Apparatus and method for predictively correcting gradient pulse shape distortions in MRI imaging - Patent Application 20070123333
Patent Information
- Application Number
- JP2024505259
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2021-08-06
- Filing Date
- 2022-07-28
- Publication Date
- 2025-07-30
AI Technical Summary
Gradient amplifiers in magnetic resonance imaging systems introduce nonlinearities that violate the linear time-invariant assumption, leading to difficulties in waveform prediction and deteriorating image quality, especially when cheaper components are used.
Implement a current sensing system to measure the gradient coil current and apply a current-to-field modulation transfer function (CGMTF) for accurate real-time output current measurements, allowing derivation of the actual k-space trajectory for image reconstruction, and use artificial intelligence to predict and correct gradient distortions.
This approach enhances image quality by accurately predicting and correcting gradient distortions, minimizing the need for continuous monitoring of the gradient amplifier output current and improving the predictability of gradient systems.
Smart Images
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Abstract
Description
[Technical field]
[0001] The present invention relates to magnetic resonance imaging, in particular to a gradient system for a magnetic resonance imaging system. The present invention also relates to a method and a computer program for operating the gradient system. [Background technology]
[0002] The gradient system 200, which has several components as shown in FIG. 2, is a key component for facilitating spatial encoding of MR signals in an MRI scanner. j 202 generates corresponding currents which are appropriately pre-emphasized by corresponding pre-emphasis units 206 and amplified by gradient amplifiers 208 driving gradient coils 210 to generate the desired magnetic field response. This process can be described in the LTI (linear time invariant) sense by a transfer function (gradient impulse response function GIRF or gradient modulation transfer function GMTF, see FIG. 2). Summary of the Invention [Problem to be solved by the invention]
[0003] However, the gradient amplifier 208 may introduce some nonlinearities that are difficult to predict and violate the strict LTI assumption. If simpler / cheaper components are used in the gradient amplifier 208, the nonlinear effects may be more severe, making it much more difficult for waveform prediction to be used in reconstruction. As a result, image quality is poor. To alleviate this problem, a basic current detection using a current detection system 212 is proposed. At the beginning of the approach, a current to field modulation transfer function (CGMTF), which describes how the gradient coil 210 converts the input current into the actual gradient magnetic field 204, needs to be incorporated into this current sensing concept, thereby enabling accurate gradient waveform prediction based on actual real-time output current measurements.
[0004] FIG. 2 shows a schematic of a state-of-the-art gradient system 200. The schematic of the gradient system shows the associated transfer functions GIRF and GMTF as well as the current-driven reconstruction approach. The gradient system 200 comprises a linear waveform pre-emphasis step 206 for eddy current compensation (ECC), a gradient amplifier 208 and respective gradient coils 210 (x, y, z). The gradient impulse response function (GIRF) or its Fourier transform, the gradient modulation transfer function (GMTF), describes the behavior of the complete gradient system 200 but cannot represent the nonlinearities occurring in the gradient amplifier 208. By measuring the gradient amplifier output current and applying a current-to-field transfer function (current to field impulse response function CGIRF, current to field modulation transfer function CGMTF), a more accurate description of the magnetic field response is obtained. If the current is measured simultaneously with imaging, an accurate k-space trajectory can be derived for image reconstruction.
[0005] So far, this concept has been scientifically tested for non-Cartesian imaging to derive the actual k-space trajectories present during MR signal sampling. For this purpose, in parallel with MR data acquisition, the currents flowing through the gradient coils 210 are also sampled synchronously. This is a simple application since the image reconstruction 216 starts after sampling the MR data, at which point the sampled currents that generate the actual gradient waveforms 204, and the k-space trajectories thus derived, can be easily made available. It should be noted that, as discussed here, the current-based correction approach is limited to readout gradients where the k-space trajectories for reconstruction can be adapted to the actual magnetic field.
