Method of calibrating a gradient amplifier, magnetic resonance apparatus and computer program product

Abstract

The invention relates to a method for calibrating gradient amplifiers of a magnetic resonance apparatus, to a magnetic resonance apparatus and to a computer program product. According to the method, the magnetic resonance apparatus comprises at least one gradient coil for generating a gradient magnetic field. At least one of the at least one gradient coil comprises at least two coil segments which are designed for generating a gradient magnetic field having a gradient along one spatial direction. The magnetic resonance apparatus comprises a gradient amplifier for each of the at least two coil segments which is designed for providing a current for the respective coil segment. A plurality of magnetic field distributions generated by at least one of the at least two coil segments, respectively, is measured, wherein, for measuring each of the plurality of magnetic field distributions, a predetermined current is provided for the at least one coil segment generating the respective magnetic field distribution by the respective gradient amplifier. The gradient amplifiers are calibrated by comparison of the plurality of measured magnetic field distributions.

Description

Technical Field

[0001] This invention relates to a method for calibrating a gradient amplifier in a magnetic resonance apparatus, a magnetic resonance apparatus, and a computer program product. Background Technology

[0002] In medical technology, the outstanding advantage of magnetic resonance (MR) imaging, also known as magnetic resonance tomography (MRI), lies in its high soft tissue contrast. In this method, a human or animal patient is typically positioned within the imaging volume of the MRI scanner. During MRI measurements, high-frequency excitation pulses are typically transmitted to the patient using the high-frequency antenna unit of the MRI scanner.

[0003] It is important to distinguish this from the magnetic field generated by the magnet coil units of a magnetic resonance imaging (MRI) device, which typically includes a static master magnetic field and / or gradient magnetic fields. The master magnetic field, also known as the B0 field, is generated by, for example, a superconducting master magnet. The gradient magnetic field is generated by the gradient coil units of the MRI device and is typically used for position encoding. A gradient coil unit may include multiple gradient coils, each designed to generate a (partial) gradient magnetic field in a specific spatial direction or along a specific gradient axis, such as X, Y, and Z. Therefore, these spatial directions typically correspond to the gradient axes of the MRI device.

[0004] The generated excitation pulses stimulate nuclear spins in the patient, thereby triggering a position-encoded magnetic resonance signal. This magnetic resonance signal is received by the magnetic resonance imaging device and used for magnetic resonance imaging reconstruction.

[0005] A gradient coil is known to be divided into multiple coil segments, wherein current is supplied to each coil segment through a gradient power amplifier (GPA), i.e., power supply.

[0006] Uniform power supply to these coil segments is necessary to generate the desired gradient magnetic field along the spatial direction. Ideally, the generated gradient magnetic field should be as linear as possible along the corresponding spatial direction and should not generate, or only generate, negligible higher spatial orders. The value of Ordnung.

[0007] However, if the currents of the individual gradient amplifiers generating the coil segments along the gradient axis are different, higher spatial order field values ​​may occur. The resulting magnetic field is no longer perfectly gradient. Possible effects include:

[0008] • Distortion in magnetic resonance imaging (MRI) occurs because typical image reconstructions are based on a strictly gradient magnetic field.

[0009] • Coupling input in Shim coils: If the generated magnetic field distribution corresponds to the magnetic field distribution of a coil that may be used for magnetic field homogenization (so-called Shim coils), the generated field may induce current in these coils and damage these coils and the electronic devices connected to them.

[0010] • Torque and force generated on components of gradient coils that may lead to excessive mechanical stress.

[0011] Gradient amplifiers typically employ current sensors that are as accurate as possible, monitoring the actual current generated and used to regulate that current. However, these current sensors can be inaccurate, for example, due to manufacturing tolerances. If the individual current sensors of a gradient amplifier supplying power to the partial coils of the gradient axis are inaccurate relative to each other, the currents they generate may also be inaccurate relative to each other, potentially leading to the aforementioned problems.

[0012] Variations in individual segments of the gradient coil (e.g., different resistances) can also lead to the superposition of partial fields generated by those segments, potentially producing unwanted higher spatial order field components. Summary of the Invention

[0013] The technical problem to be solved by the present invention is to improve the gradient magnetic field generated by gradient coils powered by multiple gradient amplifiers, especially the spatial linearity of the gradient magnetic field.

[0014] The technical problem is solved according to the present invention by a method for calibrating a gradient amplifier of a magnetic resonance apparatus, a magnetic resonance apparatus, and a computer program product.

[0015] Therefore, a method for calibrating a gradient amplifier in a magnetic resonance apparatus is proposed. Here, the magnetic resonance apparatus includes at least one gradient coil for generating a gradient magnetic field. At least one of the at least one gradient coil includes at least two coil segments designed to generate a common gradient magnetic field having a magnetic field gradient along a common spatial direction. Thus, the at least two coil segments are associated with a gradient axis corresponding to the spatial direction.

