CN110960216A - Multi-frequency holographic microwave brain imaging system and imaging method - Google Patents

Multi-frequency holographic microwave brain imaging system and imaging method Download PDF

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CN110960216A
CN110960216A CN201911021121.3A CN201911021121A CN110960216A CN 110960216 A CN110960216 A CN 110960216A CN 201911021121 A CN201911021121 A CN 201911021121A CN 110960216 A CN110960216 A CN 110960216A
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王露露
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Abstract

The application provides a multi-frequency holographic microwave brain imaging system and an imaging method, wherein the imaging method comprises the following steps: transmitting an ultra-wideband microwave signal to an object in a to-be-detected area by using a receiving-transmitting integrated microwave antenna, detecting a scattered electric field signal of the object in the to-be-detected area, and processing the scattered electric field signal of the object in the to-be-detected area to obtain a two-dimensional reconstructed image of the object in the to-be-detected area; presetting an electromagnetic numerical model of the skull according to the total visible scattering electric field of all the microwave antennas, and obtaining a two-dimensional reconstruction image of the skull according to the electromagnetic numerical model of the skull; and judging whether the two-dimensional reconstruction image of the skull is the same as the two-dimensional reconstruction image of the object in the region to be detected, if so, determining that the object in the region to be detected is a skull organ, and transmitting the two-dimensional reconstruction image of the skull to an image display module for displaying. The method and the device have the advantages that the non-contact and non-invasive effects are realized, diseases such as cerebral apoplexy can be continuously monitored, and the resolution of the brain image obtained through reconstruction is high.

Description

Multi-frequency holographic microwave brain imaging system and imaging method
Technical Field
The application belongs to the technical field of microwave imaging, and particularly relates to a multi-frequency holographic microwave brain imaging system and an imaging method.
Background
The stroke seriously harms the health of middle-aged and elderly people in China, affects the life quality, and brings heavy economic burden to families of patients and heavy medical burden to society. Early discovery, early diagnosis and early intervention are the keys for realizing the timely rescue and treatment of the stroke. Medical imaging technologies such as X-ray computed tomography, electronic Computed Tomography (CT), Magnetic Resonance Imaging (MRI), and ultrasound imaging are currently common medical imaging diagnostic technologies for stroke. However, X-ray computed tomography and CT generate ionizing radiation harmful to human health and do not allow continuous, real-time monitoring of the development of cerebral edema.
In the past decade, expert scholars have conducted a great deal of theoretical research and experimental validation work around holographic microwave imaging. The existing holographic microwave imaging technology basically adopts single-frequency domain imaging, and has a plurality of defects, such as long image scanning time, low image resolution, low sensitivity and the like. The image resolution depends to a large extent on the operating frequency range of the imaging system, the microwave antenna and antenna array configuration, and the image reconstruction algorithm. Resolution is related to the operating frequency and wavelength of the imaging system, and better image resolution can be achieved by using higher operating frequencies and shorter wavelengths. However, short wavelengths also mean smaller penetration depths, while higher operating frequencies are not suitable for imaging detection of biological tissue.
Disclosure of Invention
To overcome, at least to some extent, the problems in the related art, the present application provides a multi-frequency holographic microwave brain imaging system and method.
According to a first aspect of embodiments of the present application, there is provided a multi-frequency holographic microwave brain imaging method, comprising the steps of:
the method comprises the steps that a multi-frequency holographic microwave brain imaging system is arranged, and the multi-frequency holographic microwave brain imaging system comprises a main control module, a multi-frequency signal generation module, a switching module, a transmitting and receiving module, an image processing module and a display module, wherein the transmitting and receiving module comprises a two-dimensional microwave antenna array, and the two-dimensional microwave antenna array adopts a microwave antenna integrating receiving and transmitting;
the main control module controls the transmitting and receiving module to be switched to a transmitting state through the switching module, and controls the multi-frequency signal generating module to generate an ultra-wideband microwave signal, and the generated ultra-wideband microwave signal is transmitted to an object in an area to be detected through at least three microwave antennas which are integrated in transmitting and receiving in the transmitting and receiving module through the switching module;
the main control module controls at least three receiving and transmitting integrated microwave antennas in the transmitting and receiving module to sequentially measure scattering electric field signals of objects from a region to be detected and transmits the detected scattering electric field signals to the image processing module;
the image processing module processes the received scattered electric field signal to obtain a two-dimensional reconstruction image of an object in the area to be detected;
presetting an electromagnetic numerical model of the skull according to the total visible scattering electric field of all microwave antennas;
obtaining a two-dimensional reconstruction image of the skull according to the electromagnetic numerical model of the skull;
and judging whether the two-dimensional reconstruction image of the skull is the same as the two-dimensional reconstruction image of the object in the region to be detected, if so, determining that the object in the region to be detected is a skull organ, and transmitting the two-dimensional reconstruction image of the skull to an image display module for displaying.
In the above multi-frequency holographic microwave brain imaging method, the specific process of transmitting the ultra-wideband microwave signal is as follows:
establishing a rectangular coordinate system of an electromagnetic field area where an object is located, and determining the distance between the object and the two-dimensional microwave antenna array, the position coordinates of the microwave antennas and the number of the microwave antennas integrating receiving and transmitting;
transmitting ultra-wideband microwave signals with a preset frequency band to an object through at least three microwave antennas in a two-dimensional microwave antenna array, wherein the ultra-wideband microwave signals generate scattering electric fields in, on and around the object;
at least three microwave antennas in the two-dimensional microwave antenna array sequentially detect scattered electric field signals generated inside, on the surface and around the object;
the total electric field obtained from the incident electric field and the scattering electric field is:
Figure BDA0002247251470000021
in the formula (I), the compound is shown in the specification,
Figure BDA0002247251470000031
indicating detection by a single microwave antennaScattered electric field signals generated inside, on and around the object,
Figure BDA0002247251470000032
representing the sum of the incident electric fields emitted by the N microwave antennas,
Figure BDA0002247251470000033
is shown at Ar(xr，yr，zr) The distance vector from the microwave antenna to the object in the region to be detected,
Figure BDA0002247251470000034
indicating that it is located at Ai(xi，yi，zi) The distance vector between the microwave antenna and the object in the region to be detected, and omega represents angular frequency.
