Method And Control Unit For Compensation Of Eddy Current Induced Magnetic Fields In Magnetic Resonance Imaging

Abstract

Eddy current induced magnetic fields (MF) are compensated in a magnetic resonance imaging system. An MR-sequence (M) includes a number of gradients. A dataset includes values of an amplitude and a time constant of eddy current fields of a number of gradients on at least one gradient axis. A number of points in time within the time period of the MR-sequence are defined. A number of constant currents are calculated for a number of coils of the magnetic resonance imaging system based on the dataset. The number of constant currents is designed to compensate at least at the one defined point in time (PT1, PT2). The calculated number of constant currents are applied on the related coils prior or during the application of the MR-sequence or a section of the MR-sequence.

Claims

1. A method for compensation of eddy current induced magnetic fields in a magnetic resonance imaging system, the method comprising: providing an MR-sequence comprising a number of gradients, providing a dataset comprising values to generate a number of constant currents designed to compensate magnetic fields induced by eddy currents of the gradients of the MR-sequence, defining at least one point in time within a time period of the MR-sequence on at least one gradient axis, calculating a number of constant currents for a number of coils of the magnetic resonance imaging system based on the dataset, wherein the constant currents are designed to compensate magnetic fields induced by eddy currents of the gradients of the MR-sequence at the at least one defined point in time, applying the calculated number of constant currents on coils prior or during the application of the MR-sequence or a section of the MR-sequence.

2. The method according to claim 1, wherein the dataset comprises values of the amplitude of the eddy current fields of a number of gradients on at least one gradient axis, wherein the calculation of a number of constant currents comprises: calculating amplitudes of the magnetic fields induced by the eddy currents of the gradients of the MR-sequence, based on the dataset, calculating the constant currents for the coils of the magnetic resonance imaging system.

3. The method according to claim 2 wherein the dataset comprises a time constant, and wherein calculating the constant currents comprises calculating each constant current applicable for one of the coils, wherein the number of constant currents is designed to compensate the calculated amplitudes of the magnetic fields induced by the eddy currents of the gradients of the MR-sequence at the at least one defined point in time.

4. The method according to claim 1, wherein the dataset comprises values of a number of spatial orders of an amplitude of the eddy current fields of a number of spatially separated gradients on at least two gradient axes.

5. The method according to claim 4 wherein the amplitudes of the magnetic fields induced by the eddy currents are calculated, based on the spatial orders of the amplitude of the eddy current fields for spatial separated gradients on gradient axes, wherein the coils comprise shim coils, and wherein the constant currents for a number of the shim coils are calculated based on the spatial orders of the amplitude of the eddy current fields, for the spatial separated gradients on gradient axes, and wherein the calculated constant currents are applied to the shim coils designed to compensate the respective spatial order.

6. The method according to claim 1, wherein one of the at least one point of time is selected from the group comprising the point of time of a fat saturation pulse, the point of time of a water saturation or excitation pulse and the point of time of a k-space center.

7. The method according to claim 1, wherein the calculation of the amplitudes of the magnetic fields induced by eddy currents comprises actual parametrization of the measurement protocol for the number of slices, the thickness of the slices, the orientation of the slices, the field of view, and/or the resolution.

8. The method according to claim 1, wherein the calculation of the number of the constant currents for the coils is designed to compensate the magnetic fields at different ones of the at least one point in time, wherein an average value for each constant current is chosen concerning the compensation at every chosen of the ones of the at least one point in time.

9. The method according to claim 8, wherein two or more of the points in time are chosen and the constant currents for the coils are calculated such that for one point in time not the best compensation is chosen, but in total the best compensation for all points in time.

10. The method according to claim 9, wherein pa quality of measurements is calculated for each point in time in dependence of a degree of compensation at this point in time and after that the compensation is designed such that average or weighted average constant currents are applied so that the quality of measurements for each point in time meets at an average value.

11. The method according to claim 1, wherein after the application of the calculated number of constant currents on the related coils, there is included a predefined stabilizing time until the MR-sequence is applied.

12. The method according to claim 1, wherein the MR-sequence comprises two or more sections, wherein different sections comprise different gradients and wherein different ones of the at least one point in time are chosen in different sections, and wherein the calculation and application of the constant currents is done separately for respective gradients and the points in time of different sections of the MR-sequence.

13. The method according to claim 1, wherein the method is repeated for two or more similar MR-sequences or similar sections of an MR-sequence, wherein information of the calculated constant currents for the two or more MR-sequences or sections are stored and the stored values are applied without again calculating the constant currents in the case the respective MR-sequence or section is applied again.

