Overview of Magnetic Resonance Imaging
MRI scan (magnetic resonance imaging) does not use x-rays, sound waves, or radioactivity. Instead, this imaging procedure uses very complex magnetic properties. MRI scan applications are always increasing and its potential seems limitless.
History of MRI
The world of medical imaging was still becoming accustomed to CT scan and ultrasound when a new way of imaging by using magnets and magnetic fields was invented. Some medical professionals wondered why they needed another test; radiologists and technicians were still trying to get used to the tests they had. But, the images produced by MRI were so spectacular, especially in areas that physicians could barely see before, like the knee, the shoulder, and other areas. Pictures of the brain, neck, and lower back were now visible from three different planes (axial, sagittal, coronal), as compared to the one (axial) that CT scan provided. There was no radiation and no serious side effects. Magnetic resonance imaging 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 former 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 magnetic resonance 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 (MRA), which 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 of many surgeons who operate on narrowed or blocked arteries.