CA1111543A - Ultrasonic imaging unit - Google Patents
Ultrasonic imaging unitInfo
- Publication number
- CA1111543A CA1111543A CA338,804A CA338804A CA1111543A CA 1111543 A CA1111543 A CA 1111543A CA 338804 A CA338804 A CA 338804A CA 1111543 A CA1111543 A CA 1111543A
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- Canada
- Prior art keywords
- transducer
- elements
- array
- group
- ultrasonic
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Abstract
Abstract of the Disclosure An ultrasonic imaging unit operates on the pulse-echo principle and comprises a transducer system having a stationary array of adjacent transducer elements, in the unit 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 and/or receive echoes reflected from a discontinuity in the body, and generate an electric echo signal in response to the received echoes, there being provided an anticipatory path within a closed envelope between the transducer system and a transmission region within a body under examination.
Description
l~llS~3
- 2 -RAN 470l/96 The invention relates to a method of producing cross-sectional images using an ultrasonic imaging ~nit operat-ing on the pulse-echo principle and comprising a trans-ducer system comprising a stationary elongated array of adjacent transducer elements and having transverse 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 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 elec~ric 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 motor, a relatively heavy water bath is usually requi~ed. In addition, the extra distance travel~ed 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 shi~ted in time.
Ve/8.lO.1976 .
111~543 3 1 In a ~nown ultrasonic imaging unit of the aforementioned kind (American patent specification 3 881 466), the transducer system produces an un~ocussed ultrasonic beam and the transverse resolution is determined by the width of the transducer elements. In the known device, there is a limit to which the transverse resolution can be improved by reducing the width of the transducer elements, the limit being set by the minimum width of the ultrasonic beam.
Although the cross-sectional images produced by the known device are relatively distinct, it has been found in practice that still higher transverse resolution is desir-able for many applications.
An object of the invention, therefore, is to provide a method and an ultrasonic imaging unit which can give higher transverse resolution.
The method according to the invention is characterised in that, in order to focus the ultrasonic ~eam produced by each group of transducers, the transmitter signals applied to the electrode segments or subgroups thereof and/or the echo signals given by the electrode seqments or subgroups thereof are t~me-shifted with respect to one another, each transmltter or echo signal ~eing associated with a time shift which is a function of the distance between the corresponding electrode segment 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 timinq generator for producing a pulsed electric timing signal; a transducer system comprising a stationary elongated array of adjacent transducer e7ements and comprising transverse electrode segments adjacent one another on at least one s~e, the transducer system being used to produce an ultrasonic beam ~n response to pulsed llilS~3 1 transmitter siqnals derived from the electric timing sign21 to transmit the ultrasonic beam into a heterogeneous body, to receive echoes reflected from a discontinuity in the body, and produce an electric echo signal in response to the 6 received echo; and an element-counter selec~or device connected to the timing generator, the transducer system and an $ndicator device and used for successively and cyclically selecting groups of adjacent elements of the transducer system, generating the ultrasonic beam, applying the transmitter signals to the electrode segments of the selected group, 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-~ectional structure of the heterogeneous body.
The ultrasonic imaging unit according to the invention is characterised by a transmitter-signal generator inserted between the 2Q timing generator and the element-counter selector deYice 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 genérator, and/or an echo-signal receiver inserted between the element-counter selector de~ice 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.
~, llflS43 1 In one asp ct of the invention, there is provided:
An ultrasonic i~aging unit for producing cross-sectional images, which unit operates on the pulse-echo principle and comprises a transducer system having a stationary array of adjacent transducer elements, in which unit S successively an~ cyclically selected groups of adjacent transducer elements of the transducer system are used to produce an ultrasonic 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 heterogene~us bDdy, an~Jor to receive echo_s reflected from a discontinuity in the body, and to generate an electric echD signal in response to the received echDes in w~ich unit an anticipatory path is prDvided between th~ transducer system an~
a transmission region within a body under examination, the anticipatory path being comprised within a closed envelope, and the transducer system is arcuate in the scan plane and serves for generating a scanning beam ky n~ans of the cyclical selection of groups of adjacent transducer elements to pPrform a sector scanning in the transmission region and/or for receiving ech~es derived ~y performing a sector scanning in that region.
Some embodiments of the invention will now be described w.th reference to the acoompanying drawings, in which:
., -4a-111~5~3 1 Fig. 1 is a perspective view of the transducer system in the previously-mentioned prior-art ultrasonic imaging unit, Fig. 2 is a diagrammatic cross-section of the radiation characteristic 23 of a group of transducers according to the invention, compared with the radiation characteristic 22 of a group of transducers in the system according to Fig. 1.
Fig. 3 is a diagrammatic cross-section 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 diagrams of transmitter signals 41, 42 which, according to the invention, are applied to the electrode segments 31-34 of the group of transducers 21 in ~ Fig. 3.
; 20 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 Fig. 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-ment 38 in Fig. ~, 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, Bc 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 7 electrode segments , -, , ~ .
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 motor, a relatively heavy water bath is usually requi~ed. In addition, the extra distance travel~ed 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 shi~ted in time.
Ve/8.lO.1976 .
111~543 3 1 In a ~nown ultrasonic imaging unit of the aforementioned kind (American patent specification 3 881 466), the transducer system produces an un~ocussed ultrasonic beam and the transverse resolution is determined by the width of the transducer elements. In the known device, there is a limit to which the transverse resolution can be improved by reducing the width of the transducer elements, the limit being set by the minimum width of the ultrasonic beam.
Although the cross-sectional images produced by the known device are relatively distinct, it has been found in practice that still higher transverse resolution is desir-able for many applications.
An object of the invention, therefore, is to provide a method and an ultrasonic imaging unit which can give higher transverse resolution.
The method according to the invention is characterised in that, in order to focus the ultrasonic ~eam produced by each group of transducers, the transmitter signals applied to the electrode segments or subgroups thereof and/or the echo signals given by the electrode seqments or subgroups thereof are t~me-shifted with respect to one another, each transmltter or echo signal ~eing associated with a time shift which is a function of the distance between the corresponding electrode segment 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 timinq generator for producing a pulsed electric timing signal; a transducer system comprising a stationary elongated array of adjacent transducer e7ements and comprising transverse electrode segments adjacent one another on at least one s~e, the transducer system being used to produce an ultrasonic beam ~n response to pulsed llilS~3 1 transmitter siqnals derived from the electric timing sign21 to transmit the ultrasonic beam into a heterogeneous body, to receive echoes reflected from a discontinuity in the body, and produce an electric echo signal in response to the 6 received echo; and an element-counter selec~or device connected to the timing generator, the transducer system and an $ndicator device and used for successively and cyclically selecting groups of adjacent elements of the transducer system, generating the ultrasonic beam, applying the transmitter signals to the electrode segments of the selected group, 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-~ectional structure of the heterogeneous body.
