A method, system and program product for measuring accompanying field to eliminate residual magnetism influence

Abstract

The present invention belongs to the technical field of magnetic resonance imaging, and specifically discloses a companion field measurement method, system and program product for eliminating the influence of residual magnetism. By setting multiple groups of bipolar gradients for testing, and performing corresponding combined calculations on the test results to distinguish the second-order companion field from the first-order residual magnetism, the residual magnetism accumulated phase can be effectively removed, and the influence of residual magnetism during the measurement process can be eliminated, so as to more accurately measure and evaluate the companion field situation, such as determining the phase distribution and corresponding symmetry coefficient of the companion field, which is helpful to further compensate the companion field. At the same time, the present invention can also expand the analysis of the residual magnetism situation under a certain gradient.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnetic resonance imaging, and in particular relates to a method, system and program product for measuring an accompanying field for eliminating residual magnetism influence. Background Art

[0002] Magnetic resonance imaging (MRI) is an important medical imaging technology that uses strong magnetic fields and radio frequency pulses to image the inside of the human body. However, the presence of the accompanying field can cause the image to be affected by artifacts, reducing the quality of the imaging. In the MRI system, the accompanying field is a nonlinear magnetic field distribution caused by the gradient coil. During the sequence development process, the strength of the accompanying field needs to be reasonably evaluated to determine whether compensation is needed and what the compensation plan is.

[0003] The traditional method of measuring the accompanying field does not fully consider the influence of the residual magnetism of the system. There are magnetizable materials (such as iron choke, etc.) on the permanent magnet device. When a large gradient is applied through the gradient coil, the iron choke and other materials will also be magnetized. When the gradient of the gradient coil is turned off, the magnetized iron choke material cannot completely recover the state before magnetization and retains a certain magnetism, which is called residual magnetism. The residual magnetism in the permanent magnet system will affect the measurement of the accompanying field, resulting in a phase shift in the measurement results. If the residual magnetism is not taken into account, the accompanying field cannot be accurately evaluated.

[0004] In addition, in superconducting magnetic resonance equipment, the symmetry coefficient α of the accompanying field formula of the cylindrical gradient coil is usually set to an empirical value of 0.5 (the symmetry coefficient α can characterize the symmetry of the magnetic field generated by the z-axis gradient in the x-axis and y-axis directions, and the x-axis, y-axis and z-axis directions are the three gradient field directions of the system), but in the permanent magnet system of the flat-plate gradient coil, its empirical value is unknown and cannot be predicted in advance. Summary of the invention

[0005] The purpose of the present invention is to provide a method, system and program product for measuring an accompanying field for eliminating the influence of residual magnetism, so as to solve the above-mentioned problems existing in the prior art.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] In a first aspect, a method for measuring a companion field for eliminating residual magnetism influence is provided, comprising:

[0008] Prepending a first test gradient, a second test gradient, a third test gradient, a fourth test gradient, a fifth test gradient, a sixth test gradient and a seventh test gradient in each TR of the spin echo sequence respectively, to obtain a corresponding first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence and a seventh test spin echo sequence, wherein the first test gradient is set to be gradient-free, the second test gradient and the third test gradient are bipolar test gradients of a certain gradient axis, and the fourth test gradient, the fifth test gradient, the sixth test gradient and the seventh test gradient are bipolar test gradients of two certain gradient axes;

[0009] performing magnetic resonance scanning on the target object using a first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence, and a seventh test spin echo sequence, respectively, to obtain corresponding first test complex image data, second test complex image data, third test complex image data, fourth test complex image data, fifth test complex image data, sixth test complex image data, and seventh test complex image data;

[0010] Performing fusion calculation on the first test complex image data, the second test complex image data, and the third test complex image data to obtain first fused complex image data, and performing fusion calculation on the fourth test complex image data, the fifth test complex image data, the sixth test complex image data, and the seventh test complex image data to obtain second fused complex image data;

[0011] Determine the phase of the accompanying field self-square term according to the first fused complex image data, and determine the phase of the accompanying field cross term according to the second fused complex image data;

[0012] The phase distribution of the companion field is obtained by summing up the phase of the self-squared term of the companion field and the phase of the cross term of the companion field.

[0013] In one possible design, the second test gradient is a bipolar test gradient that is first positive and then negative on a certain gradient axis, the third test gradient is a bipolar test gradient that is first negative and then positive on a certain gradient axis, the fourth test gradient is a bipolar test gradient that is first positive and then negative and is applied simultaneously on two gradient axes, the fifth test gradient is a bipolar test gradient that is first negative and then positive and is applied simultaneously on two gradient axes, the sixth test gradient is a bipolar test gradient that is applied simultaneously on two gradient axes, the first of which is first positive and then negative, and the second of which is first negative and then positive, the seventh test gradient is a bipolar test gradient that is applied simultaneously on two gradient axes, the first of which is first negative and then positive, and the second of which is first positive and then negative, and the positive lobe and negative lobe areas of all bipolar test gradients are equal.

