MAGNETIC RESONANCE IMAGE METHOD, MAGNETIC RESONANCE IMAGE SYSTEM AND COMPUTER PROGRAM PRODUCT

Description

[0001] Magnetic resonance imaging (MRI) systems are imaging devices that use a strong external magnetic field to align the nuclear spins of an object under investigation and to excite them to precession around the corresponding orientation by applying an excitation RF pulse. The precession, or transition of the spins from this excited state to a lower-energy state, generates an alternating electromagnetic field that can be detected as an MRI signal via receiving antennas.

[0002] Using magnetic gradient fields, for example as gradient pulses, position coding can be imprinted on the signals, enabling the received MR signal to be assigned to a volume element of the object under investigation. The gradient pulses can be applied in three different spatial directions, which are, for example, perpendicular to each other and are designated X, Y, and Z directions. According to a common convention, the Z direction is parallel to the external background magnetic field, so gradient pulses in the Z direction are used to select a specific layer to image in the object and are also referred to as layer selection gradients. In the Y direction, phase coding is used as position coding, and in the X direction, frequency coding is used.

[0003] The received MR signal can then be analyzed to, for example, obtain an image of the object under investigation. Since the MR signal has a high-frequency carrier signal whose frequency is at the Larmor frequency of the nuclear spins, while the actually relevant information lies in the low-frequency range, the analysis of the MR signals for image reconstruction usually involves frequency conversion to reduce the frequency, for example by downmixing the MR signals with the Larmor frequency as the center frequency.

[0004] To receive the MR signals, for example, readout gradient pulses are switched, in particular gradient pulses in the frequency coding direction, i.e. X-The direction is involved. During the readout gradient pulses, the electronics for reading the MR signal, which in particular includes an analog-to-digital converter (ADC), are activated at corresponding readout intervals. For example, it is said that the ADC is switched.

[0005] Readout gradient pulses are, for example, approximately rectangular pulses. In reality, however, the edges of a readout gradient pulse typically extend over a period of time, possibly a very short one. A typical form of a readout gradient pulse is therefore an approximately linearly rising edge, also called a gradient ramp or ramp phase, followed by a region with approximately constant amplitude, also called a gradient flat top or plateau phase, followed in turn by an approximately linearly falling edge.

[0006] Glitches are disturbances at the edge of k-space that can arise from the simultaneous operation of an ADC and a numerically controlled oscillator (NCO). These disturbances are temporally coupled to the beginning of the respective readout period. Other disturbances can also be temporally coupled to the beginning of the respective readout period.

[0007] It is an object of the present invention to reduce interference effects in MR imaging, in particular interference effects that are temporally linked to the beginning of a respective readout period.

[0008] This problem is solved by the respective subject matter of the independent claims. Advantageous further developments and preferred embodiments are the subject matter of the dependent claims.

[0009] CN 100 346 171 C discloses a magnetic resonance imaging technique with a random delay in the readout period.

[0010] The invention is based on the idea of ​​implementing a random component in the readout or processing of the MR signals. For this purpose, various readout periods are delayed randomly or pseudorandomly and / or the center frequency for frequency conversion is shifted randomly or pseudorandomly.

[0011] According to one aspect of the invention, a method for magnetic resonance imaging (MR) is described. In this method, for each readout gradient pulse of a plurality of readout gradient pulses during a readout period assigned to the respective readout gradient pulse, an MR signal is acquired by means of a receiving antenna. Depending on the acquired MR signals, in particular all of the acquired MR signals, an MR image, in particular a spatial MR image, is generated, in particular reconstructed.

[0012] In a first variant, the start time of the respective readout period is delayed by a delay period with respect to the start time of the respective assigned readout gradient pulse, which is randomly or pseudorandomly selected from two or more predefined time values ​​for each readout gradient pulse of the multitude of readout gradient pulses.

[0013] According to a second variant, the acquired MR signals are processed to generate the MR image, the processing including a frequency conversion, in particular a frequency conversion for frequency reduction, according to a center frequency, wherein the center frequency for each readout gradient pulse of the plurality of readout gradient pulses is selected randomly or pseudorandomly from two or more predetermined frequency values.

[0014] The first variant and the second variant can also be combined, so that the start times of the readout gradient pulses are delayed by the delay duration selected randomly or pseudorandomly from the two or more specified time values, and additionally the center frequency for frequency conversion is selected randomly or pseudorandomly from two or more specified frequency values.

[0015] In particular, according to the first variant for generating the MR image, the acquired MR signals are processed, with the processing including frequency conversion according to a center frequency. However, in the first variant, the center frequency can be the same for all readout gradient pulses, or, if the first variant is combined with the second, it can be selected randomly or pseudorandomly from the two or more predefined frequency values, as described.

