2D echo planar diffusion imaging method, medium and device based on dual-frequency orthogonal gradient

Abstract

The present invention discloses a 2D echo planar diffusion imaging method, medium and device based on dual-frequency orthogonal gradients. The present invention suppresses the partial volume effect caused by cerebrospinal fluid in the ventricular region by applying a pulse gradient orthogonal to the cosine oscillation gradient. The imaging sequence includes the following steps: first, suppressing the fat signal by a fat saturation module; second, using a diffusion coding module, embedding a pair of trapezoidal cosine oscillation gradients into the left and right sides of the refocusing radio frequency pulse, and at the same time applying another pair of pulse gradients of the same length in the orthogonal direction of the oscillation gradient to suppress free water with a higher diffusion rate; finally, using the echo planar imaging coding readout method to collect signals. The diffusion imaging sequence based on dual-frequency orthogonal gradients can effectively realize the accurate measurement of the time-dependent apparent diffusion coefficient under the clinical scanning system, which is conducive to the clinical transformation of microstructure imaging technology based on time-dependent diffusion magnetic resonance.

Description

Technical Field

[0001] The present invention relates to the field of magnetic resonance technology, in particular to the field of diffusion magnetic resonance imaging. Background Art

[0002] Oscillating gradient spin echo diffusion imaging (OGSE) can set gradient magnetic fields with different oscillation frequencies to collect images with different diffusion times (t d ) of water molecules. Tissues have different apparent diffusion coefficients (ADC) at different diffusion times, which is called diffusion time dependence. The diffusion time dependence of tissues can effectively reflect their microstructural characteristics, such as cell size, membrane permeability, surface-to-volume ratio, and other key indicators for the diagnosis of brain tumors and stroke diseases. Since oscillating gradient spin echo diffusion imaging can achieve shorter diffusion times than traditional pulse gradient sequences, it plays an important role in the study of diffusion time dependence.

[0003] In the measurement of diffusion time dependence, a series of oscillating gradients (OG) with different frequencies are required. However, due to the limited gradient strength of current clinical MRI machines, it is impossible to generate an oscillating gradient magnetic field with high b value and high oscillation frequency within a specified time. In addition, the long echo time leads to low signal-to-noise ratio and limited imaging analysis rate. At low b value (<500s / mm 2 ) and low resolution (such as 2*2*5mm), the measured ADC value is easily affected by the partial volume effect of cerebrospinal fluid. Due to the high free water content in cerebrospinal fluid, its own ADC value is significantly higher than that of normal brain tissue; the ADC value of tissue near the sulcus and ventricle will be higher than the actual value due to the influence of cerebrospinal fluid, resulting in distortion of the measurement of tissue diffusion time dependence and microstructural parameters. Therefore, in clinical systems, it is urgent to develop targeted imaging sequences to overcome the influence of the partial volume effect of cerebrospinal fluid. Summary of the invention

[0004] In order to overcome the shortcomings of existing sequences and the problem of distortion in diffusion time-dependent measurement, the present invention proposes a 2D echo planar diffusion imaging sequence (2D DEFOG-EPI) based on dual-frequency orthogonal gradients to solve the problems of the above-mentioned OGSE sequence and improve the clinical practicality of the time-dependent diffusion imaging method.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients, comprising:

[0007] S1: obtaining a first 2D K-space signal by executing an imaging sequence, and reconstructing and generating a first image based on the calibrated first 2D K-space signal;

[0008] The imaging sequence includes a fat saturation module, a diffusion preparation module, and a signal readout module;

[0009] The fat saturation module is located at the beginning of the imaging sequence and is used to suppress chemical shift artifacts;

[0010] The diffusion preparation module is located after the fat saturation module and is used to inhibit free water diffusion. During the execution of the diffusion preparation module, first, at 90° x After the RF pulse slice-selective excitation, the calibration signal is collected and then the orthogonal diffusion gradient is embedded at 90°. x With 180° y Between RF pulses, the orthogonal diffusion gradient is composed of mutually orthogonal trapezoidal cosine oscillation diffusion gradient and pulse diffusion gradient, and finally at 180° y The RF pulse was followed by another application of the same orthogonal diffusion gradient;

