for K = 135 : -1 : 1
petname = sprintf('PET_I1001_PT%03d.dcm', K);
info(K) = dicominfo(petname);
rescale_slopes = [info.RescaleSlope];
If, for some reason you need to find the non-zero entries, then
slice_idx = find(rescale_slopes);
Note: the above code has a limitation that the dicominfo returned by each slice must have exactly the same set of fields. If that assumption is violated then the info(K) assignment will give you an error about assignment between dissimilar structures.