Magnetic Resonance Imaging is possible because some nuclei of the atoms contained in various tissues of the body have an electrical charge. This means that they can be influenced both by magnetic fields and by high frequency radio waves.
By happy coincidence, the nucleus of the Hydrogen atom has this property, and it's also by far the most abundant element in the body, by virtue of being present (twice) in every molecule of water (H2O).
It turns out that these nuclei, when in the presence of a magnetic field, will absorb a small amount of energy from a radio frequency (RF) transmitter, and then will re-emit that energy after the RF transmitter is turned off, analogous to the way that some fluorescent materials will 'glow in the dark' after the lights are turned off. And just as there are different types of fluorescent materials that can produce different shades of color, so also the Hydrogen that is contained in different tissues will re-emit its RF energy in different ways. More importantly, diseased tissue often behaves differently from normal tissue.
Just as the microscope is a tool that enables us to see physical properties that were previously unknown, the MRI machine enables us to discover properties of tissue that were invisible to us before its invention. The hitherto-invisible properties of tissues that are now visible with the MRI are called T1 and T2. When you have physicists naming things, that's the kind of name you end up with.
 |  |  | ||
| T1 Image | T2 Image | Proton Density Image |
In addition to these images that are based on the differences of T1 and/or T2, the MRI also can make images that are based on principles that are not unique to the MRI, such as motion within the body. The MRI can also make images that correspond very closely to the way the body appears if you were to actually cut it open and photograph it in black and white, except without the mess and the pain. Images based on a combination of these factors are often taken during a typical examination.
No comments:
Post a Comment