Effects of Diffusion Times on Diffusion-Tensor-Imaging Contrast

Govind Nair, Timothy Q Duong

Center for Comparative NeuroImaging, PsychiatryUniversity of Massachusetts Medical School, Worcester, MA 01655

Introduction

• DTI is widely used to map fiber tracks– Most DTI studies used diffusion time (tdiff) of 30-40 ms

• DTI contrast could be dependent on tdiff

• On the order of cell size, ADC is a function of tdiff

< 5 ms, highly dependent > 20 ms much slower variations

• On the order of larger than cell size, – Horsfield (1994) observed a tdiff dependence (40-800 ms) of

WM ADC in human brain

Objective

We hypothesize that – DTI contrast could be improved at longer tdiff

These hypotheses were tested on – Normal mice and– Myelin-deficient shiverer mice

Methods

• Seven wild type (wt) mice (20-30g)

• Eight Shiverer (shi) mice (20-30g)

• 1% isoflurane

• Spontaneously breathing

• Head stereotaxic device

• Respiratory rate: 100-150 bpm

• Rectal temperature: 37.5 ± 0.25°C

Methods

• 9.4T/89mm vertical bore magnet• 100 G/cm gradient (ID = 45 mm)

• STEAM sequence – TR/TE = 2500/14 ms, variable TM– 64 x 64, 15x15 mm2, 4 averages– b = 10, 1200 s/mm2 – DWI acquired at 6 directions – diffusion time = 30, 80,180, 280 ms– 10 mins per DTI axis per diffusion time

Data analysis

(Basser & Pierpaoli, 1998)

(Le Bihan 2001)

Phantom experiments

• Cross-terms at different tdiff yielded different effective b-values

• In principle, cross terms could be calculated

• Instead we adjusted the diffusion gradients to yield identical effective b values at different tdiff on a phantom

• Results were validated on two phantoms.

Phantom: NAA in DMSO and water(in image mode, identical parameters as in vivo studies)

0.00000

0.00025

0.00050

0.00075

0.00100

0.00125

0.00150

0.00175

(mm

2 /s)

30 80 180 280 30 80 180 280 ms Diffusion time

Average ADC NAA in DMSO across tdiff: 0.51 ± 0.02 x10-3 mm2/s Temp = 22o C

Trace ADC

Average ADC Water across tdiff:1.98 ± 0.02 x 10-3 mm2/sTemp = 20o C

NAA in DMSO

2 4 6 8 10 12 14 16 18 20 22

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Fra

ctio

nal A

niso

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Pixel Number (x 115m)

wt (n = 7) shi (n = 8)**

***

**** P < 0.001** P < 0.01* P < 0.05

corpus callosum hippocampus

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wt shi

* * *** ***

*

corpus callosum hippocampus

30 80 180 280 30 80 180 280 ms Diffusion time

(x

10-3

mm

2/s

)

Variation in with tdiff

0.0

0.5

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2.5 wt Shi

Variation in ADCtrace with tdiff

* ** **

corpus callosum hippocampus

30 80 180 280 30 80 180 280 ms Diffusion time

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DC

trac

e (

x 1

0-3 m

m2/s

)

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ract

ion

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otr

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y

wt shi

** *

30 80 180 280 30 80 180 280 ms Diffusion time

corpus callosum hippocampus

Variation in FA with tdiff

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me

Rat

io wt Shi

** **

corpus callosum hippocampus

Variation in VR with tdiff

30 80 180 280 30 80 180 280 ms Diffusion time

Conclusions

• Scheme to correct for cross terms for different tdiff

• In corpus callosum:– Wt: showed tdiff dependence– Shi: showed lesser tdiff dependence– // in both wt and shi were independent of tdiff

– Anisotropy of wt showed a greater dependence on tdiff than shi

• In hippocampus, FA, VR, , ADCtrace showed small tdiff dependence; there was no difference between wt and shi

Grant supportsWhitaker Foundation, RG-02-0005

American Heart Association, SDG-0430020NIH, NEI R01 EY014211

NIH, NINDS R01 NS45879

Authors wish to thank Dr. Karl Helmer for his technical assistance