Method, apparatus, and magnetic resonance scanner for correcting radiofrequency field inhomogeneity in magnetic resonance imaging

Abstract

Embodiments of the present invention disclose a method, apparatus, and magnetic resonance scanner for correcting radiofrequency field inhomogeneity in magnetic resonance imaging. The method includes: scanning a target tissue using a first pulse sequence to obtain a first MR image; acquiring a B1 + field map of the target tissue; obtaining the B1 + field map of the target tissue according to the first MR image and the B1 ‑ field map; performing B1 field inhomogeneity correction on a second MR image of the target tissue according to the B1 + field map and the B1 ‑ field map, where the second MR image is an MR image obtained by scanning the target tissue using any imaging protocol and any pulse sequence. Embodiments of the present invention optimize the B1 field inhomogeneity correction effect in MRI.

Description

Technical Field

[0001] The present invention relates to the technical field of MR (Magnetic Resonance), and particularly to a method and apparatus for correcting radio frequency field inhomogeneity in MRI (Magnetic Resonance Imaging) and a magnetic resonance scanner. Background Art

[0002] The inhomogeneity of the B1 field (radio frequency field) is one of the main problems restricting the clinical application of ultra-high field MRI. The B1 + field (transmitting radio frequency field) magnitude determines the flip angle in MR imaging, and this angle affects the intensity of the MR signal and the contrast of the MR image. The B1 — field (receiving radio frequency field) magnitude is the reception sensitivity (wherein, when using a multi-channel coil, the magnitude of the B1 — field is obtained by using a set calculation method for the reception sensitivities of multiple channel coils, and the specific calculation method is determined according to actual needs), and only affects the intensity of the MR signal. The spatial variation contrast of the MR image and the intensity of the MR signal affect diagnosis and need to be corrected.

[0003] Currently, there are mainly two methods for correcting B1 field inhomogeneity. One is that for MRI with a field strength less than or equal to 3T, B1 shimming technology is used to provide a more uniform B1 + field, which is sufficient to meet clinical applications; the other is that in MRI, image acquisition is usually performed using a coil array. The coil array consists of multiple small coils, and each coil is more sensitive to nearby signals, which will result in a higher signal at the periphery of the imaging target and a lower signal at the center of the imaging target. By comparing the signal intensities of these coil arrays with the signal intensity of the flat reception sensitivity of the body coil, the magnitude of the B1 — field of the coil array can be estimated, thereby correcting this non-uniform signal intensity distribution. However, under ultra-high field conditions, the inhomogeneity of B1 is more severe, and the above two methods cannot meet clinical use.

[0004] Parallel transmission (PTx, Parallel Transmission) can excite a more uniform flip angle by accelerating spatially selective excitation. Therefore, the problem of B1 + field inhomogeneity can theoretically be solved using parallel transmission. However, this method requires the installation of a parallel transmission system, and residual flip angle inhomogeneity still exists. In addition, under ultra-high fields, usually the body coil does not have spatially uniform sensitivity, which means that the above method cannot be applied for B1-field inhomogeneity correction. The B1 + field and B1 —The most widely used post - processing method for field correction is non - parametric non - uniform intensity normalization (N3) and its improved version N4ITK. However, when there is a large difference in the strength of the bias field and there is extremely low signal in the pre - corrected image due to the bias field effect, the assumption that the bias field (B1 + field and B1 — field) effect can be approximated as the convolution of a Gaussian function with the true tissue signal intensity distribution is not applicable.

[0005] As described above, for MRI at ultra - high fields, the inhomogeneity of the B1 field is more severe, and the above - mentioned correction methods are not applicable. Therefore, developing a method to correct the inhomogeneity of the B1 field in MR images is crucial for the clinical application of ultra - high - field MRI. Summary of the Invention

[0006] In view of this, embodiments of the present invention propose a method and device for correcting the inhomogeneity of the B1 field in MRI to optimize the correction effect of the inhomogeneity of the B1 field in MRI;

[0007] Embodiments of the present invention also propose a magnetic resonance scanner to optimize the correction effect of the inhomogeneity of the B1 field in MRI.

[0008] The technical solution of the embodiments of the present invention is implemented as follows:

[0009] A method for correcting the inhomogeneity of the radio - frequency field in magnetic resonance imaging, the method includes:

[0010] Scanning a target tissue using a first pulse sequence to obtain a first magnetic resonance (MR) image;

[0011] Obtaining an emission radio - frequency B1 + field map of the target tissue;

[0012] According to the first MR image and the B1 + field map, obtaining a received radio - frequency B1 — field map of the target tissue;

[0013] According to the B1 + field map and the B1 — field map, performing B1 field inhomogeneity correction on a second MR image of the target tissue; wherein, the second MR image is an MR image obtained by scanning the target tissue using any imaging protocol and any pulse sequence.

[0014] The obtaining the B1 + field map of the target tissue according to the first MR image and the B1 — field map includes:

[0015] For any voxel of the target tissue, based on the longitudinal relaxation time T1, transverse relaxation time T2, echo time TE, repetition time TR, flip angle of the voxel during the first scanning process, and the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map, obtain the first remaining intensity of the MR signal of the voxel during the first scanning process; wherein, the first remaining intensity of the MR signal is: the intensity of the MR signal remaining after removing the influence of the proton density, the sine value of the product of the flip angle and the absolute value of the relative intensity of the B1 + field, and the B1 — field from the MR signal of the voxel during the first scanning process; the first scanning process is: the scanning process for obtaining the first MR image.

[0016] Based on the intensity of the MR signal corresponding to the voxel on the first MR image, the proton density of the voxel, the first remaining intensity of the MR signal, the flip angle of the voxel during the first scanning process, and the absolute value of the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map, obtain the absolute value of the intensity of the B1 — field of the voxel; the absolute values of the intensities of the B1 — fields of all voxels of the target tissue constitute the B1 — field map of the target tissue.

[0017] The obtaining of the absolute value of the intensity of the B1 + field of the voxel based on the intensity of the MR signal corresponding to the voxel on the first MR image, the proton density of the voxel, the first remaining intensity of the MR signal, the flip angle of the voxel during the first scanning process, and the absolute value of the relative intensity of the B1 + field corresponding to the voxel on the B1 — field map includes:

[0018] Multiply the proton density of the voxel by the first remaining intensity of the MR signal to obtain a first product; multiply the flip angle of the voxel during the first scanning process by the absolute value of the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map to obtain a second product; obtain the sine value of the second product; multiply the first product by the sine value of the second product to obtain a third product; divide the intensity of the MR signal corresponding to the voxel on the first MR image by the third product, and use the obtained quotient value as the absolute value of the intensity of the B1 — field of the voxel.

[0019] The said based on the T1, T2, TE, TR, flip angle of the voxel during the first scanning process, and the B1 +The corresponding B1 on the field map + The relative intensity of the field, and obtaining the first remaining intensity of the MR signal of the voxel during the first scan, including:

[0020] Using the Bloch equation or the phase diagram theory to obtain T1, T2, TE, TR, flip angle, and B1 + The functional expression between the relative intensity of the field and the intensity of the MR signal, and using this functional expression for T1, T2, TE, TR, flip angle of the voxel during the first scan, and the B1 of the voxel + The corresponding B1 on the field map + The relative intensity of the field is calculated to obtain the first remaining intensity of the MR signal of the voxel during the first scan.

[0021] The first pulse sequence is: a gradient echo pulse sequence with a low flip angle;

[0022] According to the first MR image and the B1 + Field map, obtaining the B1 of the target tissue — The field map includes:

[0023] For any voxel of the target tissue, the product of the proton density of the voxel and the first remaining intensity of the MR signal of the voxel during the first scan is set as the first constant; wherein, the first remaining intensity of the MR signal is: the intensity of the MR signal remaining after removing the influence of the product of the proton density, flip angle, and the absolute value of the relative intensity of the B1 + Field, the sine value of the product of the absolute value of the relative intensity of the B1 — Field, and the first scan process is: the scan process for obtaining the first MR image;

[0024] According to the intensity of the MR signal corresponding to the voxel on the first MR image, the first constant, the flip angle of the voxel during the first scan, and the B1 + The corresponding B1 on the field map + The absolute value of the relative intensity of the field, obtaining the B1 of the voxel — The absolute value of the intensity of the B1 field of all voxels of the target tissue — The absolute value of the intensity of the B1 field constitutes the B1 field map of the target tissue — Field map.

[0025] The first pulse sequence is: a gradient echo pulse sequence with a low flip angle;

[0026] According to the first MR image and the B1 + Field map, obtaining the B1 of the target tissue — The field map includes:

[0027] For any voxel of the target tissue, the product of the proton density of the voxel and the first remaining intensity of the MR signal of the voxel during the first scanning process is set as a first constant; wherein, the first remaining intensity of the MR signal is: the intensity of the remaining MR signal after removing the influence of the product of the proton density, the sine value of the flip angle and the absolute value of the relative intensity of the B1 field, and the B1 field during the first scanning process; the first scanning process is: the scanning process for obtaining the first MR image. + field — field

[0028] According to the intensity of the MR signal corresponding to the voxel on the first MR image, the first constant, and the flip angle of the voxel during the first scanning process, obtain the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field.

