Correcting interference effects in echo plane imaging

Abstract

A method for performing magnetic resonance imaging of an examination region (ROI) by means of an echo plane sequence is described. In the method, a theoretical model is generated which depicts the vibration structure of the unit (1b) which generates the magnetic field, in which an eddy current is induced in the vibration structure by the vibration of the vibration structure, said eddy current generating a gradient disturbing magnetic field in the examination region (ROI). A gradient disturbing magnetic field is determined based on a theoretical model (MMF). In addition, an echo plane sequence is applied. A magnetic resonance signal (RX) is then received from the examination region (ROI), and magnetic resonance raw data (RD) is generated on the basis of the magnetic resonance signal. The corrected magnetic resonance raw data (KRD) is determined on the basis of the magnetic resonance raw data (RD) and on the basis of the determined gradient interference magnetic field in the examination region (ROI). Finally, magnetic resonance image data (BD) is reconstructed on the basis of the corrected magnetic resonance raw data (KRD). A magnetic resonance imaging system (1) is also described.

Description

TECHNICAL FIELD

[0001] The invention relates to a method for performing magnetic resonance imaging of an examination region by means of an echo planar sequence. The invention also relates to a magnetic resonance imaging system by means of which such a method can be performed. BACKGROUND

[0002] The magnetic resonance imaging system comprises a control device and a scanning unit. The scanning unit has a so-called magnetic field generating unit (in English: "Field Generating Unit"). The magnetic field generating unit comprises a basic field magnet, gradient coils for generating phase encoding gradients, read gradients and slice selection gradients, so-called shim coils for improving the homogeneity of the basic magnetic field and a radio frequency magnet coil, also called "Bodycoil". In German also "Korpercoil" (Bodycoil) is also called "Bodycoil". The "Bodycoil" (Bodycoil) can be used to emit and / or receive radio frequency signals.

[0003] In an echo planar sequence (abbreviation: "EPI" sequence) the read gradient has to be switched very fast to a high gradient strength and switched off again in order that a fast scan of the lines in k-space can be achieved. Here an interaction occurs between the basic field magnet and the gradient coils in which vibrations are caused which can impair the entire measurement. Furthermore, due to the fast change of the gradient, gradient induced eddy currents are generated which are generated on electrically conductive structures and in particular also on resonantly excited, electrically conductive structures of the magnetic field generating unit and which oppose the establishment of the gradient field. A distinction is made between primary eddy currents and secondary eddy currents due to the magnetic field gradient. Due to the primary eddy currents induced in the basic magnetic field, Lorentz forces are generated. If the Lorentz forces excite mechanical resonances, secondary eddy currents are generated due to the mechanical vibrations. The present patent application is directed to compensating the secondary eddy currents. Said reduction of the gradient field causes that the signal maximum is not registered in the middle of k-space but after the middle of k-space in odd phase encoding steps and before the middle of k-space in even phase encoding steps.

[0004] Due to the sawtooth lines of the signal maxima, artifacts, so-called N / 2-ghosts, in this particular case called "EPI-ghosts" or "ghost-artifacts" occur. The ghost-artifacts superimpose the image data with a shift of N / 2 in the phase-encoding direction. N / 2 here corresponds to the number of half the phase-encoding lines needed for scanning an examination region, also called "Region of Interest" and abbreviated "ROI", or a field of view, in German "Sichtfeld", also abbreviated "FOV". Even though the "EPI-ghosts" seem to be similar to aliasing artifacts caused by magnetic resonance signals of tissue outside the FOV generated by frequencies above the Nyquist frequency, the EPI-ghosts differ from the aliasing artifacts in that the "EPI-ghosts" also occur when the examined object is smaller than the FOV in the phase-encoding direction so that no frequencies above the Nyquist frequency occur.

