In an MRI equipment the stray magnetic field outside the bore of the main magnet is called fringe field. These fields are created by magnetic flux lines and the number of flux lines per unit area is called magnetic flux density. The flux density is higher at the magnetic isocenter and the density decreases as we move away from the isocenter of the magnet.
Magnetic Isocenter is the centre of the bore of the magnet in three planes (x,y,z). The magnetic field homogeneity is measured in the isocenter of the magnet. Magnetic field homogeneity is very important for clinical magnetic resonance imaging. Shimming is the process whereby the evenness of the magnetic field is optimized. Shim coils are used to make the magnetic field more homogeneous.
In a superconductive magnet the flux lines run parallel to the bore of the magnet and it is called main magnetic field. Main magnetic field is indicated by B0.
The strength of the magnetic field is measured in gauss(G) or tesla (T). Tesla is the SI unit of magnetic flux density. It was defined in 1960 in honor of inventor Nikola Tesla. The cgs unit of measurement of magnetic field is gauss . It is named after the German mathematician and physicist Carl Friedrich Gauss . One Tesla = 10,000 gauss.
Fringe field should be contained within the MRI scanning room. The typical requirement is to contain the 5G(0.5mT) flex line within the walls and ceiling of the scanning room. The stray field outside the scan room can affect patient monitoring devices and medical implants.
Fringe field is contained within the specification by shielding the flex lines.
There are two types of magnet shielding, active and passive shielding. Passive shielding is done using steel plates in the wall and ceiling. This will help the flux lines to contain within the room. An actively shielded magnet uses pairs of coils called bucking coils with equal but opposite current to the main magnetic field coils. Bucking coils counter the main field and the mechanical structure within the cryostat becomes more complex.
Effective magnet shielding in essential to prevent stray field extending outside the scan room. Shielding will also help to reduce the MR siting cost. Moving heavy metal objects may interfere with the main magnetic field if the shielding is not appropriate. Before any MRI installation an experienced engineer should conduct a site survey to find out the shielding requirement .