[0016] The magnetic resonance apparatus includes (proprietary) gradient amplifiers for each of the at least two coil segments, each gradient amplifier being designed to supply current (i.e., power) to the corresponding (i.e., assigned) coil segment. Multiple magnetic field distributions generated by at least one of the at least two coil segments are measured, wherein, to measure each of the multiple magnetic field distributions, a preset current is supplied to at least one coil segment generating the corresponding magnetic field distribution via the corresponding gradient amplifier. The gradient amplifiers are calibrated by comparing the multiple measured magnetic field distributions.

[0017] Calibrating the gradient amplifier can reduce potential nonlinearities in the gradient coil. In particular, it can fully or partially compensate for manufacturing tolerances of the gradient amplifier and / or gradient coil. Especially useful in magnetic resonance devices using more than one gradient amplifier for each gradient axis, it can correct current asymmetry.

[0018] A magnetic resonance device, for example, includes three gradient coils, each designed to generate a magnetic field gradient along a different spatial direction or along a different gradient axis, wherein the three spatial directions or gradient axes are orthogonally oriented to each other. Specifically, the magnetic resonance device includes three gradient coils: a first gradient coil for generating a gradient magnetic field with a magnetic field gradient along the X-direction, a second gradient coil for generating a gradient magnetic field with a magnetic field gradient along the Y-direction, and a third gradient coil for generating a gradient magnetic field with a magnetic field gradient along the Z-direction. The X, Y, and Z directions may be perpendicular to each other. The vectors of the gradient magnetic fields preferably have an orientation along the same spatial direction, such as the Z-direction.

[0019] Preferably, the gradient amplifier is calibrated for each of the three gradient coils, the coil segment of which is supplied with current by at least two gradient amplifiers. (If the gradient coil includes, for example, only one gradient amplifier, then the calibration recommended here is typically not required or cannot be performed on that single gradient amplifier).

[0020] Preferably, by calibrating the gradient amplifier, any possible deviation between the preset current and the actual current is leveled out, so that the deviation of the actual current is the same for all coil segments of the same gradient coil, and in particular becomes zero.

[0021] A preset current, particularly a nominal current and / or a desired current, is provided for at least one coil segment to generate the corresponding magnetic field distribution. Therefore, the preset current does not need to be the actual current provided to the at least one coil segment by a corresponding gradient amplifier. In particular, the gradient amplifier is calibrated to correct any possible deviation between the nominal current and / or desired current and the actual current (flowing through at least one coil segment). Specifically, the gradient amplifier is calibrated to smooth out any possible deviation between the nominal current and / or desired current and the actual current (flowing through the coil segment), so that the deviation is advantageously the same for all coil segments of the same gradient coil.

[0022] The preset current supplied to at least one coil segment to generate the corresponding magnetic field distribution has a preset nominal and / or desired current intensity. It does not need to be the actual current intensity supplied to the at least one coil segment via an associated gradient amplifier. Calibration of the gradient amplifier specifically corrects for any possible deviation between the nominal and / or desired current intensity and the actual current intensity (flowing through at least one coil segment). In particular, calibration of the gradient amplifier smooths out any possible deviation between the nominal and / or desired current intensity and the actual current intensity (flowing through the coil segment), thus advantageously ensuring that the deviation is the same for all coil segments of the same gradient coil.

[0023] The goal of calibration is not to make the actual current equal to the nominal current, but to adjust the current flowing through different segments of a gradient coil or a gradient axis.

[0024] Calibration of a gradient amplifier may in particular include determining at least one correction factor. This correction factor may be applied to a preset (nominal and / or desired) current, particularly multiplied by the current.

[0025] At least one of the gradient coils includes at least two coil segments. For example, a first gradient coil may include two coil segments, a second gradient coil may include two coil segments, and a third gradient coil may include two coil segments. However, a gradient coil may also include only one coil segment or two or more coil segments. However, the proposed magnetic resonance apparatus includes at least one gradient coil having more than one coil segment. For example, the first and second gradient coils may each include only one coil segment, and the third gradient coil may include four coil segments.

[0026] The coil segment preferably includes a conductor arrangement with an interface for connecting the coil element to the gradient amplifier. When current flows through the conductor arrangement, a magnetic field, particularly a gradient magnetic field, is advantageously generated. Ideally, the gradient magnetic field is one whose intensity (the numerical value of the gradient magnetic field vector) varies linearly along a spatial direction. By calibrating the gradient amplifier, it is advantageously achieved that the total and / or combined fields of the gradient magnetic fields along the gradient axes are as linear as possible.

[0027] Each coil segment preferably generates a gradient magnetic field, particularly a magnetic field with gradient components. Such a gradient magnetic field is typically not perfectly linear in reality, but may also include nonlinear components, especially contributions to higher spatial orders. Ideally, when the individual coil segments are powered to the same degree, they compensate for each other. The gradient magnetic field of coil segments with the same gradient coil preferably has a magnetic field gradient along the same spatial direction. The gradient magnetic fields generated by multiple coil segments with the same gradient coil preferably complement each other to form a composite gradient magnetic field (total field), which preferably has primarily gradient components.

[0028] Each coil segment is preferably supplied with current by a proprietary gradient amplifier. For example, if the first gradient coil has two coil segments, the magnetic resonance device includes a proprietary gradient amplifier for each of the two coil segments. Multiple gradient amplifiers belonging to multiple coil segments of the same gradient coil can also be understood as partial gradient amplifiers, since they each provide partial gradient amplification for that gradient coil or gradient axis.

