NL1010061C2 - Detection of vectorial velocity distribution. - Google Patents

Description

975231 / OGR / TL / MDO

Detection of vectorial velocity distribution

The present invention relates to a method for remotely detecting the vectorial velocity distribution in a medium, wherein a signal is sent to the medium to be examined and wherein said vectorial velocity distribution is calculated on the basis of a signal received from the medium. Such a method is particularly, but not exclusively, applicable in measuring the velocity distribution of blood in a blood vessel, for which reason the present invention will be explained hereinafter in the context of this application. It is noted that by the term "velocity distribution" it is meant that the medium to be examined does not have to move as a plug, all elements of the medium having exactly the same velocity, but that velocity variations may exist in the medium as a whole, so that the velocity components of different elements of the medium have a certain spread or distribution.

Methods of the above-described type are already known per se, for example from international patent publication WO 95/32667. This publication describes a method for determining the velocity distribution of blood in a blood vessel, whereby ultrasound pulses are transmitted to the blood vessel in a narrow beam by means of a transducer. The signals reflected by the blood are received by the transducer and passed to a signal processing device. From the reflection signals, the axial velocity component of the blood is first estimated based on the Doppler shift. 30 The transverse velocity component of the blood is then estimated. The magnitude of the velocity vector follows from the combination of these two velocity components.

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The method described in said publication has some drawbacks.

A first drawback of this known method is that for determining the velocity component of the blood in the direction of the beam axis, use is made of a statistical model to calculate the most likely value for that velocity component. This implies that assumptions are made regarding the stochastic parameters of the signal received back. This means a limitation of the applicability of the known method. An additional problem in this context is that the sound attenuation in the medium to be examined depends on the frequency of the applied sound, so that a change in spectral bandwidth of the generated signal leads to deviations in the axial calculated on the basis of the statistical model. speed component.

A second drawback of the method described in said publication is that the transverse velocity component is calculated on the basis of the rate at which intensity changes occur in the received signal. For this, however, it is necessary that the beam width at the location where the reflections occur is known J, which will generally not be the case.

A third drawback is that in order to apply this known method it is necessary that flow areas are with a laminar flow pattern.

A fourth drawback of the method described in said publication is that only an estimate is given for the magnitude of the velocity vector, but that no information is obtained from the received signal; derived with respect to the direction of the velocity vector. With respect to the axial velocity component, the transverse velocity component still has a freedom t 35 of 360 °. In the method according to said publication, it is therefore necessary that the three-dimensional direction of the velocity vector is known beforehand, for example because the orientation of the flow channel (blood vessel) to be examined is known.

The present invention aims to provide a method in which the said drawbacks are absent, or at least reduced.

In particular, the present invention aims to provide a method which is also applicable in non-laminar flow.

Furthermore, the present invention aims to provide a method by which it is possible, with as few assumptions as possible, to derive the quantities to be measured directly from the signals received back. More specifically, the present invention aims to provide a method by which it is possible to derive not only the magnitude but also the direction of the velocity vector directly from the received signals.

The present invention is based on an improved understanding of the physical processes leading to the measurable reflection signal. Based on this improved understanding, the present invention provides a model describing the reflection signal, and formulas derived therefrom, by which the transducer parameters and the quantities to be measured can be directly calculated from the measured reflection signals received.

Thus, the present invention is able to determine the quantities to be measured faster, more accurately and more consistently from the measured reflection signals, and derive more data from the observations.

More specifically, the present invention is able to derive the local beam width from the observations.

According to an important aspect of the present invention, the shape of the 1010061 4 ultrasound beam generated by the transmitter is believed to be known. This is a reasonable assumption, because that beam is fixedly associated with the respective transmitter and is reproducible under normal conditions, so that the shape of that beam can be measured in a test rig. Based on the shape of the beam, the local width of the beam can then be calculated from the signal received back itself.

An important advantage of the model proposed by the present invention is furthermore that it makes few demands on the measuring conditions and the measuring arrangement. In known models, it is necessary that the axis of the ultrasound beam is directed perpendicular to the flow direction of the medium to be examined, or that at least the angle between the axis of the ultrasound beam 15 on the one hand and the direction of flow of the investigated medium medium on the other hand is accurately known. This is not necessary in the model proposed by the present invention, and said angle can even be calculated from the measured reflection signals.

