CA1300733C - Apparatus for examining a moving object by means of ultrasound echography - Google Patents
Apparatus for examining a moving object by means of ultrasound echographyInfo
- Publication number
- CA1300733C CA1300733C CA000547067A CA547067A CA1300733C CA 1300733 C CA1300733 C CA 1300733C CA 000547067 A CA000547067 A CA 000547067A CA 547067 A CA547067 A CA 547067A CA 1300733 C CA1300733 C CA 1300733C
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- Prior art keywords
- circuit
- echoes
- output
- ultrasound
- calculating
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
- G01S15/88—Sonar systems specially adapted for specific applications
- G01S15/89—Sonar systems specially adapted for specific applications for mapping or imaging
- G01S15/8906—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques
- G01S15/8979—Combined Doppler and pulse-echo imaging systems
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
- A61B8/06—Measuring blood flow
Abstract
PHF 86.585 16 28-7-1987 ABSTRACT:
Apparatus for examining a moving object by means of ultrasound echography.
An apparatus for examining a moving object by means of ultrasound echography, enabling the measurement of the axial speed of said movement or the projection of the speed on the axis z of a beam of ultrasound excitations transmitted by an ultrasound transducer (10) with a repetition period T. The apparatus also comprises a transmission stage (20) and a stage (30) for receiving and processing the echographic signals returned to the transducer (10). The stage (30) for receiving and processing the echographic signals comprises a circuit (330) for esti-mating said axial speed Viz(t) which comprises a circuit (340) for extracting the time shift ? i(t) between two consecutive echoes ei(t) and ei+l(t), constructed for solving the equation in ?i (t):
ei+l(t) =
Apparatus for examining a moving object by means of ultrasound echography.
An apparatus for examining a moving object by means of ultrasound echography, enabling the measurement of the axial speed of said movement or the projection of the speed on the axis z of a beam of ultrasound excitations transmitted by an ultrasound transducer (10) with a repetition period T. The apparatus also comprises a transmission stage (20) and a stage (30) for receiving and processing the echographic signals returned to the transducer (10). The stage (30) for receiving and processing the echographic signals comprises a circuit (330) for esti-mating said axial speed Viz(t) which comprises a circuit (340) for extracting the time shift ? i(t) between two consecutive echoes ei(t) and ei+l(t), constructed for solving the equation in ?i (t):
ei+l(t) =
Description
ll3~0733 `` PI~F 86.585 1 28-7~1987 Apparatus for examining a moving object by means of ultra-sound echography.
The invention relates to an apparatus for exa-mining a moving object by means of ultrasound echography, enabling the measurements of the axial speed of said movement i.e. the projection of the speed on the axis "z"
of a beam of ultrasound excitations which are periodically transmitted by at least one ultrasound transducer with a repetition period "T"~ which apparatus also comprises a transmission stage and a stage for receiving and proces-sing the echographic signals returned to the transducer.
The invention can be very advantageously used for the echographic examination of moving organs, such as the heart~and blood flows The technical problem to be solved by any method and any apparatus for examining a moving object by means 15 Of ultrasound echography is that an exact as possible estimation must be made of the axial speed of the motion studied in order to obtain~ using display devices, exact images of the organs and blood flows subjected to an ultrasound eohographic examination.
For a number of years various solutions to this problem have been proposed, notably pulsed-wave ultrasound Doppler systems which are currently used for measuring the blood flow speeds in a given point, or at least the protec-tion of such speeds on the axis of the beam transmitter by 25 the ultrasound transducersO More recently, apparatus have become available which enable real-time determination of the distribution of the flow speeds along the path followed by the ultrasound wave and even across the sectional plane obtained by wsy of a scanning motion of the transducer.
30 The majority of these systems utili~e the frequency shift or the phase shift of the signal returned by the moving targets in order to derive the axial speed of the blood .' ~
~300733 .... 2 flows therefrom. For example, European published patent appli-cation No. 0~092,841, published on February 11, 1983, relates to such an apparatus, which utilizes the measurement of the phase shift between the successive echoes returned by the moving targets in response to a recurrent excitation.
However, the apparatus for carrying out this known method utilizing the phase shift are restricted by an uncertainty relation which links the axial resolution ~z and the precision of the measurement of the speed ~V/V to the wavelength ~:
~V
V ~Z 2 This relation, cited in Chapter II, section 2.3-a, of the publication "Doppler Ultrasound and Its Use in Clinical Measurement", P. AIXINSON and J. P. WOODCOCK, Academic Press, 1982, thus imposes a compromise between the axial resolution and the precision of the speed measurement; thls is incompatible with the exact measurement of a speed pro:file or a blood flow image.
In this respect, Canadian Patent Application Serial No.
524,010 filed November 27, 1986 discloses a different method of processing the echographic signal where the precision of the speed measurement is not limited by the spatial resolution. This is a time analysis method which consists in the interpretation of the returning of the ultrasound signals in terms of the shifting in time of the echographic signals after each transmission of pulse signals instead of in terms of frequency shift or phase shifto This method is based on the following principle: assume that an object moves at the axial speed Vz(t) (the axial speed is the projection of the speed on the axis "z" of an ultrasound ~30~'733 excitation beam transmitted by an ultrasound transducer with a repetition period "T"). If at the instant t=0, i.e. when the object is situated at a distance "z" from said transducer, the transducer transmits a first ultrasound excitation, the echo el(t) returned by the object will be received at the instant tl = 2z/C, where C is the propagation speed of the ultrasound wave. Subsequently, if at the instant t=T the transducer transmits a second ultrasound excitation the echo e2tt) returned by the moving object will be detected by the transducer at the instant t = T +2[z + Vz(t)T]/C; the object thus has travelled the distance Vz(t)T during the period "T". When the time origin for the second echo is taken as the instant t=T (origin of the corresponding excitation), the relation e2(t) = el[t - T (t)] is obtained, where l(t) = 2Vz(t)T/C. This relation is very general and can be written as:
ei~l(t) = ei~t - ~i(t)] (1) Ti(t) = ~Viz(t)T/C (2) Ti(t) thus being the time shift induced by the displacement of the object between the excitations "i" and "i+l".
Thus, it appears from the formula (2) that the axial speed Viz(t) can be measured on the basis of the time shift Ti(t) which is extracted Erom the formula (1) by appropriate processing.
