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Contrast Mechanism and Contrast Mechanism and Pulse SequencesPulse Sequences
Contrast Mechanism and Contrast Mechanism and Pulse SequencesPulse Sequences
Allen W. Song Allen W. Song
Brain Imaging and Analysis CenterBrain Imaging and Analysis Center
Duke UniversityDuke University
III.1 Image ContrastsIII.1 Image Contrasts
The Concept of ContrastContrast = difference in signals emitted by water protons
between different tissuesFor example, gray-white contrast is possible because T1 is
different between these two types of tissue
T2 Decay
MRSignal
T1 Recovery
MRSignal
50 ms50 ms 1 s1 s
Static Contrast Imaging Methods
1.1. Weighted by the Proton DensityWeighted by the Proton Density
2.2. Weighted by the Transverse Relaxation Times (T2 and T2*)Weighted by the Transverse Relaxation Times (T2 and T2*)
3.3. Weighted by the Longitudinal Relaxation Time (T1)Weighted by the Longitudinal Relaxation Time (T1)
Most Common Static Contrasts
The Effect of TR and TE onProton Density Contrast
0 10 20 30 40 50 60 70 80 90 1000
0.5
1
1.5
2
2.5
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
0.5
1
1.5
2
2.5
T2 Decay
MR
Sig
nal
t (ms)t (s)
MR
Sig
nal
TRTR TETE
T1 Recovery
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 256256128 volume ~1 mm resolution
Proton Density Weighted ImageProton Density Weighted Image
T2T2T2*T2*
Cars on different tracksCars on different tracks
Transverse Relaxation Times
180o turnt = TE/2
180o turnt = TE/2
TE/2
TE/2
t=0
t=TE
t=0
t=TE
Fast Spin Fast Spin
Fast Spin Fast Spin
Slow Spin Slow Spin
Slow SpinSlow Spin
Since the Magnetic Field Factor is always present, Since the Magnetic Field Factor is always present, how can we isolate it to achieve a singular T2 Contrast?how can we isolate it to achieve a singular T2 Contrast?
TE/2
TE/2
The Effect of TR and TE onT2* and T2 Contrast
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70 80 90 1000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
T2 Decay
MR
Sig
nal
MR
Sig
nal T1 Recovery
TRTR TETE
T1 ContrastT1 Contrast T2 ContrastT2 Contrast
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 ImageT2 Weighted Image
T2* Weighted ImageT2* Weighted Image
PD ImagesPD Images
T2* ImagesT2* Images
The Effect of TR and TE onT1 Contrast
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70 80 90 1000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
T1 contrast T2 contrast
T2 Decay
MR
Sig
nal
MR
Sig
nal
T1 Recovery
TRTR TETE
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 256256128 volume ~1 mm resolution
T1 Weighted ImageT1 Weighted Image
-S-So
SSo
S = SS = Soo * (1 – 2 e * (1 – 2 e –t/T1–t/T1))
S = SS = Soo * (1 – 2 e * (1 – 2 e –t/T1’–t/T1’))
Inversion Recovery to Boost T1 ContrastInversion Recovery to Boost T1 Contrast
IR-Prepped T1 Contrast
In summary, TR controls T1 weighting and In summary, TR controls T1 weighting and TE controls T2 weighting. Short T2 tissues TE controls T2 weighting. Short T2 tissues are dark on T2 images, but short T1 tissues are dark on T2 images, but short T1 tissues are bright on T1 images.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 ContrastTime-of-Flight Contrast
•Phase ContrastPhase Contrast
Time-of-Flight Contrast
No Flow
Medium Flow
High Flow
No Signal
Medium Signal
High Signal
Vessel
AcquisitionSaturation Excitation
Vessel Vessel
90o
Excitation
ImageAcquisition
RF
Gx
Gy
Gz
90o
Saturation
Time to allow fresh flow enter the slice
Pulse Sequence: Time-of-Flight Contrast
Phase Contrast (Velocity Encoding)
Externally AppliedSpatial Gradient G
Externally AppliedSpatial Gradient -G
Blood Flow v
2
0
2)()(
GvT
dtvtxGdtvtxGT T
T
Time
T2T0
90o
Excitation
Phase
Image
Acquisition
RF
Gx
Gy
Gz
G
-G
Pulse Sequence: Phase Contrast
MR AngiogramMR Angiogram
Diffusion Weighting
Dtl 2
Externally AppliedExternally AppliedSpatial Gradient Spatial Gradient GG
Externally AppliedExternally AppliedSpatial Gradient -Spatial Gradient -GG
TimeTime
TT2T2T00
322
3
2TGD
oeSS
Pulse Sequence: Gradient-Echo Diffusion Weighting
90o
Excitation
Image
Acquisition
RF
Gx
Gy
Gz
G
-G
90o
Excitation
Image
Acquisition
RF
Gx
Gy
Gz
G
180o
G
Pulse Sequence: Spin-Echo Diffusion Weighting
Diffusion Anisotropy
Determination of fMRI Using the Directionality of Diffusion Tensor
Advantages of DWI
1.1. The absolute magnitude of the diffusion The absolute magnitude of the diffusion coefficient can help determine proton pools coefficient can help determine proton pools with different mobilitywith different mobility
