My MRI's have always been done with AND without contrast. I have only had 2 CT's and neither were done with a contrast agent.

"CT contrast agents, sometimes referred to as "dyes," are used to highlight specific areas so that the organs, blood vessels, or tissues are more visible. By increasing the visibility of all surfaces of the organ or tissue being studied, they can help the radiologist determine the presence and extent of disease or injury.

Contrast agents are available in several different forms, but in general a CT contrast agent is a pharmaceutical substance. Some of the more common contrast agents used are:

Iodine

Barium

Barium sulfate

Gastrografin


Contrast agents for CT examinations are administered in four different ways:

Intravenous injection

Oral administration

Rectal administration

Inhalation-this is a relatively uncommon procedure in which xenon gas is inhaled for a highly specialized form of lung or brain imaging. The technique, xenon CT, is only available at a small number of locations worldwide and is used only for rare cases.


Intravenous Contrast
Intravenous contrast is used to highlight blood vessels and to enhance the structure of organs like the brain, spine, liver, and kidney. The contrast agent (usually an iodine compound) is clear, with a water-like consistency. Typically the contrast is contained in a special injector, which injects the contrast through a small needle taped in place (usually on the back of the hand) during a specific period in the CT exam.

Once the contrast is injected into the bloodstream, it circulates throughout the body. The CT's x-ray beam is weakened as it passes through the blood vessels and organs that have "taken up" the contrast. These structures are enhanced by this process and show up as white areas on the CT images. When the test is finished, the kidneys and liver quickly eliminate the contrast from the body."

See: http://www.ehealthmd.com/library/ctscan/CTS_contrast.html