Method and Device for Measuring Objects Using Ultrasound Echography

The invention relates to a device for the scanning of objects, notably biological tissues, by means of ultrasound echography, comprising at least one ultrasonic transducer which is connected to a transmitter stage for the repeated transmission of ultrasonic signals and to a receiver stage for receiving the ultrasonic echoes corresponding to the most important obstacles encountered by the transmitted signals in their direction of propagation, said receiver stage comprising a first processing circuit for the echoes received which consists mainly of a first amplifier which is connected to the output electrode of the ultrasonic transducer, a gain compensation device, and a display device for displaying the positions of the echoes in the scanning direction as a function of time as well as the amplitudes thereof.

A device of this kind is proposed in the previous French Patent Application No. 8119585, which corresponds to U.S. patent application Ser. No. 411,731, filed Aug. 26, 1982. Said device notably comprises a processing stage for the echoes received which consists of a first and a second processing circuit. The first processing circuit is of a known type and consists mainly of a first amplifier for the signals present on the output electrode of the transducer, a gain compensation device, and a display device. The second processing device which is connected parallel to the first processing device comprises (a) a second amplifier which is also connected to the output electrode of the transducer, (b) a group of n parallel channels which are connected to the output of the second amplifier and each of which successively comprises a band-pass filter (the pass-bands of the various filters thus obtained being substantially consecutive so that together they span approximately the pass-band of the second amplifier) and an envelope detector (consisting of a rectifier and a low-pass filter having a variable time constant), (c) an arithmetic circuit which is connected to the output of the n channels and which serves to calculate, on the basis of the output signals thereof, a parameter which is an indicator for the spread of the amplitudes of the signals of central frequency of each of the channels and which is also directly locally correlated to the mean slope (the so-called differential ultrasonic attenuation factor) of the curve of the variation of the ultrasonic attenuation as a function of the frequency in the tissues scanned, and (d) a circuit which is connected to the output of this arithmetic circuit in order to determine the value of this factor within each of the zones in the tissues scanned which are bounded by the echoes corresponding to the most important obstacles encountered, the output signals of this factor-determining circuit modulating the picture displayed on the display device.

The device thus constructed is important because of the fact that it enables quantitative information to be obtained by the local calculation of a parameter which is directly correlated to the differential ultrasonic attenuation factor; subsequently the values of this factor can be directly displayed in A-type or B-type echography. The assembly of such an apparatus, however, is complex notably due to the presence of the block of n filters; even if parts of this device are replaced by software, the calculation to be performed still require a comparatively long period of time.

It is an object of the invention to provide a device of the kind set forth which is simpler than the described, known device.

To this end, the device in accordance with the invention is characterized in that the receiver stage comprises a second processing circuit which is connected parallel to the first processing circuit and which consists of a series-connection of a control circuit for automatic control of the gain as a function of the distance of the echoes, a correction circuit for correcting diffraction effects, a selection circuit for selecting a narrow frequency band which is centred around a frequency F.sub.c within the pass-band of the transducer, a logarithmic amplifier, and a divider which divides the output of the selection circuit by a factor which is proportional to F.sub.c, whose output signal is applied to the display device.

Such a construction of the device is distinguished by the fact that, from the successive instantaneous frequency spectra of the output signal of the transducer, only narrow frequency zones are taken into account which are centred around the frequency which is most suitable for the analysis of the objects to be examined. For expressing the value of the energy in this frequency band, thanks to the fact that this value I is proportional to E.sup.-.beta.F c.sup.d (in which .beta. is the differential ultrasonic attenuation factor, F.sub.c is the central frequency of the band selected and d is the distance of the tissues), when the units are suitably chosen the characteristics I.sub.dB =f(d) will be shaped, for a constant value of .beta. and F.sub.c, as straight lines whose mean slope always directly represents the value of the ultrasonic attenuation.

The invention will be described in detail hereinafter with reference to the accompanying drawings. Therein:

FIGS. 1 through 3 show three embodiments of the device for the scanning objects in accordance with the invention.

The embodiments of the device described with reference to the figures comprise a single probe which forms a carrier for an ultrasonic transducer 10 and which can be used to obtain A-type echograms. The invention can also be used in exactly the same way when instead of only one line a complete plane section of the tissues is examined, either by means of a probe which is connected to a radar-type display device with manual displacement or with mechanical sector scanning, or by means of a linear array of p ultrasonic transducers which define a corresponding number of (p) parallel scanning directions in the tissues to be examined, said array being connected to a switching circuit whereby the echo processing device is successively connected to each activated transducer or group of transducers, or also by means of an array of transducers with electronic sector scanning which is also connected to a switching circuit and to a network of delay lines or phase shifters.

The transducer 10 is connected on the one side to a transmitter stage 50 which serves to ensure that the transducer can repeatedly transmit ultrasonic signals in an arbitrary scanning direction through the tissues to be examined and on the other side to a receiver stage which serves for the processing of the ultrasonic echoes which are received by the transducer and which correspond to the most important obstacles encountered by the transmitted signals in their propagation direction. The situation of such obstacles is defined in the echograms by the echoes of high amplitude which indicate the boundaries between the tissues for which the differential ultrasonic attenuation factors are to be determined.

