Magnetic resonance system, magnetic resonance imaging sequence and optimization method

Abstract

The invention relates to a magnetic resonance system, a magnetic resonance imaging sequence and an optimization method. The imaging sequence comprises: a radio frequency excitation pulse; the first radio frequency refocusing pulse and the second radio frequency refocusing pulse are sequentially applied after the radio frequency excitation pulse; original gradient pulses including a right side original pulse applied between the center of the first radio frequency refocusing pulse and the center of the second radio frequency refocusing pulse and a left side original pulse applied at the center of the radio frequency excitation pulse and the center of the first radio frequency refocusing pulse, the right side original pulse being applied between the center of the first radio frequency refocusing pulse and the center of the second radio frequency refocusing pulse, and the left side original pulse being applied between the center of the first radio frequency refocusing pulse and the center of the second radio frequency refocusing pulse. The left side original pulse comprises a first gradient pulse corresponding to the radio frequency excitation pulse; and a first balance pulse, the first balance pulse being located in a first time period between an end point of the first gradient pulse and a start point of the first radio frequency refocusing pulse, and the first balance pulse including a positive pulse and a negative pulse located on the first gradient axis.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of medical imaging, and in particular to a magnetic resonance (MR) imaging sequence, an optimization method of the magnetic resonance imaging sequence, and a magnetic resonance system. BACKGROUND

[0002] Magnetic resonance imaging technology has become one of the most important modern imaging diagnostic technologies due to its non-invasive, rich diagnostic information, high resolution, and other characteristics. The magnetic resonance imaging technology utilizes electromagnetic principles to generate and acquire image information by performing imaging sequences (also referred to as scan sequences or pulse sequences) that match clinical diagnostic needs. When designing, optimizing, or selecting these imaging sequences, various aspects such as clinical diagnostic needs, image quality, scan safety, scan time, and the like can be considered, for example, to perform a scan as safely and quickly as possible while improving the clarity and resolution of anatomical structure images.

[0003] Imaging sequences typically include gradient pulses, and ideal gradient pulses cause the magnetic field to change linearly. However, when actually scanning, nonlinear concomitant fields are inevitably generated at the same time as the linear gradient field, which is a potential source of artifacts in magnetic resonance imaging. For example, the concomitant fields cause unwanted phase accumulation, resulting in phase errors between echo signals, and further causing signal loss, image blurring, ghosting, and the like. SUMMARY

[0004] One aspect of the present application provides a magnetic resonance imaging sequence, comprising:

[0005] a radio frequency excitation pulse, a first radio frequency refocusing pulse and a second radio frequency refocusing pulse applied sequentially after the radio frequency excitation pulse, a primary pulse, and a first balance pulse. The primary gradient pulse includes a right primary pulse and a left primary pulse, the right primary pulse is applied between the center of the first radio frequency refocusing pulse and the center of the second radio frequency refocusing pulse, the left primary pulse is applied between the center of the radio frequency excitation pulse and the center of the first radio frequency refocusing pulse, and the left primary pulse includes a first gradient pulse corresponding to the radio frequency excitation pulse. The first balance pulse is located in a first time period between the end point of the first gradient pulse and the start point of the first radio frequency refocusing pulse, and the first balance pulse includes a positive pulse and a negative pulse located on a first gradient axis.

[0006] In another aspect, the application provides a method for optimizing a magnetic resonance imaging sequence, comprising a first step, a second step and a third step. In the first step, a right Maxwell term generated by the right original pulse and a left Maxwell term generated by the left original pulse are determined. In the second step, in response to the right Maxwell term being greater than the left Maxwell term and a first difference between the right Maxwell term and the left Maxwell term being greater than a preset value, a maximum value of a first compensation Maxwell term that can be generated in a first gradient axis is determined based on a current echo interval of the magnetic resonance imaging sequence. In the third step, a first balance pulse arranged on the first gradient axis is determined to increase the left Maxwell term, and in response to the maximum value being greater than the first difference, an amplitude of the first balance pulse is less than a maximum amplitude; in response to the maximum value being equal to the first difference, the amplitude of the first balance pulse is equal to the maximum amplitude. Wherein, the first balance pulse is located in a first time period between an end point of the first gradient pulse and a start point of the first radio frequency refocusing pulse, and the first balance pulse comprises a positive pulse and a negative pulse.

[0007] In another aspect, the application provides a magnetic resonance imaging system comprising a scanner and a processor, wherein the processor is configured to perform the method for optimizing a magnetic resonance imaging sequence of any of the above aspects or to control the scanner to perform the magnetic resonance imaging sequence of any of the above aspects.

[0008] It should be understood that the brief summary is provided merely for purposes of summarizing some concepts of the detailed description that follows and is not meant to limit the claimed subject matter in any way. Moreover, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. BRIEF DESCRIPTION OF DRAWINGS

[0009] The application will be better understood with reference to the following non-limiting description of embodiments, taken in conjunction with the accompanying drawings, in which:

[0010] Figure 1 is a schematic diagram of a magnetic resonance (MR) system according to one example embodiment;

[0011] Figure 2 is a waveform diagram of a magnetic resonance imaging sequence according to one embodiment of the application;

[0012] Figure 3 is a waveform diagram of a magnetic resonance imaging sequence according to one embodiment of the application;

[0013] Figure 4 is a waveform diagram of a magnetic resonance imaging sequence according to one embodiment of the application;

[0014] Figure 5 is a waveform diagram of a magnetic resonance imaging sequence of one embodiment of the present application;

[0015] Figure 6 is a waveform diagram of a magnetic resonance imaging sequence of one embodiment of the present application;

[0016] Figure 7 is a waveform diagram of a magnetic resonance imaging sequence of one embodiment of the present application;

[0017] Figure 8 is a flowchart of an optimization method for a magnetic resonance imaging sequence of one embodiment of the present application;

[0018] Figure 9 is a flowchart of an optimization method for a magnetic resonance imaging sequence of another embodiment of the present application; and

[0019] Figure 10 is a flowchart of an optimization method for a magnetic resonance imaging sequence of another embodiment of the present application.

[0020] The accompanying drawings illustrate components, sequences or waveforms, systems, and methods described by embodiments of the present application. Together with the following description, the drawings show and illustrate principles of the structures, methods and concepts described herein. In the drawings, the thickness and size of components can be exaggerated or otherwise modified for the sake of clarity. Well-known structures, materials or operations are not shown or described in detail in order to avoid obscuring aspects of the described components, systems, and methods. DETAILED DESCRIPTION

[0021] The specific embodiments of the present application will now be described in detail below. It should be noted that in the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present application. It will be apparent, however, to one skilled in the art that the present application can be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and apparatus are not presented in order to avoid obscuring the description of the present application. It should be noted that the description of the present application is not meant to limit the application to the specific embodiments presented herein, but rather the application is meant to be as broad as possible consistent with the present disclosure.

[0022] Unless otherwise defined, technical and scientific terms used in the claims and specification shall have the meaning commonly understood by one of ordinary skill in the art. The use of "first", "second", and "third" or similar wording in the specification and claims to describe various embodiments is merely for convenience and does not require or imply that the applications are limited to these embodiments. The articles "a", "an", and "the" preceding an element are intended to be read as meaning "one or more" unless the context of use clearly indicates otherwise. The terms "comprise", "comprising", "comprises", "include", "including", and "includes" are intended to be interpreted as specifying the presence of stated features or components rather than precluding the presence or addition of one or more other features or components. The term "coupled" and variations thereof, as used herein, are intended to mean physically, logically, or communicatively coupled or linked, and are not necessarily limited to a direct coupling or connection.