The ultrasonic imaging unit according to the invention is characterised by a transmitter-signal generator inserted between the 2Q timing generator and the element-counter selector deYice 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 genérator, and/or an echo-signal receiver inserted between the element-counter selector de~ice 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.
~, llflS43 1 In one asp ct of the invention, there is provided:
An ultrasonic i~aging unit for producing cross-sectional images, which unit operates on the pulse-echo principle and comprises a transducer system having a stationary array of adjacent transducer elements, in which unit S successively an~ cyclically selected groups of adjacent transducer elements of the transducer system are used to produce an ultrasonic 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 heterogene~us bDdy, an~Jor to receive echo_s reflected from a discontinuity in the body, and to generate an electric echD signal in response to the received echDes in w~ich unit an anticipatory path is prDvided between th~ transducer system an~
a transmission region within a body under examination, the anticipatory path being comprised within a closed envelope, and the transducer system is arcuate in the scan plane and serves for generating a scanning beam ky n~ans of the cyclical selection of groups of adjacent transducer elements to pPrform a sector scanning in the transmission region and/or for receiving ech~es derived ~y performing a sector scanning in that region.
Some embodiments of the invention will now be described w.th reference to the acoompanying drawings, in which:
., -4a-111~5~3 1 Fig. 1 is a perspective view of the transducer system in the previously-mentioned prior-art ultrasonic imaging unit, Fig. 2 is a diagrammatic cross-section of the radiation characteristic 23 of a group of transducers according to the invention, compared with the radiation characteristic 22 of a group of transducers in the system according to Fig. 1.
Fig. 3 is a diagrammatic cross-section 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 diagrams of transmitter signals 41, 42 which, according to the invention, are applied to the electrode segments 31-34 of the group of transducers 21 in ~ Fig. 3.
; 20 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 Fig. 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-ment 38 in Fig. ~, 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, Bc 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 7 electrode segments , -, , ~ .
3 - 6 -1 and used in a second embodiment of the ultrasonic imaging unit according to the invention, Fig. 9b is a cross-section showing the shape of the irradiating surface of the group of transducers 91 in Fig. 9a, Fig. 10 shows diagrams of the transmitter signals which according 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 of transducers having seven electrode segments used in a preferred embod-iment of the ultrasonic imaging unit according to the invention, ' .
Fig. llb is a cross-section through a preferred shape of the irradiating surface of the group of 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 diagram 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. lS shows diagrams of the ~iming pulse 132 gener-ated by the timing generator 131 (Fig. 13) and of the pulsed sine wave 162 derived from the timing pulse, 5~3 7 _ 1Fig. 16 is a block diagram illustrating the echo-signal receiver 143 in the device in Fig. 13, Fig. 17 illustrated the principle of a preferred embcd-iment of the element selector drive 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 sroup of transducers according to the invention and the elements thereof.
Fig. 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 of 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.
Fig. 23 is a diagram of a sound head with an arcuate transducer system, Fig. 24, 25 illustrate the production of a cylindrical wave front in two variants of the invention, 30Fig. 26 shows the use according to the invention of an arcuate transducer system for producing a "phased array", and Figs. 27, 28 illustrate the dimensioning of an arcuate transducer system according to the invention.
As Fig. 1 shows, the transducer system 11 of the known ultrasonic imaging units (U.S. Patent Specification 3 881 466) .
ll~lS43 - 8 -1 comprises a stationary elongated array of adjacend transducer elements 12. Groups of A ad~acent elements 12 are successively stimulated to produce pulses. The location of each successive group of A elements is shifted of a longitudinal distance of B
elements from the position of the immediately preceding group.
The ultrasonic beam 13 is thereby moved in the direction of arrow L, as shown by the series of chain-dotted rectangles 14 showing the instantaneous position of beam 13 after equal intervals of time. Note that each group of transducers in the known transducer system 11 generates an unfocussed ultrasonic beam 13, since all the A elements in the group of transducers are simultaneously energised so as to yield pulses. The unfocussed radiation characteristic 22 of the ultrasonic beam 13 in Fig. 1 is shown in Fig. 2.
In Fig. 1, an orthogonal system of coordinates is defined by three arrows, Q, L and S. Arrow L is along the longitudinal axis of the irradiating surface 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 cross-sections and elevations shown in the accomp-anying drawings are defined with respect to this coordinate system.
Fig. 3 is a partial cross-section showing the structure of a preferred arrangement of transducers 38 for performlng the method according to the invention.
Arrangement 38 comprises a complete electrode 36 which is earthed and one surface 37 of which is used as an irradiating surface; arrangement 38 also comprises a piezoelectric layer 35 and electrode segments 31-34, shown in rear view in Fig. 4.
It is clear from the preceding description of arranse-ment 38 that the transducer elements according to the invention can have common parts such as the piezoelectric 1 layer 35 or the complete electrode 36. The arrange~ent 38 acco-;ding to the invention càn be operated simply by providlng it wlth electrode segments on one side, which are supplied with the time-shifted transmitter signals and from which echo signals can be obtained. Thus, each electrode segment deiines a transducer element according to the invention.
The effect obtained by the invention, i.e. higher transverse resolution, is mainly due to a novel manner of operation of the transducer system. This will be ex21ained in dctail, firstly with reference to Figs. 2, 4 and 5.
Fig. 4 shows electrode segments 31-34 of a group of transducers 21 according to the invention. In order to produce an ultrasonic beam 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 focussed 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 lea~ 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 KHz and at a suitable phase angle.
~115~
1 The effect of operating group 21 according to the invention can be improved by the followins additional measures:
1) It has been found advantageous to select the fo71Owins combinations of phase lead for the outer elements of the group:
"
Transmitter signals Echo si~nals either approx. 90 approx. 45 or approx. 45 approx. 90 :
As a result of these different values of the phase lead for the transmitter and echo signals, the radiation characteristic 23 according to the invention (Fig. 2) is 1~ additionally narrowed over a certain depth.
.~ .
2) Advantageously, the transmitter and echo signals 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 welghting factor than the echo signals from the outer elements.
Advantageously, the weighting ratio is 2:1 for both the trans~itter 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 focussing in the Q direction can also be electronically produced, using a transducer arrangement as in Fig. 7, in which each of the electrode segments is divided into three parts a, b, c in the Q direction. As shown in Fig. 7, only the shaded parts of the electrode '' ~ .... ..
' ' - . ...
