A patient is placed in the bore of the MRI machine. The convention of the axes are shown below. These are the same axes as will be used throughout the MR notes.
There are several components to an MRI machine.
1. Superconducting electromagnet
It is possible to use a permanent magnet, for example the bar magnets or fridge magnets, for creating the magnetic field necessary in a MR machine. However, to generate the field strength required, it would have to be massive. So instead we use a superconducting electromagnet. This is a magnet created by coils with an electric current running through it that then creates an magnetic field in the Z direction. This superconducting electromagnet is responsible for the main permanent magnetic field (B0) and weighs approx. 6 tonnes. It is always on.
The constant electric current generates a lot of heat, which is cooled down by surrounding liquid helium (-269 degrees C). This also serves to reduce the resistance to zero as the current is transmitted through the coils.
The majority of electromagnets create a magnetic field strength of 1.5 Tesla (T) although newer machines generate magnetic fields of 3T. 1 Tesla = 10,000 gauss and the Earth's magnetic field is approx. 0.5 gauss (i.e. a 3 Tesla machine has a magnetic force of 60,000 times that of Earth).
2. Shim coils (not shown)
These lie just inside of the outer main magnet and are used to fine-tune the main magnetic field to ensure it is as uniform as possible.
3. Gradient coils
There are three sets of gradient coils orientated in the x, y and z axes used to alter the gradient of the magnetic field (the reason for this will become clear when studying "Spatial Encoding"). The coils are switched on and off rapidly, in 1 ms or less, and it is this that creates the loud noise.
4. RF (radiofrequency) coils
These coils are tuned to a particular frequency. They produce a magnetic field at right angles (XY plane) to the main magnetic field and also receive the MR signals being produced. To maximise the signal the coils have to be placed as close to the part being imaged as possible. There are several types of RF coils:
- Standard body coil (transmit and receive): permanent part of the scanner. Used to image large parts of the body
- Head coil (transmit and receive): incorporated into a helmet and used for head scans
- Surface (or local) coils (receive only): these are small coils applied as close to the area being imaged as possible e.g. arm coils, leg, orbits, lumbar spine coils etc.
- Phased array coils: multiple receiver coils that receive the signals individually but are then combined to improve the signal-to-noise ratio
- Transmit phased array coils
Now that we've covered the basics of the MR machine, we can go on to the introduction of MR physics.