[0005] Similar phenomena also occur when the excitation in the mechanical resonance of the gradient coils is vibrational. Thus, eddy currents can be generated not only directly by rapid magnetic field changes, but also by moving, electrically conductive components that are penetrated by the magnetic field. The compensation methods applied to date are not particularly effective or have other negative effects on the imaging sequences used. For example, they prolong the total acquisition time and the minimum possible echo time as well as the susceptibility to magnetic field drift. Magnetic field drift can be caused by the heating of individual components, in particular the gradient coils or so-called shim iron.

[0006] As a countermeasure, certain echo times associated with EPI sequences have been prohibited to date, in particular echo times that correspond to the intrinsic resonance of the gradient coils. However, this approach leads to a strong attenuation of the vibrations in the magnetic field: very wide ranges of echo times must be prohibited. This strongly limits the optimization of the imaging sequences.

[0007] As a further countermeasure, reference recordings are made in a stable and / or identical mechanical state of the gradient system for phase correction. Three variants for phase correction described below are known from the prior art.

[0008] In the internal phase correction also known as "3-line B0" correction, three reference lines are detected before each readout phase. The internal phase correction is illustrated in Fig. 1. The time for reading the reference lines is relatively short, but must be added to the time for the actual EPI sequence. The readout sequence for reading the three reference lines is different from the readout train of the actual EPI sequence for image acquisition. The additional time requirement is relatively small at about 5 ms (ms = milliseconds) for each EPI sequence. Overall, the internal phase correction is characterized by a short measurement time and high robustness with respect to patient movements.

[0009] In the external phase correction illustrated in Fig. 2, the entire read gradient is applied when reading the three reference lines, which is also used later for the image acquisition of the EPI sequence. In this way, the artifact suppression is particularly strong, however the time expenditure is particularly large due to the application of the entire read gradient when applying and reading the reference lines. The additional time requirement is approximately the repetition time of the EPI sequence. If a field deviation occurs between different repetitions, the correction becomes inaccurate. The external phase correction is also sensitive to patient movements. In other cases, the image quality by the external phase correction is improved compared to the internal phase correction.

[0010] In a third known method illustrated in Fig. 3, the ghost artifact is reduced based on GRAPPA reference recordings. For this purpose, two additional reference recordings are made at the beginning of the EPI sequence in order to record a GRAPPA-like kernel, by means of which the defects of the k-space trajectories scanned during the raw data acquisition should be corrected. Here, the complete read gradient is also applied during the reference recordings. The additional time requirement is here twice the repetition time of the actual EPI sequence. If a field deviation occurs between different repetitions, the correction will become inaccurate. SUMMARY

[0011] It was therefore an object to develop a method for performing magnetic resonance imaging by means of an echo planar sequence with high image quality and increased robustness with respect to movements and with reduced time expenditure compared to the methods to date.

[0012] The object is achieved by a method for performing magnetic resonance imaging of an examination region by means of an echo planar sequence according to the application and a magnetic resonance imaging system according to the application.

[0013] In the method for performing magnetic resonance imaging of an examination region by means of an echo planar sequence according to the application, a theoretical model is generated based on the echo planar sequence generation theory. In the echo planar sequence, a frequency gradient or read gradient produces a so-called echo train with a plurality, for example 128, of switches. The plurality of switches enables the data acquisition of an image within a very short time, for example 70 ms. The echo planar sequence is particularly suitable for recording dynamic physiological processes in the body of a patient. The theoretical model depicts the vibration structure of the unit generating the magnetic field, wherein eddy currents are induced in the vibration structure by the vibrations of the vibration structure, which produce a gradient interference magnetic field in the examination region. The gradient interference magnetic field is determined based on the theoretical model. Furthermore, the echo planar sequence is applied. As already set out at the outset, the secondary eddy currents, which are produced due to the mechanical vibrations produced by the primary eddy currents, are compensated by means of the method according to the application. The also mentioned primary eddy currents are preferably compensated by a pre-correction of the applied gradient fields.

