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|  MRI, short for Magnetic Resonance Imaging, has on occasion been compared to Cat Scan by the News Media since they are both computer based systems that provide "cross-sectional" views that show the human body as slices like the slices in a loaf of bread. In truth however, that is the only way they are similar, for the way they obtain the information their computers use to generate those images are totally different. CT still uses x-rays to gather it's data just like conventional radiology, but MRI uses a strong magnetic field coupled with radio waves to collect its data. To understand how a MRI unit works requires a brief lesson in physics, so some readers may wish to skip ahead a little. MRI works because the nuclei of most atoms work like tiny bar magnets in a way. This means that when placed in a strong magnetic field, they will try to line up with the field so that they are heading in the same direction. I use the word try here because these atoms do not quite succeed in perfectly lining up with the field. Instead they spin around it at a slight angle almost like a wobbling top. When rotating, or processing around like this, many atoms can be stimulated by a pulsed radio wave at a certain frequency. The frequency that will cause stimulation is different for each element, thus allowing us to stimulate only one type of atom at a time. When the radio waves are turned off, the stimulated atom emits radio waves of it's own as it relaxes back to its natural state. This information, discovered in the late 1940's and early 50's led to the development of MR Spectroscopy. In the late 60's and early 70's, it was discovered by a person named Damadian that while tumors and normal tissue both contain water that could be identified as such by MR Spectroscopy, the amount of time the water took to relax back to it's natural state was different between the two. This led to speculation that it might be possible to create images of the human body by mapping the differences in these relaxation times. And so, in 1973 the first MRI images were published. Admittedly those first images were very crude, and only large objects could be visualized, but over the past 23 years MRI has advanced to the point where it can now turn out images as fine and as detailed in their own way as CT.
Although MRI is invaluable due The second drawback is that not everyone can safely enter into the same room as a MRI unit. Since the magnetic field created by the unit can interfere with most electronic devices, and easily erase computer disks and tapes, anyone who has an implanted device made out of a magnetic metal, or is electrical in nature, such as pacemakers, insulin pumps, and aneurysm clips. For this reason, the first thing the technologist doing your exam will do is to go over a survey sheet to check for any potential problems before they will allow you to have your exam(s). Most orthopedic type implants, such as artificial hips, pins, and rods, are made of nonmagnetic metals such as aluminum, so they normally would not present a problem. Other types of implants on the other hand can be a different story. Some implants, for one reason or another are made of magnetic metals, and thus will be affected by the MRI's magnetic field. So if you or your family member do have an implant of some type, dig out the papers they should have given you on it when it was implanted, or call the surgeon who implanted it if you cannot find them. Then call the facility that will be doing your exam and discuss it with them. They will be able to tell you if your implant is safe or not. After the technologist who will be doing your exam has determined that it is safe for you to go ahead with it, he or she will ask you about the problem that lead your doctor to order an MRI, and about your past medical history to gain a better understanding of what he or she will be looking for. You will then be shown to a set of lockers where you will be asked to remove everything from your pockets, especially any metal items and items that may be harmed by a strong magnetic field, such as credit cards, ATM cards, and watches. Depending on your exam and the clothing you are wearing, you may also be asked to either partially or completely disrobe and to put on a hospital gown. If you are an inpatient instead of an outpatient, you will be disconnected from any pumps and/or other hospital equipment not cleared for use in a MRI's magnetic field. You will then be taken into the scanning room and placed on the MRI's table, and the technologist will position you in the proper position for your exam. Since most MRI scans can take a relatively long time(40 minutes or more) to complete, you will be made as comfortable as possible. Depending on what body part is to be examined, you may also be fitted with a "coil" that the MRI will use to better pick up the signals that it uses to image your body. Coils come in a variety of shapes and sizes, form long padded boards that you lay on, to tube shaped pieces that fit around knees, legs, and arms. Each one is specially designed for certain parts of the body, and which one will be used on you depends on what the doctor is looking for. For some exams, it may also be necessary to inject you with a special contrast agent that will help highlight certain items on your films. For more information on your particular exam, click on the exam listings on the side bar, or contact the facility at which you will be having your tests. | |
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to its ability to differentiate between the various tissues and organs making up the body, there are still some drawbacks to them. The first drawback is the size and weight of the units. 10 ton MRI units are hardly uncommon, and it is not unheard of for them to weigh almost 100 tons. When coupled with the strong magnetic field a MRI unit creates to perform the exams, finding a place to put an MRI unit can become a huge headache. Thus your choice of where to have your MRI done may be limited.