Echography probe and apparatus incorporating such a probe

An echography apparatus comprising a probe reconstituting mobile rings by element switching, said probe comprising a plurality of transducer elements spread over a convex coupling surface, and switching means being provided for grouping together certain transducer elements into rings.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a new type of static echographY probe and the 
process for manufacturing such a probe. The invention also relates to an 
echography apparatus incorporating such a probe. 
2. Description of the Prior Art 
The most widely used echography probes at the present time are sectorial 
sweep probes, that is to say comprising either an oscillating mobile 
assembly or several transducers mounted on a wheel and switched as they 
travel past an emission window. The qualities of these probes are their 
speed of acquisition and their fundamental simplicity which results in 
relatively simple and inexpensive signal processing means. The coupling 
surface is relatively small, so that the probe may be disposed between two 
ribs of the patient for cardiac observation. On the other hand, the life 
span of these probes is limited. 
Systems using liner arrays of transducer elements are essentially reserved 
for observing adominal regions, because of the large dimensions of the 
probe. In these systems, the elements (or groups of elements) are 
successively switched so as to provide a sweep perpendicular to the row of 
elements. The technology of linear array probes has been used for 
observations of the thoracic cage, by reducing the coupling surface of the 
probe and distributing delays (on emission as at reception) between the 
transducer elements of the array so as to reconstitute a sectorial sweep, 
i.e. so as to emit and receive in convergent directions inscribed in a 
sweep range. This technology, known under the name of phased array, 
provides a static probe whose coupling surface has sides of no more than 
20 mm. However, the processing electronic equipment is very expensive. In 
fact, the delays to be provided (by delay lines, on the reception side at 
least) may reach 10 microseconds and an acceptable control of the 
directivity is only possible if these delays are provided with a tolerance 
of 10 nanoseconds. Now, at the present time, such an accuracy is obtained 
only for delays of two to three microseconds at most. To overcome this 
problem, a frequency change may be operated, then the signals received 
converted into digital information; and predetermined delay laws may be 
applied to the digital information. The electronic circuits for operating 
the frequency change represent a considerable part of the price of the 
equipment. 
Furthermore, a type of ring transducer probe is known in which the beam is 
generated by a group of transducer elements in the form of concentric 
rings. This arrangement has the advantage of a Bessel function "antenna 
diagram" (18 dB attenuation of the secondary lobes with respect to the 
main lobe). Proposals have even been made for reconstituting such rings 
from a flat transducer element array, so as to cause movements of these 
rings providing an ultrasonic mission sweep in a predetermined direction. 
This has the drawback of creating expensive and cumbersome probes, (like 
the linear arrays). Furthermore, the coupling is mediocre. 
SUMMARY OF THE INVENTION 
The purpose of the invention is first of all to provide a static probe 
structure ensuring under all circumstances excellent coupling of the 
transducer elements with the body of the patient, with a reduced coupling 
surface for, more especially, examining the inside of the thoracic cage 
(by passing between the ribs) and with which a sectorial sweep may be 
effected, at least partially by movement of the rings. 
To this end, the invention provides then an echography probe comprising a 
mosaic of transducer elements covering at least a part of the convex 
coupling surface. 
With respect to the above described system known under the name of phased 
array, the probe of the invention has more especially the advantage of 
generating the sectorial sweep essentially by switching transducer 
elements and not exclusively by delay laws. The coupling is moreover much 
better and the secondary lobes are attenuated by 18 dB if a ring 
configuration is adopted. As will be seen further on, the invention also 
provides for several emission-reception sequences for each position of the 
rings, by defining a limited number of microangulations, using appropriate 
delay laws between the elements of the rings. However, in this case, the 
delays brought into play are much smaller and so technologically easier to 
achieve with delay lines, with the required accuracy. 
