Patent Application
20100016763
1. Field of the Invention
The present invention is generally related to fluid property sensing.
2. Description of the Related Art
Conventional ultrasound methods are known to determine fluid property status of biological fluids of a biological creature contained outside of the biological creature. For instance, ultrasonic standing waves are used to force the red blood cells to accumulate at pressure minima. By optically measuring width of resultant bands, red blood cell concentration, related to hematocrit status can be measured directly. The effects of hematocrit, shear rate, and turbulence in blood on ultrasonic Doppler spectrum, scattering of ultrasound by red blood cells, and effects of hematocrit on attenuation of ultrasonic signals transmitted through a vial containing blood have been studied.
Pulse-echo technique to measure hematocrit by measuring the attenuation of the blood contained outside of the biological creature as a function of range in front of the transducer and use of ultrasonic transducers that are in fluid contact on a surface of a liquid to sense the viscosity of the liquid have also been studied. The travel time (delay) of an acoustic signal in an external liquid sample have been used to measure temperature of the sample, while the attenuation is used to measure the viscosity of the sample.
Fluid property status of an intraluminal fluid inside a lumen of a biological creature can be assessed to a certain degree through conventional methods using ultrasonic transducers positioned outside of the biological creature. Although these conventional approaches for determining fluid property status of externally and intraluminally contained fluids using externally positioned transducers are useful, new approaches would be desirable.
As discussed herein, implementations of an intraluminal fluid property status sensing system and method locate an acoustic transducer within a lumen of a biological creature to transmit ultrasound through the intraluminal fluid to be reflected or otherwise affected by the fluid with subsequent reception by the same transducer. Reflection or interaction of the ultrasound with an intraluminal fluid depends upon one or more properties of the intraluminal fluid so can be used to determine status of such properties.
In different implementations, the fluid status system may be either permanently or temporarily implanted or inserted in a lumen of living beings for purposes of monitoring the hematocrit and/or other properties of the fluid. One specific application is in the measurement of fluid status within the blood (e.g., to measure hematocrit) and other physiological parameters from within a blood vessel or within the heart itself.
One implementation includes an external electronic component communicatively linked through a wireless connection via an RF magnetic field to an internal sensing component. With this implementation, the internal sensing component could be inserted or implanted within the heart or elsewhere in the vasculature (such as in a dialysis shunt). In another implementation, the external electronic component is wirelessly coupled via an acoustic link to the internal sensing component.
Other implementations include the internal sensing component on a catheter or cannula, with lead wires extending up the catheter or cannula and out of the body to external instrumentation. Another implementation includes the internal sensing component on a pacing lead, with wires running up the pacing lead, or connection tether of wires to an implantable defibrillator, pacemaker, monitoring device or combination device. The internal sensing component can either store in memory, algorithmically process and store in memory or telemeter the data directly out of the body, using RF or acoustic transmission.
The fluid status system permits continuous, real-time interrogation of fluid properties using ultrasonic transducers. In some applications, the internal sensing component with its sensors can be placed by insertion, implantation, otherwise in intravascular locations in animals or in the human body for the purpose of measuring fluid attenuation, temperature, and other physiologic parameters. Implementations can use a minimum number of components that allow the internal sensing component to be reduced to a size suitable for insertion as a component of or inside of a typical intravascular or intracardiac catheter diameter.
The fluid status system can be used without need to withdraw fluid from the patient. Fluid status can be sampled in a small region at a desired location within a subject. Samples of fluid status can be continuously taken and recorded to provide trending data. Also, the multiple parameters (such as fluid viscosity and temperature) can be sensed simultaneously with the same sensor.
Acoustic sensors can be designed to respond to changes in fluid properties. Several types of sensor arrangements can be used, depending upon the parameter being sensed and the type of acoustic wave that is being generated and detected. The fluid status system has further advantages compared with systems that have externally located sensors. Measurement accuracy issues due to the attenuation of the intervening tissue (between the transducer and the blood) are eliminated. Alignment issues between the transducer beam and the blood vessel are eliminated, because the sensor is in the blood. Fixed positioning inside the bloodstream should provide much more stable and repeatable data over a sequence of readings. The device is not hand-held, so user issues can be reduced or eliminated thereby enabling automated data acquisition. Furthermore, no acoustic coupling gel is needed.
A first version 100 of the intraluminal fluid status sensing system is shown in
In some implementations, the original ultrasonic signal 106a can also be wirelessly transmitted to the external component 104 as an original external signal 108a (e.g. as a radio or acoustic signal) to be used for timing information by the external component 104. Outputting the original ultrasonic signal 106a to the external component 104 as the original external signal 108a is useful in situations where the sensing component 102 is “free running”, i.e., generating pulses at some rate and at some instances in time that are not controlled by the external component 104. The external component 104 can receive and “lock onto” the original external signal 108a so that the external component 104 samples the echo signals after a delay, at the appropriate points in time. In a situation where the external component 104 is wired to the sensing component 102 (such as a catheter) the external component could control the timing of the pulses so that the original external signal is unnecessary.
The sensing component 102 sends on the received reflected ultrasonic signal 106b as a reflected external signal 108b. For hematocrit measurements in particular, orientation of the original ultrasonic signal 106a with respect to direction of the fluid flow 14 can be varied to a relatively large degree.
The sensing component 102 is electrically excited to send the original ultrasonic signal 106a into the fluid as a longitudinal acoustic wave. Liquids and gases generally support longitudinal acoustic waves. Solids support longitudinal and transverse (shear) wave types, and solids with interfaces supporting additional “surface” wave types. In solids with two surfaces (such as thin plates), additional wave types can be supported. Additionally, solids immersed in fluids can support evanescent wave fields that propagate along the boundary, with the wave's travel velocity and/or attenuation being altered by the fluid characteristics.