[0029] When the absolute values of the products of the relative intensities of the B1 + field and the intensities of the B1 — field of all voxels of the target tissue are obtained, use a preset three-dimensional spline fitting algorithm to fit the absolute values of the products of the relative intensities of the B1 + field and the intensities of the B1 — field of all voxels of the target tissue, and obtain a three-dimensional spline fitting map of the absolute values of the products of the relative intensities of the B1 + field and the intensities of the B1 — field of the target tissue.

[0030] For any voxel of the target tissue, divide the signal intensity corresponding to the voxel on the three-dimensional spline fitting map by the absolute value of the relative intensity of the B1 + field in the B1 + field mapping map of the voxel, to obtain the absolute value of the intensity of the B1 — field of the voxel; the absolute values of the intensities of the B1 — field of all voxels of the target tissue constitute the B1 — field mapping map of the target tissue.

[0031] The gradient echo pulse sequence with a low flip angle is: a gradient echo single echo sequence, a gradient echo multi-echo sequence, or an echo planar imaging sequence.

[0032] Before obtaining the first magnetic resonance MR image, it further includes:

[0033] When multiple MR images are obtained after scanning the target tissue with a first pulse sequence with a low flip angle, select the image with the lowest tissue contrast as the first MR image.

[0034] The above-mentioned according to the B1 + field map and the B1 — field map, perform B1 field inhomogeneity correction on the second MR image of the target tissue, including:

[0035] For any voxel of the target tissue, according to the T1, T2 of the voxel during the second scanning process and the relative intensity of the B1 + field corresponding to the voxel on the field map, obtain the second remaining intensity of the MR signal of the voxel during the second scanning process, where the second remaining intensity of the MR signal is: the intensity of the MR signal remaining after removing the influence of the proton density and the B1 + field in the second scanning process of the voxel, and the second scanning process is: the scanning process for obtaining the second MR image; — field in the second scanning process of the voxel, and the second scanning process is: the scanning process for obtaining the second MR image;

[0036] According to the intensity of the MR signal corresponding to the voxel on the second MR image, the absolute value of the intensity of the B1 — field of the voxel and the second remaining intensity of the MR signal, obtain the intensity of the MR signal corresponding to the voxel on the second MR image after completing the B1 inhomogeneity correction.

[0037] The above-mentioned according to the T1, T2 of the voxel during the second scanning process and the relative intensity of the B1 + field corresponding to the voxel on the field map, obtain the second remaining intensity of the MR signal of the voxel during the second scanning process, including: + field corresponding to the voxel on the field map, obtain the second remaining intensity of the MR signal of the voxel during the second scanning process, including:

[0038] Adopt the Bloch equation or the phase diagram theory to obtain the function expression between T1, T2 and the relative intensity of the B1 + field and the MR signal intensity. According to the function expression, use the function expression to calculate the T1, T2 of the voxel during the second scanning process and the relative intensity of the B1 + field corresponding to the voxel on the field map, + field to obtain the second remaining intensity of the MR signal of the voxel during the second scanning process.

[0039] A device for correcting radiofrequency field inhomogeneity in magnetic resonance imaging, the device includes:

[0040] The first magnetic resonance (MR) image acquisition module is used to scan the target tissue with the first pulse sequence to obtain the first MR image;

[0041] The radiofrequency B1 + field map acquisition module is used to acquire the B1 + field map of the target tissue;

[0042] Receive radiofrequency B1— A field map acquisition module, configured to obtain the B1 field map of the target tissue according to the first MR image and the B1 field map + ; — A correction module, configured to perform B1 field inhomogeneity correction on the second MR image of the target tissue according to the B1 field map and the B1 field map

[0043] A magnetic resonance scanner, comprising the above-mentioned RF field inhomogeneity correction device in magnetic resonance imaging + In the embodiments of the present invention, the target tissue is first scanned with a first pulse sequence to obtain a first MR image, and then the B1 field map of the target tissue is obtained according to the first MR image and the B1 field map of the target tissue, so as to obtain the B1 field map and the B1 field map of the target tissue. Thereafter, for any MR image of the same target tissue, B1 field inhomogeneity correction can be performed according to the B1 field map and the B1 field map of the target tissue. This method can be used to correct the B1 field inhomogeneity on MR images obtained by any imaging protocol or any imaging sequence, and has an obvious effect on correcting the B1 field inhomogeneity in an ultra-high field MRI system. — BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The following will, by referring to the accompanying drawings and describing the preferred embodiments of the present invention in detail, make the above and other features and advantages of the present invention clearer to those of ordinary skill in the art. In the drawings:

[0045] FIG. + is a flowchart of a method for correcting B1 field inhomogeneity in MRI provided by the first embodiment of the present invention; — FIG. + is a flowchart of a method for correcting B1 field inhomogeneity in MRI provided by the second embodiment of the present invention; — FIG. + is a flowchart of a method for correcting B1 field inhomogeneity in MRI provided by the third embodiment of the present invention; — FIG. is a flowchart of a method for correcting B1 field inhomogeneity in MRI provided by the fourth embodiment of the present invention;

[0046]

[0047] Figure 1

[0048] Figure 2

[0049] Figure 3

[0050] Figure 4

[0051] Figure 5 ​​​​​​​​​Schematic diagram of 9 slices of a PD-weighted image obtained by performing MR scanning on the target brain tissue of a healthy volunteer using the SPGR sequence in the application example of the present invention;

[0052] Figure 6 It is based on Figure 5 The PD-weighted image to obtain the B 1s ± Schematic diagram of 9 slices of the field map;

[0053] Figure 7 Schematic diagram of 9 slices of the |B1 + | field map of the target brain tissue obtained by using the presaturation TFL sequence in the application example of the present invention;

[0054] Figure 8 Schematic diagram of 9 slices of a T2-weighted image obtained by performing MR scanning on the target brain tissue using the 3D-SPACE sequence in the application example of the present invention;

[0055] Figure 9 It is the schematic diagram of the image obtained after correcting Figure 8 using the existing N4ITK method;

[0056] Figure 10 It is the schematic diagram of the image obtained after correcting Figure 8 using the method provided in the fourth embodiment of the present invention;

[0057] Figure 11 Schematic diagram of 4 slices of a T2-weighted image obtained by performing MR scanning on the target brain tissue using the 2D-TSE sequence in the application example of the present invention and an enlarged view of the dashed rectangular area of the 4th slice;

[0058] Figure 12 It is the schematic diagram of 4 slices of the image obtained after correcting Figure 11 using the method provided in the fourth embodiment of the present invention and an enlarged view of the dashed rectangular area of the 4th slice;

[0059] Figure 13 Schematic diagram of the structure of the B1 field non-uniformity correction device in MRI provided by the embodiment of the present invention.

[0060] Among them, the reference numerals are as follows:

[0061] Label Meaning 101～104 Step 201～210 Step 301～308 Step 401～410 Step 130 B1 Field Inhomogeneity Correction Device in MRI 131 First MR Image Acquisition Module 132 <![CDATA[B1 + Field mapping diagram acquisition module]]> 133 <![CDATA[B1 — Field mapping diagram acquisition module]]> 134 Correction Module Detailed implementation manners

[0062] To make the objectives, technical solutions and advantages of the present invention clearer, the following examples are given to further elaborate on the present invention in detail.

[0063] Figure 1 Flow chart of the B1 field non-uniformity correction method in MRI provided by the first embodiment of the present invention, and the specific steps are as follows:

[0064] Step 101: Scan the target tissue using a first pulse sequence to obtain a first MR image.

[0065] Step 102: Obtain the B1 + field map of the target tissue.

[0066] Obtaining the B1 + field map of the target tissue is a mature technology and will not be elaborated in this embodiment.

[0067] Step 103: According to the first MR image and the B1 + field map, obtain the B1 — field map of the target tissue.

[0068] In an alternative embodiment, this step 103 specifically includes: for any voxel of the target tissue, according to the T1 (longitudinal relaxation time), T2 (transverse relaxation time), TE (Echo Time), TR (Repetition Time), flip angle of the voxel during the first scan, and the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map, obtain the first remaining intensity of the MR signal of the voxel during the first scan; according to the intensity of the MR signal corresponding to the voxel on the first MR image, the proton density of the voxel, the first remaining intensity of the MR signal of the voxel during the first scan, the flip angle of the voxel during the first scan, and the absolute value of the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map, obtain the absolute value of the intensity of the B1 — field of the voxel; and, the absolute values of the intensities of the B1 — fields of all voxels of the target tissue constitute the B1 — field map of the target tissue. Wherein, the first remaining intensity of the MR signal is: the intensity of the MR signal remaining after removing the influence of the product of the proton density, the flip angle and the absolute value of the relative intensity of the B1 + field, and the B1 — field from the MR signal of the voxel during the first scan. The first scan process is: the scan process for obtaining the first MR image.

