What is the difference between them?
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Many years ago I worked on all three systems.
X-rays are one of the simplest forms of medical imaging. An x-ray tube converts high voltage into X-rays by sending an intense stream of electrons into a spinning tungsten disc. The X-rays produced from the disk passes through soft tissue and bones differently. This is recorded on film, special cassettes or imaging sensors as varying degrees of light, soft tissues appear darker and bones appear lighter, although this can be reversed depending on how it is used. X-rays are typically done in a fixed position but there are 2D and 3D applications where the X-ray tube is moved while imaging.
CT (Computerized Axial Tomography) scans use a similar X-ray tube, but instead of being in one location it is moved around the area of interest in a circle and a computer records the changes. Here is where mathematics are used, as the information is processed the amount of density varies based upon each angle. A fast computer takes all of this information and can determine areas that are soft tissue and other areas that are bone. A sufficiently smart program can determine specific density values and assigns a value to each ‘pixel’, where the computer arranges the information on the monitor. The more information the computer can store and process determines the density of the tissue but cannot determine the actual material of the area.
This is where an MRI (Magnetic Resonance Imaging) can excel, MRI can determine with pretty good accuracy exactly what atoms are in a precise location. An MRI consists of a very large and powerful magnet, within this magnet is a series of electrical coils (field coils) that can send and receive information from the atoms. When a person is placed in the tunnel of the MRI magnet, the atoms that make up our bodies are aligned to the big magnets magnetic field. When the field coils are turned on it temporarily shields the atoms from the big magnets magnetic field so they return to their original position. When the computer turns the field coils off the field coils start listening to the atoms, as the atoms realign to the big magnet they emit a small radio signal as they fight to stay in their normal alignment. The field coils record these signals and their positions into a large database. This is repeated many times, the field coils are energized and turned off and they listen to the atoms going back into alignment.
The neat thing is different atoms give off different radio signals when changing orientation to the big magnet, the computer recording these signals knows the difference between them and can assign a value to a precise location. After enough information is gathered the computer can make a pretty good guess as to what atoms are within your body. This information is pieced together and displayed on a monitor. Colors can be assigned to a known tissue value and the computer can generate a fair representation of what is inside.
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