CN115494437A - Hand-held low-gradient single-sided MRI device for detecting full-thickness skin - Google Patents

Abstract

The invention relates to a hand-held low-gradient unilateral nuclear magnetic resonance device for detecting full-thickness skin, belonging to the technical field of nuclear magnetic resonance. The device includes a unilateral permanent magnet structure and a radio frequency transceiver integrated coil; the unilateral permanent magnet structure is used to generate a static magnetic field, and the permanent magnet structure is obtained by using the finite element method and the sparrow search algorithm, and the direction of the magnetic field is parallel to the upper surface of the permanent magnet structure ; The radio-frequency transceiver integrated coil is used to generate an exciting radio-frequency magnetic field orthogonal to the main magnetic field, and detect the magnetic resonance echo signal generated by the sample under test arranged on the unilateral permanent magnet structure. The invention has the advantages of simple structure, small size, light weight, low cost and reliable performance, the magnetic induction intensity of the main magnetic field _B0 is less than or equal to 100mT, and the magnetic field gradient G perpendicular to the magnetic field direction in the center of the detection target area is about 2.96T/m, which can It realizes the measurement of NMR signal of whole-thickness skin tissue and facilitates bedside non-invasive detection.

Description

Hand-held low-gradient single-side MRI device for detecting full-thickness skin

technical field

The invention belongs to the technical field of nuclear magnetic resonance, and relates to a hand-held low-gradient single-sided nuclear magnetic resonance device for detecting full-thickness skin.

Background technique

In recent years, portable single-sided nuclear magnetic resonance technology has been widely used in food analysis and quality control, material science, geophysics and other fields. It has an open structure, small size, and is easy to move. It can perform non-destructive testing on objects at any position and from any angle. At the same time, it uses permanent magnets to provide the main magnetic field, which is low in price and low in energy consumption. The information given includes relaxation time T1, T2 imaging, diffusion coefficient D, and even chemical shift. Skin burns are a common disease, but currently there is no accurate and non-invasive method for judging the degree of skin burns and diagnosing skin recovery after burns. In clinical practice, it mainly depends on the subjective judgment of doctors and the personal experience of patients. This method is relatively Rough and subjective, it is easy to cause further deepening of the burn depth. Existing medical diagnostic methods such as computed tomography (CT) and magnetic resonance imaging (MRI), etc., can image human skin, but they are large in size and difficult to monitor and measure clinically in real time; existing high-frequency ultrasound imaging The system can get an image of the skin, but cannot differentiate between vascular inflammation and edema deep in the dermis. Therefore, there is an urgent need for a quantitative and accurate method to judge the burn depth and recovery degree of the skin of burn patients in order to accurately formulate treatment and rehabilitation programs. The present invention proposes a hand-held low-gradient unilateral nuclear magnetic resonance device for full-thickness skin detection, which only relies on permanent magnets to construct the main magnetic field, and is equipped with an integrated radio frequency transceiver coil to realize the measurement of magnetic resonance signals and rapid detection of full-thickness skin.

Contents of the invention

In view of this, the object of the present invention is to provide a hand-held low-gradient single-sided nuclear magnetic resonance device for detecting full-thickness skin.

To achieve the above object, the present invention provides the following technical solutions:

Hand-held low-gradient single-side MRI device for detecting full-thickness skin, the device includes:

Unilateral permanent magnet structure and integrated radio frequency transceiver coil 4;

The unilateral permanent magnet structure includes three sets of magnet structures;

The three groups of magnet structures are respectively the left magnet group 1, the middle magnet 2 and the right magnet group 3;

The left magnet group 1 and the right magnet group 3 are on the same level;

The middle magnet 2 is located between the left magnet group 1 and the right magnet group 3, and is located below the horizontal plane of the left magnet group 1 and the right magnet group;

The specifications of the left magnet group 1 and the right magnet group 3 are the same;