The output electrode of the transducer 10 is connected to the input of the amplifier 101 whose output signals are applied to the gain compensation device 102 whereby the amplitude of the echoes is compensated for in dependence of the distance (TGC), after which these signals are displayed on the display device 103 in the form of an A-type echogram along an axis which corresponds to the principal propagation direction of the transducer 10.

The receiver stage in accordance with the invention also comprises a second processing circuit 200 which is connected parallel to the first processing circuit 100 and which consists of the following elements:

(a) a control circuit 210 which is connected to the output electrode of the transducer 10 for the automatic control of the gain as a function of the distance of the echoes;

(b) a correction circuit 220 for the correction of diffraction effects;

(c) a selection circuit 230 for the selection of a frequency band which is centred around the frequency F.sub.c of the spectrum of the transducer which corresponds to the maximum power, said circuit consisting of an oscillator 231, a multiplier 232 and a low-pass filter 240;

(d) a logarithmic amplifier 250;

(e) a divider 260 which is adapted to divide by the value of the center frequency, F.sub.c.

This stage operates as follows: as soon as the corrections of the gain as a function of time and the diffraction have been performed, the frequency band of the signal thus obtained on the output of the circuit 220 is converted to a base band by mixing this signal with the output signal of the oscillator 231 in the multiplier 232, and by subsequently filtering this signal in the low-pass filter 240. Because, as has already been described, the ultrasonic energy in this frequency band is proportional to e.sup.-.beta.F c.sup.d, only the logarithmic amplifier 250 and the divider 260 need be connected to the output of the circuit 230 in order to produce a signal on the output of this divider whose amplitude is directly locally correlated to the mean slope .beta. of the curve representing the variation of the ultrasonic attenuation as a function of the frequency in the tissues scanned (this slope .beta. is referred to as the differential ultrasonic attenuation factor). It is to be noted that the frequency F.sub.c is chosen to be equal to either the frequency corresponding to the maximum power which can be delivered by the transducer or to a frequency which is lower than the frequency corresponding to this maximum power in order to take into account the fact that the tissues examined bahave as a low-pass filter.

In the present embodiment, the output signals of the gain compensation device 102 and the divider 260 modulate the image displayed on the display device 103 as follows: the image comprises on the one hand the known A-type echogram which is supplied by the first processing circuit and which is applied to a first channel Y.sub.1 of the device 103 and on the other hand a stepped curved which is displayed on a second channel Y.sub.2 and which represents the various values of the ultrasonic attenuation factor between the boundaries defined by the echogram of the channel Y.sub.1 (these boundaries correspond to the most important obstacles encountered in the tissues and are, therefore, displayed as the echoes of high amplitude of the echogram of the channel Y.sub.1).

The correction circuit 220 for the correction of the diffraction effects mainly comprises a memory 221 and a multiplier 222. This memory is controlled by a clock circuit 223 and contains the necessary correction signals for correction as a function of the distance from the transducer, said signals being applied to the multiplier 222; their values have been stored in an advance after a calibration phase. In the case of a non-focusing transducer, it is merely necessary for this calibration phase to take into account the echo signals of the selected transducer when the latter is arranged opposite a reflective metal surface which is successively situated at all distances from the transducer which correspond to the customary depths in echographic examinations or, in the more common case of a focusing transducer, the echo signals which are obtained by the use of phantoms whose ultrasonic properties are exactly known, the echo signals thus obtained enabling the storage of correction factors in the memory 221 which is a PROM memory in this embodiment.

A known control circuit for the automatic gain control comprises a controllable amplifier 211 which is controlled by the actual control circuit 212. For accurate energy measurement in accordance with the invention it is desirable to fix this gain temporarily between two instants t.sub.1 and t.sub.2 ; this represents the actual period of time required for the execution of the measurement. This result is obtained by including a time window in the control circuit 210 during which the gain variation of the amplifier is suspended.

In order to suspend the gain variation it is also possible to omit modification of the control circuit 210 and to obtain a similar effect at a later stage, either (see FIG. 2) by subtracting a signal which is proportional to the output signal of the control circuit 212 in a subtract circuit 214 from the input signal of the divider 260 after it has been passed through a second logarithmic amplifier 213 or by using two separate frequencies F.sub.c1 and F.sub.c2 (see FIG. 3). In the latter case, two identical, parallel-connected channels must be connected to the output of the control circuit 210 for the automatic gain control, each of said channels comprising a correction circuit 311, 321, respectively, for the correction of the diffraction effects (only if necessary), a selection circuit 312, 322, respectively, for the selection of a frequency band F.sub.c1, F.sub.c2, respectively, and a logarithmic amplifier 314, 324, respectively, whose outputs are connected to the inputs of the subtraction circuit 214. In that case the divider 260 is a divider adapted to divide by F.sub.c1 -F.sub.c2.

Obviously, the invention is not restricted to the described embodiments; within the scope of the invention many alternatives are feasible. When the transducer 10 and its spectrum are known, notably the construction of the circuit can be simplified by using instead of the circuit 230 a simple band-pass filter whose pass-band is centered around F.sub.c which is chosen to be equal to the F.sub.c in the foregoing. This solution is cheaper than the foregoing solution, but the circuit then suits only one given transducer.