The invention also provides a process for manufacturing an echography probe 
characterized in that it consists: 
in molding an insulating support on the internal surface of a piezoelectric 
material having a convex external surface, 
in cutting slices of substantially constant width from the assembly formed 
by said block of piezoelectric material and the insulating support, 
in partially cutting said slices at regular intervals along the directions 
perpendicular to their convex curved surfaces, by severing the whole of 
said piezoelectric material each time so as to define a curved row of 
individualized transducer elements in each slice, 
in fixing on each side of each slice a printed circuit comprising as many 
individualized conductors as there are transducer elements in said slice 
so that each conductor is in contact with a transducer element side, and 
in assembling and fixing said slices side by side in order so as to 
reconstitute a mosaic of transducer elements spaced apart over a convex 
surface. 
The invention also relates to a variant of this process in which curved 
slices of piezoelectric material are individualized before molding an 
insulating support on the concave internal surface of each slice. 
The invention finally relates also to an echography apparatus of the type 
comprising a fixed transducer probe, said probe comprising a mosaic of 
transducer elements defining a convex coupling surface and further 
comprising switching means for grouping transducer elements selectively 
together in a configuration defining approximately concentric rings and 
for causing said configuration to move in an alternating sweep and first 
means for associating a first delay law with different rings. 
This first delay law, applied to the rings, defines the focal 
characteristics an emission-reception sequence (dynamico focusing occuring 
both at an emission step and at a reception step. For increasing the 
number of lines of the reconstituted image, the echography apparatus 
advantageously comprises second means for associating additional delay 
laws with the different transducer elements of each ring. These additional 
delay laws which relate to the elements of the same ring bring into play 
shorter delays than the first law, and it is these laws which determine 
the microangulations on each side of the normal to the coupling surface 
passing through the center of the ring configuration. In other words, if 
the first law alone is applied to the rings, the firing takes place along 
this normal and the additional delay laws determine for each firing a 
given microangulation with respect to this normal. Each possible position 
of the ring configuration may then give rise to several firings and so to 
several lines of the reconstituted image.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 has been shown the end part of an echography probe 11 in 
accordance with the invention, whose coupling surface 12 (i.e. the surface 
intended to be placed in contact with the subject to be examined) is 
convex and formed partially of a mosaic of transducer elements 13. In this 
example, the general shape of the coupling surface is that of a spherical 
skull cap because it is one of the shapes which is most suitable for 
providing good coupling between the probe and the patient. However, other 
similar shapes could be suitable, as for example paraboloids or ellipsoids 
of revolution. A cylindrical convex surface may also be envisaged since 
one of the preferred methods of use of the probe (which will be described 
further on) consists in selecting and switching the transducer elements so 
as to cause an approximately concentric ring configuration to move from 
one side to the other of the probe. A cylindrical surface having a mosaic 
strip of a width equal to the diameter of the largest ring could therefore 
be suitable. 
For the same reason, the spherical skull cap, paraboloid or ellipsoid 
embodiments are not necessarily provided with a mosaic over the whole of 
their coupling surface, a mosaic strip is sufficient for the type of use 
with ring sweep operation. 
Structurally, the probe may be formed of the side by side assembly of 
slices each comprising a curved row of transducer elements, said slices 
having different mean radii of curvature. 
FIG. 2 illustrates one way of constructing such a probe. It may be 
advantageous to start with a block of piezoelectric material in the shape 
of a spherical skull cap 14 (FIG. 2a) since such shapes are currently used 
in ultrasonic techniques for different systems. An insulating support 15 
is molded against the concave face of the spherical skull cap 14 (FIG. 
2b); the techniques for molding these supports are well known to a man 
skilled in the art. Slices 17 are then cut parallel to each other from a 
median strip of the spherical skull cap (FIG. 2c) using for example a very 
fine saw 18. These slices therefore have different mean radii of 
curvature. Once the slices have been individualized, they are partially 
severed at regular intervals (2d) along directions perpendicular to their 
convex curved surface. Saw 19 is therefore adjusted so as to sever each 
time the whole of the piezoelectric material (by slightly nicking the 
insulating support) so as to define a curved row of individualized 
transducer elements 13. in each slice. Concurrently, printed circuits 20 
are manufactured (FIG. 2e) comprising as many individualized conductors 21 
as the slices comprise transducer elements. Then two printed circuits of 
this kind are fixed (for example by bonding) on each side of each slice, 
so that each conductor 21 is in contact with a side of a transducer 
element 13. Then said slices are reassembled in the same order as for 
cutting up (i.e. so as to reconstitute a mosaic of transducer elements 
distributed over a relatively regular convex surface) and they are fixed 
side by side, for example by bonding. 