[0069] In an alternative embodiment, according to the intensity of the MR signal corresponding to the voxel on the first MR image, the proton density of the voxel, the first remaining intensity of the MR signal of the voxel during the first scan, the flip angle of the voxel during the first scan, and the B1 +The corresponding B1 on the field map + The absolute value of the relative intensity of the field, and obtain the B1 of this voxel — The absolute value of the intensity of the field, including: multiplying the proton density of this voxel by the first residual intensity of the MR signal of this voxel during the first scan to obtain a first product; multiplying the flip angle of this voxel during the first scan by the B1 + The corresponding B1 on the field map + Multiply the absolute value of the relative intensity of the field to obtain a second product; obtain the sine value of the second product; multiply the first product by the sine value of the second product to obtain a third product; divide the intensity of the MR signal corresponding to this voxel on the first MR image by the third product, and use the obtained quotient value as the B1 of this voxel — The absolute value of the intensity of the field.

[0070] Step 104: According to the B1 + Field map and B1 — Field map, perform B1 field inhomogeneity correction on the second MR image of the target tissue; wherein, the second MR image is an MR image obtained by scanning the target tissue using any imaging protocol and any pulse sequence.

[0071] In an optional embodiment, this step 104 specifically includes: for any voxel of the target tissue, according to the T1 and T2 of this voxel during the second scan and the B1 + The corresponding B1 on the field map + The relative intensity of the field, obtain the second residual intensity of the MR signal of this voxel during the second scan; according to the intensity of the MR signal corresponding to this voxel on the second MR image, the B1 — The absolute value of the intensity of the field and the second residual intensity of the MR signal, obtain the intensity of the MR signal corresponding to this voxel on the second MR image after completing the B1 inhomogeneity correction. Among them, the second residual intensity of the MR signal is: the intensity of the MR signal remaining after removing the influences of the proton density and the B1 — Field of this voxel, and the second scan process is: the scan process for obtaining the second MR image.

[0072] The beneficial technical effects of the above embodiments are as follows: First, scan the target tissue using the first pulse sequence to obtain the first MR image, and then according to the first MR image and the B1 + Field map of the target tissue, obtain the B1 — Field map of the target tissue, so as to obtain the B1 + Field map and B1 — Field map of the target tissue, and then for any MR image of the same target tissue, it can be based on the B1 + Field map and B1 of the target tissue— Perform B1 field inhomogeneity correction on the field map. This method can be used to correct the B1 field inhomogeneity (also known as the bias field effect) on MR images obtained using any imaging protocol or imaging sequence, and has obvious effect on correcting the B1 field inhomogeneity in ultra-high field MRI systems.

[0073] Figure 2 The following is the flowchart of the B1 field inhomogeneity correction method in MRI provided by the second embodiment of the present invention, and its specific steps are as follows:

[0074] Step 201: Scan the target tissue using the first pulse sequence to obtain the first MR image.

[0075] Step 202: Obtain the B1 + field map of the target tissue.

[0076] Obtaining the B1 + field map of the target tissue is a mature technology, and this embodiment will not elaborate.

[0077] Step 203: For any voxel of the target tissue, according to the T1 (longitudinal relaxation time), T2 (transverse relaxation time), TE (echo time), TR (repetition time), flip angle of the voxel during the first scan, and the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map, obtain the first remaining intensity of the MR signal of the voxel during the first scan. Among them, the first remaining intensity of the MR signal of the voxel during the first scan is the intensity of the MR signal remaining after removing the influence of the proton density, the sine value of the product of the flip angle and the absolute value of the relative intensity of the B1 + field, and the B1 — field from the MR signal of the voxel during the first scan; the first scan process is: the scan process of obtaining the first MR image.

[0078] That is, in this step, it is considered that: the intensity of the MR signal is affected by three factors: one, the proton density; two, the sine value of the product of the flip angle and the absolute value of the relative intensity of the B1 + field; three, the B1 — field; four, other factors except one, two, and three.

[0079] In practical applications, the Bloch equation or the phase diagram theory can be used to obtain the functional expressions between T1, T2, TE, TR, the flip angle, and the relative intensity of the B1 + field and the intensity of the MR signal, and use this functional expression for the T1, T2, TE, TR, flip angle of the voxel during the first scan, and the B1 + field corresponding to the voxel on the B1 +Calculate the relative intensity of the field to obtain the first remaining intensity of the MR signal of the voxel during the first scan.

[0080] For example, for any voxel r on the target tissue, the first remaining intensity of the MR signal of the voxel during the first scan can be expressed as: f(T1(r), T2(r), TE, TR, α·|B1 + (r)|)

[0081] where T1(r) is the T1 of voxel r, T2(r) is the T2 of voxel r, α is the set flip angle, and B1 + (r) is the relative intensity of the B1 + field (when the true flip angle of voxel r is the same as the set flip angle, |B1 + (r)| = 1), TE is the echo time, and TR is the repetition time. The expression of f(T1(r), T2(r), TE, TR, α·|B1 + (r)|) can be obtained using the Bloch equation or the phase diagram theory.

[0082] In addition, since T1(r) and T2(r) of each pixel need to be obtained by scanning with a specific sequence, the values of T1(r) and T2(r) can be empirical constants.

[0083] Step 204: Multiply the proton density of the voxel by the first remaining intensity of the MR signal of the voxel during the first scan to obtain a first product.

[0084] For example, the first product is M0(r)·f(T1(r), T2(r), TE, TR, α·|B1 + (r)|). Where M0(r) is the proton density of voxel r.

[0085] Step 205: Multiply the flip angle of the voxel during the first scan by the absolute value of the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map to obtain a second product, and obtain the sine value of the second product.

[0086] For example, the second product = α·|B1 + (r)|.

[0087] Step 206: Multiply the first product by the sine value of the second product to obtain a third product.

[0088] For example, the third product = M0(r)·f(T1(r), T2(r), TE, TR, α·|B1 + (r)|)·sin(α·|B1 + (r)|)

[0089] Step 207: Divide the intensity of the MR signal corresponding to the voxel on the first MR image by the third product, and use the obtained quotient as the B1 of the voxel. — The absolute value of the field strength.

[0090] For example: |B1 — (r)|=S(r) / (M0(r)·sin(α·|B1 + (r)|)·f(T1(r),T2(r),TE,TR,α·|B1 + (r)|))

[0091] Among them, B1 — (r) is the B1 of voxel r — The strength of the field, S(r) is the strength of the MR signal corresponding to voxel r in the first MR image,

[0092] When the B1 of each voxel of the target tissue is obtained — The absolute value of the field intensity is then used to obtain the B1 of the target tissue. — Field map.

[0093] For any voxel on the target tissue, we have:

[0094] S(r)=M0(r)·sin(α·|B1 + (r)|)·|B1 — (r)|·f(T1(r),T2(r),TE,TR,α·|B1 + (r)|)

[0095] Where r represents any voxel on the target tissue, S(r) is the MR signal intensity of voxel r, M0(r) is the proton density of voxel r, α is the set flip angle, and B1 + (r) is the B1 of voxel r + Relative strength of the field, B1 — (r) is the B1 of voxel r — The intensity of the field, T1(r) is the T1 of voxel r, T2(r) is the T2 of voxel r, TE is the echo time, TR is the repetition time, f(T1(r),T2(r),TE,TR,α·|B1 + (r)|) is the first residual intensity of the MR signal of voxel r.

[0096] Then: |B1 — (r)|=S(r) / (M0(r)·sin(α·|B1 + (r)|)·f(T1(r),T2(r),TE,TR,α·|B1 + (r)|)

[0097] Step 208: For any voxel of the target tissue, based on the T1 and T2 of the voxel during the second scanning process and the relative intensity of the B1 corresponding to the voxel on the B1 + field map, obtain the second remaining intensity of the MR signal of the voxel during the second scanning process. Wherein, the second remaining intensity of the MR signal of the voxel during the second scanning process is: the intensity of the remaining MR signal after removing the influence of the proton density and the B1 + field in the second scanning process of the voxel, and the second scanning process is: the scanning process for obtaining the second MR image. — field in the second scanning process of the voxel, and the second scanning process is: the scanning process for obtaining the second MR image.

[0098] In practical applications, using the Bloch equation or the phase diagram theory, obtain the functional expression between the relative intensities of T1, T2, and B1 + field and the MR signal intensity. According to this functional expression, use this functional expression to calculate the relative intensities of T1 and T2 of the voxel during the second scanning process and the B1 + field corresponding to the voxel on the B1 + field map, and obtain the second remaining intensity of the MR signal of the voxel during the second scanning process.

[0099] For example: For any voxel r on the target tissue, the second remaining intensity of the MR signal of the voxel during the second scanning process can be expressed as: g(T1(r), T2(r), |B1 + (r)|)

[0100] where, T1(r) is the T1 of voxel r, T2(r) is the T2 of voxel r, and B1 + (r) is the relative intensity of the B1 + field of voxel r. The expression of g(T1(r), T2(r), |B1 + (r)|) can be obtained using the Bloch equation or the phase diagram theory.