- llllS~3 11 -1 segments are used for transmitting or rec~iving. The inner parts 32b, 33b are energised with the transmitter signal-41 and the remaining active parts are energised with the transmitter signal 42. The resulting system is electronicaliy more complicated than the transducer arrangement comprising a curved irradiating surface, but it only requires a transducer arrangement having a flat irradiating surfa~e, which is cheaper.
.
In the known transducer system 11 in Fig. 1, the ultrasonic beam 13 can be displaced by the width of a transducer element 12 after each transmitting and recept-ion 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 und 8c) the ultrasonic beam is displaced by half the width of an element in that successively selected groups ~f 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 alt rnately:
.
.
1111~43 - 12 -1 With 4 elements:
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 45 0 0 45 With 3 elements:
Element 32 33 34 (Amplitude Transmission (Phase 45 0 45 (Amplitude Reception (Phase 22,5 0 22,5 A second embodiment of 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. According 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 1~1-104 in Fig. 9a for the time-shifted echo signals 2Q2-208 (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-section in Fig. 9b. The chain line 107 in Fig. 9a shows the position of constant phase on the irradiating surface of the transducer system; for simplicity, the drawing shows a phase which varies continuously in the L direction, , . . f f `` lillS43 ~ 13 -1 instead of varying stepwise as in the present example.
In the present example the locus Oc constant phase is a set of straight lines 107, instead of being a circle as in the case of a conical wave front.
A better approximation of a conical wave front can be obtained by the embodiment of the invention illustrated initially with respect to Figs. lla, llb and 12. In this embodiment, the phase angle of the transmitter signals or time-shifted echo signals ~s a quadratic function of the position of the corresponding elements in the centre of the group of transducers, and is a linear function at the edge. A corresponding phase angle distribution in the Q direction is obtained by shaping the irradiating surface 37 as shown in Fig. llb with respect to a cross-section - of the transducer system. Surface 37 in Fig. llb is preferably 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 shown b~,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. 9a, lla have a greater area than in the embodiment in Fig. 4. This greater area results in a correspondingly greater aperture, which is required for obtaining 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 higher weighting coefficient on reception, thus improving the short-range field.
The dimensioning of groups 21 and elemen.s 31-34 as in Fig. 4 for obtaining a weakly ~ocussed ultrasonic beam 23 as in Fig. 2 will be explained initially with respect to ` ' llliS43 - 14 -1 Figs. 18 and 19. An efficient weakly-focussing group of transducers is characterised in that its width w and length 1 is 15-30 wavelengths. The radius of curvature R (Fig.
19) of the wave front is made approximately equal to half the depth of the examined body, and is preferably somewhat smaller. In the case of a group of transducers comprising four elements, the width of the individual elements is made such that the phase difference between the waves radiated by neighbourinq elements is not appre`ciably greater than 90. If these values of the radius of curvature and the phase difference are exceeded, there is a corresponding impairment in the shape of the beam and consequently in the transverse resolution. However, weak focussing according to the invention can be obtained, at least in principle, with a~phase difference between 30 and 180.
The dimensions of the transducer elements will now be illustrated with a respect to a concrete example tFigs. 18 and 19). As shown in Fig. 18, the two inner elements in the group transmit at phase 0 and the two outer elements at phase 90. From Fig. 19 and by the chord theorem, we o~tain dl = 2R ~ ... (1) 25 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 ma~e equal to 80 mm (approximately half the depth of the examined body) and ~ = O,75 mm lthis wavelength corresponds to a frequency of 2 MHz~, we o~tain 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 approximately A
~ lS43 - 15 -1 equal to the previGusly-calculated distance dl Fig. 13 is a bloc~ circuit dia~ram of an ultrasonic imaging unit according to the invention which, as shown in Fig. lla, uses 7-element groups of transducers for transmlssion and reception. The block circuit diagram in Fig. 13 shows a transducer arrangement 38 as in Fig. 3, a timing generator 131, a timing signal 132 delivered by generator 131, a transmitter-signal generator 133, transmitter signals 134 supplied by generator 133 over lines 135 to element-selector drive switches 138, an element counter and decoder 136 for controlling switch 138 and connected to timing generator 131, echo signals 142 delivered by a group of transducers, an echo signal receiver 143, the combined echo signal 144 at the output of receiver 143, a time-sensitive amplified 145, a detector 146, a signal processor 147, the output signal 148 of processor 147, an X-deflection generator 151, a deflection signal 154 given by generator 151, a Y-stage function generator 152, a stage function signal delivered by generator 152, and a reception oscillograph 156 having three inputs X, Y
and Z.
The timing generator 131 generates periodic timing pulses 132 triggering the transmission of an ultrasonic signal and the generation of the necessary sinc signals.
Four electric transmitter pulses 121-124 (see Fig. 14) are generated in the transmitter signal generator 133. Three of the signals 122, 123, 124 have a phase lead correspond-ing to a carrier-signal phase of +30, +100 and +180 compared wikh a signal 121, whose phase is denoted by 0.
These transmitter signals are supplied on lines 134. In unit 138 (the element selector drive switch) the transmitter signals are supplied to 7 supply lines, on which the transmitter signals have the phases +180, +100, +30, 3~ 0, +30, +100, +180. The element counter and decoder 136 switches the desired seven elements, either for transmission or for reception, via switch 138. After '" ' `
llilS43 - 16 -1 each pulse, the configuration in Fig. lla is shifted by one element in the L direction. At the same time, the transmitter signals are cyclically interchan~ed with the different phases on the supply lines so that each element obtains the corresponding transmitter signal having the correct phase. The echo signals 142 travel from the seven switched-on ele~ents to the echo-signal receiver 143, where the signals are variously delayed, multiplied by various weighting factors, and then added. The output signal 144 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 156. Processor 147 compresses the dynamic range of the signal delivered by detector 146.
- The X-deflection generator 151 generates a voltage which is proportional to the time which has elapsed since the last pulse was transmitted. The Y-stage funct'on generator 152 generated a voltage proportional to the position of the central axis of the switched-on group of transducers.
The construction and operation of the transmitter-signal generator 133 will be described initially with respect to Figs. 14 and lS. 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 unit~ 164-167 these signals are multiplied by the corresponding weighting factors.
Fig. 16 shows the echo-receiver 143 in detail. The echo signals 142 are multiplied by the corresponding weighting factors in weighting units 171-177. They are then delayed by phase shifters 181-185 and added in an adder 186.
The basic principle of a preferred embodiment o~ the ~ .
~ S43 - 17 -1 element selector drive switch 138 in the uni. in ~ig. 13 will be initially explained with respect to Fig. 17. For simplicity, the principle is explained in tlle case of a group of transducers containing only four elements, although the unit in Fig. 13 uses seven-ele~ent groups.