In the XYZ coordinate system, the magnetic poles of the left magnet group 1 point to the Z direction, the magnetic poles of the middle magnet 2 point to the -Y direction, and the magnetic poles of the right magnet group 3 point to the -Z direction;

The main magnetic field B ₀ from the left magnet group 1 to the right magnet group 3 is generated above the unilateral permanent magnet structure, the magnetic pole points to the Y direction, and a uniform magnetic field is generated in the geometric center area of the middle magnet, and the geometric center area of the middle magnet To detect the target area 5;

The magnetic induction intensity of the main magnetic field _B0 is less than or equal to 100mT, and the magnetic field gradient G perpendicular to the direction of the magnetic field along the center of the detection target area 5 is 2.96T/m;

The magnetic field distribution generated by the unilateral permanent magnet structure is calculated by the finite element method, and the structural parameters of the unilateral permanent magnet are optimized by the sparrow search algorithm;

The radio frequency transceiver integrated coil 4 adopts a transceiver integrated double-layer planar coil, which is designed by using the time harmonic field inverse method and the flow function method, and is used to generate an excitation radio frequency magnetic field orthogonal to the main magnetic field, and receive and set it on the detection target. The echo signal generated after the sample under test in the area is excited.

Optionally, the size of the left magnet group 1 and the right magnet group 3 is: the length is 25.4mm, the height is 12.7mm, and the width is 12.7mm; the size of the middle magnet 2 is: the length is 50.8mm, the height It is 12.7mm and the width is 25.4mm.

Optionally, the optimization of the structural parameters of the unilateral permanent magnet through the sparrow search algorithm is specifically:

The spatial position of the unilateral permanent magnet structure is expressed as an n×d matrix X:

Where n is the number of permanent magnet structures, d is the dimension of the structure parameters of the permanent magnet structure to be optimized; the position of the _ith permanent magnet structure in space is Xi = ( _xi,1 , _xi,2 ,..., _{xi ,d} ); the fitness value of the unilateral permanent magnet structure is expressed as a vector F _X :

The optimization ability of the sparrow search algorithm is based on three parts: the discoverer, the joiner and the vigilante. The three jointly search for the optimal permanent magnet structure; the position update formula of each generation of discoverers is as follows:

表示第i个永磁体结构中第j维参数于t次迭代时的信息量，α是[0,1]的随机数，R是一个随机数表示当前的预警值，R∈[0,1]，ST表示安全值，ST∈[0.5,1]，Q是服从正态分布的随机数，L是一个1×d_j的矩阵，d_j是该磁铁组第j维参数的维度，该矩阵内每个元素全部为1；Among them, t represents the current number of iterations, T represents the total number of iterations,

Indicates the amount of information of the j-th dimension parameter in the i-th permanent magnet structure at the t-th iteration, α is a random number in [0,1], R is a random number indicating the current warning value, R∈[0,1] , ST represents the safety value, ST∈[0.5,1], Q is a random number that obeys the normal distribution, L is a matrix of 1×d _j , and d _j is the dimension of the jth dimension parameter of the magnet group, the matrix Each element is all 1;

The position update formula for each generation of joiners is as follows:

是目前发现者所占据的最优位置，X_worst则表示当前全局最差的位置；A是一个1×d的矩阵，其中每个元素随机赋值为1或-1，并且A⁺＝A^T(AA^T)^-1，

表示A⁺的第j维的值；当

时，加入者将积极追随发现者向着更好的位置进行移动；当

时，适应值较低的第i个加入者需要移动到其他位置，结合exp函数特性摆脱当前较差的位置以获得更好的适应值；in,

is the best position currently occupied by the discoverer, and X _worst represents the current global worst position; A is a 1×d matrix, in which each element is randomly assigned a value of 1 or -1, and A ⁺ = A ^T ( AA ^T ) ^-1 ,

Indicates the value of the j-th dimension of A ⁺ ; when

When , the joiner will actively follow the discoverer to move to a better position; when