[0014] The magnetic resonance signals of the examination region are received and detected in response to the applied echo planar sequence, wherein magnetic resonance raw data is generated on the basis of the magnetic resonance signals. Now, on the basis of the determined gradient-like interference magnetic field in the examination region and on the basis of the magnetic resonance raw data, modified magnetic resonance raw data is determined. Due to the knowledge of the interference magnetic field, a correction of the phase response or the phase of the magnetic resonance raw data can be performed, which leads to a reduction of the N / 2-ghosts described at the beginning.

[0015] Finally, magnetic resonance image data is reconstructed on the basis of the modified magnetic resonance raw data.

[0016] In other words, in the method according to the application, a theoretical model is used first on the basis of the knowledge of the properties of the echo planar sequence and, if necessary, on the basis of the system parameters of the magnetic resonance imaging system used for the magnetic resonance imaging. The theoretical model describes the vibrations generated by the primary eddy currents. The vibrations in their turn induce a Lorentz force in the existing magnetic field, by which the secondary eddy currents are generated. On the basis of the secondary eddy currents, the interference magnetic field is generated, which is determined on the basis of the theoretical model.

[0017] In contrast to the conventional, the image correction is performed in the method according to the application on the basis of the theoretical model by means of which the interference magnetic field is determined. Here, the known system parameters of the magnetic resonance imaging system or the known system parameter values associated with them can be used for calculating the theoretical model. If necessary, the system parameters can be determined experimentally in advance. In contrast to the conventional way, the system parameter values, if any, must be determined experimentally only once and then can be used as model parameter values, if necessary, in order to calculate the interference magnetic field.

[0018] The magnetic resonance imaging system according to the application has a model unit for generating a theoretical model on the basis of the echo planar sequence to be applied, which depicts the vibration structure of the units generating the magnetic field. By the vibrations of the vibration structure, eddy currents are induced in the vibration structure, which generate a gradient-like interference magnetic field in the examination region. The model unit is also used for determining the gradient-like interference magnetic field on the basis of the theoretical model.

[0019] Part of the magnetic resonance imaging system according to the application is also a sequence unit for applying the echo planar sequence.

[0020] The magnetic resonance imaging system according to the application also comprises an input interface for receiving the magnetic resonance signals from the examination region, wherein the magnetic resonance raw data is generated on the basis of the magnetic resonance signals.

[0021] The magnetic resonance imaging system according to the application comprises a correction unit for determining the modified magnetic resonance raw data on the basis of the magnetic resonance raw data and on the basis of the determined gradient-like interference magnetic field in the examination region.

[0022] The magnetic resonance imaging system according to the application also comprises a reconstruction unit for reconstructing magnetic resonance image data on the basis of the corrected magnetic resonance raw data. The magnetic resonance imaging system according to the application shares the advantages of the method according to the application for performing magnetic resonance imaging of an examination region by means of an echo planar sequence.

[0023] Most of the components of the magnetic resonance imaging system according to the application mentioned above can be implemented completely or partially in the form of software modules in the processor of a corresponding computing system, for example by a control device of the magnetic resonance imaging system or a computer for controlling such a system. An implementation as far as possible in software has the advantage that a hitherto used computing system can also be retrofitted in a simple manner by means of a software update in order to work in the manner according to the application.

[0024] In this connection, the object is also achieved by a corresponding computer program product having a computer program which can be directly loaded into a computing system, the computer program having program segments in order to carry out the steps of the method according to the application for performing magnetic resonance imaging of an examination region by means of an echo planar sequence when the program is executed in the computing system. Such a computer program product can comprise additional components, for example documentation and / or additional components, in addition to the computer program, also hardware components, for example a hardware key for using the software (dongle, etc.), if appropriate.

[0025] For transport to a computing system or control device and / or for storage at or in a computing system or control device, a computer-readable medium, for example a memory stick, a hard disk or another transportable or fixedly installed data carrier, can be used on which the program segments of the computer program readable and executable by the computing system are stored. The computing system can for this purpose have one or more cooperating microprocessors, etc.