[0023] Reference is made to Figure 1 FIG. 1 shows a schematic diagram of an exemplary MR (Magnetic Resonance) system 100, in accordance with some embodiments. The operation of the MR system 100 is controlled by an operator workstation 110, which includes an input device 114, a control panel 116, and a display 118. The input device 114 can be a joystick, keyboard, mouse, trackball, touch activated screen, voice control, or any similar or equivalent input device. The control panel 116 can include a keyboard, touch activated screen, voice control, buttons, sliders, or any similar or equivalent control device. The operator workstation 110 is coupled to and in communication with a computer system 120 that enables the operator to control the production and viewing of images on the display 118. The computer system 120 includes multiple components that communicate with each other via an electrical and / or data connection module 122. The connection module 122 can be a direct wired connection, a fiber optic connection, a wireless communication link, or the like. The computer system 120 can include a central processing unit (CPU) 124, a memory 126, and an image processor 128. In some embodiments, the image processor 128 can be replaced by image processing functionality implemented in the CPU 124. The computer system 120 can be connected to an archival media device, permanent or backup memory, or a network. The computer system 120 can be coupled to and in communication with a separate system controller 130.

[0024] The system controller 130 includes a set of components that communicate with one another via electrical and / or data connection modules 132. The connection modules 132 can be direct wired connections, fiber optic connections, wireless communication links, etc. The system controller 130 can include a CPU 131, a sequence pulse generator 133 that communicates with the operator workstation 110, a transceiver (or RF transceiver) 135, a memory 137, and an array processor 139. In some embodiments, the sequence pulse generator 133 can be integrated into a resonance assembly 140 of the MR system 100. The system controller 130 can receive commands from the operator workstation 110 to instruct the MR scan sequence to be performed during an MR scan. The system controller 130 is also coupled to and in communication with a gradient driver system 150 that is coupled to a gradient coil assembly 142 to produce magnetic field gradients during an MR scan.

[0025] The sequence pulse generator 133 can also receive data from a physiological acquisition controller 155 that receives signals from a number of different sensors (such as electrocardiograph (ECG) signals from electrodes attached to the patient), the sensors connected to the subject or patient 170 undergoing the MR scan. The sequence pulse generator 133 is coupled to and in communication with a scan room interface system 145 that receives signals from various sensors associated with the status of the resonance assembly 140. The scan room interface system 145 is also coupled to and in communication with a patient positioning system 147 that sends and receives signals to control the movement of the patient table into the desired position for the MR scan.

[0026] The system controller 130 provides gradient waveforms to the gradient driver system 150, which includes G x , G y , and G z amplifiers, etc. Each G x , G y , and G z gradient amplifier excites a corresponding gradient coil in the gradient coil assembly 142 to produce a magnetic field gradient used for spatial encoding of the MR signals during the MR scan. The gradient coil assembly 142 is disposed within the resonance assembly 140, which also includes a superconducting magnet with superconducting coils 144 that, in operation, provide a static, uniform, longitudinal magnetic field Bo that runs through the cylindrical imaging volume 146. The resonance assembly 140 also includes an RF body coil 148 that, in operation, provides a transverse magnetic field Bi that is approximately perpendicular to Bo throughout the cylindrical imaging volume 146. The resonance assembly 140 can also include RF surface coils 149 that are used to image different anatomies of the patient undergoing the MR scan. The RF body coil 148 and the RF surface coils 149 can be configured to operate in transmit and receive mode, transmit mode, or receive mode.

[0027] A subject or patient 170 for MR scanning can be positioned within the cylindrical imaging volume 146 of the resonant assembly 140. The transceiver 135 in the system controller 130 generates RF excitation pulses that are amplified by the RF amplifier 162 and provided to the RF body coil 148 through a transmit / receive switch (T / R switch) 164.

[0028] As described above, the RF body coil 148 and the RF surface coil 149 can be used to transmit RF excitation pulses and / or receive resulting MR signals from a patient undergoing MR scanning. MR signals emitted by excited nuclei within the patient being MR scanned can be sensed and received by the RF body coil 148 or the RF surface coil 149 and transmitted back through the T / R switch 164 to the preamplifier 166. The T / R switch 164 can be controlled by a signal from the sequence pulse generator 133 to electrically connect the RF amplifier 162 to the RF body coil 148 during transmit mode and to connect the preamplifier 166 to the RF body coil 148 during receive mode. The T / R switch 164 can also enable the RF surface coil 149 to be used in transmit mode or receive mode.

[0029] In some embodiments, the MR signals sensed and received by the RF body coil 148 or the RF surface coil 149 and amplified by the preamplifier 166 are stored in the memory 137 as an array of raw k-space data for post-processing. A reconstructed magnetic resonance image can be acquired by transforming / processing this stored raw k-space data.

[0030] In some embodiments, the MR signals sensed and received by the RF body coil 148 or the RF surface coil 149 and amplified by the preamplifier 166 are demodulated, filtered, and digitized in the receive portion of the transceiver 135 and transferred to the memory 137 in the system controller 130. For each image to be reconstructed, this data is rearranged into separate k-space data arrays, and each of these separate k-space data arrays is input to the array processor 139, which is operated to Fourier transform the data into an array of image data.

[0031] The array processor 139 uses a transformation method, most commonly a Fourier transform, to create images from the received MR signals. These images are transferred to the computer system 120 and stored in the memory 126. In response to commands received from the operator workstation 110, the image data can be archived in long term storage, or can be further processed by the image processor 128 and transferred to the operator workstation 110 for presentation on the display 118.

[0032] In various embodiments, the components of computer system 120 and system controller 130 can be implemented on the same computer system or multiple computer systems. It will be appreciated that, Figure 1 The illustrated MR system 100 is for illustration. Suitable MR systems can include more, fewer and / or different components.

[0033] The system controller 130, the image processor 128 can respectively or commonly include a computer processor and a storage medium on which a program of predetermined data processing to be executed by the computer processor is recorded, for example, a program for implementing a scan process (e.g., a scan protocol, an imaging sequence), image reconstruction, image processing, etc. can be stored on the storage medium, for example, a program of a magnetic resonance imaging sequence of an embodiment of the present application and an optimization method of implementing the magnetic resonance imaging sequence of the embodiment of the present application. The above-mentioned storage medium can include, for example, a ROM, a floppy disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, or a non-volatile memory card.

[0034] The above-mentioned "imaging sequence" refers to a combination of pulses with specific amplitudes, widths, directions, and timings applied when performing a magnetic resonance imaging scan, which can generally include, for example, radio frequency pulses and gradient pulses. The radio frequency pulses can include, for example, radio frequency excitation pulses, radio frequency refocusing pulses, inversion recovery pulses, etc. The gradient pulses can include, for example, the above-mentioned gradient pulses for slice selection, gradient pulses for phase encoding, gradient pulses for frequency encoding, gradient pulses for phase offset (dephasing), gradient pulses for dispersion phase (dispersion), etc.

[0035] Generally, a plurality of scan sequences can be pre-set in a magnetic resonance system to enable selection of a sequence that is adapted to a clinical detection requirement, which can include, for example, an imaging site, image quality, etc. For example, an imaging sequence provided by any of the embodiments of the present application can be pre-set to acquire a diagnostic image with reduced blurring or artifacts.

[0036] In existing imaging sequences, a plurality of radio frequency refocusing pulses can be applied after a radio frequency excitation pulse, and a changing phase encoding gradient pulse can be applied between each adjacent two radio frequency refocusing pulses and echo signals can be acquired during this period to fill multiple rows of phase encoding lines in k-space within one repetition time (TR), thus reducing the imaging time. Such an imaging sequence includes a fast spin echo (FSE) sequence, also known as a turbo spin echo (TSE) sequence.

[0037] Magnetic resonance technology utilizes electromagnetic principles for imaging and follows Maxwell's equations as shown below:

[0038]

[0039] wherein, B is the magnetic field strength, μ0 is the magnetic permeability, J is the current density, ε0 is the dielectric constant, E is the electric field strength, t is time, and ρ is the free charge density.

[0040] wherein, since the time-varying electric field and the current density in the human body can be ignored when the human body is imaged by using the magnetic resonance system, the following formula is obtained from formula (1):

[0041]

[0042] Based on the above formula (3), (5), the main components of the magnetic field of the magnetic resonance system are obtained, as shown in the following formula:

[0043]

[0044] wherein, G x , G y , and G z are linear gradient fields applied in the frequency encoding direction, the phase encoding direction, and the slice selection direction, respectively, and x, y, and z represent coordinate values in the frequency encoding direction, the phase encoding direction, and the slice selection direction, respectively.

