FR2476360A1 - - Google Patents

Abstract

VARIABLE FOCAL ACOUSTIC LENS KIT INCLUDING A FIRST AND A SECOND ACOUSTIC LENS ARRANGED BEHIND THE OTHER AT A CERTAIN DISTANCE TO FOCUS ACOUSTIC ENERGY WAVES OF A PREDETERMINAL WAVELENGTH. THE FIRST ACOUSTIC LENS 106 HAS A GIVEN ACTUAL OPENING AND A SINGLE PREDETERMINED RELATIVELY SHORT FOCAL, WHILE THE SECOND LENS 108 HAS A MEANS FOR ELECTRONICALLY CONTROL OF ITS FOCAL AND SIMULTANEOUSLY CONTROL OF ITS FOCAL DIMENSION OR THE DEPTH SIZE, OR SIMULTANEOUSLY. DE FIELD AND THE SEPARATING POWER OF THE LENS ASSEMBLY KEEP SENSITIVELY FIXED VALUES. APPLICATION TO IMAGE FORMING DEVICES BY ECHOES OF ULTRASOUND PULSES FOR OBTAINING THE IMAGE OF A DEEP STRUCTURE.

Description

The present invention relates to a set of acoustic lenses to

  variable focal length and, more particularly, a similar set of lenses of the type that can be used in an ultrasonic pulse echo image forming device for obtaining the image of a deep structure. In this field, reference can be made to U.S. Patent No. 4,131,022. This one describes

  an impulse echo image display device

  ultrasound resonances containing an acoustic lens of fixed focal length f and actual aperture A. (The expressicn "aperture

  "real" as used in this description,

  denotes the maximum linear dimension of the cross-section of the lens, measured in the same units of length as the focal length. The "relative opening" of the lens, defined by A / f, is dimensionless.) The acoustic lens is used to focus a beam of ultrasonic energy of a predetermined wavelength X. This beam is normally at least as much that the actual opening A, so that the entire cross section of

  the acoustic lens receives ultrasonic energy.

  As described in the patent cited above, the separating power is directly proportional to the value of

  the wavelength X of the ultrasonic energy waves and is inversely

  proportional to the value of the relative opening A / f. Thus, the greater the relative openness, the greater the separating power (ie the minimum dimension of a spot that can be

  distinguished) which can be obtained by means of ultrasonic energy waves

  sound of given wavelength is high. However, while the depth of field of an acoustic lens is also

  proportional to the wavelength of ultrahigh energy waves

  sound, it is inversely proportional to the square of the opening

  relative strength. However, because of the relative wavelength

  important wave of ultrasonic energy waves propagating in a liquid (for example of the order of 1 mm), an acoustic lens having a relatively large relative opening (for example from 1/3 to 1/2) has a good power separator for a notable depth of field. Moreover, since the separating power decreases only linearly with the relative opening, while the depth of field increases as a function of the square of the decrease of the relative opening, it is possible to obtain a relatively large increase in the depth. field for a relatively moderate decrease in separating power. Reference will also be made to the US patent

  of America No. 4,138,895. This describes an electronic transducer

  acoustic device comprising a blade of piezoelectric material having a common electrode on one side and on the other side a

  central electrode surrounded by an annular electrode. The open-

  The actual size of the central and annular electrodes, taken together, is large enough to produce a beam of ultrasonic energy whose width is sufficient to substantially strike the entire aperture of a relatively large relative aperture acoustic lens. However, the central electrode considered alone produces a beam of ultrasonic energy whose width is sufficient only to strike a relatively small central portion of the actual opening of the acoustic lens. There is provided switching means for selectively using the central electrode alone or for using both the central and annular electrodes. Relatively large relative openness and, therefore, the relative depth of field

  small that can be obtained from the central and annular electrodes

  Together, they can be used to produce a relatively high separating type C-scan display of an ultrasonic image corresponding to a shallow depth of the ultrasonic wave structure. The relatively small effective relative aperture and, therefore, the relatively large depth of field afforded by the central electrode alone can be used to produce a relatively smaller, relatively smaller, B-type scanning display. an ultrasound image corresponding to a greater depth

  of the structure subjected to ultrasonic waves.

