Method and system for determining in-vitro tissue heat dose threshold value based on thermochromic film

Abstract

The invention discloses an in-vitro tissue thermal dose threshold determination method and system based on a thermochromic film, and belongs to the technical field of biomedicine ultrasonic dosimetry. The method comprises the following steps: preparing a thermochromic film with multiple color-changing temperature points, and calibrating a quantitative relationship between the hue and the temperature of the thermochromic film; constructing a sample formed by overlapping an in-vitro tissue, the thin film and a transparent phantom, and placing the sample in a focal region of a high-intensity focused ultrasound system; synchronously carrying out ultrasonic irradiation and film color-changing video acquisition; hue distribution of video frames is converted into temperature distribution through image processing, dynamic accumulation is carried out based on a thermal dose model, and a two-dimensional thermal dose distribution field is reconstructed; and finally, registering the thermal damage area after tissue irradiation with a thermal dose field, and extracting a thermal dose value on the outer side of the damage boundary to calculate a threshold value. The system comprises corresponding ultrasonic generation, sample bearing, image acquisition and control processing units. According to the invention, non-contact, real-time and two-dimensional visual measurement of the heat dose is realized, the system cost is low, and the heat dose threshold can be determined more objectively and accurately.

Description

[0001] A method and system for determining the thermal dose threshold of ex vivo tissues based on thermochromic films. Technical Field

[0002] This invention relates to the field of biomedical ultrasound dosimetry technology, and in particular to a method and system for determining the thermal dose threshold of ex vivo tissue based on a thermochromic thin film. Background Technology

[0003] High-intensity focused ultrasound (HIFU), as a revolutionary non-invasive treatment technology, has demonstrated significant clinical value in the treatment of solid tumors. It achieves target tissue thermal ablation through precise acoustic energy focusing and has been successfully applied in the clinical treatment of more than ten diseases, including uterine fibroids, prostate cancer, and liver cancer. Compared to traditional surgery and radiotherapy / chemotherapy, this technology offers advantages such as minimal trauma, rapid recovery, and repeatable treatments. Furthermore, it can activate the body's immune response, fully demonstrating the advanced nature of minimally invasive medicine. However, clinical practice has revealed significant differences in efficacy between different treatment centers: Chongqing HIFU Hospital achieved an average ablation rate of 88.3% in 1000 patients with uterine fibroids treated with the JC200 system. The core issue of this treatment heterogeneity stems from the ambiguity of the dose-response relationship of thermal ablation—that is, the quantitative relationship between acoustic energy deposition and the thermal dose threshold is not yet clear. A breakthrough in this key scientific issue will directly promote the standardization of HIFU treatment.

[0004] In thermal ablation therapy systems, besides HIFU, other energy modes include radiofrequency and microwave. Although all are minimally invasive techniques, the lack of unified dosimetric standards severely restricts their clinical application. To address this, researchers proposed the concept of thermally effective dose (TID). In vitro experiments have confirmed that this parameter can effectively predict cell death rate within the range of 40-47℃. In vivo studies using MRI thermometry have found that a TID value of 240 CEM43 can cause coagulative necrosis in prostate tissue and adenomyosis, but its applicability in the field of high-temperature ablation remains controversial. Studies have shown significant differences in thermal sensitivity among different tissues: the TID value required to achieve 99% killing of renal cell carcinoma cells and prostate cells at 55℃ ranges from 800 to 150,000 CEM43, while complete ablation of skin tissue requires more than 10,000 CEM43. This tissue-specific thermal threshold highlights the necessity of precise dose planning in HIFU treatment.

[0005] Accurate temperature monitoring is a prerequisite for determining the thermal dose threshold. Currently, most HIFU systems in China employ ultrasound-guided imaging technology, retrieving the temperature field through changes in sound velocity. Within a temperature rise range of 10-15℃, the error is only ±1℃, but accuracy drops significantly in high-temperature regions. In contrast, FDA-approved magnetic resonance thermometry (such as the ExAblate system) offers excellent spatial resolution and can generate three-dimensional temperature maps, but its spatial averaging effect leads to systematically lower measured values: experiments show that the maximum magnetic resonance temperature reading at tissue boiling (100℃) is only 73℃. Furthermore, MRI-guided systems pose a risk of claustrophobia (incidence rate 0.1-1%) and are expensive, limiting their adoption in developing countries. Among emerging temperature measurement technologies, thermochromic liquid crystal displays demonstrate unique potential due to their rapid response and two-dimensional visualization advantages. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a method and system for determining the thermal dose threshold of ex vivo tissue based on a thermochromic film. The method utilizes a thermochromic film to characterize the temperature field and calculate the biological thermal dose.

