I think 'usually' they order a CT scan only if you have symptoms, i.e. nausea, headaches, confusion, other mental symptoms, or recurrent ear problems, and that sort of stuff, but not routinely.
(and only if the symptoms cannot be explained by other causes)
I am not sure if you actually had brain mets or not, as what I said above is more true in the case when you have never been diagnosed with such already. if you have been treated for brain mets then I would think they may observe you more closely and do CT scans on a more regular basis. They may also do an MRI but I am more aware of people getting CT scans.
Hope this helps, Katrin