[0006] WO 2019 / 179797 A1 discloses a magnetic resonance imaging system with a gradient coil system having a set of gradient coils configured to generate gradients, a gradient coil amplifier, and a current sensor system configured to measure current sensor data describing currents supplied to each of the set of gradient coils. Execution of the machine executable instructions causes a processor to perform the steps of controlling the magnetic resonance imaging system with pulse sequence commands to acquire magnetic resonance imaging data, recording the current sensor data during acquisition of the magnetic resonance imaging data, calculating a corrected k-space trajectory using the current sensor data and a gradient coil transfer function, and reconstructing a corrected magnetic resonance image using the magnetic resonance imaging data and the corrected k-space trajectory.
[0007] However, if the performance of the gradient amplifier is too impaired, the performance of other gradients involved in the MR scan will also be impaired. For crusher gradients, this is often less of a concern, but is important in the case of MR signal excitation or refocusing procedures involving gradients. For optimal performance of spatially selective RF pulses, the behavior of the gradient system must be well predictable or known in advance.
[0008] Real-time feedback of the sensed selection gradient current to the RF transmitting object can help to actively steer the optimal selection, but it somehow mimics the function of a current amplifier control loop, which has very high bandwidth requirements. Such an approach is another undesirable cost driver, apart from unknown feasibility and performance.
[0009] US Pat. No. 6,377,043 B1 describes a system that allows the time variation of the magnetic field generated by a gradient coil to be derived from the measured time variation of the current through the gradient coil and from a pulse response determined and stored for the associated coil.
[0010] The paper "Time optimal control-based RFT pulse design under gradient imperfections" by C. Aigner et al. describes a gradient system imperfection model that fits into a control framework for radio frequency (RF) pulse design.
[0011] The objective is to improve control of gradient magnetic fields for high image quality and to minimize the need for continuous monitoring of gradient amplifier output currents. [Means for solving the problem]
[0012] According to the invention, this object is addressed by the subject matter of the independent claims. Preferred embodiments of the invention are set forth in the dependent claims.
[0013] Therefore, according to the present invention, there is presented a gradient system of a magnetic resonance examination system for generating a gradient magnetic field in an imaging zone of the magnetic resonance examination system, comprising at least one gradient coil for generating a gradient magnetic field when a current is supplied thereto, the gradient coil being configured to generate the at least one selected gradient magnetic field by transmitting a selected gradient pulse along an axis, the gradient system comprising a gradient coil amplifier configured to supply a current to the gradient coil, the gradient system further comprising a current sensor system configured to sample a current supplied to the gradient coil by the gradient coil amplifier, the gradient system further configured to generate an actual selected gradient pulse shape of the selected gradient magnetic field by applying a current-field modulation transfer function (CGMTF) to the sampled current, whereby the magnetic resonance examination system is able to compensate for deviations of the actual selected gradient pulse shape from an input selected gradient pulse shape by adjusting a radio frequency pulse emitted simultaneously with the selected gradient pulse.
[0014] The basic idea is that instead of monitoring only the readout gradient, the selection gradient, or other potentially important gradients used for MR signal encoding, are also monitored via current sensing. Based on the known input gradient pulse shape and the sensed output current, it can be derived how the limited amplifier performance compromises the output current. Due to non-linear effects occurring in the active amplifier control loop, these compromise effects cannot be simply modeled and must depend on the measured input. The sensed current is related to the actual selection gradient magnetic field via the current-field modulation transfer function (CGMTF) of the corresponding gradient coil channel. The transfer function is used to predict the actual selection gradient pulse shape (Gf) in the gradient coil based on the sensed current. The invention achieves to avoid the need to continuously monitor the gradient amplifier output current.
[0015] In one embodiment of the invention, the selective gradient magnetic field is a spatial readout encoding gradient magnetic field and / or a crusher gradient magnetic field and / or a slice selection gradient magnetic field and / or a slice refocusing gradient magnetic field and / or a gradient magnetic field that is spatially selective in one or more dimensions.
[0016] In another aspect, the present invention provides a magnetic resonance imaging system configured to acquire magnetic resonance imaging data from an imaging zone, the magnetic resonance imaging system comprising a gradient system as described above.
