EP0181903A1 - Method for measuring by ultrasounds flow speeds of fluids - Google Patents

Abstract

Measurement of flow velocities of a flow. The device includes: n broadband transmitter and receiver transducers (3 and 7), m oscillators (4) producing continuous sinusoidal voltages of different frequencies which are applied, simultaneously, each to a transmitter transducer, a low pass filter (16) interposed on each processing channel between the multiplier and the high-pass filter. Application to the visualization by Doppler velocimetry of the speed of the blood in a segment of a vessel.

Description

 Measurement of fluid flow velocities by ultrasound.

- TECHNICAL AREA.

The present invention relates to the field of measuring the flow velocities of a flow and, more particularly, that of measuring the blood flow in the arteries, with a view to studying the propagation of the flow wave and researching, by this means, any significant anomaly of a flow disturbance due to a local modification of the useful passage section of a vessel.

- PRIOR ART.

To measure the speed of a flow, two techniques based on Doppler velocimetry are known.

The so-called continuous ultrasonic emission technique makes it possible to measure the value of the average speed of all the particles @n movement in the ultrasonic field defined by an emitted beam. To have an image corresponding to a vessel segment, it is therefore important to provide means making it possible to successively move the beam in order to explore successive slices of a vessel segment and thus take note, slice by slice, of the value of the speed average.

The beam can be moved mechanically or electronically.

Whatever the means used to do this, we understand that the information corresponding to each slice must be stored for the duration of the exploration of the vessel segment considered, so that we can then use all of the information collected to reproduce an image of the explored spacecraft segment. This image can then be analyzed as being a rear projection of the plane of the vessel segment. It is understood that such a technique does not make it possible to obtain a real-time image of blood flow and therefore cannot be practically used to study the blood flow wave when this wave experiences significant variations within a cardiac cycle. The second technique, known as Doppler velocimetry by coded, pulsed or pseudo-random emission, makes it possible to record the velocity values at a number of points in the beam within an ultrasonic beam. To obtain an image of a vessel segment, it is necessary, as previously, to move the beam to explore the segment in successive slices and thus have a matrix of memorized points to form the image. This displacement can be produced mechanically or electronically, as in the previous technique and has the same drawbacks.

It should be noted that, if the first technique makes it possible to obtain an image which can be assimilated to a rear projection of the explored vessel segment, the second provides a sectional image according to a plane formed by the different successive lines of exploration. Given the non-rectilinear path followed by the vessels, it is understood that the second technique does not provide an image of a segment of vessel of sufficient length. French patent application n ° 81-10833 relates to a real-time measurement technique for the visualization of the flow velocities of a flow by making use of pseudo-random, coded emissions, independent of each other, which are applied simultaneously to several groups of transducers, so as to emit as many ultrasonic beams as there are groups.

In this technique, the different ultrasonic beams are substantially parallel and thus make it possible to simultaneously explore several parallel slices of a vessel segment by arranging in each slice the measurement of the speed of the flow at a certain number of points.

We could consider that this technique answers the problem posed, but it has the same disadvantage as above concerning the obtaining of a sectional image instead of a rear projection in plan. - STATEMENT OF THE INVENTION.

The object of the invention is to remedy this drawback by proposing a new measurement technique making it possible, in application to the measurement of the speed of the blood in the vessels and, mainly, of the arteries, to obtain in real time a back-projected image of the evolution of the flow wave.

Another object of the invention is to propose a new measurement technique which can be implemented simply, practically and quickly, by means of an apparatus which can be produced at an attractive cost price.

To achieve the above goals, the object of the invention is characterized in that the measurement method consists in:

- use a transmitter composed of two electrically separated wide bandwidth transducers, - generate a number m of continuous sinusoidal voltages of different frequencies,

- apply, simultaneously, to each of the n transmitting transducers one of the m sinusoidal voltages, in order to produce m ultrasonic beams of different frequencies,

- collect all the echo signals reflected by the targeted target on a receiver composed of n transducers coupled to the transmitting transducers and each associated with at least one channel for processing the echo signal, - filtering on each channel the received echo signal by a low-pass filter allowing only the reception spectral range associated with the transmission frequency of the transmitting transducer coupled to the receiving transducer considered to pass, - conform the isolated signal collected and direct it towards a frequency meter, - and supply a display by a sampler scanning successively the outputs of n frequency meters corresponding to the n receiving transducers. Various other characteristics will emerge from the description given below with reference to the appended drawings which show, by way of nonlimiting examples, embodiments of the subject of the invention.