[0014] In a possible design, the step of performing fusion calculation on the first test complex image data, the second test complex image data, and the third test complex image data to obtain first fused complex image data includes:

[0015] The first test complex image data, the second test complex image data, and the third test complex image data are substituted into a preset first fusion formula for calculation to obtain first fused complex image data. The first fusion formula is P1=(I2. / I1).×(I3. / I1), wherein P1 represents the first fused complex image data, I1 represents the first test complex image data, I2 represents the second test complex image data, I3 represents the third test complex image data, . / represents the division of image elements, and .× represents the multiplication of image elements.

[0016] In one possible design, the fusing calculation of the fourth test complex image data, the fifth test complex image data, the sixth test complex image data, and the seventh test complex image data to obtain the second fused complex image data includes:

[0017] The fourth test complex image data, the fifth test complex image data, the sixth test complex image data and the seventh test complex image data are substituted into a preset second fusion formula for calculation to obtain second fused complex image data, wherein the second fusion formula is P2=(I4. / I7).×(I5. / I6), wherein P2 represents the second fused complex image data, I4 represents the fourth test complex image data, I5 represents the fifth test complex image data, I6 represents the sixth test complex image data, I7 represents the seventh test complex image data, . / represents the division of image elements, and .× represents the multiplication of image elements.

[0018] In one possible design, determining the phase of the accompanying field self-square term according to the first fused complex image data and determining the phase of the accompanying field cross term according to the second fused complex image data includes:

[0019] Determine the image phase of the first fused complex image data, divide the image phase of the first fused complex image data by 2, and obtain the phase of the accompanying field self-squared term;

[0020] An image phase of the second fused complex image data is determined, and the image phase of the second fused complex image data is divided by 4 to obtain an adjoint field cross term phase.

[0021] In one possible design, the method further includes:

[0022] Determine the residual magnetization accumulation phase of the corresponding gradient axis according to the gradient axes of the bipolar test gradients of the second test gradient and the third test gradient, and the second test complex image data and the third test complex image data;

[0023] According to the two gradient axes of the bipolar test gradients of the fourth test gradient, the fifth test gradient, the sixth test gradient and the seventh test gradient, and the fourth test complex image data, the fifth test complex image data, the sixth test complex image data and the seventh test complex image data, the residual magnetization phases corresponding to the positive polarity gradients of the two gradient axes are determined.

[0024] In one possible design, the method further includes:

[0025] The symmetry coefficient α corresponding to the self-square term is obtained by phase fitting of the self-square term of the accompanying field, and the symmetry coefficient α corresponding to the cross term is obtained by phase fitting of the cross term of the accompanying field.

[0026] In a second aspect, a companion field measurement system for eliminating residual magnetism influence is provided, comprising a gradient setting unit, a sequence scanning unit, an image calculation unit, a phase determination unit and a measurement summary unit, wherein:

[0027] a gradient setting unit, for respectively prepending a first test gradient, a second test gradient, a third test gradient, a fourth test gradient, a fifth test gradient, a sixth test gradient and a seventh test gradient in each TR of the spin echo sequence, so as to obtain a corresponding first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence and a seventh test spin echo sequence, wherein the first test gradient is set to no test gradient, the second test gradient and the third test gradient are bipolar test gradients of a single gradient axis, and the fourth test gradient, the fifth test gradient, the sixth test gradient and the seventh test gradient are bipolar test gradients of any two gradient axes;

[0028] a sequence scanning unit, configured to perform magnetic resonance scanning on a target object using a first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence, and a seventh test spin echo sequence, respectively, to obtain corresponding first test complex image data, second test complex image data, third test complex image data, fourth test complex image data, fifth test complex image data, sixth test complex image data, and seventh test complex image data;

[0029] an image calculation unit, configured to perform fusion calculation on the first test complex image data, the second test complex image data, and the third test complex image data to obtain first fused complex image data, and perform fusion calculation on the fourth test complex image data, the fifth test complex image data, the sixth test complex image data, and the seventh test complex image data to obtain second fused complex image data;

[0030] a phase determination unit, configured to determine a phase of an accompanying field self-square term according to the first fused complex image data, and to determine a phase of an accompanying field cross term according to the second fused complex image data;

[0031] The measurement summary unit is used to summarize the phase of the accompanying field self-square term and the phase of the accompanying field cross term to obtain the phase distribution of the accompanying field.

[0032] In a third aspect, a companion field measurement system for eliminating residual magnetism influence is provided, comprising:

[0033] A memory for storing instructions;

[0034] A processor is used to read the instructions stored in the memory and execute any one of the methods described in the first aspect according to the instructions.

[0035] In a fourth aspect, a computer-readable storage medium is provided, wherein instructions are stored on the computer-readable storage medium, and when the instructions are executed on a computer, the computer executes any one of the methods described in the first aspect. In addition, a computer program product is provided, and when the computer program product is executed on a computer, the computer executes any one of the methods described in the first aspect.

