METHOD AND SYSTEM FOR DETERMINING A FLOW SPEED OF A FLUID FLOWING THROUGH AN IMPLANTED, VASCULAR ASSISTANCE SYSTEM

Abstract

The invention relates to a method for determining at least one flow parameter of a fluid (31) flowing through an implanted vascular support system (10), comprising the following steps: a) estimating the flow velocity of the fluid (31), b) carrying out a pulsed Doppler measurement using an ultrasound sensor (18) of the support system (10) in an observation window (201) inside the support system (10), wherein the observation window (201) is displaced at an observation window velocity which is determined using the flow velocity estimated in Step a), c) determining the at least one flow parameter of the fluid using at least one measurement result of the pulsed Doppler measurement or a measurement result of the pulsed Doppler measurement and the observation window velocity.

Claims

1-19. (canceled)

20. A method for determining at least one flow parameter of blood in a cardiac support system, the method comprising: estimating a flow velocity of the blood; performing a pulsed Doppler measurement using an ultrasound sensor of the cardiac support system to generate at least one Doppler measurement result, wherein, during the pulsed Doppler measurement, the ultrasonic sensor is within an observation window within a cannula-like section of the cardiac support system, and wherein the observation window is displaced at an observation window velocity that is based on the estimated flow velocity; determining the at least one flow parameter of the blood based on at least one Doppler measurement result.

21. The method of claim 20, wherein determining the at least one flow parameter of the blood comprises determining the flow velocity of the blood based on the at least one Doppler measurement result and the observation window velocity.

22. The method of claim 20, wherein the flow velocity is estimated based on an operating parameter of a flow machine of the cardiac support system.

23. The method of claim 20, further comprising determining the observation window velocity so that a Doppler shift is transformed into a range that can be displayed without ambiguity on a frequency spectrum.

24. The method of claim 20, wherein the observation window velocity corresponds substantially to the estimated flow velocity.

25. The method of claim 20, further comprising displacing the observation window by changing a time interval between an emission of an ultrasonic pulse and a start time of a measurement time interval between ultrasonic pulses.

26. The method of claim 20, wherein the observation window velocity and a sampling rate are correlated with one another.

27. The method of claim 20, further comprising determining the observation window velocity such that the at least one Doppler measurement result and a Doppler shift caused by static scatterers are spaced apart from one another on a frequency spectrum.

28. The method of claim 20, wherein determining the at least one flow parameter of the blood comprises determining the flow velocity of blood by adding together the observation window velocity and a relative velocity determined based on the at least one Doppler measurement result.

29. A cardiac support system, the cardiac support system comprising: a controller configured to determine at least one flow parameter of blood flowing through the cardiac support system, the controller comprising: a device configured to estimate a flow velocity of the blood, a device configured to carry out a pulsed Doppler measurement using an ultrasonic sensor to generate at least one Doppler measurement result, wherein, during the pulsed Doppler measurement, the ultrasonic sensor is within an observation window within a cannula-like section of the cardiac support system, and wherein the observation window is displaced at an observation window velocity that is based on the estimated flow velocity; and a device configured to determine at least one flow parameter of the blood based on at least one Doppler measurement result.

30. The system of claim 29, wherein the device configured to determine the at least one flow parameter of the blood is configured to determine a flow velocity of the blood based on at least one Doppler measurement result and the observation window velocity.

31. The system of claim 29, wherein the device configured to estimate the flow velocity of the blood is configured to estimate the flow velocity of the blood based on an operating parameter of a flow machine of the cardiac support system.

32. The system of claim 29, wherein the observation window velocity of the observation window is selected so that a Doppler shift is in a range that can be displayed without ambiguity on a frequency spectrum.

33. The system of claim 29, wherein the observation window velocity corresponds substantially to the estimated flow velocity.

