Systems and methods of monitoring the acoustic coupling of medical devices

Systems and methods for monitoring the acoustic coupling of medical devices is disclosed. An illustrative system for monitoring the acoustic coupling of an acoustic transducer attached to a patient's body includes a signal generator adapted to supply an electrical signal to the transducer, a circuit configured to measure at least one electrical parameter of the transducer, and a processor adapted to evaluate the degree of acoustic coupling of the transducer to the body based on the measured electrical signal. The processor can measure the frequency response of the acoustic transducer to the electrical signal, a time domain response of the acoustic transducer to the electrical signal, or a combination of both.

TECHNICAL FIELD

The present invention relates generally to medical devices employing acoustic transducers for transcutaneously transmitting and receiving acoustic signals from within the body. More specifically, the present invention pertains to systems and methods for monitoring the acoustical coupling of medical devices.

BACKGROUND

Acoustic transducers are utilized in a variety of medical applications for transmitting and receiving acoustic signals through the body. In cardiac rhythm management applications, for example, acoustic transducers can be used for telemetrically communicating with and powering implantable medical devices, and for providing therapy to a patient. An example telemetry system employing acoustic transducers is described, for example, in U.S. Pat. No. 7,024,248 to Penner et al., entitled “Systems and Methods For Communicating With Implantable Devices,” which is incorporated herein by reference in its entirety for all purposes. Acoustic transducers are frequently utilized in other medical fields such as medical imaging (e.g., ultrasonography) to permit non-invasive visualization of internal body structures or organs within the body.

In some cases, piezoelectric transducers are used to generate acoustic waves that can be transcutaneously transmitted into or received from the body. Such devices are typically placed in intimate contact with the patient's skin, and utilize the mechanical and electrical properties of piezoelectric materials to enable electrical to acoustic transduction. To facilitate the transmission and receipt of acoustic waves through the skin, an acoustic coupling medium (e.g., an acoustic gel) is sometimes used to reduce or eliminate the presence of air at the interface between the skin and the transducer, which due to its low acoustical impedance, can cause reflection and attenuation losses of the acoustic energy at the interface.

As a result of this property of acoustic interfaces, individuals wearing acoustic devices must often confirm the proper placement of the acoustic transducer on the skin, and in some cases must ensure that an adequate coupling medium is present on the surface of the skin to provide adequate impedance coupling at the transducer/skin interface. For untrained individuals unfamiliar with such devices, or in those cases where the device is to be placed on the skin for extended periods of time, the monitoring of the acoustic coupling may be difficult or even prohibitive. In certain settings such as in an ambulatory setting, for example, the acoustic transducer may become dislodged from the skin, requiring the individual or caregiver to reapply the transducer to reestablish the acoustic transmission.

SUMMARY

The present invention pertains to systems and methods for monitoring the acoustical coupling of medical devices. An illustrative system for monitoring the coupling of an acoustic transducer attached to a patient's body includes an acoustic transducer in communication with an implantable medical device, a signal generator adapted to supply an electrical signal to the acoustic transducer, a circuit configured to measure at least one electrical parameter of the acoustic transducer, and an evaluation module adapted to evaluate the degree of acoustic coupling of the transducer to the body based on the measured electrical parameter. In some embodiments, the evaluation module is configured to evaluate the degree of acoustic coupling by sensing a frequency parameter associated with the acoustic transducer. In other embodiments, the evaluation module is configured to evaluate the degree of acoustic coupling based on a time domain parameter associated with the acoustic transducer. In further embodiments, both a frequency parameter and a time domain parameter may be used to evaluate the degree of acoustic coupling.

