Images are created by the radio frequency signals emitted from hydrogen nuclei as they are flipped in and out of alignment with a powerful magnetic field by high frequency magnetic pulses.
Nuclei behave like small magnets as spin and have charge.
MRI subjects them to high magnetic field which causes them to align in 1 direction.
They are then turned in and out of alignment by a pulsed perpendicular field.
Causes precession – radio waves emitted which can be measured and used to form an image.
Field strengths are up to 1.5 Tesla.
Earths magnetic field is 1 Gauss.
1T = 10,000G.


MRI compatible.
Conventional equipment outside the magnetic field (beyond 0.5mT line).


Patients in a narrow tube for at least 20 mins (can be much longer).
Magnetic field
Ferromagnetic materials can become projectiles and also interfere with images produced.
Electric motors may fail (syringe drivers etc).
Currents are induced in electrical conductors causing equipment failures or heating and burns (eg ECG leads, pacemakers).
Implants and foreign bodies.
If distant equipment need long (10m) tubing (D or Es both function as a T-piece to prevent rebreathing.
Airway pressures measured at the machine may be inaccurate.
Delivered TV will be reduced by compliance in the system and compression of gases.
Side stream analysers will have an
time lag.
Safe beyond the 0.5mT line.