Ultrasonic Doppler blood flow monitoring system

Abstract: An ultrasonic Doppler blood flow monitoring system comprises: a catheter (1) to be inserted into a blood vessel (91); at least two ultrasonic transducers (10,12) provided on the catheter and arranged for transmitting ultrasonic waves toward blood flow at two different angles of incidence with respect to blood flow direction and then receiving the reflected ultrasonic waves, to obtain two ultrasonic Doppler shift frequency signals due to blood flow; and a section (20a,20b) for calculating the velocity of flowing blood on the basis of the received Doppler shift frequency signals. Therefore, even if the catheter is not placed in parallel to the blood flow direction, it is possible to continuously calculate the velocity of flowing blood accurately. Blood flow measurement by thermodilution methods is also included for calibration purposes.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an ultrasonic Doppler blood flow monitoring system, and more specifically to a blood flow monitoring system such that a catheter provided with ultrasonic transducers or vibrators is inserted into a blood vessel to transmit and receive an ultrasonic wave signal to and from the blood flowing through the blood vessel, in order to measure the velocity of flowing blood on the basis of the ultrasonic Doppler shift signals and further to calculate the amount of flowing blood. Description of the Prior Art With the advance of the medical technique, various serious diseases difficult in surgical treatment have become operable and therefore remediable recently. In the complicated operations performed for these diseases, there exists a tendency that a long operation time is required. Therefore, it is very important for the operator to monitor the cardiac functions of a patient accurately, that is, to grasp the condition of the patient at all times during a long-time operation. At present, there is well known a method of obtaining cardiac outputs (the amount of blood fed out of a heart) in accordance with thermodilution measurement method, as the method of monitoring the cardiac function of a patient during and after operation. In this thermodilution measurement method, a catheter is inserted from the vein through the right atrium of the heart to the pulmonary artery; cool water is injected into the right atrium through the catheter; and the cardiac output can be estimated on the basis of the change in temperature in the pulmonary artery. This catheter is provided with a balloon at the distal tip thereof. Therefore, when this balloon is expanded, it is possible to smoothly carry the catheter from the right atrium to the capillary portions of the pulmonary artery on the blood flow. The above-mentioned catheter is called SWAN-GANZ catheter and is widely used. As far as no vascular contrast medium is injected, the catheter itself can be easily inserted into the blood vessel without involving any risk. In the case of the above-mentioned thermodilution measurement method, however, since cooling water must be injected into the right atrium of the heart, the cardiac output is to be inspected at time intervals of about one hour at the most. As a result, there exists a problem in that it is impossible to monitor the cardiac output continuously for many hours. In other words, in the thermodilution measurement method, there exists a problem in that a sudden change of the condition of a patient under operation cannot be detected immediately and therefore appropriate countermeasures to be taken are delayed. The most important functions required by anesthetists, internists, and cardiac surgeons are to monitor the cardiac functions continuously for many hours. From this standpoint, the thermodilution measurement method is not the one which can satisfy their requirements. To overcome the above-mentioned problems involved in the thermodilution measurement method, CARDIOMETORICS Corp. has developed a system called DOPCOM/FLOWCATH (Commodity name) which can monitor cardiac functions continuously. In this monitoring system, a catheter provided with ultrasonic Doppler transducers is incorporated. The catheter is inserted into a pulmonary artery, and the ultrasonic Doppler transducers provided on the catheter emit ultrasonic waves into the blood vessel and then receive echoes returned therefrom. On the basis of the received ultrasonic Doppler signals resulted from the echoes, the inner diameter of the pulmonary artery and the velocity of flowing blood are both measured, in order to calculate the amount of flowing blood, that is, the cardiac output, so that the cardiac functions can be monitored. The prior art catheter adopted for the above-mentioned