Most types of body imaging use injectable contrast, or dyes. MRI is no different. What is different, however, is the type of contrast used, how it works and why it is used.
The contrast or dye materials used in procedures such as X-rays and CT scans work in the same way. Both use X-rays and these dyes work by preventing the X-ray photons from passing through areas where the dye has been injected. The X-ray photons, therefore, cannot reach the X-ray film. This results in different levels of density on the X-ray/CT film. The dyes have no direct physiologic impact on the body's tissue.
The contrast dye used in an MRI is fundamentally different.
MRI dyes work by altering the tissue's local magnetic field. Normal and abnormal tissue will respond differently to the slight alteration, giving doctors differing signals. These varied signals are transferred to the MRI images, allowing doctors to visualize many types of tissue abnormalities and disease processes. An MRI can do this better than possible without the contrast dye.