Title: Contrast%20Mechanism%20and%20Pulse%20Sequences
1Contrast Mechanism and Pulse Sequences
- Allen W. Song
- Brain Imaging and Analysis Center
- Duke University
2III.1 Image Contrasts
3The 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
4Static Contrast Imaging Methods
1 s
50 ms
5Most Common Static Contrasts
- Weighted by the Proton Density
- Weighted by the Transverse Relaxation Times (T2
and T2) - Weighted by the Longitudinal Relaxation Time (T1)
6The Effect of TR and TE on Proton Density Contrast
TR
TE
MR Signal
MR Signal
T2 Decay
T1 Recovery
t (ms)
t (s)
7Optimal 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
8Proton Density Weighted Image
9Transverse Relaxation Times
T2
10Since 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
11The Effect of TR and TE on T2 and T2 Contrast
TR
TE
T1 Recovery
MR Signal
MR Signal
T2 Decay
T1 Contrast
T2 Contrast
12Optimal 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
13T2 Weighted Image
14T2 Weighted Image
T2 Images
PD Images
15The Effect of TR and TE on T1 Contrast
16Optimal 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
17T1 Weighted Image
18Inversion Recovery to Boost T1 Contrast
S So (1 2 e t/T1)
So
S So (1 2 e t/T1)
-So
19IR-Prepped T1 Contrast
20In 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.
21Motion 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)
22Flow Weighting MR Angiogram
- Time-of-Flight Contrast
- Phase Contrast
23Time-of-Flight Contrast
24Pulse Sequence Time-of-Flight Contrast
25Phase Contrast (Velocity Encoding)
26Pulse Sequence Phase Contrast
90o
RF
Excitation
G
Gx
Phase Image Acquisition
-G
Gy
Gz
27MR Angiogram
28Diffusion Weighting
Externally Applied Spatial Gradient -G
Externally Applied Spatial Gradient G
T
2T
0
Time
29Pulse Sequence Gradient-Echo Diffusion Weighting
30Pulse Sequence Spin-Echo Diffusion Weighting
180o
90o
RF
G
G
Excitation
Gx
Image Acquisition
Gy
Gz
31Diffusion Anisotropy
32Determination of fMRI Using the Directionality
of Diffusion Tensor
33Advantages 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
34Fiber Tractography
35DTI and fMRI
36Perfusion Weighting Arterial Spin Labeling
Imaging Plane
Labeling Coil
Transmission
37Arterial 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
38Pulse Sequence Perfusion Imaging
EPISTAR
FAIR
39Advantages of ASL Perfusion Imaging
- It can non-invasively image and quantify
- blood delivery
- Combined with proper diffusion weighting,
- it can assess capillary perfusion
40Perfusion Contrast
41Diffusion and Perfusion Contrast
Perfusion
Diffusion
42III.2 Some of the fundamental acquisition
methods and their k-space view
43k-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
44Gradient 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
45MRI Pulse Sequence for Gradient Echo Imaging
Excitation
Slice Selection
Frequency Encoding
Phase Encoding
digitizer on
Readout
46K-space view of the gradient echo imaging
Ky
1 2 3 . . . . . . . n
Kx
47Multi-slice acquisition
Total acquisition time Number of views
Number of excitations TR
Is this the best we can do?
Interleaved excitation method
48TR
Excitation
Slice Selection
Frequency Encoding
Phase Encoding
Readout
49Spin 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
50MRI Pulse Sequence for Spin Echo Imaging
180
90
Excitation
Slice Selection
Frequency Encoding
Phase Encoding
digitizer on
Readout
51K-space view of the spin echo imaging
Ky
1 2 3 . . . . . . . n
Kx
52Fast 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.
53Echo 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
54Pulse Sequence
K-space View
55Why 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
56Spiral Imaging
57K-Space Representation of Spiral Image
Acquisition
58Why 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.
59Under very homogeneous magnetic field, images
look good
60Gradient-Recalled EPI Images Under Homogeneous
Field
61Gradient Recalled Spiral Images Under Homogeneous
Field
62However, if we dont have a homogeneous field
(That is why shimming is VERY important in fast
imaging)
63Distorted EPI Images with Imperfect x-Shim
64Distorted Spiral Images with Imperfect x-Shim