[0007] To achieve the above objectives, the present invention provides the following technical solution: The present invention provides a method for determining the thermal dose threshold of ex vivo tissue based on thermochromic films, comprising the following steps: Step 1: Fabrication and calibration of thermochromic films: Fabricate thermochromic films with multiple color-changing temperature points, and establish a quantitative relationship model between the hue value of the film image and temperature through gradient heating calibration. Step 2: Constructing the composite sample and experimental system: The isolated biological tissue, the thermochromic film, and the acoustic transparent phantom are stacked to form a sandwich-type composite sample, which is placed in the focal region of the experimental system containing a high-intensity focused ultrasound transducer, and an image acquisition device is set to face the thermochromic film. Step 3: Synchronous ultrasonic irradiation and image acquisition: Control the experimental system to perform ultrasonic irradiation on the composite sample, and simultaneously trigger the image acquisition device to record the video of the color change process of the thermochromic film due to the ultrasonic heating effect at a set frame rate; Step 4: Dynamically reconstruct the two-dimensional thermal dose distribution field: The video of the color change process is processed frame by frame. The hue distribution of each frame image is converted into a two-dimensional temperature distribution through the quantitative relationship model. Then, the thermal dose mathematical model used to quantify biological thermal damage is used for time-series cumulative calculation to obtain the two-dimensional thermal dose distribution field that evolves over time. Step 5: Determine the thermal dose threshold: Obtain a digital image of the actual thermal damage area of ​​the isolated biological tissue after ultrasonic irradiation, and spatially register it with the two-dimensional thermal dose distribution field at the final moment obtained in Step 4; extract the thermal dose value of a specified width region outside the boundary of the actual thermal damage area in the two-dimensional thermal dose distribution field, calculate its statistical value, and use it as the experimental calibration result of the thermal dose threshold for coagulative necrosis of this specific isolated biological tissue under ultrasonic irradiation.

[0008] Furthermore, in step one, the raw materials for making the thermochromic film include thermochromic powders that appear purple, blue, green, and yellow at 40°C, 50°C, 60°C, and 70°C, respectively, as well as a PVDF-HFP / DMAC matrix.

[0009] Furthermore, in step one, the gradient heating calibration uses a water bath heating pot to construct a temperature gradient from 40°C to 70°C, and the calibration nodes include 40°C, 44°C, 48°C, 52°C, 56°C, 60°C, 64°C, 68°C, and 70°C.

[0010] Furthermore, in step four, the frame-by-frame processing includes: reading video frames, obtaining region of interest (ROI) images through a cropping function; converting the ROI images from RGB color space to HSV color space, and extracting the hue channel.

[0011] Furthermore, in step four, the mathematical model for heat dose is the equivalent heat dose (TID) model, and the calculation of heat dose distribution is based on the formula: ; in, Indicates the equivalent heat dose; Indicates the total duration of thermal action; This represents the equivalent damage rate constant (usually taken as 0.25, corresponding to the tissue damage benchmark at 43℃). This represents the tissue temperature at time t; Represents the time variable in the thermal process; Furthermore, in step five, extracting the specified width region outside the boundary of the actual thermal damage region specifically includes: performing a morphological dilation operation on the digital mask of the actual thermal damage region, and performing a difference operation between the dilated mask and the original mask to obtain the mask of the specified width region outside the boundary.

[0012] The present invention provides a system for determining the thermal dose threshold of ex vivo tissue based on a thermochromic thin film, used to implement the above method, comprising: High-intensity focused ultrasound generator unit, used to generate and emit focused ultrasound waves to the focal zone; The sample unit includes a support stage and a composite sample placed thereon. The composite sample is formed by sequentially and tightly bonding an ex vivo biological tissue layer, a thermochromic film layer, and an acoustically transparent phantom layer, and is located at the focal region. The image acquisition unit is positioned as an image acquisition device capable of capturing images of the surface of the thermochromic film. The control and processing unit is used to control the high-intensity focused ultrasound generating unit and the image acquisition unit to work synchronously, and to store the image data acquired by the image acquisition device, perform calibration model calculations for the thermochromic film, reconstruct the thermal dose distribution field, and calculate the thermal dose threshold.

[0013] Furthermore, the high-intensity focused ultrasound generating unit includes a signal generator, a power amplifier, an impedance matching network, and the high-intensity focused ultrasound transducer connected in sequence.

[0014] Furthermore, the sample unit also includes a water tank for holding the coupling medium and accommodating the composite sample, and a three-dimensional motion platform for adjusting the spatial position of the composite sample.

[0015] Furthermore, the control and processing unit includes a synchronization controller and a computer. The synchronization controller is used to send synchronization trigger signals to the signal generator and the image acquisition device, and the computer is used for image and data processing.

[0016] The beneficial effects of this invention are as follows: This invention provides a method and system for determining the thermal dose threshold of ex vivo tissue based on a thermochromic thin film, belonging to the field of biomedical ultrasound dosimetry technology. The method includes: preparing a thermochromic thin film with multiple color-changing temperature points and calibrating the quantitative relationship between its hue and temperature; constructing a sample consisting of ex vivo tissue, the thin film, and a transparent phantom, and placing it in the focal region of a high-intensity focused ultrasound system; simultaneously performing ultrasound irradiation and acquiring video of the film's color change; converting the hue distribution of the video frames into a temperature distribution through image processing, and then dynamically accumulating the data based on a thermal dose model to reconstruct a two-dimensional thermal dose distribution field; finally, registering the thermally damaged area of ​​the irradiated tissue with the thermal dose field, and extracting the thermal dose value outside the damage boundary to calculate the threshold. The system includes corresponding ultrasound generation, sample carrying, image acquisition, and control processing units. This invention achieves non-contact, real-time, two-dimensional visual measurement of thermal dose, with low system cost and the ability to determine the thermal dose threshold more objectively and accurately.

