Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1998-06-29
2000-05-09
Oda, Christine K.
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
324319, 324322, 600421, G01V 300
Patent
active
06060882&
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The field of this invention is the measurement of nuclear magnetic resonance (NMR and MRI) for the purpose of determining molecular or microscopic structure, and, more particularly, a tunable, low-inductance, rf coil that has transparency to self-generated rf magnetic fields.
BACKGROUND ART
This invention pertains to improving the sensitivity and B.sub.1 homogeneity of NMR and MRI experiments, particularly on large samples at high fields, by means of novel, low-inductance "saddle" or "volume" coils for use on surfaces usually aligned with B.sub.0. Related NMR coils are described by Zens in U.S. Pat. No. 4,398,149, Hill and Zens in U.S. Pat. No. 4,517,516, and Doty in U.S. Pat. No. 4,641,098. Sensitivity optimization is reviewed by David Doty in "Probe Design and Construction" in The NMR Encyclopedia, Vol. 6, Wiley Press, 1996, and numerous coils are reviewed by James Hyde in "Surface and Other Local Coils for In Vivo Studies", Vol. 7, of the same series. A separate application discloses improved surface coils using litz foil.
The novel coils in this invention have higher quality factor Q, filling factor .eta..sub.F, homogeneity of the transverse magnetic field B.sub.1, and self-resonance f.sub.0. They may also be designed to have sharper roll-off in B.sub.1 beyond the region of interest. In high resolution NMR, this gives improved NMR lineshape by reducing sensitivity to sample material located in regions of poor B.sub.0 homogeneity, a problem previously addressed by Zens in U.S. Pat. No. 4,549,136 using susceptibility-matched plugs and by Hill in U.S. Pat. No. 4,563,648 using geometric compensation and by many others using shielding (Ad Bax, J. Magn. Reson. 65, pp. 142-145, 1985). In MRI, the improved roll-off reduces foldback from signals originating near the gradient null point.
The improvements in this invention are achieved primarily from the use of judiciously chosen, circuitous, parallel, current paths with insulated crossovers that result in (1) greatly reduced current density (typically a factor of three below that of prior-art designs of comparable inductance) along the inner edges of the conductor bands which subtend small angles with respect to the B.sub.1 direction, (2) transparency to self-generated B.sub.1, and (3) to a lesser extent, improved transparency to orthogonal rf fields. Transparency to self-generated rf fields in low-inductance coils has generally required phase-shift networks, as disclosed by Edelstein et al. in U.S. Pat. No. 4,680,548, but their birdcage design is extremely difficult to tune over a wide range of sample loading conditions or even over the narrowest of multi-nuclear ranges, such as .sup.19 F-.sup.1 H, a 6% change in frequency. Transparency to self-generated fields can also be achieved by other distributed parameter resonators, as disclosed by Hayes et al., U.S. Pat. No. 4,783,641, and again in U.S. Pat. No. 5,053,711, but these resonators all have severely limited tuning range and inferior B.sub.1 homogeneity. Moreover, the ring currents in the birdcage and induced currents in the external rf shield result in poor rf homogeneity ("hot spots" and "dark regions") for large samples.
The NMR spectroscopist often finds it necessary to observe a wide variety of nuclides, especially .sup.13 C, .sup.1 H, .sub.19 F, .sup.27 Al, .sup.29 Si, .sup.23 Na, .sup.2 H, and .sup.15 N in the study of commercially and scientifically important chemicals, and considerable interest is developing in multi-nuclear localized MR spectroscopy. Multi-nuclear tuning is readily achieved in prior art designs with sample diameters up to 15 mm at high field with multi-turn coils having inductance typically in the range of 35 to 150 nH, while fixed-frequency coils may have inductance as low as 2 nH.
Recent improvements by Kost et al. (and proprietary designs by Varian and Nalorac) in the conventional slotted-resonator have improved its homogeneity, but it is still often not adequate for large samples when the ratio of the diameter of the external rf shield to the coil diameter i
REFERENCES:
patent: 4398149 (1983-08-01), Zens
patent: 4517516 (1985-05-01), Hill
patent: 4549136 (1985-10-01), Zens
patent: 4563648 (1986-01-01), Hill
patent: 4641098 (1987-02-01), Doty
patent: 4820987 (1989-04-01), Mens
patent: 4878022 (1989-10-01), Carlson
patent: 5379768 (1995-01-01), Smalen
patent: 5481191 (1996-01-01), Rzedzian
Borsboom et al. Low-frequency quadrature mode birdcage resonator' MAGMA (united states) Mar. 1997, 5(1) pp.33-37 ISSN 0968-5243.
Leifer, Mark C. "Theory of the quadrature Eliptic Birdcage Coil" Magnetic Resonance in Medicine MRM vol. 38 pp. 726-732, Apr. 1997.
Li, Shizhe et al. "A Method to Create an Optimum Current Distribution and Homogeneous B1 Field for Elliptical Birdcage Coils" Magnetic Resonance in Medicine MRM vol. 37, pp. 600-608, Mar. 1997.
Vujcic, T. et al., "Transverse Low-Field RF Coils in MRI" Magnetic Resonance in Medicine MRM vol. 36 1997 pp. 111-116, Dec. 1996.
L. Bollinger, M.G. Prammer, and J.S. Leigh, "A Multiple-Frequency Coil with a Highly Uniform B.sub.1 Field," J. Magn. Reson., 1988, 81, 162-166.
G.J. Kost, S.E. Anderson, G.B. Matson, and C.B. Conboy, "A Cylindrical-Window NMR Probe with Extended and Tuning Range for Studies of the Developing Heart,"J. Magn. Reson., 1989, 82, 238-252.
G. Isaac, M.D. Schnall, R.E. Lenkinski, and K. Vogele, "A Design for a Double-Tuned Birdcage Coil for Use in an Integrated MRI/MRS Examination,"J. Magn. Reson., 1990, 89, 42-50.
J.R. Fitzsimmons, B.L. Beck, H.R. Brooker, "Double Resonant Quadrature Birdcage," Magn. Reson. in Med., 1993, 30, 107-114.
F.D. Doty, "Probe Design and Construction," Encyclopedia of NMR, Wiley Press, 1996.
Doty Scientific Inc.
Fetzner Tiffany A.
Oda Christine K.
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