At this stage in the manufacture of the probe, we have therefore as many 
pairs of electric conductors as there are individualized transducer 
elements. In the case when a ring sweep is desired, it should be noted 
that the delay laws applicable are the same for the transducer elements 
symmetric with respect to a plane of symmetry of the coupling surface 
perpendicular thereto and in which the desired path of the center of the 
ring configuration is inscribed. Consequently, the conductors connected to 
the transducer elements symmetrical with respect to this plane may 
advantageously be connected in parallel or in series (preferably directly 
inside the head of the probe) which reduces by half the number of wires to 
be connected to the electronic unit processing the signals. 
FIG. 3 shows a possible configuration with three concentric rings 26, 27 
and 28 (plus the central part 25); this configuration is also illustrated 
in FIG. 1 in a possible sweeping position. The central part 25 comprises 
four elements, the first ring 26 comprises 28, the second ring 27 
comprises 52 and the third ring 28 comprises 72. 
For each emission-reception or firing sequence, the electronic processing 
system must then first of all select 156 transducer elements adjacent to 
each other, for each position of the rings. The ring configuration 
occupies 14 transducer elements in the vicinity of the above mentioned 
plane of symmetry, in the direction of movement of the rings. Moreover, if 
the diameter of the coupling surface is 30 mm (supposing that it is a half 
sphere) and if the pitch for cutting up the transducer elements is 1.5 mm, 
the two slices the nearest to the plane of symmetry will have 30 or so 
elements. The number of possible positions of the ring configuration will 
therefore be 16. 
By programming a first delay law between the different rings (the central 
part 25 being assimilated to one of them), very directive focusing may be 
obtained with a beam emitted perpendicularly to the coupling surface from 
the center of the configuration. Calculation of these delays is within the 
scope of a man skilled in the art. They correspond to the compensation of 
the different propagation times of the ultrasounds emitted from different 
rings situated in different planes (since it may be considered that each 
ring is inscribed in the same plane for a spherical coupling surface) so 
that the wave front following the normal to the center of the ring 
configuration benefits from a good phase concordance, in the firing 
direction between the contributions of the different rings. These delays 
are of the order of from 1 to 3 microseconds. They are thus 
technologically feasible with a good accuracy of the order of ten 
nanoseconds. These are the longest delays which must be used. The cost 
price of corresponding delay lines is however not prohibitive and in any 
case these lines are only in a limited number (three in the example 
described). The delays are applied from the outer ring. In other words, 
the energization of the outer ring (at emission) forms the reference from 
which the different delays are counted before energization of the 
following rings. 
Considering more particularly the ring configuration of FIG. 3, the first 
above mentioned delay law may be "improved" by selecting each ring in two 
stages, since they have a "width" corresponding to two transducer 
elements. Thus different delays may be applied to the internal and 
external elements of each ring, which is tantamount to considering that 
the configuration of FIG. 3 comprises in fact six rings, although the 
shapes of these rings are then much more approximative, more especially 
close to the center. It is also possible to vary the number of rings 
depending on the desired penetration depth and also to change the number 
of rings in the same firing sequence, between emission and reception. 
However, we saw above that the number of possible positions of the ring 
configuration is only 16 in the example described. This is why, in each 
position of the rings, a certain number of microangulations may be formed 
on each side of the normal. Thus, four right hand microangulations and 
four left hand microangulations give eight additional lines for each 
position of the ring configuration, i.e. an image formed of 144 lines. 