[0017] This method solves the problems of limited temperature range for thermochromic films in existing technologies; high temperatures caused by thermocouples due to their thermal viscosity, which can easily lead to positioning errors and prevent the measurement of temperature changes in the focal region; and high cost and inability to perform real-time measurements with MRI.

[0018] This method overcomes the limitations of temperature measurement range, enabling precise characterization of the two-dimensional temperature field distribution in the target area. It significantly reduces the cost threshold for technology applications. Achieving millisecond-level temperature response rates, it can synchronously synchronize the instantaneous temperature dynamics during high-intensity focused ultrasound (HIFU) treatment, allowing for real-time synchronization of HIFU temperature changes.

[0019] Thermochromic color response correlation mechanism: Based on the temperature-color quantitative response characteristics of thermochromic films, a correlation model between heat dose and color characteristics was established. For the first time, the change of Hue value of the film was introduced into the determination of heat dose threshold in isolated bovine liver, making the results more consistent with the actual physiological reality of heat damage. Visualized thermal damage localization: The spatial distribution of thermal dose is intuitively presented through the change of film color gradient. Combined with boundary extraction technology, the range of thermal damage is locked, avoiding the problem of ambiguous area determination in traditional monitoring, and the localization accuracy is high. Multi-dimensional cross-validation strategy: Integrating thin film color response and thermal dose calculation reduces the judgment bias of a single indicator and significantly improves the objectivity and reliability of thermal dose threshold determination; Standardized process design: Develop a complete standardized process from film bonding, gradient heating, data acquisition to model calculation, reduce operational dependence, and ensure the systematic integration of repeatability and comparability of results under different experimental conditions.

[0020] The above and other objects, advantages, and features of the present invention will be more fully set forth and demonstrated through the following detailed description of specific embodiments in conjunction with the accompanying drawings. Those skilled in the art, upon referring to the following detailed description and the accompanying drawings, will be able to better understand and realize the above advantages of the present invention. Other objects, features, and advantages of the present invention will become clearer after being described in detail in the detailed description section in conjunction with the accompanying drawings. Attached Figure Description

[0021] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the following drawings are provided for illustration.

[0022] Figure 1 Flowchart of a method for determining the thermal dose threshold of ex vivo tissue based on thermochromic films; Figure 2 A schematic diagram of a system for determining the thermal dose threshold of ex vivo tissue based on thermochromic films; Figure 3 It is a thermochromic thin film; Figure 4 Heating the water bath; Figure 5 This is a schematic diagram of the experimental platform; Figure 6 For Hue curves; Figure 7This is a thermal dose threshold measurement platform; Figure 8 For thermal dose distribution; Figure 9 This refers to heat injury in isolated bovine liver. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0024] Example 1 like Figure 1 As shown in this embodiment, the method for determining the thermal dose threshold of ex vivo tissue based on thermochromic films is characterized by comprising the following steps: Step 1: Fabrication and calibration of thermochromic films: Fabricate thermochromic films containing multiple color-changing temperature points, and establish a quantitative relationship model between the hue value of the film image and temperature through gradient heating calibration. In step one of this embodiment, the raw materials for making the thermochromic film include thermochromic powders that appear purple, blue, green, and yellow at 40℃, 50℃, 60℃, and 70℃ respectively, as well as a PVDF-HFP / DMAC matrix.

[0025] In step one of this embodiment, the gradient heating calibration uses a water bath heating pot to construct a temperature gradient from 40°C to 70°C. The calibration nodes include 40°C, 44°C, 48°C, 52°C, 56°C, 60°C, 64°C, 68°C, and 70°C.

[0026] Step 2: Constructing the composite sample and experimental system: The isolated biological tissue, the thermochromic film, and the acoustic transparent phantom are stacked to form a sandwich-type composite sample, which is placed in the focal region of the experimental system containing a high-intensity focused ultrasound transducer, and an image acquisition device is set to face the thermochromic film. Step 3: Synchronous ultrasonic irradiation and image acquisition: Control the experimental system to perform ultrasonic irradiation on the composite sample, and simultaneously trigger the image acquisition device to record the video of the color change process of the thermochromic film due to the ultrasonic heating effect at a set frame rate; In this embodiment, the heat source is ultrasound, and the color change is caused by the heat generated by ultrasound. The thermochromic film is applied in the field of ultrasound biothermal effect research.

[0027] Step 4: Dynamically reconstruct the two-dimensional thermal dose distribution field: The video of the color change process is processed frame by frame. The hue distribution of each frame image is converted into a two-dimensional temperature distribution through the quantitative relationship model. Then, the thermal dose mathematical model used to quantify biological thermal damage is used for time-series cumulative calculation to obtain the two-dimensional thermal dose distribution field that evolves over time. The thermal dose mathematical model (i.e., the TID model) used in this embodiment is a biophysical model specifically designed to quantify thermal damage to biological tissues.

