Preparing a magnetic resonance imaging method in compliance with stimulus-based restrictions

Abstract

A method for preparing a magnetic resonance imaging of an examination object is described herein. In the method, a plurality of representative pulse sequence segments are generated, which are each configured with a reference gradient amplitude of a gradient pulse having a highest stimulation potential of the representative pulse sequence segment. A maximum gradient rise rate is determined for each representative pulse sequence segment, for which an allowed maximum value of the stimulation potential is not exceeded. One of the plurality of representative pulse sequence segments is determined and selected for a measurement scheme to be planned, in accordance with the gradient amplitude of the gradient pulse having the highest stimulation potential of a pulse sequence segment of a pulse sequence to which the measurement scheme is based. The pulse sequence segment of the pulse sequence to which the measurement scheme is based is adapted such that the maximum gradient rise rate pertaining to the selected representative pulse sequence segment is also not exceeded by the pulse sequence segment of the pulse sequence to which the measurement scheme is based. A magnetic resonance imaging method, a preparation device and a magnetic resonance imaging system are further described.

Description

TECHNICAL FIELD

[0001] The invention relates to a method for preparing a magnetic resonance imaging of an examination object. The invention also relates to a magnetic resonance imaging method. The invention also relates to a preparation device for a magnetic resonance imaging system. The invention also relates to a magnetic resonance imaging system. BACKGROUND

[0002] Imaging systems based on magnetic resonance measurement methods, in particular of nuclear spins, so-called magnetic resonance tomographs or magnetic resonance imaging systems, have been successfully established and proven by widespread application. It should be briefly mentioned here that the terms magnetic resonance imaging and magnetic resonance measurement or the abbreviation "measurement" are synonymous throughout the present application. This also includes, in particular, magnetic resonance measurements of abstract quantities with non-directly objective properties, such as measurement parameters in diffusion measurements. The term "Abfolge" (or series) is also used synonymously with the term "sequence" when describing a pulse sequence preset for a specific measurement.

[0003] In such image acquisition, the original orientation of the magnetic dipoles to be examined and a uniform static basic magnetic field B0 are usually superimposed with rapidly switched magnetic fields, so-called gradient fields, for the position resolution of the imaging signals. In order to determine material properties of the examination object to be depicted, a dephasing or relaxation time is determined after a magnetization deflection from the original orientation, so that different material-specific relaxation mechanisms or relaxation times can be identified. The deflection is usually achieved by means of some HF pulses (abbreviation HF for high frequency), also called excitation pulses, and the position resolution is based on a temporally determined manipulation of the deflected magnetization by means of gradient fields in a so-called measurement sequence or control sequence, which determines the exact time sequence of the HF pulses, the exact time sequence of the gradient field changes (by means of a switching sequence of the emitted gradient pulses) and the exact time sequence of the acquisition of the measurement values.

[0004] Usually, the correspondence between the measured magnetization (from which the mentioned material properties can be derived) and the position coordinates of the measured magnetization in the position space in which the examination object is located is achieved by means of an intermediate step. In this intermediate step, the acquired magnetic resonance raw data, also called k-space data, are arranged on readout points of a so-called "k-space", wherein the coordinates of the k-space are encoded as a function of the gradient fields. By means of a Fourier transformation, the amount of magnetization (in particular the transverse magnetization in the plane transverse to the above-mentioned basic magnetic field) on a specific position of the examination object can be determined from the readout point data, which calculates the signal strength in the position space from the signal strength (magnetization amount) assigned to a specific frequency (position frequency) or phase by means of a Fourier transformation.

[0005] Time-varying magnetic fields used in magnetic resonance recordings can lead to unwanted stimulation of the nervous system, in particular to peripheral nerve stimulation and cardiac stimulation. In particular due to the increasing performance of gradient systems in terms of their gradient amplitudes and gradient rise rates, limitations in terms of parameters or parameter values selected for a pulse sequence result from physiological limitations. While magnetic resonance imaging apparatuses are equipped with monitoring devices which interrupt an ongoing measurement immediately in the event of an exceedance of a specified limit value, in order to optimize the workflow it is generally checked before the start of an imaging process whether the pulse sequence used complies with, or rather conforms to, the above-mentioned limitations, and the user is given the possibility to make adjustments to the pulse sequence parameters or measurement parameters in order to comply with the above-mentioned limit values. However, the calculations to be performed for this check are quite complex in terms of modeling of the underlying physiological limitations. The increase in the amount of calculation leads to delays in the planning of a magnetic resonance imaging, which can cause problems in time-critical emergency examinations. In general, an extension of the check time reduces the patient's comfort and reduces the number of examinations per time unit, thus reducing the availability of the magnetic resonance imaging system.

[0006] Conventionally, the gradient rise rate (or gradient slew rate) is chosen to be so small throughout, i.e. for all gradient pulses of the MR measurement, that no stimulation occurs. The IEC standard (IEC is an abbreviation for "International Electrotechnical Commission") contains limits to be complied with in terms of time variation of the magnetic field and amplitude variation of the electric field. The disadvantage of this approach is that the limit values are designed to be very conservative and thus the magnetic resonance measurement can become unnecessarily slow.

[0007] The IEC standard therefore provides for a more complex modeling on the basis of measurements on a representative group of subjects in order to determine the actual stimulation limits. An example of such a modeling is the SAFE model, which determines a value from the gradient time profile of each gradient axis by means of a transfer function consisting of a number of filtering, attenuation, rectification and combination steps, which value represents the stimulation potential of the MR examination (MR is an abbreviation for "Magnetic Resonance"). However, due to the complexity of the model and in particular the dependency on the gradient time profile history, the calculation effort for checking whether the limit values are complied with is greatly increased.

[0008] For this purpose the model is typically used, for example, in such a way that the check of the stimulation limits is performed immediately at a point in time after the user has made the measurement request, i.e. after all measurement parameters have been determined. If the result of the check is that the limit values are exceeded, the system can suggest to the user to select changes to the measurement parameters, with which changes it can be possible to perform the measurement or the magnetic resonance imaging. The disadvantage of this approach is that the user learns about the limitations at a later point in time and the examination workflow is interrupted. Furthermore, the suggestions of the system often do not meet the requirements of the user, so that the user has to make manual adjustments and iteratively approach a performable measurement scheme.

[0009] Alternatively, a fixed measurement protocol can be specified, with prior checks to ensure adherence to stimulation limits. However, this deprives users of the flexibility to tailor MRI scans to the specific diagnostic needs of individual patients.

[0010] In existing techniques, it is also recommended that physiological limitations be considered when planning magnetic resonance imaging (MRI) and that the range of settable parameters be limited accordingly. Clever mathematical simplifications can significantly reduce the computational burden of complex modeling, thus ensuring that the user interface response time is generally fast enough or short enough for use in routine clinical work. However, as the number of gradient pulses to be considered during the examination increases, delays in setting measurement parameters can still occur, which can impact clinical procedures, especially in emergency situations.

[0011] In principle, the response time of the user interface can also be reduced using more powerful computers. However, in addition to the higher hardware costs, it should be noted that the complexity of modeling will also increase significantly. For example, if future developments are to take into account the location and orientation of nerve fiber bundles for stimulation, the complexity of modeling will increase dramatically. Summary of the Invention

[0012] Therefore, the technical problem to be solved by the present invention is to enable the planning and preparation of a time-saving magnetic resonance imaging method compared with conventional methods, especially considering the limitations related to stimulation and by adjusting the corresponding parameters of the pulse sequence used.

[0013] This technical problem is solved by the method for preparing an object for examination according to the invention, by the magnetic resonance imaging method according to the invention, by the preparation apparatus according to the invention, and by the magnetic resonance imaging system according to the invention.

[0014] In the method for preparing a magnetic resonance imaging of an examination object according to the application, a plurality of representative pulse sequence segments are first generated in a preparation process before the actual magnetic resonance imaging and before the adaptation of the measurement protocol for the specific magnetic resonance imaging, which are each configured with a reference gradient amplitude of the gradient pulse having the highest stimulation potential of the representative pulse sequence segment, and the stimulation potential of which is representative of a group of partially different pulse sequences. Here, the stimulation potential is a value calculated for a gradient pulse sequence on the basis of the stimulation model used, which states whether the gradient pulse sequence is allowed to be carried out on the patient. For example, the stimulation potential can correspondingly configure the probability and / or intensity of a possible stimulation to a value range between 0.0 and 1.0, wherein for values below 0.8 the measurement can be carried out without restriction; for values below 1.0 the measurement can be carried out after explicit confirmation by an operator or user; and for values greater than 1.0 the measurement is not allowed. The representative pulse sequence segments can be limited to a short sub-sequence with additional gradient pulses in a much larger pulse sequence or in a much larger range of pulse sequence types, especially if only this short sub-sequence is relevant to the calculation of the stimulation potential. This simplification is especially useful for diffusion pulse sequences, in the case of which the diffusion gradients are decisive for the stimulation.