In the above multi-frequency holographic microwave brain imaging method, the specific process of obtaining the two-dimensional reconstructed image of the object in the region to be detected is as follows:
establishing a nonlinear observation model between the electromagnetic properties such as dielectric constant and conductivity of an object in a region to be detected and a scattering electric field, wherein the nonlinear observation model comprises a total electric field model and a scattering electric field model;
and reconstructing an image of the object in the region to be detected by using the total electric field model and the scattering electric field model.
Further, the total electric field model is:
Figure RE-GDA0002382776190000035
in the formula (I), the compound is shown in the specification,
Figure BDA0002247251470000036
which is indicative of the incident electric field,
Figure BDA0002247251470000037
representing the position from the target point to the point AT(xT,yT,zT) The distance vector of the microwave antenna of (1),
Figure BDA0002247251470000038
the divergence operator is represented by a vector of vectors,
Figure BDA0002247251470000039
the expression of the green's function is,
Figure BDA00022472514700000310
Figure BDA00022472514700000311
Figure BDA00022472514700000312
representing a position vector from a field source point to any point in the cranium;
the scattered electric field model is:
Figure BDA00022472514700000313
in the formula (I), the compound is shown in the specification,
Figure BDA00022472514700000314
which represents the scattered electric field and is,
Figure BDA00022472514700000315
representing any target point in the skull to be located at AR(xR,yR,zR) The distance vector of the microwave antenna of (1),
Figure BDA00022472514700000316
represents a position vector; epsilonbDielectric constant, μ, representing backgroundbThe magnetic permeability of the background is shown,
Figure BDA00022472514700000317
Figure BDA00022472514700000318
ε represents the dielectric constant of the brain, σ represents the conductivity of the skull, σbConductivity, ω, representing the background(p)＝ 2πf(p)For working angular frequencyRate, f(p)Which is indicative of the frequency at which the signal is transmitted,
Figure BDA00022472514700000319
fminfor the minimum operating frequency, P1., P, Q1., Q, P denotes the frequency number, Q denotes the view number,
Figure BDA0002247251470000041
as a result of the total electric field,
Figure BDA0002247251470000042
further, the scattering electric field model is also expressed as:
Figure BDA0002247251470000043
in the formula (I), the compound is shown in the specification,
Figure BDA0002247251470000044
r denotes the distance between the radiation source and the target point,
Figure BDA0002247251470000045
represents a unit vector;
when a ≈ 1, b ≈ -1, the scattering electric field model is simplified as:
Figure BDA0002247251470000046
in the formula (I), the compound is shown in the specification,
Figure BDA0002247251470000047
the expression of the green's function is,
Figure BDA0002247251470000048
further, the process of reconstructing the image of the object in the region to be detected by using the total electric field model and the scattering electric field model comprises:
sequentially comparing the scattered electric fields detected by any two microwave antennas in at least three microwave antennas in the two-dimensional microwave antenna array;
obtaining information capable of reflecting amplitude and phase of dielectric property distribution in the target object according to the difference obtained by comparing every two in sequence;
obtaining the total visible scattering electric field of all the microwave antennas according to the amplitude and phase information;
and performing inverse Fourier transform on the total visible scattering electric field detected by all the microwave antennas to obtain a two-dimensional reconstruction image of the object in the region to be detected.
Furthermore, the visible scattering electric field of the scattering electric field detected by any two microwave antennas in the microwave antenna array is:
Figure BDA0002247251470000049
when the number of the microwave antennas is N, N is a natural number and N is greater than or equal to 3, and the total visible scattering electric field is the sum of the visible scattering electric fields of the N (N-1) microwave antennas, the total visible scattering electric field of all the microwave antennas is:
Figure BDA0002247251470000051
in the above multi-frequency holographic microwave brain imaging method, the electromagnetic numerical model of the skull is:
Figure BDA0002247251470000052
in the formula, epsilonbDielectric constant, μ, representing the backgroundbThe magnetic permeability of the background is shown,
Figure BDA0002247251470000053
Figure BDA0002247251470000054
ε represents the dielectric constant of the brain, σ represents the conductivity of the skull, σbWhich is indicative of the electrical conductivity of the background,
Figure BDA0002247251470000055
ω(p)＝2πf(p)is the operating angular frequency, f(p)Which is indicative of the frequency at which the signal is transmitted,
Figure BDA0002247251470000056
fminfor the minimum operating frequency, P1., P, Q1., Q, P denotes the frequency number, Q denotes the view number,
Figure BDA0002247251470000057
as a result of the total electric field,
Figure BDA0002247251470000058
Figure BDA0002247251470000059
further, the process of obtaining the two-dimensional reconstruction image of the skull according to the electromagnetic numerical model of the skull is as follows:
calculating the volume fraction of the electromagnetic numerical model of the skull, wherein the volume fraction is as follows:
Figure BDA00022472514700000510
substituting the electromagnetic numerical model of the skull into equation (14) yields:
Figure BDA00022472514700000511
in the formula (15), the reaction mixture is,
Figure BDA00022472514700000512
λbwhich represents the wavelength of operation of the light,
Figure BDA00022472514700000513
representing a unit vector in a spherical coordinate system,
Figure BDA00022472514700000514
dV＝s2sinθdθdφds；
define new parameters (l, m, n):
l＝sinθcosφ (16)
m＝sinθsinφ
Figure BDA0002247251470000061
dV can be obtained from the following formula:
dV＝s2dldmds/n (17)
substitution of formula (17) into formula (15) yields:
Figure BDA0002247251470000062
component of the baseline vector in a Cartesian coordinate System
Figure BDA0002247251470000063
Is composed of
Figure BDA0002247251470000064
Figure BDA0002247251470000065
Because the microwave antenna array is two-dimensional, the microwave antennas are arranged at the same height, and the volume of the electromagnetic numerical model of the skull is divided into:
Figure BDA0002247251470000066
the line integral of the electromagnetic numerical model of the skull along the radial coordinate n is:
Figure BDA0002247251470000067
the following two-dimensional integral versus visibility scattering function for the variable (l, m) is derived using equation (21):
Figure BDA0002247251470000068
the visibility scattering function is subjected to inverse Fourier transform to reconstruct a skull image, and a two-dimensional reconstructed image of the skull is obtained, wherein the two-dimensional reconstructed image specifically comprises the following steps:
Figure BDA0002247251470000069
the multi-frequency holographic microwave brain imaging method further comprises the following steps:
the peak signal-to-noise ratio (PSNR) was used to assess the quality of the brain images:
Figure BDA0002247251470000071
in the formula, peak represents an image peak; the MSE represents the mean square error and,
Figure BDA0002247251470000072
Figure BDA0002247251470000073
the closer the MSE is to the value zero, the better the image quality is; y represents a vector observation of the predicted variable,
Figure BDA0002247251470000074
representing vectors of nn predictions generated from samples of nn data points on all variables.
