Patent Application
20090322332
The invention in general relates to nuclear magnetic resonance (NMR) spectroscopy, and in particular to transmit/receive switch systems and methods for NMR probes.
Nuclear magnetic resonance (NMR) spectrometers typically include a superconducting magnet for generating a static magnetic field B0, and an NMR probe including one or more special-purpose radio-frequency (RF) coils for generating a time-varying magnetic field B1 perpendicular to the field B0, and for detecting the response of a sample to the applied magnetic fields. Each RF coil and associated circuitry can resonate at the Larmor frequency of a nucleus of interest present in the sample. The RF coils are typically provided as part of an NMR probe, and are used to analyze samples situated in sample tubes or flow cells.
An NMR coil may be used for both applying RF pulses to a sample and for detecting the sample's response to the applied RF pulses. In such a system, a transmit/receive switch may be employed to connect the coil to transmit circuitry during the transmission phase, and to receive circuitry during the detection phase. The transmit/receive switch protects the receive circuitry, particularly any receive circuit amplifiers, from the relatively high powers of the RF pulses applied to the coil during a transmit phase. Some conventional transmit/receive switches employ diodes formed on a silicon integrated circuit to perform the switching function. Such silicon diodes may not perform optimally as their temperature is reduced.
According to one aspect, the present invention provides a nuclear magnetic resonance (NMR) apparatus comprising a nuclear magnetic resonance radio-frequency coil, and a superconducting transmit/receive switch electrically connecting the radio-frequency coil alternatively to a transmit circuit and to a receive circuit. The transmit/receive switch includes a receive-path superconductor situated in an electrical path between the receive circuit and the radio-frequency coil. In a receive state of the switch, the receive-path superconductor is in a superconducting state, to connect the receive circuit to the radio-frequency coil. In a transmit state of the switch, the receive-path superconductor is in a normal state, to isolate the receive circuit from the radio-frequency coil.
According to another aspect, a nuclear magnetic resonance method comprises applying a set of pulses to a nuclear magnetic resonance radio-frequency coil while quenching a receive-path superconductor situated in an electrical path between the radio-frequency coil and a receive-path amplifier, and employing the receive-path amplifier to amplify a nuclear magnetic resonance response to the set of pulses while maintaining the receive-path superconductor in a superconducting state.
The foregoing aspects and advantages of the present invention will become better understood upon reading the following detailed description and upon reference to the drawings where:
In the following description, a set of elements includes one or more elements. A plurality of elements includes two or more elements. Any reference to an element is understood to encompass one or more elements. Each recited element or structure can be formed by or be part of a monolithic structure, or be formed from multiple distinct structures. The statement that a coil is used to perform a nuclear magnetic measurement on a sample is understood to mean that the coil is used as transmitter, receiver, or both. Any recited electrical or mechanical connections can be direct connections or indirect connections through intermediary circuit elements or structures. Unless otherwise qualified, the term superconductor encompasses superconductors in a superconducting state as well as superconductors in a non-superconducting (normal) state.
The following description illustrates embodiments of the invention by way of example and not necessarily by way of limitation.
To perform a measurement, a sample is inserted into coil 24. Magnet 16 applies a static magnetic field B0 to the sample held within sample container 22. Control/acquisition console 18 comprises a transmit circuit configured to apply desired radio-frequency pulses to coil 24, and a receive circuit configured to acquire data indicative of the nuclear magnetic resonance properties of the sample within coil 24. Coil 24 is used to apply radio-frequency magnetic fields B1 to the sample, and/or to measure the response of the sample to the applied magnetic fields. The RF magnetic fields are perpendicular to the static magnetic field. The same coil may be used for both applying an RF magnetic field and for measuring the sample response to the applied magnetic field.
Transmit/receive switch 40 connects RF coil 24 alternatively to a console transmit chain 52a and a console receive chain 52b. In a transmit state, switch 40 connects transmit chain 52a to coil 24, while in a receive state, switch 40 connects receive chain 52b to coil 24. Transmit chain 52a includes circuitry configured to apply NMR pulses to coil 24, while receive chain 52b includes circuitry configured to detect the response of the NMR sample within coil 24 to the applied pulses. Generally, the applied pulses have much higher powers than the detected response signals. In exemplary embodiments, the applied pulses have power levels on the order of −20 to +60 dBm, for example about 30-50 dBm, while the detected response signals have power levels many (e.g. 10) orders of magnitude lower, often on the order of −120 to −160 dBm, for example about −160 dBm. A receive amplifier 44 connects transmit/receive switch 40 to receive chain 52b. Receive amplifier 44 amplifies detected response signals received from coil 24, and sends the amplified signals to receive chain 52b.
Transmit/receive switch 40 includes a transmit-path superconducting lead 50a forming part of an electrical path between coil 24 and transmit chain 52, and a receive-path superconducting lead 50b forming part of an electrical path between coil 24 and receive chain 52b. In particular, leads 50a-b may be identical parts of a monolithically formed superconductor lead electrically connected at its ends to transmit chain 52a and receive chain 52b, and electrically connected at an internal point (e.g. at midpoint) to coil 24. The rest of the conductors shown in
A cryogenic fluid source 60 is fluidically connected to transmit/receive switch 40, coil 24 and other conductive components of probe 20 through one or more valves 62. Cryogenic fluid source 60 provides a cryogenic fluid such as cold gaseous helium to maintain superconducting leads 50a-b below their critical temperature (or temperatures, if different materials are used for the two leads). Under the critical temperature, leads 50a-b are in a superconducting state if the currents through leads are below a critical current value. A DC power source 64 is electrically connected to transmit/receive switch 40, and in particular to leads 50a-b. DC power source 64 is used to selectively quench each lead 50a-b by running a super-critical current alternatively through each lead. Quenching a lead transitions the lead from a superconducting to a non-superconducting (normal) state.
