Question: I have to have an MRI soon. Because I am very overweight, my doctor told me that I have to have something called an open MRI but that this is not as good as a "closed" one. Now I am worried that I won't get as good an examination. Why isn't the open MRI as good as the closed one?
Answer: To answer your question, you have to understand how a MRI examination works. MRI stands for magnetic-resonance imaging. In contrast to X-ray machines and computerized axial tomography (CAT) scanners that use X-rays to produce a photographic image, MRI units use magnetic energy and radio waves to generate an image. Our bodies are 60 percent water. Every water molecule contains 2 hydrogen atoms. Because hydrogen atoms are positively charged particles, if the body is placed in a strong enough magnetic field, the atoms will align themselves in the direction of that field.
In a MRI machine, your body is subjected to a strong magnetic field. After the hydrogen atoms line up, a radio frequency transmitter sends a signal into the tissue and this excites some of the atoms. When the signal is turned off, the atoms return to normal and in doing so, they send off a small radio signal of their own. This is picked up by a receiver and, using some incredibly complex mathematical equations, this information can be turned into an image by a computer. MRIs can produce incredibly detailed images of the insides of our bodies. The first MRI scan on a human was done in 1977, and that machine is now in the Smithsonian Institution.
In a closed MRI unit, your body is inserted into a tunnel so that you are actually inside the magnet. In an open unit, as the name implies, the scan is done in a more open setting. Closed MRIs produce better images than open machines, and this has much to do with the magnet's strength.
The more powerful the machine, the better the image produced. More powerful magnets more effectively align the hydrogen atoms and scanning can be performed more rapidly. The longer a scan takes, the more motion will affect the image. Patients may squirm, breathe or shift slightly, and this will make the image less clear.
Open MRIs were developed for two situations: obese patients who cannot fit into the tunnel of a closed unit and for those patients who are simply too claustrophobic to go into the narrow, enclosed tunnel of a closed unit. Patients in these categories have to accept the fact that their examination will be less than ideal as a result.
Open MRI machines are attractive because they are much less expensive than the more powerful closed units, and they are more comfortable for patients. At the current level of development, however, open units are simply not as good as closed ones, and you should always insist that your examination be done in a closed MRI unless your doctor gives you specific reasons why this is not possible or necessary.