Correction of interference effects in echoplanar imaging

Abstract

A method for performing magnetic resonance imaging (MRI) with an echo-planar sequence of a region of interest (ROI) is described. In this method, a theoretical model representing an oscillating structure of a magnetic field-generating unit (1b) is generated. Oscillation of this structure induces eddy currents, which in turn generate a gradient-like interference magnetic field in the ROI. This gradient-like interference magnetic field is determined based on the theoretical model (MMF). The echo-planar sequence is then played back. Magnetic resonance signals (RX) from the ROI are received, and raw magnetic resonance data (RD) is generated from these signals.Based on the raw magnetic resonance data (RD) and the determined gradient-like interference magnetic field in the area of ​​investigation (ROI), corrected raw magnetic resonance data (KRD) are generated. Finally, magnetic resonance image data (BD) are reconstructed based on the corrected raw magnetic resonance data (RD). A magnetic resonance imaging system (1) is also described.

Description

[0001] The invention relates to a method for performing magnetic resonance imaging with an echo-planar sequence of an area under investigation. The invention also relates to a magnetic resonance imaging system with which such a method can be carried out.

[0002] A magnetic resonance imaging (MRI) system comprises a control unit and a scan unit. The scan unit includes a field generating unit (FLU). The FLU comprises a base magnet, a gradient coil for generating phase-encoding gradients, readout gradients, and slice selection gradients, shim coils for improving the homogeneity of the base magnetic field, and a high-frequency magnet coil, also known as a body coil. A body coil can be used for transmitting and / or receiving high-frequency signals.

[0003] In echoplanar sequences (abbreviated as "EPI" sequences), the readout gradients must be switched very rapidly to high gradient strengths and then switched off again to enable fast scanning of the lines in k-space. This results in interactions between the base field magnet and the gradient coil, causing oscillations that can affect the entire measurement. Furthermore, the rapid gradient changes generate gradient-induced eddy currents, which arise on electrically conductive structures, and especially on resonantly excited, electrically conductive structures of the magnetic field generation unit, and counteract the build-up of the gradient field. A distinction is made between primary eddy currents, which are generated as a result of the magnetic field gradients, and secondary eddy currents. Lorentz forces arise due to the induced primary eddy currents in the base magnetic field.When Lorentz forces excite mechanical resonances, secondary eddy currents are generated due to these mechanical vibrations. The present patent application deals with the compensation of these secondary eddy currents. This reduction of the gradient field causes the signal maximum to be registered not in the center of k-space, but rather behind the center of k-space for odd-numbered phase encoding steps and in front of the center of k-space for even-numbered phase encoding steps.

[0004] This zigzag line of signal maxima creates artifacts, known as N / 2 ghosts, in this specific case referred to as "EPI ghosts" or "ghost artifacts." These ghost artifacts superimpose on the image data with a shift of N / 2 in the phase-encoding direction. N / 2 corresponds to half the number of phase-encoding lines required to scan the area of ​​interest (ROI) or field of view (FOV) in the phase-encoding direction.Although the “EPI ghosts” look similar to infolding artifacts caused by magnetic resonance signals from tissue outside the FOV, generated at a frequency above the Nyquist frequency, they differ from these in that the “EPI ghosts” also occur when the object under investigation is smaller than the FOV in the phase-encoding direction and therefore no frequencies above the Nyquist frequency can occur.

[0005] Similar phenomena also occur vibration-induced during excitation at the mechanical resonance of the gradient coil. Eddy currents can therefore be generated not only directly by rapid magnetic field changes, but also by moving, electrically conductive components permeated by magnetic fields. Compensation methods used so far are not particularly effective or have other negative effects on the imaging sequences. For example, they increase the overall acquisition time and the minimum possible echo time, as well as the susceptibility to magnetic field drift. Magnetic field drift can be caused by heating of individual components, especially the gradient coil or the so-called shims.

[0006] As a countermeasure, certain echo times associated with EPI sequences have been prohibited, particularly echo times corresponding to the natural resonances of gradient coils. However, due to the strong damping of the oscillation in the magnetic field, this approach necessitates prohibiting a very broad range of echo times. This severely limits the optimization of imaging sequences.

[0007] As a further countermeasure, reference measurements for phase correction were taken in the steady-state and / or identical mechanical condition of the gradient system. The three phase correction variants described below are known in the prior art.

