While different types of studies use a different material, the idea is the same. The Contrast liquid is one that will interfere with how the imaging equipment takes your images. For example, the contrast used in X-ray or CT is a liquid that will block or limit radiation in certain parts of your body. The tissues that absorb the contrast dye will appear differently on your images. Similarly, contrast dyes that are used in MRI or ultrasound exam (which are magnetic-powered machines), will interfere with the way magnetic fields interact with the parts of your body that contain it. Creating areas of brighter brights, and more defined edges on the resulting image.