[0028] In step four of this embodiment, the frame-by-frame processing includes: reading video frames, obtaining the region of interest (ROI) image through a cropping function; converting the ROI image from the RGB color space to the HSV color space, and extracting the hue channel.

[0029] In step four of this embodiment, the mathematical model for heat dose is the equivalent heat dose (TID) model, and the calculation of heat dose distribution is based on the formula: ; in, Indicates the equivalent heat dose; Indicates the total duration of thermal action; This represents the equivalent damage rate constant (usually taken as 0.25, corresponding to the tissue damage benchmark at 43℃). This represents the tissue temperature at time t; Represents the time variable in the thermal process; Step 5: Determine the thermal dose threshold: Obtain a digital image of the actual thermal damage area of ​​the ex vivo biological tissue after ultrasonic irradiation, and spatially register it with the two-dimensional thermal dose distribution field obtained in Step 4 at the final moment; extract the thermal dose value of a specified width region outside the boundary of the actual thermal damage area in the two-dimensional thermal dose distribution field, and calculate its statistical value as the experimental calibration result of the thermal dose threshold for coagulative necrosis of this specific ex vivo biological tissue under ultrasonic irradiation. In this embodiment, the actual thermal damage area is the coagulative necrosis area identified by histological sections (such as H&E staining).

[0030] The final output in this embodiment is essentially an empirically determined threshold for a specific tissue under ultrasonic irradiation conditions, calibrated through in vitro experiments. This method provides a reliable thermal dose threshold specific to particular conditions.

[0031] In step five of this embodiment, extracting the specified width region outside the boundary of the actual thermal damage region specifically includes: performing a morphological dilation operation on the digital mask of the actual thermal damage region, and performing a difference operation between the dilated mask and the original mask to obtain the mask of the specified width region outside the boundary.

[0032] like Figure 2 As shown, the thermal dose threshold determination system for ex vivo tissue based on thermochromic thin film provided in this embodiment is used to implement the above method, including a high-intensity focused ultrasound generating unit, a sample unit, an image acquisition unit, and a control and processing unit; The high-intensity focused ultrasound generating unit is used to generate and emit focused ultrasound waves to the focal region; The sample unit includes a support stage and a composite sample placed thereon. The composite sample is formed by sequentially and tightly bonding an isolated biological tissue layer, a thermochromic film layer, and an acoustically transparent phantom layer, and is located at the focal region. The image acquisition unit is positioned as an image acquisition device capable of capturing images of the surface of the thermochromic film. The control and processing unit is used to control the high-intensity focused ultrasound generating unit and the image acquisition unit to work synchronously, and to store the image data acquired by the image acquisition device, perform calibration model calculations for the thermochromic film, reconstruct the thermal dose distribution field, and calculate the thermal dose threshold.

[0033] The high-intensity focused ultrasound (HIFU) generating unit described in this embodiment includes a signal generator, a power amplifier, an impedance matching network, and the HIFU transducer, which are connected in sequence.

[0034] The sample unit described in this embodiment also includes a water tank for holding the coupling medium and accommodating the composite sample, and a three-dimensional motion platform for adjusting the spatial position of the composite sample.

[0035] The control and processing unit described in this embodiment includes a synchronization controller and a computer. The synchronization controller is used to send synchronization trigger signals to the signal generator and the image acquisition device, and the computer is used for image and data processing.

[0036] Example 2 The method for determining the thermal dose threshold of ex vivo tissue based on thermochromic films provided in this embodiment specifically includes the following steps: 1. Fabrication of thermochromic thin films: In this embodiment, a thermochromic powder that undergoes a color-to-colorless transition at a specific temperature was selected. Thermochromic microcapsule powders exhibiting purple, blue, green, and yellow colors at 40℃, 50℃, 60℃, and 70℃ respectively were chosen, with PVDF-HFP / DMAC as the matrix. The powder was mixed with a PVDF-HFP / DMAC solution (mass ratio 1:9) at a mass fraction of 10%. The mixture was stirred using a magnetic stirrer for 30 minutes to ensure initial uniform dispersion of the microcapsules. The mixture was then treated in an ultrasonic cleaner to prevent microcapsule aggregation; ultrasonic dispersion for 15 minutes was then used to break up the aggregation. The film was coated onto a glass substrate using an automatic coating machine, and a segmented curing process was performed in a constant-temperature drying oven, controlling the wet film thickness to 200 μm. The film was then placed in a drying oven and cured first at 60℃ for 1 hour, then increased to 80℃ for 2 hours, resulting in a flexible thermochromic film with uniform thickness and good mechanical properties. This method can prepare thermochromic films with stable thermal response performance in the 40–70℃ range; for example... Figure 3 As shown, Figure 3 It is a thermochromic thin film.

[0037] The thermochromic powder in this embodiment is prepared from an electron transfer organic compound system. Electron transfer organic compounds are a class of organic chromogenic systems with special chemical structures. At a specific temperature, electron transfer causes changes in the molecular structure of the organic compound, thereby achieving a color change. This chromogenic substance not only has a bright color, but can also change from "colored to colorless".

