Magnetic resonance excitation method and device and magnetic resonance imaging system

Abstract

The embodiment of the invention discloses a magnetic resonance excitation method and device and a magnetic resonance imaging system. The method comprises the following steps: acquiring a signal uniformity rate of an MR image brought by a preset time domain layer selective pulse; obtaining a region of interest of a target tissue set by a user, and obtaining the thickness of the region of interest in the layer direction; calculating the actual excitation thickness in the layer direction according to the thickness of the region of interest and the signal uniformity of the MR image brought by the preset time domain layer selective pulse; the airspace layer selective pulse is obtained in the mode that the actual excitation thickness is not smaller than the half-peak width of the airspace layer selective pulse; according to the spatial domain layer selective pulse, determining the time-varying voltage of the corresponding time domain layer selective pulse; generating a corresponding time domain layer selective pulse according to the time-varying voltage of the time domain layer selective pulse; and exciting the target tissue by using the generated time domain layer selective pulse. According to the embodiment of the invention, the phenomenon of signal non-uniformity in the MR image is eliminated.

Description

Technical Field

[0001] The present invention relates to the field of MRI (Magnetic Resonance Imaging) technology, in particular to an MR (Magnetic Resonance) excitation method, device and MRI system. Background Art

[0002] Three-dimensional imaging is very popular in clinical MRI, and considering that non-selective excitation is prone to produce wrap-around artifacts, slice-selective excitation is very popular in three-dimensional MR sequences.

[0003] The profile of a slice-selective pulse usually has a relatively long transition band, which may lead to signal inhomogeneities in the slice direction. Figure 1 Schematic diagram of the existing use of layer-selective pulses to perform MR excitation on a water phantom. Among them: 11 is a time-domain layer-selective pulse, t represents time; 12 is a water phantom, 121 and 122 are interferences on the water phantom; 13 is a spatial-domain layer-selective pulse corresponding to 11, r represents different positions in the layer direction, 13 is the intensity of the MR signal generated in the layer direction of the water phantom after 11 is applied to a uniform water phantom, 131 and 132 are the peak points of 13, 133 and 134 are the half-peak points of 13, and 135 is the FWHM (Full Width at Half Peak). Maxima, half-peak width) is the width between 133 and 134, 136 is the flat area, that is, the area between 131 and 132, 137 and 138 are transition zones, that is, the area between 133 and 131 and the area between 132 and 134. It should be noted that the broad transition zone refers to the area between the lowest point on the left side of 13 and 131 and the area between 132 and the lowest point on the right side of 13. However, since the intensity of the MR signal corresponding to the area between the lowest point on the left side of 13 and 133 and the area between 134 and the lowest point on the right side of 13 is very low, and the MR signals generated by these two areas are very weak, the transition zone usually only focuses on the area between the half-peak point and the peak point; 123 is the FOV (Field Of View) set by the user. The user usually sets the FOV according to the FWHM and sets the width of the FOV slightly larger than the FWHM; 14 is the FOV expected by the user. 123 is the MR image corresponding to the user's expectation that there is no signal unevenness and curling artifact in 14; and 15 is the MR image corresponding to the actual FOV 123. It can be seen that there are signal unevenness and curling artifacts at the left and right edges of 15, wherein the signal unevenness is caused by the transition zones 137 and 138 in 13, and the curling artifact is caused by the fact that the FOV 123 set by the user is too small, resulting in the interference objects 121 and 122 curling into the FOV 123.

[0004] In order to avoid signal inhomogeneity and kink artifacts in three-dimensional MRI, a proposed solution is to increase the layer oversampling rate. However, this method can only reduce the kink artifacts in the layer direction, but cannot improve the signal inhomogeneity in the layer direction. Figure 2 The figure is a schematic diagram of an attempt to reduce signal inhomogeneity and curling artifacts in the layer direction by increasing the layer oversampling rate. Among them, 21 is a water phantom, 211 and 212 are interferences on the water phantom; 22 is the FOV set by the user; 23 is the MR image corresponding to the user-set FOV 22 after the water phantom is excited by layer-selective pulses. It can be seen that in 23, interferences 211 and 212 are curled onto FOV 22; 24 is the enlarged FOV obtained by the user by increasing the layer oversampling rate; 25 is the MR image corresponding to FOV 24; 26 is the MR image corresponding to the area where FOV 22 is located in 25. It can be seen that there is no curling artifact in 26. It can be seen that this method does avoid the curling artifact in the MR image area corresponding to the user-set FOV 22. However, increasing the layer oversampling rate cannot alleviate the signal inhomogeneity phenomenon in MR images because: Figure 1 It can be seen that the signal inhomogeneity in MR images is caused by the transition band in the slice selective pulse, and increasing the slice oversampling rate can only expand the FOV, but cannot make any changes to the transition band of the slice selective pulse. Summary of the invention

[0005] In view of this, the embodiments of the present invention provide, on the one hand, an MR excitation method and apparatus to eliminate the signal inhomogeneity phenomenon in MR images; and, on the other hand, provide an MRI system to eliminate the signal inhomogeneity phenomenon in MR images.