34. The system of claim 29, wherein the device configured to carry out a pulsed Doppler measurement is configured to displace the observation window by changing the time interval between an emission of an ultrasound pulse and a start time of a measurement time interval from ultrasound pulse to ultrasound pulse.

35. The system of claim 29, wherein the observation window velocity and a sampling rate of the blood flowing through the cardiac support system are correlated with one another.

36. The system of claim 29, wherein the observation window velocity is determined such that the measurement result of the pulsed Doppler measurement and a Doppler shift caused by static scatterers are spaced apart from one another on a frequency spectrum.

37. The system of claim 29, wherein the device configured to determine the at least one flow parameter of the blood is configured to determine a flow velocity of the blood by adding together the observation window velocity and a relative velocity determined on the basis of the pulsed Doppler measurement.

Description

[0065] The solution presented here as well as its technical environment are explained in more detail below with reference to the figures. It is important to note that the invention is not intended to be limited by the design examples shown. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the facts explained in the figures and to combine them with other components and/or insights from other figures and/or the present description. The figures show schematically:

[0066] FIG. 1: an implanted vascular support system in a heart,

[0067] FIG. 2: a further implanted vascular support system in a heart,

[0068] FIG. 3: a sequence of a here presented method,

[0069] FIG. 4: an example Doppler frequency spectrum,

[0070] FIG. 5: a further example Doppler frequency spectrum,

[0071] FIG. 6: a detail view of a here proposed support system,

[0072] FIG. 7: example Doppler frequency spectra,

[0073] FIG. 8: further example Doppler frequency spectra,

[0074] FIG. 9: further example Doppler frequency spectra, and

[0075] FIG. 10: a system comprising an implantable vascular support system and comprising a control and/or processing device for determining at least one flow parameter of a fluid flowing through the implantable vascular support system.

[0076] FIG. 1 schematically shows an implanted vascular support system 10 in a heart 20. The support system 10 supports the heart 20 by helping to convey blood from the (left) ventricle 21 into the aorta 22. For this purpose, the support system 10 is anchored in the aortic valve 23, as exemplified by FIG. 1. At a level of support of 100%, the support system 10 (LVAD) conveys the entire blood volume flow. The level of support describes the proportion of the volume flow conveyed through the support system 10 by a conveying means, such as a pump of the support system 10, to the total volume flow of blood from the ventricle 21 to the aorta 22.

[0077] Accordingly, at a level of support of 100%, the total fluid volume flow 32 from the ventricle 21 and the fluid volume flow 31 through the support system 10 are identical. The aortic valve or bypass volume flow (not shown here; symbol: Q.sub.a) is consequently zero. The total fluid volume flow 32 can also be described as (total) cardiac output (CO, symbol: Q.sub.CO). The fluid volume flow 31 can also be referred to as a so-called pump volume flow (symbol: Q.sub.p), which quantifies only the flow through the support system 10 itself. The level of support can thus be calculated from the ratio Qp/Q.sub.CO.

[0078] As an example, the support system 10 according to FIG. 1 is a cardiac support system in aortic valve position. The cardiac support system 10 is positioned in a heart 20. Blood is withdrawn from ventricle 21 and delivered into the aorta 22. The operation of the cardiac support system 10 (pump part) produces a blood flow 31.

[0079] In support systems 10 of the type shown in FIG. 1, the blood is conveyed inside a cannula-like section or channel 200 of the (cardiac) support system 10 through the aortic valve 23 and discharged again in the region of the aorta 22. The tip of the support system 10 (which projects into the ventricle 21) is particularly preferably suitable for the integration of an ultrasound transducer, so that the blood then flows away from the ultrasound transducer into the cannula-like section or channel 200 of the (cardiac) support system 10.

[0080] FIG. 2 schematically shows a further implanted vascular support system 10 in a heart 20. As an example, the support system 10 according to FIG. 2 is a cardiac support system in apical position. The reference signs are used consistently, so that reference can be made in full to the preceding statements regarding FIG. 1.