The switching diagram shown in Fig. 17 can be used for triggering and shifting a group of four transducer 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 transducer elements are represen-ted by the corresponding electrode segments 31, 32, 33, etc. By means of a switch system 191, the transducer ele-16 ments are c~clically connected to four supply lines 192-l9S.
These four lines are connected via a switch system 196 to two supply lines 197, 198, which are supplied with the transmitter signals 41, 42 having the amplitudes and phases shown in Fig. S. 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 lgl 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 2~ position previously occupied by the upper switch (e.g.
212) for actuating the preceding group I. The uppermost switch 211 takes the position previously occupied by the lowest switch 214. The same switches can ~e used for transmission and reception, if the electronic design of .he 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 ~eception.
The advantages o~ the invention can ~e illustrated as ~ollows:
..
.
l~llS43 1 The method acco~ding to the invention makes possible to attain higher transvers resoluti^n, so as to obtain more distinct ultrasonis images.
In addition, the unit according to the invention is economic, since its expense is relatively low.
Owing to the weighting of the transmitter and echo signals according to the invention, there is an appreciable reduction in the secondary lobes of the radiation charact-eristic of an ultrasonic beam generated by a group cf transducers according to the invention.
In addition, the embodiments of the invention described hereinbefore with respect to Figs. 9a-12 produce ultrasonic beams having an approximately conical wave front, so that the ultrasonic beam is strongly focussed over a great depth.
Other advantages and properties of the invention are 2~ clear Crom 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. 2~) appears to yield better results than when the beam is m~ved 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 lungs and measures approximately 2 x 7 cm. In addition, the ribs make it difficult to obtain an image 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 10 cm in length and is inefficiently used.
.
l~iiS43 1 Known sector ;canners operate either on tr.e "plas2d array" prlnciple (J. ~isslo, OT. v. Ramm, F. L. Thurstone, "A phase array ultrasound system for cardiac imaging", Proceedings of the Second European Congress on ~ltrasonics in Medicine, Munich, 12-16 May 1975, pp. 67-74, edited by E. Kazner, M. de Vlieger, H. R. Muller, V. R. McCready, Excerpta Medica Amsterdam - Oxford 1975), or are mechanical contact scanners (cf A. Shaw, J. S. Paton, N. L. Gregory, D.
J. Wheatley, "A real time 2-dimensional ultrasonic scanner for clinical use", Ultrasonics,Januar 1976, pp. 35-40).
The following description relats to an arc scanner which operates on the same principle as a linear scanner and has the scanning range of a sector scanner.
~he main component of the arc scanner is a linear "array", the sesments of whic~ are disposed not on a straight line but on an arc. The scannable region is shown in Fig. 22. As in Figs. 20 and 21, the transducer should be assumed to be above in the drawing. If the top half of the range is used as anticipatory path re ion and only the bottom half for imaging, a system with beam rotation is obtained as in Fig. 20. The complete sound head of an electronic arc scanner is shown in ~ig. 23. An arcuate piezoceramic transducer 302 is disposed in the upper part of housing 301 and individual electrodes 303 are disposed at its top. Upwardly reflected ultrasound is destroyed in absorber 304. The lower part of the housing is lined with sound-absorbing material 305 and filled with an ultra-sound-transmitting medium 306. At the bottom, the sound head is closed by a diaphragm 307. ~he diaphragm is at the centre of the arc formed by the transducer, i.e. at the narrowest place in the scannable region (see Fig. 22).
In order to eliminate interfering multiple reflections between the diaphragm and transducer from the image, the transit time between the transducer and diaphragm should be exactly the same as between the diaphragm and the most remote o~ject which has to be imaged. If water is used ` iil~S43 1 for the anticipatory path, this means that the radius of the transducer arc must be exactly equal to the maximum depth of penetration, since the human body and ~Ja.er ha~;e approxlmately the same speed of sound (appro~. 1500 m/sec.).
The shape of the beam can be optimised in a manner very similar to linear scanning, as described hereinbefore.
If all segments of a group of transducers are operated and simultaneously switched on at the same phase, the sound beam is focused at the centre of the arc, i.e. at the diaphragm. When the depth of penetration increases the beam becomes progressively wider, so that the lateral resolution of the system becomes progressively worse.
A considerable improvement can be obtained if the focus is 1~ not at the centre of the arc but at a point located at about approx. 2/3 of the maximum imaging depth measured from membrane 307. This is achieved by suitable phase-shifting of the individual transducer elements during transmission and reception. The phase shifting here has the opposite sign (phase lag) as in the linear scanner described previously.
The reason for this is explained in Figs. 24, 25 in the case of the transmitter. 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 strongly curved wave front (continuous line) is converted into a slightly curved front (chain line). Thus, at increasing distances from the axis, the signal requires a progressively greater lag. Similar considerations apply to reception. Depending on the special dimensions of the group of transducers, the shape of the ~eam may ~e 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.
i43 1 Pre~icusly, only the shapiIlg o the bea~ in t}~e scanning direction has been discussed. ~oYJ~er , ~eak focussiny in the direction at ri$ht angles tl1ere~o may also be advantageous. Advantageously, the Loc-l point is at the same place as in the first direction, i.e. at 2/3 of the maYimum imaging depth. ~ocussin~ is obtzined either by means Oc a suitable curved transducer or an acoustic lens disposed in front of the transducer. Of course, focussin~ can be electronically produced in this direction also, as in the previously-described linear scanner, if the greater complexity of the system is allowed for.
Numerical calculations indicate that additional apodisation does not provide any further improvement of the shape of the beam. Apodization is, however, advantageous if there is no focussing in the second direction, what may be desirable for simplifying the con~truction. Apodisation can be obtained e.~. by means of segments which become progressively narrower outwards (see Fig. 28).
Electronically, the arc scanner has all the advantages of the linear scanner. Its disadvantage is that it requires a water anticipatory path, with the result that the sound head is heavy and awkward to handle and the maximum image frequency is only half that of a scanner without the~anticipatory path. ~he anticipatory path, and there-fore the sound head, can be reduced if water is replaced by a substance in which the speed of sound is lower than in water.