[0036] Beneficial effects: The present invention sets multiple groups of bipolar gradients for testing, and performs corresponding combined calculations on the test results to distinguish the second-order accompanying field from the first-order residual magnetism, which can effectively remove the accumulated phase of the residual magnetism and eliminate the influence of the residual magnetism during the measurement process, so as to more accurately measure and evaluate the accompanying field. For example, determining the phase distribution of the accompanying field and the corresponding symmetry coefficient is helpful to further compensate for the accompanying field. At the same time, the present invention can also expand the analysis of the residual magnetism under a certain gradient. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0038] Figure 1This is a schematic diagram of the steps of the method in Example 1 of the present invention;

[0039] Figure 2 It is a timing diagram of the spin echo sequence;

[0040] Figure 3 is a schematic diagram of the first test gradient;

[0041] Figure 4 is a schematic diagram of the second test gradient;

[0042] Figure 5 is a schematic diagram of the third test gradient;

[0043] Figure 6 is a schematic diagram of the fourth test gradient;

[0044] Figure 7 is a schematic diagram of the fifth test gradient;

[0045] Figure 8 is a sixth test gradient schematic diagram;

[0046] Fig. 9 This is a schematic diagram of the seventh test gradient;

[0047] Fig.10 Schematic diagram of the parameters of the bipolar gradient;

[0048] Fig.11 Schematic diagram of the system structure in Example 3 of the present invention. DETAILED DESCRIPTION

[0049] It should be noted that the description of these embodiments is used to help understand the present invention, but does not constitute a limitation of the present invention. The specific structures and functional details disclosed herein are only used to describe the exemplary embodiments of the present invention. However, the present invention can be embodied in many alternative forms, and it should not be understood that the present invention is limited to the embodiments set forth herein.

[0050] It should be understood that, unless otherwise clearly specified and limited, the term "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the embodiments can be understood according to specific circumstances.

[0051] In the following description, certain details are provided to facilitate a complete understanding of the example embodiments. However, it will be appreciated by those of ordinary skill in the art that the example embodiments may be implemented without these certain details. For example, devices may be shown in block diagrams to avoid obscuring the examples with unnecessary details. In other embodiments, well-known processes, structures, and techniques may not be shown in unnecessary detail to avoid obscuring the embodiments.

[0052] Embodiment 1:

[0053] This embodiment provides a method for measuring the accompanying field to eliminate the influence of residual magnetism. Figure 1 As shown, the method comprises the following steps:

[0054] S1. Prepend a first test gradient, a second test gradient, a third test gradient, a fourth test gradient, a fifth test gradient, a sixth test gradient and a seventh test gradient in each TR of a spin echo sequence, respectively, to obtain a corresponding first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence and a seventh test spin echo sequence, wherein the first test gradient is set to no gradient, the second test gradient and the third test gradient are bipolar test gradients of one gradient axis, and the fourth test gradient, the fifth test gradient, the sixth test gradient and the seventh test gradient are bipolar test gradients of two gradient axes.

[0055] In specific implementation, the timing of the spin echo (SE) sequence is as follows: Figure 2As shown in the figure, Gfx, Gfy, and Gfz represent the test gradients on the three gradient axes of x, y, and z respectively. The characteristics of the test gradient are: on each gradient axis, the gradient is applied in a positive and negative alternating manner, and the gradient amplitude decays from the maximum gradient value that the magnetic resonance system can reach according to a certain rule (such as the amplitude of the next gradient is 2 / 3 of the previous one, etc.) until it reaches the preset minimum threshold (such as 5% or less of the maximum gradient value). RF1 is a 90° excitation pulse, Gs is the layer selection gradient of the 90° excitation pulse, RF2 is a 180° refocusing pulse, Gsr is the layer selection gradient of the 180° refocusing pulse, the bipolar gradient Gt is the test gradient, Grp is the readout pre-dephasing gradient, Gpe is the phase encoding gradient, and Gr is the readout gradient. The time interval between two adjacent 90° RF pulse excitations is TR. Each TR corresponds to one phase encoding. It should be understood as follows: within one TR, first apply the test gradient on the three gradient axes, then start the SE sequence, first perform 90-degree layer selection excitation, then apply or not apply the bipolar test gradient as needed, then apply the readout pre-phase gradient, then perform 180-degree layer selection back focusing, then apply the current phase encoding gradient (which will change with TR to facilitate encoding imaging), and finally apply the readout gradient for data readout to obtain echo data for subsequent imaging. The pre-test gradients in each TR are the same.

