The difference is which subtractions are performed. For a file with volumes 1,2,3,4,5…
--diff would do 1-2, 3-4, 5-6…
--surdiff would do 1-2, 2-3, 3-4, 4-5…
Hence surdiff generates twice as many volumes. If you are then just producing a perfusion image it doesn’t matter which one you use. The surdiff option is largely there for functional studies where there is both rest and task activity in the same acquisition.
Michael
> On 15 Nov 2017, at 13:27, John anderson <[log in to unmask]> wrote:
>
> Dear asl experts,
> What is the difference between the flags --diff and --surrdiff in the command asl_file
>
> In asl_file's man, I see that --surrdiff was defined as: Do surround subtraction on the pairs. Please what is that means?
>
> in my pcasl data, I have 90 volume (i.e. after converting he DICOMs to nii and visualizing the nii file using fslview). When I use the command :
> asl_file --data={raw_data} --ntis=1 --iaf=tc --diff --out=diffdata --mean={output_name}
>
> This output the file "diffdata" which contain 45 volumes. When i use the flag --surrdiff in the command asl_file. the output "diffdata" file contain 90 volumes. I am a little bit confused about these differences so I appreciate any clarification
>
> Thanks!
> John
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