Quadrature RF coil for vertical field MRI systems

Details Quadrature RF coil for vertical field MRI systems Quadrature RF coil for vertical field MRI systems

1999-12-28

2002-06-11

Williams, Hezron (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

C324S307000, C324S309000

Reexamination Certificate

active

06404199

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with magnetic resonance imaging and open magnetic resonance imaging systems and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with other magnetic resonance imaging and spectroscopy systems, particularly those in which the direction of the main magnetic field is in the plane of the radio frequency coils.
Conventionally, magnetic resonance imaging procedures include disposing the patient in a substantially uniform, temporally constant main magnetic field B
0
. The resulting magnetization of the sample is manipulated with radio frequency magnetic fields which are applied to the examination region so as to resonate polarized dipoles and produce magnetic resonance signals. The signals are received and used to produce images or spectra from the sample.
The direction of the main magnetic field in a high field system is generated along the central bore of an annular magnet assembly, i.e., the B
0
field aligns with the central axis of the patient. In bore-type systems, ladder RF coils have been utilized for quadrature detection. For such systems, U.S. Pat. No. 5,898,306 by H. Liu and C. Truwit describes a ladder resonator quadrature surface coil which is a low-pass configuration. The coil consists of uniformly spaced axial conductive elements, rungs, sandwiched between two conductive end elements. The low-pass ladder coil described generates quadrature magnetic RF fields B
1
from two resonant modes of the structure. One of the B
1
fields is oriented perpendicular to the plane of the coil and the other is oriented in the direction of the end conductors, perpendicular to the rungs. By properly orienting the coil in the plane of the patient, both modes generate B
1
fields perpendicular to the B
0
field. In this orientation, the coil receives and transmits resonant RF signals in quadrature.
The Liu coil describes rung elements which are interrupted by a plurality of capacitive elements arranged in a symmetric low-pass configuration. The coil detects both even and odd current modes in the lower resonant frequencies. The tuning capacitor values are often quite small—on the order of a picofarad. This results in coil tuning instability.
In addition, a symmetrical degenerate band-pass configuration has been described by Boskamp, et al. Tuning of such a band-pass coil is achieved by adjusting capacitive elements in both the rung and end conductors. This tuning process tends to be time-consuming, hence expensive.
However, not all magnetic resonance systems employ a horizontal B
0
magnetic field. Vertical field or open magnetic resonance imaging systems typically include a pair of parallel disposed pole pieces, which are often interconnected by a ferrous flux return path. Electrical coils for inducing the vertical main magnetic field are disposed along the flux return path or at the poles. Typically, the pole pieces are positioned horizontally such that a vertical field is created therebetween. Many advantages are realized with the use of open vertical field systems, such as openness for patient comfort and greater patient accessibility for the physician.
The RF coils used to manipulate magnetization as well as receive the magnetic resonance signals are different for vertical field open systems than for bore type systems. In a vertical field system, the B
0
orientation is directed across the patient as opposed to along the long axis of the patient, from head to toe. Useful RF fields, either linear or quadrature, are oriented perpendicular to the B
0
field. Thus, vertical field systems have used solenoid coils oriented along the patient axis to detect one orientation of field and a saddle coil or Helmholtz pair to detect an orthogonal transverse field. It is also desirable to have coils that conform to the openness requirements of the magnet. Volume coils for a bore type machine are often cylindrical, similar to the magnet bore. RF coils for a vertical field system are often sheets parallel to the pole faces to maintain the desired openness of the magnet. Transmit RF coils for open systems often include a pair of butterfly coils mounted parallel to the poles of the magnet. This conforms well to the open uses of the system while providing for quadrature excitation.
Early vertical field systems had a B
0
field of 0.2 to 0.35 Tesla with hydrogen magnetic resonance at frequencies of roughly 8 to 15 MHz. As field strength increases, the resonance frequency increases proportionally, requiring different coil design techniques. At low fields, solenoids and current loop coils are often used, whereas at higher frequencies, resonant structures, such as birdcage or ladder coils, are used. Also, at low fields, the receive coil thermal noise dominates while at magnetic fields of 1 Tesla or more, patient thermal noise dominates. Consequently, in high field systems, smaller receive coils or arrays of smaller coils are used to limit the patient volume contributing to the noise. Quadrature coils are usually used at higher fields because the patient noise seen by the orthogonal coils is also orthogonal resulting in a combined receive coil signal-to-noise improvement.
The present invention contemplates a new and improved open quadrature high-pass RF surface coil which overcomes the above difficulties and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, an open magnetic resonance apparatus generates a temporally constant main magnetic field through an examination region. An RF transmitter transmits radio frequency pulses to a quadrature radio frequency coil to excite resonance in selected dipoles in the examination region such that the dipoles generate resonance signals at a characteristic resonance frequency. A quadrature radio frequency coil assembly receives the resonance signals from the resonating dipoles and a radio frequency receiver demodulates the resonance signals received from the quadrature RF coil. The quadrature RF coil includes a plurality of parallel rung elements disposed parallel to the main magnetic field. Electrical conductive ends connect the plurality of rung elements. A plurality of capacitive elements interrupt at least one of the rung elements and ends.
In accordance with another aspect of the present invention, a quadrature high-pass ladder coil for a magnetic resonance apparatus includes a central rung element which is interrupted by at least one central rung capacitive element. A plurality of rung elements are disposed parallel to and symmetrically on each side of the central rung element. Further, a pair of electrical conductive ends interconnect the rung elements where the ends are each interrupted by a plurality of capacitive elements.
In accordance with another aspect of the present invention, a method of magnetic resonance imaging in an open MRI system is provided in which a temporally constant main magnetic field is generated through an examination region and gradient magnetic fields are generated for spatial selection and position encoding. The method includes positioning a quadrature RF ladder coil parallel to the main magnetic field, said coil supporting two resonant modes. Further, the two resonant modes are matched to a common resonance frequency of the dipoles of interest. Magnetic resonance is excited in the dipoles of interest in the examination region and induced magnetic resonance signals are detected using the quadrature RF ladder coil. The detected signals which are at and above the common imaging frequency are passed to a receiver and reconstructed into an image representation.
One advantage of the present invention is that it provides a planar RF transmitter particularly adapted for use at higher fields.
Another advantage of the present invention is that it allows for quadrature excitation and/or reception within the examination volume.
Another advantage of the present invention is

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