[0056] This method requires setting multiple sets of bipolar test gradients as corresponding test gradients, such as Figures 3 to 9 As shown, it includes a first test gradient, a second test gradient, a third test gradient, a fourth test gradient, a fifth test gradient, a sixth test gradient and a seventh test gradient. Among them, the first test gradient is set to no test gradient, the second test gradient is a bipolar test gradient that is positive first and then negative on a certain gradient axis, the third test gradient is a bipolar test gradient that is negative first and then positive on a certain gradient axis, the fourth test gradient is a bipolar test gradient that is positive first and then negative applied simultaneously on two gradient axes (such as the z-axis and the x-axis, the z-axis and the y-axis, or the x-axis and the y-axis), the fifth test gradient is a bipolar test gradient that is negative first and then positive applied simultaneously on two gradient axes, the sixth test gradient is a bipolar test gradient that is negative first and then positive applied simultaneously on two gradient axes, the first of which is positive first and then negative, and the second is negative first and then positive, the seventh test gradient is a bipolar test gradient that is applied simultaneously on two gradient axes, the first of which is negative first and then positive, and the second is positive first and then negative, and the positive lobe and negative lobe areas of all bipolar test gradients are equal, that is, the integral of the gradient over time is 0. The gradient axes of the second test gradient and the third test gradient are kept consistent. For each bipolar test gradient (the fourth test gradient, the fifth test gradient, the sixth test gradient and the seventh test gradient), the two gradient axes should be kept consistent in each test, such as both acting on the x and y axes, but with differences in the positive and negative conditions and order of the bipolar gradients.

[0057] Then, a first test gradient, a second test gradient, a third test gradient, a fourth test gradient, a fifth test gradient, a sixth test gradient and a seventh test gradient are respectively pre-placed in each TR of the spin echo sequence to obtain a corresponding first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence and a seventh test spin echo sequence.

[0058] S2. Perform magnetic resonance scanning on the target object using the first test spin echo sequence, the second test spin echo sequence, the third test spin echo sequence, the fourth test spin echo sequence, the fifth test spin echo sequence, the sixth test spin echo sequence and the seventh test spin echo sequence respectively to obtain corresponding first test complex image data, second test complex image data, third test complex image data, fourth test complex image data, fifth test complex image data, sixth test complex image data and seventh test complex image data.

[0059] In specific implementation, the target object can be subjected to magnetic resonance scanning by using the first test spin echo sequence, the second test spin echo sequence, the third test spin echo sequence, the fourth test spin echo sequence, the fifth test spin echo sequence, the sixth test spin echo sequence and the seventh test spin echo sequence respectively through the corresponding gradient system (composed of gradient coils, gradient amplifiers, analog-to-digital converters, gradient controllers, etc.), and the corresponding first test complex image data, the second test complex image data, the third test complex image data, the fourth test complex image data, the fifth test complex image data, the sixth test complex image data and the seventh test complex image data are obtained. The first test spin echo sequence is used to scan and obtain the initial complex image data, the amplitude of all pixels of the initial complex image data is set to 1, the background phase is kept unchanged, and the normalized first test complex image data is obtained.

[0060] The phase of the first test complex image data only includes the background phase; the phase of the second test complex image data includes the background phase, the phase of the accompanying field self-squared term and the negative residual magnetism phase; the phase of the third test complex image data includes the background phase, the phase of the accompanying field self-squared term and the positive residual magnetism phase; the phase of the fourth test complex image data includes the background phase, the phase of the accompanying field self-squared term, the phase of the accompanying field positive cross term, the negative residual magnetism phase in the first gradient direction and the negative residual magnetism phase in the second gradient direction; the phase of the fifth test complex image data includes the background phase, the phase of the accompanying field self-squared term, the phase of the accompanying field positive cross term, the positive residual magnetism phase in the first gradient direction and the positive residual magnetism phase in the second gradient direction; the phase of the sixth test complex image data includes the background phase, the phase of the accompanying field self-squared term, the phase of the accompanying field negative cross term, the negative residual magnetism phase in the first gradient direction and the positive residual magnetism phase in the second gradient direction; the phase of the seventh test complex image data includes the background phase, the phase of the accompanying field self-squared term, the phase of the accompanying field negative cross term, the positive residual magnetism phase in the first gradient direction and the negative residual magnetism phase in the second gradient direction.

[0061] S3. Perform fusion calculation on the first test complex image data, the second test complex image data, and the third test complex image data to obtain first fused complex image data, and perform fusion calculation on the fourth test complex image data, the fifth test complex image data, the sixth test complex image data, and the seventh test complex image data to obtain second fused complex image data.

[0062] In a specific implementation, the first test complex image data, the second test complex image data, and the third test complex image data are substituted into a preset first fusion formula for calculation to obtain the first fused complex image data, wherein the first fusion formula is P1=(I2. / I1).×(I3. / I1), wherein P1 represents the first fused complex image data, I1 represents the first test complex image data, I2 represents the second test complex image data, and I3 represents the third test complex image data. At this time, the gradient axes of the second test gradient and the third test gradient remain consistent.

[0063] Substitute the fourth test complex image data, the fifth test complex image data, the sixth test complex image data and the seventh test complex image data into the preset second fusion formula for calculation to obtain the second fused complex image data, wherein the second fusion formula is P2=(I4. / I7).×(I5. / I6), wherein P2 represents the second fused complex image data, I4 represents the fourth test complex image data, I5 represents the fifth test complex image data, I6 represents the sixth test complex image data, and I7 represents the seventh test complex image data. At this time, the gradient axes of the fourth test gradient, the fifth test gradient, the sixth test gradient and the seventh test gradient maintain the same two axes, except that the positive and negative conditions evolve according to the corresponding schematic diagram.

