A tissue identification method based on multi-frequency thermoacoustic imaging combined with feature extraction

Abstract

The present invention seeks to protect a method for multi-frequency thermoacoustic imaging combined with feature extraction to achieve identification of different tissue components and contents, which belongs to the field of microwave thermoacoustic imaging. The method of the present invention is: multi-frequency microwaves are radiated to biological tissues, and the thermoacoustic signals are received by an ultrasonic transducer, and the signals are amplified and used for image reconstruction. Multi-frequency microwaves are pulsed microwaves of multiple different frequencies. During imaging, different frequencies are switched to successively excite the measured tissue, and multiple groups of thermoacoustic images under microwave excitation of different frequencies are obtained. Then, the multiple groups of thermoacoustic images are fused to obtain multi-frequency thermoacoustic images. Feature extraction is performed by selecting the area of ​​interest in the single-frequency image and identifying the distribution of different tissue components and calculating the content of different tissue components based on the differential algorithm. The present invention also provides a system for implementing the above method. The present invention is closely combined with actual clinical needs, realizes efficient identification of different tissue components and precise contents, and has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the field of microwave thermoacoustic imaging and photoacoustic imaging, comprises a system and a method for calculating the content of different tissues, and in particular relates to a tissue identification method based on multi-frequency thermoacoustic imaging combined with feature extraction. Background Art

[0002] Medical imaging plays a vital role in clinical diagnosis. Microwave thermoacoustic imaging (MTA) is a non-invasive biomedical imaging method that combines the high contrast of microwave imaging with the high resolution of ultrasound imaging. The principle of MTA can be summarized as follows: First, pulsed microwaves are used as an excitation source to irradiate biological tissue. Second, the tissue absorbs the microwave energy and expands, generating mechanical waves (or thermoacoustic signals) that radiate outward. Upon receiving the signal, the probe amplifies it. The acquisition card collects the signal. Finally, the image is reconstructed using relevant algorithms. Based on the type of electromagnetic pulse, MTA is generally divided into photoacoustic imaging and microwave-induced thermoacoustic imaging. Both can achieve high-resolution and high-contrast imaging. However, due to the high frequency of the photoacoustic excitation source in MTA, methods related to MTA typically focus on superficial tissues. On the other hand, MTA has the potential to enhance deep tissue imaging, attracting the attention of many researchers. With the continuous development of MTA technology and the expansion of its application areas, MTA has demonstrated broad prospects and unique value in clinical practice and applications.

[0003] However, conventional microwave thermoacoustic imaging relies primarily on single-frequency microwave excitation to achieve imaging. While imaging is relatively effective for tissues with strong microwave absorption, weaker tissues exhibit less pronounced imaging characteristics. This reduced microwave absorption can be attributed to two main reasons: first, the tissue's inherently low microwave absorption coefficient; and second, the tissue structure is too small, smaller than the detector's slice projection thickness. This weakens the tissue's overall microwave absorption and is affected by surrounding larger tissue, ultimately resulting in poor imaging quality.

[0004] Traditional single-frequency microwave thermoacoustic imaging has achieved a certain degree of visualization of specific biological tissue structures. However, due to its limitations, the image resolution and quantitative analysis capabilities of single-frequency excitation are significantly limited when dealing with different tissues with different microwave absorption characteristics, especially those with weak microwave absorption or mixed tissues such as fat, muscle, and blood vessels. Existing technologies have difficulty accurately distinguishing and quantifying the relative content of these different tissue components, thus limiting the technology's in-depth application in clinical diagnosis and scientific research.

[0005] In 2023, Chi Zihui et al. (Science China: Technological Sciences, 2024, 54(2):333-350) proposed a dual-frequency microwave thermoacoustic imaging method and application research. This method effectively reduces the influence of bones on imaging quality by using two short-pulse microwaves with different center frequencies to excite tissues, and combines energy calibration and weighted subtraction techniques. This technology improves the imaging capability of biological tissues with complex bone structures, and contributes to the development of traditional microwave thermoacoustic imaging technology based on single-frequency excitation. However, the main contribution of this technology is to improve the imaging quality of the bone area, and does not involve the identification of different tissue components and their contents. At present, there is a lack of relevant research, and it is impossible to analyze the data of dual-frequency microwave thermoacoustic imaging through appropriate data processing methods to achieve the identification of different tissue components and their contents. In short, although the existing technology has been improved in some aspects, it still has many shortcomings, especially in the identification and content analysis of multiple tissue components. Summary of the Invention

[0006] To overcome the limitations of existing single-frequency microwave thermoacoustic imaging in tissue composition identification and content assessment, the present invention provides a multi-frequency thermoacoustic imaging combined feature extraction method, designed to achieve efficient identification of different tissue components and precise content. This method closely aligns with actual clinical needs, using a multi-frequency excitation mechanism to assess different tissue components and content. This method verifies the accuracy of microwave thermoacoustics in assessing content in different tissues, confirms the feasibility and stability of microwave thermoacoustics in assessing tissue content, and improves the sensitivity and reliability of detecting multiple tissue components.

[0007] A multi-frequency thermoacoustic imaging system, comprising:

[0008] An image acquisition system is configured to acquire thermoacoustic signals of a sample under at least two frequency excitations, reconstruct a single-frequency thermoacoustic image using an imaging algorithm, and obtain a multi-frequency thermoacoustic image using a fusion algorithm;

[0009] The image analysis module is configured to calculate the content of different tissues in the sample through tissue thermoacoustic signals under at least two frequency excitations.

[0010] Preferably, the tissue is selected from fat, muscle or blood vessel.