14. The method according to claim 13, wherein between two different ones of the MR-sequences or sections of the MR-sequence, a pause or dummy measurements is applied: at the time of changing the constant current for the coils, and/or at the time of changing the acquisition of lesser important parts of the image dataset are performed.

15. A control system comprising a compensation processor, the control system comprising a data interface for receiving an MR-sequence comprising a number of gradients, a data interface for receiving and/or a memory for storing a dataset comprising values of an amplitude and a time constant of eddy current fields of the gradients on at least one gradient axis, or comprising values of a number of constant currents configured to compensate magnetic fields induced by the eddy currents of the gradients of the MR-sequence at least at one defined point in time, a processor configured to calculate the number of constant currents for a number of coils of a magnetic resonance imaging system based on the dataset, wherein the number of constant currents is configured to compensate the magnetic fields induced by the eddy currents of the gradients of the MR-sequence at least at the one defined point in time, a gradient system interface for applying the calculated number of constant currents on the related coils prior or during the application of the MR-sequence or a section of the MR-sequence.

16. The control system of claim 15, further comprising: a magnetic resonance imaging system comprising the related coils.

17. A computer-readable medium on which is stored program instructions that can be read and executed by a computer, the instructions comprising: providing an MR-sequence comprising a number of gradients, providing a dataset comprising values to generate a number of constant currents designed to compensate magnetic fields induced by eddy currents of the gradients of the MR-sequence, defining at least one point in time within a time period of the MR-sequence on at least one gradient axis, calculating a number of constant currents for a number of coils of the magnetic resonance imaging system based on the dataset, wherein the constant currents are designed to compensate magnetic fields induced by eddy currents of the gradients of the MR-sequence at the at least one defined point in time, and applying the calculated number of constant currents on coils prior or during the application of the MR-sequence or a section of the MR-sequence.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Other objects and features of the present embodiments will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.

[0084] FIG. 1 shows a simplified MRI system according to an embodiment;

[0085] FIG. 2 shows an exemplary MR-sequence;

[0086] FIG. 3 shows another exemplary MR-sequence;

[0087] FIG. 4 illustrates the aim of one embodiment; and

[0088] FIG. 5 shows a block diagram of the process flow of a preferred method according to one embodiment.

[0089] In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0090] FIG. 1 shows a schematic representation of a magnetic resonance imaging system 1 (MRI-system). The MRI system 1 includes the actual magnetic resonance scanner (data acquisition unit) 2 with an examination space 3 or patient tunnel in which a patient or test person is positioned on a driven bed 8, in whose body the actual examination object O is located.

[0091] The magnetic resonance scanner 2 is typically equipped with a basic field magnet system or B0 coil 4, a gradient system or gradient coils 6 as well as an RF transmission antenna system or whole body coil 5 and an RF reception antenna system or local coils 7. In the shown exemplary embodiment, the RF transmission antenna system 5 is a whole-body coil permanently installed in the magnetic resonance scanner 2, in contrast to which the RF reception antenna system 7 is formed as local coils (symbolized here by only a single local coil) to be arranged on the patient or test subject. In principle, however, the whole-body coil can also be used as an RF reception antenna system, and the local coils can respectively be switched into different operating modes.

[0092] The basic field magnet system 4 here is designed in a typical manner so that it generates a basic magnetic field in the longitudinal direction of the patient, i.e. along the longitudinal axis of the magnetic resonance scanner 2 that proceeds in the z-direction. The gradient system 6 typically includes individually controllable gradient coils in order to be able to switch (activate) gradients in the x-direction, y-direction or z-direction independently of one another. In addition, the magnetic resonance scanner 2 includes shim coils 11, which may be formed in a conventional manner.

[0093] In the body of the basic field magnet system 4, there are shown magnetic fields MF that are induced by eddy currents from applied gradients.

[0094] The MRI system 1 shown here is a whole-body system with a patient tunnel into which a patient can be completely introduced. However, in principle, embodiments can also be other MRI systems, for example with a laterally open, C-shaped housing, as well as in smaller magnetic resonance scanners in which only one body part can be positioned.