[0026] The following description contains in each case particularly advantageous design and improvement options of the application. In this connection, in particular, an embodiment of one embodiment category can also be improved analogously to an embodiment of another embodiment category. Furthermore, different features of different embodiments can also be combined into new embodiments within the scope of the application.

[0027] In one preferred design of the method according to the application for performing magnetic resonance imaging of an examination region by means of an echo planar sequence, the theoretical model comprises a mechanical model which describes the mechanical vibration behavior of the magnetic field generating unit by means of which the magnetic resonance imaging is performed. Advantageously, the vibrations of the magnetic field generating unit which are caused by the primary eddy currents during imaging are taken into account when determining the secondary eddy currents and the resulting interference magnetic field.

[0028] In a preferred variant of the method for performing magnetic resonance imaging of an examination region by means of an echo planar sequence according to the application, the determination of the corrected magnetic resonance raw data comprises a phase correction. Due to a deviation of the magnetic field, a change in the phase of the scanned magnetic resonance signal occurs. In order to correct the error, the phase of the scanned magnetic resonance signal is corrected on the basis of a theoretical model.

[0029] Furthermore, the determination of the corrected magnetic resonance raw data preferably comprises a correction of the basic magnetic field Bo. With a change in the basic magnetic field, the Larmor frequency of the excited protons changes and thus also the frequency for detecting the magnetic resonance signal. By correcting the Bo field, a movement of the image data in the image data space or position space is compensated. Since in an EPI sequence the pixel bandwidth in the read direction is different compared to the pixel bandwidth in the phase encoding direction, a distortion of the image data occurs in an EPI sequence, which is likewise compensated by correcting the basic magnetic field Bo.

[0030] In a design of the method for performing magnetic resonance imaging of an examination region by means of an echo planar sequence according to the application, the theoretical model is designed such that the eddy currents comprise a fast decaying eddy current with a time constant between 0.1 ms and 10 ms. This depends on the material and structural features of the vibrating structure. Typical values for the decay time of the eddy currents in the vibrating components of a magnetic resonance imaging system lie in the mentioned value range. Time constants shorter than 0.1 ms are negligible, since in this case the phase error is too small. Time constants longer than 10 ms would move all k-space lines and thus not cause N / 2-ghosts.

[0031] Preferably, the theoretical model of the vibrating structure comprises a damped harmonic oscillator. The model of a damped harmonic oscillator can be completely described by a small number of parameters, in particular the natural frequency and the damping. More complex vibrating structures also behave approximately like a harmonic oscillator at small deflections. Advantageously, the vibrating behavior of a magnetic resonance imaging system can be described in a simple manner in a model. Furthermore, a large number of complete analytical solutions for describing mechanical vibrations are available.

[0032] Preferably, the echo planar sequence comprises a reference measurement for phase variations. The reference measurement preferably comprises the above-mentioned internal phase correction, also called "3-line Bo" correction. The theoretical model is used in this variant to correct the reference-based internal phase correction in order to further reduce artifacts without increasing the recording time.

[0033] The reference measurement for phase variations, also referred to as PC-scan, preferably comprises a complete, preferably one-time, reference recording of the k-space. By measuring the k-space once, it is possible to determine the basic magnetic field and the linear phase response or the correction values of the phase in relation to the k-space lines. Since the clinical EPI report comprises a large number of EPI measurements, the total measurement time is hardly extended by the additional complete measurement of the k-space without phase encoding.

[0034] It is particularly preferred that the reference measurement is performed in a steady state. If, for example, a complete PC-scan is performed, the internal phase correction, also referred to as "3-line B0" correction, can be performed in the steady state, since the stabilization of the vibrating structure has already taken place during the first half of the PC-scan. If, for example, 64 k-space lines are recorded for the PC-scan, the oscillation is already in the steady state during the recording of the three middle k-space lines as reference measurement for the external phase correction, but the PC-scan is partially performed in a non-steady state. If the actual image recording is subsequently performed in the steady state, the internal phase correction can be used as a correct reference measurement, since it has also been performed in the steady state. If the PC-scan is started in a non-steady state, a time saving results.