These and other aspects, features and advantages of the present invention will be further elucidated by the following discussion of an embodiment with reference to the drawing, in which: Figure 1 schematically illustrates a specific measurement arrangement for measuring flow velocity in a flow channel; Figure 2 schematically illustrates the measuring principle on which the measuring method is based; Figure 3 shows a two-dimensional representation of an example 30 of the envelope of successively measured reflection signals;

Figure 4 shows a schematic perspective view of a transducer to define a coordinate system.

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Figure 1 schematically illustrates a measuring arrangement 10 for measuring the flow velocity in a flow channel 11. In this measuring arrangement 10, the flow channel 11 is part of a closed circuit 12, in which a medium 13 is circulated by means of a pump 16.

The measuring arrangement 10 further comprises a measuring chamber 20 within which the flow channel 11 is located. A transducer 21 is arranged in the measuring chamber 20, which transducer is arranged to generate a beam of ultrasound pulses, as will be explained in more detail. The distance from the transducer 21 to the flow channel 11 can be set within certain limits, as can the angle between the beam axis and the axis of the flow channel 11.

Figure 2 schematically illustrates the applied measuring principle. The transducer 21 generates a short-duration sound pulse 22 in the direction of the medium 13 to be examined, which comprises one or more reflection cores 14. Therefore, if the emitted sound signal 22 reaches a reflection core 14, a reflected sound signal 23 will be sent back to the transducer 21. When the reflected sound signal 23 is received by the transducer 21, it generates an electrical signal 24 representative of the shape of the audio signal 23 received by transducer 21; this electrical signal 24, as known per se, is passed on to a signal processing device 30, with which a memory 31 is associated. The signal processing device 30 samples the electrical signal 24 and stores the signal samples in the memory 31.

For the further processing of the measurement data, the signal samples are made analytical using a Hilbert transform. The reflection signals are then complex numbers. An advantage of working with analytical signals is that the correlation is not affected by the carrier frequency, as will be apparent from the following. This has eliminated a disturbing influence. Any accidental or secondary maxima in the correlation (see below) will then occur much less, or at least have a lower level 5. It will be apparent to one skilled in the art that the Hilbert transformation to analytical signal samples can be performed before the signal samples are stored in memory 31, or when the signal samples are retrieved from memory 31 for further ~ 10 processing, or in a separate intermediate step . In the following, no distinction will be made between these options.

If there is only one reflective body or reflective core 14 in the medium, the receive back audio signal 23 will normally be in the form of a single pulse, with the time between the transmission of the transmission pulse 22 and the reception of the reception pulse 23 based on the sound velocity in the relevant medium 13 is a measure of the spatial distance between the transducer 21 and the reflective body or reflective core 14. The medium 13 is a diffuse medium, i.e. it contains a plurality of randomly distributed particles 14 that can function as reflection cores. This means in practice that the signal 23 received back by the transducer 21 is a combination of many reflection pulses.

Reflections that arrive at transducer 21 almost simultaneously can interfere with each other. They can reinforce each other (constructive interference) or quench it (destructive interference). Figure 3 illustrates an example of the envelope of a set of consecutive i 'of such interference signals, in the example shown, obtained by measurement in a human carotid artery. In Figure 3, the strength of the electrical signal 24 delivered by the transducer 21, or the strength of the reflection signal 23 received by the transducer 21, is plotted as a function of time (horizontal). Since, as explained above, time corresponds to distance, and distance is in fact much more interesting to the observer than time, the horizontal scale 5 shows the distance scale. The vertical axis corresponds to the successive received reflection signals.

In Figure 3, the strength of the electrical signal 24 delivered by the transducer 21 is represented by the blackening (gray value) of a point in the figure. A strong signal is hereby represented by a dark point, while a weak signal is represented by a light point.