[0008] In internal phase correction, also known as "3-line B0" correction, three reference lines are recorded before each readout phase. Internal phase correction is used in Fig. Figure 1 illustrates this. The time required to read out the reference lines is relatively short, but it must be added to the time for the actual EPI sequence. The readout sequence for the three reference lines differs from the readout sequence for the actual EPI sequence used for image acquisition. The additional time required is comparatively small, at approximately 5 ms (ms = milliseconds) for each EPI sequence. Overall, the internal phase correction is characterized by short measurement times and high robustness against patient movement.

[0009] In external phase correction, which is Fig. As illustrated in Figure 2, when reading out the three reference lines, the entire readout gradient is generated, which is later also used for image acquisition of the EPI sequence. With this approach, artifact suppression is particularly strong; however, the time required for generating and reading out the reference lines is especially high due to the generation of the entire readout gradient. The additional time required is approximately the same as the repetition time of the EPI sequence. If field deviations occur between different repetitions, the correction can become inaccurate. External phase correction is also sensitive to patient movement. Otherwise, image quality is improved by external phase correction compared to internal phase correction.

[0010] The third known method, which is in Fig. As illustrated in Figure 3, ghost artifact reduction is based on a GRAPPA reference acquisition. For this purpose, two additional reference acquisitions are performed at the beginning of an EPI sequence to capture GRAPPA-like kernels, which are used to correct imperfections in the k-space trajectory sampled during raw data acquisition. Here, too, the complete readout gradient is played back during the reference acquisition. The additional time required is twice the repetition time of the actual EPI sequence. If field deviations occur between different repetitions, the correction can become inaccurate.

[0011] The task is therefore to develop a method for performing magnetic resonance imaging with an echo-planar sequence with high image quality and increased robustness to movements, as well as with reduced time expenditure compared to previous methods.

[0012] This problem is solved by a method for performing magnetic resonance imaging with an echoplanar sequence of an investigation area according to claim 1 and a magnetic resonance imaging system according to claim 12.

[0013] In the inventive method for performing magnetic resonance imaging with an echo-planar sequence of an examination area, a theoretical model is generated based on the echo-planar sequence. In an echo-planar sequence, a frequency gradient or readout gradient generates a so-called echo train with a plurality of, for example, 128 circuits. This plurality of circuits enables the data acquisition of an image in a very short time, for example, 70 ms. The echo-planar sequence is particularly suitable for recording dynamic physiological processes in a patient's body. The theoretical model represents an oscillating structure of a magnetic field-generating unit, wherein eddy currents are induced in the oscillating structure by oscillation, which generate a gradient-like interfering magnetic field in the examination area.Based on the theoretical model, a gradient-like interference magnetic field is determined. The echo-planar sequence is then played back. As explained earlier, the inventive method compensates for the secondary eddy currents that arise from the mechanical vibrations generated by primary eddy currents. The primary eddy currents mentioned earlier are preferably compensated by pre-distortion of the played-back gradient fields.

[0014] In response to the played echo-planar sequence, magnetic resonance signals from the area under investigation are received and recorded, generating raw magnetic resonance data based on these signals. Corrected raw magnetic resonance data are then determined based on the measured gradient-like interference magnetic field in the area under investigation and the raw magnetic resonance data. Knowing the interference magnetic field allows for corrections to the phase response and phase of the raw magnetic resonance data, leading to a reduction of the N / 2 ghosts described earlier.

[0015] Finally, magnetic resonance imaging data are reconstructed based on the corrected raw magnetic resonance data.

[0016] In other words, the method according to the invention first employs a theoretical model based on knowledge of the properties of the echo-planar sequence and, optionally, system parameters of the magnetic resonance imaging system used for magnetic resonance imaging. The theoretical model describes oscillations generated by primary eddy currents. These oscillations, in turn, cause Lorentz forces in the existing magnetic field, which generate secondary eddy currents. Based on these secondary eddy currents, a disturbance magnetic field is generated, which is determined using the theoretical model.

[0017] Unlike conventional methods, the inventive method performs image correction based on a theoretical model used to determine the interfering magnetic field. Known system parameters, or known system parameter values ​​of the magnetic resonance imaging system associated with these parameters, can be used to calculate the theoretical model. If necessary, these system parameters can be determined experimentally beforehand. In contrast to conventional approaches, these system parameter values ​​only need to be determined experimentally once, if at all, and can then be used as model parameter values ​​to calculate the interfering magnetic field.

