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Pulsed Doppler techniques: Some examples from the University of Washington
Ultrasound in Med. & Biol., Vol. 2, pp. 251-262. Pergamon Press, 1976. Printed in Great Britain.
REVIEW PULSED DOPPLER TECHNIQUES: SOME EXAMPLES FROM THE UNIVERSITY OF WASHINGTON* D. W. BAKER, D. E. S ~ r . S S ? and S. L. JOHNSON~ Center for Bioengineering,Universityof Washington,Seattle, Washington,U.S.A. (First received 6March 1975;and in/inalform 9July 1976) Key Words: Acoustics, Ultrasonics, Doppler flowmeters, Blood flow measurements, Range-gated, Blood flow
direction. The clinical applications of ultrasound to date have involved either pulsed echo or continuous wave Doppler techniques. The primary emphasis has been on the development of pulsed echo techniques for the investigation of organ shape and geometry and normal-abnormal tissue structures. Pulsed echo techniques have also been used extensively in cardiology for evaluating the dynamics of the heart motion and valve function. While these dynamic measurements are of extreme value in cardiology, they do not necessarily represent all the information that is available nor the best way to make these measurements. Pulsed Doppler techniques, on the other hand, promise the ability to make more effective measurements of dynamic cardiovascular functions. The Doppler signal processing methods are specifically sensitive to the motion of interfaces or blood flow, and conversely are insensitive and incapable of forming images or maps of stationary structures in the usual echo sense. Doppler techniques described by Baker (1970) and Peronneau et al. (1970) are relatively new and are far from being perfected. The ultimate utility of Doppler processing will take some time to evolve. The present potential applications include assessment of blood flow acceleration, velocity and volume flow rates, at many sites throughout the body. These include measurements of flow through various valves of the heart, the outflow tract, and in the peripheral vessels. Of particular interest is the use of Doppler spectral signal analysis for the localization and identification of flow disturbances resulting from anatomical defects. Ideally, it might be possible to monitor the performance of the heart as a pump in terms of cardiac output, the effectiveness of valves, or to follow the distribution of blood flow through the periphery, in vessels as small as 2 mm at depths from 2 to 5 cm. Larger vessels, such as the aorta, may be mapped over their entireties except for those areas obscured by lung or other gas containing structures or bone. While not demonstrated at this time, it is conceivable that pulsed Doppler techniques may in fact be used to develop images of myocardial motion, by methods which may have distinct advantages over pulsed echo techniques.
echo device. A typical pulsed device transmits a 1 ~ burst of 3-8 MHz ultrasound at the pulse repetition frequency (PRF). The Doppler shift produced by echoes from moving structures can be detected using a range-gated receiver. The depth at which the velocity is detected is controlled by the amount of delay in the receiver rangegate. It can be easily set from I to 15 cm depending on the PRF and is necessarily much higher than a pulsed echo device and may range as high as 15-20 kHz (Baker, 1975). The dual capability of the instrument forces a compromise in determining the best pulse repetition frequency. The maximum Doppler shift that can be detected is governed by sampling theory concepts, i.e. the PRF must be twice the highest expected Doppler shift. At the same time the depth of penetration or receiver range gate delay demands a lower PRF for increasing depth. Since the Doppler shift frequency is proportional to the cosine of the angle between the sound beam and the flow velocity vector, it is fortunate that many of the deeper vessels can only be approached at large angles (>45 °) such that the frequency shift often falls below half the pulse repetition rate requirement. Ideally one would like to detect the Doppler signal with the best possible signal to noise (s/n) ratio at every depth. Certain compromises are necessary to optimize the signal to noise ratio since the resolution and sensitivity of the system are inversely related. The value of the velocity measured by the device is the weighted average of all the velocities present in a small region called the sample volume. The location of the leading edge of the teardrop shaped sample volume is set by the receiver range-gate (Jorgensen et al., 1973). The size and shape of the samples volume depends on the duration of the transmitted sonic burst, and transducer bandwidth and beamwidth, and the receiver bandwidth. It is well known that amplifier noise is directly proportional to its bandwidth. Similarly the pulsed Doppler system noise is determined by its overall bandwidth starting from the transmitting circuits, through the transducers to the receiving circuitry. Present systems have a typical bandwidth of 600 kHz for cardiac measurements and 1.5 MHz for peripheral vascular applications.
BASIC PRINCIPLES SIGNAL OUTPUT AND ANALYSIS
A typical Pulsed Doppler instrument is an effect the combination of a continuous wave Doppler and a pulsed
The simplest pulsed Doppler has as its output a Doppler signal spectrum which is the difference in frequency between the transmitted and the received ultrasonic signal. The signal ranges from d.c. to 5 or 10 kHz depending on the flow velocities and the transmitted frequency of
*Supported by NIH Grant HL-07293. tDepartment of Surgery, University of Washington. ~tDepartment of Medicine, University of Washington. UMB.. roe 2,No. 4---A
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3-8 MHz. This signal can be analyzed in terms of the magnitude of the Doppler shift in order to determine the velocity of the flow or interface motion in the sample volume. In the simpler devices, the direction of the motion cannot be readily determined from the Doppler spectrum. When flow direction is important, the units are usually equipped with a quadrature type phase detector circuit. These units have two channels of Doppler output. The phase of one channel is shifted by 90° from the other and when flow moves towards the transducer one channel will lead the other by an additional 90° and when motion or flow is away, it will lag by 90°. It is possible therefore by noting the relative phase shift between the two channels to determine the direction as well as the velocity. The most common scheme for detecting the mean value of the Doppler signal from the quadrature outputs is to pass them through a zero crossing type analog frequency meter. This circuit provides an analog voltage proportional to the instantaneous mean value of the zero crossing interval. Since the circuit cannot function as an ideal zero crossing detector the signal to noise ratio must be 30 dB or more. However, when the signal to noise ratio is less, the device cannot discriminate between the low level noise type signal and the Doppler signal. When the zero *Kay Electric Co., Pine Brook, NJ.
