CA1110750A

CA1110750A - Electronic beam scanning for ultrasonic imaging

Info

Publication number: CA1110750A
Application number: CA265,524A
Authority: CA
Inventors: Christoph B. Burckhardt; Pierre-Andre Grandchamp; Heinz Hoffmann; Rainer Fehr
Original assignee: F Hoffmann La Roche AG
Current assignee: F Hoffmann La Roche AG
Priority date: 1975-12-01
Filing date: 1976-11-12
Publication date: 1981-10-13
Also published as: CH608103A5; SE8000837L; DE2660208B2; SE7613431L; SE445009B; JPS5268492A; DK146227C; FR2445545A1; FR2334117A1; DE2660888C3; GB1570880A; NL7612852A; FR2445545B1; DE2660208C3; DK538076A; DE2654280A1; IT1070826B; DE2660882C3; DK146227B; NL176207C

Abstract

Abstract of the Disclosure A method of producing cross-sectional images uses an ultrasonic imaging unit operating on the pulse-echo principle and comprises a transducer system having a stationary elongated array of adjacent transducer elements with trans-verse electrode segments adjacent one another on at least one side; successively and cyclically selected groups of adjacent transducer elements are used to produce an ultrasonic beam in response to pulsed electric transmitter signals applied to the electrode segments, and are also used to transmit the ultrasonic beam into a heterogeneous body, receive echoes reflected from a discontinuity in the body, and generate an electric echo signal in response to the received echoes.

Description

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P~N 47(`1I/96 The invent:ion rel.ates to a me~hod of producil-.g cross-sectional images usiny an ultrasonic imaging unit operat-ing on the pu]se~echo princip:Le and comprising a trans-ducer system comprising a stationary elongated a.rray of adjacent transducer elements and havinq transver.se elect-rode segments adjacent one another on at least one side;
in the method, successively and cyclically selected groups of adjacent transducer elements of the transducer system are used to produce an ultrasoni.c beam ln response to pulsed electric transmi-tter slc3nals appliecl to the electrode segments r ancl are also used to t.ransmit the ultrason:ic beam into a heterogeneous body, receive echocs refl.ected .from a discontinuity in the body, and genera-te an electric echo signal in response to the received echoes; the invent~
ion also relates to an ultrasonic imaging unit for perform-ing the method.

In order to produce ultrasonic images (more particul-arly for producing cross-sectional images) it is convent-ional for an ultrasonic transducer to be mechanically moved. This had various disadvantages. If the trans-ducer is moved by hand, the scanning process is lengthy and dependent on the skill of the operator. If the transducer is moved by a mo-tor, a relatively heavy water bath is

3~ usually required. In addition, the extra distance travelled through the water bath results in a reduction in the maximum possible image frequency.

In order to obviate these disadvantages; therefore, ultrasonic imaging units with electronic scanning have been developed, the ultrasonic beam being linearly shifted in time.
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-I In a krlowll ultrasonlc lmacJ:ing unit of the afore~rlltioIled kind (Am.?ri.ccln paten-t specification 3 8~l 466), the transducer system produces an unfocussed ultrason.i.c beam and the transverse reso.lution :i.s determined by the wi.dth of the transducer elements. In the known device, there is a limit to which the transverse resolutlon can be improved by reducing the width of the trarlsducer elements, -the limit being set by the minimum wid-th of the ultrasonic beam~
Al-though the cross--sectional images produced by the known device are relatively distinct, it has been Eound in practice that s-till higher transverse reso:Lution is desir-ab].e for many applicati.ons.

~n object oE the .invention, therefore, i9 to provide a method and an ul.trasonic imagirlg uni.t WhiCIl can g:i.ve higher transverse resolution.

The method according to the invention is characterised in that, i.n order to focus the ultrasonic beam (23) produced by each group of transducers 21, the transmitter signals (41, 42) applied to the electrode segments or subgroups thereof and/or the echo signals (l42) given by the electrode segments or subgroups thereof are time-shifted with respect to one another, each transmitter or echo signal being associated with a time shift whi.ch is a function of the distance between the corresponding electrode segmen-t or subgroup of segments and the centre of the group of transducers.

The invention also relates to an ultrasonic imaging unit for performing the method according to the invention, the unit comprising a timing generator for producing a pulsed electric timing signal; a transducer system comprising a stationary elongated array of adjacent transducer elements and comprising transverse electrode segments adjacent one another on at least one side~ the transducer system being used to produce an ultrasonic beam in response to pulsed ,. : ,, "
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4 _ -l transmitter si~nals derived frorn -the electrlG timi.ng signa1., to transmit the ultrasonic beam into a hete~rogeneous body, to xeceive echoes ref].ected fro:rn a discontinuity i.n the body, and produce an electric echo signal .in response to the received echoj and an element~counter selector devioe connected to the timing generatorr the transducer system and an indicator device and used for successively and cyclically selecting groups of adjacent elements of the transducer systemr generating the ultrasonic beam, applying the transmitter signals to the electrode segments of the selec-ted group, and transmitting the echo signals produced by the group to the indicator de-vice, which is used to convert the echo signals into a visible image reproducing the cross-sectional structure of the heterogeneous body.

The ultrasonic imaging unit according to the invention is characterised by a transmi.tter-signal generator inserted between the timing generator and the element-counter selector device and used to derive transmitter signals for the electrode segments or subgroup thereof of the selected group of transducers, the signals being time-shifted with respect to one another and obtained from the timing signals given by the timing generator, and/or an ~cho-signal receiver inserted between the element-counter selector device and the indicator device and used to produce a relative time shift between the echo signals deli~ered by the electrode segments or subgroup thereof of the group of transducers.
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Some embodiments of the invention will now be described with reference to the accompanying drawings, in which:

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1 F:Lg~ 1 is a perspective view of the transducer system in the previously-rnen-tioned prio:r-art ultrasonic imaging unit, Fig. 2 is a d:Lagramma-tic cross-section of the radiation charac-teristic 23 of a group of transducers according to the invention, compared with the radiakion characterist:ic 22 of a group of transducers in -the system according to Fig. 1~

Fig. 3 is a diagrammatic cross-sec-tion -through a pref-erred embodiment of an arrangement of transducers 38 in thetransducer system 11 in Fig. 1.