[0081] In support systems 10 of the type shown in FIG. 2, the blood is drawn in through a cannula-like section or channel 200 and returned to the aorta 22 through a bypass 19 outside the heart 20. In this case, the integration of an ultrasound transducer in the pump housing of the (cardiac) support system 10, looking out of the cannula-like section 200 drawing in the blood in the direction of the ventricle 21, is most suitable. In other words, this means in particular that the ultrasound transducer is disposed in the support system 10, and is oriented toward the channel 200 and toward the ventricle 21. In this case, the blood flows toward the ultrasound transducer. The method proposed here works equally well with both variants of FIG. 1 and FIG. 2, because only the movement direction of the measurement window has to be adjusted (for example in a computer program).

[0082] FIG. 3 schematically shows a sequence of a method presented here in a system for determining at least one flow parameter of a fluid flowing through an implantable vascular support system.

[0083] The method is used to determine a flow velocity of a fluid flowing through an implanted vascular support system. The shown sequence of the method steps a), b) and c) with Blocks 110, 120 and 130 is only an example and can be the result of a regular operating sequence. In Block 110, the flow velocity of the fluid is estimated. In Block 120, a pulsed Doppler measurement is carried out using an ultrasound sensor of the support system in an observation window inside the support system, whereby the observation window is displaced at an observation window velocity which is determined using the flow velocity estimated in Step a). In Block 130, the at least one flow parameter of the fluid is determined using at least one measurement result of the pulsed Doppler measurement and/or a measurement result of the pulsed Doppler measurement and the observation window velocity.

[0084] For an example illustration of the method, the following parameters are assumed: [0085] Diameter inlet or measurement region, e.g. 5 mm, [0086] Maximum blood flow to be measured, e.g. Q=9 l/min, [0087] Resulting max. blood flow velocity: v.sub.Blood,max=7.64 m/s, [0088] Speed of sound in blood, e.g. c.sub.Blood=1540 m/s, [0089] Ultrasound frequency, e.g. f.sub.0=6 MHz, [0090] Distance of ultrasound element to the beginning of the viewing window, e.g. 25 mm, [0091] Number of ultrasonic oscillation cycles per emitted ultrasound PWD pulse, e.g. 10, [0092] Resulting length of the wave packet produced by the ultrasound pulse (in distance): I.sub.Burst=c.sub.0×10/f.sub.0=2.57 mm, [0093] Resulting maximum propagation distance of ultrasound pulse: d=55.13 mm.

[0094] For a measurement directly in the direction of emission (flow direction corresponds to the primary emission direction; α=0), these specifications result in the following (expected) maximum Doppler shift:

[00004]$\begin{array}{cc}\mathrm{df}=\frac{2.Math.v_{\mathrm{Blood},\max }.Math.f_0}{c_0}=\frac{2.Math.7,64\frac{m}{s}.Math.6\mathrm{MHz}}{1540\frac{m}{s}}=59,53\mathrm{kHz}& \left(1\right)\end{array}$

[0095] The measurement should be carried out as a pulsed Doppler measurement, in which a new ultrasound pulse is emitted only when all significant echoes of an ultrasound pulse emitted immediately prior have decayed. The selection of the pulse repetition rate (PRF) to be used for this is explained in the following.

[0096] Taking into account the (Nyquist) sampling theorem (which, however, does not have to be considered here or, because only the relative velocity between the fluid and observation window has to be recorded, does not become satisfiable until the observation window is displaced), a maximum Doppler frequency of 59.53 kHz in a real-valued analysis would mean that a minimum pulse repetition rate or a minimum pulse repetition frequency of

PRF.sub.min=2.Math.df=119.06 kHz.  (2)

would have to be set.