[0095] Furthermore, the MRI system 1 has a central control device or controller 13 that is used to control the MRI system 1. This central control device 13 includes a sequence control unit or controller 14 for measurement sequence control. With this sequence control unit 14, the series of radio-frequency pulses (RF pulses) and gradient pulses can be controlled depending on a selected MR-sequence M (see FIG. 2) or, respectively, a series of multiple MR-sequences M to acquire magnetic resonance images within a measurement session. For example, such a series of MR-sequences M can be predetermined within a measurement or control protocol. Different control protocols for different measurements or measurement sessions are typically stored in a memory 19 and can be selected by an operator (and possibly modified as necessary) and then be used to implement the measurement.

[0096] To output the individual RF pulses of an MR-sequence M, the central control device 13 has a radio-frequency transmission device or transmitter 15 that generates and amplifies the RF pulses and feeds them into the RF transmission antenna system 5 via a suitable interface (not shown in detail). To control the gradient coils of the gradient system 6, the control device 13 has a gradient system interface 16. With this gradient system interface 16, for example, the shim coils 11 could also be driven, since the gradient coils are used to set the DC offset currents to shim the B0 field (i.e. to compensate the linear component of the field distortions). The sequence control unit 14 communicates in a suitable manner (for example via emission of sequence control data SD) with the radio-frequency transmission device 15 and the gradient system interface 16 to emit the MR-sequences M.

[0097] Moreover, the control device 13 has a radio-frequency reception device or receiver 17 (likewise communicating with the sequence control unit 14 in a suitable manner) in order to acquire magnetic resonance signals (i.e. raw data) for the individual measurements, which magnetic resonance signals are received in a coordinated manner from the RF reception antenna system 7 within the scope of the MR-sequences M.

[0098] A reconstruction unit or processor 18 receives the acquired raw data and reconstructs magnetic resonance image data therefrom for the measurements. This reconstruction is typically performed on the basis of parameters that may be specified in the respective measurement or control protocol. For example, the image data can then be stored in a memory 19.

[0099] Operation of the central control device 13 can take place via a terminal 10 with an input unit and a display unit 9, via which the entire MRI system 1 can thus also be operated by an operator. MR images can also be displayed at the display unit or display 9, and measurements can be planned and started by the input unit (possibly in combination with the display unit 9), and in particular suitable control protocols can be selected (and possibly modified) with suitable series of MR-sequences M as explained above.

[0100] The control unit 13 includes a compensation unit or processor 12 designed to perform the method. This compensation unit 12 includes the following components.

[0101] A data interface 20. This data interface is in this example designed to receive an MR-sequence M as well as a number of points in time PT1, PT2 within the MR-sequence M. For example, a user can choose an MR-sequence M by using the terminal 10 and manually include points in time PT1, PT2 (see FIG. 2) in the chosen MR-sequence M. However, a user could also enter the data for the following examination and the system automatically chooses an appropriate MR-sequence M stored in the memory 19 together with predefined points in time PT1, PT2.

[0102] A memory 21, where a dataset D is stored and includes values of the amplitude and the time constant of eddy current E1, E2, E3 fields of a number of gradients G on at least one gradient axis. In this memory 21, predefined points in time PT1, PT2 could also be stored.

[0103] A processor unit or processor 22 designed to calculate a number of constant currents S1, S2, S3 (see FIG. 4) for a number of coils 11 of the magnetic resonance imaging system 1 based on the dataset D (see FIG. 5). The number of constant currents S1, S2, S3 is designed to compensate magnetic fields MF induced by eddy currents E1, E2, E3 of the gradients G of the MR-sequence M at least at one defined point in time PT1, PT2.

[0104] Means 23 for applying the calculated number of constant currents S1, S2, S3 on the related coils 11 prior or during the application of the MR-sequence M. The means 23 may be an interface, such as the gradient coil interface 16, a current source, a transmitter, or other current generating device or circuit.

[0105] The MRI system 1 according to one embodiment, and in particular the control device 13, can have a number of additional components that are not shown in detail but are typically present at such systems, for example a network interface in order to connect the entire system with a network and be able to exchange raw data and/or image data or, respectively, parameter maps, but also additional data (for example patient-relevant data or control protocols).

[0106] The manner by which suitable raw data are acquired by radiation of RF pulses and the generation of gradient fields, and MR images are reconstructed from the raw data, is known to those skilled in the art and thus need not be explained in detail herein.

[0107] FIG. 2 shows a part of a TSE sequence (TSE: Turbo Spin Echo) with spectral fat saturation as an exemplary MR-sequence M. The time-axis t runs from left to the right. In the upper diagram, the RF-Pulses R are shown, in the lower diagram, gradients G are shown in the direction of the readout (here e.g. the x-axis). The other two gradient axes are not shown. In this figure, a point of time PT1 is chosen in the center of fat saturation. At this point of time PT1, compensation of eddy-current induced magnetic fields MF is advantageous, since spectral fat saturation is sensitive on fluctuations in the B0 magnetic field.