[0064] Among them, . / represents the division of image elements, .× represents the multiplication of image elements, and the image vector and vector For example:

[0065]

[0066]

[0067] . / and .× are operations directly performed on complex images. According to the rules of complex operations, the new complex image obtained by element-by-element division is: the amplitude is the quotient of the amplitudes of the original two complex images according to the elements, and the phase is the difference in the phases of the original two complex images according to the elements; the new complex image obtained by element-by-element multiplication is: the amplitude is the product of the amplitudes of the original two complex images according to the elements, and the phase is the sum of the phases of the original two complex images according to the elements. The function of phase difference can be achieved by element-by-element division, that is, subtracting the background phase, etc., and the function of phase sum can be achieved by element-by-element multiplication, that is, offsetting the positive and negative residual magnetic phases, etc.

[0068] S4. Determine the phase of the accompanying field self-square term according to the first fused complex image data, and determine the phase of the accompanying field cross term according to the second fused complex image data.

[0069] In specific implementation, the complex image phase calculated according to the first fusion formula P1 = (I2. / I1). × (I3. / I1) is the phase caused by the 2 times self-squared term, and the complex image phase calculated according to the second fusion formula P2 = (I4. / I7). × (I5. / I6) is the phase caused by the 4 times cross-term. Therefore, it is necessary to determine the image phase of the first fused complex image data, divide the image phase of the first fused complex image data by 2, and obtain the phase of the accompanying field self-squared term; determine the image phase of the second fused complex image data, divide the image phase of the second fused complex image data by 4, and obtain the phase of the accompanying field cross-term.

[0070] S5. Sum up the phase of the self-squared term of the companion field and the phase of the cross term of the companion field to obtain the phase distribution of the companion field.

[0071] In specific implementation, the phase of the accompanying field self-squared term and the phase of the accompanying field cross term are summarized to obtain the phase distribution of the accompanying field. At the same time, the residual magnetization cumulative phase of the corresponding gradient axis can be determined according to the gradient axis of the bipolar test gradient of the second test gradient and the third test gradient, and the second test complex image data and the third test complex image data; the residual magnetization phase of the positive polarity gradient of the corresponding two gradient axes is determined according to the two gradient axes of the bipolar test gradient of the fourth test gradient, the fifth test gradient, the sixth test gradient and the seventh test gradient, and the fourth test complex image data, the fifth test complex image data, the sixth test complex image data and the seventh test complex image data; the symmetry coefficient α corresponding to the self-squared term is obtained by fitting the phase of the accompanying field self-squared term, and the symmetry coefficient α corresponding to the cross term is obtained by fitting the phase of the accompanying field cross term:

[0072] 1. Adjoint field self-squared term:

[0073] like Fig.10 As shown in the figure, assuming that the starting time of the bipolar gradient is t1, the time for gradient climbing and descending is δ, the gradient platform time is τ, the gradient amplitude at the platform is G, and the bipolar gradient end time is t2, then for the phase φ caused by the self-squared term s :

[0074] ①When the gradient acts on the z-axis:

[0075]

[0076] ②When the gradient acts on the x or y axis:

[0077]

[0078] The phase caused by the uneven main magnetic field strength B0 of the gradient system on the image is recorded as the background phase φ0. At this time, since the bipolar gradient ends with a negative gradient, the residual magnetism is also mainly a smaller coaxial negative gradient, and the phase caused by it on the image is recorded as -φ h ,φ h is the positive residual magnetic gradient, then the phase φ1 of the second test complex image data is composed of:

[0079] φ1=φ s +φ0-φ h

[0080] Considering that the third test gradient is to only exchange the positive and negative order of the second test gradient bipolar gradient, that is, to change from positive gradient first and negative gradient second to negative gradient first and positive gradient second, without changing the axis of action and the duration, amplitude, etc., the phase φ2 of the third test complex image data is formed as follows:

[0081] φ2=φ s +φ0+φ h

[0082] The background phase φ0 caused by the B0 field inhomogeneity can be directly obtained by the first test complex image data obtained without applying the bipolar test gradient, and the image phase φ caused by the self-squared term of the accompanying field can be solved. s :

[0083]

[0084] The phase difference can be achieved by element-wise division, and the phase sum can be achieved by element-wise multiplication, that is, the mapping is equivalent to [(I2. / I1).×(I3. / I1)] / 2. At the same time, the phase φ accumulated by the residual magnetization of the current axis gradient can be solved according to the image phases φ1 and φ2 corresponding to the second test complex image data and the third test complex image data. h :

[0085]

[0086] The above φ s and φ h They are all distributed in space. For example, when the gradient acts on the z-axis, φ s It is distributed in a quadratic curve in the z-axis direction. When G, B0, δ, and τ are all known (B0 is the main magnetic field intensity, δ is the gradient climbing time, τ is the gradient platform time, and x and y represent the spatial position, all of which are known quantities), the phase φ caused by the self-squared term of the accompanying field is obtained by the formula in ①. s By fitting, we can get the value of the corresponding symmetry coefficient α. The same can be said for the case where the gradient acts on the x-axis and y-axis.