The material(s) and/or dimensions used for leads 50a-b may be chosen so that leads 50a-b are quenched by available current levels. In some embodiments, a width of each lead 50a-b may be on the order of 0.0001″ to 0.1″, for example between 0.001″ and 0.01″, and a height of each lead 50a-b may be between 0.01 μm (micron) and 100 μm, for example between 0.1 μm and 10 μm. The material(s) used for leads 50a-b may be chosen according to their critical currents, tolerance to magnetic fields, and suitability for patterning on a desired substrate. In some embodiments, leads 50a-b may be formed from a high-temperature superconductor having a critical temperature above the boiling point of nitrogen, for example from Yttrium Barium Copper Oxide (YBCO), a ceramic superconductor. In some embodiments, the substrate on which leads 50a-b are formed may include insulators such as sapphire, MgO, or quartz.
In some embodiments, leads 50a-b are connected to adjacent resistive metal conductors by patterning resistive metal on the ends of leads 50a-b. Suitable resistive metals for metalizing leads 50a-b and for other conductors may include gold and/or other conductive metals.
In some embodiments, the operating temperature of leads 50a-b may be between 4 K and 90 K, for example between 10 K and 25 K. While higher temperatures are generally easier to attain, some materials may have suboptimal current handling properties at higher temperatures. For example, YBCO, though superconducting around 90 K, has limited current handling at that temperature. Moreover, a lower temperature may allow achieving lower noise values. A superconducting switch may be of particular use at relatively low temperatures that would lead to degradation in the performance of conventional silicon-diode-based switches.
In some embodiments, leads 50a-b are quenched by applying an initial pulse of super-critical current. The initial pulse overloads the superconductor(s) and renders the material(s) resistive. Subsequently, lower current values may be used to maintain leads 50a-b in a resistive state. In some embodiments, the initial pulse may have current values on the order of hundreds of mA to Amperes, while the subsequent applied current may have values on the order of mA to tens of mA. Some embodiments may employ applied voltage values between 0.1 V and 100 V, for example about 5 V, and resulting current values between 1 mA and 10 A.
To set transmit/receive switch 40 to the transmit state, the DC In 1-3 voltages are controlled to set the DC current flow through transmit-path lead 50a below the critical current of lead 50a (e.g. to substantially zero), and to set the DC current flow through receive-path lead 50b above the critical current of lead 50b. Transmit-path lead 50a thus remains superconducting, connecting transmit chain 52a to coil 24, while receive-path lead 50b becomes non-superconducting, effectively isolating amplifier 44 from transmit chain 52a. The isolation protects amplifier 44, and decreases transmit pulse losses to the receive coil. In some embodiments, receive-path lead 50b may be designed to provide isolation on the order of 60-80 dB.
To set transmit/receive switch 40b to the receive state, the DC In 1-3 voltages are controlled to set the DC current flow through transmit-path lead 50a above the critical current of lead 50a, and to set the DC current flow through receive-path lead 50b below the critical current of lead 50b. Receive-path lead 50b is then superconducting, connecting amplifier 44 to coil 24, while transmit-path lead 50a is non-superconducting, effectively isolating coil 24 and amplifier 44 from transmit chain 52a. With transmit-path lead 50a resistive and not impedance matched to the NMR coil(s), RF energy from the coil(s) travels preferentially along receive path lead 50b, and little or no RF energy travels through transmit-path lead 50a. The isolation may reduce the noise reaching amplifier 44, and thus reduce the noise temperature of the NMR detection circuit.
In some embodiments, a transmit/receive switch may include only one of the transmit-path and receive-path superconductor leads described above.
Exemplary embodiments described above allow achieving relatively low operating temperatures for NMR cold probe transmit/receive switches. Transmit/receive switches using silicon elements such as diodes may not perform adequately at such temperatures. Gallium arsenide diodes may perform at lower temperatures than silicon diodes, but gallium arsenide diodes may not be as robust as silicon diodes, and may limit the power levels of transmit pulses.
Lowering operating temperatures may lower the noise temperatures of the circuits, thus allowing improved signal-to-noise ratios in some embodiments. NMR signal-to-noise ratios generally may depend on the coil filling factor, which affects how much of the NMR signal comes from the sample relative to non-sample sources, the Q parameter, which is indicative of resistive losses in the system, and by the noise temperature, which provides a baseline level of noise. If the signal-to-noise ratio is proportional to (Q*filling factor/noise temperature)̂0.5, reducing the system noise temperature may allow improved signal-to-noise ratios for a given Q and filling factor.
The above embodiments may be altered in many ways without departing from the scope of the invention. For example, one or more superconducting switches as described above may be used in conjunction with silicon-diode-based switches. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.
