An in-vivo radio frequency response modeling method for active implantable medical devices based on equivalent lumped element model

Abstract

The application discloses a kind of active implantable medical device in-vivo radio frequency response modeling method based on equivalent lumped element model, by constructing and matching the electromagnetic characteristics of the actual conductive path of AIMD and surrounding tissue, equivalent lumped element model (LEM), the key factors such as the multiscale electrical characteristics of the geometry size of implantable medical device and its in homogeneous, non-uniform even real anatomic tissue environment are parameterized characterization, so as to quickly and accurately reconstruct the radio frequency transfer function of implant.The method of the application effectively breaks through the bottleneck of the existing Tier 3 evaluation method, meets the technical requirements of the latest standard based on Tier 4 evaluation method for modeling the radio frequency transfer function of implant in the environment closely related to clinical tissue, and provides a new technical path and implementation means for efficient and accurate evaluation of the radio frequency heating risk of implantable medical device in the magnetic resonance environment.

Description

Technical Field

[0001] This invention relates to the field of magnetic resonance imaging safety technology, and more specifically, to a method for modeling the in vivo radio frequency response of active implantable medical devices based on an equivalent lumped element model. Background Technology

[0002] Magnetic resonance imaging (MRI), with its excellent soft tissue contrast, multidimensional imaging capabilities, and non-ionizing radiation, has become a core medical imaging tool in modern clinical diagnosis. With increasing global health needs, the application of active implantable medical devices (AIMDs) such as pacemakers and neurostimulators is becoming increasingly common. Simultaneously, the demand for MRI examinations among AIMD patients continues to grow. These patients often require MRI to clarify their condition during disease diagnosis and treatment. However, the electromagnetic fields generated during MRI examinations can interfere with AIMDs, potentially causing equipment malfunctions, local tissue overheating, and in severe cases, even irreversible damage to patients. How to ensure the safety of AIMD patients while allowing them to undergo MRI examinations smoothly has become a critical issue that urgently needs to be addressed in clinical practice.

[0003] To address this challenge, the International Electrotechnical Commission (IEC) Joint Working Group developed the ISO 10974 standard, proposing a four-tier (Tier 1-Tier 4) framework for assessing AIMD compatibility with MRI. Tier 3 and Tier 4 are specifically designed for assessing AIMD with slender electrical conductors. In practical applications, both approaches have significant limitations: while the Tier 4 approach can achieve high-fidelity risk assessment through full-wave electromagnetic simulation of anatomically realistic models, it relies on high-power computing clusters and incurs extremely high computational loads, making it unsuitable for efficient applications in routine clinical settings. The Tier 3 approach, based on homogeneous phantom experiments to determine the transfer function and combining in vivo incident field estimation for assessment, is more widely used clinically. However, the significant difference between homogeneous tissue models and the actual human tissue environment introduces substantial assessment uncertainty, necessitating conservative safety margins. This directly leads to some AIMD patients being unnecessarily restricted from MRI examinations. More importantly, the latest draft of ISO 10974 has proposed a Tier 4 improvement scheme, which explicitly requires the determination of transfer functions in clinically relevant heterogeneous tissue environments. However, there is currently no effective modeling method suitable for this scheme. Existing transfer function modeling techniques, whether it is the piecewise excitation method, the injection network method, or the combined technology of radiofrequency electromagnetic radiation / transceiver phase distribution, are all limited to homogeneous tissue environments. Moreover, when faced with the complex geometry of AIMD leads, they simply cannot achieve accurate modeling and cannot meet the assessment accuracy requirements in clinical heterogeneous tissue scenarios.

[0004] Therefore, there is an urgent need to propose a novel radiofrequency transfer function modeling method for AIMD that can adapt to the real heterogeneous tissue environment in clinical practice and accurately handle the complex lead structure of AIMD, thereby effectively assessing the safety risks of AIMD patients undergoing MRI examinations and providing reliable technical support for the MRI diagnosis and treatment of such patients. Summary of the Invention

[0005] The purpose of this invention is to propose a method for modeling the in vivo radiofrequency response of active implantable medical devices based on an equivalent lumped element model. This method can significantly reduce the computational complexity of ISO / TS 10974 Tier 4 assessment, while overcoming the limitations of traditional simplified tissue models in predicting radiofrequency pyrogenic power, and inability to take into account tissue heterogeneity and real clinical anatomical scenarios. This ensures the efficiency and accuracy of AIMD radiofrequency pyrogenic risk assessment.