[0038] In this embodiment, PVDF-HFP and DMAC are mixed at a mass ratio of 8:2 and then evenly coated by scraping.

[0039] The segmented curing process in this embodiment is as follows: pre-curing (40℃, 30 minutes), main curing (60℃, 45 minutes), and final curing (80℃, 15 minutes), with a total curing time of approximately 90 minutes.

[0040] 2. Calibration of the thermochromic film: A temperature gradient of 40-70℃ was constructed using a water bath heating pot, including 9 calibration nodes: 40℃, 44℃, 48℃, 52℃, 56℃, 60℃, 64℃, 68℃, and 70℃. The prepared thermochromic film was fixed in the center of the water bath heating pot. Images of the film at each calibration temperature were captured using an image acquisition device. After holding at each temperature point for 5 minutes, 3 images were acquired, and the average value was used for analysis. Figure 4 As shown, Figure 4 Heat the water bath.

[0041] The image acquisition device in this embodiment can be a high-speed camera or a CCD camera.

[0042] An image analysis program was written in MATLAB software. The transient sampling labeling method was used to first read images in batches and perform format processing, and then extract the regions of interest (ROIs) of the images. After checking the validity of the channels, the R, G, and B channel data were separated. By calculating the minimum value of the three channels, the numerator and denominator parameters, and the radian value theta, the data were converted to the HSI color space and the hue range was corrected. Finally, the hue value corresponding to each temperature was obtained, and a hue-color response model was established.

[0043] In this embodiment, the color response of the thermochromic film refers to the correspondence between the color of the film (driven by the phase change characteristics of the thermochromic capsule) and temperature under different temperature conditions. When the temperature changes, the molecular structure or optical properties of the thermochromic material change, causing the film color to exhibit an observable gradient change. This dynamic correspondence between temperature and color is the color response.

[0044] Hue value is a parameter in the HSV color space that characterizes color category (typically ranging from 0 to 1 or 0° to 360°), extracted by converting the acquired RGB image to the HSV space. Its relationship with temperature is usually achieved using a piecewise fitting model (adapted to the nonlinear response characteristics of thermochromic materials).

[0045] 3. Thermal dose threshold measurement platform: Build an advanced experimental system to achieve precise control of ultrasonic irradiation and data acquisition.

[0046] like Figure 5 As shown, Figure 5 This is a schematic diagram of the experimental platform. A pulse signal is generated by a control signal generator, which, after passing through a power amplifier and impedance matching network, excites a high-intensity focused ultrasonic transducer. The sample used in the experiment is a sandwich structure composed of ex vivo biological tissue, a thermochromic film, and a transparent phantom, placed at the focal region of the transducer. To reduce interference from reflected signals generated by the test water tank, an ultrasonic tile is placed behind the sample. The transducer focus is precisely positioned on the thermochromic film. A lateral digital image acquisition device captures the color changes of the thermochromic film in real time, and then transmits the video file to the control computer for data processing; details are as follows: A constant-temperature water bath was constructed to create a stable temperature gradient zone from 40℃ to 70℃. The thin film sample was fixed at each temperature point, and images were acquired using an industrial camera (30fps). After holding the sample at each temperature point for a sufficient time (e.g., 5 minutes), 10 frames were continuously acquired. Images were processed using a self-written MATLAB program: a fixed-size Region of Interest (ROI) was cropped, the image was converted from RGB to HSV space, the hue channel was extracted, and its average value was calculated. A polynomial fitting was performed with temperature on the x-axis and the average hue value on the y-axis to obtain the hue-T calibration curve and formula, for example: H = p1*T 2 + p2*T + p3.

[0047] The platform in this embodiment mainly includes: 1) a HIFU generation unit: composed of a function signal generator, a power amplifier, an impedance matching network, and a focusing transducer (center frequency 1.0 MHz, focal length 100 mm); 2) a sample and positioning unit: a three-dimensional precision moving platform carries a water tank, in which a "sandwich" sample consisting of an ex vivo bovine liver slice (5 mm thick), the aforementioned thermochromic film, and a transparent agar phantom (10 mm thick) tightly adhered from top to bottom, and air bubbles are removed using a coupling agent; 3) an image acquisition unit: a high-definition image acquisition device is laterally aligned with the film plane; 4) a synchronization control and processing unit: a computer controls the signal generator to start transmitting and the image acquisition device to start recording via a synchronization signal line, and stores the recorded data. The ex vivo bovine liver tissue in this embodiment is used as a model tissue.

[0048] In this embodiment, the sound pressure amplitude is 0.6-1 Vpp, the duty cycle is 30%, and the irradiation time is 10 s. Since the maximum power of the experimental power amplifier is 40 W, and to avoid material degradation caused by prolonged irradiation.

[0049] In this embodiment, the power amplifier has a gain range of 0-40W; the HIFU transducer center frequency is 2MHz; the outer diameter of the effective radiating surface of the transducer is Φ57.5mm, and the outer diameter of the outer shell is Φ65mm; the focal length geometric focal length is 60mm; and the maximum input power is 400W.