monitoring system will be described hereinbelow with reference to the attached drawings. Figs. 1(A) and (B) show the essential portion of a catheter 80 for the monitoring system. The catheter 80 is provided with a first ultrasonic transducer (ultrasonic vibrator) 81, a second ultrasonic transducer 82, and a third ultrasonic transducer 83. The first ultrasonic transducer 81 is disposed in the catheter 80 so that an ultrasonic wave can be transmitted and received at a predetermined angle with respect to the blood flow direction(V). The second and third ultrasonic transducers 82 and 83 are disposed in the catheter 80 facing away from each other so that the ultrasonic waves can be transmitted and received in the radial direction of the catheter 80. In this prior art monitoring system, the velocity of flowing blood is obtained on the basis of only the ultrasonic Doppler signal obtained by an ultrasonic wave transmitted and received in the single direction. In more details, the velocity component (v α ) in the transmission and reception direction of the first ultrasonic wave is calculated on the basis of an ultrasonic Doppler shift frequency obtained by the first ultrasonic transducer 81; the calculated velocity component (v α ) is further corrected on the basis of the angle (α) of incidence of the ultrasonic wave with respect to the blood flow direction (V) to calculate an absolute value (v) of the velocity of flowing blood. In the above-mentioned calculation, the assumption is made that the catheter 80 is disposed in parallel to the blood flow direction and therefore the angle (α) of incidence of the ultrasonic wave is equal to the installation angle of the first ultrasonic transducer 81 relative to the catheter 80. The above-mentioned relationship can be represented as follows: v = v α / cos α Further, the second and third ultrasonic transducers 82 and 83 also transmit and receive ultrasonic waves to measure the two radial distances d 1 and d 2 to the wall 91 of the blood vessel. The blood vessel diameter (D) can be expressed by an addition of the distances d 1 and d 2 and a radial distance interval d ⊘ between the second and third transducers 82 and 83 as follows: D = d 1 + d 2 + d ⊘ On the basis of the blood vessel diameter D, the cross sectional area of the blood vessel can be calculated as A = π D 2 / 4 On the basis of both the cross-sectional area (A) and the velocity (v) of flowing blood, the amount of flowing blood (Q) can be calculated as follows: Q = v × A As described above, in the DOPCOM/FLOWCATH system, the velocity of flowing blood is calculated on the assumption that the orientation of the catheter 80 is in parallel to the blood flow direction (V), and further the amount of flowing blood is calculated on the assumption that the sum of the measured radial distances d 1 and d 2 between each transducer and the blood vessel wall and the distance d ⊘ between the transducers is roughly equal to the blood vessel diameter (D). However, in a case where the catheter 80 is placed in a bent portion of the blood vessel, the orientation of the catheter is not always in parallel to the blood flow direction (V), as depicted in Fig. 2. In addition, in a case where the catheter 80 is placed in the blood vessel with being offset from the center of the blood vessel as depicted in Fig. 3, the second and third transducers 82 and 83 cannot necessarily measure the blood vessel diameter accurately. Further, the cross-sectional shape of the blood vessel is not always circular. As described above, in the prior art monitoring system, when the orientation of the catheter 80 is not in parallel to the blood flow direction (V), the angle (α) of incidence of the ultrasonic wave signal is not constant. As a result, the velocity (v) of the flowing blood calculated on the basis of the above values inevitably involves an error. In addition, when the catheter 80 is disposed with being offset from the center of the blood vessel, since the measured inner diameter of the blood vessel becomes a value (D1) smaller than the actual inner diameter (D) as depicted in Fig. 3, the cross-sectional area (A) calculated on the basis of the measured inner diameter of the blood vessel inevitably involves another error. Accordingly, the amount of flowing blood (Q) calculated as a product of the velocity (v) of the flowing blood and the cross-sectional area (A) of the blood vessel also inevitably includes an error. Further, a major part of the clinical data so far obtained is dependent upon the thermodilution measurement method, and the clinical data obtained by the DOPCOM / FLOWCATH system are still few. Therefore, it is not sufficiently reliable to use the DOPCOM / FLOWCATH system for the