I Had A Bladder Scan Done And Was Asked To

I had a bladder scan done and was asked to drink 4 glasses of water 1 hour prior to the procedure and upon doing the procedure there was no urine in the bladder then they gave me another 4 glasses of water and waited 30 min. When they did the scan I only had 156 ml of urine and with a residual of 12 ml. Where did all that water I drank go?