A method of monitoring the coupling of an acoustic transducer attached to patient's body can include providing an electrical signal to the acoustic transducer, measuring at least one electrical parameter associated with the response of the acoustic transducer to the electrical signal, and evaluating the degree of acoustic coupling of the acoustic transducer to the body based on the measured electrical parameter. In some embodiments, the electrical parameter sensed may comprise a voltage and/or current parameter associated with the acoustic transducer. In one embodiment, the electrical signal provided to the acoustic transducer is swept across a range of different frequencies, and the step of evaluating the degree of acoustic coupling of the transducer to the body includes measuring an impedance parameter at multiple frequencies. In other embodiments, evaluating the acoustic coupling includes measuring a time domain parameter associated with the response of the acoustic transducer to the electrical signal. In further embodiments, evaluating the acoustic coupling includes measuring the complex impedance of an equivalence electrical circuit modeling the acoustic transducer.

DETAILED DESCRIPTION

FIG. 1is a schematic view showing an illustrative system10for transcutaneously communicating with an implantable medical device. As shown inFIG. 1, the system10includes an external device12in acoustic communication with an implantable medical device14located within a patient's body. In certain embodiments, for example, the external device12comprises an external monitor adapted to transmit and receive acoustic signals to and from an implanted pressure sensor14that senses pressure at a location within the body. An example pressure sensor adapted to sense arterial blood pressure is disclosed, for example, in U.S. Pat. No. 6,277,078, entitled “System and Method for Monitoring A Parameter Associated With The Performance Of A Heart,” which is incorporated herein by reference in its entirety for all purposes.

The implantable medical device14can be configured to sense other physiological parameters within the body. Examples of other physiological parameters that can be sensed by the implantable medical device14include, but are not limited to, blood flow, temperature, and strain. Various electrical, chemical, and/or magnetic properties may also be sensed within the body via the implantable medical device14. Although only one implantable medical device14is shown in the illustrative system10ofFIG. 1, multiple implantable medical devices14can be in acoustic communication with the external device12and/or with other devices located outside or inside the patient's body.

The external device12includes an acoustic transducer16adapted to communicate with the implanted medical device14by transmitting an acoustic wave18transcutaneously into the body. In certain embodiments, the acoustic transducer16is configured to operate as both a transmitter and receiver. In a transmission mode of operation, the acoustic transducer16is energized via an electrical signal20, which is converted by the transducer16into acoustic energy for generating an acoustic wave18that can be received by the implantable medical device14. In a receiver mode of operation, the acoustic transducer16is configured to convert acoustic waves18transmitted by the implantable medical device14into electrical energy. In an alternative embodiment, separate acoustic transducers can be provided for transmitting and receiving acoustic waves18within the body. In one such embodiment, for example, a first acoustic transducer is used for transmitting acoustic waves into the body whereas a second acoustic transducer is used for receiving acoustic waves from within the body.

The acoustic transducer16can be held against the patient's body, or alternatively, can be coupled to the patient's body via a patch, strap, belt, or other suitable attachment means22. An acoustic coupling material24may be applied between the patient's skin26and the acoustic transducer16to facilitate the transmission of acoustic energy through the skin26. Examples of suitable acoustic coupling materials24can include hydrogel, silicone, polyurethane, or the like. An illustrative patch that can be used to couple the acoustic transducer16to the patient's skin26is described, for example, in U.S. Pat. No. 7,024,248, entitled “Systems and Methods For Communicating With Implantable Devices,” which is incorporated herein by reference in its entirety for all purposes.

The external device12is coupled to a controller28that controls the operation of the external device12, including the delivery of electrical signals20to the acoustic transducer16for monitoring the transducer16. An interface30such as a graphical user interface (GUI) may be used to monitor the status of the external device12, including the frequency and amplitude of the electrical signal20provided to the acoustic transducer16as well as the degree of acoustic coupling between the transducer16and the body. The interface30can also be used to monitor other aspects of the external device12, including the monitoring of sensor and status data transmitted from the implantable medical device14. Although the controller28and interface30are shown as separate components inFIG. 1, in other embodiments the controller28and/or monitor30may be provided as a component of the external device12, or as a component of another device located outside or inside the body.

The controller28can be linked to an external system32used to monitor the data received from the external device12, the implantable medical device14, as well as other communicating devices. In some embodiments, for example, the external system32comprises a remote patient management system such as the LATITUDE® system available from Boston Scientific of Natick, Massachusetts.