[0029] The gradient amplifier is preferably controlled by the system control unit of the magnetic resonance device. For example, the system control unit sends electrical control signals to the gradient amplifier to control it.

[0030] The magnetic field distribution preferably describes at least the strength and / or direction of the magnetic field, in this case, the gradient magnetic field, in relation to spatial location. For example, the strength and / or direction of the magnetic field is associated with spatial coordinates, such as (x, y, z). Preferably, the associated magnetic field distribution is measured for each coil segment of the magnetic resonance apparatus supplied with current by the gradient amplifier to be calibrated.

[0031] To measure the magnetic field distribution of a coil segment, a gradient amplifier supplies a preset current to the coil element. Specifically, the preset current flows through the conductor arrangement of the coil segment, thereby generating a magnetic field. Measurements of the field distribution of different coil segments are preferably performed sequentially.

[0032] Calibration of the gradient amplifier may specifically include gradient amplifier ablation. Comparison of multiple measured magnetic field distributions may specifically include analysis of the measured magnetic field distributions. Comparison of multiple measured magnetic field distributions may, for example, be performed by the system control unit of the magnetic resonance apparatus. Specifically, magnetic field distributions (generated by coil segments) along the same gradient axis are compared. Here, magnetic field distributions (generated by coil segments) along different gradient axes are not compared.

[0033] A calibrated gradient amplifier is preferably used to perform magnetic resonance measurements. In particular, a calibrated gradient amplifier can be used to record magnetic resonance signals. Advantageously, magnetic resonance imaging generated from such signals exhibits a low degree of artifacts.

[0034] Preferably, the current intensity of the preset current is the same for each gradient amplifier (nominal ground) to be calibrated. This makes it particularly easy to compare multiple measured magnetic field distributions to calibrate the gradient amplifier.

[0035] Even if the (nominal and / or desired) current intensity is the same for each gradient amplifier to be calibrated, the actual current intensity may differ. Advantageously, any discrepancies between the (nominal and / or desired) current intensity and the actual current intensity can be corrected through gradient amplifier calibration. Furthermore, calibration of the gradient amplifier effectively levels out or equalizes any potential differences between the (nominal and / or desired) current intensity and the actual current intensity of each individual coil segment.

[0036] Preferably, the preset current has a temporarily constant current intensity. For example, the current intensity is constant during a portion of a magnetic resonance sequence used to measure multiple magnetic field distributions or throughout the entire magnetic resonance sequence, i.e., also during possible gradient pulses used for position encoding (e.g., for layer selection, phase encoding, and / or frequency encoding).

[0037] However, it is also possible to consider that the preset current intensity differs among the gradient amplifiers; that is, at least one gradient amplifier to be calibrated provides a different current intensity to its associated coil segment than other gradient amplifiers to be calibrated provide to their associated coil segments. Advantageously, these different current intensities are taken into account when comparing multiple measured magnetic field distributions.

[0038] Preferably, the preset current used for each gradient amplifier to be calibrated has a current intensity greater than 1 ampere, especially several amperes. Advantageously, the preset current intensity is large enough to make the resulting field distribution measurable, for example, to achieve the measurement of the signal phase change of a magnetic resonance signal, from which the magnetic field distribution is determined.

[0039] Of course, gradient amplifiers are designed to provide additional current contributions to the corresponding coil segments beyond the preset current. These additional current contributions could be, for example, the current used to apply gradient pulses according to a (preset) magnetic resonance sequence. Thus, the preset current (used to calibrate the gradient amplifier) ​​can be understood in particular as a current bias for the current used to apply gradient pulses according to a (preset) magnetic resonance sequence.

[0040] In particular, at least two coil segments of the gradient coil can include at least three coil segments, wherein at least one of the multiple magnetic field distributions is generated (combined) by at least two coil segments of the at least three coil segments. For example, if the gradient coil of the gradient axis is divided into more than two coil segments and is therefore powered by more than two separate gradient amplifiers, and if there are a total of four gradient amplifiers for each gradient axis, a preset current can also be applied to different groups of gradient amplifiers, such as two gradient amplifiers. Then, the comparison of the resulting magnetic field distributions can be performed, in particular, group by group.

[0041] Preferably, the calibration of the gradient amplifier includes determining at least one correction factor for the gradient amplifier, wherein the gradient amplifier provides current to the corresponding coil segment in a calibrated state, taking into account at least one correction factor.

[0042] The at least one correction coefficient can be applied in particular to magnetic resonance imaging (MRI) sequences. In the calibrated state, MRI signals can be detected, from which one or more MRI images can be generated, which exhibit fewer artifacts than those without a calibrated gradient amplifier.

[0043] The at least one correction coefficient may in particular include a plurality of normalized correction coefficients. Normalization may be achieved, in particular, with respect to the sum of the plurality of correction coefficients; for example, the sum of the correction coefficients produces a preset value, such as 1.