[0016] In addition to the aforementioned readout gradient pulses, further gradient pulses, for example for layer selection, position coding and / or for scanning the k-space, as well as high-frequency pulses for excitation, refocusing, dephasing, flipping and so on of the nuclear spins can be switched according to known measurement sequences.

[0017] Furthermore, additional MR signals can be acquired using the receiving antenna in further readout periods and / or using additional receiving antennas, and the MR image can be generated depending on the acquired MR signals and the acquired additional MR signals according to reconstruction methods known in principle, except for the random or pseudo-random selection of the time values ​​and / or the frequency values.

[0018] Depending on the type of measurement sequence, the multitude of readout gradient pulses can be a multitude of consecutive readout gradient pulses within a single measurement sequence, or the multitude of readout gradient pulses can be distributed across multiple measurement sequences. It is therefore also possible that each measurement sequence contains only one of the readout gradient pulses.

[0019] A readout period begins at the corresponding start time and ends at the corresponding end time. The MR signal is only acquired or used for reconstruction during the period from the start time to the end time. The gradient amplitude of a readout gradient pulse is zero at the corresponding start time and also zero at the corresponding end time. The gradient amplitude can be different from zero between the start and end times of the readout gradient pulse. This can be due to a unipolar pulse, in which case there is no zero crossing of the gradient amplitude between the start and end times, or a bipolar pulse, in which such a zero crossing occurs.The temporal evolution of the gradient amplitude is identical for all readout gradient pulses among the multitude of readout gradient pulses. However, bipolar readout gradient pulses typically consist of two distinct gradient events, with the first and second parts of the bipolar readout gradient pulse being identical except for their opposite amplitudes.

[0020] In particular, the duration of the readout gradient pulses, i.e., the time difference between the corresponding start and end times, is the same for all readout gradient pulses within the multitude of readout gradient pulses. Similarly, the duration of all readout periods assigned to the readout gradient pulses within the multitude of readout gradient pulses, i.e., the corresponding time difference between the start and end times of each readout period, is also the same. For example, the number of measurement points, also known as sampling points, within a readout period, i.e., the corresponding number of sample points, is also the same. Ab Sample rate, the same for all readout periods.

[0021] Frequency conversion transforms the respective MR signal, in particular from a frequency band centered around a first central frequency, to a frequency band centered around a second central frequency, which is lower than the first central frequency. The first central frequency corresponds, in particular, to a Larmor frequency, and the second central frequency corresponds to the difference between the first central frequency and the center frequency used for frequency conversion. Thus, frequency conversion removes, or approximately removes, a high-frequency component of the MR signal, thereby defining an envelope of the MR signal.

[0022] The two or more predefined time values ​​themselves are not random or pseudo-random, but fixed. Only the selection of the delay duration for the individual readout periods is random or pseudo-random. To randomly or pseudo-randomly select the delay duration from the two or more predefined time values, each time value can be assigned an identifier, for example, a number from 1 to 2. N, where N corresponds to the total number of the two or more predefined time values. A random or pseudorandom number generator is then used to generate a random or pseudorandom sequence of the identifiers, for example, numbers from 1 to 10. N, The frequency is determined and assigned to the readout periods according to their chronological sequence. This applies analogously to the center frequencies in corresponding embodiments.

[0023] The number N of two or more time values ​​is not necessarily equal to the number of readout periods or the number of multiple readout gradient pulses, but is in particular smaller. Accordingly, the same delay duration can be assigned to different readout periods. The time values ​​of the two or more predefined time values ​​are, in particular, all smaller than a predefined maximum delay. The maximum delay can be chosen such that a predefined sequence of readout periods remains unchanged by the different delay durations, especially when several of the readout periods lie within a single measurement sequence. The specific selection of the maximum delay duration then depends, for example, on a desired or required time difference between the individual readout periods.In particular, the maximum delay duration is less than the respective duration of the selection periods, especially less than or equal to one tenth, one twentieth, one fiftieth or one hundredth of the duration of the selection periods.

[0024] The two or more time values ​​are each distinct from one another. One of the two or more time values ​​can also be zero. The delay duration need not necessarily be defined with respect to the start time of the respective readout gradient pulse. Rather, it can also be defined with respect to another characteristic point in time within the respective readout period, for example, with respect to a transition point between a gradient ramp and a gradient flat section, if provided for. Nevertheless, there is always a delay between the start times of the readout period and the associated readout gradient pulse.