[0011] Preferably, the multi-frequency thermoacoustic image is a dual-frequency thermoacoustic image, and the calculation formula of tissue content is:

[0012]

[0013] Among them, F and M refer to two different tissues in the sample, TA_F is the proportion of tissue F in the sample, and C F and C M are the contents of the two tissues, σ m1and σ m2 is the conductivity of tissue M at frequencies f1 and f2, σ f1 and σ f2 is the conductivity of tissue F at frequencies f1 and f2, E1 and E2 are the electric field strengths at frequencies f1 and f2, Γ f and Γ m are the Gruneisen parameters of tissue F and tissue M, respectively. A1 and A2 are the thermoacoustic signal intensity information at frequencies f1 and f2, respectively.

[0014] Preferably, the at least two frequencies are at least two frequencies having different microwave center frequencies but the same other parameters, wherein the other parameters include pulse width, pulse power and pulse repetition frequency.

[0015] Preferably, the imaging algorithm is selected from a back-projection algorithm, a delay and superposition algorithm, a filtered back-projection algorithm, a time reversal algorithm or compressed sensing.

[0016] Preferably, the fusion algorithm obtains a multi-frequency thermoacoustic image from single-frequency thermoacoustic data or images, specifically comprising: fusing multiple groups of single-frequency thermoacoustic images or directly fusing multiple groups of single-frequency thermoacoustic data, wherein the fusion adopts differential fusion, and the initial fusion weight factor is determined by the theoretical microwave absorption coefficient. After obtaining the multi-frequency thermoacoustic image, the smoothness and focus intensity of the tissue area to be highlighted are judged. If the tissue is smooth and focused, the multi-frequency thermoacoustic image obtained after fusion is directly output along with several groups of single-frequency images. If the tissue is not smooth and not focused, the weight factor is adjusted until the tissue is smooth and focused, and then one group of multi-frequency thermoacoustic images is output along with multiple groups of single-frequency thermoacoustic images.

[0017] Preferably, the image acquisition system includes: a multi-frequency microwave source, a coaxial line, an antenna, an ultrasonic transducer, an amplifier, a multi-channel data acquisition card and a computer.

[0018] Among them, the multi-frequency microwave source is used to generate pulsed microwaves of different frequencies, which are transmitted to the antenna via a coaxial cable, and the microwaves are radiated to the sample under test. The sample under test absorbs the microwave energy to generate ultrasonic waves, i.e., thermoacoustic signals. The thermoacoustic signals under the excitation of microwaves of different frequencies are detected by ultrasonic transducers respectively, and the detected signals are amplified by amplifiers. The amplified signals are collected by a multi-channel data acquisition card to obtain thermoacoustic data under excitation of different frequencies. The entire process is controlled by a computer. The single-frequency thermoacoustic image is reconstructed using an imaging algorithm based on the relationship between time domain information and spatial position, and then the single-frequency thermoacoustic data or image is fused using a fusion algorithm to obtain a multi-frequency thermoacoustic image. The frequency in the multi-frequency microwave thermoacoustic imaging refers to the microwave center frequency. The single-frequency thermoacoustic image is used to analyze biological tissues with strong absorption of single-frequency microwave energy. The area of ​​interest in the single-frequency image is selected to calculate the content of different tissue components. The multi-frequency thermoacoustic image is used to highlight tissues with weak absorption of single-frequency microwave energy and large changes in microwave energy absorption with frequency, and to detect corresponding lesions.

[0019] Preferably, the pulse microwaves of different frequencies generated by the multi-frequency microwave source are generated by the same variable-frequency microwave source, or by multiple single-frequency microwave sources; the pulse microwaves of different frequencies need to maintain consistent pulse width and peak power; or the multi-frequency microwave source has a power monitoring and real-time feedback function;

[0020] And / or, the antenna is a frequency-invariant antenna covering multiple frequency ranges or targeting multiple required radiation frequency points; or, the antenna is a wideband antenna covering multiple frequency ranges; the antenna must ensure that the field distribution of multiple frequency points used for excitation imaging remains consistent; the sample under test is excited by the antenna with a consistent field distribution;

[0021] And / or, the ultrasonic transducer, amplifier, and multi-channel data acquisition card constitute a thermoacoustic signal acquisition module, and when the multi-frequency microwave excitation is switched, the ultrasonic transducer, amplifier, multi-channel data acquisition card, antenna, and biological tissue being measured remain stationary;

[0022] And / or, the multi-frequency microwave source and the antenna constitute a microwave excitation module, wherein the multi-frequency microwave source is composed of multiple independent microwave sources that can output the same pulse width, the same repetition frequency, the same power and different frequencies, or the multi-frequency microwave source is a microwave source with a variable frequency but ensuring that other pulse microwave parameters remain unchanged, and the other pulse microwave parameters include pulse width, pulse power and pulse repetition frequency.

