Magnetic resonance imaging (MRI) is a powerful imaging technique used to investigate the body. MRI scanners use very strong magnetic fields and radio waves, which interact with protons in tissues to create a signal that is then processed to form images of the body. The protons (hydrogen atoms) can be thought of as tiny bar magnets, with a north pole and a south pole, spinning on an axis – like a planet. Normally, the protons are randomly aligned, but when a strong magnetic field is applied, the protons will align with this field.
Applying a radio wave pulse at the correct frequency excites the protons, causing them to resonate and disturbing the magnetic alignment. The excited protons release the absorbed energy as a radio frequency signal and the emission is picked up by a receiver coil in the scanner. The radio frequency that causes the protons to resonate depends on the strength of the magnetic field. In an MRI scanner, gradient electric coils are used to vary the magnetic field strength across the body. This means that different sections of the body will resonate at different frequencies. So by applying different frequencies in sequence, you can image slices of the body separately and gradually build up a picture.
When the radio source is switched off, the protons will return to their original undisturbed state (aligned with the magnetic field), emitting radio waves as they do so which are picked up by the receiver coils. Different tissues will relax at different rates, for example fat and water have different relaxation times, so the relaxation time can reveal the type of tissue being imaged. There are two relaxation times which can be measured; T1 – the time taken for the magnetic alignment to relax and T2 – the time taken for the spin to return to its resting state.
Multiple radio pulse sequences can be used to highlight or suppress certain tissue types. For example, abnormalities are not usually found within fat, so a fat suppression pulse sequence can be used to remove the signal emitted by fatty tissues, leaving just the signal from areas which are more likely to contain irregularities.
MRI scanners need incredibly strong magnetic fields; generally around 1.5 Tesla, but they can be as large as 7 Tesla. For comparison, the Earth’s magnetic field is only 0.00005 Tesla. The magnet is composed of multiple coils of conductive wire through which a current is passed to generate the magnetic field. To achieve the high field strengths required, the magnet is cooled with liquid helium to below 10 Kelvin (-442oF / -263oC). This enables superconductivity, allowing current to flow through the coils without creating electrical resistance, meaning that when a magnet is super cooled, it is capable of conducting larger currents and therefore able to produce stronger magnetic fields.
MRI was invented by Paul Lauterbur in 1971 at Stony Brook University, Long Island. The technique was then developed by Sir Peter Mansfield and the first MRI body scan of a human being was produced in 1977. Although it wasn’t until the 1980’s that the first MRI scanner capable of creating clinically useful images was produced. This machine was designed by John Mallard, who is credited for the widespread introduction of MRI, and was used to identify several conditions afflicting a test patient, including a tumor in his chest, an abnormal liver and bone cancer. The ‘discoveries concerning magnetic resonance imaging’ won Paul Lauterbur and Sir Peter Mansfield the 2003 Nobel Prize in Physiology or Medicine.
MRI is widely used in medical diagnostics and unlike X-rays and CT scans, has the great advantage of not exposing the subject to ionizing radiation. However, the effects of the high magnetic fields on the body are still unknown. MRI scanners are particularly good for neurological scanning and are excellent for visualizing small tumors, dementia, epilepsy and other conditions of the central nervous system. A scan can take between 15 and 90 minutes, depending on the size of the area and how many images are taken. The machines are incredibly noisy and can produce sounds as loud as those generated by a jet engine.
MRI scanners can be very dangerous and strict safety procedures have to be followed in the vicinity of these machines, as several fatalities have been recorded. Due to the strong magnetic fields involved, this equipment cannot be used on patients who have pacemakers which could be disrupted, or metal implants or shrapnel which could be heated and moved by the magnets during the procedure. Furthermore, ferromagnetic objects can be strongly attracted to the magnets and pose a serious projectile risk. For this reason, such objects are banned from close proximity to the scanners.