[0006] This invention provides a method for modeling the in vivo radio frequency response of active implantable medical devices based on an equivalent lumped element model, comprising the following steps: Measuring the RF transfer function of an actual AIMD: Place the AIMD in an analog medium and inject an RF excitation signal for testing. Acquire the AIMD's response signal to obtain the RF transfer function of the actual AIMD. LEM Establishment and Parameter Iterative Calibration: Obtain the geometric model of the AIMD electrode wire structure, and model the AIMD electrode wire structure as a series lumped element chain (LEM) based on the geometric model. The inductance value of each lumped element is calculated by multiplying the inductance per unit length by the element length. The equivalent capacitance between the wire and the surrounding medium is adapted by adjusting the dielectric constant of the insulating layer. The LEM is simulated to obtain the RF transfer function of the LEM. By iteratively calibrating the inductance per unit length of the LEM and the dielectric constant of the insulating layer, the amplitude deviation between the RF transfer function of the LEM and the RF transfer function of the actual AIMD is less than a preset threshold. The radio frequency transfer function of the implant in the human body is obtained by LEM: The calibrated LEM is implanted into a human body model, and simulation calculations are performed to obtain the radio frequency transfer function of the LEM in the human body model, thereby characterizing the radio frequency transfer function of the actual AIMD after implantation in the human body.

[0007] Optionally, the radio frequency transfer function of the actual AIMD is measured, including: The implantable device is placed in a simulated medium. A segmented excitation system is used, and a time-domain fiber optic electric field sensor is installed near the electrode head of the implantable device. The excitation antenna moves along the length of the wire of the implantable device in a set step size, and the total number of sampling points is set to [number missing]. Set step size = L / N , LGiven the lead wire length of the implantable device, the induced electric field near the electrode head measured by the time-domain fiber optic electric field sensor is recorded under each segmented radio frequency excitation, and the radio frequency transfer function of the actual AIMD is obtained based on the induced electric field.

[0008] Optionally, the step size is less than 10 mm.

[0009] Optionally, obtain the geometric model of the AIMD electrode wire structure, including: Import or input the electrode tip length of the implant electrode lead structure l Wire length L and wire insulation thickness t .

[0010] Optionally, the LEM can be simulated to obtain its RF transfer function, including: First, regarding the inductance per unit length... Empirical values ​​are set for the dielectric constant of the insulating layer, and then LEM simulation is carried out. The specific process is as follows: Determine the basic parameters of LEM, including the RF signal amplitude. Total length of LEM Equivalent impedance including electrodes and surrounding tissue Set the angular frequency of the radio frequency signal. And define the characteristic impedance of the LEM. With equivalent wavenumber Relationship: in This is the inductance per unit length of the LEM. Let LEM be the equivalent capacitance per unit length; let The position coordinates along the length direction of LEM (0≤ ≤ Based on the current and The value of LEM at position is calculated using the following formula. RF transfer function at: .

[0011] Optionally, by iteratively calibrating the unit length inductance and dielectric constant of the LEM, the amplitude deviation between the RF transfer function of the LEM and the RF transfer function of the actual AIMD is less than a preset threshold, including: The total number of sampling points is set to Sampling step size = L / N L is the length of the conductor. For the first Spatial coordinates of each sampling point ( =1,2,…, ), for each sampling location Extract the amplitude of the radio frequency transfer function of the actual AIMD. Amplitude of the radio frequency transfer function of LEM The positional amplitude deviation is calculated using the following formula: ; Calculate the absolute value of the deviation of all sampling points, when the following condition is met: When the LEM's RF transfer function matches the actual AIMD's RF transfer function, it is determined that the matching degree is satisfactory. If the standard is not met, the inductance per unit length is adjusted. Based on the dielectric constant of the insulating layer, the LEM is optimized, and the simulation and deviation calculation process is repeated until the maximum value of the position-by-position amplitude deviation meets the preset threshold. Finally, the inductance per unit length is determined. and the dielectric constant of the insulating layer.