[0050] 4. Thermal dose threshold measurement: Fresh ex vivo tissue was degassed and cut into uniform cuboids. The ex vivo tissue was tightly bonded to a thermochromic film, and gaps were filled using an ultrasonic coupling agent to form a sandwich structure. Irradiation was performed using different ultrasonic parameters in the experimental system. The color change of the thermochromic film was recorded synchronously using an image acquisition device. The thermal dose threshold measurement experiment in this embodiment is as follows: Irradiation and Recording: Aim the transducer focus at the thin film plane. Set the ultrasonic parameters (e.g., frequency 1.0 MHz, acoustic power 50 W, irradiation time 10 s). Start the synchronization control, execute the irradiation, and record the video.

[0051] Thermal dose field reconstruction (e.g.) Figure 8 (As shown): The video is read, the ROI is extracted frame by frame, and the Hue distribution map is calculated. This is then converted into a temperature distribution map using a calibration formula. The classic thermal equivalent dose model is employed. TID = ΣΔt * R^(Tref - T); In this case, Tref is set to 43℃. For regions with temperature T≥55℃, R=0.5 is used, and for regions with temperature 43℃≤T<55℃, R=0.25 is used. The TID increment is calculated frame by frame and accumulated to obtain the final two-dimensional TID distribution map.

[0052] 5. Reconstruction of thermal dose distribution in ex vivo tissue: Read the color-changing video of the thermochromic film, acquire the first frame image, and crop the ROI region using the imcrop function; convert the ROI region image from RGB color space to HSV color space and extract the hue channel; obtain the two-dimensional temperature distribution within each ROI region according to the Hue-color response model; calculate the thermal dose distribution of each frame using the thermal isoeffective dose (TID). ; in, Indicates the equivalent heat dose; Indicates the total duration of thermal action; This represents the equivalent damage rate constant (usually taken as 0.25, corresponding to the tissue damage benchmark at 43℃). This represents the tissue temperature at time t; Represents the time variable in the thermal process; Calculate the difference between the real-time temperature and the reference temperature, determine the temperature coefficient R based on the temperature range, and calculate the heat dose increment in combination with the single frame time interval; accumulate the heat dose increment into the equivalent heat dose distribution, and convert the accumulated heat dose into minute units; The 43℃ value used in this embodiment originates from the classic Sapareto and Dewey thermal injury model, which represents the critical temperature at which mammalian soft tissue transitions from reversible thermal stress to irreversible thermal injury. This value is suitable for assessing the similar thermal sensitivity of isolated bovine liver to human soft tissue. The temperature coefficient R represents a value that tends to 0.25 when the temperature is < 43℃ and tends to 0.5 when the temperature is > 43℃. Based on the Arrhenius thermal injury kinetic model, this value is a standardized value for assessing thermal injury in mammalian soft tissue. Preliminary experiments have verified that at this value, the correlation between TID and the degree of pathological damage in bovine liver is R² ≥ 0.92, which can accurately distinguish the damage boundary.

[0053] 6. Calculation of thermal dose threshold for isolated tissue: ImageJ is used to delineate the thermally damaged area of ​​isolated tissue; the delineated area of ​​isolated tissue is extracted as a Region of Interest (ROI); an ROI interactive system is created to read the TID distribution of isolated tissue, and the ROI is manually matched with the TID distribution by keyboard input; the ROI mask of isolated tissue is expanded by 1 pixel by morphological dilation operation, and the difference operation is performed with the original mask to obtain the mask area 1 pixel outside the ROI boundary; the pixel values ​​of the corresponding positions in the TID image are extracted based on the outer boundary mask, invalid values ​​are filtered out, and the number of pixels and the average value are calculated to obtain the thermal dose threshold.

[0054] In this embodiment, the operation of selecting a 1-pixel wide strip outside the thermal damage boundary in the thermal dose threshold calculation refers to extracting the thermal dose value from a 1-pixel wide strip outside the boundary line after locking the thermal damage region boundary through image segmentation. The core meaning is to target the edge transition zone of thermal damage – this region is neither the thermal dose saturation value inside the damage nor the low dose value of the undamaged region far outside the boundary, but rather a microscopic region where the thermal dose is in a critical state of "damage / undamage," directly reflecting the critical dose characteristics of thermal damage. The basis for selecting a 1-pixel width is that each pixel in the experimental image corresponds to an actual tissue scale of 0.03~0.05mm, which matches the microscopic physiological boundary scale of the cellular thermal damage transition zone (cell diameter 10~20μm), accurately capturing the critical dose region. Simultaneously, 1 pixel, as the smallest operable unit of a digital image, avoids diluting the critical dose by introducing low-dose signals from the undamaged region due to excessive width, and also avoids insufficient pixel samples or interference from image noise due to excessively small width (no practical operational significance). Furthermore, the uniform selection rules ensure the repeatability and comparability of threshold calculations between different experiments and samples.

[0055] The method provided in this embodiment is applicable to temperature measurement, thermal dose distribution reconstruction, and thermal dose threshold determination in focused ultrasound experiments on ex vivo tissues. This method reduces costs and achieves real-time temperature response. It can be used in animal experiments.

