Method for monitoring patient exposure in a magnetic resonance device

1. A method for monitoring patient exposure during an examination with a magnetic resonance device, the magnetic resonance device comprising a transmitter device with a high-frequency coil that is actuatable via a plurality of transmission channels, at least two power amplifiers being used to supply power to the plurality of transmission channels, wherein for each transmission channel of the plurality of transmission channels, a narrowband first measuring device configured to measure an amplitude and a phase of a voltage in a defined frequency range around a magnetic resonance frequency and a broadband second measuring device covering a wider frequency range than the first measuring device and configured to record voltage measurement values suitable for determining an overall power are used, the method comprising:

determining a coil power loss from the measured amplitude and the measured phase of the first measuring device;

determining an overall transmitted power from the voltage measurement values of the second measuring device;

determining a specific absorption rate (SAR) value describing a power entering a patient from the coil power loss and the overall transmitted power and comparing the SAR value with at least one limit value; and

terminating a transmission operation of the transmitter device when the at least one limit value is exceeded.

2. The method as claimed in claim 1 , wherein a defined frequency range of less than 500 kHz around the magnetic resonance frequency is used.

3. The method as claimed in claim 2 , wherein the defined frequency range around the magnetic resonance frequency is less than 250 kHz.

4. The method as claimed in claim 1 , wherein the first measuring device and the second measuring device are coupled via a directional coupler.

5. The method as claimed in claim 1 , further comprising determining, using the second measuring device, a forward power and a reflected power, the overall power being the difference between the forward power and the reflected power.

6. The method as claimed in claim 1 , wherein determining the coil power loss from the measured amplitude and the measured phase comprises taking a previously determined scatter parameter matrix for the high-frequency coil into account.

7. The method as claimed in claim 6 , further comprising determining the scatter parameter matrix in the context of a calibration measurement without patients.

8. The method as claimed in claim 1 , wherein determining the SAR value comprises determining the SAR value from a difference between the overall power and the coil power loss using a conversion factor.

9. The method as claimed in claim 8 , wherein the conversion factor is dependent on an exposed region of the patient, a measuring accuracy of the first measuring device and the second measuring device, or the exposed region and the measuring accuracy.

10. The method as claimed in claim 1 , wherein determining the SAR value comprises determining the SAR value as a moving average across a predetermined time window.

11. The method as claimed in claim 10 , wherein the time window is 10 seconds.

12. The method as claimed in claim 10 , wherein the time window is 360 seconds.

13. The method as claimed in claim 10 , wherein determining the SAR value comprises determining the SAR value by averaging the coil power loss and the overall power.

14. The method as claimed in claim 11 , wherein determining the SAR value comprises determining the SAR value by averaging the coil power loss and the overall power.

15. The method as claimed in claim 12 , wherein determining the SAR value comprises determining the SAR value by averaging the coil power loss and the overall power.

16. The method as claimed in claim 1 , wherein the measured amplitude, the measured phase, and the voltage measurement values are scanned and processed in a digital signal processor.

17. The method as claimed in claim 1 , wherein a number of power amplifiers corresponding to the number of transmission channels of the plurality of transmission channels are used.

18. A transmitter device for a magnetic resonance device, the transmitter device comprising:

a high-frequency coil that is accountable via a plurality of transmission channels;

at least two power amplifiers operable to supply power to the plurality of transmission channels;

a monitoring device that, for each transmission channel of the plurality of transmission channels connected downstream of the at least two power amplifiers, has a narrowband first measuring device configured to measure an amplitude and a phase of a voltage in a defined frequency range around a magnetic resonance frequency, and a broadband second measuring device covering a wider frequency range than the first measuring device and configured to record voltage measurement values suitable for determining an overall power; and

an arithmetic device configured to:

determine a coil power loss from the measured amplitude and the measured phase of the first measuring device;

determine an overall transmitted power from the voltage measurement values of the second measuring device;

determine a specific absorption rate (SAR) value describing a power entering a patient from the coil power loss and the overall transmitted power and compare the SAR value with at least one limit value; and

terminate a transmission operation of the transmitter device when the at least one limit value is exceeded.

19. The transmitter device as claimed in claim 18 , wherein the at least two power amplifiers comprise a plurality of power amplifiers corresponding to the number of transmission channels.

20. A magnetic resonance device comprising:

a transmitter device comprising:

a high-frequency coil that is accountable via a plurality of transmission channels;

at least two power amplifiers operable to supply power to the plurality of transmission channels;

a monitoring device that, for each transmission channel of the plurality of transmission channels connected downstream of the at least two power amplifiers, has a narrowband first measuring device configured to measure an amplitude and a phase of a voltage in a defined frequency range around a magnetic resonance frequency, and a broadband second measuring device covering a wider frequency range than the first measuring device and configured to record voltage measurement values suitable for determining an overall power; and

an arithmetic device configured to:

determine a coil power loss from the measured amplitude and the measured phase of the first measuring device;

determine an overall transmitted power from the voltage measurement values of the second measuring device;

determine a specific absorption rate (SAR) value describing a power entering a patient from the coil power loss and the overall transmitted power and compare the SAR value with at least one limit value; and

terminate a transmission operation of the transmitter device when the at least one limit value is exceeded.

21. A method for monitoring patient exposure during an examination with a magnetic resonance device, the method comprising:

determining a coil power loss from a measured amplitude and a measured phase of a first measuring device;

determining an overall transmitted power from voltage measurement values of a second measuring device;

determining a specific absorption rate (SAR) value describing a power entering a patient from the coil power loss and the overall transmitted power, and comparing the SAR value with at least one limit value; and

terminating a transmission operation of a transmitter device of the magnetic resonance device when the at least one limit value is exceeded.