Fig. 4 is a rear view of a group of transducers 21 according to the :invention, comprising four transducer elements.
Fig. 5 gives d.iagrams of transmitter signals 41, 42 which, according to the invention, are applied to the elec~rode segments 31-34 of the group of transducers 21 in Fig. 3.
Fig. 6 is a diagrammatic cross-section parallel to the QS plane in Fig. 1 of an irradiating surface 37 in the arrangement 38 in Fis. 3, the surface having a suitable shape for weakly focussing the ultrasonic beam in the QS
direction.

Fig. 7 is a rear view of an embodiment of the arrange-men-t 38 in Fig. 3, whereby the weak focussing in the Q
direction is obtained by means of a flat irradiating surface instead of the concave surface in Fig. 6.

Figs. 8a, 8b, 8c show an advantageous configuration of groups of transducers 71, 72, 73 which are cyclically and successively selected.
Fig. 9a is a rear view of a group of transducers 91 according to the invention comprising 11 electrode segments ;. ~: .,, , . .. . ~ , . .
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~ 6 1 and used ln a second eMbodiment of the ul~-rasoll:ic imagirlc3 unit according to the invention~

Fig. 9b is a cross-section showing the shape of 1he irradiating surEace o, -the group of trallsducers 91 in E'ig. 9a~

Fig. 10 show~ diagrams o~' the transmitl,er signals which accordiny to the invention are applied to the electrode segments 92-98 of the group of transducers 91 according to Fig. 9a, Fig. 11_ is a rear view of a group oE transducers having seven electrode segments used in a preferred embod-iment of the ultras~nic imagirlcJ unit according to the invention, Fiy. l:Lb :ls a cross-sect:ion through a preferred shape of the irradiating surface of the group oE transducers in Fig. lla, Fig. 12 shows diagrams of the transmitter signals which according to the invention are applied to the electrode segments 112 118 of the group of transducers 111 according to Fig. lla, Fig. 13 is a schematic b~ock diacJram illustrating a preferred embodiment of the ultrasonic imaging unit according to the invention, Fig. 14 is a block diagram illustrating the transmitt er-signal generator 133 in the device in Fig. 13, Fig. 15 shows diagrams of the timing pulse 132 gener-ated by the timing generator 131 ~Fig. 13) and of the pulsed sine wave 162 derived from the timing pulse, :

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~. . ~ , , Fig. 16 is a block diagra~ illus-trating the echo-signal recelve~ 143 in the dev~ce ln Fig. 13, Fig. 17 illustrated the principle o~ a preferred embod-S iment o~ the ele~ent selector dri~e switch 138 in the devicein Fig. 13. For simplicity, the principle is illustrated in the case of a group of transducers containing only four elements, although the unit in Fig. 13 comprises groups each containing 7 elements.

Figs. 18 and 19 illustrate the dimensioning of a group of transducers according to the invention and the elements thereof.

15Fig. 20 is a diagram of a region scanned by a sector-scan each line represents a position of the ultrasonic beam.

Fig. 21 is a diagram of a region scanned by linear beam displacement, each line represents a position o~ the ultrasonic beam.
Fig. 22 is a diagram of a region scanned with an arcuate transducer system (not shown) placed e.g. on top of Fig. 22, each line represents a position of the ultrasonic beam.

25Fig. 23 is a diagram of a sound head with an arcuate transducer system.

Fiys. 24, 25 illustrate the production of a cylindrical ~ave front in two variants of the invention, Fig. 26 shows the use according to the invention of an arcuate transducer system for producing a "phased array", and Figs. 27, 23 illustrate the dimensioning of an arcuate 35tran~ducer system according to the invention.

As Fig. 1 shows, the transducer system ll of the known ultrasonic imaging units ~U.5. Patent Specification 3 881 466) , :. , .. . .:

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1 comprises a stationary e:Lonc3ated array of ac~jacend transducer elements 12~ Groups of A acljacen-t elements 12 a~e successive]y stimulated to prc>duce pulses. 'I'he locat:ion of eilch successi~e group of A elements is shif-ted oF a :longi.tudinal distance of B
elements frorn the posi-tion oE the immeclia-tely precedin~ group.
The ultrasonic beam 13 is thereby movecl in the ~irection of arrow L, as shown by -the series of chain-dotted rectangl~s 14 showing the ins-tan-taneous posi.-tion of beam 13 after equal intervals o~ time. Note tha-t each group oE transducers in the known transducer system 11 generates an unfocussed ultrasonic ~eam 13, since all the A elements :in the yroup of transdueers are simultaneousl.y energised so as to yield pulses. The unfocussed radiation characteristic 22 of the ultrasonic beam 13 in Fig. 1 is shown :in F:Lg. 2.

In E'ig. 1, an orthogonal system of coordinates i.s deine~ by three arrows, Q, I, and S. Arrow L is alon~ the longitudinal axis of the irradiating surace of the transducer system 11. Arrow S is parallel to the major axis of the ultrasonic beam 13. Arrow Q is at right angle to the plane defined by arrows L and S. The positions of the eross-sections and elevations shown in the accomp-anying drawings are defined with respeet to this eoordinate system.
.25 Fig. 3 is a partial cross-secti.on showing the strueture o a preferred arraIIgement of transdueers 38 for performing the method aeeording to the invention.
Arrangement 38 eomprises a complete electrode 36 which is earthed and one surface 37 of whieh is used as an irradiating surfaee; arrangement 38 also eomprises a piezoeleetrie layer 35 and eleetrode segments 31-34, shown in rear view in Fig. 4.

3~ It is elear from the preeeding description of arrange-ment 38 that the transdueer elements aeeording to the invention ean have eommon parts sueh as the piezoeleetric : , .. . . .
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layer 35 or the camplete electrode 36. The ~rrangement 38 accord-ing to the inventi~n can ~e operated simply by providing it with electrode seg~ents on one side, which are supplied with the time-s~ted tran~mltter st~n~ls and ~rom which echo si~nals can be obtained. Thus, each electr~de segment de~ines a transducer element accordiny to the invention.

The effect o~tained by the invention, i.e. higher transverse resolution, is malnl~ due to a novel manner of operation of the transducer system. This will be explained in detail, firstly with referencP to Figs. 2, 4 and 5.