[0056] Example 3 This embodiment further illustrates the method with specific illustrations and implementation details. The specific implementation process of this method is described in detail using specific embodiments.

[0057] 1. Calibration of thermochromic films: A curve is obtained by calculating the Hue value of the thermochromic film at the corresponding temperature. For example... Figure 6 As shown, Figure 6The graph shows the Hue value-temperature calibration curve of the thermochromic film. The horizontal axis represents the temperature (range 35℃~70℃), and the vertical axis represents the Hue value of the film (range 0~1). The red scatter points in the graph are the Hue value data of the film measured at different temperatures in the actual calibration experiment, and the blue curve is the fitting result of these scatter points.

[0058] As shown in the figure, the Hue value of the film decreases monotonically as the temperature increases from 35℃ to 70℃ (e.g., the Hue value is close to 1 at 35℃ and close to 0 at 70℃). This curve is the core calibration basis for the indirect monitoring of "color → temperature" using thermochromic films. In subsequent experiments, only the Hue value of the film needs to be collected, and the corresponding temperature can be deduced by comparing it with this fitted curve. The fitting process transforms the discrete calibration data into a continuous temperature-Hue value correspondence, improving the accuracy and continuity of temperature deduction. 2. Construction of a thermal dose threshold measurement platform: The movement of the "sandwich" structure is controlled by a three-dimensional motion platform. For example... Figure 7 As shown, Figure 7 This is a thermal dose threshold measurement platform.

[0059] 3. Thermal dose calculation: A thermal dose threshold measurement platform was used to measure the temperature of isolated bovine liver tissue, obtaining a color-changing video of the temperature-sensitive film. The thermal dose distribution was calculated using the TID (Thermal Induction Distribution). This thermal dose threshold measurement platform is used to realize layered focused ultrasound experiments involving the coupling of isolated tissue and the temperature-sensitive film. Figure 8 As shown, Figure 8 This represents the distribution of thermal dose. The coordinate axes and regions in the figure are as follows: the x and y axes represent the pixel coordinates of the image, corresponding to the two-dimensional planar regions of the ex vivo bovine liver sample; colors and thermal dose: the color scale on the right represents the thermal dose value (unit: minutes·°C), with blue corresponding to the low-dose region, yellow to the medium-dose transition zone, and red to the high-dose region; Distribution characteristics: The central red area in the figure is the area with the highest thermal dose, corresponding to the energy focusing core area of ​​focused ultrasound (the area where ultrasound energy is concentrated and deposited); the surrounding area, which gradually changes from yellow to blue, is the transition area where the thermal dose decreases with increasing distance from the focusing center. Experimental relevance: This figure visually presents the spatial distribution of thermal dose in isolated bovine liver tissue under focused ultrasound irradiation—the thermal dose in the focused central region is significantly higher than that in the surrounding area, consistent with the energy focusing characteristics of focused ultrasound, and also provides a visual basis for subsequent determination of thermal damage areas (high-dose areas).

[0060] 4. Quantitative spatial mapping of coagulative necrosis areas in histological sections was performed using ImageJ, such as... Figure 9 As shown, Figure 9This image shows thermal damage to isolated bovine liver. The main area is normal isolated bovine liver tissue (presenting a reddish-brown background with relatively natural texture), while the lighter-colored area outlined in yellow is a coagulative necrosis area manually delineated using ImageJ software. This area suffers irreversible damage to its cellular structure due to the thermal effects of ultrasound irradiation, resulting in a lighter texture appearance in the slice compared to normal liver tissue. ImageJ's quantitative spatial calibration here focuses on spatially locating and quantifying the "thermal damage area" by outlining the necrotic region. This clarifies the specific distribution of the necrotic area within the liver tissue slice, providing a quantitative pathological basis for subsequent correlation of heat dose distribution and verification of the correspondence between heat dose thresholds and tissue necrosis.

[0061] 5. By spatially correlating and statistically comparing the morphological results with the calculated two-dimensional thermal dose field, the critical thermal dose threshold for inducing coagulative necrosis of tissue was finally established.

[0062] In this embodiment, damage analysis and threshold calculation are performed as follows: After irradiation, bovine liver tissue is histologically sectioned (HE stained), and the coagulative necrosis area is delineated using ImageJ software to generate a binary mask image. The histological mask image and the TID distribution map are spatially aligned using an image registration algorithm (e.g., feature point-based). To obtain the critical dose at the damage edge, the histological mask is morphologically expanded outward by one pixel to obtain a new mask. The difference between the new mask and the original mask is calculated; this difference represents a ring-shaped region "one pixel wide outside the damage boundary." All pixel values ​​of this ring-shaped region on the TID image are extracted, invalid values ​​are removed, and their arithmetic mean is calculated. For example, the average TID value is 240 CEM43, which is determined as the thermal dose threshold for coagulative necrosis in ex vivo bovine liver tissue under the experimental conditions.