According to a second aspect of the embodiments of the present application, there is also provided a multi-frequency holographic microwave brain imaging system, which includes a main control module, a multi-frequency signal generation module, a switching module, a transmitting and receiving module, an image processing module and a display module;
the main control module is used for controlling the multi-frequency signal generation module to transmit ultra-wideband microwave signals and controlling the transmitting and receiving module to switch between a transmitting state and a receiving state through the switching module;
the transmitting and receiving module is used for transmitting an ultra-wideband microwave signal to the area to be detected in a transmitting state and receiving a scattering electric field reflected from the area to be detected in a receiving state;
the image processing module is used for reconstructing a brain image according to the received scattered electric field data and transmitting the reconstructed brain image to the display module for display.
In the multi-frequency holographic microwave brain imaging system, the transmitting and receiving module adopts a two-dimensional microwave antenna array, and the two-dimensional microwave antenna array comprises N receiving and transmitting integrated microwave antennas, wherein N is a natural number and is not less than 3.
In the multi-frequency holographic microwave brain imaging system, the frequency of the ultra-wideband microwave signal is 1GHz-4 GHz.
According to the above embodiments of the present application, at least the following advantages are obtained: the utility model provides a multifrequency holographic microwave brain imaging system utilizes multifrequency signal generation module to produce ultra wide band microwave signal uninterruptedly, microwave signal space propagates to the transmission receiving module, transmission receiving module sends microwave signal to waiting to detect the region at the transmitting state, transmission receiving module detects the scattered electric field that waits to detect the region production at the receiving state, image processing module carries out the reconfiguration of brain image according to the scattered electric field data that receive, and show the brain image of reconfiguration, this application multifrequency holographic microwave brain imaging system non-contact, it is noninvasive, it is integrative to receive the signal, can carry out continuous monitoring to diseases such as cerebral apoplexy.
According to the multi-frequency holographic microwave brain imaging method, the ultra-wideband microwave signals are transmitted, the received scattered electric field data is used for brain image reconstruction, and the resolution of the brain image can be remarkably improved.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the scope of the invention, as claimed.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of the specification of the application, illustrate embodiments of the application and together with the description, serve to explain the principles of the application.
FIG. 1 is a block diagram of a multi-frequency holographic microwave brain imaging system according to an embodiment of the present disclosure;
fig. 2 is a schematic diagram of a relative position relationship between a two-dimensional microwave antenna array and a fixing device and a to-be-detected region object in a multi-frequency holographic microwave brain imaging system according to an embodiment of the present application. Wherein the content of the first and second substances,
Figure BDA0002247251470000082
showing a microwave antenna integrated with a transceiver,
Figure BDA0002247251470000081
representing an object to be detected;
fig. 3 is a schematic diagram of two-dimensional arrangement of microwave antenna arrays and positions of skull organs in a multi-frequency holographic microwave brain imaging system according to an embodiment of the present disclosure;
FIG. 4 is a flowchart of a multi-frequency holographic microwave brain imaging method according to an embodiment of the present disclosure;
FIG. 5 is a schematic diagram of a geometrical arrangement of a pair of microwave antennas in a multi-frequency holographic microwave brain imaging system according to an embodiment of the present disclosure;
FIG. 6(a) is a two-dimensional permittivity distribution image of a skull model;
FIG. 6(b) is a two-dimensional conductivity distribution image of a skull model;
FIG. 6(c) is the real part of the two-dimensional reconstructed image of the lower skull model with the working frequency of 1-4GHz microwave band;
FIG. 6(d) is the imaginary part of the two-dimensional reconstructed image of the skull model under the microwave frequency band with the working frequency of 1-4 GHz;
FIG. 7(a) is the real part of a two-dimensional reconstructed image of a skull model at an operating frequency of 1 GHz;
FIG. 7(b) is the real part of a two-dimensional reconstructed image of a skull model at an operating frequency of 2 GHz;
FIG. 7(c) is the real part of a two-dimensional reconstructed image of a skull model at a working frequency of 3 GHz;
FIG. 7(d) is the real part of the two-dimensional reconstructed image of the skull model at an operating frequency of 4 GHz.
Description of reference numerals:
1. a main control module; 2. a multi-frequency signal generation module; 3. a switching module; 4. a transmitting and receiving module; 5. An image processing module; 6. a display module; 7. and (4) a fixing device.
Detailed Description
To make the objectives, technical solutions and advantages of the embodiments of the present application more apparent, the spirit of the present disclosure will be clearly described in the following drawings and detailed description, and any person skilled in the art who knows the embodiments of the present application can change and modify the technology taught by the present application without departing from the spirit and scope of the present application.
The illustrative embodiments and descriptions of the present application are provided to explain the present application and not to limit the present application. Additionally, the same or similar numbered elements/components used in the drawings and the embodiments are intended to represent the same or similar parts.
As used herein, "first," "second," …, etc., are not specifically intended to mean in a sequential or chronological order, nor are they intended to limit the application, but merely to distinguish between elements or operations described in the same technical language.
With respect to directional terminology used herein, for example: up, down, left, right, front or rear, etc., are simply directions with reference to the drawings. Accordingly, the directional terminology used is intended to be illustrative and is not intended to be limiting of the present teachings.
As used herein, the terms "comprising," "including," "having," "containing," and the like are open-ended terms that mean including, but not limited to.
As used herein, "and/or" includes any and all combinations of the described items.
References to "plurality" herein include "two" and "more than two"; reference to "multiple groups" herein includes "two groups" and "more than two groups".
As used herein, the terms "substantially", "about" and the like are used to modify any slight variation in quantity or error that does not alter the nature of the variation. In general, the range of minor variations or errors that may be modified by such terms may be 20% in some embodiments, 10% in some embodiments, 5% in some embodiments, or other values. It should be understood by those skilled in the art that the aforementioned values can be adjusted according to actual needs, and are not limited thereto.
Certain words used to describe the present application are discussed below or elsewhere in this specification to provide additional guidance to those skilled in the art in describing the present application.