[0015] The representative pulse sequence segments are assigned to specific pulse sequence types. These pulse sequence types can include specific types of excitation, encoding and readout, but also pulse sequences for examining different types of measurement variables, for example pulse sequences for representing objects such as bones, organs, tendons, muscles, etc. or for representing dynamic processes of liquids, especially blood, or liquid diffusion, blood flow, other liquid flow, etc.

[0016] As will be explained in detail later, further distinctions can also be made for each pulse sequence type. In particular, different representative pulse sequence segments can be determined for different pulse sequences having different polarities or different polarity characteristics.

[0017] As already mentioned, each representative pulse sequence segment is also configured with a reference gradient amplitude of one or more gradient pulses having the highest stimulation potential and thus dominating the stimulation behavior of the entire representative pulse sequence segment.

[0018] Different representative pulse sequence segments with different quantitative or gradient pulse sequence parameter values, such as gradient pulse durations, distances between gradient pulses and gradient amplitudes, can also be generated. However, as a rule, in order to limit the amount of computation, the gradient rise rates for different values of the above-mentioned parameters, such as gradient pulse durations or distances between gradient pulses, are calculated on a test basis during precomputation, but the smallest permissible gradient rise rate representing all permissible values of these parameters is then stored together with the common pulse sequence segment representing these different values.

[0019] Thus, in this preparation process, a maximum gradient rise rate is determined for each representative pulse sequence segment, for which maximum gradient rise rate, or rather in the case of which maximum gradient rise rate, the permissible maximum value of the stimulation potential is not exceeded. If none of the stored representative pulse sequences is sufficiently close to the current pulse sequence in terms of its sequence parameter values, in particular with respect to the gradient amplitude values, or if the permissible stimulation limit is to be utilized as fully as possible, interpolation can also be carried out between two maximum gradient rise rates of different representative pulse sequence segments, if necessary, in order to obtain a more precise result for the maximum gradient rise rate.

[0020] As will be explained in detail later, this preparation process, or precomputation of this type, is always carried out in time before the actual preparation of a specific magnetic resonance imaging and the associated adjustment of the respective measurement protocol for the specific magnetic resonance imaging of the examination object.

[0021] The above-mentioned representative pulse sequence segments and the corresponding maximum gradient rise rates are preferably stored in a database in tabular form, which is accessed directly before the start of the magnetic resonance imaging, if necessary, that is, in particular when the measurement protocol for the specific examination object is actually prepared and planned. The partial computation effort for calculating the permissible pulse sequence is thus generalized and carried out in advance by the two general preparation steps described above. In this way, a scale effect is advantageously achieved, since this computation only needs to be carried out once, and the maximum gradient rise rates configured to the representative pulse sequence segments and stored can be accessed again at any time when the measurement protocol is planned and adjusted.

[0022] The next step is to determine or adjust the measurement protocol in preparation and planning of the specific magnetic resonance imaging. This step can include an editing by the user, but also an automatic adjustment of the measurement protocol by the magnetic resonance imaging system or by a suitable computer program for adjusting the measurement protocol. An automatic editing by the system can be important, in particular in a (partially) automated measurement planning. The system preferably determines the number of layers or the extension size and arrangement of the layers, the so-called "Field of View" (FoV) which is required to cover the anatomy to be examined. The adjustment of the position and orientation of the examination region ROI (ROI stands for "Region of Interest") can also be achieved automatically based on landmarks (in English: "Auto Align").

[0023] In the actual preparation and planning of the specific magnetic resonance imaging of a specific examination object, one of the plurality of representative pulse sequence segments is determined and selected from a database for the measurement protocol to be planned for the current magnetic resonance imaging to be performed, according to the gradient amplitudes of the gradient pulses with the highest stimulation potential of the pulse sequence segments of the pulse sequence to which the measurement protocol is based. This determination is made according to the pulse sequence type and, if necessary, also on the basis of one or more pulse sequence parameters or pulse sequence parameter values of the pulse sequence to which the measurement protocol (Messprotokoll) is based. The representative pulse sequence segment does not necessarily have to be identical to the pulse sequence to which the measurement protocol is based. It is sufficient if the representative pulse sequence segment is representative in terms of the physiological limits of the pulse sequence to which the measurement protocol is based.

[0024] This means that, if the selected representative pulse sequence segment complies with the physiological limits at a certain value of a sequence parameter, in particular the gradient rise rate, this value should always also apply to the pulse sequence to which the measurement protocol is based. The selected pulse sequence segment is representative if the pulse sequence segment to be compared, which has the strongest stimulation in the pulse sequence to which the measurement protocol is based, and its pulse sequence parameters, in particular the gradient amplitude, the gradient duration, the gradient distance and the gradient polarity, respectively, have values or characteristics which are covered by the representative pulse sequence segment in such a way that the maximum rise rate which is assigned to the representative pulse sequence segment also applies to the specific value of the pulse sequence parameter of the pulse sequence to which the measurement protocol is based. If the respective values are not exactly the same, it can also be possible that the determined maximum gradient rise rate of the representative pulse sequence segment can be expected to be less than the maximum permissible gradient rise rate of the pulse sequence to which the measurement protocol is based. In this case, the determined value of the maximum gradient rise rate derived from the representative pulse sequence segment can not be optimal for the pulse sequence to which the measurement protocol is based, but ensures compliance with the physiological limits.

[0025] Finally, the step of adjusting the pulse sequence on the basis of which the measurement scheme is prepared is carried out when planning the measurement scheme of the magnetic resonance imaging, such that the maximum gradient rise rate assigned to the selected representative pulse sequence segment is not exceeded by the pulse sequence or the pulse sequence segment of the magnetic resonance imaging pulse sequence which corresponds to or is assigned to the representative pulse sequence segment. Advantageously, in this phase, in which usually little time remains for the calculation, for example in a medical emergency, time-consuming calculations of the maximum rise rate of the pulse sequence of the current measurement scheme are not necessary, since the required data are already present in a stored manner and only need to be identified or associated with the pulse sequence of the current measurement scheme and selected. The method steps can be carried out fully automatically, but also partially in a dialog with the user, for example, the user can adjust certain scheme parameters himself when planning a specific measurement.

[0026] In the magnetic resonance imaging method according to the application, first of all the method according to the application for preparing a magnetic resonance imaging of an examination object is applied. That is to say, in particular a measurement scheme for a magnetic resonance imaging of an examination object is planned and determined.

[0027] Subsequently the magnetic resonance imaging for examination purposes thus generally comprises the acquisition of raw data of the examination object by applying a pulse sequence corresponding to the measurement scheme.

[0028] Finally, as is usual, image data or other types of spatially distributed measurement data of the examination object, preferably measurement values characterizing the diffusion motion of water molecules, are reconstructed on the basis of the acquired raw data.

[0029] It is advantageous, in particular in diffusion imaging, that gradient pulses with a high amplitude can be used in order to leave an imprint of the current spatial position of the nuclear spins in the form of the signal phase on the nuclear spins in as short a time as possible and to detect the change in position at a later point in time. Shorter encoding times are required here in order to reduce signal losses due to relaxation effects, in order to record an image of sufficient quality for diagnosis as quickly as possible. For shorter encoding times, the amplitude of the diffusion gradient should therefore be as high as possible and the rise rate should also be as high as possible. Physiological limits should be observed on the other hand. With the aid of the method according to the application, a time-saving optimized or at least approximately optimized rise rate and amplitude of the diffusion gradient for a specific measurement can be determined.

[0030] The preparation apparatus for a magnetic resonance imaging system according to the application is designed to control the magnetic resonance imaging system with the aid of the magnetic resonance imaging method according to the application. The preparation apparatus according to the application comprises a representative data generation unit for generating a plurality of representative pulse sequence segments, which are each configured with a reference gradient amplitude of the gradient having the highest stimulation potential of the representative pulse sequence segment, and the stimulation potential of which representative pulse sequence segments represents a group of partially different pulse sequences.

[0031] The part of the preparation device according to the application further comprises a rise determination unit for determining for each representative pulse sequence segment a maximum gradient rise rate for which the allowed maximum of the stimulation potential is not exceeded.

