Bandwidth expansion in magnetic resonance
A magnetic resonance imaging system includes a primary magnet and a secondary magnet operable to produce magnetic fields within a sample being imaged. The MRI system further includes at least one RF coil that is operable to receive electromagnetic frequencies from the sample. The RF coil is formed from tubing that serves as a cooling conduit through which flows a cooling fluid provided by a cooling source. The cooling fluid cools the RF coils to improve imaging of the sample.
1. A method comprising:
positioning an object to be imaged in relation to an MR imaging apparatus and an RF receive coil used in connection with the MR imaging apparatus;
adjusting a characteristic of a preamplifier, which is electrically connected to the receive coil, via a control signal generated by a computer of the MR imaging apparatus to reduce an effective Q of the receive coil;
using the MR imaging apparatus and the RF coil to conduct an MR imaging examination of the object; and
repeating the steps of positioning, adjusting, and using for a plurality of objects.
2. The method of claim 1 wherein the characteristic is an input impedance.
3. The method of claim 1 wherein the step of adjusting includes adjusting a feedback provided to a stage of the preamplifier.
4. The method of claim 3 wherein the stage is an input stage which includes a transconductance amplifier.
5. The method of claim 3 wherein the feedback comprises series feedback.
6. The method of claim 5 wherein the feedback is provided through a tapped inductor arrangement.
7. The method of claim 3 wherein the feedback comprises shunt feedback.
8. The method of claim 7 wherein the feedback is provided through a tapped capacitor arrangement.
9. The method of claim 3 wherein adjusting the feedback includes adjusting a gain of an amplifier disposed in a feedback loop.
10. The method of claim 9 wherein the amplifier is an operational amplifier.
11. The method of claim 3 wherein adjusting the feedback includes adjusting a phase response of an amplifier disposed in a feedback loop.
12. The method of claim 1 wherein adjusting includes generating an RF signal at a plurality of frequencies and determining an amplitude of a signal produced by the preamplifier at each of the frequencies.
13. The method of claim 1 wherein the method further comprises adjusting the resonant frequency of the receive coil.
14. A magnetic resonance imaging apparatus comprising:
a magnet which generates a main magnetic field in an examination region;
a gradient coil which generates a time varying gradient magnetic field in the examination region;
a transmit coil which generates radio frequency signals so as to excite magnetic resonance in nuclei disposed in the examination region;
a receive coil which receives magnetic resonance signals generated by the nuclei and produces an electrical output signal in response thereto;
an amplifier which amplifies the electrical output signal, wherein the amplifier has an input impedance;
an input impedance adjustment circuit operatively connected to the amplifier and adapted to adjust the input impedance of the amplifier;
a computer operatively connected to the input impedance adjustment circuit to selectively adjust the input impedance of the amplifier.
15. The apparatus of claim 14 wherein the receive coil has a resonant frequency and further including a tuning circuit operatively connected to the receive coil and adapted to adjust the resonant frequency; wherein the computer is operatively connected to the tuning circuit to selectively adjust the resonant frequency.
16. The apparatus of claim 14 wherein the input impedance adjustment circuit includes a feedback circuit having at least one of a variable resistance and a variable reactance.
17. The apparatus of claim 16 wherein the feedback circuit provides a shunt feedback.
18. The apparatus of claim 16 wherein the feedback circuit provides a series feedback.
19. The apparatus of claim 14 wherein the amplifier includes a HEMT FET.
20. The apparatus of claim 14 wherein the amplifier includes first and second amplifier stages.
21. The apparatus of claim 20 wherein the second stage is configured as a non-inverting amplifier.
22. The apparatus of claim 21 wherein the input impedance adjustment circuit varies at least one of the gain and phase response of the second amplifier stage.
23. The apparatus of claim 22 wherein the first and second amplifier stages are arranged in a cascode configuration.
24. A circuit for amplifying a radio frequency signal received by an RF coil of an MR system, the circuit comprising:
an amplifier;
circuitry operatively connected to the amplifier, wherein the circuitry is adapted to adjust an effective Q of the RF coil in response to a control signal generated by a processor of the MR system.
25. The circuit of claim 24 wherein the circuitry includes a variable reactance (Cp) for adjusting the effective Q of the RF coil in response to the control signal.
26. The circuit of claim 24 wherein the circuitry includes a variable resistance (Rp) for adjusting the effective Q of the RF coil in response to the control signal.
27. The apparatus of claim 26 wherein the circuitry includes an amplifier.
28. The apparatus of claim 24 wherein the amplifier includes first and second amplifier stages, and wherein the circuitry varies at least one of a gain and a phase response of the second amplifier stage.