Image display process by ultrasounds from an alignment of transducer elements

A process of utilizing ultrasound for acquiring data representative of the internal portions of a structure and the display of the data thus obtained. This process includes the emitting of sequences of ultrasound to the structure under consideration and receiving therefrom corresponding echoes. For each sequence two groups of transducer elements are selected for generating ultrasound beams of possible N different focal characteristics sequentially. The ultrasound beams alternate from one group to the other except when a given beam corresponds to the most distant focal zone. In that case, the succeeding emission is carried out without alternating, i.e., the emission is generated from the same group.

BACKGROUND OF THE INVENTION 
The invention concerns an image display process which utilizes ultrasounds 
from an alignment of transducer elements; it concerns, more particularly, 
a sequential arrangement of shots of ultrasound beams to a structure to 
obtain the best possible compromise between the quality of the image thus 
obtained and the rapidity with which the process is achieved. 
BRIEF DESCRIPTION OF THE PRIOR ART 
It is known that an image along a chosen section of a structure can be 
formed by generating a succession of ultrasound beams from an emitting 
strip subdivided into a plurality of transducers comprising piezo-electric 
elements, disposed side by side, and by collecting and processing the 
echoes sent back by the structure towards the same elements. Thus, the 
transducers are used sequentially for emitting and receiving. The 
sequential emitting and receiving consists in generating the ultrasound 
beams in such a way as to explore the structure line by line and to use 
the echoes received, or a portion thereof, as video signals of a 
television display. In order to obtain a suitable lateral resolution 
(resolution in the direction parallel to the strip), it is necessary that 
each beam emitted has a section as small as possible and that this section 
is as constant as possible along all the exploration depths. This 
difficulty was partially overcome by generating, at each time, each beam 
by exciting a group of adjacent elements with a predetermined delay law; 
that is, the two extreme elements being excited first, the two adjacent 
elements are excited next, and so on up to the central elements of the 
group. The delay law chosen allows the process to obtain a beam narrow 
enough on a certain part of its length to give rise to a good lateral 
definition, at least along one part of the required exploration depth. It 
is also known that an image of a structure can be formed by varying the 
focal pattern of the beam at emission for a given shot. This can be 
advantageously done by decomposing the plane section into several parallel 
bands, or focal zones, and causing the bands (focal zones) to correspond 
to the emission the ultrasound beams having the respective optimal focal 
characteristics for these zones. Thus, if the cut to be visualized is 
divided into four focal zones, it will be necessary to generate for each 
line of the final image four beams having different focal characteristics; 
that is, the one line of the final image is reconstituted from the echoes 
corresponding to the "useful" portions of the narrow sections of the four 
beams having been emitted according to the direction of the line. It is 
understood that in these conditions, acquisition time of the image is 
multiplied by the number of focal zones chosen. In other words, as this 
acquisition time is dependent on the propagation speed of the ultrasonic 
waves in the structure, it is necessary to await the time necessary for 
receiving the echoes between two consecutive shots. To increase the rhythm 
of the images, it was proposed in European patent application No. 0 031 
510 that ultrasonic waves not be emitted at a constant period but rather, 
be emitted immediately after acquisition of the echoes issuing from the 
focal zone previusly explored. This process has the drawback of an 
important recurrence diaphony during the successive shots, i.e. bursts, of 
beams since the echoes corresponding to one given shot hereinafter to be 
interchangeable with burst are still very significant at the moment of 
acquisition of the echoes of the following burst, and consequently, the 
former bursts are superimposed on the later shots. Thus, in practice, it 
is thus necessary to wait a supplementary time interval between the 
bursts. This causes a decrease of the rhythm of the images with respect to 
what could be hoped for from such an image formation process. 
