METHOD FOR AUTOMATICALLY COMPENSATING EDDY CURRENTS IN A MAGNETIC RESONANCE APPARATUS

Abstract

Methods for automatically compensating eddy currents in a magnetic resonance apparatus include determining modified magnetic resonance sequence data by a compensation computing unit and performing a magnetic resonance measurement in which a gradient generating system generates magnetic field gradients based on the modified magnetic resonance sequence data. The determining of the modified magnetic field gradient includes: receiving original magnetic resonance sequence data of a predetermined magnetic resonance sequence; computing eddy current information about eddy currents that would be produced in the magnetic resonance apparatus by applying the original magnetic resonance sequence data; computing, based on the computed eddy current information, at least one eddy current compensation gradient pulse for compensating the eddy currents; generating modified magnetic resonance sequence data by inserting the at least one eddy current compensation gradient pulse into the original magnetic resonance sequence data; and outputting the modified magnetic resonance sequence data to the gradient generating system.

Claims

1. A method for automatically compensating eddy currents in a magnetic resonance apparatus, the method comprising: determining a modified magnetic resonance sequence data by a compensation computing unit as an intermediate layer of the magnetic resonance apparatus; and performing a magnetic resonance measurement in which a gradient generating system generates magnetic field gradients in the magnetic resonance apparatus based on the modified magnetic resonance sequence data, wherein the determining of the modified magnetic resonance sequence data comprises: receiving original magnetic resonance sequence data of a predetermined magnetic resonance sequence; computing eddy current information about eddy currents that would be produced in the magnetic resonance apparatus by applying the original magnetic resonance sequence data; computing, based on the computed eddy current information, at least one eddy current compensation gradient pulse for compensating the eddy currents; generating the modified magnetic resonance sequence data by inserting the at least one eddy current compensation gradient pulse into the original magnetic resonance sequence data; and outputting the modified magnetic resonance sequence data to the gradient generating system of the magnetic resonance apparatus.

2. The method of claim 1, wherein the modified magnetic resonance sequence data is determined at least in part during the magnetic resonance measurement.

3. The method of claim 2, wherein the modified magnetic resonance sequence data is determined in real time and/or on-the-fly.

4. The method of claim 3, wherein the original magnetic resonance sequence data of the predetermined magnetic resonance sequence is provided from a system control unit of the magnetic resonance apparatus, and wherein the original magnetic resonance sequence data of the predetermined magnetic resonance sequence remains unchanged in the system control unit.

5. The method of claim 4, wherein the predetermined magnetic resonance sequence comprises a diffusion-weighted sequence.

6. The method of claim 5, wherein the eddy currents are computed based on at least one parameter specific to a type of the magnetic resonance apparatus.

7. The method of claim 1, wherein the modified magnetic resonance sequence data is determined in real time and/or on-the-fly.

8. The method of claim 1, wherein the original magnetic resonance sequence data of the predetermined magnetic resonance sequence is provided from a system control unit of the magnetic resonance apparatus, and wherein the original magnetic resonance sequence data of the predetermined magnetic resonance sequence remains unchanged in the system control unit.

9. The method of claim 1, wherein the predetermined magnetic resonance sequence comprises a diffusion-weighted sequence.

10. The method of claim 1, wherein the eddy currents are computed based on at least one parameter specific to a type of the magnetic resonance apparatus.

11. The method of claim 1, wherein the determining of the modified magnetic resonance sequence data comprises detecting at least one fat saturation pulse in the original magnetic resonance sequence data, and wherein the at least one eddy current compensation gradient pulse is inserted into the original magnetic resonance sequence data before the at least one fat saturation pulse.

12. The method of claim 11, wherein the predetermined magnetic resonance sequence comprises applying gradient pulses to a plurality of axes of the gradient generating system of the magnetic resonance apparatus, and wherein the computing of the at least one eddy current compensation gradient pulse takes into account the gradient pulses so far applied in the magnetic resonance measurement to the plurality of axes of the gradient generating system of the magnetic resonance apparatus.

