Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1994-04-04
1995-06-13
Arana, Louis
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
324318, G01V 300
Patent
active
054246466
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an RF drive circuit used in MR apparatus, and more particularly to an RF drive circuit capable of maintaining a prescribed amplitude of the spin-inducing RF revolving magnetic field regardless of the load impedance.
BACKGROUND ART
FIG. 5 shows an example of conventional RF drive circuit of MR apparatus. The RF drive circuit 51 has its input terminal IN supplied with an RF pulse signal having a prescribed envelope. The RF pulse signal is conducted through a distributor 2 and fed to a variable-gain amplifier 3. The variable-gain amplifier 3 operates in unison with a power amplifier 4 to amplify the RF pulse signal to produce a drive voltage of a prescribed voltage level, which is fed to a quadrature hybrid 5. The quadrature hybrid 5 produces a 0.degree.-component drive voltage and a 90.degree.-component drive voltage from the input drive voltage, and applies the resulting drive voltages to a 0.degree.-port and 90.degree.-port of an RF coil 6. By being energized by the two-component drive voltages, the RF coil 6 generates an RF revolving magnetic field in its internal space.
A voltage monitor means 52 monitors the output voltage of the power amplifier 4 and feeds the result to an envelope extracting circuit 53, which extracts the envelope of the monitored device voltage. Another envelope extracting circuit 11 connected to the distributor 2 extracts the envelope of the RF pulse signal provided by the distributor 2.
A comparator 12 compares the envelopes of the RF pulse signal and output voltage of the power amplifier 4, and controls the gain of the variable-gain amplifier 3 in accordance with the comparison result so that the output voltage of the power amplifier 4 has the prescribed value regardless of the load condition of the coil (the impedance of the patient who lies in the internal space of the RF coil 6).
The foregoing conventional RF drive circuit 51 merely controls the output voltage of the power amplifier 4 to the prescribed level, but it does not control the balance of the voltages applied to the 0.degree.-port and 90.degree.-port of the RF coil 6. Therefore, if the RF coil 6 loses the impedance balance due to different impedances of the 0.degree.-port and 90.degree.-port, the quadrature balance, i.e., the condition of RF revolving magnetic field generation, varies, resulting in a varying flip angle for spin induction.
DISCLOSURE OF THE INVENTION
Accordingly, an object of the present invention is to provide an RF drive circuit of MR apparatus capable of controlling the drive voltage of the RF coil, inclusive of the correction of the quadrature balance.
The invention resides in an RF drive circuit for quadratically driving the RF coil of MR apparatus, and it comprises a voltage monitor means for monitoring the voltages on the 0.degree.-port and 90.degree.-port of the RF coil, a feedback voltage generation means for producing, from the monitored voltages on the 0.degree.-port and 90.degree.-port, a feedback voltage that corresponds to a component of a revolving magnetic field formed in the RF coil, with the magnetic field component being in the field revolving direction and coupling with a spin system, and a drive voltage control means for controlling, based on the feedback voltage, the drive voltage to be applied to the RF coil.
The inventive RF drive circuit of MR apparatus operates to monitor the voltages on the 0.degree.-port and 90.degree.-port of the RF coil, produce a feedback voltage that corresponds to the component, which is along the field revolving direction and couples with the spin system, of the revolving magnetic field formed in the RF coil, and control the drive voltage to be applied to the RF coil based on the feedback voltage. Consequently, even if the RF coil loses the impedance balance of the 0.degree.-port and 90.degree.-port, the imbalance is corrected automatically and the RF magnetic field that contributes to the spin induction can be stabilized.
BRIEF EXPLANATION OF THE DRAWINGS
FIG. 1 is a block diagram of the RF dri
REFERENCES:
patent: 4763074 (1988-08-01), Fox
patent: 4777440 (1988-10-01), Van Heelsbergen
patent: 5051700 (1991-09-01), Fox
patent: 5166620 (1992-11-01), Panosh
Arana Louis
GE Yokogawa Medical Systems Limited
Kojima Moonray
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