[0018] The magnetic resonance imaging system according to the invention comprises a model unit for generating a theoretical model, which represents an oscillating structure of a magnetic field-generating unit, based on a displayed echo planar sequence. Oscillations of the oscillating structure induce eddy currents, which generate a gradient-like interfering magnetic field in a study area. The model unit also serves to determine the gradient-like interfering magnetic field based on the theoretical model.

[0019] Part of the magnetic resonance imaging system according to the invention is also a sequence unit for playing back the echoplanar sequence.

[0020] The magnetic resonance imaging system according to the invention further comprises an input interface for receiving magnetic resonance signals from the area under investigation, wherein raw magnetic resonance data are generated based on the magnetic resonance signals.

[0021] The magnetic resonance imaging system according to the invention comprises a correction unit for determining corrected magnetic resonance raw data based on the magnetic resonance raw data and on the basis of the determined gradient-like interference magnetic field in the area under investigation.

[0022] The magnetic resonance imaging system according to the invention also comprises a reconstruction unit for reconstructing magnetic resonance image data based on the corrected raw magnetic resonance data. The magnetic resonance imaging system according to the invention shares the advantages of the inventive method for performing magnetic resonance imaging with an echo-planar sequence of an examination area.

[0023] A large proportion of the aforementioned components of the magnetic resonance imaging system according to the invention can be implemented wholly or partially as software modules in a processor of a corresponding computing system, e.g., by a control unit of a magnetic resonance imaging system or a computer used to control such a system. A largely software-based implementation has the advantage that even previously used computing systems can be easily retrofitted by means of a software update to operate in the manner of the invention.

[0024] Therefore, the problem is also solved by a corresponding computer program product with a computer program that can be directly loaded into a computer system, containing program sections to execute the steps of the inventive method for performing magnetic resonance imaging with an echo-planar sequence of an examination area when the program is executed in the computer system. Such a computer program product may, in addition to the computer program, optionally include additional components such as documentation and / or additional components, including hardware components such as hardware keys (dongles, etc.) for using the software.

[0025] For transport to the computer system or control unit and / or for storage on or in the computer system or control unit, a computer-readable medium, such as a memory stick, a hard drive, or other portable or permanently installed data carrier, can be used, on which the program sections of the computer program that can be read and executed by a computer system are stored. The computer system may, for example, have one or more cooperating microprocessors or similar components for this purpose.

[0026] The dependent claims and the subsequent description each contain particularly advantageous embodiments and further developments of the invention. In particular, the claims of one claim category may also be further developed analogously to the dependent claims of another claim category. Furthermore, within the scope of the invention, the various features of different embodiments and claims may also be combined to form new embodiments.

[0027] In a preferred embodiment of the inventive method for performing magnetic resonance imaging with an echo-planar sequence of an investigation area, the theoretical model comprises a mechanical model that describes the mechanical vibration behavior of a magnetic field generation unit with which the magnetic resonance imaging is performed. Advantageously, the vibrations of the magnetic field generation unit caused by the primary eddy currents during imaging are taken into account when determining the secondary eddy currents and the resulting interfering magnetic field.

[0028] In a preferred embodiment of the inventive method for performing magnetic resonance imaging with an echo-planar sequence of an area of ​​investigation, the determination of the corrected raw magnetic resonance data includes phase correction. Due to a deviation in the magnetic field, a change in the phase of a sampled magnetic resonance signal occurs. To correct this error, the phase of the sampled magnetic resonance signals is corrected based on the theoretical model.

[0029] Furthermore, the determination of the corrected magnetic resonance raw data preferably includes a correction of the fundamental magnetic field B0. Changing the fundamental magnetic field alters the Lamor frequency of the excited protons and thus also the frequency at which magnetic resonance signals are acquired. Correcting the B0 field compensates for any shift of the image data in the image data space or spatial space. Since the pixel bandwidth in the readout direction differs from the pixel bandwidth in the phase-encoding direction in an EPI sequence, an additional distortion of the image data occurs in an EPI sequence, which is also compensated for by correcting the fundamental magnetic field B0.

[0030] In one embodiment of the inventive method for performing magnetic resonance imaging with an echo-planar sequence of a study area, the theoretical model is designed such that the eddy currents comprise rapidly decaying eddy currents with a time constant between 0.1 and 10 ms. This depends on the material and the structural characteristics of the oscillating structure. Typical values ​​for decay times of eddy currents in oscillating components of a magnetic resonance imaging system lie within this range. Time constants shorter than 0.1 ms are negligible because the phase error is too small in this case. Time constants longer than 10 ms would shift all k-space lines and thus not cause an N / 2 ghost.