- BRIEF DESCRIPTION OF THE DRAWINGS.

Fig. 1 is a block diagram of the apparatus according to the invention.

Figs. 2 and 3 are diagrams representing a characteristic of the subject of the invention.

Fig. 4 is a block diagram illustrating an elementary arrangement of the apparatus according to FIG. 1.

Figs. 5 and 6 are diagrams showing a characteristic of one of the elements of the invention in relation to FIGS. 2 and 3.

Figs. 7 to 10 are schematic views illustrating images of speed measurement using the invention.

- BEST WAY TO CARRY OUT THE INVENTION. According to the invention, the method and the device propose, to measure in real time and obtain an image of the flow velocities of a flow, in particular of blood inside a vessel 1, to use a transmitter 2 composed of n wide bandwidth transducers 3. These different transducers 3 are each connected to a generator 4 providing a continuous sinusoidal voltage of natural frequency.

As an example, fig. 1 shows a transmitter composed of eight transmitter transducers 3 ₁ to 3 ₈ which are thus supplied by eight oscillator generators 4 ₁ to 4 ₈ . The voltages supplied by the generators 4 ₁ to 4 ₈ are applied simultaneously to the corresponding transmitting transducers 3 ₁ to 3 ₈ , so that the latter produce as many parallel ultrasonic beams 5 ₁ to 5 ₈ coming to be reflected on the red blood cells contained in the ship segment 1 to be explored. The transmitter 2 is completed by a receiver 6 composed of as many receiver transducers 7 ₁ to 7 _{8 as} there are transducers transmitters 3 ₁ to 3 _{8 to} which they are respectively coupled, so as to each collect the echo signal of the corresponding incident beam. It must be considered that, in certain embodiments, the transmission and reception functions can also be provided by a single transducer for each beam.

According to a characteristic of the invention, the oscillator generators 4 ₁ to 4 ₈ are chosen to emit DC voltages having different frequencies, separated from each other by a difference at least equal to 4% of the base frequency . In an exemplary embodiment, this difference is chosen at 8% of a base frequency fixed at 5 MHz for the oscillator 4 ₁ . Thus, the emission frequency of the oscillator generator 4 ₂ is fixed at 5 MHz plus 40 KHz, and so on. This allows, as illustrated in fig. 2, to separate the different frequencies f ₁ to f ₈ by a sufficient difference to avoid overlapping or overlapping the different Doppler spectra S ₁ to S ₈ which are attached to them and whose spread is generally between plus or minus 10 KHz each. Fig. 2 shows the Doppler spectra S ₁ to S ₈ received, for example, by the receiver transducer 7 ₁ due to the simultaneous emission of the emission frequencies f ₁ to f ₈ by the transducers 3 ₁ to 3 ₈ . This figure shows the decreasing influence of the Doppler spectra, in relation to the distance separating the receiver transducer 7 ₁ from the successive transducers 7 ₂ to 7 ₈ . Fig. 3 shows the Doppler spectra S ₃ to S ₈ received by the receiver transducer 7 ₃ due to the simultaneous emission of the frequencies f ₁ to f ₈ by the transmitter transducers 3 ₁ to 3 ₈ . The comparison with fig. 2 makes it possible to consider the influence of the other transducers as a function of their position and their distance.

Since the different receiver transducers 7 ₁ to 7 ₈ are influenced by the simultaneous return echo signals corresponding to the transmission frequencies applied simultaneously to the transmitter transducers 3 _{1 to} 3 ₈ , it is understood that it is necessary to be able to isolate for each receiver the echo signal corresponding exactly to the original ultrasonic beam which gave birth to it by reflection on the particles of blood and the walls of the vessel. To this end, the invention recommends using in the treatment path 8 ₁ to 8 ₈ attached to each receiver transducer 7, a filter capable of eliminating all the echo signals not corresponding to the ultrasonic beam emitted by the transmitter transducer to which it is coupled.