[0101] If the target tissue is scanned in a parallel transmission mode, then for any voxel r on the target tissue, the second remaining intensity of the MR signal of the voxel during the second scanning process can be expressed as:

[0102] g(T1(r), T2(r), B 1,1 + (r), B 1,2 + (r), B 1,3 + (r),......, B 1,n-1 + (r), B 1,n + (r))

[0103] where n is the total number of transmission channels, and B 1,k + (r) is the relative intensity of the B1 field of voxel r for transmission channel k, where 1 ≤ k ≤ n. +

[0104] Step 209: Obtain the product of the absolute value of the intensity of the B1 field of this voxel and the second remaining intensity of the MR signal of this voxel during the second scanning process, to obtain a fifth product. —

[0105] For example: Fifth product = |B1 — (r)| · g(T1(r), T2(r), |B1 + (r)|)

[0106] where B1 — (r) is the intensity of the B1 field of voxel r. —

[0107] Step 210: Divide the intensity of the MR signal corresponding to this voxel on the second MR image by the fifth product, and use the obtained quotient value as the intensity of the MR signal corresponding to this voxel on the second MR image after B1 inhomogeneity correction is completed.

[0108] For example: I cor (r) = I(r) / (|B1 — (r)| · g(T1(r), T2(r), |B1 + (r)|))(1)

[0109] where I(r) is the intensity of the MR signal corresponding to voxel r on the second MR image, and I cor (r) is the intensity of the MR signal corresponding to voxel r on the second MR image after B1 inhomogeneity correction is completed.

[0110] It should be noted that in the above formula, the values of T1(r) and T2(r) are taken as empirical constant values.

[0111] When the B1 field inhomogeneity correction of the intensity of the MR signal corresponding to each voxel of the target tissue is completed, a second MR image with B1 field inhomogeneity correction completed is obtained.

[0112] Formula (1) is obtained through the following process:

[0113] For the second MR image, for any voxel r on the target tissue, there is:

[0114] I(r) = |B1 — (r)| · g(T1(r), T2(r), |B1​​​+ (r)|)·M0(r)

[0115] When there is no B1 field inhomogeneity, |B1 + (r)|) = 1, so:

[0116] I cor (r) = (I(r) / |B1 — (r)|)·(g(T1(r), T2(r), 1) / g(T1(r), T2(r), |B1 + (r)|))

[0117] In many cases, T1 mapping and T2 mapping cannot be obtained. Therefore, empirical constant values of T1 and T2 are usually used to replace T1(r) and T2(r). At the same time, since only the relative value of I cor (r) needs to be considered, g(T1(r), T2(r), 1) can be replaced by a constant. Most simply, this constant can be 1, so:

[0118] I cor (r) = (I(r) / |B1 — (r)|)·(1 / g(T1(r), T2(r), |B1 + (r)|)) = I(r) / (|B1 — (r)|·g(T1(r), T2(r), |B1 + (r)|), that is, the formula (1) is obtained.

[0119] Formula (1).

[0120] In an alternative embodiment, the first pulse sequence in step 101 is: a gradient echo pulse sequence with a low flip angle;

[0121] Step 103 specifically includes: for any voxel of the target tissue, setting the product of the proton density of the voxel and the first remaining intensity of the MR signal of the voxel in the first scanning process as a first constant; according to the intensity of the MR signal corresponding to the voxel on the first MR image, the first constant, the flip angle of the voxel in the first scanning process, and the absolute value of the relative intensity of the B1 + field mapping of the voxel, obtaining the absolute value of the intensity of the B1 + field of the voxel. The absolute values of the intensities of the B1 — fields of all voxels of the target tissue constitute the B1 — field mapping of the target tissue. Among them, the first remaining intensity of the MR signal is: the sine value of removing the product of the proton density, the flip angle, and the absolute value of the relative intensity of the B1 — field from the MR signal of the voxel in the first scanning process, B1 + field— The intensity of the MR signal remaining after the influence of the proton density, flip angle, and B1 field on the three, the first scanning process is: the scanning process of obtaining the first MR image.

[0122] Figure 3 The flowchart of the B1 field non-uniformity correction method in MRI provided by the third embodiment of the present invention is as follows:

[0123] Step 301: Scan the target tissue using a gradient echo pulse sequence with a low flip angle to obtain a first MR image.

[0124] In this step, scanning the target tissue using a gradient echo pulse sequence with a low flip angle aims to obtain a first MR image with low tissue contrast. Among them, the gradient echo pulse sequence with a low flip angle is, for example: GRE (Gradient Recalled Echo) single echo sequence, GRE multi-echo sequence, or EPI (Echo Planar Imaging) sequence, etc. The specific size of the low flip angle can be determined according to the pulse sequence used. For example, for the GRE sequence, the low flip angle used can be 5°. Generally, the low flip angle in this embodiment refers to a flip angle not greater than 10°. In practical applications, TE, TR, or / and the flip angle can be adjusted to reduce tissue contrast.

[0125] Step 302: Obtain the B1 + field map of the target tissue.

[0126] Step 303: For any voxel of the target tissue, set the product of the proton density of the voxel and the first remaining intensity of the MR signal of the voxel in the first scanning process as the first constant. Among them, the first remaining intensity of the MR signal of the voxel in the first scanning process is: the intensity of the MR signal of the voxel in the first scanning process minus the product of the proton density, the sine value of the product of the flip angle and the absolute value of the relative intensity of the B1 + field, and the influence of the B1 — field on the three, the first scanning process is: the scanning process of obtaining the first MR image.

[0127] Since the tissue contrast of the first MR image is made as low as possible by selecting scanning parameters, therefore, for any voxel of the target tissue, the product of the proton density of the voxel and the first remaining intensity of the MR signal of the voxel in the first scanning process is close to a constant. Here, let this constant be the first constant. It should be noted that the specific value of the first constant is not important because the intensity of the B1 — field of each voxel in the B1 — field map of the target tissue is already a relative value of the B1 —The relative value of the field strength. In practical applications, for the convenience of calculation, the first constant can be directly taken as 1.

[0128] That is, for any voxel r on the target tissue, M0(r)·f(T1(r), T2(r), TE, TR, α·|B1 + (r)|)≈constant. Usually, M0(r)·f(T1(r), T2(r), TE, TR, α·|B1 + (r)|)≈1, so in this step, M0(r)·f(T1(r), T2(r), TE, TR, α·|B1 + (r)|)=1.

[0129] Among them, M0(r) is the proton density of voxel r, T1(r) is the T1 of voxel r, T2(r) is the T2 of voxel r, TE is the echo time, TR is the repetition time, α is the set flip angle, and B1 + (r) is the B1 of voxel r + field's relative intensity, and f(T1(r), T2(r), TE, TR, α·|B1 + (r)|) is the first remaining intensity of the MR signal of voxel r.

[0130] Step 304: Multiply the first constant, the flip angle of this voxel in the first scan process, and the absolute value of the relative intensity of the B1 + field corresponding to this voxel on the B1 + field mapping diagram to obtain the fourth product.

[0131] Since a low flip angle is used, sin(α·|B1 + (r)|)≈α·|B1 + (r)|, then the formula S(r)=M0(r)·sin(α·|B1 + (r)|)·|B1 — (r)|·f(T1(r), T2(r), TE, TR, α·|B1 + (r)|)≈α·|B1 + (r)|·|B1 — (r)|.

[0132] Among them, r represents any voxel on the target tissue, S(r) is the MR signal intensity of voxel r, M0(r) is the proton density of voxel r, α is the set flip angle, and B1 + (r) is the B1 of voxel r + field's relative intensity, and B1 — (r) is the B1 of voxel r —The intensity of the field, T1(r) is the T1 of voxel r, T2(r) is the T2 of voxel r, TE is the echo time, TR is the repetition time, and f is the first residual intensity of the MR signal of voxel r.

[0133] Step 305: Divide the intensity of the MR signal corresponding to the voxel on the first MR image by the fourth product, and use the obtained quotient as the B1 of the voxel — The absolute value of the intensity of the field.

[0134] That is: when the first constant is 1, |B1 — (r)| = S(r) / (α·|B1 + (r)|)

[0135] When the absolute value of the B1 — field intensity of each voxel of the target tissue is obtained, the B1 — field map of the target tissue is obtained.

[0136] Step 306: For any voxel of the target tissue, according to the T1 and T2 of the voxel in the second scanning process and the relative intensity of B1 + field corresponding to the voxel on the B1 + field map, obtain the second residual intensity of the MR signal of the voxel in the second scanning process. Among them, the second residual intensity of the MR signal of the voxel in the second scanning process is: the intensity of the MR signal remaining after removing the influence of the proton density and the B1 — field in the second scanning process. The second scanning process is: the scanning process for obtaining the second MR image.

