Resonant circuit-based blood vessel monitors and related systems and methods

Abstract

Systems and methods for control and signal processing in variable inductance resonant circuit blood vessel monitoring devices are disclosed, including using sensor signal amplitudes to determine and interpret sensed parameters.

Description

[0001] Related Applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 344,409, filed on May 20, 2022, entitled “Resonant Circuit-Based Monitors and Related Systems and Methods,” which is incorporated herein by reference.

[0003] field

[0004] The present disclosure relates to improvements in wireless vascular monitors, and in particular to resonant circuit-based vascular monitors and related systems and methods.

[0005] background

[0006] A resonant circuit (RC) based sensor is one in which the resonant frequency changes due to a change in a physical parameter in the surrounding environment, which causes a change in the resonant frequency produced by the circuit within the device. When the circuit is excited, the change in resonant frequency can be detected as a "ring-back" signal, which indicates the parameter or change sensed therein. As is well known, the basic resonant circuit includes an inductor and a capacitor. In most available RC sensing devices, the change in resonant frequency is caused by a change in the capacitance of the circuit. The plates of a capacitor move together or apart in response to changes in pressure, thereby providing a pressure sensor, which is a well-known example of such a device. Less commonly, the change in resonant frequency is based on a change in the inductance of the circuit.

[0007] The applicant has filed multiple patent applications that disclose new RC monitoring devices that use variable inductance for monitoring intravascular dimensions and determining physiological parameters such as patient fluid status based thereon. See, for example, PCT / US2017 / 063749, filed on November 29, 2017, entitled “Wireless Resonant Circuit and Variable Inductance Vascular Implants for Monitoring Patient Vasculature and Fluid Status and Systems and Methods Employing Same” (published as WO2018 / 102435), and PCT / US2019 / 034657, filed on May 30, 2019, entitled “Wireless Resonant Circuit and Variable Inductance Vascular Monitoring Implants and Anchoring Structures Therefore” (published as WO2019 / 232213), each of which is incorporated herein by reference, which discloses many different embodiments and techniques related to such devices.

[0008] Although these previous disclosures represent advances in the art, improvements can still be made to the control and signal processing of such devices. Accordingly, the present disclosure provides solutions to some of the unique problems described herein that were only encountered after the introduction and testing of the new RC monitoring devices described above.

[0009] Public Overview

[0010] In one implementation, the present disclosure relates to a method for controlling a wireless resonant circuit sensor, the sensor including a variable inductance coil that changes resonant frequency in response to changes in a monitored physical parameter and, when excited, produces a ringback signal having a signal amplitude related to the physical parameter. The method includes outputting an excitation signal selected to produce a ringback signal from the sensor; receiving the ringback signal from the sensor at a receiving amplifier; comparing the amplitude of the ringback signal of the sensor to the dynamic range of the receiving amplifier; and reducing the receiving amplifier gain when the compared amplitude is at or exceeds the amplitude dynamic range of the receiving amplifier. If the amplitude is at the limit of the dynamic range, the receiver gain can be reduced to return the system to its linear range.

[0011] In another implementation, the present disclosure is directed to a method for characterizing a resonant circuit sensor to correlate a sensor output with a measured physical parameter, wherein the sensor includes a variable inductance coil that changes a resonant frequency in response to changes in the physical parameter by generating a ringback signal having a signal amplitude that can be correlated to the physical parameter when excited. The method includes: determining, prior to placement in a patient, signal amplitude data versus a physical parameter value within a range of signal amplitudes for at least one of the sensors; and creating a signal amplitude characterization curve for at least one sensor by plotting a curve with the signal amplitude data using a curve fitting or interpolation technique.

[0012] In yet another implementation, the present disclosure relates to a method for controlling a wireless resonant circuit sensor, the sensor including a variable inductance coil that changes the resonant frequency in response to changes in a monitored physical parameter and generates a ringback signal of a frequency or amplitude related to the physical parameter when excited. The method includes outputting a sensor excitation signal at an initial transmission frequency; receiving a ringback signal at the ringback frequency from the sensor in response to the sensor excitation signal; determining the difference between the transmission frequency and the ringback frequency; when the difference between the transmission frequency and the ringback frequency is below a predetermined threshold, periodically repeating output, reception, and determination; when the difference meets or exceeds the predetermined threshold, changing the transmission frequency of the sensor excitation signal to a new transmission frequency that matches the ringback frequency of the last received ringback signal; and periodically repeating output at the new transmission frequency, and then repeating reception and determination. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] For the purpose of illustrating the invention, the accompanying drawings show several aspects of one or more embodiments of the invention. However, it should be understood that the invention is not limited to the precise arrangements and instrumentalities shown in the accompanying drawings, in which:

[0015] Figure 1 is a schematic system overview of an embodiment of a wireless vascular monitoring system employing a resonant circuit based sensor implant.

[0016] Figure 2 is a block diagram of an embodiment of a control system for the wireless vascular monitoring system disclosed herein.

[0017] Figure 3A , Figure 3B and Figure 3C Shown are signals obtained in in vivo preclinical experiments using a prototype RC-WVM system as disclosed herein.

[0018] Figure 4A and Figure 4BAn exemplary ringback signal received in a bench top test via a control system receiver-amplifier module is shown without and with transmit-to-receive stimulus signal leakage according to embodiments disclosed herein.

[0019] Figure 5 is an example of a sensor characterization curve.

[0020] Figure 6 is an example of an amplitude versus area characterization curve.

[0021] Figure 7 is an example of an empirically derived transmit efficacy curve.

[0022] Figure 8 is an example of a graph of frequency versus time for a fixed transmit frequency system, where the average transmit frequency is presented as a dashed line and the average resonant frequency is presented as a solid line.

[0023] Fig. 9 is an example of a graph of the frequency variation of a dynamically adjusted frequency system over time, wherein the average transmit frequency is presented as a dashed line and the average resonant frequency is presented as a solid line.

[0024] Detailed Description

[0025] The unique physiology of the inferior vena cava (IVC) presents some unique challenges in attempting to detect and interpret changes in the size of the IVC caused by changes in the patient's fluid status. For example, the IVC wall in the typical monitoring area (i.e., between the hepatic vein and the renal vein) is relatively compliant compared to other blood vessels, which means that changes in the volume of the vessel can result in different relative distance changes between the anterior-posterior wall compared to the lateral-medial wall. Therefore, it is very typical that changes in fluid volume will result in contradictory changes in the geometry and movement of the vessel; that is, as blood volume decreases, the IVC tends to become smaller and collapse with respiration, and as blood volume increases, the IVC tends to become larger and collapse less with respiration. The applicant has developed new wireless sensor implants and related systems and methods to address these challenges and provide a clinically effective wireless vascular monitor ("WVM"). In one such embodiment, the WVM includes a resonant circuit ("RC-WVM") configured as a coil that can be implanted in the patient's vasculature. Detailed examples of embodiments of the RC-WVM system and method are disclosed in particular in U.S. Patent No. 11,206,992, entitled “Wireless Resonant Circuit and Variable Inductance Vascular Monitoring Implants and Anchoring Structures Therefore,” granted to the applicant on December 28, 2021 (U.S. Patent Application No. 17 / 018,194 filed on September 11, 2020), which is incorporated herein by reference in its entirety.