Fig. 4 shows a processing channel 8 comprising, for a pair of transmitter 3 and receiver 7 transducers, an amplifier 9, the output of which is connected, like that of the corresponding oscillator generator 4, to the input of a multiplier 10. The processing channel 8 also includes a high-pass filter 11 intended to filter the signals corresponding to the wall echoes, as well as a threshold conformator 12 connected to a frequency meter 13. The frequency meters 13 of the different channels are connected to a scrutinizing sampler 14 common supplying a display means 15. FIG. 4 shows that the channel 8 can be double and include a branch 8a when the velocimeter attached to each pair of transducers 3 and 7 is designed to provide, in addition to the frequency signal, information corresponding to the direction of flow.

According to the invention, filtering is ensured by interposing a filter 16 between the multiplier 10 and the high-pass filter 11. The filter 16 is of the low-low type so as to leave no residue, for the treatment by the route considered, than the signal corresponding to the Doppler S spectrum assigned to this channel.

From fig. 2, the echo signals processed by the multiplier 10 are, as is known in Doppler measurement, translated, so as to match the frequency considered with the origin, as illustrated in FIGS. 5 and 6 can be analyzed by comparison with FIGS. 2 and 3. These figures show that, if the echo signal corresponding to the receiver transducers 7 ₁ and 7 _{3 is} taken into account respectively, it is therefore necessary to take charge, for the channel 8 ₁ , of the Doppler spectrum S. and for channel 8 ₃ the Doppler spectrum S ₃ , without allowing the parasitic frequencies corresponding to the Doppler spectra of the adjacent frequencies to pass. The filter 16 is designed so as to allow a spectral range corresponding to the area defined by the trace T which must be determined to introduce an attenuation such that the Doppler spectrum S ₂ corresponding to the frequency f ₂ - f ₁ for the channel 8 ₁ and S ₄ + S ₂ corresponding to the frequencies f ₄ - f ₃ and f ₈ - f ₂ so that the channel 8 ₃ is obscured. To this end, according to the invention, the filter 16 of each channel 8 is of a type capable of introducing a minimum attenuation of 80 decibels between the spectral domain S to be retained and the closest spectral domain. In relation to the frequency difference, each filter according to the invention is chosen of the ButterWorth type of order 7. By the above means, each channel 8 ensures the selection and the processing of the echo signal corresponding exactly to the ultrasonic beam emitted by the transmitter coupled to the receiver transducer considered and thus provides, after filtering of the parasitic signals due to the wall echoes and shaping, a signal which can be directly used via the frequency meter to supply the display means 15.

Since the different transmitting transducers 3 ₁ to 3 ₈ thus simultaneously transmit ultrasonic emissions at different frequencies, but the processing channels attached to the receiving transducers are able to take into account for processing only the echo signal corresponding to the beam transmitter to which it is assigned, it becomes possible to have access, in real time, to all the echo signals which thus make it possible to have global information for measuring the average speeds in the slices of segment 1 corresponding to the beams 5 ₁ to 5 ₈ .

This global information thus makes it possible to have a real-time image of the flow wave to obtain, for example, in the case where the array of transducers 3 and 7 is placed along the axis of the vessel segment, eight lines continuous L ₁ to L ₈ visualizing the flow flow in the eight successive sections of the vessel segment as illustrated in fig. 7, in which the abscissa represents the time scale t.

Fig. 8 represents another possibility of visualization in which the abscissa represents the scale of space e. In this case, the visualization is obtained by a representation based on a gray scale or pseudo-colors. This figure illustrates a example according to which the eight local flow velocities are substantially identical, which corresponds to a substantially constant segment of the vessel.

On the other hand, fig. 9 illustrates an example according to which the velocities increase from one section to another, which corresponds to a segment of vessel whose section decreases in the direction of flow.

When the emitter 2 is composed of one or more bars arranged perpendicular to the axis of the vessel segment, it becomes possible to visualize the presence or absence of a flow and to obtain, as illustrated by the fig. 10, a rear projection in plan of the targeted segment.

Depending on the number of transmitter transducers 3 making up transmitter 2, it is possible to consider that the parasitic influence of simultaneous transmissions can be neglected as soon as they are sufficiently distant from a given receiver transducer. In practice, this is the case when the transducers are separated by an interval corresponding to eight or ten juxtaposed transducers.

