Ulrrasound rn Med. & Bd. Printed in the U.S.A.
Vol. 17, No. 5, pp. 497-SOO,
0301-5629/91 $3.00 + .CO 0 1991 Pergamon Press plc
1991
aoriginal Contribution A NEW ULTRASONIC METHOD FOR FLUID PROPERTY MEASUREMENTS STEPHANO.DYMLING,
HANSW.PERSSON,THOMAS ~~~IOELLLINDSTR~M
G.HERTZ
Department of Electrical Measurements, Lund Institute of Technology, P.O. Box 118, S-221 00, Lund, Sweden Abstract-A new ultrasonic method for fluid property measurements is described. The method uses ultrasound to generate acoustic streaming in a fluid. The resulting flow velocity will vary due to several viscous parameters of the fluid. If there are acoustic scatterers in the fluid, it will be possible to monitor the resulting flow velocity with au ultrasound Doppler device. The same acoustic energy is then utilized both to generate the acoustic streaming in the medium and to measure the resulting flow. The method seems to be very well suited for monitoring biological processes. Of special interest, for measurements on contaminated blood or on fluids in a fermentation process, is the method’s ability to investigate liquids in a completely closed vessel. Key Words: Acoustic streaming, Fluid property, Ultrasonic sensor, Viscosity.
INTRODUCTION
absolute measuring accuracy, a possible solution can be to find a sensor which is relatively specific and gives a consistent output from time to time for a given process condition. The present article describes such a new ultrasonic sensor for the rapid and nondisturbing measurement of some viscous parameters of enclosed biological fluids.
Electronic process monitoring and control is of vital importance to many areas of modern society, ranging from science to industry. The rapid development of semiconductor electronics over recent years has produced a wide range of powerful, fast, cheap, and thereby widely available personal computers. These can, in many applications, directly replace the minicomputer systems of yesterday, and it can therefore be premised that the future use and development of process monitoring and control systems will be more limited due to lack of appropriate measurement sensors than to shortage in available computer power. One can exemplify with the need to measure and/or characterize a number of process parameters in liquids and gases, i.e., fluid media. While it is relatively easy to select suitable sensors to measure certain physical parameters like pressure and temperature, it seems to be much more difficult to find sensors for biological liquids with complex characteristics, like flowability, particle content, clot formation, stickiness, etc. It can be even more difficult to find a suitable sensor to monitor certain of the many biotechnical processes, like fermentation, which takes place in a closed process vessel. In some instances, it might be difficult even to define the characteristics of the process we want to monitor. Instead of sensors with high Address all correspondence
METHODS Two well-known physical effects, “acoustic streaming” and the “Doppler” effect, are prerequisites for the new ultrasound sensor. Acoustic streaming is the ability of acoustic energy to induce time-independent flow in a fluid. The effect is illustrated in Fig. 1. A signal generator drives the ultrasound transducer to generate acoustic energy which is transmitted into a fluid medium. The acoustic energy generates a flow in the medium, i.e., acoustic streaming. The subject has been thoroughly treated by Nyborg (1965) and the maximum streaming velocity, v,,,, is shown to be
(1) where (Yis the absorption coefficient of the fluid, I the transmitted sound intensity, Ythe radius of the sound beam, c the speed of sound in the fluid, 1 the viscosity of the fluid, and Y is a geometric constant.