[0063] The embodiments described above are merely preferred embodiments for fully illustrating the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A method for determining the thermal dose threshold of ex vivo tissue based on thermochromic films, characterized in that, Includes the following steps: Step 1: Fabrication and calibration of thermochromic films: Fabricate thermochromic films with multiple color-changing temperature points, and establish a quantitative relationship model between the hue value of the film image and temperature through gradient heating calibration. Step 2: Constructing the composite sample and experimental system: The isolated biological tissue, the thermochromic film, and the acoustic transparent phantom are stacked to form a sandwich-type composite sample, which is placed in the focal region of the experimental system containing a high-intensity focused ultrasound transducer, and an image acquisition device is set to face the thermochromic film. Step 3: Synchronous ultrasonic irradiation and image acquisition: Control the experimental system to perform ultrasonic irradiation on the composite sample, and simultaneously trigger the image acquisition device to record the video of the color change process of the thermochromic film due to the ultrasonic heating effect at a set frame rate; Step 4: Dynamically reconstruct the two-dimensional thermal dose distribution field: The video of the color change process is processed frame by frame. The hue distribution of each frame image is converted into a two-dimensional temperature distribution through the quantitative relationship model. Then, the thermal dose mathematical model used to quantify biological thermal damage is used for time-series cumulative calculation to obtain the two-dimensional thermal dose distribution field that evolves over time. Step 5: Determine the thermal dose threshold: Obtain a digital image of the actual thermal damage area of ​​the isolated biological tissue after ultrasonic irradiation, and spatially register it with the two-dimensional thermal dose distribution field at the final moment obtained in Step 4; The thermal dose value of a specified width region outside the boundary of the actual thermal damage area in the two-dimensional thermal dose distribution field is extracted, and its statistical value is calculated as the experimental calibration result of the thermal dose threshold for coagulative necrosis of this specific ex vivo biological tissue under ultrasonic irradiation.

2. The method for determining the thermal dose threshold of ex vivo tissue based on thermochromic films as described in claim 1, characterized in that, In step one, the raw materials for making the thermochromic film include thermochromic powders that appear purple, blue, green, and yellow at 40℃, 50℃, 60℃, and 70℃ respectively, as well as a PVDF-HFP / DMAC matrix.

3. The method of ex vivo tissue thermal dose threshold determination based on thermochromic thin-film of claim 1, wherein, In step one, the gradient heating calibration uses a water bath to construct a temperature gradient from 40°C to 70°C. The calibration nodes include any one or more combinations of 40°C, 44°C, 48°C, 52°C, 56°C, 60°C, 64°C, 68°C, and 70°C.

4. The method of ex vivo tissue thermal dose threshold determination based on thermochromic film of claim 1, wherein, In step four, the frame-by-frame processing includes: reading video frames, obtaining the region of interest (ROI) image through a cropping function; converting the ROI image from the RGB color space to the HSV color space, and extracting the hue channel.

5. The method of ex vivo tissue thermal dose threshold determination based on thermochromic film of claim 1, wherein, In step four, the mathematical model for heat dose is the equivalent heat dose TID model, and the calculation of heat dose distribution is based on the formula: ； wherein, represents the equivalent thermal dose; represents the total duration of the thermal action; represents the equivalent damage rate constant; represents the tissue temperature at time t; represents the time variable during the thermal action.

6. The method of ex vivo tissue thermal dose threshold determination based on thermochromic thin-film of claim 1, wherein, In step five, extracting the specified width region outside the boundary of the actual thermal damage region specifically includes: performing a morphological dilation operation on the digital mask of the actual thermal damage region, and performing a difference operation between the dilated mask and the original mask to obtain the mask of the specified width region outside the boundary.

7. A system for ex vivo tissue thermal dose threshold determination based on thermochromic films for implementing the method according to any one of claims 1 to 6, characterized in that, include: High-intensity focused ultrasound generator unit, used to generate and emit focused ultrasound waves to the focal zone; The sample unit includes a support stage and a composite sample placed thereon. The composite sample is formed by sequentially and tightly bonding an ex vivo biological tissue layer, a thermochromic film layer, and an acoustically transparent phantom layer, and is located at the focal region. The image acquisition unit is positioned as an image acquisition device capable of capturing images of the surface of the thermochromic film. The control and processing unit is used to control the high-intensity focused ultrasound generating unit and the image acquisition unit to work synchronously, and to store the image data acquired by the image acquisition device, perform calibration model calculations for the thermochromic film, reconstruct the thermal dose distribution field, and calculate the thermal dose threshold.

8. The in-vitro tissue thermal dose threshold determination system based on thermochromic thin-film of claim 7, wherein, The high-intensity focused ultrasound generating unit includes a signal generator, a power amplifier, an impedance matching network, and the high-intensity focused ultrasound transducer connected in sequence.

9. The in-vitro tissue thermal dose threshold determination system based on thermochromic thin-film of claim 7, wherein, The sample unit also includes a water tank for holding the coupling medium and accommodating the composite sample, and a three-dimensional motion platform for adjusting the spatial position of the composite sample.

10. The in vitro tissue thermal dose threshold determination system based on thermochromic thin-film of claim 7, wherein, The control and processing unit includes a synchronization controller and a computer. The synchronization controller is used to send synchronization trigger signals to the signal generator and the image acquisition device, and the computer is used for image and data processing.

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