[0026] In the course of working with the RC-WVM embodiments described in the above applications, applicants have developed a number of new embodiments as disclosed herein that further improve the accuracy and usability of the RC-WVM implants, systems, and methods as previously described. These new embodiments are described below after a basic overview discussion of an example of the RC-WVM system and its operation.

[0027] Figure 1An overview of an RC-WVM system 10 to which embodiments disclosed herein apply is provided. As shown therein, such a system may generally include an RC-WVM implant 12 configured for placement in the inferior vena cava (IVC) of a patient, a control system 14, an antenna module 16, and one or more remote systems 18, such as a processing system, a user interface / display, a data storage device, etc., which communicate with a control and communication module via one or more data links 26. The data link 26 may be a wired or remote / wireless data link. In many implementations, the remote system 18 may include a computing device and a user interface used as an external interface device, such as a laptop, a tablet, or a smart phone.

[0028] The RC-WVM implant 12 typically includes a variable inductance, constant capacitance, resonant LC circuit formed as a collapsible and expandable coil structure that, when located at a monitoring location within the patient's IVC, moves with the IVC wall as the IVC wall expands and contracts due to changes in fluid volume. The variable inductance is provided by the implant's coil structure so that as the size of the coil (e.g., the area enclosed by the coil or "sensing area") changes as the IVC wall moves, the inductance also changes. The capacitive elements of the circuit can be provided by discrete capacitors or by specially designed inherent capacitance of the implant structure itself. When an excitation signal is directed to the RC-WVM implant, the resonant circuit generates a "ringback" signal at a frequency that is characteristic of the circuit. The characteristic frequency changes based on the change in size of the inductor (i.e., the coil) as it moves with the vessel wall. Because the inductance value depends on the geometry of the implant, which changes as described above based on dimensional changes of the IVC in response to fluid status, heart rate, etc., the ringback signal can be interpreted by the control system 14 to provide information about the geometry of the IVC, thereby providing information about fluid status and other physiological information such as respiration and heart rate.

[0029] The control system 14 includes functional modules for signal generation, signal processing and power supply, such as signal generation, signal processing and power supply, which generally include excitation and feedback monitoring ("EFM") circuits and are shown as modules 20, which include signal generation modules 20a and receiver-amplifier modules 20b, such as Figure 2 ) and a communication and data acquisition module 22 to facilitate communication and data transmission to various external or remote systems 18 via data links 26 and optionally other local or cloud-based networks 28. After analyzing the signals received from the RC-WVM implant 12, the results can be manually or automatically communicated to the patient, caregiver, medical professional, health insurance company, and / or any other desired and authorized party via the external or remote system 18 in any suitable manner (e.g., verbally, by printing a report, by sending a text message or email, or otherwise). Figure 2As shown, components of the control system 14 may include: a transmit / receive (T / R) switch 92, a transmitter tuning and matching circuit 94, a receiver tuning and matching circuit 96, a direct digital synthesizer (DDS) 98, an anti-aliasing filter 100, a preamplifier 102, an output amplifier 104, a single-ended to differential input amplifier (SE to DIFF) 106, a variable gain amplifier (VGA) 108, a filter amplifier (e.g., an active bandpass filter-amplifier) ​​110, an output filter (e.g., a passive high-order low-pass filter) 112, a high-speed analog-to-digital converter (ADC) 114, a microcontroller 116, and a communication submodule 118. Signal identification, selection, and other signal processing functions after amplification and filtering may be embedded within the microcontroller 116, or may be performed in an external interface device 18, such as an external computing system executing program instructions for performing the steps disclosed herein.

[0030] The antenna module 16 is connected to the control system 14 via a power and communication link 24, which can be a wired or wireless connection. The antenna module 16 generates a properly shaped and oriented magnetic field around the RC-WVM implant 12 based on the signal provided by the signal generation module 20a of the control system 14, so as to excite the resonant circuit as described above. The antenna module 16 thus provides both a receiving function / antenna and a transmitting function / antenna. In some embodiments, the transmitting and receiving functions are performed by a single antenna, which is switched between the transmitting and receiving modes, for example, by a transmit / receive switch 92 (which can be a single-pole double-throw switch). In other embodiments, each function is performed by a separate antenna. The antenna module 16 can also optionally include an input bandpass filter to reduce noise (e.g., noise generated by intermodulation) and improve signal quality. The input bandpass filter can also help improve immunity to external electromagnetic interference.

[0031] As will be appreciated by those skilled in the art, the optimal excitation of the LC resonant circuit occurs when the excitation signal is transmitted at the natural frequency of the circuit. However, in the RC-WVM implant 12 as described herein, since the RC-WVM sensor size varies according to its intended use, the natural frequency of the circuit at any given time is a priori unknown. In one embodiment, a typical sensor is suitable for the IVC diameter of a patient nominally in the range of about 14mm to about 28mm. This means that the total sensor diameter range will be from slightly less than about 14mm to slightly greater than 28mm, so as to detect changes in the IVC size above and below the nominal size range. When the sensor diameter is at the lower end of the size range, such as below about 19mm or even below about 15mm, the amplitude of the ringback signal that can be generated by the sensor will be relatively low due to the reduced inductive coupling, and therefore may pose a challenge to detection and accurate signal analysis. Another challenge in determining an appropriate excitation signal may be imposed by regulatory requirements, which generally require that any such signal have limited bandwidth and power. These challenges can be addressed in a variety of ways.

[0032] In one embodiment, the excitation signal provided by the signal generation module 20a and delivered by the antenna module 16 can be configured as a predefined transmit pulse (e.g., a single frequency burst) to excite the RC-WVM sensor. In this embodiment, since smaller sensor diameters produce lower ringback signal amplitudes, the transmit pulse frequency is selected to optimally excite the sensor assuming that the sensor is in a smaller diameter range. In an alternative, the transmit pulse frequency can be selected assuming that the sensor is at its smallest diameter, which will have the lowest ringback signal amplitude and therefore require optimal excitation to ensure that the ringback signal is at a sufficiently detectable level to obtain a reliable reading. The same predefined transmit pulse frequency is used to excite the sensor for the duration of the signal measurement, for example, 60 seconds. However, as the blood vessel expands, the optimal excitation frequency changes and the amplitude of the ringback signal may decrease, resulting in a less reliable reading.