to Hans W. Persson. 497
498
Ultrasound in Medicine and Biology
Fig. 1. Acoustic streaming caused by a sound wave
propagating in a fluid medium. The velocity of a fluid flow can be measured using the Doppler technique, i.e., by measuring the change in frequency of sound that is reflected by a moving object. This change in frequency is called the Doppler shift (Atkinson and Woodcock 1982) and its magnitude, Af; is given by
kvf Af=c_,
(2)
where k is a constant that depends on the measurement setup, v the velocity ofthe reflecting object, fthe frequency of the transmitted sound and c is the velocity of sound in the fluid (Atkinson and Woodcock 1982). The general idea of the new sensor is to use ultrasound to generate acoustic streaming in a fluid and then to measure the velocity of the flow. The resulting flow velocity will vary due to several parameters of the fluid and of the sound beam, as indicated in Eqn. (1). If there are acoustic scatterers in the fluid, it will be possible to monitor the resulting flow velocity with an ultrasound Doppler device. The same acoustic energy is then utilized both to generate the acoustic streaming in the medium and to measure the resulting flow. It should be possible to use the velocity of the scatterers in the medium as an indicator of viscous properties of the medium, if all other parameters of importance are known to be constant. A possible measurement system setup is shown in Fig. 2. A signal generator with properly selected output power and frequency is connected to an ultrasound Doppler transmitter transducer. The continuous-wave (CW) ultrasound transducer consists of two separate transducer elements, one for transmission and one for reception of ultrasound waves. The generated acoustic energy is propagated into the medium through the wall of the vessel and acoustic streaming is induced. Part of the acoustic energy will be reflected by the moving particles in the suspension and the reflected ultrasound energy is frequency-shifted due to the Doppler effect. The reflected ultrasound signal is detected by the receiver transducer and then amplified. The frequency-shifted part of the received signal, the
Volume 17, Number 5, 1991
Doppler shift, is obtained by demodulating the received signal, which can be done by multiplication with the signal from the signal generator in a mixer and subsequent low-pass filtering. The exact magnitude of the Doppler shift can then be estimated by means of a frequency analyzer. A condition of great practical interest is the presence of particles in the medium. A “particle,” to be understood, is a small domain with an acoustic impedance differing from the acoustic impedance of the surrounding medium, i.e., particles, gas bubbles or drops of another fluid in a liquid, or drops of liquid in a gas. The moving scatterers in the medium can be accelerated to different velocities in the medium, e.g., due to the flow profile of the streaming or due to differences in size or acoustical impedance among the scatterers. These particles will also be affected by a radiation force, resulting in a motion relative to the ultrasound transducer dependent on the flow of the medium, as well as the individual movements of the different particles relative to the surrounding medium. The intensity of the Doppler-shifted signal is a direct measure of the part of the applied acoustic energy that has been reflected from the moving scatterers in the medium. This intensity depends on parameters like the equivalent acoustical cross-section of the scatterers, their size and number. If the acoustical cross-section is known, e.g., from a reference fluid, the intensity of the Doppler signal can be used as a
’
Fluid medium
Mixer FFT analyzer
Fig. 2. Measurement system setup for the ultrasonic fluid property sensor. A signal generator drives an ultrasound transmitting transducer to generate acoustic streaming in the medium. Sound scattered by particles in the moving medium is detected by the receiving transducer. After amplification and demodulation, the signal is analyzed and presented on a frequency analyzer of Fast-Fourier Transform type (FFT).
499
Fluid property measurements 0 S. 0. DYMLING et a/.
measure of the number of moving scatterers in the medium. The resulting Doppler shift will then be made up of many separate frequency components of different amplitudes, where each amplitude is dependent on the number of scatterers and their acoustical crosssections. It can then be of interest to know the frequency content of the Doppler signal as well as their respective amplitudes. This can easily be achieved if the frequency analysis is performed in real-time by a spectrum analyzer like a Fast-Fourier Transform (FFT) analyzer. Standardization of the Dopplershifted signal by division, with the amplitude of the nonshifted, reflected ultrasound signal, will further increase the possible reproducibility of the measurements. INSTRUMENTATION A single glance could indicate sweeping similarities between the conventional CW ultrasound Doppler instrument designed for noninvasive blood flow measurement in clinical medicine and the new fluid property ultrasound sensor. In fact, could the standard CW ultrasound Doppler be used as sensor? This is generally not the case. To avoid artifacts from other moving structures in the body, e.g., breathing movements, commercially available ultrasound Doppler instruments are equipped with steep highpass filters which efficiently remove all Doppler signals corresponding to blood flow velocities lower than ~5 cm/s. The instrumentation setup for the experimental verification of the ultrasonic fluid property sensor is schematically shown in Fig. 2. The fluid was contained in an open vessel with an acoustically transpar-