  The present invention provides an improvement of varying the focal length in a set of acoustic lenses comprising a first and second acoustic lens separated to focus acoustic energy waves of a predetermined wavelength. According to the invention, the first acoustic lens has a single relatively short predetermined focal length, while the second acoustic lens is electrically adjusted to present any one of a predetermined number of different preselected distinct focal lengths. Each of these distinct focal lengths is substantially larger than the predetermined focal length of the first lens

acoustic.

  Such a set of acoustic lenses with variable focal distance, or with variable focal length, is particularly intended for use in an ultrasonic pulse echo image forming device for giving images of a deep structure hit by a pulse of ultrasonic energy wave of a predetermined wavelength. In this case, a time control means responsive to the instant of emission of an ultrasonic energy pulse is coupled to the second lens to sequentially select each of different distinct focal lengths for the period of time in which Ultrasonic echoes are received by successive parts of the deep structure subjected to ultrasonic waves. This provides an effective resultant depth of field for the lens array that embraces the full depth of the deep structure including images.

must be obtained.

  The following description, designed as an illustration

  of the invention, aims to give a better understanding of its features and advantages; it is based on the appended drawings, among which: FIG. 1 is a block diagram of a device

  ultrasound pulse echo image display

  sound system containing a set of acoustic lenses with variable focal length constituting an embodiment of the invention; FIGS. 2a and 2b are respectively views

  in section and in plan of an embodiment of the trans-

  multi-electrode conductor of the electro-

  of the lens assembly of Figure 1; FIG. 3 is a block diagram of one embodiment of the coupling portion of the electronic acoustic lens of FIG. 1; FIGS. 3a and 3b show part of FIG. 3 and serve to explain its operation; and FIG. 4 is a diagram used to explain the operation of the invention and showing an example of the respective focal lengths of a focal acoustic lens.

  variable using the principles of the invention.

  By way of illustration, a device 50 for imaging B-type ultrasound images is shown in FIG. 1. However, it will be understood that the invention can be applied to an ultrasound imaging device using both type B scanning and type C scanning. It is also possible to apply the invention to an ultrasonic imaging device using only C-type scanning, which uses the set of Variable focal length acoustical lenses of the invention to obtain a sufficient depth of field of view sufficient to form images of all points within a deep structure with good focus. In FIG. 1, the set 100 of acoustic lenses with variable focal length, which constitutes the subject of the invention, is used to strike the deep structure to be studied 102 by means of a focused scanning beam of pulsed pulsed waves. ultrasound energy of a predetermined wavelength moving in a medium 104 for propagating ultrasonic waves. As shown in FIG. 1, the assembly 100 comprises a real acoustic lens 106 and an electronic acoustic lens 108 arranged one behind the other at a certain separation distance. The electronic acoustic lens 108 comprises a time control means 110, a coupling means 112 and a multi-transducer 114.

electrodes.

  Preferably, the actual acoustic lens, that is to say material, which consists of a solid material (such as polystyrene) of lower refractive index

  to that of the surrounding environment 104 of ultrasonic wave propagation

  sound (such as water), is held in a fixed position. In this case, the structure 102 is scanned using a

  beam of ultrasonic energy either by displacement of the transducer

  114 only, or via a scanning means

  auxiliary, such as Risley prisms (not shown).

  However, it is also envisaged according to the invention to move both the material lens 106 and the electronic lens 108 to effect the scanning of the structure 102 by means of a

beam of ultrasonic energy.

  The time control means 110 includes a clock that intermittently applies a control pulse to a source 116 of pulses of a B-type ultrasound imaging device 50. The pulse source 116 includes high frequency oscillator which applies an ultrasonic frequency pulse (e.g. 1.5 Kiz) through the coupling means 112 to the multi-electrode transducer 114 in response to each control pulse

  from the time control means 110.

  The respective structures of the multi-electrode transducer 114 and the coupling means 112 (of which embodiments are shown in Figures 2a, 2b and 3, 3a and 3b) will be discussed in detail below. For the moment, it suffices to say that, in response to the application of an ultrasonic frequency pulse by the source of pulses 116, the multi-electrode transducer 114 emits a beam of pulsed pulses of energy.

  ultrasound device having a focal length selected from a predetermined number

  pre-selected distinct focal lengths, each of the distinct focal lengths being substantially greater than the relatively short predetermined focal length of the

  material acoustic lens 106.

