Keevil, who was part of the lobbying process, says a working party is currently trying to re-word the directive in such a way that MRI will not be affected. If they don’t succeed, we could be forced back to poorer quality imaging of soft tissues using harmful ionising radiation, which surely cannot be in anyone’s interest.
How MRI works
MRI scanners contain either permanent magnets or electromagnets (some of which are superconducting) typically able to produce a magnetic field of 1.5 tesla, which is 30,000 times greater than the Earth’s magnetic field. The human body is almost 65 per cent water, and MRI images are obtained from the proton within each hydrogen atom in this water.
Each of these protons spins about an imaginary axis like a spinning top, which makes the proton magnetic. The proton spins points in random directions until a body is placed in a scanner. Under the influence of the high magnetic field created by the scanner magnet, the proton spins align either parallel or anti-parallel to the field. Slightly more protons position themselves parallel to the field, creating a net magnetisation.
A radio frequency pulse (a time-varying magnetic field) is directed at the area of the body that is to be studied. This changes the magnetisation of the protons in that region. Once this pulse is switched off, the protons relax into the state they were in before it was turned on.
A conducting coil works as an antenna and detects – via electromagnetic induction – the small changes in magnetic field that the relaxation produces. As the type of tissue surrounding a proton influences its behaviour after the radio frequency pulse, computer software can analyse the detected signals and build up a detailed image of the body that shows different types of tissue. Graphical processors, originally developed by the computer gaming industry, are now used by some MRI systems to reconstruct and display images quickly.
© IET. Reproduced with permission.