[0025] Both the random or pseudorandom delay of the readout periods and the random or pseudorandom shift of the center frequency for frequency conversion cause incoherence between the data from different readout periods used for reconstruction, particularly in k-space. This results in disturbances due to phenomena linked to the beginning of the respective readout period—that is, phenomena that always occur simultaneously with the start time of the readout period, or always at a specific time after the start time of the readout period, or within a limited period after the start time of the readout period—being smeared in k-space. This, in turn, leads to the corresponding artifacts and disturbances being less pronounced in the reconstructed MR image in spatial space.

[0026] It should be noted that the delay in the readout periods, or the shift in the center frequency, is specifically taken into account during the generation of the reconstructed MR image, so that the data required for reconstruction, particularly near the k-space center, of the individual sub-k-spaces are aligned in the readout direction. In other words, the acquired data are specifically directed to the location in k-space defined by the readout gradient pulse and then used accordingly for reconstruction. Since the disturbances are temporally linked to the beginning of the respective readout period, the resulting artifacts are therefore located at different positions in k-space for different readout periods.

[0027] According to at least one embodiment of the second variant or a combination of the first and the second variant, the frequency conversion includes a respective downmixing of the detected MR signals according to the center frequency.

[0028] In particular, the corresponding MR signal is fed to a mixer, which also receives a reference signal with the center frequency. The reference signal can be generated, for example, by means of a local oscillator. The reference signal is, in particular, a sine wave or, approximately, a sine wave with the center frequency. Such down-conversion is a component of conventional methods for processing MR signals to reconstruct the MR image. In contrast to conventional methods, however, the corresponding embodiments according to the invention do not use the same center frequency, namely, in particular, the Larmor frequency, but rather the center frequency is determined randomly or pseudorandomly, as described above. This allows the desired incoherence according to the invention to be advantageously achieved by adapting existing electronic components or supplementing them.

[0029] According to at least one embodiment, the respective center frequency is determined as the sum of a predetermined Larmor frequency and a frequency difference, wherein the respective frequency difference is selected randomly or pseudorandomly from two or more predetermined frequency difference values.

[0030] By randomly or pseudorandomly selecting the frequency difference, the center frequency is also randomly or pseudorandomly selected. Depending on the value of the frequency difference, the resulting center frequency can be higher or lower than the Larmor frequency. The frequency differences can therefore be positive or negative. In this way, the desired incoherence according to the invention can be achieved in a controlled manner.

[0031] The absolute values ​​of the two or more predefined frequency difference values ​​are each smaller than a predefined maximum frequency difference. The maximum frequency difference is smaller than the Larmor frequency, for example, less than or equal to one-tenth, one-twentieth, one-fiftieth, or one-hundredth of the Larmor frequency. This ensures that the effect originally intended by the frequency conversion—namely, the extraction of the envelope of the corresponding MR signal—is still achieved to a good approximation.

[0032] According to at least one embodiment, in particular according to the first variant or a combination of the first with the second variant, a number of the readout gradient pulses of the plurality of readout gradient pulses is greater than a number of the two or more time values.

[0033] This allows the desired incoherence to be achieved according to the invention; in particular, the individual time values ​​can be defined differently enough from each other to achieve the incoherence without introducing a potentially excessive time shift for individual readout periods.

[0034] The number of readout gradient pulses can be, for example, in the range [5, 500], [10, 500], or [20, 200]. The number of two or more time values ​​can then be, for example, in the range [3, 50] or [3, 20], whereby it is always ensured that the number of two or more time values ​​is less than the number of readout gradient pulses. The number of two or more frequency values ​​can also be, for example, in the range [3, 50] or [3, 20], whereby it is always ensured that the number of two or more frequency values ​​is less than the number of readout gradient pulses.

[0035] According to at least one embodiment, the readout gradient pulses of the plurality of readout gradient pulses follow one another within a single measurement sequence, in particular directly one after the other.

[0036] For example, a high-frequency excitation pulse is switched, and the readout gradient pulses of the multitude of readout gradient pulses are all switched after the high-frequency excitation pulse and before another high-frequency excitation pulse is switched.

[0037] In such measurement sequences with a large number of readout gradient pulses, temporally defined or temporally limited phenomena that lead to interference effects can, for example, occur more frequently, so that the invention is particularly advantageous here.

[0038] According to at least one embodiment, the readout gradient pulses of the plurality of readout gradient pulses within the measurement sequence have alternating polarity.

[0039] The readout gradient pulses are therefore, in particular, unipolar gradient pulses with alternating polarity within the measurement sequence.

[0040] In such measurement sequences with a large number of readout gradient pulses, temporally defined or temporally limited phenomena that lead to interference effects can, for example, occur more frequently, so that the invention is particularly advantageous here.

[0041] According to at least one embodiment, the measurement sequence is performed as an echoplanar imaging (EPI) scan or as a segmented EPI scan.