actual operation. In summary, the prior art method of measuring the amount of flowing blood by use of the DOPCOM / FLOWCATH system involves the following problems: (1) Since the calculated velocity of flowing blood and the calculated cross-sectional diameter of the blood vessel are likely to include errors, respectively, the amount of flowing blood calculated on the basis of these values is not sufficiently accurate. (2) The clinical data obtained so far by this system are few, and therefore the clinical data enough to monitor the cardiac functions during operation are not yet accumulated. A similar apparatus and a method for measuring volumetric flow of liquid is known from EP 0 363 156 A3. This system comprises three ultrasonic transducers, wherein one transducer is provided to measure the velocity of the flowing blood and the other two transducers are disposed in the catheter facing away from each other so that the ultrasonic wave can be transmitted and received in the radial direction of the catheter for measuring the cross-sectional area of the vessel through which the blood is flowing. In this system the velocity of flowing blood is calculated on the basis of an ultrasonic Doppler signal obtained by an ultrasonic wave transmitted and received in a single direction. Thus, the velocity component in the transmission and reception direction of the ultrasonic wave is calculated on the basis of an ultrasonic Doppler-shift frequency obtained by a single ultrasonic transducer. The calculated velocity component is further corrected on the basis of the angle of incidence of the ultrasonic wave with respect to the blood flowing direction to calculate an absolute value of the velocity of the flowing blood. Furthermore, from EP 0 363 156 A3 it is known to correct an angular uncertainty due to the variability of the human anatomy wherein with the use of two transducers the angular position error can actually be measured and used to correct the volumetric flow computation. Thereby an angle error is calculated on the basis of two beams which intersect in a single range gate. The angle error is then inserted in the formula for the volumetric flow, so that the angle error of the volumetric flow calculation is compensated. SUMMARY OF THE INVENTION Therefore it is the object of the present invention to provide an ultrasonic Doppler blood flow monitoring system, which can measure the velocity of flowing blood continuously without being susceptible to change in the angle of incidence of the ultrasonic wave with respect to the blood flow direction. The object is solved by an ultrasonic Doppler blood flow monitoring system with the features of claim 1. According to the ultrasonic Doppler blood flow monitoring system of the present invention, since the amount of flowing blood can be measured on the basis of the ultrasonic Doppler shift frequency signals without being susceptible to change in the angle of incidence of each ultrasonic wave with respect to the blood flow direction, even if the catheter is not placed in parallel to the blood flow direction at a bent portion of the blood vessel, it is possible to continuously measure the velocity of flowing blood accurately. In the ultrasonic Doppler blood flow monitoring system, the two ultrasonic transducers are arranged in such a way that two ultrasonic waves are transmitted and received at two different angles of incidence with respect to the blood flow direction. Further, the two ultrasonic transducers are arranged in such a way that the two ultrasonic waves transmitted from the respective ultrasonic transducer toward the blood flow are perpendicular to each other and not intersected with each other. The object, construction and advantages of the present invention will be apparent from the following description with reference to the attached drawings. Constructions not in accordance with the invention are merely described for illustration. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(A) is an illustrative side view showing a catheter of the prior art ultrasonic Doppler blood flow monitoring system, which is inserted into a blood vessel; Fig. 1(B) is an illustrative cross-sectional view showing the same catheter shown in Fig. 1(A); Fig. 2 is a similar illustrative side view for assistance in explaining the problem involved in the catheter of the prior art ultrasonic Doppler blood flow monitoring system; Fig. 3 is an illustrative cross-sectional view for assistance in explaining the problem involved in the catheter of the prior art ultrasonic Doppler blood flow monitoring system; Fig. 4 is a block diagram showing an entire system configuration of the ultrasonic Doppler blood flow monitoring system according to the