The electrical properties of biological tissue are closely related to the health of physiological systems. When diseases such as cerebral edema and cerebral apoplexy occur, the dielectric property (usually expressed by dielectric constant) of biological tissues is changed remarkably. The dielectric properties of different types of biological tissues are obviously different. For example, the dielectric constant of biological tissues with high water content such as bones and tumors is higher than that of biological tissues with low water content such as fat, and the difference provides a feasible physical basis for detecting the physiological and pathological states of living biological tissues by microwave biological imaging. When the multi-frequency holographic microwave imaging detects the stroke in the brain, the image reconstruction is carried out by measuring the distribution of electric fields at the head, the surface and the periphery under the microwave excitation action, and then the important characteristics of the dielectric constant distribution, the scattering electric field and the like of the brain tissue are obtained.
As shown in fig. 1, the multi-frequency holographic microwave brain imaging system of the present application includes a main control module 1, a multi-frequency signal generating module 2, a switching module 3, a transmitting and receiving module 4, an image processing module 5, and a display module 6.
The main control module 1 is connected with the multi-frequency signal generation module 2 and the switching module 3, the multi-frequency signal generation module 2 is connected with the transmitting and receiving module 4 through the switching module 3, the transmitting and receiving module 4 is connected with the image processing module 5 through the switching module 3, and the image processing module 5 is connected with the display module 6.
Wherein, the multi-frequency signal generation module 2 adopts a network analyzer. In particular, a microwave network analyzer, model Keysight N5242B PNA-X, may be used, which can generate microwave signals in the frequency range 10MHz-26.5 GHz.
The transmitting and receiving module 4 adopts a two-dimensional microwave antenna array, and the two-dimensional microwave antenna array comprises N receiving and transmitting integrated microwave antennas, wherein N is a natural number and is more than or equal to 3. The microwave antenna integrated with the transceiver can transmit the microwave signals generated by the multi-frequency signal generating module 2 to the skull and also can receive scattered electric field signals in, on and around the skull.
Specifically, the two-dimensional microwave antenna array comprises 16 microwave antennas integrated with transceiving. The 16 microwave antennas integrating receiving and transmitting are uniformly distributed in a circular shape and are positioned on the same horizontal plane. The vertical distance d between the two-dimensional microwave antenna array and the skull is far larger than one working wavelength lambda of the two-dimensional microwave antenna arraybA distance of (d > λbThe skull is located in the far field region of the two-dimensional microwave antenna array.
The microwave antenna integrating the receiving and transmitting adopts a horn antenna, an electric dipole antenna, a patch antenna or an open waveguide antenna, a dielectric antenna and the like, and the size of each microwave antenna is determined according to the working frequency and the used material.
In order to reduce signal coupling and improve detection sensitivity, the gap between the skull and the microwave antenna array and the gap between adjacent microwave antennas in the microwave antenna array are filled with mediums capable of reducing electromagnetic noise, such as saline water, coconut oil and the like.
The working frequency of the multi-frequency holographic microwave brain imaging system is ultra-wideband, and the optimal working frequency range is 1GHz-4 GHz.
As shown in fig. 2 and 3, the multi-frequency holographic microwave brain imaging system further includes a fixing device 7, the fixing device 7 is used for fixing the skull of the human body, the two-dimensional microwave antenna array is vertically arranged at the bottom of the fixing device 7, and the vertical distance d between the two-dimensional microwave antenna array and the skull is much larger than a working wavelength λ of the two-dimensional microwave antenna arraybA distance of (d > λb。
When the multi-frequency holographic microwave brain imaging system works, the main control module 1 controls the multi-frequency signal generation module 2 to uninterruptedly generate ultra-wideband microwave signals, the transmitting and receiving module 4 is switched to a transmitting state through the switching module 3, and at least 3 microwave antennas in the transmitting and receiving module 4 sequentially transmit the ultra-wideband microwave signals to the skull.
The main control module 1 switches the transmitting and receiving module 4 to a receiving state through the switching module 3 under a microwave irradiation environment, scattered electric fields generated inside, on the surface and around the skull are sequentially received by at least 3 microwave antennas and transmitted to the image processing module 5, the image processing module 5 carries out brain image reconstruction according to received scattered electric field data, and transmits reconstructed brain images to the display module 6 for display.
As shown in fig. 4, based on the above multi-frequency holographic microwave brain imaging system, the present application further provides a multi-frequency holographic microwave brain imaging method, which includes the following steps:
s1, a multi-frequency holographic microwave brain imaging system is provided, which comprises a main control module 1, a multi-frequency signal generating module 2, a switching module 3, a transmitting and receiving module 4, an image processing module 5 and a display module 6. The transmitting and receiving module 4 includes a two-dimensional microwave antenna array, and the two-dimensional microwave antenna array adopts a microwave antenna integrated with a transmitting and receiving body.
S2, emitting an ultra-wideband microwave signal to an object in the area to be detected;
the main control module 1 controls the transmitting and receiving module 4 to switch to a transmitting state through the switching module 3, and controls the multi-frequency signal generating module 2 to generate an ultra-wideband microwave signal, and the generated ultra-wideband microwave signal is continuously transmitted to an object in an area to be detected through at least three microwave antennas which are integrated in transmitting and receiving in the transmitting and receiving module 4 through the switching module 3. The object in the region to be detected generates a scattering electric field under microwave irradiation.
S3, measuring the scattered electric field signal of the object in the region to be detected;
the main control module 1 controls at least three integrated microwave antennas in the transmitting and receiving module 4 to sequentially measure the scattered electric field signals of the object from the region to be detected, and transmits the detected scattered electric field signals to the image processing module 5.
S4 and the image processing module 5 processes the received scattered electric field signal to obtain a two-dimensional reconstructed image of the object in the region to be detected.
And S5, presetting an electromagnetic numerical model of the skull according to the total visible scattering electric field of all the microwave antennas.
And S6, obtaining a two-dimensional reconstruction image of the skull according to the electromagnetic numerical model of the skull.
S7, judging that if the two-dimensional reconstruction image of the skull is the same as the two-dimensional reconstruction image of the object in the region to be detected, the object in the region to be detected is the skull organ, and transmitting the two-dimensional reconstruction image of the skull to the image display module 6 for displaying.