[0045] It can be seen that the magnetic field of the magnetic resonance system is formed by the static magnetic field B0 and the gradient magnetic field, wherein the gradient magnetic field provides the required linear gradient field, and also includes the accompanying field (or Maxwell field, Maxwell term, etc.), and the main part of the accompanying field is represented by the following formula:

[0046]

[0047] The inventors found that when the gradient amplitude is large after the first radio frequency re-focus pulse (right side), it is difficult to balance the right side Maxwell term, and by applying a balance pulse before the first re-focus pulse, the compensation effect is better.

[0048] The inventors found that when the gradient amplitude is large after the first radio frequency re-focus pulse (right side), it is difficult to balance the right side Maxwell term, and by applying a balance pulse before the first re-focus pulse, the compensation effect is better.

[0049] In practical applications, the gradient pulses of the fast spin echo sequence can be adjusted in pulse shape (e.g. either of the pulse amplitude, pulse duration) to achieve different clinical applications. The inventors further found that, for example, if the gradient pulse applied after the first radio frequency refocusing pulse (e.g. located at the right side of the sequence diagram) generates a too large concomitant field, even if the gradient amplitude before the first radio frequency refocusing pulse is boosted to the maximum amplitude allowed by the system, the concomitant field cannot be completely compensated and thus corresponding artifacts are brought. Further compensation of the concomitant field by prolonging the duration of the balance pulse at this time will cause the echo spacing (ESP) to increase, which can introduce problems of image blurring and scan time prolongation.

[0050] In the fast spin echo sequence, the ESP mentioned above refers to the larger one of the first time interval and the second time interval, wherein the first time interval is twice the time interval between the center of the radio frequency excitation pulse and the center of the first radio frequency refocusing pulse, and the second time interval is the time interval between the center of the first radio frequency refocusing pulse and the center of the second radio frequency refocusing pulse.

[0051] The first embodiment of the present application provides a magnetic resonance imaging sequence, which includes a radio frequency excitation pulse, a first radio frequency refocusing pulse and a second radio frequency refocusing pulse applied in sequence after the radio frequency excitation pulse, a raw gradient pulse, and a first balance pulse. The raw gradient pulse includes a right side raw pulse and a left side raw pulse, the right side raw pulse is applied between the center of the first radio frequency refocusing pulse and the center of the second radio frequency refocusing pulse, the left side raw pulse is applied between the center of the radio frequency excitation pulse and the center of the first radio frequency refocusing pulse, and the left side raw pulse includes a first gradient pulse corresponding to the radio frequency excitation pulse. The first balance pulse is located in a first time period between the end point of the first gradient pulse and the start point of the first radio frequency refocusing pulse, and the first balance pulse includes a positive pulse and a negative pulse located on a first gradient axis.

[0052] In some embodiments, the left side raw pulse includes a second gradient pulse applied on the first gradient axis after the radio frequency excitation pulse, wherein the second gradient pulse forms at least part of the positive pulse or the negative pulse of the first balance pulse.

[0053] In some embodiments, the first balance pulse lasts throughout the first time period.

[0054] In some embodiments, the raw gradient pulse includes a raw gradient pulse arranged on the first gradient axis and a raw gradient pulse arranged on a second gradient axis, wherein the raw gradient pulse of the second gradient axis has a larger load than the raw gradient pulse of the first gradient axis.

[0055] The following will be described in conjunction with Figures 2 to 4A first embodiment of the present application will be described.

[0056] Figure 2 A waveform diagram of a magnetic resonance imaging sequence of one embodiment of the present application is shown, which includes a radio frequency excitation pulse RF1, a first radio frequency refocusing pulse RFR1 and a second radio frequency refocusing pulse RFR2 applied in succession after the radio frequency excitation pulse RF1, a primary gradient pulse and a first balance pulse.

[0057] The radio frequency excitation pulse RF1 is a 90 degree radio frequency pulse. The first radio frequency refocusing pulse RFR1 and the second radio frequency refocusing pulse RFR2 can be 180 degree or less than 180 degree radio frequency pulses, which are used to perform phase refocusing of dephased proton populations and to generate a corresponding echo signal (not shown in the figure) after the end of the refocusing pulses. Although only two radio frequency refocusing pulses are shown in the figure, it is understood by the skilled person that more radio frequency refocusing pulses can be present after the second radio frequency refocusing pulse.

[0058] The primary gradient pulse includes a right side (to the right of the first radio frequency refocusing pulse RFR1) primary pulse and a left side (to the left of the first radio frequency refocusing pulse RFR1) primary pulse, which right side primary pulse is applied between the center of the first radio frequency refocusing pulse RFR1 and the center of the second radio frequency refocusing pulse RFR2. The left side primary pulse is applied between the center of the radio frequency excitation pulse RF1 and the center of the first radio frequency refocusing pulse RFR1. The left side primary pulse includes a first gradient pulse G01 corresponding to the radio frequency excitation pulse RF1, which first gradient pulse G01 can be, for example, a gradient pulse for slice selection, which first gradient pulse G01 is emitted simultaneously with the radio frequency excitation pulse RF1 (e.g. the end of the plateau of G01 is the same as the end of RF1). The first balance pulse is located in a first time period T1 between the end of the first gradient pulse G01 and the start of the first radio frequency refocusing pulse RFR1.

[0059] A right side Maxwell term Bm1 is generated between the first radio frequency refocusing pulse RFR1 and the second radio frequency refocusing pulse RFR2, which is half of the Maxwell term generated by the right side primary pulse (an echo signal is generated in the middle of the first and second radio frequency refocusing pulses, the first half of the right side primary pulse generates a phase accumulation error for the echo signal), i.e. a concomitant field is generated to the right (or after) of the first radio frequency refocusing pulse RFR1. In particular, the right side primary pulse can include a gradient pulse G1 arranged on a first gradient axis Gz and a gradient pulse G2 arranged on a second gradient axis. The right side Maxwell term Bm1 is half of the sum of a right side first Maxwell term generated by the gradient pulse G1 and a right side second Maxwell term generated by the gradient pulse G2.

[0060] The left original pulse generates a left Maxwell term Bm0, and when the right Maxwell term Bm1 is greater than the left Maxwell term Bm0, a corresponding artifact can occur due to a first difference between the two.

[0061] The first balance pulse is located in a first time period, for example, time period T1, between the end of the first gradient pulse G01 and the start of the first radio frequency refocusing pulse RFR1, and the first balance gradient includes a positive pulse Gz1 and a negative pulse Gz2.

[0062] By setting the first balance pulse, the left Maxwell term Bm0 is increased to eliminate or reduce the first difference, and thus to eliminate or reduce the corresponding artifact.

[0063] As shown in Figure 2 The left original pulse can also include a second gradient pulse G02 applied on the first gradient axis Gz after the radio frequency excitation pulse RF1, which forms at least a part of the positive pulse Gz1 or the negative pulse Gz2 of the first balance pulse. The second gradient pulse G02 as an original pulse, its original main function can not be to compensate for the accompanying field, but has other functions, however, it plays a certain compensation function because it contributes a part of the initial left Maxwell term Bm0. After determining the first balance pulse, the second gradient pulse G02 can be taken as a whole as a part of the first balance pulse (for example, increasing the width and / or amplitude of the second gradient pulse G02 to form the positive pulse Gz1 of the first balance pulse).

[0064] When the first balance gradient is set on the first gradient axis, the first gradient axis can be fully loaded in the time period T1 (all time periods of T1 have pulses set on the first gradient axis, which can include the second gradient pulse G02 and the first balance pulse). For example, in the time period T1, the blank time period outside the time period occupied by the left original pulse can be used to set additional first balance pulses (as shown in the shaded part with slashes in Figure 2 ).

