The present invention relates generally to fabrication of coils for use in nuclear magnetic resonance (NMR) spectrometers and, particularly, to a method of fabricating such coils employing beam techniques to deposit and/or pattern the coils directly onto cylindrical dielectric tubes.
NMR spectroscopy is a valuable tool for chemical analysis. Conventional NMR spectrometers use radio frequency (RF) detection coils ranging from several millimeters to tens of centimeters in diameter to couple energy to sample volumes. In general, the coils couple energy, in the form of a rapidly alternating magnetic field, both into and out of the samples of interest. The coil transmits energy to the sample for exciting it from its equilibrium state to an excited state and receives energy from the sample as it relaxes from the excited state to the equilibrium state. Coil quality affects the observed signal-to-noise ratio and, thus, determines in large part the chemical sensitivity and resolution of the spectrometer.
Conventional coils are made from copper or silver to minimize electrical loss. A drawn wire or foil is wound or formed into the appropriate shape, usually on the surface of a thin, dielectric tube. If the demands of chemical resolution are not particularly severe, as in solid-state NMR, conductor materials thick enough to be self-supporting without the dielectric tube are typically used. In order to further improve the chemical resolution, measures are often taken to reduce the effective susceptibility of the wire or foil. Such measures include coating a paramagnetic material with a diamagnetic one (e.g., plating a copper wire with a layer of rhodium). Outside the wire or foil, the magnetic effects of the two materials will tend to cancel each other.
Although refinements of the general approach for manufacturing RF detection coils are known, further improvements are still desired. For example, a coil for use in NMR can be patterned onto a flexible circuit board. Wrapping the circuit board around a rigid dielectric cylinder forms the coil. This allows for better control of the conductor positions. The limited radius of curvature of the flexible circuit board, however, limits the minimum radius that can be used. Further, constructing a coil in this manner may require overlapping the dielectric and bonding the electrical traces together where the two ends of the board are joined together. In doing so, magnetic distortions and RF losses may result.
Those skilled in the art are also familiar with suspending an NMR coil in a material that matches the susceptibility of the foil or wire conductor. Use of a large container of liquid or solid plug of material allows the magnetic discontinuity to be moved far from the sample region. Note that the matching material should be free of any nuclei desired for NMR observation. To allow better access for pulsed field gradient coils, temperature control, and so forth, the container or plug must be fairly small, which requires that the shape of the container or plug be contrived to reduce the production of magnetic distortion.
Oxide superconductive RF coils have been patterned directly onto flat, dielectric substrates. By patterning the superconductor into thin strips parallel to the direction of current flow, the magnetic field distortion produced by persistent currents is minimized. This also enables the use of the material in high-resolution NMR applications. The fine resolution available with photolithography also allows the use of interdigital capacitors with high capacitance density. In turn, low resonant frequencies may be achieved using a single-layer process and no lossy external connections. However, flat coils have a much lower filling factor for the cylindrical samples required for NMR analysis and, without use of a superior conductor, yield much poorer chemical sensitivity and have an undesirable RF field profile.
U.S. Pat. Nos. 5,276,398, 5,466,480, 5,565,778, 5,594,342, and 5,619,140, the entire disclosures of which are incorporated herein by reference, disclose magnetic resonance probe coils and methods of making such coils.
In view of the foregoing, a more flexible and effective method for manufacturing relatively small NMR coils and NMR microcoils is desired.
The invention meets the above needs and overcomes the deficiencies of the prior art by providing an method of fabricating, or constructing, a detection coil for use in an NMR spectrometer. Among the several objects and features of this invention is the provision of such method that permits construction of a coil having signal-to-noise ratio characteristics for improving chemical sensitivity and resolution of the spectrometer; the provision of such method that reduces the effective susceptibility of the coil; the provision of such method that reduces magnetic distortions and RF losses; the provision of such method that permits construction of relatively small detection coils; the provision of such method that achieves a resonant frequency applicable to the particular design; and the provision of such method that is economically feasible and commercially practical.
Briefly described, a method embodying aspects of the invention includes steps for fabricating a detection coil for use in NMR spectroscopy. The method includes depositing a layer of conductive material on an outer surface of a generally cylindrical tube of dielectric material and then patterning the layer into a solenoid around the outer surface of the dielectric tube.
In another embodiment, a method of fabricating a detection coil includes the steps of depositing a first layer of material on an outer surface of a generally cylindrical tube of dielectric material and depositing a second layer of material on an outer surface of the first layer. In this method, one or more of the first and second layers of materials are a conductive material. The method also includes patterning the deposited first and second layers into a solenoid around the outer surface of the dielectric tube.
Yet another embodiment of the invention is directed to a method of fabricating a detection coil for use in NMR spectroscopy. The method includes depositing a layer of conductive material on an outer surface of a generally cylindrical tube of dielectric material and patterning the layer into an interdigital capacitor around the outer surface of the dielectric tube. The capacitor has a longitudinal tie bar portion and a plurality of fingers connected to and extending from the tie bar.
A detection coil embodying aspects of the invention includes a generally cylindrical tube of dielectric material having first and second film layers of conductive material deposited on its outer surface. The first and second layers each have a magnetic susceptibility of opposite sign with respect to the other and are patterned to form a solenoid around the outer surface of the dielectric tube.
Alternatively, the invention may comprise various other methods and systems.