Magnetic Resonance Imaging (MRI) does not use of X-rays or other forms of ionizing radiation. Instead, as the name suggests, it takes advantage of the magnetic properties of the atoms in our body and only uses a large magnet and radiofrequency pulses.
MRI scanners make use of the fact that the nuclei of certain atoms (like hydrogen, common in the water in our bodies) behave like tiny magnets. These atoms have a "nuclear spin," and they can align with a magnetic field.* The most obvious feature of an MRI scanner is the big "tube," which is the magnet that generates this static magnetic field. During a scan, radiofrequency pulses specifically tuned to the field and timed precisely in milliseconds stimulate these nuclei. This quickly flips the alignment of the nuclei, which then realign with the big magnetic field and send out their own radio signals as they do so. These signals are picked up by antennae in the scanner and the data is gathered in a computer. Sophisticated software then unpacks and translates these signals to reconstruct an image of the body being scanned. Each tissue type (bone, muscle, blood, etc.) looks different because the radio signals emitted from these tissues are expressed and received differently.
The techniques to obtain and process MR data have been refined to the point that specific tissues can now be viewed in isolation and rendered in 3D. One specialty of investigators at UCAIR is the imaging of blood vessels in the brain and heart. Images such as the one below are used to identify brain aneurysms so that they may be monitored or removed surgically. Click on the image to see a 3-D rotational view of the blood vessels. A balloon-like cranial aneurysm will come into view as the image turns.
A video description of how an MRI scanner works can be found here, provided by Siemens Corporation describing safety procedures.
A detailed explanation of MRI for the scientifically inclined can be found here
* The strength of a magnetic field is measured in Tesla. One Tesla is equal to 10,000 Gauss. The earth's magnetic field, at the earth's surface, varies between 0.3 and 0.6 Gauss. Most clinical MRI scanners use a magnetic field of 1.5Tesla (T). Here in the Department of Radiology we have both 1.5 and 3T scanners. In general, the higher the magnetic field strength of the scanner, the more detail can be seen in the MR image.