|  Home  |  Doctors  | Students  |  Organizations |  Health & Fitness  |  News  | Message Boards  |  About Us  |


 


















































 






 











Patrons Doctor
     
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Back to List
Magnetic Resonance Imaging
MRI doesn't use x-rays or even sound. No radioactivity. It uses magnetic properties that are so complex as to defy understanding. Its applications increase daily, and its potential seems limitless.

Historical Perspective

The world of medical imaging was still becoming accustomed to CT and ultrasound when the word got out that there was going to be a new way of imaging by using magnets and magnetic fields. Some in the profession wondered why they needed another test; radiologists and technicians were still trying to get used to what they had. But the images were so spectacular, especially in areas that physicians could barely see before, like the knee, the shoulder, and extremities, in general. Pictures of the brain, the neck, and the lower back were now visible from three different planes (axial, sagittal, coronal), as compared to the one (axial) that CT provided. There was no radiation and no serious side effects. Magnetic resonance revolutionized medical imaging.

How Magnetic Resonance Imaging Works

Countless articles and books have been written to explain MRI imaging to the nonphysicist. The reader is directed to them for more in-depth and accurate information. The textbook by a member of this site’s editorial board, David Stark, M.D., is a good place to start (and probably finish). Basically, the patient is placed in a tube where various magnetic fields are applied to the body. The way the body responds to those fields and how it relaxes when the magnetic field is removed is noted and sent to a computer along with information about where the interactions occurred. Myriads of these points are sampled and fed into a computer that processes the information and creates an image.

An interesting feature of MRI imaging is that flowing things have a distinctive appearance on MRI scans (similar to Doppler ultrasound). Flowing structures cause "flow voids," which appear as black holes on the scans. There are computers powerful enough to extract information about a given flow void, such as in the carotid arteries in the neck. The computer does this for each and every slice, of which there are many, and can put together images of the vessel causing the flow void. The images look just like someone had injected dye, as in an angiogram. This is Magnetic Resonance Angiography, or MRA. The good news about MRA is that it offers another way of looking at vascular structures in the body. For example, in cases where the aorta is injured by arteriosclerosis, aging, or trauma, MRA can provide exquisite images. Resolution can be somewhat of a problem, however, for small structures, such as the carotid arteries. MRA images are good, but the angiogram remains the method of choice for many surgeons who operate on narrowed or blocked arteries (see Carotid Artery Disease).

When is MRI used?

MRI has become the gold standard in imaging the extremities, especially the joints. Some departments use CT as the screening test for imaging the brain, but most consider MRI the best choice for imaging the upper spine and lower back. CT and MRI are equally useful for the neck. For chest and abdomen, CT continues to dominate, though MRI is continually coming up with new applications. Ultrasound is still the way to examine the female pelvis, though MRI has made inroads in imaging the uterus and ovaries, and CT is helpful in certain cases. As for the scrotal contents, ultrasound use is prevalent in most departments, while MRI and radionuclide imaging are strong alternatives in certain circumstances.

Risks and Potential Complications

During an MRI, you are placed inside an enormously powerful magnet, so it makes sense that you will not have anything on your person that could be even remotely attracted to this magnet. Remember, this is a huge magnet. There are stories, perhaps untrue, about entire forklifts being sucked into one of these magnets that were accidentally turned on during installation. So obviously you won't bring your keys in, but what about those little pieces of metal inside your body that you may or may not know about. What about very small metal filings that may have penetrated your eyeball years ago, or the tiny clips that were used years ago to tie off an aneurysm in your brain, and so on? For the full list, you will need to talk to a representative of the MRI facility at which you plan to have your study. The MRI technologist will be able to give you a more complete list as well as answer any questions that you may have.

As is the case in CT, your MRI may require the injection of intravenous contrast. The most commonly used dye is gadolinium, which basically does to MRI what iodine does to CT. The good news is that unlike the CT dye, with its rare complication, complications from gadolinium are even rarer, and gadolinium can be injected by a skinny needle that hurts less.

What actually happens?

This can be an unpleasant test. If you are obese, you may be in for some additional trouble. More often than not you will be confined in a tight space within the magnet and asked not to move. Some patients are too claustrophobic to undergo this procedure. The space is confining and noisy. Some departments give patients earphones with music to help block the noise and distract them. The newer MRI gantries have a much wider bore and are much less confining.
If you receive contrast, the technologist, nurse, or radiologist will generally inject it through a small caliper needle that should not be particularly painful. 

Back to List