crosser is adjusted so that it triggers just above the noise level, its output will usually be somewhat lower than the true frequency spectrum average. Spectral analysis of the raw Doppler signal to determine the velocity spectrum and consequently the mean frequency has been used for some time (Strandness et al., 1967). Spectral analysis usually involves tape recording signals and then passing them through an analyzer. The process takes a considerable amount of time and the results are not available until after the original study. To circumvent this problem, techniques are being developed for rapid real time spectral analysis. From this type of analysis it should be possible to derive the true average velocity at any instant of time. Figure 1 shows two types of spectral analysis currently in use plus one new method. The top two lines are the output from a "Kay" analyzer.* The upper trace shows 3 dB intensity contours, the middle trace omits the contours and the bottom trace is the output of a circuit which provides the instantaneous real time spectrum all as a function of time. The two upper traces took two to three minutes each to analyze tape recordings of the raw signal. The lower trace came directly from the output of the Doppler device without tape recording. None of these spectral displays indicate the direction of flow. Determining the best analytical techniques for dealing
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Pulsed Doppler techniques: some examplesfrom the Universityof Washington with the Doppler spectrum is best accomplished using computer techniques to evaluate the characteristics of the spectrum and to learn the best approach to the proceSSing. Figure 2 shows a spectral plot of blood flow detected from the pulmonary artery using a range-gated pulsed Doppler• The signals were recorded on tape and later processed on a DEC* PDP-12 computer• The analysis uses an adaptation of a Fast Fourier Transform algorithm (FFT). A transform is made every 20 ms on each of the device's quadrature outputs, the relative phase angle, term by term of each of these transforms is used to determine if there is flow toward or away from the transducer. Transforms for flow toward the transducer are shown above the line of "0'"s and flow away is shown below the line. Figure 3 shows the transform and the relative phase angle between the quadrature outputs at point "A" on Fig. 2. The computer implemented Fast Fourier Transform is expected to be a very useful tool in evaluating the dynamics of flow and valve motion in the heart. This is a particularly difficult and important problem to solve since the transducer is usually set on the surface of the skin and the blood flow or valve motion is measured from within the structures which are themselves moving. This is evident in many of the spectral plots where a vessel may move towards the transducer at the same instant as the flow is moving away from the transducer. Analytical techniques to separate out these various motions are needed to cope with errors which may result. While the digital computer and the Fast Fourier Transform may not become a standard part of an ultrasonic device, their use will point the way towards the best analytical techniques and possibly the best electronic processors• QUANTITATION
OF BLOOD FLOW TRANSCUTANEOUSLY
The measurement of volume blood flow in a rigorous manner involves solving a classical equation common to engineering. It is as follows with modification for a Doppler device. Q = V A cos 0, *Digital Equipment Corp., Maynard, MA.
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lumen from which the average velocity is calculated. The second approach involves using a pulsed Doppler having a range gate or sample volume whose length is equal to the slant diameter of the vessel and is positioned to give the instantaneous average directly (Baker and Daigie, 1975). The three-dimensional orientation of the velocity vector with respect to the transducer, the longitudinal axis of the vessel and the plane representing the cross sectional area must be measured. The cross sectional area at the measurement site must also be determined to compute the volume rate. Solving this problem, provided there is sufficient medical justification, will no doubt require a multidimensional instrument capability. This would include real time Bmode imaging with simultaneous superimposed real time Doppler all with interactive graphic displays and supporting digital processors. The Duplex System described by Barber et al. (1974 and 1974a) is an example of this type of ultrasonic system. SOME ~
APPLICATIONS OF gXISTING DEVICES
The circulatory system has been divided into two general categories for clinical evaluation. The central vascular area, heart and great vessels, falls within the sphere of cardiology and the remaining vessels are considered peripheral circulation. This division is done primarily to separate long range (or depth) from short range devices. Measurement of flow and valve dynamics in cardiology is probably the more diificult of the two. Following are selected examples of the use of pulsed Doppler techniques for the assessment of valvular disease. The first is shown schematically in Fig. 4. The transducer is located in the supersternal notch and the beam is oriented towards the arch of the aorta. The figure shows the transducer in position and the corresponding A-scan echo pattern produced. The lower oscilloscope trace shows the receiver time compensated gain (TCG) function. The negative pulse located at 3.8 cm on the TCG function is the location of the Doppler sample volume in the arch of the aorta. At this location, two spectral plots were made for comparison. These are shown in Fig. 5. The upper diagram is from the Kay spectrograph which is shown in terms of 3 dB intensity contours. The lower graph illustrates the digital computer Fast Fourier Transform techniques. There are obvious differences since these are identical signals. Reverse flow is demonstrated in the transform, the wave form is more peaked, and there is less noise in the signal. Because there