Furthermore, efforts were made to reduce the recurrence diaphony problems 
by proceeding to the acquisition of the image by ultrasound bursts 
separated from one another in the section, for example, by selecting two 
groups of transducers to generate two beams separated by about half the 
length of the strip and by acquiring the useful echoes from focalized 
bursts carried out alternately from two groups. Such a process is 
described, for example, in French patent application No. 81-12 843 filed 
in the name of the applicant. 
SUMMARY OF THE INVENTION 
The present invention proposes, among others, an advantageous combination 
of earlier solutions set out hereinabove, as well as a sequential 
arrangement particularly allowing the simplifying of the control software 
and switchable means necessary to suitably excite the different transducer 
elements while minimizing the time losses due to the commutations between 
the two groups of selected elements. 
In this spirit, the invention concerns a display process which includes the 
emitting of sequences of ultrasound bursts to a structure and receiving 
therefrom corresponding echoes. The emitting and receiving are performed 
from an alignment of transducer elements coupled to a structure. The 
display process, for emitting, includes the generating of predetermined 
sequences of focalized beams from, each time, two groups of transducer 
elements selected respectively in two sections of the alignment, the 
groups differing by at least one element at each sequence in such a way as 
to realize a sweeping of such beams. Each sequence comprises a 
predetermined number and series of emitting beams from elements of the two 
selected groups. The beams are shaped according to N possible focal 
patterns associated respectively to N areas or focal zones of the 
alignment, N being a chosen integer, wherein the beams are essentially 
generated alternately from one group to the other except when one given 
beam corresponds to the focal zone positioned the furthest from the 
alignment. 
Thus, when the acquisition of the useful data is proceeded with the 
furthest positioned focal zone, it is possible to emit the following beam 
from the same group. Preferably, each emission corresponding to the 
furthest positioned focal zone is followed by an emission of substantially 
the same direction but with a focal pattern corresponding to the focal 
zone adjacent to the same furthest positioned focal zone. 
Another important data of the problem is the number of lines constituting 
the image. It is obvious that in order to improve the sharpness of the 
image it is necessary to increase the number of lines. A first solution 
consists, for a strip of a given length, in increasing the number of 
piezo-electric transducer elements. Given that these elements are obtained 
by cutting ceramic with a fine saw, it is obvious that the reduction in 
length of the piezo-electric elements encounters difficulties imposed by 
technological limits. In order to increase the number of lines, it has 
been proposed to emit beams not according to a direction perpendicular to 
the alignment of the transducers but in directions inclined in the section 
plane and symmetrical with respect to this normal direction. This process 
is, for example, described in U.S. Pat. No. 4,070,905. Thus, a single 
group of elements is capable of obtaining several adjacent lines of the 
image. The invention also concerns the incorporation of a technique known 
as "microangulation" in the sequential burst arrangement process. This 
means that, for each group of elements chosen at each sequence, bursts are 
emitted during the sequence, according to the two possible directions, 
substantially symmetrical with respect to a normal direction of the 
alignment and forming with it a small angle. It is advantageous to combine 
this microangulation process with the focalization mode of the beams since 
the microangulation is carried out by the same means as focalization, i.e. 
by consequently adapting the aforesaid delay laws between the elements of 
a selected group. On the other hand, the microangulation technique is 
improved by the fact that the emitting angle is that much smaller as the 
corresponding focal zone is distanced further from the alignment of 
transducers.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 represents an echography sonde formed essentially of an alignment 11 
of piezo-electric transducer elements 12. This sonde is placed in front of 
the structure the section of which it is required to observe. The two 
halves of this section represent the general aspect of the ultrasound 
beams that can be emitted from the strip with different focal patterns. It 
will be noted that these beams only have the required fineness on one part 
of their length; that is, the required fineness is substantially smaller 
than the depth d of the image required. This is the reason why the section 
to be displayed has been cut into N areas (N=4, in the example) I, II, III 
and IV in the form of bands parallel to alignment 11 and more and more 
distanced, respectively, from it. These areas will be called "focal zones" 
hereinafter and the focal zones will always be numbered in increasing 
order from this strip. As it is known to give the beam a required focal 
pattern N focal dispositions are associated to N focal zones, 
respectively, in such a way that, as much as possible, the useful portion 
of the narrow section of the beam is situated in the corresponding focal 
zone. Thus, on FIG. 1, the ultrasound beam to the left of the drawing is 
the beam which has been selected for the first focal zone I, while the 
right-hand beam is that which has been selected for the fourth focal zone 
IV. The manner of generating such beams is known. It is necessary to 
excite not one, but a group of adjacent elements 12 with some 
predetermined delays between the elements. 