[0087] 2. Adjoint field cross terms:

[0088] Assume that the starting time of the bipolar gradient is t1, the time for gradient climbing and descending is δ, the gradient platform time is τ, the gradient amplitude at the platform is G, and the bipolar gradient ends at t2. The cross term is more complicated because when performing the test scan of the cross term, self-square terms will inevitably appear; the cross term requires the use of two-axis gradients at the same time. Taking the x-axis and z-axis gradients as an example, the following four test gradients are combined to eliminate the background phase φ0 and the residual magnetization phase φ h , self-squared phase φ s To obtain the pure cross-term phase φ c :

[0089] φ1=φ0+φ s,x +φ s,z +φ c,xz -φ h,x -φ h,z

[0090] φ2=φ0+φs,x +φ s,z +φ c,xz +φ h,x +φ h,z

[0091] φ3=φ0+φ s,x +φ s,z -φ c,xz +φ h,x -φ h,z

[0092] φ4=φ0+φ s,x +φ s,z -φ c,xz -φ h,x +φ h,z

[0093] In the above formulas, φ i (i=1,2,3,4), respectively represent the phases of the fourth test complex image data, the fifth test complex image data, the sixth test complex image data and the seventh test complex image data, φ0 represents the image background phase caused by the uneven B0 field, φ s,x and φ s,z They represent the image phase caused by the self-squared terms of the x-axis and z-axis, respectively, h,x and φ h,z Respectively represent the image phase caused by the remanent magnetization of the positive gradient of the x-axis and z-axis, φ c,xz It represents the image phase caused by the adjoint field cross term when the positive x-axis gradient and the positive z-axis gradient act simultaneously. The cross term adjoint field phase of the two axes can be solved by combining phase addition and subtraction:

[0094]

[0095] Phase difference can be achieved by element-wise division, and phase sum can be achieved by element-wise multiplication, that is, the mapping is equivalent to (I4. / I7).×(I5. / I6)] / 2. It can be seen that φ i In the four equations (i=1,2,3,4), there is the same φ0+φ s,x +φ s,z term, so it can be regarded as a whole and only as one unknown number. In addition, there is still φ c,xz ,φ h,x ,φ h,z Three unknowns, a total of four unknowns and four phase equations, so we can use these four equations and the phase φ of the corresponding complex image i (i=1,2,3,4) Solving these four unknowns can also simultaneously calculate the x-axis and z-axis positive gradient image phase φ caused by the residual magnetization under the current test gradient h,x and φh,z .

[0096] According to the phase distribution φ caused by the adjoint field cross term calculated previously c,xz , combined with the corresponding theoretical formula (in the form of an inverse proportional function in the xoz plane)

[0097]

[0098] The fitting value of the symmetry coefficient α can be obtained by fitting (where B0 is the main magnetic field intensity, δ is the gradient climbing time, τ is the gradient platform time, and x and z are the corresponding spatial positions, all of which are known quantities).

[0099] The method of this embodiment can effectively eliminate the influence of residual magnetism during the accompanying field measurement process, so as to more accurately measure and evaluate the accompanying field, determine the phase distribution and corresponding symmetry coefficient of the accompanying field, and help to further compensate for the accompanying field. At the same time, the present invention can also expand the analysis of the residual magnetism under a certain gradient.

[0100] Embodiment 2:

[0101] This embodiment provides a companion field measurement system for eliminating residual magnetism influence, including a gradient setting unit, a sequence scanning unit, an image calculation unit, a phase determination unit and a measurement summary unit, wherein:

[0102] a gradient setting unit, for respectively prepending a first test gradient, a second test gradient, a third test gradient, a fourth test gradient, a fifth test gradient, a sixth test gradient and a seventh test gradient in each TR of the spin echo sequence, so as to obtain a corresponding first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence and a seventh test spin echo sequence, wherein the first test gradient is set to be gradient-free, the second test gradient and the third test gradient are bipolar test gradients of a certain gradient axis, and the fourth test gradient, the fifth test gradient, the sixth test gradient and the seventh test gradient are bipolar test gradients of two certain gradient axes;

[0103] a sequence scanning unit, configured to perform magnetic resonance scanning on a target object using a first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence, and a seventh test spin echo sequence, respectively, to obtain corresponding first test complex image data, second test complex image data, third test complex image data, fourth test complex image data, fifth test complex image data, sixth test complex image data, and seventh test complex image data;

[0104] an image calculation unit, configured to perform fusion calculation on the first test complex image data, the second test complex image data, and the third test complex image data to obtain first fused complex image data, and perform fusion calculation on the fourth test complex image data, the fifth test complex image data, the sixth test complex image data, and the seventh test complex image data to obtain second fused complex image data;

[0105] a phase determination unit, configured to determine a phase of an accompanying field self-square term according to the first fused complex image data, and to determine a phase of an accompanying field cross term according to the second fused complex image data;

[0106] The measurement summary unit is used to summarize the phase of the accompanying field self-square term and the phase of the accompanying field cross term to obtain the phase distribution of the accompanying field.

