Hi Scott,
I am not sure it is that simple. The apparent diffusion coefficient at
some bvalue is NOT a simple function of the ADC at another bvalue.
It will depend on many factors: gradient orientation, degree of
restriction within the tissue, the change in the diffusion time
relative to a change in gradient strength, etc.
The second reason listed above means that any empirical relation you
might observe between ADCs at different bvalues is going to be subject-
dependent.
Cheers,
Saad.
On 10 Mar 2009, at 05:01, Scott Kolbe wrote:
> Dear FSLers
>
> I have some patient data which was half collected at b=1000 and half
> at b=2000 - sequence changed but we didn't notice :(
>
> I scanned two controls with both b-values, registered the
> diffusivity images, then fitted curves to scatterplots for voxel-
> wise measures of MD and L1-3 between the two scans (screen grab
> attached for MD in one of the controls). I would like to correct the
> diffusivity values for the b=2000 group to match the b=1000 group so
> i can study the diffusivities for all patients.
>
> I am interested to know if anyone has done this before? i get very
> good agreement between the curve fits between the two control
> subjects but i'm not sure how many more i should double scan.
>
> also, are there any problems with correcting the diffusivity values
> like this that I have not foreseen? i understand that the SNR is
> different but does this prohibit diffusivity correction?
>
> cheers
> Scott
>
> --
> ========================
> Scott Kolbe
> MS Imaging Group
> Howard Florey Institute &
> Centre for Neuroscience
> University of Melbourne
> VIC, Australia, 3010.
>
> ph: +61 3 8344 1887
> email: [log in to unmask]
> website: www.neuroimaging.org.au/index.php?id=383
>
>
>
>
> --
> ========================
> Scott Kolbe
> MS Imaging Group
> Howard Florey Institute &
> Centre for Neuroscience
> University of Melbourne
> VIC, Australia, 3010.
>
> ph: +61 3 8344 1887
> email: [log in to unmask]
> website: www.neuroimaging.org.au/index.php?id=383
>
> <MD_power_fit.jpg>
Saad Jbabdi
Oxford University FMRIB Centre
JR Hospital, Headington, OX3 9DU, UK
+44 (0) 1865 222545 (fax 717)
www.fmrib.ox.ac.uk/~saad
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