[0011] The signal readout module is located after the diffusion preparation module and is used for parallel acquisition of 2D K-space signals;

[0012] S2: removing all trapezoidal cosine oscillation diffusion gradients in the imaging sequence, and then re-executing the imaging sequence after removing the trapezoidal cosine oscillation diffusion gradients to obtain a second 2D K-space signal, and reconstructing and generating a second image based on the calibrated second 2D K-space signal;

[0013] S3: Obtain a diffusion-weighted image by calculating an apparent diffusion coefficient according to the reconstructed first image and the second image.

[0014] Based on this solution, each step can further provide the following preferred implementations. It should be noted that the technical features in each preferred implementation can be combined with each other without conflict. Of course, these preferred implementations can also be implemented by other methods that can achieve the same technical effects, which does not constitute a limitation.

[0015] As a preferred embodiment of the first aspect, the processing flow in the fat saturation module is as follows:

[0016] S111, firstly applying a gradient magnetic field in the X, Y, and Z axes of the magnetic resonance device to eliminate the transverse magnetization vector;

[0017] S112, then applying a radio frequency pulse with a flip angle of 110° to selectively excite protons in the corresponding frequency band of fat, and transforming the magnetization vector of the fat tissue from longitudinal to transverse;

[0018] S113. Finally, a gradient magnetic field having a direction opposite to that of the gradient magnetic field in S111 but equal in magnitude is applied in the three axes of X, Y, and Z, thereby eliminating the transverse magnetization vector of the fat tissue to achieve the effect of suppressing the fat signal.

[0019] As a preferred embodiment of the first aspect, the processing flow in the diffusion preparation module is as follows:

[0020] S121, applying a layer-selective gradient magnetic field along the Z-axis direction, and simultaneously applying a radio frequency pulse with a flip angle of 90° in the X-axis direction, so as to excite an imaging area at a specified position and layer thickness;

[0021] S122, applying a trapezoidal cosine oscillation diffusion gradient and a pulse diffusion gradient along a preset diffusion direction, wherein the magnetic field application directions of the two diffusion gradients are orthogonal to each other and have the same duration;

[0022] S123, at the half echo time point, applying a slice-selective gradient magnetic field identical to that in S121 along the Z-axis direction, and applying a Gaussian radio frequency pulse with a flip angle of 180° along the Y-axis direction, thereby achieving signal convergence;

[0023] S124. Repeatedly apply two diffusion gradients with the same waveform as in S122 in a preset diffusion direction according to a preset diffusion intensity until all preset diffusion direction and intensity combinations are completed.

[0024] As a preferred embodiment of the first aspect, the processing flow in the signal reading module is as follows:

[0025] S131, firstly, applying a gradient magnetic field in the X and Y axes to move the initial position of signal acquisition from the center of K-space to the edge position;

[0026] S132 , then gradually adjust the gradient magnetic fields applied in the X and Y axis directions, scan the signal points in the K-space along the Cartesian coordinate system, and complete the required K-space filling.

[0027] As a preferred embodiment of the first aspect, the image reconstruction process of the first image and the second image is as follows:

[0028] S21, using the Navigator calibration signal collected after 90° radio frequency pulse layer selective excitation, calibrating the phase fluctuation between the odd and even row signals in the 2D K-space signal collected in the signal readout module;

[0029] S22: Based on the 2D K-space signal calibrated by S21, the GRAPPA technology is used to supplement the missing signals in the parallel acquisition process, and finally the image is reconstructed through two-dimensional Fourier transform.