[0137] In practical applications, using the Bloch equation or the phase diagram theory, obtain the functional expressions between T1, T2, and the relative intensity of B1 + field and the MR signal intensity. According to the functional expressions, use the functional expressions to calculate the T1 and T2 of the voxel in the second scanning process and the relative intensity of B1 + field corresponding to the voxel on the B1 + field map, and obtain the second residual intensity of the MR signal of the voxel in the second scanning process.

[0138] For example: for any voxel r on the target tissue, the second residual intensity of the MR signal of the voxel in the second scanning process can be expressed as: g(T1(r), T2(r), |B1 + (r)|)

[0139] Among them, T1(r) is the T1 of voxel r, T2(r) is the T2 of voxel r, and B1 + (r) is the B1 of voxel r +The relative strength of the field. g(T1(r),T2(r),|B1 + The expression of (r)|) can be obtained using the Bloch equation or phase diagram theory.

[0140] If the target tissue is scanned in parallel emission mode, for any voxel r on the target tissue, the second residual intensity of the MR signal of the voxel in the second scanning process can be expressed as:

[0141] g(T1(r),T2(r),B 1,1 + (r),B 1,2 + (r),B 1,3 + (r),......,B 1,n-1 + (r),B 1,n + (r))

[0142] Where n is the total number of transmission channels, B 1,k + (r) is the B1 of voxel r for transmission channel k + The relative strength of the field, 1≤k≤n.

[0143] Step 307: Get the B1 of the voxel — The product of the absolute value of the intensity of the field and the second residual intensity of the MR signal of the voxel during the second scanning process yields a fifth product.

[0144] For example: The fifth product = |B1 — (r)|·g(T1(r),T2(r),|B1 + (r)|)

[0145] Among them, B1 — (r) is the B1 of voxel r — The strength of the field.

[0146] Step 308: Divide the intensity of the MR signal corresponding to the voxel on the second MR image by the fifth product, and use the obtained quotient as the intensity of the MR signal corresponding to the voxel on the second MR image after the B1 inhomogeneity correction is completed.

[0147] For example: cor (r)=I(r) / (|B1 — (r)|·g(T1(r),T2(r),|B1 + (r)|)

[0148] Where I(r) is the intensity of the MR signal corresponding to voxel r on the second MR image, I cor(r) is the intensity of the MR signal corresponding to voxel r on the second MR image after the B1 inhomogeneity correction is completed.

[0149] It should be noted that, in practical applications, in the above formula, the values of T1(r) and T2(r) are empirical constant values.

[0150] When the B1 field inhomogeneity correction of the intensity of the MR signal corresponding to each voxel of the target tissue on the second MR image is completed, the second MR image with the B1 field inhomogeneity correction completed is obtained.

[0151] It should be noted that, in most cases, no additional scanning process is required to obtain the first MR image. + When performing field mapping, a pre-saturated TFL (Turbo Flash, fast low-angle excitation) sequence is usually used for scanning. At this time, the tissue contrast of the MR image obtained is low, and this MR image can be used as the first MR image. Alternatively, when performing normal imaging scanning on the target tissue, if a low flip angle gradient echo pulse sequence is used, the MR image obtained at this time can be directly used as the first MR image. If multiple MR images are obtained at the same time, the MR image with the lowest tissue contrast is selected as the first MR image.

[0152] In an optional embodiment, the first pulse sequence in step 101 is: a gradient echo pulse sequence with a low flip angle;

[0153] Step 103 specifically includes: for any voxel of the target tissue, setting the product of the proton density of the voxel and the first residual intensity of the MR signal of the voxel in the first scanning process as a first constant; obtaining the B1 of the voxel according to the intensity of the MR signal corresponding to the voxel on the first MR image, the first constant and the flip angle of the voxel in the first scanning process. + The relative strength of the field and B1 — The absolute value of the product of the field strength; when the B1 of all voxels of the target tissue is obtained + The relative strength of the field and B1 — When the absolute value of the product of the intensity of the field is calculated, the B1 of all voxels of the target tissue is calculated using a preset three-dimensional spline fitting algorithm. + The relative strength of the field and B1 — The absolute value of the product of the field intensity is fitted to obtain the B1 of the target tissue + The relative strength of the field and B1 — A three-dimensional spline fitting diagram of the absolute value of the product of the intensity of the field; for any voxel of the target tissue, the signal intensity corresponding to the voxel on the three-dimensional spline fitting diagram is divided by the signal intensity of the voxel in B1 + The corresponding B1 on the field map+ The absolute value of the relative intensity of the field to obtain B1 of the voxel — The absolute value of the intensity of the field, B1 of all voxels of the target tissue — The absolute values of the intensities of the field constitute B1 of the target tissue — field map. Among them, the first residual intensity of the MR signal is: the sine value of the product of the proton density, flip angle and the absolute value of the relative intensity of the B1 + field removed from the voxel during the first scanning process, and the intensity of the MR signal remaining after the influence of these three factors, B1 — field. The first scanning process is: the scanning process of obtaining the first MR image;

[0154] Figure 4 The flowchart of the B1 field non-uniformity correction method in MRI provided by the fourth embodiment of the present invention is as follows:

[0155] Step 401: Scan the target tissue using a gradient echo pulse sequence with a low flip angle to obtain a first MR image.

[0156] In this step, scanning the target tissue using a gradient echo pulse sequence with a low flip angle aims to obtain a first MR image with low contrast. Among them, the gradient echo pulse sequence with a low flip angle is such as: GRE single echo sequence, GRE multi-echo sequence or EPI sequence, etc. The specific size of the low flip angle can be determined according to the adopted pulse sequence. For example, for the GRE sequence, the low flip angle adopted can be 5°. Generally, the low flip angle in this embodiment refers to a flip angle not greater than 10°. In practical applications, the tissue contrast can be reduced by adjusting TE, TR or / and the flip angle.

[0157] Step 402: Obtain the B1 + field map of the target tissue.

[0158] Step 403: For any voxel of the target tissue, set the product of the proton density of the voxel and the first residual intensity of the MR signal of the voxel during the first scanning process as the first constant. Among them, the first residual intensity of the MR signal of the voxel during the first scanning process is: the sine value of the product of the proton density, flip angle and the absolute value of the relative intensity of the B1 + field removed from the voxel during the first scanning process, and the intensity of the MR signal remaining after the influence of these three factors, B1 — field. The first scanning process is: the scanning process of obtaining the first MR image.

[0159] Since the tissue contrast of the first MR image is made as low as possible by selecting scanning parameters, for any voxel of the target tissue, the product of the proton density of the voxel and the first remaining intensity of the MR signal of the voxel during the first scanning process is close to a constant. Here, let this constant be the first constant. It should be noted that the specific value of the first constant is not important because the B1 of the target tissue — The B1 of each voxel in the field map — The intensity of the field is inherently a relative value with respect to the intensity of the B1 of other voxels — In practical applications, for the convenience of calculation, the first constant can be directly set to 1.

[0160] That is, for any voxel r on the target tissue, M0(r)·f(T1(r), T2(r), TE, TR, α·|B1 + (r)|)≈constant. Usually, M0(r)·f(T1(r), T2(r), TE, TR, α·|B1 + (r)|)≈1, then in this step, M0(r)·f(T1(r), T2(r), TE, TR, α·|B1 + (r)|)=1.

[0161] Among them, M0(r) is the proton density of voxel r, T1(r) is the T1 of voxel r, T2(r) is the T2 of voxel r, TE is the echo time, TR is the repetition time, α is the set flip angle, and B1 + (r) is the relative intensity of the B1 field of voxel r + field, and f(T1(r), T2(r), TE, TR, α·|B1 + (r)|) is the first remaining intensity of the MR signal of voxel r.

[0162] Step 404: Multiply the first constant by the flip angle of the voxel during the first scanning process to obtain a second constant.

[0163] If the first constant is 1, then the second constant is equal to the flip angle α.

[0164] Step 405: Divide the intensity of the MR signal corresponding to the voxel on the first MR image by the second constant, and use the obtained quotient value as the absolute value of the product of the relative intensity of the B1 field of the voxel and the intensity of the B1 field. + field and the intensity of the B1 — field.

[0165] That is: if the second constant is equal to the flip angle α, then B1 ± (r)=|B1 — (r)|·|B1 +(r)| = S(r) / α. Wherein, S(r) is the MR signal intensity corresponding to voxel r in the first MR image.

[0166] Step 406: When the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field for all voxels of the target tissue is obtained, a preset three-dimensional spline fitting algorithm is used to fit the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field for all voxels of the target tissue, and a three-dimensional spline fitting graph of the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field for the target tissue is obtained.

[0167] The three-dimensional spline fitting algorithm is a mature technology. In the embodiments of the present invention, when performing three-dimensional spline fitting on B1 ± (r), multi-resolution control points can be used. For example: the distance between adjacent control points can be any two or all three of 22 mm (millimeters), 44 mm, and 88 mm.

