An MRI machine is a giant magnetic cube with a horizontal tube running through the magnet from front to back. This tube is known as the bore of the magnet, and the patient, lying on his or her back, slides into the bore on a special table. MRI scanners vary in size and shape, and newer models have some degree of openness around the sides, but the basic design is the same.

The biggest and most important component in an MRI system is the magnet. The MRI suite can be a very dangerous place if strict precautions are not observed. Metal objects can become dangerous projectiles if they are taken into the scan room. Prior to allowing a patient into the scan room, he or she is thoroughly screened for metal objects. If patients have implants inside them it can also be very dangerous for them to be in a strong magnetic field. Metallic fragments in the eye, pacemakers and aneurysm clips in the brain can be particularly dangerous since the magnet can move them.

Most orthopedic implants are fine because they are firmly embedded in bone. Metal staples in most parts of the body are safe once they have been in a patient for six weeks when enough scar tissue has formed to hold them in place.

Besides the main magnet in the MRI, there are three "gradient" magnets. These are very low strength compared to the main magnetic field.The main magnet immerses the patient in a stable and very intense magnetic field, and the gradient magnets create a variable field. The rest of an MRI system consists of a very powerful computer system, some equipment that transmits RF (radio frequency) pulses into the patient's body while they are in the scanner, and other secondary components.

When placed in a magnetic field, the hydrogen atoms in the water in our bodies have a strong tendency to line up with the direction of the magnetic field. The MRI machine applies an RF (radio frequency) pulse that is specific to hydrogen. The system directs the pulse toward the area of the body to be examined. The RF pulse forces hydrogen protons to spin at a particular frequency and in a particular direction. The three gradient magnets are arranged in such a manner inside the main magnet that when they are turned on and off very rapidly they alter the main magnetic field on a very local level. When the RF pulse is turned off, the hydrogen protons begin to slowly return to their natural alignment within the field and release their excess stored energy. When they do this, they give off a signal that the coil picks up and sends to the computer system which turns the data into a picture.

MRI contrast works by altering the local magnetic field in the tissue being examined. Normal and abnormal tissue will respond differently to this slight alteration.

Advantages:

• MRI systems do not use ionizing radiation like XRay and CT
• MRI contrast materials have a very low incidence of side effects
• MRI can image in any plane without the patient ever moving.

Disadvantages:

• There are many people who cannot safely be scanned with MRI
• There are many claustrophobic people in the world who won't tolerate the enclosure
• The machine makes a tremendous amount of noise during a scan that sounds like a continual, rapid hammering
• MRI scans require patients to hold very still for extended periods of time
• MRI systems are very expensive, and therefore the exams are also very expensive

Note: The image above shows an MRI machine with a super-imposed image of a scan done on the neck.