present invention; Fig. 5 is an enlarged perspective view showing a distal portion of the catheter not according to the present invention; Fig. 6 is an illustration showing the state where the distal portion of the catheter reaches the pulmonary artery; Figs. 7(A) and (B) are side views for assistance in explaining arrangements of the ultrasonic transducers of the ultrasonic Doppler blood flow monitoring system not according to the present invention; Fig 7(C) shows the arrangement according to the present invention. DETAILED DESCRIPTION Fig. 4 shows an entire system configuration. A catheter 1 insertable into a blood vessel is provided with two ultrasonic transducers (vibrators) 10 and 12, each of which includes a single transducer element. Two transmitters 14a and 14b each for transmitting an ultrasonic transducer exciting signal are connected to the two ultrasonic transducers 10 and 12, respectively. In response to the exciting signals transmitted by the transmitters 14a and 14b, each of the ultrasonic transducers 10 and 12 transmits an ultrasonic wave into a blood vessel, respectively and further receives a reflected ultrasonic wave (echo) reflected from a blood cell in a blood of high-speed moving blood flow within the vessel, respectively. In this case, the frequency of the transmitted ultrasonic wave is different from that of the reflected ultrasonic wave due to the Doppler effect. The difference between the frequencies is called Doppler shift. The reflected ultrasonic wave with the Doppler shift is converted into an electrical signal in each transducer 10, 12. Thus generated ultrasonic Doppler shift frequency signals in the transducers 10 and 12 are fed to two receivers 16a and 16b connected to the two transducers 10 and 12, respectively, and further given to two Doppler shift frequency calculation sections 20a and 20b (included in a blood flow velocity measuring section 18) both connected to the receivers 16a and 16b, respectively. Here, both the Doppler shift frequency calculation sections 20a and 20b are the same in configuration. Therefore, only the calculation section 20a will be described hereinbelow in further details. In the Doppler shift frequency calculation section 20a, a quadrature detector 22 detects the received signal to detect only a Doppler shift frequency signal from the reflected ultrasonic wave, and transmits the detected Doppler shift frequency signal to a sample hold circuit 24. In this processing, the detected output is divided into two channels, and processed on the basis of the two reference wave signals having a 90 degree phase difference between the both, for instance as follows: The sample hold circuit 24 extracts a Doppler shift frequency signal of any given depth from the output signal of the quadrature detector 22, and transmits the extracted signal to a high-pass filter 26 and then to a low-pass filter 28, in sequence. Here, the Doppler detection output signal includes not only a signal reflected from a high-speed moving blood flow but also an unnecessary signal (referred to as clutter) reflected from a low-speed moving substance (e.g. heart wall). Therefore, the signal extracted by the sample hold circuit 24 is passed through the high-pass and low-pass filters 26 and 28 connected in series, to remove the clutter, before being transmitted to two A/D convertors 30 arranged in parallel to each other. The A/D convertors 30 convert the two analog signals from each of which clutter is removed, respectively into two digital signals, and transmit the respective digital signals of two channels to a phase calculator 32. The phase calculator 32 obtains a phase of the digital signal for each channel, and transmits thus obtained phase signal to a phase difference calculator 34. The phase difference calculator 34 calculates a phase difference between the two on the basis of the phases of the respective channels, and thereby calculates the velocity component in the ultrasonic wave transmit/receive direction in every pulse on the basis of the calculated phase difference. The phase difference calculator 34 transmits thus obtained data to an average circuit 36. The average circuit 36 calculates an average of the velocity components based on the velocity component data which have been obtained in each pulse, and then transmits the calculated results to an absolute value calculator 38. The absolute value calculator 38 calculates an absolute value of the velocity of flowing blood on the basis of the average of the velocity components calculated based on the velocity component in each pulse on the basis of the signals obtained by the respective ultrasonic transducers 10 and 12, and outputs the calculated result to the display section 