WO1998045727A1

WO1998045727A1 - Acoustic imaging system

Info

Publication number: WO1998045727A1
Application number: PCT/FR1998/000684
Authority: WO
Inventors: Daniel Billet
Original assignee: Thomson Marconi Sonar S.A.S.
Priority date: 1997-04-04
Filing date: 1998-04-03
Publication date: 1998-10-15
Also published as: FR2761781B1; FR2761781A1

Abstract

The invention concerns acoustic imaging systems. It consists in producing delays required for the reception signals of each of the sensors (201) of such a system using a set (203) of cells (207) with current memory. Said cells are posted and read by means of two offset registers (204, 205) wherein circulates a single bit per register. The space between the bits delays the signal introduced in the set, to form channels. The system enables the use of very compact acoustic cameras or ultrasonograph probes.

Description

ACOUSTIC IMAGING SYSTEM

The present invention relates to acoustic imaging systems which make it possible to obtain images by processing the echoes received from the objects of which one wishes to make the image. These echoes are obtained from an acoustic signal, generally ultrasonic, emitted by the system towards these objects. This basic technique can be applied in different uses, in particular in sonars, which are at the origin of this branch of the technique, and in medical sonographs, which represent a particularly important evolution of this technique initially limited to sonar. The sonar technique, which originated from the omnidirectional listening of the echoes of a signal itself emitted in a roughly omnidirectional manner, has evolved towards on the one hand a transmission concentrated in a given sector, and on the other apart from a fine processing of the echoes received to form a large number of channels, preferably juxtaposed in deposit and in site. The information relating to the temporal positions of the echoes in each of the channels thus makes it possible to define space-time boxes, identified by a distance from a site and an azimuth, corresponding to a fragment of the insonified space. By listing all the boxes associated with a continuous echo, we can reconstruct a three-dimensional image of the insonified space. In this image, the phenomena at the origin of the echoes generally correspond to solid objects immersed in the marine environment and whose size, shape and location are thus determined.

This processing is currently carried out digitally, which requires a significant processing capacity, in particular to constitute the channels defined above.

Indeed, by taking for example the case of an acoustic camera intended for underwater exploration comprising for example a two-dimensional reception antenna formed of contiguous sensors constituting a retina, and an acoustic projector located for example in the middle of this retina, we are led to form from the signals received on the retina a set of juxtaposed pathways which make it possible to obtain for each emission of the acoustic projector an image of the environment insonified by this projector.

By limiting itself to the case of a two-dimensional image, each channel corresponds to an image point. The number of channels that it is necessary to form from the reception signals from the retina sensors is equal to the ratio between the insonified angular field and the desired angular definition.

We know that to form a path in a given direction we must perform the operation defined by the formula:

N ∑tf,. s, (t - τ i) (1)

; = 1

OR

N is the number of transducers

If (t) is the signal received by the transducer i at time t ai is a weighting coefficient v, is the delay to be applied to put the signals back in phase in the direction of the channel to be formed.

Of course this processing must be carried out on the echoes coming from a single emission before the arrival of the echoes corresponding to the following emission. The duration of the treatment is therefore determined by the interval between two successive transmissions of the insonification signal. In addition, this transmission rate, corresponding to the image rate, depends largely on the speed of the carrier vehicle on which the imaging system is mounted. Indeed, considering, by way of example, an imaging system mounted on a towed or autonomous underwater vehicle sailing at a speed of 5 knots at 5 m above the sea floor with an opening of 30 x 30 ° , it is necessary to obtain a frame per second in order not to have a coverage hole.

In such a system, the above-mentioned circular reception antenna, for example, is formed by 800 square sensors with a side of 4 mm, the transmitter operating at 500 kHz. Under these conditions, the Fresnel distance is 0.55 m, and given the height above the bottom it is not necessary to focus the channels, which simplifies the treatment to a certain extent and reduces the volume . With those characteristics, to obtain an angular resolution of 1.5 x 1.5, one must form 400 channels per second.

For this, it is necessary to digitize the signals of the 800 sensors with a sampling frequency at least equal to 8 times the carrier frequency, in order to obtain an accuracy on the delays of at least λ / 8 in order to be able to form correctly the channels without interpolation.

