Method of and device for measuring an ultrasound scatter function

A biological or other material is scanned along a line or plurality of lines by at least one transducer, and the signals received are separated into n substantially equal, consecutive frequency bands after which the envelope of the signals in each frequency band is determined. Each of the signal envelopes, being proportional to the power spectrum of the echographic signal, is divided by a dividing signal which is proportional to the power spectrum of a phantom which has the same attenuation as the object being examined, the logarithm being determined of the signal resulting from this division, after which the following operations are performed in the channels a, b, . . . , i, . . . n on the basis of the signals thus obtained: (a) calculating the logarithm of a frequency which is representative of the passband of each channel; (b) storing in a memory and/or displaying a function of the ratio of the scatter function of the object scanned and the scatter function of the phantom as a function of the frequency mentioned sub (a); as a representation of the object's scatter function.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to a method of scanning objects by means of 
ultrasound echography, involving the repeated transmission of ultrasound 
signals by means of at least one ultrasound transducer and the reception 
of the ultrasound echoes which correspond to the principal obstacles 
encountered by the transmitted signals in their propagation direction, the 
signals received being separated into n frequency bands which are 
substantially equal and consecutive so that they span approximately all 
frequencies of the signals received, the envelope of the signals in each 
frequency band being subsequently determined. 
The invention also relates to a device for performing such a method, 
including at least one ultrasound transducer which is connected to a 
transmitter stage for the repeated transmission of ultrasound signals and 
to a receiver stage for receiving the ultrasound echoes which correspond 
to the principal obstacles encountered by the transmitted signals in their 
propagation direction, said receiver stage including at least: 
(A) an amplifier which receives the output signal of the transducer; 
(B) a group of n channels which are connected in parallel to the output of 
the amplifier and each of which successively includes: 
(1) a bandpass filter, the group of n filters thus formed being such that 
their respective passbands are substantially equal and consecutive so that 
they span substantially the passband of the transducer; 
(2) an envelope detector which is identical for each channel and which 
includes a rectifier which is succeeded by a lowpass filter. Such a device 
can be used, for example for the non-destructive testing of materials and 
the scanning of biological tissues. 
A conventional ultrasound echographic image is obtained by the detection of 
the envelope of the echoes produced in the tissues by an ultrasound beam. 
Because the most important echoes arise at the boundaries of the organs, 
these images mainly show the contours thereof. All information concerning 
the phase (and hence the frequency) of the signal is then lost. Such 
information can be related to suitable indicators of some indistinct 
diseases which are difficult to diagnose by other means. 
It is an object of the invention to provide a method of and a device for 
scanning objects by means of ultrasound echography in which the 
frequency-dependency of the scatter function of the object scanned can be 
determined. This method enables a quantitative determination of the 
scatter function of the object examined which can be characterized by this 
function so that the pathological condition thereof can be determined. 
To this end, the method in accordance with the invention is characterized 
in that each of the signal envelopes, being proportional to the power 
spectrum of the echographic signal, is divided by a dividing signal which 
is proportional to the power spectrum of a phantom having the same 
attenuation as the object scanned, the logarithm of the signal resulting 
from this division being determined, after which the following operations 
are performed in the channels a, b, . . . , i, . . . , n on the basis of 
the signals thus obtained; 
(a) calculating the logarithm of the central frequency f.sub.i of the 
passband associated with each channel or of another frequency which is 
representative of this passband; 
(b) storing in a memory and/or displaying a function of the ratio of the 
scatter function of the object examined and that of the phantom as a 
function of the frequency mentioned with reference to the calculation 
performed sub (a) as a representation of the subject's scatter function; 
The device for performing the method in accordance with the invention is 
characterized in that the receiver stage also includes, connected behind 
the envelope detector; in each channel: 
(3) a divider circuit, a first input of which receives the output signal of 
the corresponding envelope detector and a second input of which receives a 
dividing signal which originates from a first memory which is controlled 
by a clock circuit which is activated by the clock of the transmitter 
stage; 
(4) a logarithmic amplifier; the receiver stage also including an 
arithmetic circuit which is connected to the output of the n channels and 
which performs the following operations on the basis of the n output 
signals of the channels: 
(a) calculating the logarithm of the central frequency f.sub.i of the 
passband associated with each channel or of another frequency which is 
representative of this passband; 
(b) storing in a memory and/or displaying a function of the ratio of the 
scatter function of the object scanned and that of the phantom as a 
function of the frequency mentioned with reference to the calculation 
performed sub (a) as a representation of the subject's scatter function.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The analysis of the frequency-dependency of the scatter function of the 
object examined is performed in accordance with the invention by 
comparison of mean power spectra. It is known that for a time slot w and 
the instant .tau. the formula for a mean power spectrum can be written as: 
EQU S.sub.w (.tau.,f)=.vertline.G(f).vertline..sup.2 
.multidot..vertline.U(f).vertline..sup.2 
.multidot.D(.tau.f).multidot.e.sup.-2 .alpha.(f)c (1) 
In this equation .vertline.G(f).vertline..sup.2 depends only on the 
transmitted signal and the acoustic and electrical properties of the 
transducer; D(.tau., f) denotes the filter effect of the diffraction 
caused by the geometry of the transducer; .alpha.(f) and c denote the 
attenuation and the velocity of the ultrasound waves in the object 
examined; and U(F) denotes the scatter function of the object. 