[0107] Embodiment 3:

[0108] This embodiment provides a companion field measurement system for eliminating residual magnetism influence, such as Fig.11 As shown, at the hardware level, it includes:

[0109] Data interface, used to establish data connection between the processor and the external data terminal;

[0110] A memory for storing instructions;

[0111] The processor is used to read the instructions stored in the memory and execute the accompanying field measurement method for eliminating the residual magnetism influence in embodiment 1 according to the instructions.

[0112] Optionally, the system further includes an internal bus, through which the processor, the memory and the data interface can be interconnected, and the internal bus can be an ISA (Industry Standard Architecture) bus, a PCI (Peripheral Component Interconnect) bus or an EISA (Extended Industry Standard Architecture) bus, etc. The bus can be divided into an address bus, a data bus, a control bus, etc.

[0113] The memory may include, but is not limited to, random access memory (RAM), read only memory (ROM), flash memory, first input first output (FIFO) and / or first in last out (FILO) memory, etc. The processor may be a general-purpose processor, including a central processing unit (CPU), a network processor (NP), etc.; it may also be a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components.

[0114] Embodiment 4:

[0115] This embodiment provides a computer-readable storage medium, on which instructions are stored, and when the instructions are executed on a computer, the computer executes the accompanying field measurement method for eliminating residual magnetism in Embodiment 1. The computer-readable storage medium refers to a carrier for storing data, which may include but is not limited to a floppy disk, an optical disk, a hard disk, a flash memory, a USB flash drive, and / or a memory stick, etc., and the computer may be a general-purpose computer, a special-purpose computer, a computer network, or other programmable devices.

[0116] This embodiment also provides a computer program product, which, when running on a computer, executes the accompanying field measurement method for eliminating residual magnetism influence in Embodiment 1. The computer may be a general-purpose computer, a special-purpose computer, a computer network or other programmable device.

[0117] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for measuring a companion field to eliminate the influence of residual magnetism, characterized in that: include: A first test gradient, a second test gradient, a third test gradient, a fourth test gradient, a fifth test gradient, a sixth test gradient and a seventh test gradient are respectively pre-placed in each TR of the spin echo sequence to obtain a corresponding first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence and a seventh test spin echo sequence, wherein the first test gradient is set to no gradient, the second test gradient is a bipolar test gradient which is first positive and then negative on a certain gradient axis, and the third test gradient is a bipolar test gradient that is first negative and then positive on a certain gradient axis, the fourth test gradient is a bipolar test gradient that is first positive and then negative and is applied simultaneously on two certain gradient axes, the fifth test gradient is a bipolar test gradient that is first negative and then positive and is applied simultaneously on two certain gradient axes, the sixth test gradient is a bipolar test gradient that is applied simultaneously on two certain gradient axes, the first of which is first positive and then negative, and the second of which is first negative and then positive, the seventh test gradient is a bipolar test gradient that is applied simultaneously on two certain gradient axes, the first of which is first negative and then positive, and the second of which is first positive and then negative, and the positive lobe and negative lobe areas of all bipolar test gradients are equal; performing magnetic resonance scanning on the target object using a first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence, and a seventh test spin echo sequence, respectively, to obtain corresponding first test complex image data, second test complex image data, third test complex image data, fourth test complex image data, fifth test complex image data, sixth test complex image data, and seventh test complex image data; Performing fusion calculation on the first test complex image data, the second test complex image data, and the third test complex image data to obtain first fused complex image data, and performing fusion calculation on the fourth test complex image data, the fifth test complex image data, the sixth test complex image data, and the seventh test complex image data to obtain second fused complex image data; Determine the phase of the accompanying field self-square term according to the first fused complex image data, and determine the phase of the accompanying field cross term according to the second fused complex image data; The phase distribution of the companion field is obtained by summing up the phase of the self-squared term of the companion field and the phase of the cross term of the companion field.

2. The accompanying field measurement method for eliminating residual magnetism influence according to claim 1, characterized in that: The step of performing fusion calculation on the first test complex image data, the second test complex image data, and the third test complex image data to obtain first fused complex image data includes: The first test complex image data, the second test complex image data, and the third test complex image data are substituted into a preset first fusion formula for calculation to obtain first fused complex image data. The first fusion formula is P1=(I2. / I1).×(I3. / I1), wherein P1 represents the first fused complex image data, I1 represents the first test complex image data, I2 represents the second test complex image data, I3 represents the third test complex image data, . / represents the division of image elements, and .× represents the multiplication of image elements.

