MRI machines operate through high powered magnets in two groups, a stable field and a variable field. The stable field consists of a main magnet of one of three types:resistive, permanant, or superconducting. A resistive magnet contains coils of wire that an electric current passes through to produce a magnetic field. This type of magnet requires a great deal of electricity to maintain the magnetic field, usually up to 50 kilowatts. The permanant magnet is always magnetic, so it doesn't require electricity like in the resistive magnet. However, permanent magnets are extremely heavy. The superconducting magnet is a different version of the traditional resistive magnet. The coils of this magnet are coated in liquid helium and kept at a temperature of 452.4 degrees below zero. This extreme cold allows the coils to generate a magnetic field with much less electricity than regualr wire coils. Because it requires less electricity and wieghs much less, the superconducting magnet is the most common main magnet used in an MRI. The variable field is produced through three gradient magnets. They are not as powerful as the main magnet, and are much smaller.
To understand how these magnets produce MRI images, a few basic concepts
in the spin properties of atoms must be explained. Hydrogen atoms are
the best examples of spin properties, and they are also the type of atom examined
in an MRI. This is because the body is mainly made up of water; therefore hydrogen atoms.
A hydrogen atom without an external field will spin on its own in random alignment. Think of
a pile of pencils. If you pick up the pile, every pencil will pointing in a different direction.
In an external, or applied, magnetic field , the atoms will align parrallel to the applied field.
This would be like setting the pencils down on a table and lining them all up so they are parallel.
In an MRI, the main magnet produces the applied magnetic field that aligns
the hydrogen atoms in the patient's body parallel to the field.
When the magnetic field is turned off, the hydrogen atoms return to regular alignment.
In making this shift, the atoms release energies that are detected through the coils
in an MRI machine.
A good example of this concept is a compass needle. The needle of a compass typically points north, as it
is aligned with the earth's magnetic field. This is like the hydrogen atoms which natural spin on their
own alignment. However, it is possible to pull the needle around with your fingers
so that it points north. This is like the magnets in an MRI machine that force the hydrogen atoms
to align with their magnetic field. This shift in alignment uses energy; in this case your fingers
moving the needle, an then the needle moving back to it's stable alignment when you let go.
In the same way, the shift in hydrogen atoms uses energy too. The change in energy that is detected can be used with mathematical
equations and computed to produce an image. Technicians in an MRI laboratory do not calculate these signals, as the
calculations have taken years of research to determine. Rather, computers in the laboratory do the calculations for them,
producing images on the computer screen that the tecnician can manipulate.