METHOD FOR DETERMINING A B0 MAP

Abstract

A method for determining a B.sub.0 map for, for example, performing an imaging magnetic resonance measurement using a magnetic resonance apparatus, includes measuring an original magnetic field distribution in a measurement volume of the magnetic resonance apparatus, and computing a final B.sub.0 map that describes a magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus by setting a shim state. The magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus by setting the shim state differs from the original magnetic field distribution.

Claims

1. A method for computing a final B.sub.0 map for performing an imaging magnetic resonance measurement using a magnetic resonance apparatus, the method comprising: measuring an original magnetic field distribution in a measurement volume of the magnetic resonance apparatus; and computing a final B.sub.0 map that describes a magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus by setting a shim state, wherein a magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus by setting the shim state differs from the original magnetic field distribution.

2. The method of claim 1, further comprising setting the shim state based on the original magnetic field distribution.

3. The method of claim 3, wherein setting the shim state based on the original magnetic field distribution comprises setting the shim state based on a first B.sub.0 map that is determined from the original magnetic field distribution.

4. The method of claim 1, further comprising computing control signals using the final B.sub.0 map, the control signals being for transmission of a radio frequency (RF) pulse by at least one transmit antenna of the magnetic resonance apparatus.

5. The method of claim 4, wherein the at least one transmit antenna comprises a plurality of transmit antennas.

6. The method of claim 2, wherein setting the shim state comprises determining electric shim currents for at least one magnetic field coil of the magnetic resonance apparatus based on the original magnetic field distribution, wherein the computing of the final B.sub.0 map is carried out based on the original magnetic field distribution and the determined electric shim currents for the at least one magnetic field coil of the magnetic resonance apparatus.

7. The method of claim 6, wherein the computing of the final B.sub.0 map is carried out based on a first B.sub.0 map that is determined from the original magnetic field distribution,

8. The method as claimed in claim 6, further comprising computing an additional magnetic field distribution brought about in the measurement volume of the magnetic resonance apparatus by the determined shim currents, wherein the additional magnetic field distribution is added to the original magnetic field distribution in the measurement volume of the magnetic resonance apparatus.

9. The method of claim 1, wherein computing the final B.sub.0 map comprises: receiving the original magnetic field distribution or a B.sub.0 map as input data of a trained function; applying the trained function to the input data; and outputting the final B.sub.0 map as output data of the trained function.

10. The method of claim 9, wherein receiving the original magnetic field distribution or the B.sub.0 map as the input data of the trained function comprises receiving a first B.sub.0 map that is determined from the original magnetic field distribution as the input data of the trained function.

11. The method of claim 2, wherein setting the shim state comprises determining electric shim currents for at least one magnetic field coil of the magnetic resonance apparatus, and wherein computing the final B.sub.0 map comprises: computing a provisional B.sub.0 field based on the original magnetic field distribution and the determined electric shim currents for the at least one magnetic field coil; receiving the provisional B.sub.0 map as input data of a trained function; applying the trained function to the input data; and outputting the final B.sub.0 map as output data of the further trained function.

12. The method of claim 11, wherein computing the provisional B.sub.0 map comprises: computing an additional magnetic field distribution brought about in the measurement volume of the magnetic resonance apparatus by the determined electric shim currents; and adding the additional magnetic field distribution to the original magnetic field distribution in the measurement volume of the magnetic resonance apparatus.

13. The method of claim 1, further comprising performing an imaging magnetic resonance measurement using the magnetic resonance apparatus and the final B.sub.0 map.

14. A system control unit for a magnetic resonance apparatus, the system control unit comprising: a processor configured to: receive an original magnetic field distribution in a measurement volume of the magnetic resonance apparatus; compute a final B.sub.0 map that describes a magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus by setting a shim state, wherein a magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus by setting the shim state differs from the original magnetic field distribution.

15. A magnetic resonance apparatus comprising: a system control unit comprising: a processor configured to: measure an original magnetic field distribution in a measurement volume of the magnetic resonance apparatus; set a shim state in the measurement volume of the magnetic resonance apparatus based on the measured original magnetic field distribution, wherein a magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus by setting the shim state differs from the original magnetic field distribution; and compute a final B.sub.0 map that describes a magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus by the setting of a shim state.

16. The magnetic resonance apparatus of claim 15, wherein the magnetic resonance apparatus is configured to perform an imaging magnetic resonance measurement using the final B.sub.0 map.