I
---
\
EXP
0 Hz AVERAGE
BW: (a)
250 3.63
ent wall. A crystal-controlled signal generator (Philips PM 5 193) is used to generate a highly stable sine-wave of a selected frequency. The ultrasound transducers are locally manufactured from lead zincronate titanate (PZ 27, Ferroperm A/S, Denmark). After amplification, the received ultrasound signal is demodulated in a RF-mixer (Mini-Circuits ZFM-3) by mixing the signal with a reference signal from the signal generator. The output signal from the mixer is low-pass filtered to remove undesired mixing products, whereafter subsequent frequency analysis is performed in a digital spectrum analyzer of FFT-type (Hewlett-Packard 3582A). Examples ofthe resulting frequency spectra are shown in Fig. 3(a) and (b). To demonstrate its method of application, we have used the new ultrasonic property sensor to a common complex biological fluid: milk. Milk is chosen as being a liquid well adapted to this measurement method and it has a relatively predictable process of turning sour. Fresh, it is easy flowing, but after a few hours in room temperature, it will change its viscous state. Finally, it contains a large number of suitable ultrasound scatterers [fat emulsified in the form of fat drops, up to 0.003 mm in diameter in homogenized milk (Walstra and Jennes 1984)] to facilitate ultrasound Doppler signals from the moving liquid. The frequency of the transmitted sound was 10 MHz, and the amplitude of the signal generator used to drive the ultrasound transducer was adjusted to generate an acoustic intensity of about 100 mW/cm2 in the container, resulting in a maximum acoustic streaming velocity in the order of 1 cm/s in fresh milk. Using these parameters, the Doppler equation [eqn. (2)] will predict a Doppler shift of 133 Hz, i.e., roughly 10e5 of the transmitted ultrasound frequency.
Hz
/ Hz
\ EXP
0 Hz AVERAGE
BW:
250 Hz / 3.83 Hz
04
Fig. 3. Doppler spectra obtained from milk in an open container. (a) fresh milk; (b) non-fresh, “sour milk.”
Ultrasound in Medicine and Biology
0
2
4
Time
ii
8
IO
[hours]
Fig. 4. Milk in an open container at room temperature. In order to monitor the freshness of the milk, the relative maximum velocity of the acoustic streaming is presented as a function of time. Note the drastic change in viscous state after about 8 h.
RESULTS Figure 3(a) shows a demodulated Doppler frequency spectrum from a measurement on a container filled with fresh milk. After 10 h in room temperature, a coarse inspection shows distinct changes in the appearance of the milk: the milk has turned sour. The corresponding Doppler frequency spectrum is showed in Fig. 3(b). It is immediately evident that the acoustic streaming is greatly reduced in sour milk. To get an even better understanding of the souring process in milk, a computer was used to monitor the relative maximum acoustic streaming velocity over time. The results are shown in Fig. 4. As can be seen in this experiment, a drastic change in the viscous state of the milk took place after about 8 h in room temperature.
Volume 17, Number 5, 1991
The exact outcome of the measurements depends of a number of parameters: ultrasound frequency and intensity, dimension and shape of the ultrasound transducer, viscosity and acoustic attenuation of the current fluid, particle content and so on. However, for a specific measurement situation most of these specifications can be considered to be constant, and a change in the process will give rise to a fairly specific and reproducible output signal. The new ultrasound fluid property sensor can, therefore, be used to survey many different liquids: blood, fermentation process fluids, and industrial process fluids are only a few possible examples. If the measurement container is provided with an acoustically transparent wall, it will even be possible to investigate the liquids in a completely enclosed vessel. The new sensor is highly sensitive to liquids with a viscosity similar to water. Therefore, it will be an excellent complement to existing cylinder viscosimeters which are optimal at higher viscosities. A recent study of acoustic streaming, induced by a conventional clinical ultrasound scanner (Star&t et al. 1989), indicates that streaming can be induced in any fluid path of reasonable length in viva This might suggest a future possibility of noninvasive measurement of fluid property inside the human body. REFERENCES Atkinson, P.; Woodcock, J. P. Doppler ultrasound and its use in clinical measurements. London: Academic Press; 1982:26. Nyborg, W. L. Acoustic streaming. In: Mason, W. P., ed. Physical acoustics (vol. IIB). New York: Academic Press; I965:265-33 1. Starr&, H. C.; Duck, F. A.; Humphrey, V. F. An experimental investigation of streaming in pulsed diagnostic ultrasound beams. Ultrasound Med. Biol. 15:363-373: 1989. Walstra, P.; Jennes, R. Dairy chemistry and physics. New York: John Wiley & Sons; 1984:256.