[0033] In another embodiment, the frequency sweep function can be used to transmit the excitation signal more reliably at or near the optimal frequency. In one example, the signal generation module 20a performs the frequency sweep function by outputting a preset number of transmit pulses (in one example, five transmit pulses are used) in a predefined frequency sequence within the expected implant natural frequency range. The ringback sensor signal captured during the frequency sweep function is processed by the receiver-amplifier module 20b, the communication and data acquisition module 22, and the optional external device 18. All ringback signals (corresponding to the preset number of transmit pulses) are received and processed. From the resonant frequencies detected in the preset number of transmit pulses sent, a frequency with the highest amplitude is selected as the optimal transmit frequency. The optimal excitation frequency is then used as an excitation transmit pulse to excite the sensor within the duration of the signal measurement (e.g., 60 seconds). Note that, depending on the size of the sensor when the transmit pulse is scanned, all ringback signals from the preset number of transmit pulses can be detected, and any ringback signal can be used as the optimal resonant frequency.

[0034] In the frequency sweep method explained above, the system selects the frequency with the highest amplitude detected during execution of the frequency sweep function. As explained, the amplitude of the resonant frequency generated depends on the size (e.g., area or diameter) of the IVC at the monitoring location, with larger sizes producing larger signal amplitudes. With this approach, the system may therefore tend to select excitation frequencies that are more preferred for larger sensor sizes. Subsequently, during signal acquisition, when the size of the vessel decreases (e.g., due to respiratory failure), the excitation may become suboptimal, potentially resulting in low or insufficient signal quality when the vessel collapses. Other optional excitation frequency determination methods can be utilized to address this issue.

[0035] In one such another alternative embodiment, a two-tier approach is used to determine the excitation frequency. First, an initial excitation frequency is determined using, for example, the frequency sweep function described above. Therefore, the signal generation module 20a is configured to transmit at a frequency determined by means of the frequency sweep function during an initial observation period, which should be long enough to cover at least one respiratory cycle. During this period, the sensor resonant frequency is evaluated, and the highest detection frequency is then selected as the excitation frequency for the remainder of the signal measurement. This approach can be advantageous in selecting a higher frequency, corresponding to a smaller sensor area (which may be the worst case for signal quality), and can therefore provide a more reliable excitation.

[0036] The limitations of the method described in the previous paragraph are envisioned when considering the case of significant collapse of the IVC due to respiration. In this case, since the initial frequency sweep will tend to pick up resonant frequencies corresponding to larger sensor / vessel sizes, the resonant frequency of the sensor can deviate significantly from the excitation frequency when the IVC reaches its maximum level of collapse, resulting in suboptimal excitation. This, coupled with the reduced amplitude of the sensor response (due to the small sensor area), can lead to unreliable resonant frequency detection (due to low signal quality) and potentially incorrect excitation frequency determination.

[0037] To overcome this problem, a further improvement can be employed in which the system repeatedly performs the above frequency sweep function during a period of predefined length, which should be long enough to cover at least one breathing cycle. Since the excitation frequency is sequentially varied between predefined frequencies (including the frequency corresponding to the smallest sensor area), a more optimal excitation is achieved in the case of large IVC collapse and small sensor. As with the above method, the system selects the highest observed resonant frequency as the excitation frequency for the remainder of the signal measurement.

[0038] In another implementation, the frequency of the excitation signal is dynamically adjusted during signal acquisition. In one embodiment, the amplitude or signal-to-noise ratio (SNR) of the response signal from the RC-WVM sensor is continuously or periodically monitored (for each sample). If the signal amplitude is detected to be below a predefined threshold (e.g., due to a greater collapse of the IVC), a new frequency sweep is performed (using any of the methods described previously), allowing retuning to the latest sensor resonant frequency.

[0039] In another embodiment, the output frequency of the signal generation module 20a is continuously adjusted after each measurement point. In this case, the resonant frequency of the sensor is calculated for each sample acquired between sample acquisitions. Thus, the excitation frequency for the next sample is adjusted to the most recently measured resonant frequency. As long as the sampling rate of the system is faster than the kinetics of the IVC collapse, this approach will always ensure optimal excitation.

[0040] The embodiments described above require a signal processing algorithm for frequency detection that can be executed in real time in the communication and data acquisition module 22. A Fast Fourier Transform (FFT) can be used for the purpose. However, if a high resolution of the detected IVC size is required, the length of the required FFT may result in a high computation time and is therefore not suitable for frequency determination between sample acquisitions. Alternatively, a variation of the conventional FFT may be used, such as the Zoom FFT. This technique allows the analysis to be focused on a given portion of the frequency spectrum, in this way reducing the length of the FFT and therefore its computation time, without compromising the resolution of the detected frequencies.

[0041] Assuming that the amount of RF power that can be transmitted via antenna 16 will be subject to restrictions imposed by applicable regulations intended to ensure efficient use of the spectrum, determining the optimal transmission frequency using any of the above methods is key to providing effective excitation of the RC-WVM sensor. As an additional means of minimizing the intended RF emission level, the dependency between the RC-WVM sensor area and the sensor response signal strength can be considered. As previously described, a larger sensor area will generally result in a larger mutual inductance (and therefore magnetic field coupling) between the antenna 16 and the RC-WVM sensor. With this in mind, the signal generation module 20a can be controlled in a manner that adjusts the output RF power according to the output frequency. In particular, the maximum power is transmitted when the detected resonant frequency of the sensor is at the high end of the expected sensor bandwidth, which corresponds to the minimum sensor area and therefore has the weakest response. Therefore, as the frequency decreases, the output power decreases monotonically, thereby facilitating compliance with applicable radio regulations.

[0042] In another implementation, the amplitude of the RC-WVM sensor response signal is monitored and the output of the transmitter is dynamically adjusted, for example, to achieve a constant signal amplitude (similar to an automatic gain control application). As described in the previous paragraph, this approach can allow for tighter control of the transmitted RF power. Additionally, this approach provides a means of ensuring that the amplitude of the received signal does not result in saturation of the receiver stage, which would otherwise result in inaccuracies in the signal processing algorithms subsequently used to determine the sensor principal components.

[0043] Figure 3A , Figure 3B and Figure 3C Examples of signals from in vivo testing are shown, respectively, the original ringback signal, the detection of the resonant frequency, and the conversion to the IVC size using the reference characterization curve. Figure 3A The raw ringback signal showing the decay of the resonant response of the RC-WVM implant over time is shown in the time domain. The modulation of the implant geometry results in a change in the resonant frequency due to the change in the shape of the IVC, which can be seen as the difference between two different plotted traces. Figure 3B Shows the Figure 3A The RC-WVM implant signal is converted into the frequency domain and plotted over time. Figure 3A The resonant frequency of the IVC is determined (e.g., using a fast Fourier transform) and plotted over time. Larger, slower modulations of the signal (i.e., three broader peaks) indicate respiratory-induced motion of the IVC wall, while faster, smaller modulations overlaid on the signal indicate motion of the IVC wall in response to the cardiac cycle. Figure 3C Shown in Figure 3AThe frequency modulation plotted in is converted to a graph of sensor area versus time. (In this case, the conversion is based on a characterization curve determined by benchtop testing over a range of sample diameter cavities following standard lab / test procedures.) Thus, Figure 3C The changes in IVC size at the monitoring location in response to the respiratory and cardiac cycles are shown.