[0042] In an EPI scan, the entire k-space of a given excited slice is sampled within a single measurement sequence. In a segmented EPI scan, several consecutive measurement sequences are used to... k- to scan the space of an excited layer.

[0043] In such measurement sequences with a large number of readout gradient pulses, temporally defined or temporally limited phenomena that lead to interference effects can, for example, occur more frequently, so that the invention is particularly advantageous here.

[0044] According to at least one embodiment, each readout gradient pulse of the plurality of readout gradient pulses has a first ramp phase, a plateau phase following the first ramp phase, and a second ramp phase following the plateau phase. The start time of the respective readout period lies within the first ramp phase and / or the end time of the respective readout period lies within the second ramp phase.

[0045] The plateau phase can also be referred to as the gradient flat region, and the ramp phases can be referred to as gradient ramps. During the plateau phase, the amplitude of the readout gradient pulse is, in particular, constant or approximately constant, equal to a plateau value. The sign of the gradient amplitude during the plateau phase corresponds to the polarity of the readout gradient pulse, or, in the case of bipolar readout gradient pulses, to the polarity of the corresponding part of the readout gradient pulse. During the first ramp phase, the gradient amplitude changes, in particular, from zero to the plateau value, for example, linearly or approximately linearly. During the second ramp phase, the gradient amplitude changes from the plateau value to zero, in particular linearly or approximately linearly.

[0046] Since the start time of the readout period lies within the first ramp phase and / or the end time of the readout period lies within the second ramp phase, ramp sampling is used. This is particularly advantageous because it provides more time for signal acquisition than if signal acquisition were only performed during the plateau phase.

[0047] According to at least one embodiment, the amplitude of the readout gradient pulse changes linearly or approximately linearly from zero to the plateau value during the first ramp phase, is constant or approximately constant equal to the plateau value during the plateau phase, and changes linearly or approximately linearly from the plateau value to zero during the second ramp phase.

[0048] This makes it particularly easy to generate gradient pulses.

[0049] Approximately linear and approximately constant can be understood to mean that a linear or constant gradient amplitude is targeted or set, but the actual course may deviate to the usual extent due to tolerances or other variations.

[0050] According to a further aspect of the invention, an MR imaging system is described. The MR imaging system comprises a gradient coil arrangement, a receiving antenna, a signal acquisition unit connected to the receiving antenna, and at least one control unit. The at least one control unit is configured to drive the gradient coil arrangement to generate a plurality of readout gradient pulses and to drive the signal acquisition unit to acquire an MR signal received by the receiving antenna for each readout gradient pulse of the plurality of readout gradient pulses during a respective readout period. The MR imaging system comprises an evaluation unit configured to generate, and in particular reconstruct, an MR image based on the acquired MR signals.

[0051] According to a first variant, at least one control unit is set up to delay a start time of the respective readout period with respect to a start time of the respective readout gradient pulse by a delay duration and to select the delay duration for each readout gradient pulse randomly or pseudorandomly from two or more predefined time values.

[0052] According to a second variant, at least one control unit is set up to control the signal acquisition unit, to perform a frequency conversion according to a center frequency for the respective processing of the acquired MR signals, and to select the center frequency for each of the readout gradient pulses randomly or pseudorandomly from two or more predetermined frequency values.

[0053] In various embodiments, the first and second variants of the MR imaging system can also be combined.

[0054] The at least one control unit includes, in particular, at least one arithmetic unit. An arithmetic unit can be understood to be, in particular, a data processing device containing a processing circuit. The arithmetic unit can therefore, in particular, process data to perform arithmetic operations. This may also include operations to perform indexed accesses to a data structure, for example, a lookup table (LUT).

[0055] The computing unit may, in particular, contain one or more computers, one or more microcontrollers, and / or one or more integrated circuits, for example, one or more application-specific integrated circuits (ASICs), one or more field-programmable gate arrays (FPGAs), and / or one or more systems on a chip (SoCs). The computing unit may also contain one or more processors, for example, one or more microprocessors, one or more central processing units (CPUs), one or more graphics processing units (GPUs), and / or one or more signal processors, in particular one or more digital signal processors (DSPs). The computing unit may also include a physical or virtual array of computers or other units of the aforementioned type.

[0056] In various embodiments, the computing unit includes one or more hardware and / or software interfaces and / or one or more storage units.

[0057] A storage unit can be volatile data storage, for example as dynamic random access memory (DRAM) or static random access memory (SRAM), or as non-volatile data storage, for example as read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), flash memory or flash EEPROM, ferroelectric random access memory (FRAM), or magnetoresistive random access memory.It can be designed as MRAM (magnetoresistive random access memory) or as phase-change random access memory, PCRAM (phase-change random access memory).