Fig. 4 shows electrode segments 31-34 of a group of trans-ducers 21 according to the invention. In order to produce an ultrasonic bea~ according to the invention, transmitter signals 41, 42 which are time-shifted relative to one another as shown in Fig. 5 are applied to the electrode segments 31-34, the transmitter signals for the outer segments 31, 34 having a phase lead. In this manner a weakly focu~sed ultrasonic beam 23 is produced (Fig.
2).

In a preferred embodiment of the invention, a time shift is produced not only between the transmitter signals but also between the echosignals received by the individual elements of the group of transducers. The group of transducers 21 shown in Fig.4 has four elements for transmitting and receiving, the transmitted signals and the time-shifted echo signals of the outer elements having a phase lead of 90. According to the invention, the phase lead is defined with respect to a period (360) of the high-frequency carrier signal (e.g. 2 MHz), which is supplied to the electrode segments of the successive groups of transducers in pulses at a repetition frequency of e.g. 2 XHz and at a suitable phase angle.

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: : : ' -I The effec-t of opera-ting group 21 accvrcling to -the invention can be improved by the fol:Lowiny addit:icnc measures:

1) It has been ound advanlageous to select t:he follow:irlc~
combirlations o~ phase lead for the outer elemen-ts of the grollp:

- Transmi_ter sign~s Echo si~l.s either approx. 90 approx. 45 -iO or approx. 45 approx. 90 As a result of these diferent values of the phase lead for the transmitter and echo s;gnals, the radiation characteristic 23 accord:ing to the invention (Fig. 2) is adclitionally narrowed over a certaln depth.

2) Advantageously, the transmitter and echo s:ignals are weighted. As shown in Fig. 5, the inner electrode segments 32, 33 are supplied with a transmitter signal having a higher amplitude aO. Similarly, the echo signals received from the inner segments are multiplied by a higher weighting factor than the echo signals from the outer elements.

Advantageously, the weighting ratio is 2:1 for both the transmitter and the echo signals.

3) Advantageously, weak focussing is also produced in the Q direction in Fig. 1, e.g. by using a transducer arrangement comprising a slightly curved irradiating surface 37 (see Fig. 6).

The weak focussiny in the Q direction can also be electronically produced, using a transducer arrangement 3~ as in ~ig. 7, in which each o the electrode segments is divided into three parts a, _, c in the Q direction. As shown in Fig. 7, only the shaded parts of the electrode : ; ::. : ~
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segments are used ~r transmitting or receivin~. The inner parts 32~, 33b ~re ~nergised ~ith the transmitter signal 41 and the remaining ~ctive part~ axe energised with the transmitter signal 42. The resulting s~stem is electronically more compli-cated than the transducer arrangement comprising a curved irradi-ating surface, but it only requires a transclucer arrangement having a flat irradiating surface, which is cheaper.

In the known transducer system 11 in ~ig. 1, the ultrasonic lQ beam 13 can be displaced by the width of a transducer element 12 after each transmitting and reception period. However, the number of lines in the image and the resolution can be increased if the ultrasonic beam is displaced by a smaller amount each time e.g. by half the width of an element. The same result, of course, can be obtained by halving the width of the element, but the result is to double the number of elements and correspondingly increase the complexity.

In a preferred embodiment of the invention (Figs. 8a, 8b and 8_)the ultrasonic beam is displaced by half the width of an element in that successively selected groups of transducers 71, 72, 73 alternately contain an even and an odd number of elements, the successive groups being alternately formed by reducing the number of segments in one direction and increasing the number of segments in the opposite direction. The amplitudes and phases of the transmitter signals or the time-shifted echo signals are selected so that the shape of the ultrasonic beam remains substantially uniform, independently of the number of elements in the group of transducers. The following relations of amplitudes and phase give very similar beam shapes, e.g. when 4 and 3 elements are used alternately:

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Element 31 32 33 34 ~Amplitude 0,5 1 1 0,5 Transmission (Phase 90 0 0 90 (Amplitude 0,5 1 1 0,5 Reception (Phase ~5~ 0 0~ 45 Wi-th 3 elements:
Elemen-t 32 33 34 (Amplitude Transmission ~Phase 45 0 ~5 (Arnp~itude Reception (I'hase 22,5 0 22,5 ~ second embodiment oE the :invention will be described initially with respect to Figs. 9a, 9b and 10. It is known (Swiss Patent Specification 543 313) that the ultrasonic beam can be efficiently focussed over a considerable depth if ; an ultrasonic wave having a conical wave front is radiated.
A wave front of this kind is radiated e.g. by a conical ultrasonic transducer. Accordin~ to the invention, a conical irradiating surface can be approximated if the phase angle ~ is made to increase in linear manner with the distance between the transducer elements 92-98 and the centre of the group of transducers, in the case of the transmitter signals 101-104 in Fig. 9a for the time-shifted echo signals 202-208 (Fig. 16). Fig. 10 shows the linear increase in the phase angle ~. A linear increase in the phase angle of the reflected ultrasonic waves is also obtained in the Q direction by shaping the irradiating surface 37 as shown in cross~sec-tion in Fig. 9bo The chain line 107 in Fig. 9a shows the position of constant phase on the irradiating surface of the transducer system; for sirnplicity, the drawing shows a phase which varies continuously in the L d:irection, .
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1 instead of varylIlg siepwise as :in L:he preserlt e.~ample.
:[n the prese~llt e~ample the ]ocus of constant pl1ase i5 a set oE straight ].ines 107, instead of bei.ng a clrcle as in the case of a coniccll wave :ront.

A better approxi.mation of a conlcal wave :Eront can be obtained by the embodiment of the invention i]lustrated initially with respec-t to Figs. lla, llb and 12. In thi.s embodiment, the phase angle of the tra.nsmitter siynals or time-shif-tecl echo signals is a quadratic function of the position of the correspondiny el.ements i.n the centre of the group of transducers, allc1 is a linear Eunct:ion at the edge. ~ corrc-~spond:inc,J phase arlcJ:l.e di.stribut.:i.on :i.n the Q direct:i.on is obtai.ned by shap:i.ng the irrac'l:Lati.ng sllr:~clce 34 as shown i.n ~ig. l:lb w:ith respect to a cross-sect:Lon of the transducer system. Line 37 in Fi.g. llb is preferab:Ly a hyperbola. A curve of this kind is circular in the central region 127 and linear at the edge. The improvement obtained with this embodiment is sho~l by the fact that the locus of constant phase 106 shown in Fig. lla has rounded corners.