Canadian Patent Application Serial No. 524,010 proposes an extraction method for the time shift Ti(t) which utilizes intercorrelation functions, the desired time shift being that for which the intercorrelation function between two successive echoes ei(t) and ei+l(t) is maximum. Even though this method offers the 13~733 3a advantage of providing an exact axial speed Viz(t), it has the drawback that it necessitates the use of a complex device for performing the method, which device includes not only a trans-mission stage and a receiving and processing stage for the echographic signals returned to the transducer, but also numerous correlation circuits, as many averaging circuits and an inter-polation circuit in the form of a microprocessor or wired logic.
The general technical problem to be solved in accordance with the invention is to propose an apparatus for examining an object by ultrasound echography which achieves exact measurement of the axial speed without restrictions due to the axial resolution as well as ease of execution by means of simple electronic circuits. In a ~3 1)~733 specific embodiment in accordance with the invention for blood flows, it is found that the signal returned by the blood is very weak in compariæon with the fixed echoes. The proposed apparatus should, therefore, enable the extraction of the desired signal, in spite of the presence of these parasitic fixed echoes.
To achieve this, the present invention provides apparatus for examining a moving object which has a velocity by means of ultrasound echography to enable the measurement of a projaction of the velocity of said object on an axis of a beam of ultrasound pulses which propagate toward the object with a propagation velocity, C, comprising: means which periodically transmit said ultrasound pulses toward the object using a repetition period, T; and means which receive and process echographic signals which include a plurality of echoes of said pulses whiah are returned to the apparatus from the object wherein as an improvement said means which receive and process include: a circuit for estlmating said projection of the velocity, Viz(t), which circuit comprises~ first calculating means for producing an output corresponding to a time ~hlft, ri(t), between two successive echoes ei(t) and ei~l(t) in said echographic signal which functlon to solve for ri( t) the equation:
P n ~ ei (t) ~ i (t) where ei(n)(t) is a derivative of order n with respect to time of the echo ei(t) and second calculating means connected to multiply the output o~ the first calcula~ing means by C/2T in order to produce an output representing Viz(t).
By way of example said development limited to the order 1 is performed, the time shift ~.(t) then being given by:
~i(t) = [ei(t) - ei~l(t)] /ei(l)(t) ~4) With respect to this formula it is to be noted that the sign of ritt) and hence the sign of Viz(t) can thus be determined, so that the direction of the axial speed can be obtained.
Actually a negative time æhift represents a motion toward~ the transducer, whilst a positive shift is characteristic of a motion B
~3~733 20104-~376 away from the transducer. The method relating to the use of the formula l4) can be executed in a substantially simplified form by means of a device such as said extraction circuit for the time shift ri(t~ which comprises a delay line, a subtractor, a circuit for calculating first derivatives with 4a B
PHF 86.585 5 28-7-1987 respect to time, and a divider. In addition to its sim-plicity, this device also offers the advantage of having a completely analog construction.
In the specific embodiment for blood flows, the circuit for eliminating the fixed echoes and for attenu-ating the echoes relating to slow motions, comprising a subtractor for two consecutive echographic lines. The signal resulting from this difference is subsequently processed by the circuit for estimating the axial speed.
lo As will be described in detail hereinafter, the subtrac-tion of two consecutive echographic lines enables the re-moval of the fixed echoes and the reduction of the effects of the echoes corresponding to slow motions.
The invention will be descr;bed in detail here-inafter with reference to the accompanying diagrammaticdrawing.
Fig. 1 shows an embodiment of a device in accor-dance with the invention.
Fig. 2 shows a specific embodiment of the trans-mission stage of the device shown in fig. 1.
Fig. 3 shows a preferred embodiment of a circuitfor estimating the axial speed.
Fig. 4 shows a circuit diagram of a circuit for suppressing the fixed echoes and for attenuating the echoes relating to slow motions.
Fig. 5 shows a diagram giving a law for limiting the time shift values ~i(t).
Fig~ 6 shows a discriminator circuit of the device shown in fig. 1.
Fig. 1 shows a diagram of a device for examining a moving object by means of ultrasound echography, enab~ing the measurement of the axial speed of said motion, i.e.
the projection of the speed on the axis "z" of a beam of ultrasound excitations periodically transmitted by an ultrasound transducer 10 with a repetition period "T".
The device also comprises a transmission stage 20, a stage 30 for receiving and processing echographic signals returned to the transducer 10, as well as a device 40 for 1300~7~3 PHF 86.585 6 28-7-1987 mechanical scanning control of the transducer. Instead of this transducer~ however, an array of transducers could be used which are then associated with an electronic scanning control device.
In the embodiment which is shown in greater de-tail in fig. 2, the transmission stage 2û comprises a gene-rator 21 for electric excitation signals which are applied to the transducer 10 which converts these signals into trains of periodic pulsed ultrasound signals. This trans-mission is controlled by clock signals which are present on a connection 102 and which are supplied with a repeti-tion frequency "F" (for example in the order of magnitude of 5 kHz) which is determined by a sequencer which succes-sively comprises an oscillator 22, (in this case having a frequency of 32 MHz)o and a frequency divider 23. The di~ider supplies the clock signals on the connection 102 as well as other control signals on the connections 104 and 106 with a frequency of 1 kHz and 16 MHz, respectively7 in the present example. The control signals on the connec-tion 104 control notabLy the device 40 for the scanningof the transducer. A separator 24 between the transducer stage 20 and the receiviny and processing stage 30 is in-serted between the transducer 10 and said stages 20, 30 so that the receiving circuits cannot be overloaded by the transmitted signals.
The receiving and processing stage 30 comprises, connected to the output of the separator 24, a high-fre-quency amplifier 300 with gain compensation as a function of depth, followed by two processing channels 301 and 302 which are connected in parallel. The channel 301 is of a conventional type and in this case comprises a series con-nection of an envelope detector 410, a logarithmic compres-sion amplifier 311, a storage and scan conversion device 370 enables the formation of grey scale images of cross-sections of the objects examined according to the principleof conventional echography.
