Page 250 - Biomedical Engineering and Design Handbook Volume 2, Applications

DESIGN OF MAGNETIC RESONANCE SYSTEMS 229

Display

Computer

Gradient
RF exciter Receiver
amplifiers

RF power
Preamplifier
amplifier
G X G Y G Z

RF transmit RF receive
coil coil

Magnet

FIGURE 8.1 Components comprising a MR system are illustrated.

8.2 MR MAGNET CHARACTERISTICS

Static magnetic fields of MR scanners are generated either by resistive electromagnets, permanent
magnets, or (more commonly) by superconducting magnets. Superconducting magnets are usually the
least massive. Superconducting magnets use cryogens. When superconducting magnets quench
(i.e., when they warm up and are no longer superconducting), proper venting must prevent asphyxiation
hazards for developing. In addition, mechanical design must prevent magnet damage from quenches.
Typically, the static magnetic field is parallel to the floor and aligned with the long (superior/
inferior) axis of the patient. However, there are systems where the static magnetic field is along the
anterior/posterior axis of the patient and some where the static field is along the left/right patient
axis. While patients are typically horizontal, in some magnets the patient may be vertical. Most
superconducting magnets have horizontal openings for the patient and at field strengths of 0.5 to
3 T. Most vertical magnets are permanent or resistive, though there are vertical superconducting
magnets as well. Vertical magnets currently have field strengths up to 0.7 T.
Magnetic fringe fields from large, strong, unshielded magnets used in MR could require large
areas to accommodate siting. To alleviate this problem, passive shielding can be achieved using
ferromagnetic materials arranged as numerically determined. Often, magnets are actively shielded
(sometimes in addition to some passive shielding). Bucking coils that oppose the static magnetic
field are added to increase the rate the static magnetic field diminishes with distance. Actively
shielded magnets decrease siting costs.
Many superconducting magnets employ recirculation devices to prevent loss of cryogens. Such
systems have lower operating costs.