[0006] A magnetic resonance (MR) excitation method, the method comprising:

[0007] Acquire the signal uniformity of the MR image brought by the preset time-domain layer selective pulse;

[0008] Obtain the region of interest of the target tissue set by the user, and obtain the thickness of the region of interest in the layer direction;

[0009] Calculating the actual excitation thickness in the layer direction according to the thickness of the region of interest and the signal uniformity of the MR image brought by the preset time-domain layer selective pulse;

[0010] Obtaining a spatial layer selective pulse in such a manner that the actual excitation thickness is not less than the half maximum width FWHM of the spatial layer selective pulse;

[0011] According to the obtained spatial layer selective pulse, determine the voltage of the temporal layer selective pulse corresponding to the spatial layer selective pulse that changes with time;

[0012] generating a corresponding time-domain layer selective pulse according to the voltage of the time-domain layer selective pulse varying with time;

[0013] The generated time-domain slice-selective pulses are used to excite the target tissue.

[0014] The method of obtaining the signal uniformity of the MR image brought by the preset time-domain layer selective pulse comprises:

[0015] The uniform water phantom is excited by using a preset time-domain layer-selective pulse to obtain an MR image;

[0016] Find areas of signal inhomogeneity in MR images;

[0017] According to the signal inhomogeneity region in the MR image, the signal uniformity of the MR image brought by the preset time-domain layer selective pulse is calculated.

[0018] The method of calculating the signal uniformity of the MR image brought by the preset time-domain layer selective pulse according to the signal inhomogeneous area in the MR image includes:

[0019] Calculating the ratio of the size of the signal inhomogeneity region in the MR image to the total size of the MR image, and using the ratio as the signal inhomogeneity rate of the MR image caused by the preset time-domain layer selective pulse;

[0020] According to the signal inhomogeneity of the MR image brought by the preset time-domain slice selective pulse, the signal uniformity of the MR image brought by the preset time-domain slice selective pulse is calculated.

[0021] The step of searching for an area with inhomogeneous signals in an MR image comprises:

[0022] The maximum signal intensity is found in the MR image, the minimum uniform signal intensity is calculated according to the maximum signal intensity and the preset weight, and the area with the signal intensity less than the minimum uniform signal intensity is found in the MR image, wherein the area with the signal intensity less than the minimum uniform signal intensity is the signal inhomogeneous area.

[0023] The preset weight satisfies: greater than or equal to 0.6 and less than or equal to 0.8.

[0024] After the actual excitation thickness in the layer direction is calculated, the method further comprises:

[0025] Calculating the difference between the actual excitation thickness and the thickness of the region of interest, and calculating the layer overexcitation rate according to the difference and the thickness of the region of interest;

[0026] It is determined whether the layer oversampling rate set by the user is not less than the layer overexcitation rate. If so, the layer oversampling rate set by the user is kept unchanged; otherwise, the layer oversampling rate input by the user is adjusted to be equal to the layer overexcitation rate.

[0027] After the target tissue is excited by the generated time-domain layer selective pulse, the method further comprises:

[0028] The actual encoding FOV is determined according to the layer oversampling rate set by the user and the field of view FOV set by the user. The excited MR signal is encoded according to the actual encoding FOV to obtain an MR image.

[0029] A magnetic resonance (MR) excitation device, comprising:

[0030] A signal uniformity acquisition module is used to acquire the signal uniformity of the MR image brought by the preset time-domain layer selective pulse;

[0031] The pulse design module is used to obtain the region of interest of the target tissue set by the user, and obtain the thickness of the region of interest in the layer direction; calculate the actual excitation thickness in the layer direction according to the thickness of the region of interest and the signal uniformity of the MR image brought by the preset time-domain layer-selective pulse; obtain the spatial layer-selective pulse in a manner that the actual excitation thickness is not less than the half-peak width FWHM of the spatial layer-selective pulse; determine the voltage of the time-varying ...

[0032] The excitation module is used to generate a corresponding time-domain layer selective pulse according to the voltage of the time-domain layer selective pulse that changes with time, and use the generated time-domain layer selective pulse to excite the target tissue.

[0033] The signal uniformity acquisition module acquires the signal uniformity of the MR image brought by the preset time-domain layer selective pulse, including:

[0034] The uniform water phantom is excited by using a preset time-domain layer-selective pulse to obtain an MR image;

[0035] Find areas of signal inhomogeneity in MR images;

[0036] According to the signal inhomogeneity region in the MR image, the signal uniformity of the MR image brought by the preset time-domain layer selective pulse is calculated.

[0037] The signal uniformity acquisition module calculates the signal uniformity of the MR image brought by the preset time-domain layer selective pulse according to the signal inhomogeneous area in the MR image, including:

[0038] Calculating the ratio of the size of the signal inhomogeneity region in the MR image to the total size of the MR image, and using the ratio as the signal inhomogeneity rate of the MR image caused by the preset time-domain layer selective pulse;

[0039] According to the signal inhomogeneity of the MR image brought by the preset time-domain slice selective pulse, the signal uniformity of the MR image brought by the preset time-domain slice selective pulse is calculated.

[0040] The signal uniformity acquisition module searches for signal inhomogeneity areas in the MR image, including:

[0041] The maximum signal intensity is found in the MR image, the minimum uniform signal intensity is calculated according to the maximum signal intensity and the preset weight, and the area with the signal intensity less than the minimum uniform signal intensity is found in the MR image, wherein the area with the signal intensity less than the minimum uniform signal intensity is the signal inhomogeneous area.