40. Fig. 5 shows in detail the distal portion of a catheter 1 not according to the invention. As shown in the drawing, the catheter 1 is provided with two ultrasonic transducers 10 and 12 for measuring the velocity of flowing blood. The two transducers 10 and 12 are so arranged that two ultrasonic waves are emitted at two different angles of incidence with respect to the blood flow direction, respectively. In this arrangement, each ultrasonic wave has the same frequency. The frequency is preferably to be set from 5MHz to 20MHz. However, it is theoretically possible to constitute the transducers so as to have different frequencies with each other. Further, the catheter 1 is provided with a balloon 42 for allowing the catheter 1 to be easily inserted into the pulmonary artery 95 through the right atrium 93 of the heart so as to be carried by the blood flow. This balloon 42 is formed of an expansible and contractible material such as rubber and is normally brought into tight contact with the catheter 1. When a working fluid such as air is injected from the outside into the balloon 42 through a lumen formed in the catheter, the balloon 42 is inflated so as to form a large diameter portion near the distal tip of the catheter 1 as shown in Fig. 5. Therefore, when the distal tip of the catheter 1 is inserted into the jugular vein or the femoral vein and then the balloon 42 is inflated, the catheter 1 is carried to the heart by the blood flow, that is, through the right atrium 93 and the right ventricle 94 up to the pulmonary artery 95. Thereafter, the balloon 42 is deflated to fix the catheter 1 in position. Fig. 6 shows the state where the catheter 1 is fixed at a predetermined position. Under these conditions, the ultrasonic transducers 10 and 12 provided on the catheter 1 are located in the pulmonary artery 95 so as to transmit ultrasonic waves to the blood flow and further to receive the ultrasonic waves reflected from the blood flow. In the above-mentioned arrangement, the two ultrasonic transducers 10 and 12 are arranged on the catheter 1 in such a way that the angles (α, α + ) of incidence of the two ultrasonic waves transmitted from the respective transducers 10 and 12 with respect to the blood flow direction (V) are different from each other as depicted in Fig. 7(A). Further, the reflected ultrasonic wave signals received by these transducers 10 and 12 are converted into electrical signals, and then fed to the respective receivers 16a and 16b. The two frequencies of these two reflected ultrasonic wave signals are shifted respectively due to the Doppler shift effect in response to the velocity of flowing blood. These frequency shifts are calculated by the two Doppler shift frequency calculation sections 20a and 20b. independently. The relationship between the Doppler shift frequencies (Δf 1 , Δf 2 ) and the absolute value of the blood flow velocity (v) can be represented as follows: Δf 1 = ( 2 f c / c ) v cos α Δf 2 = ( 2 f c / c ) v cos (α + ) where f c : Frequency of transmitted ultrasonic wave c : Sound velocity of a living body v : Absolute value of velocity of flowing blood α : Angle of incidence of transmitted ultrasonic wave to the blood flow direction  : An angle between two ultrasonic waves Therefore, by eliminating α by use of the two equations (1) and (2), the following equation can be obtained. v = { c / (2 f c sin )} × { (Δf 1 ) 2 - 2Δf 1 Δf 2 cos  + (Δf 2 ) 2 } 1/2 On the basis of the above equation (3), the absolute value calculator 38 calculates the absolute value of the velocity (v) of flowing blood without being susceptible to change in the angle of incidence of the ultrasonic waves. In this arrangement, since the ultrasonic waves are transmitted and received in the two different directions, it is possible to calculate the absolute value of the velocity (v) of flowing blood even when the catheter 1 is placed in a bent portion of the blood vessel and therefore the orientation of the catheter 1 is not in parallel to the direction of the blood flow. As described above, in order to measure the absolute value of the velocity of flowing blood, irrespective of the angle of incidence of the ultrasonic waves with respect to the blood flow direction, at least two ultrasonic waves are transmitted and received to and from the flowing blood along two different directions. Therefore, the arrangement of the two transducers 10 and 12 in the catheter 1 as shown in Fig. 7(A) is sufficient to measure the absolute velocity of flowing blood. To facilitate the manufacturing process and the precision management of the catheter 1, however, it is preferable to arrange the two transducers 10 and 12 in intersectional positional relationship at right angles with