  The deep structure 102, in response to the received ultrasound energy, echoes, a portion of which returns, via the material acoustic lens 106, to the multi-electrode transducer 114, which gives rise to detected electrical signals. The coupling means 112, under the control of the time control means 110, applies, via an image forming electronic circuit 118, to the type B scanning display device of the image forming device, that those signals that occur for a certain period of time after the application of ultrasonic energy to the deep structure 102. It is essential that the entire depth of the structure 102 is within the depth of field of the lens assembly 100 so that an image of the entire deep structure 102 can be formed on the B-scan display device 120. However, the structure 102 has a depth which is greater than the depth of field of any desired single focal length of the lens assembly 100. In order to produce the desired total depth of field of the lens assembly 100, the coupling means 112 sequentially selects, under control of the means 110 of

  time control, each of the different focal distances differs

  the resultant effective depth of field for the variable focus acoustic lens assembly 100 which embraces the entire depth of the deep structure 102. To allow a better understanding of how this is achieved, reference will now be made to Figures 2a, 2b, 3, 3a and 3b. As shown in FIGS. 2a and 2b, the transducer

  114 with several electrodes comprises a blade of piezo material

  200 whose faces are covered by several electrodes. More specifically, a common electrode 202 covers one of the two opposite faces of the blade 200, while the other

  of these two faces is substantially covered by several electri-

  separated spatially distributed contiguous trodes 204, 206, 208, 210, 212 and 214. As shown in FIGS. 2a and 2b, the electrode 204 is a central electrode with a circular cross-section of

  preselected diameter and is surrounded by annular electrodes

  The electrodes 204 to 214 substantially cover the entire cross-section defined by the outer diameter of the outermost electrode 214 (i.e. inner diameter of each annular electrode is substantially equal to the outer diameter of the electrode it surrounds). As indicated in FIGS. 2a and 2b, the radial widths (that is the difference between the inner and outer radii of the annular electrodes 206 to 214) are different from one another and are different from the diameter of the central electrode 204. The reason for these radial widths

  different will be discussed below.

  As shown in FIG. 2a and FIG. 3, the ccmmune electrode 202 is connected to the earth potential, and each of the electrodes 204 to 214 is separately connected to the coupling means 112. The coupling means 112, as shown in FIG. FIG. 3 comprises an isolation circuit 300 acting as a transmission-reception switch. The circuit 300 allows ultrasonic frequency pulses from the source of pulses

  116 are applied to each of the electrodes 204 to 214 of the trans-

  114, but prevents the signals received by the electrodes 204 to 214 from being sent back to the source of pulses 116. By way of example, the isolation circuit 300 may consist of a threshold switch (of the type described in cited patent No. 4,138,895) formed of semiconductor diodes polarized in opposite directions. Such a threshold switch allows the ultrasonic frequency pulses from the source of pulses 116, which pulses have absolute values of amplitude greater than the threshold voltage of a semiconductor (for example 0.7 V). , be

  each applied in parallel to all the electrodes 204 to 214.

  However, since the amplitude of the received signal is always much smaller than the threshold voltage, the received signals are isolated

  vis-à-vis the source of pulses 116.

  If, as has been assumed in the example given above, the ultrasonic frequency pulses are applied in parallel to all the electrodes 204 to 214 of the transducer 114, the function of this transducer is to emit a front of plane wave

  of ultrasound energy in the propagation medium 104, that is,

  say that the focal length of the electronic lens 108 is in this case infinite. The coupling means 112 also comprises a switching means and phase shift circuits which couple any selected combination of electrodes 204 to 214, via an adder 302, to a common terminal 304 (located at the output of the adder). 302) connected to the electronic circuit 118 of