[0042] The device further comprises:

[0043] a layer over-excitation rate calculation module, used to calculate the difference between the actual excitation thickness and the thickness of the region of interest, and calculate the layer over-excitation rate according to the difference and the thickness of the region of interest;

[0044] The layer oversampling rate adjustment module is used to determine whether the layer oversampling rate set by the user is not less than the layer overexcitation rate. If so, the layer oversampling rate set by the user is kept unchanged; otherwise, the layer oversampling rate input by the user is adjusted to be equal to the layer overexcitation rate.

[0045] The device further comprises:

[0046] The FOV adjustment and encoding module is used to determine the actual encoding FOV according to the layer oversampling rate set by the user and the field of view FOV set by the user, and encode the excited MR signal according to the actual encoding FOV to obtain the MR image.

[0047] A magnetic resonance imaging (MRI) system comprises any one of the above-mentioned magnetic resonance (MR) excitation devices.

[0048] In an embodiment of the present invention, by acquiring the signal uniformity of the MR image brought by the time-domain layer selective pulse to be used, the actual excitation thickness in the layer direction is calculated according to the thickness of the region of interest in the layer direction and the signal uniformity of the MR image brought by the time-domain layer selective pulse, so that the actual excitation thickness in the layer direction is not less than the FWHM of the spatial layer selective pulse, a spatial layer selective pulse is obtained, the voltage of the corresponding time-domain layer selective pulse that varies with time is determined according to the spatial layer selective pulse, and the corresponding time-domain layer selective pulse is generated according to the voltage of the time-domain layer selective pulse that varies with time, and the generated time-domain layer selective pulse is used to excite the target tissue, so that: the flat area of ​​the spatial layer selective pulse can cover the region of interest in the layer direction, so that there is no signal unevenness in the MR imaging area corresponding to the region of interest, thereby improving the quality of the MR image. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings so that the above and other features and advantages of the present invention will be more apparent to those skilled in the art. In the accompanying drawings:

[0050] Figure 1 A schematic diagram of the existing method of using layer-selective pulses to perform MR excitation on a water phantom;

[0051] Figure 2 A schematic diagram of existing attempts to reduce signal inhomogeneity and warping artifacts in the slice direction by increasing the slice oversampling rate;

[0052] Figure 3 Schematic diagram of excitation when the excitation thickness in the layer direction is equal to the thickness of the user's region of interest and is greater than the thickness of the user's region of interest;

[0053] Figure 4A A flow chart of an MR excitation method provided by an embodiment of the present invention;

[0054] Figure 4B A flow chart of obtaining the signal uniformity of an MR image brought about by a preset time-domain layer selective pulse provided by an embodiment of the present invention;

[0055] Figure 5 Schematic diagram of the process of calculating the signal inhomogeneity rate of MR images caused by time-domain slice selective pulses;

[0056] Figure 6 A schematic diagram for comparing spatial-domain layer-selective pulses and FOVs when the brain is subjected to three solutions, namely, the existing solution without layer oversampling and layer overexcitation, the existing solution with layer oversampling, and the solution with layer overexcitation and layer oversampling according to an embodiment of the present invention;

[0057] Figure 7 It is a schematic diagram for comparing the spatial domain layer selective pulses, FOV and the obtained MR images under three schemes: the existing scheme without layer oversampling and layer overexcitation, the existing scheme with layer oversampling, and the scheme with layer overexcitation and layer oversampling according to the embodiment of the present invention;

[0058] Figure 8 It is a schematic diagram for comparing MR images obtained by adopting the existing solution without slice oversampling and slice excitation and by adopting the solution with slice overexcitation and slice oversampling according to the embodiment of the present invention in this application example;

[0059] Fig. 9 A schematic structural diagram of an MR excitation device 90 provided in one embodiment of the present invention.

[0060] The reference numerals are as follows:

[0061]

[0062]

[0063]

[0064] DETAILED DESCRIPTION

[0065] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail with reference to the following embodiments.

[0066] The inventors analyzed the existing schemes that attempt to reduce the signal inhomogeneity and the wrap-around artifacts in the layer direction by increasing the layer oversampling rate and found that the scheme can only eliminate the wrap-around artifacts in the FOV set by the user, but cannot eliminate the signal inhomogeneity in the FOV set by the user. The reason is that the scheme only expands the FOV, and the signal inhomogeneity in the MR image is caused by the transition band in the layer selective pulse. Increasing the layer oversampling rate cannot make any changes to the transition band of the layer selective pulse.