[0030] As a preferred embodiment of the first aspect, in S3, the apparent diffusion coefficient of each pixel in the diffusion weighted image is calculated according to the pixel values ​​at corresponding positions in the first image and the second image, and the calculation formula of the apparent diffusion coefficient ADC is as follows:

[0031] ADC=-log(S DEFOG / S PGSE ) / b

[0032] Where: S DEFOG , S PGSE are the pixel values ​​of the first image and the second image respectively, and b is the diffusion intensity corresponding to the trapezoidal cosine oscillation diffusion gradient.

[0033] In a second aspect, the present invention provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the 2D planar echo diffusion imaging method based on dual-frequency orthogonal gradients as described in any of the schemes in the first aspect can be implemented.

[0034] In a third aspect, the present invention provides a magnetic resonance imaging device, which includes a magnetic resonance scanner and a control unit, wherein a computer program is stored in the control unit, and when the computer program is executed, the 2D planar echo diffusion imaging method based on dual-frequency orthogonal gradients as described in any scheme in the first aspect can be implemented.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The present invention proposes a 2D echo planar diffusion imaging sequence based on dual-frequency orthogonal gradients, and for the first time uses a diffusion preparation module that combines a cosine oscillation diffusion gradient with a pulse gradient. Compared with the traditional 2D OGSE-EPI sequence, the 2D DEFOG-EPI sequence suppresses the influence of cerebrospinal fluid on the ADC value measurement of the area near the ventricles and sulci by adding a pulse gradient orthogonal to the cosine oscillation diffusion gradient. On the other hand, it shortens the overall duration of the sequence scan and improves the image signal-to-noise ratio by using the GRAPPA method. Experiments have shown that the use of the 2D DEFOG-EPI sequence can more accurately measure the diffusion time dependence of the area affected by cerebrospinal fluid. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is the timing diagram of 2D DEFOG-EPI sequence.

[0038] Figure 2 The b0 diagram and the ADC diagram at each oscillation frequency are obtained using the 2D DEFOG-EPI sequence and the 2D DW-EPI sequence respectively.

[0039] Figure 3 The ADC values ​​of the hippocampus were calculated on 6 volunteers using 2D DEFOG-EPI sequence and 2D DW-EPI sequence at different diffusion frequencies. DETAILED DESCRIPTION

[0040] As attached Figure 1 , is a sequence timing diagram of the present invention. The following is a method proposed by the present invention and combined with an embodiment to demonstrate its specific technical effects, so that those skilled in the art can better understand the essence of the present invention.

[0041] In a preferred implementation of the present invention, a 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients is provided, the method comprising the following steps:

[0042] Step 1: Obtain a first 2D K-space signal by executing an imaging sequence, and reconstruct and generate a first image based on the calibrated first 2D K-space signal.

[0043] The imaging sequence in the present invention is a 2D echo-planar diffusion imaging sequence based on dual-frequency orthogonal gradients (hereinafter referred to as 2D DEFOG-EPI sequence). The 2D DEFOG-EPI sequence includes a fat saturation module, a diffusion preparation module and a signal readout module. The timing diagram of the sequence is shown in FIG. Figure 1 As shown, the specific processing flow in the three modules is introduced below:

[0044] 1.1) Fat saturation module

[0045] The fat saturation module is located at the beginning of the imaging sequence to suppress chemical shift artifacts. In this implementation, the processing flow in the fat saturation module is as follows:

[0046] 1.1.1) First, at the beginning of the sequence operation, a gradient magnetic field of a certain intensity is applied simultaneously in the three axial directions of X, Y, and Z to eliminate the magnetization vector remaining in the lateral direction;

[0047] 1.1.2) Then, a Gaussian radio frequency pulse with a flip angle of 110° is applied to selectively excite protons in the corresponding frequency band of fat, flipping a large number of longitudinal magnetization vectors in the fat tissue to transverse. After waiting for a short period of time, a small amount of residual magnetization vectors in the fat tissue are further converted from longitudinal to transverse.

