Transcript Contrast Mechanism and Pulse Sequences

Contrast Mechanism and
Pulse Sequences
Allen W. Song
Brain Imaging and Analysis Center
Duke University
III.1 Image Contrasts
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
Signal
MR
Signal
T2 Decay
T1 Recovery
50 ms
1s
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
TE
2.5
2.5
2
2
MR Signal
MR Signal
TR
1.5
T1 Recovery
1
T2 Decay
1
0.5
0.5
0
1.5
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
t (s)
0
10
20
30
40
50
60
70
80
90
100
t (ms)
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
Proton Density Weighted Image
Transverse Relaxation Times
T2*
T2
Cars on different tracks
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
t=0
Fast Spin
Fast Spin
TE/2
t=TE
Slow Spin
Slow Spin
TE/2
t=0
Slow Spin
t=TE
TE/2
Slow Spin
180o turn
t = TE/2
180o turn
t = TE/2
The Effect of TR and TE on
T2* and T2 Contrast
TE
1
1
0.9
0.9
0.8
0.8
0.7
0.7
MR Signal
MR Signal
TR
T1 Recovery
0.6
0.6
0.5
0.5
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
T2 Decay
0.4
0
0.2
0.4
0.6
0.8
1
1.2
T1 Contrast
1.4
1.6
1.8
2
0
0
10
20
30
40
50
60
T2 Contrast
70
80
90
100
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]
T2 Weighted Image
T2* Weighted Image
T2* Images
PD Images
The Effect of TR and TE on
T1 Contrast
TR
TE
1
1
0.9
0.9
0.8
0.8
0.7
T1 Recovery
0.6
MR Signal
MR Signal
0.7
0.5
0.4
0.3
0.2
T2 Decay
0.5
0.4
0.3
0.2
T1 contrast
0.1
0
0.6
T2 contrast
0.1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0
0
10
20
30
40
50
60
70
80
90
100
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
T1 Weighted Image
Inversion Recovery to Boost T1 Contrast
So
S = So * (1 – 2 e –t/T1)
S = So * (1 – 2 e –t/T1’)
-So
IR-Prepped T1 Contrast
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.
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)
Flow Weighting: MR Angiogram
•Time-of-Flight Contrast
•Phase Contrast
Time-of-Flight Contrast
Saturation
Acquisition
Excitation
No Flow
No
Signal
Medium
Flow
Medium
Signal
High
Flow
High
Signal
Vessel
Vessel
Vessel
Pulse Sequence: Time-of-Flight Contrast
90o
Time to allow fresh
flow enter the slice
90o
RF
Excitation
Gx
Saturation
Gy
Gz
Image
Acquisition
Phase Contrast (Velocity Encoding)
T
2T
   G( x  vt)dt   G( x  vt)dt
0
T
 GvT 2
Blood Flow v
Externally Applied
Spatial Gradient -G
Externally Applied
Spatial Gradient G
T
0
2T
Time
Pulse Sequence: Phase Contrast
90o
RF
Excitation
G
Gx
-G
Phase
Image
Gy
Acquisition
Gz
MR Angiogram
Diffusion Weighting
l
S  So e
2Dt
2
 D 2G 2T 3
3
Externally Applied
Spatial Gradient -G
Externally Applied
Spatial Gradient G
0
T
2T
Time
Pulse Sequence: Gradient-Echo Diffusion Weighting
Excitation
RF
90o
G
Gx
G
-
Image
Gy
Gz
Acquisition
Pulse Sequence: Spin-Echo Diffusion Weighting
180o
90o
RF
Excitation G
G
Gx
Image
Gy
Gz
Acquisition
Diffusion Anisotropy
Determination of fMRI Using
the Directionality of Diffusion Tensor
Advantages of DWI
1. 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
Fiber Tractography
DTI and fMRI
D
A
B
C
Perfusion Weighting:
Arterial Spin Labeling
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
Gz
Odd
Scan
Alternating opposite
Distal Inversion
Even
Scan
180o
90o
180o
RF
Gx
Image
Gy
Gz
Alternating
Proximal Inversion
Odd Scan
Even Scan
Advantages of ASL Perfusion Imaging
1. It can non-invasively image and quantify
blood delivery
2. Combined with proper diffusion weighting,
it can assess capillary perfusion
Perfusion Contrast
Diffusion and Perfusion Contrast
Diffusion
Perfusion
III.2 Some of the fundamental acquisition
methods and their k-space view
k-Space Recap
Equations that govern k-space trajectory:
Kx = /2p 0t Gx(t) dt
Ky = /2p 0t Gx(t) dt
S (k x , k y )  

I ( x, y )e
 i 2p ( k x x  k y y )
dxdy
These equations mean that the k-space coordinates
are determined by the area under the gradient waveform
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
MRI Pulse Sequence for Gradient Echo Imaging
Excitation
Slice
Selection
Frequency
Encoding
Phase
Encoding
digitizer on
Readout
K-space view of the gradient echo imaging
Ky
1
2
3
.
.
.
.
.
.
.
n
Kx
Multi-slice acquisition
Total acquisition time =
Number of views * Number of excitations * TR
Is this the best we can do?
Interleaved excitation method
TR
Excitation
……
Slice
Selection
……
Frequency
Encoding
……
Phase
Encoding
readout
Readout
readout
readout
Spin Echo Imaging
Signal is generated by radiofrequency pulse
refocusing mechanism (the use of 180o pulse )
It doesn’t 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
MRI Pulse Sequence for Spin Echo Imaging
90
180
Excitation
Slice
Selection
Frequency
Encoding
Phase
Encoding
digitizer on
Readout
K-space view of the spin echo imaging
Ky
1
2
3
.
.
.
.
.
.
.
n
Kx
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.
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
Pulse Sequence
...
...
K-space View
...
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
Spiral Imaging
RF
t = TE
t=0
Gx
Gy
Gz
K-Space Representation of
Spiral Image Acquisition
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.
Under very homogeneous magnetic field, images look good …
Gradient-Recalled EPI Images Under Homogeneous Field
Gradient Recalled Spiral Images Under Homogeneous Field
However, if we don’t have a homogeneous field …
(That is why shimming is VERY important in fast imaging)
Distorted EPI Images with Imperfect x-Shim
Distorted Spiral Images with Imperfect x-Shim