DIFFUSION MRI
PI Dr. Kwan-Jin Jung
TA Priti Albal
Dave Frank & Maggie Mahan
12 July 2012
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
2
Disclaimer
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
3
Phantom Analysis
4
PHANTOM
• Model for extracellular restricted
diffusion
• Made of
interdigitating
polyester yarns
– 1 leg = 25 bundles
– 1 bundle = 8800, 10
mm fibers
5
DTI, b800, Direction Comparison
6 Directions
All tracts
Center ROI
Bottom
left leg ROI
50 Directions
Tracts
216
Mean FA
0.546
Tracts
58
Mean FA
0.556
Tracts
20
Mean FA
0.559
Tracts
376
Mean FA
0.445
Tracts
214
Mean FA
0.453
Tracts
72
Mean FA
0.446 6
DTI, V50, b-value Comparison
b = 800 s/mm2
All Tracts
Center ROI
Bottom
left leg ROI
Tracts
376
Mean FA
0.445
b = 2000 s/mm2
Tracts
458
Mean FA
0.408
Tracts
214
Mean FA
0.453
Tracts
70
Mean FA
0.414
Tracts
72
Mean FA
0.446
Tracts
82
Mean FA
0.430
7
DTI/HARDI comparison
(ROI placed on center crossing)
DTI, V50, b = 2000 s/mm2
HARDI, V50, b = 2000 s/mm2
8
Number of fibers resolved
Phantom Orientation
1000
900
800
700
600
500
400
300
200
100
0
Whole
Center
Single
Parallel
Orthogonal
Parallel
Orthogonal
Transverse
Transverse
Coronal
Coronal
9
Phantom Susceptibility
Bo
10
Human Analysis
11
Motion Correction in Diffusion Imaging
1. Registration of intensity
2. Rotation of the diffusion vector
Starting designed
vector position
Actually scanned
Registration
and vector rotation
12
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
13
Global Comparison
No Motion Correction
Motion Correction
14
Cingulate Comparison
No MC
Tracts = 762
Mean FA = 0.418
MC
Tracts = 1208
Mean FA = 0.493
15
fMRI Paradigm
• Interested in using dorsolateral prefrontal
cortex as a mask in tracking analysis
• DLPFC elicited by memory encoding (cue)
Rest 18s
Retention
8s
Cue 2s
Probe 2s
16
Group Activation for Cue
ROI: Dorsolateral Prefrontal Cortex
R
L
17
2 Patterns of Activation
Participant 1
Participant 2
18
Crossing Fibers
(HARDI, 128 Directions, b2000)
Participant 1
Participant 2
19
Crossing Problem
20
Ventral-Rostral Fiber Redirection
21
Due to Rostral U-Fibers?
U-Fibers
U-Fibers?
22
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
23