[0048] 1.1.3) Finally, a gradient magnetic field of the same magnitude but opposite in direction to the gradient magnetic field applied in 1.1.1) is applied in the three axes of X, Y, and Z, thereby eliminating the transverse magnetization vector of the fat tissue and achieving the effect of suppressing the fat signal.

[0049] 1.2) Diffusion preparation module

[0050] After the fat saturation module, the diffusion preparation stage is entered. Therefore, the corresponding diffusion preparation module is located after the fat saturation module. During the execution of the diffusion preparation module, the first step is to x After the RF pulse slice-selective excitation, the calibration signal is collected and then the orthogonal diffusion gradient is embedded at 90°. x With 180° y Between RF pulses (i.e., embedded 90° x -180° y In the RF pulse sequence), the orthogonal diffusion gradient is composed of mutually orthogonal trapezoidal cosine oscillation diffusion gradient and pulse diffusion gradient, and finally at 180° y The same orthogonal diffusion gradient is performed again after the RF pulse. In this implementation, the processing flow in the diffusion preparation module is as follows:

[0051] 1.2.1) First, a layer-selective gradient magnetic field is applied along the Z-axis direction, and a Gaussian radio frequency pulse with a flip angle of 90° is applied in the X-axis direction in conjunction with the layer-selective gradient magnetic field in the Z-axis direction, so as to excite the imaging area of ​​the specified position and layer thickness;

[0052] 1.2.2) Then, a trapezoidal cosine oscillation diffusion gradient of a certain frequency and a pulse diffusion gradient are applied along a preset diffusion direction, wherein the magnetic field application directions of the two diffusion gradients are orthogonal to each other and have the same duration;

[0053] 1.2.3) Then, at half the echo time (TE / 2), a slice-selective gradient magnetic field identical to the slice-selective gradient magnetic field applied in 1.2.1) is applied along the Z-axis direction, and a Gaussian radio frequency pulse with a flip angle of 180° is applied along the Y-axis direction, thereby achieving signal refocusing;

[0054] 1.2.4) Finally, in the preset diffusion direction, two diffusion gradients with the same two diffusion gradient waveforms (including amplitude, shape, duration, etc.) as those applied in 1.2.2) are repeatedly applied according to the preset diffusion intensity until all preset diffusion direction and intensity combinations are completed.

[0055] In this implementation, the duration of a single trapezoidal cosine oscillation gradient is δ, the interval time of the oscillation gradient before and after the 180° focusing pulse is Δ, and the oscillation gradient application direction B1 and the pulse gradient application direction B2 are as follows:

[0056]

[0057] 3) Signal readout module

[0058] After diffusion preparation, the signal can be read out in 2D-EPI mode through the signal readout module. Therefore, the signal readout module is located after the diffusion preparation module and is the last module of the 2D DEFOG-EPI sequence, which is used to collect 2D K-space signals in parallel. In this implementation, the processing flow in the signal readout module is as follows:

[0059] 1.3.1) 2D-EPI encoding acts in the X and Y directions: First, a gradient magnetic field of a certain strength is applied in the X and Y axial directions to move the initial position of signal acquisition from the center of K-space to the edge position;

[0060] 1.3.2) Then gradually adjust the gradient magnetic field applied in the X and Y axes, scan the signal points in the K-space along the Cartesian coordinate system, and complete the required K-space filling. In specific implementation, the signal acquisition work of each area in the K-space can be completed by slightly adjusting the phase encoding gradient in the Y direction and greatly adjusting the frequency encoding gradient in the X direction.

[0061] In addition, in this implementation, in order to save sequence scanning time, parallel imaging can be used along the Y-axis direction to speed up the acquisition speed. The time interval from EPI encoding to the center of K-space to the 180° focusing pulse is half the echo time (TE / 2).

[0062] For the target object, the above 2D DEFOG-EPI sequence is completely executed once to obtain a set of 2D K-space signals, which are recorded as the first 2D K-space signals.

[0063] The first 2D K-space signal can be reconstructed into a first image after calibration, and the reconstruction process can be implemented by existing technologies. In the present invention, an image reconstruction module can be set to implement the calibration and reconstruction functions, which is specifically described as follows:

[0064] 4) Image reconstruction module: After acquiring the K-space signal, image reconstruction can be performed in the image reconstruction module. During image reconstruction, the first 2D K-space signal needs to be calibrated using the calibration signal acquired during the execution of the 2D DEFOG-EPI sequence, and then the calibrated signal is reconstructed into the first image. The calibration signal can be a Navigator calibration signal acquired after 90° RF pulse layer selective excitation, or a Navigator calibration signal acquired in the signal readout module, and there is no restriction on this. In this implementation, the processing flow in the image reconstruction module is as follows:

[0065] 1.4.1) Firstly, the phase deviation between the K-space odd-numbered line and the even-numbered line signals in the first 2D K-space signal is calibrated according to the Navigator acquired after the 90° RF pulse slice selective excitation;

[0066] 1.4.2) Secondly, since parallel acquisition acceleration is used in the signal readout module, some missing signals will appear in the K-space. Therefore, for the first 2D K-space signal calibrated in 1.4.1), the GRAPPA technology needs to be used to calculate and complete the missing sampling point signals in the parallel acquisition process in the K-space, so as to obtain a complete K-space signal; finally, the complete K-space signal is subjected to a two-dimensional Fourier transform to complete the image reconstruction, and the dual-frequency orthogonal gradient diffusion image reconstructed here is recorded as the first image I DEFOG .

[0067] The parallel imaging technology of GRAPPA in the present invention can effectively shorten the sequential scanning time and obtain high-quality images with a higher signal-to-noise ratio.

[0068] In the present invention, the first image I is obtained DEFOG Finally, the apparent diffusion coefficient needs to be calculated to obtain the diffusion-weighted image. However, the way the 2D DEFOG-EPI sequence calculates the apparent diffusion coefficient weighted image is different from the traditional sequence. It not only requires dual-frequency orthogonal gradient diffusion images, but also requires the acquisition of images of the control group.

[0069] Step 2: remove all trapezoidal cosine oscillation diffusion gradients in the 2D DEFOG-EPI sequence, then re-execute the imaging sequence after removing the trapezoidal cosine oscillation diffusion gradients, obtain a second 2D K-space signal, and reconstruct and generate a second image based on the calibrated second 2D K-space signal.

[0070] It should be noted that after all the trapezoidal cosine oscillation diffusion gradients in the 2D DEFOG-EPI sequence are removed here, the orthogonal diffusion gradients in the diffusion preparation module of the imaging sequence only include the pulse diffusion gradients. In actual implementation, the trapezoidal cosine oscillation diffusion gradients can be removed by setting the amplitudes of all the trapezoidal cosine oscillation diffusion gradients in the 2D DEFOG-EPI sequence to 0. Of course, it can also be achieved by setting another imaging sequence, which is not limited to this. The 2D K-space signal obtained after the imaging sequence after removing the trapezoidal cosine oscillation diffusion gradient is executed, that is, the second 2D K-space signal.

[0071] Similarly, the second 2D K-space signal can also be reconstructed into a second image after calibration, and the reconstruction process can be implemented by existing technologies. In the present invention, the aforementioned image reconstruction module can be used to implement the calibration and reconstruction functions, that is, in the image reconstruction module, the second 2D K-space signal is calibrated using the Navigator collected after 90° RF pulse layer selective excitation, and then the calibrated signal is reconstructed into a second image.

[0072] It should be noted that the processing flow in the image reconstruction module for reconstructing the second image here is essentially the same as the flow for reconstructing the first image in step 1. The only difference is that the calibration signal is replaced by the Navigator calibration signal collected in the imaging sequence after the trapezoidal cosine oscillation diffusion gradient is removed in step 2, and the first 2D K-space signal is replaced by the second 2D K-space signal collected in step 2. The specific processes are as follows:

[0073] 2.1) First, the phase deviation between the K-space odd-numbered line and even-numbered line signals in the second 2D K-space signal is calibrated according to the Navigator calibration signal collected after the 90° RF pulse layer selective excitation;

[0074] 2.2) For the second 2D K-space signal calibrated in 2.1), the GRAPPA technology is used to complete the missing sampling point signals in the parallel acquisition process in the K-space through calculation, so as to obtain a complete K-space signal; finally, the complete K-space signal is subjected to a two-dimensional Fourier transform to complete the image reconstruction, and the control group image reconstructed here is recorded as the second image I PGSE .

[0075] Step 3: Obtain a diffusion-weighted image by calculating the apparent diffusion coefficient based on the reconstructed first image and the second image. In this implementation, the apparent diffusion coefficient of each pixel in the diffusion-weighted image is calculated based on the pixel value of the corresponding position in the first image and the second image, and the calculation formula of the apparent diffusion coefficient ADC is as follows:

[0076] ADC=-log(S DEFOG / S PGSE ) / b

[0077] Where: S DEFOG , S PGSE are the pixel values ​​of the first image and the second image respectively, and b is the diffusion intensity corresponding to the trapezoidal cosine oscillation diffusion gradient in the 2D DEFOG-EPI sequence.

[0078] It should be noted that the apparent diffusion coefficient ADC in the above diffusion weighted image is calculated in pixels, that is, the pixel values ​​at the corresponding positions in the first image and the second image are calculated by the above calculation formula to obtain the ADC value of the position, and then written into the diffusion weighted image. Of course, in actual calculation, the ADC value calculation of all pixels in the diffusion weighted image can be realized in batches through the overall image calculation.

[0079] The following is based on the 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients shown in steps 1 to 3 in the above preferred implementation method, and applies it to a specific embodiment to demonstrate its technical effect so that those skilled in the art can better understand the essence of the present invention.

[0080] Example

[0081] The 2D-EPI diffusion imaging sequence method based on dual-frequency orthogonal gradients shown in steps 1 to 3 above was tested on 6 healthy young volunteers. The specific parameters are introduced below: the magnetic resonance scanning equipment used in the test was a Siemens Prisma 3T scanner (maximum gradient of 80mT / m, maximum switching rate of 200mT / m), and the scanning coil used was a 64-channel head coil.

[0082] Experiment: To compare the performance of the 2D DEFOG-EPI sequence of the present invention and the conventional 2D echo planar diffusion imaging sequence (2DDW-EPI sequence) in measuring the diffusion time dependence of the hippocampus region, four groups of experiments with different diffusion times were designed. The pulse diffusion gradient was used at 0 Hz, and the oscillating diffusion gradient was used for scanning at 20 Hz, 40 Hz, and 60 Hz, and the b value was 400 s / mm. 2 The 2D DEFOG-EPI sequence is based on the 2D DW-EPI sequence and has an additional diffusion gradient direction that is orthogonal, has an equal duration, and a b value of 400 s / mm. 2 The other imaging parameters were the same: 6 diffusion gradient directions, 2 repeated scans, FOV = 220 × 200 mm, and image resolution of 2 × 2 mm 2 , slice thickness is 4 mm, interval coefficient is 50%, inter-slice scanning mode is alternating scanning, repetition time (TR) is 5100 ms, echo time is 1300 ms. The readout parameters are: twice accelerated GRAPPA factor, with 24 reference phase lines, partial Fourier factor = 75%, bandwidth = 1976 Hz, scanning time = 87 s.

[0083] Attached Figure 2 The b0 and ADC images obtained using the 2D DEFOG-EPI sequence and the 2D DW-EPI sequence are shown. The hippocampus region is outlined on the b0 image.

[0084] Attached Figure 3 The ADC values ​​of the hippocampus measured at different frequencies (60Hz, 40Hz, 20Hz, 0Hz) using the 2D DEFOG-EPI sequence and the 2D DW-EPI sequence are shown. In the 2D DEFOG-EPI sequence, it can be clearly seen that the ADC value of the hippocampus increases with the increase of diffusion frequency, while the 2D DW-EPI sequence has no diffusion time-dependent effect.

[0085] In addition, based on the same inventive concept, in other embodiments, a computer-readable storage medium may be provided, wherein a computer program is stored on the storage medium. When the computer program is executed by a processor, the computer program is used to implement the 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients as shown in steps 1 to 3 above.

[0086] It should be noted that the imaging sequence, image reconstruction module and calculation program of the apparent diffusion coefficient designed in steps 1 to 3 can all be stored in a storage medium in the form of a corresponding software program. The above-mentioned storage medium may include a random access memory (RAM) or a non-volatile memory (NVM), such as at least one disk storage. The above-mentioned processor may be a general-purpose processor, including a central processing unit (CPU), a network processor (NP), etc.; it may also be a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components. Of course, the device should also have the necessary components to realize the operation of the program, such as a power supply, a communication bus, etc. In addition, with the widespread application of cloud servers, the above-mentioned software program can also be carried on the cloud platform to provide corresponding services, so the computer-readable storage medium is not limited to the form of local hardware.

[0087] It should be noted that when the above-mentioned computer program is executed by the processor, the process of executing the imaging sequence and obtaining the first 2D K-space signal and the second 2D K-space signal actually requires the external magnetic resonance scanner to provide hardware support, that is, when the computer program is executed by the processor, it will generate corresponding instructions for controlling the magnetic resonance scanner, and then control the magnetic resonance scanner to obtain the corresponding 2D K-space signal.

[0088] Therefore, based on the same inventive concept, in other embodiments, a magnetic resonance imaging device can also be provided, which includes a magnetic resonance scanner and a control unit, wherein a computer program is stored in the control unit, and when the computer program is executed, the magnetic resonance scanner can be controlled to thereby implement the 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients as shown in steps 1 to 3 above.

[0089] The hardware structure of the magnetic resonance scanner and control unit mentioned above can be implemented by a conventional magnetic resonance imaging system. The magnetic resonance scanner should include a magnet part and a magnetic resonance spectrometer part. The control unit should contain the necessary data processing and image reconstruction software for magnetic resonance imaging. The computer program can be read by the control unit and used to control the magnetic resonance scanner. The control unit can be a separate unit, or the control unit integrated in the magnetic resonance scanner itself can be directly used, so that the magnetic resonance scanner can directly output the diffusion weighted image results without adding an additional control unit.

[0090] It should be noted that the above-described embodiment is only a preferred solution of the present invention, but it is not intended to limit the present invention. A person skilled in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent conversion falls within the protection scope of the present invention.

Claims

1. A 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients, characterized in that: include: S1: obtaining a first 2D K-space signal by executing an imaging sequence, and reconstructing and generating a first image based on the calibrated first 2D K-space signal; The imaging sequence includes a fat saturation module, a diffusion preparation module, and a signal readout module; The fat saturation module is located at the beginning of the imaging sequence and is used to suppress chemical shift artifacts; The diffusion preparation module is located after the fat saturation module and is used to inhibit free water diffusion. During the execution of the diffusion preparation module, first, at 90° x After the RF pulse slice-selective excitation, the calibration signal is collected and then the orthogonal diffusion gradient is embedded at 90°. x With 180° y Between RF pulses, the orthogonal diffusion gradient is composed of mutually orthogonal trapezoidal cosine oscillation diffusion gradient and pulse diffusion gradient, and finally at 180° y The RF pulse was followed by another application of the same orthogonal diffusion gradient; The signal readout module is located after the diffusion preparation module and is used for parallel acquisition of 2D K-space signals; S2: removing all trapezoidal cosine oscillation diffusion gradients in the imaging sequence, and then re-executing the imaging sequence after removing the trapezoidal cosine oscillation diffusion gradients to obtain a second 2D K-space signal, and reconstructing and generating a second image based on the calibrated second 2D K-space signal; S3: Obtain a diffusion-weighted image by calculating an apparent diffusion coefficient according to the reconstructed first image and the second image.

2. The 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients according to claim 1, characterized in that: The processing flow in the fat saturation module is as follows: S111, firstly applying a gradient magnetic field in the X, Y, and Z axes of the magnetic resonance device to eliminate the transverse magnetization vector; S112, then applying a radio frequency pulse with a flip angle of 110° to selectively excite protons in the corresponding frequency band of fat, and transforming the magnetization vector of the fat tissue from longitudinal to transverse; S113. Finally, a gradient magnetic field having a direction opposite to that of the gradient magnetic field in S111 but equal in magnitude is applied in the three axes of X, Y, and Z, thereby eliminating the transverse magnetization vector of the fat tissue to achieve the effect of suppressing the fat signal.

3. The 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients according to claim 1, characterized in that: The processing flow in the diffusion preparation module is as follows: S121, applying a layer-selective gradient magnetic field along the Z-axis direction, and simultaneously applying a radio frequency pulse with a flip angle of 90° in the X-axis direction, so as to excite an imaging area at a specified position and layer thickness; S122, applying a trapezoidal cosine oscillation diffusion gradient and a pulse diffusion gradient along a preset diffusion direction, wherein the magnetic field application directions of the two diffusion gradients are orthogonal to each other and have the same duration; S123, at the half echo time point, applying a slice-selective gradient magnetic field identical to that in S121 along the Z-axis direction, and applying a Gaussian radio frequency pulse with a flip angle of 180° along the Y-axis direction, thereby achieving signal convergence; S124. Repeatedly apply two diffusion gradients with the same waveform as in S122 in a preset diffusion direction according to a preset diffusion intensity until all preset diffusion direction and intensity combinations are completed.

4. The 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients according to claim 1, characterized in that: The processing flow in the signal readout module is as follows: S131, firstly, applying a gradient magnetic field in the X and Y axes to move the initial position of signal acquisition from the center of K-space to the edge position; S132 , then gradually adjusting the gradient magnetic fields applied in the X and Y axis directions, scanning the signal points in the K-space along the Cartesian coordinate system, and completing the required K-space filling.

5. The 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients according to claim 1, characterized in that: The image reconstruction process of the first image and the second image is as follows: S21, using the Navigator calibration signal collected after 90° radio frequency pulse layer selective excitation, calibrating the phase fluctuation between the odd and even row signals in the 2D K-space signal collected in the signal readout module; S22: Based on the 2D K-space signal calibrated by S21, the GRAPPA technology is used to supplement the missing signals in the parallel acquisition process, and finally the image reconstruction is completed through two-dimensional Fourier transform.

6. The 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients according to claim 1, characterized in that: In S3, the apparent diffusion coefficient of each pixel in the diffusion weighted image is calculated according to the pixel values ​​at the corresponding positions in the first image and the second image, and the calculation formula of the apparent diffusion coefficient ADC is as follows: ADC=-log(S DEFOG / S PGSE ) / b Where: S DEFOG , S PGSE are the pixel values ​​of the first image and the second image respectively, and b is the diffusion intensity corresponding to the trapezoidal cosine oscillation diffusion gradient.

7. A computer-readable storage medium, characterized in that: The storage medium stores a computer program, and when the computer program is executed by a processor, the 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients as claimed in any one of claims 1 to 6 can be implemented.

8. A magnetic resonance imaging device, characterized in that: The invention comprises a magnetic resonance scanner and a control unit, wherein a computer program is stored in the control unit, and when the computer program is executed, the 2D echo planar diffusion imaging method based on dual-frequency orthogonal gradients as claimed in any one of claims 1 to 6 can be implemented.

Images