Thus, FIG. 2a shows the delay law between the elements of a group of 
sixteen (the delay sigma being expressed as ordinate) in order to generate 
a beam perpendicular to the alignment 11. First of all, extreme elements 1 
and 16 of the group are simultaneously excited. Next, elements 2 and 15 
are excited simultaneously. Following that, elements 3 and 14 and so on up 
to two elements 8 and 9 of the center of the group are successively 
excited simultaneously. Furthermore, the number of elements to be excited 
in a group in order to obtain a given focal disposition is different 
according to the focalization depth required, i.e. according to the focal 
zone involved. As a general rule, in each group, a beam having a greater 
number of transducer elements (and an associated delay law) is generated 
corresponding to a focal zone distanced apart from alignment 11 than when 
a beam is generated corresponding to a focal zone close to this same 
alignment. In this spirit, the number of elements of a group in the 
context of the present description is the number of elements necessary to 
generate a beam associated to the most distant focal zone. Thus, FIG. 1 
shows, marked in black, two groups of sixteen selected elements G and G' 
(for a given operating sequence that will be explained hereinafter) in two 
respective sections A, B of the alignment 11 each representing one half of 
the strip. Group G is used, according to the example, to generate a beam 
corresponding to a focal zone I; and in this case, only the eight central 
elements are effectively excited with suitable delays. On the other hand, 
group G' is used, according to the example, to generate a beam 
corresponding to a focal zone IV; and in this case, sixteen elements of 
the group are effectively excited with another adapted delay law. 
Therefore, in the specific example of FIG. 1, a selected group G or G' is 
always constituted by sixteen elements; and as N=4 (number of focal 
zones), four respective focal dispositions of the beam are associated to 
these four zones with sixteen elements being used to generate the beam 
associated to the fourth focal zone, twelve elements to generate the beam 
associated to the third focal zone, and eight elements to generate the 
beams respectively associated to the first and second focal zones. The 
elements that are not used for a particular focal disposition are those of 
the ends of the group so that the elements in use always form an assembly 
of adjacent elements. 
Furthermore, with the aim of doubling the line densities for the same 
number of transducer elements, the ultrasound bursts along two possible 
directions from each group of elements G, G' selected for a determined 
sequence were conducted. More specifically, the two possible directions 
are substantially symmetrical with respect to the direction normal at the 
strip and define a small angle (.+-..alpha..sub.1 or .+-..alpha..sub.2) 
with respect to it. Angle .alpha. is small enough (it was deliberately 
accentuated on FIG. 1) so that upon display of the corresponding echoes, 
it is admissible that these echoes be displayed along two adjacent lines 
of the television receiver on which the image is reconstituted. This 
microangulation can be obtained by simple modification of the delay law 
defining the focal pattern. 
FIG. 2b illustrates the transformation of the delay law of FIG. 2a in order 
to obtain a beam of substantially the same focal pattern emitted with a 
small angle .alpha. with respect to a direction normal to alignment 11. It 
will thus be understood that the microangulation of the beams is 
technologically realized with the same means as those that determine the 
required focal disposition, namely in practice, adjustable delay lines. 