[0065] As shown in Figure 2 The second gradient axis Gx can also have original gradient pulses, for example, the third gradient pulse G03 which will be described below.

[0066] The first balancing pulse is used to generate a first compensating Maxwell term Bm2 as a compensating field to compensate for the difference between the right-side Maxwell term Bm1 and the current left-side Maxwell term Bm0. In some embodiments, there can have been a preceding gradient pulse between the radio frequency excitation pulse RF1 and the first radio frequency refocusing pulse RFR1 to generate an initial left-side Maxwell term, in which case Bm0 is greater than 0. By the first balancing pulse, the compensating field is generated in advance of the first radio frequency refocusing pulse RFR1, and with the inversion effect of the radio frequency refocusing pulse, the compensating field compensates for the concomitant field generated after the first radio frequency refocusing pulse RFR1 to eliminate image artifacts that can be caused by the concomitant field. Also, the first balancing pulse includes a positive pulse Gz1 and a negative pulse Gz2 on the first gradient axis, and compared to applying a balancing pulse in only a single direction, applying the positive pulse and the negative pulse can generate a larger Maxwell quadratic term to compensate for more (or larger) concomitant field within a limited echo spacing and system parameter limitations (e.g., maximum gradient pulse amplitude that the magnetic resonance system can apply).

[0067] In some embodiments, such as in an ideal case, the difference between the right-side Maxwell term Bm1 and the left-side Maxwell term Bm0 can be completely compensated for by the first compensating Maxwell term Bm2. This allows the balancing pulse to be applied on only a single gradient axis to compensate for the concomitant field. For example, without increasing the echo spacing ESP of the FSE sequence, the balancing pulse is applied on only the first gradient axis Gz to generate the compensating field. This can be achieved, for example, by applying gradient pulses in both positive and negative directions (e.g., the positive pulse Gz1 and the negative pulse Gz2). In some embodiments, when the difference between the right-side Maxwell term Bm1 and the left-side Maxwell term Bm0 is small, or when the first time period T1 is long enough, the first compensating Maxwell term Bm2 generated by the first balancing pulse can also be small accordingly, in which case the amplitude of the first balancing pulse can be small, such as less than the maximum amplitude allowed by the magnetic resonance system, or at least one of the positive pulse and the negative pulse of the first balancing pulse can be less than the maximum amplitude.

[0068] In some embodiments, the first balancing pulse is sustained throughout the first time period T1, i.e., the waveforms of the positive pulse Gz1 and the negative pulse Gz2 of the first balancing pulse occupy (or share, cover) the entire T1. This allows a larger compensating field to be provided to compensate for the concomitant field to a larger extent, which can reduce artifacts, or, in the case of a limited ESP (or a limited time period T1), to provide more margin for other gradient axes to design the waveforms of the balancing gradients on the other gradient axes to generate a compensating field comparable to the concomitant field together with the balancing gradients of the other gradient axes as much as possible.

[0069] When the positive pulse Gz1 and the negative pulse Gz2 (the first balance pulse is full load) with the maximum amplitude are continuously provided in the whole T1, the maximum first compensation Maxwell term Bm2 is provided, which compensates the accompanying field in the corresponding first gradient axis Gz to the maximum. However, as mentioned above, the amplitude of the positive pulse Gz1 and the negative pulse Gz2 can not be the maximum amplitude. When occupying the whole time period T1 and emitting the maximum amplitude to make the left Maxwell term greater than the right Maxwell term, the amplitudes of the positive pulse Gz1 and the negative pulse Gz2 can be proportionally reduced to make the difference between the two Maxwell terms within ± preset value.

[0070] As mentioned above, the original gradient pulse includes the original gradient pulse provided on the first gradient axis and the original gradient pulse provided on the second gradient axis, and in the embodiment of the present application, the load of the original gradient pulse provided on the first gradient axis is less than the load of the original gradient pulse provided on the second gradient axis, so that when the first balance pulse (for example, the first balance pulse has the maximum pulse amplitude and the first balance pulse is continuously provided in the whole time period T1) is provided on the first gradient axis, the excessive balance gradient provided on the second gradient axis can be avoided, thereby avoiding further increasing the burden of the second gradient axis. Therefore, the first gradient axis mentioned above can be the gradient axis with relatively small burden (the overall amplitude of the gradient pulse on the first gradient axis is small and / or the duration is small), and the second gradient axis can be the gradient axis with relatively large burden.

[0071] In some embodiments, the magnetic resonance imaging sequence further includes a phase encoding gradient axis (not shown in the figure) of the logical axis, which corresponds to the axial direction of the physical axis of the magnetic resonance system. For example, the first gradient axis can be the slice selection gradient axis of the logical axis, and the second gradient axis can be the frequency encoding gradient axis of the logical axis. In the embodiment of the present application, the phase encoding direction (or the phase encoding gradient axis) of the logical axis corresponds to the z-axis direction of the physical axis of the magnetic resonance system, i.e. the axial extension direction of the scan cavity of the magnetic resonance system. In this way, better image quality can be obtained.

[0072] Although Figure 2 In the embodiment of the present application, the slice selection gradient axis Gz is taken as the first gradient axis, and the frequency encoding gradient axis Gx is taken as the second gradient axis. However, when the gradient load of the frequency encoding gradient axis Gx is smaller, the frequency encoding gradient axis Gx can also be taken as the first gradient axis.

[0073] Figure 3 The waveform diagram of the magnetic resonance imaging sequence of another embodiment of the present application is shown, which is similar to Figure 2The waveform of the left side original pulse is similar to that of the right side original pulse, except that the left side original pulse can include a gradient pulse G2 set on the first gradient axis and a gradient pulse G3 set on the second gradient axis. The gradient pulse on the second gradient axis satisfies the condition of flow compensation. Those skilled in the art understand that by setting the gradient pulse satisfying the condition of flow compensation, the flow artifact is eliminated or reduced.

[0074] Figure 3 In the embodiment, the first balance pulse set on the first gradient axis (including a positive pulse Gz1 and a negative pulse Gz2) is sufficient to compensate for the difference between the right side Maxwell term Bm1 and the left side Maxwell term Bm0, so that no gradient pulse is set between the radio frequency excitation pulse RF1 and the first radio frequency refocusing pulse RFR1 on the second gradient axis Gx for additional compensation.

[0075] When the flow compensation condition can be satisfied by setting the right side original pulse on the second gradient axis Gx, the left side original pulse can also not be set on the second gradient axis Gx.

[0076] Figure 4 A waveform diagram of a magnetic resonance imaging sequence of another embodiment of the present application is shown, which is similar to Figure 2 The waveform of the left side original pulse is similar to that of the right side original pulse, except that the left side original pulse can include a gradient pulse G2 set on the first gradient axis and a gradient pulse G3 set on the second gradient axis. The gradient pulse on the second gradient axis satisfies the condition of flow compensation. Those skilled in the art understand that by setting the gradient pulse satisfying the condition of flow compensation, the flow artifact is eliminated or reduced.

[0077] Figure 4 In the embodiment, the first balance pulse set on the first gradient axis (including a positive pulse Gz1 and a negative pulse Gz2) is sufficient to compensate for the difference between the right side Maxwell term Bm1 and the left side Maxwell term Bm0, so that no gradient pulse is set between the radio frequency excitation pulse RF1 and the first radio frequency refocusing pulse RFR1 on the second gradient axis Gx for additional compensation. Figure 4 The first balance pulse in the embodiment includes a positive pulse Gz3 and two negative pulses Gz4, Gz5, wherein the negative pulses Gz4 and Gz5 have the same shape, and the waveform area of the positive pulse Gz3 is equal to the sum of the waveform areas of the two negative pulses Gz4 and Gz5. Under the premise of satisfying the condition of flow compensation, by setting a positive pulse and a negative pulse at the same time, the accompanying field is better compensated. Similar to Figure 2 , Figure 3 The first balance pulse is sufficient to compensate for the difference between the right side Maxwell term Bm1 and the left side Maxwell term Bm0, so that no balance pulse is set between the radio frequency excitation pulse RF1 and the first radio frequency refocusing pulse RFR1 on the second gradient axis Gx for additional compensation, or only an initial third gradient pulse G03 is provided.

