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I would say that anything below 25 directions will *NOT* be able to come up with a sensible probabilistic solution since your bvecs will greatly impact the probabilities of fiber orientation. Thus, I think you will never be able to use 6-dir data for probabilistic tractography. However, Tim may have a stronger and more informed opinion about that, and at least dtifit should be ok to use.
Best regards-
Andreas
 

	-----Urspr√ľngliche Nachricht----- 
	Von: Russ Poldrack [mailto:[log in to unmask]] 
	Gesendet: Mi 08.12.2004 01:30 
	An: [log in to unmask] 
	Cc: 
	Betreff: Re: [FSL] dealing with multiple DTI runs
	
	

	Thanks Andreas.  Regarding the using a 6-direction data with bedpost,
	is this an absolute statement (i.e, "One will never be able to use
	6-direction data to do probabilistic tracking") or a relative statement
	("One needs to be careful with the output of bedpost using 6-direction
	data")? If the latter, can you suggest some metrics for robustness?
	
	cheers
	russ
	
	On Dec 7, 2004, at 4:00 PM, Automatic digest processor wrote:
	
	>
	> hi russ-
	>
	> 6 directions is generally too few for a robust probabilistic solution.
	> w/r to 1, i would tend to average the coregistered /
	> eddy-current-corrected runs and perform further computations on the
	> average. (simply because it would be less time consuming.)
	> w/r to 2, yes - flirt can register to the b0-image (corrratio puts up
	> well with the liquor being bright on b0 but dark on the other images),
	> i.e you can simply put it first to use it as the default reference
	> volume.
	>
	> best regards-
	> andreas
	>
	---
	Russell A. Poldrack, Ph.D.
	Assistant Professor of Psychology, UCLA
	Franz  Hall, Box 951563
	Los Angeles, CA 90095-1563
	email: [log in to unmask]
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