In many organic liquids, and also in many silicone rubbers, the speed of sound is about lO00 m/sec. This means that the anticipatory path can be reduced 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 considerable reduction in the sound head can be obtained if th~ arc scanner is used not as a sound head ll~lS43 - 22 -1 but as a signal processor Cor a "pnased arra~". This possibiii~y is shown in Fig. 26. A group of transducers 401 comprising a number of segments of an arcua'e transducer 402 transmits an ultrasonic beam 403 ~hich, at the centre of the arc, stri~es a "phased array" 4~4 whose segments are disposed parallel to the segments of the arcuate transducer 404. ~y means of the phased array, the sound field is detected in segments in a phase-sensitive manne~r and transmitted to a second "phased array"
405; which forms the actual sound head, reconstructs the sound field at the site of the first "phased array" and radiates a corresponding ultrasound beam 406. Of course, the device can also be operated in the reverse direction, and is therefore suitable-for transmission and reception.
Advantageously, a transmitting and a receiving intermediate amplifier is disposed between the two "phased arrays" in each segment. For simplicity, these amplifiers were omitted in Fig. 26.
At this point it should be noted that the sound field radiated by the second "phased array" 405 need not be identical with the field detected by the first "phased array" 404. The phase and amplitude of the signals from each segment can be varied by the aforementioned inter-26 mediate amplifier. In addition, the second "phase array"
405 can be given a shape different from the first, thus altering the sound field. This provides an additional means of improving the focussing of the sound beam and thus improving the lateral 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 deflection can be obtained relatively easily by digital means, but complicated delay lines and switches have hitherto been required for llllS43 - 23 -reception. It is there ore bet~_r to use a lybrid solution, in which the "phased array" is directi-~ operated during transmission and the arc scanner is used only as a received-sisnal processor.
Finally, we shall described a simple example of an arc scanner for cardiolosical applications (Figs. 27 and 28).
The data for thls signal are 2S follows:
Frequency 2 MHz max. depth of penetration 15 cm angle to be scanned 50-60 number of segments 64 phases to be used 0, 90 anticipatory path medium water focussing in one direction only.
Under these boundary conditions, an optimisation process was carried out, 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 lS cm, a width B of 2 cm and an arc length 17,6 cm, corresponding to an angle e = 67,2. The transducer is divided into 64 segments having a width S - 2,75 mm. 12 elements are used simultaneously for transmission and reception. One such group is shown in Fig. 28. The edges of the individ-ual elements 411 are formed by arcs of a circle. This shape results in the desired apodisation and improvement of the beam shape. During transmission and reception, 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 transducer. At the same time, the signal amplitudes of the outer six elements during transmission and reception are multiplied by a factor of 0,5 and the signal amplitudes Oc '~; ' '.
llllS43 - 2~ -the inner six elements are mu3tiplled bl a actor cf unlty.
By means of this ~ransducer, a resolut-on of a~ le~st
Fig. llb is a cross-section through a preferred shape of the irradiating surface of the group of 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 diagram 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. lS shows diagrams of the ~iming pulse 132 gener-ated by the timing generator 131 (Fig. 13) and of the pulsed sine wave 162 derived from the timing pulse, 5~3 7 _ 1Fig. 16 is a block diagram illustrating the echo-signal receiver 143 in the device in Fig. 13, Fig. 17 illustrated the principle of a preferred embcd-iment of the element selector drive 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 sroup of transducers according to the invention and the elements thereof.
Fig. 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 of 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.
Fig. 23 is a diagram of a sound head with an arcuate transducer system, Fig. 24, 25 illustrate the production of a cylindrical wave front in two variants of the invention, 30Fig. 26 shows the use according to the invention of an arcuate transducer system for producing a "phased array", and Figs. 27, 28 illustrate the dimensioning of an arcuate transducer system according to the invention.
As Fig. 1 shows, the transducer system 11 of the known ultrasonic imaging units (U.S. Patent Specification 3 881 466) .
ll~lS43 - 8 -1 comprises a stationary elongated array of adjacend transducer elements 12. Groups of A ad~acent elements 12 are successively stimulated to produce pulses. The location of each successive group of A elements is shifted of a longitudinal distance of B
elements from the position of the immediately preceding group.
The ultrasonic beam 13 is thereby moved in the direction of arrow L, as shown by the series of chain-dotted rectangles 14 showing the instantaneous position of beam 13 after equal intervals of time. Note that each group of transducers in the known transducer system 11 generates an unfocussed ultrasonic beam 13, since all the A elements in the group of transducers are simultaneously energised so as to yield pulses. The unfocussed radiation characteristic 22 of the ultrasonic beam 13 in Fig. 1 is shown in Fig. 2.
In Fig. 1, an orthogonal system of coordinates is defined by three arrows, Q, L and S. Arrow L is along the longitudinal axis of the irradiating surface 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 cross-sections and elevations shown in the accomp-anying drawings are defined with respect to this coordinate system.
Fig. 3 is a partial cross-section showing the structure of a preferred arrangement of transducers 38 for performlng the method according to the invention.
Arrangement 38 comprises a complete electrode 36 which is earthed and one surface 37 of which is used as an irradiating surface; arrangement 38 also comprises a piezoelectric layer 35 and electrode segments 31-34, shown in rear view in Fig. 4.
It is clear from the preceding description of arranse-ment 38 that the transducer elements according to the invention can have common parts such as the piezoelectric 1 layer 35 or the complete electrode 36. The arrange~ent 38 acco-;ding to the invention càn be operated simply by providlng it wlth electrode segments on one side, which are supplied with the time-shifted transmitter signals and from which echo signals can be obtained. Thus, each electrode segment deiines a transducer element according to the invention.
The effect obtained by the invention, i.e. higher transverse resolution, is mainly due to a novel manner of operation of the transducer system. This will be ex21ained in dctail, firstly with reference to Figs. 2, 4 and 5.
Fig. 4 shows electrode segments 31-34 of a group of transducers 21 according to the invention. In order to produce an ultrasonic beam 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 focussed 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 lea~ 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 KHz and at a suitable phase angle.
~115~
1 The effect of operating group 21 according to the invention can be improved by the followins additional measures:
1) It has been found advantageous to select the fo71Owins combinations of phase lead for the outer elements of the group:
"
Transmitter signals Echo si~nals either approx. 90 approx. 45 or approx. 45 approx. 90 :
As a result of these different values of the phase lead for the transmitter and echo signals, the radiation characteristic 23 according to the invention (Fig. 2) is 1~ additionally narrowed over a certain depth.
.~ .
2) Advantageously, the transmitter and echo signals 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 welghting factor than the echo signals from the outer elements.
Advantageously, the weighting ratio is 2:1 for both the trans~itter 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 focussing in the Q direction can also be electronically produced, using a transducer arrangement as in Fig. 7, in which each of the electrode segments is divided into three parts a, b, c in the Q direction. As shown in Fig. 7, only the shaded parts of the electrode '' ~ .... ..
' ' - . ...
