WO2007069155A1

WO2007069155A1 - Doppler detection of pulsatile blood flow

Info

Publication number: WO2007069155A1
Application number: PCT/IB2006/054670
Authority: WO
Inventors: John Petruzzello; Eric Cohen-Solal; Benoit Dufort; Balasundara Raju
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-12-14
Filing date: 2006-12-07
Publication date: 2007-06-21
Also published as: CN101326447A; EP1963884A1; JP2009519079A

Abstract

Pulsed-wave Doppler is a well-known ultrasound technique used to quantify blood flow in the circulatory system. By using short bursts of ultrasound and measuring the frequency shift of the returning echo, it is possible to estimate blood velocity in a narrow volume inside the body. Arterial blood flow shows a characteristic pattern that is modulated by the beating heart. However, veins shows a flow that is more unpredictable in nature, continuous most of the time, but sometimes pulsatile. In some occasions, the vein is collapsed and no flow can be detected. This makes detection and differentiation of the veins and arteries difficult because of the unpredictable flow in the veins. The present invention overcomes this unpredictable nature of blood flow by modulating the flow into a recognizable pattern that can be identified with Doppler ultrasound techniques . Arteries and veins can be differentiated since they can show different flow modulation patterns depending on the pressure applied.

Description

DOPPLER DETECTION OF PULSATILE BLOOD FLOW

This invention relates to the detection of blood flow by Doppler methods and, in particular, to assisted ultrasonic Doppler detection of pulsatile blood flow.

Ultrasonic Doppler diagnosis is performed when a clinician desires to acquire information about the flow of blood of a patient. The display of flow velocity may be done by means of a spectral Doppler display in which velocities are displayed graphically, or by a color Doppler display in which velocities are displayed in shades or hues of color. In some cases the detection of the flow alone can be used to locate the blood vessels containing the flow as described in US Pat. 5,474,073, "ULTRASONIC DIAGNOSTIC SCANNING FOR THREE DIMENSIONAL DISPLAY, " by Schwartz et al . By imaging the sources of Doppler flow signals the locations of the vessels containing the flow can be inferred. Such techniques are useful when ascertaining the effectiveness of organ transplants or other surgical repair of the vascular system.

The use of the Doppler flow signals to locate blood vessels can be applied to other diagnostic or surgical situations when greater locational precision is required. For instance identification of the center of the lumen of a blood vessel can be needed when subtle characteristics of laminar flow are being explored, or placement of a catheter or needle in a blood vessel is necessary. However the flow of blood in the venous system can limit the ability of Doppler detection. Buffered as they are by the capillary bed from the direct pressure of the left ventricle, the flow rate in veins can exhibit very low pressure and be uneven and low in velocity. In some circumstances veins with these characteristics can virtually collapse. These low flow conditions with little or no discernable pulsatility can make identification of the vessels by Doppler means difficult if not impossible. Accordingly it is desirable to be able to locate low flow vessels such as veins with Doppler ultrasound even under these adverse conditions.

In accordance with the principles of the present invention an apparatus and method are provided for inducing pulsatile flow in blood vessels which assists detection of the flow by ultrasonic Doppler techniques. As used herein, the term "pulsatile" means a series of one or more periodic or aperiodic changes in flow rate. Blood flow in a vessel is occluded as by applying pressure to the blood vessel. While the pressure is applied pressure is developed from blood flow to the occlusion. When the pressure is released and the vessel is again open the occluded blood flow will suddenly surge through the vessel. This sudden change in flow is more readily detected by Doppler ultrasound and the source of the change in flow identifies the location of the blood vessel. In an example illustrated below, an automated blood pressure pump and cuff are utilized to apply a sequence of pressure occlusions and releases which produce a unique pulsatile flow pattern in a blood vessel which can be readily distinguished from the arterial pulsatile flow produced by the heart.

In the drawings :

FIGURE 1 illustrates in block diagram form an ultrasound system constructed in accordance with the principles of the present invention in conjunction with a pressure cuff that occludes peripheral blood vessels. FIGURE 2 illustrates several Doppler displays of an ultrasound system.

FIGURES 3a, 3b, and 3c are graphical illustrations of the control signal for a pressure cuff and Doppler flow characteristics of the pulsatility of an occluded vessel and an unoccluded artery.