[0168] Step 407: For any voxel of the target tissue, divide the signal intensity corresponding to the voxel on the three-dimensional spline fitting graph by the absolute value of the relative intensity of the B1 + field corresponding to the voxel on the B1 + field mapping graph to obtain the absolute value of the intensity of the B1 — field of the voxel.

[0169] That is, |B1 — (r)| = B 1s ± (r) / |B1 + (r)|

[0170] Wherein, B1 — (r) is the intensity of the B1 — field of voxel r, and B 1s ± (r) is the signal intensity corresponding to voxel r on the three-dimensional spline fitting graph.

[0171] When the absolute value of the intensity of the B1 — field of each voxel of the target tissue is obtained, the B1 — field mapping graph of the target tissue is obtained.

[0172] Step 408: For any voxel of the target tissue, according to the T1 and T2 of the voxel in the second scanning process and the B1 + field corresponding to the voxel on the B1 +The relative intensity of the field, and obtain the second remaining intensity of the MR signal of the voxel during the second scan. Wherein, the second remaining intensity of the MR signal of the voxel during the second scan is: after removing the influence of the proton density and B1 — field in the second scan process, the intensity of the remaining MR signal. The second scan process is: the scan process of obtaining the second MR image.

[0173] In practical applications, using the Bloch equation or the phase diagram theory, obtain the relative intensity of T1, T2, and B1 + field and the functional expression between the MR signal intensity. According to this functional expression, use this functional expression to calculate the T1, T2 of the voxel during the second scan and the B1 + field corresponding to the voxel on the B1 + field relative intensity, and obtain the second remaining intensity of the MR signal of the voxel during the second scan.

[0174] For example: For any voxel r on the target tissue, the second remaining intensity of the MR signal of the voxel during the second scan can be expressed as: g(T1(r), T2(r), |B1 + (r)|)

[0175] Wherein, T1(r) is the T1 of voxel r, T2(r) is the T2 of voxel r, and B1 + (r) is the relative intensity of the B1 + field of voxel r. The expression of g(T1(r), T2(r), |B1 + (r)|) can be obtained using the Bloch equation or the phase diagram theory.

[0176] If the target tissue is scanned in a parallel transmission mode, then for any voxel r on the target tissue, the second remaining intensity of the MR signal of the voxel during the second scan can be expressed as:

[0177] g(T1(r), T2(r), B 1,1 + (r), B 1,2 + (r), B 1,3 + (r),......, B 1,n-1 + (r), B 1,n + (r))

[0178] Wherein, n is the total number of transmission channels, and B 1,k + (r) is the B1 of voxel r for transmission channel k +Relative intensity of the field, 1 ≤ k ≤ n.

[0179] Step 409: Obtain B1 of the voxel — Multiply the absolute value of the intensity of the field and the second remaining intensity of the MR signal of the voxel during the second scanning process to obtain a fifth product.

[0180] For example: Fifth product = |B1 — (r)| · g(T1(r), T2(r), |B1 + (r)|)

[0181] where B1 — (r) is the intensity of B1 of voxel r — field.

[0182] Step 410: Divide the intensity of the MR signal corresponding to the voxel on the second MR image by the fifth product, and use the obtained quotient value as the intensity of the MR signal corresponding to the voxel on the second MR image after B1 inhomogeneity correction is completed.

[0183] For example: I cor (r) = I(r) / (|B1 — (r)| · g(T1(r), T2(r), |B1 + (r)|)

[0184] where I(r) is the intensity of the MR signal corresponding to voxel r on the second MR image, and I cor (r) is the intensity of the MR signal corresponding to voxel r on the second MR image after B1 inhomogeneity correction is completed.

[0185] It should be noted that in practical applications, in the above formula, the values of T1(r) and T2(r) adopt empirical constant values.

[0186] When the B1 field inhomogeneity correction of the intensity of the MR signal corresponding to each voxel of the target tissue is completed, a second MR image with B1 field inhomogeneity correction completed is obtained.

[0187] In the above second to fourth embodiments, since the estimation of the B1 - field map is added and the B1 field inhomogeneity is corrected through physical calculation without relying on an image model, the correction method is more stable, thus further optimizing the B1 field inhomogeneity correction effect in MRI.

[0188] The following gives the experimental results of the embodiments of the present invention, and this experiment is carried out on a 7T MRI system:

[0189] Figure 5Nine slices of a PD (Proton Density) weighted image obtained by performing an MR scan on the target brain tissue of a healthy volunteer using an SPGR (Spoiled Gradient Recalled echo) sequence. Imaging parameters: TE = 2.41 ms (milliseconds), TR = 20 ms, flip angle = 5°, FOV (Field of View) = 220 mm (millimeters) × 220 mm × 220 mm, imaging matrix = 80 × 80 × 80.

[0190] Figure 6 Nine slices of the B Figure 5 field map obtained according to the PD weighted image of 1s ± and the fourth embodiment of the present invention. The B 1s ± field map refers to a three-dimensional spline fitting map obtained by performing three-dimensional spline fitting on the B1 ± field map.

[0191] Figure 7 Nine slices of the |B1 + | field map of the target brain tissue obtained using a presaturated TFL sequence, with the slice positions being the same as Figure 6 . Imaging parameters: FOV = 220 mm × 220 mm × 220 mm, imaging matrix = 64 × 64 × 30.

[0192] Figure 8 Nine slices of a T2 weighted image obtained by performing an MR scan on the target brain tissue using a 3D-SPACE (3D Sampling Perfection with Application optimized Contrasts using different flip angle Evolution) sequence. Imaging parameters: TE / TR = 118 / 4000, ETL (Echo Train Length) = 119, FOV = 212 mm × 150 mm × 170 mm, imaging matrix = 318 × 224 × 256.

[0193] Figure 9 An image obtained by correcting Figure 8 using the existing N4ITK method, with the positions of the nine slices being the same as Figure 8 . It can be seen from the nine slices of Figure 9 that for most brain regions, the signal inhomogeneity has been improved, but in regions with a lower B1 field strength, the signal inhomogeneity has not been improved.

[0194] Figure 10 The image obtained after calibration using the method provided in the fourth embodiment of the present invention for Figure 8 has the same positions of 9 slices as Figure 8 . Among them, the function g(T1(r), T2(r), |B1 + (r)|) is obtained using the phase diagram theory. As can be seen from Figure 10 the 9 slices of

[0195] Figure 11 : The image intensity is more uniform, and the signal voids in the temporal lobe and cerebellum are covered. Figure 11 As can be seen from

[0196] Figure 12 : There are signal voids near the temporal lobe and the inferior lobe of the cerebellum. Figure 11 The image obtained after calibration using the method provided in the fourth embodiment of the present invention for Figure 11 has the same positions of 4 slices as + . Among them, the function g(T1(r), T2(r), |B1 Figure 12 As can be seen from

[0197] Figure 13 : The uniformity of the image is greatly improved. + field mapping diagram acquisition module 132, B1 — field mapping diagram acquisition module 133, and calibration module 134, where:

[0198] The first MR image acquisition module 131 is configured to scan a target tissue using a first pulse sequence to obtain a first MR image.

[0199] B1 + field mapping diagram acquisition module 132 is configured to acquire a B1 + field mapping diagram of the target tissue.

[0200] B1 — The B1 field map acquisition module 133 is configured to obtain the B1 field map of the target tissue according to the first MR image acquired by the first MR image acquisition module 131 and the B1 field map acquired by the B1 field map acquisition module 132 + field map acquired by the B1 field map acquisition module 132 + field map to obtain the B1 field map of the target tissue — field map.

[0201] The correction module 134 is configured to perform B1 field inhomogeneity correction on the second MR image of the target tissue according to the B1 field map acquired by the B1 field map acquisition module 132 and the B1 field map acquired by the B1 field map acquisition module 133; wherein, the second MR image is an MR image obtained by scanning the target tissue using any imaging protocol and any pulse sequence. + field map acquired by the B1 field map acquisition module 132 + field map and the B1 — field map acquired by the B1 field map acquisition module 133 — field map, and perform B1 field inhomogeneity correction on the second MR image of the target tissue; wherein, the second MR image is an MR image obtained by scanning the target tissue using any imaging protocol and any pulse sequence.