17. In a non-transitory computer-readable storage medium that stores instructions executable by a system control unit of a magnetic resonance apparatus to compute a final B.sub.0 map for performing an imaging magnetic resonance measurement using a magnetic resonance apparatus, the instructions comprising: measuring an original magnetic field distribution in a measurement volume of the magnetic resonance apparatus; and computing a final B.sub.0 map that describes a magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus by setting a shim state, wherein a magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus by setting the shim state differs from the original magnetic field distribution.

18. The non-transitory computer-readable storage medium of claim 17, wherein the instructions further comprise setting the shim state based on the original magnetic field distribution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Further advantages, features, and details of the present embodiments appear in the embodiments described below and follow from the drawings. Corresponding parts are denoted by the same reference signs in all the figures, in which:

[0044] FIG. 1 is a schematic diagram of one embodiment of a magnetic resonance apparatus;

[0045] FIG. 2 is a block diagram of a first embodiment of a method;

[0046] FIG. 3 is a block diagram of a second embodiment of the method;

[0047] FIG. 4 is a block diagram of a third embodiment of the method;

[0048] FIG. 5 is a block diagram of a fourth embodiment of the method; and

[0049] FIG. 6 is a block diagram of a method for training a trained function.

DETAILED DESCRIPTION

[0050] FIG. 1 shows schematically one embodiment of a magnetic resonance apparatus 10. The magnetic resonance apparatus 10 includes a patient placement zone 14 for accommodating a patient 15. In the present exemplary embodiment, the patient placement zone 14 is shaped as a cylinder and is enclosed in a circumferential direction cylindrically by a magnet unit 11. In principle, however, the patient placement zone 14 may have a different design. The patient 15 may be moved into the patient placement zone 14 by a patient positioning apparatus 16 of the magnetic resonance apparatus 10. The patient positioning apparatus 16 includes for this purpose a patient couch 17 that is configured to be able to move inside the patient placement zone 14.

[0051] The magnet unit 11 includes a main magnet 12 for producing a powerful main magnetic field 13 that, for example, is constant over time. For example, the main magnetic field 13 and the interaction thereof with the patient 15 results in a magnetic field distribution B.sub.0 in a measurement volume inside the patient placement zone 14. This magnetic field distribution B.sub.0 may be represented in the form of a B.sub.0 map. The magnet unit 11 further includes a gradient coil unit 18 for generating magnetic field gradients that are used for spatial encoding during imaging. The gradient coil unit 18 is controlled by a gradient control unit 19 of the magnetic resonance apparatus 10.

[0052] The magnet unit 11 also includes a radiofrequency antenna unit 20 that, in the present exemplary embodiment, is in the form of a body coil that is fixedly integrated in the magnetic resonance apparatus 10. The radiofrequency antenna unit 20 is controlled by a radiofrequency antenna control unit 21 of the magnetic resonance apparatus 10 and radiates radiofrequency magnetic resonance sequences into an examination space that is largely formed by a patient placement zone 14 of the magnetic resonance apparatus 10. A B.sub.1 distribution and excitation of atomic nuclei is thereby established in the main magnetic field 13 produced by the main magnet 12. Magnetic resonance signals are produced by relaxation of the excited atomic nuclei. The radiofrequency antenna unit 20 is configured to receive the magnetic resonance signals.

[0053] The magnetic resonance apparatus 10 includes a system control unit 22 for controlling the main magnet 12, the gradient control unit 19, and the radiofrequency antenna control unit 21. The system control unit 22 centrally controls the magnetic resonance apparatus 10 (e.g., implementing a predetermined imaging gradient echo sequence). In addition, the system control unit 22 includes an analysis unit (not presented in further detail) for analyzing the magnetic resonance signals captured during the magnetic resonance examination. In addition, the magnetic resonance apparatus 10 includes a user interface 23 that is connected to the system control unit 22. Control data such as imaging parameters, for example, and reconstructed magnetic resonance images may be displayed to medical personnel on a display unit 24 (e.g., on at least one monitor) of the user interface 23. In addition, the user interface 23 includes an input unit 25 that may be used by the medical operating personnel to enter data and/or parameters during a measurement procedure.

[0054] Typically, a series of calibration measurements are performed before the start of an imaging magnetic resonance measurement (e.g., the measurement of the magnetic resonance signals from which magnetic resonance images may be reconstructed, such as clinical and/or diagnostic magnetic resonance images). Depending on the application, these are used, for example, to determine an optimum fundamental frequency of undisturbed protons, to implement preferred shim settings, or to allow dependencies on receive-coil profiles to be taken into account.