[0097] In the case of the implanted, vascular support systems in focus here, however, the following maximum pulse repetition rate PRF.sub.max results from the geometric consideration (maximum propagation distance of the ultrasound pulse) or the geometric boundary conditions in the support system and the resulting transit time of all relevant signal components:

[00005]$\begin{array}{cc}{\mathrm{PRF}}_{\max }=\frac{c_{\mathrm{Blood}}}{d}=27.93\mathrm{kHz}& \left(3\right)\end{array}$

[0098] Thus the maximum pulse repetition rate of the pulsed Doppler measurements here (or for the support systems in focus) is less than twice the maximum occurring Doppler shift.

[0099] These boundary conditions lead to a violation of the sampling theorem and consequently to an ambiguity of the measurement results, which can be remedied by an evaluation or method (displacing the observation window) as described in the following sections.

[0100] First, however, to illustrate the problems that occur with these boundary conditions, the resulting ambiguity is illustrated in FIGS. 4 and 5 (which can advantageously be avoided with the method presented here). FIG. 4 schematically shows an example Doppler frequency spectrum 40. FIG. 4 shows a Doppler shift at a volume flow of 3 l/min and a pulse repetition rate of approx. 25 kHz. The primary frequency component 41 (peak) is below the carrier frequency at approx. 0 Hz. FIG. 5 schematically shows a further example Doppler frequency spectrum 40. FIG. 5 shows a Doppler shift at a volume flow of 3 l/min and a pulse repetition rate of approx. 20 kHz. The primary frequency component 41 (peak) is at approx. +8 kHz. This illustrates in particular that different frequencies are output at different PRFs and the identical volume flow and, as a result, a volume flow set by the pump cannot be determined unambiguously without the use of the invention described here. At 20 kHz PRF, for example, the peak is at 3l/min at a frequency of approx. 8 KHz, which would in particular correspond to a velocity of 0.77 m/s or a volume flow of 0.9 l/min. However, the actual volume flow (to be measured) is 3 l/min in this example. These measurements were also carried out at an 8 MHz ultrasound frequency.

[0101] An example method in the sense of the solution proposed here, in which respective, ambiguous measurement results can advantageously be avoided, is described in the following sections.

[0102] For this purpose, it is proposed that the observation window be displaced at an observation window velocity, which is determined using an estimated flow velocity of the fluid (here the blood). This advantageously allows the Doppler shift to be transformed into a range that can be displayed without ambiguity using the selected ultrasound frequency and PRF. In connection with the displacement of the observation window, it is particularly advantageous if the radial cross-sections of the blood flow velocities are unchanged over a specific range (a few centimeters) in axial extension to the ultrasound element. The described method can be used in cardiac support systems of different types, for example in systems in aortic valve position as shown as an example in FIG. 1 or, for example, also in apically placed systems as shown as an example in FIG. 2.

[0103] An ultrasound-based pump volume flow measurement is usually based on one or more ultrasound transducers integrated into the support system and an optionally spatially offset (electronic) control and/or processing device, which can also be referred to as a measuring unit. The spatially offset control and/or processing device can be placed implanted and also placed extracorporeally connected by a transcutaneous lead. Together with the implantable vascular support system, it then forms a system for determining at least one flow parameter of the fluid flowing through the implantable vascular support system.

[0104] The described embodiments of FIG. 1 and FIG. 2 in particular require a pulsed Doppler measurement method (pulsed wave Doppler) in order to be able to position the observation window (the measurement region or the measurement window) along the main beam direction of the ultrasound transducer. The task of the control and/or processing device and/or the measuring unit is to produce suitable ultrasound pulses to be emitted by the ultrasound transducer or transducers, receive and amplify received scattered ultrasound energy (reflections, echo), and process the received signals to calculate a Doppler frequency spectrum.

[0105] Given the sufficiently known speed of sound in blood, the selection of the position of the observation window usually takes place via time intervals. After the emission of an ultrasound pulse, reflections from scatterers (e.g. blood cells) are immediately received directly in front of the transducer. Then, as the wavefront advances further, reflections from more distant regions are received. In a pulsed Doppler method, the received signals are processed only in a specific time interval temporally spaced apart from the time of emission of the ultrasound pulse.