[0044] As will be appreciated by those of ordinary skill, Figure 3A-3C Accurate and reliable interpretation of the complex signals shown in requires good signal fidelity and confidence for both the excitation signal and the ringback signal from the RC-WVM. Therefore, the embodiments disclosed herein provide solutions to potential problems to help ensure the best possible signal fidelity and confidence.

[0045] One way that signal fidelity may be compromised is when defective hardware within the control system causes inaccurate readings. Therefore, a mechanism is needed to verify the accuracy of the data generated by the system. In one embodiment, data accuracy can be verified by reading a known frequency signal generated by the signal generation module 20a with a receiver-amplifier module 20b and confirming that the output of the system matches the known input. Therefore, in an embodiment, a known fixed frequency and amplitude signal portion is included in the captured signal to allow offline verification of the raw data file. Once the transmission cycle begins, the receiver-amplifier 20b and the communication and data acquisition submodule 22 start capturing the generated signal together. The transmission signal amplitude is large, so a small leakage signal is generated by the transmit / receive (T / R) switch 92 that reaches the receiver channel. Since the latter has a very large gain, the resulting signal at the output of the receiver can be detected and processed to determine its frequency, which is known a priori because the transmitter has been programmed to generate such a frequency. In another alternative, by allowing the transmit / receive switch 92 to temporarily leak a known excitation signal from the transmitting side to the receiving side when switching from transmitting to receiving, a known or fixed frequency signal portion can be included in the sensor raw data capture.

[0046] In this way, when the receiver-amplifier module 20b begins to acquire the received signal, the first part of the signal is the known frequency part. Figure 4A and Figure 4B A brief signal leak is described. Figure 4A The ringback signal that may be received by the control system after the RC-WVM sensor is excited by a signal from the transmit side in typical operation without any signal leakage through the T / R switch 92 is shown. Figure 4AThe signal in Figure 1 starts at maximum amplitude on the left when the RC-WVM coil is first excited and decays over time as energy is dissipated. Note that in this example, the ringback signal starts at time 14 μs, which represents the time delay for the transmit signal to be sent and excite the sensor. (The excitation signal begins at time 0, which is Figure 4A Not shown in , but in Figure 4B .) Figure 4B The signal in shows the received signal when leakage through the switch is allowed, as described in the above embodiment. The leakage portion of the signal (LS) starts at about time zero because there is no delay waiting for the sensor to be energized. Then, by limiting the leakage signal (LS) to the time before the expected sensor ringback signal, the leakage signal does not interfere with the sensor reading, but at the same time provides a known frequency verification signal that can be checked against the control system output.

[0047] In one embodiment, the process of providing the leakage signal as a known frequency hardware verification signal may include the following steps:

[0048] 1. The RF transmitter outputs a known pulse via the antenna to excite the sensor.

[0049] 2. The transmit / receive switch is configured to allow signal leakage from the transmit side to the receive side. When the transmitter is activated, the receiver electronics begins capturing receiver data.

[0050] 3. The transmit / receive switch completely changes the antenna connection to the receiver electronics to detect the sensor RF response.

[0051] 4. The receiver electronics continues to capture the sensor signal via the ADC.

[0052] 5. The captured ADC data is stored in the microcontroller and sent to the laptop for long term storage. This data now includes the transmit portion of the transmit / receive cycle within the data packet. The data packet also includes the frequency programmed into the RF transmitter.

[0053] 6. The data can then be verified by comparing the frequency and amplitude of the transmitted portion of the data signal data to the programmed frequency and predefined thresholds for expected amplitude.

[0054] Another problem that may be encountered with systems of the type described herein is interference from background noise. Excessive electromagnetic noise or external electromagnetic interference from nearby equipment can cause the system to detect readings that are unrelated to the sensor signal. During normal operation, the system attempts to detect signals caused by the sensor in response to the excitation signal transmitted to the sensor during the transmission cycle. A sufficiently strong external signal may couple into the system and mask the sensor signal, which may result in incorrect measurements.

[0055] According to the present disclosure, this problem can be solved by providing a mechanism for evaluating electromagnetic background noise before starting measurement. In one embodiment, the system operates in normal mode, that is, the transmission mode is turned on, and a known test frequency that is sufficiently far away from the expected sensor bandwidth / excitation frequency is transmitted. In this way, the sensor is not excited and therefore does not generate a ringback signal response. Then, the control system switches to the receiver mode in normal operation and records any received signal. Since the sensor does not respond (due to the "detuned" transmission frequency), the received signal is entirely composed of background electromagnetic noise. Then, appropriate corrections or adjustments can be used in signal processing based on the detected background noise. In one option, the control system evaluates the power of the maximum component of the background noise signal. This process is repeated a predefined number of times, and an average value is obtained for a more consistent metric. The calculated signal level is then defined as the background noise.

[0056] The background noise assessment process as described above is not limited to before starting sensor signal recording. In other embodiments, the background noise assessment can also be performed at different stages or multiple points in the sensor signal acquisition process to mitigate the risks associated with intermittent noise sources or increased noise coupling due to patient movement, etc.

[0057] After evaluating the background noise, the sensor signal is identified by frequency scanning. Once the sensor response signal is detected, its amplitude is evaluated and the resulting value is compared to the previously measured background noise amplitude, effectively calculating the signal-to-noise ratio (SNR). A minimum threshold level is established for the SNR. Any SNR below this limit indicates that external interference is high enough to inhibit reliable measurement. This in turn can alert the user to change position or eliminate any potential sources of interference in order to continue using the system.

[0058] Combined with the above Figure 3A and Figure 3C The use of characterization curves to convert the raw signal output of an RC-WVM sensor into physiologically relevant readings about blood vessel size and size changes is discussed. Generally speaking, the art understands that characterizing raw sensor signals to provide physiologically relevant readings useful to health care providers. However, RC-WVM sensors as described herein can present unique characterization problems because their characteristic inductance is intentionally varied by design. In addition, the inductance and capacitance characteristics that define the resonant circuit vary due to variability in sensor manufacturing. To address these challenges of characterization of RC-WVM sensors, many new and different approaches can be used.

