MRI - How it works
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.