DIFFUSION MRI

DIFFUSION MRIPI Dr. Kwan-Jin Jung

TA Priti AlbalDave Frank & Maggie Mahan

12 July 2012

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Outline• Phantom Analysis

– Orientation– Direction – b-value – Tensor vs HARDI models

• Human Analysis– Motion correction in DWI– fMRI paradigm & ROI mask creation– Tracking

• Activation patterns• Crossing• Limitations

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During this presentation we will use the words “tracts” and “fibers”

These refer to the trace of the principal diffusion directions

We do not presume to know the physiological constructs these may or may not reflect

Disclaimer

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Phantom Analysis

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PHANTOM• Model for extra-

cellular restricted diffusion

• Made of interdigitating polyester yarns– 1 leg = 25 bundles– 1 bundle = 8800, 10

mm fibers

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DTI, b800, Direction Comparison6 Directions 50 Directions

All tracts

Center ROI

Bottom left leg ROI

Tracts216Mean FA 0.546

Tracts376Mean FA0.445

Tracts 20Mean FA 0.559

Tracts 58Mean FA 0.556

Tracts72Mean FA0.446

Tracts214Mean FA0.453

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DTI, V50, b-value Comparisonb = 800 s/mm2 b = 2000 s/mm2

All Tracts

Center ROI

Bottom left leg ROI

Tracts376Mean FA0.445

Tracts214Mean FA0.453

Tracts72Mean FA0.446

Tracts458Mean FA0.408

Tracts70Mean FA0.414

Tracts82Mean FA0.430

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DTI, V50, b = 2000 s/mm2 HARDI, V50, b = 2000 s/mm2

DTI/HARDI comparison(ROI placed on center crossing)

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Parallel Orthogonal Parallel OrthogonalTransverse Transverse Coronal Coronal

0100200300400500600700800900

1000

WholeCenterSingle

Num

ber o

f fibe

rs re

solv

edPhantom Orientation

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Phantom Susceptibility

Bo

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Human Analysis

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Motion Correction in Diffusion Imaging

1. Registration of intensity2. Rotation of the diffusion vector

Actually scannedStarting designed vector position

Registrationand vector rotation

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Motion Correction Procedure

1. Acquire diffusion-weighted (DW) images

2. Estimate the diffusion tensor

3. Simulate DW images using tensor

4. Motion detection by comparing the acquired

and simulated DW images

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Global Comparison

No Motion Correction Motion Correction

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Cingulate Comparison

No MC MC

Tracts = 762Mean FA = 0.418

Tracts = 1208Mean FA = 0.493

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fMRI Paradigm• Interested in using dorsolateral prefrontal

cortex as a mask in tracking analysis• DLPFC elicited by memory encoding (cue)

2s

8s

ProbeCue

Rest 18s

Retention

2s

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Group Activation for CueROI: Dorsolateral Prefrontal Cortex

R L

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2 Patterns of ActivationParticipant 2Participant 1

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Crossing Fibers

Participant 2Participant 1

(HARDI, 128 Directions, b2000)

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Crossing Problem

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Ventral-Rostral Fiber Redirection

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Due to Rostral U-Fibers?

U-Fibers?U-Fibers

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Take Home Messages

• Diffusion imaging dependent on:• ROI, b-value, # of directions, & orientation

• Diffusion imaging particularly susceptible to subject motion

• Subject comparisons in diffusion imaging not trivial

• Tracking is a useful exploratory technique