Method for measuring characteristics of living tissue by ultrasonic waves

Abstract

ultrasonic waves of at least three independent frequency bands having different center frequencies are transmitted into a living body from its skin surface and reflected waves are analyzed, by which living tissue characteristics are measured. The reflected waves from various depths in the living body are received, their frequency components are separately extracted and energies of the received reflected waves are obtained, thereby obtaining an attenuation coefficient inclination and a space inclination of a frequency power exponent of a reflection coefficient of the living body.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method for measuring characteristic parameters of living tissues by transmitting ultrasonic waves into a living body and analyzing reflected waves therefrom. More particularly, the invention pertains to a method for measuring the frequency dependency of the reflection coefficient and that of the attenuation coefficient of the living tissue separately of each other.

Conventional systems for obtaining tissue characteristics by analyzing reflected ultrasonic waves of plural frequencies have been proposed by Iinuma (Japanese Patent "Kokai" No. 38490/74) and Nakagawa (Japanese Patent Publication No. 24798/77). With these systems, however, their operations are based on sound pressure waveforms, so that when the ultrasonic waves have a wide frequency band, like pulses, accurate measurements are impossible under the influence of the phase relationships of respective frequency components, pulse overlapping of continuous reflected waves and phase cancellation in a receiving sensor.

The abovesaid prior art systems can be employed in the case where the living body is composed of several kinds of tissues, an ultrasonic reflector of a definite, approximately smooth surface exists at the boundary between adjacent tissues and the reflection factor and the transmission factor of the ultrasonic reflector have no frequency dependence. Such reflection is called specular reflection.

With recent technological progress, however, it has become possible to measure a weak reflection from the tissue between boundaries. In general, the tissue has such a microstructure that cells, capillary vessels, lymphatic vessels, muscular fibers and so forth intertwine complicatedly. A typical size of such a tissue is nearly equal to or smaller than the wavelength of ultrasonic waves. On account of this, reflected waves from the microstructure are accompanied by complex interference owing to phase dispersion and pulse overlapping, introducing in a B-mode tomogram a speckled pattern commonly referred to as "speckle". It has been proven experimentally that reflection from the tissue (backward scattering) has a frequency characteristic such that its power reflection coefficient is proportional to the nth power of the frequency, and that the value of n is a characteristic value (a parameter) representing the tissue. It has been reported that n=2.2 in the liver and n=3.3 in the myocardium.

Systems for obtaining the tissue characteristics in such a case have been proposed by Hayakawa in references 1* and 2* and by others. *1. "Theory of Reflecting Ultrasonic Computer Tomograph Using Plural Frequencies", Proceedings of the 37th meeting of Japan Society of Ultrasonics in Medicine, in Japanese *2. "Multifrequency echoscopy for quantitative acoustical characterization of living tissues", J. Acoust. Soc. Am. 96 (6), June 1981.

Noting the energy value of ultrasonic waves, the system *1 conducts a second order differentiation of an attenuation coefficient by the natural logarithm of the frequency (δ² α/δ(lnf)²) and a first order differentiation in the direction of depth, by which "a second order differentiated value of the attenuation coefficient of the ultrasonic waves by the natural logarithms of their frequencies" is obtained as a tissue characteristic parameter. According to the system *2, energy (or power) values of the ultrasonic waves are obtained through utilization of three frequencies f₁, f₂ and f₃ and, as a difference value, "the second order differentiated value of the attenuation coefficient of the ultrasonic waves by the natural logarithm of their frequencies" is obtained in the form of a parameter. As experimentally ascertained, it is indicated that, when the attenuation coefficient is proportional to the first power of the frequency, as experimentally ascertained, the abovesaid parameter ##EQU1## becomes proportional to the attenuation constant α.

The abovesaid Hayakawa system requires complex processing corresponding to the second order differentiation by the natural logarithm of the frequencies, and hence is difficult J oni is a maximum numberτi40 ln G₁ ·A₁² of the ROM 178 are applied to a subtractor 175, wherein a subtraction lnE₁ -lnG₁ ·A₁² is carried out to output ##EQU22## which is stored in a memory 176.

Similar processing is performed for the reflected signal received at a time ti+1 after Δt to obtain ##EQU23## and a difference between this and ##EQU24## at the time ti stored in the memory 176 is obtained by a subtractor 177. The difference thus obtained is a differentiated (differenced) value at Δz. This becomes the output of the calculating unit 17-1 and represents the following quantity: ##EQU25## Likewise, the calculating unit 17-2 provides the following output: ##EQU26##

Reference numerals 18-1, 18-2, 18-3, . . . signify subtractors. The subtractor 18-1 subtracts the output of the calculating unit 17-2 from the output of the calculating unit 17-1. The subtractor 18-2 subtracts the output of the calculating unit 17-3 from the output of the calculating unit 17-2. The other subtractors operate in a similar manner. In a similar manner, the following subtractors operate.

