I was also going to say that clear fluids should present no problem, so water and/or gin/vodka shouldn't be too problematic, so long as they're not going to do anything else to you afterwards depending on what they find.
I recently had an ultrasound for abdominal pain - they wouldn't let me even drink water because they thought it might be appendicitis (it was) but I was allowed to suck ice cubes (and was on a saline drip). I don't know how much alcohol you need to take in to prevent the DTs, but you might be able to do an alcohol/water mix ice cube or several, and suck them as you need them if there is a chance that you will need further intervention after the ultrasound - alcohol is absorbed in the mouth too, so that might work to prevent the DTs. (I ALSO don't know how much you'd need to water 40% alcohol to get it TO freeze in the first place!)