Vol. 17, No. 5, pp. 497-SOO,
0301-5629/91 $3.00 + .CO 0 1991 Pergamon Press plc
1991
aoriginal Contribution A NEW ULTRASONIC METHOD FOR FLUID PROPERTY MEASUREMENTS STEPHANO.DYMLING,
HANSW.PERSSON,THOMAS ~~~IOELLLINDSTR~M
G.HERTZ
Department of Electrical Measurements, Lund Institute of Technology, P.O. Box 118, S-221 00, Lund, Sweden Abstract-A new ultrasonic method for fluid property measurements is described. The method uses ultrasound to generate acoustic streaming in a fluid. The resulting flow velocity will vary due to several viscous parameters of the fluid. If there are acoustic scatterers in the fluid, it will be possible to monitor the resulting flow velocity with au ultrasound Doppler device. The same acoustic energy is then utilized both to generate the acoustic streaming in the medium and to measure the resulting flow. The method seems to be very well suited for monitoring biological processes. Of special interest, for measurements on contaminated blood or on fluids in a fermentation process, is the method’s ability to investigate liquids in a completely closed vessel. Key Words: Acoustic streaming, Fluid property, Ultrasonic sensor, Viscosity.
INTRODUCTION
absolute measuring accuracy, a possible solution can be to find a sensor which is relatively specific and gives a consistent output from time to time for a given process condition. The present article describes such a new ultrasonic sensor for the rapid and nondisturbing measurement of some viscous parameters of enclosed biological fluids.
Electronic process monitoring and control is of vital importance to many areas of modern society, ranging from science to industry. The rapid development of semiconductor electronics over recent years has produced a wide range of powerful, fast, cheap, and thereby widely available personal computers. These can, in many applications, directly replace the minicomputer systems of yesterday, and it can therefore be premised that the future use and development of process monitoring and control systems will be more limited due to lack of appropriate measurement sensors than to shortage in available computer power. One can exemplify with the need to measure and/or characterize a number of process parameters in liquids and gases, i.e., fluid media. While it is relatively easy to select suitable sensors to measure certain physical parameters like pressure and temperature, it seems to be much more difficult to find sensors for biological liquids with complex characteristics, like flowability, particle content, clot formation, stickiness, etc. It can be even more difficult to find a suitable sensor to monitor certain of the many biotechnical processes, like fermentation, which takes place in a closed process vessel. In some instances, it might be difficult even to define the characteristics of the process we want to monitor. Instead of sensors with high Address all correspondence
METHODS Two well-known physical effects, “acoustic streaming” and the “Doppler” effect, are prerequisites for the new ultrasound sensor. Acoustic streaming is the ability of acoustic energy to induce time-independent flow in a fluid. The effect is illustrated in Fig. 1. A signal generator drives the ultrasound transducer to generate acoustic energy which is transmitted into a fluid medium. The acoustic energy generates a flow in the medium, i.e., acoustic streaming. The subject has been thoroughly treated by Nyborg (1965) and the maximum streaming velocity, v,,,, is shown to be
(1) where (Yis the absorption coefficient of the fluid, I the transmitted sound intensity, Ythe radius of the sound beam, c the speed of sound in the fluid, 1 the viscosity of the fluid, and Y is a geometric constant.