[0108] FIG. 3 shows a part of an EPI diffusion-sequence (EPI: Echo Planar Imaging) with spectral fat saturation as another exemplary MR-sequence M. In the upper diagram, the RF-Pulses R are shown, in the lower diagram, gradients G are shown in the direction of measurement (here e.g. the x-axis). The other two gradient axes are not shown. In this figure, two different points of time PT1, PT2 are chosen, first the center of fat saturation and second the center of the EPI measuring gradients. At these points of time PT1, PT2, compensation of eddy-current induced magnetic fields MF is advantageous. Especially the compensation in the k space center is advantageous to prevent spatial distortions.

[0109] It should be noted that typically a series of preparation scans are applied before the actual measurement. The RF- and gradient pulses are applied, however, no data is recorded or the recorded data is not used for reconstruction (i.e. the data is maybe used for calibration). As a result, the eddy current induced magnetic fields MF are already in steady state during the relevant points of time PT1, PT2. These preparation scans are not necessarily part of the MR-sequence M. In fact, the preparation scans could be used for calibration, however, the constant currents are not necessarily applied before the preparation scans.

[0110] FIG. 4 illustrates the aim of one embodiment. In this figure, three diagrams are shown, wherein the first diagram A shows an exemplary MR-sequence M that is a diffusion sequence that is similar to the MR-sequence M shown in FIG. 3. As single point in time PT2 the k-space center is chosen (dash-dotted line). It should be noted that this point in time is not positioned right in the center of the echo train since there are asymmetries due to partial fourier. The RF-Pulses R are shown dashed, one gradient axis G (read gradient) is shown solid.

[0111] The second diagram B shows calculated eddy currents E1, E2, E3 for three spatial orders (i.e., corresponding to different spatial base functions). It can be seen that the three spatially different eddy currents have different amplitudes, signs and time constants. In the chosen point of time PT1, all of these eddy currents E1, E2, E3 are different from zero and hence contribute to B0 inhomogeneities.

[0112] The third diagram C shows a superposition of the eddy currents and the static compensation. Constant currents S1, S2, S3 have been calculated for the shim coils 11 (see e.g. FIG. 1). These constant currents generate constant compensation fields which can be represented in the figure as offsets (arrows) to the eddy currents E1, E2, E3 in all three spatial orders. It can be seen that with this compensation in the chosen point of time PT1, there is not distortion any more.

[0113] FIG. 5 shows a block diagram of the process flow of a preferred method for compensation of eddy current E1, E2, E3 induced magnetic fields MF in a magnetic resonance imaging system 1.

[0114] In act I, an MR-sequence M is provided. The MR-sequence includes a number of gradients G. Concerning the MRI system shown in FIG. 1, this MR-sequence M could be provided by the terminal 10 or the memory 19 of the control unit 13.

[0115] In act II, a dataset D is provided. The dataset D includes values of the amplitude and the time constant of eddy current E1, E2, E3 fields of a number of gradients G on at least one gradient axis.

[0116] In act III, a number of points in time PT1, PT2 are defined within the time period of the MR-sequence M. FIGS. 2, 3 and 4 show exemplary points in time PT1, PT2.

[0117] In act IV, a number of constant currents S1, S2, S3 is calculated. This is done here with two steps.

[0118] First, amplitudes of magnetic fields MF induced by eddy currents M1, M2, M3 of the gradients of the MR-sequence M are calculated, based on the dataset D.

[0119] Second, a number of constant currents S1, S2, S3 for a number of coils 11 of the magnetic resonance imaging system 1 are calculated. Each constant current S1, S2, S3 is applicable for one of these coils 11. The number of constant currents S1, S2, S3 is designed to compensate the calculated amplitudes of magnetic fields MF induced by eddy currents E1, E2, E3 of the gradients G of the MR-sequence M at least at one defined point in time PT1, e.g. as shown in FIG. 4.

[0120] In act V, the calculated number of constant currents S1, S2, S3 is applied on the related coils 11 prior or during the application of the MR-sequence M.

[0121] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. For the sake of clarity, it is to be understood that the use of a or an throughout this application does not exclude a plurality, and comprising does not exclude other steps or elements. The mention of a unit or a module does not preclude the use of more than one unit or module.