[0023] The present invention also provides a multi-frequency microwave thermoacoustic imaging method based on the above system, comprising the following steps:

[0024] Step 1: When the multi-frequency is dual-frequency, for biological tissues with weak microwave absorption under single-frequency microwave excitation, a frequency range is selected in which the theoretical microwave absorption coefficient changes greatly while the microwave absorption coefficient of surrounding tissue changes less, and two boundary frequency points of this frequency range are selected as two excitation frequency points for dual-frequency microwave thermoacoustic imaging;

[0025] Step 2: Turn on the device and set parameters for initialization;

[0026] Step 3: Place the sample to be tested into the coupling liquid;

[0027] Step 4: Use a computer to trigger the multi-frequency microwave source to generate a pulsed microwave with a center frequency, and trigger the data acquisition card to start working: the pulsed microwave with a center frequency is radiated to the biological tissue to be measured through the antenna. The tissue generates a thermoacoustic signal based on the thermoacoustic effect. The thermoacoustic signal is detected by the ultrasound detector, converted into an electrical signal by the ultrasound detector, and collected by the data acquisition card. The signal is then stored in the computer and processed for single-frequency thermoacoustic image reconstruction;

[0028] Step 5: Keep all experimental conditions unchanged;

[0029] Step 6: Use a computer to trigger the multi-frequency microwave source to generate pulsed microwaves of another center frequency, and trigger the data acquisition card to start working: the pulsed microwave pulses of another center frequency are radiated to the biological tissue to be measured through the antenna. The tissue generates ultrasound, i.e., thermoacoustic signals, based on the thermoacoustic effect. The thermoacoustic signals are detected by the ultrasound detector, converted into electrical signals by the ultrasound detector, and collected by the data acquisition card. The signals are then stored in the computer and processed for single-frequency thermoacoustic image reconstruction. Thus, the thermoacoustic imaging of this single-frequency excitation is completed. According to actual needs, microwaves of two or more frequencies can be generated to excite the thermoacoustic signals.

[0030] Step 7: Use the fusion algorithm to post-process the completed single-frequency thermoacoustic image, or directly process the single-frequency thermoacoustic data stored in the computer;

[0031] Step 8: Export images, where single-frequency thermoacoustic images are used to analyze biological tissues that have strong single-frequency microwave energy absorption and detect corresponding lesions, while multi-frequency thermoacoustic images are used to highlight tissues that have weak single-frequency microwave energy absorption but large changes in microwave energy absorption with frequency changes and detect corresponding lesions.

[0032] The present invention also provides a method for calculating tissue content based on the above system, comprising the following steps:

[0033] Step A: Collect the electric field intensity of a reference object that is insensitive to changes in the microwave center frequency at each excitation frequency as the normalization base of the multi-frequency microwave thermoacoustic signal;

[0034] Step B: Acquiring multiple single-frequency thermoacoustic images;

[0035] Step C: Calculate the normalized electric field intensity of the single-frequency thermoacoustic image;

[0036] Step D: Extract tissue features from the single-frequency thermoacoustic image to obtain the area where the tissue of interest is located.

[0037] Step E: Calculate the content of different tissues based on the selected area where the tissue of interest is located.

[0038] The technical solution of the present invention is as follows: The method for evaluating the content of different tissues using multi-frequency microwave thermoacoustic imaging technology comprises the following steps:

[0039] 1. System Construction: Design and construct a multi-frequency microwave thermoacoustic imaging system, which includes a multi-frequency microwave source, antenna, ultrasonic transducer, amplifier and multiplexer, multi-channel data acquisition card, and computer control system.

[0040] 2. Microwave generation and transmission: A multi-frequency microwave source generates a series of pulsed microwave signals with different center frequencies, which are transmitted sequentially to the antenna via a coaxial cable.

[0041] 3. Microwave radiation and thermoacoustic signal generation: The antenna radiates the received microwaves of different frequencies into the biological tissue being tested. The tissue absorbs the microwave energy and converts it into an ultrasonic signal, which is a thermoacoustic signal.

[0042] 4. Thermoacoustic signal detection and amplification: The array ultrasonic detector sequentially captures the thermoacoustic signals generated under different frequency excitations and sends them to the array amplification circuit for signal enhancement processing.

[0043] 5. Signal acquisition and recording: The amplified thermoacoustic signals are collected and stored in real time by a multi-channel data acquisition card, forming multiple sets of thermoacoustic data sets at different frequencies.

[0044] 6. Image reconstruction: Based on the relationship between the time domain information and spatial position of the thermoacoustic signal, imaging algorithms such as back-projection algorithm, time reversal algorithm or compressed sensing are used to reconstruct the image of each set of single-frequency thermoacoustic data to generate multiple single-frequency thermoacoustic images.

[0045] 7. Image fusion and analysis: The obtained single-frequency thermoacoustic images are integrated using a fusion algorithm to obtain high-resolution and high-contrast multi-frequency thermoacoustic images.

[0046] 8. Calculation of tissue content:

[0047] ① Calculation of normalized electric field intensity: Based on the characteristics of the thermoacoustic signal at each frequency, the normalized electric field intensity at different frequencies is first calculated on the single-frequency image.

[0048] ② Feature extraction: Extract tissue features from single-frequency thermoacoustic images, including:

[0049] Edge detection: used to identify tissue boundaries and ensure clear edge contours.

[0050] Texture analysis: used to assess the homogeneity and structural characteristics within tissues.

[0051] Morphological processing: used to remove noise and enhance important details in images.

[0052] Statistical analysis: used to quantify characteristic parameters, such as mean gray value, standard deviation, etc.

[0053] ③ Tissue content calculation: Select the area where the tissue of interest is located in the single-frequency thermoacoustic image, extract the characteristic parameters within the area, and calculate the fat content. The fat tissue content is calculated based on the characteristic parameters and the result is compared with the actual fat content for verification.

[0054] 9. Identification of lesions and tissue characteristics: Analyze relevant lesions based on the quantified content of different tissue components; at the same time, by comparing multi-frequency fusion images, highlight those tissue areas and corresponding pathological conditions that cause significant changes in microwave energy absorption with frequency changes.