[0059] In one embodiment, a sensor characterization curve is created by sequentially passing the RC-WVM sensor through a series of increasingly larger tubes of known area and recording the corresponding frequencies, such as Figure 5A unique curve can then be generated from these area-frequency measurements using a number of methods. For example, a curve fitting method can be employed in which a curve is fitted to the original data by minimizing the error between the fit and the original data. Curve fitting can be performed using a number of different fit types, including but not limited to exponential and logarithmic fits based on the following function:

[0060] Logarithm: y = c1 ln(x) + c2

[0061] Exponent: y = c1·ex / c2+c3.

[0062] In another example, interpolation can be used, where a curve is created by interpolating between the recorded area-frequency data. Many interpolation methods can be used, including linear interpolation functions such as:

[0063] Linear interpolation:

[0064] In addition to the curve type selected, characterization curves can be generated from single sensor specific area-frequency data or from average area-frequency data from a batch of sensors.

[0065] Typically, each RC-WVM sensor characterization curve is determined in a cleanroom during sensor manufacturing. However, these curves can shift slightly after the manufacturing and sterilization process. Since sensors for clinical use cannot be re-characterized after sterilization, sensor / batch specific manufacturing curves can only be created before sterilization. Optionally, reference characterization curves can also be generated from independent sensors that are not intended for clinical use after sterilization, as long as they are manufactured and sterilized in a similar manner to the clinical sensors, they will be used as a reference.

[0066] In another embodiment, greater characterization accuracy can be achieved as follows. First, during manufacturing, area versus frequency data is determined for each sensor. Before or after sterilization, a characterization curve is created from the sensor or batch specific area-frequency data by curve fitting or interpolation as described above. Then, sensor measurements are taken and the results are converted to IVC size using the characterization curve created in the previous steps. Thus, by using sensor or batch specific characterization curves, measurement errors caused by manufacturing variability are minimized. Use of predetermined characterization curves allows for more accurate measurements over a larger range of sizes and can avoid the need for in vivo calibration for imaging modalities such as intravascular ultrasound (IVUS), which have other inherent accuracy issues.

[0067] In another alternative embodiment, information about the amplitude of the response signal induced by the resonant sensor is extracted from the sensor trace. Changes in the cross-sectional area of ​​the sensor's inductive element affect the amount of magnetic flux captured by the sensor, which in turn affects the amplitude of the signal induced in the sensor coil. Therefore, the sensor response signal amplitude can provide additional information about parameters such as the sensor cross-sectional area, which in some cases can be complementary to information obtained from the natural frequency of the resonant circuit embedded in the sensor.

[0068] Using the extracted signal amplitude information, an amplitude characterization curve can be established to map the amplitude of the received signal to the sensor cross-sectional area. As an example, this can be done by manipulating the sensor into different configurations (where the cross-sectional area is known) while determining the resulting sensor response signal amplitude detected by the system. Such an amplitude characterization curve can be used to further characterize the accuracy of the sensor response and interpretation of the received signal.

[0069] Figure 6 An example of area as a function of an amplitude characterization curve is shown. For a given sensor response signal, the amplitude and frequency of the resonant frequency component can be determined. These two parameters can be used together to determine the cross-sectional area of ​​the sensor. Using both amplitude and frequency characterizations can help to enhance the robustness of the area estimate. As an example, situations may occur where external interference corrupts or affects the accuracy of the area estimate based on the sensor frequency. In this case, area detection based on amplitude can be used as a secondary mechanism that allows detection or potentially correction of the estimated area.

[0070] Although signal amplitude may be used as described above, in an in vivo environment for application of the systems described in the present disclosure, the detected sensor signal amplitude may be affected by other functional parameters that may be independent of the sensor geometry and may introduce errors when attempting to extract sensor information from its response signal amplitude. To improve accuracy, such other functional parameters may be accounted for by techniques as described below.

[0071] Saturation of Detected Signals - Implantable sensors as described in the present disclosure can be expected to operate over a relatively wide range of vessel sizes and shapes. As discussed, the amplitude of the response signal elicited by the sensor is directly related to the cross-sectional area of ​​the vessel in which the sensor is implanted, with the amplitude being lowest when the area of ​​the vessel is at the smaller end of the scale. The receiver elements used in the system to detect the sensor response signal should be able to provide sufficient amplification for the worst-case amplitude of the sensor response signal. However, as the area (and therefore the sensor response signal amplitude) increases, a point will eventually be reached where the linearity of the receiver amplifier will not be maintained if the same level of amplification is applied, resulting in saturation and clipping of the output signal. The effects described will affect the fidelity of the sensor signal amplitude determination, leading to potential errors in decoding the information contained in the amplitude.

[0072] To counteract the effects described in the previous paragraphs, the gain of the receiver amplifier can be adjusted in response to the dynamic behavior of the implanted sensor. This can be accomplished in several ways, including but not limited to:

[0073] The amplitude of the sensor response signal is evaluated based on the dynamic range of the receiver amplifier and analog-to-digital converter. If the amplitude is at the limit of the dynamic range, the receiver gain can be reduced to return the system to its linear range. This gain reduction can be achieved by using a variable gain amplifier (VGA) ( Figure 2 108), the gain of which can be achieved, for example, by a microcontroller ( Figure 2 116) is modified, for example, by using a control voltage generated by an internal digital-to-analog converter (DAC). Safety thresholds may be used to ensure that gain is adjusted before saturation occurs (e.g., when the detected signal amplitude rises and approaches the upper limit of the dynamic range, a gain reduction is triggered when the signal amplitude reaches a predetermined threshold). These thresholds may be selected as a compromise between the likelihood of saturation and the effective dynamic range of the receiver stage (e.g., using a low threshold will reduce the probability of saturation, particularly due to rapidly changing signals, but will result in the use of a very limited dynamic range, thereby reducing the effective resolution of the analog-to-digital conversion stage). As an example, for a 12-bit ADC (where the maximum number of ADC counts is 4095), the receiver gain control function may be implemented such that the receiver gain is reduced when the peak of the amplified response signal from the RC-WVM sensor reaches a count corresponding to 75% of the maximum ADC count (i.e., 3064).

[0074] A second, lower threshold value may be added to the above method such that when the sensor signal amplitude drops below this threshold value, the gain is increased so that a larger portion of the dynamic range of the receiver amplifier is occupied, in this way ensuring optimal use of the resolution of the analog-to-digital converter. As an example, for a 12-bit ADC (where the maximum number of ADC counts is 4095), the receiver gain control function may be implemented such that when the peak value of the amplified response signal from the RC-WVM sensor falls below the count corresponding to 25% of the maximum ADC count (i.e., 1024), the receiver gain is increased.

[0075] The optimal receiver gain can be calculated based on the sensor resonant frequency. In particular, there is an inverse correlation between the cross-sectional area of ​​the sensor and its resonant frequency, with lower frequencies corresponding to larger areas, which in turn results in larger signal amplitudes, as previously mentioned. On this basis, a frequency to gain mapping can be created that will allow the receiver gain to be adjusted in response to the detected sensor resonant frequency.

