MRI Fringe Fields and Shielding

Fringe Fields and Shielding

Walls, floors and ceilings cannot contain static magnetic fields. The stray magnetic field outside the bore of the magnet is known as the fringe field. All magnets have a fringe field to some extent and these fields must be taken into account when installing a magnet.

Fringe fields can be compensated for by the use of magnetic field shielding which may be active or passive. Passive shielding is the more expensive alternative using iron plates to restrict the field lines. Some manufacturers offer actively shielded magnets that reduce the fringe field to about 30m². Active shielding partially cancels the field outside the main magnet coils thus reducing the magnitude of the fringe field. The 0.5mT isomagnetic line is taken as the critical cut-off limit.

Superconducting Magnet

Superconducting Magnet

The main components of an MRI system are the superconducting magnet, the gradient system, the RF system and the computer system. The magnet produces a strong, static field and the radiofrequency transmit and receive coils excite and detect the MR signal. The magnetic field gradients localise the MR signal and the computer system facilitates scanner control, image display and archiving. This chapter will describe each of these components in turn. Figure 1-1 below shows the general layout of an MRI system.

 

MRI Magnet Types

Magnet Types

The magnet is the main component of any MR system and there are four different types of magnets capable of MRI:

·         Superconducting magnets

·         Air-cored resistive magnets

·         Iron-cored electromagnets

·         Permanent magnets

 
Superconducting magnets are by far the most common type and the three main manufacturers are Siemens, GE and Philips.

Definition of MRI

Definition of MRI

Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualize internal structures of the body in detail. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body.
An MRI scanner is a device in which the patient lies within a large, powerful magnet where the magnetic field is used to align the magnetization of some atomic nuclei in the body, and radio frequency magnetic fields are applied to systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field detectable by the scanner—and this information is recorded to construct an image of the scanned area of the body. Magnetic field gradients cause nuclei at different locations to process at different speeds, which allows spatial information to be recovered using Fourier analysis of the measured signal. By using gradients in different directions 2D images or 3D volumes can be obtained in any arbitrary orientation.

 

MRI provides good contrast between the different soft tissues of the body, which makes it especially useful in imaging the brain, muscles, the heart, and cancers compared with other medical imaging techniques such as  computed tomography (CT) or X-rays. Unlike CT scans or traditional X-rays, MRI does not use ionizing radiation.