A reconstruction method for planar echo imaging

Abstract

The application discloses a reconstruction method of planar echo imaging, comprising the following steps: S1, collecting planar echo data and multi-echo gradient echo data of a region to be imaged respectively; S2, performing inverse geometric correction on the multi-echo gradient echo data to generate corrected data; and S3, performing image reconstruction according to the corrected data and the planar echo data to generate an imaging image. The application has the beneficial effect that the correction of the planar echo image is realized by collecting multi-echo gradient echo data and calculating a B0 field, the problem of possible artifacts in the calibration process of the planar echo imaging in the prior art is avoided, and the image of the planar echo imaging is clearer. Meanwhile, the multi-echo gradient echo data is used for coil sensitivity correction, the scanning speed is higher, and the calibration time can be effectively shortened.

Description

Technical Field

[0001] This invention relates to the field of magnetic resonance imaging technology, and more specifically to a method for reconstructing planar echo imaging. Background Technology

[0002] For magnetic resonance imaging (MRI), users (such as doctors or researchers) typically want to complete scans in a short time while maintaining image quality. Echo planar imaging (EPI) is the fastest MRI method currently available. It is widely used in rapid brain and heart imaging, cardiac cine, magnetic resonance angiography, and functional brain MRI (including brain functional activity, brain perfusion, and diffusion-weighted MRI). For parallel imaging reconstruction using EPI, a necessary step is to first calibrate the coil sensitivity, requiring the acquisition of calibration data. Current methods include using EPI or non-EPI sequences (such as the common FLASH sequence, short for Fast low angle shot magnetic resonance imaging, a type of fast gradient echo sequence) to acquire calibration data.

[0003] However, existing technologies often have the following problems: 1. EPI-based calibration scans may produce artifacts because EPI sequences have a low signal-to-noise ratio when used as calibration data and may contain EPI-related artifacts, such as Nyquist artifacts and breathing artifacts; 2. Calibration based on non-EPI sequences (such as ordinary FLASH) can provide higher SNR and no EPI-related artifacts, but the geometric deformation of non-EPI sequences and EPI sequences is different (the geometric deformation of FLASH sequences is generally negligible, while EPI sequences have obvious geometric deformation), which cannot provide consistent pixel position matching, and therefore may also cause artifacts. Summary of the Invention

[0004] To address the aforementioned problems in the existing technology, a reconstruction method for planar echo imaging is provided.

[0005] The specific technical solution is as follows:

[0006] A reconstruction method for planar echo imaging, comprising:

[0007] Step S1: Acquire planar echo data and multi-echo gradient echo data for the area to be imaged;

[0008] Step S2: Perform reverse geometric correction on the multi-echo gradient echo data to generate correction data;

[0009] Step S3: Reconstruct the image based on the correction data and the planar echo data to generate an imaging image.

[0010] Preferably, step S2 includes:

[0011] Step S21: Generate a relative distribution map of the static magnetic field based on the multi-echo gradient echo data;

[0012] Step S22: Based on the static magnetic field relative distribution map, add geometric deformation to the multi-echo gradient echo data to generate the correction data.

[0013] Preferably, in step S21, the method for generating the relative distribution map of the static magnetic field includes:

[0014] Step S211: Upsample the multiple echo images formed based on the multi-echo gradient echo data using the K-space zero-filling method;

[0015] Step S212: Calculate the phase difference between the upsampled echo images and perform two-dimensional median filtering on the echo images;

[0016] Step S213: According to the formula: Calculate the relative distribution diagram of the static magnetic field;

[0017] Wherein: ΔB is the relative distribution diagram of the static magnetic field. The phase difference, First echo image The second echo image is represented by γ, where γ is the gyromagnetic ratio and ΔTE is the difference between the magnetic field components of the first and second echoes. l is the coil channel index, conj(*) represents the conjugate operation, and Arg(*) represents the calculation of the phase angle of the complex image.

[0018] Preferably, step S22 includes:

[0019] Based on the formula: Calculate the correction data;

[0020] Where, E(x,k) y ) represents the correction data, ΔB(x,y) represents the relative distribution map of the static magnetic field, G(x,y) represents the multi-echo gradient echo data, U represents the upsampling factor, and N represents the multi-echo gradient echo data. y For the phase encoding number, Δt y This is the echo spacing.

[0021] Preferably, step S3 includes:

[0022] Step S31: Perform coil sensitivity calibration on the planar echo data based on the calibration data;

[0023] Step S32: Generate an imaging image from the planar echo data after coil sensitivity calibration using a preset image reconstruction method.

[0024] Preferably, the image reconstruction method is the SENSE reconstruction method, the GRAPPA reconstruction method, or the SPIRiT iterative reconstruction method.

[0025] Preferably, the reconstruction method further includes:

[0026] Step S4: Perform geometric correction on the imaging image based on the static magnetic field relative distribution map to generate a geometrically corrected image.

[0027] Preferably, the method of step S4 includes:

[0028] Step S41: Invert the relative distribution diagram of the static magnetic field;

[0029] Step S42: According to the formula: The geometric deformation in the imaging image is eliminated to generate the geometrically corrected image.

[0030] Where F(x,k) y ) is the geometrically corrected image, ΔB1(x,y) is the relative distribution map of the inverted static magnetic field, H(x,y) is the imaging image, U is the upsampling factor, and N y For the phase encoding number, Δt y This is the echo spacing.

[0031] The above technical solution has the following advantages or beneficial effects: By acquiring multi-echo gradient echo data and calculating the B0 field, the planar echo image is corrected, avoiding the artifact problem that may occur during the calibration process of planar echo imaging in the prior art, resulting in a clearer planar echo image. Simultaneously, the use of multi-echo gradient echo data for coil sensitivity correction results in a faster scanning speed, effectively shortening the calibration time. Attached Figure Description

[0032] Embodiments of the invention will be described more fully with reference to the accompanying drawings. However, the drawings are for illustration and explanation only and do not constitute a limitation on the scope of the invention.

[0033] Figure 1 This is an overall schematic diagram of an embodiment of the present invention;

[0034] Figure 2 This is a schematic diagram of the coil sensitivity calibration process in an embodiment of the present invention;

[0035] Figure 3 This is a schematic diagram of sub-step S2 in an embodiment of the present invention;

[0036] Figure 4 This is a schematic diagram of sub-step S21 in an embodiment of the present invention;

[0037] Figure 5 This is a schematic diagram of the relative distribution of the static magnetic field in an embodiment of the present invention;

[0038] Figure 6 This is a multi-echo gradient echo image that has not undergone reverse geometric correction in this embodiment of the invention;

[0039] Figure 7 This is a multi-echo gradient echo image that has undergone reverse geometric correction in this embodiment of the invention;

[0040] Figure 8 This is a schematic diagram of sub-step S3 in an embodiment of the present invention;

[0041] Figure 9 This refers to planar echo imagery in existing technologies;

[0042] Figure 10 This refers to the planar echo image in this embodiment of the invention;

[0043] Figure 11 This is a schematic diagram of a method according to another embodiment of the present invention;

[0044] Figure 12 This is a schematic diagram of step S4 in another embodiment of the present invention. Detailed Implementation

[0045] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0047] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0048] This invention includes:

[0049] A reconstruction method for planar echo imaging, such as Figure 1 As shown, it includes:

[0050] Step S1: Acquire planar echo data and multi-echo gradient echo data for the area to be imaged;

[0051] Step S2: Perform reverse geometric correction on the multi-echo gradient echo data to generate correction data;

[0052] Step S3: Reconstruct the image based on the correction data and planar echo data to generate an imaging image.

[0053] As an optional implementation, the number of multi-echo gradient echoes is at least 2.

[0054] Specifically, in one embodiment, the present invention employs a two-stage multi-echo gradient echo sequence to calibrate the planar echo data. For example... Figure 2 As shown, in this embodiment, the MRI device acquired two echo patterns using a multi-echo gradient echo sequence. Images and Image. Based on Images and The image can be used to calculate the relative distribution map of the static magnetic field in the multi-echo gradient echo sequence, i.e., the B0 field map. Subsequently, based on the relative distribution map of the static magnetic field and the effective echo spacing of the echo-planar imaging (EPI), the [data / data] can be analyzed. The image undergoes inverse geometric correction, that is, in Add geometric deformations to the image for corresponding planar echo imaging, and then apply the deformations... The image is downsampled to its original matrix size to calibrate the sensitivity of the coils during planar echo imaging, thus achieving a planar echo imaging reconstruction process similar to traditional methods. By employing multi-echo gradient echo scanning, the scanning time is shorter, effectively reducing the calibration time for planar echo imaging and improving overall imaging efficiency.

[0055] As an optional implementation, the parameters of the planar echo data include: repetition time TR = 2000ms, echo time TE = 30ms, flip angle = 90 degrees, and effective echo spacing = 0.578 / 2 = 0.289ms.

[0056] As an optional implementation, multi-echo gradient echo data is acquired before or after the acquisition of planar echo data.

[0057] As an optional implementation, the parameters of the multi-echo gradient echo data include: echo time TE = 6 / 12ms, repetition time TR = 410ms, flip angle = 30 degrees, and matrix size = 96×96.

[0058] As an optional implementation, the field of view (FOV) and the layer angle are kept consistent for both planar echo data and multi-echo gradient echo data.

[0059] In a preferred embodiment, such as Figure 3 As shown, step S2 includes:

[0060] Step S21: Generate a relative distribution map of the static magnetic field based on the multi-echo gradient echo data;

[0061] Step S22: Based on the relative distribution map of the static magnetic field, add geometric deformation to the multi-echo gradient echo data to generate correction data.

[0062] As an optional implementation, correction data is generated by downsampling the geometrically deformed echo image to the original matrix size. By downsampling the echo image to the original matrix size, the pixel positions of the echo image can correspond to the pixel positions of the planar echo imaging, thereby enabling the echo image to be used for the correction of planar echo imaging.

[0063] Specifically, after generating the relative distribution map of the static magnetic field based on the multi-echo gradient echo data, it is necessary to perform an echo image of the first multi-echo gradient echo in X-Ky space, i.e. Figure 2 In The image incorporates geometric deformations corresponding to planar echo imaging. These deformations are designed to match the coil sensitivity of planar echo imaging, thereby enabling… The image can be used for coil sensitivity calibration.

[0064] In a preferred embodiment, such as Figure 4 As shown, the method for generating the relative distribution map of the static magnetic field in step S21 includes:

[0065] Step S211: Upsample multiple echo images formed based on multi-echo gradient echo data using the K-space zero-filling method;

[0066] Step S212: Calculate the phase difference between the upsampled echo images and perform two-dimensional median filtering on the echo images;

[0067] Step S213: According to the formula: Calculate the relative distribution of the static magnetic field;

[0068] Where: ΔB is the relative distribution of the static magnetic field. For phase difference, This is the first echo image. The second echo image is represented by γ, where γ is the gyromagnetic ratio and ΔTE is the difference between the magnetic field components of the first and second echoes. l is the coil channel index, conj(*) represents the conjugate operation, and Arg(*) represents the calculation of the phase angle of the complex image.

[0069] As an optional implementation, when the number of echoes is greater than 2, the phase difference is calculated by least-squares fitting based on the phases of the multiple echo images.

[0070] As an optional implementation, when performing least-squares fitting, the signal intensities of multiple echo images are weighted separately.

[0071] As an optional implementation, after generating the static magnetic field relative distribution map, the static magnetic field relative distribution map is expanded and eroded to remove interference from signal blank areas.

[0072] Specifically, such as Figure 5 As shown, the above steps enable the calculation of data based on multi-echo gradient echo data, such as... Figure 5 The static magnetic field relative distribution diagram is shown below. Then, the static magnetic field relative distribution diagram is superimposed on a graph such as... Figure 6 On the multi-echo gradient echo image shown, inverse geometric correction is then performed on the multi-echo gradient echo image to form a result as shown in the figure. Figure 7 The image shown is a multi-echo gradient echo image after reverse geometric correction.

[0073] In a preferred embodiment, step S22 includes:

[0074] Based on the formula: Calculate the correction data, where E(x,k) y ) represents the correction data, ΔB(x,y) represents the relative distribution of the static magnetic field, G(x,y) represents the multi-echo gradient echo data, U represents the upsampling factor, and N represents the multi-echo gradient echo data. y For the phase encoding number, Δt y This is the echo spacing.

[0075] In a preferred embodiment, such as Figure 8 As shown, step S3 includes:

[0076] Step S31: Perform coil sensitivity calibration on the plane echo data based on the calibration data;

[0077] Step S32: Generate an imaging image from the plane echo data after coil sensitivity calibration using a preset image reconstruction method.

[0078] In a preferred embodiment, the image reconstruction method is the SENSE reconstruction method, the GRAPPA reconstruction method, or the SPIRiT iterative reconstruction method.

[0079] Specifically, when the image reconstruction method is selected as the SENSE reconstruction method, the coil sensitivity calibration process is performed in the image space. First, the coil sensitivity distribution map is estimated by the ESPIRiT method. Then, the SENSE equation system is established and solved based on the estimated coil sensitivity distribution map. The specific process is based on existing technology (Pruessmann KP, Weiger M, Scheidegger MB, et al. SENSE: sensitivity encoding for fast MRI[J]. Magnetic Resonance in Medicine: An Official Journal of the International Society for Magnetic Resonance in Medicine, 1999, 42(5): 952-962.), so it will not be described in detail here.

[0080] Specifically, when the image reconstruction method is GRAPPA reconstruction, the coil sensitivity is performed in the K-space, the least squares method is used to obtain the K-space interpolation factor, then interpolation is performed in the K-space, and the image is reconstructed by Fourier transform. The specific process is the existing technology (Griswold MA, Jakob PM, Heidemann RM, et al. Generalized autocalibrating partially parallel acquisitions (GRAPPA)[J]. Magnetic Resonance in Medicine: An Official Journal of the International Society for Magnetic Resonance in Medicine, 2002, 47(6): 1202-1210.), which will not be elaborated here.

[0081] Furthermore, such as Figure 9 and Figure 10 As shown, both embodiments above employ the GRAPPA reconstruction method with an acceleration factor of 2 to reconstruct the planar echo sequence image, and display the output image at a brightness of X10. It is clear from the figure that... Figure 6 In the image, the location indicated by the arrow shows obvious residual aliasing, which significantly affects the final image quality. Figure 7 In this study, because the coil sensitivity was calibrated using a static magnetic field relative distribution map, there is no residual aliasing at the location indicated by the arrow, resulting in significantly better imaging performance than existing technologies.

[0082] Specifically, when the image reconstruction method is the SPIRiT iterative reconstruction method, the coil sensitivity calibration is performed in the K space, followed by iterative reconstruction. The specific process is the existing technology (Lustig M, Pauly J M.SPIRiT:iterative self-consistent parallel imaging reconstruction from arbitrary k-space[J].Magnetic resonance in medicine,2010,64(2):457-471.), which will not be elaborated here.

[0083] In a preferred embodiment, such as Figure 11 As shown, the reconstruction method also includes:

[0084] Step S4: Perform geometric correction on the imaging image based on the relative distribution map of the static magnetic field to generate a geometrically corrected image.

[0085] Specifically, once the relative distribution map of the static magnetic field is obtained, it can be used to remove geometric deformation in the planar echo data, thereby achieving better image quality.

[0086] In a preferred embodiment, such as Figure 12 As shown, the method in step S4 includes:

[0087] Step S41: Invert the relative distribution diagram of the static magnetic field;

[0088] Step S42: According to the formula: Eliminate geometric deformation in the imaging image to generate a geometrically corrected image.

[0089] Where F(x,k) y H(x,y) is the geometrically corrected image, ΔB1(x,y) is the inverted relative distribution map of the static magnetic field, H(x,y) is the imaging image, U is the upsampling factor, and N is the geometrically corrected image. y For the phase encoding number, Δt y This is the echo spacing.

[0090] Specifically, once the relative distribution map of the static magnetic field is obtained, the plane echo data can be corrected based on the currently calculated static magnetic field to remove the geometric deformation caused by the static magnetic field.

[0091] The beneficial effects of this invention are as follows: by acquiring multi-echo gradient echo data and calculating the B0 field, the planar echo image is corrected, avoiding the artifact problem that may occur during the calibration process of planar echo imaging in the prior art, resulting in a clearer planar echo image. Simultaneously, the use of multi-echo gradient echo data for coil sensitivity correction allows for faster scanning speeds, effectively shortening the calibration time.

[0092] The above are merely preferred embodiments of the present invention and are not intended to limit the implementation methods and protection scope of the present invention. Those skilled in the art should recognize that any equivalent substitutions and obvious changes made based on the description and illustrations of the present invention should be included within the protection scope of the present invention.

Claims

1. A reconstruction method of planar echo imaging, characterized in that, The method comprises the steps of: Step S1: acquiring planar echo data and multi-echo gradient echo data of a region to be imaged respectively; Step S2: performing inverse geometric correction on the multi-echo gradient echo data to generate corrected data; Step S3: performing image reconstruction according to the corrected data and the planar echo data to generate an imaging image; The step S2 comprises: Step S21: generating a static magnetic field relative distribution map according to the multi-echo gradient echo data; Step S22: adding geometric deformation to the multi-echo gradient echo data according to the static magnetic field relative distribution map to generate the corrected data; In the step S21, after the static magnetic field relative distribution map is generated, inflation and erosion are performed on the static magnetic field relative distribution map to remove the interference of signal blank areas.

2. The reconstruction method of claim 1, wherein, In the step S21, the method for generating the static magnetic field relative distribution map comprises: Step S211: upsampling a plurality of echo images formed based on the multi-echo gradient echo data based on a K-space zero padding method; Step S212: calculating the phase difference between the upsampled echo images, and performing two-dimensional median filtering on the echo images; Step S213: Calculate the relative distribution map of the static magnetic field according to the formula: Calculate the relative distribution map of the static magnetic field. wherein: is the relative distribution map of the static magnetic field, is the phase difference, is the first echo image, is the second echo image, is the spin-lattice ratio, is the echo time difference between the first and second echoes; is the coil channel index, conj(*) denotes the conjugate operation, and Arg(*) denotes the calculation of the phase angle of a complex image.

3. The reconstruction method of claim 1, wherein, The step S22 comprises: Based on the formula: the correction data is calculated; wherein is the corrected data, is the relative distribution map of the static magnetic field, is the multi-echo gradient echo data, is an upsampling factor, is a number of phase encodings, is an echo spacing, is a gyromagnetic ratio.

4. The reconstruction method of claim 1, wherein, The step S3 comprises: Step S31: performing coil sensitivity calibration on the planar echo data according to the corrected data; Step S32: generating an imaging image by using a preset image reconstruction method on the planar echo data after coil sensitivity calibration.

5. The reconstruction method of claim 4, wherein, The image reconstruction method is a SENSE reconstruction method, a GRAPPA reconstruction method or a SPIRiT iterative reconstruction method.

6. The reconstruction method of claim 1, wherein, The reconstruction method further comprises: Step S4: performing geometric correction on the imaging image according to the static magnetic field relative distribution map to generate a geometrically corrected image.

7. The reconstruction method of claim 6, wherein, The method of the step S4 comprises: Step S41: performing negation on the static magnetic field relative distribution map; Step S42: According to the formula: Eliminate the geometric deformation in the imaging image, generate the geometric correction image; wherein, is the geometrically corrected image, is the negated relative magnetostatic field distribution map, is the imaged image, is the up-sampling factor, is the number of phase encodings, is the echo spacing, is the gyromagnetic ratio.

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