As appears from fig. 1, the second channel 302 1~0~)~33 PHF 86.585 7 28-7-1987 of said receiving and processing stage 30 for the echo-graphic signals comprises a circuit 330 for estimating the axial speed which enables, on the basis of a time analysis of the signal, the time shift -~i(t) induced by the motion of the object between two successive echoes ei(t) and ei~l(t) to be extracted by means of a limited development of the relation (1):
ei~l(t) = ei ~t ~ i( )~
linking the two echoes, or (t) = ~ (-1) ei(n) (t) . ri (t) (3) n=0 where ei+1(t) can be measured directly and the derivatives ei(n)(t) can be electronically calculated. T~e relation (3) represents an equation in ~i(t) which need only be solved in order to extract the desired time shift ~i(t).
The axial speed Viz(t) is derived therefrom by applying the following formule:
Viz(t) _1~i(t)C/2T (5) where C represents the propagation speed of the ultrasound wave.
Thereforet in accordance with fig. 3, the circuit 330 for estimating the axial speed comprises on the one hand a circuit 340 for extracting the time shift ri(t) which is constructed so as to solve the equation (3) and on the other hand a circuit 350 for multiplication by C/2T.
In practice the time shift ~i(t) is very small with respect to the characteristic variation times of the echographic signal. For example~ application of the for-mule (2) with T = 20D/us~ C = 1500 m/s and Viz(t) = 5 cm/s (blood flows, movements of the heart walls) leads to:
~Ci(t) = 13.3 ms.
For an echographic signal which is centred around 4 MHz, the time shift is effectively smaller than one tenth of the period of the signal, so 250 ns. Conse-PHF 86.585 8 28-7-1987 quently, a development limited to the first order of the relation (1) is justified With n = 1, the equation (3) is then written as:
ei~l(t) - ei(t) _ ei( )(t) l~i(t) (6) or ~i(t) = [ei(t) ei~l(t)] /ei( )(t) (4) The circuit 340 for extracting the time shift as shown in fig. 3 is a completely analog embodiment for calculating the expression (6). To this endf the circuit 34û is composed of a delay line 341, a subtractor 342, a circuit 343 for calculating the first derivative with respect to time 7 and a divider 344. A delay line having the required qualities, i.e. long delay (200/us), high stability (250 ps), a wide dynamic range (79 dB) and a large passband (4 MHz), is de~cribed in FR-A-2 415 391 (P~IF 78-502).
However, its digital implementation i9 also conceivable. The calculation of the derivative, however, requires a Few precaution9. Actually, when the echographic signal is sampled in the step ~T of the signal 106 (60 ns in the described example), the points obtained are situ-ated too far from one another for obtaining a satisfacto-ry estimation of the derivative by means of the relation:
ei( )(k ~T) = [ei((k~ T) - ei(k ~T)] / ~T.
The solution is to shift, in an analog manner, the echo ei(k ~T) by a period "~ " which is small-.with re-spect to ~T ( ~ = 5 ns, for example), followed by sub-tracting ei(l< ~T - ~) from eik ~T and, finally, by divi-ding the difference obtained by ~, in accordance with the formule:
ei (k ~T) = [ei(k ~I) - ei(k ~T - ~)] / ~
In order to reduce the estimation noise, it is advantageous in practice to place the mean value of the time shift l~i(t), calculated by the general relation (3) and notably by the relation (4), on a time window having .:
~3~0733 PHF 86.5R5 9 28-7-1987 a width W in order to evaluate a mean time shift ri(t) which is defined by:
t+W
r i(t) W Jt l~i(U) du.
This operation is performed by an integrator/
averaging circuit 332 which is connected between the circuit 340 for extracting the time shift and the circuit 35û for multiplication by C/2T. Thus, a corresponding mean axial speed Viz(t) is deduced as follows:
Viz(t) = ~i(t)C/2T, After calculation of ~i(t) and before multipli-cation by C/2T, a mean value of ri(t) can also be formed for N consecutive excitations at a rhy~hm which is given 15 by the signal lû4; in this case this results in a mean value over N = 5 excitations. This mean value is formed by the circuit 333 shown in fig. 3; thus on the output of this circuit there is obtained:
~(t) ~ N k~l Tk(t) so that the axial speed is:
Vz(t) _ ~(t) C/2T.
However~ in order to remain within the validi-25 ty range of the limited development defined by the relation(6) which assumes ri(t) te be sufficiently small, it is advantageous to limit the values of ri(t)~ represented by a distribution function f(r ) which is anulled when the absolute value of ~r exceeds a maximum value 1~maX-30 Fig. 5 shows an example of such a distribution funcion:when ~r is between ~ ~max and~max, f(~ ) = r, and when ¦~I> rmaxJ f(~ ) = 0. lrmaxmay be taken to equal one tenth of the period P of the echographic siynal, so 25 ns for P = 250 ns. In that case the mean time shift lri(t) 35 is given by:
- (t) 1 J f [ lri(u)] du.
,. ., ~
PHF 86.5~5 10 28-7-1987 To this end, the circuit 330 for estimating the axial speed comprises a circuit 331 for limiting the time shift values which is situated between the circuit 340 for ex-tracting the time shift and the integrator/averaging circuit 332.
The output signal of the circuit 330 for esti-mating the axial speed is thus validated or not by a dis-criminator ciruit 360 as shown in fig. 6, a~ter which tbe values thus confirmed are applied to the display device 312 via the colour encoding device 370.
The presence of the discriminator ciruit 360 is indispensable. Actually, if the successive echographic lines obtained in the rhythm of the excitations initiated by the signal 102 at the frequency F = l/T are supplied by perfectly fixed taryets, the result of the difference between these two lines will only be noise. Generally speaking, an echngraphic line can be described as follows:
yi(t) = q(t) ~ ei(t) where q(t) is the signal caused by fixed targets and e (t) is the echo produced by the moving ob~ect~
The diFference di(t) between two consecutive lines thus amounts tos i ) Yi~l(t) ~ Yi(t) - ei~l(t) _ ei(t) (7) If the echographic lines yi(t) originate only from fixed targets during a given time interval, it appears from (7) that di(t) - 0 except for noise. Thus, the re-sult supplied by the circuit 330 for estimating the axial speed which processes this noise does not indicate a 30 speed zero, so that it is necessary tovalidate this result or not. To this end, the circuit }60 comprises, connected in series, a multiplier 361 which receives the output sig-nal di(t) of the subtractor 342 on both its inputs and which squares the difference signal. An integrator 362 en-35 ableq calculation of the local energy on a window havinga width W' (possibly equal to W) in accordance with the formule: t~W' 2 Ei(t) - ~ di (u)du.