[0012] Optionally, the preset threshold is set to below 1.5 dB.

[0013] Optionally, after completing the establishment and parameter iterative calibration of the LEM, the method further includes: comparing the radio frequency transfer function of the LEM obtained by simulation under different tissues with the radio frequency transfer function of the actual AIMD obtained by measurement in the tissue, wherein the different tissues include adipose tissue and blood tissue.

[0014] Optionally, in adipose and blood tissue scenarios, the positional amplitude deviation between the actual AIMD radio frequency transfer function and the LEM radio frequency transfer function is stably limited to within 1.2 dB.

[0015] In summary, the present invention has at least one of the following beneficial effects: 1. This invention designs a complete technical process of "actual measurement - model establishment and calibration - multi-tissue comparison - human simulation," supported by media and professional equipment conforming to ISO 10974 standards. It provides a precise benchmark for LEM establishment, ensuring the reliability of model construction and subsequent evaluation, and offering a systematic technical path for AIMD radiofrequency transfer function analysis. Compared with existing Tier 3 assessment methods, which suffer from insufficient accuracy in predicting radiofrequency heating power and difficulty in simultaneously considering tissue heterogeneity and real clinical anatomical scenarios, this invention effectively overcomes these bottlenecks. It meets the latest standard's technical requirements for modeling the radiofrequency transfer function of implants in environments closely related to clinical tissues based on Tier 4 assessment methods. This provides a novel technical path and implementation method for efficient and accurate assessment of radiofrequency heating risks of implantable medical devices in a magnetic resonance imaging (MRI) environment.

[0016] 2. This invention constructs an equivalent lumped element model (LEM) that matches the actual conductive path of the AIMD and the electromagnetic properties of the surrounding tissue. This model parametrically characterizes key factors such as the geometry of the implantable medical device and its multi-scale electrical properties in homogeneous, non-homogeneous, and even real anatomical tissue environments. This enables rapid and accurate reconstruction of the implant's radio frequency transfer function. The LEM parameters are optimized through an iterative calibration mechanism (deviation threshold of 1.5 dB). By combining comparative verification with two types of tissue scenarios, namely fat and blood, the matching degree between the LEM and the actual AIMD's transfer function in different media is ensured. This effectively solves the problem of insufficient adaptability of simplified models and is more in line with the diverse tissue environments in clinical practice.

[0017] 3. The LEM constructed in this invention is a full-wave model, which can take into account the complete propagation physical characteristics of electromagnetic waves. The parameters do not need to be adapted for different embedding media and frequencies. Compared with traditional high-resolution models, it greatly simplifies the process of obtaining the transfer function. At the same time, the calibration parameters can be adjusted according to the changes in the implant wires, improving the evaluation efficiency and adaptation flexibility.

[0018] 4. This invention can characterize the radiofrequency heating transfer function of various actual AIMDs by adjusting the LEM model to match implants of different sizes and shapes without reconstructing the model. Combined with standardized measurement and calibration steps, it reduces evaluation uncertainty, adapts to the diverse clinical needs of implantable devices, and has great potential for clinical translation and promotion.

[0019] 5. This invention uses tissue simulation media conforming to ISO 10974 standards for field testing, which meets industry standards and realizes the whole chain evaluation from in vitro standard environment to human simulation environment, satisfying the standardization and clinical relevance requirements of AIMD radio frequency transfer function modeling. Attached Figure Description

[0020] Figure 1 This is a flowchart of a method according to an embodiment of the present invention; Figure 2 A schematic diagram of the experimental platform built for an embodiment of the present invention: (a) a clinical cardiac pacemaker implantation lead with a length of 520 mm; (b) a schematic diagram of the experimental device, including a pacemaker lead, a homogeneous tissue simulation medium conforming to ISO 10974 standard, a segmented excitation system, a time-domain fiber optic electric field sensor near the implanted electrode head, and a robotic arm equipped with an excitation antenna. Figure 3 This is a schematic diagram of the LEM according to an embodiment of the present invention; Figure 4 This is a comparison chart of the measured radio frequency transfer function of AIMD and LEM for clinical pacemaker leads according to an embodiment of the present invention. Figure 5This is a comparison of the radio frequency transfer function of AIMD and LEM measured in homogeneous blood tissue (a) and adipose tissue environment (b) according to an embodiment of the present invention. Figure 6 This is a schematic diagram of the implant wires and electrodes in a human anatomical model according to an embodiment of the present invention; Figure 7 This refers to the radio frequency transfer function of the LEM in the human anatomical model of this invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention.