Thus the output of the subtractor 18-1 provides the difference between Eqs. (14) and (15): ##EQU27## This is the left side of Eq. (4) as shown in Eq. (6). The order of calculation by the calculating units 17-1 and 17-2 and the calculation by the subtractor 18-1 is reverse from the order of calculations described previously but, in this case, it does not matter mathematically.

The output of the subtractor 18-2 similarly provides the left side of Eq. (5).

Reference numeral 19-1 indicates an algebraic calculator which receives the outputs of the subtractors 18-1 and 18-2 and solves from Eqs. (4) and (5) a simultaneous equation with β(z) and ##EQU28## as the unknowns. Certain constants α₁1 and α₁2 determined by the frequencies f₁ and f₂ are multiplied by the outputs of the subtractors 18-1 and 18-2 and then added together to obtain β(z). Other constants α₂1 and α₂2 are likewise multiplied by the outputs of the subtractors 18-1 and 18-2 and then added together to obtain ##EQU29## It is convenient to calculate the constants α₁1, α₁2, α₂1 and α₂2 from the frequencies f₁ and f₂ in advance and to prestore them in the algebraic calculator 19-1.

Reference numeral 20 designates an arithmetic mean circuit which comprises an adder 21 for adding the outputs of the algebraic calculators 19-1, 19-2, . . . and a divider 22 for dividing the output of the adder 21 by the number N of inputs to the adder 22. The arithmetic mean circuit 20 obtains an arithmetic mean value of the N β(z) or ##EQU30## values respectfully obtained from all the frequency components of the output from the DFFT circuit 16.

Reference numeral 23 identifies a shift register which comprises L stacked registers 23-1, 23-2, . . . 23-L for storing the output of the arithmetic mean circuit 20. At first, the output of the arithmetic mean circuit 20 for the depth i is written into the register 23-1 and when the output of the arithmetic mean circuit 20 for the next depth goes into the register 23-1, the content of the register 23-1 is shifted to the register 23-2. In this way, upon each occurrence of inputting new data into the register 23-1, previous data are shifted upward through successive registers in the shift register 23. In consequence, L data are stored in the shift register 23, with the oldest data in the register 23-L and the latest one in the register 23-1.

Reference numeral 24 denotes an arithmetic mean circuit for obtaining an arithmetic mean value of L data. The arithmetic mean circuit 24 is also comprised of an adder 25 for adding L outputs from the registers 23-1 to 23-L and a divider 26 for dividing the output of the adder 25 by L. The outputs of the registers 23-1 to 23-L are added together by the adder 25 and its output is applied to the divider 26, wherein it is divided by L to obtain the arithmetic mean.

The output of the arithmetic mean circuit 24 provides, for each scanning, a mean value of (L×ZN)β(z)'s or ##EQU31## over the depths z₁, z₂, z₃, . . . Z_L is obtained, and the mean value is stored in a memory. By scanning the same tissue M times at certain time intervals, obtaining a measured value for each scanning, storing it and averaging the values for the same depth z_i in all the measurements, it is possible to obtain a mean value of L×M×N samples for each depth z_i.

While the above description has been given of a method for executing statistical processing with the last calculated value β(z) or ##EQU32## the statistical processing can be applied to intermediate results and this may sometimes make the subsequent calculations easy. This can be achieved, for example, by executing statistical processing of the outputs of the DFFT circuit 16 in connection with frequency for M-time scanning of L points to remove the influence of the spectrum scalloping and executing again statistical processing with a last calculated value.

In the foregoing embodiment the frequency components f₁, f₂, f₃, . . . correspond to the outputs of the DFFT circuit 16 in a sequential order but, by a suitable selection of the outputs of the DFFT circuit 16 in a manner to form a geometric or arithmetic progression as described previously, the calculating circuits of the algebraic calculators 19-1, 19-2, . . . can be simplified although the number of N's decreases.

By scanning one sectional area of a living body in successive scanning directions so that, for instance, β(z) may be obtained as a function of each of the depths z₁, z₂, . . . z_i and z_i+1 as a mean value of the L×M×N measured values for each scanning direction, and then displaying the resulting values on the corresponding scanning lines of a CRT, it is possible to obtain a distribution diagram of β(z) or ##EQU33## on the sectional area of the living body. This is very useful for detecting an abnormal tissue as of a cancer.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

Claims

1. A method for measuring the internal characteristics of a body, comprising:

transmitting pulses of ultrasound pressure waves into said body, each said pulse comprising ultrasound pressure waves of at least three frequencies;

selectively receiving as different respective return signals the corresponding ultrasound pressure waves reflected from different ranges of depths in said body;

determining values corresponding to the relative energy of respective frequency components in each said return signal corresponding to said at least three frequencies; and

processing said values to determine information on spatial variation of the reflection coefficient at different depths in said body, wherein said processing includes effectively forming ratios of said relative energy of said frequency components in each said return signal for respective different pairs of said at least three frequencies.