- llllS~3 11 -1 segments are used for transmitting or rec~iving. The inner parts 32b, 33b are energised with the transmitter signal-41 and the remaining active parts are energised with the transmitter signal 42. The resulting system is electronicaliy more complicated than the transducer arrangement comprising a curved irradiating surface, but it only requires a transducer arrangement having a flat irradiating surfa~e, which is cheaper.
.
In the known transducer system 11 in Fig. 1, the ultrasonic beam 13 can be displaced by the width of a transducer element 12 after each transmitting and recept-ion 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 und 8c) the ultrasonic beam is displaced by half the width of an element in that successively selected groups ~f 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 alt rnately:
.
.
1111~43 - 12 -1 With 4 elements:
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 45 0 0 45 With 3 elements:
Element 32 33 34 (Amplitude Transmission (Phase 45 0 45 (Amplitude Reception (Phase 22,5 0 22,5 A second embodiment of 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. According 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 1~1-104 in Fig. 9a for the time-shifted echo signals 2Q2-208 (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-section in Fig. 9b. The chain line 107 in Fig. 9a shows the position of constant phase on the irradiating surface of the transducer system; for simplicity, the drawing shows a phase which varies continuously in the L direction, , . . f f `` lillS43 ~ 13 -1 instead of varying stepwise as in the present example.
In the present example the locus Oc constant phase is a set of straight lines 107, instead of being a circle as in the case of a conical wave front.
A better approximation of a conical wave front can be obtained by the embodiment of the invention illustrated initially with respect to Figs. lla, llb and 12. In this embodiment, the phase angle of the transmitter signals or time-shifted echo signals ~s a quadratic function of the position of the corresponding elements in the centre of the group of transducers, and is a linear function at the edge. A corresponding phase angle distribution in the Q direction is obtained by shaping the irradiating surface 37 as shown in Fig. llb with respect to a cross-section - of the transducer system. Surface 37 in Fig. llb is preferably 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 shown b~,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. 9a, lla have a greater area than in the embodiment in Fig. 4. This greater area results in a correspondingly greater aperture, which is required for obtaining 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 higher weighting coefficient on reception, thus improving the short-range field.
The dimensioning of groups 21 and elemen.s 31-34 as in Fig. 4 for obtaining a weakly ~ocussed ultrasonic beam 23 as in Fig. 2 will be explained initially with respect to ` ' llliS43 - 14 -1 Figs. 18 and 19. An efficient weakly-focussing group of transducers is characterised in that its width w and length 1 is 15-30 wavelengths. The radius of curvature R (Fig.
19) of the wave front is made approximately equal to half the depth of the examined body, and is preferably somewhat smaller. In the case of a group of transducers comprising four elements, the width of the individual elements is made such that the phase difference between the waves radiated by neighbourinq elements is not appre`ciably greater than 90. If these values of the radius of curvature and the phase difference are exceeded, there is a corresponding impairment in the shape of the beam and consequently in the transverse resolution. However, weak focussing according to the invention can be obtained, at least in principle, with a~phase difference between 30 and 180.
The dimensions of the transducer elements will now be illustrated with a respect to a concrete example tFigs. 18 and 19). As shown in Fig. 18, the two inner elements in the group transmit at phase 0 and the two outer elements at phase 90. From Fig. 19 and by the chord theorem, we o~tain dl = 2R ~ ... (1) 25 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 ma~e equal to 80 mm (approximately half the depth of the examined body) and ~ = O,75 mm lthis wavelength corresponds to a frequency of 2 MHz~, we o~tain 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 approximately A
~ lS43 - 15 -1 equal to the previGusly-calculated distance dl Fig. 13 is a bloc~ circuit dia~ram of an ultrasonic imaging unit according to the invention which, as shown in Fig. lla, uses 7-element groups of transducers for transmlssion and reception. The block circuit diagram in Fig. 13 shows a transducer arrangement 38 as in Fig. 3, a timing generator 131, a timing signal 132 delivered by generator 131, a transmitter-signal generator 133, transmitter signals 134 supplied by generator 133 over lines 135 to element-selector drive switches 138, an element counter and decoder 136 for controlling switch 138 and connected to timing generator 131, echo signals 142 delivered by a group of transducers, an echo signal receiver 143, the combined echo signal 144 at the output of receiver 143, a time-sensitive amplified 145, a detector 146, a signal processor 147, the output signal 148 of processor 147, an X-deflection generator 151, a deflection signal 154 given by generator 151, a Y-stage function generator 152, a stage function signal delivered by generator 152, and a reception oscillograph 156 having three inputs X, Y
and Z.
The timing generator 131 generates periodic timing pulses 132 triggering the transmission of an ultrasonic signal and the generation of the necessary sinc signals.
Four electric transmitter pulses 121-124 (see Fig. 14) are generated in the transmitter signal generator 133. Three of the signals 122, 123, 124 have a phase lead correspond-ing to a carrier-signal phase of +30, +100 and +180 compared wikh a signal 121, whose phase is denoted by 0.
These transmitter signals are supplied on lines 134. In unit 138 (the element selector drive switch) the transmitter signals are supplied to 7 supply lines, on which the transmitter signals have the phases +180, +100, +30, 3~ 0, +30, +100, +180. The element counter and decoder 136 switches the desired seven elements, either for transmission or for reception, via switch 138. After '" ' `
llilS43 - 16 -1 each pulse, the configuration in Fig. lla is shifted by one element in the L direction. At the same time, the transmitter signals are cyclically interchan~ed with the different phases on the supply lines so that each element obtains the corresponding transmitter signal having the correct phase. The echo signals 142 travel from the seven switched-on ele~ents to the echo-signal receiver 143, where the signals are variously delayed, multiplied by various weighting factors, and then added. The output signal 144 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 156. Processor 147 compresses the dynamic range of the signal delivered by detector 146.
- The X-deflection generator 151 generates a voltage which is proportional to the time which has elapsed since the last pulse was transmitted. The Y-stage funct'on generator 152 generated a voltage proportional to the position of the central axis of the switched-on group of transducers.
The construction and operation of the transmitter-signal generator 133 will be described initially with respect to Figs. 14 and lS. 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 unit~ 164-167 these signals are multiplied by the corresponding weighting factors.
Fig. 16 shows the echo-receiver 143 in detail. The echo signals 142 are multiplied by the corresponding weighting factors in weighting units 171-177. They are then delayed by phase shifters 181-185 and added in an adder 186.
The basic principle of a preferred embodiment o~ the ~ .
~ S43 - 17 -1 element selector drive switch 138 in the uni. in ~ig. 13 will be initially explained with respect to Fig. 17. For simplicity, the principle is explained in tlle case of a group of transducers containing only four elements, although the unit in Fig. 13 uses seven-ele~ent groups.
