MRI contrast agents help in making certain objects more clear than the rest. About 20% of the scans ordered in USA, use MRI, with contrast agents, injected in the portion to be probed.
Reasons for Using Contrast Agents
With contrast agents, MRI can give better images in certain cases, when the doctor needs more accurate and detailed images of certain body parts and tissues. If one compares, MRI with and without contrast, then the latter is non-invasive, while the former is an invasive procedure.
Following are the instances where MRI, along with contrasting agents, is generally ordered:
- These agents are used for better imaging of cancerous tumors, for precision surgery.
- Other than scans for cancer, it is also used to look for infections and inflammations, which are not easily visible through other imaging techniques or testing procedures.
- Structural abnormalities in various internal parts of the body are made more visible through an MRI scan, which can help doctors in coming up with a better and more accurate diagnosis.
- Magnifying and visualizing blood vessels to detect abnormalities, is often a reason for ordering MRI scans.
- Brain MRI, with contrasting agents, is often used before and after neurosurgery procedures for diagnosis, as well as treatment evaluation. The major diseases which necessitate a scan with contrast agents are cancerous brain tumors, meningeal disease, CNS infections, and inflammatory diseases.
- Diagnosis of spinal problems necessitates a scan with contrasting agents.
- Abnormalities in the gastrointestinal tract are also probed by using contrasting agents, administered orally.
Chemicals Used as Contrast Agents
Use of the agents is an established procedure now. Depending upon the area which is being highlighted, different types are used. There are two main types:
► Intravascular MRI Contrast Agents: They are used in imaging of blood vessels and are mostly chelated compounds of gadolinium like Omniscan, Magnevist, Multihance, Optimark, Vasovist, and Prohance. Iron oxide based agents, administered intravascularly, are Super Paramagnetic Iron Oxide and Ultrasmall Supermagnetic Iron Oxide.
These compounds are used in nanoparticle-sized colloidal form and have provided excellent contrast imaging, in cases of liver tumor probes. The product names of various iron oxide based agents are Combidex, Endorem, Sinerem, Cliavist, and Resovist. Chelate compounds of Manganese are also used as agents in an MRI scan.
► Oral MRI Contrast Agents: These are used for MRI scans of the gastrointestinal tract. Gadolinium and manganese chelate compounds are also used for oral administration.
Some natural products are also used to provide contrast and improve clarity of MRI scans. With a particularly high manganese concentration, green tea and blueberry tea are also used. A chemical called Perflubron is often used as an agent, for gastrointestinal scans.
All these substances, used as contrast agents, are paramagnetic. Magnetic resonance imaging is dependent on the magnetic properties of matter and paramagnetic substances help in differentiating between different tissues, when injected intravenously or orally.
Side effects of these agents are rare as they are disposed off through feces. One known side effect is nephrogenic systemic fibrosis, caused by exposure to gadolinium. It is a problem which people suffering from kidney or renal failure are vulnerable to. Other than that, these agents can be safely administered and any drastic side effects are very rare.