When , the i-th joiner with a lower fitness value needs to move to another position, and get rid of the current poor position in combination with the exp function characteristics to obtain a better fitness value;

The position update formula for each generation of vigilantes is as follows:

是当前的全局最优位置；β作为步长控制参数，是服从均值为0，方差为1的正态分布的随机数；K∈[-1,1]是一个随机数，f_i则是当前磁铁组的适应值；f_g和f_w分别是当前全局最佳和最差的适应值；ε是最小的常数，以避免分母出现零；当f_i>f_g时，表示警戒者处于边缘位置，适应值较低，需向中心位置靠拢；当f_i＝f_g时，表示处于中间位置永磁体结构的适应值较低，该永磁体结构需要去其他位置来提高适应值；among them

is the current global optimal position; β, as the step size control parameter, is a random number that obeys the normal distribution with mean value 0 and variance 1; K∈[-1,1] is a random number, and f _i is the current The fitness value of the magnet group; f _g and f _w are the current global best and worst fitness values respectively; ε is the smallest constant to avoid zero in the denominator; when f _i > f _g , it means the vigilant is at the edge , the fitness value is low, and it needs to move closer to the central position; when f _i =f _g , it means that the fitness value of the permanent magnet structure in the middle position is low, and the permanent magnet structure needs to go to other positions to increase the fitness value;

During the magnetic field simulation, the FOV is sliced along the Z-axis according to a certain step length, and N ₁ slices are obtained; the two central axes on each slice are sampled at the same step length, and N ₂ magnetic inductions are obtained on each central axis Intensity B _i , the calculation formula of uniformity P _i on any axis is as follows:

Among them, B _c is the magnetic induction intensity at the midpoint of the tangent plane; the uniformity P _xi and P _yi of the two central axes in a tangent plane are calculated by formula (6), and the uniformity H ₁ is expressed as follows:

Calculate the uniformity B _h of each section:

Among them, B _max is the maximum value of the magnetic induction intensity in the section, and B _min is the minimum value of the magnetic induction intensity in the section; after solving the B _h of each section through the formula (8), the uniformity H ₂ is obtained:

Combining formulas (7) and (9), the evaluation function H of the structural fitness value of the permanent magnet is as follows:

H＝(H ₁ +H ₂ )·(|G-2|+|G-3|) (20)

where G is the magnetic field gradient along the Z-axis of the detection target area. In order to keep the G low gradient of 2-3T/m in the design of the permanent magnet structure, the formula (10) uses (|G-2|+|G -3|) to evaluate the fitness value.

Optionally, the magnet grades of the left magnet group 1 and the right magnet group 3 are NdFe1, and the magnet grade of the middle magnet 2 is NdFe2;

The distance between the left magnet group 1 and the right magnet group 3 is d1; the distance between the bottom surface of the left magnet group 1 and the right magnet group 3 and the upper bottom surface of the middle magnet 2 in the Z-axis direction is d2, and the left magnet group The inner distance between 1 and the right magnet group 3 is d3, the inner distance between the middle magnet 2 is d4, and the width of the middle magnet 2 is W;

Optionally, the unilateral permanent magnet structure is five NdFeB magnets, and the integrated radio frequency transceiver coil is designed using the time harmonic field inverse method and the current function method, and is designed to be orthogonal to the main magnetic field according to the distribution of the main magnetic field. and related matching radio frequency field; assume that the length of the wiring area of the radio frequency transceiver integrated coil is Lx, and the width is Ly; The current density component in the main magnetic field B0; the current density component is expressed as,

P _mn is the coefficient to be obtained;

According to the current continuity principle, the flow function of the current density on the y=0 plane is expressed as:

Then the magnetic field component of each point in the ROI space of the target area is detected by the Biosafal theorem

R represents the distance from the field point to the source point, and S ₀ represents the plane where the current density is located;

In the formula, (x, y, z) represents the coordinates of any field point in the ROI, and (x ₀ , y ₀ , z ₀ ) represents the coordinates of any source point on the current density plane;

