Magnetic resonance imaging (MRI)

The idea of magnetic resonance imaging (MRI) was first described in 1938 by Rabi and colleagues. Since then a large amount of work has been invested in improving the image quality and acquisition speed, making it a very common imaging technique within hospitals.

The main benefit of MRI over other techniques, such as X-Ray imaging or computer tomography (CT), lies in the fact that MRI does not use ionising radiation. Ionising radiation can cause an electron to be knocked out of its orbit around a nucleus, which can then inflict damage to the patient's DNA. The human body can repair such damage to a certain extent, however, it is advised not to be exposed too often to this kind of radiation. One should keep in mind, however, that each imaging technique has its benefit. Therefore, when the doctor decides which technique to use, he or she weighs the potential risk against the benefits, making both X-Ray and CT important imaging techniques for patient care.

MRI on the other hand does not utilise ionising radiation and can subsequently be used for relatively safe and repeatable scans, in particular at the early stages of life. In general an MRI machine is just like a sophisticated camera that is somewhat similar to those found on modern mobile phones. Unlike the camera on your mobile phone, however, the MRI machine acquires different information which comes from the inside of your body, rather than from the surface. The secret behind the images acquired lies within the name, magnetic resonance imaging.

MRI vs Camera

An MRI machine is first and foremost a gigantic magnet, which is always turned on, just like the earth's magnetic field. The MRI machine, however, can be about 100'000 times more powerful.

MRI

The trick lies in the small molecules within our body, for example hydrogen (water), which are like a compass in the earth's magnetic field. The compass needles (spins) align themselves with respect to the magnetic field inside the MRI, just like the compass needles aligns itself to the magnetic field of the earth.

A second magnetic field then tips the compass needles away from their position, which makes them start to oscillate around their preferred direction (parallel to the MRI's magnetic field). The oscillation is large at the beginning, but with time becomes smaller and smaller, ultimately bringing the compass needles back into their original position. As the oscillation becomes smaller, energy is sent out which is then detected by the MRI and makes up the image.

Taking the image takes longer than with a regular camera (usually around 30 minutes). During that time, it is important to lie as still as possible. Similar to a real camera, once the object in the picture moves, the image becomes blurry.

Blurry images

And just like a real camera, the MRI makes sounds while taking the image. Unfortunately it is a lot louder than the normal *click* sound of a camera. Therefore, when going into an MRI scanner, the patient uses ear protection. The sound an MRI scanner makes while taking the picture can be different every time, depending on what part of the body is going to be imaged. This is one example of the noise a patient might hear.