MRI: a short guide for patients and families

Steven Pavlakis MD, Maimonides Medical Center, Brooklyn NY
Michael Segal MD PhD, SimulConsult

MRI is a test that takes pictures of the body using magnetic fields and radio waves.  The patient lies on a table that extends into a large tube inside a powerful magnet.  Pictures, such as that of the brain shown here,  are generated from computer analysis of the signals generated. 

MRI stands for Magnetic Resonance Imaging.   A “picture” of the body is produced using a magnetic field and radio waves to align atoms in the body to produce signals that can be measured and assembled into an image.  MRI is the safest and best test for medical imaging in most medical situations, and its use is limited chiefly by its cost.

No side affects of the magnetic field and radio waves on the body are known, as compared to CT scans where there is exposure to a small amount of X-rays.  Since the effect of MRI has not been studied in detail during fetal gestation the test is discouraged during pregnancy, as are CT scans.  Patients with metal in their body from medical implants or fragments of metal from injuries should discuss the details with the MRI doctors and staff.  Some implants, such as aneurysm clips, can be very dangerous in an MRI, while some others, such as some implants in bone, can be OK.  Metal objects in the MRI room can be dangerous since they can get drawn into the magnet, acting as projectiles.  Metal recording media such as music tapes or credit card strips can be erased by the powerful magnet.  

In some cases an injection of “contrast dye” is given, which can enhance certain features in the MRI image. Typically the material injected is a gadolinium compound, which is very safe.  Allergic reactions to this material are very rare, much rarer than reactions to CT contrast dye. A related test is MRA (magnetic resonance angiography), used to image blood vessels with far less risk that previous techniques.

For the MRI test, the patient is placed on a table which is pushed into the tube containing the magnet.  This does not hurt but some patients feel “closed in”, particularly when the head is being imaged. For most patients, closing the eyes upon entering the tube reduces feelings of being closed in.  One can ask for special MRI-safe headphones to listen to music on CDs or radio stations, but do not bring in your own tapes since they can be erased by the magnet. Imaging can take anywhere from 15 to 45 minutes, depending on tests being done and the equipment. The machine makes repetitive banging noises; if one is bothered by noise, the headphones and music will help.

MRI and MRA are used to evaluate internal structure and look for infection, tumors, blood vessel abnormalities such as aneurysms, stroke, multiple sclerosis and many other genetic and acquired conditions. Most patients will have normal studies, but for others, the information gleaned from MRI may be helpful to well-being and on occasion be life-saving.