Note that the radiating groups of transducers in the embodiments in Figs. 9ar ].la have a greater area than in the embodiment in Fig. 4. This greater area results in a correspondinyly greater aperture, which is requ.ired for obtaininy better resolution.

Advantageously in the last-mentioned embodiments, as in the others, the inner .part of the radiating group of transducers transmits at a higher amplitude and the echo signals received there are multiplied by a hiyller weighting coefficient on recep-tion, thus improving the short-range field.

The dimensioniny of yroups 21 and elements 31-34 as in Fig. 4 for obtaining a weakly ,focussed ultrasonic beam 23 as in Fig. 2 will be explained initially with respect to Fl~s. l~ n~'J 1~ , e:rf:icicilc wea~ rc)~lp o tr~ .;d~ s i.~i c~ c,~ s~ :L3~ t ~ ]l ~ a~ld l~glh is 15 ~ 0 w~v~ 1 OJ~C~ L !ls, r;~]~ ~^ac'i~,ls Or Cl!I^`iJcl tU~-C R ~ Fig.
19) of the W-,i~O fron~, i.s m~,de api-ro~ e1y equ~]. to half the depth of t'i~e ex.ml]-!ed body, a~,d is prefe3clbly somewl1at smalle3. In the case of a group o.- trar,~iduce~s co,-;ip.~ ing four elements, tl1e wicl.l1 oE t}~e jndi~idual elen,erlts is made such that tl1e pl~ase diference bec~.7een the waves xadiated by neigl1bouring elenie,nts is not appreci.ah]y greater than 90. If these valuec; of the radi.us o curvature and the phase difEel-ence are e~ceeded, there is a corresponding impairment .in the shape oE the beam and con.c;equent].y .in thc-~ trarlsve1.sc resc)lut.io11. Ilowe~vcr, wea~ fc~cussi.rlg according to tl1e invelltion carl be ob~a~ ed, at least: .in princ:Lple, with a pllac:e diffefel1ce betwec,n 30 and 'L80.

The dimensions of the transducer elements will now he illustra-ted with a respect to a concrete example (Figs. 18 and l9). As shown in Fig. 18, the two inner elemen-ts in the group transmit at phase 0 and the two outer elements at phase 90. From Fig. 19 and by the chord theorem, we ohtain dl2 = 2R b (1) in which dl = the lateral shift leading to the desired phase shift of 90, R = radius of curvature of the wave front, and ~ = the distance corresponding to a phase shift of 90.
In the present case ~ = 4 (2) with ~ = wavelength.

If R is made equal to 80 mm (approximately half the depth of the examined body) and ~ = 0,75 mm (this wavelength corresponds to a frequency of 2 M~z), we obtain dl 5,48 mm.
If the element is at a distance d2 = 6 mm from the centre of the group of transducers~ This value of d2 is approximatel.y ~....
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-I e~ual. to th~ prev:lous:Ly~ca:Lcul.ated cii.stance clL~

YigO 13 is a b:Lock circlli-t dicl~ram o:E an ul-trasonic imaging unit accor-~l:ing to -the :i.n-ven~io]l which, as shown in Fig. lla, Usei'~ 7-element. c3roups of trani~ducers Eor transmission and recepti.oll, The b].oc~ circll1.t diag:ram in Fi.g. 13 shows a transducer arrangemerl-t 38 as in Fig, 3, a timing generator 131, a tirning si.gnal 132 delivered by gene]-ator :l31., a -transmi-tter~signa]. gerlerator 133, transmitter signals 134 supplied by generator 133 over lines 135 to elemen-t-selector drive switches 138, an element counte:r and deeoder 136 for eontrolling swi.tch ].38 and eonnected to timing generator 131, echo s:igna].s 1.42 de].i.vered by a group o:E transclucers, an echo signal reeeive:r 143, the combined eeho slgnal :l44 at the outpu', ~6 Of .reee,tver 143, a t:ime-serlsitive~ ampli:Eied :l.45, a cletector 146, a slcJnal proeessor 147, the ou~-;put s:i.gnal.:l.48 of proce~.ssor 147, an X-deflection generator ].51, a defleeti.orl si.gnal 154 given by generator 151, a Y-stage furletion generator 152, a stage funetion signal delivered by generator 152, and a reception oscillograph 156 having three inpu-ts X, Y
and Z.

The timing generator 131 generates periodic timing pulses 132 triggering the transmission of an ultrasonic siynal and the generation of the necessary sinc signals.
Four eleetrie transmitter pulses :l21 ~24 (see I;'ig. 14) are generated in the transmitter signal generator 133. Three of the signals 122, 123, 124 have a phase lead correspond-ing to a earrier-signal phase of +30, +100 and +180 eompared with a signal 121, whose phase is denoted by 0.
These transmitter signals are supplied on lines 134. In unit 138 (the element seleetor drive sw:iteh) the transmitter signals are supp].ied to 7 supply lines, on which the transmitter signals have the phases ~180, +100, +30, 3~ 0, +30, ~100, ~180. The element eounter and deeoder 136 switches the desired seven elements, either for : transmissi.on or for recep-tion, via switch 138. After ' - ~ , :, ..

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1 each p~lse, ~he con~iguration in Flg. :L]a ls shifted by one elernen-t in the L directic)rl. ~t t:he same time, the transmitter signa]s are cyclically i,nterchar~ ed witll the difEerent phases on the supply lines so that each elenlent obtains the corresponding transmitter signal havincJ -the correct pllase. I~he echo signals 142 travel from tlle se~en switched-on element:s to the echo-signal receiver lfi3, where the signals are variously delayed, multiplied by various weigh-tiny factors, and then added. The outpu-t signal 144 10 of receiver 143 travels through a time-sensitive amplifier 145, which compensates the attenuation of the body tissue.
The signal is then rectified in detector 146 and travels via processor 147 to the Z input of the reproduction oscillograph ]56. Processor 147 compresses the dynamic 15 range of the signal dellvered by cletector 146.

Thè X-deflect:ion generator 151 generates a voltage which is proportional to the time which has elapsed since the last pulse was transmitted. The Y-stage function generator 152 20 generated a voltage proportional to the position of the central axis of the switched-on group of transducers.

The construction and operation oE the transmitter-signal generator 133 will be described initially with respect to 25 Figs. 14 and 15. The timing pulse 132 triggers a pulsed high-frequency generator 161 whose output signal 162 (a pulsed carrier signal) is delayed in the tapped delay line 163 so as to produce four signals having the phases 0, 30, 100 and 180. In weighting unibs 164-167 these signals 30 are multiplied by the corresponding weighting factors.