[0017] In another aspect, the present invention provides a method of operating a gradient system for a magnetic resonance examination system generating magnetic gradient fields in an imaging zone of the magnetic resonance examination system, comprising the steps of providing a gradient system as described above, sampling currents supplied to a gradient coil by a gradient coil amplifier with a current sensor system, obtaining an actual selected gradient pulse shape by applying a current-field modulation transfer function CGMTF to the sampled current, predicting gradient distortions of the actual selected gradient pulse shape based on a known input selected gradient pulse shape, and correcting the gradient distortions of the actual selected gradient pulse shape by the magnetic resonance examination system by adapting a radio frequency pulse shape emitted simultaneously with the actual selected gradient pulse.
[0018] In one embodiment of the invention, obtaining the actual selected gradient pulse shape by applying a current-magnetic-field modulation transfer function (CGMTF) to the sampled currents comprises obtaining the actual selected gradient pulse shape by convolution according to the following formula: G f (t)=I(t)*CGMTF(t) Here, G f (t) is the actual selection gradient pulse shape, I(t) is the sampled current, and CGMTG(t) is the current-field modulation transfer function of the desired orders (at least the 0th and 1st orders).
[0019] In another embodiment of the invention, the current magnetic field modulation transfer function is pre-recorded prior to applying the function to the actual selected gradient pulse shape.
[0020] In one embodiment of the present invention, the step of predicting the gradient distortion of the actual selected gradient pulse shape based on the known input selected gradient pulse shape is performed by artificial intelligence.
[0021] In another embodiment of the present invention, predicting the gradient distortion of the actual selected gradient pulse shape by artificial intelligence includes using a trained convolutional neural network that returns the actual selected gradient pulse shape from parameter values representing the desired gradient pulse shape.
[0022] In one embodiment of the present invention, parameter values are selected from a list including maximum gradient strength and / or slew rate and / or intensity and / or variations when using variable rate selective excitation (VERSE).
[0023] In another embodiment of the invention, the step of sampling the current supplied by the gradient coil amplifiers to each of the set of gradient coils by a current sensor system comprises sampling the current during the start-up phase of the magnetic resonance imaging system using an initial sequence dummy shot, or during a short pre-scan involving the gradient pulses.
[0024] In one embodiment of the present invention, the step of correcting gradient distortions of the actual selected gradient pulse shape by adapting the radio frequency pulse shape emitted simultaneously with the actual selected gradient pulse is performed in addition to correcting gradient distortions of the readout gradient of the gradient system.
[0025] In another aspect, the present invention provides a computer program product comprising instructions, which when executed by a computer, cause the computer to carry out the method described by the method steps above.
[0026] In a further aspect, the present invention provides a computer program product comprising instructions which, when executed by a computer, cause the computer to control a magnetic resonance imaging system to correct gradient distortions of an actual selected gradient pulse shape by adapting a radio frequency pulse shape emitted simultaneously with the actual selected gradient pulse.
[0027] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter, but such embodiments do not necessarily represent the full scope of the invention, and therefore reference should be made to the claims and this specification for interpreting the scope of the invention. [Brief description of the drawings]
[0028] [Figure 1] 1 illustrates an example of a magnetic resonance imaging system in accordance with an embodiment of the present invention. [Diagram 2] FIG. 1 shows a schematic diagram of a gradient system according to the state of the art. [Diagram 3] FIG. 1 illustrates a schematic of a gradient system according to an embodiment of the present invention. [Figure 4] FIG. 13 illustrates RF excitation applied to a sample current waveform in accordance with an embodiment of the present invention. [Diagram 5] 4 is a flow chart showing the operation of the gradient system according to FIG. 3; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] FIG. 1 shows an example of a magnetic resonance imaging system 100 with a magnet 104 according to an embodiment of the present invention. The magnet 104 is a cylindrical superconducting magnet with a bore 106 passing through it. Different types of magnets can be used, for example, both split cylindrical magnets and so-called open magnets. Split cylindrical magnets are similar to standard cylindrical magnets, except that the cryostat is split into two sections to allow access to the magnet's isoplane, and such magnets can be used, for example, in conjunction with charged particle beam therapy. An open magnet has two magnet sections, one above the other, with a space between them large enough to accommodate a subject, and the arrangement of the two magnet sections resembles a Helmholtz coil. Open magnets are popular because they provide less confinement for the subject. Inside the cryostat of the cylindrical magnet is a collection of superconducting coils. Within the bore 106 of the cylindrical magnet 104 is an imaging zone 108, where the magnetic field is strong and uniform enough to perform magnetic resonance imaging. A region of interest 109 is shown within the imaging zone 108. Magnetic resonance imaging data is typically acquired for the region of interest. A subject 118 is shown supported by a subject support 120 such that at least a portion of the subject 118 is within the imaging zone 108 and the region of interest 109.
