A patient is placed in the bore of the MRI machine. The convention of the axes is shown above. These are the same axes as will be used throughout the MR notes.
There are several components to an MRI machine.
1. Superconducting electromagnet
A magnetic field, such as that created by a permanent magnet e.g. a bar magnet or fridge magnet you may have at home, is necessary for an MR machine to function. The problem is that for a bar magnet to create 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 a 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 and liquid helium (-269°C) is used to cool down the system. The helium also serves to reduce the resistance to the current being transmitted through the coils to zero.
The majority of electromagnets create a magnetic field strength of 1.5 Tesla (T) or 3T. Some newer machines can generate fields of 7T. 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 reading about “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, 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.