The switching diagram shown in Fig. 17 can be used for triggering and shifting a group of four transducer 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 transducer elements are represen-ted by the corresponding electrode segments 31, 32, 33, etc. By means of a switch system 191, the transducer ele-16 ments are c~clically connected to four supply lines 192-l9S.
These four lines are connected via a switch system 196 to two supply lines 197, 198, which are supplied with the transmitter signals 41, 42 having the amplitudes and phases shown in Fig. S. 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 lgl 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 2~ position previously occupied by the upper switch (e.g.
212) for actuating the preceding group I. The uppermost switch 211 takes the position previously occupied by the lowest switch 214. The same switches can ~e used for transmission and reception, if the electronic design of .he 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 ~eception.
The advantages o~ the invention can ~e illustrated as ~ollows:
..
.
l~llS43 1 The method acco~ding to the invention makes possible to attain higher transvers resoluti^n, so as to obtain more distinct ultrasonis images.
In addition, the unit according to the invention is economic, since its expense is relatively low.
Owing to the weighting of the transmitter and echo signals according to the invention, there is an appreciable reduction in the secondary lobes of the radiation charact-eristic of an ultrasonic beam generated by a group cf transducers according to the invention.
In addition, the embodiments of the invention described hereinbefore with respect to Figs. 9a-12 produce ultrasonic beams having an approximately conical wave front, so that the ultrasonic beam is strongly focussed over a great depth.
Other advantages and properties of the invention are 2~ clear Crom 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. 2~) appears to yield better results than when the beam is m~ved 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 lungs and measures approximately 2 x 7 cm. In addition, the ribs make it difficult to obtain an image 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 10 cm in length and is inefficiently used.
.
l~iiS43 1 Known sector ;canners operate either on tr.e "plas2d array" prlnciple (J. ~isslo, OT. v. Ramm, F. L. Thurstone, "A phase array ultrasound system for cardiac imaging", Proceedings of the Second European Congress on ~ltrasonics in Medicine, Munich, 12-16 May 1975, pp. 67-74, edited by E. Kazner, M. de Vlieger, H. R. Muller, V. R. McCready, Excerpta Medica Amsterdam - Oxford 1975), or are mechanical contact scanners (cf A. Shaw, J. S. Paton, N. L. Gregory, D.
J. Wheatley, "A real time 2-dimensional ultrasonic scanner for clinical use", Ultrasonics,Januar 1976, pp. 35-40).
The following description relats to an arc scanner which operates on the same principle as a linear scanner and has the scanning range of a sector scanner.
~he main component of the arc scanner is a linear "array", the sesments of whic~ are disposed not on a straight line but on an arc. The scannable region is shown in Fig. 22. As in Figs. 20 and 21, the transducer should be assumed to be above in the drawing. If the top half of the range is used as anticipatory path re ion and only the bottom half for imaging, a system with beam rotation is obtained as in Fig. 20. The complete sound head of an electronic arc scanner is shown in ~ig. 23. An arcuate piezoceramic transducer 302 is disposed in the upper part of housing 301 and individual electrodes 303 are disposed at its top. Upwardly reflected ultrasound is destroyed in absorber 304. The lower part of the housing is lined with sound-absorbing material 305 and filled with an ultra-sound-transmitting medium 306. At the bottom, the sound head is closed by a diaphragm 307. ~he diaphragm is at the centre of the arc formed by the transducer, i.e. at the narrowest place in the scannable region (see Fig. 22).
In order to eliminate interfering multiple reflections between the diaphragm and transducer from the image, the transit time between the transducer and diaphragm should be exactly the same as between the diaphragm and the most remote o~ject which has to be imaged. If water is used ` iil~S43 1 for the anticipatory path, this means that the radius of the transducer arc must be exactly equal to the maximum depth of penetration, since the human body and ~Ja.er ha~;e approxlmately the same speed of sound (appro~. 1500 m/sec.).
The shape of the beam can be optimised in a manner very similar to linear scanning, as described hereinbefore.
If all segments of a group of transducers are operated and simultaneously switched on at the same phase, the sound beam is focused at the centre of the arc, i.e. at the diaphragm. When the depth of penetration increases the beam becomes progressively wider, so that the lateral resolution of the system becomes progressively worse.
A considerable improvement can be obtained if the focus is 1~ not at the centre of the arc but at a point located at about approx. 2/3 of the maximum imaging depth measured from membrane 307. This is achieved by suitable phase-shifting of the individual transducer elements during transmission and reception. The phase shifting here has the opposite sign (phase lag) as in the linear scanner described previously.
The reason for this is explained in Figs. 24, 25 in the case of the transmitter. 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 strongly curved wave front (continuous line) is converted into a slightly curved front (chain line). Thus, at increasing distances from the axis, the signal requires a progressively greater lag. Similar considerations apply to reception. Depending on the special dimensions of the group of transducers, the shape of the ~eam may ~e 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.
i43 1 Pre~icusly, only the shapiIlg o the bea~ in t}~e scanning direction has been discussed. ~oYJ~er , ~eak focussiny in the direction at ri$ht angles tl1ere~o may also be advantageous. Advantageously, the Loc-l point is at the same place as in the first direction, i.e. at 2/3 of the maYimum imaging depth. ~ocussin~ is obtzined either by means Oc a suitable curved transducer or an acoustic lens disposed in front of the transducer. Of course, focussin~ can be electronically produced in this direction also, as in the previously-described linear scanner, if the greater complexity of the system is allowed for.
Numerical calculations indicate that additional apodisation does not provide any further improvement of the shape of the beam. Apodization is, however, advantageous if there is no focussing in the second direction, what may be desirable for simplifying the con~truction. Apodisation can be obtained e.~. by means of segments which become progressively narrower outwards (see Fig. 28).
Electronically, the arc scanner has all the advantages of the linear scanner. Its disadvantage is that it requires a water anticipatory path, with the result that the sound head is heavy and awkward to handle and the maximum image frequency is only half that of a scanner without the~anticipatory path. ~he anticipatory path, and there-fore the sound head, can be reduced if water is replaced by a substance in which the speed of sound is lower than in water.
In many organic liquids, and also in many silicone rubbers, the speed of sound is about lO00 m/sec. This means that the anticipatory path can be reduced 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 considerable reduction in the sound head can be obtained if th~ arc scanner is used not as a sound head ll~lS43 - 22 -1 but as a signal processor Cor a "pnased arra~". This possibiii~y is shown in Fig. 26. A group of transducers 401 comprising a number of segments of an arcua'e transducer 402 transmits an ultrasonic beam 403 ~hich, at the centre of the arc, stri~es a "phased array" 4~4 whose segments are disposed parallel to the segments of the arcuate transducer 404. ~y means of the phased array, the sound field is detected in segments in a phase-sensitive manne~r and transmitted to a second "phased array"
405; which forms the actual sound head, reconstructs the sound field at the site of the first "phased array" and radiates a corresponding ultrasound beam 406. Of course, the device can also be operated in the reverse direction, and is therefore suitable-for transmission and reception.
