Note: Descriptions are shown in the official language in which they were submitted.
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
,
-, , ~ .
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
1111543 9
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~
-- 10 --
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.
.
- 19 -
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~
