Magnetic resonance imaging system, main magnetic field correction method, and storage medium thereof

Abstract

Embodiments of the present invention provide a magnetic resonance imaging system, a main magnetic field correction method, and a storage medium therefor. Specifically, a magnetic resonance imaging system, a main magnetic field correction method for the magnetic resonance imaging system, and a computer-readable storage medium are provided. The method includes: acquiring an estimated image of a phantom based on a first imaging sequence, the first imaging sequence having a variable resonant frequency; pre-correcting the main magnetic field based on the estimated image; acquiring a scanned image of the phantom based on the pre-corrected main magnetic field; and determining whether the quality of the scanned image is within an acceptable range. If not, returning to the step of acquiring the estimated image.

Description

Technical Field

[0001] Embodiments disclosed in the present invention relate to medical imaging technology, and more particularly to a magnetic resonance imaging system, a main magnetic field correction method for a magnetic resonance imaging system, and a computer-readable storage medium. Background Art

[0002] Magnetic resonance imaging (MRI) is a medical imaging modality that can obtain images of the human body without using X-rays or other ionizing radiation.

[0003] MRI uses a magnet with a strong magnetic field to generate a static magnetic field (or main magnetic field) B0. When the part of the human body to be imaged is positioned in the static magnetic field B0, the nuclear spins associated with the hydrogen nuclei in the human tissue become polarized, causing the tissue in the part to be imaged to produce a longitudinal magnetization vector on a macroscopic scale and be in a state of equilibrium. When a radio frequency field B1 that intersects with the direction of the static magnetic field B0 is applied, the direction of proton rotation changes, the longitudinal magnetization vector decays, and the tissue in the part to be imaged produces a transverse magnetization vector on a macroscopic scale. After the radio frequency field B1 is removed, the longitudinal magnetization intensity gradually returns to the equilibrium state, and the transverse magnetization vector decays in a spiral shape until it returns to zero. During the decay of the transverse magnetization vector, a free induction decay signal is generated. This free induction decay signal can be collected as a magnetic resonance signal, and based on the collected signal, the tissue image of the part to be imaged can be reconstructed.

[0004] To ensure the quality of magnetic resonance imaging, the main magnetic field in the examination space (eg, the scanning cavity defined by the main magnet) must at least have good uniformity. This requires uniformity correction of the main magnetic field, also known as shimming.

[0005] A traditional calibration method involves measuring the magnetic field strength within the scanning cavity using magnetometers. This requires mounting multiple magnetometers on a bracket or similar device, installing the bracket within the scanning cavity, and then manually rotating the bracket to move the magnetometers to the desired location within the scanning volume to measure the magnetic field strength. Based on the measured magnetic field strength, shimming is then performed, for example, by adding shims to the main magnet to adjust the magnetic field strength at specific locations.

[0006] This type of measurement equipment is expensive, complex, difficult to operate, and time-consuming. Furthermore, particularly in remote areas, multiple medical institutions often share a single measurement device, which can be expensive to transport, prone to damage during transportation, and increases communication and waiting time between institutions.

[0007] The correction data of the main magnetic field can also be obtained by a phantom-based imaging method. However, when the main magnetic field has poor uniformity, the uniformity correction is very complicated. Summary of the Invention

[0008] In one aspect, the present invention provides a main magnetic field correction method for a magnetic resonance imaging system, comprising:

[0009] acquiring an estimated image of the phantom based on a first imaging sequence having a variable resonant frequency;

[0010] pre-correcting the main magnetic field based on the estimated image;

[0011] acquiring a scanning image of the phantom based on the pre-calibrated main magnetic field; and

[0012] Determine whether the quality of the scanned image is within an acceptable range; if not, return to the step of obtaining the estimated image of the phantom.

[0013] On the other hand, the step of obtaining the estimated image includes:

[0014] imaging the phantom based on a plurality of radio frequency excitation pulses to obtain a plurality of images to be synthesized, wherein the plurality of radio frequency excitation pulses have different frequencies; and

[0015] The plurality of images to be synthesized are synthesized to generate the estimated image.

[0016] On the other hand, the first imaging sequence is a MAVRIC sequence.

[0017] On the other hand, the scan image is acquired based on the pre-corrected main magnetic field and a second imaging sequence.

[0018] On the other hand, the step of determining whether the quality of the scanned image is within an acceptable range includes comparing the scanned image with a pre-stored standard image to determine whether the degree of deformation of the phantom in the scanned image is within an acceptable range.

[0019] On the other hand, if the quality of the scanned image is within an acceptable range, the pre-corrected main magnetic field is precisely shimmed.

[0020] On the other hand, the precise shimming includes:

[0021] Acquiring a first shim image and a second shim image of the phantom based on a third imaging sequence, wherein the first shim image and the second shim image have a phase offset therebetween; and

[0022] Shimming correction is performed on the pre-corrected main magnetic field based on a phase difference between the first shim image and the second shim image.