Referring first to FIGURE 1, an ultrasound system constructed in accordance with the principles of the present invention is shown in block diagram form. An ultrasound probe 10 includes an ultrasonic transducer array 12 which transmits ultrasonic waves and receives ultrasonic echo signals, all under control of an acquisition beamformer 14. The received echo signals may be at the same frequency as the transmit frequency, or at a higher or lower harmonic of the transmit frequency. Control of the transducer transmission and processing of the received echo signals is provided by an acquisition beamformer 14. The transmitted and received beams may be steered over a planar region for two dimensional imaging or steered over a volumetric region for three dimensional imaging. The coherent echo signals may be detected and processed for B mode display, may be coupled to Doppler processors 16 and 18 for spectral and/or colorflow display, or may be used for both B mode and Doppler display as described in U.S. Patent 6,139,501 (Roundhill et al . ) The processed B mode and Doppler signals are coupled to an image processor 22 where they are processed for display in the desired image format and are then displayed on an image display 26. Sequences of real time images may be captured and stored in a Cineloop memory 24 in r.f., estimate, native, or composite display form, from which they may be replayed for more detailed analysis or reprocessing.

FIGURE 2 illustrates an image display showing the types of images which may be produced by a Doppler -A -

ultrasound system. FIGURE 2 shows a colorflow Doppler image 40 which is used to image the blood flow velocities of the portion of a vessel 50 which is inside a color box 42. The colorflow image is produced by overlaying a B mode image of tissue structure with a color Doppler image of flow over the same region as the image, thereby producing and overlay of two spatially corresponding images. In the example of FIGURE 2 the color overlay is not laid over the entire B mode image but only the color box area 42, obviating the need to gather Doppler information outside the color box 42 and thereby increasing the frame rate of display. A spectral analysis of flow at a point in the anatomy shown in the colorflow image 40 is initiated by positioning a sample volume 52 over the center of the blood vessel 50. A flow direction cursor 54 is set to be aligned with the direction of blood flow for angle correction. Preferably the flow direction cursor setting and angle correction are performed automatically as described in U.S. Patent 6,464,637 (Criton et al . ) Doppler data is acquired from the sample volume location 52 at a high rate and used to produce a spectral Doppler display 72, shown at the bottom of the drawing. Each vertical line in the spectral display indicates the range of instantaneous velocity values at the sample volume location as plotted along the horizontal time axis of the spectral display. The peak velocity values 70 and the mean velocity values 62 may be automatically traced by the ultrasound system as described in US Pat. 5,287,753 (Routh et al . ) Adjacent to the colorflow image 40 on the display screen is a color bar 60, which depicts the mapping of the flow colors to a range of velocity values. In this illustration positive (with reference to the probe) velocities extend from green (G) to yellow (Y) in color and negative velocities extend from light blue (LB) to dark blue (DB) , where the zero velocity point between green and yellow is the color baseline .

In accordance with the principles of the present invention the ultrasound system of FIGURE 1 includes apparatus for occluding blood flow and thereby artificially inducing pulsatile flow. A pressure controller 20 is coupled to the acquisition beamformer 14, the spectral and colorflow Doppler processors 16 and 18, the image processor 22, and an inflation pump 28. The connections to the other areas of the ultrasound system provide time markers indicating the times at which pressure is applied and/or relieved by pressure cuff 80 attached to a patient. Cuff pressure is increased to occlude a vessel in the limb to which the cuff is attached by actuating the pump 28 to pump air into the cuff. When the cuff pressure is to be released a valve in the pressure line to the cuff 80 is opened under control of the pressure controller 20 to relieve the pressure. While this example illustrates the use of an air-inflated pressure cuff as may commonly be used with an automated blood pressure monitor, fluids other than air may alternatively be used. A fluid which is less compressive than air can enable more rapid application of pressure to the limb, but requires a fluid reservoir for its closed system when pressure is relieved. An air system is also immune from the possibility of troublesome seals in the pump and leaks .

While the example of FIGURE 1 illustrates communication and synchronization between the occlusion system and the rest of the ultrasound system, this need not be the case. The occlusion system including the pressure controller 20, the pump 28, and the pressure cuff 80 can be entirely separate and independent from the ultrasound system. The separate system can be free- running, periodically inflating and deflating the cuff at a rate and times of occurrence which are completely independent of the operation of the ultrasound system.

In operation the cuff 80 is slipped onto a limb of the patient and the ultrasound probe 10 is placed at a point where diagnosis is to be performed on a vessel passing through or connected to a vessel passing through the cuff 80. In the example of FIGURE 1 the point 52 where such a vessel is to be examined is in the arm distal to the location of the cuff 80. With the Doppler probe in place the pressure controller 20 controls the pump 28 to periodically inflate and deflate the cuff 80, thereby producing an artificially induced modulation of the flow velocity in the vessel being examined. A typical control signal for the pump 28 is shown in FIGURE 3a, which is a simple on-off square wave 102. On each positive-going transition of the square wave 103 the pump inflates the cuff 80 to wholly or partially occlude the flow of blood to and from vessels in the limb distal to the cuff, and on each negative-going transition such as 104 and 106 the pressure valve is opened, allowing the occluded vessels to open and pent-up blood pressure to surge through the formerly occluded vessels. This will cause a momentary increase in flow velocity through the formerly occluded vessels which can be detected by Doppler ultrasound as shown in FIGURE 3b by velocity peaks 114 and 116 corresponding in time to the negative-going inflation pressure transitions 104 and 106. These momentary, artificially induced flow peaks in the Doppler waveform of FIGURE 3b enable the formerly occluded vessels to be more readily distinguished than before as seen by the flow state of the Doppler waveform before (to the left of) the first velocity peak 114.

