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Hi,
I wouldn't expect any bias in the orientation estimates if you fit more fibre populations than necessary, thanks to the A.R.D prior used in bedpostx.
Since bedpostx handles model complexity for you, it is always worth trying to fit the more complex crossing fibre model, I think.

On the other hand, I have been playing with a 12 directions data set where bedpostx could fit nice crossing fibres in several white matter regions. 

Saad.


On 2 Jun 2008, at 12:42, Juergen Haenggi wrote:

Dear FSL experts

In bedpostx we can select the number of fibres modelled per voxel. Because
our DTI data were acquired with only 15 different diffusion directions. This
is in contrast to the 60 directions used in the Behrens et al., NeuroImage,
34, 144-155, 2007 paper, where two fibres per voxel were modelled.

Should we modelled only one fibre per voxel in bedpostx because there are
only 15 different diffusion directions? Does results could be biased when
modelling tot many (e.g. three) fibres per voxel?

Thanks in advance
Best regards
Juergen

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Juergen Haenggi
Ph.D. (Dr. des.)
Division of Neuropsychology
Institute of Psychology
University of Zurich
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http://www.psychologie.uzh.ch/neuropsy/ (website)
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Saad Jbabdi, 
Postdoctoral Research Assistant,  
Oxford University FMRIB Centre

FMRIB, JR Hospital, Headington, Oxford  OX3 9DU, UK
+44 (0) 1865 222545  (fax 222717)
[log in to unmask]    http://www.fmrib.ox.ac.uk/~saad
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