Bandwidth expansion in magnetic resonance
An apparatus for use in a magnetic resonance examination includes a radio frequency receive coil ( 50 ) and adjustable tuning circuitry ( 51 ) for adjusting the resonant frequency of the receive coil. The apparatus also includes a preamplifier ( 29 ) which amplifies signals generated by the receive coil. An adjustable feedback circuit ( 31 ) alters an effective Q of the receive coil.
1. An apparatus comprising:
a receive coil which receives RF signals and produces an electrical output signal in response thereto;
a preamplifier which amplifies the electrical output signal, wherein the preamplifier has an input impedance; and
adjustable feedback circuitry, including:
a variable resistance; and
a variable reactance,
wherein the adjustable feedback circuitry is operatively connected to the preamplifier and automatically adjusts the input impedance of the preamplifier so as to alter an effective Q of the receive coil.
2. The apparatus of claim 1 , further comprising:
a computer operatively connected to the adjustable feedback circuitry so as to selectively adjust the input impedance of the preamplifier.
3. The apparatus of claim 2 , wherein in 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.
4. The apparatus of claim 3 , wherein the computer is operatively connected to the tuning circuit so as to selectively adjust the resonant frequency.
5. The apparatus of claim 1 , wherein the receive coil impedance varies as a function of the frequency of the RF signals.
6. The apparatus of claim 2 , wherein the RF signals have an expected signal bandwidth, and the computer adjusts the input impedance to a value that is approximately equal to a mean of the coil impedance extremes over the expected signal bandwidth.
7. The RF apparatus of claim 1 , wherein the feedback circuitry provides a shunt feedback.
8. The apparatus of claim 1 , wherein the feedback circuitry provides a series feedback.
9. The apparatus of claim 1 , wherein the preamplifier includes first and second amplifier stages.
10. The apparatus of claim 9 , wherein the second stage is configured as a non-inverting amplifier.
11. The apparatus of claim 10 , wherein the variable resistance and the variable reactance vary a gain and a phase response of the second amplifier stage.
12. The apparatus of claim 9 , wherein the first and second amplifier stages are arranged in a cascade configuration.
13. A method comprising:
adjusting feedback circuitry of a preamplifier which is electrically connected to an RF receive coil, the feedback circuitry including a variable resistance and a variable reactance, and wherein adjusting includes adjusting at least one of the variable resistance and the variable reactance so as to adjust the effective Q of the receive coil.
14. The method of claim 13 , wherein adjusting includes adjusting both the variable resistance and the variable reactance.
15. The method claim 13 , wherein the preamplifier includes a transconductance amplifier.
16. The method of claim 13 , wherein the feedback is provided through one of a tapped inductor or a tapped capacitor.
17. The method of claim 13 , wherein adjusting the feedback circuitry includes adjusting a gain of an amplifier disposed in a feedback loop.
18. The method of claim 13 , wherein adjusting the feedback circuitry includes adjusting a phase response of an amplifier disposed in a feedback loop.
19. The method of claim 1 , wherein adjusting includes using a computer to adjust at least one of the variable resistance and the variable reactance.
20. An apparatus comprising:
a computer; and
an RF receive portion having feedback circuitry, including a variable resistance and a variable reactance, wherein the computer automatically adjusts at least one of the variable resistance and the variable reactance to adjust an effective Q of the apparatus.