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This is a reference to whether the subtraction of pairs of labeled (tag) and control images has been done before submitting to oxford_asl. This is something ideally you would determine from your acquisition, but a quick way to check is to look at the data. If the images look like (noisy) perfusion images as they are (even better take the mean over all the volumes) then it has already been differenced. Otherwise it is most likely to be ‘tag-control’ ordering - as that is the most common. You can play with the data using asl_file, or alternatively load it into the (new) GUI and that allows you to preview the data and what happens once you have done the label control subtraction. Michael > On 15 Nov 2017, at 01:22, John anderson <[log in to unmask]> wrote: > > Dear ASL experts, > My question is very basic, I highly appreciate any input!! I would like to learn from your experience to enrich my knowledge! > > The flag --iaf in the command "asl_file" represent the ASL data form, and there are three options: difference data, tag-control and control-tag. > > I highly appreciate if you can define in few words what each form means. If the data is PCASL, then how can I check its form? > > Thanks > John