Spatial homogenization of the field strength of radiofrequency pulses

Patent metadata

Field	Value
Publication number	US-9952298-B2
Application number	US-201414570081-A
Country	US
Kind code	B2
Filing date	Dec 15, 2014
Priority date	Dec 17, 2013
Publication date	Apr 24, 2018
Grant date	Apr 24, 2018

Abstract

A method and a device are provided for spatial homogenization of the field strength of voltages of radiofrequency pulses of a number of RF transmitters, wherein (a) the measured complex voltages, induced by the electromagnetic field of the antenna, of a plurality of field probes, which are disposed in the vicinity of the antenna, are respectively superposed with a fitting phase shift and used for establishing the new desired homogenized voltages of the radiofrequency pulses of the antenna by a complex transfer function, and/or (b) output signals of at least two directional couplers on the RF feed lines of the antenna are superposed in a respectively fitting phase-shifted manner and these are used to establish complex impedances or scattering parameters of a complex scattering matrix of the antenna, which are used for establishing the new desired homogenized voltages of the radiofrequency pulses of the antenna.

First claim

The invention claimed is:

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.

Assignees

• Eberler Ludwig
• Nistler Juergen
• Siemens Ag

Inventors

• Eberler Ludwig
• Nistler Jürgen

Classifications

• G01R33/3607 Primary

  RF waveform generators, e.g. frequency generators, amplitude-, frequency- or phase modulators or shifters, pulse programmers, digital to analog converters for the RF signal, means for filtering or attenuating of the RF signal · CPC title

• G01R33/28

  Details of apparatus provided for in groups G01R33/44 - G01R33/64 · CPC title

• G01R33/583

  Calibration of signal excitation or detection systems, e.g. for optimal RF excitation power or frequency (G01R33/246 takes precedence) · CPC title