is no adequate method at the present time for making a separate instantaneous independent calibration of these signals, we are unable to assess the accuracy of either plot. The second example shows the transducer positioned in the third left intercostal space to intercept the aortic outflow tract at the location of the aortic valve (Fig. 6). Flow through a normal aortic valve is relatively smooth and is dimcult to detect with the beam in the position shown. When a significant amount of stenosis is present eddies or "turbulence" makes it possible to detect Doppler signals even at right angles to the flow. Usually the pulsed Doppler transducer is positioned with the aid of a pulsed echo device. The echo pattern detected from the aortic valve using pulsed echo techniques has been well documented. When the proper location has been found for the pulsed echo transducer then the Doppler transducer is moved into the same position and its range-gate set to approximately the same depth. At this point, the operator listens to the Doppler signals, observes the A-scan echo pattern and then is able to position the sample volume directly in the center of the outflow tract. In this example the motion of the cusp of the valve will produce a definite "click-like" sound which the operator can easily identify. The stenotic jet produces a "hiss-like" sound to the observer. The true spectral characteristics are not apparent, however, until the spectral analysis is accomplished as shown in Fig. 7. It is apparent that the spectrum is quite broad, probably due to the turbulent nature of the jet. The Kay spectrograph once again gives a different impression than the transform. This is primarily due to the fact that the Kay spectrum is folded over at the zero frequency point and does not indicate the direction. The transform shows that the flow has velocity components both plus and minus which are probably caused by swirling and eddying and the sudden turbulent nature of the jet. The diameter of the jet can be estimated by scanning the range-gate across the orifice and noting the depth at which the signal first appears and the depth at which the signal disappears. The difference in these two ranges will be the diameter of the jet. Results to date do not indicate a close agreement when compared with area calculations derived from pressure gradient techniques. This is probably due to the fact that stenotic jets are not necessarily round. Aortic valve insufficiency can be evaluated by placing the sample volumes in the left ventricular outflow tract (Ward et al.).The location of the probe is shown in Fig. 8(a). In this position we should AO
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transform to identify the spectral broadening and swishing sound due to the leaky valve during diastole. The fourth example shows the sample volume in the pulmonary artery, Fig. 9. The vessel exhibits a great deal of motion with respect to the transducer which is illustrated in the transform plot (Fig. 10). Notice blood flow moving
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away from the transducer shown in the negative direction (below zero base line) on the transform and at the same instant a simultaneous motion of the vessel in the opposite direction (above zero base line). Following the systolic interval there is a reverse flow component flowing towards the transducer. A Kzy spectrogram does not reveal the details of this complex motion. A transform plot from pulmonary valve stenos/s is shown in Fig. 11. In this case there was a jet of very high velocity and of a narrow spectral (small velocity gradient) width. These examples demonstrate the kinds of signals that one can derive from heart valves using pulsed Doppler techniques. They can provide qualitative yes,no type conclusions as to the presence of stenosis or insufficiency in the valve. In some cases an approximation of the jet diameter may be sensed to give a measure of the degree of involvement. Quantitation of the velocities through heart valves, will require measuring the angle between the flow vector and the sound beam axis. The detection and assessment of blood flow impairment in peripheral vascular disease involves a study of the geometry and shape of the blood vessels.at the site of interest and the measurement of flow rates, whether velocity or volume flow, through that site. These studies
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may involve serial measurenents along the course of a vessel at various branch points or in straight segments for comparison from one leg to the other at the same site. Since vessel geometry, i.e. narrowing, is an indication of the degree of stenosis at the location of a plaque it is an essential parameter to measure. Flow measurements cannot be made in a quantitative sense without some knowledge of this geometry and cross sectional shape• A method for imaging blood vessels using pulsed Doppler techniques has been devised. Figure 12 shows the basic concept. The ultrasonic transducer is fixed to a short scan arm which can pivot in two directions about a point. The motion of the transducer beam is confined to a plane or segment of a spherical shell. It can be moved in either the X- or Y-directions or rotated in an angle 0. The value of these displacements is sensed using potentiometers and their outputs are used to position a spot on a cathode ray tube. The plane of the cathode ray tube corresponds to the scan plane of the transducer. Scanning commences with the range gate set for a certain depth R. As the scanning progresses a spot will be stored at each (X, ¥, 0) location that the flow velocity exceeds a specified threshold value• The range R is then varied and the scan continued until a full image is built up on the cathode ray tube memory.