During operation, the controller28can be used to control, energize, and/or otherwise communicate with the implantable medical device14. In some embodiments, for example, the controller28can be tasked to activate the implantable medical device14by transmitting one or more acoustic waves18into the body that are received by an acoustic transducer34coupled to implantable medical device14. Upon excitation from the acoustic waves18, the implantable medical device14may wake-up from an initial, sleep state and transition to an active, powered state and take one or more measurements within the body and/or perform some other designated task within the body. The data sensed by the implantable medical device14can then be transmitted to the external device12for further analysis.

In some embodiments, the acoustic transducer16comprises a piezoelectric transducer having a number of terminal leads electrically connected to the controller28. Piezoelectric materials are characterized in their ability to generate an electrical potential in response to an applied mechanical stress. Example piezoelectric materials suitable for use in piezoelectric transducers are piezo-ceramics such as lead zirconate titanate (PZT). As discussed further herein, the mechanical to electrical coupling provided by these materials enables the sensing of the mechanical environment at the transducer/body interface by sensing various parameters associated with the electrical signal20generated by the controller28.

The acoustic transducer16can be modeled as a linear, four terminal device with one portion of the device existing in the electrical realm and another portion existing in the mechanical-acoustic realm. The piezoelectric properties of the transducer material link the electrical and mechanical-acoustic portions of the device together via a linear relationship. The mechanical-acoustic portion of this relationship can be modeled using an electrical analogy where the force of the acoustic waves exerted on the transducer face represents a voltage whereas a volumetric movement of the face represents a current. The acoustic transducer16can thus be modeled as an electrical circuit having a number of resistors, capacitors, and inductors.

FIG. 2is a block diagram showing an equivalent electrical circuit36for the acoustic transducer16ofFIG. 1. As shown inFIG. 2, the electrical circuit36comprises an electrical portion38and a mechanical-acoustic portion40separated from each other via a dividing line42, which represents the electromechanical link provided by the piezoelectric material. The electrical portion38denotes the electrical dynamics of the circuit36, and includes a set of terminals44,46which represent the terminal leads of the acoustic transducer16. The clamped electrical capacitance of the acoustic transducer16, in turn, is represented in the circuit36as a capacitor C0, which for some piezoelectric transducers is formed by electrodes deposited on each side of a piezoelectric material. The capacitance C0of the acoustic transducer16is typically large based on the high dielectric coefficient of the piezoelectric material.

The mechanical-acoustic portion40of the circuit36represents the mechanical dynamics of the system. An inductor LMrepresents the effective mass of the acoustic transducer16, where the mechanical inertia of the transducer16opposes acceleration in the same way as inductance opposes a change of current. A capacitor CMrepresents the elastic force of the acoustic transducer16, where an applied voltage stores charge in the same way as an applied force effectively stores displacement. A resistor RM, in turn, represents the frictional losses associated with the acoustic transducer16. The link42between the electrical portion38and the mechanical-acoustic portion40of the circuit36is established by the piezoelectric effect of the acoustic transducer16.

The mechanical-acoustic portion40of the circuit36is closed by a load impedance RL, which represents the impedance of the medium coupling the acoustic transducer16to the patient's body. For an acoustic transducer operating in air, for example, the load impedance RLis very low, and is thus essentially a short circuit. This is due to the relative softness of air relative to water since only a small amount of force (voltage) is required to induce a velocity (current) in the air molecules surrounding the transducer surface. When the acoustic transducer16is acoustically coupled to the patient's body, however, less acoustic energy is reflected at the transducer/body interface, resulting in a greater amount of energy entering into the patient's body. This results in an increase in the load resistance RLfrom the zero load state occurring when the acoustic transducer16is operating in air.

While the load impedance RLis modeled as a pure resistance in the circuit36ofFIG. 2, the impedance RLwill normally have an inductive component as well since the vibrations on the transducer surface normally carry an additional mass of water that moves back and forth with the surface, thus adding to the inertial mass of the transducer. This additional mass may be negligible relative to the mass represented by LM, or can be considerable depending on the design of the transducer.