[0055] For some types of imaging magnetic resonance measurements, a B.sub.0 distribution in the object under examination after shimming has been carried out is to be known, with the B.sub.0 distribution generally being different before and after the shimming. This may be the case, for example, if a measurement uses dynamic or multichannel RF pulses, for the computation of which of the B.sub.0 and/or the B.sub.1 distribution is to be known.

[0056] Different embodiments of a method are described below in which, based on a first B.sub.0 map measured before a shimming procedure, a final B.sub.0 map for the state after the shimming procedure is computed using algorithm. It is thereby possible to dispense with measuring the B.sub.0 map after shimming. The embodiments are characterized, for example, in that the final B.sub.0 map may be computed from shim currents used and/or may be ascertained by a trained function (e.g., a deep-learning algorithm). The features mentioned below of any method acts that arise in more than one embodiment and have the same reference sign may also be applied to embodiments that are described without mention of these features.

[0057] FIG. 2 shows one embodiment of a method for computing a final B.sub.0 map (e.g., for performing an imaging magnetic resonance measurement using the magnetic resonance apparatus 10). In S10, an original magnetic field distribution in a measurement volume of the magnetic resonance apparatus 10 is measured (e.g., a first B.sub.0 map that describes an original magnetic field distribution in a measurement volume of the magnetic resonance apparatus 10). This measurement may be performed using a 3-echo method. The first B.sub.0 map may be determined based on the measured original magnetic field distribution, and therefore, the first B.sub.0 map may also be regarded as a measured B.sub.0 map.

[0058] In S20, a shim state is set based on the first B.sub.0 map. The magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus 10 by the setting of the shim state differs from the original magnetic field distribution. The resulting magnetic field distribution may be more homogeneous than the original magnetic field distribution. For the purpose of setting the shim state, the gradient control unit 19, for example, controls the gradient coil unit 18 in a suitable manner to perform 1st-order shimming. The magnetic resonance apparatus 10 may also include dedicated shim coils (not shown in FIG. 1) that may be used to compensate for (e.g., to shim) higher orders as well.

[0059] In S30, a final B.sub.0 map is computed. The final B.sub.0 map describes the magnetic field distribution produced in the measurement volume of the magnetic resonance apparatus 10 by the setting of the shim state. The computing may be carried out in the system control unit 22, for example. In S40, the imaging magnetic resonance measurement is performed. For example, this involves measuring magnetic resonance signals from which magnetic resonance images of the patient may be reconstructed. The computed final B.sub.0 map is used to perform this measurement (e.g., to compute dynamic and/or multichannel RF pulses). In contrast with conventional imaging magnetic resonance examinations, by computing the final B.sub.0 map, it is possible to dispense with the time-consuming measurement of a B.sub.0 map in the shim state.

[0060] FIGS. 3 to 5 show different ways of computing the final B.sub.0 map.

[0061] According to FIG. 3, setting the shim state in S20 includes determining electric shim currents for at least one magnetic field coil of the magnetic resonance apparatus 10 based on the original magnetic field distribution in S21. The at least one magnetic field coil may be, for example, the aforementioned gradient coil unit 18 and/or a dedicated shim coil.

[0062] The final B.sub.0 map is then computed based on the original magnetic field distribution and the determined shim currents for the at least one magnetic field coil of the magnetic resonance apparatus. Thus, the computing of the final B.sub.0 map in S30 includes computing, in S32, an additional magnetic field distribution created in the measurement volume of the magnetic resonance apparatus 10 by the shim currents determined in S21. Since the shim currents that are used for the imaging magnetic resonance measurement performed in S40 are known anyway, these may be simulated, for example, and the additional magnetic field distribution resulting therefrom may be computed.

[0063] In addition, the original magnetic field distribution measured in S10 is provided in S31. In S33, the original field distribution and the additional magnetic field distribution are added, and the final B.sub.0 map is ascertained from the result of the addition. This resultant final B.sub.0 map may then be a close approximation to the state during shimming.

[0064] As described below in greater detail, FIG. 4 shows an embodiment of the method depicted in FIG. 2, in which the final B.sub.0 map is ascertained by a trained function (e.g., a deep-learning method). The method includes an algorithm that was set in a training phase and in which a sufficient number of pairs of first a measured B.sub.0 map before the shimming and second a measured B.sub.0 map after the shimming were used in order to train the trained function and/or a neural network. After the training phase, the algorithm may be used such that the algorithm provides, from a B.sub.0 map measured before the shimming, an expected B.sub.0 map after the shimming.