[0035] It is likewise particularly preferred that the echo planar sequence comprises an initial reference measurement for identifying drift effects. In order to determine the so-called drift effects, it can be advantageous to perform an additional reference measurement by applying a reference gradient before the imaging and to use the result thereof for correcting the system parameter values or for correcting the model-determined response behavior of the magnetic resonance imaging system. In the described advantageous variant, the comparatively small time requirement for the reference measurement is combined with a significantly improved image quality due to the model-calculated interference fields. The so-called drift effects, which can lead to errors in particular in time-extended measurements, are advantageously compensated.

[0036] In one design variant of the method for performing a magnetic resonance imaging of an examination region by means of an echo planar sequence according to the application, a reference recording is first performed, which preferably comprises a "3-line B0" correction or an internal phase correction. In the recording, a deviation of the basic magnetic field and preferably a deviation of the phase response or a deviation of the phase and preferably also a temporal echo spacing are determined. The temporal echo spacing describes the temporal spacing between two successive echoes excited by the radio frequency signal of the echo planar sequence. Due to the described interference effects, the temporal echo spacing can change slightly.

[0037] Subsequently, a mechanical model of the magnetic field generating unit of the magnetic resonance imaging system used to perform the imaging is generated based on the echo time intervals as a theoretical model. The mechanical model depicts the mechanical vibration behavior of the magnetic field generating unit in the form of a harmonic oscillator. Based on the model, the contribution of the eddy currents due to the vibrations to the generation of the basic magnetic field and the phase response or phase of the magnetic resonance signal can be estimated in a model manner as a shift in time in the time domain as a function of the k-space line and the excitation frequency.

[0038] The already estimated deviations of the basic magnetic field and the already estimated deviations of the phase response of the magnetic resonance signal are extended corrected based on the model and the corrected values for the basic magnetic field and the phase response or phase in this way are calculated as a function of the i-th k-space line.

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

[0040] In one particularly preferred variant of the method for performing magnetic resonance imaging of an examination region by means of an echo planar sequence according to the application, a phase-variation scan recorded completely or in steady state is additionally used once before the determination of the theoretical model and the correction values related to the k-space line are determined based on the measurement data produced in this way. The measurement data produced in this way are obtained from the examination region. The values produced in this way are used as support values for the theoretical model of the magnetic field generating unit.

[0041] In one particularly preferred variant of the method for performing magnetic resonance imaging of an examination region by means of an echo planar sequence according to the application, the echo planar sequence comprises a reference measurement for the phase variation and the echo time intervals are determined based on the reference measurement, based on which the theoretical model is determined. Advantageously, it is possible to

[0042] In one preferred design of the method for performing magnetic resonance imaging of an examination region by means of an echo planar sequence according to the application, the echo planar sequence has a reference measurement for the phase variation, which comprises a scan of at least three reference lines. Preferably, the reference measurement has exactly three reference lines. The recording of only three reference lines in the read direction makes it possible to save time for the reference measurement compared to a reference measurement which covers the entire k-space of the imaging. The three reference lines in the read direction at least make it possible to determine coarse information about the phase variation between the individual reference lines. BRIEF DESCRIPTION OF DRAWINGS

[0043] The application is explained in more detail below with reference to embodiments and with reference to the accompanying drawings. The drawings show:

[0044] Fig. 1 shows a schematic diagram of an internal phase correction pulse sequence,

[0045] Fig. 2 shows a schematic diagram of a pulse sequence diagram with external phase correction,

[0046] Fig. 3 shows a schematic diagram of a pulse sequence diagram with phase correction based on GRAPPA reference recordings,