' '''' :: :- ` ' . . .
~ 13~0733 PHF ~6 585 11 28-7-1987 A circuit 364, 365 for calculating the mean value is formedg as in the case of the circuit 333, by an accumulator which comprises an adder 364 and a delay line 365 which introduces a delay T (or a multiple of T), and ena~les the formation of the mean value of the local en-ergy over M activations, that is to say (M-1) differences in accordance with the expression:
i-M-l E(t) 1 ~ Ei(t) (12) The value thus obtained is applied to a validation circuit which comprises a comparator 461 which receives on a first input the ouput siQnal of the accumulator 364, 365 (or directly that of the summing device 362 in the case where 15 the circuit for calculating the mean value is not provi-ded) and on a second output 462 a reference voltage which forms a threshold. The output signal of the comparator is logic O or 1~ depending on whether the voltage received on its first input is lower than or higher than, respec-2U tively~ the refence threshold ~N(t) which is proportionalto the level of the noise Nt). A multiplier 463, a first input of which receives the output signal Vz(t) of the circuit 330~ applies this signal, denotsd hereinafter as V'z(t) on an output or simply supplies the values zero, 25 depending on whether the validation signal applied to a second input by the comparator 461 is 1 or ~ respectively.
Actually, outside the true flow zones the mean energy cal~
culated on the ouput of the circuit 364, 365 is that of noise only, and can in principle be measured alone, in the absence of excitation, in order to determine the appro-priate threshold value; thus, N(t) is also given by:
t+W' 2 ~ di (u)du outside any excitation. The effective threshold level is 35 thus determined by the coefficient ~ to be chosen by the operator. However, in the presence of signals returned by the moving targets, the mean energy of the signal di(t) exceeds that of the noise alons, thus authorizing the 13(~0733 PHF 86.585 12 28-7-1987 validation of the signals supplied by the circuit 330 for estimating the axial speed.
It is to be noted that the value of ~ can also be used for the display in order to establish a limit be-tween colour display and display in grey; if E(t) exceedsthe threshold ~N(t), the display will be in colour.
However, it will be grey if E(t) is below the threshold.
The output signal of the discriminator circuit 360 is applied to the device for storage, scan conversion and colour encoding 370 which also receives; prior to dis-play, the output signal of the amplifier 311 of the proces-sing channel 301. A deviee of this kind is described, for example in European Patent Application EP-A 0 100 094.
Fig. 3 of this document actually shows, connected between the terminals A, B, C and ER, EG, EB, an example of the circuits which can be used, the terminal A receiving the conventional echographic signal and the terminals B and C
receiving the parameters which are characteristic of the moving device 312 thus enable real-time display of flows 20 or displacements superposed on the conventional echogra-phic reflection image.
Fig, 4 shows the diagram of a circuit 320 for suppressing the fixed echoes and for attenuating the echoes relating to slow movements, which is particularly 25 neces~ary in the case of examination of blood flows.
The digital circuit 320 for suppressing the -echoes as shown in fig. 4 itself comprises, in the present embodiment~ an analog-to-digital converter 321 whose out-put is connected on the one hand directly to the negative 30 input of a subtractor 322 and on the other hand to the positive input of this subtractor via a delay circuit 323.
The delay introduced by the circuit 323 could amount to several periods T = l/F, but is preferably chosen to be as small as possible and equal to T.
The subtractor 322 thus form~ the difference di(t) between two successive echographic lines yi(t) and yi~l(t). Therefore, if the circuit 320 is present, its output can be connected directly to the input of discrimi-. :, -~ ~ 300733 PHF 86.585 13 2B-7-1987 nator circuit 360 which also requires di(t) to be known.
On the other hand~ because the repetition relation ei+l(t) - ei ~t ~ ~i( )]
is also verified by di(t), the output signal of the cir-cuit 320 serves as an input signal for the circuit 330 for estimating the axial speed.
The circuit 320 is provided for the elimination of all fixed echoes, notably those whose occurrence is 10 caused by reflection of the ultrasound waves from the walls of vessels where the flows being studied occur. The presence of fixed echoes is distrubing because their am-plitude (in the order of ~40 dB in the case of blood flows) is much higher than that of the useful signals, that is 15 to say the signals returned by the moving targets. The circuit 320 is also controlled, via the connection 106, by the frequency divider 23 of the sequencer which supplies this circuit with the sampling control signal having a frequency of 16 MHZ.
The invention relates to an apparatus for exa-mining a moving object by means of ultrasound echography, enabling the measurements of the axial speed of said movement i.e. the projection of the speed on the axis "z"
of a beam of ultrasound excitations which are periodically transmitted by at least one ultrasound transducer with a repetition period "T"~ which apparatus also comprises a transmission stage and a stage for receiving and proces-sing the echographic signals returned to the transducer.
The invention can be very advantageously used for the echographic examination of moving organs, such as the heart~and blood flows The technical problem to be solved by any method and any apparatus for examining a moving object by means 15 Of ultrasound echography is that an exact as possible estimation must be made of the axial speed of the motion studied in order to obtain~ using display devices, exact images of the organs and blood flows subjected to an ultrasound eohographic examination.
For a number of years various solutions to this problem have been proposed, notably pulsed-wave ultrasound Doppler systems which are currently used for measuring the blood flow speeds in a given point, or at least the protec-tion of such speeds on the axis of the beam transmitter by 25 the ultrasound transducersO More recently, apparatus have become available which enable real-time determination of the distribution of the flow speeds along the path followed by the ultrasound wave and even across the sectional plane obtained by wsy of a scanning motion of the transducer.
30 The majority of these systems utili~e the frequency shift or the phase shift of the signal returned by the moving targets in order to derive the axial speed of the blood .' ~
~300733 .... 2 flows therefrom. For example, European published patent appli-cation No. 0~092,841, published on February 11, 1983, relates to such an apparatus, which utilizes the measurement of the phase shift between the successive echoes returned by the moving targets in response to a recurrent excitation.