Taking into account that the samples of the channel signals must be obtained at a rate equal to at least twice the reception frequency, in order to comply with Shannon's theorem, the computing power necessary to comply with all these conditions is therefore substantially equal to 320 Gigaops per second.

We know that today an electronic card in known format known as "double Europe" allows to obtain a computing power of about 1 Gigaops per second. Under these conditions, several hundred type cards are required.

"Double Europe" to obtain the computing power necessary for the formation of the channels, without counting the computing power necessary to then process the channel signals in order to obtain the image proper, and possibly that necessary to process automatically this image.

The volume, and therefore the cost, of the electronic equipment necessary to carry out these operations is particularly important under these conditions. In addition, as all the processing is done digitally, it is necessary to digitize the signals received directly at each transducer, which implies having an analog / digital encoder per transducer. The number of these encoders is then considerable, 800 in the example above for example, which causes, in addition to problems of space and cost, significant heat dissipation problems. To overcome these drawbacks, the invention proposes an acoustic imaging system, of the type comprising a set of sensors 201 intended to receive acoustic signals reflected by the object intended to be imaged in order to deliver in response electrical signals corresponding to these signals. acoustic, processing means to apply to these electrical signals delays determined to form a set of reception channels, mainly characterized in that these processing means comprise for each sensor a voltage / current converter 202 making it possible to convert said electrical signals into intensity, a set 203 of cells 207 with current memory connected in parallel at input and at output to receive said intensity signals, a write register 204 and a read register 205 controlled by a clock of period T _H and the stages of which are respectively connected to the write and read control inputs of said cells, and means 206 for injecting respectively into each of these two registers a single bit which circulates in the registers and which controls the writing and reading of the cells; the two unique bits being separated by a number n of stages between the 2 registers, to establish a delay of the output signal equal to a value n T.

According to another characteristic, to add the output signals S S of the assemblies 203 in order to obtain successive channel signals, these outputs are connected together two by two according to a tree dichotomy.

According to another characteristic, the sensors 201 are arranged according to a two-dimensional retina so as to form the reception system of an underwater acoustic camera which makes it possible to obtain an image from the set of channels formed by said means of treatment.

According to another characteristic, the sensors are transducers 402 of an ultrasound probe 401 intended to operate according to a series of “shots” obtained by the selection of a series of subsets 405 of the transducers which are offset with each shot at least one sensor to allow scanning of a sectoral area 404 described by the succession of shots, and in that for each shot a dynamic focusing 403 is carried out on transmission and on reception; this focusing being obtained by delays.

Other features and advantages of the invention will appear clearly in the following description, given with reference to the appended figures which represent:

- Figure 1, the diagram of a switched current memory cell; - Figure 2, the diagram of a device according to the invention for obtaining delays analogically;

- Figures 3 and 4, detailed partial diagrams of the device of Figure 2; - Figure 5, the diagram of a device for adding the output signals of the devices of Figure 2; and

- Figure 6, the sectional diagram of a medical ultrasound probe for implementing the invention.

Switched current memory cells are known devices of which the simplest diagram is shown in FIG. 1.

A node 101 is supplied by a bias current J by a current source 102. On this node are connected switches

103 and 104 which operate under the control of so-called phase signals φi and φ ₂ to bring the input current i to the node and to output the output current - i therefrom.

A field effect transistor 105 connects this node to ground. The gate of this transistor is itself connected on the one hand to ground via a storage capacitor 107 and on the other hand to node 101 via a switch 106 controlled by the signal φ- i. The operation of this device is as follows:

In a first step the transistor 105 is blocked and the switches 103 and 106 are closed. The sum of the currents i and J then charges the capacitor 107. This capacitor charges to a potential V which depends on the characteristics of the elements used and which is such that it polarizes the transistor 105 so that it becomes passing and conducting the entire current i + J.

In a second step, the switches 103 and 106 are open and the switch 104 is closed. At this time, the capacitor 107, remaining charged, continues to impose the passage through the transistor 105 of a current equal to i + J. As the switch 103 is open, the current complement to obtain this sum, since J, supplied by the current source 102, is constant, arrives by the switch 104, which therefore causes an equal current to flow on the output connection have .

