the apparatus for scanning by means of ultrasonic echography as shown in fig1 comprises , in a conventional manner , an ultrasonic transducer 10 which is connected on the one hand to a transmission stage 20 and on the other hand to a receiving and processing stage 30 , as well as a mechanical scan control device 40 for the transducer . instead of this single transducer there is preferably used an array of transducers which are then connected to an electronic scan control device . the transmission stage 20 comprises an electric excitation signal generator whose signals are applied to the transducer 10 which , in known manner , converts the signals into periodic trains of ultrasonic pulsed signals , transmission being controlled by clock signals supplied at a recurrent frequency f = 1 / t , for example in the order of 5 khz , determined by an internal sequencer of the transducer 10 . in accordance with the invention , the transmission stage 20 comprises first means for transmitting the trains of pulsed signals with two alternating , neighbouring recurrent periods t 1 and t 2 . these first means , whose concept per se is known to those skilled in the art , are formed , for example by a clock generator 21 which applies a squarewave signal of the frequency 1 / t 0 , for example 1 / t 0 = 27 . 5 khz , to a logic circuit 22 , the two outputs of which are connected to two counters 23 and 24 , respectively . the counters 23 and 24 are coupled end - around and the first counter 23 counts to n and the second counter 24 counts to p ( for example , n = 5 and p = 6 ). the output of the counter 23 is connected to a second input 25 of the logic circuit 22 and the output of the counter 24 is connected to a third input 26 of the circuit 22 . a pulse received on the input 25 deactivates the counter 23 so as to activate the counter 24 , and a pulse received on the input 26 has the reverse effect . on the other hand , each of the outputs of these counters are connected to an input of an exclusive - or gate 27 , the output of which is connected to the input of a generator 28 for generating electric excitation signals . the counters 23 and 24 are , for example of the type 74f 160a or 74f 168 manufactured by philips . the output signal of the generator 28 is applied , via a separator 29 , to the transducer 10 which converts those signals into periodic trains of ultrasonic pulsed signals comprising from 4 to 10 pulses . the separator 29 between the transmission stage 20 and the receiving and processing stage 30 , inserted between the transducer 10 , the generator 28 and said stage 30 in a customary manner , prevents the overloading of the receiving circuits by the transmission signals . for the values chosen as described above : t 0 = 36 . 4 μs , n = 5 and p = 6 , the values : nt 0 = 182 μs ( 1 / t 1 = 5 . 5 khz ) and pt 0 = 218 μs ( 1 / t 2 = 4 . 58 khz ) are obtained for the alternating recurrent periods of the pulses . these period values are to be compared with that currently adopted for t in the prior art : t = 200 μs ( 1 / t = 5 khz ). the device 40 is controlled by a clock signal which is applied to an input 41 and which has a frequency of , for example 1 khz . in the most frequent case where the member 10 is a transducer array , the device 40 serves to produce , upon transmission , a distribution rule for distribution from one elementary transducer to another so as to focus the ultrasonic wave in the depth direction in conformity with a straight line which is substantially perpendicular to the medium ( the body ) scanned . the receiving and processing stage 30 in known manner comprises , connected to the output of the separator 29 , and rf amplifier ( not shown ) with gain compensation as a function of the depth , followed by a processing channel which comprises a cascade connection of a fixed echo suppression circuit , a digital sample memory , a correlation - interpolation circuit , and a validation circuit . in accordance with the invention , this processing channel ( shown in fig2 ) has given specific aspects . the fixed echo suppression circuit 31 , always required because of the very high amplitude of the echos from the blood vessel walls , should include second means adapted to double the recurrent period t 1 and t 2 as will be described hereinafter with reference to the fig5 and 6 . the output of the circuit 31 is not a single output as in the prior art , but a double output and for input samples s i , the output samples wherefrom fixed echos have been removed are not samples d i but even output samples , for example on one output , so d 2k ( for i = 2k ), and odd output samples on the other output , so d 2k + 1 ( for i = 2k + 1 ). thus , on one output samples appear which relate to the period t 1 and on the other output samples appear which relate to the period t 2 . the memory 32 for storing samples ( fig2 ) is a double memory which , for a given pulse train , stores in a first half the samples d i of the even excitations which are associated , for example with the recurrent period t 1 , and in a second half the samples d i of the odd excitations which are associated with the recurrent period t 2 . the