[0044] Preferably, the measurement of multiple magnetic field distributions is achieved by magnetic resonance methods, such as a dual-echo method and / or a single-echo method with different echo times. In particular, the measurement of the multiple magnetic field distributions involves detecting a magnetic resonance signal using a magnetic resonance apparatus, wherein the multiple magnetic field distributions are determined based on the magnetic resonance signal. Advantageously, the measurement of the multiple magnetic field distributions is achieved without the use of external sensors, but solely using devices typically available in clinical operation of magnetic resonance apparatuses.

[0045] For example, in the dual-echo method, the magnetic field distribution can be calculated from the phase difference between the echoes. For this purpose, magnetic resonance sequences based on gradient echoes or spin echoes can be used, for example. Advantageously, a preset current ensures that a magnetic field distribution is generated through a corresponding gradient amplifier, and then the effect of said magnetic field distribution is measured. Other possible gradient pulses in the magnetic resonance sequence can be utilized, in particular, as is commonly done.

[0046] Advantageously, these magnetic resonance signals, i.e., those capable of generating multiple magnetic field distributions, can be detected with appropriate, and especially high, readout bandwidth. This allows for the advantageous realization that the preset current used to generate the corresponding magnetic field distribution causes only small or negligible spatial distortion in the relevant region.

[0047] The appropriate readout bandwidth depends in particular on the magnitude of the applied current and the field distribution of each individual segment. The readout bandwidth is preferably high enough that no significant distortion (Verzerrungen) is formed in the direction of the readout gradient of the magnetic resonance sequence used due to the field distribution generated by the current. Preferably, the readout bandwidth is at least 100 Hz / pixel, especially at least 300 Hz / pixel, and particularly at least 500 Hz / pixel.

[0048] Advantageously, measurements of multiple magnetic field distributions are achieved via magnetic resonance sequences, wherein phase encoding is performed along the direction in which the corresponding gradient amplifier produces the largest magnetic field change or the magnetic field change most relevant to the analysis.

[0049] Preferably, the direction of the phase encoding is the same as the spatial direction of the magnetic field gradient, which is generated by at least two coil segments whose magnetic field distribution is measured to calibrate the associated gradient amplifier. Therefore, the direction of the phase encoding is preferably chosen such that it points in the same direction as the corresponding gradient axis. This is because distortion due to magnetic field variations typically does not occur along the phase encoding direction. Therefore, if a coil segment, for example, produces a field variation in the X direction (i.e., if it is a segment of the X gradient coil), then the phase encoding direction should point in the X direction.

[0050] Preferably, the comparison of the multiple measured magnetic field distributions includes determining the degree of comparison, particularly a similarity measure. The similarity measure preferably describes the similarity of the magnetic field distributions. Here, for example, the degree to which the corresponding magnetic field distributions differ from each other in certain regions can be quantified. Advantageously, the gradient amplifier is calibrated with the aim of maximizing the similarity measure, i.e., influencing the corresponding magnetic field distributions so that the magnetic field distributions are as similar as possible in certain regions.

[0051] Preferably, multiple measured magnetic field distributions are compared, taking into account the segmentation of the gradient coil, and in particular the local layout of at least two coil elements of the gradient coil.

[0052] The segmentation of gradient coils can take into account, in particular, the direction of the theoretical magnetic field generated by the corresponding coil segments of the gradient coil.

[0053] The local layout of the coil segments can be determined, in particular, by the location and / or region of the coil segments within the magnetic resonance apparatus.

[0054] Preferably, the comparison of multiple measured magnetic field distributions includes comparing at least two of the magnetic field distributions at a predetermined geometric location. Such a location can be, for example, a point, a line, and / or a plane. Preferably, this geometric location is where the two magnetic field distributions should ideally be identical.

[0055] For example, if the gradient coils used for the gradient axis X are segmented such that each individual coil segment ideally produces the same magnetic field distribution in the central transverse layer (i.e., at Z=0), then the magnetic field distributions produced by the associated gradient amplifiers can be directly compared at Z=0 as a measure of similarity. For example, the orientation of the magnetic field distribution produced in the X direction can be analyzed.

[0056] Preferably, the at least two coil segments and / or the magnetic field distribution generated by the at least two coil segments have (local) symmetry, wherein the comparison of the plurality of measured magnetic field distributions is achieved taking into account the symmetry.

[0057] For example, the symmetry of two coil segments can be described by mirror images of the first magnetic field distribution generated by the first coil segment and the second magnetic field distribution generated by the second coil segment at a point, a line, or a plane.

[0058] Preferably, the measurements of the plurality of magnetic field distributions and the calibration of the gradient amplifier are performed repeatedly, wherein the measurements of the plurality of magnetic field distributions are performed in consideration of the previously performed calibrations.

[0059] Preferably, these steps are iterated repeatedly until convergence of the result is determined. For example, a convergence criterion can be used as an interruption criterion for the iteration, which requires that the determined correction coefficient differs from the value calculated in the previous iteration by no more than a specific value or a specific coefficient.

[0060] Preferably, the at least one correction coefficient is changed for application in subsequent iterations, particularly in specific steps. For example, the change from one iteration to the next is 0.5%. Preferably, the similarity metric thus determined is compared with the similarity metric from previous iterations. In particular, iterations can be performed repeatedly until the maximum similarity between the field distributions produced by the individual gradient amplifiers is determined.

