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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
T2 Decay MR Signal T1 Recovery MR Signal 1 s 50 ms
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Most Common Static Contrasts
Weighted by the Proton Density Weighted by the Transverse Relaxation Times (T2 and T2*) Weighted by the Longitudinal Relaxation Time (T1)
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The Effect of TR and TE on Proton Density Contrast
MR Signal MR Signal T1 Recovery T2 Decay t (s) t (ms)
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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 256256128 volume ~1 mm resolution
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Proton Density Weighted Image
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Transverse Relaxation Times
Cars on different tracks 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 t=0 180o turn t = TE/2 Fast Spin Fast Spin TE/2 t=TE Slow Spin Slow Spin TE/2 t=0 180o turn t = TE/2 Slow Spin TE/2 Slow Spin t=TE
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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 T1 contrast T2 contrast T2 Decay MR Signal T1 Recovery TR TE
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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 256256128 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
No Flow Medium Flow High Flow No Signal Medium Signal High Vessel Acquisition Saturation Excitation
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Pulse Sequence: Time-of-Flight Contrast
Excitation Image Acquisition RF Gx Gy Gz Saturation Time to allow fresh flow enter the slice
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Phase Contrast (Velocity Encoding)
Externally Applied Spatial Gradient G Spatial Gradient -G Blood Flow v Time T 2T
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Pulse Sequence: Phase Contrast
RF Excitation G Gx Phase Image Acquisition -G Gy Gz
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MR Angiogram
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Diffusion Weighting T 2T Externally Applied Externally Applied
Spatial Gradient G Externally Applied Spatial Gradient -G T 2T Time
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Pulse Sequence: Gradient-Echo Diffusion Weighting
Excitation Image Acquisition RF Gx Gy Gz G -G
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Pulse Sequence: Spin-Echo Diffusion Weighting
RF Excitation G G 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 A B C D
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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
Gx Gy Gz Image 90o 180o Alternating opposite Distal Inversion Odd Scan Even Alternating Proximal Inversion Odd Scan Even Scan 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
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III.2 Some of the fundamental acquisition
methods and their k-space view
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Kx = g/2p 0t Gx(t) dt Ky = g/2p 0t Gx(t) dt 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 readout readout readout 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 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
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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 2T2* [80 ms] Must make gradients sweep back and forth, doing all frequency and phase encoding steps in quick succession Can acquire low resolution 2D images per second
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Pulse Sequence K-space View ...
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Allows highest speed for dynamic contrast
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 t = TE RF Gx Gy Gz t = 0
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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 don’t 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
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