[0031] Preferably, the theoretical model of the oscillating structure comprises a damped harmonic oscillator. The damped harmonic oscillator model can be completely described by a few parameters, particularly the natural frequency and the damping. Even more complex oscillating structures behave approximately like a harmonic oscillator at small displacements. Advantageously, the vibrational behavior of a magnetic resonance imaging system can be easily described using this model. Furthermore, a wide range of closed-form analytical solutions are available that describe the mechanical vibration.

[0032] Preferably, the echoplanar sequence includes a reference measurement for phase change. This reference measurement preferably includes the aforementioned internal phase correction, also known as the "3-line B0" correction. In this variant, the theoretical model serves to correct this reference-based internal phase correction in order to further reduce artifacts without increasing the acquisition time.

[0033] Preferably, the reference measurement for phase change, also known as a PC scan, comprises a complete, preferably single, reference acquisition of the k-space. A single measurement of the space allows for the determination of k-space line-dependent correction values ​​for the fundamental magnetic field and the linear phase response or phase. Since a clinical EPI protocol includes a large number of EPI measurements, a single additional complete k-space measurement without phase coding hardly increases the overall measurement time.

[0034] Reference measurements are preferably performed in a steady state. For example, if a complete PC scan is performed, the internal phase correction, also known as "3-line B0" correction, can be carried out in a steady state because the vibrating structure begins to settle during the first half of the PC scan. If, for example, 64 k-space lines are acquired for a PC scan, the vibration is already in a steady state during the acquisition of the three middle k-space lines as a reference measurement for external phase correction, but the PC scan is partially performed in an unsteady state. If the actual image acquisition is then performed in a steady state, the internal phase correction can be used as a correct reference measurement because it was also performed in a steady state. Starting the PC scan in an unsteady state results in a time saving.

[0035] The echoplanar sequence also preferably includes an initial reference measurement for detecting drift effects. To identify these drift effects, it can be advantageous to perform an additional reference measurement prior to imaging by applying reference gradients and to use the results to correct system parameter values ​​or the modeled response behavior of the magnetic resonance imaging system. This advantageous approach combines the relatively short time required for the reference measurement with significantly improved image quality due to the modeled interference field. Furthermore, it advantageously compensates for drift effects, which can lead to errors, especially in time-extended measurements.

[0036] In one embodiment of the inventive method for performing magnetic resonance imaging with an echo-planar sequence of an area of ​​investigation, a reference acquisition is first performed, which preferably includes a "3-line B0" correction or an internal phase correction. During this acquisition, a deviation of the background magnetic field and, more preferably, a deviation of the phase response or phase, and preferably also a temporal echo-to-echo interval, are determined. This temporal echo-to-echo interval indicates the time interval between two successive echoes that were excited by a radio frequency signal of an echo-planar sequence. Due to the described interference effects, this temporal echo-to-echo interval can vary slightly.

[0037] Subsequently, a mechanical model of the magnetic field generation unit of a magnetic resonance imaging system, used for imaging, is generated based on the echo-to-echo interval as a theoretical model. This mechanical model represents the mechanical oscillation behavior of the magnetic field generation unit in the form of a harmonic oscillator. Based on this model, the contribution of eddy currents due to the oscillations to the generation of the fundamental magnetic field can be estimated, and the phase response, or phase, of the magnetic resonance signal can be calculated as a time shift in the time domain as a function of a k-space line and the excitation frequency.

[0038] Based on this model, an extended correction of the already estimated deviation of the fundamental magnetic field and the already estimated deviation of the phase response of the magnetic resonance signal is made, and corrected values ​​for the fundamental magnetic field and the phase response or phase are calculated as a function of the i-th k-space line.

[0039] Finally, phase correction of the raw magnetic resonance data, and thus also of the image data, is performed based on the corrected field and phase data.

[0040] In a particularly preferred embodiment of the inventive method for performing magnetic resonance imaging with an echo-planar sequence of a study area, a one-time use of a complete or steady-state phase-change scan is performed prior to the determination of the theoretical model, and the k-space-line-dependent correction values ​​are determined based on the measurement data thus generated. The generated measurement data were obtained from the study area. The values ​​thus generated are used as support values ​​for the theoretical model of the magnetic field generation unit.

