Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-04-23
2001-05-22
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000, C324S300000
Reexamination Certificate
active
06236206
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to nuclear magnetic resonance (“NMR”) RF coils of a bird cage design, and more particularly, to a NMR birdcage coil being globally tunable by a movable structure.
BACKGROUND OF THE INVENTION
With contemporary technology, medical and/or chemical samples can be analyzed by a wide variety of methods. However, if an analysis of diverse chemical constituents and/or spatial distributions of such constituents are desired of a sample, the method most widely used is the practice of magnetic resonance.
In the practice of magnetic resonance phenomena, RF radiation is applied to a sample by a surrounding structure and resulting resonant signals are induced in the same or another surrounding structure for analysis. The structure may be a helical coil, saddle coil, resonant cavity, or a birdcage resonator. The latter structure is the object of the present work, wherein it is desired to obtain selected resonant frequencies in a birdcage type structure to facilitate studies of large or small samples.
In general, birdcage coils are constructed to act as either a low pass or a high pass structure. As illustrated in
FIG. 1A
, a conventional low pass birdcage coil provides at least one capacitive element
2
electrically coupled along each conductive leg
4
. In contrast, a conventional high pass birdcage coil provides a capacitive element
2
electrically coupled between each conductive leg
4
as illustrated in FIG.
1
B. With both structures, the conductive legs are typically supported by, or deposited upon a non-conductive material.
Functionally, the bird cage structure may be regarded as a periodic structure which closes on itself Periodic elements of the structure produce phase shifts which must aggregate to some integer multiple of 2&pgr; when summed over the closed loop.
Geometrically, the resonator has cylindrical symmetry and it is desired that the RF current in each leg be proportional to sin k&thgr;/2&pgr; and/or cos k&thgr;/2&pgr;, where &thgr; is the azimuthal angle about the cylindrical axis and k is an integer defining resonant mode. The mode k=1 provides a uniform RF field distribution within the coil structure. Quadrature operation of the coil is realized when two RF drives of with relative phases of &pgr;/2 are connected to the coil at two points displaced in phase by &pgr;/2 about the phase distribution along the periphery of the coil.
The birdcage coil is tuned as closely as possible to the desired frequency by adjusting the capacitive elements (
2
in
FIG. 1
) equally. In current designs, the final tuning adjustment is achieved by the discrete tuning of a single capacitor within the birdcage structure. However, by adjusting only a single capacitor, the user will only be able to obtain the desired frequency without maintaining the electrical symmetry of the structure. Therefore, optimum RF distribution will not be achieved within the structure.
Even if the birdcage coil can be adjusted with all capacitive elements
2
equal in the absence of a sample or load, insertion of a sample or load will require tuning the structure yet again to compensate for a decrease in frequency caused by the inherent dielectric properties of a typical load or sample. As illustrated in
FIG. 2A
, a change in frequency due to introduction of a load results in a phase error which must be compensated by the single tuning capacitor to restore the correct total phase shift of &pgr;/2. The result of phase error is an inhomogeneous B
1
field as appears in the corresponding
FIGS. 2B through 2D
. It is sometimes useful to be able to tune a birdcage structure for observation of different nuclear species, e.g.,
1
H and
19
F. A tuning adjustment between such resonant frequencies could not be accomplished with variation of a single capacitance without completely destroying the uniformity of the RF field.
It would be advantageous to develop a birdcage structure which maintains the symmetry of the azimuthal distribution of phase shift around the structure as it is tuned to accommodate different loads or to observe different nuclear species. The desired structure provides a homogeneous field under all tuning conditions and preserves the symmetry necessary for quadrature operation.
SUMMARY OF THE INVENTION
In the present invention, a globally tunable birdcage coil is provided having a plurality of leg conductors disposed spatially from an axis and parallel therewith. The plurality of leg conductors define an active volume for receiving a load. The tuning structure maintains RF quadrature isolation within the active volume while the load is analyzed.
In a first embodiment, a tuning arrangement is provided for controlling magnetic field homogeneity within a birdcage coil over varying loading and coupling conditions. The birdcage coil includes a plurality of leg conductors disposed spatially from an axis and parallel therewith. A tuner structure is positioned adjacent the leg conductors and disposed about the axis for simultaneously tuning each of the leg conductors by varying the distance between ring conductors, each said ring conductor spaced from, and surrounding the respective end rings of a centrally disposed birdcage coil.
In another embodiment, a pair of coaxial birdcage coils is arranged with provision for the relative rotation of said coils.
REFERENCES:
patent: 4694255 (1987-09-01), Hayes
patent: 4939465 (1990-07-01), Biehl et al.
patent: 4992737 (1991-02-01), Schnur
patent: 5144240 (1992-09-01), Mehdizadeh et al.
patent: 5466480 (1995-11-01), Zhou et al.
patent: 6011395 (2000-01-01), Leifer et al.
Su et al, Journal of Magnetic Resonance Series B 110, 210-212 (1996).*
Article by Dardzinski et al., entitled “A Birdcage Coil Tuned by RF Shielding for Application at 9.4T”, published inJournal of Magnetic Resonanceon Mar. 1, 1998, vol. 131, pp. 32-38.
Article by Pimmel et al., entitled “A Hybrid Bird Cage Resonator for Sodium Observation at 4.7T”, published inMagnetic Resonance in Medicineon Mar. 1, 1992, vol. 24, No. 1, pp. 158-162.
Article by Jeong et al., entitled “A Simple Inductive Tuning of Birdcage Coil, with Wide Range of Tuning”,Proceedings of the International Society for Magnetic Resonance in Medicine,Fifth Scientific Meeting and Exhibition, Vancouver, B.C., Canada, Apr. 12-18, 1997, vol. 3, p. 1499.
Article by Pimmel et al., entitled “Hybrid Bird Cage Resonator,”Proceedings International Society for Magnetic Resonance in Medicinein 1997, p. 527.
Hartman Spencer C.
Wong Wai Ha
Berkowitz Edward H.
Oda Christine
Shrivastav Brij B.
Varian Inc.
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