13. The method of claim 1, wherein the predetermined magnetic resonance sequence comprises applying gradient pulses to a plurality of axes of the gradient generating system of the magnetic resonance apparatus, and wherein the computing of the at least one eddy current compensation gradient pulse takes into account the gradient pulses so far applied in the magnetic resonance measurement to the plurality of axes of the gradient generating system of the magnetic resonance apparatus.

14. The method of claim 1, wherein the determining of the modified magnetic resonance sequence data comprises estimating a magnetic field perturbation that would be produced in the magnetic resonance apparatus by the eddy currents when applying the original magnetic resonance sequence data, and wherein the at least one eddy current compensation gradient pulse is computed and inserted into the original magnetic resonance sequence data only when the magnetic field perturbation exceeds a specified threshold value.

15. The method of claim 1, wherein the determining of the modified magnetic resonance sequence data comprises identifying in the original magnetic resonance sequence data a time segment in which the magnetic resonance apparatus does not switch any gradient pulses or transmit any radiofrequency (RF) pulses, and wherein the at least one eddy current compensation gradient pulse is inserted into the original magnetic resonance sequence data in the identified time segment.

16. The method of claim 1, wherein the original magnetic resonance sequence data has at least one dedicated placeholder for the inserting of computed eddy current compensation gradient pulses.

17. A compensation computing unit for determining modified magnetic resonance sequence data for performing a magnetic resonance measurement by a magnetic resonance apparatus, the compensation computing unit comprising: at least one processor configured to: receive original magnetic resonance sequence data of a predetermined magnetic resonance sequence; compute eddy current information about eddy currents that would be produced in the magnetic resonance apparatus by applying the original magnetic resonance sequence data; compute, based on the computed eddy current information, at least one eddy current compensation gradient pulse for compensating the eddy currents; generate modified magnetic resonance sequence data by inserting the at least one eddy current compensation gradient pulse into the original magnetic resonance sequence data; and output the modified magnetic resonance sequence data to a gradient generating system of the magnetic resonance apparatus for performing the magnetic resonance measurement.

18. A magnetic resonance apparatus comprising: a compensation computing unit, wherein the magnetic resonance apparatus is configured to: receive original magnetic resonance sequence data of a predetermined magnetic resonance sequence; compute eddy current information about eddy currents that would be produced in the magnetic resonance apparatus by applying the original magnetic resonance sequence data; compute, based on the computed eddy current information, at least one eddy current compensation gradient pulse for compensating the eddy currents; generate modified magnetic resonance sequence data by inserting the at least one eddy current compensation gradient pulse into the original magnetic resonance sequence data; and output the modified magnetic resonance sequence data to a gradient generating system of the magnetic resonance apparatus for performing a magnetic resonance measurement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Further advantages, features, and details of the disclosure appear in the exemplary embodiments described below and follow from the drawings. Corresponding parts are denoted by the same reference signs in all the figures, in which:

[0046] FIG. 1 is a schematic diagram of an example of a magnetic resonance apparatus.

[0047] FIG. 2 is a flow diagram of an example of a proposed method.

[0048] FIG. 3 is a sequence diagram containing an example of an inserted eddy current compensation gradient pulse.

DETAILED DESCRIPTION

[0049] FIG. 1 shows schematically a magnetic resonance apparatus 10. The magnetic resonance apparatus 10 includes a magnet unit 11, which contains a main magnet 12 for generating a powerful main magnetic field 13, which in particular is constant over time. The magnetic resonance apparatus 10 also includes a patient placement region 14 for accommodating a patient 15. In the present exemplary embodiment, the patient placement region 14 is shaped as a cylinder and is enclosed in a circumferential direction cylindrically by the magnet unit 11. In principle, however, it is conceivable that the patient placement region 14 has a different design. The patient 15 may be moved into the patient placement region 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, which is configured to be able to move inside the patient placement region 14.

[0050] The magnet unit 11 further includes a gradient coil unit 18 for generating magnetic field gradients, which 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. The gradient coil unit 18 and the gradient control unit 19 are parts of a gradient generating system. The magnet unit 11 also includes a radiofrequency antenna unit 20, which 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 magnetic resonance sequences into an examination space, which is formed by a patient placement region 14 of the magnetic resonance apparatus 10. Excitation of atomic nuclei thereby occurs in the main magnetic field 13 produced by the main magnet 12. Magnetic resonance signals are generated by relaxation of the excited atomic nuclei. The radiofrequency antenna unit 20 is configured to receive the magnetic resonance signals. The radiofrequency antenna unit 20 and the radiofrequency antenna control unit 21 are parts of a radiofrequency generating system.