The sensing component 102 then receives the reflected ultrasonic signal 106b as echoes from the flowing fluid 14 to convert back into an electrical signal. The time required for the original ultrasonic signal 106a to travel from the sensing component 102 to an ensemble of the scatterers 12 as scatterers (i.e., red blood cells and other blood constituents) and back as the reflected ultrasonic signal 106b to the sensing component is used to “range gate” the original ultrasonic signal and the resultant reflected ultrasonic signal.
A series of range gates 109 (also known as range cells) are depicted for attenuation measurements involved with determination of hematocrit and other fluid status. The external component 104 processes signals received from the sensing component 102 in a delayed fashion as is known in the art provide the range gating. Delays of fixed intervals are built into signal processing by the external component 104 so that various ranges of the reflected ultrasonic signal 106b distanced from one another are accounted for in fluid status determination by the external component.
Fluid status measurement, such as involving attenuation measurement, results from the measure of two (or more) echo amplitudes of the reflected ultrasonic signal 106b at different distance ranges. If attenuation of the original ultrasonic signal 106a and the reflected ultrasonic signal 106b were substantially absent, then amplitude of that portion of the reflected ultrasonic signal 106b received by the transducer 118 would change as a function of range distance of reflection of the original acoustic signal 106a occurring from the transducer, due to beam-spreading (diffraction) effects as a function of the range distance. Factors involved include aperture size, shape, and ultrasonic frequency. This effect related solely to no attenuation by the fluid 14 would be characterized for a given design, and used by the external component 104 to compensate raw measurements.
Attenuation is typically measured in dB/cm (or dB/mm) at a given frequency. Sometimes it is specified as the attenuation slope (e.g., dB/cm-MHz). If the echo amplitude is measured at two successive ranges, as A1 and A2, spaced apart by a distance Z in cm, then the “raw” attenuation is:
20*log(A2/A1)/Z in units of dB/cm.
This attenuation value (which is negative, as A2 is always smaller than A1), would then be adjusted by adding the diffraction compensation value, in order to arrive at the attenuation in the fluid itself.
The attenuation value, in dB/cm, increases with ultrasonic signal frequency. This increase is nearly linear over a narrow frequency range, and thus the attenuation slope (dB/cm-MHz) is sometimes used. This may be useful in hematocrit measurements, and if needed, it could be accomplished by stepping the oscillator frequency through several points within the passband of the transducers.
Attenuation is a combination of absorption and scattering. In blood and tissue—generally ˜90% absorption and 10% scattering. If the scattering were high, then one would not be able to make useful ultrasonic images.
Averaging would be used to improve the signal-to-noise ratio of the amplitude measurement for each range. On successive pulses, the echoes from the first range would be averaged together, and the echoes from the second range would be averaged together. The values from different ranges would not be averaged with each other.
The range gates 109 also allow for measurement of Doppler signals at multiple locations across the vessel, to obtain a flow profile, if desired. For Doppler measurements, the original ultrasonic signal 106a are aligned with direction of the fluid flow 14 improves measurement sensitivity and accuracy.
As is known, signal attenuation per unit length is used to compute fluid attenuation, which can be used to compute fluid properties such as viscosity. Viscosity can be directly related to other fluid properties, such as the concentration of cells within the fluid. If the fluid is blood, the concentration of red blood cells (termed hematocrit) can thus be measured. If the fluid is urine (instead of blood), the concentration of cells or electrolytes can be determined from the attenuation.
An exemplary version of the sensing component 102 is shown in
For 10 MHz operation, a transmit burst for the electrical signal from the oscillator 110 and subsequent original acoustic signal 106a could be 10 or 20 cycles long, so the switch 114 would be closed for 1 or 2 microseconds. The transducer 118 sends the received reflected ultrasonic signal 106b to the amplifier 120 and on to the transmitter 124 to transmit as the reflected external signal 108b when the control 112 switches the switch 122 appropriately. When the control 112 appropriately switches the switch 122, the electrical signal from the oscillator 110 is also sent to the transmitter 124 to convert and transmit as the original external signal 108a to the external component 104.
An exemplary version of the external component 104 is shown in
The signal processing of the microprocessor 104 includes sampling the echo waveform amplitude as a function of range in front of the transducer 118. The transmit burst of the electrical signal from the oscillator 110 to generate the original acoustic signal 106a is used to produce a trigger signal, so that the analog-digital converter 136 samples with appropriate timing with respect to the transducer 118. Consequently, one or more analog-digital samples can be included within each of the range gates 109.
As echo amplitude fluctuates over time due to instantaneous variation in arrangement of scatterers within the original acoustic signal 106a, the values within each of the range gates 109 are averaged over a series of successive pulse-echo events, in order to obtain the amplitude data needed to calculate the attenuation of the signal in the fluid. Once the attenuation is determined by the microprocessor 140, the fluid property status 142 (such as hematocrit value for blood or cell/electrolyte concentration for urine) can be outputted.
In some implementations, the microprocessor 140 process echo amplitude of pulse-echo events at two or more ranges to derive attenuation values of the original acoustic signal 106a. When the intraluminal fluid 14 is blood the attenuation value can be processed by the microprocessor 140 to derive hematocrit of the blood. The sensing component 102 can be positioned within the lumen 10 to be in direct contact with the intraluminal fluid 14. The sensing component 102 can be implanted or inserted into a living being either through a natural or artificial surgically created hole in the living body.
A second version 200 of the fluid status system is depicted in
From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.
This application claims the benefit of U.S. Provisional Application No. 61/081,674, filed Jul. 17, 2008, and incorporates by reference the U.S. Provisional Application herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61081674 | Jul 2008 | US |