[0047] Figure 4 A flow chart is shown which illustrates a method for performing magnetic resonance imaging by means of an echo planar sequence according to one embodiment of the application,

[0048] Figure 5 A flow chart is shown which illustrates a method for performing magnetic resonance imaging by means of an echo planar sequence according to one alternative embodiment of the application,

[0049] Figure 6 A schematic diagram of a magnetic resonance imaging system according to one embodiment of the application is shown,

[0050] Figure 7 A diagram is shown which illustrates a measured deviation between two k-space scan directions in relation to the number of gradient pulses applied,

[0051] Figure 8 An image diagram of a reference recording with artifacts in relation to the number of gradient pulses applied is shown. DETAILED DESCRIPTION

[0052] In Fig. 1 a schematic diagram of a pulse sequence diagram of an echo planar sequence with conventional internal phase correction is illustrated. The pulse sequence diagram shows a row of radio frequency excitation pulses HF in the uppermost row. In the second row marked by "RX", the pulse sequence diagram shows a first read pulse ADC ref and a row of read radio frequency pulses ADC for the magnetic resonance signal to be imaged. In the third row, the read gradient G x is shown not only for the reference recording G ref (Gref is also referred to as reference gradient below) but also for the imaging G x (Gx has the meaning of a gradient in the read direction here, which is applied simultaneously with the time window in which the read radio frequency pulses ADC are detected). In the fourth row, the phase encoding gradient G y is illustrated. In the fifth row, the slice selection gradient G z is shown. As already mentioned, the time required for the true reference measurement is very short, for which reason the reference gradient G ref does not correspond exactly to the gradient G x applied in the read direction for imaging.

[0053] In Fig. 2 a schematic diagram of a conventional external phase-corrected pulse sequence diagram is illustrated. In external phase correction, the gradient G x ref has been applied in the read direction during the reference measurement ref . The reference gradient G ref corresponds to the gradient G x applied in the read direction simultaneously with the read RF pulse ADC x . In this way, the interference signal generated in the reference measurement by the gradient G ref applied in the read direction also in the real imaging is detected simultaneously with the first read pulse ADC ref together, so that the determination of the interference effect and its compensation is improved compared to the time-saving method shown in Fig. 1. However, the external phase correction requires more time compared to the internal phase correction shown in Fig. 1.

[0054] In Fig. 3 a schematic diagram of a conventional phase-corrected pulse sequence diagram based on GRAPPA reference recordings is shown. In the conventional correction method shown in Fig. 3 two reference gradients G ref are applied in the read direction. The reference measurements or read pulses ADC ref are considered for the calculation of the kernel, which is considered together in the image reconstruction and should compensate for the interference effects that have occurred in the reference measurement.

[0055] In Figure 4 a flowchart 400 is shown, which illustrates a method for performing magnetic resonance imaging of an examination region by means of an echo planar sequence according to one embodiment of the application.

[0056] In step 4.I a reference recording Int PC SC is first made, which corresponds to the "3-line B0" correction or internal phase correction reference measurement illustrated in Fig. 1. In the recording, in addition to the deviation dB0 of the basic magnetic field and the change dPH of the phase response or the change in phase, the echo spacing T EE in time is also determined.

[0057] In step 4.II a mechanical model MMF of the magnetic field generating unit is generated on the basis of the echo spacing T EE in time. The mechanical model MMF depicts the mechanical vibration behavior of the magnetic field generating unit in the form of a harmonic oscillator. On the basis of the model the contribution MMF-B 0,i of the eddy currents to the generation of the basic magnetic field B0 due to the vibrations can be estimated in a model-like manner and the estimated phase response or the estimated phase MMF-PH i can be calculated as a temporal offset in the time domain as a function of the k-space line and the excitation frequency.

[0058] Based on the model, in step 4.III, the already in step 4.I determined deviations dB0 of the basic magnetic field and the in step 4.I determined changes dPH of the phase response are extended corrected EXT-PC and the values for the basic magnetic field B 0,i and the phase response PH i or the phase are corrected in this way.