[0061] It is preferable to interpolate and / or average specific correction coefficients in different iterative cycles to determine the final correction coefficients. For example, interpolation can be performed from iterations with the highest and second-highest similarity metrics. This method is particularly suitable for cases where the gradient coil of a gradient axis is divided into two segments, i.e., there are two coil segments.

[0062] In addition, a magnetic resonance device is proposed, which is designed to perform the previously described method.

[0063] The advantages of the proposed magnetic resonance apparatus essentially correspond to the advantages of the methods used to calibrate the gradient amplifiers of the magnetic resonance apparatus, which have been detailed above. The features, advantages, or alternative implementations mentioned herein can be adapted to magnetic resonance apparatuses and vice versa.

[0064] Magnetic resonance devices may, in particular, include a system control unit adapted to compare multiple measured magnetic field distributions and / or calibrate gradient amplifiers. For this purpose, the system control unit may, for example, include one or more processors and / or storage modules.

[0065] A computer program product is also proposed, which includes a program and is directly loadable into the memory of a programmable system control unit of a magnetic resonance apparatus, having a programmmittel to implement the previously described methods when the program is implemented in the system control unit of the magnetic resonance apparatus. Attached Figure Description

[0066] Other advantages, features, and details of the invention will become apparent from the embodiments described below and from the accompanying drawings. Corresponding parts are provided with the same reference numerals in all the drawings.

[0067] In the attached diagram:

[0068] Figure 1 A magnetic resonance device is shown;

[0069] Figure 2 A gradient coil is shown that generates a magnetic field gradient extending in the X direction;

[0070] Figure 3 This illustrates a method for calibrating a gradient amplifier in a magnetic resonance apparatus;

[0071] Figure 4 Other possible aspects of methods for calibrating gradient amplifiers in magnetic resonance devices are shown;

[0072] Figure 5-6 An example of an element of a magnetic resonance sequence is shown, which is suitable for measuring the distribution of a magnetic field generated on a gradient amplifier by a preset current. Detailed Implementation

[0073] exist Figure 1 The image schematically shows a magnetic resonance imaging (MRI) device 10. The MRI device 10 includes a magnet unit 11 having a main magnet 12 for generating a strong and, particularly, time-constant, main magnetic field 13. Furthermore, the MRI device 10 includes a patient receiving area 14 for accommodating a patient 15. In this embodiment, the patient receiving area 14 is designed as cylindrical and cylindrically surrounded in the circumferential direction by the magnet unit 11. However, in principle, different designs for the patient receiving area 14 are always conceivable. The patient 15 can be pushed into the patient receiving area 14 via a patient support device 16 of the MRI device 10. For this purpose, the patient support device 16 has a patient table 17 designed to be movable within the patient receiving area 14.

[0074] The magnetic unit 11 also includes a gradient coil unit 18 for generating a magnetic field gradient, which is used for position encoding during imaging. The gradient coil unit 18 includes three gradient coils (not shown here), each capable of generating a magnetic field gradient in a separate spatial direction. Figure 2 An exemplary illustration shows gradient coils 18x used to generate a magnetic field gradient in the X direction. The gradient coil unit 18 is controlled by a gradient control unit 19 of the magnetic resonance device 10. The gradient control unit 19 includes a gradient amplifier (not shown here) for supplying current to the three gradient coils. Figure 2 Two gradient amplifiers 191x and 192x are exemplarily shown for supplying current to the gradient coil 18x. Furthermore, the magnetic unit 11 includes a high-frequency antenna unit 20, which in this embodiment is designed to be fixedly integrated into the body coil of the magnetic resonance apparatus 10. The high-frequency antenna unit 20 is controlled by the high-frequency antenna control unit 21 of the magnetic resonance apparatus 10 and transmits high-frequency magnetic resonance sequences into the examination space formed substantially by the patient reception area 14 of the magnetic resonance apparatus 10. Therefore, the main magnetic field 13 generated by the main magnet 12 induces the excitation of atomic nuclei. Magnetic resonance signals are generated through the relaxation of the excited atomic nuclei. The high-frequency antenna unit 20 is designed to receive magnetic resonance signals.

[0075] The magnetic resonance imaging (MRI) apparatus 10 includes a system control unit 22 for controlling the main magnet 12, the gradient control unit 19, and the high-frequency antenna control unit 21. The system control unit 22 centrally controls the MRI apparatus 10, for example, by executing a predetermined imaging gradient echo sequence. Furthermore, the system control unit 22 includes an analysis unit (not shown in more detail) for analyzing the MRI signals detected during an MRI examination. Additionally, the MRI apparatus 10 includes a user interface 23 connected to the system control unit 22. Control information, such as imaging parameters, and reconstructed MRI images can be displayed to a medical operator on a display unit 24 of the user interface 23, for example, on at least one monitor. Furthermore, the user interface 23 has an input unit 25 through which the medical operator can input information and / or parameters during measurement.