[0023] On the other hand, the third imaging sequence includes a radio frequency excitation pulse, a radio frequency refocusing pulse, a slice selection gradient pulse, a phase encoding pulse, a first frequency encoding pulse and a second frequency encoding pulse, wherein the first frequency encoding pulse and the second frequency encoding pulse are adjacent and have a preset interval time, wherein the first frequency encoding pulse is used to generate first echo data to generate the first uniform field image, and the second frequency encoding pulse is used to generate second echo data to generate the second uniform field image.

[0024] Another aspect of the present invention provides a main magnetic field correction method for a magnetic resonance imaging system, comprising:

[0025] performing a first imaging sequence with a variable resonant frequency on a phantom to acquire a set of images of the phantom, and performing a synthesis process on the set of images to generate an estimated image of the phantom;

[0026] pre-correcting the main magnetic field based on the estimated image;

[0027] Scanning the phantom based on the pre-calibrated main magnetic field to acquire a scan image;

[0028] determining whether the degree of deformation of the phantom in the scanned image is lower than a preset threshold;

[0029] If not, the multiple resonance frequencies of the first imaging sequence are adjusted and the process returns to the step of performing the first imaging sequence on the phantom; if yes, the pre-calibrated main magnetic field is precisely shimmed.

[0030] Another aspect of the present invention provides a computer-readable storage medium, wherein the computer-readable storage medium includes a stored computer program, wherein when the computer program is executed, any of the above methods is executed.

[0031] Another aspect of the present invention provides a magnetic resonance imaging system, comprising:

[0032] a scanner for scanning an object to obtain image data of the object, wherein the object includes a phantom;

[0033] an image reconstructor for reconstructing an image of the object based on the image data;

[0034] an image analysis processor for acquiring correction data of the main magnetic field and image quality judgment data based on the image of the object; and

[0035] A controller is used to control the scanner, the image reconstructor and the image analysis processor to execute any one of the above methods.

[0036] It should be understood that the above brief description is provided to introduce some concepts further described in the detailed description in a simplified form. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is solely defined by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages mentioned above or in any section of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The present invention will be better understood from the following description of non-limiting embodiments with reference to the accompanying drawings, in which:

[0038] Figure 1 FIG2 shows a schematic structural diagram of a magnetic resonance imaging system according to an embodiment of the present invention;

[0039] Figure 2 A flow chart of a main magnetic field correction method for a magnetic resonance imaging system according to an embodiment of the present invention is shown;

[0040] Figure 3 An example of a phantom image with large deformation generated based on a main magnetic field with poor uniformity is shown;

[0041] Figure 4 An example of a scanned image of a phantom generated based on a pre-calibrated main magnetic field according to an embodiment of the present invention is shown;

[0042] Figure 5 An example of a standard image of a phantom is shown;

[0043] Figure 6 A flow chart of a main magnetic field correction method according to another embodiment of the present invention is shown;

[0044] Figure 7 shows an example of a third imaging sequence according to an embodiment of the present invention;

[0045] Figure 8 A flow chart of a main magnetic field correction method according to another embodiment of the present invention is shown;

[0046] Figure 9 FIG. 4 is a flow chart showing a magnetic resonance imaging system according to another embodiment of the present invention. DETAILED DESCRIPTION

[0047] Figure 1A schematic diagram of the structure of a magnetic resonance imaging system according to one embodiment of the present invention is shown. The magnetic resonance imaging system includes a scanner 110, an image reconstructor 120, an image analysis processor 130, and a controller 140. The controller 140 is coupled to the scanner 110 to control the operation of the scanner 110, for example, controlling the scanner 110 to perform a scanning sequence on a subject 16 to acquire image data of the subject 16. The controller 140 is also configured to communicate with the image reconstructor 120 to control the image reconstructor 120 to reconstruct an image of the subject 16 based on the image data acquired by the scanner. The controller 140 is also configured to communicate with the image analysis processor 130 to control the image analysis processor 130 to analyze or process the reconstructed image or raw image data of the subject. Such analysis or processing may include, for example, acquiring correction / shimming data for the main magnetic field based on the image of the subject, and may also include, for example, determining / evaluating the quality of the image of the subject.

[0048] When scanning and imaging the human body, the object 16 may be a human anatomical structure to be diagnosed; when performing main magnetic field evaluation or correction / uniform field analysis, the object 16 may be a phantom. The controller 140 may control the scanner 110 to acquire image data of the phantom, and control the image reconstructor to reconstruct the image of the phantom based on the image data of the phantom. The controller 140 may also control the image analysis processor 130 to evaluate the uniformity of the main magnetic field in the space where the phantom is located based on the image of the phantom, and acquire correction data / uniform field data of the main magnetic field based on the evaluation results.

[0049] Specifically, the controller 140 can send sequence control signals to relevant components of the scanner 110 (such as the radio frequency generator and gradient coil driver described below) through a sequence generator (not shown in the figure), so that the scanner 110 executes a preset imaging sequence. In one embodiment, the imaging sequence may include one or more imaging sequences for main magnetic field uniformity correction. In an embodiment of the present invention, the main magnetic field uniformity correction may include pre-correction and may further include precise correction / field shimming.

