What Is MRI?

Siemens Trio MRI Scanner
The "3T" Siemens Trio MRI Scanner (technical specifications)

MRI scanners make use of the fact that the nuclei of certain atoms behave as if they were small (very small) magnets. Such nuclei are said to possess "nuclear spin," and they can align with a magnetic field (lower energy) or opposed to a magnetic field (higher energy) *. 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, time-varying magnetic fields stimulate these nuclei - for medical imaging, the hydrogen nuclei of water - to flip their alignment. After the pulse of radio-frequency magnetic fields, the nuclei realign and send out radio signals as they do so. These signals are picked up by antennae in the scanner and translated by computer into an image of the body part being scanned. The manner in which the realignment process takes place is very sensitive to the environment of each nucleus. Since the environment is dictated by the tissue in which the hydrogen atom resides, different tissues "look" different to the scanner. MRI does not require the use of X-rays or other forms of ionizing radiation.

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 cranial blood vessels (below). Images such as this are used to identify cranial 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.