  image formation. To avoid complicating the description of the func-

  switching device, the one represented on the

  FIG. 3 is functionally indicated as comprising a

  a three-way controller 306 which couples the electrode 204 to a first input of the adder 302, a two-way switch 308 which couples the electrode 206 to a second input of the adder 302, a two-way switch 310 which couples the electrode 208 to a third input of the adder 302, a one-direction switch 312 which couples the electrode 212 to a fifth input of the adder 202, and a one-direction switch 314 which couples the electrode 214 to a sixth input of the adder 302. (As shown in FIG. 3, the electrode 210 is connected directly to a fourth input of the adder 302.) Functionally, this switching means also comprises a control circuit 316. switching circuit which selectively actuates the switches 306, 308, 310, 312 and 314 based on time control signals applied thereto by the time control circuit 110. It will be understood that, in practice, the switching means normally comprises a plurality of normally closed electronic gates which are selectively

  opened in any one of a variety of predetermined combinations

  terminated according to signals applied by the time control circuit 110. However, the operation of these electronic gates is equivalent to that of the switches 306, 308,

  310, 312 and 314 described in detail below.

  The coupling means 112 also comprises a plurality of phase shift circuits. More specifically, when the two-way controller 310 is in its upper position, any signal appearing on the electrode 208 is applied to the third input of the adder 302 without any phase shift. But when the switch 310 is in its lower position, any signal appearing on the electrode 208 is applied to the third input of the adder 302 with a quarter-cycle phase delay of the ultrasonic frequency, this delay being provided by the phase shift circuit 318. In the same way, any signal appearing on the electrode 206 is applied to the second input of the adder 302 without phase shift when the two-way switch 308 is in its upper position, or

  well with a quarter-cycle phase shift of the ultrasonic frequency

  sound, this phase shift being provided by the phase shift circuit 320

  II, when the switch is in its lower position.

  In addition, any signal appearing on the electrode 204 is applied to the first input of the adder 302 without phase shift when the three-way switch 306 is in its upper position, or with a phase shift of one quarter of a cycle. the ultrasonic frequency, this phase shift being provided by a phase shift circuit 322, when the switch 306 is in its average position, or with a half-cycle phase shift of the ultrasonic frequency, this phase shift being provided by the circuit

  324 phase shift IV, when the switch 306 is in its posi-

lower level.

  The invention makes use of certain known optical relationships, which will now be discussed. First, the resulting focal length f of a lens assembly consisting of a first focal lens f1 disposed in alignment with a second focal lens f is given by the following relationship

= Y1 (1)

  Secondly, a material imaging lens displaces the incident wavefront by an amount that varies continuously in inverse proportion to the distance separating

  each point of incidence of the wavefront of the axis of the lens.

  An electronic lens, of the type shown in FIGS. 2a, 2b, 3, 3a and 3b, uses a finite number of discrete electrodes associated with a discrete finite number of electrical phase shifting circuits to simulate the function of a lens. hardware. Such a lens would require an infinite number of electrodes placed infinitely close to each other (which is impossible) for a material lens to be reproduced exactly. However, as is known (according to the Rayleigh condition), a lens in which the phase shift varies discontinuously as a function of the distance from the axis nevertheless produces an image, without any appreciable degradation, insofar as the phase shift produced for each wavefront incidence point does not deviate from that which would be provided by the continuously variable phase shift of a material lens of

  more than a quarter wavelength of the incident wave (ie

  say a quarter cycle of the ultrasonic frequency). An electronic lens of the type shown in FIGS. 1, 2a, 2b, 3, 3a and 3b is designed to respect this Rayleigh condition. It can be shown that an electron lens comprising a transducer with n electrically separated concentric electrodes on one of its faces has a focal length f2 and satisfies the Rayleigh condition for an incident energy wavefront.

  ultrasonic wavelength x if the outer radii respec-

  R1 to r of respective n electrodes obey the following equations f2X 1/2 ri =] (2-1) 2f2 \ '1/2 r2 2 (2-2) nf2S 1/2 r 2 (2-n) n 2 Since the object of the invention is to selectively produce any one of several different focal lengths, a different set of ray electrodes r1 to r is required for each different focal length. In addition, it is necessary that the relative openness of the lens has a value

  substantially constant (i.e. substantially independent of

  chosen focal length). To satisfy these conditions, the transducer of the electronic lens must have a sufficient number of contiguous concentric electrodes on one side.

  of its faces, each electrode having an appropriate radial width

  asked, in order to be able to give each of the different games of

  radii for each of the different focal lengths by selective connection

  at least some electrodes in one or more groups, each group being formed of one or more electrodes connected in parallel. This causes the radial width of some of the electrodes to be relatively small. The actual size of the radial widths of the electrodes depends on the respective dimensions of the

  different focal lengths desired.

