Method and device for determining a magnetic resonance system activation sequence

1. A method for determining a magnetic resonance system activation sequence, the magnetic resonance system activation sequence comprising a multichannel pulse train having a plurality of individual HF pulse trains emitted in a parallel manner by the magnetic resonance system by way of different independent high-frequency transmit channels, the method comprising:

calculating, with a control sequence determination device, the multichannel pulse train based on a predefined target magnetization using an HF pulse optimization, calculating the multichannel pulse train using the HF pulse optimization comprising optimizing a setpoint deviation of an HF local exposure value from an HF global exposure value, optimizing the setpoint deviation comprising optimizing a ratio of the HF local exposure value to the HF global exposure value to a predefined value; and

changing, subsequent to optimizing the setpoint deviation, parameters relating to HF exposure such that the HF local exposure value is increased when the HF local exposure value is below a predefined setpoint value.

2. The method as claimed in claim 1 , wherein the HF local exposure value is based on a combination of different local HF exposure values in different volume units.

3. The method as claimed in claim 1 , wherein a local HF exposure value is based on a correlation of the plurality of individual HF pulse trains of the multichannel pulse train, a sensitivity matrix, or the correlation of the plurality of individual HF pulse trains of the multichannel pulse train and the sensitivity matrix, the sensitivity matrix representing the dependency of an HF exposure on a current HF transmit amplitude in a respective volume unit for different volume units of an examination object.

4. The method as claimed in claim 2 , wherein the HF local exposure value comprises a predefined standard of an HF local exposure vector.

5. The method as claimed in claim 1 , further comprising a plurality of method stages, with the HF local exposure value being optimized with respect to the setpoint deviation from the HF global exposure value using different methods in different method stages of the plurality of method stages.

6. The method as claimed in claim 1 , wherein a target function used within the HF pulse optimization is predefined such that the target function comprises at least one HF local exposure value of an examination object, the at least one HF local exposure value being a function of the magnetic resonance system activation sequence.

7. The method as claimed in claim 6 , wherein the target function is a function of a deviation of the HF local exposure value from the HF global exposure value.

8. The method as claimed in claim 1 , wherein calculating the multichannel pulse train comprises calculating the multichannel pulse train on the basis of a predefined k space gradient trajectory, the predefined k space gradient trajectory being optimized in an HF exposure optimization using a parameterizable function with respect to the HF local exposure value.

9. The method as claimed in claim 8 , wherein a sampling speed of the predefined k space gradient trajectory is set during the emission of HF pulses such that the sampling speed is reduced at points where an expected HF voltage is higher than a predefined HF voltage and increased at points where the expected HF voltage is lower than the predefined HF voltage.

10. The method as claimed in claim 1 , wherein an HF pulse form is varied so that HF pulses are extended over time at points where an expected local HF exposure value is higher than a predetermined local HF exposure value and compressed over time at points where the expected local HF exposure value is lower than the predetermined local HF exposure value.

11. The method as claimed in claim 1 , further comprising recalculating the multichannel pulse train to emit repetition pulses.

12. A method for operating a magnetic resonance system, the magnetic resonance system having a plurality of independent high-frequency transmit channels, the method comprising:

determining an activation sequence, the activation sequence comprising a multichannel pulse train having a plurality of individual HF pulse trains emitted in a parallel manner by the magnetic resonance system by way of different independent high-frequency

transmit channels,

wherein the determining comprises:

calculating the multichannel pulse train based on a predefined target magnetization using an HF pulse optimization, calculating the multichannel pulse train using the HF pulse optimization comprising optimizing a setpoint deviation of an HF local exposure value from an HF global exposure value, optimizing the setpoint deviation comprising optimizing a ratio of the HF local exposure value to the HF global exposure value to a predefined value; and

changing, subsequent to optimizing the setpoint deviation, parameters relating to HF exposure such that the HF local exposure value is increased when the HF local exposure value is below a predefined setpoint value; and

operating the magnetic resonance system using the activation sequence.