[0030] The magnetic resonance imaging system 100 includes a gradient system 102, which is the primary spatial encoding system used in magnetic resonance imaging. The gradient system 102 includes several components. Within the magnet bore 106 is at least one gradient coil 110 used for preliminary magnetic resonance imaging data acquisition to spatially encode magnetic spins within the imaging zone 108 of the magnet 104. The magnetic field gradient coil 110 is connected to a magnetic field gradient coil amplifier 112. The gradient coil 110 can include, for example, three separate coils for spatial encoding in three orthogonal spatial directions. A gradient magnetic field power supply supplies current to the gradient magnetic field coil. The current supplied to the gradient coil 110 is controlled as a function of time and can be ramped or pulsed. The gradient magnetic field coil 110 can represent, in one embodiment of the invention, three separate orthogonal gradient magnetic field coils for generating a gradient magnetic field within the imaging zone 108. These are typically oriented as shown by axes 122, 123, and 124. Axis 124 is aligned with the axis of magnet 104, which is typically called the z-axis. 122 and 123 are the x-axis and y-axis, respectively. They are orthogonal to each other and to z-axis 124.
[0031] The magnetic field gradient coil amplifier 112 is configured to supply current to the magnetic field gradient coil. The magnetic field gradient coil amplifier 112 is shown as having a current sensor system 113 for measuring the current supplied to the magnetic field gradient coil 110. The current sensor system 113 may be, for example, part of the magnetic field gradient coil amplifier 112 or may be integrated into the magnetic field gradient coil 110. Adjacent to the imaging zone 108 is a radio frequency coil 114 for manipulating the orientation of the magnetic spins in the imaging zone 108 and also for receiving radio transmissions from the spins in the imaging zone 108. The radio frequency antenna may have multiple coil elements. The radio frequency antenna may be referred to as a channel or an antenna. The radio frequency coil 114 is connected to a radio frequency transceiver 116. The radio frequency coil 114 and the radio frequency transceiver 116 may be replaced with separate transmitting and receiving coils and separate transmitters and receivers. It is understood that the radio frequency coil 114 and the radio frequency transceiver 116 are representative examples. The radio frequency coil 114 is also intended to represent a dedicated transmit antenna and a dedicated receive antenna. Similarly, the transceiver 116 can represent separate transmitters and receivers. The radio frequency coil 114 may also have multiple receive / transmit elements, and the radio frequency transceiver 116 can have multiple receive / transmit channels. For example, when a parallel imaging technique such as SENSE is implemented, the radio frequency coil 114 will have multiple coil elements.
[0032] The transceiver 116 and the gradient controller 112 are shown connected to a hardware interface 128 of a computer system 126. The computer system further includes a processor 130 in communication with the hardware system 128, a memory 134, and a user interface 132. The memory 134 may be any combination of memory accessible to the processor 130. This may include, such as main memory, cache memory, and non-volatile memory such as flash RAM, a hard drive, or other storage devices. In some examples, the memory 134 may be considered a non-transitory computer readable medium. The memory 134 is shown as including machine executable instructions 140. The machine executable instructions 140 enable the processor 130 to control the operation and functionality of the magnetic resonance imaging system 100. The machine executable instructions 140 may also enable the processor 130 to perform various data analysis and computational functions.