2. The diffusion direction can indicate fiber tracks2. The diffusion direction can indicate fiber tracks
ADCADC AnisotropyAnisotropy
Fiber Tractography
DTI and fMRI
AB
C
D
Perfusion Weighting: Arterial Spin Labeling
TransmissionTransmission
Imaging PlaneImaging Plane
Labeling CoilLabeling Coil
AlternatingAlternatingInversionInversion
Pulsed LabelingPulsed Labeling
AlternatingAlternatingInversionInversion
Imaging PlaneImaging Plane
FAIRFAIRFlow-sensitive Alternating IRFlow-sensitive Alternating IR
EPISTAREPISTAREPI Signal Targeting with Alternating RadiofrequencyEPI Signal Targeting with Alternating Radiofrequency
Arterial Spin Labeling Can Also Be Achieved Without Additional Coils
RF
Gx
Gy
Gz
Image
90o 180o
Alternating oppositeDistal Inversion
OddScan
EvenScan
180o
RF
Gx
Gy
Gz
Image
90o180o180o
AlternatingProximal Inversion Odd Scan
Even Scan
Pulse Sequence: Perfusion Imaging
FA
IRF
AIR
EP
IST
AR
EP
IST
AR
Advantages of ASL Perfusion Imaging
1.1. It can non-invasively image and quantifyIt can non-invasively image and quantify blood deliveryblood delivery2.2. Combined with proper diffusion weighting, Combined with proper diffusion weighting, it can assess capillary perfusionit can assess capillary perfusion
Perfusion ContrastPerfusion Contrast
PerfusionPerfusionDiffusionDiffusion
Diffusion and Perfusion ContrastDiffusion and Perfusion Contrast
III.2 III.2 Some of the fundamental acquisition Some of the fundamental acquisition methods and their k-space viewmethods and their k-space view
k-Space Recap
Kx = Kx = /2/200ttGx(t) dtGx(t) dt
Ky = Ky = /2/200ttGx(t) dtGx(t) dt
Equations that govern k-space trajectory:Equations that govern k-space trajectory:
These equations mean that the k-space coordinatesThese equations mean that the k-space coordinatesare determined by the area under the gradient waveformare determined by the area under the gradient waveform
dxdyeyxIkkS ykxkiyx
yx )(2),(),(
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 fieldSignal 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
digitizer ondigitizer on
ExcitationExcitation
SliceSliceSelectioSelectionnFrequencyFrequency EncodingEncoding
PhasePhase EncodingEncoding
ReadoutReadout
K-space view of the gradient echo imaging
Kx
Ky
123.......n
Multi-slice acquisition
Total acquisition time =Total acquisition time = Number of views * Number of excitations * TRNumber of views * Number of excitations * TR
Is this the best we can do?Is this the best we can do?
Interleaved excitation methodInterleaved excitation method
readoutreadout
ExcitationExcitation
SliceSliceSelectioSelectionnFrequencyFrequency EncodingEncoding
PhasePhase EncodingEncoding
ReadoutReadout
readoutreadout readoutreadout
…………
…………
…………
TRTR
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 fieldSignal 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
digitizer ondigitizer on
ExcitationExcitation
SliceSliceSelectioSelectionnFrequencyFrequency EncodingEncoding
PhasePhase EncodingEncoding
ReadoutReadout
9090 180180
K-space view of the spin echo imaging
Kx
Ky
123.......n
Fast Imaging Sequences
How fast is “fast imaging”?How fast is “fast imaging”?
In principle, any technique that can generate an entire image In principle, any technique that can generate an entire image with sub-second temporal resolution can be called fast imaging.with sub-second temporal resolution can be called fast imaging.
For fMRI, we need to have temporal resolution on the order of For fMRI, we need to have temporal resolution on the order of a few tens of a few tens of msms to be considered “fast”. Echo-planar imaging, to be considered “fast”. Echo-planar imaging, spiral imaging can be both achieve such speed.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 2T2* [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 ImagingSpiral Imaging
t = TERFRF
GxGx
GyGy
GzGz
t = 0
K-Space Representation of Spiral Image Acquisition
Why Spiral?
• More efficient More efficient kk-space trajectory to improve-space trajectory to improve throughput.throughput.• Better immunity to flow artifacts (no gradient atBetter immunity to flow artifacts (no gradient at the center of k-space)the center of k-space)• Allows more room for magnetization preparation,Allows more room for magnetization preparation, such as diffusion weighting.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