[0050] Those skilled in the art will appreciate that the aforementioned "imaging sequence" refers to a combination of pulses with specific power, amplitude, width, direction, and timing applied during an MRI scan. These pulses may typically include, for example, radio frequency pulses and gradient pulses. The radio frequency pulses may include, for example, radio frequency transmit pulses used to excite proton resonance in the human body. The gradient pulses may include, for example, slice selection gradient pulses, phase encoding gradient pulses, and frequency encoding gradient pulses.

[0051] In one example, the scanner 110 may include a main magnet assembly 111 , a bed 112 , a radio frequency generator 113 , a radio frequency transmission coil 114 , a gradient coil driver 115 , a gradient coil assembly 116 , a radio frequency power amplifier 119 , and a data acquisition unit 117 .

[0052] The main magnet assembly 111 generally includes an annular superconducting magnet defined in a housing, and the annular superconducting magnet is installed in an annular vacuum container. The annular superconducting magnet and its housing define a cylindrical space surrounding the object 16, such as Figure 1 The scanning cavity 18 shown is the main magnetic field space. The scanning cavity 18 defines the imaging region, or at least a portion of the imaging region, of the magnetic resonance imaging system. The main magnet assembly 111 generates a constant magnetic field, such as the main magnetic field B0, along the Z direction of the scanning cavity 18. Typically, the more uniform portion of the main magnetic field B0 is formed in the central region of the main magnet.

[0053] The couch 112 is used to carry the subject 16 and, in response to control by the controller 140, moves along the Z direction to enter and exit the scanning cavity 18. For example, in one embodiment, the imaging volume of the subject 16 can be positioned in the central region of the scanning cavity where the magnetic field strength is relatively uniform, so as to facilitate scanning and imaging of the imaging volume of the subject 16. For example, a phantom can be positioned at a suitable position in the main magnetic field space to facilitate measurement of the magnetic field strength in the main magnetic field space.

[0054] The MRI system uses the formed main magnetic field B0 to transmit static magnetic pulse signals to the object 16 (eg, a phantom) placed in the scanning cavity 18 , so as to order the precession of protons in the resonance volume within the object 16 and generate a longitudinal magnetization vector.

[0055] The RF generator 113 and the RF power amplifier 119 can serve as part of an RF transmission chain, wherein the RF generator 113 is used to respond to a control signal from the controller 140 to generate an RF pulse. The RF pulse is typically a small RF signal with relatively low power. The small RF signal can be amplified by the RF power amplifier 119 and then applied to the RF transmitting coil 114.

[0056] The RF transmit coil 114 may be a body coil that can be connected to a transmit / receive (T / R) switch. Controlling the transmit / receive switch allows the RF transmit coil 114 to switch between transmit and receive modes. In transmit mode, the RF transmit coil 114 responds to the RF excitation pulses and transmits a RF field B1 perpendicular to the static magnetic field B0 toward the subject 16, thereby exciting the nuclei within the subject 16 and converting the longitudinal magnetization vector into a transverse magnetization vector. In receive mode, the body coil can receive magnetic resonance signals from the subject 16.

[0057] When the radio frequency excitation pulse ends, a free induction decay signal is generated in the process that the transverse magnetization vector of the object 16 gradually returns to zero.

[0058] The gradient coil driver 115 is configured to respond to a gradient pulse control signal or a shim control signal sent by the controller 140 to provide appropriate current / power to the gradient coil assembly 116 .

[0059] The gradient coil assembly 116 , on the one hand, forms a changing magnetic field in the imaging space to provide three-dimensional position information for the magnetic resonance signal, and on the other hand, is used to generate a compensation magnetic field for the main magnetic field B0 to shim the main magnetic field B0 .

[0060] The gradient coil assembly 116 may include three gradient coils, each configured to generate magnetic field gradients tilted along three mutually perpendicular spatial axes (e.g., the X-axis, the Y-axis, and the Z-axis). More specifically, the gradient coil assembly 116 applies magnetic field gradients in a slice selection direction (e.g., the Z-axis) to select a slice within the imaging volume. Those skilled in the art will appreciate that a slice is any one of a plurality of two-dimensional slices distributed along the Z-axis within the three-dimensional imaging volume. The "Z-axis" typically extends from the head to the feet of the patient when positioned on the bed 112. During imaging, the RF transmit coil 114 transmits RF excitation pulses to the slice within the imaging volume, thereby exciting the slice. The gradient coil assembly 116 applies magnetic field gradients in a phase encoding direction (e.g., the Y-axis) to phase encode the magnetic resonance signals of the excited slice. The gradient coil assembly 116 applies gradient fields in a frequency encoding direction (e.g., the X-axis) of the subject 16 to frequency encode the magnetic resonance signals of the excited slice.

[0061] The data acquisition unit 117 is used to respond to the data acquisition control signal of the controller 140 to acquire the above-mentioned magnetic resonance signals (for example, received by the body coil or the surface coil). In one embodiment, the data acquisition unit 117 may include, for example, a radio frequency preamplifier, a phase detector, and an analog / digital converter, wherein the radio frequency preamplifier is used to amplify the magnetic resonance signal, the phase detector is used to perform phase detection on the amplified magnetic resonance signal, and the analog / digital converter is used to convert the phase-detected magnetic resonance signal from an analog signal to a digital signal.