[0055] The key advantage and innovative effect of this invention lies in the use of multi-frequency microwave thermoacoustic imaging technology. Based on the differential absorption patterns of microwave energy at different frequencies by biological tissues, we carefully select multiple frequency ranges with significant absorption differences as excitation sources. Specifically, for tissue types that exhibit weak microwave absorption under single-frequency excitation but exhibit significant variations in absorption coefficient within specific frequency ranges, we design the system to use the boundary frequencies of these frequency ranges as the core parameters for multi-frequency excitation.

[0056] During experimental operation, the system sequentially stimulates the biological tissue under test using a series of pulsed microwave signals of varying frequencies. The thermoacoustic signal generated by the tissue at each frequency is detected by an ultrasonic transducer, amplified by an amplifier, and transmitted to a data acquisition card for real-time imaging on a computer. For each selected frequency, a corresponding single-frequency thermoacoustic image is reconstructed and fused into a multi-frequency thermoacoustic image.

[0057] Among them, each set of single-frequency images is used to deeply analyze biological tissues that have strong microwave energy absorption at this frequency and their related lesions; by integrating all single-frequency thermoacoustic images through a multi-frequency fusion algorithm, a high-contrast multi-frequency thermoacoustic image can be obtained. This image can significantly highlight those tissue structures and their potential lesions that have weak microwave absorption at a single frequency but have large fluctuations in absorption characteristics with frequency changes.

[0058] Furthermore, the present invention also enables the calculation and quantitative assessment of the content of biological tissue components. By analyzing multiple single-frequency thermoacoustic imaging data, the content of different tissue components (such as fat, muscle, and blood vessels) within a sample can be accurately determined, enhancing the ability to detect the internal structure and functional state of complex biological tissues.

[0059] In summary, compared with traditional single-frequency thermoacoustic imaging technology, the multi-frequency microwave thermoacoustic imaging system proposed in this invention shows excellent performance in revealing tissue structural characteristics with weak microwave absorption but strong frequency dependence and in quantifying tissue components, greatly improving imaging quality and lesion recognition rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 It is a schematic diagram of a dual-frequency microwave thermoacoustic imaging system according to a preferred embodiment of the present invention.

[0061] Figure 2 1 is a schematic diagram of the overall process of the tissue identification method in an embodiment of the present invention.

[0062] Figure 3 This is the step for executing multi-frequency microwave thermoacoustic imaging in an embodiment of the present invention.

[0063] Figure 4 4 is a flow chart for calculating the content of different components in tissues provided in an embodiment of the present invention.

[0064] Figure 5 This is a real picture of pork belly.

[0065] Figure 6 This is an example diagram of tissue content calculation. DETAILED DESCRIPTION

[0066] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0067] Example

[0068] This invention aims to address the limitations of existing microwave thermoacoustic imaging technology by proposing a multi-frequency microwave thermoacoustic imaging method to calculate the content of different tissues. The core of this method and system lies in its ability to generate high-resolution images with multiple frequency information. Its unique feature is its ability to effectively highlight tissue structures that have weak energy absorption at a single frequency but exhibit significant absorption characteristics with frequency variations, thereby enabling the calculation of different tissue content.

[0069] As a preferred embodiment, this embodiment provides a multi-frequency microwave thermoacoustic imaging system, which includes:

[0070] An image acquisition system is configured to acquire thermoacoustic signals of a sample under at least two frequency excitations, reconstruct a single-frequency thermoacoustic image using an imaging algorithm, and obtain a multi-frequency thermoacoustic image using a fusion algorithm;

[0071] The image analysis module is configured to calculate the content of different tissues in the sample through tissue thermoacoustic signals under at least two frequency excitations.

[0072] Image acquisition system such as Figure 1 As shown, it includes a microwave excitation module, an ultrasonic coupling module, a data acquisition module, and a real-time imaging module. Specifically, it comprises a multi-frequency microwave source, an antenna, an ultrasonic detector, an amplifier and a multiplexer, a data acquisition card, and a computer. The multi-frequency microwave source is used to generate pulsed microwaves of different frequencies, which are radiated to the biological tissue being measured through the antenna. After absorbing the microwave energy, the biological tissue contracts and expands to generate thermoacoustic signals. The thermoacoustic signals under the excitation of microwaves of different frequencies are successively detected by the ultrasonic transducer, amplified by the amplifier, and transmitted to the multi-channel acquisition card, thereby obtaining multiple sets of thermoacoustic data under different frequency excitations, which can be imaged in real time on a computer. The entire process is controlled by a computer. After data acquisition, single-frequency thermoacoustic images are reconstructed using algorithms such as back-projection algorithms, time reversal algorithms, and compressed sensing algorithms based on the relationship between time domain information and spatial position. Then, a fusion algorithm is used to obtain multi-frequency thermoacoustic images from the single-frequency thermoacoustic data or images. The frequency in the multi-frequency microwave induction refers to the microwave center frequency. The single-frequency thermoacoustic image is used to analyze biological tissues that have strong single-frequency microwave energy absorption and detect corresponding lesions and calculate different tissue contents, while the multi-frequency fusion image is used to highlight tissues that have weak single-frequency microwave energy absorption but large changes in microwave energy absorption with frequency changes and detect corresponding lesions.

[0073] The present invention is based on the bioelectromagnetic law that the microwave absorption coefficient between biological tissues changes with frequency. For tissues with weak microwave absorption under single-frequency microwave excitation, a frequency range in which the microwave absorption coefficient changes greatly while the microwave absorption coefficient of surrounding tissues changes less is selected, and then the boundary frequency point of this frequency range is selected as the excitation frequency point for multi-frequency microwave thermoacoustic imaging.

