Gradient impulse response function mapping

1. A magnetic resonance imaging system comprising:

a magnetic resonance imaging magnet for generating a main magnetic field for orientating the magnetic spins of nuclei of a subject located within an imaging volume,

at least one magnetic field gradient system for generating a gradient magnetic field for spatial encoding of a magnetic resonance signal of spins of nuclei within the imaging volume, the gradient system comprising a gradient amplifier and a gradient coil,

a radio-frequency system comprising a coil array with a plurality of coil elements configured for acquiring magnetic resonance data using parallel imaging,

a non-transitory computer readable memory storing machine executable instructions and pulse sequence commands, wherein the pulse sequence commands are configured for controlling the magnetic resonance imaging system to acquire the magnetic resonance data according to a parallel imaging protocol,

a processor for controlling the magnetic resonance imaging system, wherein execution of the machine executable instructions causes the processor to control the magnetic resonance imaging system to:

acquire by the coil elements first magnetic resonance data simultaneously from a first group of passive local probes using a first set of the pulse sequence commands, wherein the first group of passive local probes comprises a plurality of passive local probes located spaced apart from each other;

disentangle contributions to the first magnetic resonance data from the individual local probes using the parallel imaging protocol,

calculate for the magnetic resonance imaging system a gradient impulse response function of the gradient system using the first magnetic resonance data from the local probes;

determine a correction for compensating a deviation of the behavior of the gradient system from a predefined behavior using the gradient impulse response function;

apply the correction for generating magnetic resonance images of a subject, wherein generating the magnetic resonance images comprises acquiring second magnetic resonance data from the subject with a second set of the pulse sequence commands by the coil elements and reconstructing the magnetic resonance images of the subject using the second magnetic resonance data.

2. The magnetic resonance imaging system of claim 1 , wherein the first group of passive local probes comprises at least three passive local probes and wherein the calculation of the gradient impulse response function is at least up to second order contributions.

3. The magnetic resonance imaging system of claim 1 , wherein the passive local probes comprise a plurality of virtual δ-probes excited spatially selectively and spaced apart from each other within a common physical probe arranged within the magnetic resonance imaging system.

4. The magnetic resonance imaging system of claim 3 , wherein the common physical probe is a phantom probe.

5. The magnetic resonance imaging system of claim 3 , wherein the common physical probe is the subject of which the magnetic resonance images are to be generated.

6. The magnetic resonance imaging system of claim 1 , wherein the passive local probes comprise a plurality of physical phantom probes which are located spaced apart from each other within the magnetic resonance imaging system.

7. The magnetic resonance imaging system of claim 6 , wherein the physical phantom probes have a spherical shape and are arranged at the corners of a regular polyhedron.

8. The magnetic resonance imaging system of claim 6 , wherein the first set of pulse sequence commands is configured for a spatially non-selective excitation of the passive local probes and signal encoding of the resulting first magnetic resonance data based on parallel imaging.

9. The magnetic resonance imaging system of claim 6 , wherein, for calculating low frequency contributions to the gradient impulse function, the first set of the pulse sequence commands is configured for repeatedly exciting the passive local probes by repeatedly applying radio frequency pulses when acquiring the first magnetic resonance data.

10. The magnetic resonance imaging system of claim 9 , wherein the magnetic resonance imaging system comprises a second group of passive local probes, wherein the second group of passive local probes comprises a plurality of physical phantom probes which are located spaced apart from each other within the magnetic resonance imaging system, and wherein the acquiring of the first magnetic resonance data by repeatedly exciting the passive local probes comprises subsequently exciting the first and second group of passive local probes in an interleaved fashion.

11. The magnetic resonance imaging system of claim 10 , wherein the passive local probes of the first and second group of passive local probes comprise controllable shielding structures and wherein the group of passive local probes being excited is selected by controlling the shielding structures.

12. The magnetic resonance imaging system of claim 1 , wherein the first set of pulse sequence commands is configured for a spatially selective excitation of the passive local probes and signal encoding of the resulting first magnetic resonance data based on parallel imaging.

