An MRI scan works by using a powerful magnet, radio waves, and a computer to create detailed images. Our bodies are made up of millions of hydrogen atoms (the human body is 80% water), which are magnetic. When your body is placed in the magnetic field, these atoms align with the field, much like a compass points to the North Pole. A radio wave “knocks down” the atoms and disrupts their polarity. The sensor detects the time it takes for the atoms to return to their original alignment.
In essence, MRI measures the water content (or fluid characteristics) of different tissues, which is processed by the computer to create a black and white image. The image is highly detailed and can show even the smallest abnormality. A very high spatial resolution and the ability to distinguish soft tissues are the main advantages of MRI. However, sometimes insufficient contrast is observed, and the use of contrast agents is of great assistance in many applications of MRI helping radiologists to make a more precise diagnosis.
The most commonly used compounds for contrast enhancement contain gadolinium, which has magnetic properties. Such MRI contrast agents shorten the relaxation times of nuclei within body tissues following oral or intravenous administration. In MRI scanners, sections of the body are exposed to a very strong magnetic field causing primarily the hydrogen nuclei (“spins”) of water in tissues to be polarized in the direction of the magnetic field. Contrast agent may be injected into your bloodstream to enhance certain tissues. It circulates through the blood stream and is absorbed in certain tissues, which then stand out on the scan.
Gadolinium contrast agent consists of complex molecules, arrangements of atoms held together by chemical bonds. The chemical bonds are made between a gadolinium ion and a carrier molecule (a chelating agent). A chelating agent prevents the toxicity of gadolinium while maintaining its contrast properties.
How is gadolinium contrast agent given to me? What are the risks of gadolinium contrast agent injections?
Gadolinium contrast will be given to you by intravenous injection into a vein in your arm. Most patients do not notice any sensations, although a few patients will report a cold feeling in the arm during the injection, which is of no significance. An even smaller number (between about 1 and 4 in 100) will notice mild nausea or headache. Vomiting can occur, but this is rare (less than 1 in 100 injections). Less often, in approximately 1 in 1000 patients, an itchy skin rash might appear a few minutes after the injection. This appears to be due to a mild allergy. It usually settles down by itself within an hour or so, but rarely it might be a warning sign of a more serious allergic reaction developing.
Gadolinium contrast agent is generally very safe. In patients with normal kidney function, most of the gadolinium injected (over 90%) is passed out in the urine within 24 hours.
Nephrogenic systemic fibrosis
Nephrogenic systemic fibrosis (NSF) is a rare disease resulting in skin contractures (or localised skin thickening and tightening) and internal organ damage. It has occurred with some gadolinium-based contrast agents in a minority of patients with pre-existing severe kidney function abnormalities. There are some forms of gadolinium contrast for which there seem to be lower risks of NSF than for others. These low-risk forms are used in patients with less severe renal disease if the likely benefit (better diagnosis) justifies the very low likelihood of subsequent NSF. Even in those with end-stage kidney disease, the risk of NSF developing after a single injection of a lower risk agent is believed to be well under 1 in 100 injections.
For this reason, you will be asked questions about possible kidney disease as part of the safety screening before the MRI scan. If you have kidney disease, please advise your referring doctor before the procedure. You, your doctor and the MRI radiologist can discuss whether or not the possible benefits of a gadolinium injection outweigh the possible risks in your case.
Recently, it has been recognised that very small amounts of at least some forms of gadolinium contrast agent (about 1% of the injected dose) are retained in the tissues, mostly in the bones, with tiny amounts in the brain. This seems to be more likely with the same forms of gadolinium contrast that have a higher risk for NSF. At this stage, there are no known adverse effects from these very small amounts of retained gadolinium. This finding has made radiologists more careful to recommend gadolinium contrast only where it is likely to assist the diagnosis.