[0033] FIG. 2 shows diagrammatically a gradient system according to the state of the art already described in the introduction of this specification.
[0034] 3 shows a schematic of a gradient system 300 according to one embodiment of the present invention. The gradient system schematic shows the associated transfer functions GIRF, GMTF, and the current-driven correction approaches CGIRF, CGMTF. Significant imperfections in the gradient system 300 (e.g., gradient amplifier 308) can cause the selected gradient G S(here a trapezoid) is distorted. Measuring the output current of the gradient amplifier 308 and applying the current-to-field transfer functions CGIRF, CGMTF gives a more accurate description of the field response. These distortions can be predicted or measured and the shape of the RF pulse can be appropriately adapted to compensate for deviations in the gradient fields, thus avoiding adverse effects on the slice definition. Instead of monitoring only the readout gradient, the selection gradient is also monitored via current sensing. Based on the known input waveform 302 and the sensed output current, conclusions can be drawn about how the limited amplifier performance compromises the output current. Due to non-linear effects occurring in the active amplifier control loop, these compromise effects cannot be simply modeled and must depend on the measured input. The sensed currents can be related to the actual selection gradient field 304 via the current-to-field transfer function (CGMTF) for the corresponding gradient channel. Using the pre-recorded transfer function CGMTF, the actual selection gradient field Gf in the gradient coil 310 is predicted based on the sensed currents via a simple convolution operation according to: G' S (t)=I(t)*CGMTF(t)
[0035] In one embodiment of the present invention, G S The prediction of (t) is trained using artificial intelligence. Given a known desired input gradient Gi(t), the actual gradient G' realized in the bore is SBy comparing (t) and adding some descriptive parameters to the desired gradient (maximum gradient strength, slew rate, strength, variation in case of using VERSE, etc.), a simple convolutional network can be trained that can predict the actual one based on the nominal input gradient waveform and its basic parameters. The necessary training data can be generated during the initial MRI system installation with some test scans or during the initial testing phase of the final system. This learning can be performed on each MR system individually or on a single use of identical hardware in the factory, thereby applying the trained AI model to the entire system fleet. The gradient waveforms thus predicted can then be used in the RF design process, for example for low tip angles (small tip angles), to achieve the target spatial selection profile.
[0036] In another embodiment of the present invention, the selected gradient current is monitored during an initial sequence dummy shot, which is commonly applied to achieve a steady state of the MR signal in clinical scans. Feedback it to the RF pulse design and change the RF waveform on the fly during this dummy phase to achieve a better slice definition for the next RF pulse taking into account the gradient fields actually applied can correct the corresponding RF problem accordingly. It is clear that these adaptations can be implemented for any spatially selective RF pulse used for MR excitation, refocusing, or for example for proper magnetization preparation. It is further noted that this applies to RF pulses that are spatially selective in one or more dimensions, including application to one or more transmit coils.
[0037] The proposed AI learning and dummy shot evaluation methods can also be used to better predict the actual gradients during signal readout and thus provide a way to eliminate nonlinear effects without continuously reading out the output currents or direct gradient fields during imaging.
[0038] 4 shows a diagram of RF excitation adapted to a sampled current waveform according to an embodiment of the present invention. FIG. 4(a) shows a selected gradient G S FIG. 4(b) shows a typical excitation RF pulse applied in the presence of . The resulting RF waveform and slice profile represent the Fourier pairs for the small tip angle excitation case. FIG. 4(b) shows the distortion of the gradient waveform G' S The selected gradients for the case of 10 ...
