Magnetic Resonance Imaging (MRI), invented in the early 1980s, uses an enormous magnet instead of radiation to see structures inside the body. It often shows better detail than CT Scans, and is the only way to visualize nerves bundles in and around the brain and spinal cord. But it takes lots longer to perform (and sometimes to schedule), is more expensive, so in many cases CT scans are preferred because they’re easier and quicker to obtain.
A dye called gadolinium is often injected during MRIs, to help with diagnosis. It’s safer for the kidneys than the dye used with CT scans. However, people with very poor kidney function, especially those on dialysis, can get severe skin and muscle thickening all over the body, and should not receive it. There’s also an MR-Angiogram, using a different dye, which can see blood how circulates in different parts of the body.
An MRI cannot be performed on persons with iron substances in their bodies. These include various implanted heart devices like pacemakers, and certain clips used in surgery (like for brain aneurysms). Artificial joints and coronary artery stents are safe. Metal from bullets or shrapnel may sometime preclude MRIs.
It’s also important to ask if patients ever spent time in machine shops, where steel was hammered or cut. Such people may have the tiniest of metal fragments in their eyes, causing no problems, but which would be catastrophic if whipped to & fro by the magnet. Metal in medicated patches can cause burns, and all jewelry should obviously be removed.
Rare people are too claustrophobic to tolerate the 30-45 minutes inside an MRI machine (if they move around, it ruins the test). Sometimes a tranquilizer taken ahead of time can help. There are also open MRIs, but not many available; these may also be necessary for patients weighing over 350 pounds.
The preference for CT scans over MRIs in many cases is a touchy issue, because the former employ a fair bit of radiation, which can cause cancer years later (see Radiation Risks from X-Rays). MRIs don’t use any. But insurance companies opt for CT’s because they’re cheaper, and clinicians order them because they’re easier and quicker to obtain. If everyone were to get MRIs instead, the wait would be very long. Avoiding radiation is most important for young people (who have a long life ahead in which they can develop cancer), and for anyone who will need very regular tests throughout the future.
Finally, as for all tests, there’s a risk of happening to find abnormalities we weren’t looking for, and which may not mean anything (medical term is “incidentaloma”). But since it’s hard to know for sure, we wind up doing more tests just for those, like biopsies (with their own risks); results usually turn out to be negative. Also, MRI’s are often false-positive, finding conditions that don’t exist. For example, studies show that over 95% of healthy, pain-free adults have abnormalities on MRI’s of the knee, shoulder, and back (88% of the neck). So if we find something on a patient with pain, it in no way proves that it’s the cause!