MRI magnet with reduced fringe field

Details MRI magnet with reduced fringe field MRI magnet with reduced fringe field

2000-07-06

2004-01-27

Arana, Louis (Department: 2859)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

C324S319000

Reexamination Certificate

active

06683456

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the diagnostic imaging arts. It finds particular application in controlling flux fields in conjunction with open MRI scanners and will be described with particular reference thereto. It will be appreciated, however, that the present invention is also applicable to bore and other types of magnetic systems with different flux return paths or no flux return paths.
In magnetic resonance imaging, a uniform magnetic field is created through an examination region in which a subject to be examined is disposed. With open magnetic systems, the main magnetic field is generated vertically between upper and lower pole pieces. A series of radio frequency (RF) pulses and magnetic field gradients are applied to the examination region to excite and manipulate magnetic resonances. Gradient magnetic fields are conventionally applied to encode spatial position and other information in the excited resonance. The magnetic resonance signals are then processed to generate two or three dimensional image representations of a portion of the subject in the examination region.
In an open system, the field does not only exist between the poles. There is also a flux return path through which the main magnetic field returns forming closed loops. Often, a ferrous flux return path provides a low resistance flux return path. With a ferrous flux return path, there is still a fringe magnetic field from flux that is returning through the air rather than the flux return path. For safety reasons, the examination room, and associated viewing and control rooms, are configured and shielded such that technicians are not subject to a field greater than 5 Gauss (500 &mgr;T). With conventional systems, the fringe fields have been reduced by increasing the cross section of the ferrous return path or ferrous sheathing in the walls. Depending on factors such as the size of the main magnetic field and the size of the room, the amount of ferrous shielding ranges from 10 to 200 tons of iron. Increasing the iron in the return path reduces distance from the examination region to the 5 Gauss line.
One direction in which control of the 5 Gauss line has proved elusive is in the vertical direction above and below the magnet assembly. In larger field magnets, the 5 Gauss line extends into the floor above the magnetic resonance suite (over 4 meters with a 0.5 T main field). The cost of the added iron and the structure to support it often limit installation sites to the ground floor with the floor above, if any, off limits for human occupancy.
In bore type systems, active shielding of the magnetic field reduces the fringe field. However, the active shielding technology is not particularly amenable with open systems. In bore systems, it is difficult to limit the 5 Gauss line along the axis of the bore.
The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with the present invention, a magnetic resonance apparatus includes a first magnet assembly which generates a temporally constant main magnetic field through an examination region and generates a fringe field that extends peripherally outward from the main magnetic field. At least a second magnet assembly is disposed to reshape at least a portion of the fringe field. An RF transmitter transmits radio frequency pulses to an RF coil to excite resonance in selected dipoles. An RF coil assembly receives resonance signals from the resonating dipoles. A radio frequency receiver demodulates the resonance signals and a processor processes them.
In accordance with a more limited aspect of the present invention, the second magnet assembly includes permanent magnet material.
In accordance with another aspect of the present invention, an improvement is provided in a magnetic resonance apparatus which generates a temporally constant magnetic field through an examination region and which generates a fringe field beyond the examination region. The improvement includes permanent magnets positioned to move at least portions of the fringe field closer to the examination region.
In accordance with another aspect of the present invention, a magnetic method is provided. A main magnetic field is generated through a region of interest and a fringe field is concurrently generated around the region of interest. Permanent magnets are positioned to shape and contain the fringe field.
One advantage of the present invention is that it reduces the amount of shielding needed to contain the fringe magnetic field.
Another advantage of the present invention is that it allows for open MR systems with smaller, if any, ferrous flux return paths.
Another advantage of the present invention is that it allows for a less massive MR unit that is not restricted by weight to the ground floor of buildings.
Another advantage of the present invention is that it allows for MR system integration into existing buildings rather than constructing rooms specifically designed for them.
Yet another advantage of the present invention is that it can be used with C-shaped magnets, H-shaped magnets, four-poster magnets, open systems and permanent magnets.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.


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