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 –T 1 –T 2 –T 2 * 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 –T 1 relaxation –T 2 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 T 1 –?? T 2 or T 2 * –??

FMRI – Week 4 – Contrast Scott Huettel, Duke University Generating T 1 -weighted Images

FMRI – Week 4 – Contrast Scott Huettel, Duke University T 1 -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 T 1 –Intermediate TR, short TE T 2 or T 2 * –??

FMRI – Week 4 – Contrast Scott Huettel, Duke University Inversion Recovery (IR-prep)

FMRI – Week 4 – Contrast Scott Huettel, Duke University IR-Prepped T 1 -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 T2 Cars on the same track Indianapolis = 2.5mi Darlington = 1.4mi T 2 : spin-spin relaxation T 2 * : 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 T 2 -weighted Images

FMRI – Week 4 – Contrast Scott Huettel, Duke University Overview of Imaging Parameters Proton-Density-Weighted –Long TR, short TE T 1 –Intermediate TR, short TE T 2 or T 2 * –Long TR, intermediate TE

FMRI – Week 4 – Contrast Scott Huettel, Duke University PD Images T2* Images T 2 * = Sensitivity to Field Inhomogeneity Susceptibility Artifacts

FMRI – Week 4 – Contrast Scott Huettel, Duke University Types of Contrast Static Contrast –Proton Density –T 1 –T 2 –T 2 * 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 Perfusion Diffusion 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) GxGx 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. GyGy

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 A B 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 T 2 *-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

Gradient Problems

FMRI – Week 4 – Contrast Scott Huettel, Duke University Magnetic Field Inhomogeneity