I had my gallbladder removed in 2010 and once I got to the ER cuz of the massive pain I was in they did an ultrasound that day.. Didn't have to fast but maybe cuz it was an emergency at that time. If u have time to prepare and know to fast at least call ur dr's office before procedure to answer ur question and ease ur mind.. Good luck
Preferably yes. Ultrasound works with sound waves, water is a good substance to transmit sound more clearly. So, in addition to drinking a lot of water, it is better to have a full bladder during the examination, just don't pee right before.