One aspect of matching the radio frequency field with the main magnetic field is that the radio frequency field B ₁ vector generated by the radio frequency coil should be orthogonal to the main magnetic field B ₀ vector at each height generated by the unilateral magnet, that is:

B ₁ ⊥ B ₀

expands to

B _0x B _1x +B _0y B _1y +B _0z B _1z = 0 (15)

Another aspect of matching the radio frequency field with the main magnetic field is that the magnetic field modulus of the radio frequency field B ₁ should be proportional to the magnetic field modulus of the main magnetic field B ₀ produced by the unilateral magnet; The spin hydrogen protons can flip to the same plane at the same time, and then under the condition that the RF field matches the main magnetic field, a high-resolution correlation spectrum and a high signal-to-noise ratio can be obtained; this must satisfy

||B ₁ ||∝||B ₀ || (16)

expands to the equation

The ratio of the k-value radio frequency field B ₁ to the main magnetic field B ₀ generated by the unilateral magnet, usually a few thousandths;

Substitute the current density trigonometric function series expression into the magnetic field expression to obtain the relationship between the i-th target field point and the l-th current density source point, i=1,...,Q, l=m× N+n; l=1,..., M×N, expressed as:

have

P _mn =P _l ,K _mn,i =K _l,i ,D _mn,i =D _l,i ,H _mn,i =H _l,i ;

Among them, the values of M, N and Q are determined by computer memory resources according to the actual situation; a specific target radio frequency field is generated, and the length and width are respectively -L _x /2≤x≤L _x /2, -L _y /2≤y≤L P _l is calculated in the finite current density plane of _y /2, and the two orthogonal components J _x (x,z) and J _z (x,z) of the current density in the plane are calculated according to P _l , that is, the harmonic field target Field method, and then use the stay function method to calculate the actual wiring trajectory with the contour of the flow function.

Optionally, the unilateral nuclear magnetic resonance device further includes an aluminum shell, and the unilateral permanent magnet structure and the integrated radio frequency transceiver coil are both arranged in the aluminum shell.

The beneficial effect of the present invention is that the small nuclear magnetic resonance device for superficial skin detection provided by the present invention has simple structure, small size, light weight and reliable performance, and can realize signal measurement of nuclear magnetic resonance, which is convenient for on-site non-invasive detection.

Other advantages, objects and features of the present invention will be set forth in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the investigation and research below, or can be obtained from It is taught in the practice of the present invention. The objects and other advantages of the invention may be realized and attained by the following specification.

Description of drawings

In order to make the purpose of the present invention, technical solutions and advantages clearer, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

Fig. 1 is the overall schematic diagram of the hand-held low-gradient unilateral nuclear magnetic resonance device for detecting full-thickness skin of the present invention;

Fig. 2 is a schematic diagram of the initial structure of the permanent magnet and variables to be optimized;

Fig. 3 is a flow chart of optimizing the permanent magnet structure by the sparrow search algorithm;

4 is a schematic diagram of the magnetic induction intensity along the Z-axis direction at the center of the detection target area;

5 is a schematic diagram of the equipotential surface of the magnetic induction intensity in the detection target area;

Fig. 6 is a schematic diagram of the orthogonal and correlated matching between the radio frequency field and the main magnetic field in the detection target area.

Fig. 7 is a schematic diagram of the structure of the radio frequency coil.

Reference signs: 1-left magnet group; 2-middle magnet; 3-right magnet group; 4-radio frequency transceiver integrated coil; 5-detection target area.

detailed description

Embodiments of the present invention are described below through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation modes, and various modifications or changes can be made to the details in this specification based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the diagrams provided in the following embodiments are only schematically illustrating the basic concept of the present invention, and the following embodiments and the features in the embodiments can be combined with each other in the case of no conflict.