[0058] According to at least one embodiment, the signal acquisition unit has a mixer which is configured to downmix the acquired MR signals to the center frequency for frequency conversion.

[0059] According to at least one embodiment, the MR imaging system has a high-frequency coil arrangement, wherein the at least one control unit is configured to control the high-frequency coil arrangement and the gradient coil arrangement in such a way that the readout gradient pulses of the plurality of readout gradient pulses in a measurement sequence, in particular a single measurement sequence, follow one another, for example, immediately one after the other.

[0060] According to at least one embodiment, the at least one control unit is configured to control the high-frequency coil arrangement and the gradient coil arrangement in such a way that the readout gradient pulses of the plurality of readout gradient pulses within the measurement sequence have alternating polarity.

[0061] According to at least one embodiment, the at least one control unit is configured to control the high-frequency coil arrangement and the gradient coil arrangement in such a way that the measurement sequence is performed as an EPI recording, i.e. as an EPI measurement sequence, or as a measurement sequence of a segmented EPI recording.

[0062] In some embodiments, at least one control unit may also include or partially include the signal acquisition unit and / or the evaluation unit.

[0063] Further embodiments of the MR imaging system according to the invention follow directly from the various configurations of the method according to the invention, and vice versa. In particular, individual features and corresponding explanations as well as advantages relating to the various configurations of the method according to the invention can be transferred analogously to corresponding configurations of the MR imaging system according to the invention. In particular, the MR imaging system according to the invention is configured or programmed to carry out a method according to the invention. In particular, the MR imaging system according to the invention carries out the method according to the invention.

[0064] According to a further aspect of the invention, a computer program with commands is provided. When the commands are executed by an MR imaging system according to the invention, in particular by the at least one control unit of the MR imaging system, the commands cause the MR imaging system to perform an MR imaging method according to the invention.

[0065] The instructions can be provided, for example, as program code. This program code can be provided, for example, as binary code or assembly language, and / or as source code in a programming language such as C, and / or as a program script, such as Python.

[0066] According to another aspect of the invention, a computer-readable storage medium is specified that stores a computer program according to the invention.

[0067] The computer program and the computer-readable storage medium are each computer program products containing the commands.

[0068] Further features of the invention will become apparent from the claims, the figures, and the description of the figures. The features and combinations of features mentioned above in the description, as well as the features and combinations of features mentioned below in the description of the figures and / or shown in the figures, may be encompassed by the invention not only in the combinations specified, but also in other combinations as defined by the claims.

[0069] The invention is explained in more detail below with reference to specific embodiments and associated schematic drawings. In the figures, identical or functionally equivalent elements may be designated with the same reference numerals. The description of identical or functionally equivalent elements is not necessarily repeated with respect to different figures.

[0070] The figures show: FIG 1 a schematic representation of an exemplary embodiment of the MR imaging system according to the invention; FIG 2 a schematic representation of an MR image in k-space; FIG 3 a schematic representation of an MR image in spatial space; FIG 4 a schematic representation of a measurement sequence according to an exemplary embodiment of the MR imaging method according to the invention; FIG 5 a schematic representation of a measurement sequence according to a further exemplary embodiment of the MR imaging method according to the invention; FIG 6 a schematic representation of a measurement sequence according to a further exemplary embodiment of the MR imaging method according to the invention; FIG 7 a schematic representation of a measurement sequence according to a further exemplary embodiment of the MR imaging method according to the invention;and FIG. 8 a schematic representation of a frequency conversion according to a further exemplary embodiment of the inventive method for MR imaging.

[0071] In FIG 1 An exemplary embodiment of the MR imaging system 1 according to the invention is shown schematically.

[0072] The MR imaging system 1 comprises an MR scanner with a field magnet 3, which can generate a static magnetic field for aligning the nuclear spins of an object 8 to be imaged. The MR scanner can have a patient tunnel 2, which encompasses an imaging area, wherein the static magnetic field is highly homogeneous within the imaging area with respect to its magnetic field strength and / or its absolute value. The object 8 can be positioned on a patient table 7, which can be movable to change the position of the object 8.

[0073] The field magnet 3 can, for example, be a superconducting magnet, i.e., an electromagnet with a superconducting coil. In this way, static magnetic fields of up to 3T or more can be achieved. For lower field strengths, permanent magnets or electromagnets with normal-conducting coils can also be used.

[0074] The MR scanner further comprises a gradient coil array 5, which can generate variable magnetic fields along each of the three spatial dimensions X, Y, Z and superimpose the variable magnetic fields with the base magnetic field to achieve position encoding. The gradient coils of the gradient coil array 5 can be normal-conducting coils. In particular, the gradient coils can include one or more gradient coils for generating a magnetic field gradient in the X direction, one or more gradient coils for generating magnetic field gradients in the Y direction, and one or more gradient coils for generating magnetic field gradients along the Z direction.