[0202] In an optional embodiment, the B1 — field map acquisition module 133 is specifically configured to:[[]]

[0203] For any voxel of the target tissue, according to the T1, T2, TE, TR, flip angle of the voxel during the first scan and the relative intensity of the B1 field corresponding to the voxel on the B1 field map, obtain the first remaining intensity of the MR signal of the voxel during the first scan; according to the intensity of the MR signal corresponding to the voxel on the first MR image, the proton density of the voxel, the first remaining intensity of the MR signal, the flip angle of the voxel during the first scan and the absolute value of the relative intensity of the B1 field corresponding to the voxel on the B1 field map, obtain the absolute value of the intensity of the B1 field of the voxel; and, the absolute values of the intensities of the B1 fields of all voxels of the target tissue constitute the B1 field map of the target tissue. Wherein, the first remaining intensity of the MR signal is: the intensity of the MR signal remaining after removing the influence of the product of the proton density, the flip angle and the absolute value of the relative intensity of the B1 field of the voxel during the first scan, and the first scan process is: the scan process for obtaining the first MR image. + field corresponding to the voxel on the B1 field map + field map to obtain the first remaining intensity of the MR signal of the voxel during the first scan; according to the intensity of the MR signal corresponding to the voxel on the first MR image, the proton density of the voxel, the first remaining intensity of the MR signal, the flip angle of the voxel during the first scan and the absolute value of the relative intensity of the B1 field corresponding to the voxel on the B1 field map + field corresponding to the voxel on the B1 field map + field map, obtain the absolute value of the intensity of the B1 field of the voxel; and, the absolute values of the intensities of the B1 fields of all voxels of the target tissue constitute the B1 field map of the target tissue. — field map. Wherein, the first remaining intensity of the MR signal is: the intensity of the MR signal remaining after removing the influence of the product of the proton density, the flip angle and the absolute value of the relative intensity of the B1 field of the voxel during the first scan, and the first scan process is: the scan process for obtaining the first MR image. — field map of the target tissue. — field map. Wherein, the first remaining intensity of the MR signal is: the intensity of the MR signal remaining after removing the influence of the product of the proton density, the flip angle and the absolute value of the relative intensity of the B1 field of the voxel during the first scan, and the first scan process is: the scan process for obtaining the first MR image. + field map to obtain the first remaining intensity of the MR signal of the voxel during the first scan; according to the intensity of the MR signal corresponding to the voxel on the first MR image, the proton density of the voxel, the first remaining intensity of the MR signal, the flip angle of the voxel during the first scan and the absolute value of the relative intensity of the B1 field corresponding to the voxel on the B1 field map — field map, obtain the absolute value of the intensity of the B1 field of the voxel; and, the absolute values of the intensities of the B1 fields of all voxels of the target tissue constitute the B1 field map of the target tissue. Wherein, the first remaining intensity of the MR signal is: the intensity of the MR signal remaining after removing the influence of the product of the proton density, the flip angle and the absolute value of the relative intensity of the B1 field of the voxel during the first scan, and the first scan process is: the scan process for obtaining the first MR image.

[0204] In an optional embodiment, the B1 — field map acquisition module 133 according to the intensity of the MR signal corresponding to the voxel on the first MR image, the proton density of the voxel, the first remaining intensity of the MR signal, the flip angle of the voxel during the first scan and the B1+ The corresponding B1 on the field map + The absolute value of the relative intensity of the field, and obtain the B1 of this voxel — The absolute value of the intensity of the field, including: multiplying the proton density of this voxel by the first remaining intensity of the MR signal to obtain a first product; multiplying the flip angle of this voxel during the first scan process by the B1 + The corresponding B1 on the field map + The absolute value of the relative intensity of the field to obtain a second product; obtaining the sine value of the second product; multiplying the first product by the sine value of the second product to obtain a third product; dividing the intensity of the MR signal corresponding to this voxel on the first MR image by the third product, and taking the obtained quotient value as the B1 of this voxel — The absolute value of the intensity of the field.

[0205] In an alternative embodiment, B1 — The field map acquisition module 133 obtains the first remaining intensity of the MR signal of this voxel during the first scan process according to the T1, T2, TE, TR, flip angle of this voxel during the first scan process and the B1 + The corresponding B1 on the field map + The relative intensity of the field, including: using the Bloch equation or the phase diagram theory to obtain the functional expression between T1, T2, TE, TR, flip angle and the relative intensity of B1 + And the intensity of the MR signal, and using this functional expression to calculate the T1, T2, TE, TR, flip angle of this voxel during the first scan process and the B1 + The corresponding B1 on the field map + The relative intensity of the field to obtain the first remaining intensity of the MR signal of this voxel during the first scan process.

[0206] In an alternative embodiment, the first pulse sequence adopted by the first MR image acquisition module 131 is: a gradient echo pulse sequence with a low flip angle;

[0207] B1 — The field map acquisition module 133 is specifically used for:

[0208] For any voxel of the target tissue, setting the product of the proton density of this voxel and the first remaining intensity of the MR signal of this voxel during the first scan process as a first constant; according to the intensity of the MR signal corresponding to this voxel on the first MR image, the first constant, the flip angle of this voxel during the first scan process and the B1 + The corresponding B1 on the field map + The absolute value of the relative intensity of the field, and obtain the B1 of this voxel —The absolute value of the field strength. Among them, the first remaining intensity of the MR signal is: the intensity of the MR signal remaining after removing the influence of the proton density, the sine value of the product of the flip angle and the absolute value of the relative intensity of the B1 + field, and the B1 — field from the voxel during the first scanning process. The first scanning process is: the scanning process for obtaining the first MR image.

[0209] In an alternative embodiment, the first pulse sequence adopted by the first MR image acquisition module 131 is: a gradient echo pulse sequence with a low flip angle;

[0210] B1 — The B1 field map acquisition module 133 is specifically configured to:

[0211] For any voxel of the target tissue, set the product of the proton density of the voxel and the first remaining intensity of the MR signal of the voxel during the first scanning process as the first constant; according to the intensity of the MR signal corresponding to the voxel in the first MR image, the first constant, and the flip angle of the voxel during the first scanning process, obtain the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field; when obtaining the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field for all voxels of the target tissue, use a preset three-dimensional spline fitting algorithm to fit the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field for all voxels of the target tissue to obtain a three-dimensional spline fitting map of the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field; for any voxel of the target tissue, divide the signal intensity corresponding to the voxel in the three-dimensional spline fitting map by the absolute value of the relative intensity of the B1 + field corresponding to the voxel in the B1 + field map to obtain the absolute value of the B1 — field of the voxel. Among them, the first remaining intensity of the MR signal is: the intensity of the MR signal remaining after removing the influence of the proton density, the sine value of the product of the flip angle and the absolute value of the relative intensity of the B1 + field, and the B1 — field from the voxel during the first scanning process. The first scanning process is: the scanning process for obtaining the first MR image.

[0212] In an alternative embodiment, the low flip angle pulse sequence adopted by the first MR image acquisition module 131 is: a gradient echo single echo sequence, a gradient echo multi echo sequence, or an EPI sequence.

[0213] In an optional embodiment, before the first MR image acquisition module 131 obtains the first magnetic resonance MR image, it further includes: when a plurality of MR images are obtained after scanning the target tissue with a first pulse sequence with a low flip angle, an image with the lowest tissue contrast is selected as the first MR image.

[0214] In an optional embodiment, the correction module 134 is specifically used for:

[0215] For any voxel of the target tissue, according to the T1 and T2 of the voxel in the second scanning process and the voxel in B1 + The corresponding B1 on the field map + The relative strength of the field is used to obtain the second residual strength of the MR signal of the voxel in the second scanning process; according to the strength of the MR signal corresponding to the voxel on the second MR image, the B1 of the voxel — The absolute value of the intensity of the field and the second residual intensity of the MR signal are used to obtain the intensity of the MR signal corresponding to the voxel on the second MR image after the B1 inhomogeneity correction is completed. The second residual intensity of the MR signal is: the absolute value of the intensity of the field and the second residual intensity of the MR signal of the voxel in the second scanning process after the proton density, B1 — The intensity of the remaining MR signal after the influence of the two fields, the second scanning process is: a scanning process for obtaining a second MR image.

[0216] In an optional embodiment, the correction module 134 calculates the value of the voxel in the second scanning process according to the T1 and T2 of the voxel and the B1 + The corresponding B1 on the field map + The relative strength of the field, obtaining the second residual strength of the MR signal of the voxel in the second scanning process, including: using the Bloch equation or phase map theory to obtain T1, T2 and B1 + The function expression between the relative intensity of the field and the MR signal intensity is used to calculate the T1 and T2 of the voxel in the second scanning process and the B1 of the voxel according to the function expression. + The corresponding B1 on the field map + The relative intensity of the field is calculated to obtain the second residual intensity of the MR signal of the voxel in the second scanning process.

[0217] The embodiment of the present invention further provides an MR scanner, comprising the B1 field inhomogeneity correction device 130 in MRI as described above.

[0218] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for correcting radiofrequency field inhomogeneity in magnetic resonance imaging, characterized in that, The method includes: Scanning a target tissue with a first pulse sequence to obtain a first magnetic resonance image; Obtain the transmitted radiofrequency field B1 of the target tissue + Field map; Based on the first magnetic resonance image and the B1 + field map, obtain the received radiofrequency field B1 — field map of the target tissue composed of the absolute value of the intensity of the B1 — field; According to the B1 + field map and the B1 — field map, perform B1 field inhomogeneity correction on the second magnetic resonance image of the target tissue; wherein, the second magnetic resonance image is a magnetic resonance image obtained by scanning the target tissue using any imaging protocol and any pulse sequence.