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This image can either be a longitudinal section or a cross section depending on the orientation of the scanning plane with respect to the vessel. An example, taken at the branch of the common femoral, superficial femoral, and profunda is shown in Fig. 13. The particular example shown illustrates a saphenous vein grafted at the point of the bifurcation. The Doppler image revealed an intrusion into the vessel lumen at the point of the graft attachment. At the time of surgery it was found that the vessel was partially occluded due to a closure along the suture line. Figure 14 shows an ultrasonic arteriogram of another left femoral artery with the corresponding X-ray arteriogram for comparison. In this image is an occluded superficial femoral which is apparent in the ultrasonic image. The granularity of the ultrasonic image is due to the pulsatile flow and the time spent in scanning the vessel. Further evidence of plaque detection from another case is shown on the example (Fig. 15) or the popfiteal artery. The left image (A) shows the intrusion of the plaque into the lumen. On the right (B) is the normal vessel. A further application of ultrasonic imaging is used in evaluating flow through saphenous vein
coronary artery by-passes. In this illustration (Fig. 16) we see the coronary arteriogram on the left (A) and ultrasonic image (B). Flow wave forms are depicted for the aorta in (C) and from the graft (B). It has been possible to image as many as 65% of a series of 49 patients (Gould et al., 1972). Flow signals alone or with imaging could be detected in 95% of these. The examples in cardiology and peripheral vascular disease shown here have utilized either spectral analysis techniques for evaluating flow or the geometric imaging concepts used in peripheral vascular disease. Ultimately, these two features need to be incorporated into one instrument in order to arrive at a quantitative measure of flow. Figure 17 shows an ultrasonic artcriograph of the femoral profunda bifurcation. Superimposed on the image are vectors or lines corresponding to the direction of sound beam for each measurement. Corresponding wave forms from each location are shown. The objectives of this procedure are to determine the angle between the sound beam vector and the flow velocity vector which can be used later to calculate the volume flow rate through the various branches. These same techniques can be used in
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cardiology; in Fig. 18 we see an image of the arch of the aorta. In the drawing below the image, the position of the sound beam vector is depicted. The flow velocity can then be estimated in order to calculate the angle theta (0). To date, these procedures have not been adequately or thoroughly evaluated to determine if they are an effective method for arriving at quantitative measurement of flow. In summary, this paper has demonstrated the use of pulsed Doppler techniques for assessing blood flows through valves of the heart and the arch of the aorta, for the purpose of assessing cardiac performance. Techniques for spectral analysis have been demonstrated using Fourier transform algorithms and a digital computer. A
type of real time spectral analysis has been developed to give the same type of analysis. Pulsed Doppler techniques are being used for developing images of longitudinal and cross sections of vessels based on the position of flow. These images can reveal the presence of plaques, intrusions or complete occlusions. When the two methods are combined with a superimposed sound beam vector on the flow image display it may be possible to compute the intervening angle. The next steps, which may be required to arrive at a quantitative volume or velocity flow measurement will involve the ability to assess the full 3dimensional geometry at the point of interest and to do further signal processing to smooth out variations in cross
Pulsed Doppler techniques: some examples from the University of Washington
261
Fig. 16. Coronary artery by-pass grafts can be imaged in many patients. The blood flow velocity can be detected and recorded by positioning the pulsed Doppler sample volume within the desired vessel as determined from the flow image. The timing of the flow velocity through the graft E is used to confirm its presence. The arteringram shows the same graft lower left A. Flow velocity in the aorta is shown in D for comparison.
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AXIS Fig. 18. It is possible to image the arch of the aorta using a high sensitivity moderate resolution pulsed Doppler. The sound beam flow angle 0 may be estimated. sectional area fluctuations or motion of the vessel. Finally, studies will have to be performed to assess the accuracy of these techniques. REEgRgNC~ Baker, D. W. (1970) Pulsed ultrasound Doppler blood flow sensing. IEEE Trans. Sonics Ultrasonics 17, 65. Baker, D. W. and Daigle, R. E. (1975) Noninvasive ultrasonic flowmetry. NATO-ASICardiovascular Flow Dynamics Conference. Houston. Barber, F. E., Baker, D. W., Nation, A. W., Strandness, D. E. and Reid, J. M. (1974) Ultrasonic duplex echo Doppler scanner. IEEE Trans. Biomed. Engng 21, 109-113. Barber, F. E., Baker, D. W., Strandness, D. E., Ofstad, J. M. and Mahler, G. D. (1974a) Duplex scanner II: for simultaneous imagingof artery tissues and flow. 1974 Ultrasonics Symposium Proceedings. IEEE Cat. No. 74 CHO 896-ISU, pp. 744--748.
Gould, L. K., Mozersky, D. J., Hokansen, D. E. et al. (1972) A non-invasive technique for determining patency of saphenous vein coronary by-pass grafts. CircJdation 46, 595-600. Jorgensen, J. E., Campan, D. N. and Baker, D. W. (1973)Physical characteristics and mathematical modeling of the pulsed ultrasonic flowmeter. Med. Biol. Engng 11, 404-421. Peronneau, P., Hinglais, J., Pellet, M. and Leger, F. (1970). V~locimetre sanquin par effect Doppler a'~missionultra-sonore puls~e. L'onde Electrique 50, 369-386. Strandness, D. E., Schultz, R. D., Sumner,D. S. and Rushmer,R. F. (1967) Ultrasonic flow detection: A useful technique in the evaluation of peripheral vascular disease. Am. I. Surg. 113, 311-320. Ward, J. M., Baker, D. W., Rubenstein, S. A., Johnson, S. L. Detection of aortic insufficiency by pulsed Doppler echocardiography. (In preparation).