[0076] Figure 2 The gradient coil 18x of the gradient coil unit 18 and two gradient amplifiers of the gradient control unit 19 are shown exemplarily. The gradient coil 18x is designed to generate a gradient magnetic field with a magnetic field gradient along the X direction. The gradient coil unit 18 may also include corresponding gradient coils for generating a gradient magnetic field with a magnetic field gradient along the Y direction and / or for generating a gradient magnetic field with a magnetic field gradient along the Z direction. The vectors of these gradient magnetic fields are generally oriented along the direction of the main magnetic field 13.

[0077] The gradient coil 18x comprises two coil segments 181x and 182x. The two coil segments 181x and 182x are designed to generate gradient magnetic fields with a magnetic field gradient along the X-direction. The gradient control unit 19 of the magnetic resonance device includes gradient amplifiers for each of the two coil segments 181x and 182x, namely gradient amplifier 191x for coil segment 181x and gradient amplifier 192x for coil segment 182x. Gradient amplifier 191x is designed to supply current to coil segment 181x; gradient amplifier 192x is designed to supply current to coil segment 182x.

[0078] exist Figure 3 The diagram schematically illustrates an exemplary method for calibrating gradient amplifiers 191x and 192x of a magnetic resonance imaging (MRI) device 10. In S10, multiple magnetic field distributions are measured, each generated by coil segments 181x and 182x. Therefore, a first magnetic field distribution generated by coil segment 181x is measured, and a second magnetic field distribution generated by coil segment 182x is measured. To measure each of the two magnetic field distributions, the coil segment generating the corresponding magnetic field distribution is supplied with a preset current by the corresponding gradient amplifier. Thus, to measure the first magnetic field distribution, coil segment 181x is supplied with a preset current by gradient amplifier 191x, and to measure the second magnetic field distribution, coil segment 182x is supplied with a preset current by gradient amplifier 192x.

[0079] Measuring the magnetic field distribution generated by gradient coils using conventional magnetic resonance methods is difficult because, in conventional methods, the magnetic field distribution is also used for position encoding, and deviations from the ideal linear orientation will cause distortion of the coordinate system in which position encoding is performed. Therefore, it is impossible, for example, to measure the field distribution generated by pulses passing through gradient coils using conventional magnetic resonance methods (so-called field mapping or B0 mapping methods) that measure the spatial distribution of the magnetic field. Because the corresponding gradients used in position encoding or image reconstruction are assumed to be linear, the measured field distribution will also appear linear.

[0080] Instead, it is suggested that conventional magnetic resonance methods be used to measure the magnetic field distribution generated by pre-set currents on individual gradient amplifiers. These pre-set currents can be used as a supplement to currents that may function for magnetic field homogenization (BO-Shimming) and are applied to the gradient pulses of the corresponding magnetic resonance sequence.

[0081] If a gradient coil of a gradient axis is divided into more than two segments (and...) Figure 2 (The situation shown is different) and therefore powered by more than two separate gradient amplifiers, these currents can also be applied to different groups of gradient amplifiers; for example, if there are a total of four gradient amplifiers for each gradient axis, then the current is applied to two gradient amplifiers. In this case, the following representation applies similarly, except that instead of powering individual gradient amplifiers, it powers groups of gradient amplifiers.

[0082] This method is exemplarily based on Figure 4 Explanation. First, in S11, a preset current, especially a current bias ΔI, is applied to one of the N gradient amplifiers of the magnetic resonance device 10. This current can be several amperes. The current bias ΔI is then superimposed on the other currents I(GPA1), I(GPA2), ..., I(GPA3) that need to be provided by the corresponding gradient amplifiers. N The current bias applied to one (partial) gradient amplifier can function throughout the entire magnetic resonance sequence to measure the field distribution, or it can function during a portion of the magnetic resonance sequence. The current bias is chosen such that the field distribution generated by the current bias in the magnetic resonance sequence used achieves a measurable effect, such as a change in the phase of the measured signal.

[0083] exist Figure 5 and Figure 6 The diagram shows exemplary elements of a magnetic resonance sequence suitable for measuring the magnetic field distribution generated by a current bias on a gradient amplifier. Figure 5The current bias ΔI only works during a portion of the magnetic resonance sequence, according to Figure 6 The current bias ΔI plays a role throughout the magnetic resonance sequence. Gradients used for position encoding (especially layer selection, phase encoding, and / or readout / frequency encoding) are not shown. The figure labels ADC1 and ADC2 denote the data acquisition windows, where the echoes are shifted by time ΔTE.

[0084] In S12, the field distribution generated by this current bias is measured individually. This is done for all gradient amplifiers along the gradient axis (if there is more than one gradient amplifier) ​​(or for each group of gradient amplifiers, which is feasible in cases where it is divided into more than two segments, see the discussion above). Thus, for example, N field distributions are measured here, where, in each measurement, the current of one of the total N gradient amplifiers is modified by the value ΔI.