The electrical circuit36depicted inFIG. 2exhibits a resonance. When the load resistance RLis low (e.g., when the acoustic transducer is operating in air and exhibits little loss), the acoustic transducer16exhibits a series resonance (i.e., a maximum of the conductance) at a frequency of approximately fR≈1/√{square root over (2πLMCM)}. At this frequency fR, the motional inductance and capacitance cancel each other such that the resonance is a purely mechanical resonance. The value of the real part of the conductance at resonance is 1/RM. The width of the real part of the conductance is approximately Δf≈fR√{square root over (2πfRRMCM)}. Because of the existence of the electrical capacitance C0, there is also a second resonant condition at

fA=fR⁢C0+CMC0.
This resonance is a parallel resonance (i.e., a maximum of the resistance), and is a combined mechanical and electrical effect.

The behavior of the impedance curves for the electrical circuit36changes based on the degree of acoustic coupling provided between the acoustic transducer16and the transducer/body interface. Any dissipative portion of the radiation load of the acoustic transducer16adds to RMwhile any inductive portion adds to LM. Thus, in the presence of sufficient acoustic coupling, the resonance of the circuit36will tend to decrease in height and increase in width. The resonance frequency fRwill also tend to decrease in the presence of sufficient acoustic coupling.

WhileFIG. 2depicts an illustrative circuit36modeling the acoustic transducer16ofFIG. 1, it should be understood that other circuits may be used to model the transducer and coupling behavior. A more complex equivalence circuit could involve, for example, multiple resonances in close proximity as well as changes in circuit element values (e.g., LM) depending on frequency. Another equivalence circuit can include a matching circuit between the terminal leads and a measuring device. The matching circuit may comprise, for example, any combination of inductors, capacitors, transformers, and resistors, whether in parallel, series, or a combination of both. In use, the matching circuit can be used to match the resulting electrical impedance to that of the driving circuit so as to enhance the efficiency and/or sensitivity of the system. While the presence of a matching circuit may complicate the behavior of the impedance, the impedance is still sensitive to the presence of acoustic coupling, and therefore can be used to monitor the electrical impedance of the acoustic transducer16.

The impedance characteristics of the acoustic transducer16can be further understood in terms of its time-domain characteristics. The impedance characteristics of the acoustic transducer16can be expressed in the Laplace domain in the following form:

Z⁡(s)=V⁡(s)I⁡(s),I⁡(s)=V⁡(s)⁢Z-1⁡(s),V⁡(s)=I⁡(s)⁢Z⁡(s).
In the time-domain, this can be expressed as:
I(s)=V(t)Y(t),V(t)=I(t)Z(t)
where Z(t) is the inverse Laplace transform of Z(s), and Y(t) is the inverse Laplace transform of Z−1(s). Thesymbol in the above expression denotes a convolution. For the illustrative equivalence circuit36depicted inFIG. 2, for example, the Laplace space impedance of the acoustic transducer16can be written in the following form:

Z⁡(s)=s-1⁢s2+ω02⁢τ⁢⁢s+ω02C0⁢s2+τω02⁢C0⁢s+ω02⁡(C+C0),⁢τ=(RM+RL)⁢CM,ω02=1(LM+LL)⁢CM
To obtain I(t), for example, Y(t) must thus be evaluated. Since Z(s) is a rational function of s, the inverse Laplace transform of its reciprocal has the form of a sum of decaying exponentials. There is one decaying exponential for every root of the numerator of Z(s), with the s value corresponding to the root placed in the exponent. This can be expressed generally as:

Y⁡(t)=∑n⁢⁢An⁢exp⁡(sn⁢t)
For the illustrative equivalence circuit36ofFIG. 2, for example, there would be two roots at:

s1,2=-ω02⁢τ2±ⅈω0⁢1-ω02⁢τ22
Each root donates one exponential, which oscillates at a frequency which is close to ω0/2π, and decays with the following time constant:

ω02⁢τ2=RM+RL2⁢(LM+LL)
The roots always appear as either real roots or conjugate pairs, since the resulting time-domain kernel Z(t) or Y(t) are always real.

