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| MR-System | East Tips |
Magnetic Field Homogeneity
The magnetic field should be uniform all over its extent to get correct information (signal) from the
patient. Even though the magnetic field is more or less uniform there might be minor
inhomogeneity. The process is necessary as the magnetic field is prone to become
inhomogeneous because of the difficulty of winding a perfect coil and the presence of metal
within the environment. Shimming can be active or passive. Passive shimming is done by
keeping metal pieces, called shim plates, in the field to oppose the inhomogeneity. Active
shimming is done by passing a current through the gradient coils, which generate small
magnetic field gradients superimposed on the main magnetic field (B0). These coils are called
shim coils. Shim coils make the field homogeneous by adding from the field at desired
points. Shim coils can be resistive windings located within the room temperature
compartment of the magnet bore or superconducting windings located within the helium
vessel. Homogeneity is expressed as parts per million (ppm). Homogeneity of 10 ppm may be
sufficient for routine spin-echo imaging. However, for proton spectroscopy, a highly
homogeneous field of 0.1 ppm is required to be able to detect metabolites with smaller
chemical shift differences.
Shielding
The stray magnetic field outside the bore of the magnet is known as the fringe field. This stray
field can cross conventional walls, floors, or ceilings and can potentially be harmful to patients
with pacemakers, monitoring devices, and other magnetically activated devices present
nearby. Fringe field and its effects can be prevented by either passive shielding or active
shielding. Passive shielding is done by lining the wall of the MR room with steel or copper. This
the shielded chamber is called a ‘Faraday cage’.Active shielding uses an additional solenoid
magnet outside the cryogen bath that restricts the Bo lines to an acceptable
location. However, active shielding is an expensive alternative.
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