In the step S2, the specific process of transmitting the ultra-wideband microwave signal includes:
s21, establishing a rectangular coordinate system of the electromagnetic field area where the object is located, and determining the distance between the object and the two-dimensional microwave antenna array, the position coordinates of the microwave antennas and the number of the microwave antennas integrated with transceiving.
The two-dimensional microwave antenna array is vertically arranged at the bottom of the object, and the distance between the two-dimensional microwave antenna array and the object is far larger than one working wavelength.
The two-dimensional microwave antenna array comprises N microwave antennas which are uniformly distributed in a circular shape.
S22, emitting an ultra-wideband microwave signal of a preset frequency band to the object through at least three microwave antennas in the two-dimensional microwave antenna array, where the ultra-wideband microwave signal generates a scattering electric field inside, on the surface, and around the object, and the scattering electric field can be regarded as a frequency domain harmonic electromagnetic field. The ultra-wideband microwave signal can generate incidence, reflection and refraction phenomena when passing through an object.
As shown in fig. 5, when a rectangular coordinate system oyx is established with the center of the two-dimensional microwave antenna array as the origin, the direction perpendicular to the paper surface and outward as the X-axis direction, the horizontal direction and the right direction as the Y-axis direction, and the vertical direction as the Z-axis direction, the coordinate of a point P on the object in the region to be detected is P (X, Y, Z), and the plane passing through the point P and parallel to the plane OXY is a plane nml. Wherein, point Ai(xi，yi，zi) At and point Aj(xj，yj，zj) The microwave antennas which are integrated with each other for receiving and transmitting are arranged at the positions.
The ultra-wideband microwave signal emitted to an object is represented as an incident electric field of the object in the application, and the specific form of the incident electric field is as follows:
Figure BDA0002247251470000131
in the formula (1), the reaction mixture is,
Figure BDA0002247251470000132
indicating the type of guided wave as TE10A represents the aperture length of the microwave antenna, B represents the aperture width of the microwave antenna, and h represents the amplitude of the electromagnetic wave(p，q)(theta, phi) represents a radiation pattern,
Figure BDA0002247251470000133
representing vector polarization, εbDielectric constant, μ, representing the backgroundbDenotes the magnetic permeability, ω, representing the background(p)＝2πf(p)，ω(p)The frequency of the operating angle is represented,
Figure BDA0002247251470000134
representing from target point to location AT(xT，yT，zT) The distance vector of the microwave antenna is located, theta represents the included angle between the connecting line of a point on the object in the region to be detected and the original point and the Z axis, phi represents the included angle between the connecting line of the projection of the point on the object in the region to be detected on the OXY plane and the original point and the X axis, p represents the frequency number, and q represents the view number.
Further, when the number N of the microwave antennas is a natural number and N is greater than or equal to 3, the incident electric field is the sum of the incident electric fields emitted by the N microwave antennas, that is:
Figure BDA0002247251470000135
in the formula (2), the reaction mixture is,
Figure BDA0002247251470000136
is shown at Ai(xi，yi，zi) The distance vector from the microwave antenna to the object in the region to be detected, and omega represents the angular frequency.
S23, sequentially detecting scattered electric field signals generated inside, on and around the object by at least three microwave antennas in the two-dimensional microwave antenna array, where the scattered electric field signals generated inside, on and around the object detected by a single microwave antenna are:
Figure BDA0002247251470000137
the total electric field is then:
Figure BDA0002247251470000138
that is, the ultra-wideband microwave signal emitted toward the object needs to satisfy the total electric field represented by equation (4).
In step S4, the specific process of obtaining the two-dimensional reconstructed image of the object in the region to be detected includes:
s41, establishing a nonlinear observation model between the electromagnetic properties such as dielectric constant and conductivity of the object in the region to be detected and the scattering electric field;
according to the working mechanism that different scattering strengths of different biological tissues in the intracranial part of an object are induced after an ultra-wideband microwave signal with a specific frequency band penetrates through the surface of the object, a nonlinear observation model between electromagnetic properties such as dielectric constant and conductivity of the object and a scattering electric field is established. The nonlinear observation model includes a total electric field model and a scattered electric field model.
And establishing a characterization model for describing the internal structure of the object based on the number and distribution arrangement shape of the microwave antennas.
Wherein, the total electric field model is:
Figure RE-GDA0002382776190000141
in the formula (5), the reaction mixture is,
Figure BDA0002247251470000142
which is indicative of the incident electric field,
Figure BDA0002247251470000143
representing the position from the target point to the point AT(xT,yT,zT) The distance vector of the microwave antenna at (a),
Figure BDA0002247251470000144
the divergence operator is represented by a vector of vectors,
Figure BDA0002247251470000145
the expression of the green's function is,
Figure BDA0002247251470000146
Figure BDA0002247251470000147
representing a distance vector from the field source point to any point in the skull.
The scattered electric field model is:
Figure BDA0002247251470000148
in the formula (6), the reaction mixture is,
Figure BDA0002247251470000149
which represents the scattered electric field and is,
Figure BDA00022472514700001410
representing any target point in the skull to be located at AR(xR,yR,zR) The distance vector of the microwave antenna of (1),
Figure BDA00022472514700001411
represents a position vector; epsilonbDielectric constant, μ, representing backgroundbThe magnetic permeability of the background is shown,
Figure BDA00022472514700001412
Figure BDA00022472514700001413
ε represents the dielectric constant of the brain, σ represents the conductivity of the skull, σbConductivity, ω, representing the background(p)＝ 2πf(p)Is the operating angular frequency, f(p)Which is indicative of the frequency at which the signal is transmitted,
Figure BDA00022472514700001414
fminfor the minimum operating frequency, P1., P, Q1., Q, P denotes the frequency number, Q denotes the view number,
Figure BDA00022472514700001415
as a result of the total electric field,
Figure BDA00022472514700001416
further, the scattering electric field model can be described by the following formula:
Figure BDA00022472514700001417
in the formula (7), the reaction mixture is,
Figure BDA0002247251470000151
r denotes the distance between the scattering source and the object point,
Figure BDA0002247251470000152
representing a unit vector.
Further, let a ≈ 1 and b ≈ -1, the scattering electric field model can be described as:
Figure BDA0002247251470000153
in the formula (I), the compound is shown in the specification,
Figure BDA0002247251470000154
the expression of the green's function is,
Figure BDA0002247251470000155
and (3) combining the internal electric field effect model and the external electric field effect model to obtain a nonlinear observation model, wherein the nonlinear observation model is a scattering electric field model represented by the formula (8).