[0022] This invention provides a method for modeling the in vivo radio frequency response of active implantable medical devices based on an equivalent lumped element model. By simplifying the electromagnetic parameter characterization of the helical wire, a simplified model that accurately represents the electromagnetic behavior of the real implant is constructed. The effectiveness of this model in MRI compatibility assessment is ensured through multi-scenario verification, overcoming the limitations of traditional full-wave simulation's high computational load and insufficient accuracy of traditional simplified models. The flowchart of this invention is shown below. Figure 1 As shown, the specific method includes the following steps: Measuring the RF transfer function of an actual AIMD: Place the AIMD in an analog medium and inject an RF excitation signal for testing. Acquire the AIMD's response signal to obtain the RF transfer function of the actual AIMD. LEM Establishment and Parameter Iterative Calibration: Obtain the geometric model of the AIMD electrode wire structure, and model the AIMD electrode wire structure as a series lumped element chain (LEM) based on the geometric model. The inductance value of each lumped element is calculated by multiplying the inductance per unit length by the element length. The equivalent capacitance between the wire and the surrounding medium is adapted by adjusting the dielectric constant of the insulating layer. The LEM is simulated to obtain the RF transfer function of the LEM. By iteratively calibrating the inductance per unit length of the LEM and the dielectric constant of the insulating layer, the amplitude deviation between the RF transfer function of the LEM and the RF transfer function of the actual AIMD is less than a preset threshold. Preferably, the method further includes comparing the RF transfer function of the LEM obtained by simulation under different tissues with the RF transfer function of the actual AIMD obtained by measurement in the same tissue. The different tissues include adipose tissue and blood tissue.

[0023] The radio frequency transfer function of the implant in the human body is obtained by LEM: The calibrated LEM is implanted into a human body model, and simulation calculations are performed to obtain the radio frequency transfer function of the LEM in the human body model, thereby characterizing the radio frequency transfer function of the actual AIMD after implantation in the human body.

[0024] The measurement of the radio frequency transfer function of the actual AIMD includes: a. Experimental setup: according to... Figure 2 As shown in (a) and (b), an experimental platform was built that includes a homogeneous tissue simulation medium conforming to ISO 10974, a segmented excitation system (piX, ZMT AG, Zurich), a time-domain fiber electric field sensor (deployed near the electrode head), and a robotic arm with an excitation antenna. b. Test subject: Selected commercial pacemaker implantation leads (520mm in length, 1mm fixed electrode tip length, 1mm helix radius, and 0.5mm insulation layer thickness). c. Data Acquisition: In this embodiment, a 520mm long clinical cardiac pacemaker implantation lead was used for the experiment. It was placed in a homogeneous TSM medium (conductivity 0.47 S / m, relative permittivity 78) conforming to ISO 10974 standards. The radio frequency transfer function of the actual AIMD was measured at 64MHz (a commonly used low-frequency band for MRI). The total number of sampling points was set to [number missing]. 52, set step size = L / N , L The implantable device has a lead wire length of 520mm and a step size of 10mm. The robotic arm moves the antenna to a designated segment position, and the segmented excitation system excites the antenna to output a 64MHz single-frequency radio frequency signal. After stabilizing for 100ms, a time-domain fiber optic electric field sensor is triggered to collect the induced electric field near the implant electrode tip. After completion, the arm moves to the next segment and repeats the above process. Under each segmented radio frequency excitation, the acquisition software records the time-domain waveform of the induced electric field near the electrode tip and converts it into the frequency-domain amplitude of the induced electric field near the electrode tip, which is the radio frequency transfer function (RF function) with the implanted device (implant) in this environment. Then, the implant is removed, and the above steps are repeated to obtain the RF transfer function without the implant in this environment. The final actual AIMD RF transfer function is obtained by subtracting the measured RF transfer function with the implant from the measured RF transfer function without the implant in this environment. The measured actual AIMD RF transfer function is shown below. Figure 4 As shown by the blue curve in the figure, the left figure represents the amplitude value, and the right figure represents the phase value.