This cell therefore makes it possible to memorize the value of the input current and to restore it at the output with a change of sign. According to the invention, a set of cells of this type is used according to the diagrams shown in FIGS. 2 to 4 to obtain the delays necessary for the formation of the channels of an acoustic imaging system. Figure 2 shows the overall diagram, Figure 3 the control connections, and Figure 4 the input and output connections of the current signals to be stored.

Each of the transducers 201 is thus connected to a known device 202 which makes it possible to convert the output signal of this transducer, which is essentially in voltage, into a current signal. This signal is then applied to the input of a set 203 composed of N current memory cells 207, of a type similar to that described with respect to FIG. 1, and which are put in parallel at input and at output.

The input control signals φ of each cell are obtained from a shift register 204, of digital type, formed for example by N flip-flops 209 connected in series. The outputs of these N flip-flops are also connected respectively to the storage control inputs of the N cells of the assembly 203.

A register 205, identical to register 204, makes it possible to obtain the signals φ ₂ for controlling the output of the cells of the assembly 203. The registers 204 and 205 operate in circulating memory by circulating at the rate of a clock "H" bits "1" introduced into these registers by a logic unit 206. When a bit 1 arrives in one of the flip-flops 208, it starts storing in the corresponding cell 207 the current signal supplied by the device 202. When a bit 1 arrives in one of the flip-flops 209, it restores the current memorized in the corresponding cell 207 on the output S.

This logic unit introduces at a suitable time a single bit "1" in each of the registers 204 and 205 in such a way that by scrolling through the registers, they cause the sampling and memorization of the sensor signal and then its reading at the instants necessary for get the delay you want. For this, the bit contained in the write register and the bit contained in the read register are separated by a distance equal to n stages. If then the period of the clock H is equal to T _H , the delay τ brought to the signal at the output S is equal to: τ = nT _H (2) A set of analog current signals S is therefore obtained on the output S, sampled at the clock frequency corresponding to input signals delayed with respect to each other by a value determined by the positions of the bits at 1 controlled by the logic unit 206. To modify these delays, it suffices to modify the positions of the write and read bits so as to obtain the differences between these bits corresponding to the desired delays. This modification is obtained at the level of the logic unit, which operates on the basis of the control signals which are sent to it by a central processor. This modification can therefore be obtained both as easily and as quickly as in a digital processing, while requiring only a much lower computational load, since these computations relate only to the determination of the delays to be obtained and not the actual training of these on the signals themselves. To obtain the channel signal, these different signals must then be summed according to formula 1. As these signals are represented by currents, the summation can be carried out very simply by adding all the currents by simple connection of the outputs of the assemblies 203 between them. In order to avoid reducing the bandwidth, due to the parasitic capacities existing at the level of each wire, the invention proposes, as shown in FIG. 5, to add these signals according to a tree dichotomy system where these are taken successively two by two. As shown in FIG. 5, limited to four signals Si to S, first add the signals Si and S _2, and S ₃ and S ₄ , then add the two signals thus obtained to obtain the signal of channel S _v . To obtain the weighting determined by the coefficients ai, we will use for example resistors, or preferably cells with current mirror of known type.

The number of stages of the assemblies 203 is determined by the maximum angle that the most depressed channels must form with the normal to the reception network. Referring to the digital example described above, this angle is equal to 15 °, which corresponds for the values described to a maximum delay equal to 22 μs for the transducers located at the ends of the reception network. The clock frequency, meanwhile, corresponds to the sampling frequency necessary to obtain good channel formation, ie 8 times the carrier. It is therefore equal for the values described above at 4 MHz. Under these conditions, the number of stages necessary to obtain this delay with this frequency is equal to 88. Taking into account the technology existing at present, this value is sufficiently small so that a single assembly comprising these 88 stages of cells and its related organs, such as the writing and reading registers and the logical addressing unit, in a monolithic structure.

As there are 800 transducers in this example, 800 assemblies are therefore required with their auxiliary circuits. The technology currently existing makes it possible to group these assemblies by 16 in an ASIC type circuit which can be contained in a CERQUAD 100 type housing whose surface is of the order of 4 cm ² .

