BET works for other modalities, too. I've used it on FDG successfully,
but you need to adjust the -f parameter. Otherwise, it depends on what
your tracer is, if you don't have cortical uptake it can be difficult
and you may want to create your brain mask using only early (flow)
frames of the study. SPM's apriori masks are also an option.
For CT, a simple threshold will usually do the job.
Hope this helps,