Fig. 16 shows the echo-receiver 143 in detailO The echo signals 142 are multiplied by the corresponding weighting factors in weighting units 171-177. They are the delayed by 35 phase shifters 181-185 and added in an adder 186.

- The basic principle of a preferred embodimen-t of the .
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~, , : :, 1 elemen'c selector drive switch 138 in the unit in Fig r 13 will be initially eY~p]ained wlth respect to Fig. 17. ~or simplicity, the principle is explain~d in the case of a group of -transducers containing only Eour element~
although ~he uni~ in Fig. 13 uses seveII-elemen-t groups.
The switching diagram shown in Fig. 17 can be used for triggering and shifting a group of four transdl1cer element.
The two inner elements of each group (e.g. 32 and 33 in group I) are triggered with the transmitter'signal 41 as in Fig. 5 and the two outer elements (e.g. 31 and 34 in group I) are triggered with the transmitter signal 42 in Fig. 5. In Fig. 17, the transdwcer elemen-ts are represen ted by the corresponding electrode segments 31, 32, 33, ~'' etc. By means of a switch system 191, the transducer ele-ments are cyclica~ly connected to four supply lines 192~195.
These ~our lines are connected via a switch system 196 to two supply lines 197, 198, which are supplied with the transmltter signcl:Ls 41, 42 having the amplitudes and phases shown in Fig. 5. Fig. 17 shows switch positions for two successive groups of transducers I (continuous lines) and II (chain lines). The means of controlling the switch system 191 needs no explanation. In the switch system 196, in order to actuate a new group II, each switch (e.g. 213) is placed in the same position as the position previously occupied by the upper switch (e.g.
212) or actuating the preceding group I. The uppermost switch 211 takes the position previously occupied by the lowest switch 214. The same switches can be used for transmission and reception, if the electronic design of the switch system is suitable. If different electronic switches are required for transmitting and reception, the circuit in Fig. 17 can be duplicated, using separate supply lines for transmission and reception.

The advantages of the invention can he illustrated as follows:

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1 Tl-e method accord:ing to the inverlt;oll rnakes possible to attain hiqher transv-exs resolut~on, so as to obtain more distlnct ult,rasonic images.

In addit~on, the unit according -to the inveni:iorl is economic, slnce J tS expense is rela-tively :Low.

Owing to the weig-hting of the transmitter and echo signals according to the invention, there is an appreciclble reduction in the secondary lobes of the radia-tion charact-eristic of an ultrasonic beam generated by a group of transducers according to the invcntion.

In addition, the embodiments oE the invention described hereinbefore with respect to Figs. 9~-12 produce ultrasonic beams having an approximately conical wave Eront, 90 that the ultrasonic bèam is skrongly :Eocussecl over a gre,(lt clepth.

Other advantages and properties of the inven-tion are clear from the previous description of preferred embodiments.

The following description relates to variants of the invention for rotating the beam and thereby scanning in sectors.

In cardiology, for example, ultrasonic imaging in which the beam is rotated (Fig~ 20) appears to yield better results than when the beam is moved in linear manner (Fig. 21). The reason is the small acoustic window through which the image has to be obtained. It is limited by the sternum and lunys and measures approximately 2 x 7 cm~ In addition, the ribs make it difficult to obtain'an lmage of the heart. A sector scanner requires an aperture of only a few square cm and is therefore the most suitable, whereas a linear scanner is usually over l0 cm in length and is inefficiently used.

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_ L9 1 Knowr sc-~etor scanner.s operate e:i.-the3~ on the "phased array" prillciple (;rr Ki..sslo, OTo v. Ramm, F. l, Thurstone;
"A phase clrray ultrac;ollrid system for card.iac lmagil-c3", Procee~d.Lnc3s of -t:he '~econd European Congress on Ult.rasonjcs in Medic.ine, MUI1:iC11, 12^-16 May 1975, pp. 67--7fi, edi.tecl by E. Kazner, M. de Vl:ieger, Ho R. Miille:r, V. R. McCready, ~xeerpta Medi.ca Amsterdam - Oxford 1975), or a.re mechanical eontact scanners (cf A. Shaw, J. S. Paton, N. L. Gregory, D.
J. Wheatley, "A real -time 2-dimensional u:L.t.rason:ic scanner Eor elinical use", Ultrasonies,Januar :L976, pp. 35-40).
The Eollowing descr.ipti.on relats to an arc scanner which operates on the same principle as a linear scanner and has the seanning range of a seetor seanner.

16 The ma:in eomponellt of the are seanne.r is a linc-!ar "a:rray", the segmen-ts o:E whieh are disposed not on a strclight line but on an are. The seannable region :is shown in Fig. 22. As in Figs. 20 and 21, the transdueer shoul.d be assumed to be above in the drawing. If the top 2~ halE of the range is used as anti.cipatory path reyion and only the bottom halE for imag.iny, a system with beam rotation is obtained as in Fig. 20. The eomplete sound head of an eleetronie are seanner is sho~l in Fig. 23. An a.reuate piezoeeramie transdueer 302 is disposed in the upper part o housing 301 and individual eleetrodes 303 are disposed at i.ts top. I~pwardly reEleeted ultrasound is destroyed in absorhe.r 304. The lower part of the housing is line~d with sound-absorbing materi.al 305 and :Eilled with an ultra-sound~transmitting medium 306. At -the bottom, the sound head is elosed by a diaphragm 307. The diaphra~m is at the eentre of the are formed by the transdueer, i.e. at the narrowest plaee i.n the seannable region (see Fig. 22).
In order to eliminate interfering multiple refleetions between the di.aphragm and transdueer from the image, the transit time between the transdueer and diaphragm should be exaetly the same as between the diaphragm and the mos-t remote objeet whieh has to be imaged. If water :is used , .

