You may see the term contrast enhancement used often in reports of CT scans and MRI. Contrast enhancement allows the radiologist to see normal from abnormal areas better. The contrast enhances the ability to see normal from abnormal. This allows a more accurate diagnosis to be made. The confidence level is much higher even for the case of normal tests if contrast is given.
Contrast enhancement can only be detected after a liquid substance called a contrast agent is given through your vein. Contrast changes the way the organs and abnormal areas look like on the imaging test that your doctor ordered. The abnormal areas are highlighted and easier to see.
Many disease states are better seen after contrast. One example is cancer. It is easier to see on tests after contrast is given through your vein. Cancer will be highlighted in many cases because it has more blood supply then normal surrounding tissues. Many benign lesions such as liver masses also have more contrast enhancement or a brighter appearance because they tend to accumulate the the contrast more then the surrounding liver.
One of the major downsides of contrast agents is the possibility of allergic reactions. Most commonly these are minor. Hives or other minor reactions are most common. More severe reactions are reported in rare cases. The contrast will eventually be eliminated by your body and is not permanent.
People who have poorly functioning kidneys can develop further worsening of function in the case of contrast used for ct. Your kidney function will be measured in some cases before you get a CT scan. A rare complication of getting gadolinium or MR contrast agents when you have poorly functioning kidneys is neurogenic systemic fibrosis which will cause scarring of your skin and organs. This is very uncommon with the newer contrast agents.
Overall, the benefits of getting contrast agents for CT and MRI outweighs any potential dangers. Majority of patients either have no complications or minor transient reactions. Contrast helps to get you the correct diagnosis or the peace of mind that comes with a normal exam that is truly normal.