[0062] The digitized magnetic resonance signals may be received as raw image data by the image reconstructor 120, which may reconstruct a two-dimensional slice image of the object 16 based on the digitized magnetic resonance signals. Specifically, the image reconstructor 120 may communicate with the controller 140 to perform the above image reconstruction.

[0063] The image analysis processor 130 can perform any necessary image analysis and processing on the reconstructed image or raw image data. For example, image analysis can be used to determine image quality, determine the uniformity of the main magnetic field, and further obtain correction data for the main magnetic field. Another example is image processing to improve image contrast, uniformity, clarity, brightness, etc. Specifically, the image analysis processor 130 can perform the above-mentioned image analysis and processing based on communication with the controller 140.

[0064] The magnetic resonance imaging system may further include a display 150 , which may be used to display an operation interface and various data or images generated during data processing.

[0065] The magnetic resonance imaging system further includes a console 160, which may include user input devices such as a keyboard and a mouse. The controller 140 may generate control signals in response to control commands generated by a user via the console 160, an operation panel / buttons provided on the main magnet housing, and some automatic detection results to control the scanner 110, the image reconstructor 120, the image analysis processor 130, the display 150, etc. to perform corresponding operations.

[0066] In one embodiment, the controller 140, the image reconstructor 120, and the image analysis processor 130 may each or jointly include a computer and a storage medium, on which a predetermined control program and data processing program to be executed by the computer are recorded. For example, the storage medium may store programs for implementing imaging scanning, image reconstruction, image processing, etc., for example, it may store programs for implementing the main magnetic field correction method according to an embodiment of the present invention. The storage medium may 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.

[0067] The above magnetic resonance imaging system is described as only one example. In other embodiments, the device may have various modified forms.

[0068] Based on the above description, in the process of imaging the object, ensuring that the main magnetic field is sufficiently uniform helps to reduce artifacts and obtain higher image quality. Therefore, the main magnetic field can usually be calibrated through the imaging data of the phantom. When the main magnetic field itself has good uniformity, the deformation of the generated phantom image will be small, and it is also easier to perform main magnetic field correction based on such an image. However, when the uniformity of the main magnetic field is poor, the deformation of the phantom in the generated image is too large (such as Figure 2 As shown in FIG, it is difficult to determine the source of the inhomogeneity (for example, which positions of the main magnetic field are inhomogeneous). In this case, a large number of complex calculations or attempts may be required to obtain more accurate main magnetic field correction data.

[0069] Figure 3 A flow chart 300 of a main magnetic field calibration method according to an embodiment of the present invention is shown.

[0070] In step S310, an estimated image of the phantom is acquired based on a first imaging sequence with a variable resonant frequency. The phrase "with a variable resonant frequency" may mean that the first imaging sequence includes a plurality of sequentially applied radio frequency transmit pulses, wherein the frequencies of the plurality of radio frequency transmit pulses differ. Because the phantom is excited by transmit pulses with different resonant frequencies, multiple sets of corresponding image data are generated. Each set of image data may better represent some parts of the phantom (where the resonant frequency of the main magnetic field is closer to the corresponding resonant frequency) while poorly representing other parts (where the resonant frequency of the main magnetic field is more deviated from the resonant frequency).

[0071] Therefore, the image data generated at these multiple resonant frequencies can be processed to pre-estimate the complete image data of the phantom. This estimation differs from the scanned image obtained by image reconstruction based on a single complete scan data in that such an estimation may differ in various aspects due to different image processing algorithms. In embodiments of the present invention, the estimated image can be generated by synthesizing the multiple sets of images (or image data). This synthesis process can include synthesizing different portions (e.g., portions with better imaging quality) from the multiple sets of image data, and further performing an amplitude-weighted interpolation algorithm between two adjacent sets of data.

[0072] In one embodiment, step S310 may include: imaging the phantom based on multiple radio frequency excitation pulses to obtain multiple images to be synthesized, and synthesizing the multiple images to be synthesized to generate the estimated image. The multiple radio frequency excitation pulses have different frequencies.

[0073] As an example, the first imaging sequence may be MAVRIC (Multi Acquisition with Variable Resonance Image Combination). Those skilled in the art will appreciate that this sequence can be used to image human tissue with implants to avoid metal artifacts in human tissue images. However, in embodiments of the present invention, this sequence is used to generate phantom images, which are then further utilized to rapidly calibrate the main magnetic field, as described below. Furthermore, in step S310, other imaging sequences with variable resonant frequencies may also be used to generate multiple images corresponding to different resonant frequencies.

[0074] In step S310, the scanner 110 may be configured to execute the first imaging sequence on the phantom to obtain multiple sets of image data of the phantom. The image reconstructor 120 may then reconstruct multiple sets of images based on the multiple sets of image data. The image analysis processor 130 may then synthesize the multiple sets of images to generate the estimated image. In other embodiments, the image reconstructor 120 may reconstruct the multiple sets of images and perform image synthesis processing. The image analysis processor 130 may also synthesize and reconstruct the estimated image directly based on the multiple sets of image data. Alternatively, the image analysis processor 130 may synthesize the multiple sets of image data, and then the image reconstructor 120 may reconstruct the estimated image based on the synthesized image data.