13. The magnetic resonance imaging system of claim 12 , wherein the spatially selective excitation of the passive local probes comprises applying multi-dimensional or multi-band excitation pulses.

14. The magnetic resonance imaging system of claim 1 , wherein the acquiring of the first magnetic resonance data comprises measuring a first set of magnetic resonance data from the passive local probes with the gradient field being applied to the passive local probes and a second set of magnetic resonance data from the passive local probes without applying the gradient field to the passive local probes, wherein off-resonance contributions are subtracted from the first magnetic resonance data, wherein the subtracting comprises subtracting the second set of magnetic resonance data from the first magnetic resonance data.

15. The magnetic resonance imaging system of claim 1 , wherein the correction is applied to the generation of the magnetic fields when acquiring the second magnetic resonance data and/or to the reconstruction of the magnetic resonance images using the second magnetic resonance data.

16. A computer program product comprising machine executable instructions stored on a non-transitory computer readable for execution by a processor of a magnetic resonance imaging system for controlling the magnetic resonance imaging system, wherein the magnetic resonance imaging system comprises a magnetic resonance imaging magnet for generating a main magnetic field for orientating the magnetic spins of nuclei of a subject located within an imaging volume, at least one magnetic field gradient system for generating a gradient magnetic field for spatial encoding of a magnetic resonance signal of spins of nuclei within the imaging volume, the gradient system comprising a gradient amplifier and a gradient coil, a radio-frequency system comprising a coil array with a plurality of coil elements configured for acquiring magnetic resonance data using parallel imaging, wherein execution of the machine executable instructions causes the processor to control the magnetic resonance imaging system to:

acquire by the coil elements first magnetic resonance data simultaneously from a plurality of passive local probes using a first set of the pulse sequence commands, wherein the local probes are located spaced apart from each other;

disentangle contributions to the first magnetic resonance data from the individual local probes using the parallel imaging protocol,

calculate for the magnetic resonance imaging system a gradient impulse response function of the gradient system using the first magnetic resonance data from the local probes;

determine a correction for compensating a deviation of the behavior of the gradient system from a predefined behavior using the gradient impulse response function;

apply the correction for generating magnetic resonance images of a subject, wherein generating the magnetic resonance images comprises acquiring second magnetic resonance data from the subject with a second set of the pulse sequence commands by the coil elements and reconstructing the magnetic resonance images of the subject using the second magnetic resonance data.

17. A method for operating a magnetic resonance imaging system, wherein the magnetic resonance imaging system comprises a magnetic resonance imaging magnet for generating a main magnetic field for orientating the magnetic spins of nuclei of a subject located within an imaging volume at least one magnetic field gradient system for generating a gradient magnetic field for spatial encoding of a magnetic resonance signal of spins of nuclei within the imaging volume, the gradient system comprising a gradient amplifier and a gradient coil, a radio-frequency system comprising a coil array with a plurality of coil elements configured for acquiring magnetic resonance data using parallel imaging, a non-transitory computer readable memory storing machine executable instructions and pulse sequence commands, wherein the pulse sequence commands are configured for controlling the magnetic resonance imaging system to acquire the magnetic resonance data according to a parallel imaging protocol, a processor for controlling the magnetic resonance imaging system, wherein the method comprises:

acquiring by the coil elements first magnetic resonance data simultaneously from a plurality of passive local probes using a first set of the pulse sequence commands, wherein the local probes are located spaced apart from each other;

disentangling contributions to the first magnetic resonance data from the individual local probes using the parallel imaging protocol,

calculating for the magnetic resonance imaging system a gradient impulse response function of the gradient system using the first magnetic resonance data from the local probes;

determining a correction for compensating a deviation of the behavior of the gradient system from a predefined behavior using the gradient impulse response function;

applying the correction for generating magnetic resonance images of a subject, wherein generating the magnetic resonance images comprises acquiring second magnetic resonance data from the subject with a second set of the pulse sequence commands by the coil elements and reconstructing the magnetic resonance images of the subject using the second magnetic resonance data.