However, the apparatus for carrying out this known method utilizing the phase shift are restricted by an uncertainty relation which links the axial resolution ~z and the precision of the measurement of the speed ~V/V to the wavelength ~:
~V
V ~Z 2 This relation, cited in Chapter II, section 2.3-a, of the publication "Doppler Ultrasound and Its Use in Clinical Measurement", P. AIXINSON and J. P. WOODCOCK, Academic Press, 1982, thus imposes a compromise between the axial resolution and the precision of the speed measurement; thls is incompatible with the exact measurement of a speed pro:file or a blood flow image.
In this respect, Canadian Patent Application Serial No.
524,010 filed November 27, 1986 discloses a different method of processing the echographic signal where the precision of the speed measurement is not limited by the spatial resolution. This is a time analysis method which consists in the interpretation of the returning of the ultrasound signals in terms of the shifting in time of the echographic signals after each transmission of pulse signals instead of in terms of frequency shift or phase shifto This method is based on the following principle: assume that an object moves at the axial speed Vz(t) (the axial speed is the projection of the speed on the axis "z" of an ultrasound ~30~'733 excitation beam transmitted by an ultrasound transducer with a repetition period "T"). If at the instant t=0, i.e. when the object is situated at a distance "z" from said transducer, the transducer transmits a first ultrasound excitation, the echo el(t) returned by the object will be received at the instant tl = 2z/C, where C is the propagation speed of the ultrasound wave. Subsequently, if at the instant t=T the transducer transmits a second ultrasound excitation the echo e2tt) returned by the moving object will be detected by the transducer at the instant t = T +2[z + Vz(t)T]/C; the object thus has travelled the distance Vz(t)T during the period "T". When the time origin for the second echo is taken as the instant t=T (origin of the corresponding excitation), the relation e2(t) = el[t - T (t)] is obtained, where l(t) = 2Vz(t)T/C. This relation is very general and can be written as:
ei~l(t) = ei~t - ~i(t)] (1) Ti(t) = ~Viz(t)T/C (2) Ti(t) thus being the time shift induced by the displacement of the object between the excitations "i" and "i+l".
Thus, it appears from the formula (2) that the axial speed Viz(t) can be measured on the basis of the time shift Ti(t) which is extracted Erom the formula (1) by appropriate processing.
Canadian Patent Application Serial No. 524,010 proposes an extraction method for the time shift Ti(t) which utilizes intercorrelation functions, the desired time shift being that for which the intercorrelation function between two successive echoes ei(t) and ei+l(t) is maximum. Even though this method offers the 13~733 3a advantage of providing an exact axial speed Viz(t), it has the drawback that it necessitates the use of a complex device for performing the method, which device includes not only a trans-mission stage and a receiving and processing stage for the echographic signals returned to the transducer, but also numerous correlation circuits, as many averaging circuits and an inter-polation circuit in the form of a microprocessor or wired logic.
The general technical problem to be solved in accordance with the invention is to propose an apparatus for examining an object by ultrasound echography which achieves exact measurement of the axial speed without restrictions due to the axial resolution as well as ease of execution by means of simple electronic circuits. In a ~3 1)~733 specific embodiment in accordance with the invention for blood flows, it is found that the signal returned by the blood is very weak in compariæon with the fixed echoes. The proposed apparatus should, therefore, enable the extraction of the desired signal, in spite of the presence of these parasitic fixed echoes.
To achieve this, the present invention provides apparatus for examining a moving object which has a velocity by means of ultrasound echography to enable the measurement of a projaction of the velocity of said object on an axis of a beam of ultrasound pulses which propagate toward the object with a propagation velocity, C, comprising: means which periodically transmit said ultrasound pulses toward the object using a repetition period, T; and means which receive and process echographic signals which include a plurality of echoes of said pulses whiah are returned to the apparatus from the object wherein as an improvement said means which receive and process include: a circuit for estlmating said projection of the velocity, Viz(t), which circuit comprises~ first calculating means for producing an output corresponding to a time ~hlft, ri(t), between two successive echoes ei(t) and ei~l(t) in said echographic signal which functlon to solve for ri( t) the equation:
P n ~ ei (t) ~ i (t) where ei(n)(t) is a derivative of order n with respect to time of the echo ei(t) and second calculating means connected to multiply the output o~ the first calcula~ing means by C/2T in order to produce an output representing Viz(t).
By way of example said development limited to the order 1 is performed, the time shift ~.(t) then being given by:
~i(t) = [ei(t) - ei~l(t)] /ei(l)(t) ~4) With respect to this formula it is to be noted that the sign of ritt) and hence the sign of Viz(t) can thus be determined, so that the direction of the axial speed can be obtained.
Actually a negative time æhift represents a motion toward~ the transducer, whilst a positive shift is characteristic of a motion B
~3~733 20104-~376 away from the transducer. The method relating to the use of the formula l4) can be executed in a substantially simplified form by means of a device such as said extraction circuit for the time shift ri(t~ which comprises a delay line, a subtractor, a circuit for calculating first derivatives with 4a B
PHF 86.585 5 28-7-1987 respect to time, and a divider. In addition to its sim-plicity, this device also offers the advantage of having a completely analog construction.
In the specific embodiment for blood flows, the circuit for eliminating the fixed echoes and for attenu-ating the echoes relating to slow motions, comprising a subtractor for two consecutive echographic lines. The signal resulting from this difference is subsequently processed by the circuit for estimating the axial speed.
lo As will be described in detail hereinafter, the subtrac-tion of two consecutive echographic lines enables the re-moval of the fixed echoes and the reduction of the effects of the echoes corresponding to slow motions.
The invention will be descr;bed in detail here-inafter with reference to the accompanying diagrammaticdrawing.
Fig. 1 shows an embodiment of a device in accor-dance with the invention.
Fig. 2 shows a specific embodiment of the trans-mission stage of the device shown in fig. 1.
Fig. 3 shows a preferred embodiment of a circuitfor estimating the axial speed.
Fig. 4 shows a circuit diagram of a circuit for suppressing the fixed echoes and for attenuating the echoes relating to slow motions.
Fig. 5 shows a diagram giving a law for limiting the time shift values ~i(t).
Fig~ 6 shows a discriminator circuit of the device shown in fig. 1.
