Radio-frequency coil signal chain for a low-field MRI system
Methods and apparatus for reducing noise in RF signal chain circuitry for a low-field magnetic resonance imaging system are provided. A switching circuit in the RF signal chain circuitry may include at least one field effect transistor (FET) configured to operate as an RF switch at an operating frequency of less than 10 MHz. A decoupling circuit may include tuning circuitry coupled across inputs of an amplifier and active feedback circuitry coupled between an output of the amplifier and an input of the amplifier, wherein the active feedback circuitry includes a feedback capacitor configured to reduce a quality factor of an RF coil coupled to the amplifier.
1. A switching circuit configured to be coupled to a radio-frequency (RF) coil of a low-field magnetic resonance imaging system, the switching circuit comprising:
at least one field effect transistor (FET) configured to operate as an RF switch, wherein the at least one FET has a parasitic drain-source capacitance of less than 15 picoFarads.
2. The switching circuit of claim 1 , wherein the at least one FET comprises at least one gallium nitride (GaN) FET.
3. The switching circuit of claim 2 , wherein the at least one GaN FET comprises a first GaN FET and a second GaN FET, wherein the first GaN FET and the second GaN FET are arranged to receive a same gate voltage.
4. The switching circuit of claim 1 , wherein the at least one FET has an on-state resistance of less than 1 ohm.
5. The switching circuit of claim 1 , further comprising:
driving circuitry configured to apply a gate voltage to the at least one FET, wherein the driving circuitry includes at least one isolation element configured to isolate a voltage source from the at least one FET.
6. The switching circuit of claim 5 , wherein the at least one isolation element comprises a transformer.
7. The switching circuit of claim 6 , wherein the transformer is an air core transformer.
8. The switching circuit of claim 5 , wherein the at least one isolation element comprises at least one resistor.
9. The switching circuit of claim 5 , wherein the driving circuitry comprises an AC voltage source coupled to a gate of the at least one FET via a transformer.
10. The switching circuit of claim 5 , wherein the driving circuitry comprises a diode coupled between the at least one isolation element and a gate of the at least one FET, wherein the diode is configured to rectify an input AC voltage to provide a DC voltage at the gate of the at least one FET.
11. The switching circuit of claim 5 , wherein the driving circuitry comprises an inductor coupled to the RF coil, wherein the inductor is configured to drive a gate of the at least one FET based on a transmit pulse sensed by the inductor.
12. A drive circuit configured to apply a gate voltage to at least one field-effect transistor (FET) configured to operate as a radio-frequency switch in a low-field magnetic resonance imaging system, the drive circuit comprising:
at least one isolation element configured to isolate a voltage source from the at least one FET, wherein the at least one isolation element comprises a transformer.
13. The drive circuit of claim 12 , wherein the transformer is an air core transformer.
14. The drive circuit of claim 12 , further comprising:
an AC voltage source coupled to a gate of the at least one FET via the transformer.
15. The drive circuit of claim 12 , further comprising:
a diode coupled between the at least one isolation element and a gate of the at least one FET, wherein the diode is configured to rectify an input AC voltage to provide a DC voltage at the gate of the at least one FET.
16. The drive circuit of claim 12 , further comprising:
an inductor coupled to the RF coil, wherein the inductor is configured to drive a gate of the at least one FET based on a transmit pulse sensed by the inductor.
17. A magnetic resonance imaging (MRI) system comprising:
at least one radio-frequency (RF) coil; and
at least one field effect transistor (FET) configured to operate as an RF switch for the at least one RF coil, wherein the at least one FET has a parasitic drain-source capacitance of less than 15 picoFarads.
18. The MRI system of claim 17 , wherein the at least one FET is configured to operate as the RF switch at an operating frequency of less than 10 MHz.
19. The MRI system of claim 17 , further comprising:
driving circuitry configured to apply a gate voltage to the at least one FET, wherein the driving circuitry includes at least one isolation element configured to isolate a voltage source from the at least one FET.
20. The MRI system of claim 19 , wherein the at least one isolation element comprises a transformer.