[0078] Further, the left side original pulse further includes a fourth gradient pulse applied on the first gradient axis after the radio frequency excitation pulse RF1, for example Figure 4The first balance pulse (including one positive pulse Gz3 and two negative pulses Gz4, Gz5) is located in a third time period T3, which is a time period in the first time period T1 during which the fourth gradient pulse is not applied, for example, the third time period T3 is a blank time period between the start of the first time period T1 and the start of the fourth gradient pulse.

[0079] The second embodiment of the present application provides a magnetic resonance imaging sequence similar to the first embodiment, further comprising a second balance pulse located on the second gradient axis, the second balance pulse is located in a second time period T2, which is located between the end of the radio frequency excitation pulse RF1 (i.e. the end of the plateau of the first gradient pulse G01) and the start of the first radio frequency refocusing pulse RFR1. Those skilled in the art understand that the plateau of the pulse of the magnetic resonance imaging sequence refers to a time period during which the waveform amplitude remains unchanged.

[0080] The second balance pulse includes at least one of a positive pulse and a negative pulse.

[0081] In some embodiments, the third gradient pulse G03 described above forms at least part of the positive pulse or the negative pulse of the second balance pulse (when forming the second balance pulse, the start point of the third gradient pulse G03 can be moved within the second time period T2).

[0082] In some embodiments, the second balance pulse can include one positive pulse and two negative pulses located on both sides of the one positive pulse, and the sum of the waveform areas of the two negative pulses is equal to the waveform area of the one positive pulse.

[0083] The second balance pulse is used to further compensate for the difference between the right side Maxwell term and the left side Maxwell term, so that the absolute value of the difference between the final two side Maxwell terms is less than or equal to a preset value, wherein the left side Maxwell term is the sum of the Maxwell terms generated by all gradient pulses in the time period from the center of the radio frequency excitation pulse RF1 to the center of the first radio frequency refocusing pulse RFR1, including the first compensation Maxwell term generated by the first balance pulse, the second compensation Maxwell term generated by the second balance pulse, and the sum of the Maxwell terms generated by other left side original gradient pulses. The right side Maxwell term is half of the Maxwell term generated by the right side original pulse.

[0084] The following will be described in conjunction with Figures 5 to 7 The second embodiment of the present application is described.

[0085] Figure 5A waveform diagram of a magnetic resonance imaging sequence according to another embodiment of the application is shown, which comprises a radio frequency excitation pulse RF1, a first radio frequency rephasing pulse RFR1 and a second radio frequency rephasing pulse RFR2 applied in succession after the radio frequency excitation pulse RF1, a primary gradient pulse, a first balancing pulse and a second balancing pulse. The primary gradient pulse comprises a left primary pulse and a right primary pulse.

[0086] The primary gradient pulse can comprise a gradient pulse G1 arranged on a first gradient axis and a gradient pulse G2 arranged on a second gradient axis.

[0087] The first balancing pulse is for generating a first compensating Maxwell term Bm2 as a compensating field to compensate for a difference (first difference) between the right Maxwell term Bm1 and a current (or initial) left Maxwell term Bm0. The first balancing pulse comprises a positive pulse Gz1 and a negative pulse Gz2 arranged on the first gradient axis.

[0088] The positive pulse Gz1 and the negative pulse Gz2 are sustained throughout T1 and have a maximum amplitude that the system can allow, which results in providing a maximum first compensating Maxwell term Bm2, providing more margin for the other gradient axes to design the waveform of the balancing gradients on the other gradient axes to produce a compensating field comparable to the accompanying field together with the balancing gradients of the other gradient axes as much as possible.

[0089] Figure 5 The sequence waveform shown is similar to Figure 2 The difference is that a second balancing pulse is included, which can be arranged on the second gradient axis Gx and located within a second time period T2. As Figure 5 The second balancing pulse in this embodiment comprises a positive pulse Gx1 and a negative pulse Gx2 arranged on the second gradient axis Gx, as shown, however, in other embodiments the second balancing pulse can comprise only a positive pulse or a negative pulse. In some embodiments, the positive pulse Gx1 can comprise a third gradient pulse G03 as at least a part of the left primary pulse, i.e. the third gradient pulse G03 can be part of the positive pulse Gx1. The left Maxwell term Bm0 is increased via the first balancing pulse to a new left Maxwell term Bm0', wherein:

[0090] Bm0' = Bm0 + Bm2

[0091] In some embodiments, when the second balancing pulse only includes the positive pulse Gx1, it can be necessary to obtain a sufficient compensation field by increasing the pulse amplitude or the duration (which makes it necessary to superimpose the increased duration to the current ESP, making the ESP larger), which increases the pulse burden of the second gradient axis and can cause image blurring problems. However, it is also possible to generate the required compensation field, for example, the second compensation Maxwell term Bm3, by setting the negative pulse Gx2, without the need to increase the ESP (or a smaller increase in the ESP) or without the need to increase the pulse amplitude (or a smaller increase in the pulse amplitude). Ideally, the difference between the right-side Maxwell term Bm1 and the initial left-side Maxwell term Bm0 can be completely compensated via the first compensation Maxwell term Bm2 and the second compensation Maxwell term Bm3, that is: (Bm1-Bm0) is equal to or approximately equal to (Bm2+Bm3). However, if the above-mentioned difference (Bm1-Bm0) cannot be compensated even if both the first compensation Maxwell term Bm2 and the second compensation Maxwell term Bm3 reach the maximum under the current echo interval, due to the limited time period allowed for adding the balancing pulse, the maximum value of the first compensation Maxwell term Bm2 (for example, setting the full-load first balancing pulse) can be re-determined based on the increased echo interval (correspondingly, the time periods T1 and T2 are also increased), when there is still an unbalanced field, the second compensation Maxwell term (for example, by setting the second balancing pulse) is determined based on the increased echo interval to compensate for the difference in the Maxwell field on both sides of the first radio frequency refocusing pulse, and so on until a suitable balancing pulse (for example, only including the first balancing pulse or further including the second balancing pulse) and echo interval are determined.

[0092] Figure 6 A waveform diagram of a magnetic resonance imaging sequence of another embodiment of the present application is shown, which includes a radio frequency excitation pulse RF1, a first radio frequency refocusing pulse RFR1 and a second radio frequency refocusing pulse RFR2 applied in sequence after the radio frequency excitation pulse RF1, an original gradient pulse and a first balancing pulse.

[0093] The original gradient pulse can include a gradient pulse G3 set on a first gradient axis and a gradient pulse G4 set on a second gradient axis. Among them, the gradient pulse on the second gradient axis satisfies the condition of flow compensation

[0094] The first balancing pulse is used to generate a first compensation Maxwell term Bm2 as a compensation field to compensate for the difference between the right-side Maxwell term Bm1 and the initial left-side Maxwell term Bm0. The first balancing pulse includes a positive pulse Gz1 and a negative pulse Gz2 on the first gradient axis Gz.

[0095] The positive pulse Gz1 and the negative pulse Gz2 are continuous throughout the T1 and have the maximum amplitude that the system can allow, which makes it possible to provide the maximum first compensation Maxwell term Bm2, providing more margin for other gradient axes to design the waveform of the balanced gradient on the other gradient axes to produce as much as possible a compensation field with the balanced gradient of the other gradient axes that is equivalent to the accompanying field.

[0096] The magnetic resonance imaging sequence of the embodiment further comprises a second balancing pulse, which is arranged on the second gradient axis Gx and is located within the second time period T2, for example between the radio frequency excitation pulse RF1 and the first radio frequency refocusing pulse RFR1. The magnetic resonance imaging sequence of the embodiment is similar to the sequence shown in Figure 5 The difference includes that the second balancing pulse is a symmetric pulse, specifically, the symmetric pulse comprises a positive pulse Gx3 and two negative pulses Gx4 and Gx5 symmetrically arranged on both sides of the pulse Gx3, the directions of the positive pulse Gx3 and the negative pulses Gx4 and Gx5 are opposite, and the total area of the positive pulse Gx3 and the two negative pulses Gx4 and Gx5 is 0. The symmetric pulse compensates the accompanying field while satisfying the flow compensation condition for the second gradient axis Gx.