In the example of FIGURE 3a it is seen that the cuff is inflated and deflated at a rate which is distinctly different from the pulsatile blood flow produced by the pumping of the heart. FIGURE 3c is a Doppler waveform 120 of arterial blood flow typically produced by a Doppler ultrasound system. The regular nature of the pulsatility of the velocity peaks with each heartbeat is clearly seen in this waveform. The distinctly different pulsatility pattern of the pressure cuff induced peaks of FIGURE 3b illustrate how the artificially induced pulsatility of a vein, for instance, can be clearly distinguished from the regular pulsatility of an artery. In this example the artificial pulsatility of FIGURE 3b differs from the heartbeat pulsatility both in frequency and in regularity. Either can be sufficient to distinguish the artificial waveform, as can a change in flow intensity which can be detected by Doppler power processing.

There are several different approaches and several different modes in which to use a system of the present invention. In one example, the pressure device 80 is situated proximally and the ultrasound probe 10, 52 is situated distally relative to the heart. This configuration can be used in three different modes.

In a first mode, the position of a blood vessel is desired, either a vein or an artery. When sufficient pressure above systolic pressure is applied by the device 80, the blood flow in the vessels (both veins and arteries) is temporarily stopped. When the pressure is released, the blood rushes back into the veins and arteries, displaying a characteristic velocity peak when measured with a Doppler instrument focused on one of the vessels. Normal flow resumes in the veins and arteries until pressure is applied again. The cycle is then repeated. An algorithm that can identify the velocity peaks of the Doppler waveform as described above is used to locate the ultrasound signal coming from the center of a vessel. Due to the laminar flow of blood the highest peak velocity will be found at the center of the vessel. When the pressure is applied in a known pattern, it is easy to correlate the velocity peaks with the applied pattern.

In a second mode, the position of a vein is desired. When suitable pressure below systolic pressure but above venous pressure is applied, , the blood flow in the veins is temporarily stopped, but the flow in the arteries is relatively unaffected. When the pressure is released, the blood rushes back into the veins, displaying a characteristic velocity peak when measured with a Doppler instrument focused on one of the veins. Normal flow resumes in the veins until pressure is applied again and the cycle is repeated. An algorithm that can identify the Doppler velocity peaks is used to identify the ultrasound signal coming from the center of a vein. Since the pressure is applied in a known pattern, it is easy to correlate the velocity peaks in the veins with the applied pattern. The arteries will display the characteristic heartbeat pattern, which is different from the applied pressure pattern, and will be differentiated by the algorithm.

In a third mode, the position of an artery is desired. When sufficient pressure above systolic pressure is applied, the blood flow in the arteries and veins is temporarily stopped. When the pressure is released partially (below systolic pressure, but above venous pressure), the blood rushes back into the arteries, but not the veins which remain occluded, and the blood flow displays a characteristic velocity peak when measured with a Doppler instrument focused on an artery. Flow returns in the arteries until pressure is applied again, at which time the cycle is repeated. An algorithm that can identify the velocity peaks is used to identify the ultrasound signal coming from the center of an artery. Since the pressure is applied in a known pattern, it is easy to correlate the velocity peaks in the arteries with the applied pattern. The veins will not display any pattern, since their flow is stopped, and will be differentiated by the algorithm.

In a second exemplary application of the present invention, the pressure device 80 is situated distally and the ultrasound probe 10 is situated proximally relative to the heart. In this case, when pressure is applied, blood in the veins is accelerated for a short period and then the flow returns to normal. When the pressure is released, no significant change is detected, until the pressure is applied again and the cycle repeats.

The nature of the ultrasound system will depend on the amount of information required on the vessel position. If only the lateral position on the skin surface is needed without depth information, then a continuous-wave (CW) ultrasound system can be used. If the depth of the vein center is also required, then a pulsed-wave (PW) ultrasound system is required for variable depth scanning and identification. The PW system can be used with different depth gating to identify the depth of the vessel. In order to find the position of the vessel on the skin surface, different transducer configurations can be used. The simplest one consists of a single transducer moved mechanically on the patient limb. Another configuration would use an array of transducers that are selected or scanned electronically. A combination of both is also possible, where an array is moved mechanically. A suitable transducer configuration is described in US Pat. 6,575,914 (Rock et al . ) See also US patent application number 60/737,909, filed November 17, 2005 by Ayati et al . and entitled "CPR GUIDED BY VASCULAR FLOW MEASUREMENT."