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1 for the arlticipatory patll, -tnis mc~arls ~ha-l Ihe racl:Lus o:r t}le trallsC!l.lCer aLC mus t be e~;ac-tly equ.~L to tlle maxirnllm deE)t:h of pene-trat:k~ll, s:Lnce -the h~ JIi body ancl water havc~
app:coximately the sallle speed o; sound (approx~ 1 5no m~sec ).
. 1 The shape oE the beam can be op-t:imisecl in a manne~-very similar to linear scann:ing, as c1escl-ibed herejnbeEore.
If a:Ll segmen-ts oE a group of transducers are opel-ated and simultaneously switched on at the same phase, the sound beam is focused at the centre of the arc, i.e. at the cliaphragm. When the dcp-th of penetration increases the beam becomes pxogressive]y widerr so that the latera:L
resolution of the system becomes procJressively worse.
A considerable improvement can be ob-ta:ined i:E the ocus is not at the centre of the arc but at a po:int :located at about approx. 2/3 oE the max:ilmlm i.mag:in~J deptl-l measur(!d from membrane 307. Th:is :is achieved by sui~able phac;e-slli[t:in(3 of the individual transducer elements during transmission and rec~ption. Tlle phase shif-ting here has the opposite SiCJn (phase lag) as in -the linear scanner described previously.

The reason for this is explained in Figs. 24, ~5 in the case of the transmi~ter. In the linear scanner (Fig. 24) an originally flat wave front (continuous line) is converted into a cylindrical front (chain line). At increasing distances from the beam axis, the!signal needs a correspondingly large phase lead. In the case of the arc scanner (Fig. 25), on the other hand, a stron~ly curved wave front (continuous line) :Ls converted into a slightly curved front (chain line). Thus, at increasing distances from the axis, the signal re~uires a progressively greater lag. Similar considerations apply to reception. Depending on the special dimensions of the group o-f transducers, the shape of the beam may be further improved by apodisation, e.g. by attenuating the amplitudes of the outer elements during transmission and reception. More particularly, the number of different phases used for focussing is critical.

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l Pîev:ioucly, only -t:.he~ shapin~ ol the beam :i.n -the scalirli.nc3 direc-t:iorl has beell diseu-;sed l.~owevel- , WC`(-$}`
focussil1cJ in the direct:iol1 at rl~h-t anyles thel-~to may also be advanta~eous. ~c1--7~lnt~1geous.Ly, the foeal po:i.nt i.s at the same pl.ace as :in the :Eirst d:i.reeti.orl, i.e. at ~/3 of the max:i.mur[l imaging depth. l;ocussin~ :Ls ob-ta:il1ed either by means of a suitable c-u:rved tra~lsd~icer or an acol~stic lens disposed in fxont of the -transdueex. O:E course, foeussing ean be electronica].ly p.rodueed in -this di.rection also, as in the previously-deseribed linear scanner, if the greater complexity o:E the sy.sten1 is allowed fol^.
Numerieal ealeulations indieate that additiollal apodi.sation does not provi.de any further improvement of the shape of the beam. ~podization is, howeve:r, advclntacJeolls if there is no foeuss:ing i.n -the second di.reeti.on, whclt may be de~i.:rahle for s:implifyinq l:he eon~t..ruct:i.ol1. ~pod:i.sati.on can be obt:a:i.necl e.~J. by rneclns oi seglllerlts whieh beeome prog1:esc;:ive.l.y nar:cower outwarcls (see l~':ig. 28).

Eleetronieally, the are seanner has all the advanta~es of the linear seanner. Its disadvantage is that it requires a water antieipatory path, with.the result that the sound head is heavy ancL awkward to handle and the maximum image frequeney i.s only hal.f tha-t of a scanner without the anticipatory path. The ant-ieipat:ory path, and there-fore the sound head, ean be reduced if water is replaeed by a substanee in whi.eh the speed of sound is lower than :in wat.er.
In many organic li.quids, and also in many silicone rubbers, the speed of sound is about lO00 m/see. This means that the anticipatory path ean be redueed by l/3 and -the volume of the sound head can be reduced by at least half/ but the reflection is amplified and the sound beam is refracted at the interface between the anticipatory path re~ion and the body tissue.
A further eonsiderable reduetion in the sound head ean be obtained if the are seanner is used no-t as a sound head ,, ,. ~ , : ;

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1 but as a signaL processc)r for a "phac;ed ar:Lay".. ~h:i.. s posslbilit.y ls showll in Fi.y. 26. ~ gxoup of tranc;~ ce:Ls 401 comprisi.lly a nllmber of sec~mellts of arl a:,..cucate transducer ~02 -transmits an ultrclsollic bearn 403 whi.cll, at the centre of the arc, s-trikes a "phased crray" 40~ whose segmen-ts are disposed parall.el. ~o the segJnents of the arcuate transducer ~0~. By meclns of the phased array, the sound field is detected ln segments i.n a phase-sensitive manner and transmitted to a second ''phc~sed arxay"
405~ which forms the actual sound head, reconstruc-ts the sound field at the site of the first "phased ar~ay" and radiates a corresponding ultrasound beam 406~ Of course, the device can al.so be operated in the reverse direction, and i.s therefore suitable for transmisC;i.on anc1 reception.
16 ~dvanta.geous:L~, a transmi.tt:Lnq arld a rcce:iv:ing isrtermediclte arnpli~:Ler is d:isposed between the two "pllased a:rxays" i.rl each segmerlt. For si.mpl.icity, these ampliElers were om:i-tted in Fig. 26.

At this point it should be noted that the sound fi.eld radiated by the second "phased array" 405 need not be identical with the field detected by the firs-t "phased array" 404 The phase and amplitude of the signals from each segment can be varied by the aforementioned inter-mediate ampliEier. In addition, the second "phase array"
405 can be given a shape d:ifferent frorrl the first, thus altering the sound field. Thi.s provides an additional means of improving the Eocussing of the sound beam and thus improving the la-teral resolution of the system.
The advantage of this device~ compared with a tradit-ional "phased array" system, is that the sound beam is angularly deflected by using simple means. Strictly speaking, this applied mainly to operation as a receiver.
During transmission, angular deflecti.on can be obtained - relatively easi.ly by digital means, but cornplicated delay lines and switches have hitherto been required for . . .

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-I receptlon~ It is -t~)eref-'ore b~-tter to use ~ hybLid solut:ion, :i~l wh;ch t]le "phased arrcLy" is dlrectly operatec~
during transmisslon and -the axc scanll-er is used on:ly as a recelved-signal, processor.

