Teensy Bit of Physics
Before you run away, I know. Physics = arghhh. But, unfortunately, a very basic understanding of the physics is sort of essential to understanding just how we get those pretty pictures on MRI.
Luckily, we can break it down into easy to understand concepts and give you the bare bones you need to be able to understand MRI a bit better. It’s all for your own good!
It's all about magnets
The most abundant molecule in the human body is hydrogen which consists of a single proton and neutron within the nucleus. A hydrogen atom can act like a small magnet with a positive and negative pole. Each hydrogen atom spins around its axis. An MRI scanner works by manipulating these hydrogen atoms with superimposed magnetic fields. The way that the hydrogen atoms react depends on the field applied and the surrounding environment and the reaction of the atoms is returned as electromagnetic waves that comprise the signal received. In this way, we can make different materials return different signals based on the way the hydrogen atoms react to the applied magnetic fields.
An MRI machine is basically a very strong magnet that is always on. This is why it is never a good idea, and sometimes even fatal, to enter an MRI room with anything metallic. The superimposed magnetic fields are then applied based on the sequence we want to run.
This explains why there are so many artifacts in MRI, especially related to metal. Metal alters the local magnetic field interfering with the signal the hydrogen atoms return. It also explains why we have to be careful with metallic implants. The applied magnetic fields can cause the metal to react, move and heat up. For large fixed implants, such as hip replacements, this isn’t such a problem. But for small foreign bodies, such as metal in the eye, this can cause serious complications.
If you would like more information on the physics behind MRI you can find it here.