[0032] The preparation device according to the application further comprises a selection unit for determining and selecting one of the plurality of representative pulse sequence segments as the measurement scheme to be planned for the current magnetic resonance imaging to be performed depending on the gradient amplitudes of the gradient pulses having the highest stimulation potential of the pulse sequence segments of the pulse sequence on which the measurement scheme is based.

[0033] The preparation device according to the application preferably further has an adaptation unit (or adjustment unit). The adaptation unit is provided for adjusting the pulse sequence segments of the pulse sequence on which the measurement scheme of the magnetic resonance imaging is based when planning the measurement scheme such that the maximum gradient rise rate assigned to the selected representative pulse sequence segment is not exceeded. The adjustment preferably comprises an adjustment of the scheme parameters of the measurement scheme to be planned which adjustment causes the maximum gradient rise rate assigned to the selected representative pulse sequence segment to be not exceeded. As will be explained in detail later, the adjustment particularly preferably comprises an adjustment of the overall pre-set and preferably displayed scheme parameter range from which the values of the scheme parameters can be selected without exceeding the limits or violating the limits for the neural stimulation.

[0034] Parameters which can be set by the user are generally referred to as scheme parameters which define the measurement in terms of the depicted geometry or its contrast properties. Scheme parameters which have an influence on the time sequence and amplitudes of the gradient pulses are important for the stimulation. In diffusion imaging, diffusion encoding gradients have a high stimulation potential: the shape and amplitude are influenced, for example, by the diffusion encoding mode selected by the user, the selection of the diffusion directions or the diffusion weighting. The time available for diffusion encoding (pre-set directly by the user or indirectly by the duration of the echo time TE and the other parts of the measurement sequence which need to be applied within the echo time TE) also plays a role. Scheme parameters which define the gradient pulses for the position encoding (such as the spatial resolution or the size of the imaging region) can also be important for the stimulation.

[0035] The preparation device according to the application has the same advantages as the method for preparing a magnetic resonance imaging of an examination object according to the application.

[0036] The magnetic resonance imaging system according to the application has the preparation device according to the application. The magnetic resonance imaging system according to the application has the same advantages as the method for preparing a magnetic resonance imaging of an examination object according to the application.

[0037] The largely software-based implementation has the advantage that an existing magnetic resonance imaging system or a control device thereof can be retrofitted in a simple manner by a software update in order to work in the manner according to the application.

[0038] Most of the above-mentioned method steps of the method for preparing a magnetic resonance imaging of an examination object according to the application or of the magnetic resonance imaging method according to the application, in particular the generation of the plurality of representative pulse sequence segments, the determination of the maximum gradient rise rate, the determination and selection of one of the representative pulse sequence segments and the adaptation of the pulse sequence segments of the magnetic resonance imaging, can be implemented in whole or in part in the form of a software module in a processor of a corresponding computing system, for example by a control device of a magnetic resonance imaging system or a computer for controlling such a magnetic resonance imaging system.

[0039] The largely software-based implementation has the advantage that a currently already used computing system can also be retrofitted in a simple manner by a software update in order to work in the manner according to the application. In this respect, the technical problem is also solved 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 for preparing a magnetic resonance imaging of an examination object according to the application or of the magnetic resonance imaging method according to the application when the program is executed in the computing system. In addition to the computer program, such a computer program product can include additional components, for example documentation and / or additional components and hardware components, for example a hardware key for the use of the software (dongle, etc.), if necessary.

[0040] For transport to a computing system or control device and / or for storage on or in a computing system or control device, a computer-readable medium, for example a memory stick, a hard disk or another removable or fixedly installed data carrier, can be used on which program segments of the computer program which are readable and executable by the computing system are stored. The computing system can for this purpose have one or more cooperating microprocessors or similar devices.

[0041] The following description contains particularly advantageous design solutions and extensions of the application respectively. Furthermore, different features of different embodiments can also be combined into new embodiments within the scope of the application.

[0042] In a preferred variant of the method for preparing a magnetic resonance imaging of an examination object according to the application, the parameter value range from which the user can select the parameter value of the to-be-adjusted protocol parameter for the to-be-planned measurement protocol is displayed to the user in accordance with the maximum gradient rise rate. The to-be-adjusted protocol parameter is a protocol parameter that has an influence on the stimulation potential of the pulse sequence on which the measurement protocol is based. Advantageously, the user receives information about the permissible protocol parameter values and does not have to "try and error" in order to find permissible and he wishes parameter values for the protocol parameters.

[0043] Very particularly preferably, in the adjustment step the value range of the planning presets for the measurement protocol for the permissible protocol parameter values is modified such that the determined maximum gradient rise rate is not exceeded. For example, the protocol parameter value range can initially have been preset by technical limits of the magnetic resonance imaging system. Now, this protocol parameter value range is further limited, if necessary, in order not to exceed the limit determined for the current pulse sequence of the maximum gradient rise rate. Advantageously, by the range modification the parameter values initially entered by the user are prevented from being outside this modified range and having to correct their input afterwards.

[0044] In the adjustment step, the user can first select a first parameter value for a first parameter to be adjusted. This first protocol parameter can be selected from a plurality of to-be-adjusted protocol parameters. Subsequently, the parameter value range for a second protocol parameter is preferably displayed to the user in accordance with the parameter value selected for the first parameter and in accordance with the maximum gradient rise rate. Of course, this approach can be applied to any number of different to-be-adjusted protocol parameters, i.e. especially more than two different protocol parameters. That is, during the adjustment of the measurement protocol, the user advantageously obtains "real-time information" that control his other behavior in selecting the protocol parameters such that the limit on the stimulation potential is adhered to. Thus, the user does not need to find "permissible" protocol parameter values for the second protocol parameter by trial and error, but rather the permissible parameter range for the second protocol parameter is automatically determined based on the representative pulse sequence segments stored in the database and the limits on the gradient rise rate attributed to these representative pulse sequence segments and displayed to the user.

[0045] Preferably, in the above scenario, upon selection of the first parameter value a smaller "conflict-free" parameter value range is displayed to the user, for example in green, which does not need to be compensated by an appropriate selection of the second parameter value, and a marginal region is displayed around this smaller parameter value range, for example in red, from which the user can select the first parameter value, but which first parameter value so selected has to be compensated by an appropriate selection of the second parameter value in order to satisfy the stimulation limit.

[0046] It can also occur that the user intentionally does not adhere to the preset value range when selecting a first protocol parameter value, because a specific value outside the value range is decisive for the success of his check. In order to still adhere to the stimulation limit, the value of a second protocol parameter from the plurality of protocol parameters to be adjusted is then advantageously automatically adjusted depending on the selected parameter value for the first parameter, so that the maximum gradient rise rate of the selected representative pulse sequence segment is still adhered to. Advantageously, the stimulation limit is adhered to even if the user does not follow the automatically recommended suggestion for the first protocol parameter value.

[0047] In a preferred design of the method for preparing a magnetic resonance imaging according to the application, the representative pulse sequence segment comprises one of the following gradient pulse sequences:

[0048] - a pair of gradient pulses of the same polarity (or a gradient pulse pair with the same polarity);

[0049] - a pair of gradient pulses of different polarity;

[0050] - a series of gradient pulses of the same polarity (or a gradient pulse series with the same polarity), wherein the duration is preferably a characteristic time constant of the model of the representative pulse sequence segment;

[0051] - a series of gradient pulses of alternating polarity, wherein the duration is preferably a characteristic time constant of the model of the representative pulse sequence segment.

[0052] The above variants of different pulse sequence types are in particular relevant to the design of pulse sequences for diffusion imaging.

[0053] Since the different stimulation potential values result from the characteristic properties of the above-mentioned gradient pulse sequences, it is particularly useful and effective to perform separate pre-calculation of the maximum allowed gradient rise rate for these different types of gradient pulse sequences. In the planning phase of a specific measurement protocol, the pre-calculated values for the stored representative gradient pulse segments can be used depending on the specific type of the gradient pulse sequence used for the specific measurement protocol, which more precisely match the gradient pulse sequences of the same type provided by the specific measurement protocol than without differentiation according to these specific types.

[0054] For example, depending on the selected setting of the measurement scheme, different types of representative pulse sequence segments can be relevant. For example, for a first setting of the measurement scheme, identically polar representative pulse sequence segments can be used, so that monopolar representative pulse sequence segments can be preferred for determining suitable scheme parameter values, while for a second setting of the measurement scheme for a second measurement, pulse sequences with alternating polarity should be used. Thus, in this second setting, representative pulse sequence segments are preferred for determining suitable scheme parameter values, if the physiological limits depend on this particular feature, respectively.

[0055] It is thus particularly preferred that in the method according to the application, such representative pulse sequence segments are selected, which have a pulse sequence segment in terms of stimulation potential that is comparable to the pulse sequence on which the measurement scheme is based. In this connection, "comparable" shall mean that the stimulation potential of the comparable pulse sequence segment of the pulse sequence on which the measurement scheme is based is not higher, at least not significantly higher, than the stimulation potential of the representative pulse sequence segment. In particular, in the case of gradient pulses relevant for stimulation, the comparable representative pulse sequence segment preferably has the same pulse sequence type or the same pulse sequence structure as the pulse sequence segment of the pulse sequence on which the measurement scheme is based. Precomputations that have already been performed for the representative pulse sequence segment in terms of compliance with stimulation limits can advantageously be transferred to the pulse sequence on which the measurement scheme is currently based, without having to perform these precomputations anew.

[0056] The representative pulse sequence segment preferably comprises a plurality of gradient pulses, wherein at least one first gradient pulse comprises a reference gradient amplitude. Furthermore, in this variant, at least one second gradient pulse comprises a gradient amplitude, which is different from the reference gradient amplitude and has a significantly smaller degree of influence on the stimulation potential than a gradient pulse with the reference gradient amplitude. If the second gradient pulse is taken into account when calculating the stimulation potential and the maximum gradient rise rate, a safer limit for the maximum gradient rise rate is formed than if the second gradient pulse is not taken into account.

[0057] It should again be expressly pointed out here that the method according to the application also has variants in which the representative pulse sequence segment does not comprise said second gradient pulse. In these variants, the calculation of the stimulation potential is particularly simple, since in this case only the first gradient pulse is important in the calculation of the stimulation potential.

[0058] In certain aspects, the second gradient pulse has an amplitude that can be determined individually depending on the pulse sequence type or the scheme parameter. The representative pulse sequence segment can comprise a plurality of gradient pulses, wherein at least one of these gradient pulses has an individually determined amplitude. In contrast, the other gradient pulses can have a reference gradient amplitude. For example, the second gradient pulse with the individually determined amplitude can have an amplitude G n = a n * Gref wherein G ref is a reference gradient amplitude of the first gradient pulse, a n is a scaling factor determined for the second gradient pulse.

[0059] It is advantageously possible to select a particularly suitable representative pulse sequence segment on the basis of the pulse sequence type or on the basis of the pulse sequence parameters pursuant to which the current measurement scheme is based, wherein the desired gradient amplitude values of one or more gradients of the pulse sequence are also taken into account in determining the maximum gradient slew rate.

[0060] To this end, it is preferably possible to store different maximum gradient slew rates in the database for a plurality of different combinations of gradient amplitudes, and it is possible to select a data set from the database having the desired gradient amplitude value or values of the reference gradient amplitude, or if necessary to interpolate between two data sets if the desired gradient amplitude value or values of the reference gradient amplitude lie between two stored values, in order to obtain a more precise limiting value for the gradient slew rate.

[0061] In addition to the reference gradient amplitude, it is also possible to take into account the amplitudes of other gradients in determining the table of representative pulse sequence segments and the corresponding assigned maximum gradient slew rates. For example, the maximum gradient slew rate of a representative pulse sequence segment can be calculated in accordance with a combination of the reference gradient amplitude of the diffusion gradient and the imaging gradient amplitude of the same pulse sequence segment.

[0062] The generation of the plurality of representative pulse sequence segments is preferably carried out in the development of a computer program for magnetic resonance imaging of a magnetic resonance imaging system. Advantageously, the created data is available at any time in the database, and additional time for creating the data does not have to be taken into account in the application case.

[0063] Alternatively, the generation of the plurality of representative pulse sequence segments can also be carried out at the start of the magnetic resonance imaging system. This alternative is advantageous if the device configuration changes, for example the gradient system changes or modeling parameters change, and the representative pulse sequence segments are representative for a specific configuration. This variant therefore relates in particular to an adjustment of hardware parameters.

[0064] The generation of the plurality of representative pulse sequence segments for a specific pulse sequence is preferably carried out before the start of a specific computer program for magnetic resonance imaging using the specific pulse sequence. For example, software modules for different pulse sequences can exist as so-called sequence modules, which are parameterized specifically. In this variant, the precomputation of the representative pulse sequence segments can be carried out in accordance with the selection of the software module, i.e. only after the selection of the software module. The method can be advantageous if the representative pulse sequence segments should be parameterized differently in accordance with the software module.

[0065] In a variant of the method for preparing a magnetic resonance imaging which can be implemented particularly simply according to the application, the determination of the maximum gradient rise rate is carried out in such a way that the maximum gradient rise rate is not exceeded even in the case of the maximum gradient amplitude. In this variant, only one maximum gradient rise rate needs to be calculated and stored for each parameter set, since the limitation of the gradient rise rate applies to any gradient amplitude.

[0066] As already mentioned, it can be useful to determine the maximum gradient rise rate for a representative pulse sequence segment in accordance with a reference gradient amplitude which is less than the maximum gradient amplitude and to calculate and store the maximum gradient rise rate for different reference gradient amplitudes accordingly. The load limit can advantageously be better utilized and the user is provided with a wider range of parameter values for setting the measurement protocol, if necessary.

[0067] Preferably, a representative pulse sequence segment is selected for the measurement protocol to be planned in accordance with the selection of the parameter values of the protocol parameters to be adjusted for the measurement protocol to be planned. Different representative pulse sequences can be relevant in accordance with the setting of the measurement protocol. For example, for a first setting a gradient sequence with the same polarity can be used. While for a second setting a sequence with alternating polarity is employed. If the physiological limit depends on this feature, respectively adapted representative pulse sequence segments can be selected automatically.

[0068] However, in the case of complex stimulation models, for example the SAFE model, it is generally not possible to draw conclusions about the case in which gradients are set simultaneously on a plurality of gradient axes from the knowledge of the minimum permissible gradient rise rate of the gradient pulses on one axis.

[0069] In the case of such a pulse sequence, separate pre-calculation can then be carried out for all directions applied within the MR imaging range and those gradient rise rates with the greatest stimulation potential are used. The direction combinations used are generally known in advance. However, if the user can define the direction combinations, the pre-calculation can also be carried out only when these directions are defined.

[0070] In general, different gradient pulse profiles G x (t) are applied in magnetic resonance imaging on three gradient axes G y (t) and G z (t). In the simplest case, the stimulation potential resulting from such a pulse sequence can be described by a stimulation model which considers all axes independently of one another: Thus, the stimulation potential results from the individual consideration of each of the three axes.

[0071] In a particularly precise design according to the method of the invention, the pre-computation for representative pulse sequence segments is performed independently for each of the three axes. In this variant, the differences in pulse sequences in different directions are fully taken into account. Suitable representative pulse sequence segments are configured for specific segments of the pulse sequence to be planned for each axis, and the pre-computed maximum gradient ascent rate is calculated and applied separately for each axis, taking into account the current gradient magnitude associated with that axis.

[0072] To reduce computational workload, it is preferable to configure the same representative pulse sequence segment for all axes for further simplification, and / or configure the same gradient ascent rate for all axes, preferably the lowest gradient ascent rate among all three axes.

[0073] However, the SAFE model, which is frequently used in practice, incorporates stimulus contributions from all three axes, thus preventing the independent consideration of the three axes when applying the SAFE model. In this case, it is preferable to extend the pre-computation for each representative pulse sequence segment in such a way that at least one reference gradient magnitude is considered for each axis. Therefore, the pre-computation is performed for each 3-tuple (G... ref,x G ref,y G ref,z Provide a suitable maximum gradient ascent rate S max However, this significantly increases the cost of pre-computation.

[0074] Especially in the case of diffusion or dispersion measurements, the relevant directions of the diffusion-encoded gradient are usually known at the pre-computation time point. This is because users typically select a set of directions for diffusion measurements (each set can contain one to hundreds of directions), which are applied sequentially during the measurement process. These directions can be "fixed" pre-defined in the program code: in this case, pre-computation can be performed during software development or when the program code is loaded. In some cases, users can "freely" pre-define these directions, for example, in the form of a file containing direction information: in this case, pre-computation can be performed once upon activation, such as when the file is loaded.

[0075] In this variant, the pre-calculation within the scope of the method according to the invention is preferably performed as follows:

[0076] First, for the measurement to be planned, there are N known direction vectors (R) to be recorded. i,x ,R i,y ,R i,z ), where i comes from {1, N}, and N is a natural number. For each of the N direction vectors, the maximum gradient ascent rate is determined within the pre-computed range using the stimulation model, i.e., for each representative pulse sequence segment and based on the reference gradient magnitude G. ref Determined. Therefore, according to the correspondence Gx = R i,x G ref , G y = R i,y G ref and G z = R i,z G ref , internally using reference gradient pulses on all three axes.