[0076] Excitation Signal Effectiveness - Another item that affects the total signal amplitude is the effectiveness of the excitation signal, which is defined as the amount of energy of the excitation signal transmitted by the control system that is effectively captured by the resonant circuit of the RC-WVM sensor. As discussed in paragraph 0016 above, the maximum energy transfer to the sensor resonant circuit occurs when the frequency of the excitation signal is equal to the resonant frequency of the sensor. Any difference between the two will result in a reduction in the effectiveness of the excitation signal, and thus a reduction in the amplitude of the response signal induced by the RC-WVM sensor. This can lead to inaccurate area estimates (for example, due to the reduced amplitude, the sensor area may be assumed to be smaller than it actually is, which is actually the result of inefficient sensor excitation). To mitigate this problem, a transmit effectiveness function can be derived that takes the form of a transmit efficiency coefficient factor (which will be a number between 0 and 1), and the coefficient is a function of the difference between the transmit signal frequency and the receive signal frequency (where the coefficient is equal to 1 when the transmit signal frequency and the receive signal frequency are the same). By understanding the launch efficacy, one can consider the impact of suboptimal RC-WVM sensor excitation on the resulting signal amplitude and thus eliminate or reduce this confounding factor, which ultimately leads to improved reliability of the amplitude of the RC-WVM sensor-evoked response signal as a predictor of sensor cross-sectional area.

[0077] In one embodiment, the effect of suboptimal sensor excitation can be accounted for by adjusting the measured signal amplitude upward by the inverse of the transmit performance function. This can be done by multiplying the measured signal amplitude by the inverse of the performance function (in other words, dividing by the performance function to give an adjusted signal amplitude). The adjusted signal amplitude is then used instead of the measured signal amplitude to determine the sensing parameter, such as cavity area, using the amplitude characterization curve. For example, if the measured signal amplitude is 70 a.u. and the performance function is 0.7, the adjusted signal amplitude is 100 a.u. (i.e., 70 a.u. / 0.7).

[0078] For example, by calculating the spectrum of the transmitted signal and the frequency response curve of the sensor and by moving the transmitted signal frequency within the expected transmission range, the transmission performance function can be theoretically derived. In a possible implementation, the spectrum of the transmitted signal is calculated (for example, using Fourier transform). The amplitude of the spectrum is then sampled at the resonant frequency of the RC-WVM sensor. For a transmitted signal consisting of a predefined number of sinusoidal pulses (whose spectrum will correspond to a sine function), the following equation can be used to calculate the performance based on the RC-WVM sensor resonant frequency (f_rx) and the transmission (f_tx) frequency:

[0079]

[0080] Alternatively, for example, Figure 7 As shown, the transmit effectiveness function can be derived empirically by taking the sensor signal for a given area, starting with the best alignment between the transmit and receive frequencies, and subsequently moving the transmit frequency up and down while evaluating the impact on the amplitude of the sensor response signal. Figure 7 Examples of empirically derived launch efficacy curves are presented.

[0081] Dynamic transmit signal frequency adjustment based on signal performance - An alternative method for transmit signal frequency determination includes dynamic frequency adjustment based on the difference between the detected sensor resonant frequency and the excitation signal transmit frequency. In this technique, the system performs an initial frequency scan to transmit at a set of discrete frequencies within the expected range of the sensor resonant frequency. As previously disclosed, the initial transmit frequency is derived using this method. The frequency of the sensor response signal is then monitored at each acquisition, and the difference between the detected frequency and the transmit frequency is calculated. Thus, a threshold can be established so that the transmit frequency will remain unchanged as long as the calculated difference between the frequencies of the transmit and receive signals is below the established threshold. If the frequency difference between the transmit and receive signals crosses the established threshold, the transmit frequency is readjusted to match the last detected sensor resonant frequency.

[0082] In order to achieve dynamic transmission signal frequency adjustment, the signal generation circuit should be able to generate a variable frequency signal, which is described above in conjunction with frequency scanning (see paragraph 0018). Figure 2 In one example implementation based on system 14 illustrated in the block diagram, there is a feedback loop that allows the system to detect the sensor resonant frequency (e.g., with the aid of a Zoom FFT as discussed in paragraph 0025) and reconfigure the frequency of the DDS in the transmitter circuit to match the detected sensor frequency.

[0083] Figure 8 and Fig. 9 An example where the transmit frequency is fixed and another example where the transmit frequency is dynamically adjusted to approximate the resonant frequency of the RC-WVM sensor are shown, respectively. By adjusting the transmit frequency, the system will tend to operate close to the maximum transmit efficiency (as defined above), thereby producing the maximum signal amplitude, which in turn leads to an increase in the reliability of the amplitude of the response signal induced by the RC-WVM sensor as a predictor of the sensor cross-sectional area.

[0084] The threshold for the transmit signal frequency update can be defined as a compromise between the bandwidth of the transmit excitation signal and the expected range of sensor resonant frequency changes caused by, for example, IVC collapse due to breathing. In one example, such a predetermined threshold can be set to a specific value of the difference between the sensor resonant frequency and the excitation signal transmit frequency, such as a difference of about 25 kHz or greater. In another example, the predetermined threshold can be set based on a minimum transmit performance function, such as a transmit performance function of 0.7 or less. Following this approach, optimal or near-optimal excitation can be achieved while minimizing the number of transmit frequency changes and maintaining quasi-fixed frequency operation.

[0085] Additional features, advantages, and limitations of the embodiments disclosed herein are listed in the following numbered subparagraphs:

[0086] 1. A method and system for validating a sensor signal received from a resonant circuit based sensor, comprising including a known fixed frequency and amplitude portion of a signal within an output signal captured from the sensor to allow validation of raw data received from the sensor, wherein the validation can optionally be performed offline.

[0087] 2. A method and system for determining an optimal transmit frequency for exciting a resonant circuit sensor, comprising outputting a plurality of predefined transmit pulses to excite the sensor within an expected sensor frequency range; determining the highest amplitude sensor signal received corresponding to the optimal excitation frequency; and exciting the sensor at the determined optimal transmit frequency for a duration of signal measurement, wherein the duration can optionally be approximately 60 seconds.

[0088] 3. A method and system for characterizing a size-related output signal of a sensor, comprising determining size versus frequency data of the sensor during sensor manufacturing; creating a sensor or batch-specific characterization curve of the size-frequency data by curve fitting or interpolation before or after sterilization of the corresponding one or more sensors; measuring with the sensor; using the created characterization curve to convert the sensor results to a desired size; minimizing size measurement errors caused by manufacturing variability by using the sensor or batch-specific characterization curve; wherein, optionally, the use of a predetermined characterization curve allows accurate measurements over a large size range.

