If someone needs a ct scan of abdomen and pelvis due to cramps and diarrhea but can't have oral or iv contrast is an ultrasound better. or is ct better even without contrast? how accurate is a ct scan with no contrast?

the answer to the first part of your question is a CT is more accurate than an ultrasound for the purposes of your problem. A CT with contrast is the most accurate study, you will not get an accurate evaluation of the bowel without contrast . Unless you are allergic to oral contrast I would try that, if not CT without contrast or an MRI might be a better choice