[0077] It is generally desirable to measure all diffusion directions with the same timing of the gradient pulses in one measurement. Therefore, for each representative pulse sequence segment and for each reference gradient amplitude the minimum maximum gradient rise rate of all directions is stored.

[0078] Thus finally, for each set of direction vectors of each representative pulse sequence segment a separate precomputation is obtained which allows a fast determination of the maximum gradient rise rate for a given reference gradient amplitude.

[0079] The stimulation potential can depend on the relative orientation between the gradient direction and the patient's anatomy. For example, the stimulation potential of the gradient can be stronger along the dorso-ventral direction. Depending on the way the patient is supported in the tunnel-shaped magnetic resonance scanner, this can be the physical y-gradient when supported on the belly or back, or the physical x-gradient when the patient is supported on the side.

[0080] To cover these cases, when precomputing the stimulation values for a representative pulse sequence segment, in addition to the direction combinations, also the various permutations of the gradient axes can be considered.

[0081] If the direction combinations are determined in physical coordinates xyz, i.e. independent of the orientation of the layer of measurement, in the case of a tunnel-shaped magnetic resonance scanner with a main magnetic field along the horizontal z-axis, these correspond to the permutations of xy and yx, which correspond to the back support and the side support, respectively, taking into account the permutations of xy and yx.

[0082] If the direction information is not preset in physical coordinates (i.e. the xyz coordinate system of the gradient coils), but in logical coordinates (i.e. the prs coordinate system of the imaging, p = phase, r = readout direction, s = layer direction), the precomputation can be performed using a "worst case" assumption. Since the patient is supported in a physical axis orientation (supine, prone, lateral), one of the axes has the highest stimulation potential. Thus, different correspondences of the prs coordinates with respect to the xyz coordinates can be considered at precomputation time, and the result with the minimum maximum gradient rise rate is stored. Thus, for the direction vectors (R i,p , R i,r R i,s ), in the precomputation the following is considered in the order as mentioned above:

[0083] G x = Ri,p G ref , G y = R i,r G ref , G z = R i,s G ref

[0084] G x = R i,p G ref , G y = R i,s G ref , G z = R i,r G ref

[0085] G x = R i,r G ref , G y = R i,p G ref , G z = R i,s G ref

[0086] G x = R i,r G ref , G y = R i,s G ref , G z = R i,p G ref

[0087] G x = R i,s G ref , G y = R i,p G ref , G z = R i,r G ref

[0088] G x = R i,s G ref , G y = R i,r G ref , G z = R i,p G ref .

[0089] If the direction combination is determined in the logical coordinates prs, i.e. in relation to the layer orientation measured, all the permutations prs, psr, rps, rsp, spr, srp are taken into account, and the result with the lowest maximum gradient rise rate of the representative pulse sequence segments is then stored.

[0090] If the (known) direction vectors are determined in the prs coordinates, and none of the direction vectors extends completely parallel to the p-axis, r-axis or s-axis, then in fact the "worst possible" layer tilt must be taken into account. Namely, the tilt for which the gradient vector i,xyz = sqrt(R i,p 2 + R i,r 2 + R i,s 2 ) * G ref is applied exactly along the x-axis, y-axis or z-axis:

[0091] G x = G i,xyz ; G y = 0; G z = 0

[0092] G x = 0; G y = G i,xyz ; G z = 0

[0093] G x = 0; G y = 0; G z = G i,xyz .

[0094] If an open MR scanner with a basic field along the vertical y-axis is used, the following possibilities exist:

[0095] - determination of the direction combination in the physical coordinates xyz, i.e. independently of the layer orientation measured;

[0096] - determination of the direction combination in the logical coordinates prs, i.e. in relation to the layer orientation measured, in which all the permutations prs, psr, rps, rsp, spr, srp are taken into account;

[0097] - if the patient bed can be rotated in steps of 90° about the vertical axis, then even for the direction combination in the physical coordinates xyz, all the permutations xyz, xzy, yxz, yzx, zxy, zyx must be taken into account.

[0098] If rotation is not only possible in steps of 90°, the situation becomes more complex and it is not sufficient to consider the permutations prs, psr, rps, rsp, spr, srp (for similar reasons as when considering layer tilting in a tunnel-shaped scanner).

[0099] For an exact consideration, one has to consider the amplitudes in the horizontal xz-plane first: G i,xz = sqrt(R i,x 2 + R i,z 2 ) * G ref and subsequently the permutations resulting from different possibilities of patient support.

[0100] As already mentioned several times in brief, in creating a representative gradient pulse sequence, the influence of other gradients can be taken into account. For example, in the case of unipolar diffusion encoding, an imaging gradient with inverted polarity can be applied between, before or after the encoding pulses of the same polarity. This can be taken into account in the precomputation, for example in such a way that in the calculation of the stimulation potential, an additional gradient with inverted polarity and an amplitude G img representing the imaging gradient is added in the time interval A between the diffusion gradients.

[0101] A safety margin can be used when precomputing the maximum gradient rise rate S max , for example a gradient rise rate S of 5% less. In this way, the influence of inaccuracies in the precomputation can be taken into account, for example because not all conceivable combinations of the duration D and the interval A of the gradient G are actually tested.

[0102] When precomputing the maximum gradient rise rate, a monotonic dependency between the gradient rise rate and the gradient amplitude can be ensured. For example, if the precomputation for a higher gradient amplitude results in a faster rise rate than the precomputation for a lower gradient amplitude, the former can be set to a slower value. In addition to the improved consistency, from this a dependency S max (G) and the calculation of the maximum permissible gradient amplitude G(S max ) from the rise rate S max arises. This knowledge can help to calculate a specific gradient time course within a sequence.

[0103] If a program for adjusting a measurement scheme should determine a trapezoidal gradient with the maximum torque M = 0 T dt M(t) within a preset time length T in order to be able to determine a value range for the scheme parameters, the maximum permissible gradient amplitude G(S max) can be helpful. If the maximum possible gradient amplitude cannot be known immediately due to complex hardware modeling, a "Trial and Error" strategy can be applied. Here, for different ramp-up rates the associated maximum allowed gradient amplitudes can be determined according to G(S max ) and the optimal gradient shape can be determined by trial and error.

[0104] Precomputation can be accelerated using appropriate measures. For example, the modeling can be designed such that the stimulation potential increases with increasing gradient amplitude. In this case, precomputation can start with a high gradient amplitude and then consider smaller gradient amplitudes successively. Once a gradient ramp-up rate S that is in line with the hardware performance limits is possible for a certain parameterization of the representative pulse sequence segment, the same value can be assumed for all smaller gradient amplitudes G.

[0105] To additionally speed up precomputation it can be provided that only those representative pulse sequence segments are considered in all direction combinations of the pulse sequence that are expected to have the highest stimulation potential. What is involved here is to limit the number of directions to be investigated. In all direction combinations have the same length |G| = sqrt(G x 2 +G y 2 +G z 2 This simplification can be applied in the case where all direction combinations have direction vectors of the same length |G| = sqrt(G

[0106] For example, if the directions are defined in physical xyz coordinates, those directions are considered that have the largest gradient field components along the x, y and z axes. Which of the three axes has the strongest stimulation depends on the orientation of the body or the examination object. This aspect is taken into account by permutation combinations. If for two directions the same maximum component value results, other components can also be used to select the directions to be considered, for example the direction with the largest y component among the directions with the largest x component. In this way, at most six directions need to be considered, namely the largest x component with the largest y component in the ambiguous case, the largest x component with the largest z component in the ambiguous case, the largest y component with the largest x component in the ambiguous case, the largest y component with the largest z component in the ambiguous case, the largest z component with the largest x component in the ambiguous case and the largest z component with the largest y component in the ambiguous case. By the considerations of this section the number of directions that actually need to be considered in precomputation can be limited to at most six. But by this the permutation combinations need to be considered for each of these six directions (R i,x ,R i,y ,R i,z ).

[0107] The pre-calculation can also be performed for different stimulation potential limits. For example, the IEC standard defines a first limit for the "controlled boost operation mode" (class 1 controlled mode) and a second limit for the "normal operation mode" (normal mode), which is 80% of the first limit. Measurements performed in the "controlled boost operation mode" require a user confirmation that the patient is more likely to be stimulated. It can be provided within the scope of the present invention that the pre-calculation is performed for the operation mode in which the activation is currently in. Alternatively, the pre-calculation can be performed for both operation modes and the appropriate value is applied respectively. The selection of the pre-calculation can also be performed independently of the actual operation mode, for example based on the parameters of the measurement protocol.

