In addition to precession, atoms also have a separate rotational axis. The RF pulse causes rotation of atoms to be in phase (i.e. rotate at similar speeds). MRI scanners use this concept to perform time weighting. T1 weighting examines the relaxation time of atoms according to their magnetic vector alignment. T2 weighting examines the loss of rotational phase as atoms return to the ground state. Water appears bright on T2- but dark on T1-weighted images.
Applied science
What hazards does the MRI pose and how are they minimised?
The MRI has little direct effect, but poses several indirect hazards.
Ferromagnetic objects are attracted to magnetic fields. Gas cylinders, hypodermic needles, etc., become dangerous projectiles within the MRI. Intracranial clips may dislodge, and pacemakers and metallic heart valves may malfunction. Classification of equipment dictates their scope of use within the MRI:
MR safe: non-magnetic and non-conductive equipment that is safe to use anywhere within the scanner room.
MR conditional: may contain MR-sensitive components, but can be used within a set proximity of the scanner. The 5-gauss line is the perimeter around the scanner where static magnetic field strength is >5 gauss, and is often the proximity limit for MR conditional devices. A 50-gauss line is also present in some scanners, although less common.
MR unsafe: should not be brought into the scanner room.
Electromagnetic induction in standard monitoring cables interferes with monitoring equipment, and may cause burns to patients. To avoid this, monitoring electrodes are made of paramagnetic material (such as nickel), and cables are encased in a Faraday’s cage, which provides shielding. The pulse oximeter uses fibre-optic cables to transmit signals to an analyser located outside the room. The cables pass through a special hole in the wall called the wave guide.
Acoustic noise produced by the MRI can be up to 85 dB, which may impair hearing and masks audible alarms. Ear protection is essential for individuals entering the room while scanning. Cooling, used to maintain magnet efficiency, may accelerate heat loss from patients leading to hypothermia. Finally, an emergency shut-down function (quenching) discharges the liquid helium into the atmosphere. If an individual were exposed to this, asphyxiation and frostbite may occur.
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