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FMRI – Week 4 – Contrast Scott Huettel, Duke University
MR Contrast
FMRI Graduate Course (NBIO 381, PSY 362)
Dr. Scott Huettel, Course Director
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Review: Image Formation
• Every image can be constructed from spatial frequency information– i.e., sinusoidal gratings of particular
frequency, orientation, and phase
• Images vary in the contributions from different spatial frequencies
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Spatial Image = Combination of Spatial
Frequencies
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Review: Image Formation
• Every image can be constructed from spatial frequency information– i.e., sinusoidal gratings of particular
frequency, orientation, and phase• Images vary in the contributions from
different spatial frequencies• By using magnetic gradients, we
change the relative precession phase of protons across space, according to spatial freq.
FMRI – Week 4 – Contrast Scott Huettel, Duke University
k space = spatial frequency
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Contrast
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Types of Contrast
• Static Contrast– Proton Density – T1
– T2
– T2*
• Motion Contrast– Flow– Perfusion– Diffusion
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Defining “Contrast”
• The intensity difference between different quantities being measured by an imaging system.
• The physical quantity measured in an image.
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Quick Self-Assessment
• Define the following– TR– TE
– T1 relaxation
– T2 relaxation
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Creating Proton-Density-Weighted Images
Proton density contrast measures, quite simply, how many protons are present in a
voxel.
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Proton-Density-Weighted Imaging
Gradient-Echo Imaging (GRE)
Flip Angle
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Overview of Imaging Parameters
• Proton-Density-Weighted– Long TR, short TE
• T1
– ??
• T2 or T2*– ??
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Generating T1-weighted Images
FMRI – Week 4 – Contrast Scott Huettel, Duke University
T1-weighted Imaging
Spin-Echo Imaging (SE)
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Overview of Imaging Parameters
• Proton-Density-Weighted– Long TR, short TE
• T1
– Intermediate TR, short TE
• T2 or T2*– ??
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Inversion Recovery (IR-prep)
FMRI – Week 4 – Contrast Scott Huettel, Duke University
IR-Prepped T1-Weighted Images
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Generating T2- (and T2*-)weighted Images
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Thought problem: What can cause nearby protons to lose phase
coherence?(They must precess at different rates… what could cause
that to happen?)
Indianapolis = 2.5mi
Darlington = 1.4mi
FMRI – Week 4 – Contrast Scott Huettel, Duke University
T2Cars on the same track
Indianapolis = 2.5mi
Darlington = 1.4mi
T2 : spin-spin relaxation T2* : spin-spin interactions + local field
inhomogeneities
FMRI – Week 4 – Contrast Scott Huettel, Duke University
How can we compensate for the fact that local field inhomogeneities cause
some spins to precess faster than others?
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Phase Compensation via Spin-Echo
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Creating T2-weighted Images
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Overview of Imaging Parameters
• Proton-Density-Weighted– Long TR, short TE
• T1
– Intermediate TR, short TE
• T2 or T2*– Long TR, intermediate TE
FMRI – Week 4 – Contrast Scott Huettel, Duke University
PD ImagesPD Images
T2* ImagesT2* Images
T2* = Sensitivity to Field Inhomogeneity
Susceptibility Artifacts
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Types of Contrast
• Static Contrast– Proton Density – T1
– T2
– T2*
• Motion Contrast– Flow (e.g., Time of Flight)– Perfusion– Diffusion
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Time of Flight (ToF) Flow Imaging
Angiogram
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Perfusion Imaging
Perfusion: The irrigation of tissue through blood delivery (typically through capillaries).
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Perfusion Contrast
FMRI – Week 4 – Contrast Scott Huettel, Duke University
PerfusionPerfusionDiffusionDiffusion
Reduced Perfusion following Stroke
By adding diffusion weighting (to eliminate the effects of flow), perfusion imaging can be made
specific to capillaries.
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Diffusion (in homogeneous medium)
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Core Approach of Diffusion Tensor Imaging (DTI)
Gx
Apply alternating, opposite gradients along one direction.
Measure signal.
Apply alternating, opposite gradients along a different
direction. Measure signal.
Repeat for a total of 6+ directions. The amplitude of the
signal across these directions constitutes the diffusion tensor.
Gy
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Pulse Sequences for DTI
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Diffusion Tensors
FMRI – Week 4 – Contrast Scott Huettel, Duke University
ADC FA
Measures Provided by Diffusion Imaging
Tracts
Apparent Diffusion
Coefficient (ADC): Information
about the relative mobility of protons.
Fractional Anisotropy (FA):
Information about the
constraints on proton mobility.
Tractography: Information about the
directionality of proton mobility
across the brain.
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Isotropic and Anisotropic Diffusion
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Reduced Anisotropic Diffusion in Older Adults
OlderYounger
Data from Madden et al. (2007)
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Madden et al. (in press)
Older (60-85y) and younger (18-27y) adults
made categorical judgments.
We modeled information accumulation (i.e., drift rate) using a model for each subject’s response
time.
Age-related variance in information accumulation was reduced dramatically, when integrity of two fiber tracts was included in the
model.
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Fiber Tracking (Tractography)
FMRI – Week 4 – Contrast Scott Huettel, Duke University
AB
C
D
Integrating DTI and fMRI
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Common Fast Imaging Sequences
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Echo-Planar Imaging (EPI)
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Gradient Artifacts in EPI Images
None
X
Y
Z
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Spiral Imaging
Shown is a spiral-out sequence.
We could also use spiral-in or spiral-in-and-out
sequences.
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Interpolation of Spiral Images
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Gradient Artifacts in Spiral Images
None
X
Y
Z
FMRI – Week 4 – Contrast Scott Huettel, Duke University
EPI vs. Spiral Imaging
• Both can acquire 64*64 resolution T2*-weighted images within about 40ms (~20slices/s) on BIAC scanners
• EPI Properties– Generally fast, simple, efficient– Covers k-space in Cartesian coordinates– Subject to flow effects (gradient at k=0)
• Spiral Properties– Very fast, uses both gradients simultaneously– Shorter time between images (spiral-in)– Easier on gradients– Requires interpolation in k-space– Minimizes flow effects (no gradient at k=0)– Can easily add preparatory gradients (spiral-out)
FMRI – Week 4 – Contrast Scott Huettel, Duke University
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Gradient Problems
FMRI – Week 4 – Contrast Scott Huettel, Duke University
Magnetic Field Inhomogeneity
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