[0108] The method according to the present invention can also be combined with other methods for testing the stimulation limit. For example, the present method can be combined with the method described in German patent application 10 2022 213 903.8. In the method described in 10 2022 213 903.8, the user receives direct feedback when entering the protocol parameter values, which feedback states whether the entered values comply with the neurostimulation limit. Advantageously, on the basis of the pre-calculation performed according to the present invention, the checking process of the protocol parameter values can be greatly accelerated, so that the communication of the user when determining the measurement protocol via the operating system interface achieves an acceptably short waiting time.

[0109] The method according to the present invention can preferably be used to determine the gradient rise rate of the diffusion gradient, while the stimulation potential of the imaging gradient, for example the stimulation potential of the echo planar imaging, is only checked at the beginning of the actual imaging. The method according to the present invention ensures that the ramp duration of the diffusion gradient is optimized and the executability within the stimulation limit is guaranteed already when editing the protocol parameters. This limits the solution space in the post-check at the beginning of the measurement, so that suitable measurement parameter change options can be quickly suggested to the user. BRIEF DESCRIPTION OF DRAWINGS

[0110] The present invention is explained in more detail below on the basis of embodiments in conjunction with the drawings. In the drawings:

[0111] Figure 1 A view of a pulse sequence for diffusion measurement with a monopolar gradient series according to one embodiment of the present invention is shown;

[0112] Figure 2 A view of a pulse sequence for diffusion measurement with a bipolar gradient series according to one embodiment of the present invention is shown;

[0113] Figure 3 A view of a pulse sequence for diffusion measurement with an oscillating encoding gradient series according to one embodiment of the present invention is shown;

[0114] Figure 4 A view of a pulse sequence for diffusion measurement with an oscillating encoding gradient series according to one embodiment of the present invention is shown;Figure 1 a representative pulse sequence segment of the gradients of the pulse sequence shown;

[0115] Figure 5 showing a pulse sequence for Figure 2 a representative pulse sequence segment of the gradients of the pulse sequence shown;

[0116] Figure 6 showing a pulse sequence for Figure 3 a representative pulse sequence segment of the gradients of the pulse sequence shown;

[0117] Figure 7 showing a representative pulse sequence segment of a pulse sequence for diffusion measurement with additional imaging gradients;

[0118] Figure 8 showing a flow chart illustrating a method for preparing a magnetic resonance image of an examination object according to one embodiment of the application;

[0119] Figure 9 showing a schematic diagram of a preparation device according to one embodiment of the application;

[0120] Figure 10 showing a schematic diagram of a magnetic resonance imaging system according to one embodiment of the application. DETAILED DESCRIPTION

[0121] In Figure 1 a view of a pulse sequence 1 for diffusion measurement according to one embodiment of the application is shown in which a high-frequency pulse RF and a readout window AF in which a high-frequency measurement signal is received, the pulse sequence also having a series of monopolar gradients G diff,x , G diff,y , G diff,z are applied. These pulses and signals are plotted over time t, respectively. In diffusion measurement, a series of gradient pulses of very high amplitude is used to imprint the current spatial position on the nuclear spins in the form of a signal phase in as short a time as possible and to detect the change in position at a later point in time. A short encoding time is required here in order to reduce the signal loss caused by relaxation effects, in particular due to transverse relaxation, so that an image of sufficient quality for diagnosis is recorded as quickly as possible.

[0122] Different series of diffusion gradient pulses G diff may be used depending on the clinical question and the body region, for example Figure 1The diagram shows a pulse sequence with "unipolar" encoding (gradient pulses of the same polarity). Such a series with specific encoding can, for example, serve as the basis for the gradient pulse sequence in a measurement scheme used in actual measurements. While the entire measurement process or the current pulse sequence associated with that process may contain additional gradients, in many cases the stimulation potential is determined by the diffusion gradient, so considering this sub-segment is sufficient. Figure 1 In the diagram, the first row shows the excitation high-frequency pulse RF1 and the refocusing high-frequency pulse RF2. Additionally, the first row shows the acquisition window AF, in which the HF signal is received from the inspection area as the echo signal. The second row shows the two single-pole encoded diffusion gradients G in the x-direction. diff,x The first gradient switches after the excitation high-frequency pulse RF1, and the second gradient switches after the refocusing high-frequency pulse RF2. Figure 1 In the third and fourth rows, the diffusion gradient G diff,y and G diff,z The gradient is switched between the y and z directions. There is a time interval between the diffusion gradients in one direction, during which the refocusing high-frequency pulse RF2 is used. The gradients in different directions have the same duration but partially different amplitudes. For clarity, additional gradient pulses for position encoding are not shown.

[0123] Figure 2 A visual representation of a pulse sequence for diffusion measurement according to an embodiment of the present invention is shown. Figure 2 The pulse sequence has a bipolar diffusion gradient series. That is, each diffusion gradient G diff,x G diff,y G diff,z Or their amplitudes all change their signs.

[0124] Figure 3 A view is shown of a pulse sequence for diffusion measurement according to an embodiment of the present invention, the pulse sequence having an oscillatingly encoded gradient G. diff,x G diff,y G diff,z The series. In this variant, the diffusion gradient G diff,x G diff,y G diff,z Its sign changes multiple times after activation and refocusing.

[0125] from Figures 1 to 3The pulse sequences underlying the actual measurement schemes shown can be seen that a shorter duration of the gradient ramps, i.e. a higher rise rate S, allows to achieve a shorter time interval between the excitation module and the readout module AF. Since the diffusion gradients have a higher amplitude, the stimulation potential rises significantly with shorter ramps. Therefore, when preparing an MR imaging, the stimulation potential and the allowed maximum gradient rise rate S of a representative pulse sequence segment belonging to these pulse sequences is calculated. From the allowed maximum gradient rise rate, the allowed ramp duration R can also be determined according to R = G / S, wherein R is the ramp duration, G is the gradient amplitude and S is the gradient rise rate. Furthermore, as an alternative to the gradient rise rate S, the rise time T = 1 / S can also be calculated.

[0126] Figure 4 The representative pulse sequence segments of the gradients of the pulse sequences shown are illustrated Figure 1 The representative pulse sequence segments of the gradients of the pulse sequences shown are illustrated Figure 4 with a duration D, a time interval A and a gradient amplitude G ref of the selectable parameters.

[0127] For Figure 4 For the monopolar representative pulse sequence segments shown, a brief explanation of the pre-calculation of the allowed ramp parameters or ramp parameter values described above. At the point of the pre-calculation, neither the exact time course of the two gradients nor their actual amplitudes are known. Therefore, the rise rate S is determined for different gradient amplitudes G and the lower limit min of the maximum allowed rise rate of the individual separate series is determined as the maximum rise rate S of the entire set of possible series for a plurality of possible time courses max . In the subsequent editing of the current measurement scheme, the representative pulse sequence segments can be found from the database with the parameters of the current measurement scheme and their allowed rise rates S max are used as a limit for determining the current measurement scheme.

[0128] For diffusion measurements, for example, the duration of the diffusion encoding gradients is known to be in the range of 5 ms to 50 ms. Furthermore, for a specific type of gradient coil, the time constants of the SAFE model are known, which are in the range of 1 ms to 10 ms. For this case, it can be sufficient, for example, to consider gradient pulse durations of 1 ms to 30 ms and gradient pulse intervals of 0 ms to 30 ms. With this parameter range, the cumulative effect of the successive ramps and possible compensation effects is taken into account.

[0129] With regard to the contribution to the stimulation that depends only on the gradient rise amount, a series of rising and falling ramps with shorter time intervals can provide a greater contribution. This cumulative effect is taken into account by gradient pulses with shorter duration and shorter intervals. This effect exists, for example, in the case of a monopolar gradient pulse series with a shorter duration D and a shorter interval A.

[0130] Regarding the stimulus contribution that includes an ascending sign in the calculation, a preceding descending slope may reduce the amount of a subsequent ascending slope. However, this compensatory effect is reduced by longer gradient pulses with larger time intervals. Therefore, such a contribution is effective in the case of gradient pulse series with longer durations D and longer intervals A.

[0131] In the pre-calculation, for different combinations of the duration D and time interval A of the unipolar gradient pulse, the maximum gradient ascent rate S for a specific gradient amplitude G is pre-calculated. max (G,D,A). For a specific gradient magnitude G = G ref Determine the common upper bound S of the gradient ascent rate. max (G)=minD,A(S max (G,D,A) serves as a lower bound for any values ​​of the gradient duration D and the time interval A. In the subsequent editing of the measurement scheme, in the simplest case, the maximum gradient magnitude G of the system can be conservatively used. max Maximum rate of increase S max (G max Alternatively, a pulse sequence for imaging can be planned using a value related to the actual gradient magnitude G, with the maximum gradient ascent rate S for that related value stored in a database. max (G) Alternatively, the maximum gradient ascent rate S used for this correlation value can be calculated by interpolating two adjacent table values ​​of the maximum gradient ascent rate. max (G).