[0075] Furthermore, the MR scanner can have one or more transmitting coils 4 to generate radio-frequency pulses for exciting nuclear spins to cause precession in the corresponding magnetic field. The transmitting coils 4 can also function as receiving antennas to receive MR signals in response to the nuclear spin excitation and precession resonance. Alternatively or additionally, one or more dedicated receiving coils 6, for example, body coils, arranged in the immediate vicinity of the object 8, can be used.

[0076] The MR imaging system 1 further comprises a control system that may include one or more computing units and additional electronic components for controlling the MR scanner and / or for processing the MR signals. The control system has an evaluation unit 9, a signal acquisition unit 10, and at least one control unit 11, 12, for example, a high-frequency controller 11 and a gradient controller 12. The gradient controller 12 is configured, for example, to supply the gradient coils with variable currents via leads, which can provide the desired gradient fields, in particular in the form of gradient pulses, in a temporally coordinated manner within the imaging area. The high-frequency controller 11 is configured, for example, to generate high-frequency pulses, for example, excitation pulses, with predefined temporal profiles, amplitudes, and spectral power distribution to excite a magnetic resonance of the nuclear spins.

[0077] A method according to the invention can be carried out using the MR imaging system 1. In the figures FIG 4 bis FIG 8 Details of various exemplary embodiments of such a method according to the invention are shown schematically.

[0078] The gradient control 12 is configured to control the gradient coil arrangement 5 to generate a plurality of readout gradient pulses 19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f and to control the signal acquisition unit 10, for each readout gradient pulse 19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f during a respective readout period 20a, 20b, 20c, 22a, 22b, 22c, 22d, 22e, 22f, to acquire an MR signal received by one of the receiving antennas 4, 6. The evaluation unit 9 is configured to generate an MR image based on the acquired MR signals. The gradient control 12 is configured to generate the MR image by performing a frequency conversion according to a center frequency for the respective processing of the acquired MR signals in order to downmix the MR signals.

[0079] The gradient control 12 is configured to delay a start time of the respective readout period 20a, 20b, 20c, 22a, 22b, 22c, 22d, 22e, 22f with respect to a start time of the respective readout gradient pulse 19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f by a delay duration and to select the delay duration for each of the readout gradient pulses 19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f randomly or pseudorandomly from two or more predefined time values.

[0080] Alternatively or additionally, the signal acquisition unit 10 is configured to randomly or pseudorandomly select the center frequency for each of the readout gradient pulses 19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f from two or more predefined frequency values.

[0081] This results in an incoherence between the individual evaluation periods 20a, 20b, 20c, 22a, 22b, 22c, 22d, 22e, 22f, which smears artifacts 14 in k-space and thus at least partially compensates for corresponding artifacts 17 in local space.

[0082] FIG 2 schematically shows an exemplary k-space recording 13 with stripe-shaped artifacts 14 that may occur, in particular if a method according to the invention is not used. FIG 3 schematically shows a related MR image 15 of an object 16 in the local space with corresponding artifacts 17.

[0083] In the characters FIG 4 , FIG 5 und FIG 6 Schematic sequence diagrams for three readout gradient pulses 19a, 19b, 19c and corresponding readout periods 20a, 20b, 20c are shown according to an exemplary embodiment of a method for MR imaging according to the invention. The readout gradient pulses 19a, 19b, 19c correspond, for example, to different measurement sequences. During the associated readout periods 20a, 20b, 20c, an MR signal is received and an MR image is generated, at least in part, depending on the MR signals received during the readout periods 20a, 20b, 20c. In the upper half of the FIG 4 , FIG 5, FIG 6 The readout period 20a, 20b, 20c is shown as a function of time t. The lower half of the graph shows the gradient amplitude GX in the readout direction and the frequency coding direction, respectively. For example, a dephasing gradient pulse 18a, 18b, 18c, particularly with negative polarity, is applied, followed by a corresponding readout gradient pulse 19a, 19b, 19c with opposite polarity, particularly positive polarity. A gradient echo signal is generated while the readout gradient pulses 19a, 19b, 19c are being applied. Corresponding layer selection gradients in the Z-direction and gradients for phase coding, and so on, in the Y-direction are not shown for clarity.