Each time that two groups such as G and G' have been selected, a burst 
sequence the number of which is equal to four times the number of the 
focal zones (this by reason of the above-mentioned microangulation) is 
ordered. The echoes recovered from this sequence reconstitute four lines 
of the image localized by pairs and situated respectively opposite 
sections A and B of alignment 11 at the center of the selected groups. Any 
new sequence is initialized by changing one element, each time, in each 
group in such a way as to sweep progressively all the strip. Thus, for 
example, for the sequence that follows for which groups G and G' are 
selected, it will be necessary to remove the element furthest to the left 
of each group (looking at FIG. 1) and to add one element to the right of 
each group in order to define the two new groups involved for this new 
sequence, and so on until the groups are displaced along the length of a 
half strip. 
According to one important characteristic of the invention, the succession 
of bursts in a sequence is such that the beams are generated essentially 
alternately from one group to another, except when one given beam 
corresponds to a focal zone IV. When that occurs the succeeding shot is 
carried out from the same group and, according to a preferred sequence 
embodiment, each emission corresponding to the focal zone IV is followed 
by the emission of a beam of substantially the same direction but with a 
focal pattern corresponding to focal zone III. 
Of course, this process is valid with or without microangulation, i.e. the 
sequence is foreseen so as to allow acquisition of two lines (shots 
perpendicular to the alignment) or four lines (shots .+-..alpha. on either 
side of the normal). For example, in the case of an image reconstruction 
of four focal zones a possible succession of shots is set out. Consider 
FIG. 3, where, for the sake of clarity, only the direction of the bursts 
(microangulation .+-..alpha.) has been represented and where the ordered 
numerals of the bursts are indicated in each corresponding focal zone near 
the "useful" section of the associated beam. The characteristics of each 
beam emitted by the letter G or G' with subscript can thus be resumed. In 
this case, G or G' designates the group from which the beam is emitted, 
the first subscript indicates the row of the focal zone from the alignment 
and the second subscript (a or b) indicates the sense of the 
microangulation, i.e. the beams of substantially the same orientation in 
each group. Consequently, if the subscript a is allocated to a 
microangulation directed towards the left of FIG. 3 and the subscript b to 
a microangulation directed towards the right, the succession of bursts in 
a sequence is the following: 
G.sub.1a, G'.sub.4a, G'.sub.3a, G.sub.1b, G'.sub.4b, G'.sub.3b, G.sub.4a, 
G.sub.3a, G'.sub.1a, G.sub.4b, G.sub.3b, G'.sub.1b, G.sub.2a, G'.sub.2a, 
G.sub.2b, G'.sub.2b. 
FIG. 4 schematizes in the same way an optimal strategy of shots in the case 
where N=3 and in this case, each sequence comprises the following shots: 
G.sub.1a, G'.sub.3a, G'.sub.2a, G.sub.1b, G'.sub.3b, G'.sub.2b, G.sub.3a, 
G.sub.2a, G'.sub.1a, G.sub.3b, G.sub.2b, G'.sub.1b. 
It appears that a more rapid acquisition of the image can be obtained by 
diminishing the number of focal zones since four lines of a sequence are 
acquired with simply twelve bursts against sixteen previously. The 
counterpart and this rapid acquisition is obviously a lesser lateral 
definition in certain areas of the image, or a smaller exploration depth. 
Furthermore, FIG. 5 schematizes a strategy of bursts according to the 
principle of the invention in the case where N=2, each sequence thus 
comprises the following bursts: 
G.sub.2a, G.sub.1a, G'.sub.2a, G'.sub.1a, G.sub.2b, G.sub.1b, G'.sub.2b, 
G'.sub.1b. 
This strategy of bursts revealed the most efficient sequence in order to 
allow acquisition of the image as rapidly as possible at the given 
diaphony rate. Indeed, other than the time separation of the bursts, a 
spatial separation is realized at about half the length of the strip. 