[0132] for Figure 5 and Figure 6 The two representative bipolar or oscillatory pulse sequence segments shown can be operated in the same manner as... Figure 4 The representative unipolar pulse sequence segments shown are very similar.

[0133] As in Figure 5 The view Figure 5 As shown, in the case of bipolar coding, there will be gradient duration D and time interval A, and can be +G. ref or -G ref The gradient of the amplitude is used to determine the representative pulse sequence segment of the bipolar pulse sequence.

[0134] As in Figure 6 The view Figure 6In the case of oscillating encoding, a series of alternating gradient pulses with a duration D is considered, which series has no interruptions, i.e. a time interval A = 0. In this case, the number of gradients considered for determining the stimulation potential SP of a representative pulse sequence segment depends on the time constant τ of the model on which the calculation of the stimulation potential is based, in the present embodiment the relevant time constant τ of the SAFE model. As many gradient pulses as can be applied within three times the longest time constant τ can be considered, for example.

[0135] Figure 7 The view of a representative pulse sequence segment of a pulse sequence for diffusion measurement is shown Figure 7 , the pulse sequence having additional imaging gradients (shown by dashed lines).

[0136] Figure 7 The imaging gradients shown are inserted into the monopolar pulse sequence and have an opposite polarity compared to the diffusion gradients (drawn with solid lines) of the pulse sequence and have an amplitude G img , which is different from the amplitude G ref of the diffusion gradients. This modification can also be considered when pre-computing the maximum gradient rise rate S max , in that representative pulse sequence segments with additional imaging gradients are generated, which additional imaging gradients are taken into account in the pre-computation of the stimulation and the maximum gradient rise rate.

[0137] The imaging gradients can be applied between two diffusion gradients, for example, to select a layer. Depending on the image orientation, the gradient can have a different sign than the subsequent or preceding diffusion gradient and thus increase the stimulation potential.

[0138] If this actual pulse sequence is configured as a "representative" pulse sequence segment to the pulse sequence segment in Figure 4 , the stimulation potential can be underestimated by the "representative" pulse sequence segment; therefore Figure 4 the representative pulse sequence segment shown would no longer be "representative".

[0139] For this case, Figure 7 the representative pulse sequence segment shown would be configured as a "representative pulse sequence segment".

[0140] Figure 8 A flowchart 800 illustrating a method for preparing a magnetic resonance image of an examination object according to one embodiment of the application is shown.

[0141] In step 8.I, a plurality of representative pulse sequence segments R-PSA is generated, which representative pulse sequence segments are respectively configured with a reference gradient amplitude G refand the stimulation potential SP of these representative pulse sequence segments represents a set of partially different pulse sequences.

[0142] In step 8. II, a maximum gradient rise rate S is determined for each representative pulse sequence segment R-PSA and for at least one reference gradient amplitude. max For this maximum gradient rise rate, the allowed maximum value of the stimulation potential SP is not exceeded. But usually a complete table with different gradient amplitude values G is generated for which the maximum gradient rise rate S is determined respectively. max (G).

[0143] Steps 8. I and 8. II are performed once in the pre-calculated range of representative pulse sequence segments R-PSA and their maximum gradient rise rates S max .

[0144] In step 8. III, the measurement protocol P is opened for editing by the user or for automatic adjustment by the magnetic resonance imaging system. In step 8. III, the number and extension dimensions of the layers required to cover the examination region ROI are determined automatically or partially automatically, if necessary, of the so-called field of view (FoV = Sichtfeld). The position and orientation of the examination region ROI can also be adjusted automatically based on landmarks.

[0145] In step 8. IV, one of the representative pulse sequence segments R-PSA is determined and selected for the measurement protocol P to be planned for the magnetic resonance imaging to be performed at present, depending on the characteristics or type of the pulse sequence PS on which the measurement protocol P is based. Different types of representative pulse sequence segments R-PSA can be selected depending on the settings of the measurement protocol. For example, for a first setting of the measurement protocol P, representative pulse sequence segments R-PSA of the same polarity can be selected (see Figure 1 , Figure 4 ). While for a second setting of the measurement protocol, representative pulse sequence segments R-PSA of alternating polarity are selected (see Figure 2 , Figure 3 , Figure 5 , Figure 6 ). Thereby, the table pre-calculated for the representative pulse sequence segments R-PSA can be used to determine the maximum allowed gradient rise rate S max (G, R-PSA) depending on the gradient amplitudes G of the gradient pulses of the pulse sequence PS on which the measurement protocol P is based.

[0146] In step 8. V, the pulse sequence segments M-PSA of the pulse sequence of the planned magnetic resonance imaging are adjusted so that the determined maximum gradient rise rate S max is not exceeded.

[0147] Figure 9A schematic diagram showing a preparation device 90 according to an embodiment of the application is shown.

[0148] The preparation device 90 has a representative data generation unit 91. The representative data generation unit 91 is provided for generating a plurality of representative pulse sequence segments R-PSA having at least one gradient pulse and generally a plurality of gradient pulses. The amplitude of the at least one gradient pulse that causes the highest stimulation potential is preset by a reference gradient amplitude G ref scaling.

[0149] Furthermore, the preparation device 90 comprises a rise determination unit 92 for determining for each representative pulse sequence segment R-PSA a maximum gradient rise rate S max for which the allowed maximum of the stimulation potential SP is not exceeded.

[0150] The preparation device 90 also has a database 92a that stores the generated representative pulse sequence segments R-PSA together with the maximum gradient rise rates S ref for different reference gradient amplitudes G max and makes them available for later planning of a specific measurement scheme P of a magnetic resonance imaging.

[0151] Part of the preparation device 90 is also a selection unit 93 for determining and selecting one of the representative pulse sequence segments R-PSA for a measurement scheme P to be planned of a magnetic resonance imaging to be carried out at present, depending on the highest gradient amplitude G of the pulse sequence PS on which the measurement scheme P is based.

[0152] The preparation device 90 also has an adaptation unit 94 for adjusting the scheme parameters PP of the measurement scheme P to be planned. The adjustment is carried out in such a way that the maximum gradient rise rate S max attributed to the selected representative pulse sequence segment R-PSA is not exceeded.

[0153] Figure 10 A rough schematic diagram showing a magnetic resonance imaging system 100 (in the following: MR device) according to the application is shown. The MR device comprises on the one hand an actual magnetic resonance scanner 102 having an examination space 103 or patient passage into which an examination object O or in the present case a patient or subject can be moved on a couch 108, for example a specific organ to be imaged being located in the examination object or patient or subject.

[0154] The magnetic resonance scanner 102 is usually equipped with a main magnetic field system 104, a gradient system 106, and an HF transmit antenna system 105 and an HF receive antenna system 107. In the embodiment shown, the HF transmit antenna system 105 is a whole body coil permanently installed in the magnetic resonance scanner 102, while the HF receive antenna system 107 is composed of local coils to be arranged on the patient or subject (in the case shown only a single local coil is represented in the middle). Figure 10 In principle, however, the whole body coil 105 can also be used as the HF receive antenna system, and the local coil 107 can be used as the HF transmit antenna system, provided that these coils can be switched into different operating modes, respectively.

[0155] The MR device 100 also has a central control device 110 for controlling the MR device 100. The central control device 110 comprises a sequence control unit 114 for pulse sequence control. The time sequence of high-frequency pulses (HF pulses) and gradient pulses is determined by this sequence control unit in accordance with a selected imaging sequence PS. Such an imaging sequence PS can be preset, for example, within a measurement or control scheme P. Usually, different control schemes P for different measurements are stored in a memory 119 and can be selected (and, if necessary, also changed) by an operator and then used for performing a measurement.

[0156] For outputting individual high-frequency pulses, the central control device 110 has a high-frequency transmit device 115 which generates and amplifies the high-frequency pulses and feeds them into the HF transmit antenna system 105 via a suitable interface (not shown in detail). For controlling the gradient coils of the gradient system 106, the control device 110 has a gradient system interface 116. The sequence control unit 114 communicates with the high-frequency transmit device 115 and the gradient system interface 116 in a suitable manner, for example by sending sequence control data SD, in order to send the pulse sequence PS. The control device 110 also has a high-frequency receive device 117 (also communicating with the sequence control unit 114 in a suitable manner) in order to acquire the magnetic resonance signals received by the HF receive antenna system 107 in a coordinated manner. A reconstruction unit 118 receives the acquired data after demodulation and digitization as raw data or k-space data RD and reconstructs MR image data BD therefrom. These image data BD can then be stored, for example, in a memory 119 or displayed via a display unit 109.