[0041] In a particularly preferred embodiment of the inventive method for performing magnetic resonance imaging with an echo-planar sequence of an area of ​​investigation, the echo-planar sequence comprises a reference measurement for phase change, and a temporal echo-to-echo interval is determined based on the reference measurement, on the basis of which the theoretical model is derived. Advantageously, the true

[0042] In a preferred embodiment of the inventive method for performing magnetic resonance imaging with an echo-planar sequence of an examination area, the echo-planar sequence includes a reference measurement for phase change, which scans at least three reference lines. Preferably, the reference measurement includes exactly three reference lines. Acquiring only three reference lines in the readout direction saves time for the reference measurement compared to a reference measurement that covers the entire k-space of the image. Three reference lines in the readout direction allow at least the determination of a rough indication of a phase change between the individual reference lines.

[0043] The invention is explained in more detail below with reference to the accompanying figures and exemplary embodiments. The figures show: Fig. 1 a schematic representation of a pulse sequence diagram of an internal phase correction, Fig. 2 a schematic representation of a pulse sequence diagram of an external phase correction, Fig. 3 a schematic representation of a pulse sequence diagram of a phase correction, which is based on a GRAPPA reference recording, Fig. 4 a flowchart illustrating a method for performing magnetic resonance imaging with an echo planar sequence according to an embodiment of the invention, Fig. 5 a flowchart illustrating a method for performing magnetic resonance imaging with an echoplanar sequence according to an alternative embodiment of the invention, Fig. 6 a schematic representation of a magnetic resonance imaging system according to an embodiment of the invention, Fig. 7 a graph illustrating a measured deviation between two k-space scanning directions as a function of the number of output gradient pulses, Fig. 8 image representations of a reference recording with artifacts that depend on the number of gradient pulses played.

[0044] In Fig. Figure 1 is a schematic representation of a pulse sequence diagram of an echo-planar sequence with conventional internal phase correction. The pulse sequence diagram shows a series of high-frequency excitation pulses (HF) in the top row. In the second row, labeled "RX", the pulse sequence diagram shows an initial readout pulse (ADC). ref , which represents a reference signal, and a series of readout radio frequency pulses ADC, which represent the magnetic resonance signals of the imaging. In the third line, readout gradients G are shown. x both for the reference recording Gref (Gref is also referred to as the reference gradient in the following) as well as for imaging G x (Gx here represents a gradient in the readout direction, which is output simultaneously with the time window of the ADC's readout high-frequency pulse acquisition). The fourth line shows phase encoding gradients G. y This is illustrated. The fifth line shows layer selection gradients G. z shown. As already mentioned, the time required for the actual reference measurement is very short, but the reference gradient G corresponds to ref not exactly the gradient G used for imaging x in the direction of selection.

[0045] In Fig. Figure 2 illustrates a schematic representation of a pulse sequence diagram of a conventional external phase correction. In external phase correction, a gradient G is applied during the reference measurement. xin the reading direction as reference gradient G ref , which corresponds to the gradient G output simultaneously with the readout high-frequency pulse ADC x in the readout direction. In this way, the gradient G, which is also used in the actual imaging, is used in the reference measurement. x Interference signals generated in the readout direction with the first readout pulse ADC generated by a high-frequency reference signal ref immediately captured, so that the determination of interference effects and their compensation is easier compared to the time-saving method used in Fig. As shown in 1, it is improved. However, the external phase correction takes more time than the one shown in Fig. 1 shows internal phase correction.

[0046] In Fig. Figure 3 shows a schematic representation of a pulse sequence diagram of a conventional phase correction based on a GRAPPA reference image. In the Fig. The three conventional correction methods shown use two reference gradients G. ref Output in the readout direction. The reference measurements or the readout pulses ADC. ref are used to calculate kernels that are incorporated into image reconstruction and are intended to compensate for interference effects that already occur in the reference measurements.

[0047] In Fig. Figure 4 shows a flowchart 400 illustrating a method for performing magnetic resonance imaging with an echoplanar sequence of an area of ​​investigation according to an embodiment of the invention.

[0048] In step 4.I, a reference recording Int PC SC is first performed, which is the reference measurement of the in Fig. Figure 1 illustrates the “3-line B0” correction or internal phase correction. In this recording, in addition to a deviation dB0 of the fundamental magnetic field and a change dPH of the phase response or a change in phase, a temporal echo-to-echo interval T is also taken into account. EE determined.