[0039] In one embodiment of the present invention, the case of sensing the gradient waveform during the start-up phase can be considered. This is basically shown in FIG. 4. For simplicity, a slice section along the z-direction is considered. The selected gradient G S While Iz is being generated, the current Iz flowing in the z-gradient coil 110, 310 is sampled by the current sensor system 113, 312. The sampled current is convolved with the CGMTF obtained according to the following equation: G' S (t) = Iz(t) * CGMTFz(t)
[0040] The CGMTF may be measured once, for example, at the end of the installation of the MR system, to obtain the actual gradient magnetic field G' generated by the gradient coils 110, 310. Scan be obtained. If the RF excitation pulse belongs to the small tip angle class, the Pauly approach can be used to calculate the new corrected RF pulse. In this excitation case, the refocusing lobe must be taken into account, although there is no RF present. This should correspond to half the integral of the selection gradient in the small tip angle approximation to facilitate the refocusing of the excited transverse magnetization and to maintain the SNR. Note that the same formula holds when using a gradient selection gradient consisting of all three basic gradient components. In this case, the convolution given in the above formula must include more components of the sampled current (at least x, y, z) and the corresponding transfer function (CGMTF). Note that the CGMTF has even more components than the basic gradients. Apart from the B0 term, higher order terms (including cross terms) can be considered to adequately predict the real magnetic field from the measured currents.
[0041] 5 shows a flow chart illustrating a method of operating the gradient system 102, 300 according to FIG. 3 for a magnetic resonance examination system 100 for generating magnetic gradient fields in the imaging zone 108 of the magnetic resonance examination system 100. First, in step 500, the gradient system 102, 300 as described above is provided. Next, in step 502, the current supplied to the gradient coil by the gradient coil amplifier 112, 308 is sampled by the current sensor system 113, 312. In step 504, the actual selected gradient pulse shape G' is determined by applying a current-field modulation transfer function (CGMTF) to the sampled current. S In step 506, the actual selected gradient pulse shape G' is obtained. S The gradient distortion of the input selection gradient pulse shape G S Finally, in step 508, the actual selected gradient pulse G' S The gradient distortion of the actual selection gradient pulse G' S At the same time, this is compensated for by the magnetic resonance examination apparatus 100 by adapting the shape of the emitted radio frequency pulses.
[0042] While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered as illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be interpreted as limiting the scope. Moreover, for the sake of clarity, not all components in the drawings have been labeled with reference signs. [Explanation of symbols]
[0043] Magnetic resonance imaging system 100 Gradient System 102 Magnet 104 Magnet bore 106 Imaging Zone 108 Areas of interest 109 Gradient coil 110 Magnetic field gradient coil amplifier 112 Current Sensor System 113 High Frequency Coil 114 Transceiver 116 Subject 118 Specimen support 120 x-axis 122 y-axis 123 z-axis 124 Computer Systems 126 Hardware Interface 128 Processor 130 User Interface 132 Computer Memory 134 Machine Executable Instructions 140 Gradient System 200 Input Waveform 202 Gradient magnetic field expansion 204 Pre-emphasis 206 Gradient Amplifier 208 Gradient coil 210 Current Sensor System 212 AD converter 214 Reconstruction 216 Reconstructed image 218 Gradient impulse response function GIRF Gradient Modulation Transfer Function GMTF Current magnetic field impulse response function CGIRF Current-magnetic-field modulation transfer function CGMTF Gradient System 300 Input gradient pulse shape 302 Actual Gradient Pulse Shape 304 Pre-emphasis 306 Gradient Amplifier 308 Gradient coil 310 Current Sensor System 312 A / D converter 314 RF Adaptation 316 RF Excitation Applied to Sampled Current Waveforms 318 Radio Frequency Pulse RF Selection gradient G S Corrected radio frequency pulse RF' Actual selection gradient G' S Providing a gradient system 500 Current Sampling 502 Acquiring the actual selected gradient pulse shape 504 Prediction of gradient distortions of actual selective gradient pulse shapes 506 Correction of gradient distortions in actual selection gradient pulse shapes 508
Claims
1. A gradient system for a magnetic resonance imaging system for generating a gradient magnetic field within an imaging zone of the magnetic resonance imaging system, at least one gradient coil configured to generate at least one selected gradient magnetic field by transmitting a selected gradient pulse, a gradient coil; at least one RF adaptation unit; wherein the selected gradient magnetic field is a spatial readout encoding gradient magnetic field and / or a crusher gradient magnetic field and / or a slice selection gradient magnetic field and / or a slice refocusing gradient magnetic field and / or a spatially selective gradient magnetic field in one or multiple dimensions; the gradient system has a gradient coil amplifier configured to supply current to the gradient coil; the gradient system further has a current sensor system configured to sample the current supplied to the gradient coil by the gradient coil amplifier; the gradient system is further configured to generate an actual selected gradient pulse shape of the selected gradient magnetic field by applying a current magnetic field modulation transfer function to the sampled current, whereby the magnetic resonance imaging system can compensate for the deviation of the actual selected gradient pulse shape from the input selected gradient pulse shape by adjusting, by the RF adaptation unit, a high-frequency pulse emitted simultaneously with the selected gradient pulse; the current magnetic field modulation transfer function describes how the gradient coil converts an input current into an actual gradient magnetic field, a gradient system.