[0025] LEM establishment and parameter iterative calibration include: a. LEM Construction: Based on the physical parameters of the actual AIMD 3D model, LEMs corresponding to the spiral wires of various lengths are constructed to ensure that the model parameters match the geometry and material properties of the real wires. Structural parameters of the actual AIMD implant wires are collected: length 520mm, electrode tip length fixed at 1mm, spiral radius 1mm, insulation layer thickness 0.5mm. Figure 3This is a schematic diagram of the LEM. Based on the collected structural parameters, the initial LEM was established using the Finite-Difference Time-Domain (FDTD) computing platform Sim4Life V9.0 (ZMT AG, Zurich). The LEM mesh resolution was set to 0.2mm × 0.2mm × 0.2mm. The spiral implanted wire was modeled as a lumped element chain LEM, which is equivalent to a series of lumped elements. The inductance value of each lumped element was calculated by multiplying the inductance per unit length by the element length. The equivalent capacitance between the spiral implanted wire and the surrounding medium was adjusted by adjusting the dielectric constant of the insulation layer.

[0026] First, select initial parameter values ​​to be calibrated. Set empirical values ​​for inductance per unit length and dielectric constant as initial values, setting them to 40 nH / m and 15, respectively. Determine the basic parameters of the LEM: RF signal amplitude. Total length of LEM L Equivalent impedance including electrodes and surrounding tissue Set the angular frequency of the radio frequency signal. And define the characteristic impedance of the LEM. The correlation with the equivalent wavenumber k is: in This is the inductance per unit length of the LEM. The capacitance per unit length of LEM.

[0027] LEM RF transfer function calculation: Let The position coordinates along the length direction of LEM (0≤ ≤ Based on the current and The value of LEM at position is calculated using the following formula. RF transfer function at: b. LEM parameter calibration: The LEM was iteratively simulated, and the RF transfer function obtained from the LEM was compared and calibrated with the RF transfer function of the actual AIMD model obtained above.

[0028] Calculation of amplitude deviation between the RF transfer function of LEM and actual AIMD: In the RF transfer function of both... The total number of sampling points is 52, and the sampling step size is the same as the experimental test step size. L / N All are 10mm; For the first Spatial coordinates of each sampling point ( =1,2,…, For each sampling location The experimentally measured radio frequency transfer function of AIMD RF transfer function with LEM Perform amplitude deviation calculation: Calculate the absolute value of the deviation for all sampling points, when If the LEM's position-by-position transfer function amplitude in this scenario is determined to match the actual AIMD to clinical application-grade requirements, with no local deviation exceeding the standard, then the LEM's inductance per unit length can be adjusted. The dielectric constant (capacitance value) is used to change its radio frequency transfer function, so that the maximum amplitude deviation between the radio frequency transfer function of the LEM and the actual AIMD meets the preset threshold.

[0029] By iteratively adjusting the inductance per unit length and the dielectric constant of the insulating layer, and changing their parameter values, calibration is completed when the deviation of their RF transfer functions is less than or equal to a preset threshold of 1.5 dB. Finally, the inductance per unit length and the dielectric constant of the insulating layer are determined. Ultimately, the dielectric constant of the insulating layer and the inductance can be locked at 20 and 53 nH / m, respectively, to ensure model consistency. Figure 4 The figure shows an experimental comparison of the radiofrequency transfer function of AIMD and LEM measured in clinical pacemaker leads. The radiofrequency transfer functions of the two are very well matched.