Advantageously, a transmitting and a receiving intermediate amplifier is disposed between the two "phased arrays" in each segment. For simplicity, these amplifiers were omitted in Fig. 26.
At this point it should be noted that the sound field radiated by the second "phased array" 405 need not be identical with the field detected by the first "phased array" 404. The phase and amplitude of the signals from each segment can be varied by the aforementioned inter-26 mediate amplifier. In addition, the second "phase array"
405 can be given a shape different from the first, thus altering the sound field. This provides an additional means of improving the focussing of the sound beam and thus improving the lateral 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 deflection can be obtained relatively easily by digital means, but complicated delay lines and switches have hitherto been required for llllS43 - 23 -reception. It is there ore bet~_r to use a lybrid solution, in which the "phased array" is directi-~ operated during transmission and the arc scanner is used only as a received-sisnal processor.
Finally, we shall described a simple example of an arc scanner for cardiolosical applications (Figs. 27 and 28).
The data for thls signal are 2S follows:
Frequency 2 MHz max. depth of penetration 15 cm angle to be scanned 50-60 number of segments 64 phases to be used 0, 90 anticipatory path medium water focussing in one direction only.
Under these boundary conditions, an optimisation process was carried out, 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 lS cm, a width B of 2 cm and an arc length 17,6 cm, corresponding to an angle e = 67,2. The transducer is divided into 64 segments having a width S - 2,75 mm. 12 elements are used simultaneously for transmission and reception. One such group is shown in Fig. 28. The edges of the individ-ual elements 411 are formed by arcs of a circle. This shape results in the desired apodisation and improvement of the beam shape. During transmission and reception, 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 transducer. At the same time, the signal amplitudes of the outer six elements during transmission and reception are multiplied by a factor of 0,5 and the signal amplitudes Oc '~; ' '.
llllS43 - 2~ -the inner six elements are mu3tiplled bl a actor cf unlty.
By means of this ~ransducer, a resolut-on of a~ le~st
4 mm is obtained in the scanning plane in the entire useful regior. Owing to the absence OL focussing, the resolution in the direction perpendlcular thereto is lower by a factor of 1,5. As already mentioned, improved resolution in this direction also can be obtained by addit-ional focussing.
3~
3~
Claims (7)
1. An ultrasonic imaging unit for producing cross-sectional images, which unit operates on the pulse-echo principle and comprises a transducer system having a stationary array of adjacent transducer elements, in which unit successively and cyclically selected groups of adjacent transducer elements of the transducer system are used to produce an ultrasonic beam in response to pulsed electric transmitter signals applied to the transducer elements to transmit the ultrasonic beam, substan-tially in a scan plane, into a heterogeneous body, and/or to receive echoes reflected from a discontinuity in the body, and to generate an electric echo signal in response to the received echoes in which unit an anticipatory path is provided between the transducer system and a transmission region within a body under examination, the anticipatory path being comprised within a closed envelope, and the transducer system is arcuate in the scan plane and serves for generating a scanning beam by means of the cyclical selection of groups of adjacent transducer elements to perform a sector scanning in the transmission region and/or for receiving echoes derived by performing a sector scanning in that region.
2. A unit according to Claim 1, wherein, in order to weakly focus the ultrasonic beam produced by each group of transducers, the transmitter signals applied to the transducer elements and/or the echo signals from the transducer elements are time-shifted with respect to one another, the phase angle (O) of the transmit signal or of 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 elements, the transmitter signal and/or the time-shifted echo signal of the transducer element which is at the greater distance from the centre of the group of elements has a phase lag with respect to the corresponding signal of the other elements.
3. A unit according to Claim 1, wherein the elements of the transducer system comprise a radiating surface which becomes progressively narrower in the direction at right angles to the scanning direction and outwardly from the longitudinal axis of the transducer system.
4. A unit according to Claim 3, wherein the edges of the individual elements are formed by arcs.
5. A unit according to Claim 1, wherein the medium used for transmitting ultrasonic waves between the transducer system and the body under investigation is a substance in which the speed of sound is lower than in water.
6. A unit according to Claim 1, comprising a second and a third array of transducer elements, each element of the second array being electrically connected to a corresponding element of the third array, in which unit the centre of the second array is placed at the centre of the arc corresponding to the shape of the transducer system, the elements of the second array substantially face the elements of the transducer system and the second array is arranged to receive an ultrasonic beam generated by a group of elements of the transducer system and to transmit electrical signals corresponding to said beam to the third array, which thereupon radiates a corresponding ultrasonic beam, or to receive electrical signals corresponding to echo waves received by the third array and to transmit an ultrasonic beam corresponding to the latter electrical signals to a group of transducer elements of the transducer system.
7. A unit according to Claim 1, wherein the distance between the arcuate transducer device and the focus of the ultrasonic field produced thereby is approximately equal to the length of the anticipatory path in the transmission medium plus approximately 2/3 of the maximum imaging depth.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA338,804A CA1111543A (en) | 1975-12-01 | 1979-10-31 | Ultrasonic imaging unit |
Applications Claiming Priority (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CH1555575A CH594252A5 (en) | 1975-12-01 | 1975-12-01 | Resolution of electronically controlled ultrasonic scanner - is improved by phase control focussing with outlying members of transducer group advanced or retarded (NL 3.6.77) |
| CH15555/75 | 1975-12-01 | ||
| CH12074/76 | 1976-09-23 | ||
| CH1207476A CH608103A5 (en) | 1975-12-01 | 1976-09-23 | |
| CA338,804A CA1111543A (en) | 1975-12-01 | 1979-10-31 | Ultrasonic imaging unit |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1111543A true CA1111543A (en) | 1981-10-27 |
Family
ID=27166473
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA338,804A Expired CA1111543A (en) | 1975-12-01 | 1979-10-31 | Ultrasonic imaging unit |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1111543A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113195052A (en) * | 2018-10-22 | 2021-07-30 | 瓦伦西亚理工大学 | Method for producing lens and ultrasonic device containing lens |
-
1979
- 1979-10-31 CA CA338,804A patent/CA1111543A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113195052A (en) * | 2018-10-22 | 2021-07-30 | 瓦伦西亚理工大学 | Method for producing lens and ultrasonic device containing lens |
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