[0059] In step 4.IV, the phase correction PC of the magnetic resonance raw data and thus also of the image data is now carried out based on the corrected data B 0,i , PH i .

[0060] A flowchart 500 is shown in Figure 5 which illustrates a method for carrying out magnetic resonance imaging by means of an echo planar sequence according to an alternative embodiment of the application.

[0061] In the embodiment illustrated in Figure 5 , in step 5.II, in addition, a complete or in steady state recorded phase change scan PC-S is used once and the correction values B 0i , PH i related to the k-space lines are determined based on the thus resulting measurement data. The resulting measurement data are obtained from the examination region ROI. The values produced in this way are used in step 5.III as support values for the mechanical model MMF of the magnetic field generating unit. Steps 5.IV and 5.V correspond to steps 4.III and 4.IV.

[0062] A schematic diagram of a magnetic resonance imaging system 1 according to an embodiment of the application is shown in Figure 6 . The magnetic resonance imaging system 1 has a model unit 3a which is configured to generate a theoretical model MMF which depicts the vibration structure of the unit which generates the magnetic field. Here, by the vibrations of the vibration structure eddy currents are induced in the vibration structure which cause a gradient-like interference magnetic field in the examination region ROI. The model unit 3a is therefore also configured, inter alia, to determine the gradient-like interference magnetic field based on the vibration of the vibration structure calculated in a model-like manner and the eddy currents derived therefrom of the theoretical model. Based on the eddy currents or the derived interference magnetic field, the changes of the basic magnetic field B 0,i and the phase response PH i of the magnetic resonance signal are derived.

[0063] Part of the magnetic resonance imaging system 1 is also a sequence unit 1 a for applying an echo planar sequence to generate radio frequency signals HF by means of which the magnetic resonance signals RX are excited. The magnetic resonance imaging system 1 also comprises a magnetic field generating unit 1 b which generates a basic magnetic field B0, gradient fields G x , G ref and radio frequency fields on the basis of the applied echo planar sequence.

[0064] The magnetic resonance imaging system 1 also comprises an input interface 2 for receiving the magnetic resonance signals RX from the examination region ROI, wherein magnetic resonance raw data RD are generated on the basis of the magnetic resonance signals RX.

[0065] Part of the magnetic resonance imaging system 1 is also a correction unit 3 for determining corrected magnetic resonance raw data KRD on the basis of the magnetic resonance raw data RD and a deviation determined by a model unit 3 a on the basis of the basic magnetic field B 0,i and a phase response PH i of the magnetic resonance signals RX.

[0066] The magnetic resonance imaging system 1 also comprises a reconstruction unit 4 for reconstructing magnetic resonance image data BD on the basis of the corrected magnetic resonance raw data KRD.

[0067] In Figure 7 a diagram is illustrated which shows a measured deviation PKPK between two k-space scanning directions in relation to the number PZ of applied bipolar gradient pulses. The deviation PKPK relates to the basic magnetic field in Hz and the deviation between the peaks. In Figure 7 the scenario illustrated in , the ghosting in the x-direction and the y-direction is located at approximately 15 cm outside the isocenter. After the number PZ of 20 bipolar gradient pulses, the stabilization process ends, which is illustrated by the horizontal course of the PKPK curve at the value PZ = 20.

[0068] Figure 8 In Figure 8 an image illustration of a reference recording with artifacts is illustrated which are related to the number of applied gradient pulses. As can be seen in , the artifacts decrease after the number PZ of approximately 10 gradient oscillations.

[0069] Finally, it is again noted that the methods and devices described in the foregoing are only preferred embodiments of the application and that the application can be varied by the person skilled in the art without departing from the scope of the application as intended by the embodiments. For the sake of completeness, it should also be noted that the use of the indefinite article "a" or "an" does not exclude that the features involved can also occur multiple times. Likewise, the term "unit" does not exclude that it is composed of multiple parts, which can be spatially distributed if necessary. Irrespective of the grammatical gender of a particular term, both men and women with a male or female gender identity are included.