[0106] The spatial distance of the observation window to the transducer plane of the ultrasound transducer can be selected or set via the selection of the time interval. The spatial extent of the observation window along the main beam direction of the ultrasound transducer can be selected or set via the length of the time interval.

[0107] A pulsed Doppler measurement usually consists of a large number of individual ultrasound pulses, i.e. a rapid sequence of emission and reception times with the frequency PRF (pulse repetition frequency). In this context, the PRF is in particular the duration from emission pulse to emission pulse. Changing the time interval between emission and measuring time interval from pulse to pulse, results in a moving observation window. In other words, this also means that, in order to displace the observation window, the time interval between an emission of an ultrasound pulse and a starting point of a measuring time interval has to be changed from ultrasound pulse to ultrasound pulse.

[0108] FIG. 6 schematically shows a detail view of a here proposed support system 10. The illustration of FIG. 6 relates to an example of a structure of a cardiac support system 10, in which a method proposed here can be used.

[0109] The ultrasound element 18 here represents the ultrasound sensor 18 and radiates in the direction of the blood flow velocity. In the region of an inlet cage 12 (provided with openings) of the support system 10, the inflowing blood 31 does not exhibit a constant flow profile yet. In the further course downstream in the regions 202 and 204, however, the radial flow profile is largely constant. The observation window 201 can thus advantageously be displaced in this region at the observation window velocity V.sub.Gate. In the embodiments shown in FIG. 1 and FIG. 2, the regions 202 and 204 can be located in the channel 200, for example.

[0110] If, for example, as shown in the following equation (4), a flow velocity of v.sub.Blood=3 m/s away from the piezo element of the ultrasound sensor 18 is to be measured in a fixed observation window at a PRF of 25 kHz and an ultrasound frequency of f.sub.0=4 MHz, a Doppler shift of −15.58 kHz will result. At the given PRF of 25 kHz and the evaluation of positive and negative velocities, this Doppler shift can no longer be displayed in the negative part of the Doppler spectrum and is therefore displayed as 9.42 kHz in the positive frequency domain of the spectrum.

[0111] However, if (as proposed here) the observation window 201 is moved with a displacement velocity of v.sub.Gate=1.75 m/s away from the piezo element of the ultrasound sensor 18, for example, the resulting (or relative) flow velocity to be transformed is reduced; here for example reduced to 3 m/s−1.75 m/s=1.25 m/s. At a PRF of 25 kHz, the resulting Doppler shift of −6.49 kHz can be displayed in the Doppler spectrum without ambiguity (see the following equation (7)).

[00006]$\begin{array}{cc}{f}_{d,\mathrm{wrapped}}=\frac{-2.Math.v_{\mathrm{Blood}}.Math.f_0}{c_0}& \left(4\right)\\ \frac{-2.Math.3\frac{m}{s}.Math.4\mathrm{MHz}}{1540\frac{m}{s}}& \left(5\right)\\ =-15,58\mathrm{kHz}& \left(6\right)\\ f_{d,\mathrm{trackingdoppl}}=\frac{-2.Math.\left(v_{\mathrm{Blood}}-v_{\mathrm{Gate}}\right).Math.f_0}{c_0}& \left(7\right)\\ =\frac{-2.Math.\left(3\frac{m}{s}-1,75\frac{m}{s}\right).Math.4\mathrm{MHz}}{1540\frac{m}{s}}& \left(8\right)\\ =-6,49\mathrm{kHz}& \left(9\right)\end{array}$

[0112] This is an example of how and that the observation window velocity can be determined such that a Doppler shift is transformed into a range that can be displayed without ambiguity.

