“What happens if you eat before a CT scan with contrast?”

It depends on what part of the body is being scanned. Usually it is the abdomen that requires no food prior to a CT scan to get a better view. Yours in health, Dr. Cipriano

The contrast lights up the hollow viscous organs,oral contrast lights up the inside of them so if you eat something prior to, it can obscure the picture, make it hard to tell if there's a mass or obstruction or just a sammich. It could also cause "artifact" on the image and make the test hard to read and when doing a test like that we try to limit technical errors. Also, if you need a CT scan, you probably don't feel well and you have to lay on your back in a tube in a room by yourself and God forbid something happens, and you vomit. We have to wait until they roll the bed back to us before we can try to help you. So if you aspirate water it's not as life threatening as having food stuck in your lungs that we now can't get out.... So kinda a few reasons.

Nothing

Obscures picture and less accurate.

Occasionally, the contrast may cause nausea and vomiting, much more rare with modern contrast agents. If the stomach is the area of primary concern it is nice to have the stomach clear of food so the gastric mucosa can be carefully evaluated.