Finally, we shall desc:~ ,ed a .simple exan-lple of an arc scanner for cardiologi,cal applications (Figs~ 27 and 2S).
The data f'or this signal are as follows:

'i Frequency 2 MEIz max. depth of penetration 15 cm angle to he scanned 50-60 numbe~r of segments 64 phases to be used 0, 90 an-ticipatory path med:ium water focuss:ing in one dLrection only.

Under these boundary conditions, an optimisation process was carried ou-t, with reference to sound fields calculated by computer, and yielded the following dimensions.

As shown in Fig. 27, the transducer system 302 forms a portion of a cylinder. It has a radius R of 15 cm, a width B of 2 crn and an arc lengtll 17,6 cm, corresponding to an angle O = 67,2. The transducer is divided into 64 segmellts having a width S = 2,75 mm. 12 elements are used simul-taneously for transmission and reception. One such group is shown in Fig. 28. The edcles of the individ-ual elements 411 are formed by arcs of a circle. This shape results in the desired apodisation and improvement of the bearn shape. During transmission and recep-tion, the signals from the outer 6 elements are made to lag 90 behind the signals for the inner six elements. This corresponds to focussing at a point about 25 cm from the ; 3~ transducer. At the same time, the signal ampli-tudes of the outer six elements during transmission and reception are multiplied by a factor of 0,5 and the signal amp~Litudes of : ., ;
: ;, : ,. . . .

e illner ':,i.X el.el[le`ll``C'; cl:LC' mu'.tipL:ieci by a factc)~ oL llil.it~,7.

By me~ s of thi.s trallscluceI, a reso:LutJ on o:f ai- :Least ~I mm i.s obtai.ned :i n the scclnn:i.n~ pl.ane in the ent:i re useful rey:i.on. Owi.rlcj -to the a.bsence o:E :focus.si.ng, the resol.u-t:Lon :i.n the clirection perpellc~i:i.c,lllc~r thereto i s lo-~ieî
hy a factor of l,5. As already ment:ioneci, i.mproved resolution in lhis ciirection c^tLSo can be obtalnecl hy acldLt-ional focussing.

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Claims

What we claim is :

1. A method of producing cross-sectional images using an ultrasonic imaging unit operating on the pulse-echo princi-ple and comprising a transducer system having a stationary array of adjacent transducer elements, in which method successively and cyclically selected groups of adjacent trans-ducer elements of the transducer system are used to produce an ultrasonic beam in response to pulsed electric trans-mitter signals applied to the transducer elements, to trans-mit the ultrasonic beam, substantially in a scan plane, into a heterogeneous body, to receive echoes reflected from a discontinuity in the body, and to generate an electric echo signals in response to the received echoes, the transmitter signals applied to the transducer elements and/or the echo signals given by the transducer elements being time-shifted with respect to one another, each transmitter or echo signal being associated with a time shift which is a function of the distance between the corresponding transducer element and the centre of the group of transducer elements such that, in the case of adjacent transducer elements, the transmitter signal and/or the time-shifted echo signal of the transducer element at the greater distance from the middle of the group of transducer elements has a phase lead with respect to the corresponding signal of the other element, which method comprises weakly focussing the ultrasonic beam and/or the corresponding reception characteristic in the scan plane and over the examined depth within the body by means of said time-shifting of the transmitter and/or echo signals, and weighting the amplitude of the transmitter and/or echo signals, each transmitter or echo signal of an transducer ele-ment being assigned a weighting factor determined by a function of the distance between the transducer element and the centre of the group of transducers.

2. A method of producing cross-sectional images using an ultrasonic imaging unit operating on the pulse-echo principle and comprising a transducer system having a stationary array of adjacent transducer elements, in which method successively and cyclically selected groups of adjacent transducer elem-ents of the transducer system are used to produce an ultra-sonic beam in response to pulsed electric transmitter signals applied to the transducer elements, to transmit the ultrasonic beam, substantially in a scan plane, into a heterogeneous body, to receive echoes reflected from a discontinuity in the body, and to generate an electric echo signal in response to the received echoes, the trans-mitter signals applied to the transducer elements and/or the echo signals given by the transducer elements being time-shifted with respect to one another, each transmitter or echo signal being associated with a time shift which is a function of the distance between the corresponding trans-ducer element and the centre of the group of transducer elements such that in the case of adjacent transducer ele-ments, the transmitter signal and/or the time shifted echo signal of the transducer element at the greater distance from the middle of the group of transducer elements has a phase lead with respect to the corresponding signal of the other element, which method comprises aspherically focussing the ultrasonic beam and/or the corresponding reception charac-teristic in the scan plane by means of said time shifting of the transmitter and/or echo signals, aspherically focussing the ultrasonic beam and/or the corresponding reception characteristic also in planes perpendicular to the scan plane, and weighting the amplitude of the transmitter and/or echo signals, each transmitter or echo signal of a trans-ducer element being assigned a weighting factor determined by a function of the distance between the transducer element and the centre of the group of transducers.

3. An ultrasonic imaging unit for producing cross-sect-ional images, the unit comprises: a timing generator for producing a pulsed electric timing signal; a transducer sys-tem comprising a stationary array of adjacent transducer elements, the transducer system being used to produce an ultrasonic beam in response to pulsed transmitter signals derived from the electric timing signal, to transmit the ultrasonic beam, substantially in a scan plane, into a heter-ogeneous body, to receive echoes reflected from discontinuity in the body, and produce an electric echo signal in response to the received echoes; an element-counter selector device connected to the timing generator, the transducer system and an indicator device and used for successively and cyclically selecting groups of adjacent elements of the transducer sys-tem, applying the transmitter signals to the transducer ele-ments of the selected group for generating the ultrasonic beam, and transmitting the echo signal produced by the group to the indicator device, which is used to convert the echo signals into a visible image reproducing the cross- sectional structure of the heterogeneous body; transmitter-signal gen-erator means inserted between the timing generator and the element-counter selector device for deriving transmitter sig-nals for the transducer elements or elements sub-assemblies of the selected group of transducer elements, the transmit-ter signals obtained from the timing signal given by the timing generator being time-shifted with respect to one another; and, echo-signal receiver means inserted between the element-counter selector device and the indicator dev-ice for producing a relative time shift between the echo signal delivered by the transducer elements or elements sub-assemblies of the group of transducers; the phase angle (.PHI.) of the transmitter signals or the time-shifted echo signals being so determined by a function of the distance between the corresponding transducer element and the centre of the group of transducer elements that, in the case of adjacent transducer elements, the transmitter signal and/or the time-shifted echo signal of the transducer element at the greater distance from the middle of the group of trans-ducer elements has a phase lead with respect to the corr-esponding signal of the other element; in which unit the time-shifted signals serve for generating an ultrasonic beam and/or a corresponding reception characteristic which is weakly focussed over the examined depth within the body, the transmitter and/or the receiver means including means for weighting the transmitter and/or echo signals to optimize the shape of the ultrasonic beam and/or the reception charac-teristic, each transmitter or echo signal from a transducer element being assigned a weighting factor determined by a function of the distance between the transducer element and the centre of the group of transducer elements.