Each horizontal line of measurement points in Figure 3 represents the envelope of the electrical response of transducer 21, after transmitting a single transmit pulse 22 through transducer 21. During a measurement procedure, transmitting a single transmit pulse 22 through transducer 21 becomes multiple repeated times (with a predetermined repetition frequency); the electrical responses of the transducer 21 (a plurality of horizontal lines of measuring points) associated with the successive transmission pulses 20 22 are shown one below the other; serial numbers of those transmit pulses are placed along the left axis of figure 3.

In general, it can be recognized in figure 3 that the constructive and destructive interference pattern (response to a transmission pulse) of the reflection signal changes as a function of time. This is a result of changing the mutual positions of the individual reflection cores 14. More specifically, the individual reflection cores are displaced due to the flow of the medium 13, and the mutual relationship between the reflection cores will change. These changes occur on a time scale greater than the repetition period of the transmit pulses. This relatively slow aspect of the change is reflected in the relatively large vertical dimensions of the spots in the graphical representation of Figure 3. More specifically, this vertical length is indicative of the transverse component of the flow rate. The tilt of those spots implies a horizontal displacement of the interference pattern as a function of time, which corresponds to the axial component of the flow of the medium.

In the following, a model proposed by the present invention will be derived for interpreting the measurement signals. The formulas used for this are collected at the end of the description.

For defining a coordinate system, reference is made to Figure 4. Figure 4 shows a transducer array 121, comprising a plurality of transducers 21 arranged side by side. Each transducer 21 can emit a sound beam 22, the beam axis 25 of which is assumed to be the Z axis. The X axis is defined by a connecting line of the transducers 21, so that the sound beams of all the transducers 21 are located together in the XZ plane. The Y axis is defined perpendicular to the ij plane defined by the transducers 21. In the following derivation, it will be assumed that use is made of two different transducers 21i and 212 of the transducer array 121, the distance in the X direction between those two transducers being denoted as ΔΧ and in the Y- direction as ΔΥ; with the described coordinate system ΔΥ is equal to zero. These two transducers 21x and 212 are chosen so that their 30 sound beams partially overlap in space. This achieves an initial correlation level between the measurement signals RFX provided by the first transducer 211 and the measurement signals RF2 provided by the second transducer 212. 1 01 0061 9 This initial correlation level is used in determining the motion components.

Assuming that the local distribution of the reflection cores 14 within the medium 13 is constant during the passage of a sound wave, the first reflection signal RF1 (0 (t)) received by transducer 21x (where t is the time after emitting a sound pulse through that transducer) are described by formula (1), where g (x, y, z) denotes the (three-dimensional) spatial distribution of the reflection cores 14 in the medium 13, and ft (x, y, z) the three-dimensional sensitivity function of the transducer in question The sensitivity function describes the three-dimensional pressure wave volume that contributes to the reflection signal at time t Furthermore, although the distribution of the reflection cores 14 in the medium 13 is stochastic, it is assumed that their movement is coherent within the three-dimensional sensitivity function.

Assume that the array 121 emits sound pulses with 20 spaced ΔΤΕ? between successive transmit pulses. The magnitude of the displacements of the medium in the X, Y and Z directions occurring during that time distance will be denoted as Sv, SY and Sz, respectively. The corresponding velocity components are Vx = SxMTep, VY = SY / hTEP and Vz = Sz / ATep, respectively.

The (T + 1) -th captured reflection signal RF2; T (t) of the other transducer 212 can then be described by formula (2). The term PnT + p12 in formula (2) is the time difference (unit ΔΤΕΡ) between the transmit pulses associated with the reflection signals RF1f0 (t) and RF2 / T (t). The variable pu is the time difference (unit ΔΤΕΡ) between two successive reflection signals captured with the same transducer. If every emitted sound pulse results in a captured signal, pn = 1. However, if a number of n reflection signals is to be ignored between two reflective signals to be captured, p11 = n + 1. The variable p12 is the time difference (unit ΔΤΕΡ) between the mth reflection signal captured by transducer 211 and the mth reflection signal captured by transducer 212. If both transducers capture their signals simultaneously, p12 = 0.

It should be noted that the derivation now given is valid for measurements performed using two transducers 21χ and 212. However, the derivation is equally valid if only one transducer is used / the formulas simplify then because ΔΧ = ΔΥ = 0 and p12 = 0, so that RFX is equal to RF2.