Nevertheless, according to the invention, there is a derogation from this 
general principle when proceeding with the acquisition of a portion of 
line in the most distant focal zone. Indeed, it was found that the echoes 
created beyond this last focal zone have undergone such an attenuation 
that they cannot interfere with other echoes corresponding to beams 
subsequently generated and focalized on other focal zones. This is the 
reason for which, after having acquired the echoes corresponding to a last 
focal zone along a given direction, it is possible to acquire immediately 
the echoes corresponding to the next focal zone in the same direction, 
thus without passing from one group of elements to the other. This permits 
the saving each time (i.e. four times per sequence) of commutation time 
from one group to the other, with only the delay law being modified. Under 
these conditions the known emission recurrence principle with a much lower 
diaphony rate can be applied, especially in view of European patent 
application No. 0 031 510 where a given beam is emitted after a time 
shorter than the time necessary for an echo, formed at the limit of the 
most distant focal zone, to return to alignment 11 to be collected. In 
most cases, each new beam can be emitted at the end of an interval of, 
theoretically, the shortest time, i.e. at the end of the time necessary 
for the return of an echo formed at the limit of the focal zone 
corresponding to the preceding beam. In practice, it will always be 
possible, except if this focal zone is the closest to the alignment 11 
(zone I). In this case (i.e. after the shots G.sub.1a, G.sub.1b, G'.sub.1a 
and G'.sub.1b, for N=4) it will often be necessary, in order to conserve a 
low diaphony rate, to delay by a predetermined supplementary time interval 
the moment of the emission of the beam that follows the emission of a beam 
corresponding to the closest focal zone. This supplementary time will be 
however less long, other things being equal, by reason of the spatial 
separation of the consecutive beams generated successively from two 
selected groups G and G'. Return times are, in all cases, easy to 
determine since they depend on known parameters such as the propagation 
speed of the ultrasounds in the structure to be displayed (function of the 
ultrasound frequency chosen) and the limits of the different focal zones. 
It should be noted from this point of view that the boundaries of the 
different focal zones can only be taken into consideration as data at 
reception while only retaining as significant the signals received between 
the two instants which corresponding respectively to the return times of 
the echoes formed at the limits of the focal zone chosen (temporary 
windowing). 
According to another characteristic of the invention, the microangulation 
value is different in function for the focal zone involved. More 
specifically, the microangulation is smaller as the corresponding focal 
zone is positioned further from alignment 11 (.alpha.1&gt;.alpha.2, cf, FIG. 
1). Indeed, as the image is reconstituted from a sweeping of parallel 
lines on the displaying screen, it is preferable to adapt per each time, 
the microangulation to the distance of the focal zone. This is to avoid 
interference of the corresponding image at the overlapping of the data 
collected, by ensuring that the emitting beam strays as little as possible 
from the direction normal to the alignment in its "useful" portion. For 
example, for each focal zone it is possible to choose a value of the 
microangulation, exemplified by converging shots (of types a and b) in two 
directions from two adjacent groups shifted by only one transducer 
element, crossing only at the limit or beyond the focal zone. 
FIG. 6 shows the general arrangement of an image system intended to operate 
the process defined hereinabove. Each transducer element 12 is connected 
to an emitter 30, known per se, transmitting at each excitation, an 
impulse of duration corresponding to the half-period of the ultransonic 
frequency chosen. The transducer element is further coupled to controlled 
receiving circuit 32 (controlled analog switches, for example). Since the 
selected groups are formed of sixteen elements, sixteen control circuits 
34 are required. Each of the control circuits, when addressed, energizes 
certain transducer element with a predetermined delay. Under these 
conditions, for a strip of 79 elements (according to an embodiment 
currently preferred), it is possible to associate five transducer elements 
to each control circuit 34. For example, on FIG. 6, the first, 
seventeenth, thirty-third, forty-ninth and sixty-fifth elements have been 
regrouped. The five corresponding emitters 30 are connected to the outputs 
of a demultiplexer 36 and the five inputs of the receiving control 
circuits 32 are connected to the outputs of a demultiplexer 38 (the 
connections of control circuits 32 are shown in broken lines). The 
identical signal outputs of the five receiving circuits 32 are all 
connected to the input of a preamplifier 40. The demultiplexers are 
addressed in order to excite one emitter among five at each shot and to 
validate simultaneously and solely the corresponding receiving circuit 32. 