[0030] Comparison of the radio frequency transfer functions of actual AIMD and LEM under different tissues, including: Implanted medical devices exist in various tissue environments with different dielectric properties. Since the LEM model constructed in this invention is a full-wave model, it can take into account the complete propagation physical characteristics of electromagnetic waves; therefore, its parameters do not need to be adapted for different embedding media. To verify this hypothesis, this method was designed for numerical verification in homogeneous adipose tissue and blood tissue environments. Under the above verification conditions, the radio frequency transfer function of the actual AIMD obtained through experiments and the radio frequency transfer function of the LEM obtained through simulation were compared and evaluated.

[0031] Based on the aforementioned specific experimental operations, modeling and simulation parameters, and mesh conditions, the actual radio frequency transfer function of AIMD was obtained through experiments in homogeneous adipose tissue and blood tissue environments. Furthermore, the radio frequency transfer function of LEM under these tissue conditions was obtained through simulation of LEM. Finally, the obtained radio frequency transfer functions were compared.

[0032] Figure 5The image shows a comparison of the radiofrequency transfer function (RFT) of the actual helical implantation device (AIMD) and the LEM in homogeneous adipose tissue and blood tissue environments, respectively. (a) shows implantation in blood tissue, and (b) shows implantation in adipose tissue. The corresponding RFT curves of the two devices show high agreement in different tissue environments. In both adipose and blood tissue scenarios, the positional amplitude deviation between the RFT of the actual AIMD and the LEM is consistently limited to within 1.2 dB, demonstrating the excellent adaptability of the LEM and fully validating its effectiveness and accuracy in different tissue environments.

[0033] The transfer function of the implant in the human body is obtained through LEM, including: The LEM, after parameter calibration, was implanted into the high-resolution anatomical human model Ella V3. Figure 6 Based on the advantage of the LEM full-wave model, which can take into account the complete propagation physical characteristics of electromagnetic waves, simulation calculations can be carried out without additional parameter adjustments to adapt to the complex tissue medium of the human body. Finally, the radio frequency heating transfer function of LEM in the human body model is obtained. Figure 7 This allows for the precise characterization of the radiofrequency heating transfer function of AIMDs in complex, multi-media tissue environments after implantation in the human body, providing a direct reference for evaluating the radiofrequency characteristics of AIMDs in clinical in vivo scenarios. Modeling and simulating the LEM (Low-Intensity Modulation) enables the rapid and accurate acquisition of the radiofrequency transfer function of the implant in relevant clinical tissue environments.

[0034] Compared with the limitations of existing Tier 3 assessment methods, such as poor accuracy in predicting radiofrequency thermal power and difficulty in simultaneously addressing the dual needs of tissue heterogeneity and real-world clinical anatomical scenarios, the method of this invention effectively overcomes these bottlenecks. It not only meets the technical requirements of the latest standard for modeling the radiofrequency transfer function of implants in clinically relevant tissue environments based on Tier 4 assessment methods, but also provides a new technical path and implementation method for the efficient and accurate assessment of radiofrequency thermal risks of implantable medical devices in magnetic resonance imaging environments, thus providing solid technical support for the assessment of AIMD radiofrequency characteristics in clinical in vivo scenarios.

