In which I describe the components of an MRI scanner...
So following on from last week – other than a lot of money to pay for it all*, what do you need for an operational MRI scanner?
*Scanners are often upward of several million pounds.
A Large Magnet
In order to make a significant number of the spins in your body align in one direction a very powerful magnet is needed. The strength of these magnets is measured by a large unit called the Tesla (symbol: T). To get an appreciation of how strong that is there is another unit called the gauss which is also used to measure magnetic fields. 1 Tesla = 10,000 gauss and a normal bar magnet is around 100 gauss.
The majority of MRI systems in hospitals and research institutions are superconducting magnets. A superconducting magnet is one where the material has been cooled to a very low temperature (4.2 Kelvin or -268.95°C). Here the material has no resistance and a massive current can be run through it generating a very large magnetic field. Many of these superconducting magnets are made of niobium-titanium. These magnets are very expensive and the magnet is the main cost in an MRI system.
The gradient coils are used to generate slight variations in the magnetic field of the scanner. These variations in the x, y & z directions allow for three dimensional imaging by encoding location information into the signal†.
They can also be used to compensate for slight inconsistencies in the static field the magnet produces. This process, known as shimming, improves image quality.
†More on this in a few weeks.
The radiofrequency (RF) coil (or coils) transmit and/or receive an RF pulse into/from the patient. A transmit pulse causes the spins in the patient to move from equilibrium and as they relax back they emit a signal which is detected by the coil.
Besides the magnet which needs to be kept at a low temperature using liquid helium the gradient coils also need to be cooled. The constant switching of current in the coils causes them to heat up and break if they are not cooled. This is usually done using a water based liquid cooling system.
Control Station & Software
The scanner needs to be controlled by a computer which will run manufacturer specific software. The workstation is where the MR technician/scientist will select the scans to be run and where the data will be reconstructed and stored. Research scanners come with many more options.
A Large Room and/or Shielding
The strong magnetic field spreads out from the magnet and will attract ferrous‡ objects as well as affecting electronics/credit cards etc. One solution to this problem is just to place the magnet in a very large room bigger than the stray field and warn everyone that there is a strong magnetic field in there. For larger bore scanners though the stray field can be very large and shielding is used to reduce it. Active shielding is built into the magnet itself and reduces the stray field outside the bore. Passive shielding, i.e. placing the magnet in a faraday cage room, just stops the stray field extending outside the room.