Referring again to FIG. 3, in which the ring configuration is centered on 
an orthonormed reference x o y, where the axis x' o x1 designates the 
sweep direction and where the different elements are shown by FIGS. 1, 2, 
3, etc. . . . positively and by FIGS. 1', 2', 3', etc. . . . negatively 
along this axis and by letters A, B, C, etc. . . . positively and A', B', 
C', etc. . . . negatively along the axis y' o y, the order of energization 
of the different elements may be the following for a "left hand" 
microangulation considering the drawing: 
RING 28: 
B7 and B'7-A7 and A'7-D6 and D'6-C6 and C'6-B6 and B'6-A6 and A'6-F5 and 
F'5-E5 and E'5-D5 and D'5-C5 and C'5-F4 and F'4-E4 and E'4-G3 and G'3-F3 
and F'3-G2 and G'2-F2 and F'2-G1 and G'1-F1 and F'1-G1' and G'1'-F1' and 
F'1'-G2' and G'2'-F2' and F'2'-G3' and G'3'-F3' and F'3'-F4' and F'4'-E4' 
and E'4'-F5' and F'5'-E5' and E'5'-D5' and D'5'-C5' and C'5'-D6' and 
D'6'-C6' and C'6'-B6' and B'6'-A6' and A'6'-B7' and B'7'-A7' and A'7'- 
RING 27: 
B5 and B'5-A5 and A'5-D4 and D'4-C4 and C'4-B4 and B'4-A4 and A'4-E3 and 
E'3-D3 and D'3-C3 and C'3-E2 and E'2-D2 and D'2-E1 and E'1-D1 and D'1'-E1' 
and E'1'-D1' and D'1'-E2' and E'2'-D2' and D'2'-E3' and E'3'-D3' and 
D'3'-C3' and C'3'-D4' and D'4'-C4' and C'4'-B4' and B'4'-A4' and A'4'-B5' 
and B'5'-A5' and A'5'- 
RING 26 
B3 and B'3-A3 and A'3-C2 and C'2-B2 and B'2-A2 and A'2-C1 and C'1-B1 and 
B'1-C1' and C'1'-B2' and B'2'-A2' and A'2'-B3' and B'3'-A3' and A'3'. 
CENTRAL T 25: 
A1 and A'1 - A1' and A'1'. 
For a "right hand" microangulation the elements need to be energized in the 
reverse order. The simultaneously selected elements are those which are 
interconnected in the probe head, as mentioned above. 
So 35 delays are counted for the outer ring 28, 25 delays for ring 27, 11 
delays for ring 26 and one for the central part 25, i.e. a total of 72 
delays. 
The values of these delays depend on the desired microangulation. Use may 
therefore be made of a set of programmable delay lines and a switching 
matrix for associating the elements concerned (for a ring configuration) 
with the delays which are assigned thereto. This arrangement will be 
described further on. The calculation of the delays is within the scope of 
a man skilled in the art. They correspond simply to the compensation of 
the different propagation times of the ultrasounds emitted from different 
elements so that the wave front in the direction of the desired 
microangulation benefits from a good phase concordance between the 
contributions of the transducer elements. 
One possible example of an echography apparatus capable of operating with 
the above described probe will now be described. This apparatus comprises 
a first group 30 of delay lines (These lines provide a few relatively long 
delays, intended to be applied between the rings), a grouping matrix 31 
for associating the delays of group 30 with the different rings, a second 
group 32 of programmable delay lines (72 in number according to the 
example if FIG. 3) and a switching matrix 33 interconnected between the 
delay lines of group 32 and the different transducer elements (grouped 
together symmetrically in pairs) of the mosaic. The system further 
comprises a summing amplifier 34 grouping together the reception signals 
at the outputs of the delay line group 30 as well as at an independent 
access of matrix 31 (connection 31a) corresponding to the outer ring to 
which no delay is applied at this level. An ultrasonic signal emitter 35 
is also connected to the delay lines of group 30 and to connection 31a. 