[0067] The inventor thus came up with the idea that since signal inhomogeneity always occurs at the edge of the excitation area (i.e., in the edge excitation layer), the signal inhomogeneity in the user-set FOV can be eliminated by increasing the excitation thickness in the layer direction (i.e., the thickness of the excitation area in the layer direction). This is because: when the excitation thickness in the layer direction increases, since the signal inhomogeneity is always located at the edge of the excitation area, there will be no signal inhomogeneity in the user-set FOV area in the middle of the excitation area (i.e., in the middle excitation layer). Figure 3 Schematic diagram of excitation when the excitation thickness in the layer direction is equal to the thickness of the user's region of interest (or user's focus area) and is greater than the thickness of the user's region of interest. In practical applications, the FOV set by the user is usually equal to or slightly greater than the user's region of interest, where:

[0068] 31 is a schematic diagram of MR excitation when the excitation thickness in the slice direction is equal to the thickness of the region of interest. 311 is the region of interest and also the excitation region. 312 is the spatial slice selective pulse determined according to 311. 3121 and 3122 are the peak points of 312. 3123 and 3124 are the half-peak points of 312. In practical applications, the spatial slice selective pulse is usually designed based on the principle that the FWHM of the spatial slice selective pulse is equal to the thickness of the region of interest. Therefore, it can be seen that the FWHM of 312 is equal to the thickness of 311, that is, at this time, the excitation thickness in the slice direction is equal to the thickness of the region of interest. Then, due to the existence of transition zones 3125 and 3126, the obtained MR image will have signal inhomogeneity at the edge in the slice direction.

[0069] 32 is a schematic diagram of excitation when the excitation thickness in the layer direction is greater than the thickness of the region of interest. 321 is the excitation region, 322 is the spatial layer selective pulse, 3221 and 3222 are the peak points of 322, 3223 and 3224 are the half-peak points of 322, and it can be seen that the FWHM of 322 is greater than the thickness of 311, wherein the transition zones 3225 and 3226 of 322 are located outside the region of interest, and the flat region 3227 at the center of 322 covers 311, that is, at this time, the excitation thickness in the layer direction is greater than the thickness of the region of interest, then although the existence of the transition zones 3225 and 3226 will cause the edge of the obtained MR image to still have signal inhomogeneity, but there is no signal inhomogeneity in the middle area of ​​the MR image, and the middle area of ​​the MR image is the region of interest, that is, there is no signal inhomogeneity in the region of interest.

[0070] It can be seen from 32 that when the flat area of ​​the spatial layer selective pulse can cover the region of interest, there is no signal inhomogeneity in the region of interest. At this time, the signal inhomogeneity area excited by the transition zone area of ​​the spatial layer selective pulse is located outside the region of interest.

[0071] Based on the above analysis, the inventors have come up with the following MR excitation scheme to eliminate signal inhomogeneity in MR images:

[0072] Figure 4A The flowchart of the MR excitation method provided in one embodiment of the present invention, the specific steps are as follows:

[0073] Step 401: Acquire the signal uniformity of the MR image brought by the preset time-domain layer selective pulse.

[0074] Figure 4B A flowchart for obtaining a signal uniformity rate of an MR image brought by a preset time-domain layer selective pulse provided by an embodiment of the present invention specifically includes the following steps 4011-4013:

[0075] Step 4011: Use a preset layer-selective pulse to excite the uniform water phantom to obtain an MR image.

[0076] Step 4012: Search for signal inhomogeneity areas in the MR image.

[0077] In an optional embodiment, this step 4012 specifically includes: finding the maximum signal intensity in the MR image, calculating the minimum uniform signal intensity according to the maximum signal intensity and the preset weight, and finding the area in the MR image where the signal intensity is less than the minimum uniform signal intensity, wherein the area where the signal intensity is less than the minimum uniform signal intensity is a signal inhomogeneous area. Specifically, the minimum uniform signal intensity can be obtained by multiplying the maximum signal intensity by the preset weight. Among them, 0.6≤preset weight≤0.8, preferably, the preset weight=0.7.

[0078] Step 4013: Calculate the signal uniformity of the MR image brought by the preset time-domain layer selective pulse according to the signal inhomogeneity region in the MR image.

[0079] In an optional embodiment, this step 4013 specifically includes: calculating the ratio of the size of the signal inhomogeneity region in the MR image to the total size of the MR image, and using the ratio as the signal inhomogeneity rate of the MR image caused by the preset time-domain layer selective pulse; and calculating the signal uniformity rate of the MR image caused by the preset time-domain layer selective pulse according to the signal inhomogeneity rate of the MR image caused by the preset time-domain layer selective pulse. Specifically, the signal uniformity rate of the MR image caused by the preset time-domain layer selective pulse can be obtained by subtracting the signal inhomogeneity rate of the MR image caused by the preset time-domain layer selective pulse from 1.

[0080] Figure 5 Schematic diagram of the process for calculating the signal inhomogeneity of MR images caused by a preset time-domain slice-selective pulse.

[0081] Wherein: 51 is the spatial domain layer selective pulse corresponding to the preset time domain layer selective pulse, 511 and 512 are the peak points of 51, 513 and 514 are the half-peak points of 51; 52 is the uniform water model, 521 is the FOV. It should be noted that when calculating the signal inhomogeneity rate of the MR image brought by the preset time domain layer selective pulse, the imaging area we focus on is the area corresponding to the FWHM, because the intensity of the MR signal in the area from the lowest point of 51 to the half-peak point is very low. In practical applications, this area is usually not concerned; 53 is the MR image corresponding to FOV 521; 54 is the signal relative intensity change curve of 53 in the layer direction, wherein the signal intensity of 53 in the layer direction is normalized to the interval [0,1], and the signal relative intensity R of 53 in the layer direction is obtained. s , the horizontal coordinate r of 54 is the different positions of 53 in the layer direction, and the vertical coordinate R sis the relative signal strength, here, the preset weight is set to 0.7, and the size of the area 531 at the left edge where the relative signal strength is less than 0.7 is S1, the size of the area 532 at the right edge where the relative signal strength is less than 0.7 is S2, and the total size of 53 is S, then the signal inhomogeneity rate of the MR image caused by the preset time-domain layer selective pulse = (S1+S2) / S. Among them, the size of the area corresponding to 541 is S-(S1+S2), and the size of the area corresponding to 542 is S.