[0074] Multi-frequency microwaves are pulsed microwaves of multiple different frequencies. During the experiment, they are used to stimulate the biological tissue under test through antennas in succession. The generated thermoacoustic signals are detected by ultrasonic detectors in succession. The detected thermoacoustic signals are amplified by amplifiers and collected by data acquisition cards in succession. Image reconstruction of the thermoacoustic signals detected under the excitation of microwaves of different frequencies can obtain multiple groups of thermoacoustic images under the excitation of microwaves of different frequencies. The multiple groups of thermoacoustic images are fused to obtain a group of multi-frequency thermoacoustic images.

[0075] Multi-frequency microwaves can be generated by a single variable-frequency microwave source or by multiple single-frequency microwave sources. The pulse width and peak power of the generated microwaves must be consistent. The antenna can be a non-frequency-variable antenna covering a dual-frequency range or multiple desired radiation frequencies, or a broadband antenna covering multiple frequencies. In both cases, the field distribution at the multiple frequencies used for excitation imaging must remain consistent. When switching between multiple-frequency microwave excitations, both the antenna and the tissue being examined must remain stationary, ensuring that all experimental conditions, except for the excitation microwave frequency, remain unchanged.

[0076] The entire multi-frequency microwave thermoacoustic imaging system consists of a microwave excitation module, an ultrasonic coupling module, a data acquisition module, and a real-time imaging module. The microwave excitation module includes a multi-frequency microwave excitation source and an antenna. The ultrasonic coupling module includes an ultrasonic transducer, a coupling medium, and biological tissue. The coupling medium acts as both a microwave transmission medium from the radiating antenna to the tissue and a ultrasonic transmission medium from the tissue to the ultrasonic transducer, affecting the transmission efficiency of both microwave energy and ultrasonic waves. The data acquisition module includes a data acquisition card, an amplifier, and a multiplexer. The real-time imaging module includes a computer, which can be used to view thermoacoustic images and store thermoacoustic signal data in real time.

[0077] Single-frequency thermoacoustic image reconstruction can utilize traditional microwave thermoacoustic imaging algorithms, such as backprojection, time reversal, and compressed sensing, ensuring that the same algorithm is used for the corresponding single-frequency thermoacoustic data. Multi-frequency fusion algorithms are based on single-frequency thermoacoustic imaging and primarily involve differential fusion, where the weighting coefficients are determined based on a combination of the theoretical microwave absorption coefficient and the smoothness or focusing of the tissue being extracted and reconstructed for highlighting. Multi-frequency fusion algorithms can be performed on reconstructed single-frequency thermoacoustic images or directly on the single-frequency thermoacoustic data.

[0078] The multi-frequency microwave source can be composed of independent microwave sources that can output the same pulse and the same power but different frequencies, or it can be a microwave source with variable frequency but ensuring that other pulse microwave parameters remain unchanged. Among them, the variable frequency microwave source is preferred.

[0079] The antenna can be a wideband antenna covering multiple frequency ranges, but the frequency point should not be at the boundary frequency point of the wideband antenna as much as possible. It can also be a non-frequency-variable antenna targeting multiple required radiation frequency points or covering multiple frequency ranges, among which the non-frequency-variable antenna is preferred.

[0080] Pulsed microwaves, except for frequency, remain unchanged in other parameters. They excite the tissue being measured through coaxial lines of the same length and with consistent loss parameters and antennas with consistent field distribution. The information reflected by multi-frequency images relies more on changes in the tissue's intrinsic microwave absorption coefficient and reduces the impact of changes in external field distribution.

[0081] The ultrasonic coupling module detects the thermoacoustic signals under different frequency excitations, and the data acquisition module uses multiplexing technology to ensure real-time imaging, thereby avoiding jitter interference during in vivo imaging.

[0082] This system can perform in vivo, ex vivo, and living body imaging. When performing multi-frequency imaging to collect thermoacoustic signals under different microwave frequency excitations, other experimental conditions except the frequency should remain unchanged as much as possible.

[0083] The multi-frequency fusion algorithm can be performed on the reconstructed single-frequency thermoacoustic image or directly on multiple sets of single-frequency thermoacoustic data. The resulting thermoacoustic image includes multiple sets of single-frequency thermoacoustic images and one set of multi-frequency thermoacoustic image. Multi-frequency fusion is preferably performed on the reconstructed multiple sets of single-frequency thermoacoustic images.

[0084] The overall process of the present invention is as follows Figure 2 As shown, specifically:

[0085] Multi-frequency microwave thermoacoustic imaging methods such as Figure 3 As shown, it includes the following steps:

[0086] Step 1: For biological tissues that have weak microwave absorption under single-frequency excitation and are expected to be highlighted by multi-frequency thermoacoustic imaging, first determine the frequency range in which their theoretical microwave absorption coefficient changes significantly while the microwave absorption coefficient of surrounding tissues is relatively stable, and select a representative boundary frequency point within this range as one of the multiple excitation frequency points for multi-frequency microwave thermoacoustic imaging.

[0087] Step 2: Start and initialize all relevant equipment and set system parameters to meet experimental requirements;

[0088] Step 3: firmly fix the biological tissue to be tested in an appropriate position;

[0089] Step 4: Use a computer program to control the multi-frequency microwave source to generate a pulsed microwave pulse at the first center frequency, and simultaneously activate the data acquisition card. The microwave pulse of this specific frequency is precisely radiated into the biological tissue to be measured via the antenna. The tissue generates an ultrasonic signal (i.e., a thermoacoustic signal) based on the thermoacoustic effect. This thermoacoustic signal is captured by the ultrasound detector and converted into an electrical signal. The signal is then collected by the data acquisition card and stored in the computer for corresponding single-frequency thermoacoustic image reconstruction processing, thus completing the thermoacoustic imaging construction at the first frequency.