The correlation between the initial reflection signal RF1 (0 (t)) captured with the first transducer 21x and the depth shifted (T + 1) -th reflection signal RF2 (T (t + Z / fs)) captured with the second transducer 212, denoted as R (T, Z), is defined by formula (3), where T denotes a time difference (unit ΔΤΕΡ), Z denotes a spatial distance difference, 20 fs is the sampling frequency of the reflection signal, * denotes complex conjugate, and ( ) denotes mathematical expectation.

By substituting formulas (1} and (2) in formula (3), the correlation R (T, Z) can now be written as formula (4).

Assuming that the reflection cores 14 are δ-correlated in space, the expectation of the spatial distribution of the displaced reflection cores 30 can be written as formula (5), where δ is the Dirac delta function and (g2) is the mean squared value is of the spatial distribution of the reference nuclei. Thus, the 1010061 11 correlation function of formula (4) can be simplified to formula (6).

Since the two sensitivity functions ft * and f-tZ / fs of formula (6) are very close together in space, they are assumed to have the same shape.

In the following, an analytical description will first be given of the sensitivity function of a transducer. The shape of the sensitivity function in the axial direction can be derived from the spectral distribution of the sound signal, while in the XY plane local characteristics of the sound beam limit the limits of the sensitivity function. These contributions to the sensitivity function are independent of each other and will be discussed independently.

The spectral power density P (f) of an (analytical) reflection signal has an approximate Gaussian form, and can be written in normalized form as formula (7), where P (f) is the normalized (P (fc) = 1) power at a given frequency f, fc is a central carrier frequency, and BWEq is the equivalent bandwidth of the spectral distribution. By equivalent bandwidth is meant the width of a rectangular distribution having the same area as the curve defined by the actual spectral distribution, the height of the equivalent function being equal to the height of the original function at fc. For a Gaussian curve, the equivalent bandwidth can be written as formula (8), where BWdB denotes the bandwidth at a specified level relative to the maximum in decibels.

Consider a reflection signal from a single reflection core located at a distance d (= 0.5 * c * td, where c is the sound velocity in the medium) 1 01 0061 12 for the transducer ^. Such a reflection signal, if the spectral distribution satisfies formula (7), can be written as formula (9), where ψ is any initial phase. The normalized axial sensitivity-5 function ft (z) can then be written as formula (10), where zt (= 0.5 * c * t) is the axial distance associated with t seconds after transmission of the transmit pulse.

In the directions perpendicular to the axis of the beam, the beam dimensions contribute to the sensitivity function 10 in the XY plane. In these directions, the sensitivity functions can be approximated with Gaussian functions according to formulas (11) and (12). In it, wxdB (zt) and wydB (zt) indicate the beam width in the x and y directions, respectively, at depth zt, at a specified level in dB relative to the peak value.

Since the total sensitivity function f, (x, y, z) can be written as ft (x, y, z) = ft (x) -ft (y) -ft (z), it can now be done by multiplying the formulas (10), (11), and (12) are written as formula (13).

20 Formula (13) is only valid for flat waves, for which all pressure waves in the XY plane are in phase. However, in focused transducers, curvature of the emitted wavefront occurs due to path length differences (AL) between the sound waves emitted from the center of the transducer and from the edges. This curvature can be approximated parabolically by formula (14), the curvature at position (X, Y, zt) being determined by the extra path length ALX and ALY in the x and y directions, respectively.

The sensitivity function ft ’(X / y / Z) corrected for this curvature can be written as. j formula (15).

This modified sensitivity function ft '(x, y, z) can be written according to formula (16) as a product of independent x, y, and z components.

1010061 13

Thus, the correlation function R (T, Z) of formula (6) can be decomposed into x, y, and z components of formula (17). Here RX (T, Z) is the x portion (transverse direction) of the correlation function, RY (T, Z) is the 5 y portion (height direction) of the correlation function, and Rz (T, Z) the z part (axial direction) of the correlation function.

