Four hour mammogram appointment??

Hi, I'm going to the breast clinic for a mammogram next week. I've been told to set aside 4 hours for the appointment and I don't know why? I've looked and seen a mammogram is 15minutes and you don't get results the same day. I'm at 9am booking so don't think it's in case of delays? I don't know what else it is for though? Can anyone please shed any light? Thank you.