[0075] By adopting multiple resonant frequencies, as much image data as possible under different main magnetic field strengths is obtained. The image data under different field strengths are then synthesized. In the synthesized image, the phantom can maintain its basic shape without excessive deformation. Even if the uniformity of the main magnetic field itself is poor, the generation of images with excessive distortion due to the poor uniformity can be avoided, which facilitates further main magnetic field correction.

[0076] In step S320, the main magnetic field is pre-calibrated based on the estimated image. Since the deformation of the phantom in the estimated image may exist, but the degree of deformation is small, the pre-calibration can be performed by relying on a traditional image-based magnetic field correction method. For example, the pre-calibration may include: obtaining main magnetic field correction data based on the estimated image, such as an adjusted main magnetic field map. The main magnetic field correction data can be used to guide the operator to perform main magnetic field pre-calibration, for example, as one of the methods, adding or reducing shims at appropriate locations. In this step, the frequency distribution of the estimated image can be analyzed by the image analysis processor 130 to obtain the pre-calibration data of the main magnetic field.

[0077] In step S330, a scanned image of the phantom based on the pre-corrected main magnetic field is obtained. In step S340, it is determined whether the quality of the scanned image is within an acceptable range. If so, the pre-correction is ended. If not, the process returns to step S310 until the quality of the scanned image obtained based on the current main magnetic field is within an acceptable range.

[0078] In step S330, the corresponding components of the scanner 110 may be caused to execute a second imaging sequence to generate image data of the phantom, and the image processor 120 may reconstruct a scanned image of the phantom based on the image data. The second imaging sequence may include a single-frequency radio frequency excitation pulse. As just one example, the second imaging sequence may be a spin echo (SE) sequence. Those skilled in the art will appreciate that other imaging sequences may also be used to scan the phantom in the pre-calibrated main magnetic field to obtain a scanned image thereof.

[0079] In step S340, the image analysis processor 130 may determine whether the image quality of the scanned image of the phantom is within an acceptable range. Figure 4 ) and pre-stored standard images (such as Figure 5 The standard image is compared with the phantom in the scanned image (as shown) to determine whether the degree of deformation of the phantom in the scanned image is within an acceptable range, or whether the degree of deformation of the phantom in the scanned image is below a preset threshold. If the shape similarity of the phantom in the two images is high and the degree of deformation of the phantom is below the preset threshold, the quality of the scanned image is considered acceptable and the main magnetic field is relatively uniform after pre-calibration, and pre-calibration can be terminated. Conversely, if there is a significant difference in the shape of the phantom in the two images, the quality of the scanned image is considered unacceptable and further pre-calibration of the main magnetic field is required. The standard image can be stored in a memory, and the memory can be integrated with the scanner, image reconstructor 120, image analysis processor 130, or controller 140.

[0080] In one embodiment, a suitable frequency variation range and frequency value can be set for multiple RF transmit pulses of the first imaging sequence. For example, the frequency of a set of RF excitation pulses of the first imaging sequence can be 63.75 MHz, 63.77 MHz, 63.79 MHz, 63.81 MHz, 63.83 MHz, 63.85 MHz, 63.87 MHz, 63.89 MHz, 63.91 MHz, 63.93 MHz, 64.95 MHz, 63.97 MHz, or 63.81 MHz, 63.82 MHz, 63.83 MHz, 63.84 MHz, 63.85 MHz, 63.86 MHz, 63.87 MHz, 63.88 MHz, 63.89 MHz, 63.90 MHz, 64.91 MHz, or 63.92 MHz. By setting a suitable set of resonant frequencies, more accurate B0 field data can be obtained with a limited number of RF transmit pulses, thereby improving the accuracy of pre-correction.

[0081] Figure 6 A flowchart 600 showing a main magnetic field calibration method according to another embodiment of the present invention may include: Figure 3 The steps shown are as follows. Furthermore, in step S650, after the pre-calibration is completed, that is, if the quality of the scanned image of the object is within an acceptable range, the pre-calibrated main magnetic field is precisely shimmed. Because magnetic field correction is performed based on an estimated image during pre-calibration, the accuracy of the obtained main magnetic field correction data may be insufficient due to the presence of estimated data rather than actual scanned data in the estimated image. Therefore, performing precise shimming after pre-calibration can further avoid such problems.

[0082] The above-mentioned precise shimming step may specifically include: acquiring a first shimming image and a second shimming image of a phantom based on a third imaging sequence, wherein the first shimming image and the second shimming image have a phase offset; and performing shimming correction on the pre-corrected main magnetic field based on the phase difference between the first shimming image and the second shimming image.