Fig. 1 shows a diagram of a device for examining a moving object by means of ultrasound echography, enab~ing the measurement of the axial speed of said motion, i.e.
the projection of the speed on the axis "z" of a beam of ultrasound excitations periodically transmitted by an ultrasound transducer 10 with a repetition period "T".
The device also comprises a transmission stage 20, a stage 30 for receiving and processing echographic signals returned to the transducer 10, as well as a device 40 for 1300~7~3 PHF 86.585 6 28-7-1987 mechanical scanning control of the transducer. Instead of this transducer~ however, an array of transducers could be used which are then associated with an electronic scanning control device.
In the embodiment which is shown in greater de-tail in fig. 2, the transmission stage 2û comprises a gene-rator 21 for electric excitation signals which are applied to the transducer 10 which converts these signals into trains of periodic pulsed ultrasound signals. This trans-mission is controlled by clock signals which are present on a connection 102 and which are supplied with a repeti-tion frequency "F" (for example in the order of magnitude of 5 kHz) which is determined by a sequencer which succes-sively comprises an oscillator 22, (in this case having a frequency of 32 MHz)o and a frequency divider 23. The di~ider supplies the clock signals on the connection 102 as well as other control signals on the connections 104 and 106 with a frequency of 1 kHz and 16 MHz, respectively7 in the present example. The control signals on the connec-tion 104 control notabLy the device 40 for the scanningof the transducer. A separator 24 between the transducer stage 20 and the receiviny and processing stage 30 is in-serted between the transducer 10 and said stages 20, 30 so that the receiving circuits cannot be overloaded by the transmitted signals.
The receiving and processing stage 30 comprises, connected to the output of the separator 24, a high-fre-quency amplifier 300 with gain compensation as a function of depth, followed by two processing channels 301 and 302 which are connected in parallel. The channel 301 is of a conventional type and in this case comprises a series con-nection of an envelope detector 410, a logarithmic compres-sion amplifier 311, a storage and scan conversion device 370 enables the formation of grey scale images of cross-sections of the objects examined according to the principleof conventional echography.
As appears from fig. 1, the second channel 302 1~0~)~33 PHF 86.585 7 28-7-1987 of said receiving and processing stage 30 for the echo-graphic signals comprises a circuit 330 for estimating the axial speed which enables, on the basis of a time analysis of the signal, the time shift -~i(t) induced by the motion of the object between two successive echoes ei(t) and ei~l(t) to be extracted by means of a limited development of the relation (1):
ei~l(t) = ei ~t ~ i( )~
linking the two echoes, or (t) = ~ (-1) ei(n) (t) . ri (t) (3) n=0 where ei+1(t) can be measured directly and the derivatives ei(n)(t) can be electronically calculated. T~e relation (3) represents an equation in ~i(t) which need only be solved in order to extract the desired time shift ~i(t).
The axial speed Viz(t) is derived therefrom by applying the following formule:
Viz(t) _1~i(t)C/2T (5) where C represents the propagation speed of the ultrasound wave.
Thereforet in accordance with fig. 3, the circuit 330 for estimating the axial speed comprises on the one hand a circuit 340 for extracting the time shift ri(t) which is constructed so as to solve the equation (3) and on the other hand a circuit 350 for multiplication by C/2T.
In practice the time shift ~i(t) is very small with respect to the characteristic variation times of the echographic signal. For example~ application of the for-mule (2) with T = 20D/us~ C = 1500 m/s and Viz(t) = 5 cm/s (blood flows, movements of the heart walls) leads to:
~Ci(t) = 13.3 ms.
For an echographic signal which is centred around 4 MHz, the time shift is effectively smaller than one tenth of the period of the signal, so 250 ns. Conse-PHF 86.585 8 28-7-1987 quently, a development limited to the first order of the relation (1) is justified With n = 1, the equation (3) is then written as:
ei~l(t) - ei(t) _ ei( )(t) l~i(t) (6) or ~i(t) = [ei(t) ei~l(t)] /ei( )(t) (4) The circuit 340 for extracting the time shift as shown in fig. 3 is a completely analog embodiment for calculating the expression (6). To this endf the circuit 34û is composed of a delay line 341, a subtractor 342, a circuit 343 for calculating the first derivative with respect to time 7 and a divider 344. A delay line having the required qualities, i.e. long delay (200/us), high stability (250 ps), a wide dynamic range (79 dB) and a large passband (4 MHz), is de~cribed in FR-A-2 415 391 (P~IF 78-502).
However, its digital implementation i9 also conceivable. The calculation of the derivative, however, requires a Few precaution9. Actually, when the echographic signal is sampled in the step ~T of the signal 106 (60 ns in the described example), the points obtained are situ-ated too far from one another for obtaining a satisfacto-ry estimation of the derivative by means of the relation:
ei( )(k ~T) = [ei((k~ T) - ei(k ~T)] / ~T.
The solution is to shift, in an analog manner, the echo ei(k ~T) by a period "~ " which is small-.with re-spect to ~T ( ~ = 5 ns, for example), followed by sub-tracting ei(l< ~T - ~) from eik ~T and, finally, by divi-ding the difference obtained by ~, in accordance with the formule:
ei (k ~T) = [ei(k ~I) - ei(k ~T - ~)] / ~
In order to reduce the estimation noise, it is advantageous in practice to place the mean value of the time shift l~i(t), calculated by the general relation (3) and notably by the relation (4), on a time window having .:
~3~0733 PHF 86.5R5 9 28-7-1987 a width W in order to evaluate a mean time shift ri(t) which is defined by:
t+W
r i(t) W Jt l~i(U) du.
This operation is performed by an integrator/
averaging circuit 332 which is connected between the circuit 340 for extracting the time shift and the circuit 35û for multiplication by C/2T. Thus, a corresponding mean axial speed Viz(t) is deduced as follows:
Viz(t) = ~i(t)C/2T, After calculation of ~i(t) and before multipli-cation by C/2T, a mean value of ri(t) can also be formed for N consecutive excitations at a rhy~hm which is given 15 by the signal lû4; in this case this results in a mean value over N = 5 excitations. This mean value is formed by the circuit 333 shown in fig. 3; thus on the output of this circuit there is obtained:
~(t) ~ N k~l Tk(t) so that the axial speed is:
Vz(t) _ ~(t) C/2T.