time diagram of fig3 diagrammatically illustrates the transmission of a pulse train . along the time axis there is plotted the instant which marks the start of each pulse , said instant being denoted by the rank number of the pulse within the train : 0 , 1 , 2 , . . . , i , . . . or , on the basis of the parity : 0 , 1 , 2 , . . . , 2k , 2k + 1 , . . . . for example , as is shown , the pulses of even rank mark the start of a period t 1 between two successive pulses , and those of odd rank mark the start of a period t 2 . according to the prior art , each train comprises approximately n = 10 pulses ( 10 activations ) which serve to form a mean value over n - n values for each velocity point , n being the number of coefficients of the filter for eliminating fixed echos . in conformity with the invention , given that said mean value is formed twice over n / 2 - n values as will appear from the following description , it is advantageous to increase the number n to , for example 15 . however , the value of n may not be increased too much , because otherwise the image rate is decreased for the case where the measured velocity profile is displayed on a monitor . the correlation - interpolation circuit 33 operates on the basis of 1 - bit correlation as described in detail in the cited french patent application 2 590 790 . in accordance with the invention , the circuit 33 comprises two 1 - bit correlator assemblies . the known correlation circuit comprises , in a conventional manner , a delay line with a delay of one recurrent period t , enabling reception of two consecutive signals d i ( t ) and d i + 1 ( t ) at the same time . a correlator assembly in the circuit 33 processes , using a delay line with a delay of one recurrent period t 1 , two consecutive even ( or odd ) signals d 2k ( t ) and d 2k + 2 ( t ) and the other correlator assembly processes , using a delay line with a delay of one recurrent period t 2 , two consecutive odd ( or even ) signals d 2k + 1 ( t ) and d 2k + 3 ( t ), i . e . one correlator processes , for a first correlation function , two successive rows of the first half of the memory 32 , each row corresponding to one activation , whereas the other correlator processes , for a second correlation function , two successive rows of the second half . in order to apply sample signals to the circuit 33 , the addressing and reading of the memory 32 may be sequential , using a single data output , by reading a row of a given rank of the first half , followed by the reading of the same rank of the second half , as shown in fig2 or even simultaneous on two outputs , each of which extracts one half of the memory . in each correlator , 2i + 1 delay lines shift one of the two signals with respect to the other signal by an amount u j = jδt , j being an integer number taking the values - i , - i + 1 , . . . , - 1 , 0 , 1 , . . . , i - 1 , i , and δt being the sampling step , for example 50 ns . finally , 2i + 1 correlators supply 2i + 1 sample values of each correlation function , thus defining a correlation window so that : an example of such correlation functions is given in the fig4 a and 4b , where i is greater than 4 . on the basis of these two functions , a desired velocity point is obtained for the value of the abscissa u o = τ , which corresponds to the maximum correlation peak ( principal peak ), which value is theoretically the same for the two functions . in practice , each correlation function is stored and averaged , so as to achieve better immunity against noise , with the subsequent functions which correspond to the subsequent pairs of activations for the same depth z o associated with t o . however , for several reasons it may occur that the principal peak is not very distinct from secondary peaks : first of all , the correlation function is centered as regards frequency and oscillates in time with the mean frequency of the echographic signal . on the other hand , in the presence of noise , as is always the case , the signals are decorrelated and the correlation peak value may decrease to the same order as that of the secondary peaks created by the oscillation of the correlation function . moreover , the sampling of the calculation of this function may also induce an ambiguity as regards the choice of the maximum sample before the interpolation , enabling the maximum of each correlation peak to be obtained . in accordance with the invention , the search for the correlation peak is not restricted to a zone having the width of the period of the echographic signal ( as expressed by the above relation ( 2 )) and comparison of the correlation functions of the fig4 a and 4b enables determination , with certainty , of the principal correlation peak from among several apparently possible peaks , as will be described hereinafter . the two correlation signals of the fig4 a and 4b have the same period as the transmitted signal , but correspond to different pulse repetition frequencies ( prf ). consequently , the principal correlation peaks in each signal correspond to different time shifts ( abscissa u ), so : τ 1 = 2 v / c t 1 and τ 2 = 2 v / c t 2 , v being the velocity value , searched . the scale of the time shifts between the two correlations is unified by extending them by a factor p for the first correlation ( abscissa u &# 39 ;) and n for the second correlation ( abscissa u &# 34 ;), said correlations being superimposed as shown in fig4 c . thus , each signal contains a correlation peak which has been shifted with respect to o by a period : u &# 39 ; o ˜ u &# 34 ; o ˜ 2 v / c np t o wherefrom the velocity v can be readily deduced . the selection of this peak is performed by retaining the peak which is common to the two correlations , the other peaks having been shifted by the expansion . preferably , in order to ensure that only the significant peaks are considered , a thresholding to 50 % of the maximum is imposed in advance on the correlation functions . in order to ensure that the peaks other than the principal peaks are all shifted two - by - two , it is necessary that the numbers n and p are mutual prime numbers . actually , all possibilities of measurement of τ 1 and τ 2 are expressed by : where τ 1 and τ 2 denote the estimated values of τ 1 and τ 2 , respectively , and q , r are integer numbers . by replacing τ 1 and τ 2 by their values in the preceding relations , the following equation is obtained : in order to ensure that this equation has only one solution ( except for the multiples of q and r ), it is necessary that : the modulo to be applied to τ 1 and τ 2 is , therefore , n / f o and p / f o , respectively . therefrom there is deduced ( for example , on the basis of τ 2 ): ## equ5 ## from comparison with the relation ( 2 ) it appears that the limit velocity which can thus be measured no longer relates to either t 1 or t 2 ( as it does to t in the prior art ), but relates to t 0 , so a limit velocity multiplied by n ( n being the smaller one of the two numbers n and p ). the right - hand part of the diagram of fig2 relates to a conventional situation and is described in french patent application 2 662 265 in the name of applicant . after determination of each value of the mean velocity of the profile analysed , this value is applied , via a conductor 34 , to a validation circuit 35 which is also referred to as a segmentation circuit and which receives directly from the circuit 33 , via a conductor 36 , the averaged value of the principal correlation peak and , via an amplitude calculation circuit 37 , an estimate of the amplitude ( energy ) of the signal for the point of the profile considered . the velocity value is validated and supplied at the output of the circuit 35 only if the values of the two other parameters ( correlation and amplitude ) exceed predetermined thresholds . the elimination of the fixed echos is customarily performed by forming successive differences or more complex combinations . in the case of a recurrent transmission with a period t , the successive reception signals are subjected to the following treatment : s 1 being delayed by t with respect to s 2 which is the signal received , or in a more refined manner : d . sub . 1 = 2 s . sub . 1 - s . sub . 2 - s . sub . 3 ( three coefficients : 2 , - 1 and - 1 ), s 1 having been delayed by 2t and s 2 having been delayed by t with respect to s 3 which is the signal received . in these two cases , each of the delay lines has a delay equal to the recurrent period of the transmission t . it is to be noted that for these two examples , if the relation : this means that the value of τ can be extracted by time correlation of the signals d in which the common ( fixed ) parts of the signals s have been eliminated . in accordance with the invention , this step for the suppression of fixed echos must be adapted because a double recurrence is used . in the signal transformation to be performed , it is necessary to preserve the recurrence relation of the delayed signals with respect to one another . it is to be noted that a periodicity , referred to as a general periodicity , exists in the transmission diagram of two alternating recurrences , which periodicity is equal to : t 1 + t 2 as indicated in fig3 and also that the fact that overlapping occurs between one overall period and the next overall period complicates but not obstructs the elimination of the fixed echos . as appears from fig5 the new signals d wherefrom the fixed echos have been removed are generated as follows in the case of a single difference ( two coefficients ): the case involving three coefficients , for example ( 2 , - 1 , - 1 ), is shown in fig6 :
an echograph includes a transmission stage , a circuit for suppressing fixed echos , and a correlation - interpolation circuit . the the transmission stage transmits a pulsed signal having two neighboring , alternating recurrent periods t 1 and t 2 , the fixed echo suppression circuit being adapted to double the recurrent period and comprises two outputs having the index even and odd , the correlation - interpolation circuit performing , for each velocity , two distinct correlations , one of which is associated with the period t 1 whereas the other is associated with the period t 2 , after which these two correlations are compared so as to derive a non - ambiguous velocity value therefrom .