  In particular, the equations (2-1) to (2-n) show that the respective dimensions of the radii ri to r vary in direct function of the dimension of the focal length f Thus, when the number of focal lengths that can be chosen

  the electronic lens is important, the minimum dimensions

  The radial widths of the concentric electrodes of the electronic lens that is needed to produce these focal lengths increase. It is for this reason that the invention, according to equation (1), combines a relatively long focal length electronic acoustic lens with a relatively fixed focal length material acoustic lens to produce a set of focal acoustic lenses.

  relatively short variable. Moreover, the value n and, consequently,

  the total number of electrodes required is reduced, since the relative opening of the relatively long focal length electronic acoustic lens becomes only small relative to the desired relative opening of the lens assembly

  acoustics with relatively short focal lengths.

  By way of illustration, and in order to explain the operation of the invention, it is assumed that the ultrasonic energy waves have a wavelength λ of 1 mm. It is also assumed that the set 100 of acoustic lenses with variable focal length is designed to have a focal length to choose between 150 mm, 200 mm and 250 mm, for a relatively constant relative opening of 1/3. It is also assumed that the material acoustic lens 106 has a fixed focal length of 250 mm and an actual aperture which is at least as large

  the maximum effective aperture of the electrically

  108, so that the actual opening presented by the electronic acoustic lens 108 determines the actual opening of the set 100 of acoustic lenses with variable focal length. With these assumptions, one can use equation (1) to determine the different relatively long focal lengths required for the electronic acoustic lens 108. More specifically, with a material lens 106 of focal length 250 mm, it

  the respective focal lengths of the electro-

  108 are equal to 375 mm, 1000 mm, or infinity to produce respective focal lengths of 150 mm, 200 mm, or

  250 mm for the set of lenses 100. Moreover, since the opening

  the actual set of the lens assembly 100 is determined by the actual aperture of the electronic lens 108, the latter must have respective actual apertures of 50 mm, 66.7 mm

  or 83.3 mm for the respective focal lengths of 150 mm, -

  mm or 250 mm of the lens assembly 100, so that the lens assembly 10 always has a relative opening

substantially equal to 1/3.

  The radial widths and the respective outer radii of the electrodes 204 to 214 which make it possible to choose, according to the setting of the switches 306 to 314, any one of the three supposed focal lengths of the electronic lens 108 (and their corresponding actual openings ) are

  given in Table I below.

TABLE I

  Electrode Radial Width Outer Radius (mm) (mm)

204 13.69 13.69

206 5.67 19.36

208 3.00 22.36

210 1.36 23.72

212 7.90 31.62

214 10> 05 41.67

  More specifically, the switching control circuit 316 places the switches 306 to 314 in any one of three given combinations of switching settings, each different combination corresponding to one of the three focal lengths (and the actual opening corresponding) of the electronic lens 108. FIG. 3 shows a first given combination of settings of the switches 306 to 314, which is the combination that makes it possible to obtain a focal length of 375 mm for the electronic lens 108. In this first combination, the one-direction switches 314 and 312 are open, so that the signals from the electrodes 214 and 212 are not applied to the adder 302. The two-way switch 310 is in its upper position, so that the signals present at the electrode 208 are applied directly to the adder 302. The two-position switch 308 is in its lower position, if although the signals present on

  the electrode 206 are applied to the adder 302 through the

  intermediate phase shift circuit 320 with a quarter-cycle phase shift of the ultrasonic frequency. The three-way switch 306 is in its lower position, so that the signals present on the electrode 204 are applied via the phase shift circuit 324 to the adder 302 with a half-cycle phase shift of the ultrasonic frequency. Table II below shows the conditions assumed for this first switching combination. The

  outer radius of 13.69, 19.36 and 23.72 mm respectively (

  Table II) correspond to the respective values of r1, r2 and r3 of equations (2-1) to (2-n), when the quantities f2 and X are replaced by 375 mm and 1 mm, respectively. In addition, the actual aperture of the electronic lens 108 in its first switching combination is 47.44 mm (i.e. twice the outermost radius of 23.72 mm). This value is substantially equal to the actual opening serving as a criterion,

which is 50 mm.