A first embodiment of the device in accordance with the invention is 
intended for the analysis of an object, having a known attenuation, by 
means of data derived by means of a phantom having the same attenuation. 
In the described example the phantom was formed by a gel containing 
graphite spheres and having a known back-scatter function (this is because 
these spheres are Rayleigh scattering bodies when the power spectrum of 
the scattered wave varies as the fourth power of the frequency). For this 
phantom the frequency dependency of the scatter function of the object, 
U.sub.p (f) is known and constant in the frequency range from 1 to 10 MHz. 
At any point of an echographic line U(f) can be determined for which the 
following formula holds good: 
##EQU1## 
in which U.sub.P (f) and U(f) are the scatter functions of the phantom 
(known) and the object scanned (not known, to be determined), 
respectively, and S.sub.wp (.tau.,f) and S.sub.w (.tau.,f) are the 
formulas for the mean power spectrum for the phantom and the object 
scanned, respectively. 
This determination of the scatter function can be realized by means of the 
device described with reference to FIG. 1. The present device includes a 
single probe which forms the carrier for an ultrasound transducer 10 and 
which is suitable for obtaining A-type echograms of objects such as 
biological tissues. It will be apparent that the invention can be used in 
exactly the same way when instead of only one line a complete flat slice 
of the tissues is scanned, either by means of a manually displaceable 
probe or a probe involving so-called sectorial mechanical angular 
displacement connected to a radar-type display screen, or by means of a 
linear array of p ultrasound transducers which define a corresponding 
number of p parallel scanning directions in the object to be examined, 
said array being connected to a switching network for successively 
switching over the echo processing device to each active transducer or 
groups of transducers, or also by means of an array of transducers with 
so-called sectorial electronic scanning, said array also being connected 
to a switching network for switching over the processing circuit and also 
to a network of delay lines or phase shifters. 
The transducer 10 is connected on the one side to a transmitter stage 20 
for the repeated transmission of ultrasound signals in an arbitrary 
scanning direction through the tissues to be examined by means of the 
transducer, and on the other side to a receiver stage for the processing 
of the ultrasound echoes which are received by the transducer and which 
correspond to the principal obstacles encountered by the transmitted 
signals in their propagation direction. The situation of these obstacles 
is defined in the echogram by the echoes of high amplitude which denote 
the boundaries between tissues for which the frequency-dependency of the 
back-scatter coefficient is to be determined. This association is 
generally obtained by means of a selection circuit 40 which ensures that 
either the transmitter stage or the receiver stage is exclusively 
connected to the transducer (a selection circuit of this kind is 
mentioned, for example in U.S. Pat. No. 4,139,834). The circuit 40 
prevents the transmitted signals from being affected by the signals 
received and prevents the signals received from being masked by the 
signals transmitted. 