[0097] And, since the second gradient axis Gx has a greater burden relative to the first gradient axis Gz, after the first balancing pulse (comprising the positive pulse Gz1 and the negative pulse Gz2) arranged on the first gradient axis Gz is preliminarily compensated, the second balancing pulse (symmetric pulse) is used for compensation, so that the second balancing pulse does not need to have an excessively long duration or amplitude, avoiding the image problems caused by increasing or excessively increasing the ESP, or the heating problems caused by excessive burden of the gradient axis, thereby avoiding prolonging the repetition time (TR) of the sequence in order to reduce the heating.

[0098] Figure 7 A waveform diagram of a magnetic resonance imaging sequence of another embodiment of the application is shown, which comprises a radio frequency excitation pulse RF1, a first radio frequency refocusing pulse RFR1 and a second radio frequency refocusing pulse RFR2 applied in sequence after the radio frequency excitation pulse RF1, original gradient pulses and a first balancing pulse.

[0099] The gradient pulses can comprise gradient pulses G5 arranged on the first gradient axis Gz and gradient pulses G6 arranged on the second gradient axis. The gradient pulses on the first gradient axis Gz satisfy the flow compensation condition.

[0100] The first balancing pulse is arranged on the first gradient axis Gz and is located within the first time period T1. The first balancing pulse is used to generate the maximum first compensation Maxwell term Bm2 (the maximum value of the first compensation Maxwell term) as a compensation field to compensate for the difference (the first difference) between the right Maxwell term Bm1 and the original left Maxwell term Bm0.

[0101] The first balancing pulse is continuous throughout the T1 of the first gradient axis and has the maximum amplitude that the system can allow, which makes it possible to provide the maximum first compensation Maxwell term Bm2, providing more margin for the other gradient axes to design the waveforms of the balancing gradients on the other gradient axes to produce as much as possible a compensation field with the accompanying field together with the balancing gradients of the other gradient axes.

[0102] The magnetic resonance imaging sequence of the embodiment can further include a second balancing pulse arranged on the second gradient axis Gx within a second time period T2 between the radio frequency excitation pulse RF1 and the first radio frequency refocusing pulse RFR1, which can include at least one of a positive pulse Gx1 and a negative pulse Gx2, for example. The magnetic resonance imaging sequence of the embodiment is similar to the sequence shown in FIG. 1, and the difference includes that the first balancing pulse is a symmetric pulse, specifically, the first balancing pulse includes a third pulse Gz3 and two fourth pulses Gz4, Gz5 symmetrically arranged on both sides of the third pulse Gz3, the directions of the third pulse Gz3 and the fourth pulses Gz4, Gz5 are opposite, the two fourth pulses are of the same shape, and the area sum of the third pulse Gz3 and the fourth pulses Gz4, Gz5 is 0, satisfying the flow compensation condition. Figure 4 、 Figure 6 The sequence shown in FIG. 1 further includes a second balancing gradient arranged on the second gradient axis Gx, which includes the positive pulse Gx1 and the negative pulse Gx2 described above. Due to the limited echo interval, the first balancing pulse can not be able to generate enough left Maxwell field (even if it reaches the maximum value of the first compensation Maxwell field Bm2) due to the limited duration, so after the preliminary compensation by arranging the first balancing pulse (symmetric pulse) on the first gradient axis Gz, the second compensation Maxwell field Bm3 is obtained by the second balancing pulse, avoiding the image problems caused by excessive increase of ESP, or the heating problems caused by excessive burden of the gradient axis, and further avoiding the extension of the repetition time (TR) of the sequence in order to reduce the heating.

[0103] And, Figure 7 The sequence shown in FIG. 1 further includes a second balancing gradient arranged on the second gradient axis Gx, which includes the positive pulse Gx1 and the negative pulse Gx2 described above. Due to the limited echo interval, the first balancing pulse can not be able to generate enough left Maxwell field (even if it reaches the maximum value of the first compensation Maxwell field Bm2) due to the limited duration, so after the preliminary compensation by arranging the first balancing pulse (symmetric pulse) on the first gradient axis Gz, the second compensation Maxwell field Bm3 is obtained by the second balancing pulse, avoiding the image problems caused by excessive increase of ESP, or the heating problems caused by excessive burden of the gradient axis, and further avoiding the extension of the repetition time (TR) of the sequence in order to reduce the heating.

[0104] The first balance pulse in the magnetic resonance imaging sequence of any of the above embodiments can be determined based on the echo interval, for example, based on the current echo interval, the maximum compensation Maxwell term that can be generated by the first gradient axis can be calculated, that is, the maximum value of the first compensation Maxwell term described above, and the difference between the right and left side associated fields needs to be compensated, so by obtaining the difference between the maximum value of the first compensation Maxwell term and the associated field difference to be compensated, the corresponding first balance pulse can be set. For example, so that the first balance pulse is full at time period T1, and when the difference is small, the positive pulse and the negative pulse of the first balance pulse are set to have a small amplitude, otherwise, the positive pulse and the negative pulse of the first balance pulse can be made to have the maximum amplitude. Or, when the first gradient axis already has a left original pulse (second gradient pulse G02), and when the difference is small, an additional negative pulse (or a symmetric pulse with a small amplitude) is set, and when the difference is large, the second gradient pulse G02 is adjusted to the maximum amplitude and its width is appropriately increased, and a negative pulse with the maximum amplitude is set (or a symmetric pulse with the maximum amplitude is set).

[0105] The third embodiment of the present application will be described below. Figures 8 to 10 The third embodiment of the present application will be described below.

[0106] The third embodiment of the present application provides an optimization method of a magnetic resonance imaging sequence, based on which the magnetic resonance imaging sequence of any of the above embodiments can be obtained. The magnetic resonance imaging sequence includes a radio frequency excitation pulse RF1, a first radio frequency refocusing pulse RFR1 and a second radio frequency refocusing pulse RFR2 applied in sequence after the radio frequency excitation pulse RF1, and an original gradient pulse. The radio frequency excitation pulse RF1, the first radio frequency refocusing pulse RFR1 and the second radio frequency refocusing pulse RFR2, and the original gradient pulse can be similar to the corresponding pulses in Figures 2 to 7

[0107] Figure 8 A flowchart of one embodiment of the optimization method is shown, which includes a first step 801, a second step 803 and a third step 804. In the first step 801, a right Maxwell term Bm1 generated by a right original pulse and a left Maxwell term Bm0 generated by a left original pulse are determined. The right or left Maxwell term can be determined by the simplified formula of formula (7), for example, by the following formula (8).

[0108]

[0109] ​In some embodiments, the right-side Maxwell term Bm1 can be determined to be the minimum Maxwell term after appropriate adjustment of the original pulse between the center of the first radio frequency refocusing pulse RFR1 and the center of the second radio frequency refocusing pulse RFR2, which adjustment can include, for example, reducing the gradient pulse amplitudes as much as possible while still meeting the clinical imaging requirements. The left-side Maxwell term Bm0 determined initially (from the left-side original pulse) can be the initial left-side Maxwell term resulting from any initial left-side gradient pulses (including, for example, a portion of the first gradient pulse G01, the second gradient pulse G02, the third gradient pulse G03, a portion of the fourth gradient pulse, etc.) between the radio frequency excitation pulse RF1 center and the first radio frequency refocusing pulse RFR1 center.

