MRI | Summary

Purpose:
The MRI is a very accurate diagnostic instrument it is usually used for the following diagnostic tests:
Diagnosing multiple sclerosis.
Diagnosing tumors of the pituitary gland and brain.
Diagnosing infections in the brain, spine or joints.
Visualizing torn ligaments in the wrist, knee and ankle.
Visualizing shoulder injuries.
Diagnosing tendonitis.
Evaluating masses in the soft tissues of the body.
Evaluating bone tumors, cysts and bulging or herniated discs in the spine.
Diagnosing strokes in their earliest stages.

Description of the instrument:
The biggest and most important component in an MRI system is the magnet.
There are three basic types of magnets used in MRI systems:
Resistive magnets: consist of many windings or coils of wire wrapped around a cylinder or bore through which an electric current is passed. This causes a magnetic field to be generated. It is the cheapest magnet to buy but because the resistance of the wires it requires a huge amount of electricity to operate it. And its have a low magnetic field about 0.3 tesla, more then that will take too much electricity.

A permanent magnet: Its magnetic field is always there and always on full strength, so it costs nothing to maintain the field. The major drawback is that these magnets are extremely heavy: They weigh many, many tons so it is complicated and expensive to build them. Its also have a low magnetic field about 0.4 tesla more then that it will be to big and heavy.

Superconducting magnets: the most commonly used. It is similar to the resistive magnet, but the difference is that the wire is cooled by liquid helium, that brings to a huge reduce in the resistance of the wire, so we don’t need so much electricity. It is expensive but we can reach 2 tesla with it.

RF transmitter: RF pulses are usually applied through a coil. MRI machines come with many different coils designed for different parts of the body: knees, shoulders, wrists, heads, necks and so on. They also collect the energy release of the hydrogen atoms and send it to the computer.

Computer system: receives a mathematical data that is converted, through the use of a Fourier transform, into a picture that we can put on film. That is the "imaging" part of MRI.





Scientific principle
Usually the atoms are randomly spinning in every direction. When a magnetic field is applied all of the hydrogen protons will align with the magnetic field in one direction or the other. The vast majority cancels each other out, but few from millions of the atoms will not cancel each other. We concern only about the hydrogen atoms because they have a large magnetic momentum so they have a strong tendency to line up with the direction of the magnetic field.
The machine applies an RF pulse toward the examined area in the body, the few hydrogen atoms (that do not had another atom that will cancel them) absorb the energy and begins to spin in another direction and in a specific frequency.
When the RF pulse is turned off, the hydrogen protons begin to slowly (relatively speaking) return to their natural alignment within the magnetic field and release their excess stored energy. When they do this, they give off a signal that the coil now picks up and sends to the computer system.

Operation:
There is a horizontal tube running through the magnet from front to back. This tube is known as the bore of the magnet. The patient, lying on his back, slides into the bore on a special table. Whether or not the patient goes in head first or feet first, as well as how far in the magnet they will go, is determined by the type of exam to be performed. Once the body part to be scanned is in the exact center of the magnetic field, the scan can begin. The test can take more then 20 minutes.

Safety features:
An MRI scan is a painless radiology technique, which has the advantage of avoiding x-ray radiation exposure. There are no known side effects of an MRI scan.
Patients who have any metallic materials within the body must notify to their doctor before the examination. Metallic chips, materials, surgical clips, or foreign material (artificial joints, metallic bone plates, or prosthetic devices, etc.) can significantly distort the images obtained by the MRI scanner. Patients who have heart pacemakers, metal implants, or metal chips or clips in or around the eyeballs cannot be scanned with an MRI because of the risk that the magnet may move the metal in these areas. Similarly, patients with artificial heart valves, metallic ear implants, bullet fragments, and chemotherapy or insulin pumps should not have MRI scanning.
Some people can experience a claustrophobic sensation during the procedure. Therefore, patients with any history of claustrophobia should notify the MRI stuff.






1. http://www.medicinenet.com/MRI_Scan/
2. http://electronics.howstuffworks.com/mri.htm