to Hans W. Persson. 497
498
Ultrasound in Medicine and Biology
Fig. 1. Acoustic streaming caused by a sound wave
propagating in a fluid medium. The velocity of a fluid flow can be measured using the Doppler technique, i.e., by measuring the change in frequency of sound that is reflected by a moving object. This change in frequency is called the Doppler shift (Atkinson and Woodcock 1982) and its magnitude, Af; is given by
kvf Af=c_,
(2)
where k is a constant that depends on the measurement setup, v the velocity ofthe reflecting object, fthe frequency of the transmitted sound and c is the velocity of sound in the fluid (Atkinson and Woodcock 1982). The general idea of the new sensor is to use ultrasound to generate acoustic streaming in a fluid and then to measure the velocity of the flow. The resulting flow velocity will vary due to several parameters of the fluid and of the sound beam, as indicated in Eqn. (1). If there are acoustic scatterers in the fluid, it will be possible to monitor the resulting flow velocity with an ultrasound Doppler device. The same acoustic energy is then utilized both to generate the acoustic streaming in the medium and to measure the resulting flow. It should be possible to use the velocity of the scatterers in the medium as an indicator of viscous properties of the medium, if all other parameters of importance are known to be constant. A possible measurement system setup is shown in Fig. 2. A signal generator with properly selected output power and frequency is connected to an ultrasound Doppler transmitter transducer. The continuous-wave (CW) ultrasound transducer consists of two separate transducer elements, one for transmission and one for reception of ultrasound waves. The generated acoustic energy is propagated into the medium through the wall of the vessel and acoustic streaming is induced. Part of the acoustic energy will be reflected by the moving particles in the suspension and the reflected ultrasound energy is frequency-shifted due to the Doppler effect. The reflected ultrasound signal is detected by the receiver transducer and then amplified. The frequency-shifted part of the received signal, the
Volume 17, Number 5, 1991
Doppler shift, is obtained by demodulating the received signal, which can be done by multiplication with the signal from the signal generator in a mixer and subsequent low-pass filtering. The exact magnitude of the Doppler shift can then be estimated by means of a frequency analyzer. A condition of great practical interest is the presence of particles in the medium. A “particle,” to be understood, is a small domain with an acoustic impedance differing from the acoustic impedance of the surrounding medium, i.e., particles, gas bubbles or drops of another fluid in a liquid, or drops of liquid in a gas. The moving scatterers in the medium can be accelerated to different velocities in the medium, e.g., due to the flow profile of the streaming or due to differences in size or acoustical impedance among the scatterers. These particles will also be affected by a radiation force, resulting in a motion relative to the ultrasound transducer dependent on the flow of the medium, as well as the individual movements of the different particles relative to the surrounding medium. The intensity of the Doppler-shifted signal is a direct measure of the part of the applied acoustic energy that has been reflected from the moving scatterers in the medium. This intensity depends on parameters like the equivalent acoustical cross-section of the scatterers, their size and number. If the acoustical cross-section is known, e.g., from a reference fluid, the intensity of the Doppler signal can be used as a
’
Fluid medium
Mixer FFT analyzer
Fig. 2. Measurement system setup for the ultrasonic fluid property sensor. A signal generator drives an ultrasound transmitting transducer to generate acoustic streaming in the medium. Sound scattered by particles in the moving medium is detected by the receiving transducer. After amplification and demodulation, the signal is analyzed and presented on a frequency analyzer of Fast-Fourier Transform type (FFT).
499
Fluid property measurements 0 S. 0. DYMLING et a/.
measure of the number of moving scatterers in the medium. The resulting Doppler shift will then be made up of many separate frequency components of different amplitudes, where each amplitude is dependent on the number of scatterers and their acoustical crosssections. It can then be of interest to know the frequency content of the Doppler signal as well as their respective amplitudes. This can easily be achieved if the frequency analysis is performed in real-time by a spectrum analyzer like a Fast-Fourier Transform (FFT) analyzer. Standardization of the Dopplershifted signal by division, with the amplitude of the nonshifted, reflected ultrasound signal, will further increase the possible reproducibility of the measurements. INSTRUMENTATION A single glance could indicate sweeping similarities between the conventional CW ultrasound Doppler instrument designed for noninvasive blood flow measurement in clinical medicine and the new fluid property ultrasound sensor. In fact, could the standard CW ultrasound Doppler be used as sensor? This is generally not the case. To avoid artifacts from other moving structures in the body, e.g., breathing movements, commercially available ultrasound Doppler instruments are equipped with steep highpass filters which efficiently remove all Doppler signals corresponding to blood flow velocities lower than ~5 cm/s. The instrumentation setup for the experimental verification of the ultrasonic fluid property sensor is schematically shown in Fig. 2. The fluid was contained in an open vessel with an acoustically transpar-