[0049] In step 4.II, the temporal echo-to-echo interval T is used. EE A mechanical model, MMF, of the magnetic field generation unit is generated. The mechanical model MMF represents the mechanical oscillation behavior of the magnetic field generation unit in the form of a harmonic oscillator. Based on this model, the contribution MMF-B can be... 0,i The eddy currents due to the oscillations for generating the fundamental magnetic field B0 can be estimated using a model, and an estimated phase response or phase MMF-PH can be obtained. icalculated as a time shift in the time domain depending on a k-space line and the excitation frequency.

[0050] Based on this model, in step 4.III an extended correction EXT-PC is made to the deviation dB0 of the fundamental magnetic field already determined in step 4.I and the change dPH of the phase response determined in step 4.I, and thus corrected values ​​for the fundamental magnetic field B are obtained. 0,i and the phase response PH i or the phase is calculated as a function of the i-th k-space row.

[0051] In step 4.IV, a phase correction PC of the raw magnetic resonance data and thus also of the image data is performed based on the corrected data B. 0,i , PH i .

[0052] In Fig. Figure 5 shows a flowchart 500 illustrating a method for performing magnetic resonance imaging with an echoplanar sequence according to an alternative embodiment of the invention.

[0053] At the in Fig. In the illustrated embodiment 5, step 5.II additionally involves a one-time use of a complete phase change scan PC-S or one recorded in steady state and a determination of the k-space line-dependent correction values ​​B. 0i , PH i Based on the measurement data thus generated. The generated measurement data were obtained from the investigation area ROI. The values ​​thus generated are used in step 5.III as support values ​​for the mechanical model MMF of the magnetic field generation unit. Steps 5.IV and 5.V correspond to steps 4.III and 4.IV.

[0054] In Fig. Figure 6 shows a schematic representation of a magnetic resonance imaging system 1 according to an embodiment of the invention. The magnetic resonance imaging system 1 has a model unit 3a, which is configured to generate a theoretical model MMF representing an oscillating structure of a magnetic field-generating unit. Eddy currents are induced in the oscillating structure by its oscillation, causing a gradient-like interference magnetic field in the area of ​​investigation (ROI). The model unit 3a is therefore also specifically configured to determine the gradient-like interference magnetic field based on the model-calculated oscillation of the oscillating structure and the resulting eddy currents of the theoretical model. Based on the eddy currents and the resulting interference magnetic field, a change in the background magnetic field B occurs. 0,i and the phase response PHi the magnetic resonance signals.

[0055] Part of the magnetic resonance imaging system 1 is a sequence unit 1a for playing back an echo-planar sequence to generate high-frequency signals (HF) that excite magnetic resonance signals (RX). The magnetic resonance imaging system 1 also includes a magnetic field generation unit 1b, which generates a basic magnetic field B0 and gradient fields G based on the played-back echo-planar sequence. x , G ref and generates high-frequency fields.

[0056] The magnetic resonance imaging system 1 also includes an input interface 2 for receiving the magnetic resonance signals RX from the examination area ROI, whereby raw magnetic resonance data RD are generated based on the magnetic resonance signals RX.

[0057] Part of the magnetic resonance imaging system 1 is also a correction unit 3 for determining corrected magnetic resonance raw data KRD based on the magnetic resonance raw data RD and on the basis of the deviations of the basic magnetic field B determined by the model unit 3a. 0,i and the phase response PH i the magnetic resonance signals RX.

[0058] The magnetic resonance imaging system 1 also includes a reconstruction unit 4 for reconstructing magnetic resonance image data BD based on the corrected magnetic resonance raw data KRD.

[0059] In Fig. Figure 7 illustrates a measured deviation PKPK between two k-space scanning directions as a function of the number PZ of the output bipolar gradient pulses. The deviation PKPK refers to the deviation of the fundamental magnetic field in Hz and from peak to peak. In the Fig. In the illustrated scenario 7, a phantom is placed approximately 15 cm outside the isocenter in both the x and y directions. After a number of PZ of 20 bipolar gradient pulses, a transient process is complete, which is illustrated by the horizontal position of the PKPK curve at the value PZ = 20.