[0084] The readout gradient pulses 19a, 19b, 19c each have the same shape and duration, i.e., in particular, the same amplitude profile. The durations of the readout periods 20a, 20b, 20c are also the same. The respective start time of the readout period 20a, 20b, 20c is delayed by a delay period Δt relative to the start time of the corresponding readout gradient pulse 19a, 19b, 19c. The delay periods Δt are randomly or pseudorandomly selected from two or more predefined time values. In the example of the FIG 4 Δt is equal to a given nominal delay time t0. In the example of the FIG 5 Δt is smaller than t0 and in the example the FIG 6 Δt is greater than t0.

[0085] In the examples of FIG 4 , FIG 5, FIG 6 The readout gradient pulses 19a, 19b, 19c each exhibit a first ramp phase 21a, followed by a plateau phase 21b, and then a second ramp phase 21c. For example, the start time of the readout periods 20a, 20b, 20c is in the first ramp phase 21a, and the end time is in the second ramp phase 21c. Thus, ramp sampling is performed.

[0086] Artifacts that arise, for example, due to disturbances that are temporally linked to the beginning of the respective readout period, such as glitches, are partially or completely compensated by the random or pseudorandom delay of the readout gradient pulses 19a, 19b, 19c.

[0087] In the FIG 7 A schematic sequence diagram of an EPI measurement sequence according to a further exemplary embodiment of a method according to the invention is shown. Here, following a dephasing gradient pulse 23, a plurality of directly successive unipolar readout gradient pulses 24a, 24b, 24c, 24d, 24e, 24f with alternating polarity are provided. The shape of the individual readout gradient pulses 24a, 24b, 24c, 24d, 24e, 24f corresponds, for example, to the shape of the FIG 4 bis FIG 6 described form. Here too, each of the readout gradient pulses 24a, 24b, 24c, 24d, 24e, 24f is assigned a readout period 22a, 22b, 22c, 22d, 22e, 22f, whereby ramp sampling is also performed, for example. For the delay durations Δt, four different values ​​t0, t1, t2, t3 are provided here, for example. For the successive readout gradient pulses 24a, 24b, 24c, 24d, 24e, 24f or their assigned readout periods 22a, 22b, 22c, 22d, 22e, 22f, one of the values ​​t0, t1, t2, t3 is selected randomly or pseudorandomly. In the example of the FIG 7 The delay period Δt for the first displayed readout period 22a is equal to t0, for the second readout period 22b is equal to t2, for the third readout period 22c is equal to t3, for the fourth readout period 22d is equal to t1, for the fifth readout period 22e is equal to t2 and for the sixth readout period 22f it is again t1.

[0088] Alternatively or additionally to the random delay of the start times of the readout periods 20a, 20b, 20c, 22a, 22b, 22c, 22d, 22e, 22f, the center frequency for frequency conversion to downmix the respective MR signals can be selected randomly or pseudorandomly, in particular around a predefined Larmor frequency. This is shown schematically in FIG 8 The MR signal is acquired in frequency band 25, which is centered, for example, around the Larmor frequency f0. By downconverting using the center frequency Δf, which in the example shown is smaller than the Larmor frequency f0, frequency band 25 is converted to a lower frequency band 26, centered around the center frequency (f0 - Δf). By randomly or pseudorandomly selecting the value for Δf, the desired incoherence according to the invention is also achieved or enhanced.

[0089] As described, particularly with regard to the figures, the invention makes it possible to reduce interference effects in MR imaging.

[0090] In various embodiments, the start times of the ADC acquisition periods in the individual k-space lines are slightly shifted relative to each other to achieve an incoherent distribution of potential artifacts. In particular, this results in a smearing of the artifact energy in k-space, which consequently leads to a reduction of artifacts in the reconstructed MR image.

[0091] In various implementations, ramp sampling is performed. During reconstruction, a so-called gridding method is typically used. In this process, the data values ​​acquired on the gradient ramps are interpolated back onto a Cartesian grid, since subsequent reconstruction steps, such as a Fourier transform, generally expect Cartesian input data. Within the gridding method, the shifted acquisitions are then interpolated onto the Cartesian grid according to their actual starting point.

[0092] In various embodiments, the number of data points acquired during a readout period can be reduced compared to a nominal value, for example, to account for limitations on data acquisition due to adjacent gradients on other axes. In other embodiments, the readout periods can also be slightly extended compared to a nominal duration to still have the full number of data points available.

[0093] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

Claims

1. Method for magnetic resonance imaging, MRI, wherein for each readout gradient pulse (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) of a plurality of readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) during a respective readout period (20a, 20b, 20c, 22a, 22b, 22c, 22d, 22e, 22f), an MR signal is acquired by means of a receiving antenna (4, 6) and an MR image is generated as a function of the acquired MR signals, wherein - a start time of the respective readout period (20a, 20b, 20c, 22a, 22b, 22c, 22d, 22e, 22f) is delayed relative to a start time of the respective readout gradient pulse (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) by a delay duration which is randomly or pseudo-randomly selected from two or more predefined time values for each readout gradient pulse (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f); and / or - to generate the MR image, the acquired MR signals are each processed, wherein the processing includes a frequency conversion based around a centre frequency selected randomly or pseudo-randomly from two or more predetermined frequency values for each readout gradient pulse (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f).