[0157] The central control device 110 can be operated via a terminal having an input unit 111 and a display unit 109, so that the entire MR device 100 can also be operated by an operator via this terminal. MR images can also be displayed on the display unit 109, and the measurement can be planned and initiated, if necessary in combination with the display unit 109, via the input unit 111, and in particular a suitable control scheme with suitable measurement sequences can be selected and, if necessary, modified as described above.

[0158] Figure 10 The preparation device 90 already described in Figure 9 is also shown. The preparation device 90 is provided for modifying the measurement scheme P with the scheme parameters PP such that the maximum gradient rise rate S max is not exceeded. In order to adapt the measurement scheme P, firstly a value range of one or more scheme parameters is determined, within which the pulse sequence segment M-PSA considered to be modified does not exceed the previously determined maximum gradient rise rate S max . The previously determined value range is displayed before or during the change of the relevant scheme parameter.

[0159] The MR device 100 and in particular the control device 110 according to the application can also have a number of other components which are not shown here in detail but are generally present on such devices, for example a network interface for connecting the entire device to a network and enabling the exchange of raw data RD and / or image data BD or parametric maps and other data, for example patient-related data or control schemes.

[0160] How suitable raw data RD can be acquired by irradiation of high-frequency pulses and generation of gradient fields and how magnetic resonance images BD can be reconstructed therefrom is basically known to the person skilled in the art and is not explained in detail here.

[0161] It can be seen from the above description that the application effectively provides the possibility of improving the method for preparing a magnetic resonance imaging of an examination object and the magnetic resonance imaging method in terms of the required duration.

[0162] It should be noted here that the features of all embodiments or of the extended designs disclosed in the drawings can be used in any combination.

[0163] It should finally be reiterated that the methods and devices described in detail above are only embodiments, and the person skilled in the art can modify the basic principles within a wide range, without departing from the scope of the invention. For the sake of completeness, it should also be pointed out that the use of the indefinite article "a" does not exclude that a plurality of the relevant features can be present. Likewise, the term "unit" does not exclude that it consists of a plurality of components, which can also be spatially distributed, if necessary. Irrespective of the grammatical gender of the specific terms used, persons of male, female or other gender identity are included.

Claims

1. A method for preparing a magnetic resonance imaging of an examination object (O), comprising: a general preparation step, which is carried out in advance, comprising: - generating a plurality of representative pulse sequence segments (R-PSA) which are each configured with a reference gradient amplitude (G ref ) of the gradient pulses having the highest stimulation potential (SP) of the representative pulse sequence segment (R-PSA), and whose stimulation potential (SP) represents a group of partially different pulse sequences; - determining a maximum gradient rise rate (S max ) for each representative pulse sequence segment (R-PSA) for which the allowed maximum of the stimulation potential (SP) is not exceeded; and a subsequent preparation step for the examination object, comprising: - determining and selecting one of a plurality of representative pulse sequence segments (R-PSA) for a measurement scheme (P) to be planned for a magnetic resonance imaging to be carried out at present, in dependence on the gradient amplitudes (G) of the gradient pulses of the pulse sequence segment (M-PSA) of the pulse sequence (PS) having the highest stimulation potential (SP) of the pulse sequence segment (M-PSA) of the pulse sequence (PS) to which the measurement scheme (P) is to be adapted; - adapting the pulse sequence segment (M-PSA) of the pulse sequence (PS) to which the measurement scheme (P) is to be adapted, such that the maximum gradient rise rate (Smax) assigned to the selected representative pulse sequence segment (R-PSA) is also not exceeded by the pulse sequence segment (M-PSA) of the pulse sequence (PS) corresponding to the representative pulse sequence segment (R-PSA).

2. The method of claim 1, wherein, In the adjusting step, the value range of the planning preset for the measurement scheme (P) for the allowed scheme parameter value (PP) is modified such that the determined maximum gradient rise rate (S max ) is not exceeded.

3. The method according to any of the preceding claims, wherein, According to the maximum gradient ascent rate (S max ) a range of parameter values is displayed to the user from which the user can select a parameter value for a scheme parameter (PP) to be adjusted for the measurement scheme (P) to be planned.

4. The method according to any of the preceding claims, wherein, In case of multiple scheme parameters (PP) to be adjusted, the value of a second scheme parameter is adjusted according to the parameter value selected for the first parameter such that the maximum gradient rise rate (S max ) is obeyed.

5. The method according to any one of the preceding claims, wherein, At least one of the plurality of representative pulse sequence segments (R-PSA) comprises one of the following sequences of gradient pulses: - a pair of gradient pulses having the same polarity; - a pair of gradient pulses having different polarities; - a series of gradient pulses having the same polarity; - a series of gradient pulses having alternating polarities.

6. The method according to any one of the preceding claims, wherein, The pulse sequence (PS) to which the measurement scheme (P) is to be adapted has a stimulation potential (SP) which is not higher than the pulse sequence segment (M-PSA) of the selected representative pulse sequence segment (R-PSA).

7. The method according to any one of the preceding claims, wherein, The generation of the plurality of representative pulse sequence segments (R-PSA) is already carried out in the development of a computer program for magnetic resonance imaging of a magnetic resonance imaging system (100).

8. The method of any one of claims 1 to 6, wherein, The generation of the plurality of representative pulse sequence segments (R-PSA) is carried out at the start of the magnetic resonance imaging system (100).

9. The method of any one of claims 1 to 6, wherein, The generation of the plurality of representative pulse sequence segments (R-PSA) for a specific pulse sequence (PS) of the measurement scheme (P) to be planned is carried out before the start of a computer program for magnetic resonance imaging using the specific pulse sequence (PS).

10. The method of any of the preceding claims, wherein, determination of the maximum gradient rise rate (S max ) is made so that it is not exceeded even in the case of the maximum gradient amplitude (G max ).

11. A magnetic resonance imaging method, having the following steps: - applying a method according to any one of claims 1 to 10; - acquiring raw data (RD) of an examination object (O); - reconstructing image data (BD) of the examination object (O) on the basis of the acquired raw data (RD).

12. A preparation device (90), comprising: - a representative data generating unit (91) for generating a plurality of representative pulse sequence segments (R-PSA) in a general preparatory procedure carried out in advance, which representative pulse sequence segments are each configured with a reference gradient amplitude (G ref ) of the gradient pulse having the highest stimulation potential (SP) of the representative pulse sequence segment (R-PSA), and the stimulation potential (SP) of which representative pulse sequence segments is representative of a group of partially different pulse sequences (PS); - an ascent determination unit (92) for determining, in a general preparation procedure carried out in advance, for each representative pulse sequence segment (R-PSA) a maximum gradient ascent rate (S max ) for which an allowed maximum value of the stimulation potential (SP) is not exceeded; - a selection unit (93) for determining and selecting one of a plurality of representative pulse sequence segments (R-PSA) for a measurement scheme (P) to be planned for a magnetic resonance imaging to be carried out at present, in dependence on the gradient amplitudes (G) of the gradient pulses of the pulse sequence segment (M-PSA) of the pulse sequence (PS) having the highest stimulation potential (SP) of the pulse sequence segment (M-PSA) of the pulse sequence (PS) to which the measurement scheme (P) is to be adapted, in a subsequent preparation process for an examination object. - an adaptation unit (94) for adapting, in a subsequent preparation process for the examination object, a pulse sequence segment (M-PSA) of the pulse sequence (PS) on which the measurement scheme (P) is based, such that the maximum gradient rise rate (S max ) assigned to the selected representative pulse sequence segment (R-PSA) is not exceeded by the pulse sequence segment (M-PSA) of the pulse sequence (PS) on which the measurement scheme (P) is also based, which corresponds to the representative pulse sequence segment (R-PSA).

13. A magnetic resonance imaging system (100) having a preparation device (90) according to claim 12.

14. A computer program product comprising a computer program directly loadable into the memory means of a control device (110) of a magnetic resonance imaging system (100), which computer program comprises program code sections for carrying out all the steps of the method according to any one of claims 1 to 11 when the program is executed in the control device (110).

15. A computer readable medium on which program sections are stored, which program sections are executable by a computing unit for carrying out all the steps of the method according to any one of claims 1 to 11 when the program sections are executed by the computing unit.

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