A typical MRI scanner is a complex system that utilizes advanced digital and analog electronics and expensive magnet technology. A simplified block diagram of a typical system is provided in Fig. 14. The system is centered around a computer system that interacts with a purposefully designed system controller. The system controller is responsible for coordinating each of the subsystems to produce pulse sequences and to acquire the MRI signal. The gradient system controller creates magnetic field gradient pulses by application of direct electric current to a set of gradient coils that are positioned around the subject within the magnet. The RF transmitter system is essentially a radio transmitter that is responsible for creating the RF pulses by appropriate application of alternating electric current to the RF coil. The RF receiver system is responsible for detecting the FID or spin echo signals induced in the RF coil by the movement of the magnetization.
The magnet is a relatively expensive system component. It often accounts for a large fraction ofthe system cost. The majority of current MRI systems use superconducting magnet technology because this provides adequate field strength, superior field homogeneity, and field stability with minimal operating costs. Despite the popularity of superconducting MRI magnets, permanent magnet technologies and conventional electromagnet technologies are also used in some commercial MRI scanners. The requirement for
Figure 14 MRI scanner system design.
a strong magnetic field within the head necessitates that the subject be ''inside'' the magnet aperture (or bore). In addition, the subject's head must be placed within a RF coil, and this must be placed within a set of magnetic field gradient coils. Therefore, the RF coils and the gradient coils must also be placed within the magnet aperture (Fig. 15). In order to perform an imaging examination, the subject lies on the bed. The bed is raised to the level of the RF coil, and the RF coil is placed around the head. The subject is then moved into the magnet so the head is located approximately 1 m within the magnet bore.
mean that effects do not exist. The current negative findings, however, do suggest that any possible health effects are subtle at best. Because of the possibility of unknown effects on human development, MRI is generally not used during pregnancy.
The MRI environment does present some hazards. Probably the greatest potential hazard is being struck by a magnetic object while in the MRI scanner. MRI magnets create a fringe field around them that is capable of levitating and attracting (at great speed) iron and steel objects. The majority of MRI magnets use superconducting technology and these magnets are
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