Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-02-24
2001-11-13
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06316941
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical diagnostic imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention may find further application in quality control inspections, spectroscopy, and the like.
Conventionally, magnetic resonance systems generate a strong, temporally constant main magnetic field, commonly denoted Bo, in a free space or bore of a magnet. This main magnetic field polarizes the nuclear spin system of an object. Nuclear spins of the object then possess a macroscopic magnetic moment vector preferentially aligned with the direction of the main magnetic field. In a superconducting annular magnet, the B
0
magnetic field is generated along the longitudinal axis of the cylindrical bore, which is assigned to be the z-axis. In an open system, the B
0
magnetic field is oriented vertically between a pair of pole pieces, which is assigned to be the z-axis.
To generate a magnetic resonance signal, the polarized spin system is excited at resonance by applying a radio frequency (RF) magnetic field B
1
, with a vector component perpendicular to that of the B
0
field. In a transmission mode, the radio frequency coil is pulsed to tip the magnetization of the polarized sample away from the z-axis. As the magnetization precesses around the z-axis, the precessing magnetic moment generates a magnetic resonance signal at the Lamor frequency which is received by the same or another radio frequency coil in a reception mode.
Birdcage coils are often used to excite and/or receive magnetic resonance signals, especially in horizontal field or bore-type MRI systems, because of the good B
1
, uniformity over a large field of view. In bore-type systems, the axis of the birdcage coil is typically aligned with the z-axis and the resident resonant current that generates the circularly polarized B
1
field is sampled as an orthogonal pair of transverse modes.
However, aligning the coil axis with the B
0
, field in a bore-type machine can be problematic. Often, patients experience feelings of claustrophobia during head imaging applications due to the proximity of the birdcage coil to the patient's face. In addition, aligning the coil axis with the horizontal B
0
field hampers fMRI applications, which require additional space near the patient's face for devices to stimulate the visual senses.
The present invention contemplates a new and improved radio frequency birdcage coil which overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a magnetic resonance apparatus includes a main magnet which generates a temporally constant and substantially uniform main magnetic field along a main magnetic field axis through an examination region. A radio frequency (RF) transmitter and an RF volume coil assembly perform at least one of exciting magnetic resonance dipoles within the examination region and receiving magnetic resonance signals from the resonating dipoles. A receiver receives and demodulates the magnetic resonance signals. The RF volume coil assembly includes a pair of end rings interrupted by a plurality of reactive elements. The end rings are disposed in displaced parallel planes along a coil axis which is orthogonal to the main magnetic field axis. A plurality of spaced rungs electrically interconnect the end rings to define a generally cylindrical volume. The end rings and rungs are supported by a dielectric former which has separable portions. The RF volume coil further comprises a conductive loop which is inductively coupled to the end rings.
In accordance with another aspect of the present invention, a method for magnetic resonance imaging of a portion of a patient disposed along a subject axis includes opening an RF volume coil having a coil axis along a hinged portion parallel to the coil axis. The method further includes introducing a portion of the subject to be examined into the RF volume coil and closing the hinged RF volume coil to enclose the portion of the subject to be examined within the coil. An examined portion of the subject is positioned in an examination region with the coil axis orthogonal to the subject axis and a main magnetic field is generated parallel to and along the subject axis through the examination region. RF signals are transmitted into the examined subject portion to induce magnetic resonance in nuclei and magnetic resonance signals are received using the RF volume coil. At least a first resonant mode signal and a second resonant mode signal are extracted from the RF volume coil and reconstructed into an image representation.
In accordance with another aspect of the present invention, a pull-apart radio frequency birdcage coil includes a pair of conductive end rings snapped together at a detachment point by conductive fasteners. The end rings are disposed in parallel planes along a coil axis. A plurality of spaced rungs electrically interconnect the end rings, both of which are mounted onto a former having an opening to receive a portion of a subject to be examined. A loop conductor is releasably attached to the former and slidably disposed along a coil axis to adjust the inductive coupling with the end rings. A first sampling port electrically connected to the loop conductor samples a first resonant mode directed along the coil axis. A second sampling port electrically connected to one end ring samples a second resonant mode orthogonal to the coil axis, such that the RF birdcage coil operates as a quadrature coil.
One advantage of the present invention is that it reduces patient claustrophobia.
Another advantage of the present invention is that it provides quadrature capability using an end-ring mode and sinusoidal mode.
Another advantage of the present invention resides in its applicability to fMRI applications.
Yet another advantage of the present invention resides in a full volume RF coil offering good B
1
uniformity regardless of B
0
field orientation.
Other 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.
REFERENCES:
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patent: 5075624 (1991-12-01), Bezjak
patent: 5144240 (1992-09-01), Mehdizaddeh et al.
patent: 5274332 (1993-12-01), Jaskolski et al.
patent: 6100691 (2000-08-01), Yeung
“An Efficient, Highly Homogeneous Radiofrequency Coil for Whole-Body NMR Imaging at 1.5 T” by Cecil E. Hayes, et al.
Journal of Magnetic Resonance 63, 622-628 (1985). Copyright 1985 by Academic Press, Inc.
“Electromagnetic Analysis and Design in Magnetic Resonance Imaging” by Jianming Jin.
Dept. Of Electrical and Computer Engineering, Univ. Of Ill. At Urbana—Champaign, Urbana, IL. Copyright 1999 by CRC Press LLC.
“The Theory of the Bird-Cage Resonator” by James Tropp.
Journal of Magnetic Resonance 82, 51-62 (1989). Copyright 1989 by Academic Press, Inc.
U.S. application No. 09/398,650, Fujita et al., filed Sep. 1999.
Braum William O.
DeMeester Gordon D.
Fujita Hiroyuki
Arana Louis
Fay Sharpe Fagan Minnich & McKee LLP
Marconi Medical Systems, Inc.
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