Multicoil low-field nuclear magnetic resonance detection and imaging apparatus and method
A multicoil NMR detection and imaging apparatus allows multicoil NMR detection and imaging to be performed efficiently at low operating frequencies. The apparatus comprises an AC voltage generator, a transmit switching circuit, a coil switching network, an array of two or more detection coils, a set of receive switching circuits with one switching circuit for each detection coil, and a set of preamplifier circuits with input impedance substantially greater than the impedance of each respective detection coil at the intended operating frequency. The AC generator produces an alternating current waveform that is routed through one of more detection coils during transmit mode while the preamplifier circuits are isolated from the detection coil(s). During receive mode the AC generator is isolated from the detection coils to prevent noise from the transmitter from degrading the quality of received signals.
1. A method for performing low-field NMR measurements with a Nuclear Magnetic Resonance (NMR) apparatus comprising an AC voltage generator, at least two detection coils wherein one or more of the detection coils serve as transmitting coils in a transmit mode and wherein one or more of the detection coils serve as detecting coils in a receive mode, at least two receive switching circuits, and at least two preamplifier circuits, the method comprising:
in a transmit mode, generating, with the AC voltage generator, an alternating voltage waveform with an operating frequency, while allowing current from the AC voltage generator to flow through the one or more transmitting coils, and while maintaining the receive switching circuits in an opened state, isolating the preamplifier circuits from the alternating voltage waveform;
the one or more transmitting coils creating an alternating magnetic field Bl which tips the NMR spin magnetization in a sample located within the low-field away from an axis of a static magnetic field BO, and sets the spin magnetization precessing about the BO field axis;
in a receive mode, closing the receive switching circuits, inhibiting current flow through the detection coils, and detecting NMR induced voltages on the detection coils with electronics comprising the preamplifier circuits, wherein one or more of the preamplifier circuits has an input impedance greater than the impedance of a detection coil at the operating frequency.
2. The method for performing low-field NMR measurements of claim 1 , further comprising using a coil switching network to select one or more detection coils for use as transmitting coils in the transmit mode.
3. The method for performing low-field NMR measurements of claim 2 , wherein the coil switching network comprises at least two switches that enable the selection of current directions for a transmitting coil, and further comprising selecting a current direction for a transmitting coil.
4. The method for performing low-field NMR measurements of claim 2 , wherein the coil switching network comprises two or more active coil switches, and further comprising controlling the active coil switches.
5. The method for performing low-field NMR measurements of claim 1 , further comprising reducing the dynamic range of a detected NMR signal using a filter.
6. The method for performing low-field NMR measurements of claim 1 , wherein one or more of the detection coils that serve as transmitting coils in transmit mode are also utilized as detecting coils in receive mode.
7. The method for performing low-field NMR measurements of claim 1 , wherein closing the receive switching circuits inhibits mutual coupling between at least two detection coils.
8. The method for performing low-field NMR measurements of claim 1 , further comprising opening a transmit switching circuit after generating the alternating voltage waveform, wherein the transmit switching circuit is configured to enable current flow between the AC voltage generator and the transmitting coils in transmit mode, and to isolate the AC voltage generator from the detection coils in receive mode.
9. The method for performing low-field NMR measurements of claim 1 , wherein the AC voltage generator comprises an AC signal generation device and a power amplifier, wherein the AC signal generation device generates a low-voltage version of an NMR excitation waveform, wherein the power amplifier transforms the low-voltage waveform into a current waveform, and wherein the current waveform is routed through one of more transmitting coils during transmit mode.
10. The method for performing low-field NMR measurements of claim 9 , wherein the AC signal generation device comprises a computer-controlled digital output device.
11. The method for performing low-field NMR measurements of claim 1 , wherein the receive switching circuits comprise actively controlled receive relays, with one pair of relays for each detection coil, and further comprising controlling the actively controlled receive relays.
12. The method for performing low-field NMR measurements of claim 11 , further comprising opening the receive relays during transmit mode to isolate high voltages and currents on the transmitting coils in transmit mode from the preamplifier circuits.
13. The method for performing low-field NMR measurements of claim 1 , wherein one or more of the preamplifier circuits comprise signal conditioning and amplification elements.
14. The method for performing low-field NMR measurements of claim 1 , wherein one or more of the preamplifier circuits include a step-up voltage transformer that, combined with other circuitry in a preamplifier circuit, provides an input impedance configured to inhibit current flow through the detection coils in the receive mode.
15. The method for performing low-field NMR measurements of claim 14 , wherein the step-up voltage transformer is included in a first stage of one or more of the preamplifier circuits.
16. The method for performing low-field NMR measurements of claim 1 , further comprising a Q-damping circuit configured to lower the Q-value of each detection coil immediately after a transmit pulse.
17. The method for performing low-field NMR measurements of claim 1 , wherein one or more of the receive switching circuits includes a pair of switching devices, each switching device connected to a side of a detection coil.
18. The method for performing low-field NMR measurements of claim 1 , wherein one or more of the receive switching circuits includes a switching device connected to a side of a detection coil, and wherein another side of the detection coil is connected to a common ground.