TABLE he

  First switching combination 1. Lens set 100 (design criteria) 2. Material lens 106

3. Electronic lens 108 -

  (design criteria) 4. Electrodes Radial effective width 204 13, 69 mm 206 5.67 mm 208-, 210 4.36 mm - Focal distance - Actual aperture Relative aperture - Focal distance - Actual aperture ai - Focal distance - Actual aperture Ray. outside 13.69 mm 19.36 mm 23.72 mm mm 1/3 250 mm minus 83.3 mm 375 mm mm phase shift 1/2 cycle 1/4 cycle o Figure 3a shows a second given combination of switch settings 306 to 314, this combination operating the choice of a focal length of 1000 mm for the electronic lens 108. In this second combination, the single-direction switch 314 is open, so that the signals

  present on the electrode 214 are not applied to the addi-

  302. The one-way switch 312 is closed, so that the signals on the electrode 212 are directly applied to the adder 302. The two-way switch 310 is in its lower position. so that the signals present on the electrode 208 are applied to the adder 302 via the phase shift circuit 318 with a quarter-cycle phase shift of the ultrasonic frequency. The two-way switch 308 is in its lower position, so that the signals present on the electrode 206 are applied to the adder 306 via the phase shift circuit 320 with a quarter-cycle phase shift of the ultrasonic frequency. . The three-way switch 306 is in its middle position, so that the signals present on the electrode 204 are applied via the phase-shift circuit 322 to the adder

  302 with a quarter-cycle phase shift of the ultrasonic frequency

  sound. Table III below shows the conditions assumed for this second switching combination. The respective outer radii of 22.36 and 31.62 mm (presented in Table III) correspond to the respective values r1

  and r2 equations (2-1) to (2-n), when replacing the

  f2 and X respectively by 1000 mm and 1 mm. In addition, the actual opening of the electronic lens 108 in this

  second switching combination is 63> 24 mm (i.e.

  say twice the outer radius of 31.62 mm). This value is substantially equal to the actual opening of 66.7 mm

  constituting the design criterion.

- TABLE III

  Second switching combination 1. Lens assembly 100 Focal distance 200 mm (design criteria) - Actual aperture 66.7 mm - Relative aperture 1/3 2. Material lens 106 - Focal length 250 mm - Actual aperture at least 83.3 mm 3. Electronic lens 108 Focal length 1000 mm (design criteria) - Actual aperture 66.7 mm 4. Electrodes Width Radius Actual radial phase shift 204+ 206 + 208 22.36 mm 22.36 mm 1/4 cycle 210+ 212 9,26 mm 31> 62 0 FIG. 3b shows the third combination of settings of the switches 306 to 314, this combination operating the

  selection of an infinite focal length for the electronic lens.

  In this third combination, the single-direction switches 314 and 312 are closed, so that the signals on the electrodes 214 and 212 are directly applied to the adder 302. The two-way switches 310 and 308 are in their upper positions, so that the signals present on the electrodes 208 and 206 are directly applied to the adder 302. The three-way switch 306 is in its upper position, so that the signals

  present on the electrode 204 are directly applied to the addi-

302.

  Table IV below shows the conditions assumed for this third switching combination. Since in this third switching combination all the electrodes 204 to 214 are directly coupled to the adder 302, a plane wavefront of received signals is detected by the

  electronic lens 108 (i.e., the electronic lens

  108 has, in this third combination, an infinite focal length f2). So, in the third combination, all

  the electrodes 204 to 214 are effectively connected in parallel

  and electrically operated as if they formed a single electrode with a circular cross section with a radius of 41.67 mm (equal to the outer radius of the outermost electrode 214). The outer radius of 41.67 mm of the electrode 214 is the one chosen to give the electronic lens 108 an actual opening of 83.34 mm (that is to say twice the radius of 41.67 mm), what constitutes the value necessary for that

  the lens assembly 100 has the desired relative opening of 1/3.