By combining formulas (6) and (17), the x-component RX (T, Z) of the correlation function can be written as formula (18). This can be simplified as indicated in formula (19). Here C3x (zt) is a negative constant which is a function of the characteristics of the transverse sound beam at depth zr.

Similarly, the y component Ry (T, Z) of the correlation function can be simplified as indicated in formula (20), and the z component R, (T, Z) of the correlation function can be written as formula (21 ). By filling in the axial part of formula (16), formula (21) can be rewritten to formula (22).

By writing formula (17), and after normalization (R (0,0) = 1), the correlation function can be written as the analytical model of formula (23).

The correlation of the reflection signals is affected by noise. Assuming that the noise in a reflection signal also has a Gaussian spectral distribution, and since the noise between successive reflection signals is uncorrelated, the correlation function can be rewritten as formula (24), where the argument of R (T, Z) can be written as formula (25) and 30 where the magnitude | R (T, Z) | can be written as formula (26), where S is the normalized (S + N = 1) signal power and NM is the normalized noise power 1010061-14. If ΔΧ = Ο, ΔΥ = Ο, ρ12 = Ο and Τ = Ο, the normalized noise power is N, and in all other cases, the normalized noise power is 0.

The foregoing has described a theoretical model describing a correlation function for the received reflection signals. In the following it will be explained how, conversely, data can be derived if the correlation function is known in a practical situation. In practice, however, values 10 will first have to be found for that correlation coefficient R (T, Z) at different values of T and Z. More specifically, those values must be calculated from the received reflection signals. This can be done by way of example as follows.

The received reflection signals are sampled; the consecutive sample points are indicated by the = sequence number ζ. The first sample point of each captured reflection signal is indicated by ζ = 1. The consecutive j 20 sound pulses are distinguished by the sequence number τ.

Thus, the ζth sample point of a reflection signal after a τth sound pulse is designated as RF (x, C). Before processing, the reflection signals are made analytical via a Hilbert transform. Such a transformation can be written as formula (27), where A (f) is the analytical frequency spectrum associated with the frequency spectrum S (f) of an actual signal, as will be explained below.

Assume that S (f) is the frequency spectrum corresponding to the sampled reflection signals RF (x ^) (Fourier transform). This frequency spectrum can be considered as a set of frequency components fA with j relative strength Si (complex numbers). This also contains negative frequency components, which are eliminated in the analytical process by the respective magnitudes Sj. to be replaced by magnitudes Α <_ = 0. To preserve the power of the signals, the respective magnitudes 5 Sj_ are doubled for the positive frequency components, i.e. they are replaced by magnitudes Ai = 2Si. The magnitude associated with frequency f = 0 is not changed. By subsequently applying an inverse Fourier transform, an analytical signal RFA is obtained from RF.

10 Because two transducers are used, there are also two of these two-dimensional matrices. Each corresponds to one of the two transducers.

If only one transducer is used, both matrices are equal.

15 In order to determine stable correlation coefficients, averaging is required. To this end, an identical data window is defined in both matrices with NZ sample points in the ζ direction and NT sample points in the τ direction. The data window associated with the 20 signals captured with transducer 21x is designated Wi (x ^). The data window associated with transducer 212 is denoted by νι2 (τ, ζ). For the first sample point of the first signal segment in the data windows, τ = 1 and ζ = 1.

The correlation coefficient R (T, Z) of signals from both data windows, the difference between the sequence numbers of the signals in both data windows being T and between the sample points Z, is defined according to formula (28).

When calculating the correlation coefficients 30 R (T, Z) from the sampled analytical reflection signals, a compromise must be found between precision of the correlation coefficients on the one hand and resolution in space and time on the other. A better resolution is obtained by taking smaller values for NT and / or NZ, while a better precision of the correlation coefficients 1010061 16 is obtained by taking larger values for NT and / or NZ.

In the foregoing, a derivation of a model has been described that describes the two-dimensional correlation function R (T, Z) 5 in the presence of noise (formulas 24 to 26), or if noise may be neglected (formula 23). Based on this model, the signal parameters and beam shape parameters can be calculated from the measurement data taking into account the estimated correlation coefficients, as will be described below. Here the estimated value of a parameter x will be indicated as k. In addition to the presented formulas for estimating the parameters, other formulas for estimating the same parameters can also be derived from the model. However, these formulas will lead to the same result.

The central frequency ƒ .. can be estimated according to formula (29), where Zx * Z2. The best results are obtained when Zx = 1 and Z2 = 0.

Based on the arguments of the correlation coefficients, the axial movement S_ can be estimated with formula (30), where Τχ * T2. The best results are obtained when Tx = 1 and T2 = 0.

An important aspect of formula (30) is that it is a relatively simple formula that takes relatively little computation time. However, this formula also has a limitation, like any formula based on the argument of a complex number, the so-called "aliasing" effect, caused by the fact that the argument of a complex number can only take values between - 180 ° and + 180 °.

In practice, this means an upper limit for the met. using formula (30) estimable speed.

; The present invention also provides a solution to this limitation by providing the alternative formula (31) 1010061 for the axial movement S1, where Z1 # Z2 * Z3 * Z1 and T1 'O. This formula (31) is based on the magnitude of the correlation coefficients, and therefore has no upper limit caused by aliasing. A drawback of this formula (31), however, is that it is very sensitive to variations in the amplitude of the reflection signals and is therefore somewhat less reliable. The present invention therefore proposes to combine both formulas (30) and (31) with each other, by first estimating the velocity using formula (31) in a first approximation, thus obtaining an order of magnitude, and then by using formula (30) to estimate the velocity in a second, more accurate approximation, eliminating the aliasing problem because the magnitude is known.

The equivalent bandwidth BWEQ of the reflection signal can be estimated by formula (32), where Z1 * Z2 * Z3 * Zlr and to avoid influence of noise it is better to choose T * 0.

The normalized signal power S of formulas (33) and (34), where Ο ^ Τ ^ Τ ^ Ο, ΔΧ = 0, ΔΥ = 0, and p12 = 0, can only be estimated for the signals captured with the individual transducers. The normalized signal powers of the signals captured with transducers 21L and 212 are denoted S: and S2, respectively.

The beam constants in both transverse directions C3x (z ..) and CBy {zt) can be estimated with formulas (35) and (36), where § = the average power of the signals captured with both transducers. CBx (zt) can be determined if the measurement setup is arranged in such a way that there are two transducers positioned in the XZ plane, that is to say ΔΥ = 0 while their mutual X-distance ΔΧ is known. So this is the measurement situation illustrated in Figure 4. Using formula (37), an estimate can then be calculated for the displacement Sx in the X direction, and from this the X component Vx of the velocity factor can be calculated.

CBy (zt) can be determined if the measurement setup is arranged so that there are two transducers positioned in the YZ plane, i.e. 10 holds ΔΧ = 0 while their mutual Y-distance ΔΥ is known. Using formula (38), an estimate can then be calculated for the displacement SY in the Y direction, from which the Y-component VY of the velocity vector can be calculated.

The size and direction of the transverse velocity vector are then known by combining Vx and VY.

The magnitude of S2 can be estimated using formulas (30) and (31), from which the axial velocity component follows.

The possibility described above assumes that one has at least three transducers, two of which are arranged in the XZ plane, and two of which are arranged in the YZ plane. However, if only two transducers are available, only one of the 25 beam constants can be determined. If the measurement setup is arranged such that those transducers are positioned in the XZ plane (the measurement situation illustrated in Figure 4), CBx can be estimated using formula (36), and Sx can be estimated using the formula (37), where § = / 1 ^ is the average power of the signals captured with both transducers. CBy can; 1010061 19 however, one can calculate if there is a fixed relationship between CBx and CBy. This is the case, for example, if the sound beams of those transducers have a known symmetry, such that it is possible to write 5 CBx = eCBy, where e is a known or measurable form factor. When measuring with transducers that have a circular-symmetrical sound beam, e = 1. In other cases a calibration curve for the missing beam constant must be measured, as will be clear to a person skilled in the art. Once C3y is known, SY can be estimated using formula (39). Out and Sx and SY follow the horizontal and vertical components of the velocity vector, so that magnitude and direction of the transverse component of the velocity vector are known.

The magnitude of 5Z can be estimated using formulas (30) and (31), from which the axial velocity component then follows.

Of course the same applies, mutatis mutandis, if the measuring arrangement is arranged such that those transducers are positioned in the YZ plane. The movement in the Y direction, if ΔΧ = 0 and C3y (zt) has already been estimated, can be estimated using formula (38), where ê = ^ s.s2 is the average power of the signals captured with both transducers.

25 When measuring with only one transducer, all the above formulas simplify because pi2 = 0, Δ-Χ = 0, and ΔΥ = 0. However, it is then only possible to determine the central frequency, the axial velocity component, the bandwidth, the normalized signal-30 power (and thus the signal-to-noise ratio), and the quantity CBx (zt) Sx2 + CBy (zt) SY2 (formula 39) directly to be determined. The magnitude of Sz can be estimated using formulas (30) and (31), from which the axial velocity component then follows.

However, it is then not possible to estimate a beam-5 constant based on the measurement signals, and so it is not possible to determine Sx and SY. Only when measuring with a circular symmetrical sound beam, it is possible, even after a calibration procedure, to estimate the size of the transverse velocity component.

Since then the beam constant is the same for all transverse directions, CSx (zt) = CBy (zt) = Cs (zt), in which case Cgx (z ^) Sx "^ Cgy (z ^ -) Sy- = Cg (z ^) (Sx + S) = CetZtiSr ^ T-2, where the movement is in the XY plane The term CB (zt) can be determined by calibration.

It should be noted, however, that with a single beam the magnitude of the movement perpendicular to the beam axis can be determined, but not its direction in the plane perpendicular to the beam axis.

It will be apparent to one skilled in the art that the scope of the present invention is not limited to the examples discussed above, but that various modifications and modifications thereof are possible without departing from the scope of the invention as defined in the appended conclusions.

For example, it is possible that instead of the said transducer, use is made of a combination of a separate transmitter and a separate detector.

Furthermore, the scope of the present invention is not limited to the discussed example of blood flow in a blood vessel. Rather, the present invention is applicable to the remote detection of the velocity of an arbitrary collection of reflection cores that behave as a more or less cohesive entity: for example, water droplets in a cloud may be envisaged.

Furthermore, a Hilbert transformation can be performed in a different way than the example described.

Furthermore, the definition of different data sets can be carried out before or after recording the 10 signal samples, and before or after making the reflection signals analytical.

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1010061

Claims (17)

A method for remotely detecting the distribution of the vectorial velocities in a diffuse medium (13), comprising the following steps: (a) transmitting a pulse-shaped sound signal (22) by means of at least one suitable transmitter 5 (21) ) to the medium (13); (b) detecting a reflection sound signal (23) caused by the medium (13) by means of at least one suitable detector (21); (C) converting the reflection sound signal (23) detected by the detector (21) into an electric signal (24), the strength of that electric (signal (24) being always representative of the current sound strength); (D) sampling the electrical signal (24) to provide consecutive signal samples RF (t ^), wherein ζ denotes a sequence number for the consecutive signal samples, and τ is an integer denoting the processing number captured signal; (e) registering the signal samples RF (t ^) in a memory (31); (f) repeating steps (a) through (e) for consecutive values of τ, whereby the signal samples RF (τ, ζ) are grouped into two data sets, denoted RF1 (x, C) and RF2 (t, C), respectively; (g) making analytical the reflection signals RFx (r ^) and RF2 thus obtained (τ, ζ) through 1010061 for example a Hilbert transform for it obtaining analytical measurement signals RF1a {t ^) and ΚΕ ^ ίτ, ζ); (h) defining in each data set RF1a (t ^) and RF ^ t ^) the analytical reflection signals of mutually identical data windows wx (t, C) and w2 (t ^); (i) calculating correlation coefficients R (T, Z) between these two data windows ν ^ ίτ, ζ) and ν2 (τ, ζ); (j) calculating velocity components from the correlation coefficients thus calculated; 10 wherein the axial component of the vectorial velocity in the medium (13) is calculated using formula (31) based on the magnitudes of the correlation coefficients. The method of claim 1, wherein a first reflection measurement is made using a first transmitter / detector combination (21L) to obtain a first set of measurement points νιι (τιζ), a second reflection measurement is made using a second transmitter / detector combination (212) for obtaining a second set of measuring points w2 (t ^), and wherein the correlation is performed between the first measuring points obtained on the one hand by means of the first transmitter / detector combination (21x) and the second measuring points obtained by means of the second transmitter / detector combination (212) on the other hand. The method of claim 2, wherein the two said sets of measurement points are obtained by alternately operating said first and second transmitter / detector combinations (21x; 212). The method of claim 1, wherein a first reflection measurement is made using one (0061 transmitter / detector combination (21) to obtain a first set of measurement points ν ^ ίτ, ζ), a second reflection measurement is made using the same transmitter / detector combination (21) to obtain a second set of measurement points w2 (t ^), the correlation being performed between the first measurement points obtained by the one transmitter / detector combination (21) on the one hand, and the second measuring points obtained by means of the same transmitter / detector combination (21) on the other. The method of claim 4, wherein the two said sets of measurement points are obtained by alternately taking measurements for the first set and for the second set with said one transmitter / detector combination (21). The method according to any of the preceding claims, wherein first the magnitude of the axial component of the vectorial velocity of the medium (13) is calculated using formula (31) based on the magnitudes of the correlation coefficients, and then a more accurate estimate of the axial component of the vectorial velocity of the medium (13) using formula (30) is calculated based on the arguments of the correlation coefficients, taking into account the calculated using formula (31) magnitude to eliminate aliasing effects. 7. Process according to any of the foregoing; claims, wherein a single sound beam with a circular symmetrical shape is used, so that for the beam constants CBx (zt) = CBy (zt) = CB (zt), wherein the shape function CB (zt) is determined by means of calibration; and where the quantity CgU ^ S ^ 2 = CBx (zt) Sx2 + CBy (zt) SY2 = CB (zt) (Sx2 + SY2), where the movement is in the XY plane, 5 is calculated using formula ( 39) thus to provide a transverse component of the vectorial velocity in the medium (13). The method of any one of claims 1 to 6, using two transducers (21x 10 and 212) defining an XZ plane; wherein an X component of the vectorial velocity in the medium (13) is calculated using formula (37); wherein M is defined by formula (35); where § = V (δχ§2)> 15 where and §2 are defined by formula (34); and wherein K is defined by formula (33) and (32). Method according to claim 8, wherein the shape of the sound beams (22) emitted by the transmitters (21) has an XY symmetry, so that öBx = eÖBy, where e is a form factor, which is for example equal to 1 if the sound beam (22) emitted by the transmitter (21) is circular symmetrical; where CBx is calculated using formula (36); where ÖBy is calculated from the relationship CBx = eCBy; And wherein a Y component of the vectorial velocity in the medium (13) - is calculated using formula (39). The method of claim 8, wherein a relationship is determined for CBy by calibration; where CBx is calculated using formula (36); 1010061 and wherein a Y component of the vectorial velocity in the medium (13) is calculated using formula (39). The method of any one of claims 1 to 6, using three or more transducers, two of those transducers (21 ^ and 21z) defining an XZ plane, and two of those transducers defining a YZ define plane; wherein an X component of the vectorial velocity in the medium (13) is calculated using formula (37); 10 wherein M is defined by formula (35); where § = V (§1§2); wherein and ê2 are defined by formula (34); and - wherein K is defined by formula (33) and (32); and wherein the Y component of the vectorial velocity in the medium (13) is calculated using formula (38). The method of claim 11, wherein the beam constants CBx and CBy are calculated using formula (36). The method according to any of the preceding claims, wherein in step (g) the modification of the frequency spectrum is described by formula (27). A method according to any preceding claim, wherein the calculation of the correlation function R (T, Z) is described by formula (28). i I 1010061 i The method of any of the preceding claims, wherein the center frequency is calculated according to formula (29). A method according to any of the preceding 5 claims, wherein the equivalent bandwidth is calculated according to formula (32). The method of any preceding claim, wherein the transmitter and detector are combined in a single transducer. - Λ -; ". i 'j *