[0082] Step 402: Acquire the region of interest of the target tissue set by the user, and acquire the thickness of the region of interest in the layer direction.

[0083] Step 403: Calculate the actual excitation thickness in the slice direction according to the thickness of the region of interest in the slice direction and the signal uniformity of the MR image brought by the preset time-domain slice selective pulse.

[0084] Specifically, the thickness of the region of interest in the slice direction is divided by the signal uniformity of the MR image brought by the preset time-domain slice-selective pulse to obtain the actual excitation thickness in the slice direction.

[0085] Step 404: Obtain a spatial layer selective pulse in such a way that the actual excitation thickness in the layer direction is not less than the FWHM of the spatial layer selective pulse.

[0086] Step 405: According to the obtained spatial layer selective pulse, determine the voltage of the temporal layer selective pulse corresponding to the spatial layer selective pulse that varies with time.

[0087] Step 406: Generate a corresponding time domain layer selective pulse according to the voltage of the time domain layer selective pulse that varies with time.

[0088] Step 407: Use the generated time-domain layer selective pulse to excite the target tissue.

[0089] It should be noted that for each type of time-domain layer selective pulse, step 401 only needs to be executed once. After the signal uniformity of the MR image brought about by the time-domain layer selective pulse of this type is obtained, when the time-domain layer selective pulse of this type is used for MR excitation thereafter, the actual excitation thickness in the layer direction of this MR excitation can be directly calculated using the signal uniformity of the MR image brought about by the time-domain layer selective pulse of this type.

[0090] In the above embodiment, by acquiring the signal uniformity of the MR image brought by the time-domain layer selective pulse to be used, and then calculating the actual excitation thickness in the layer direction according to the thickness of the region of interest in the layer direction and the signal uniformity of the MR image brought by the time-domain layer selective pulse, the spatial layer selective pulse is obtained in a manner that the actual excitation thickness in the layer direction is not less than the FWHM of the spatial layer selective pulse, and the time-varying voltage of the corresponding time-domain layer selective pulse is determined according to the spatial layer selective pulse, and the corresponding time-domain layer selective pulse is generated according to the time-varying voltage of the time-domain layer selective pulse, and the generated time-domain layer selective pulse is used to excite the target tissue, so that: the flat area of ​​the spatial layer selective pulse can cover the region of interest, so that there is no signal unevenness in the MR imaging area corresponding to the region of interest, thereby improving the quality of the MR image.

[0091] In an optional embodiment, in step 403, after calculating the actual excitation thickness in the layer direction, the method further includes: calculating the difference between the actual excitation thickness in the layer direction and the thickness of the region of interest in the layer direction, and calculating the layer over-excitation rate based on the difference and the thickness of the region of interest in the layer direction; judging whether the layer oversampling rate set by the user is not less than the layer over-excitation rate, and if so, keeping the layer oversampling rate set by the user unchanged; otherwise, adjusting the layer oversampling rate input by the user to be equal to the layer over-excitation rate.

[0092] The layer overexcitation rate is calculated according to the difference and the thickness of the region of interest in the layer direction, which can be obtained by dividing the difference by the thickness of the region of interest in the layer direction to obtain the layer overexcitation rate.

[0093] In an optional embodiment, after step 407, the method further includes: determining an actual encoding FOV according to a layer oversampling rate set by a user and a FOV set by a user, encoding the excited MR signal according to the actual encoding FOV, and obtaining an MR image.

[0094] In the above embodiment, by comparing the layer excitation rate and the layer oversampling rate, the layer oversampling rate is always not less than the layer excitation rate, so that there is neither signal unevenness nor warping artifacts in the FOV set by the user.

[0095] Figure 6 The diagram is a comparison diagram of the spatial-domain layer-selective pulses and FOVs used when the brain is subjected to the existing non-layer oversampling and layer overexcitation, the existing layer oversampling, and the layer overexcitation and layer oversampling of the embodiment of the present invention. Among them:

[0096] The two figures in the first column correspond to the existing scheme without layer oversampling and layer overexcitation, 60 is the brain, 611 is the spatial layer selective pulse, 6111 and 6112 are the half-peak points of 61, and 612 is the FOV;

[0097] The two figures in the second column correspond to the existing scheme for layer oversampling, where the spatial layer selective pulse is the same as 611, and 622 is the FOV; it can be seen that the FOV is expanded;

[0098] The two figures in the third column correspond to the solution of both layer overexcitation and layer oversampling in the embodiment of the present invention, wherein 631 is a spatial layer selective pulse, 6311 and 6312 are the half-peak points of 631. Since layer overexcitation is performed, the flat area of ​​631 is wider than that of 611, and the flat area needs to cover the FOV 612 set by the user. 632 is the actual FOV. Since layer oversampling is performed and the layer oversampling rate is not less than the layer overexcitation rate, the width of 632 must be not less than FWHM.