TABLE IV

  Third switching combination 1. Set of lenses 100 (design criteria) 2. Material lens 106 3. Electronic lens 108 4. Effective electrodes

204 + 206

+ 208 + 210

  +212 Radial width 41.67 nm - Focal distance - Actual aperture Relative aperture - Focal distance - Actual aperture - Focal distance - Actual aperture Outer radius R 41.67 mm 250 mm 83.3 nm 1/3 250 mm at least 83, 3 mm infinite 83.3 mm phase shift -O cycle As shown in FIG. 4, the front surface 400 of the deep structure 102, of which images must be formed, is located 125 mm from the effective main plane 402 of the set of acoustic lenses, while the rear surface 404 of the deep structure 102 is 275 mm from the main plane 402. Thus, the total depth of the deep structure 102

is 150 mm.

  Following the emission of each of the ultrasonic energy pulses, the time control circuit 110

  actuates the switching means so as to maintain the addi-

  302 not connected to the transducer 114, until a given first instant for which echoes from the front surface 400 are received by the transducer 114. At this first instant, the time control circuit 110 activates the switching means so as to place it in the first given switching combination, so that the set 100 of acoustic lenses has a focal length of 150 mm. This first combination is maintained until a second instant for which echoes from 175 mm of the main plane 402 are received by the transducer 114. At this second given instant, the time control circuit 110 activates the switching means. so as to place it in the second given switching combination, so that the set 100 of acoustic lenses then has a focal length of mm. This second combination is maintained until a third instant, for which echoes from

  225 mm of the main plane 402 are received by the transducer 114.

  At this third instant, the time control circuit 110 activates the switching means so as to place it in the third switching combination, so that the lens assembly 100 has a focal length of 250 mm. The time control circuit maintains the switching means in the third combination until echoes from 275 mm from the main plane 402 are received by the transducer 114. It is necessary that all echoes received by the transducer 114 , which come from points inside the deep structure 102, remain within the depth of field of the set 100 of acoustic lenses with variable focal length, so that they can be imaged at home in the display device 120 scanning type B. It can be seen in Figure 4 that this condition is satisfied both

  that the depth of field of the set 100 of acoustic lenses

  ticks for each selected focal length is at least 25 mm.

  As is known, the depth of field of an acoustic lens is equal to the ratio of four times the wavelength

  ultrasonic energy waves squared the relative opening.

  The assumed value for the wavelength is 1 mm and the relative aperture is assumed to be 1/3. These values

  supposed to produce a depth of field exceeding 25 mm.

  In the preferred embodiment of the coupling means 112 shown in FIG. 3, the electronic lens 108 has an infinite focal length during the emission of the pulses of ultrasonic energy. This is desirable to simplify the structure of the isolation circuit 300. However, it will be recognized that

  the isolation circuit 300 may comprise circuit circuits

  and the electronic lens 108 has a desired non-infinite wavelength (according to equations (2-1) to (2-n)). Of course, those skilled in the art will be able to imagine, from the device, which, by way of illustration and not limitation, has just been given

  description, various variants and modifications not leaving

of the scope of the invention.

Claims (8)