Claims

1. A method for performing magnetic resonance imaging on a region of interest (ROI) using an echo-plane sequence, the method comprising the following steps: - Based on the aforementioned echo plane sequence generation theoretical model (MMF), which depicts the vibrational structure of the unit (1b) generating the magnetic field, eddy currents are induced in the vibrational structure through vibration, and these eddy currents generate a gradient-type interfering magnetic field in the inspection area (ROI). -The gradient disturbance magnetic field is determined based on the theoretical model (MMF). - Apply the aforementioned echo plane sequence - Magnetic resonance signals (RX) are received from the examined region (ROI), wherein raw magnetic resonance data (RD) is generated based on the magnetic resonance signals (RX). - The corrected raw magnetic resonance data (KRD) is determined based on the generated raw magnetic resonance data (RD) and the determined gradient interference magnetic field in the region of inspection (ROI). - Reconstruct magnetic resonance image data (BD) based on the modified raw magnetic resonance data (KRD).

2. The method according to claim 1, The theoretical model (MMF) includes a mechanical model that describes the mechanical vibration properties of the unit (1b) used to generate the magnetic field for performing the magnetic resonance imaging.

3. The method according to claim 1 or 2, The modified raw magnetic resonance data (KRD) includes phase correction.

4. The method according to any one of the preceding claims, The eddies mentioned therein include rapidly decaying eddies with a time constant between 0.1 ms and 10 ms.

5. The method according to any one of the preceding claims, The theoretical model (MMF) of the vibrating structure mentioned above includes a damped harmonic oscillator.

6. The method according to any one of the preceding claims, The echo plane sequence includes a reference measurement (PC-S) for phase changes.

7. The method according to claim 6, The reference measurement (PC-S) used for phase change includes a complete reference record in k-space.

8. The method according to claim 6 or 7, The reference measurement (PC-S) used for phase change is performed in a steady state.

9. The method according to any one of the preceding claims, The echo plane sequence includes an initial reference measurement for identifying the drift effect.

10. The method according to any one of the preceding claims, The echo plane sequence includes reference measurements for phase changes, and the time-series echo interval (T) is determined based on the reference measurements. EE Based on the time interval (T) EE Determine the theoretical model (MMF).

11. The method according to any one of the preceding claims, The echo plane sequence includes a reference measurement for phase change, which includes a scan of at least three reference lines.

12. A magnetic resonance imaging system (1), the magnetic resonance imaging system (1) having: - Model unit (3a), which is used to generate a theoretical model (MMF) based on the echo plane sequence to be applied, the theoretical model (MMF) depicting the vibration structure of the unit (1b) generating the magnetic field, wherein eddy currents are induced in the vibration structure by vibration of the vibration structure, the eddy currents generating a gradient interference magnetic field in the inspection area (ROI), and is used to determine the gradient interference magnetic field based on the theoretical model (MMF). - Sequence unit (1a), the sequence unit (1a) being used to apply the echo plane sequence, - Input interface (2), the input interface (2) being used to receive magnetic resonance signals (RX) from the inspection area (ROI), wherein raw magnetic resonance data (RD) is generated based on the magnetic resonance signals (RX). - Correction unit (3), the correction unit (3) is used to determine the corrected magnetic resonance raw data (KRD) based on the original magnetic resonance data (RD) and based on the determined gradient interference magnetic field in the inspection area (ROI), - Reconstruction unit (4), which is used to reconstruct magnetic resonance image data (BD) based on the modified raw magnetic resonance data (KRD).

13. A computer program product comprising instructions that, when executed by a computer, cause the computer to perform the steps of the method according to any one of claims 1 to 11.

14. A computer-readable storage medium comprising instructions that, when executed by a computer, cause the computer to perform the steps of the method according to any one of claims 1 to 11.

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