How Can A Vaginal Ultrasound Accurately Assess A Prolapsed Bladder And Two Cysts When Excessive Water Consumption Leads To A Protruded Bladder?

I have a prolapsed bladder which is outside the vagina. I amt o have a vaginal ultra sound and was told to drink a litre of water. The bladder is vagina a full bladder will make it difficult to already sticking outside my vagina. a cat scan found two cysts. how can they see everything if my bladder is full?