REVIEW PULSED DOPPLER TECHNIQUES: SOME EXAMPLES FROM THE UNIVERSITY OF WASHINGTON* D. W. BAKER, D. E. S ~ r . S S ? and S. L. JOHNSON~ Center for Bioengineering,Universityof Washington,Seattle, Washington,U.S.A. (First received 6March 1975;and in/inalform 9July 1976) Key Words: Acoustics, Ultrasonics, Doppler flowmeters, Blood flow measurements, Range-gated, Blood flow
direction. The clinical applications of ultrasound to date have involved either pulsed echo or continuous wave Doppler techniques. The primary emphasis has been on the development of pulsed echo techniques for the investigation of organ shape and geometry and normal-abnormal tissue structures. Pulsed echo techniques have also been used extensively in cardiology for evaluating the dynamics of the heart motion and valve function. While these dynamic measurements are of extreme value in cardiology, they do not necessarily represent all the information that is available nor the best way to make these measurements. Pulsed Doppler techniques, on the other hand, promise the ability to make more effective measurements of dynamic cardiovascular functions. The Doppler signal processing methods are specifically sensitive to the motion of interfaces or blood flow, and conversely are insensitive and incapable of forming images or maps of stationary structures in the usual echo sense. Doppler techniques described by Baker (1970) and Peronneau et al. (1970) are relatively new and are far from being perfected. The ultimate utility of Doppler processing will take some time to evolve. The present potential applications include assessment of blood flow acceleration, velocity and volume flow rates, at many sites throughout the body. These include measurements of flow through various valves of the heart, the outflow tract, and in the peripheral vessels. Of particular interest is the use of Doppler spectral signal analysis for the localization and identification of flow disturbances resulting from anatomical defects. Ideally, it might be possible to monitor the performance of the heart as a pump in terms of cardiac output, the effectiveness of valves, or to follow the distribution of blood flow through the periphery, in vessels as small as 2 mm at depths from 2 to 5 cm. Larger vessels, such as the aorta, may be mapped over their entireties except for those areas obscured by lung or other gas containing structures or bone. While not demonstrated at this time, it is conceivable that pulsed Doppler techniques may in fact be used to develop images of myocardial motion, by methods which may have distinct advantages over pulsed echo techniques.
echo device. A typical pulsed device transmits a 1 ~ burst of 3-8 MHz ultrasound at the pulse repetition frequency (PRF). The Doppler shift produced by echoes from moving structures can be detected using a range-gated receiver. The depth at which the velocity is detected is controlled by the amount of delay in the receiver rangegate. It can be easily set from I to 15 cm depending on the PRF and is necessarily much higher than a pulsed echo device and may range as high as 15-20 kHz (Baker, 1975). The dual capability of the instrument forces a compromise in determining the best pulse repetition frequency. The maximum Doppler shift that can be detected is governed by sampling theory concepts, i.e. the PRF must be twice the highest expected Doppler shift. At the same time the depth of penetration or receiver range gate delay demands a lower PRF for increasing depth. Since the Doppler shift frequency is proportional to the cosine of the angle between the sound beam and the flow velocity vector, it is fortunate that many of the deeper vessels can only be approached at large angles (>45 °) such that the frequency shift often falls below half the pulse repetition rate requirement. Ideally one would like to detect the Doppler signal with the best possible signal to noise (s/n) ratio at every depth. Certain compromises are necessary to optimize the signal to noise ratio since the resolution and sensitivity of the system are inversely related. The value of the velocity measured by the device is the weighted average of all the velocities present in a small region called the sample volume. The location of the leading edge of the teardrop shaped sample volume is set by the receiver range-gate (Jorgensen et al., 1973). The size and shape of the samples volume depends on the duration of the transmitted sonic burst, and transducer bandwidth and beamwidth, and the receiver bandwidth. It is well known that amplifier noise is directly proportional to its bandwidth. Similarly the pulsed Doppler system noise is determined by its overall bandwidth starting from the transmitting circuits, through the transducers to the receiving circuitry. Present systems have a typical bandwidth of 600 kHz for cardiac measurements and 1.5 MHz for peripheral vascular applications.
BASIC PRINCIPLES SIGNAL OUTPUT AND ANALYSIS
A typical Pulsed Doppler instrument is an effect the combination of a continuous wave Doppler and a pulsed
The simplest pulsed Doppler has as its output a Doppler signal spectrum which is the difference in frequency between the transmitted and the received ultrasonic signal. The signal ranges from d.c. to 5 or 10 kHz depending on the flow velocities and the transmitted frequency of
*Supported by NIH Grant HL-07293. tDepartment of Surgery, University of Washington. ~tDepartment of Medicine, University of Washington. UMB.. roe 2,No. 4---A
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3-8 MHz. This signal can be analyzed in terms of the magnitude of the Doppler shift in order to determine the velocity of the flow or interface motion in the sample volume. In the simpler devices, the direction of the motion cannot be readily determined from the Doppler spectrum. When flow direction is important, the units are usually equipped with a quadrature type phase detector circuit. These units have two channels of Doppler output. The phase of one channel is shifted by 90° from the other and when flow moves towards the transducer one channel will lead the other by an additional 90° and when motion or flow is away, it will lag by 90°. It is possible therefore by noting the relative phase shift between the two channels to determine the direction as well as the velocity. The most common scheme for detecting the mean value of the Doppler signal from the quadrature outputs is to pass them through a zero crossing type analog frequency meter. This circuit provides an analog voltage proportional to the instantaneous mean value of the zero crossing interval. Since the circuit cannot function as an ideal zero crossing detector the signal to noise ratio must be 30 dB or more. However, when the signal to noise ratio is less, the device cannot discriminate between the low level noise type signal and the Doppler signal. When the zero *Kay Electric Co., Pine Brook, NJ.
