One of Skipping Hearts’ members posted about having a cardiac MRI in the near future and she made a comment about how she was nervous about receiving the contrast dye.

Perhaps some statistical data about the contrast dye used for cardiac MRIs and it’s potential side effects and comparing them to Computed Tomography (CT scan) would be helpful for those looking into having the procedure.

First, a brief explanation of the difference between a CT (”Cat”) scan and an MRI.

Computed Tomography (CT) Scan

A CT scan is essentially an x-ray with enhanced detail. Like an x-ray, a CT scan shoots microscopic photons into your body, and those photons penetrate your body and internal organs at different rates. The photons move more slowly through dense material like bone, and faster through less dense material like skin and muscle. The photons exit the other side of your body and expose a film much like a traditional film camera, creating a negative. The negative is then processed to give you the familiar look of an x-ray. Contrast for a CT scan is designed to slow those x-ray photons down so that doctors can get a better look at tissues and organs that normally are harder to see via X-ray.

There are two kinds of contrast used for CT scan. One is a barium compound in liquid form that the patient drinks a couple hours before the scan. It’s sometimes flavored to make it easier to drink. The other is an iodine compound that is injected intravenously.^[1] The iodine compound is the one associated with sensations of warmth and a metallic taste in the mouth. These sensations quickly fade as the kidneys and liver eliminate the contrast after the IV is stopped.

Less frequently, adverse reactions such as nausea and/or itching occur. These reactions occur in approximately 5% of patients.^[2] Anaphylaxis is a very rare reaction, but does occur.

Magnetic Resonance Imaging

With an MRI, the magnetic field generated by the apparatus affects the cells in the portion of your body that’s inside the machine. Specifically, it excites hydrogen atoms inside your body, and those atoms emit energy when exited. That energy is then captured as an image, not unlike an X-ray.^[3]

When greater detail of certain kinds of tissue are needed, a contrast material is introduced to those tissues via injection. As you might imagine, since the MRI machine works by using magnetic forces, the contrast material is designed to help that imaging system by altering the magnetic properties of the tissues being examined. Because different types of tissue respond in different ways to the contrast material, doctors are able to get much clearer distinction between tissue types.

The contrast most often used for cardiac MRIs is gadolinium based and has (a possibly undeserved) reputation as being dangerous to the general population. Side effects and adverse reactions from MRI contrast agents are less common than their CT counterparts, and most often is a mild headache. However, one extremely rare adverse reaction from gadolinium-based contrast has been nephrogenic systemic fibrosis (NSF), a disease of the skin and internal organs. The effects of the disease can be mild, but they can also be debilitating and even fatal.^[4]

However, the odds of developing this late-onset adverse reaction are extremely remote.

Between the early 80’s and 2006, 200 million patients worldwide received gadolinium-based contrast agent with their MRI. Of those, 150 developed nephrogenic systemic fibrosis. That’s less than one in a million chance of having that reaction.^[5]

And of those 150, 90% had had one particular brand of that contrast: gadodiamide (trade name Omniscan).^[5][6] There are currently five varieties of gadolinium contrast available in the United States.^[7] Let’s be conservative and assume gadodiamide has 50% of the MRI contrast market worldwide, making it a coin flip whether you have gadodiamide or one of the other 4 contrast agents available injected into your body. That essentially cut your odds for developing nephrogenic systemic fibrosis to one in three million.

Even better: to date there are no known cases of gadodiamide-induced nephrogenic systemic fibrosis in patients with normal kidney function. ^[8] If you have normal kidney function, your odds just dropped to zero. That’s right, zero.*

So breathe easy, good luck on your cardiac MRI, and make sure to let us know about your results.

*If you are on dialysis, you should consult your physician about the type of contrast being used, as every documented case of NSF was in patients in various stages of renal failure.

References

Cardiac MRI image courtesy of radiologyinfo.ca