2. The method according to claim 1, wherein Based on the first magnetic resonance image and the B1 + field map, obtaining the B1 — field map of the target tissue, including: For any voxel of the target tissue, based on the longitudinal relaxation time T1, transverse relaxation time T2, echo time TE, repetition time TR, flip angle of the voxel in the first scanning process, and the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map, obtain the first remaining intensity of the magnetic resonance signal of the voxel in the first scanning process; wherein, the first remaining intensity of the magnetic resonance signal is: the remaining intensity of the magnetic resonance signal after removing the influence of the proton density, the sine value of the product of the flip angle and the absolute value of the relative intensity of the B1 + field, and the B1 — field in the first scanning process of the voxel; the first scanning process is: the scanning process for obtaining the first magnetic resonance image. Based on the intensity of the magnetic resonance signal corresponding to the voxel on the first magnetic resonance image, the proton density of the voxel, the first remaining intensity of the magnetic resonance signal, the flip angle of the voxel during the first scan, and the absolute value of the relative intensity of the B1 + field on the field map corresponding to the voxel, obtain the absolute value of the B1 + field intensity of the voxel. The absolute values of the B1 — field intensities of all voxels of the target tissue constitute the B1 — field map of the target tissue. — ​ 3. The method according to claim 2, wherein Based on the intensity of the magnetic resonance signal corresponding to the voxel on the first magnetic resonance image, the proton density of the voxel, the first remaining intensity of the magnetic resonance signal, the flip angle of the voxel during the first scan, and the absolute value of the relative intensity of the B1 + field on the field map, obtain the absolute value of the intensity of the B1 + field of the voxel, including: — ​ Multiply the proton density of the voxel by the first remaining intensity of the magnetic resonance signal to obtain a first product; multiply the flip angle of the voxel during the first scan by the absolute value of the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map to obtain a second product; obtain the sine value of the second product; multiply the first product by the sine value of the second product to obtain a third product; divide the intensity of the magnetic resonance signal corresponding to the voxel on the first magnetic resonance image by the third product, and use the obtained quotient value as the absolute value of the intensity of the B1 — field of the voxel.

4. The method according to claim 2, wherein According to the T1, T2, TE, TR, flip angle of the voxel in the first scanning process and the relative intensity of the B1 field corresponding to the voxel on the B1 field map, obtaining the first residual intensity of the magnetic resonance signal of the voxel in the first scanning process, including: + The B1 + field, obtaining the first residual intensity of the magnetic resonance signal of the voxel in the first scanning process, including: Using the Bloch equation or the phase diagram theory, the function expressions between the relative intensities of T1, T2, TE, TR, flip angle, and B1 field and the intensity of the magnetic resonance signal are obtained. Using this function expression, the T1, T2, TE, TR, flip angle of the voxel during the first scanning process and the relative intensity of B1 field corresponding to the voxel on the B1 field mapping diagram are calculated to obtain the first residual intensity of the magnetic resonance signal of the voxel during the first scanning process. + + + ​​​ 5. The method according to claim 1, wherein The first pulse sequence is: a gradient echo pulse sequence with a low flip angle; Based on the first magnetic resonance image and the B1 + field map, obtaining the B1 — field map of the target tissue, including: For any voxel of the target tissue, the product of the proton density of the voxel and the first remaining intensity of the magnetic resonance signal of the voxel during the first scanning process is set as a first constant; wherein, the first remaining intensity of the magnetic resonance signal is: after removing the influence of the product of the proton density, the sine value of the flip angle and the absolute value of the relative intensity of the B1 + field, and the B1 — field from the magnetic resonance signal remaining during the first scanning process, and the first scanning process is: the scanning process for obtaining the first magnetic resonance image; Based on the intensity of the magnetic resonance signal corresponding to the voxel on the first magnetic resonance image, the first constant, the flip angle of the voxel during the first scan, and the absolute value of the relative intensity of the B1 + field on the field map corresponding to the voxel, obtain the absolute value of the intensity of the B1 + field of the voxel. The absolute values of the intensities of the B1 — fields of all the voxels of the target tissue constitute the B1 — field map of the target tissue. The absolute values of the intensities of the B1 — fields of all the voxels of the target tissue constitute the B1 — field map of the target tissue. The absolute values of the intensities of the B1 — field map.

6. The method according to claim 1, characterized in that, The first pulse sequence is: a gradient echo pulse sequence with a low flip angle; Based on the first magnetic resonance image and the B1 + field map, obtaining the B1 — field map of the target tissue, including: For any voxel of the target tissue, the product of the proton density of the voxel and the first remaining intensity of the magnetic resonance signal of the voxel during the first scanning process is set as a first constant; wherein, the first remaining intensity of the magnetic resonance signal is: the intensity of the remaining magnetic resonance signal after removing the influences of the proton density, the sine value of the product of the flip angle and the absolute value of the relative intensity of the B1 + field, and the B1 — field from the magnetic resonance signal of the voxel during the first scanning process, and the first scanning process is: the scanning process for obtaining the first magnetic resonance image; Obtain the absolute value of the product of the relative intensity of the B1 field and the intensity of the B1 field of the voxel according to the intensity of the magnetic resonance signal corresponding to the voxel on the first magnetic resonance image, the first constant, and the flip angle of the voxel during the first scan process. + field and the B1 — field; When obtaining the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field for all voxels of the target tissue, a preset three-dimensional spline fitting algorithm is used to fit the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field for all voxels of the target tissue, and a three-dimensional spline fitting graph of the absolute value of the product of the relative intensity of the B1 + field and the intensity of the B1 — field for the target tissue is obtained; For any voxel of the target tissue, divide the signal intensity corresponding to the voxel on the three-dimensional spline fitting graph by the absolute value of the relative intensity of the B1 + field corresponding to the voxel on the B1 + field mapping graph to obtain the absolute value of the intensity of the B1 — field of the voxel. The absolute values of the intensities of the B1 — fields of all voxels of the target tissue constitute the B1 — field mapping graph of the target tissue.

7. The method according to claim 5 or 6, characterized in that The gradient echo pulse sequence with a low flip angle is: a gradient echo single echo sequence, a gradient echo multi - echo sequence, or an echo planar imaging sequence.

8. The method according to any one of claims 5 or 6, characterized in that Before obtaining the first magnetic resonance (MR) image, it further includes: When multiple MR images are obtained after scanning the target tissue with the first pulse sequence with a low flip angle, selecting the image with the lowest tissue contrast as the first magnetic resonance image.

9. The method according to any one of claims 1 to 6, characterized in that The said according to the said B1 + field map and the said B1 — field map, perform B1 field inhomogeneity correction on the second magnetic resonance image of the said target tissue, including: For any voxel of the target tissue, according to the longitudinal relaxation time T1 and transverse relaxation time T2 of the voxel during the second scanning process, and the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map, obtain the second residual intensity of the magnetic resonance signal of the voxel during the second scanning process, where the second residual intensity of the magnetic resonance signal is: the intensity of the magnetic resonance signal remaining after removing the influence of the proton density and the B1 — field during the second scanning process; the second scanning process is: the scanning process for obtaining the second magnetic resonance image. Based on the intensity of the magnetic resonance signal corresponding to the voxel on the second magnetic resonance image, the absolute value of the intensity of the B1 — field, and the second remaining intensity of the magnetic resonance signal, obtain the intensity of the magnetic resonance signal corresponding to the voxel on the second magnetic resonance image after B1 inhomogeneity correction is completed.

10. The method according to claim 9, wherein According to the longitudinal relaxation time T1, the transverse relaxation time T2 of the voxel during the second scanning process, and the relative intensity of the B1 + field corresponding to the voxel on the B1 + field map, obtaining a second remaining intensity of the magnetic resonance signal of the voxel during the second scanning process, including: Using Bloch equations or phase diagram theory, the relative intensities of T1, T2, and B1 + fields and the functional expression between the relative intensities of the magnetic resonance signal intensity are obtained. According to this functional expression, the T1 and T2 of the voxel during the second scan and the B1 + corresponding to the voxel on the B1 + field mapping diagram are calculated to obtain the second residual intensity of the magnetic resonance signal of the voxel during the second scan.

11. A radiofrequency field inhomogeneity correction device (80) in magnetic resonance imaging, characterized in that, The apparatus (80) includes: A first magnetic resonance (MR) image acquisition module (81) for scanning a target tissue with a first pulse sequence to obtain a first magnetic resonance image; Transmit RF B1 + A field map acquisition module (82) for acquiring the B1 + field map of the target tissue; Receive RF B1 — A field map acquisition module (83) for obtaining, according to a first magnetic resonance image and the B1 + field map, the B1 — field map composed of the absolute values of the intensities of the B1 — field of the target tissue; Calibration module (84) for performing B1 field inhomogeneity calibration on the second magnetic resonance image of the target tissue according to the B1 + field map and the B1 — field map.

12. A magnetic resonance scanner, characterized in that, Including the radio - frequency field inhomogeneity correction apparatus (80) in magnetic resonance imaging as described in claim 11.

Images