[0085] Subsequently, in S21, the corresponding field distributions F1, F2, ... F along the same gradient axis are... N Comparisons are made. For example, if the first gradient coil of gradient coil unit 18 along the X gradient axis includes a field distribution F... 1x and F 2x Given two coil segments, then the field distribution F 1x and F 2x Comparisons are made. For example, if the second gradient coil of gradient coil unit 18 along the Y-axis includes a field distribution F... 1y and F 2y The two (other) coil segments, then these field distributions F 1y and F 2y Comparisons are made. For example, if the third gradient coil of gradient coil unit 18 along the Z-gradient axis includes a field distribution F... 1z and F 2z The two (other) coil segments, then these field distributions F 1z and F 2z To be compared.

[0086] In S22, the field distributions F1, F2, ... F N Determine the correction factors K1, K2, ..., K for the current or current supply of each individual gradient amplifier. N The correction coefficients ensure that the total field and / or combined field of the gradient magnetic field along the gradient axis are as linear as possible in the corresponding spatial direction.

[0087] To measure the magnetic field distribution in S10, S11, and / or S12, conventional magnetic resonance methods for measuring magnetic field distribution, such as the dual-echo method, can be used, where the field distribution is calculated from the phase difference between the echoes. For this purpose, sequences based on gradient echoes or spin echoes can be used, for example. A current bias ΔI of the gradient amplifier can be applied, for example, during the entire magnetic resonance sequence or during a portion of the magnetic resonance sequence. This current bias ensures the effectiveness of generating a field distribution through the corresponding (partial) gradient amplifier and then measuring that field distribution. Other gradient pulses in the magnetic resonance sequence (e.g., with associated currents I(GPA1), I(GPA2), ..., I(GPA...)...) N As usual, this pulse powers the coil segments of all gradient amplifiers or gradient axes respectively. Therefore, the current bias of the gradient amplifier that needs to be checked separately can be added to the current used to generate the other gradient pulses. This sequence can be designed to be used for resolution in one, two, or three dimensions.

[0088] Figure 4 An example of a sequence of elements that can be used to measure the field distribution generated by a current ΔI is shown. This example illustrates a dual-echo method embedded in a gradient echo sequence (where the gradients used for position encoding and for generating the echoes are not shown). Similarly, other sequences, such as spin echo sequences, and other implementations, such as single-echo methods with different echo times, can also be used. In this example, the current ΔI generates a field distribution B0(r), which generates a spatially altered phase difference ΔΦ(r) = γΔTE B0(r) between the echoes (here, γ is the gyroscope ratio and ΔTE is the time interval between the echoes). By measuring this phase difference, the generated field distribution B0(r) can be reconstructed.

[0089] Advantageously, this selection of sequence parameters ensures that the current bias produces only small or negligible spatial distortions in the relevant regions. This can be achieved, for example, by using a higher readout bandwidth or by selecting the phase encoding direction in coordinates where the corresponding gradient amplifier produces the largest or most relevant field change to the analysis.

[0090] To compare the magnetic field distributions measured in S20, S21, and / or S22, different methods can be considered, particularly to compare the resulting field distributions F1, F2, ..., F... N Similarity measures can be applied to compare magnetic field distributions generated by corresponding gradient amplifiers. For example, this can quantify the degree to which corresponding magnetic field distributions deviate from each other in certain regions.

[0091] Therefore, it is advantageous to consider specific segmentation of the gradient axis. For example, (such as...) Figure 2(As shown) The gradient coil used to generate the gradient can be segmented along the x-axis in such a way that each individual coil segment 181x, 182x ideally produces the same magnetic field distribution F in the central transverse plane (i.e., at Z=0). i Then, as a similarity measure, the magnetic field distributions generated by the individual gradient amplifiers 191x and 192x can be directly compared at Z=0; then, for example, the magnetic field curves generated in the X direction can be analyzed. If this magnetic field curve is in multiple magnetic field distributions F i If the values ​​are different, then, for example, the correction factor for the current used for each individual gradient amplifier 191x, 192x can be calculated from this curve so that the two gradient amplifiers (at Z=0 in this case) produce the same magnetic field distribution.

[0092] Depending on the geometry or segmentation of the coil, other comparison metrics, especially similarity metrics, can also be used. The following segments of a gradient coil can also be considered, where the currents on each individual gradient amplifier produce magnetic field distributions that differ from each other in certain symmetries. Therefore, these symmetries can also be taken into account when comparing the magnetic field distributions produced by each individual gradient amplifier.

[0093] The field distribution F generated in S21 i For example, in S22, a set of correction coefficients K is calculated. i This set of correction factors is ultimately applied to the current of each individual gradient amplifier. The goal of the correction factors, in particular, is to produce the most similar or equivalent field distributions possible for each individual gradient amplifier 191x, 192x, when using the comparison or similarity metric employed in S21. Advantageously, the total field along the gradient axis (by powering all gradient amplifiers) is made as linear as possible in the relevant spatial directions. In one design scheme, the correction factor K... i The sum is normalized, for example, K1 + K2 + ... + K N =1.

[0094] exist Figure 4 The method shown can be performed iteratively until convergence of the results is determined. Here, for example, the correction coefficients determined in previous steps can be applied in the current iteration to measure the current bias ΔI of each individual gradient amplifier. As a criterion for stopping the iteration, a convergence criterion can be used, for example, requiring the determined correction coefficients K to be within a certain range. i The value calculated in the previous iteration does not differ from a specific value or a specific coefficient.