R E V E N D I C AT 1 0 N S   A set of acoustic zoom lenses comprising a first and a second acoustic lens disposed one behind the other at a distance to focus acoustic energy waves of a predetermined wavelength, characterized in that that: the first acoustic lens (106) has a single relatively short predetermined focal length; and the second acoustic lens (108) comprises means (110, 112) for electronically controlling the focal length of the second acoustic lens so that it has any one selected from a predetermined number of different preselected distinct focal lengths, each of said distinct focal lengths   being substantially larger than said predetermined focal length.   2. set of acoustic lenses according to claim 1, characterized in that: said set of lenses has a certain relative opening (1/3); and the respective values of the distinct focal lengths (150, 200, 250 mm) are chosen in advance so that, for said predetermined wavelength (1 mm), the respective field depths (25 mm) of said set of lenses to different   focal points are in contiguous or overlapping relation-   mation. 3. Set of acoustic lenses according to claim 1 or 2, characterized in that: the first acoustic lens is made of a material having an index of refraction vis-à-vis said acoustic energy waves that it propagates which is different from that of the surrounding environment; said second acoustic lens comprises a   electroacoustic transducer (114) having a piezoelectric blade   electrical device (200) having on one of its faces a plurality of spatially distributed distinct electrodes (204 to 212); and said means for electronically controlling the focal length comprises signal application means (112) having phase shifting means (318 to 324) and switching means (306, 308, 310, 312, 314, 316) selectively connecting any combinations of certain combinations of said electrodes at a common terminal (304) so as to select a corresponding focal length of said predetermined number of focal lengths   different preselected different.   4. set of acoustic lenses according to claim 3, characterized in that: the first lens has a given actual opening; said electrodes comprise a central electrode (204) having a circular cross-section of a diameter chosen in advance, this central electrode being surrounded by a number of contiguous annular electrodes (208 to 214), the outside diameter of that of electrodes which is the outermost defining for the second lens a maximum actual opening which is not greater than said actual opening   given, each of the annular electrodes having an integer diameter   chosen in advance and an outside diameter chosen in advance; said switching means, in the case of the   selection of the largest of said distinct focal lengths   coupled, couples all the electrodes to the common terminal and thus gives some numerical aperture to said lens assembly; said switching means, in the case of selecting a separate preselected focal length not being the longest, couples a certain given combination of electrodes which is formed of a certain annular electrode (210) not being that the outermost and an electrode (204, 206, 208) surrounded by this certain annular electrode not being the outermost, to said common terminal; and the diameters of the electrodes are chosen in advance so that said combinations substantially give the whole   of lenses said certain numerical aperture.   An acoustic lens assembly according to claim 4, characterized in that said electrode combination comprises an inner group (204) of one or more contiguous electrodes and an outer group (212, 214) of one or more electrodes. contiguous; in response to the selection of said distinct preselected focal length being not the longest, said switching means comprises means (306) which respectively connect said electrodes of said internal group in parallel to the common terminal and connect in parallel said electrodes of the group external (312, 314) to the common terminal; and said phase-shifting means comprise means (322, 324) which phase-shift signals at the frequency of said acoustic energy waves which are applied between said electrodes of the internal group and the common terminal of a substantially whole number of fourths of wavelength with respect to signals at the frequency of said acoustic energy waves being applied between said electrodes   of the external group and the common terminal.   6. set of acoustic lenses according to claim 5, characterized in that: the electrodes of the inner and outer groups are in contiguous relationship; and said phase-shifting means comprise means (322) which phase out said signals applied between the electrodes of the internal group and the common terminal of substantially a quarter of a wavelength with respect to the signals applied between the   electrodes of the external group and the common terminal.   An acoustic lens assembly according to claim 5, characterized in that said given combination of electrodes further comprises an intermediate group (206, 208) of one or more contiguous electrodes located between the electrodes of the internal group and the electrodes the external group in relation of contiguity with them; in response to the selection of said distinct focal length   pre-selected is not the longest, the means of   tion comprises means (310, 308) which connect in parallel said electrodes of the intermediate gate to the common terminal; and the phase-shifting means comprise means (318, 320 and 322) which respectively phase shift signals at the frequency of said acoustic energy waves which are respectively applied (1) between the electrodes of said intermediate group (206, 208) and the common bound and (2) between   electrodes of the inner group (204) and the common terminal, sensi-   (1) a quarter of a wavelength and substantially (2) a quarter of a wavelength with respect to signals at the frequency of sound energy waves being applied.   between said outer group electrodes and the common terminal.   An ultrasonic pulse echo image forming apparatus (Fig. 1) for forming deep structure images (102, Figs. 1 and 4) receiving a pulse of ultrasonic energy of a predetermined wavelength, characterized by a set of variable focus acoustic lenses of the type set forth in claim 2 disposed at a distance from said deep structure so as to receive ultrasound echoes of said deep structure for a period of time after receipt of said pulse of ultrasonic energy by the deep structure; the respective values of said distinct focal lengths of said second acoustic lens being chosen so that, for said predetermined wavelength, the respective depths of field of said set of lenses at focal lengths   respective distinct parts have distinct parts   contiguous to the total depth of the deep structure of which images must be formed; and time control means (110) coupled to said second lens and responsive to the instant of emission of said ultrasonic energy pulse sequentially selecting each of said different distinct focal lengths during successive portions of said certain time interval so as to to give to said lens assembly an effective resulting depth of field which includes all the depth of the   deep structure of which we must form images.