[0035] The above are all preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for modeling the in vivo radio frequency response of an active implantable medical device based on an equivalent lumped element model, characterized in that, Includes the following steps: Measuring the RF transfer function of an actual AIMD: Place the AIMD in an analog medium and inject an RF excitation signal for testing. Acquire the AIMD's response signal to obtain the RF transfer function of the actual AIMD. LEM Establishment and Parameter Iterative Calibration: Obtain the geometric model of the AIMD electrode wire structure, and model the AIMD electrode wire structure as a series lumped element chain (LEM) based on the geometric model. The inductance value of each lumped element is calculated by multiplying the inductance per unit length by the element length. The equivalent capacitance between the wire and the surrounding medium is adapted by adjusting the dielectric constant of the insulating layer. The LEM is simulated to obtain the RF transfer function of the LEM. By iteratively calibrating the inductance per unit length of the LEM and the dielectric constant of the insulating layer, the amplitude deviation between the RF transfer function of the LEM and the RF transfer function of the actual AIMD is less than a preset threshold. The radio frequency transfer function (RFT) of the implant in the human body is obtained through the LEM: The calibrated LEM is implanted into a human body model, and simulation calculations are performed to obtain the RFT of the LEM in the human body model. This characterizes the RFT of the actual AIMD implanted in the human body. The RFT of the LEM is obtained through simulation, including: First, regarding the inductance per unit length... Empirical values ​​are set for the dielectric constant of the insulating layer, and then LEM simulation is carried out. The specific process is as follows: Determine the basic parameters of LEM, including the RF signal amplitude. Total length of LEM Equivalent impedance including electrodes and surrounding tissue Set the angular frequency of the radio frequency signal. And define the characteristic impedance of the LEM. With equivalent wavenumber Relationship: in The inductance per unit length of LEM. Let LEM be the equivalent capacitance per unit length. make The position coordinates along the length direction of LEM Based on the current and The value of LEM at position is calculated using the following formula. RF transfer function at: 。 2. The method for modeling the in vivo radio frequency response of active implantable medical devices based on an equivalent lumped element model according to claim 1, characterized in that, Measuring the RF transfer function of an actual AIMD includes: The implantable device is placed in a simulated medium. A segmented excitation system is used, and a time-domain fiber optic electric field sensor is installed near the electrode head of the implantable device. The excitation antenna moves along the length of the wire of the implantable device in a set step size, and the total number of sampling points is set to [number missing]. Set the step size to L / N, where L is the length of the lead wire of the implantable device. Record the induced electric field near the electrode head measured by the time-domain fiber optic electric field sensor under each segmented radio frequency excitation, and obtain the radio frequency transfer function of the actual AIMD based on the induced electric field.

3. The method for modeling the in vivo radio frequency response of active implantable medical devices based on an equivalent lumped element model according to claim 2, characterized in that, The step size is less than 10 mm.

4. The method for modeling the in vivo radio frequency response of active implantable medical devices based on an equivalent lumped element model according to claim 1, characterized in that, Obtain the geometric model of the AIMD electrode wire structure, including: Import or input the electrode head length l, wire length L, and wire insulation layer thickness t of the implant electrode lead structure.

5. The method for modeling the in vivo radio frequency response of active implantable medical devices based on an equivalent lumped element model according to claim 1, characterized in that, By iteratively calibrating the unit length inductance and dielectric constant of the insulating layer of the LEM, the amplitude deviation between the RF transfer function of the LEM and the RF transfer function of the actual AIMD is made less than a preset threshold, including: The total number of sampling points is set to Sampling step size = L / N, where L is the wire length. For the first Spatial coordinates of each sampling point For each sampling location Extract the amplitude of the radio frequency transfer function of the actual AIMD. Amplitude of the radio frequency transfer function of LEM The positional amplitude deviation is calculated using the following formula: ； Calculate the absolute value of the deviation of all sampling points, when the following condition is met: When the LEM's RF transfer function matches the actual AIMD's RF transfer function, it is determined that the matching degree is satisfactory. If the requirement is not met, the inductance per unit length is adjusted. Based on the dielectric constant of the insulating layer, the LEM is optimized, and the simulation and deviation calculation process is repeated until the maximum value of the position-by-position amplitude deviation meets the preset threshold. Finally, the inductance per unit length is determined. and the dielectric constant of the insulating layer.

6. The method for modeling the in vivo radio frequency response of active implantable medical devices based on an equivalent lumped element model according to claim 5, characterized in that, The preset threshold is set to below 1.5dB.

7. The method for modeling the in vivo radio frequency response of an active implantable medical device based on an equivalent lumped element model according to claim 5 or 6, characterized in that, After completing the establishment and parameter iterative calibration of the LEM, the method further includes: comparing the radio frequency transfer function of the LEM obtained by simulation under different tissues with the radio frequency transfer function of the actual AIMD obtained by measurement in the tissue, wherein the different tissues include adipose tissue and blood tissue.

8. The method for modeling the in vivo radio frequency response of an active implantable medical device based on an equivalent lumped element model according to claim 7, characterized in that, In adipose and blood tissue scenarios, the positional amplitude deviation between the actual AIMD radio frequency transfer function and the LEM radio frequency transfer function is consistently limited to within 1.2 dB.

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