FIG. 3is a block diagram showing an illustrative system48for monitoring the acoustic coupling of an acoustic transducer16attached to a patient's body. As shown inFIG. 3, the system48includes a signal generator50adapted to generate a time-varying electrical signal20that can be applied across the terminal leads44,46of the acoustic transducer16. In certain embodiments, for example, the signal generator50provides a sinusoidal electrical signal20across the terminal leads44,46at a desired frequency, or across a range of frequencies. The signal20generated by the signal processor50is passed through an ammeter54, which measures the current across the terminal leads44,46. The signal20is further fed to a voltmeter56, which measures the voltage differential across the terminal leads44,46. In some embodiments, the ammeter54and voltmeter56are configured to measure both the amplitude and the phase of the measured signal.

The measured current and voltage signals are fed to respective analog-to-digital (A/D) converters58,60, which convert the measured analog signals into corresponding digital signals62,64. The digitized signals62,64are then fed to an evaluation module66such as a processor or an analog or digital decision circuit that analyzes the frequency of the electrical signal20generated by the signal generator50and the current and voltage signals62,64outputted by the A/D converters58,60. Using the frequency, current, and voltage inputs, the evaluation module66then evaluates the complex impedance of the acoustic transducer16according to the following equation:

Z⁡(f)=〈V⁡(t)⁢exp⁡(-2⁢πⅈ⁢⁢f⁢⁢t)〉T〈I⁡(t)⁢exp⁡(-2⁢πⅈ⁢⁢f⁢⁢t)〉T
In the above equation, the angular brackets denote an average over a time period sufficiently large to provide the desired frequency resolution. A similar result can be obtained using other representations in lieu of the above equation, however. In one alternative, for example, the absolute value and phase of the impedance can be expressed as follows:

Z2=〈V⁡(t)2〉T〈I⁡(t)2〉T,cos⁢⁢ϕ=〈V⁡(t)⁢I⁡(t)〉T〈V⁡(t)2〉T⁢〈I⁡(t)2〉T
which may be computationally easier and faster to perform in a microprocessor since it uses only real arithmetic.

In the embodiment ofFIG. 3, the evaluation module66is configured to evaluate the degree of acoustic coupling by sweeping the electrical signal20across a frequency range, and at each frequency or at certain frequencies, measuring the complex impedance associated with the acoustic transducer16. The sweeping of the frequency can be accomplished, for example, via a control signal68from the evaluation module66that adjusts the frequency of the electrical signal20generated by the signal generator50, either across a continuum of frequencies or at multiple, discrete frequencies. The frequency span will typically be in the vicinity of the resonance frequency of the acoustic transducer16, and as such, will typically vary based on the resonance characteristics of the transducer16.

In another embodiment, the electrical signal20comprises a wideband signal simultaneously containing a range of frequencies. For example, the electrical signal20may comprise noise produced using a random number generator, which may also be filtered to the desired frequency range using a software or hardware filter. In such embodiment, the evaluation module66constructs the frequency-dependent complex impedance curve by passing the received voltage and current signals through a filter bank, such as a Fourier Transform or fast Fourier Transform (FFT), and processes each frequency component independently to construct the impedance curve. An average over several of these random excitations may also be performed in order to improve the accuracy.

The evaluation module66is configured to analyze the resultant impedance curve to determine whether a sufficient degree of acoustic coupling is present at the transducer/body interface. In certain embodiments, for example, the evaluation module66determines the frequency at which maximal conductance occurs, and the width of the conductance peak. This can be further understood with respect to the graph70inFIG. 4, which shows the frequency (in Hz) versus conductance (in Siemens) for two acoustic coupling scenarios. The solid conductance curve72in the graph70may represent, for example, the conductance of the acoustic transducer16as a function of frequency when poor acoustic coupling exists. The dashed conductance curve74, in turn, may represent the conductance of the acoustic transducer16when sufficient acoustic coupling exists.