[0095] In another possible iterative design scheme, the correction coefficient K iThe changes are made in fixed steps (e.g., + / - 0.5%) to be applied in subsequent iterations. The result of the similarity metric or comparison metric is then compared with the corresponding value from the previous iteration. This method can be particularly advanced until the maximum similarity between the field distributions produced by the individual gradient amplifiers is determined.

[0096] Furthermore, in particular, the final set of correction coefficients can be interpolated, for example, from iterations of “best” and “second best” values ​​with comparative metrics. This method is especially applicable if a gradient coil on a gradient axis is divided into two coil segments.

[0097] The basis proposed Figure 3 and Figure 4 The method described for calibrating the gradient amplifiers 191x and 192x of the magnetic resonance device 10 utilizes the current differences of the individual (partial) gradient amplifiers along the gradient axis to compensate for the differences in the coil segments 181x and 182x.

[0098] Possible methods include specifying a current sensor in the calibration gradient amplifier or measuring the actual generated field distribution using an external sensor. Unlike these methods, such a sensor is not necessary when applying the proposed method. Therefore, the method is easy to use and can also be readily applied to already installed magnetic resonance imaging (MRI) devices for example, to verify adjustments to the correction coefficients and / or to determine new correction coefficients during maintenance, such as when replacing equipment components. Advantageously, the method can be easily integrated into adjustments already performed using magnetic resonance methods commonly employed in MRI devices.

[0099] Finally, it should be reiterated that the methods and magnetic resonance devices described in the above-described details are merely embodiments, and those skilled in the art can modify the embodiments in many different ways without departing from the scope of the invention. Furthermore, the use of the indefinite article "a" does not preclude the possibility that a particular feature may exist multiple times. Similarly, the term "unit" does not preclude the possibility that a component may consist of multiple interacting sub-components, which may, if necessary, be distributed in space.

Claims

1. A method for calibrating a gradient amplifier in a magnetic resonance apparatus. in, The magnetic resonance device includes at least one gradient coil for generating a gradient magnetic field. Wherein, at least one of the at least one gradient coil comprises at least two coil segments, the coil segments being designed to generate a gradient magnetic field having a magnetic field gradient along a spatial direction. The magnetic resonance device includes a gradient amplifier for each of the at least two coil segments, the gradient amplifier being designed to provide current to the corresponding coil segment. The method includes a. Measure the distributions of multiple magnetic fields generated by at least one of the at least two coil segments. In order to measure each of the plurality of magnetic field distributions, a preset current is provided to at least one coil segment that generates the corresponding magnetic field distribution through a corresponding gradient amplifier. b. The gradient amplifier is calibrated by comparing multiple measured magnetic field distributions.

2. The method according to claim 1, in, By calibrating the gradient amplifier, the deviation between the preset current and the actual current is smoothed out.

3. The method according to claim 1 or 2, in, The calibration of the gradient amplifier includes: Determine at least one correction coefficient for the gradient amplifier. The gradient amplifier provides current to the corresponding coil segment in a calibrated state, taking into account at least one correction factor.

4. The method according to claim 1 or 2, in, The measurement of the plurality of magnetic field distributions includes detecting magnetic resonance signals using a magnetic resonance device, wherein the plurality of magnetic field distributions are determined based on the magnetic resonance signals.

5. The method according to claim 1 or 2, in, The comparison of the multiple measured magnetic field distributions includes: Determine a similarity metric that describes the similarity of magnetic field distributions.

6. The method according to claim 1 or 2, in, The at least two coil segments have a local layout and / or segmentation. The comparison of the multiple measured magnetic field distributions is achieved taking into account the local layout and / or segmentation of the at least two coil segments.

7. The method according to claim 1 or 2, in, The at least two coil segments and / or the magnetic field distribution generated by the at least two coil segments are symmetrical. The comparison of the multiple measured magnetic field distributions is achieved while taking into account the symmetry.

8. The method according to claim 1 or 2, in, Steps a and b are repeated. The measurement of the plurality of magnetic field distributions in step a. is performed taking into account the calibration performed previously in step b.

9. The method according to claim 3, in, Steps a and b are repeated. The measurement of the multiple magnetic field distributions in step a. is achieved taking into account the calibration performed previously in step b. The at least one correction coefficient is changed for application in subsequent iterations.

10. The method according to claim 9, wherein, The at least one correction coefficient is changed in a determined step for application in subsequent iterations.

11. The method according to claim 8, in, If the similarity metric determined by comparing the multiple measured magnetic field distributions exceeds a preset value and / or coefficient, then the repeated execution of steps a. and b. is interrupted.

12. The method according to claim 2, in, By calibrating the gradient amplifier, the deviation between the preset current and the actual current is leveled out, so that the deviation of the actual current is the same for all coil segments of the same gradient coil.

13. A magnetic resonance apparatus designed to perform the method according to any one of claims 1 to 12.

14. A computer program product comprising a program and capable of being directly loaded into the memory of a programmable system control unit of a magnetic resonance apparatus, having program means for implementing the method according to any one of claims 1 to 12 when the program is implemented in the system control unit of the magnetic resonance apparatus.

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