Measuring water inside a body using ultrasound

Hi guys, first post here.

I am working on a project, and I wish to measure the amount of urine inside a bladder using ultrasound. I would like to know how realistic is that? What sensor should I use? And how much accuracy or noise should I expect?

Thank you.

Hospitals have had such devices for a long time, for just that purpose. Accurate to few ml, I think.

Probably pretty expensive, as 3D imaging tech is involved.