2. The method of claim 1, said processing involving forming the differences in time in numbers corresponding to said values for adjacent ranges of depth for each respective return signal, and between numbers corresponding to said values for pairs of said frequency components for each respective return signal.

3. The method of claim 2, wherein the reflection coefficient is given by δ=bfⁿ, and the determining of the variation in the reflection coefficient is in terms of the spatial variation of the frequency power exponent n, b being a constant and f the frequency of the respective ultrasonic waves.

4. The method of claim 2, said processing providing also information on spatial variation of the frequency inclination β of the attenuation coefficient α=βf in said body, f being the frequency of the respective ultrasonic waves.

5. The method of claim 2 or 4, said processing comprising taking the a logarithm of each said value of each respective return signal, prior to forming said differences, and subsequently, after forming said differences, solving algebraically between the respective differences, or of selected pairs of said frequency components, to provide the respective information.

6. The method of claim 5, wherein said at least three frequencies define a geometric progression.

7. The method of claim 5, wherein said at least three frequencies define an arithmetic progression.

8. The method of claim 5, comprising averaging the respective results of said solving for said information for different respective pairs of said frequency components.

9. The method of claim 5, comprising performing said processing of said return signals for a plurality of adjacent depths, in said body, and averaging the respective results thus obtained for said information.

10. The method of claim 1 or 4, said processing occurring on a real time basis.

11. The method of claim 1, comprising providing said pulses of ultrasound pressure waves for transmission into said body as broadband pulses including said at least three frequencies, and performing Fourier transformation of each of said return signals for said determining of said values corresponding to the energy of the respective frequency components.

12. A device comprising:

means for transmitting pulses of ultrasonic waves of at least three frequencies into a body under test, and for determining values corresponding to the energies of respective frequency components of corresponding ultrasonic waves reflected from selected ranges of depths in the interior of said body;

means for processing said values to provide information on spatial variation of the reflection coefficient δ=bfⁿ within said body, wherein b is a constant, f is the frequency of the respective ultrasonic waves and n is a number.

13. The device of claim 12, said means for processing comprising:

a plurality of calculating units for processing respective ones of said values corresponding to energy of the respective frequency components;

a plurality of subtractors having as inputs the outputs of respective pairs of said calculation units,

a plurality of algebraic units having as inputs the respective outputs of two of said subtractors, and

an arithmetic mean circuit having as inputs the outputs of said algebraic units.

14. The device of claim 13, comprising:

a register for storing in successive stages the successive outputs of said arithmetic means circuit, each said stage providing a respective output of the content stored therein; and

an averaging circuit for averaging the respective outputs of the stages of said register.

15. The device of claim 12, comprising means for providing said values as the logarithm of said energies.

16. The device of claim 12, said processing means including means for providing information on spatial variation of the frequency inclination β of the attenuation coefficient α=βf, where f is the frequency of the respective ultrasonic waves.

17. A method for determining internal characteristics of a body from ultrasonic pulses transmitted into the body, each ultrasonic pulse including pressure waves having at least three frequencies, the ultrasonic pulses reflected with a reflection coefficient from a range of depths and received as reflected signals, said method comprising the steps of:

(a) determining energy values corresponding to energies in the reflected signals, each energy value corresponding to one of the frequencies in one of the ultrasonic pulses; and

(b) identifying a spatial variation of the reflection coefficient using relationships of the energy values corresponding to different frequencies of the ultrasonic pulses reflected from substantially identical depths.

18. A method as recited in claim 17, wherein the relationships of the energy values correspond to ratios of the energy values of adjacent frequencies in the reflected signals received simultaneously. 19. A method as recited in claim 18, wherein said identifying in step (b) comprises the steps of:

(b1) calculating logarithms of energy values determined in step (a) for adjacent depths and adjacent frequencies;

(b2) generating corrected energy logarithms from the logarithms of the energy values in dependence upon respective ones of the adjacent depths;

(b3) calculating a number representing differences between the corrected energy logarithms of the adjacent frequencies and adjacent depths;

(b4) repeating steps (b1)-(b3) for all sets of the adjacent frequencies and all of the adjacent depths within the range of depths; and

(b5) determining the spatial variation of the reflection coefficient using the number calculated in step (b3) for each of the sets of the adjacent frequencies. 20. A method as recited in claim 19, wherein step (b) further comprises the step of (b6) averaging the spatial variation of the reflection coefficient over the at least three frequencies and the range of depths.