• signal-to-noise ratio
• Applicable to most gradient hardware

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Spiral Imaging
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K-Space Representation of Spiral Image
Acquisition
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Why Spiral?

• More efficient k-space trajectory to improve
• throughput.
• Better immunity to flow artifacts (no gradient at
• the center of k-space)
• Allows more room for magnetization preparation,
• such as diffusion weighting.

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Under very homogeneous magnetic field, images
look good
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Gradient-Recalled EPI Images Under Homogeneous
Field
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Gradient Recalled Spiral Images Under Homogeneous
Field
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However, if we dont have a homogeneous field
(That is why shimming is VERY important in fast
imaging)
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Distorted EPI Images with Imperfect x-Shim
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Distorted Spiral Images with Imperfect x-Shim