Site hosted by Angelfire.com: Build your free website today!

MRI - How it works

MRI introduction MRI History MRI how it works MRI patient info

MRI does not use x-rays to form images of the body instead it relies on magnetism and the use radio waves. Below is a step by step account of how an image of the body is produced by MR scanning.

Step1
The MRI scanner is a large cylinder shaped machine, which contains a large and very powerful magnet, 10,000 – 30,000 times stronger than the earth’s magnetic field. The patient is inserted into the scanner using a moveable bed.

Step 2
The body contains hydrogen atoms, mostly in water and fat molecules. The MRI scanner uses the magnet to create a magnetic field to align the hydrogen atoms. The hydrogen atoms, which are small magnets themselves, line up with the magnetic field, as a compass aligns itself with the earth’s magnetic field.

Step 3
Next the MRI scanner directs pulses of radio waves of a particular radiofrequency at the patient. The radio waves knock the hydrogen atoms out of alignment.

Step 4
The hydrogen atoms then re-align themselves with the magnetic field. As they do so they emit radio signals (the earlier absorbed radio waves).

Step 5
Receivers in the MRI scanner pick up these radio signals and send the information to the computer.

Step 6
The computer analyses the information and produces a picture based on the location and strength of the incoming signals.

Step 7
The MRI image shows differences in the water content and distribution between tissues, as water is the most abundant source of hydrogen atoms in the body. Tissues that emit strong MR signals appear white in MRI scans for example fatty tissues, whereas those that emit little or no signal appear black, for example bone.

Depending upon which tissue or part of the body is under investigation, it is possible to alter how fast the pulses of radio waves are emitted to optimize the MRI image produced. MRI scanning is very useful, as it is possible to take cross-sectional images of the body in any plane.
 

MRI showing two slices of the brain


MRI is used to visualise soft tissues. It produces extremely detailed pictures and is mostly the choice investigation to view the brain and spinal cord. It is also used frequently to view joint problems and to image the heart and blood vessels.
 

MRI CT Refrences