[0113] A previously performed estimation of the flow velocity of the blood through the support system is in particular a basis for a corresponding determination of the observation window velocity here. This estimation is advantageously based on a previously performed ultrasound measurement (e.g. with a fixed observation window) using the ultrasound sensor 18 of the support system 10. However, this is only an example. The estimation could, for example, also be based on an empirical value, for example based on the age of the patient and/or the severity of the patient's disease.

[0114] FIG. 7 schematically shows example Doppler frequency spectra. The Doppler frequency spectra shown can, for example, result from the use of the method presented here.

[0115] FIG. 7 illustrates Doppler spectra at a blood flow velocity of 3 m/s at an ultrasound frequency of 4 MHz with an unfocused piezo element having a diameter of 6 mm and a PRF of 25 kHz. FIG. 7a illustrates the aliased (ambiguity-fraught) Doppler spectrum of a measurement with an observation window at a fixed distance of 25 mm from the piezo element. In contrast, FIG. 7b illustrates the non-aliased (unambiguous) Doppler spectrum with an observation window shifted 15 mm to 25 mm from the piezo element at a displacement velocity (observation window velocity) of 1.75 m/s.

[0116] Two deflections or peaks can furthermore be seen in each of the Doppler spectra shown in FIG. 7, namely a peak resulting from the Doppler shift caused by the aortic wall (non-moving scatterer) 42 and a peak caused by reflection on moving scatterers (e.g., blood cells) 43. In FIGS. 7 and 8, the solid lines describe results of a Fourier transformation and the dashed lines describe results of the so-called Welch method.

[0117] FIG. 7 illustrates how aliasing can be prevented by the method described here. FIG. 7b further shows how displacing the observation window on the right side of the spectrum results in a second peak 42 beyond 0 Hz. This peak 42 (which describes the Doppler shift of the non-moving scatterer aortic wall, for example) results from the relative movement of the observation window to the stationary tissue, e.g. the aortic wall, and thus shows the Doppler frequency of the observation window or the Doppler frequency which affects the observation window velocity.

[0118] FIG. 7b also shows that the peak widths of the two peaks (in comparison to the peak widths in FIG. 7a) change due to the movement of the observation window. Peak 42, which is caused by reflection on the stationary tissue of the aortic wall, widens. In contrast, peak 43, which is caused by scatterers (such as blood cells) moving at the blood flow velocity, narrows.

[0119] FIG. 8 schematically shows other example Doppler frequency spectra. The Doppler frequency spectra shown can, for example, result from the use of the method presented here.

[0120] The deterioration of the signal-to-noise ratio (SNR), which can be seen in FIG. 7, is a consequence of a mismatch between the observation window velocity and the sampling rate of the received signal, which causes the observation window to jitter. Reducing the jitter, and thereby improving the SNR in the spectrum, can, for example, be achieved by adapting the sampling frequency to the observation window velocity, resampling the received signal and/or oversampling.

[0121] The following equation (10) shows how the observation window velocity of the observation window and the sampling rate can be adapted to one another in a particularly advantageous manner. FIG. 8 illustrates the aforementioned possibilities for improving the SNR.

[0122] Equation 10 shows how the velocity of the observation window can be selected in a particularly advantageous manner in order to maximize the SNR at the given speed of sound in blood c.sub.0, a given PRF and a given sampling rate f.sub.s.

[00007]$\begin{array}{cc}{v}_{\mathrm{Gate}}=n.Math.\frac{\mathrm{PRF}.Math.c_0}{2.Math.f_S}n\in Z& \left(10\right)\end{array}$

[0123] This is an example that, and, if applicable, of how, the observation window velocity and a sampling rate can be adapted to one another.

[0124] FIGS. 8a, 8b and 8c respectively show a Doppler spectrum, after the use of the method described here, at a flow velocity of 3 m/s, when using a non-focused piezo element having a diameter of 4 mm, an ultrasound frequency of 8 MHz and a PRF of 40 kHz. In each of the measurements, the result of which is illustrated in FIGS. 8a, 8b and 8c, the observation window moves from a distance of 15 mm to a distance of 30 mm from the piezo element.