Each control circuit 34 further comprises shaping means 42 (trigger, for 
example) of a control impulse, which is transmitted by a connection 43. 
The output of the shaping means 42 is applied to the input of an adder 44 
that also receives the output signal of the preamplifier 40. The output of 
this adder 44 is connected to the input of a first delay line 46 with 
eight outputs respectively connected to the eight inputs of a first 
multiplexer 48. The output of this latter is connected to the input of a 
second delay line 50 having seven outputs respectively connected to the 
seven inputs of a second multiplexer 52. The available delays at the 
outputs of line 46 go from 0 to 70 nanoseconds per step of 10 nanoseconds 
while the delays available at the outputs of line 50 are from 0 to 480 
nanoseconds per step of 80 nanoseconds. It is thus possible to obtain any 
delay between 0 and 550 nanoseconds, with a precision of 10 nanoseconds, 
by the simple addressing of multiplexers 48 and 52. The ouput of the 
multiplexer 52 is connected, on the one hand, to an input of an adding 
amplifier SOM, that has as its other inputs the corresponding outputs of 
the other circuits 34, and, on the other hand, to the actuation input 60 
of an impulse generator 62, which elaborates at each shot an excitation 
impulse delayed by a required time interval. The excitation impulse is 
transmitted to one of the five emitters 30 via demultiplexer 36. The 
duration of this impulse (which is equal, as indicated hereinabove, to a 
half-period of the ultrasonic frequency used) is determined by control 
computer means (connection 64). Generator 62 is defined as a kind of 
programmable duration monostable. An address generator 66 controls the two 
multiplexers 48 and 52 (addressing connections 67 and 68). The control 
computer means comprises a program-memory Mp. The instructions 
corresponding to a given sequence resulting from the combination of 
parameters are chosen by the operator from program memory Mp, through the 
intermediary of a control assembly, such as for example a keyboard K 
connected to the processor Pr. The program-memory controls the address 
generator 66 as well as the demultiplexers 36 and 38 of each circuit 34. 
The parameters displayed by the operator are: the frequency of the 
ultrasonic waves, the number of focal zones and their limits. From these 
parameters, the processor organizes a certain number of instructions of 
the program-memory for governing the rhythm of the ultrasonic shots, the 
successive delay laws, the useful portions to be retained for display, the 
wait time between two shots, the gain corrections in terms of the depth of 
the focal zone, taking into account the attenuation of these echoes and 
the values of microangulation, etc. For the excitation of each transducer 
element, the processor emits a burst order (transmitted on the connection 
43) that is delayed in terms of the previous positioning of the 
multiplexers 48 and 52. This order actuates the impulse generator 62, the 
duration of this impulse being controlled by the program-memory Mp via 
coupling 64. At reception, the echoes transmitted by the validated circuit 
32 undergo the same delays (amplifier 40 connected to the input of the 
delay line 46) before being summed by the SOM amplifier, thus realizing a 
"focalization" at the reception. When the elements of the group must be 
inhibited for the focalization in a close focal zone of the strip, the 
address generator 66 grounds the output stage of the multiplexer 52, so 
that generator 62 is not actuated. The output signals of the SOM amplifier 
are processed, especially "windowed", prior to being used as video signals 
of a television receiver on which the image is reconstituted line by line. 
Of course, the present invention is in no way limited to the specific 
process described hereinabove and even less to the positioning of the 
software that has been described, which admits a large number of variants 
available to the man skilled in the art. It comprises all the technical 
equivalents, if these are understood as being comprised in the following 
claims.