Wave transmission lines and networks – Resonators
Reexamination Certificate
2000-12-27
2003-03-18
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Resonators
C324S318000, C324S322000
Reexamination Certificate
active
06535084
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to magnetic resonance (MR) imaging or spectroscopy systems. More particularly, the present invention relates to a scheme for designing a radio frequency (RF) coil assembly for transmitting and/or receiving signals in MR imaging or spectroscopy systems.
In recent years, MR imaging and spectroscopy have developed into a joint modality capable of studying relatively large sized objects of interest, such as, human anatomy. MR images depicting parameters associated with nuclear spins (for example, protons associated with water in tissue) can provide information about the amount, type, and state of various tissues in the imaging region. MR spectroscopy permits the study of chemical processes, for example, in live organisms. The use of MR to produce images and spectroscopic studies of relatively large sized objects of interest is made possible, in part, by specifically designed system components, such as, RF coil assemblies.
The MR phenomenon occurs in atomic nuclei having an odd number of protons or neutrons. Due to the spins of protons and neutrons, each nucleus associated with such a proton exhibits a magnetic moment. When an object of interest composed of such nuclei is subjected to a uniform or static magnetic field (polarizing field B
o
, along the z direction in a Cartesian coordinate system denoted as x, y, and z), the individual magnetic moments of the spins in the nuclei tend to align with this polarizing field; but may also be made to precess about it at their characteristic Larmor frequency. The Larmor frequency, also referred to as the angular precession frequency T, is given by the Larmor equation T=&ggr;B, where &ggr; is a gyromagnetic ratio characteristic of each active MR isotope and B is the magnetic field acting upon the nuclear spins (polarizing field B
o
). Thus, the Larmor frequency is dependent on the strength of the applied static magnetic field and on the characteristics of the nuclei comprising the object of interest.
The orientation of the magnetic moments produce a net magnetization M in the direction along polarizing field B
o
. Magnetization M, however, may be perturbed by the application of magnetic fields oscillating at or near the Larmor frequency. Such magnetic fields (referred to as an excitation field B
1
) are applied in a direction orthogonal to the direction of polarizing field B
o
by means of radio frequency (RF) pulses through a coil connected to an RF transmitting apparatus. Under the influence of this RF excitation, magnetization M rotates or “flips” at a certain flipping angle in the direction of excitation field B
1
. In MR studies, it is typically desirable to apply RF pulses of sufficient magnitude and duration to rotate or “flip” magnetization M into a plane perpendicular to the direction of polarizing field B
o
(i.e., into the x-y plane, also referred to as a transverse plane) to produce a net transverse magnetic moment M
t
. When RF excitation ceases, the nuclear moments that are rotated into the transverse plane nutate back toward the direction of polarizing field B
o
. The vector sum of the moments of individual spins forms a precessing bulk magnetization that can be sensed by an RF coil. The signals emitted by the excited spins and received by the RF coil, also referred to as MR signals, are representative of the magnetic field and the particular chemical environment in which the nuclei are situated. MR signals are then suitably processed to produce an MR image or spectrum.
When MR signals are utilized to produce MR images, magnetic field gradients (G
x
, G
y
, and G
z
) are also utilized to encode spatial information into these MR signals. Typically, the object to be imaged is scanned by a sequence of measurement cycles in which these gradient waveforms vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the MR image using one of well-known reconstruction techniques.
RF coils used in whole-body MR studies can be a type of RF resonators known as transverse electromagnetic (TEM) resonators. Such an TEM resonator can be configured to be an RF receiver, transmitter, or transceiver. Presently, an TEM resonator is typically an open-ended structure and is configured to accommodate therein a part or all of a patient to be studied. The TEM resonator is cylindrical in shape, although, it need not be circular in cross-section. An TEM resonator includes a plurality of identical linear antenna elements, a pair of conductive annular rings, a cylindrical conductive shield, and capacitance.
The plurality of antenna elements are disposed in cylindrical array inside the shield, the linear axes of the antenna elements parallel with the cylindrical axis of the shield. One end of all of the antenna elements are held in place by one of the annular rings and the other end of all the antenna elements are held in place by the other one of the annular rings. The annular rings (also referred to as the annuli) are positioned inside and close to the ends of the shield. A certain amount of capacitance, either lumped or distributed, is also provided by discrete capacitors placed between the ends of each of the antenna elements and the annular rings, or by the device itself, respectively. An electrical circuit is thereby formed by each of the antenna elements, the pair of annular rings, the shield, and the capacitance, in which the shield and annular rings serve as a single ground plane and the shield contains the electromagnetic field within the cavity formed by the device.
Physical dimensions associated with the TEM resonator and of the various components comprising the TEM resonator comprise the geometric parameters of the TEM resonator. To a certain extent, the geometric parameters of TEM resonators are constrained by use and system requirements. For example, the diameter of the TEM resonator, and accordingly, the diameter of the shield and annular rings, is required to be of a certain minimum dimension to accommodate an object of interest, such as, a patient. The degree of mechanical symmetry and robustness desired may also dictate some geometric parameters. Material properties or cost considerations may further dictate geometric parameters.
In addition to geometric parameter constraints, TEM resonators also have electrical requirements, such as, having to operate at a principal or useful resonant mode frequency in order to receive the MR signals emitted from the object of interest. This principal or useful resonant mode frequency (also referred to as the operating frequency) is a resonant frequency associated with the overall device and should be the same as the Larmor frequency. In practice, however, designing an TEM resonator to operate at the desired principal resonant frequency is not a simple task. The design process involves balancing and determining numerous parameters, such as, geometry, material characteristics, electrical properties, number of subcomponents, etc., that satisfy the constraints discussed above.
Accordingly, designing an TEM resonator presently involves a long and laborious process of trial and error that typically takes several weeks to complete. The design process includes deciding on the physical dimensions or geometric parameters and fully constructing a working prototype of the desired TEM resonator based thereon. Then electrical parameters associated with the working prototype are measured, especially the resonant frequencies of the device. These measured parameters are used to construct a next iteration of the working prototype. This iterative process continues till the design parameters necessary to achieve the desired principal resonant frequency have been empirically determined.
Such iterative determinations are necessary due to the complex relationship between the design parameters and because not all of the design parameters can be determined directly through numerical or analytical calculations. For example, the principal resonant mode frequency is
GE Medical Systems Global Technology Company LLC
Horton Carl B
Takaoka Dean
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