[0090] Step 5: Keep the experimental environment and settings unchanged to ensure the consistency of all parameters;

[0091] Step 6: The computer program instructs the multi-frequency microwave source to generate pulsed microwaves at another center frequency, and simultaneously activates the data acquisition card. A microwave pulse of another specific frequency is also transmitted through the antenna to the biological tissue to be tested. The ultrasonic signal generated by the thermoacoustic effect is detected by the ultrasonic detector and converted into an electrical signal. This data is again captured by the acquisition card and stored in the computer, where a single-frequency thermoacoustic image reconstruction operation is performed on the thermoacoustic signal at this frequency.

[0092] Step 7: Post-process and fuse the single-frequency thermoacoustic images of the biological tissue to be measured using a multi-frequency image fusion algorithm, or directly perform fusion operation processing on the single-frequency thermoacoustic data at different frequencies stored in the computer;

[0093] Step 8: The final image output is a single-frequency thermoacoustic image of each frequency, which is used to analyze and identify biological tissues with strong microwave energy absorption at a single frequency and their pathological characteristics; while the multi-frequency fusion image focuses on displaying tissue structures with weak microwave absorption under single-frequency excitation but with large differences in absorption characteristics with frequency changes and their corresponding pathological information, thereby achieving all-round and high-precision visualization of biological tissue components and pathological detection.

[0094] like Figure 3 As shown in the figure, this method of calculating the content of different tissues by multi-frequency microwave thermoacoustic imaging is as an example. Assume that the frequency is dual-frequency, the two tissues to be measured (i.e., tissue F and tissue M described in the Summary of the Invention) are fat (equivalent to tissue F) and muscle (equivalent to tissue M), and the thermoacoustic pressure generated by the heat source H(r, t) follows the following equation:

[0095]

[0096] in represents the sound pressure distribution, c represents the speed of light, β represents the volume expansion coefficient, c p is the specific heat capacity.

[0097] The equation for the thermoacoustic pressure leads to the forward problem:

[0098]

[0099] where r-r' is the distance between the source point and the field point, and t represents time.

[0100] The heat source is modeled as the product of two factors: the spatial distribution of energy absorption A(r), which is characteristic of the object being imaged, and the temporal illumination function I(t). Because the ultrasound transducer array has a finite bandwidth, the recorded thermoacoustic measurements is the convolution of the induced pressure and the impulse response h(t) of the ultrasound transducer array.

[0101]

[0102] When the object of interest is heated with a pulse of microwave radiation, the power deposition per unit volume is expressed as

[0103]

[0104] Where ω represents the microwave angular frequency, ε0 ​​and ε r ″ represent the dielectric constant and relative permittivity in vacuum, respectively. Represents the microwave electric field.

[0105] The electrical conductivity of the tissue is σ=ωε0ε″ r , so Therefore, the thermoacoustic signal intensity TAS obtained from the tissue is the product of the deposited energy of the tissue and the Grüneisen parameter Γ.

[0106]

[0107] Based on the thermoacoustic signals collected by dual-frequency microwave excitation with strong absorption coefficients for liver fat (fat) and non-fat tissue (set as muscle), the relative concentrations of fat and muscle can be obtained:

[0108]

[0109] In the above formula, and C Muscle Represents fat and muscle content, σ Fat (f) and σ Muscle (f) is the microwave absorption coefficient of Fat and Muscle, and TAS is the measured thermoacoustic signal intensity. Based on the data obtained from dielectric property measurement, if multi-frequency excitation is used for imaging, according to the formula:

[0110]

[0111] f n Represents the nth frequency. If dual-frequency excitation is used, the formula for calculating the proportion of fat tissue in the sample is:

[0112]

[0113] When a microwave source with center frequencies of f1 and f2 (e.g., f1 = 1 GHz, f2 = 3 GHz) is selected to excite tissue, if only the effects of fat and muscle are considered, the absorption intensity at the two frequencies can be expressed as

[0114]

[0115] Where CFAT , C MUSCLE Represent the content of Fat and Muscle respectively, σ is the conductivity of the tissue, is the intensity of the electric field, and Γ is the Gruneisen parameter of the tissue.

[0116] Assuming that the intensity of the thermoacoustic signal is proportional to the absorption coefficient of the tissue, the simplified calculation can be obtained:

[0117]

[0118] Where σ m1 and σ m2 is the electrical conductivity of muscle tissue at frequencies f1 and f2, σ f1 and σ f2 is the conductivity of fat at frequencies f1 and f2, E1 and E2 are the electric field strengths at frequencies f1 and f2, Γ f and G m are the Gruneisen parameters of fat and muscle, respectively. A1 and A2 are the thermoacoustic signal intensity information at frequencies f1 and f2, respectively.

[0119] like Figure 4 As shown, the method for calculating the content of different tissues by multi-frequency microwave thermoacoustic imaging includes the following steps:

[0120] Step A: When collecting thermoacoustic signals, a saline tube reference that is insensitive to changes in the microwave center frequency is placed next to the tissue being tested. This serves as a standardization tool to measure and calculate the electric field intensity at each excitation frequency.

[0121] Step B: Using an imaging algorithm to reconstruct the collected thermoacoustic signals to obtain multiple single-frequency thermoacoustic images, which correspond to tissue response characteristics at different microwave frequencies.

[0122] Step C: Based on the obtained single-frequency thermoacoustic image, select the location of the brine pipe in the image and calculate the normalized electric field intensity, assuming that the electric field at each frequency is uniformly distributed.