[0099] Figure 7 The figure is a schematic diagram comparing the spatial layer selective pulses, FOV and the obtained MR images under the three schemes of the existing scheme without layer oversampling and layer overexcitation, the existing scheme with layer oversampling and the embodiment of the present invention with layer overexcitation and layer oversampling.

[0100] The three figures in the first column correspond to the existing scheme without layer oversampling and layer overexcitation, 711 is the spatial layer selective pulse, 7111 and 7112 are the half-peak points of 711, 70 is the uniform water model, 712 is the FOV, and 713 is the MR image; it can be seen that there is signal inhomogeneity at the edge of 713;

[0101] The three figures in the second column correspond to the existing layer oversampling scheme, where the spatial layer selective pulse is the same as 711, 722 is the FOV, and 723 is the MR image; it can be seen that there is still signal unevenness at the edge of 723; it should be noted that the leftmost and rightmost sides of 723 are difficult to distinguish with the naked eye due to low brightness;

[0102] The three figures in the third column correspond to the solution of performing both layer overexcitation and layer oversampling according to the embodiment of the present invention, wherein 731 is a spatial layer selective pulse, 7311 and 7312 are the peak points of 711, 7313 and 7314 are the half-peak points of 711, and due to layer overexcitation, the flat area of ​​731 is wider than that of 711, and the flat area needs to cover the FOV 712 set by the user, 732 is the actual FOV, and due to layer oversampling, and the layer oversampling rate is not less than the layer overexcitation rate, the width of 732 must be not less than the FWHM, and 733 is the MR image; it can be seen that there is still signal unevenness at the edge of 733, but there is no signal unevenness within the FOV 712 set by the user, and since the actual FOV 732 is larger than the FOV 712 set by the user, there is substantially no wrap-around artifact within the FOV 712.

[0103] In order to verify the technical effect of the embodiment of the present invention, the following application examples are given:

[0104] In this example, a 3D GRE (Gradient Echo) sequence is used to perform a water phantom experiment on a 3T (Tesla) magnetic resonance system equipped with a 16-channel head / neck coil, wherein the shape of the uniform water phantom is spherical. The imaging parameters are: TE (Time Echo) = 40 ms, TR (Time Repetition) = 49 ms, FOV = 230x230 mm, thickness of each layer = 1 mm, image / layer = 60, basic resolution = 256, phase resolution = 100%, layer oversampling rate = 100%, wherein the layer oversampling rate is set to 100% because the layer oversampling rate is set to be large to ensure that the layer oversampling does not reduce the signal inhomogeneity caused by the pulse transition band.

[0105] Figure 8 Schematic diagram of comparison of MR images obtained by adopting the existing solution without layer oversampling and layer excitation and the solution with layer overexcitation and layer oversampling according to the embodiment of the present invention in this application example; wherein: 80 is a spherical uniform water phantom;

[0106] First, the FWHM of the spatial domain 3D GRE sequence is set to be equal to the thickness of the water model in the layer direction, and then the temporal domain 3D GRE sequence corresponding to the spatial domain pulse is used to scan layers 1 to 60 to obtain MR images a1 to a60. The excitation area is shown in 801, where the MR images corresponding to layers 1 to 4, 57 to 60 are shown in a1 to a4, a57 to a60 in 82. It can be seen that the signal clarity of the MR images a1, a2, a59, and a60 corresponding to the edge layers 1, 2, 59, and 60 is significantly lower than that of the MR images a3, a4, a57, and a68 corresponding to the relatively middle layers 3, 4, 57, and 58. This is because the transition zone of the pulse causes the signal inhomogeneity at the edge layer. It can also be seen that the signal inhomogeneity problem cannot be solved by unilaterally increasing the layer oversampling rate, because, in this example, the layer oversampling rate = 100%, which has been set very high.

[0107] Then, according to the signal inhomogeneity rate of 0.2 of the MR image brought by the pre-acquired 3D GRE sequence, the FWHM of the spatial 3DGRE pulse is set equal to the thickness of the water phantom in the layer direction / 0.8, and then the time domain 3DGR sequence corresponding to the spatial pulse is used to scan layers 1 to 60 to obtain MR images b1~b60, among which the MR images corresponding to layers 1~4, 57~60 are shown as b1~b4, b57~b60 in 83, and the signal clarity of the MR images b1, b2, b59, b60 corresponding to the edge layers 1, 2, 59, 60 is not much different from the MR images b3, b4, b57, b58 corresponding to the relatively middle layers 3, 4, 57, 58, showing a normal signal level, and the signal inhomogeneity phenomenon is greatly reduced.

[0108] Fig. 9 The structure diagram of the MR excitation device 90 provided in one embodiment of the present invention mainly includes: a signal uniformity acquisition module 91, a pulse design module 92 and an excitation module 93, wherein:

[0109] The signal uniformity acquisition module 91 is used to acquire the signal uniformity of the MR image brought by the preset time-domain layer selective pulse.