[0110] In the second step 803, in response to the right-side Maxwell term Bm1 being greater than the left-side Maxwell term (Bm0) and the first difference (Bm1-Bm0) between the right-side Maxwell term Bm1 and the left-side Maxwell term (Bm0) being greater than a preset value (e.g., 0 or a positive number close to 0), the maximum value (Bm2 max ) of the first compensation Maxwell term Bm2 that can be generated on the first gradient axis Gz is determined based on the current echo spacing ESP of the magnetic resonance imaging sequence. Specifically, when the echo spacing is determined, the first time period T1 is determined, assuming that a left-side gradient pulse is set on the first gradient axis Gz to completely cover the first time period T1 and has a maximum amplitude (the maximum transmit amplitude allowed by the system), and the maximum value (Bm2 max

[0111] When the difference between the right-side Maxwell term Bm1 and the left-side Maxwell term Bm0 is less than or equal to the preset value, the optimization method can be ended, for example, without setting an additional balancing gradient, or the initial left-side gradient pulse can be maintained or adjusted to balance between the duration and the amplitude (e.g., to reduce the Maxwell term by reducing the amplitude and increasing the duration, as allowed by the current ESP).

[0112] In the third step 805, a first balancing pulse is determined to be set on the first gradient axis Gz to increase the left-side Maxwell term Bm0, for example, the first balancing pulse generates the first compensation Maxwell term Bm2 to increase the left-side Maxwell term: Bm0' = Bm0 + Bm2, where the first balancing pulse is located at or within the first time period T1, and the first balancing pulse includes a positive pulse and a negative pulse, where the maximum value (Bm2 max ​The amplitude of the first balance pulse is smaller than the maximum amplitude if the first difference (Bm1-Bm0) is greater than the first difference (Bm1-Bm0). The amplitude of the first balance pulse is equal to the maximum amplitude if the first difference (Bm1-Bm0) between the maximum values is equal to the first difference (Bm1-Bm0). Specifically, when the flow compensation condition does not need to be met, the first balance pulse can include one positive pulse and one negative pulse (Gz1 and Gz2), while when the first gradient axis Gz needs to meet the flow compensation condition, the first balance pulse can include a symmetric pulse, for example, including the third pulse Gz3 and the two fourth pulses Gz4, Gz5 described above.

[0113] Further, in response to the maximum value Bm2 max The amplitude of the first balance pulse is equal to the maximum amplitude if the first difference (Bm1-Bm0) is less than the first difference (Bm1-Bm0). By setting the first balance pulse with the maximum amplitude, the maximum first compensation Maxwell term is obtained, i.e. the maximum value Bm2 max The difference between the two side Maxwell terms can be maximized to reduce artifacts caused by the accompanying field.

[0114] Since the maximum value Bm2 max The first balance pulse with the maximum amplitude cannot completely compensate for the difference even if the first difference (Bm1-Bm0) is less than the first difference (Bm1-Bm0), so further compensation can be made by setting the second balance pulse, which will be described below in conjunction with Figure 9 、 Figure 10 .

[0115] In some embodiments, the original gradient pulse can include an original gradient pulse set on the first gradient axis Gz and an original gradient pulse on the second gradient axis. The second gradient axis Gx has a heavier pulse burden than the first gradient axis Gz, and most of the right side Maxwell term Bm1 is brought by the original gradient pulse on the second gradient axis.

[0116] Therefore, the first balance pulse that brings the maximum first compensation Maxwell term Bm2 is set on the first gradient axis Gz, avoiding the second gradient axis from increasing more burden, thereby avoiding the need to reduce the risk of overheating by prolonging the repetition time TR (e.g. increasing the waiting time with pulses on the original TR) when the gradient axis generates too much heat.

[0117] Figure 9 A flowchart showing another embodiment of the optimization method is shown, which includes a first step 801, a second step 803 and a third step 805, Figure 9 The method shown is similar to the method of Figure 8 The difference includes that when the maximum value Bm2 maxThe first balance pulse has a maximum amplitude and thus generates the maximum value Bm2 of the first compensating Maxwell term when the first difference (Bm1-Bm0) is less than the second difference (Bm2-Bm0) max The method further comprises a fourth step 901 and a fifth step 903. In the fourth step 901, the first difference is updated based on the increased left-side Maxwell term, for example, the maximum value Bm2 of the newly generated first compensating Maxwell term max is added to the initial left-side Maxwell term Bm0 to obtain the updated left-side Maxwell term Bm0', i.e.,

[0118] Bm0' = Bm2 max + Bm0

[0119] The first difference is updated to (Bm1-Bm0').

[0120] It is noted that when the first balance gradient comprises original pulses (e.g. the second gradient pulse G02), the newly generated left-side Maxwell term Bm2, or the maximum value Bm2 max , only comprises the left-side Maxwell term generated by the newly added first balance gradient, but not the left-side Maxwell term generated by the original pulses. For example, if the positive pulse Gz1 (with the maximum amplitude) is entirely the originally existing second gradient pulse G02, which generates the initial left-side Maxwell term Bmz1, the positive pulse Gz1 is optimized as the first balance gradient in its entirety, and a new negative pulse Gz2 is added, which generates a new left-side Maxwell term Bmz2, then:

[0121] Bm2 max = Bmz2

[0122] For another example, if the positive pulse Gz1 comprises the second gradient pulse G02, which generates the initial left-side Maxwell term Bmz0, the positive pulse of the other part (e.g. denoted as Gz1 new ) generates a new left-side Maxwell term Bmz1 new , the positive pulse Gz1 is optimized and a new negative pulse Gz2 is added, which generates a new left-side Maxwell term Bmz2, then:

[0123] Bm2 max = Bmz1 new + Bmz2

[0124] The above examples also apply to the case when the second balance gradient also comprises original pulses, which will not be repeated here.

[0125] In the fifth step 903, a second balancing pulse is determined based on the updated first difference (Bm1-Bm0') and the current echo interval to further increase the left Maxwell term. For example, the second balancing pulse generates a second compensation Maxwell term Bm3, specifically, the second balancing pulse is set on the second gradient axis Gx and located within a second time period T2. As described in the above embodiments, the second balancing pulse can include at least one of a positive pulse and a negative pulse, and can also include a symmetric pulse when the flow compensation condition is required to be met on the second gradient axis Gx.

[0126] The second balancing pulse can not be full loaded, for example, less than the maximum amplitude allowed by the system, and only lasts for part of the second time period T2, instead of occupying the entire second time period T2. However, when the second compensation Maxwell term Bm3 cannot completely compensate for the updated first difference (Bm1-Bm0'), it can be necessary to increase the amplitude or width (duration) of the second balancing gradient.

[0127] If the maximum second compensation Maxwell term Bm3 generated by setting a full loaded second balancing pulse still cannot completely compensate for the above updated first difference (Bm1-Bm0'), it is necessary to adjust the current ESP to obtain a larger compensation space, which can be described below in combination with Figure 10 .

[0128] Figure 10 A flow chart showing another embodiment of the optimization method is shown, which includes the first step 801, the second step 803, the third step 805, the fourth step 901 and the fifth step 903. Figure 10 The method shown is similar to the method of Figure 9 , the difference including further comprising a sixth step 905 and a seventh step 907. In the sixth step 905, the first difference is updated based on the left Maxwell term increased in the fifth step, specifically, the left Maxwell term already updated via the first balancing pulse is further updated based on the left second Maxwell term Bm3, for example, represented by the following formula:

[0129] Bm0' = Bm0 + Bm2 max + Bm3

[0130] The first difference is: (Bm1-Bm0').

[0131] In the seventh step 907, in response to the first difference between the right Maxwell term and the left Maxwell term Bm0' updated via the second balance pulse still being greater than the preset value, the echo spacing ESP is increased and the first step 801 is returned until the first difference is not greater than the preset value. The optimization method of the magnetic resonance imaging sequence of the present embodiment can be iteratively repeated until the difference between the two side Maxwell fields is completely compensated, and on this basis, a smaller echo spacing (or a smaller degree of increase in the current echo spacing) is determined to avoid image problems caused by too large echo spacing.