As can be seen by a comparison of the two conductance curves72,74, the maximal conductance G1during poor acoustic coupling tends to be greater than the maximum conductance G2when sufficient acoustic coupling is present. The width of the conductance curve74when sufficient coupling is present also tends to be greater than the width of the conductance curve72during poor acoustic coupling. A decrease in frequency from f1to f2also occurs when sufficient acoustic coupling is present.

The evaluation module66can be configured to analyze the frequency f1,f2of maximum conductance G1,G2and the width of the conductance curves72,74in order to determine whether the acoustic coupling is within a desired range. For conductance curve72, for example, the evaluation module60may analyze the frequency f1associated with the peak conductance G1along with the width of the curve72from a nominal conductance value G0to the peak conductance G1, and then compare these values against predetermined threshold peak and width values to determine whether the acoustic coupling is sufficient. As an example, for some ultrasonic transducers the frequency may decrease from a frequency f1of about 44 kHz when uncoupled to a frequency f2of about 40 kHz when strongly coupled, causing a corresponding decrease in peak conductance from a first conductance value G1of about 0.01 Siemens to a second conductance value G2of about 0.0015 Siemens. In such case, a threshold for determining the coupling may comprise, for example, 0.003 Siemens. The particular frequency shift, conductance, and threshold values will typically vary, however, depending on the resonance characteristics of the acoustic transducer16. For example, the frequency shift from f1to f2may vary from a relatively small shift for heavy acoustic transducers to a relatively large shift for lightweight, membrane type transducers.

In an alternative embodiment, the evaluation module66may use the measured complex impedance curves to extract equivalent electrical model parameters such as that described with respect to the equivalence electrical circuit36ofFIG. 2. For example, for an acoustic transducer16with the equivalent model shown inFIG. 2, the value of the load resistance RM+RLcan be determined by the reciprocal of the conductance at the resonance peak. The coupling threshold criteria can then be set as the load resistance value that exceeds a predetermined load resistance value. By way of example and not limitation, for certain ultrasonic transducers the load resistance may shift from an initial load resistance value of about 50Ω when uncoupled to a second load resistance value at or above 400Ω when coupled. In such case, a threshold for determining the coupling may comprise, for example, a load resistance exceeding about 300Ω. In a similar manner, the mechanical equivalent inductance LMcould increase beyond a threshold value during adequate acoustic coupling, and can further serve as a coupling criteria, either alone or together with other extracted parameters and/or components.

In certain embodiments, the threshold coupling values comprise preprogrammed values contained within the controller28used to control the operation of the acoustic transducer16. In other embodiments, the threshold coupling values may be fed to the controller28via the interface30, from the patient management system32, and/or from another device in communication with the controller28. Since the maximum conductance value and the width of the conductance peak are indicators of the degree of acoustic coupling, these parameters can then be analyzed to determine whether sufficient coupling exists at the interface between the acoustic transducer16and the body.

Once the evaluation module66analyzes the maximum conductance and width parameters and compares these values against threshold peak and width values, the controller28may then output a signal to the patient via the interface30informing the patient of the current status of the acoustic coupling. The notification can occur visually (e.g., via a visual indicator or message on a computer monitor), audibly (e.g., via an audible sound indicating that the coupling is sufficient or insufficient), using a haptic indicator such as a vibration, or a combination of the above. In some embodiments, the controller28may also send a signal to the patient management system32informing a caregiver of the current status of the acoustic coupling. For example, the controller28may output a signal to the patient management system32in the event poor acoustic coupling is detected for a particular period of time (e.g., for a period of more than two hours). This information can then be used by a caregiver to determine whether corrective action may be required.

FIG. 5is a block diagram showing another illustrative system76for monitoring the acoustic coupling of an acoustic transducer16attached to a patient's body. The system76is similar to the system48ofFIG. 3, but omits the voltmeter used for measuring the voltage differential across the transducer terminal leads44,46. Instead, the evaluation module66is configured to substitute the measured voltage signal with an a priori known voltage signal (e.g. ±5V) from the signal generator50. In certain embodiments, for example, the processor66may be pre-programmed with a known voltage output level from the signal generator50. In other embodiments, the signal processor50may feed a signal to the evaluation module66that can be used to ascertain the voltage output level from the signal generator50.

