Hi Donald - good to hear from you. How's oz?
> Back to the main problem: I'm really surprised about the lack of
> contrast in the DWIs. Given the expected 3-fold loss in average ADC
> (according to a couple of papers), the b=4500 should give an image
> vaguely similar to what you would see in-vivo at b=1500. Have you
> measured the ADC using these images? What's the signal intensity ratio
> between the b=0 and the DWIs?
This is interesting - the b0/b4500 ratio is around 5 in cortical grey
matter; around 2.5 in sub-cortical grey matter; around 1.7 in white matter
parallel, and around 1.4 in white matter perpendicular. Something
different is definitely happening to diffusion in myelinated areas.
>
> And as Chris says, there's also the effect of temperature to consider:
> the couple of papers that I'm aware of both did their imaging at room
> temperature. If the imaging was done at a much lower temperature -
> straight out of the fridge, say - then the ADC is going to be a lot
> lower again.
>
I think it was at room temperature, so I hope this isn't the problem.
Thanks so much everybody - this is really useful.
I think we're going to have to do some more piloting to see if we start
seeing more orientation info at higher b-values (and with a few more
directions). I'll definitely let you know how it goes.
T
> Best of luck!
>
> Donald.
>
>
> On Mon, 2005-10-31 at 10:55 +0000, Sean Deoni wrote:
> > Hi guys,
> >
> >
> > Somewhat related to your question Tim, i.e. would it be possible in
> > the case of myelin break-down? Do you get normal FA values in maturing
> > brain? Really not familiar with the literature with regards to DTI in
> > 0-2 year old infants, but that might be the same situation as you have
> > here, with little or no myelin.
> >
> >
> > SD
> >
> > ----------
> >
> > "They keep creating new ways to celebrate mediocrity." Mr. Incredible
> >
> >
> > Sean Deoni
> >
> > Centre for Neuroimaging Sciences
> >
> > Institute of Psychiatry, King's College London
> >
> >
> > +44 020 7919 3069
> >
> > [log in to unmask]
> >
> >
> >
> >
>
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Tim Behrens
Centre for Functional MRI of the Brain
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