However~ in order to remain within the validi-25 ty range of the limited development defined by the relation(6) which assumes ri(t) te be sufficiently small, it is advantageous to limit the values of ri(t)~ represented by a distribution function f(r ) which is anulled when the absolute value of ~r exceeds a maximum value 1~maX-30 Fig. 5 shows an example of such a distribution funcion:when ~r is between ~ ~max and~max, f(~ ) = r, and when ¦~I> rmaxJ f(~ ) = 0. lrmaxmay be taken to equal one tenth of the period P of the echographic siynal, so 25 ns for P = 250 ns. In that case the mean time shift lri(t) 35 is given by:
- (t) 1 J f [ lri(u)] du.
,. ., ~
PHF 86.5~5 10 28-7-1987 To this end, the circuit 330 for estimating the axial speed comprises a circuit 331 for limiting the time shift values which is situated between the circuit 340 for ex-tracting the time shift and the integrator/averaging circuit 332.
The output signal of the circuit 330 for esti-mating the axial speed is thus validated or not by a dis-criminator ciruit 360 as shown in fig. 6, a~ter which tbe values thus confirmed are applied to the display device 312 via the colour encoding device 370.
The presence of the discriminator ciruit 360 is indispensable. Actually, if the successive echographic lines obtained in the rhythm of the excitations initiated by the signal 102 at the frequency F = l/T are supplied by perfectly fixed taryets, the result of the difference between these two lines will only be noise. Generally speaking, an echngraphic line can be described as follows:
yi(t) = q(t) ~ ei(t) where q(t) is the signal caused by fixed targets and e (t) is the echo produced by the moving ob~ect~
The diFference di(t) between two consecutive lines thus amounts tos i ) Yi~l(t) ~ Yi(t) - ei~l(t) _ ei(t) (7) If the echographic lines yi(t) originate only from fixed targets during a given time interval, it appears from (7) that di(t) - 0 except for noise. Thus, the re-sult supplied by the circuit 330 for estimating the axial speed which processes this noise does not indicate a 30 speed zero, so that it is necessary tovalidate this result or not. To this end, the circuit }60 comprises, connected in series, a multiplier 361 which receives the output sig-nal di(t) of the subtractor 342 on both its inputs and which squares the difference signal. An integrator 362 en-35 ableq calculation of the local energy on a window havinga width W' (possibly equal to W) in accordance with the formule: t~W' 2 Ei(t) - ~ di (u)du.
' '''' :: :- ` ' . . .
~ 13~0733 PHF ~6 585 11 28-7-1987 A circuit 364, 365 for calculating the mean value is formedg as in the case of the circuit 333, by an accumulator which comprises an adder 364 and a delay line 365 which introduces a delay T (or a multiple of T), and ena~les the formation of the mean value of the local en-ergy over M activations, that is to say (M-1) differences in accordance with the expression:
i-M-l E(t) 1 ~ Ei(t) (12) The value thus obtained is applied to a validation circuit which comprises a comparator 461 which receives on a first input the ouput siQnal of the accumulator 364, 365 (or directly that of the summing device 362 in the case where 15 the circuit for calculating the mean value is not provi-ded) and on a second output 462 a reference voltage which forms a threshold. The output signal of the comparator is logic O or 1~ depending on whether the voltage received on its first input is lower than or higher than, respec-2U tively~ the refence threshold ~N(t) which is proportionalto the level of the noise Nt). A multiplier 463, a first input of which receives the output signal Vz(t) of the circuit 330~ applies this signal, denotsd hereinafter as V'z(t) on an output or simply supplies the values zero, 25 depending on whether the validation signal applied to a second input by the comparator 461 is 1 or ~ respectively.
Actually, outside the true flow zones the mean energy cal~
culated on the ouput of the circuit 364, 365 is that of noise only, and can in principle be measured alone, in the absence of excitation, in order to determine the appro-priate threshold value; thus, N(t) is also given by:
t+W' 2 ~ di (u)du outside any excitation. The effective threshold level is 35 thus determined by the coefficient ~ to be chosen by the operator. However, in the presence of signals returned by the moving targets, the mean energy of the signal di(t) exceeds that of the noise alons, thus authorizing the 13(~0733 PHF 86.585 12 28-7-1987 validation of the signals supplied by the circuit 330 for estimating the axial speed.
It is to be noted that the value of ~ can also be used for the display in order to establish a limit be-tween colour display and display in grey; if E(t) exceedsthe threshold ~N(t), the display will be in colour.
However, it will be grey if E(t) is below the threshold.
The output signal of the discriminator circuit 360 is applied to the device for storage, scan conversion and colour encoding 370 which also receives; prior to dis-play, the output signal of the amplifier 311 of the proces-sing channel 301. A deviee of this kind is described, for example in European Patent Application EP-A 0 100 094.
Fig. 3 of this document actually shows, connected between the terminals A, B, C and ER, EG, EB, an example of the circuits which can be used, the terminal A receiving the conventional echographic signal and the terminals B and C
receiving the parameters which are characteristic of the moving device 312 thus enable real-time display of flows 20 or displacements superposed on the conventional echogra-phic reflection image.
Fig, 4 shows the diagram of a circuit 320 for suppressing the fixed echoes and for attenuating the echoes relating to slow movements, which is particularly 25 neces~ary in the case of examination of blood flows.
The digital circuit 320 for suppressing the -echoes as shown in fig. 4 itself comprises, in the present embodiment~ an analog-to-digital converter 321 whose out-put is connected on the one hand directly to the negative 30 input of a subtractor 322 and on the other hand to the positive input of this subtractor via a delay circuit 323.
The delay introduced by the circuit 323 could amount to several periods T = l/F, but is preferably chosen to be as small as possible and equal to T.
The subtractor 322 thus form~ the difference di(t) between two successive echographic lines yi(t) and yi~l(t). Therefore, if the circuit 320 is present, its output can be connected directly to the input of discrimi-. :, -~ ~ 300733 PHF 86.585 13 2B-7-1987 nator circuit 360 which also requires di(t) to be known.
On the other hand~ because the repetition relation ei+l(t) - ei ~t ~ ~i( )]
is also verified by di(t), the output signal of the cir-cuit 320 serves as an input signal for the circuit 330 for estimating the axial speed.