The pressure controller 20 can have. several functions. First it delivers a signal to the pressure cuff 80 in order to apply adequate pressure depending on whether the position of an artery or a vein is desired. The change in pressure should be done in such a way which limits the tissue movement in the patient limb, since patient motion can interfere with the Doppler flow signal. In the example of FIGURE 1 the pressure controller is coupled to the ultrasound system to enhance system performance. The connection to the beamformer 14 causes the beamformer to increase it transmission rate (PRF) at the time of a pressure transition to improve the precision with which a velocity change can be detected. The connection 56 to the spectral and colorflow Doppler processors 16 and 18 causes optimization of the Doppler processing at these times. The coupling of the pressure controller to the image processor enables the times of velocity modulation to be colored or otherwise enhanced in the scrolling spectral display 72. While, as mentioned above, these connections, synchronism, and enhancement are optional, they may be employed in a constructed system in order to improve the localization of the blood vessel under scrutiny.

In addition, the pressure controller or other part of the system should provide feedback to the user on the localization of the vessel under scrutiny. This can be done as simply as a light, or more complex display such as giving the coordinates of the vessel and/or vessel center, or displaying an image and spectrogram traces with a location marked as shown in FIGURE 2.

While the examples above illustrate the use of visual localization it will be appreciated that an audio- augmented system, or simply audio alone, can be used. For instance, a single or paired Doppler transducer can be moved over the skin of the patient and the resulting modulated Doppler signal played as a Doppler tone. The user moves the transducer over the body while the modulation is underway until a maximum tone intensity or pitch informs the user that the vessel with flow modulation has been located.

Claims

WHAT IS CLAIMED IS:

1. A system for detecting flow in a blood vessel comprising : an occluding device which is operable to periodically or aperiodically occlude a blood vessel; an ultrasound transducer operable to receive Doppler signals from a blood vessel with blood flow affected by the blood flow of the periodically or aperiodically occluded vessel; and a Doppler processor coupled to the ultrasound transducer; and an output device coupled to the Doppler processor which produces a signal identifying a blood flow velocity corresponding to the periodic or aperiodic occlusion.

2. The system of Claim 1, wherein the output device comprises an ultrasound display.

3. The system of Claim 2, wherein the ultrasound display displays a spectral Doppler waveform.

4. The system of Claim 2, wherein the ultrasound display displays a color Doppler image.

5. The system of Claim 1, wherein the occluding device comprises an inflatable pressure cuff.

6. The system of Claim 5, wherein the occluding device further comprises a pump coupled to the pressure cuff.

7. The system of Claim 6, wherein the occluding device further comprises a controllable pressure relief valve .

8. The system of Claim 7, wherein the occluding device further includes a pump controller which causes the pump and the pressure relief valve to periodically or aperiodically inflate and deflate the pressure cuff.

9. The system of Claim 5, wherein the pressure cuff is inflated with a fluid which is less compressible than air .

10. The system of Claim 1, wherein the Doppler process operates to produce Doppler velocity or Doppler power estimates.

11. The system of Claim 1, wherein the ultrasound transducer further comprises a mechanically scanned transducer probe.

12. The system of Claim 1, wherein the ultrasound transducer further comprises an electronically scanned transducer probe.

13. The system of Claim 1, wherein the ultrasound transducer further comprises a manually scanned transducer probe .

14. The system of Claim 1, wherein the occluding device is situated proximally and the ultrasound transducer is situated distally relative to the heart.

15. The system of Claim 1, wherein the occluding device is situated distally and the ultrasound transducer is situated proximally relative to the heart.

16. A method of detecting the location of a blood vessel comprising: directing an ultrasound transducer toward a region of interest in a body; modulating the flow of blood to or from the region of interest; receiving and Doppler processing signals from the ultrasound transducer; and identifying the flow modulation by use of the Doppler processed signals.

17. The method of Claim 16, wherein modulating the flow of blood further comprises periodically or aperiodically occluding a blood vessel in the body.

18. The method of Claim 17, wherein identifying the flow modulation further comprises correlating the time of occurrence of the occluding with the Doppler processed signals .

19. The method of Claim 18, wherein correlating further comprises identifying a peak velocity component of the Doppler processed signals.

20. The method of Claim 17, wherein periodically or aperiodically occluding further comprises inflating and deflating a pressure cuff on a limb of the body.

21. The method of Claim 16, wherein the flow of blood is modulated at a point of the body having a positional relation to the heart; and wherein receiving Doppler ultrasound is performed at a location on the body which is situated distally relative to the point of blood flow modulation.

22. The method of Claim 16, wherein the flow of blood is modulated at a point of the body having a positional relation to the heart; and wherein receiving Doppler ultrasound is performed at a location on the body which is situated proximally relative to the point of blood flow modulation.