I
---
\
EXP
0 Hz AVERAGE
BW: (a)
250 3.63
ent wall. A crystal-controlled signal generator (Philips PM 5 193) is used to generate a highly stable sine-wave of a selected frequency. The ultrasound transducers are locally manufactured from lead zincronate titanate (PZ 27, Ferroperm A/S, Denmark). After amplification, the received ultrasound signal is demodulated in a RF-mixer (Mini-Circuits ZFM-3) by mixing the signal with a reference signal from the signal generator. The output signal from the mixer is low-pass filtered to remove undesired mixing products, whereafter subsequent frequency analysis is performed in a digital spectrum analyzer of FFT-type (Hewlett-Packard 3582A). Examples ofthe resulting frequency spectra are shown in Fig. 3(a) and (b). To demonstrate its method of application, we have used the new ultrasonic property sensor to a common complex biological fluid: milk. Milk is chosen as being a liquid well adapted to this measurement method and it has a relatively predictable process of turning sour. Fresh, it is easy flowing, but after a few hours in room temperature, it will change its viscous state. Finally, it contains a large number of suitable ultrasound scatterers [fat emulsified in the form of fat drops, up to 0.003 mm in diameter in homogenized milk (Walstra and Jennes 1984)] to facilitate ultrasound Doppler signals from the moving liquid. The frequency of the transmitted sound was 10 MHz, and the amplitude of the signal generator used to drive the ultrasound transducer was adjusted to generate an acoustic intensity of about 100 mW/cm2 in the container, resulting in a maximum acoustic streaming velocity in the order of 1 cm/s in fresh milk. Using these parameters, the Doppler equation [eqn. (2)] will predict a Doppler shift of 133 Hz, i.e., roughly 10e5 of the transmitted ultrasound frequency.
Hz
/ Hz
\ EXP
0 Hz AVERAGE
BW:
250 Hz / 3.83 Hz
04
Fig. 3. Doppler spectra obtained from milk in an open container. (a) fresh milk; (b) non-fresh, “sour milk.”
Ultrasound in Medicine and Biology
0
2
4
Time
ii
8
IO
[hours]
Fig. 4. Milk in an open container at room temperature. In order to monitor the freshness of the milk, the relative maximum velocity of the acoustic streaming is presented as a function of time. Note the drastic change in viscous state after about 8 h.
RESULTS Figure 3(a) shows a demodulated Doppler frequency spectrum from a measurement on a container filled with fresh milk. After 10 h in room temperature, a coarse inspection shows distinct changes in the appearance of the milk: the milk has turned sour. The corresponding Doppler frequency spectrum is showed in Fig. 3(b). It is immediately evident that the acoustic streaming is greatly reduced in sour milk. To get an even better understanding of the souring process in milk, a computer was used to monitor the relative maximum acoustic streaming velocity over time. The results are shown in Fig. 4. As can be seen in this experiment, a drastic change in the viscous state of the milk took place after about 8 h in room temperature.
Volume 17, Number 5, 1991
The exact outcome of the measurements depends of a number of parameters: ultrasound frequency and intensity, dimension and shape of the ultrasound transducer, viscosity and acoustic attenuation of the current fluid, particle content and so on. However, for a specific measurement situation most of these specifications can be considered to be constant, and a change in the process will give rise to a fairly specific and reproducible output signal. The new ultrasound fluid property sensor can, therefore, be used to survey many different liquids: blood, fermentation process fluids, and industrial process fluids are only a few possible examples. If the measurement container is provided with an acoustically transparent wall, it will even be possible to investigate the liquids in a completely enclosed vessel. The new sensor is highly sensitive to liquids with a viscosity similar to water. Therefore, it will be an excellent complement to existing cylinder viscosimeters which are optimal at higher viscosities. A recent study of acoustic streaming, induced by a conventional clinical ultrasound scanner (Star&t et al. 1989), indicates that streaming can be induced in any fluid path of reasonable length in viva This might suggest a future possibility of noninvasive measurement of fluid property inside the human body. REFERENCES Atkinson, P.; Woodcock, J. P. Doppler ultrasound and its use in clinical measurements. London: Academic Press; 1982:26. Nyborg, W. L. Acoustic streaming. In: Mason, W. P., ed. Physical acoustics (vol. IIB). New York: Academic Press; I965:265-33 1. Starr&, H. C.; Duck, F. A.; Humphrey, V. F. An experimental investigation of streaming in pulsed diagnostic ultrasound beams. Ultrasound Med. Biol. 15:363-373: 1989. Walstra, P.; Jennes, R. Dairy chemistry and physics. New York: John Wiley & Sons; 1984:256.