crosser is adjusted so that it triggers just above the noise level, its output will usually be somewhat lower than the true frequency spectrum average. Spectral analysis of the raw Doppler signal to determine the velocity spectrum and consequently the mean frequency has been used for some time (Strandness et al., 1967). Spectral analysis usually involves tape recording signals and then passing them through an analyzer. The process takes a considerable amount of time and the results are not available until after the original study. To circumvent this problem, techniques are being developed for rapid real time spectral analysis. From this type of analysis it should be possible to derive the true average velocity at any instant of time. Figure 1 shows two types of spectral analysis currently in use plus one new method. The top two lines are the output from a "Kay" analyzer.* The upper trace shows 3 dB intensity contours, the middle trace omits the contours and the bottom trace is the output of a circuit which provides the instantaneous real time spectrum all as a function of time. The two upper traces took two to three minutes each to analyze tape recordings of the raw signal. The lower trace came directly from the output of the Doppler device without tape recording. None of these spectral displays indicate the direction of flow. Determining the best analytical techniques for dealing
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Pulsed Doppler techniques: some examplesfrom the Universityof Washington with the Doppler spectrum is best accomplished using computer techniques to evaluate the characteristics of the spectrum and to learn the best approach to the proceSSing. Figure 2 shows a spectral plot of blood flow detected from the pulmonary artery using a range-gated pulsed Doppler• The signals were recorded on tape and later processed on a DEC* PDP-12 computer• The analysis uses an adaptation of a Fast Fourier Transform algorithm (FFT). A transform is made every 20 ms on each of the device's quadrature outputs, the relative phase angle, term by term of each of these transforms is used to determine if there is flow toward or away from the transducer. Transforms for flow toward the transducer are shown above the line of "0'"s and flow away is shown below the line. Figure 3 shows the transform and the relative phase angle between the quadrature outputs at point "A" on Fig. 2. The computer implemented Fast Fourier Transform is expected to be a very useful tool in evaluating the dynamics of flow and valve motion in the heart. This is a particularly difficult and important problem to solve since the transducer is usually set on the surface of the skin and the blood flow or valve motion is measured from within the structures which are themselves moving. This is evident in many of the spectral plots where a vessel may move towards the transducer at the same instant as the flow is moving away from the transducer. Analytical techniques to separate out these various motions are needed to cope with errors which may result. While the digital computer and the Fast Fourier Transform may not become a standard part of an ultrasonic device, their use will point the way towards the best analytical techniques and possibly the best electronic processors• QUANTITATION
OF BLOOD FLOW TRANSCUTANEOUSLY
The measurement of volume blood flow in a rigorous manner involves solving a classical equation common to engineering. It is as follows with modification for a Doppler device. Q = V A cos 0, *Digital Equipment Corp., Maynard, MA.
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lumen from which the average velocity is calculated. The second approach involves using a pulsed Doppler having a range gate or sample volume whose length is equal to the slant diameter of the vessel and is positioned to give the instantaneous average directly (Baker and Daigie, 1975). The three-dimensional orientation of the velocity vector with respect to the transducer, the longitudinal axis of the vessel and the plane representing the cross sectional area must be measured. The cross sectional area at the measurement site must also be determined to compute the volume rate. Solving this problem, provided there is sufficient medical justification, will no doubt require a multidimensional instrument capability. This would include real time Bmode imaging with simultaneous superimposed real time Doppler all with interactive graphic displays and supporting digital processors. The Duplex System described by Barber et al. (1974 and 1974a) is an example of this type of ultrasonic system. SOME ~
APPLICATIONS OF gXISTING DEVICES
The circulatory system has been divided into two general categories for clinical evaluation. The central vascular area, heart and great vessels, falls within the sphere of cardiology and the remaining vessels are considered peripheral circulation. This division is done primarily to separate long range (or depth) from short range devices. Measurement of flow and valve dynamics in cardiology is probably the more diificult of the two. Following are selected examples of the use of pulsed Doppler techniques for the assessment of valvular disease. The first is shown schematically in Fig. 4. The transducer is located in the supersternal notch and the beam is oriented towards the arch of the aorta. The figure shows the transducer in position and the corresponding A-scan echo pattern produced. The lower oscilloscope trace shows the receiver time compensated gain (TCG) function. The negative pulse located at 3.8 cm on the TCG function is the location of the Doppler sample volume in the arch of the aorta. At this location, two spectral plots were made for comparison. These are shown in Fig. 5. The upper diagram is from the Kay spectrograph which is shown in terms of 3 dB intensity contours. The lower graph illustrates the digital computer Fast Fourier Transform techniques. There are obvious differences since these are identical signals. Reverse flow is demonstrated in the transform, the wave form is more peaked, and there is less noise in the signal. Because there