[0060] In Fig. Figure 8 illustrates image representations of a reference recording with artifacts that depend on the number of generated gradient pulses. As shown in Fig. As can be seen in 8, the artifacts decrease after a number of PZ of approximately 10 gradient oscillation periods.

[0061] Finally, it should be noted once again that the methods and devices described above are merely preferred embodiments of the invention and that the invention can be varied by a person skilled in the art without departing from the scope of the invention, insofar as it is defined by the claims. For the sake of completeness, it should also be noted that the use of the indefinite articles "a" or "an" does not preclude the possibility that the features in question may be present multiple times. Likewise, the term "unit" does not preclude the possibility that it consists of several components, which may also be spatially distributed. Regardless of the grammatical gender of a particular term, persons of male, female, or other gender identities are included.

Claims

[1] Method for performing magnetic resonance imaging with an echo-planar sequence of a region of interest (ROI), comprising the steps: - Generating a theoretical model (MMF) representing an oscillating structure of a magnetic field generating unit (1b), wherein eddy currents are induced in the oscillating structure by oscillation, which generate a gradient-like perturbation magnetic field in the area of ​​investigation (ROI), based on the echoplanar sequence, - Determining the gradient-like disturbance magnetic field based on the theoretical model (MMF), - Playback of the echoplanar sequence, - Receiving magnetic resonance (RX) signals from the area of ​​investigation (ROI), generating raw magnetic resonance (RD) data based on the magnetic resonance (RX) signals, - Determining corrected magnetic resonance raw data (MRR) based on the generated magnetic resonance raw data (MR) and on the basis of the determined gradient-like interference magnetic field in the investigation area (ROI), - Reconstructing magnetic resonance imaging (MRI) data based on corrected MRI raw data (CRD). [2] Method according to claim 1, wherein the theoretical model (MMF) comprises a mechanical model which describes the mechanical vibration behavior of the magnetic field generating unit (1b) with which the magnetic resonance imaging is performed. [3] Method according to claim 1 or 2, wherein the determination of the corrected magnetic resonance raw data (MRD) comprises phase correction. [4] Method according to any of the preceding claims, wherein the eddy currents comprise rapidly decaying eddy currents with a time constant between 0.1 and 10 ms. [5] Method according to any of the preceding claims, wherein the theoretical model (MMF) of the oscillating structure comprises a damped harmonic oscillator. [6] Method according to any of the preceding claims, wherein the echo planar sequence includes a reference measurement (PC-S) for phase change. [7] Method according to claim 6, wherein the reference measurement for phase change (PC-S) comprises a complete reference recording of k-space. [8] Method according to claim 6 or 7, wherein the reference measurement for phase change (PC-S) is performed in steady state. [9] Method according to any of the preceding claims, wherein the echoplanar sequence comprises an initial reference measurement for the detection of drift effects. [10] Method according to any of the preceding claims, wherein the echo-planar sequence comprises a reference measurement for phase change and a time echo-to-echo interval (T) is calculated on the basis of the reference measurement. EE) is determined, on the basis of which the theoretical model (MMF) is determined. [11] Method according to any of the preceding claims, wherein the echoplanar sequence comprises a reference measurement for phase change, which includes a sampling of at least three reference lines. [12] Magnetic resonance imaging system (1), comprising: - a model unit (3a) for generating a theoretical model (MMF) representing an oscillating structure of a magnetic field-generating unit (1b), wherein eddy currents are induced in the oscillating structure by oscillation, which generate a gradient-like disturbance magnetic field in the area of ​​investigation (ROI), based on an echo planar sequence to be played back and for determining the gradient-like disturbance magnetic field based on the theoretical model (MMF), - a sequence unit (1a) for playing back the echoplanar sequence, - an input interface (2) for receiving magnetic resonance signals (RX) from the investigation area (ROI), wherein raw magnetic resonance data (RD) are generated based on the magnetic resonance signals (RX), - a correction unit (3) for determining corrected magnetic resonance raw data (MRD) based on the magnetic resonance raw data (RD) and on the basis of the determined gradient-like interference magnetic field in the investigation area (ROI), - a reconstruction unit (4) for reconstructing magnetic resonance image data (BD) based on the corrected magnetic resonance raw data (KRD). [13] Computer program product comprising instructions which, when the program is executed by a computer, cause it to perform the steps of the method according to any one of claims 1 to 11. [14] Computer-readable storage medium comprising instructions which, when executed by a computer, cause it to perform the steps of the method according to claims 1 to 11.

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