One does absolutely have to make sure that when you average your directions
are pointing the same way.
The question is: if you get the maths right and you want to average your
images, is it conceptually correct to rotate bvecs according to
registrations and average them or not (whether or not it makes any practical
difference at all).
Peace,
Matt.
-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
Of Saad Jbabdi
Sent: Friday, July 22, 2011 8:19 AM
To: [log in to unmask]
Subject: Re: [FSL] averaging rotated bvecs
I guess that's what Matt meant when he said "if you get the maths right"
Saad.
On 22 Jul 2011, at 14:03, Pablo Velasco wrote:
> Hi Daniela,
>
> Intuitively, I don't think this is correct: if you average two images
taken with diffusion gradients in exact opposite directions, the average of
the diffusion vectors will be zero. However, the average of the images will
not. I guess if your rotated vectors are pointing almost along the same
direction, the error you will get from averaging directions and images will
be very small.
>
> Why don't you just do the analysis on a dataset with 72 directions?
>
> -Pablo
>
--
Saad Jbabdi
University of Oxford, FMRIB Centre
JR Hospital, Headington, OX3 9DU, UK
(+44)1865-222466 (fax 717)
www.fmrib.ox.ac.uk/~saad
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