Hi Diana,
Unless you tell 3dcalc what type to impose on the output
dataset, it will assume the type of the first input dataset.
The Mask dataset is probably of type "byte", which is an
unsigned type, with values from 0 to 255.
So you have (at least) two easy choices:
1. Put the ABCD_FIM dataset first on the command line.
2. Explicitly tell it to output a short dataset, probably
using "-datum short" and "-fscale".
I hope this helps,
- rick