In such a case, to facilitate the construction of the device, it becomes possible to use n transmitting transducers and to generate a number of different frequencies m less than the number n. In such a case, the same frequency can then be applied to two transmitting transducers whose receiving transducers are separated by a number of intermediate transducers equal to m − 2.

To facilitate the construction of the transmitter 2 and the receiver 6, it is possible, within the meaning of the invention, to constitute the various transducers from a piezoelectric plate, such as barium zirconate, with a wide band bandwidth which is sectioned or cut into as many elementary plates which thus have all the same physical characteristics.

- POSSIBILITY OF INDUSTRIAL APPLICATION.

The invention finds an interesting application in the measurement of blood flow in the arteries in order to highlight any significant anomaly of a flow disturbance.

Claims

 CLAIMS:

1 - Real-time measurement method for visualizing the flow velocities of a flow by using an ultrasonic velocimeter with Doppler effect, characterized in that: - uses a transmitter composed of n transducers with wide bandwidth , electrically separated,

- generates an in number of continuous sinusoidal voltages of different frequencies, - applies, simultaneously, to each of the n emitting transducers one of the m sinusoidal voltages, in order to produce m ultrasonic beams of different frequencies,

- collects all of the echo signals reflected by the targeted target on a receiver composed of n transducers coupled to the transmitting transducers and each associated with at least one echo signal processing channel,

- filters on each channel the echo signal received by a low-pass filter allowing only the reception spectral range associated with the transmission frequency of the transmitter transducer coupled to the receiver transducer considered to pass,

- complies with the isolated signal collected and directs it to a frequency meter,

- And feeds a display device by a sampler successively scanning the outputs of the n frequency meters corresponding to the n receiver transducers. 2 - Method according to claim 1, characterized in that each transmitter transducer - receiver transducer pair is associated with two measurement channels providing information relating to the frequency and the direction of flow and in that each of the channels comprises a filter low pass. 3 - Method according to claim 1, characterized in what is used, for each processing channel of the echo signals collected, a low-pass filter introducing a minimum attenuation of 80 decibels between the spectral domain associated with the Doppler signal sought and the other spectral domains. 4 - Method according to claim 1 to 3, characterized in that the frequencies of the ultrasonic emissions are separated from each other by a difference at least equal to 4% of the base frequency and in that the filters pass then are respectively of order 7. 5 - Method according to claim 4, characterized in that the basic frequency is 5 MHz.

6 - Method according to claim 1, characterized in that one uses a transmitter composed of n transducers and in that one generates a number of emissions m equal to n. 7 - Method according to claim 1, characterized in that one uses a transmitter composed of n transducers and in that one generates a number of transmissions m different from n and in that two receiving transducers receiving the same frequency of are separated from each other by a number of receiving transducers equal to m minus 2.

8 - Real-time measuring device for visualizing the flow velocities of a flow using an ultrasonic Doppler velocimeter, of the type comprising at least one oscillator (4), a transmitter and a receiver composed of transducers separated, at least one processing channel (8) associated with each of the receiving transducers and comprising a high frequency amplifier (9), a multiplier (10), a high pass filter (11), a shaper (12) and a frequency meter (13) connected to a scrutinizing sampler (14) preceding a display means (15), characterized in that it comprises:

- n transmitting and receiving transducers (3 and 7) with wide bandwidth,

- m oscillators (4) producing continuous sinusoidal voltages of different frequencies which are applied simultaneously to each an emitting transducer, a low-pass filter (16) interposed on each processing channel between the multiplier and the high-pass filter. 9 - Measuring device according to claim 8, characterized in that each low-pass filter introduces a minimum attenuation of 80 decibels between the spectral domain associated with the Doppler signal sought and the other spectral domains.

10 - Measuring device according to claim 8 or 9, characterized in that it comprises oscillators producing continuous sinusoidal voltages whose frequencies are separated from each other by a difference at least equal to 4% of the base frequency and in that the low-pass filters are of order 7.

11 - Measuring device according to one of claims 8 to 10, characterized in that it comprises a transmitter made up of n transducers, a generator group emitting a number of voltages m different from n and in that it comprises n transducers receivers arranged in such a way that two receiver transducers coupled to two transducers emitting ultrasonic waves of the same frequency are separated from each other by a number of receiver transducers equal to m minus 2.