[0110] The pulse design module 92 is used to obtain the region of interest of the target tissue set by the user and the thickness of the region of interest in the layer direction; calculate the actual excitation thickness in the layer direction according to the thickness of the region of interest in the layer direction and the signal uniformity of the MR image brought by the preset time-domain layer-selective pulse; obtain the spatial layer-selective pulse in a manner that the actual excitation thickness in the layer direction is not less than the FWHM of the spatial layer-selective pulse; and determine the voltage of the time-varying ...

[0111] The excitation module 93 is used to generate a corresponding time-domain layer selective pulse according to the voltage of the time-domain layer selective pulse that varies with time, and use the generated time-domain layer selective pulse to excite the target tissue.

[0112] In an optional embodiment, a signal uniformity acquisition module acquires the signal uniformity of an MR image brought about by a preset time-domain layer selective pulse, including: using a preset time-domain layer selective pulse to excite a uniform water model to obtain an MR image; searching for a signal inhomogeneous area in the MR image; and calculating the signal uniformity of the MR image brought about by the preset time-domain layer selective pulse based on the signal inhomogeneous area in the MR image.

[0113] In an optional embodiment, the signal uniformity acquisition module 91 calculates the signal uniformity of the MR image brought by the preset time-domain layer selective pulse according to the signal inhomogeneity area in the MR image, including: calculating the ratio of the size of the signal inhomogeneity area in the MR image to the total size of the MR image, and using the ratio as the signal inhomogeneity of the MR image brought by the preset time-domain layer selective pulse; and calculating the signal uniformity of the MR image brought by the preset time-domain layer selective pulse according to the signal inhomogeneity of the MR image brought by the preset time-domain layer selective pulse.

[0114] In an optional embodiment, the signal uniformity acquisition module 91 searches for signal inhomogeneity areas in the MR image, including: searching for the maximum signal intensity in the MR image, calculating the minimum uniform signal intensity based on the maximum signal intensity and a preset weight, and searching for an area in the MR image where the signal intensity is less than the minimum uniform signal intensity, wherein the area where the signal intensity is less than the minimum uniform signal intensity is a signal inhomogeneity area.

[0115] In an optional embodiment, the above device 90 further comprises: a layer over-excitation rate calculation module 94 and a layer over-sampling rate adjustment module 95, wherein:

[0116] A layer overexcitation rate calculation module 94 is used to calculate the difference between the actual excitation thickness in the layer direction and the thickness of the region of interest in the layer direction, and calculate the layer overexcitation rate according to the difference and the thickness of the region of interest in the layer direction;

[0117] The layer oversampling rate adjustment module 95 is used to determine whether the layer oversampling rate set by the user is not less than the layer overexcitation rate. If so, the layer oversampling rate set by the user is kept unchanged; otherwise, the layer oversampling rate input by the user is adjusted to be equal to the layer overexcitation rate.

[0118] In an optional embodiment, the above-mentioned device 90 further includes: a FOV adjustment and encoding module 96, which is used to determine the actual encoding FOV according to the layer oversampling rate set by the user and the FOV set by the user, and encode the excited MR signal according to the actual encoding FOV to obtain an MR image.

[0119] An embodiment of the present invention further provides an MRI system, which includes the MR excitation device 90 as described in any of the above embodiments.

[0120] Those skilled in the art will appreciate that the features described in the various embodiments and / or claims of the present disclosure may be combined and / or combined in a variety of ways, even if such combinations or combinations are not explicitly described in the present application. In particular, without departing from the spirit and teachings of the present application, the features described in the various embodiments and / or claims of the present application may be combined and / or combined in a variety of ways, and all of these combinations and / or combinations fall within the scope disclosed in the present application.

[0121] Specific embodiments are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and core ideas of the present application, and is not used to limit the present application. For those skilled in the art, changes can be made in the specific implementation methods and application scopes according to the ideas, spirits and principles of the present application, and any modifications, equivalent substitutions, improvements, etc. made by them should be included in the scope of protection of the present application.

Claims

1. A magnetic resonance (MR) excitation method, characterized in that: The method includes: Acquire the signal uniformity of the MR image brought by the preset time-domain layer selective pulse; Obtain the region of interest of the target tissue set by the user, and obtain the thickness of the region of interest in the layer direction; Calculating the actual excitation thickness in the layer direction according to the thickness of the region of interest and the signal uniformity of the MR image brought by the preset time-domain layer selective pulse; Obtaining a spatial layer selective pulse in such a manner that the actual excitation thickness is not less than the half maximum width FWHM of the spatial layer selective pulse; According to the obtained spatial layer selective pulse, determine the voltage of the temporal layer selective pulse corresponding to the spatial layer selective pulse that changes with time; generating a corresponding time-domain layer selective pulse according to the voltage of the time-domain layer selective pulse varying with time; The generated time-domain slice-selective pulses are used to excite the target tissue.

2. The method according to claim 1, characterized in that: The method of obtaining the signal uniformity of the MR image brought by the preset time-domain layer selective pulse comprises: The uniform water phantom is excited by using a preset time-domain layer-selective pulse to obtain an MR image; Find areas of signal inhomogeneity in MR images; According to the signal inhomogeneity region in the MR image, the signal uniformity of the MR image brought by the preset time-domain layer selective pulse is calculated.