The receiver stage of the described embodiment includes a first processing 
circuit 100 for processing the ultrasound echoes received, which circuit 
consists of a first amplifier 101 (actually a preamplifier), a gain 
compensation device 102, an envelope detector 103 which has a rectifying 
and a smoothing function, and a display device 104. The output electrode 
of the transducer 10 is connected to the input of the amplifier 101 whose 
output signals are applied to the device 102 for compensating for the 
amplitude of the echoes as a function of the distance, to the device 103, 
and subsequently to the display device 104 so as to be displayed in the 
form of an A-type echogram on an axis which corresponds to the principal 
propagation direction of the transducer 10. The receiver stage also 
includes a second processing circuit which is connected parallel to the 
first processing circuit 100 and which itself is composed of the following 
elements: 
(A) a second amplifier 210 which also receives the output signal of the 
transducer 10; 
(B) a group of n channels 220a and 220n which are connected in parallel to 
the output of the amplifier 210 and each of which successively includes: 
(1) a bandpass filter 221a to 221n, the group of filters thus obtained 
being such that the respective passbands thereof are substantially equal 
and consecutive so that together they span approximately the passband of 
the transducer; 
(2) an envelope detector 222a to 222n which is identical for each channel 
and which includes a rectifier and a subsequent lowpass filter having a 
time constant which is preferably adjustable to a value which is larger 
than the mean time interval between the echoes of low amplitude which 
correspond to two adjacently situated scatter points in order to reduce 
the noise inherent of the biological object and the inhomogeneities 
thereof. 
Subsequent to the associated envelope detector, each channel of the 
receiver stage also includes: 
(3) a divider circuit 230a to 230n, a first input of which receives the 
output signal of the associated envelope detector in order to calculate 
the expression (S.sub.w (.tau.,f)/S.sub.wp (.tau.,f)).sup.1/2, a second 
input thereof receiving a dividing signal from a memory 240 which is 
controlled by a clock circuit 225 which is activated by the clock of the 
transmitter stage; 
(4) a logarithmic amplifier 226a to 226n; 
(C) an arithmetic circuit 300 which is connected to the output of the n 
channels 220a to 220n and which performs the following operations on the 
basis of the n output signals of said channels: 
(a) calculating the logarithm of the central frequency f.sub.i of the 
passband associated with each channel (or of another frequency which is 
representative of this passband); 
(b) storing in a memory and/or displaying (in logarithmic coordinates in 
FIG. 2) the ratio of the scatter function of the object examined and that 
of the phantom as a function of the frequency mentioned with reference to 
the calculation performed sub (a); 
(c) calculating the scatter function of the object scanned. 
By hypothesis this scatter function can be expressed in the polynominal 
form U(f)=a f.sup.b. The representation sub (b) is actually a curve which 
is parallel to the curve log.sub.10 (U(f)/U.sub.p (f)) (as a function of 
log.sub.10 f) and the local slope of this curve is the value b-b.sub.p, in 
which b.sub.p is the value of b in the case of the phantom. Therefore, in 
the case of the phantom only the value b.sub.p of b need be added to this 
formula in order to obtain a formula which is proportional to U(f) and 
hence suitable for determining the frequency-dependency of the scatter 
function of the object examined. 
The expression thus found which is proportional to U(f) represents the 
desired parameter which is made known to the user either directly by 
display (on the device 104) or which is stored in the memory for 
interpretation and later use. 
It will be apparent that the invention is not restricted to the described 
embodiment for which many alternatives are feasible. Particularly, it is 
to be noted that the memory 240 is either a programmable read-only memory 
(PROM) or a random access memory (RAM) and that it is loaded as follows, 
irregardless of whether the transducer is a focussing type or not. A slice 
of the phantom is selected which is situated at the front of the phantom 
(with respect to the device) and at a distance Z on the main axis of 
propagation, the phantom bearing against the transducer. In this position 
the energy spectrum of the echographic signal is determined by means of a 
transmitted signal which is assumed to remain the same thereafter; 
subsequently, this determination is repeated at the same distance Z but 
for other positions (realized by way of displacements perpendicularly to 
the main axis of propagation) in order to obtain a mean energy spectrum. 
For example, a mean spectrum is determined from 100 spectra around the 
same position. Subsequently, the same determination of the mean energy 
spectrum is repeated for different distances Z between the device and the 
phantom. After that, for all successive positions along the axis Z there 
are calculated the correction values whereby the output signals of the 
circuits 230a to 230n are subsequently divided; these correction values 
are written into the memory 240. It is also to be noted that the amplifier 
210 of the described embodiment has a fixed gain; should this amplifier be 
replaced by a type with automatic gain control as a function of the 
distance, the gain should be temporarily adjusted to a fixed value in 
order to execute the measurements. Such a result can be obtained by means 
of a time slot which inhibits the variation of the gain between two 
instants which correspond to the slice of tissues for which the 
measurements are performed.