FIG. 6is a block diagram showing another illustrative system78for monitoring the acoustic coupling of an acoustic transducer16attached to a patient's body. In the illustrative embodiment ofFIG. 6, the signal generator50supplies a square-wave electrical signal20to the acoustic transducer16. The current and voltage signals80,82sensed by the ammeter54and voltmeter56are fed to respective low-pass filters84,86prior to being digitized, which eliminates the harmonics within the signals80,82. The resulting signals88,90sent to the evaluation module66thus contain only the fundamental sine-wave constituents within the signals80,82.

While filtering of the signals80,82can be performed using separate low-pass filters84,86, in other embodiments the filtering can be performed by the current and voltage meters54,56, or by the evaluation module66. In one embodiment, for example, low-pass filtering of the digitized current and voltage signals88,90may be performed in software using the evaluation module66.

In some embodiments, the impedance calculations are performed in hardware rather than in software. In certain embodiments, for example, the measured current and voltage signals80,82may be multiplied using an analog multiplier, and then averaged together using an integrator. Alternatively, and in other embodiments, the measured current and voltage signals80,82may be downshifted to baseband using an analog to digital mixer, which can be configured to separate each of the signals80,82into their phase and quadrature components before lowpass filtering. The resulting signals may be at a much lower frequency than the original current and voltage signals80,82, and may thus be better suited for analysis by processors with lower computational and sampling capabilities.

FIG. 7is a circuit diagram showing another illustrative system92for monitoring the acoustic coupling of an acoustic transducer attached to a patient's body. In the embodiment ofFIG. 7, the system92includes a checking circuit94configured to evaluate the degree of acoustic coupling in the time domain rather than in the frequency domain. The circuit94includes an operational amplifier96selectively coupled to the acoustic transducer16through a switch98(S1). The operational amplifier96includes a gain resistor100(R1). In some embodiments, the operational amplifier96further includes a number of feedback resistors102,104(R2,R3) forming a positive feedback loop. The feedback loop can be used to compensate for the non-zero resistance of the switch98. In those cases where the switch resistance is non-negligible, the resistors102and104can be selected such that the ratio of resistor102and resistor104(i.e., R3/R2) is equal to the ratio between the resistance of switch98and resistor100(i.e., S1/R1). This ensures that the zero volt condition is imposed directly on the terminal leads44,46of the acoustic transducer16rather than at the output of the switch98, thus compensating for any additional dissipation that would otherwise be caused by the resistance of the switch98.

In an initial state shown inFIG. 7, the switch96is initially toggled to apply an excitation voltage VEXto the acoustic transducer16, thus charging the transducer16to that voltage VEX. When the switch96is toggled to its second position at time t=0, the operational amplifier96forces the voltage on the acoustic transducer16to zero by means of the feedback resistors102,104. This, in turn, imposes a step function voltage excitation on the acoustic transducer16. The current through the acoustic transducer16then responds according to its time-domain admittance kernel Y(t) based on the following expression:

The convolution of a function with a step function returns the integral of the function evaluated at time t=0. Thus, the current is in the form of a sum of decaying exponentials as shown in the following expression:

I⁡(t)=∑n⁢⁢VEX⁢An⁢sn-1⁢exp⁡(sn⁢t)
The above current flows through the resistor100(R1), causing the output voltage VOUTof the circuit94to be:

VOUT=R1⁢I⁡(t)=∑n⁢⁢VEX⁢An⁢R1⁢sn-1⁢exp⁡(sn⁢t)
The output voltage106(VOUT) is then subsequently fed to a processor and analyzed to determine the time-domain characteristics of the transducer response.