Wherein, the accompanying drawings are for illustrative purposes only, and represent only schematic diagrams, rather than physical drawings, and should not be construed as limiting the present invention; in order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings may be omitted, Enlargement or reduction does not represent the size of the actual product; for those skilled in the art, it is understandable that certain known structures and their descriptions in the drawings may be omitted.

In the drawings of the embodiments of the present invention, the same or similar symbols correspond to the same or similar components; , "front", "rear" and other indicated orientations or positional relationships are based on the orientations or positional relationships shown in the drawings, which are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the referred devices or elements must It has a specific orientation, is constructed and operated in a specific orientation, so the terms describing the positional relationship in the drawings are for illustrative purposes only, and should not be construed as limiting the present invention. For those of ordinary skill in the art, the understanding of the specific meaning of the above terms.

Please refer to Figures 1 to 7. Figure 1 is an overall schematic diagram of a handheld low-gradient unilateral nuclear magnetic resonance device for detecting full-thickness skin. As shown in Figure 1, the handheld low-gradient single-sided nuclear magnetic resonance device for detecting full-thickness skin The unit includes:

Unilateral permanent magnet structure and integrated radio frequency transceiver coil 4;

The unilateral permanent magnet structure includes three sets of magnet structures;

The three groups of magnet structures are respectively the left magnet group 1, the middle magnet 2 and the right magnet group 3;

The left magnet group 1 and the right magnet group 3 are on the same level;

The middle magnet 2 is located between the left magnet group 1 and the right magnet group 3, and is located below the horizontal plane of the left magnet group 1 and the right magnet group;

The specifications of the left magnet group 1 and the right magnet group 3 are the same;

In the XYZ coordinate system, the magnetic poles of the left magnet group 1 point to the Z direction, the magnetic poles of the middle magnet 2 point to the -Y direction, and the magnetic poles of the right magnet group 3 point to the -Z direction;

The main magnetic field B ₀ from the left magnet group 1 to the right magnet group 3 is generated above the unilateral permanent magnet structure, the magnetic pole points to the Y direction, and a uniform magnetic field is generated in the geometric center area of the middle magnet, and the geometric center area of the middle magnet To detect the target area 5;

The magnetic induction intensity of the main magnetic field _B0 is less than or equal to 100mT, and the magnetic field gradient G perpendicular to the direction of the magnetic field along the center of the detection target area 5 is 2.96T/m;

The magnetic field distribution produced by the permanent magnet structure is calculated by the finite element method, and the parameters of the permanent magnet structure are optimized by the sparrow search algorithm; the radio frequency transceiver integrated coil 4 adopts a transceiver integrated double-layer planar coil, and the time harmonic field inverse method and The current function method is used to generate the excitation radio frequency magnetic field orthogonal to the main magnetic field, and receive the echo signal generated after the excitation of the tested sample set in the detection target area.

The initial structure of the permanent magnet is composed of three sets of six magnets, each set of two magnets, the three sets of magnets are arranged in a similar shape, the left and right sets of magnets have the same specifications and grades, the magnetic poles point to the opposite, and the left set of magnetic poles point vertically upward (Z direction), the right group is vertically downward (-Z direction); the magnetic pole of the middle group points horizontally and is in the same direction as the magnetic field generated by the left and right two groups of magnets at this position (-Y direction), and its brand specification can be compared with that of the left and right The two sets of magnets are inconsistent. When the permanent magnet is initialized, we stipulate that the size of the four magnets in the left and right magnet groups is: length , height , and width 12.7mm; the size of the middle group magnet is: length , height , and width 25.4mm. Therefore, the main magnetic field B0 from the left magnet group to the right magnet group (Y direction) is generated above the center of the magnet structure. The initial structure of the permanent magnet needs to be optimized with 7 parameters including: the grade of the left and right two magnets NdFe1, the grade of the middle magnet NdFe2, the distance d1 between the left and right two magnets, the bottom surface of the left and right magnets and the upper bottom surface of the middle magnet in the Z-axis direction The spacing d2, the inner spacing d3 of the two left and right magnet groups, the inner spacing d4 of the middle magnet group and the width W of the middle magnet. The initial structure of the permanent magnet and the variables to be optimized are shown in Figure 2.