[0125] In FIGS. 8a and 8b, the observation window moved at a (observation window) velocity of 1.75 m/s. A sampling rate of 20 MHz was used for FIG. 8a, and a sampling rate of 100 MHz was used for FIG. 8b. FIG. 8c illustrates the SNR with an adapted sampling rate of 20 MHz and a displacement velocity of the observation window of 1.54 m/s.

[0126] When using the method described here, it is advantageously possible to achieve another goal, namely the reduction of the spectral widening of the sought frequency peak at high blood flow velocities. This additional effect can usually not be achieved when using evaluation methods (with a fixed observation window) that do not work according to the method described here. Based on these narrower frequency peaks in the Doppler spectrum caused by the flow velocity of the blood, the accuracy of the determination (estimation) of the primary velocity component can be improved significantly.

[0127] Displacing the observation window at v.sub.Gate, the roughly estimated flow velocity of the moving scatterers (such as blood cells) in the blood, prolongs the dwell time in the observation window for all moving scatterers for which |v.sub.Blood−v.sub.Gatel|<v.sub.Blood. This can advantageously lead to an improvement of the SNR (amplitude) of √{square root over ((N))} due to the integration gain in the subsequent Fourier transformation, whereby N corresponds to the number of samples while the scatterer is in the observation window.

[0128] For the static scatterers, e.g. the aortic wall, for which the condition |v.sub.Blood−v.sub.Gate|<v.sub.Blood is not fulfilled, the scatterers no longer move in the observation window during the entire observation period as in a conventional evaluation. By using the method described here, this duration is shortened significantly, in particular as a function of the flow velocity of the blood or the velocity of the observation window. This can also be described in other words as follows: In the case of a stationary window and “one” stationary scatterer, the entire wave train is reflected on it.

[0129] Consequently, if the observation duration is selected to be less than/equal to the pulse duration of the wave train, a portion of the pulse is reflected on it during the entire observation period. This long dwell time in the observation window (time domain) produces a narrow-band peak in the spectrum (frequency domain). Moving the window shortens the dwell time, and the peak in the spectrum becomes more broad-banded. As shown in FIG. 7, the resulting reduction of the integration gain (in comparison to known methods) leads to a widening of the frequency peak caused by static scatterers (which is now no longer at 0 Hz) and to a smearing of the signal energy in the spectrum.

[0130] An additional special advantage of the method described here is that the displacement velocity of the observation window (observation window velocity) can be freely selected within certain limits. If, for example, the static scatterers observed at v.sub.Gate, which experience a Doppler shift of

[00008]$\begin{array}{cc}{f}_{d,\mathrm{start}}=\frac{2.Math.v_{\mathrm{Gate}}.Math.f_0}{c_0}& \left(11\right)\end{array}$

are in the same frequency domain as the sought Doppler shift caused by the blood flow (not two, but only one peak is detected in the spectrum), the displacement velocity of the observation window can advantageously be changed slightly, so that the sought frequency peak is no longer covered by the significantly stronger frequency peak caused by static scatterers. This effect is shown schematically in FIG. 9. The reduction of the spectral widening is not taken into account in this schematic illustration.

[0131] FIG. 9 schematically shows other example Doppler frequency spectra. The Doppler frequency spectra shown can, for example, result from the use of the method presented here.

[0132] By slightly changing the displacement velocity v.sub.Gate of the observation window, the covering of the sought frequency peak caused by the blood flow velocity by the frequency peak caused by the static scatterers can be eliminated.

[0133] In FIG. 9a, the sought Doppler frequency of the flow velocity 44 is covered. In FIG. 9b, the sought Doppler frequency of the flow velocity 44 is no longer covered. The displacement velocity of the observation window (observation window velocity) was changed slightly to do this. This is an example that, and, if applicable, of how, the observation window velocity can be determined such that the measurement result of the pulsed Doppler measurement and a Doppler shift caused by static scatterers are spaced apart from one another.

[0134] The solution presented here in particular enables one or more of the following advantages: [0135] PWD-based flow velocity or volume flow measurement is possible even with a large distance between the measurement window and the ultrasound transducer. [0136] Resolution of the geometrically caused ambiguity of the Doppler shift due to geometric boundary conditions in the support system (VAD). [0137] Reduction of the spectral widening. [0138] Increase in the accuracy of the Doppler frequency estimation. [0139] More accurate determination of the flow velocity. [0140] Preventing the sought Doppler frequency shift from being covered by the frequency peak in the Doppler spectrum caused by static scatterers, e.g. the aortic wall.

[0141] The system 45 shown in FIG. 10 comprises an implantable vascular support system 10 and includes a control and/or processing device 46 for determining at least one flow parameter of a fluid 31 flowing through the implantable vascular support system 10. The control and/or processing device 46 is connected to the implantable vascular support system 10 by a transcutaneous lead and can be placed extracorporeally. It should be noted that the control and/or processing device 46 can in principle also be designed to be implanted in the human body.

[0142] In the control and/or processing device 46, there is a device 48 for estimating the flow velocity of the fluid 31. The control and processing device 46 comprises a device 50 for carrying out a pulsed Doppler measurement using an ultrasound sensor 18 shown in FIG. 6 in an observation window 201 shown in FIG. 6 inside the support system 10, whereby the observation window 201 is displaced at an observation window velocity which is determined using the estimated flow velocity.

[0143] The device 50 for carrying out a pulsed Doppler measurement is designed to displace the observation window 201 by changing the time interval between an emission of an ultrasound pulse and a measuring time interval from ultrasound pulse to ultrasound pulse.

[0144] The control and processing device 46 further comprises a device 52 for determining the at least one flow parameter of the fluid using at least one measurement result of the pulsed Doppler measurement or a measurement result of the pulsed Doppler measurement and the observation window velocity. The device 52 for determining the at least one flow parameter of the fluid is designed to determine a flow velocity of the fluid using a measurement result of the pulsed Doppler measurement and the observation window velocity by adding together the observation window velocity and a relative velocity determined on the basis of the pulsed Doppler measurement.

[0145] The device 48 for estimating the flow velocity of the fluid 31 is used to estimate the flow velocity of the fluid 31 on the basis of an operating parameter of a flow machine of the support system 10.

[0146] The observation window velocity of the observation window 201 of the device for carrying out a pulsed Doppler measurement is designed to transform a Doppler shift into a range that can be displayed without ambiguity, whereby the observation window velocity corresponds substantially to a flow velocity estimated in the device 48 for estimating the flow velocity of the fluid 31.

[0147] In the system, the observation window velocity and a sampling rate of the fluid 31 flowing through the implanted vascular support system 10 are adapted to one another. The observation window velocity is determined such that the measurement result of the pulsed Doppler measurement and a Doppler shift caused by static scatterers are spaced apart from one another.

LIST OF REFERENCE SKINS

[0148] 10 Support system [0149] 12 Inlet cage [0150] 18 Ultrasound sensor [0151] 19 Bypass [0152] 20 Heart [0153] 21 Left ventricle [0154] 22 Aorta [0155] 23 Aortic valve [0156] 31 Fluid volume flow/blood flow [0157] 32 Total fluid volume flow [0158] 40 Doppler frequency spectrum [0159] 41 Primary frequency component [0160] 42 Peak due to Doppler shift [0161] 43 Peak due to moving scatterers [0162] 44 Flow velocity [0163] 45 System [0164] 46 Control and/or processing device [0165] 48 Device for estimating the flow velocity [0166] 50 Device for carrying out a pulsed Doppler measurement [0167] 52 Device for determining a flow parameter [0168] 200 Channel [0169] 201 Observation window