[0083] Specifically, the third imaging sequence includes a radio frequency excitation pulse, a radio frequency refocusing pulse, a slice selection gradient pulse, a phase encoding pulse, a first frequency encoding pulse, and a second frequency encoding pulse, wherein the first frequency encoding pulse and the second frequency encoding pulse are adjacent and have a preset interval time, wherein the first frequency encoding pulse is used to generate first echo data to generate a first shim image, and the second frequency encoding pulse is used to generate second echo data to generate a second shim image. After the first frequency encoding pulse ends, the inhomogeneity of the main magnetic field will cause different degrees of phase shift of the protons, and such phase difference will be reflected in the corresponding image. Therefore, a second shim image can be generated by a second encoding pulse with a preset delay compared to the first frequency encoding pulse, and the main magnetic field map is calculated based on the phase difference between the two shim images to generate shim data.

[0084] Figure 7 An example of a third imaging sequence is shown. It should be understood that the illustrated sequence is only a portion of a sequence for performing shim scanning. For example, only a sequence within one repetition time (TR) is shown. Those skilled in the art will understand that the repetition time refers to the time interval between two adjacent radio frequency transmission pulses of the imaging sequence. Figure 7 The sequence shown may also contain other pulses, which may be located between any two adjacent pulses. The examples of the present invention are only for illustration. Figure 7 The timing relationship between the pulses in FIG. 1 is defined as follows, but the timing relationship between these pulses and other pulses not shown is not defined.

[0085] like Figure 7As shown, within each repetition time of the third imaging sequence, two echoes E1 and E2 are continuously acquired to obtain first image data and second image data, respectively. The term "continuously" may refer to the fact that no other sequence pulses are executed during the sequential execution of the two echoes. As an example, the third imaging sequence includes a radio frequency transmit pulse 71, which may have a 90-degree flip angle and is used to control the radio frequency transmit coil of the magnetic resonance imaging system to transmit a radio frequency signal to the patient. The radio frequency signal has a preset resonant frequency to excite the resonance of protons in the tissue of interest. Following the radio frequency transmit pulse 71, a radio frequency refocusing pulse 72 is also included, which is used to phase-refocus the transverse magnetization vector generated after the radio frequency excitation pulse ends. Optionally, the third imaging sequence further includes a slice selection gradient pulse 73 emitted simultaneously with the RF transmit pulse 71 and the refocusing pulse 72, which is used to control the Z-direction gradient coils of the magnetic resonance imaging system to apply a gradient to the static magnetic field B0, thereby providing position information in the Z direction for the first image data and the second image data. The slice selection gradient pulse 73 is followed by a phase encoding gradient pulse 74, which is used to control the Y-direction gradient coils to apply a gradient to the static magnetic field B0, thereby providing position information in the Y direction for the first image data and the second image data. The phase encoding gradient pulse 74 is followed by frequency encoding gradient pulses 75 and 76, also known as read gradient pulses, which are used to control the X-direction gradient coils of the magnetic resonance imaging system to apply a gradient to the static magnetic field B0, thereby providing position information in the X direction for the first image data and the second image data, respectively. Accompanying / responding to the frequency encoding gradient pulses 75 and 76, a first echo E1 and a second echo E2 are generated, respectively, i.e., free induction decay signals generated by the spins of excited protons in the human body. A time difference ΔTE is present between the frequency encoding gradient pulses 75 and 76 and their corresponding first echo E1 and second echo E2.

[0086] In an embodiment of the present invention, the second imaging sequence for generating a scanned image of a phantom may be similar to the third imaging sequence, except that each repetition time of the second imaging sequence contains only one frequency encoding gradient pulse and one corresponding echo.

[0087] Figure 8 A flowchart 800 is shown of a main magnetic field calibration method according to another embodiment of the present invention. This embodiment has similar principles and implementations to the above-described embodiments.

[0088] In step S810 , a first imaging sequence with a variable resonance frequency is performed on a phantom to acquire a set of images of the phantom, and a synthesis process is performed on the set of images to generate an estimated image of the phantom;

[0089] In step S820, pre-correction of the main magnetic field is performed based on the estimated image;

[0090] In step S830, the phantom is scanned based on the pre-calibrated main magnetic field to obtain a scan image;

[0091] In step S840, it is determined whether the deformation degree of the phantom in the scan image exceeds a preset threshold. If so, the multiple resonance frequencies of the first imaging sequence are adjusted and the process returns to step S810. If not, the pre-calibrated main magnetic field is precisely shimmed.

[0092] Embodiments of the present invention may also provide a main magnetic field correction device for a magnetic resonance imaging system, a magnetic resonance imaging system, and a computer-readable storage medium. The principles and examples of the main magnetic field correction method of the embodiment of the present invention have been described in detail. The magnetic resonance system, main magnetic field correction device, and computer-readable storage medium of the embodiment of the present invention can be used to execute the method of any of the above embodiments, which has the same inventive concept as the method.

[0093] Figure 9 A block diagram of a magnetic resonance imaging system according to an embodiment of the present invention is shown, wherein the components can be Figure 1 The corresponding components shown have similar structures and working principles. Figure 9 As shown, the system may include a scanner 910 , an image reconstructor 920 , an image analysis processor 930 , and a controller 940 .

[0094] The scanner 910 is configured to scan an object, including a phantom, to obtain image data of the object. For example, the scanner 910 may execute a first imaging sequence to obtain estimated image data of the phantom, execute a second imaging sequence to obtain scanned image data of the phantom, and execute a third imaging sequence to obtain first shim image data and second shim image data of the phantom.