FIGS. 8A and 8Bare graphs108,110showing the output voltage versus time for the illustrative circuit94ofFIG. 7during two different acoustic coupling scenarios. The first graph108inFIG. 8Amay represent, for example, the output voltage106aof the circuit94when the acoustic transducer16is operating in air whereas the second graph110inFIG. 8Bmay represent the output voltage106bof the circuit94when the transducer16is operating in water. As can be seen by a comparison of the two graphs108,110, the output voltage106awhen the acoustic transducer16is operating in air decays at a slower rate than the output voltage106bduring in-water operation. In addition, for in-air operation, the output voltage106aexhibits two different frequencies, which can be observed inFIG. 8Aas the beat observed in the amplitude. In comparison, and as shown inFIG. 8B, the acoustic transducer exhibits a less pronounced beat frequency during in-water operation than during in-air operation.

In use, the circuit94can be used to evaluate the degree of acoustic coupling between the acoustic transducer16and the patient's body by measuring the decay time of the output voltage106. In some embodiments, for example, the degree of acoustic coupling can be determined by estimating the amplitude envelope of the output voltage106, and then calculating a decay rate associated with the envelope. An example decay rate for an ultrasonic transducer sufficiently coupled to the body, for instance, may be less than about 1 ms, although other decay rates are possible depending on the type of transducer employed. In some embodiments, the amplitude envelope detection can be performed by analog circuitry such as an RMS detector or a diode followed by a low-pass filter. Alternatively, and in other embodiments, the output voltage106can be sampled directly into a processor, and the decay rate calculation performed in software.

In certain embodiments, the output voltage106may be sampled into a processor adapted to run an algorithm that directly evaluates the frequency and decay of each constituent decaying exponent separately. In such case, the output voltage106may comprise the sum of several exponentials, where only the decay rate of some of the exponentials depends on the acoustic coupling. This may occur, for example, when the acoustic transducer16also includes a matching circuit that imposes a strong electrical resonance that is insensitive to the acoustic coupling, but which is sensitive to the underlying mechanical resonance of the circuit. In this situation, an analysis of the decay time for the relevant exponential or exponentials that exhibit sensitivity to the acoustic coupling rather than determining the decay time of an amplitude envelope may be utilized.

An example algorithm that can be used to decompose a signal into a sum of decaying exponentials is the Prony algorithm, which uses raw data to generate a polynomial whose roots are related to the frequency and decay rate of the exponentials. An algorithm can be used to root the polynomial to find the position of the amplitude using any variety of known rooting methods. The degree of acoustic coupling can then be determined by examining those roots that are affected by the coupling. Typically, the presence of the acoustic coupling will cause the real portion of these roots, which signifies their decay rate, to increase. The presence of sufficient acoustic coupling can then be determined when the real portion of the roots crosses a predetermined threshold. In some cases, the imaginary part of the root position will change as well due to the additional mass that results when the acoustic transducer is sufficiently coupled to the patient's body, which causes the mechanical resonance frequency to decrease. Thus, in some embodiments, the change in imaginary root position is also used as an indication of the degree of acoustic coupling that is present.

In another embodiment, the output voltage106from the circuit94may be sampled into a processor adapted to run an algorithm that evaluates the degree of acoustic coupling without requiring an explicit rooting of the polynomial in a complex plane. An example means to accomplish this utilizes the Caucy Argument Principle, which states that for a given meromorphic function ƒ(z) in the complex plane:

12⁢π⁢Im⁢∮C⁢f′⁡(z)f⁡(z)⁢ⅆz=Nzeros-Npoles
In the above expression, C is the contour surrounding the region of interest in the complex plane, and Nzerosand Npolesare the number of zeros and poles, respectively, enclosed by the contour C. The f′(z) term in the above expression denotes the derivative with respect to z, “f(z)” is set to be the Prony polynomial, and the contour C is the contour enclosing the region in the complex space where a coupling-sensitive zero is expected to be when the transducer is operating in air. By performing the contour integration using these parameters, a result of approximately 1 would be returned if a zero lies within the contour region, thus indicating that the acoustic transducer16is not adequately coupled. Conversely, the relevant zero will wander to a different location and the integral will return a result of approximately zero when the acoustic transducer16is sufficiently coupled. Based on the result from this integration, a determination of the degree of acoustic coupling can thus be made.