S42, reconstructing an image of the object in the region to be detected by using the total electric field model and the scattering electric field model;
s421, comparing the scattering electric fields detected by any two microwave antennas in at least three microwave antennas in the two-dimensional microwave antenna array in sequence;
the visible scattering electric field of the scattering electric field detected by any two microwave antennas in the microwave antenna array is as follows:
Figure BDA0002247251470000156
in the formula (9), complex conjugation is represented, < > represents average time, and scattering electric field is visible
Figure BDA0002247251470000157
Including being located at point Ai(xi，yi，zi) And point Aj(xj，yj，zj) The phase delay and/or amplitude difference of the information collected by the microwave antenna.
In a two-dimensional microwave antenna array, any two are located at point a, as shown in fig. 5i(xi，yi，zi) At the point Aj(xj，yj，zj) The visibility of the scattered electric field from any point P (x, y, z) in the skull detected by the microwave antenna to the microwave antenna can be represented by equation (9).
S422, obtaining information capable of reflecting the amplitude and the phase of the dielectric attribute distribution in the target object according to the difference obtained by pairwise comparison;
and S423, obtaining the total visible scattering electric field of all the microwave antennas according to the amplitude and phase information.
Sequentially calculating the visible scattering electric fields of any two microwave antennas according to the following formula,
Figure BDA0002247251470000161
when the number of the microwave antennas is N, N is a natural number and N is greater than or equal to 3, and the total visible scattering electric field is the sum of the visible scattering electric fields of the N (N-1) microwave antennas, the total visible scattering electric field of all the microwave antennas is:
Figure BDA0002247251470000162
s424, performing inverse Fourier transform on the visible scattering electric fields detected by all the microwave antennas to obtain a two-dimensional reconstruction image of the object in the region to be detected, wherein the two-dimensional reconstruction image comprises the following steps:
Figure BDA0002247251470000163
in step S5, the preset electromagnetic numerical model of the skull according to the total visible scattering electric field of all the microwave antennas is:
Figure BDA0002247251470000164
in formula (13), εbDielectric constant, μ, representing the backgroundbThe magnetic permeability of the background is shown,
Figure BDA0002247251470000165
Figure BDA0002247251470000166
ε represents the dielectric constant of the brain, σ represents the conductivity of the skull, σbWhich is indicative of the electrical conductivity of the background,
Figure BDA0002247251470000167
ω(p)＝2πf(p)is the operating angular frequency, f(p)Which is indicative of the frequency at which the signal is transmitted,
Figure BDA0002247251470000168
fminfor the minimum operating frequency, P1., P, Q1., Q, P denotes the frequency number, Q denotes the view number,
Figure BDA0002247251470000169
as a result of the total electric field,
Figure BDA00022472514700001610
Figure BDA00022472514700001611
in the step S6, the specific process of obtaining the two-dimensional reconstructed image of the skull according to the electromagnetic numerical model of the skull is as follows:
calculating the volume fraction of the electromagnetic numerical model of the skull, wherein the volume fraction is as follows:
Figure BDA0002247251470000171
substituting equation (13) into equation (14) yields:
Figure BDA0002247251470000172
in the formula (15), the reaction mixture is,
Figure BDA0002247251470000173
λbwhich represents the wavelength of operation of the light,
Figure BDA0002247251470000174
representing a unit vector in a spherical coordinate system,
Figure BDA0002247251470000175
dV＝s2sinθdθdφds。
define new parameters (l, m, n):
Figure BDA0002247251470000176
dV can be obtained from the following formula:
dV＝s2dldmds/n (17)
substitution of formula (17) into formula (15) yields:
Figure BDA0002247251470000177
component of the baseline vector in a Cartesian coordinate System
Figure BDA0002247251470000178
Is composed of
Figure BDA0002247251470000179
Figure BDA00022472514700001710
Because the microwave antenna array is two-dimensional, the microwave antennas are arranged at the same height, and the volume of the electromagnetic numerical model of the skull is divided into:
Figure BDA0002247251470000181
the line integral of the electromagnetic numerical model of the skull represented by equation (13) along the radial coordinate n is:
Figure BDA0002247251470000182
the following two-dimensional integral versus visibility scattering function for the variable (l, m) is derived using equation (21):
Figure BDA0002247251470000183
the visibility scattering function is subjected to inverse Fourier transform to reconstruct a skull image, and a two-dimensional reconstructed image of the skull is obtained, wherein the two-dimensional reconstructed image specifically comprises the following steps:
Figure BDA0002247251470000184
the two-dimensional reconstruction image of the skull obtained by the formula (23) is the same as the two-dimensional reconstruction image of the object in the region to be detected obtained by the formula (12), and the object in the region to be detected is the skull organ; a two-dimensional image of a three-dimensional skull may be obtained by inverse fourier transform visibility scatter function reconstruction.
In the step S2, at least three microwave antennas in the two-dimensional microwave antenna array are used to sequentially and uninterruptedly transmit the ultra-wideband microwave signal to the skull, and at least three microwave antennas are used to sequentially receive the scattered electric field signals from the inside, surface and periphery of the skull, wherein the distance d between the two-dimensional microwave antenna array and the skull is far greater than a working wavelength λbI.e. d > lambdab。
In step S5, when the skull is nonmagnetic and conductive in the microwave irradiation environment, the scattered electric field signals from the inside, surface and periphery of the skull acquired by any one of the microwave antennas can be calculated by the electromagnetic numerical model of the skull represented by formula (13).
In the above step S4, a time series of at least one electromagnetic property of the object is formed based on the scattered electric fields received by at least two of the at least three microwave antennas, and the difference of the scattered electric fields detected by the at least two microwave antennas is calculated to calculate the visibility function, and the image of the object is reconstructed by processing the visibility function.
The spatial resolution of the reconstructed brain image is affected by the microwave operating frequency, bandwidth, scanning speed and sampling frequency. To quantitatively evaluate the imaging results, the peak signal-to-noise ratio (PSNR) can be used to evaluate the quality of brain images:
Figure BDA0002247251470000191
in equation (24), peak represents an image peak, which may be defined by a user or determined by an image range of the image itself (e.g., a uint8 image, whose image range is 0 to 255); the MSE represents the mean square error and,
Figure BDA0002247251470000192
the closer the MSE is to the value zero, the better the image quality is; y represents a vector observation of the predicted variable,
Figure BDA0002247251470000193
representing vectors of nn predictions generated from samples of nn data points on all variables.