[0095] The image reconstructor 920 is configured to reconstruct a corresponding image based on the image data.

[0096] Image analysis processor 930 is configured to obtain correction data and image quality assessment data for the main magnetic field based on an image of the subject. For example, image analysis processor 930 synthesizes the estimated image data to generate an estimated image, obtains pre-correction data for the main magnetic field based on the estimated image, determines whether the scanned image is acceptable, and obtains accurate shimming data for the main magnetic field based on the phase difference between the first shimmed image and the second shimmed image.

[0097] The controller 940 is used to control the scanner 910 , the image reconstructor 920 and the image analysis processor 930 to execute the magnetic field correction method of any of the above embodiments.

[0098] The computer-readable storage medium of the embodiment of the present invention includes a stored computer program, wherein when the computer program is executed, the magnetic field correction method of any of the above embodiments is executed.

[0099] The magnetic field correction device according to the embodiment of the present invention may include a first control module, a pre-correction module, a second control module, and a judgment module.

[0100] The first control module is used to control the magnetic resonance imaging system to obtain an estimated image of a phantom based on a first imaging sequence with a variable resonant frequency. For example, the first control module controls the magnetic resonance imaging system to execute the first imaging sequence to obtain multiple sets of image data, and reconstructs and synthesizes the multiple sets of image data to obtain the estimated image.

[0101] The pre-correction module is used to pre-correct the main magnetic field of the magnetic resonance imaging system based on the estimated image;

[0102] The third control module is configured to control the magnetic resonance imaging system to acquire a scanned image of the phantom based on the pre-calibrated main magnetic field, for example, by executing a second imaging sequence on the pre-calibrated phantom to acquire scanned data of the phantom, and reconstructing the scanned data to obtain a scanned image.

[0103] The judgment module is used to determine whether the quality of the scanned image is within an acceptable range. If not, the first imaging module is used to control the magnetic resonance imaging system again to obtain the estimated image, the pre-correction module is used to pre-correct the main magnetic field again, and the second control module is used to control the magnetic resonance imaging system again to obtain a scanned image of the phantom.

[0104] Optionally, the first control module includes:

[0105] a second control unit, configured to control the magnetic resonance imaging system to image the phantom based on a plurality of radio frequency excitation pulses to acquire a plurality of images to be synthesized, wherein the plurality of radio frequency excitation pulses have different frequencies; and

[0106] The image synthesis unit is used to perform synthesis processing on the multiple images to be synthesized to generate the estimated image.

[0107] Optionally, the device further includes an adjustment module for adjusting the multiple resonant frequencies of the first imaging sequence when the quality of the scanned image is not within an acceptable range, so that the first control module controls the magnetic resonance imaging system to reacquire an estimated image of the phantom based on the adjusted resonant frequencies.

[0108] Optionally, the second control module is used to control the magnetic resonance imaging system to acquire a scanning image of the phantom based on the pre-corrected main magnetic field and the second imaging sequence.

[0109] Optionally, the device may further include a precise shimming module, which performs precise shimming on the pre-corrected main magnetic field if the quality of the scanned image is within an acceptable range.

[0110] Optionally, the precise shimming module includes:

[0111] a first control unit configured to control the magnetic resonance imaging system to acquire a first shim image and a second shim image of the phantom based on a third imaging sequence, wherein the first shim image and the second shim image have a phase offset; and

[0112] The precise correction unit is configured to perform shimming correction on the pre-corrected main magnetic field based on a phase difference between the first shimming image and the second shimming image.

[0113] Optionally, the third imaging sequence includes a radio frequency excitation pulse, a radio frequency refocusing pulse, a slice selection gradient pulse, a phase encoding pulse, a first frequency encoding pulse and a second frequency encoding pulse, wherein the first frequency encoding pulse and the second frequency encoding pulse are adjacent and have a preset interval time, wherein the first frequency encoding pulse is used to generate first echo data to generate the first uniform field image, and the second frequency encoding pulse is used to generate second echo data to generate the second uniform field image.

[0114] Optionally, the judgment module is configured to compare the scanned image with a pre-stored standard image to determine whether the degree of deformation of the phantom in the scanned image is within an acceptable range.

[0115] Experiments have shown that after the main magnetic field correction is performed based on the embodiment of the present invention, the uniformity of the main magnetic field is very close to that after the main magnetic field correction based on the magnetometer. However, the embodiment of the present invention does not require complex structures and operations. Even for a main magnetic field with very poor uniformity, and through pre-correction, a very non-uniform main magnetic field can be quickly corrected to an acceptable level (for example, making it easier to subsequently use traditional correction methods based on phantom imaging to perform precise field shimming), avoiding the use of complex calculations and making it easier to obtain a uniform main magnetic field.

