afni.nimh.nih.gov

Where is the b0 image in the 4d DWI volume? If it is the nth volume in the 4D array (we start counting at 0), then what you want is:

3dcalc -a original.nii'[n]' -expr 'a' -prefix your_nth_volume

The example you are suggesting from FSL seems geared for extracting values in a certain range. There is another syntax for that of course. Consider the output of 3dcalc -help, or -h_view, for more details.