Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1998-08-28
2001-04-17
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
Reexamination Certificate
active
06218838
ABSTRACT:
BACKGROUND OF THE INVENTION
The present application relates to the magnetic resonance arts. It finds particular application in conjunction with diagnostic imaging at surgical sites and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application in other magnetic imaging, spectroscopy, and therapy applications.
Early magnetic resonance imaging systems were based on solenoid magnets. That is, a series of annular magnets were placed around a bore through which a magnetic field was generated longitudinally. A patient was selectively moved axially along a horizontal central axis of the bore to be positioned for imaging. Magnetic resonance imaging systems with solenoid magnets tended to be claustrophobic to the patient. Moreover, access to the patient for surgical, minimally invasive procedures, physiological tests, equipment, and the like was limited and awkward.
To provide for patient access and reduce the claustrophobic effect in patients, open or vertical field magnets have been devised. Open magnets typically include a ferrous flux return path in the form of a “C”, “H”, or four-poster arrangement. The flux return paths have an open gap within which the patient is disposed for imaging. Due to the difference in the susceptibility of the flux return path and the air in the patient gap, there tends to be non-linearity and other magnetic flux errors in the patient receiving gap. In order to generate a more uniform magnetic flux field through the gap, a large ferrous pole piece is typically positioned at the ends of the flux return path on either side of the patient receiving gap. The pole pieces are shaped and contoured, as appropriate, to generate a more uniform magnetic flux between the pole pieces. Typically, a heavy ferrous ring, known as a Rose ring, is positioned along the circumference of the pole piece to drive the magnetic flux towards the center of the pole piece and the patient receiving gap.
Although the use of pole pieces has been successful, there are drawbacks. First, in magnetic resonance imaging, magnetic field gradients are generated across the imaging volume. The gradient coils are positioned between the pole pieces and the patient. When the gradient coils include shield coils, the space occupied by the self-shielded gradient coils is even larger. The physical space occupied by the gradient coils exasperates the tradeoff between the desire to have a large patient receiving gap for better patient access and less claustrophobia, and the desire to position the pole pieces closer together for a more uniform magnetic field. Second, the pole pieces are typically thick ferrous disks with a diameter about 2-3 times the height of the patient receiving gap. The massive metal pole pieces raise difficult engineering design problems to provide for their stable support with a minimal blocking of patient access.
This application provides a new and improved magnetic resonance imaging system which overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with the present invention, a magnetic resonance imaging system includes a pair of ferrous, Rose rings disposed parallel to each other on opposite sides of an imaging volume. A magnetic flux return path extends from a point adjacent one of the Rose rings, remotely around the imaging volume, to a point adjacent the other Rose ring. A magnetic flux source causes a magnetic flux through the imaging volume, between the rings, and through the magnetic flux return path. A plurality of magnetized rings are mounted in a location surrounded by the ferrous Rose rings to modify the magnetic flux passing through the imaging volume.
In accordance with another aspect of the present invention, a magnetic resonance system includes a source of magnetic flux. At least one annular, ferrous Rose ring concentrates the magnetic flux across an imaging volume. A plurality of permanent magnets are disposed in the magnetic flux adjacent the imaging volume to adjust a magnetic flux density in the imaging volume.
In accordance with a more limited aspect of the present invention, physical structures associated with the magnetic flux source, the Rose rings, the flux return path, and the permanent magnets cooperatively interact to optimize uniformity of the magnetic flux in the imaging volume.
One advantage of the present invention is that it facilitates the design of open magnets with stronger magnetic fields.
Another advantage of the present invention is that it improves magnetic field homogeneity.
Another advantage of the present invention resides in improved patient access.
Another advantage of the present invention resides in a reduction of potential eddy currents.
Yet another advantage of the present invention resides in the reduced pole mass.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.
REFERENCES:
patent: 4710741 (1987-12-01), McGinley
patent: 5162768 (1992-11-01), McDougall et al.
patent: 5436607 (1995-07-01), Chari et al.
patent: 5550472 (1996-08-01), Richard et al.
patent: 5647361 (1997-07-01), Damadian
DeMeester Gordon D.
Kinanen Ilmari
McGinley John V. M.
Young Ian R.
Fay Sharpe Fagan Minnich & McKee LLP
Oda Christine
Picker International Inc.
Vargas Dixomara
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