[0065] For example, in S35, the first B.sub.0 map is received as input data of a trained function (e.g., by the system control unit 22). In S36, the trained function is applied to the input data. In S37, the final B.sub.0 map is output as output data of the trained function.

[0066] FIG. 5 shows an embodiment of the method depicted in FIG. 2, which may be regarded as a combination of aspects of the embodiments depicted in FIG. 3 and FIG. 4. In one embodiment, a trained function (e.g., a deep-learning method) may learn a difference between a B.sub.0 map computed using a method according to FIG. 3, and an actually measured B.sub.0 map, and thus improve the method.

[0067] For example, computing the final B.sub.0 map in S30 includes computing a provisional B.sub.0 field in S31 based on the original magnetic field distribution and the shim currents determined in S21 for the at least one magnetic field coil.

[0068] For example, in order to compute a provisional B.sub.0 map, an additional magnetic field distribution brought about in the measurement volume of the magnetic resonance apparatus by the determined shim currents is computed in S33. In S32, the original magnetic field distribution is provided. In S34, the additional magnetic field distribution is added to the original magnetic field distribution in the measurement volume of the magnetic resonance apparatus 10, resulting in the provisional B.sub.0 map.

[0069] In S35*, the provisional B.sub.0 map is received as input data of a further trained function. In S36*, the further trained function is applied to the input data. In S37*, the final B.sub.0 map is output as output data of the further trained function.

[0070] FIG. 6 shows training a trained function. In S50, input training data is received. According to the embodiment shown in FIG. 4, the input training data may include, for example, a plurality of first training B.sub.0 maps, each of which describes an original magnetic training field distribution in a measurement volume of a training magnetic resonance apparatus.

[0071] According to the embodiment shown in FIG. 5, the input training data may include, for example, a plurality of provisional training B.sub.0 maps. Each provisional training B.sub.0 map of the plurality of provisional training B.sub.0 maps may be computed as follows. A first training B.sub.0 map is measured, where the first B.sub.0 map describes an original magnetic training field distribution in a measurement volume of a training magnetic resonance apparatus. Electric training shim currents for at least one magnetic field coil of a training magnetic resonance apparatus are determined based on the original magnetic training field distribution for the purpose of setting a shim state. The provisional training B.sub.0 map is computed based on the original magnetic training field distribution and the determined training shim currents for the at least one magnetic field coil of the training magnetic resonance apparatus.

[0072] In S60, output training data is received. According to the embodiments shown in FIGS. 4 and 5, the output training data may include, for example, a plurality of final training B.sub.0 maps, each of which describes a measured magnetic training field distribution in the measurement volume of the training magnetic resonance apparatus in a training shim state in which the measured magnetic training field distribution is different from the original magnetic training field distribution.

[0073] In S70, a trained function is trained based on the first training data and the final training data (e.g., based on the plurality of first training B.sub.0 maps or the plurality of provisional training B.sub.0 maps, and the plurality of final training B.sub.0 maps).

[0074] The acts S50, S60, and S70 may be performed, for example, by a system control unit of the training magnetic resonance apparatus and by a dedicated computing unit (e.g., a dedicated computer). The training data may be acquired by the same magnetic resonance apparatus 10 that is then also used to acquire the data to which the trained function is applied. It is also possible, however, to use other magnetic resonance apparatuses for acquiring the training data.

[0075] To summarize, the proposed embodiments may be used to determine a B.sub.0 map after the shimming from a measured B.sub.0 map before the shimming, with the result that the time for measuring the final B.sub.0 map may be saved. The computing may be performed, for example, based on the shim currents used and/or a trained function or a trained network. Thus, compared with conventional methods, calibration time before the start of the measurement may be saved, thereby speeding up the entire measurement. This may also help towards acceptance of methods using dynamic and/or multichannel RF pulses.

[0076] The methods described in detail above and the presented magnetic resonance apparatus are merely embodiments that may be modified by a person skilled in the art in many ways without departing from the scope of the invention. In addition, the use of the indefinite article “a” or “an” does not rule out the possibility of there also being more than one of the features concerned. Likewise, the term “unit” does not exclude the possibility that the components in question consist of a plurality of interacting sub-components that may also be spatially distributed if applicable.

[0077] The elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent. Such new combinations are to be understood as forming a part of the present specification.

[0078] While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.