How MRI Works

On July 3, 1977, an event took place that would forever alter the landscape of modern medicine. Outside the medical research community, this event made scarcely a ripple at first. This event was the first MRI exam ever performed on a human being.

It took almost five hours to produce one image. The images were, by today's standards, quite ugly. Dr. Raymond Damadian, a physician and scientist, along with colleagues Dr. Larry Minkoff and Dr. Michael Goldsmith, labored tirelessly for seven long years to reach this point. They named their original machine " Indomitable " to capture the spirit of their struggle to do what many said could not be done.

This machine is now in the Smithsonian Institution. As late as 1982, there were but a handful of MRI scanners in the entire United States. Today there are thousands. We can image in seconds what used to take hours.

The Basic Idea
If you have ever seen an MRI machine, you know that the basic design used in most is a giant cube . The cube in a typical system might be 7 feet tall by 7 feet wide by 10 feet long (2 m by 2 m by 3 m), although new models are rapidly shrinking. There is a horizontal tube running through the magnet from front to back. This tube is known as the bore of the magnet. The patient, lying on his or her back, slides into the bore on a special table. Whether or not the patient goes in head first or feet first, as well as how far in the magnet they will go, is determined by the type of exam to be performed. MRI scanners vary in size and shape, and newer models have some degree of openness around the sides, but the basic design is the same. Once the body part to be scanned is in the exact center or isocenter of the magnetic field, the scan can begin.

In conjunction with radio wave pulses of energy, the MRI scanner can pick out a very small point inside the patient's body and ask it, essentially, "What type of tissue are you?" The point might be a cube that is half a millimeter on each side. The MRI system goes through the patient's body point by point, building up a 2-D or 3-D map of tissue types. It then integrates all of this information together to create 2-D images or 3-D models .

MRI provides an unparalleled view inside the human body. The level of detail we can see is extraordinary compared with any other imaging modality. MRI is the method of choice for the diagnosis of many types of injuries and conditions because of the incredible ability to tailor the exam to the particular medical question being asked. By changing exam parameters, the MRI system can cause tissues in the body to take on different appearances. This is very helpful to the radiologist (who reads the MRI) in determining if something seen is normal or not. We know that when we do "A," normal tissue will look like "B" -- if it doesn't, there might be an abnormality. MRI systems can also image flowing blood in virtually any part of the body. This allows us to perform studies that show the arterial system in the body, but not the tissue around it. In many cases, the MRI system can do this without a contrast injection , which is required in vascular radiology.