3. The method according to claim 2, characterized in that The method of calculating the signal uniformity of the MR image brought by the preset time-domain layer selective pulse according to the signal inhomogeneous area in the MR image includes: Calculating the ratio of the size of the signal inhomogeneity region in the MR image to the total size of the MR image, and using the ratio as the signal inhomogeneity rate of the MR image caused by the preset time-domain layer selective pulse; According to the signal inhomogeneity of the MR image brought by the preset time-domain slice selective pulse, the signal uniformity of the MR image brought by the preset time-domain slice selective pulse is calculated.

4. The method according to claim 2 or 3, characterized in that: The step of searching for an area with inhomogeneous signals in an MR image comprises: The maximum signal intensity is found in the MR image, the minimum uniform signal intensity is calculated according to the maximum signal intensity and the preset weight, and the area with the signal intensity less than the minimum uniform signal intensity is found in the MR image, wherein the area with the signal intensity less than the minimum uniform signal intensity is the signal inhomogeneous area.

5. The method according to claim 4, characterized in that The preset weight satisfies: greater than or equal to 0.6 and less than or equal to 0.

8.

6. The method according to any one of claims 1 to 3, characterized in that: After the actual excitation thickness in the layer direction is calculated, the method further comprises: Calculating the difference between the actual excitation thickness and the thickness of the region of interest, and calculating the layer overexcitation rate according to the difference and the thickness of the region of interest; It is determined whether the layer oversampling rate set by the user is not less than the layer overexcitation rate. If so, the layer oversampling rate set by the user is kept unchanged; otherwise, the layer oversampling rate input by the user is adjusted to be equal to the layer overexcitation rate.

7. The method according to claim 6, characterized in that After the target tissue is excited by the generated time-domain layer selective pulse, the method further comprises: The actual encoding FOV is determined according to the layer oversampling rate set by the user and the field of view FOV set by the user. The excited MR signal is encoded according to the actual encoding FOV to obtain an MR image.

8. A magnetic resonance (MR) excitation device (90), characterized in that: The device (90) comprises: A signal uniformity acquisition module (91) is used to acquire the signal uniformity of the MR image brought by the preset time-domain layer selective pulse; A pulse design module (92) is used to obtain a region of interest of a target tissue set by a user, and obtain the thickness of the region of interest in a layer direction; calculate the actual excitation thickness in the layer direction according to the thickness of the region of interest and the signal uniformity of the MR image brought by the preset time-domain layer-selective pulse; obtain a spatial layer-selective pulse in such a way that the actual excitation thickness is not less than the half-maximum width FWHM of the spatial layer-selective pulse; and determine the voltage of the time-varying ... The excitation module (93) is used to generate a corresponding time-domain layer selective pulse according to the voltage of the time-domain layer selective pulse that changes with time, and use the generated time-domain layer selective pulse to excite the target tissue.

9. The device (90) according to claim 8, characterized in that The signal uniformity acquisition module (91) acquires the signal uniformity of the MR image brought by the preset time-domain layer selective pulse, including: The uniform water phantom is excited by using a preset time-domain layer-selective pulse to obtain an MR image; Find areas of signal inhomogeneity in MR images; According to the signal inhomogeneity region in the MR image, the signal uniformity of the MR image brought by the preset time-domain layer selective pulse is calculated.

10. The device (90) according to claim 9, characterized in that The signal uniformity acquisition module (91) calculates the signal uniformity of the MR image brought by the preset time-domain layer selective pulse according to the signal inhomogeneity area in the MR image, including: Calculating the ratio of the size of the signal inhomogeneity region in the MR image to the total size of the MR image, and using the ratio as the signal inhomogeneity rate of the MR image caused by the preset time-domain layer selective pulse; According to the signal inhomogeneity of the MR image brought by the preset time-domain slice selective pulse, the signal uniformity of the MR image brought by the preset time-domain slice selective pulse is calculated.

11. The device (90) according to claim 9 or 10, characterized in that The signal uniformity acquisition module (91) searches for a signal inhomogeneity region in the MR image, including: The maximum signal intensity is found in the MR image, the minimum uniform signal intensity is calculated according to the maximum signal intensity and the preset weight, and the area with the signal intensity less than the minimum uniform signal intensity is found in the MR image, wherein the area with the signal intensity less than the minimum uniform signal intensity is the signal inhomogeneous area.

12. The device (90) according to any one of claims 8 to 10, characterized in that The device (90) further comprises: a layer overexcitation rate calculation module (94), used for calculating the difference between the actual excitation thickness and the thickness of the region of interest, and calculating the layer overexcitation rate according to the difference and the thickness of the region of interest; The layer oversampling rate adjustment module (95) is used to determine whether the layer oversampling rate set by the user is not less than the layer overexcitation rate. If so, the layer oversampling rate set by the user is kept unchanged; otherwise, the layer oversampling rate input by the user is adjusted to be equal to the layer overexcitation rate.

13. The device (90) according to claim 12, characterized in that The device (90) further comprises: The FOV adjustment and encoding module (96) is used to determine the actual encoding FOV according to the layer oversampling rate set by the user and the field of view FOV set by the user, and encode the excited MR signal according to the actual encoding FOV to obtain an MR image.

14. A magnetic resonance imaging (MRI) system, characterized in that: The MRI system comprises the magnetic resonance MR excitation device (90) as claimed in any one of claims 8 to 13.

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