[0089] 4. A method and system for evaluating electromagnetic background noise in a sensor system, comprising operating the sensing system in a normal mode, such as with a transmitter engaged, and transmitting a test frequency that is sufficiently far from the expected sensor bandwidth so as not to excite the sensor and cause a sensor response; switching the sensor to a receiver mode and recording a received signal with the sensing system, wherein the received signal consists of background electromagnetic noise; evaluating the power of the largest component of the background noise signal; optionally repeating the process a predefined number of times to obtain an average value; and defining a calculated signal level, which is then defined as background noise.

[0090] 5. A method for controlling a wireless resonant circuit sensor, the sensor comprising a variable inductance coil, the variable inductance coil changing the resonant frequency in response to changes in a monitored physical parameter, and generating a ringback signal at a frequency related to the physical parameter when excited. The method comprises: outputting at least one excitation frequency sweep, the excitation frequency sweep comprising a preset number of transmit pulses at a predefined frequency within an expected implant resonant frequency range; receiving a ringback signal for each of the sequentially output transmit pulses; transmitting at least one initial transmit pulse within a predetermined initial period, wherein the at least one initial transmit pulse comprises one of the following: a pulse frequency corresponding to the highest amplitude ringback signal received from at least one frequency sweep; or a plurality of excitation frequency sweeps; receiving a plurality of test ringback signals in response to at least one initial transmit pulse transmitted within the initial period; identifying an initial ringback signal corresponding to a preferred excitation pulse frequency; and selecting the preferred excitation pulse frequency as a measurement transmit pulse frequency; outputting a measurement transmit pulse at the measurement transmit pulse frequency in a subsequent measurement period.

[0091] 6. A control system for a wireless resonant circuit sensor, the sensor comprising a variable inductance coil, the variable inductance coil changing the resonant frequency in response to changes in a monitored physical parameter, and generating a ringback signal at a frequency related to the physical parameter when excited. The control system comprises: a transmit / receive switch configured to control the transmission of signals to and reception of signals from an antenna; a signal generation module configured to generate an excitation signal, wherein the transmit / receive switch controls the transmission of the generated signal to the antenna; and a receiver-amplifier module configured to receive and process the ringback signal, the ringback signal being received by the antenna and transmitted to the receiver-amplifier module via the transmit / receive switch in communication with a processor configured to execute program instructions, characterized in that the system is configured to: output at least one excitation frequency sweep, the excitation frequency sweep comprising predefined frequencies within an expected implant resonant frequency range A preset number of transmit pulses; receiving a ringback signal for each transmit pulse in the sequentially output transmit pulses; transmitting at least one initial transmit pulse within a predetermined initial period, wherein the at least one initial transmit pulse includes one of the following: a pulse frequency corresponding to the highest amplitude ringback signal received from at least one frequency scan; or multiple excitation frequency scans; receiving multiple test ringback signals in response to at least one initial transmit pulse transmitted within the initial period; identifying the initial ringback signal corresponding to the preferred excitation pulse frequency; selecting the preferred excitation pulse frequency as the measurement transmit pulse frequency; and outputting the measurement transmit pulse at the measurement transmit pulse frequency within a subsequent measurement period.

[0092] 7. A method for characterizing a resonant circuit sensor to correlate the sensor output to a measured physical parameter, wherein the sensor includes a variable inductance coil that changes resonant frequency in response to changes in a physical parameter by generating a ringback signal at a frequency that can be related to the physical parameter when excited. The method comprises: determining physical parameter value versus frequency data for at least one sensor over a range of parameter values ​​and frequencies prior to placement in a patient; and creating a characterization curve for at least one sensor by plotting a curve with the data using a curve fitting or interpolation technique.

[0093] 8. A method for evaluating electromagnetic background noise before outputting an excitation signal for measuring with a resonant circuit sensor, wherein the sensor includes a variable inductance coil that changes its resonant frequency in response to changes in a physical parameter by generating a ringback signal at a frequency that can be related to the physical parameter when excited. The method includes transmitting a predetermined test pulse at a test frequency, wherein the test frequency is selected to be sufficiently far away from an expected sensor excitation frequency so as not to excite the sensor; receiving the test signal with a sensor ringback signal receiver, wherein the received test signal is composed of the test pulse and background electromagnetic noise; defining the background electromagnetic noise based on the received test signal as a signal component different from the known test pulse; and modulating the signal processing of the received measurement ringback signal to eliminate or reduce the effect of the defined background electromagnetic noise.

[0094] 9. A method for verifying a sensor signal in a resonant circuit sensor, wherein the sensor includes a variable inductance coil that changes resonant frequency in response to changes in a physical parameter by generating a ringback signal at a frequency that can be related to the physical parameter when excited. The method includes transmitting a known fixed frequency and fixed amplitude signal; capturing the known signal as a portion of a captured signal that includes the ringback signal generated by the sensor; comparing the captured known signal portion to the transmitted known signal; and verifying the sensor ringback signal when the captured known signal portion matches the transmitted known signal within predetermined limits.

[0095] The foregoing is a detailed description of illustrative embodiments of the present invention. Note that in this specification and the claims appended hereto, unless otherwise expressly stated or indicated, conjunction language such as used in the phrases "at least one of X, Y, and Z" and "one or more of X, Y, and Z" should be understood to mean that each item in the conjunction list may be expressed as any number excluding all other items in the list or as any number in conjunction with any or all other items in the conjunction list, and each item may also be expressed as any number. Applying this general rule, in the foregoing examples where the conjunction list consists of X, Y, and Z, the conjunction phrases shall each include: one or more of X; one or more of Y; one or more of Z; one or more of X and one or more of Y; one or more of Y and one or more of Z; one or more of X and one or more of Z; and one or more of X, one or more of Y, and one or more of Z.

[0096] Various modifications and additions may be made without departing from the spirit and scope of the present invention. The features of each of the various embodiments described above may be appropriately combined with the features of other described embodiments so as to provide a variety of feature combinations in associated new embodiments. In addition, although the foregoing describes a plurality of separate embodiments, the content described herein is merely an illustration of the application of the principles of the present invention. In addition, although the specific methods herein may be illustrated and / or described as being performed in a particular order, it is within the technical scope of the art that the sequencing is highly variable to implement aspects of the present disclosure. Therefore, this specification is intended to be made by way of example only and does not otherwise limit the scope of the present invention.

[0097] Exemplary embodiments have been disclosed above and shown in the accompanying drawings. Those skilled in the art will appreciate that various changes, omissions and additions may be made to the embodiments specifically disclosed herein without departing from the spirit and scope of the present invention.

Claims

1. A method for controlling a wireless resonant circuit sensor, the sensor comprising a variable inductance coil, the variable inductance coil varying a resonant frequency in response to changes in a monitored physical parameter and generating a ringback signal having a signal amplitude related to the physical parameter when excited, the method comprising: outputting from the sensor an excitation signal selected to generate the ringback signal; receiving the ringback signal from the sensor at a receiving amplifier; comparing the amplitude of the ringback signal of the sensor with the dynamic range of the receiving amplifier; When the compared amplitude is at or exceeds the amplitude dynamic range of the receive amplifier, the receive amplifier gain is reduced, and if the amplitude is at the limit of the dynamic range, the receiver gain can be reduced to bring the system back to the linear range of the system.