After the initial completion of the permanent magnet structure, the sparrow search algorithm is used to optimize the structure of all permanent magnet structures. The sparrow search algorithm optimizes the permanent magnet structure process as shown in Figure 3.

The magnetic induction intensity along the Z-axis direction in the center of the detection target area is shown in Fig. 4 .

The magnetic field in the detection target area with the optimal structure of the permanent magnet has the characteristics of high uniformity, and the distribution of the equipotential surface of the magnetic induction intensity is shown in Figure 5.

Since the main magnetic field of the predetermined detection target area in the present invention is parallel to the skin surface and attenuated along its vertical skin depth direction, in order to ensure that the radio frequency field B1 is orthogonal to the main magnetic field _B0 , the direction of the radio frequency field must be the same as the vertical direction of the skin . Therefore, the radio frequency coil is designed using the time harmonic field inverse method and the flow function method, and a matching radio frequency field orthogonal to and related to the main magnetic field is designed according to the distribution of the main magnetic field. Fig. 6 is a schematic diagram of the orthogonal and correlated matching between the radio frequency field and the main magnetic field in the detection target area. Fig. 7 is a schematic diagram of the structure of the radio frequency coil, since the structure of the double-layer coil is the same, only one layer of the coil structure is shown.

Finally, it is noted that the above embodiments are only used to illustrate the technical solutions of the present invention without limitation. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be carried out Modifications or equivalent replacements, without departing from the spirit and scope of the technical solution, should be included in the scope of the claims of the present invention.

Claims (6)

1. The hand-held low-gradient unilateral nuclear magnetic resonance device for detecting full-thickness skin is characterized in that: the device includes: Unilateral permanent magnet structure and radio frequency transceiver integrated coil (4); The unilateral permanent magnet structure includes three sets of magnet structures; The three sets of magnet structures are respectively the left magnet set (1), the middle magnet (2) and the right magnet set (3); The left magnet group (1) and the right magnet group (3) are on the same horizontal plane; The middle magnet (2) is located between the left magnet group (1) and the right magnet group (3), and is located below the horizontal plane of the left magnet group (1) and the right magnet group; The specifications of the left magnet group (1) and the right magnet group (3) are the same; In the XYZ coordinate system, the magnetic poles of the left magnet group (1) point to the Z direction, the magnetic poles of the middle magnet (2) point to the -Y direction, and the magnetic poles of the right magnet group (3) point to the -Z direction; The main magnetic field B ₀ from the left magnet group (1) to the right magnet group (3) is generated above the unilateral permanent magnet structure. The magnetic pole points to the Y direction, and a uniform magnetic field is generated in the geometric center area of the middle magnet. The middle magnet The geometric center area of is the detection target area (5); The magnetic induction intensity of the main magnetic field _B0 is less than or equal to 100mT, and the center of the detection target area (5) is 2.96T/m along the magnetic field gradient G perpendicular to the direction of the magnetic field; The magnetic field distribution generated by the unilateral permanent magnet structure is calculated by the finite element method, and the structural parameters of the unilateral permanent magnet are optimized by the sparrow search algorithm; The radio-frequency transceiving integrated coil (4) adopts a transceiving-integrated double-layer planar coil, which is designed by using the time-harmonic field inverse method and the flow function method, and is used to generate an excitation radio-frequency magnetic field orthogonal to the main magnetic field, and the receiver is set at The echo signal generated after the excitation of the tested sample in the target area is detected. 2. The hand-held low-gradient unilateral nuclear magnetic resonance device for detecting full-thickness skin according to claim 1, characterized in that: the size of the left magnet group (1) and the right magnet group (3) is: length The size of the middle magnet (2) is 50.8mm in length, 12.7mm in height and 25.4mm in width. 3. The hand-held low-gradient unilateral NMR device for detecting full-thickness skin according to claim 1, wherein: said optimization of the unilateral permanent magnet structural parameters by the sparrow search algorithm is specifically: The spatial position of the unilateral permanent magnet structure is expressed as an n×d matrix X:

Where n is the number of permanent magnet structures, d is the dimension of the structure parameters of the permanent magnet structure to be optimized; the position of the _ith permanent magnet structure in space is Xi = ( _xi,1 , _xi,2 ,..., _{xi ,d} ); the fitness value of the unilateral permanent magnet structure is expressed as a vector F _X :

The optimization ability of the sparrow search algorithm is based on three parts: the discoverer, the joiner and the vigilante. The three jointly search for the optimal permanent magnet structure; the position update formula of each generation of discoverers is as follows:

Among them, t represents the current number of iterations, T represents the total number of iterations,

Indicates the amount of information of the j-th dimension parameter in the i-th permanent magnet structure at the t-th iteration, α is a random number in [0,1], R is a random number indicating the current warning value, R∈[0,1] , ST represents the safety value, ST∈[0.5,1], Q is a random number that obeys the normal distribution, L is a matrix of 1×d _j , and d _j is the dimension of the jth dimension parameter of the magnet group, the matrix Each element is all 1; The position update formula for each generation of joiners is as follows:

in,

is the best position currently occupied by the discoverer, and X _worst represents the current global worst position; A is a 1×d matrix, in which each element is randomly assigned a value of 1 or -1, and A ⁺ = A ^T ( AA ^T ) ^-1 ,

Indicates the value of the j-th dimension of A ⁺ ; when

When , the joiner will actively follow the discoverer to move to a better position; when

When , the i-th joiner with a lower fitness value needs to move to another position, and get rid of the current poor position in combination with the exp function characteristics to obtain a better fitness value; The position update formula for each generation of vigilantes is as follows:

among them

is the current global optimal position; β, as the step size control parameter, is a random number that obeys the normal distribution with mean value 0 and variance 1; K∈[-1,1] is a random number, and f _i is the current The fitness value of the magnet group; f _g and f _w are the current global best and worst fitness values respectively; ε is the smallest constant to avoid zero in the denominator; when f _i > f _g , it means the vigilant is at the edge , the fitness value is low, and it needs to move closer to the central position; when f _i =f _g , it means that the fitness value of the permanent magnet structure in the middle position is low, and the permanent magnet structure needs to go to other positions to increase the fitness value; During the magnetic field simulation, the FOV is sliced along the Z-axis according to a certain step length, and N ₁ slices are obtained; the two central axes on each slice are sampled at the same step length, and N ₂ magnetic inductions are obtained on each central axis Intensity B _i , the calculation formula of uniformity P _i on any axis is as follows:

Among them, B _c is the magnetic induction intensity at the midpoint of the tangent plane; the uniformity P _xi and P _yi of the two central axes in a tangent plane are calculated by formula (6), and the uniformity H ₁ is expressed as follows:

Calculate the uniformity B _h of each section:

Among them, B _max is the maximum value of the magnetic induction intensity in the section, and B _min is the minimum value of the magnetic induction intensity in the section; after solving the B _h of each section through the formula (8), the uniformity H ₂ is obtained:

Combining formulas (7) and (9), the evaluation function H of the structural fitness value of the permanent magnet is as follows: H＝(H ₁ +H ₂ )·(|G-2|+|G-3|) (10) where G is the magnetic field gradient along the Z-axis of the detection target area. In order to keep the G low gradient of 2-3T/m in the design of the permanent magnet structure, the formula (10) uses (|G-2|+|G -3|) to evaluate the fitness value. 4. The hand-held low-gradient unilateral NMR device for detecting full-thickness skin according to claim 3, characterized in that: the magnet grades of the left side magnet group (1) and the right side magnet group (3) are NdFe1 , the middle magnet (2) is NdFe2; The distance between the left magnet group (1) and the right magnet group (3) is d1; the lower bottom surface of the left magnet group (1) and the right magnet group (3) and the upper bottom surface of the middle magnet (2) are on the Z axis The spacing in the direction is d2, the inner spacing of the left magnet group (1) and the right magnet group (3) is d3, the inner spacing of the middle magnet (2) is d4, and the width of the middle magnet (2) is W; In the formula (1), d is the dimension of the structural parameters of the permanent magnet structure to be optimized, one parameter is 1 dimension, two parameters are 2 dimensions, and three parameters are 3 dimensions. 5. The hand-held low-gradient unilateral nuclear magnetic resonance device for detecting full-thickness skin according to claim 4, characterized in that: the unilateral permanent magnet structure is five NdFeB magnets, and the radio frequency transceiver integrated coil adopts The time harmonic field inverse method and the flow function method are used to design, and the matching radio frequency field orthogonal to and related to the main magnetic field is designed according to the distribution of the main magnetic field; the length of the wiring area of the radio frequency transceiver integrated coil is set to be Lx, and the width is Ly; The surface current density in the plane where the integral coil is located is divided into two components: the current density component parallel to the main magnetic field B0 and the current density component perpendicular to the main magnetic field B0; the current density component is expressed as,

P _mn is the coefficient to be obtained; According to the current continuity principle, the flow function of the current density on the y=0 plane is expressed as:

Then the magnetic field component of each point in the ROI space of the target area is detected by the Biosafal theorem

R represents the distance from the field point to the source point, and S ₀ represents the plane where the current density is located;

In the formula, (x, y, z) represents the coordinates of any field point in the ROI, and (x ₀ , y ₀ , z ₀ ) represents the coordinates of any source point on the current density plane; One aspect of matching the radio frequency field with the main magnetic field is that the radio frequency field B ₁ vector generated by the radio frequency coil should be orthogonal to the main magnetic field B ₀ vector at each height generated by the unilateral magnet, that is: B ₁ ⊥ B ₀ expands to B _0x B _1x +B _0y B _1y +B _0z B _1z = 0 (15) Another aspect of matching the radio frequency field with the main magnetic field is that the magnetic field modulus of the radio frequency field B ₁ should be proportional to the magnetic field modulus of the main magnetic field B ₀ produced by the unilateral magnet; The spin hydrogen protons can flip to the same plane at the same time, and then under the condition that the RF field matches the main magnetic field, a high-resolution correlation spectrum and a high signal-to-noise ratio can be obtained; this must satisfy ||B ₁ ||∝||B ₀ || (16) expands to the equation

The ratio of the k-value radio frequency field B ₁ to the main magnetic field B ₀ generated by the unilateral magnet, usually a few thousandths; Substitute the current density trigonometric function series expression into the magnetic field expression to obtain the relationship between the i-th target field point and the l-th current density source point, i=1,...,Q, l=m× N+n; l=1,..., M×N, expressed as:

have

P _mn =P _l ,K _mn,i =K _l,i ,D _mn,i =D _l,i ,H _mn,i =H _l,i ; Among them, the values of M, N and Q are determined by computer memory resources according to the actual situation; a specific target radio frequency field is generated, and the length and width are respectively -L _x /2≤x≤L _x /2, -L _y /2≤y≤L P _l is calculated in the finite current density plane of _y /2, and the two orthogonal components J _x (x,z) and J _z (x,z) of the current density in the plane are calculated according to P _l , that is, the harmonic field target Field method, and then use the stay function method to calculate the actual wiring trajectory with the contour of the flow function. 6. The hand-held low-gradient unilateral nuclear magnetic resonance device for detecting full-thickness skin according to claim 5, characterized in that: the unilateral nuclear magnetic resonance device also includes an aluminum shell, a unilateral permanent magnet structure and an integrated radio frequency transceiver coil All are set in the aluminum shell.

Images