[0116] As used herein, "modules," "units," "controllers," "processors," and the like may be in the form of software, hardware, or a combination of software and hardware, and may include circuitry configured to perform one or more tasks, functions, or steps discussed herein. The "controller," "control unit," "control module," and "processor" used herein are not intended to necessarily be limited to a single processor or computer. For example, they may include multiple processors, ASICs, FPGAs, and / or computers, which may be integrated into a common housing or unit, or distributed across various units or housings. The depicted "controller," "control unit," "control module," and "processor" include memory. The memory may include one or more computer-readable storage media. For example, the memory may store images (e.g., estimated images, scanned images, standard images, first shim images, second shim images), system information (e.g., main magnetic field correction data), algorithms or processes for executing any of the above-described embodiments, and the like. Furthermore, the process flows and / or flowcharts (or aspects thereof) discussed herein may represent one or more sets of instructions stored in the memory for directing the main magnetic field shimming process.

[0117] As used herein, an element or step recited in the singular and preceded by the word "one" or "an" should be understood as not excluding the plural number of the element or step, unless such exclusion is explicitly stated. In addition, reference to "one embodiment" of the present invention is not intended to be interpreted as excluding the existence of additional embodiments that simultaneously incorporate the described features. Moreover, unless explicitly stated to the contrary, an embodiment "comprising," "including," "having" an element or multiple elements having a specific property may include additional such elements that do not have that property. The terms "including" and "in which" are used as the plain language equivalents of the corresponding terms "comprising" and "wherein." In addition, in the appended claims, the terms "first," "second," and "third," etc., are used only as labels and are not intended to impose numerical requirements or specific positional order on their objects.

[0118] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the relevant art to practice the invention, including making and using any device or system and performing any covered method. The scope of patent protection for the invention is defined by the claims and may include other examples known to those skilled in the art. If they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements that do not differ substantially from the literal language of the claims, then such other examples are intended to fall within the scope of the claims.

Claims

1. A main magnetic field correction method for a magnetic resonance imaging system, comprising: acquiring an estimated image of a phantom based on a first imaging sequence, the first imaging sequence having a variable resonant frequency; pre-correcting the main magnetic field based on the estimated image; acquiring a scanning image of the phantom based on the pre-calibrated main magnetic field; and Determining whether the quality of the scanned image is within an acceptable range, and if not, returning to the step of obtaining the estimated image; The step of obtaining the estimated image includes: imaging the phantom based on a plurality of radio frequency excitation pulses to obtain a plurality of images to be synthesized, wherein the plurality of radio frequency excitation pulses have different frequencies; and The plurality of images to be synthesized are synthesized to generate the estimated image.

2. The method according to claim 1, wherein The first imaging sequence is a MAVRIC sequence.

3. The method according to claim 1, wherein The scan image is acquired based on the pre-corrected main magnetic field and a second imaging sequence.

4. The method according to claim 1, wherein The step of judging whether the quality of the scanned image is within an acceptable range includes comparing the scanned image with a pre-stored standard image to judge whether the degree of deformation of the phantom in the scanned image is within an acceptable range.

5. The method according to claim 1, wherein Also includes: If the quality of the scanned image is within an acceptable range, the pre-corrected main magnetic field is precisely shimmed.

6. The method according to claim 5, wherein: The precise shimming includes: Acquiring a first shim image and a second shim image of the phantom based on a third imaging sequence, wherein the first shim image and the second shim image have a phase offset therebetween; and Shimming correction is performed on the pre-corrected main magnetic field based on a phase difference between the first shim image and the second shim image.

7. The method according to claim 6, wherein: The third imaging sequence includes a radio frequency excitation pulse, a radio frequency refocusing pulse, a slice selection gradient pulse, a phase encoding pulse, a first frequency encoding pulse and a second frequency encoding pulse, wherein the first frequency encoding pulse and the second frequency encoding pulse are adjacent and have a preset interval time, wherein the first frequency encoding pulse is used to generate first echo data to generate the first uniform field image, and the second frequency encoding pulse is used to generate second echo data to generate the second uniform field image.

8. A main magnetic field correction method for a magnetic resonance imaging system, comprising: performing a first imaging sequence having a plurality of radio frequency excitation pulses on a phantom to acquire a set of images of the phantom, and synthesizing the set of images to generate an estimated image of the phantom, wherein the plurality of radio frequency excitation pulses have different frequencies; pre-correcting the main magnetic field based on the estimated image; Scanning the phantom based on the pre-calibrated main magnetic field to acquire a scan image; determining whether the degree of deformation of the phantom in the scanned image is lower than a preset threshold; If not, the frequencies of the multiple excitation pulses of the first imaging sequence are adjusted and the process returns to the step of performing the first imaging sequence on the phantom; if yes, the pre-calibrated main magnetic field is precisely shimmed.

9. A computer-readable storage medium comprising a stored computer program, wherein: When the computer program is executed, the method according to any one of claims 1 to 8 is performed.

10. A magnetic resonance imaging system comprising: a scanner for scanning an object to obtain image data of the object, wherein the object includes a phantom; an image reconstructor for reconstructing an image of the object based on the image data; an image analysis processor for acquiring correction data of the main magnetic field and image quality judgment data based on the image of the object; and A controller for controlling the scanner, the image reconstructor and the image analysis processor to execute the method according to any one of claims 1 to 8.

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