[0132] Embodiments of the present application can also provide a magnetic resonance system comprising a scanner and a processor, wherein the processor performs the optimization method of the magnetic resonance imaging sequence of any of the above embodiments or controls the scanner to perform the magnetic resonance imaging sequence of any of the above embodiments. The magnetic resonance system of the present embodiment can comprise Figure 1 The processor in the magnetic resonance system shown can be provided in the computer system 120 or the system controller 130, for example.

[0133] In addition to any modifications indicated previously, those skilled in the art can design many other variations and alternative arrangements without departing from the spirit and scope of the present description, and the appended claims are intended to cover such modifications and arrangements. Therefore, although the information has been described above in detail and specifically with reference to the presently preferred and most advantageous aspects, it will be apparent to one of ordinary skill in the art that many modifications, including but not limited to form, function, operation, and use, can be made without departing from the principles and concepts set forth herein. Also, as used herein, the examples and embodiments are intended to be illustrative only, and should not be interpreted in any way as limiting.

[0134] The purpose of providing the above specific embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive, but the present application is not limited to these specific embodiments. Those skilled in the art should understand that various modifications, equivalent replacements, and changes, etc. can also be made to the present application, as long as these changes do not violate the spirit of the present application, and should be within the protection scope of the present application.

Claims

1. A magnetic resonance imaging sequence comprising: a radio frequency excitation pulse; a first radio frequency rephasing pulse and a second radio frequency rephasing pulse applied sequentially after the radio frequency excitation pulse; a primary gradient pulse, wherein the primary gradient pulse comprises a right side primary pulse and a left side primary pulse, the right side primary pulse is applied between a center of the first radio frequency rephasing pulse and a center of the second radio frequency rephasing pulse, the left side primary pulse is applied between a center of the radio frequency excitation pulse and the center of the first radio frequency rephasing pulse, the left side primary pulse comprises a first gradient pulse corresponding to the radio frequency excitation pulse; and, a first balance pulse, the first balance pulse is located in a first time period between an end of the first gradient pulse and a start of the first radio frequency rephasing pulse, and the first balance pulse comprises a positive pulse and a negative pulse located on a first gradient axis.

2. The magnetic resonance imaging sequence of claim 1, wherein, the left side primary pulse comprises a second gradient pulse applied on the first gradient axis after the radio frequency excitation pulse, wherein the second gradient pulse forms at least a portion of the positive pulse or the negative pulse of the first balance pulse.

3. The magnetic resonance imaging sequence of claim 2, wherein, the first balance pulse is continuous throughout the first time period.

4. The magnetic resonance imaging sequence of claim 1, wherein, at least one of the positive pulse and the negative pulse of the first balance pulse has a maximum pulse amplitude allowed to be emitted by a magnetic resonance system.

5. The magnetic resonance imaging sequence of claim 1, wherein, the pulse amplitudes of the positive pulse and the negative pulse of the first balance pulse are both less than the maximum pulse amplitude allowed to be emitted by the magnetic resonance system.

6. The magnetic resonance imaging sequence of claim 1, wherein, the primary gradient pulse comprises a primary gradient pulse disposed on the first gradient axis and a primary gradient pulse disposed on a second gradient axis, a load of the primary gradient pulse on the second gradient axis is greater than a load of the primary gradient pulse on the first gradient axis.

7. The magnetic resonance imaging sequence of claim 6, wherein, the magnetic resonance imaging sequence further comprises a second balance pulse disposed on the second gradient axis, the second balance pulse is located in a second time period, the second time period is located between an end of the radio frequency excitation pulse and the start of the first radio frequency rephasing pulse.

8. The magnetic resonance imaging sequence of claim 7, the second balance pulse comprises at least one of a positive pulse and a negative pulse.

9. The magnetic resonance imaging sequence of claim 8, wherein, the left side primary pulse comprises: a third gradient pulse applied on the second gradient axis within the second time period, wherein the third gradient pulse forms at least a portion of the positive pulse or the negative pulse of the second balance pulse.

10. The magnetic resonance imaging sequence of claim 1, wherein, the first balance pulse comprises one positive pulse and two negative pulses located on both sides of the one positive pulse, and a sum of waveform areas of the two negative pulses is equal to a waveform area of the one positive pulse.

11. The magnetic resonance imaging sequence of claim 10, wherein, the left side primary pulse comprises a fourth gradient pulse applied on the first gradient axis after the radio frequency excitation pulse, wherein the first balance pulse is located in a third time period, the third time period is a time period within the first time period during which the fourth gradient pulse is not applied.

12. The magnetic resonance imaging sequence of claim 7, wherein, the second balance pulse comprises one positive pulse and two negative pulses located on both sides of the one positive pulse, and a sum of waveform areas of the two negative pulses is equal to a waveform area of the one positive pulse.

13. The magnetic resonance imaging sequence of claim 6, wherein, The first gradient axis is a slice selection gradient axis on a logical axis, and the second gradient axis is a frequency encoding gradient axis on the logical axis.

14. The magnetic resonance imaging sequence of claim 13, wherein, A phase encoding gradient axis on a logical axis is further included, which corresponds to an axial direction of a physical axis of the magnetic resonance system.

15. The magnetic resonance imaging sequence of claim 1, comprising a fast spin echo sequence.

16. A method of optimizing a magnetic resonance imaging sequence, the magnetic resonance imaging sequence comprising: a radio frequency excitation pulse, a first radio frequency refocusing pulse and a second radio frequency refocusing pulse applied sequentially after the radio frequency excitation pulse, and a raw gradient pulse, the raw gradient pulse comprising a right raw pulse and a left raw pulse, the right raw pulse being applied between a center of the first radio frequency refocusing pulse and a center of the second radio frequency refocusing pulse, the left raw pulse being applied between a center of the radio frequency excitation pulse and the center of the first radio frequency refocusing pulse, the left raw pulse comprising a first gradient pulse corresponding to the radio frequency excitation pulse, the method comprising: a first step of determining a right Maxwell term generated by the right raw pulse and a left Maxwell term generated by the left raw pulse; a second step of, in response to the right Maxwell term being greater than the left Maxwell term and a first difference between the right Maxwell term and the left Maxwell term being greater than a preset value, determining a maximum value of a first compensation Maxwell term that can be generated on a first gradient axis based on a current echo interval of the magnetic resonance imaging sequence; a third step of determining a first balancing pulse disposed on the first gradient axis to increase the left Maxwell term, and, in response to the maximum value being greater than the first difference, an amplitude of the first balancing pulse being less than a maximum amplitude; in response to the maximum value being equal to the first difference, the amplitude of the first balancing pulse being equal to the maximum amplitude, wherein the first balancing pulse is located in a first time period between an end of the first gradient pulse and a start of the first radio frequency refocusing pulse, and the first balancing pulse comprises a positive pulse and a negative pulse.

17. The method of claim 16, wherein, in response to the maximum value being less than the first difference, the amplitude of the first balancing pulse being equal to the maximum amplitude, and the method further comprising: a fourth step of updating the first difference based on the increased left Maxwell term; and a fifth step of determining a second balancing pulse to further increase the left Maxwell term based on the updated first difference and the current echo interval, wherein the second balancing pulse is disposed on a second gradient axis and located between an end of the radio frequency excitation pulse and the start of the first radio frequency refocusing pulse.

18. The method of claim 17, wherein, Further comprising: a sixth step of updating the first difference based on the increased left Maxwell term in the fifth step; a seventh step of, in response to the right Maxwell term being greater than the left Maxwell term and the first difference being greater than the preset value, increasing the echo interval and returning to the first step.

19. The method of claim 16, wherein, The original gradient pulses include original gradient pulses disposed on the first gradient axis and original gradient pulses on a second gradient axis, the original gradient pulses of the second gradient axis having a greater load than the original gradient pulses of the first gradient axis.

20. A magnetic resonance system comprising: a scanner; and, a processor for performing an optimization method of a magnetic resonance imaging sequence according to any one of claims 16 to 19 or for controlling the scanner to perform a magnetic resonance imaging sequence according to any one of claims 1 to 15.

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