2. Method according to claim 1, wherein the readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) of the plurality of readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) are consecutive in a measurement sequence.

3. Method according to claim 2, wherein the readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) of the plurality of readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) have alternating polarity within the measurement sequence.

4. Method according to one of claims 2 or 3, wherein the measurement sequence is performed as an echo-planar imaging recording or as a measurement sequence of a segmented echoplanar imaging recording.

5. Method according to one of the preceding claims, wherein - each readout gradient pulse (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) of the plurality of readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) has a first ramp phase (21a), a plateau phase (21b) following the first ramp phase (21a), and a second ramp phase (21c) following the plateau phase (21b); and - the start time of the respective readout period (20a, 20b, 20c, 22a, 22b, 22c, 22d, 22e, 22f) is within the first ramp phase (21a) and / or an end time of the respective readout period (20a, 20b, 20c, 22a, 22b, 22c, 22d, 22e, 22f) is within the second ramp phase (21c).

6. Method according to claim 5, wherein an amplitude of the readout gradient pulse (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) - changes linearly or approximately linearly from zero to a plateau value during the first ramp phase (21a); - is constantly or approximately constantly equal to the plateau value during the plateau phase (21b); and - changes linearly or approximately linearly from the plateau value to zero during the second ramp phase (21c).

7. Method according to one of the preceding claims, wherein - a number of the readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) of the plurality of readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) is greater than the number of the two or more time values; and / or - the number of the readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) of the plurality of readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) is greater than the number of the two or more frequency values.

8. Method according to one of the preceding claims, wherein the frequency conversion comprises a respective downmixing of the acquired MR signals based around the centre frequency.

9. Method according to one of the preceding claims, wherein the respective centre frequency is determined as a sum of a predefined Larmor frequency and a frequency difference, wherein the respective frequency difference is selected randomly or pseudo-randomly from two or more predefined frequency difference values.

10. MR imaging system (1) having a gradient coil array (5), a receiving antenna (4, 6), a signal acquisition unit (10) connected to the receiving antenna (4, 6), at least one control unit (11, 12) which is designed to activate the gradient coil array (5) to generate a plurality of readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) and to activate the signal acquisition unit (10) to acquire an MR signal received by means of the receiving antenna (4, 6) for each readout gradient pulse (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) of the plurality of readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) during a respective readout period (20a, 20b, 20c, 22a, 22b, 22c, 22d, 22e, 22f), and an evaluation unit (9) which is designed to generate an MR image based on the acquired MR signals, wherein the at least one control unit (11, 12) is designed to - delay a start time of the respective readout period (20a, 20b, 20c, 22a, 22b, 22c, 22d, 22e, 22f) relative to a start time of the respective readout gradient pulse (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) by a delay duration and to select the delay duration for each of the readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) randomly or pseudo-randomly from two or more predefined time values; and / or - to activate the signal acquisition unit (10) to perform frequency conversion based around a centre frequency for respective processing of the acquired MR signals, and to select the centre frequency for each of the readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) randomly or pseudo-randomly from two or more predetermined frequency values.

11. MR imaging system (1) according to claim 10, wherein the signal acquisition unit (10) has a mixer which is designed to mix down the acquired MR signals based around the centre frequency for frequency conversion.

12. MR imaging system (1) according to one of claims 10 or 11, having a radiofrequency coil array (4), wherein the at least one control unit (11, 12) is designed to control the radiofrequency coil array (4) and the gradient coil array (5) such that the readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) of the plurality of readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) are consecutive in a measurement sequence.

13. MR imaging system (1) according to claim 12, wherein the at least one control unit (11, 12) is designed to control the radiofrequency coil array (4) and the gradient coil array (5) such that the readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) of the plurality of readout gradient pulses (19a, 19b, 19c, 24a, 24b, 24c, 24d, 24e, 24f) have alternating polarity within the measurement sequence.

14. MR imaging system (1) according to one of claims 12 or 13, wherein the at least one control unit (11, 12) is designed to control the high frequency coil array (4) and the gradient coil array (5) such that the measurement sequence is performed as an echo-planar imaging recording or as a measurement sequence of a segmented echo-planar imaging recording.

15. Computer program product comprising commands which, when executed by an MR imaging system (1) according to one of claims 10 to 14, cause the MR imaging system (1) to carry out a method according to one of claims 1 to 9.