respect to each other, as in the arrangement not according to the invention shown in Fig. 7 (B) and in the embodiment of the present invention shown in Fig. 7(C). In this case, since the angle  between the two transducers is 90 degrees, the above equation (3) can be simplified as follows: v = { c / (2 f c ) } × { (Δf 1 ) 2 + (Δf 2 ) 2 } 1/2 In the above-mentioned arrangements of the two transducers, it is impossible to match the respective sample points of the two transducers with each other. However, since the object of this system is to measure the velocity of flowing blood within the blood vessel, the depth of the sample point is about 1 cm at the most. Therefore, when the transducer of a 1-mm square is used, it is possible to reduce the distance between the two sample points of the respective transducers as small as 2 cm or less. If the distance between the two sample points can be reduced to such a small extent, it is possible to consider that the direction of the blood flow is roughly the same, and therefore there arises no specific problem even if the velocity of flowing blood is calculated in accordance with the above equation (4). Further, in the case of the transducer arrangement as shown in Fig. 7(B), two ultrasonic waves transmitted from the two transducers intersect with each other. Therefore, it is necessary to activate the two transducers alternately so as to transmit and receive the respective ultrasonic waves in order to avoid interference between the ultrasonic waves. On the other hand, in the case of the transducer arrangement of the present invention shown in Fig. 7(C), although the distance between the two sample points is further increased, since the two ultrasonic waves transmitted from the traducers, respectively do not intersect with each other, it is possible to transmit and receive the two ultrasonic waves simultaneously, so that it is possible to measure a higher velocity of flowing blood, as compared with the case as shown in Figs. 7(A) and (B). As described above, in the system according to the present invention, since the absolute value of the velocity of flowing blood can be measured without being susceptible to change in the angle of incidence of the ultrasonic waves transmitted from the ultrasonic transducers with respect to the blood flow direction, it is possible to measure the velocity of flowing blood at a high accuracy, even when the catheter 1 is placed in a bent portion of a blood vessel, and thereby not arranged in parallel to the blood flow direction.

Claims: An ultrasonic Doppler blood flow monitoring system, comprising: a catheter (1) to be inserted into a blood vessel; a pair of ultrasonic transducers (10, 12) provided on said catheter (1) for transmitting ultrasonic waves toward blood flow flowing in the blood vessel In a certain direction and receiving the ultrasonic waves which are reflected therefrom and Doppler-shifted by the blood flow; and calculating means (18) for calculating a velocity of the blood flow on the basis of ultrasonic Doppler-shifted frequency signals obtained from the received ultrasonic waves by said ultrasonic transducers (10, 12); characterized in that said ultrasonic transducers (10, 12) are provided for obtaining information necessary for measuring the velocity of the blood flow flowing in the blood vessel, and said ultrasonic transducers (10, 12) are arranged on said catheter (1) such that said ultrasonic waves are transmitted toward two spaced sampling points in the blood flow through an angle of 90° defined therebetween without intersecting with each other thereby these ultrasound waves are transmitted toward the blood flow from different two directions with different angles of α and α + 90° with respect to the blood flow direction, wherein said calculating means calculates the velocity of the flowing blood in accordance with the following equation 3 using the ultrasonic Doppler-shifted frequence signals obtained from said pair of ultrasonic transducers and represented by the following equations 1 and 2 without being susceptible to the angular relationship between the blood flowing direction and the longitudinal direction of the catheter: Δf 1 = (2f c /c) v cos α Δf 2 = (2f c /c) v cos (α + 90°) v = {c/(2f c )} x {(Δf 1 ) 2 + (Δf 2 ) 2 } 1/2 where f c : Frequency of transmitted ultrasonic wave c : Sound velocity of a living body v : Absolute value of velocity of flowing blood α : Angle of transmitted ultrasonic wave with respect to the blood flow direction

Inventors: AKAMATSU SHIGERU, KONDO YUJI, AKAMATSU, SHIGERU, KONDO, YUJI
Applicants: AKAMATSU SHIGERU, ALOKA CO LTD, AKAMATSU, SHIGERU, ALOKA CO., LTD.
Application year: 1992
Main classification: A61B8
IPC codes: A61B5, A61B8