Dear FSL experts, I am playing with some resting-state pASL dataset using oxford_asl. I typed in this in my command line: oxford_asl -i input -s struct -t high2standard.mat -c M0 --tis 0.7,1.8, it gave me this error message" *Error: Insufficient volumes in calibration image (to make a mask), please check M0.nii.gz*". My M0 only includes 1 volume. Does oxford_asl require M0 have certain number of volumes? Also my input data was preprocessed ASL data using feat (high-pass filter, motion correction, and perfusion subtraction turned on). I was wondering how does perfusion subtraction done in feat? Was it simply subtracting control from tag ( or the other way around)? Thanks very much for your help ahead of time.