[0123] Step D: In each single-frequency thermoacoustic image, the region of interest is selected and feature extraction is performed. Based on the extracted feature parameters, the fat tissue content in the target region is calculated and compared with the actual fat content for verification.

[0124] Step E: Integrate the quantitative data on the content of different tissue components obtained from the above quantitative analysis, and combine it with the rich information provided by multi-frequency fusion images to deeply analyze the tissue structure and pathological conditions, thereby providing a more comprehensive and accurate imaging basis for clinical diagnosis, treatment decision-making, and scientific research.

[0125] like Figure 6 As shown, Figure 5 Dual-frequency microwave thermoacoustic imaging and content calculation of pork belly. Figure 6 (a)(c)(e)(g) are the thermoacoustic images and calculation process of fat and muscle at 1 GHz microwave frequency. Figure 6 (b)(d)(f)(h) show the thermoacoustic images and the calculation process for fat and muscle at a 3 GHz microwave frequency. The program calculated the fat content to be 55.676% and the muscle content to be 44.324%. In reality, the fat content is (1÷1.8)×100%=55.56%, and the muscle content is (0.8÷1.8)×100%=44.44%. The program's calculation results are consistent with the actual calculation results, which is in line with expectations. This phenomenon strongly demonstrates the feasibility of multi-frequency microwave thermoacoustic imaging technology in quantifying the content of different biological tissues. Because this technology can capture and analyze the differences in tissue absorption of microwave energy at multiple frequencies, the acquired thermoacoustic image data can be processed by a carefully designed algorithm to accurately simulate and measure the absorption characteristics of different tissue components within the target area. When the program's calculation results maintain a high degree of consistency with the actual experimental values, it means that the multi-frequency microwave thermoacoustic imaging method we use has good stability and accuracy, which is sufficient for application in clinical and scientific research fields to conduct quantitative analysis and lesion detection of different tissue types such as fat, muscle, and blood vessels, thereby greatly promoting the advancement and development of medical imaging technology and biological tissue research.

[0126] The systems, devices, modules, or units described in the above embodiments may be implemented by computer chips or entities, or by products having certain functions. A typical implementation device is a computer. Specifically, the computer may be, for example, a personal computer, a laptop computer, a cellular phone, a camera phone, a smartphone, a personal digital assistant, a media player, a navigation device, an email device, a game console, a tablet computer, a wearable device, or a combination of any of these devices.

[0127] It should also be noted that the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, commodity, or apparatus that includes a series of elements includes not only those elements but also other elements not explicitly listed, or includes elements inherent to such process, method, commodity, or apparatus. In the absence of further limitations, an element defined by the phrase "comprises a ..." does not exclude the presence of other identical elements in the process, method, commodity, or apparatus that includes the element.

[0128] The above embodiments should be understood as merely illustrating the present invention and not as limiting the scope of protection of the present invention. After reading the contents of the present invention, technicians may make various changes or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A multi-frequency thermoacoustic imaging system, characterized in that: include: An image acquisition system is configured to acquire thermoacoustic signals of a sample under at least two frequency excitations, reconstruct a single-frequency thermoacoustic image using an imaging algorithm, and obtain a multi-frequency thermoacoustic image using a fusion algorithm; An image analysis module is configured to calculate the content of different tissues in the sample through tissue thermoacoustic signals under at least two frequency excitations; The multi-frequency thermoacoustic image is a dual-frequency thermoacoustic image, and the calculation formula of tissue content is: in, F and M Refers to two different tissues in the sample, TA_F For organizations F The proportion in the sample, and are the contents of the two tissues, and For organizations M In frequency and The conductivity under and For organizations F In frequency and The conductivity under and For the frequency and The electric field strength under and Organizations F and organizations M The Gruneisen parameter, and At frequencies and Thermoacoustic signal intensity information under The fusion algorithm obtains a multi-frequency thermoacoustic image from single-frequency thermoacoustic data or images, specifically comprising: fusing multiple sets of single-frequency thermoacoustic images or directly fusing multiple sets of single-frequency thermoacoustic data, wherein the fusion adopts differential fusion, and the initial fusion weight factor is determined by the theoretical microwave absorption coefficient. After obtaining the multi-frequency thermoacoustic image, the smoothness and focus strength of the tissue area to be highlighted are judged. If the tissue is smooth and focused, the multi-frequency thermoacoustic image obtained after fusion is directly output along with several sets of single-frequency images. If the tissue is not smooth and focused, the weight factor is adjusted until the tissue is smooth and focused, and then one set of multi-frequency thermoacoustic images is output along with multiple sets of single-frequency thermoacoustic images. The image acquisition system includes: a multi-frequency microwave source, a coaxial line, an antenna, an ultrasonic transducer, an amplifier, a multi-channel data acquisition card and a computer. Among them, the multi-frequency microwave source is used to generate pulsed microwaves of different frequencies, which are transmitted to the antenna via a coaxial cable, and the microwaves are radiated to the sample under test. The sample under test absorbs the microwave energy to generate ultrasonic waves, i.e., thermoacoustic signals. The thermoacoustic signals under the excitation of microwaves of different frequencies are detected by ultrasonic transducers respectively, and the detected signals are amplified by amplifiers. The amplified signals are collected by a multi-channel data acquisition card to obtain thermoacoustic data under excitation of different frequencies. The entire process is controlled by a computer. The single-frequency thermoacoustic image is reconstructed using an imaging algorithm based on the relationship between time domain information and spatial position, and then the single-frequency thermoacoustic data or image is fused using a fusion algorithm to obtain a multi-frequency thermoacoustic image. The frequency in the multi-frequency microwave thermoacoustic imaging refers to the microwave center frequency. The single-frequency thermoacoustic image is used to analyze biological tissues with strong absorption of single-frequency microwave energy. The area of ​​interest in the single-frequency image is selected to calculate the content of different tissue components. The multi-frequency thermoacoustic image is used to highlight tissues with weak absorption of single-frequency microwave energy and large changes in microwave energy absorption with frequency, and to detect corresponding lesions.

