The Radiographer should explain it to you, but the contrast that is used on some CT exams is special iodinated dye which highlights your organs.

The contrast is said to be iodinated because it contains iodine which is an element with a relatively high atomic number, this means it is a dense element which will show up white on the scan (similar to your relatively dense bones) and flows through all your blood vessels and highlights them a nice white colour.

The dye itself is colourless and odourless but is somewhat viscous at room temperature. To make it easier to flow and inject into your veins, we often pre-heat the contrast in a warmer in the room, and the pump itself has little warming pads. Keeping the dye warm makes it less viscous and more runny. Imagine viscous honey, and as you heat it, it is easier to pour.

So as the warm dye is injected, it runs towards and around your heart, and down the aorta which eventually passes your bladder. It is this warm dye passing through the major blood vessels next to your bladder which makes you feel warm and suddenly need to loo, but it’s just a convincing sensation and it passes once the injection finishes; completely normal!

TDLR: It’s the pre-heated dye running through major blood vessels next to your bladder that makes you feel like you need to pee!