In order to verify the effectiveness of the multi-frequency holographic microwave brain imaging method provided by the application, a numerical simulation model of the multi-frequency holographic microwave brain imaging system is established through an MATLAB platform and is used for simulating the intensity of scattering electric fields of different biological tissues in a skull when brain tumors occur.
FIG. 6(a) is a two-dimensional permittivity distribution image of a skull model; FIG. 6(b) is a two-dimensional conductivity distribution image of a skull model; FIG. 6(c) is the real part of the two-dimensional reconstructed image of the skull model under the microwave frequency band with the working frequency of 1GHz-4 GHz; FIG. 6(d) is the imaginary part of the two-dimensional reconstructed image of the lower skull model with the operating frequency of the microwave band of 1GHz-4 GHz.
FIG. 7(a) is a real part of a two-dimensional reconstructed image of a lower skull model with a working frequency of 1GHz microwave frequency band; FIG. 7(b) is a real part of a two-dimensional reconstructed image of a lower skull model with a working frequency of 2GHz microwave band; FIG. 7(c) is a real part of a two-dimensional reconstructed image of a lower skull model with a working frequency of 3GHz microwave band; FIG. 7(d) is a real part of a two-dimensional reconstructed image of a lower skull model with a working frequency of 4GHz microwave band.
Experimental results show that the reconstructed image of the skull model can clearly display different tissues of the brain, including tumor cells, under the microwave frequency band with the working frequency of 1GHz-4 GHz.
The foregoing is merely an illustrative embodiment of the present application, and any equivalent changes and modifications made by those skilled in the art without departing from the spirit and principles of the present application shall fall within the protection scope of the present application.

Claims (10)

1. A multi-frequency holographic microwave brain imaging method is characterized by comprising the following steps:
the method comprises the steps that a multi-frequency holographic microwave brain imaging system is arranged, and the multi-frequency holographic microwave brain imaging system comprises a main control module, a multi-frequency signal generation module, a switching module, a transmitting and receiving module, an image processing module and a display module, wherein the transmitting and receiving module comprises a two-dimensional microwave antenna array, and the two-dimensional microwave antenna array adopts a microwave antenna integrating receiving and transmitting;
the main control module controls the transmitting and receiving module to be switched to a transmitting state through the switching module, and controls the multi-frequency signal generating module to generate an ultra-wideband microwave signal, and the generated ultra-wideband microwave signal is transmitted to an object in an area to be detected through at least three microwave antennas which are integrated in transmitting and receiving in the transmitting and receiving module through the switching module;
the main control module controls at least three receiving and transmitting integrated microwave antennas in the transmitting and receiving module to sequentially measure scattering electric field signals of objects from a region to be detected and transmits the detected scattering electric field signals to the image processing module;
the image processing module processes the received scattered electric field signal to obtain a two-dimensional reconstruction image of the object in the region to be detected;
presetting an electromagnetic numerical model of the skull according to the total visible scattering electric field of all microwave antennas;
obtaining a two-dimensional reconstruction image of the skull according to the electromagnetic numerical model of the skull;
and judging whether the two-dimensional reconstruction image of the skull is the same as the two-dimensional reconstruction image of the object in the region to be detected, if so, determining that the object in the region to be detected is a skull organ, and transmitting the two-dimensional reconstruction image of the skull to an image display module for displaying.
2. The multi-frequency holographic microwave brain imaging method according to claim 1, wherein the specific process of transmitting ultra-wideband microwave signals is as follows:
establishing a rectangular coordinate system of an electromagnetic field area where an object is located, and determining the distance between the object and the two-dimensional microwave antenna array, the position coordinates of the microwave antennas and the number of the microwave antennas integrating receiving and transmitting;
transmitting ultra-wideband microwave signals with a preset frequency band to an object through at least three microwave antennas in a two-dimensional microwave antenna array, wherein the ultra-wideband microwave signals generate scattering electric fields in, on and around the object;
at least three microwave antennas in the two-dimensional microwave antenna array sequentially detect scattered electric field signals generated inside, on the surface and around the object;
the total electric field obtained from the incident electric field and the scattering electric field is:
Figure FDA0002247251460000011
in the formula (I), the compound is shown in the specification,
Figure FDA0002247251460000021
representing scattered electric field signals generated inside, on and around the object and detected by a single microwave antenna,
Figure FDA0002247251460000022
representing the sum of the incident electric fields emitted by the N microwave antennas,
Figure FDA0002247251460000023
is shown at Ai(xi，yi，zi) The distance vector between the microwave antenna and the object in the region to be detected, and omega represents angular frequency.
3. The multi-frequency holographic microwave brain imaging method according to claim 1 or 2, wherein the specific process of obtaining the two-dimensional reconstructed image of the object in the region to be detected is as follows:
establishing a nonlinear observation model between the electromagnetic properties such as dielectric constant and conductivity of an object in a region to be detected and a scattering electric field, wherein the nonlinear observation model comprises a total electric field model and a scattering electric field model;
and reconstructing an image of the object in the region to be detected by using the total electric field model and the scattering electric field model.