3. The accompanying field measurement method for eliminating residual magnetism influence according to claim 1, characterized in that: The fusing calculation is performed on the fourth test complex image data, the fifth test complex image data, the sixth test complex image data and the seventh test complex image data to obtain second fused complex image data, including: The fourth test complex image data, the fifth test complex image data, the sixth test complex image data and the seventh test complex image data are substituted into a preset second fusion formula for calculation to obtain second fused complex image data, wherein the second fusion formula is P2=(I4. / I7).×(I5. / I6), wherein P2 represents the second fused complex image data, I4 represents the fourth test complex image data, I5 represents the fifth test complex image data, I6 represents the sixth test complex image data, I7 represents the seventh test complex image data, . / represents the division of image elements, and .× represents the multiplication of image elements.

4. The accompanying field measurement method for eliminating residual magnetism influence according to claim 1, characterized in that: The method of determining the phase of the accompanying field self-square term according to the first fused complex image data and determining the phase of the accompanying field cross term according to the second fused complex image data comprises: Determine the image phase of the first fused complex image data, divide the image phase of the first fused complex image data by 2, and obtain the phase of the accompanying field self-squared term; An image phase of the second fused complex image data is determined, and the image phase of the second fused complex image data is divided by 4 to obtain an adjoint field cross term phase.

5. The accompanying field measurement method for eliminating residual magnetism influence according to claim 1, characterized in that: The method further comprises: Determine the residual magnetization accumulation phase of the corresponding gradient axis according to the gradient axes of the bipolar test gradients of the second test gradient and the third test gradient, and the second test complex image data and the third test complex image data; According to the two gradient axes of the bipolar test gradients of the fourth test gradient, the fifth test gradient, the sixth test gradient and the seventh test gradient, and the fourth test complex image data, the fifth test complex image data, the sixth test complex image data and the seventh test complex image data, the residual magnetization phases corresponding to the positive polarity gradients of the two gradient axes are determined.

6. The accompanying field measurement method for eliminating residual magnetism influence according to claim 1, characterized in that: The method further comprises: The symmetry coefficient α corresponding to the self-square term is obtained by phase fitting of the self-square term of the accompanying field, and the symmetry coefficient α corresponding to the cross term is obtained by phase fitting of the cross term of the accompanying field.

7. A companion field measurement system for eliminating residual magnetism influence, characterized in that: It includes a gradient setting unit, a sequence scanning unit, an image calculation unit, a phase determination unit and a measurement summary unit, wherein: a gradient setting unit, for respectively prepending a first test gradient, a second test gradient, a third test gradient, a fourth test gradient, a fifth test gradient, a sixth test gradient and a seventh test gradient in each TR of the spin echo sequence, so as to obtain a corresponding first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence and a seventh test spin echo sequence, wherein the first test gradient is set to be gradient-free, the second test gradient is a bipolar test gradient which is first positive and then negative on a certain gradient axis, and the third test gradient is a bipolar test gradient which is first positive and then negative on a certain gradient axis. The test gradient is a bipolar test gradient that is first negative and then positive on a certain gradient axis, the fourth test gradient is a bipolar test gradient that is first positive and then negative and is applied simultaneously on two certain gradient axes, the fifth test gradient is a bipolar test gradient that is first negative and then positive and is applied simultaneously on two certain gradient axes, the sixth test gradient is a bipolar test gradient that is applied simultaneously on two certain gradient axes, the first of which is first positive and then negative, and the second of which is first negative and then positive, the seventh test gradient is a bipolar test gradient that is applied simultaneously on two certain gradient axes, the first of which is first negative and then positive, and the second of which is first positive and then negative, and the positive lobe and negative lobe areas of all bipolar test gradients are equal; a sequence scanning unit, configured to perform magnetic resonance scanning on a target object using a first test spin echo sequence, a second test spin echo sequence, a third test spin echo sequence, a fourth test spin echo sequence, a fifth test spin echo sequence, a sixth test spin echo sequence, and a seventh test spin echo sequence, respectively, to obtain corresponding first test complex image data, second test complex image data, third test complex image data, fourth test complex image data, fifth test complex image data, sixth test complex image data, and seventh test complex image data; an image calculation unit, configured to perform fusion calculation on the first test complex image data, the second test complex image data, and the third test complex image data to obtain first fused complex image data, and perform fusion calculation on the fourth test complex image data, the fifth test complex image data, the sixth test complex image data, and the seventh test complex image data to obtain second fused complex image data; a phase determination unit, configured to determine a phase of an accompanying field self-square term according to the first fused complex image data, and to determine a phase of an accompanying field cross term according to the second fused complex image data; The measurement summary unit is used to summarize the phase of the accompanying field self-square term and the phase of the accompanying field cross term to obtain the phase distribution of the accompanying field.

8. A companion field measurement system for eliminating residual magnetism influence, characterized in that: include: A memory for storing instructions; A processor is used to read the instructions stored in the memory and execute the accompanying field measurement method for eliminating residual magnetism influence as described in any one of claims 1-6 according to the instructions.

9. A computer program product, characterized in that When the computer program product is run on a computer, the accompanying field measurement method for eliminating residual magnetism influence as described in any one of claims 1 to 6 is executed.

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