is no adequate method at the present time for making a separate instantaneous independent calibration of these signals, we are unable to assess the accuracy of either plot. The second example shows the transducer positioned in the third left intercostal space to intercept the aortic outflow tract at the location of the aortic valve (Fig. 6). Flow through a normal aortic valve is relatively smooth and is dimcult to detect with the beam in the position shown. When a significant amount of stenosis is present eddies or "turbulence" makes it possible to detect Doppler signals even at right angles to the flow. Usually the pulsed Doppler transducer is positioned with the aid of a pulsed echo device. The echo pattern detected from the aortic valve using pulsed echo techniques has been well documented. When the proper location has been found for the pulsed echo transducer then the Doppler transducer is moved into the same position and its range-gate set to approximately the same depth. At this point, the operator listens to the Doppler signals, observes the A-scan echo pattern and then is able to position the sample volume directly in the center of the outflow tract. In this example the motion of the cusp of the valve will produce a definite "click-like" sound which the operator can easily identify. The stenotic jet produces a "hiss-like" sound to the observer. The true spectral characteristics are not apparent, however, until the spectral analysis is accomplished as shown in Fig. 7. It is apparent that the spectrum is quite broad, probably due to the turbulent nature of the jet. The Kay spectrograph once again gives a different impression than the transform. This is primarily due to the fact that the Kay spectrum is folded over at the zero frequency point and does not indicate the direction. The transform shows that the flow has velocity components both plus and minus which are probably caused by swirling and eddying and the sudden turbulent nature of the jet. The diameter of the jet can be estimated by scanning the range-gate across the orifice and noting the depth at which the signal first appears and the depth at which the signal disappears. The difference in these two ranges will be the diameter of the jet. Results to date do not indicate a close agreement when compared with area calculations derived from pressure gradient techniques. This is probably due to the fact that stenotic jets are not necessarily round. Aortic valve insufficiency can be evaluated by placing the sample volumes in the left ventricular outflow tract (Ward et al.).The location of the probe is shown in Fig. 8(a). In this position we should AO
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transform to identify the spectral broadening and swishing sound due to the leaky valve during diastole. The fourth example shows the sample volume in the pulmonary artery, Fig. 9. The vessel exhibits a great deal of motion with respect to the transducer which is illustrated in the transform plot (Fig. 10). Notice blood flow moving
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away from the transducer shown in the negative direction (below zero base line) on the transform and at the same instant a simultaneous motion of the vessel in the opposite direction (above zero base line). Following the systolic interval there is a reverse flow component flowing towards the transducer. A Kzy spectrogram does not reveal the details of this complex motion. A transform plot from pulmonary valve stenos/s is shown in Fig. 11. In this case there was a jet of very high velocity and of a narrow spectral (small velocity gradient) width. These examples demonstrate the kinds of signals that one can derive from heart valves using pulsed Doppler techniques. They can provide qualitative yes,no type conclusions as to the presence of stenosis or insufficiency in the valve. In some cases an approximation of the jet diameter may be sensed to give a measure of the degree of involvement. Quantitation of the velocities through heart valves, will require measuring the angle between the flow vector and the sound beam axis. The detection and assessment of blood flow impairment in peripheral vascular disease involves a study of the geometry and shape of the blood vessels.at the site of interest and the measurement of flow rates, whether velocity or volume flow, through that site. These studies
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may involve serial measurenents along the course of a vessel at various branch points or in straight segments for comparison from one leg to the other at the same site. Since vessel geometry, i.e. narrowing, is an indication of the degree of stenosis at the location of a plaque it is an essential parameter to measure. Flow measurements cannot be made in a quantitative sense without some knowledge of this geometry and cross sectional shape• A method for imaging blood vessels using pulsed Doppler techniques has been devised. Figure 12 shows the basic concept. The ultrasonic transducer is fixed to a short scan arm which can pivot in two directions about a point. The motion of the transducer beam is confined to a plane or segment of a spherical shell. It can be moved in either the X- or Y-directions or rotated in an angle 0. The value of these displacements is sensed using potentiometers and their outputs are used to position a spot on a cathode ray tube. The plane of the cathode ray tube corresponds to the scan plane of the transducer. Scanning commences with the range gate set for a certain depth R. As the scanning progresses a spot will be stored at each (X, ¥, 0) location that the flow velocity exceeds a specified threshold value• The range R is then varied and the scan continued until a full image is built up on the cathode ray tube memory.