4. An ultrasonic imaging unit for producing cross-section-al images, the unit comprising: a timing generator for prod-ucing a pulsed electric timing signal; a transducer system comprising a stationary array of adjacent transducer ele-ments, the transducer system being used to produce an ultrasonic beam in response to pulsed transmitter signals derived from the electric timing signal, to transmit the ultrasonic beam, substantially in a scan plane, into a heterogeneous body, to receive echoes reflected from a dis-continuity in the body, and produce an electric echo signal in response to the received echoes; an element-counter sel-ector device connected to the timing generator, the trans-ducer system and an indicator device and used for success-ively and cyclically selecting groups of adjacent elements of the transducer system, applying the transmitter signals to the transducer elements of the selected group for gen-erating the ultrasonic beam, and transmitting the echo signals produced by the group to the indicator device, which is used to convert the echo signals into a visible image reproducing the cross-sectional structure of the heterog-eneous body; transmitter-signal generator means inserted between the timing generator and the element-counter sel-ector device for deriving transmitter signals for the trans-ducer elements of the selected group of transducer elements, the transmitter signals obtained from the timing signal given by the timing generator being time-shifted with respect to one another; and echo-signal receiver means inserted between the element-counter selector device and the indicat-or device for producing a relative time shift between the echo signal delivered by the transducer ementnts of the group of transducer elements; the phase angle (.PHI.) of the transmit-tor signals of the time-shifted echo signals being so deter-mined by a function of the distance between the corresponding transducer element and/or the centre of the group of trans-ducer elements that, in the case of adjacent transducer elements, the transmitter signal and/or the time-shifted echo signals of the transducer element at the greeter distance from the middle of the group of transducer elements has a phase lead with respect to the corresponding signal of the other element; which unit is characterized in that the time-shifted signals serve for generating an ultrasonic beam and/or a reception characeristic which is aspherically focussed in the scan plane, and the intersection of the radiating surface of the transducer system with any plane perpendicular to the scan plane and parallel to the transmitted ultrasonic beam has such a curvature that the radiating surface aspherically fucusses the ultrasonic beam and the corresponding reception charac-teristic also in said plane perpendicular to the scan plane, the transmitter and/or the receiver means including means for weighting the transmitter and/or echo signals to optimize the shape of the ultrasonic beam and/or the reception characteri-stic, each transmitter or echo signal from a transducer ele-ment being assigned a weighting factor determined by a function of the distance between the transducer element and the centre of the group of transducer elements.

5. A unit according to Claim 4, wherein the phase angle time-shifted transmitter signals and/or of the time-shifted echo signals increases stepwise in linear manner with the distance of the corresponding transducer element from the centre of the group of transducer elements.

6. A unit according to Claim 4, wherein the phase angle of the time-shifted transmitter signals or of the time-shifted echo signals increases stepwise and approximately in accordance with a hyperbolic function with the distance between the corresponding transducer element and the centre of the group of transducer elements.

7. A unit according to Claim 4, wherein the phase angle of the time-shifted transmitter signals or of the time-shifted echo signals increases stepwise with the distance between the corresponding transducer element and the centre of the group of transducer elements, the increase being quadratic towards the centre of the group transducer elements and lin-ear at the edge regions of the group.

8. A unit according to Claim 3 or 4, wherein the transmit-ter signal generator delivers transmitter signals of diff-erent amplitudes, and the transmitter signals having the higher amplitudes are applied to the inner transducer ele-ments of each selected group.

9. A unit according to Claim 3 or 4, wherein the echo-signal receiver has a weighting circuit which is used to associate various weighting factors with the amplitude of the echo signals delivered by-the transducer elements, the echo signals from the inner transducer elements being given higher weighting factors.

10. A unit according to Claim 3 or 4, wherein the groups of transducer elements successively connected by the element-counter selector device alternately contain an even and an odd number of transducer elements, successive groups being formed alternately by reducing the number of elements in one direction and increasing the number of elements in the opp-osite direction.

11. A unit according to Claim 3, wherein the intersection of the radiating surface of the transducer system with any plane perpendicular to the scan plane and parallel to the transmitted ultrasonic beam has such a curvature that the radiating surface weakly focusses the ultrasonic beam and the corresponding reception characteristic also in said plane perendicular to the scan plane.

12. A unit according to Claim 4, wherein the radiating sur-face of the transducer system appears in transverse section as an approximately V-shaped line.

13. A unit according to Claim 12, wherein the V-shaped line is made up of two straight segments.

14. A unit according to Claim 13, wherein the V-shaped line is approximately hyperbolical.

15. A unit according to Claim 4 or 11, wherein the trans-ducer elements are segmented along their longitudinal axis into a top, a middle and a bottom part, the top and bottom parts of the outer elements of the radiating group of trans-ducer elements is not used either for transmission or recept-ion, and the transmitter signals for the top and bottom parts of the inner transducer elements are phase-shifted compared with the transmitter signals for the middle parts of the same electrode segments and/or have a lower amplitude.

16. A unit according to Claim 3 wherein the time shaft between transmitter signals for adjacent transducer elements and/or the time shift between echo signals, time-shifted with respect to one another, from adjacent transducer elements at different distances from the centre of the group of transducer elements corresponds to a phase shift of a high-frequency carr-ier wave contained in each transmitter or echo signal, the absolute value of the phase shift lying in the region between 30° and 180°.

17. A unit according to Claim 16, wherein the phase shift has an absolute value of 90°.

18. A unit according to Claim 16, wherein the time shift be-tween the transmitter signals and the time shift between the the echo signals correspond to different phase shirts.

19. A unit according to Claim 18, having the following comb-ination of the absolute values of the phase shift between the transmitter signals and between the echo signals, time-shifted with respect to one another, from adjacent transducer elements: