Spatial homogenization of the field strength of radiofrequency pulses

1. A method for spatial homogenization of field strength of voltages of radiofrequency pulses of a number of radiofrequency (RF) transmitters, which are emitted by an antenna of a magnetic resonance measurement apparatus during a magnetic resonance measurement of an object to be examined, wherein test radiofrequency pulses are emitted in advance during a test measurement, the method comprising:

superposing, by a computer unit, in a phase-shifted manner during the test measurement, output signals of a plurality of directional couplers on RF feed lines of the antenna;

establishing, by the computer unit, complex impedances or scattering parameters of a complex scattering matrix of the antenna with the superposed output signals;

measuring the magnetic resonance of the object by the magnetic resonance measurement apparatus; and

establishing, during the measuring, new desired homogenized excitation voltages of radiofrequency pulses of the antenna using the established complex impedances or scattering parameters of the complex scattering matrix.

2. The method as claimed in claim 1 , wherein the scattering parameters or the complex impedances relate to a measurement or calibration plane situated in a vicinity of an output of the amplifiers of the number of RF transmitters.

3. The method as claimed in claim 2 , wherein lengths of feed lines from the number of RF transmitters to the antenna are taken into account in the directional couplers as additional complex impedances of the antenna.

4. The method as claimed in claim 3 , wherein the radiofrequency pulses are transmitted from the number of RF transmitters to the antenna via feed contacts on the antenna, wherein the feed contacts are respectively disposed offset from one another by a same angular value.

5. The method as claimed in claim 4 , wherein the voltages measured by the input impedances of the directional couplers are returned in a closed control loop to an RF receiver, and desired excitation voltage pulses of the number of RF transmitters are established in the computer unit and applied to the number of RF transmitters.

6. The method as claimed in claim 2 , wherein a desired measurement position of each object to be examined comprises:

a) determining the complex scattering matrix in the form of an N×N matrix, where N is a number of channels of feed contacts of the feed lines of the number of RF transmitters on the antenna;

b) transforming the scattering matrix in the reference plane of the feed contacts of the number of RF transmitters on the antenna to provide a transformed scattering matrix;

c) setting a desired excitation of the RF voltage pulses of the number of RF transmitters on a basis of a mode of the antenna;

d) calculating necessary transformed voltages using the following equation:

Utx -trans=inv( E+S )· Utx - soll , wherein:

Utx-trans=the necessary transformed voltages;

E is an identity matrix;

S is the scattering matrix; and

Utx-soll=an intended value of the voltages of the RF transmitters,

e) transforming the voltages from a reference plane back into a measurement and calibration plane to calculate the desired excitation voltage pulses of the number of RF transmitters.

7. The method as claimed in claim 6 , wherein lengths of feed lines from the number of RF transmitters to the antenna are taken into account in the directional couplers as additional complex impedances of the antenna.

8. The method as claimed in claim 1 , wherein lengths of feed lines from the number of RF transmitters to the antenna are taken into account in the directional couplers as additional complex impedances of the antenna.

9. The method as claimed in claim 8 , wherein the radiofrequency pulses are transmitted from the number of RF transmitters to the antenna via feed contacts on the antenna, wherein the feed contacts are respectively disposed offset from one another by a same angular value.

10. The method as claimed in claim 1 , wherein the radiofrequency pulses are transmitted from the number of RF transmitters to the antenna via feed contacts on the antenna, wherein the feed contacts are respectively disposed offset from one another by a same angular value.

11. The method as claimed in claim 1 , wherein the voltages measured by the input impedances of the directional couplers are returned in a closed control loop to an RF receiver, and desired excitation voltage pulses of the number of RF transmitters are established in the computer unit and applied to the number of RF transmitters.

12. The method as claimed in claim 11 , wherein an actual value of the field strength of the voltages of the radiofrequency pulses is a controlled variable of the closed control loop,

wherein one or more of an intended value of the field strength or the complex input impedances of the antenna are reference variables of the closed control loop, and

wherein the desired excitation voltage pulses of the number of RF transmitters are manipulated variables of the closed control loop.

13. The method as claimed in claim 12 , wherein tolerances of actual RF voltage pulses in relation to the desired excitation voltage pulses are less than 10% of the calibration or the test measurement.

14. The method as claimed in claim 11 , wherein tolerances of actual RF voltage pulses in relation to the desired excitation voltage pulses are less than 10% of the calibration or the test measurement.

15. A device for spatial homogenization of field strength of voltages of radiofrequency pulses of a number of radiofrequency (RF) transmitters in a magnetic resonance measurement apparatus, the device comprising:

a test signal measurement apparatus comprising a plurality of directional couplers on RF feed lines of an antenna, and a computer unit configured to:

superpose, in a fitting phase-shifted manner, output signals from a plurality of directional couplers on the RF feed lines of the antenna,

establish, using the superposed output signals, complex input impedances of the antenna, and

establish, using the complex input impedances, new desired homogenized voltages of radiofrequency pulses of the antenna.

16. A magnetic resonance imaging (MRI) device, the MRI device comprising:

a device for spatial homogenization of field strength of voltages of radiofrequency pulses of a number of radiofrequency (RF) transmitters in a magnetic resonance measurement apparatus, the device comprising:

a test signal measurement apparatus comprising a plurality of directional couplers on RF feed lines of an antenna, and a computer unit configured to:

superpose, in a fitting phase-shifted manner, output signals from a plurality of directional couplers on the RF feed lines of the antenna, and

establish, using the superposed output signals, complex input impedances of the antenna, and

establish, using the complex input impedances, new desired homogenized voltages of radiofrequency pulses of the antenna.