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This image can either be a longitudinal section or a cross section depending on the orientation of the scanning plane with respect to the vessel. An example, taken at the branch of the common femoral, superficial femoral, and profunda is shown in Fig. 13. The particular example shown illustrates a saphenous vein grafted at the point of the bifurcation. The Doppler image revealed an intrusion into the vessel lumen at the point of the graft attachment. At the time of surgery it was found that the vessel was partially occluded due to a closure along the suture line. Figure 14 shows an ultrasonic arteriogram of another left femoral artery with the corresponding X-ray arteriogram for comparison. In this image is an occluded superficial femoral which is apparent in the ultrasonic image. The granularity of the ultrasonic image is due to the pulsatile flow and the time spent in scanning the vessel. Further evidence of plaque detection from another case is shown on the example (Fig. 15) or the popfiteal artery. The left image (A) shows the intrusion of the plaque into the lumen. On the right (B) is the normal vessel. A further application of ultrasonic imaging is used in evaluating flow through saphenous vein
coronary artery by-passes. In this illustration (Fig. 16) we see the coronary arteriogram on the left (A) and ultrasonic image (B). Flow wave forms are depicted for the aorta in (C) and from the graft (B). It has been possible to image as many as 65% of a series of 49 patients (Gould et al., 1972). Flow signals alone or with imaging could be detected in 95% of these. The examples in cardiology and peripheral vascular disease shown here have utilized either spectral analysis techniques for evaluating flow or the geometric imaging concepts used in peripheral vascular disease. Ultimately, these two features need to be incorporated into one instrument in order to arrive at a quantitative measure of flow. Figure 17 shows an ultrasonic artcriograph of the femoral profunda bifurcation. Superimposed on the image are vectors or lines corresponding to the direction of sound beam for each measurement. Corresponding wave forms from each location are shown. The objectives of this procedure are to determine the angle between the sound beam vector and the flow velocity vector which can be used later to calculate the volume flow rate through the various branches. These same techniques can be used in
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cardiology; in Fig. 18 we see an image of the arch of the aorta. In the drawing below the image, the position of the sound beam vector is depicted. The flow velocity can then be estimated in order to calculate the angle theta (0). To date, these procedures have not been adequately or thoroughly evaluated to determine if they are an effective method for arriving at quantitative measurement of flow. In summary, this paper has demonstrated the use of pulsed Doppler techniques for assessing blood flows through valves of the heart and the arch of the aorta, for the purpose of assessing cardiac performance. Techniques for spectral analysis have been demonstrated using Fourier transform algorithms and a digital computer. A
type of real time spectral analysis has been developed to give the same type of analysis. Pulsed Doppler techniques are being used for developing images of longitudinal and cross sections of vessels based on the position of flow. These images can reveal the presence of plaques, intrusions or complete occlusions. When the two methods are combined with a superimposed sound beam vector on the flow image display it may be possible to compute the intervening angle. The next steps, which may be required to arrive at a quantitative volume or velocity flow measurement will involve the ability to assess the full 3dimensional geometry at the point of interest and to do further signal processing to smooth out variations in cross
Pulsed Doppler techniques: some examples from the University of Washington
261
Fig. 16. Coronary artery by-pass grafts can be imaged in many patients. The blood flow velocity can be detected and recorded by positioning the pulsed Doppler sample volume within the desired vessel as determined from the flow image. The timing of the flow velocity through the graft E is used to confirm its presence. The arteringram shows the same graft lower left A. Flow velocity in the aorta is shown in D for comparison.
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AXIS Fig. 18. It is possible to image the arch of the aorta using a high sensitivity moderate resolution pulsed Doppler. The sound beam flow angle 0 may be estimated. sectional area fluctuations or motion of the vessel. Finally, studies will have to be performed to assess the accuracy of these techniques. REEgRgNC~ Baker, D. W. (1970) Pulsed ultrasound Doppler blood flow sensing. IEEE Trans. Sonics Ultrasonics 17, 65. Baker, D. W. and Daigle, R. E. (1975) Noninvasive ultrasonic flowmetry. NATO-ASICardiovascular Flow Dynamics Conference. Houston. Barber, F. E., Baker, D. W., Nation, A. W., Strandness, D. E. and Reid, J. M. (1974) Ultrasonic duplex echo Doppler scanner. IEEE Trans. Biomed. Engng 21, 109-113. Barber, F. E., Baker, D. W., Strandness, D. E., Ofstad, J. M. and Mahler, G. D. (1974a) Duplex scanner II: for simultaneous imagingof artery tissues and flow. 1974 Ultrasonics Symposium Proceedings. IEEE Cat. No. 74 CHO 896-ISU, pp. 744--748.
Gould, L. K., Mozersky, D. J., Hokansen, D. E. et al. (1972) A non-invasive technique for determining patency of saphenous vein coronary by-pass grafts. CircJdation 46, 595-600. Jorgensen, J. E., Campan, D. N. and Baker, D. W. (1973)Physical characteristics and mathematical modeling of the pulsed ultrasonic flowmeter. Med. Biol. Engng 11, 404-421. Peronneau, P., Hinglais, J., Pellet, M. and Leger, F. (1970). V~locimetre sanquin par effect Doppler a'~missionultra-sonore puls~e. L'onde Electrique 50, 369-386. Strandness, D. E., Schultz, R. D., Sumner,D. S. and Rushmer,R. F. (1967) Ultrasonic flow detection: A useful technique in the evaluation of peripheral vascular disease. Am. I. Surg. 113, 311-320. Ward, J. M., Baker, D. W., Rubenstein, S. A., Johnson, S. L. Detection of aortic insufficiency by pulsed Doppler echocardiography. (In preparation).