The system described uses then the delay lines and the matrices 31 and 33 
not only for emitting but also for reception but a variant could be 
envisaged in which these matrices and delay lines would be used only for 
reception and where the emission delays would be provided by a control 
logic coupled to a plurality of emitters, each emitter being directly 
connected to a pair of symmetrical transducer elements. 
The switching matrix 33 may be formed from an assembly of analog 
multiplexers connected in cascade, such that any pair of the transducer 
elements of the mosaic may be connected to any delay line of group 32. If 
we refer again to the preceding example, matrix 33 will comprise 210 
accesses on the probe side and 72 accesses on the delay line group 32 
side. Groups of analog multiplexers of the DG507 type, commercialized by 
SILICONIX, could for example be used connected in cascade. Each of these 
units comprises 16 analog switches connected together so as to have 16 
inputs and a common output. Switching of the switches is controlled by an 
integrated decoder, with five inputs, receiving coded digital information. 
For each access of delay line group 32, a first stage of such units may be 
provided in number sufficient for connection to all the pairs of 
transducer elements, assembled in groups of 16, and a second stage (a 
single unit) combining at its inputs the outputs of the first stage, the 
output of the second stage being connected to one of the delay lines of 
group 32. 
These latter are programmable, that is to say that the value of the delays 
may be modified. A basic structure of such a delay line is shown in FIG. 
5. It is subdivided into two lines 36, 37 with multiple outputs (for 
example eight), each output corresponding to a predetermined delay. Line 
36 supplies a range of "short" delays whereas line 37 supplies a range of 
"long" delays. Two analog multiplexers 38 and 39 with eight inputs and one 
output have their inputs connected respectively to the outputs of lines 36 
and 37. The output of multiplexer 38 is connected to the input of line 37. 
The structure of the grouping matrix 31 is very simple. 
Its role is in fact only to "recognize" the elements belonging to the 
different rings. It is therefore only a static grouping matrix, which 
determines four groups among the accesses to the delay lines of group 32 
and connects three of them to the three delay lines of group 30, 
respectively and the fourth to the summing amplifier 34 and to the 
ultrasonic emitter 35. The delay lines of group 30 do not need to be 
programmable. 
The delay lines are programmed at each emission-reception sequence by 
adding a delay value to a line 36 and a delay value to a line 37, and so 
on for each of the 72 programmable delay lines of group 32. These delay 
values depend on the desired microangulation. The role of matrix 33 is to 
select all the elements corresponding to a given position of the ring 
configuration on the mosaic and to "associate" them with the different 
delays. 
For that, the apparatus is completed by a programmable memory 40 (PROM) 
into which the program for addressing matrix 33 and the delay line group 
32 is written once and for all. Sequencing of the reading of this memory 
is controlled by a microprocessor 41 which also controls switching on of 
the emitter 35 (pilot connection 42). Amplifier 34 adds together the 
signals representative of the echos received and to which the same delay 
laws as at emission have been applied (focusing at reception). The output 
signals of amplifier 34 (output S) are processed, more especially 
"windowed" before being used as video signals in a television receiver on 
which the image is reconstituted line by line. 
Memory 40 contains all the orders for successive addressing of matrix 33 
and the delay line group 32 for complete scanning of the ring 
configuration on the surface of the probe. In other words, an 
emission-reception sequence is generated after positioning of the analog 
multiplexers of the matrix 33 selecting the position of the ring 
configuration on the mosaic and after programming the different delay 
lines of group 32, depending on the desired microangulation value. Matrix 
33 remains in this state for nine firings (four microangulations to the 
right, four microangulations to the left and one normal to the surface). 
The delays are modified, still by partial reading of memory 40 after each 
firing. Then memory 40 drives the switching matrix 33 so as to cause the 
ring configuration to progress in the direction of the sweep, by a 
distance corresponding to the width of a transducer element and the 
microangulation sequence begins again. These operations are renewed until 
a complete image of 144 lines has been acquired in a complete sweep.