To program the delays, microcontroller type circuits will then be used, which are capable of controlling 8 independent ASICs of this type. These 8 ASICs, the control microcontroller and the associated circuits, then occupy an area of approximately 100 cm ² . All of the circuits necessary to obtain all of the delays using the device according to the invention can then be grouped together on a maximum of 4 cards in double Europe format. This figure is to be compared with that of 320 cards for a fully digital solution and underlines the interest of the invention. The invention is not limited to sonar imaging systems, but it also extends to all acoustic imaging systems, in particular to imaging systems used in medical ultrasound systems. These systems are used to obtain a three-dimensional image of the human body from a probe comprising an acoustic antenna operating at a frequency of a few megahertz. Unlike the camera described above, as shown in FIG. 6, a scanning of the space is carried out by carrying out a series of successive focused emission-receptions each called “TIR”.

The probe shown in section in this figure comprises a body 401 whose front emissive part is curved and has on its face internally a set of aligned emission transducers 402, 128 in number for example. On each shot, a subset 405 of contiguous transducers is selected which makes it possible to perform transmission and reception, 32 transducers for example. Dynamic focusing on transmission and reception is carried out from the transducers of the subassembly. Thus we focus on several zones 403 during a shot.

On the next shot, the sub-assembly of transducers is shifted by 1 transducer, so as to obtain an offset of the radius swept during the shot in a direction BT perpendicular to BR. With the figures cited above, a total scan of the area 404 to be explored is therefore carried out in 96 shots.

To obtain for example a depth of 20 cm for the field 404 to be explored, the shots must be spaced at least 0.13 milliseconds. Under these conditions, in order to obtain the radial scanning with a distance resolution equal for example to 2 millimeters, one is led to change 200 times the delays brought to the signals received by the transducers during the reception time of the echoes corresponding to the shooting. These delay changes must therefore be made in 0.65 μs, which the invention makes it possible to carry out without any problem. If we perform this processing digitally, we would be led, as in the case of the camera, to use a very large volume of electronics, which would necessarily be located outside the probe and would also require a very large number of connecting wires between the transducers and this processing electronics. Using the invention, we are led to use 128 sets of delays whose number of cells is a function of the minimum distance at which we want to focus the reception channel.

By using for example a current frequency of 3MHz, transducers with the step of the wavelength, that is to say 0.5 mm, and a focusing at a minimum distance of 20 mm, the maximum delay is equal to 1.78 microseconds. This delay requires using, if we also sample the carrier 8 times, 43 cells per set.

Then using the same technology as that described above, we are led to take 8 ASICs comprising 16 delay lines, which corresponds to an electronic surface of approximately 65 square centimeters. for the group. This electronics can therefore be easily integrated into the body of the probe, which in particular makes it possible to limit the number of connecting wires and the flow of information to the ultrasound system to which this probe is connected.

Claims

1 - Acoustic imaging system, of the type comprising a set of sensors (201) intended to receive acoustic signals reflected by the object intended to be imaged in order to deliver in response electrical signals corresponding to these acoustic signals, processing means to apply specific delays to these electrical signals to form a set of reception channels, characterized in that these processing means comprise for each sensor a voltage / current converter (202) making it possible to convert said electrical signals into intensity, a set ( 203) of cells (207) with current memory connected in parallel at input and at output to receive said intensity signals, a write register (204) and a read register (205) controlled by a clock of period T _H and the stages of which are respectively connected to the write and read control inputs of said cells, and of means (206 ) to inject respectively into each of these two registers a single bit which circulates in the registers and which controls the writing and reading of the cells; the two unique bits being separated by a number n of stages between the 2 registers, to establish a delay of the output signal equal to a value n T _H.

2 - System according to claim 1, characterized in that to add the output signals (Si- S) of the assemblies (203) in order to obtain successive channel signals, these outputs are connected together two by two according to a tree dichotomy .

3 - System according to any one of claims 1 and 2, characterized in that the sensors (201) are arranged in a two-dimensional retina so as to form the reception system of an underwater acoustic camera which allows d '' obtain an image from all the channels formed by said processing means.

4 - System according to any one of claims 1 and 2, characterized in that the sensors are transducers (402) of a probe ultrasound system (401) intended to operate according to a series of “shots” obtained by the selection of a series of sub-assemblies (405) of the transducers which are offset with each shot of at least one sensor to allow scanning a sectoral area (404) described by the succession of shots, and in that for each shot a dynamic focusing (403) is carried out on transmission and on reception; this focusing being obtained by delays.