2. The multi-frequency thermoacoustic imaging system according to claim 1, characterized in that: The tissue is selected from fat, muscle or blood vessel.

3. The multi-frequency thermoacoustic imaging system according to claim 1, characterized in that: The at least two frequencies are at least two frequencies having different microwave center frequencies but the same other parameters, wherein the other parameters include pulse width, pulse power and pulse repetition frequency.

4. The multi-frequency thermoacoustic imaging system according to claim 1, characterized in that: The imaging algorithm is selected from a back-projection algorithm, a delay and superposition algorithm, a filtered back-projection algorithm, a time reversal algorithm or compressed sensing.

5. The multi-frequency thermoacoustic imaging system according to claim 1, characterized in that: The pulse microwaves of different frequencies generated by the multi-frequency microwave source are generated by the same variable-frequency microwave source, or by multiple single-frequency microwave sources; the pulse microwaves of different frequencies need to maintain the same pulse width and peak power; or the multi-frequency microwave source has a power monitoring and real-time feedback function; And / or, the antenna is a frequency-invariant antenna covering multiple frequency ranges or targeting multiple required radiation frequency points; or, the antenna is a wideband antenna covering multiple frequency ranges; the antenna must ensure that the field distribution of multiple frequency points used for excitation imaging remains consistent; the sample under test is excited by the antenna with a consistent field distribution; And / or, the ultrasonic transducer, amplifier, and multi-channel data acquisition card constitute a thermoacoustic signal acquisition module, and when the multi-frequency microwave excitation is switched, the ultrasonic transducer, amplifier, multi-channel data acquisition card, antenna, and biological tissue being measured remain stationary; And / or, the multi-frequency microwave source and the antenna constitute a microwave excitation module, wherein the multi-frequency microwave source is composed of multiple independent microwave sources that can output the same pulse width, the same repetition frequency, the same power and different frequencies, or the multi-frequency microwave source is a microwave source with a variable frequency but ensuring that other pulse microwave parameters remain unchanged, and the other pulse microwave parameters include pulse width, pulse power and pulse repetition frequency.

6. A multi-frequency microwave thermoacoustic imaging method based on the system according to any one of claims 1 to 5, characterized in that: The following steps are involved: Step 1: When the multi-frequency is dual-frequency, for biological tissues with weak microwave absorption under single-frequency microwave excitation, a frequency range is selected in which the theoretical microwave absorption coefficient changes greatly while the microwave absorption coefficient of surrounding tissue changes less, and two boundary frequency points of this frequency range are selected as two excitation frequency points for dual-frequency microwave thermoacoustic imaging; Step 2: Turn on the device and set parameters for initialization; Step 3: Place the sample to be tested into the coupling liquid; Step 4: Use a computer to trigger the multi-frequency microwave source to generate a pulsed microwave with a center frequency, and trigger the data acquisition card to start working: the pulsed microwave with a center frequency is radiated to the biological tissue to be measured through the antenna. The tissue generates a thermoacoustic signal based on the thermoacoustic effect. The thermoacoustic signal is detected by the ultrasound detector, converted into an electrical signal by the ultrasound detector, and collected by the data acquisition card. The signal is then stored in the computer and processed for single-frequency thermoacoustic image reconstruction; Step 5: Keep all experimental conditions unchanged; Step 6: Use a computer to trigger the multi-frequency microwave source to generate pulsed microwaves of another center frequency, and trigger the data acquisition card to start working: the pulsed microwave pulses of another center frequency are radiated to the biological tissue to be measured through the antenna. The tissue generates ultrasound, i.e., thermoacoustic signals, based on the thermoacoustic effect. The thermoacoustic signals are detected by the ultrasound detector, converted into electrical signals by the ultrasound detector, and collected by the data acquisition card. The signals are then stored in the computer and processed for single-frequency thermoacoustic image reconstruction. Thus, the thermoacoustic imaging of this single-frequency excitation is completed. According to actual needs, microwaves of two or more frequencies can be generated to excite the thermoacoustic signals. Step 7: Use the fusion algorithm to post-process the completed single-frequency thermoacoustic image, or directly process the single-frequency thermoacoustic data stored in the computer; Step 8: Export images, where single-frequency thermoacoustic images are used to analyze biological tissues that have strong single-frequency microwave energy absorption and detect corresponding lesions, while multi-frequency thermoacoustic images are used to highlight tissues that have weak single-frequency microwave energy absorption but large changes in microwave energy absorption with frequency changes and detect corresponding lesions.

7. A method for calculating tissue content based on the system according to any one of claims 1 to 5, characterized in that: The following steps are involved: Step A: Collect the electric field intensity of a reference object that is insensitive to changes in the microwave center frequency at each excitation frequency as the normalization base of the multi-frequency microwave thermoacoustic signal; Step B: Acquiring multiple single-frequency thermoacoustic images; Step C: Calculate the normalized electric field intensity of the single-frequency thermoacoustic image; Step D: extracting tissue features from the single-frequency thermoacoustic image to obtain the region where the tissue of interest is located; Step E: Calculate the content of different tissues based on the selected area where the tissue of interest is located.

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