2. A magnetic resonance imaging system configured to acquire magnetic resonance imaging data from an imaging zone, the magnetic resonance imaging system having the gradient system according to claim 1.
3. A method of operating a gradient system used in a magnetic resonance imaging system, the gradient system being used to generate a gradient magnetic field within an imaging zone of the magnetic resonance imaging system, the method comprising: providing the gradient system according to claim 1; sampling, by a current sensor system, the current supplied to the gradient coil by a gradient coil amplifier; acquiring an actual selected gradient pulse shape by applying a current magnetic field modulation transfer function to the sampled current. Predicting a gradient distortion of the actual selection gradient pulse shape based on a known input selection gradient pulse shape; Correcting the gradient distortion of the actual selection gradient pulse shape by the magnetic resonance examination system by emitting an adapted high-frequency pulse shape simultaneously with the actual selection gradient pulse; A method comprising:
4. The step of obtaining the actual selection gradient pulse shape by applying the current magnetic field modulation transfer function to the sampled current comprises a step of obtaining the actual selection gradient pulse shape by convolution according to the formula G' Ｓ G(t) = I(t) * CGMTF(t) Here, G' Ｓ (t) is the actual selection gradient pulse shape, I(t) is the sampled current, and CGMTG(t) is the current-magnetic field modulation transfer function that describes how the gradient coil converts the input current into the actual gradient magnetic field. The method according to claim 3, comprising a step of obtaining the actual selection gradient pulse shape by convolution according to.
5. The method according to claim 3, wherein the current magnetic field modulation transfer function is pre-recorded before applying the function to the current sampled to obtain the actual selection gradient pulse shape.
6. The method according to claim 3, wherein the step of predicting the gradient distortion of the actual selection gradient pulse shape based on the known input selection gradient pulse shape is performed by artificial intelligence.
7. The method according to claim 6, wherein the step of predicting the gradient distortion of the actual selection gradient pulse shape by the artificial intelligence comprises using a trained convolutional neural network that returns the actual selection gradient pulse shape from parameter values representing a desired gradient pulse shape.
8. The method according to claim 7, wherein the parameter values are selected from a list including variations when using maximum gradient intensity and / or slew rate and / or intensity and / or variable rate selective excitation.
9. The step of sampling the current supplied to the gradient coil by the gradient coil amplifier by the current sensor system comprises sampling the current during an initial sequence dummy shot during a startup phase of the magnetic resonance imaging system or during a short preliminary scan with the gradient pulse. The method according to any one of claims 4 to 8.
10. The step of correcting the gradient distortion of the actual selection gradient pulse shape by the magnetic resonance examination system by adapting the high-frequency pulse shape emitted simultaneously with the actual selection gradient pulse is performed in addition to correcting the gradient distortion of the readout gradient of the gradient system. The method according to any one of claims 4 to 8.
11. A computer program having instructions for causing a magnetic resonance imaging system to be controlled by the computer to correct the gradient distortion of the actual selection gradient pulse shape by adapting the high-frequency pulse shape emitted simultaneously with the actual selection gradient pulse according to the method according to any one of claims 3 to 8 when the computer program is executed by the computer.