2. The method according to claim 1, wherein: The reducing the receive amplifier gain includes reducing the gain to within a linear range of the receive amplifier.

3. The method according to claim 1 or claim 2, wherein: The reducing the receiving amplifier gain includes reducing the gain when the ringback signal amplitude increases and reaches a predetermined amplitude threshold.

4. The method according to any one of claims 1 to 3, wherein: The receive amplifier includes an amplifier circuit including an analog-to-digital converter (ADC); and The receiver gain control function is implemented to reduce the receive amplifier gain when the peak value of the amplified response signal from the sensor reaches a count corresponding to a predetermined percentage of the maximum ADC count.

5. The method of any one of claims 1-4, further comprising adjusting a receiver amplifier gain in response to a detected sensor resonant frequency based on a predetermined mapping of sensor frequency to gain.

6. The method according to any one of claims 1 to 5, further comprising: adjusting the received ringback signal amplitude by a transmit performance function to provide an adjusted ringback signal amplitude; The adjusted ringback signal amplitude is compared to a physical parameter, namely an amplitude correlation, to determine a value of the physical parameter based on the correlation.

7. The method according to claim 6, wherein: The transmit performance function is calculated to be equal to: where f rx is the sensor resonant frequency, and f tx is the excitation signal transmission frequency.

8. The method according to claim 6, wherein: The transmit performance function is determined based on an empirically derived transmit frequency performance curve.

9. The method according to any one of claims 1 to 8, further comprising: determining physical parameter versus amplitude data for at least one of the sensors prior to placement in a patient; Based on the physical parameter versus amplitude data, creating an amplitude versus physical parameter characterization curve for the at least one sensor by curve fitting or interpolation; Taking measurements with the sensor; and The sensor measurements are converted to values ​​of the physical parameter using the characterization curve.

10. The method according to claim 9, wherein: The at least one sensor comprises a sensor batch, and the amplitude data comprises batch-specific parameter-to-amplitude data.

11. The method of claim 9 or claim 10, further comprising minimizing physical parameter measurement errors caused by sensor manufacturing variability by using sensor or sensor batch specific characterization curves.

12. A method according to any preceding claim, wherein: The resonant circuit sensor is configured for placement in a patient's vascular system and the physical parameter is a blood vessel dimension.

13. The method according to claim 12, wherein: The sensor is specifically configured for placement in the vena cava, and the vessel dimension is the area of ​​the vena cava.

14. The method of claim 13, further comprising correlating the measured area of ​​the vena cava with the patient's fluid status.

15. A method for characterizing a resonant circuit sensor to correlate a sensor output to a measured physical parameter, wherein the sensor includes a variable inductance coil that changes resonant frequency in response to changes in the physical parameter by generating a ringback signal having a signal amplitude that can be correlated to the physical parameter when excited, the method comprising: determining physical parameter value versus signal amplitude data for at least one of the sensors within a range of parameter values ​​and signal amplitudes prior to placement in a patient; and A signal amplitude characterizing curve for the at least one sensor is created by plotting a curve using the signal amplitude data using a curve fitting or interpolation technique.

16. The method according to claim 15, wherein: The physical parameter is an internal blood vessel lumen size, the internal blood vessel lumen size including the area of ​​the lumen, the sensor is implantable within the blood vessel lumen and is capable of expanding and contracting with the blood vessel lumen, wherein the determination comprises sequentially placing the sensor in a series of progressively increasing or decreasing tubes of known size and recording the corresponding ringback signal amplitude when excited in each tube of different size.

17. The method according to claim 16, further comprising: During manufacturing, determining a blood vessel size versus signal amplitude data set for each sensor in a sensor batch; and Prior to sterilizing the sensors, the characterization curve is created from the size-amplitude data of the sensor batch by curve fitting or interpolation.

18. The method according to any one of claims 15 to 17, wherein: The ringback signal also includes a frequency that can be correlated to the physical parameter, and the method further includes: determining physical parameter value versus frequency data for said at least one of said sensors within a range of parameter values ​​and frequencies prior to placement within a patient; creating a frequency characterization curve for said at least one sensor by plotting a curve using said frequency data using a curve fitting or interpolation technique; and A sensor output of said at least one of said sensors is correlated to a measurement parameter based on both the signal amplitude characterization and the frequency characterization.

19. A method for controlling a wireless resonant circuit sensor, the sensor comprising a variable inductance coil, the variable inductance coil varying a resonant frequency in response to changes in a monitored physical parameter and generating a ringback signal at a frequency or amplitude related to the physical parameter when excited, the method comprising: outputting a sensor excitation signal at an initial transmission frequency; receiving the ringback signal at a ringback frequency from the sensor in response to the sensor excitation signal; determining a difference between the transmit frequency and the ringback frequency; when the difference between the transmission frequency and the ringback frequency is below a predetermined threshold, periodically repeating the outputting, receiving and determining; When the difference meets or exceeds the predetermined threshold, changing the transmit frequency of the sensor excitation signal to a new transmit frequency that matches the ringback frequency of the last received ringback signal; and The outputting at the new transmission frequency is repeated periodically, and then the receiving and determining are repeated.

20. The method according to claim 19, wherein: Outputting the sensor excitation signal at the initial transmission frequency comprises: outputting at least one sensor excitation signal frequency sweep, the at least one sensor excitation signal frequency sweep comprising a preset number of transmit pulses at a predefined frequency within an expected sensor resonant frequency range; receiving a ringback signal of each transmission pulse in the sequentially output transmission pulses; At least one initial transmission pulse is transmitted within a predetermined initial period, wherein the at least one initial transmission pulse comprises one of the following: a pulse frequency corresponding to a highest amplitude ringback signal received from said at least one frequency scan; or A plurality of said excitation signal frequency scans; receiving a plurality of test ringback signals in response to at least one initial transmit pulse transmitted during the initial period; identifying an initial ringback signal corresponding to a preferred excitation signal pulse frequency; Selecting the preferred excitation signal pulse frequency as the measurement transmission pulse frequency; and In a subsequent measurement cycle, the sensor excitation signal at the initial frequency is output as a measurement transmission pulse at the measurement transmission pulse frequency.

21. The method according to claim 19 or 20, wherein: The predetermined threshold is a numerical value.

22. The method according to claim 21, wherein: The value is 25 kHz or more.

23. The method according to claim 19 or 20, wherein: The predetermined threshold is a function of transmit performance.

24. The method according to claim 23, wherein: The emission efficiency function is 0.7 or less.

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