[0051] 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, for instance in particular implementing a predetermined magnetic resonance sequence, for example, a 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 acquired during the magnetic resonance examination. In addition, the magnetic resonance apparatus 10 includes a user interface 23, which is connected to the system control unit 22. Control data such as imaging parameters, for instance, and reconstructed magnetic resonance images may be displayed to medical personnel on a display unit 24, (e.g., at least one monitor), of the user interface 23. In addition, the user interface 23 includes an input unit 25, which may be used by the medical operating personnel to enter data and/or parameters during a measurement procedure.

[0052] Between the system control unit 22 and the gradient control unit 19 is a compensation computing unit 26 arranged as an intermediate layer for automatically compensating eddy currents. The compensation computing unit 26 may include both hardware and software elements. Any data, in particular magnetic resonance sequence data of a predetermined magnetic resonance sequence, may be provided from the system control unit 22 of the magnetic resonance apparatus 10 to the compensation computing unit 26, without this data being modified in any way in the control unit 22.

[0053] The compensation computing unit 26 is integrated in modular form in the magnetic resonance apparatus 10. The compensation computing unit 26 may be configured such that on removing the compensation computing unit 26 from the magnetic resonance apparatus 10, the gradient generating system may continue to be operated without further limitations apart from the then missing eddy current compensation. FIG. 2 shows a corresponding method for automatically compensating eddy currents in the magnetic resonance apparatus 10. In S10, the compensation computing unit 26 determines modified magnetic resonance sequence data.

[0054] S10 includes a plurality of acts for this purpose. In S11, the system control unit 22 receives original magnetic resonance sequence data of a predetermined magnetic resonance sequence. This magnetic resonance sequence may include different sequence modules. The sequence modules may include RF pulses, which are output by the radiofrequency generating system, and/or gradient pulses, which are output by the gradient generating system. The system control unit 22 and/or the compensation computing unit 26 have suitable interfaces for transferring the magnetic resonance sequence data.

[0055] In S12, eddy current information about eddy currents that would be produced in the magnetic resonance apparatus by applying the original magnetic resonance sequence data is computed in the compensation computing unit 26.

[0056] In particular, the accumulated eddy currents during the run-through of the predetermined magnetic resonance sequence, i.e., during the magnetic resonance measurement, are computed and/or tracked. This is done, for example, by analyzing the gradient profiles of the received original magnetic resonance sequence data and computing the eddy-current induced field perturbations as well using modeling and the filter functions (e.g. decay constants of eddy currents) that are known for the type of the magnetic resonance apparatus 10. An exponential decay of the eddy currents generated by a gradient ramp may serve as the basis therefor, as has been described already above.

[0057] The accumulation of the eddy currents for computing the eddy current information involves in particular an accumulation over time. The computing of the intensity of the eddy currents may be performed solely using decay constants or time constants, regardless of the spatial variation. In the end, the eddy currents are compensated by the output of the eddy current compensation gradient pulse, which in turn generates eddy currents. This eddy current compensation gradient pulse advantageously generates eddy currents of the reverse polarity in order to compensate the accumulated eddy currents. Advantageously, there is no need to compute the spatial distribution thanks to the fact that this eddy current compensation gradient pulse may generate the same spatial distribution of eddy currents as the original gradients of the sequence.

[0058] In S13, at least one eddy current compensation gradient for compensating the eddy currents is computed on the basis of the eddy current information computed in S12.

[0059] In particular, when the event or sequence module to be output next in the magnetic resonance sequence data is a fat saturation pulse (which may be detected really easily from the frequency of the fat saturation pulse, for instance), an eddy current compensation gradient pulse is computed automatically, e.g., taking into account all the gradient trajectories previously run through in the measurement on the individual axes. For the algorithm, it is irrelevant here in which repetition, or after which DWI block, it is located, because all these magnetic field gradients have been captured previously in the computation.