The circuit 320 is provided for the elimination of all fixed echoes, notably those whose occurrence is 10 caused by reflection of the ultrasound waves from the walls of vessels where the flows being studied occur. The presence of fixed echoes is distrubing because their am-plitude (in the order of ~40 dB in the case of blood flows) is much higher than that of the useful signals, that is 15 to say the signals returned by the moving targets. The circuit 320 is also controlled, via the connection 106, by the frequency divider 23 of the sequencer which supplies this circuit with the sampling control signal having a frequency of 16 MHZ.
Claims (9)
1. Apparatus for examining a moving object which has a velocity by means of ultrasound echography to enable the measurement of a projection of the velocity of said object on an axis of a beam of ultrasound pulses which propagate toward the object with a propagation velocity, C, comprising: means which periodically transmit said ultrasound pulses toward the object using a repetition period, T; and means which receive and process echographic signals which include a plurality of echoes of said pulses which are returned to the apparatus from the object wherein as an improvement said means which receive and process include: a circuit for estimating said projection of the velocity, Viz(t), which circuit comprises, first calculating means for producing an output corresponding to a time shift, ?i(t), between two successive echoes ei(t) and ei+1(t) in said echographic signal which function to solve for ?i (t) the equation:
ei+l(t) where ei(n)(t) is a derivative of order n with respect to time of the echo ei(t) and second calculating means connected to multiply the output of the first calculating means by C/2T in order to produce an output representing Viz(t).
ei+l(t) where ei(n)(t) is a derivative of order n with respect to time of the echo ei(t) and second calculating means connected to multiply the output of the first calculating means by C/2T in order to produce an output representing Viz(t).
2. Apparatus as claimed in claim 1 wherein the first calculating means comprise a delay line connected to receive said echoes at an input; a subtractor connected to subtract said echoes from an output of said delay line; means for calculating the derivative of said output to said delay line; and divider means connected to receive an output of said means for calculating the derivative and for dividing said output by an output of said subtractor.
3. Apparatus as claimed in claim 2 wherein said circuit for estimating further comprises an integrator/averaging circuit connected in cascade between the output of the first calculating means and an input of the second calculating means.
4. Apparatus as claimed in claim 3 further comprising means for limiting time shift values ?i(t) connected to cascade between the output of the divider means and an input of the integrator/averaging circuit.
5. The apparatus of any one of claims 1, 2, 3 or 4 further comprising a discriminator circuit connected to process the output of said second calculating means, said discriminator circuit comprising: means for squaring the difference di(t) between two successive echoes; summing means which act on a window having a width W' which calculates a value E(t) = di2(u)du; and validation circuit means which compare E(t) with a threshold, said threshold being proportional to a noise level.
6. The apparatus of claim 5 further comprising means for eliminating echoes attributable to objects which are fixed or are moving slower than the moving object which is being examined before said echoes are applied to an input of said circuit for estimating said axial velocity.
7. The apparatus of claim 1, 2, 3 or 4 further comprising means for eliminating echoes attributable to objects which are fixed or are moving slower than the moving object which is being examined before said echoes are applied to an input of said circuit for estimating said axial velocity.
8. A method for examining moving objects by means of ultrasound echography comprising the steps of: directing pulses of ultrasound energy which propagate with a velocity C towards said moving object; receiving echoes of said ultrasound energy from said object and producing electrical signals characteristic thereof; processing said electrical signals to extract a time shift ?i(t) between two successive echoes Ei(t) and Ei+1(t) to solve for ?i(t) the equation ei+1(t) where ei(n)(t) is a derivative of order n with respect to time of the echo ei(t); and calculating said axial velocity by multiplying said value of ?i(t) by C/2T.
9. The method of claim 8 further comprising the step of rejecting from said calculation of ?i(t) echoes which originate from objects which are fixed or are moving slower than the moving objects which are being examined.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR8613140 | 1986-09-19 | ||
| FR8613140A FR2604081A1 (en) | 1986-09-19 | 1986-09-19 | ULTRASONIC ULTRASONIC ULTRASONIC EXPLORATION DEVICE FOR MOVING ORGANS AND BLOOD FLOWS |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1300733C true CA1300733C (en) | 1992-05-12 |
Family
ID=9339113
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000547067A Expired - Lifetime CA1300733C (en) | 1986-09-19 | 1987-09-16 | Apparatus for examining a moving object by means of ultrasound echography |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4853904A (en) |
| EP (1) | EP0261736B1 (en) |
| JP (1) | JPS6377437A (en) |
| CA (1) | CA1300733C (en) |
| DE (1) | DE3785409T2 (en) |
| FR (1) | FR2604081A1 (en) |
| IL (1) | IL83920A (en) |
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| FR2655260A1 (en) * | 1989-12-01 | 1991-06-07 | Philips Electronique Lab | DEVICE FOR MEASUREMENT AND VISUALIZATION BY ULTRASONIC ECHOGRAPHY OF PHYSIOLOGICAL PARAMETERS OF A BLOOD FLOW. |
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-
1986
- 1986-09-19 FR FR8613140A patent/FR2604081A1/en not_active Withdrawn
-
1987
- 1987-09-15 EP EP87201800A patent/EP0261736B1/en not_active Expired - Lifetime
- 1987-09-15 DE DE87201800T patent/DE3785409T2/en not_active Expired - Fee Related
- 1987-09-15 US US07/097,224 patent/US4853904A/en not_active Expired - Fee Related
- 1987-09-16 JP JP62229930A patent/JPS6377437A/en active Pending
- 1987-09-16 CA CA000547067A patent/CA1300733C/en not_active Expired - Lifetime
- 1987-09-16 IL IL83920A patent/IL83920A/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| IL83920A (en) | 1992-01-15 |
| DE3785409D1 (en) | 1993-05-19 |
| DE3785409T2 (en) | 1993-10-21 |
| FR2604081A1 (en) | 1988-03-25 |
| EP0261736B1 (en) | 1993-04-14 |
| JPS6377437A (en) | 1988-04-07 |
| US4853904A (en) | 1989-08-01 |
| IL83920A0 (en) | 1988-02-29 |
| EP0261736A1 (en) | 1988-03-30 |
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| Date | Code | Title | Description |
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| MKLA | Lapsed |