4. The multi-frequency holographic microwave brain imaging method according to claim 3, wherein the total electric field model is:
Figure FDA0002247251460000024
in the formula (I), the compound is shown in the specification,
Figure FDA0002247251460000025
which is indicative of the incident electric field,
Figure FDA0002247251460000026
representing the position from the target point to the point AT(xT,yT,zT) The distance vector of the microwave antenna of (1),
Figure FDA0002247251460000027
the divergence operator is represented by a vector of vectors,
Figure FDA0002247251460000028
the expression of the green's function is,
Figure FDA0002247251460000029
Figure FDA00022472514600000210
Figure FDA00022472514600000211
representing a position vector from a field source point to any point in the cranium;
the scattered electric field model is:
Figure FDA00022472514600000212
in the formula (I), the compound is shown in the specification,
Figure FDA00022472514600000213
which represents the scattered electric field and is,
Figure FDA00022472514600000214
representing any target point in the skull to be located at AR(xR,yR,zR) The distance vector of the microwave antenna of (1),
Figure FDA00022472514600000215
represents a position vector; epsilonbDielectric constant, μ, representing the backgroundbThe magnetic permeability of the background is shown,
Figure FDA00022472514600000216
Figure FDA00022472514600000217
ε represents the dielectric constant of the brain, σ represents the conductivity of the skull, σbConductivity, ω, representing the background(p)＝2πf(p)Is the operating angular frequency, f(p)Which is indicative of the frequency at which the signal is transmitted,
Figure FDA0002247251460000031
fminfor the minimum operating frequency, P1., P, Q1., Q, P denotes the frequency number, Q denotes the view number,
Figure FDA0002247251460000032
as a result of the total electric field,
Figure FDA0002247251460000033
5. the multi-frequency holographic microwave brain imaging method according to claim 4, wherein said scattered electric field model is further represented as:
Figure FDA0002247251460000034
in the formula (I), the compound is shown in the specification,
Figure FDA0002247251460000035
r denotes the distance between the scattering source and the object point,
Figure FDA0002247251460000036
represents a unit vector;
when a ≈ 1, b ≈ -1, the scattering electric field model is simplified as:
Figure FDA0002247251460000037
in the formula (I), the compound is shown in the specification,
Figure FDA0002247251460000038
the expression of the green's function is,
Figure FDA0002247251460000039
6. the multi-frequency holographic microwave brain imaging method according to claim 3, wherein the process of reconstructing the image of the object in the region to be detected by using the total electric field model and the scattered electric field model comprises:
sequentially comparing the scattered electric fields detected by any two microwave antennas in at least three microwave antennas in the two-dimensional microwave antenna array;
obtaining information capable of reflecting the amplitude and the phase of the dielectric property distribution in the target object according to the difference obtained by comparing every two in sequence;
obtaining the total visible scattering electric field of all the microwave antennas according to the amplitude and phase information;
and performing inverse Fourier transform on the total visible scattering electric field detected by all the microwave antennas to obtain a two-dimensional reconstruction image of the object in the region to be detected.
7. The multi-frequency holographic microwave brain imaging method according to claim 6, wherein the visible scattering electric field of the scattering electric field detected by any two microwave antennas in the array of microwave antennas is:
Figure FDA0002247251460000041
when the number of the microwave antennas is N, N is a natural number and N is greater than or equal to 3, and the total visible scattering electric field is the sum of the visible scattering electric fields of the N (N-1) microwave antennas, the total visible scattering electric field of all the microwave antennas is:
Figure FDA0002247251460000042
8. the multi-frequency holographic microwave brain imaging method according to claim 1 or 2, wherein the electromagnetic numerical model of the skull is:
Figure FDA0002247251460000043
in the formula, epsilonbDielectric constant, μ, representing the backgroundbThe magnetic permeability of the background is shown,
Figure FDA0002247251460000044
Figure FDA0002247251460000045
ε represents the dielectric constant of the brain, σ represents the conductivity of the skull, σbWhich is indicative of the electrical conductivity of the background,
Figure FDA0002247251460000046
ω(p)＝2πf(p)is the operating angular frequency, f(p)Which is indicative of the frequency at which the signal is transmitted,
Figure FDA0002247251460000047
fminfor the minimum operating frequency, P1, q 1,q, p denotes the number of frequencies, Q denotes the number of views,
Figure FDA0002247251460000048
as a result of the total electric field,
Figure FDA0002247251460000049
Figure FDA00022472514600000410
the process of obtaining the two-dimensional reconstruction image of the skull according to the electromagnetic numerical model of the skull is as follows:
calculating the volume fraction of the electromagnetic numerical model of the skull, wherein the volume fraction is as follows:
Figure FDA00022472514600000411
substituting the electromagnetic numerical model of the skull into equation (14) yields:
Figure FDA0002247251460000051
in the formula (15), the reaction mixture is,
Figure FDA0002247251460000052
λbwhich represents the wavelength of operation of the light,
Figure FDA0002247251460000053
representing a unit vector in a spherical coordinate system,
Figure FDA0002247251460000054
dV＝s2sinθdθdφds；
define new parameters (l, m, n):
Figure FDA0002247251460000055
dV can be obtained from the following formula:
dV＝s2dldmds/n (17)
substitution of formula (17) into formula (15) yields:
Figure FDA0002247251460000056
component of the baseline vector in a Cartesian coordinate System
Figure FDA0002247251460000057
Is composed of
Figure FDA0002247251460000058
Figure FDA0002247251460000059
Because the microwave antenna array is two-dimensional, the microwave antennas are arranged at the same height, and the volume of the electromagnetic numerical model of the skull is divided into:
Figure FDA00022472514600000510
the line integral of the electromagnetic numerical model of the skull along the radial coordinate n is:
Figure FDA00022472514600000511
the following two-dimensional integral versus visibility scattering function for the variable (l, m) is derived using equation (21):
Figure FDA0002247251460000061
the visibility scattering function is subjected to inverse Fourier transform to reconstruct a skull image, and a two-dimensional reconstructed image of the skull is obtained, wherein the method specifically comprises the following steps:
Figure FDA0002247251460000062
9. the multi-frequency holographic microwave brain imaging method according to claim 1 or 2, further comprising the steps of:
the peak signal-to-noise ratio (PSNR) was used to assess the quality of the brain images:
Figure FDA0002247251460000063
in the formula, peak represents an image peak; the MSE represents the mean square error and,
Figure FDA0002247251460000064
Figure FDA0002247251460000065
the closer the MSE is to the value zero, the better the image quality is; y represents a vector observation of the predicted variable,
Figure FDA0002247251460000066
representing vectors of nn predictions generated from samples of nn data points on all variables.
10. A multi-frequency holographic microwave brain imaging system is characterized by comprising a main control module, a multi-frequency signal generation module, a switching module, a transmitting and receiving module, an image processing module and a display module;
the main control module is used for controlling the multi-frequency signal generation module to transmit ultra-wideband microwave signals and controlling the transmitting and receiving module to switch between a transmitting state and a receiving state through the switching module;
the transmitting and receiving module is used for transmitting an ultra-wideband microwave signal to the area to be detected in a transmitting state and receiving a scattering electric field reflected from the area to be detected in a receiving state;
the image processing module is used for reconstructing a brain image according to the received scattered electric field data and transmitting the reconstructed brain image to the display module for display;
the transmitting and receiving module adopts a two-dimensional microwave antenna array, and the two-dimensional microwave antenna array comprises N receiving and transmitting integrated microwave antennas, wherein N is a natural number and is more than or equal to 3;
the frequency of the ultra-wideband microwave signal is 1GHz-4 GHz.
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