[0060] The eddy current compensation gradient pulse is advantageously designed here such that the compensation eddy currents it generates compensates in full or in part the eddy-current induced field perturbations already present. In particular, it may also be specified, for example, up to what proportion (e.g., 50% or up to a certain defined tolerable field perturbation), the existing field perturbations are compensated. In particular, this may reduce the necessary duration and/or the required amplitude of the eddy current compensation gradient pulse.

[0061] In S14, modified magnetic resonance sequence data is generated by inserting the eddy current compensation gradient pulse into the original magnetic resonance sequence data. Thus selective intervention and adaptation compared with the original gradient profile of the sequence is performed in order to achieve eddy current compensation.

[0062] This may involve identifying in the original magnetic resonance sequence data a time segment in which the magnetic resonance apparatus does not apply any sequence modules, in particular does not switch any gradient pulses or transmit any RF pulses. The eddy current compensation gradient pulse is then inserted into the original magnetic resonance sequence data in the identified time segment.

[0063] Advantageously, the eddy current compensation gradient pulse is inserted into a “break” in the sequence, so that the timing of the sequence does not have to be changed. No sequence modules other than the eddy current compensation gradient pulse may be output in this “break.” This is illustrated using an example shown in FIG. 3.

[0064] This figure shows a time series of a segment of a magnetic resonance sequence, more precisely of an echo planar (EPI) DWI sequence. The original magnetic resonance data includes here an RF excitation pulse RFe, a first diffusion gradient pulse Gd1, an RF refocusing pulse RFr, and a second diffusion gradient pulse Gd2, followed by a plurality of sequence modules of an EPI readout EPIro, and by an RF fat saturation pulse RFfs, finally followed again by an RF excitation pulse RFe. In S14, it is detected that no sequence modules are applied between the end of the EPI readout EPIro and the fat saturation pulse RFfs. In addition, it may also be detected that the sequence module RFfs is a fat saturation pulse. The eddy current compensation gradient Gecc computed in S13 is now inserted in this “empty” time segment before the fat saturation pulse RFfs, resulting then in the modified magnetic resonance sequence data.

[0065] It is proposed as an option that the predetermined magnetic resonance sequence may keep dedicated placeholders for the eddy current compensation gradient pulse, at the position of which the eddy current compensation gradient may be inserted. These placeholders may be flagged as such by a flag, for example, whereby the advantageous position would also be specified directly, and it would be possible to dispense with the previously described identification of suitable time segments.

[0066] In S15, the modified magnetic resonance sequence data is output to the gradient generating system, in particular to the gradient control unit 19, of the magnetic resonance apparatus 10. The gradient control unit 19 in turn controls the radiofrequency antenna unit 20 and outputs, in S20, the eddy current compensation gradient Gecc in accordance with the modified magnetic resonance sequence data.

[0067] The adaptation shown in FIG. 2 of the predetermined magnetic resonance sequence is advantageously performed at least in part in real time and/or on-the-fly during the magnetic resonance measurement, indicated by the arrow between S20 and S11. Thus, the latest original magnetic resonance sequence data of the predetermined magnetic resonance sequence is continuously provided to the compensation computing unit 26, which data is then modified in S10. This modification is advantageously performed in an automated manner and independently of the sequence type of the predetermined magnetic resonance sequence.

[0068] A fundamental advantage of the proposed method is that it is independent of the specific sequence or the sequence type. Advantageously, the method is implemented and managed just once centrally, while no changes, or only minimal changes, are needed in the sequences.

[0069] Finally, it should be reiterated that the methods described in detail above and the presented compensation computing unit and magnetic resonance apparatus are merely exemplary embodiments, which may be modified by a person skilled in the art in many ways without departing from the scope of the disclosure. 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 include a plurality of interacting sub-components, which may also be spatially distributed if applicable.

[0070] It is to be understood that 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 disclosure. Thus, whereas the dependent claims appended below depend on 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, and that such new combinations are to be understood as forming a part of the present specification.

[0071] While the disclosure has been illustrated and described in detail with the help of the embodiments, the disclosure is not limited to the disclosed examples. Other variations may be deduced by those skilled in the art without leaving the scope of protection of the claimed disclosure.