MRI contrast
When it is difficult to differentiate two types of tissue, because the signal intensity they produce is so similar, a contrast agent is added to one of them in order to distinguish it from the other tissue. An MRI contrast agent works by affecting the time it takes for the hydrogen atoms to return to their original energy state. This increases the difference in the signal intensity from the two types of tissue (the one with and the other without the contrast agent), thus increasing the degree of contrast on the image.
It is possible to produce an MRI contrast agent having properties that make it organ-specific, while others may give more general contrast enhancement in various tissues
The body treats MRI contrast agents in much the same way as it does X-ray CM, i.e. the CM becomes evenly distributed in the blood and is then washed out by the kidneys. The main differences lie in the MRI technique per se, and the fact that both the concentration and the volume of the CM used in MRI are much less than those used in X-ray. MRI agents also differ chemically from X-ray CM. Most MRI contrast media contain the heavy metal gadolinium (chemical symbol Gd), which may be said to be "encapsulated" within a ligand (a substance which easily forms chemical complexes), and this makes the Gd less toxic and also ensures that the CM remains in the bloodstream. As is the case with the X-ray technique known as computed tomography (CT), MRI also takes dynamic images. This means that a series of images of the same area of the body is taken at intervals ranging from a few seconds to a few minutes apart. The signal intensity is then measured at selected points on each image and diagrams of the intensity at these points are produced. The diagrams can indicate whether a tissue change is benign or malignant.
Contrast-enhanced MRI also enables much smaller tissue changes to be detected. On an MRI image taken without CM, tumours from about 1-2 centimetres in size and larger will show up fairly clearly. After a CM injection, however, it is not uncommon to discover that in addition to one or a few larger tumours, several smaller tumours are also present.
Because of the high quality of the images produced with MRI, this technique is especially useful in examinations of the brain and other parts of the central nervous system. It is also used in breast cancer examinations.
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