13. A control sequence determination device for determining a magnetic resonance system activation sequence, the magnetic resonance system activation sequence comprising a multichannel pulse train with a plurality of individual HF pulse trains to be emitted in a parallel manner by the magnetic resonance system by way of different independent high-frequency transmit channels, the control sequence determination device comprising:

an input interface configured to detect a target magnetization;

an HF pulse optimization unit configured to calculate the multichannel pulse train on the basis of a predefined target magnetization in an HF pulse optimization; and

a control sequence output interface,

wherein the HF pulse optimization unit is configured to perform an optimization with respect to a setpoint deviation of an HF local exposure value from an HF global exposure value in the HF pulse optimization, and

wherein the HF pulse optimization unit is further configured to change, subsequent to the optimization of the setpoint deviation, parameters relating to HF exposure such that the HF local exposure value is increased when the HF local exposure value is below a predefined setpoint value.

14. A magnetic resonance system comprising:

a plurality of independent high-frequency transmit channels;

a gradient system;

a control device configured to emit a multichannel pulse train with a plurality of parallel individual HF pulse trains by way of the plurality of independent high-frequency transmit channels to perform a desired measurement based on a predefined activation sequence;

a control sequence determination device configured to determine the predefined activation sequence and transfer the predefined activation sequence to the control device, the control sequence determination device comprising:

an input interface configured to detect a target magnetization; and

an HF pulse optimization unit configured to calculate the multichannel pulse train on the basis of a predefined target magnetization in an HF pulse optimization; and

a control sequence output interface,

wherein the HF pulse optimization unit is configured to perform an optimization with respect to a setpoint deviation of an HF local exposure value from an HF global exposure value in the HF pulse optimization, and

wherein the HF pulse optimization unit is further configured to change, subsequent to the optimization of the setpoint deviation, parameters relating to HF exposure such that the HF local exposure value is increased when the HF local exposure value is below a predefined setpoint value.

15. A non-transitory computer readable medium comprising computer readable instructions that, when executed by a control sequence determination device, cause the control sequence determination device determine a magnetic resonance system activation sequence, the magnetic resonance system activation sequence comprising a multichannel pulse train having a plurality of individual HF pulse trains emitted in a parallel manner by the magnetic resonance system by way of different independent high-frequency transmit channels, the computer readable instructions comprising:

calculating the multichannel pulse train based on a predefined target magnetization using an HF pulse optimization,

wherein a setpoint deviation of an HF local exposure value from an HF global exposure value is optimized in the HF pulse optimization, parameters relating to HF exposure being changeable such that, subsequent to the optimization of the setpoint deviation, the HF local exposure value is increased when the HF local exposure value is below a predefined setpoint value.

16. The method as claimed in claim 3 , wherein the HF local exposure value comprises a predefined standard of an HF local exposure vector.

17. The method as claimed in claim 2 , wherein a target function used within the HF pulse optimization is predefined such that the target function comprises at least one HF local exposure value of an examination object, the at least one HF local exposure value being a function of the magnetic resonance system activation sequence.

18. The method of claim 1 , wherein optimizing the setpoint deviation of the HF local exposure value from the HF global exposure value comprises optimizing a setpoint deviation of a maximum HF local exposure value from the HF global exposure value.

19. The method of claim 1 , wherein calculating the multichannel pulse train using the HF pulse optimization comprises weighting importance of the setpoint deviation of the HF local exposure value from the HF global exposure value relative to homogeneity of magnetization.

20. The method of claim 19 , wherein the weighting comprises identifying a predefined weighting factor applied to the setpoint deviation of the HF local exposure value from the HF global exposure value.

21. The method of claim 19 , wherein the weighting comprises optimizing a weighting factor applied to the setpoint deviation of the HF local exposure value from the HF global exposure value.