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CROSS REFERENCE TO
RELATED APPLICATION
This application is a continuation in part of an application entitled "Transaxial Compression Technique for Sound Velocity Estimation," Ser. No. 7/438,695, filing date Nov. 11, 1989.
Applicant incorporates said application Ser. No. 7/438,695 by reference herein and claims the benefit of said application for all purposes pursuant to 37 C.F.R. .sctn.1.78.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to methods and apparatus for performing ultrasonic diagnosis of a target body. More particularly, the invention pertains to methods and apparatus for measuring compressibility or compliance in a target body. The
invention is directed towards techniques for enhancing the accuracy of such measurements in compressible or compliant targets, particularly the human body, using one or more ultrasonic transducers in pulse-echo mode.
2. Description of Related Art
Traditional ultrasonic diagnosis is achieved by transmitting ultrasonic energy into a target body and generating an image from the resulting echo signals. A transducer is used to both transmit the ultrasonic energy and to receive the echo
signals. During transmission, the transducer converts electrical energy into mechanical vibrations. Acquired echo signals produce mechanical oscillations in the transducer which are reconverted into electrical signals for amplification and recognition.
A plot or display (e.g., on an oscilloscope, etc.) of the electrical signal amplitude versus echo arrival time yields an amplitude line (A-line) or echo sequence corresponding to a particular ultrasonic transmission. When the A-line is displayed
directly as a modulated sinusoidal pattern at radio frequency ("RF"), it is typically referred to as an RF or "undetected" signal. For imaging, the A-line is often demodulated to a non-RF or "detected" signal.
Ultrasound techniques have been extensively used in the field of diagnostic medicine as a non-invasive means of analyzing the properties of tissue in vivo (i.e., living). A human or animal body represents a nonhomogeneous medium for the
propagation of ultrasound energy. Acoustic impedance changes at boundaries of regions having varying densities and/or sound speeds within such a target body. At such boundaries, a portion of the incident ultrasonic beam is reflected. Inhomogeneities
within the tissue form lower level scatter sites that result in additional echo signals. Images may be generated from this information by modulating the intensities of pixles on a video display in proportion to the intensity of echo sequence segments
from corresponding points within the target body.
Conventional imaging techniques are widely used to evaluate various diseases within organic tissue. Imaging provides information concerning the size, shape, and position of soft tissue structures using the assumption that sound velocity within
the target is constant. Qualitative tissue characterization is carried out by interpretation of the grey scale appearance of the sonograms. Qualitative diagnosis largely depends on the skill and experience of the examiner as well as characteristics of
the tissue. Images based only on relative tissue reflectivity, however, have limited use for quantitative assessment of disease states.
Techniques for quantitative tissue characterization using ultrasound are needed for more accurate diagnosis of disorders. In recent years many significant developments have been achieved in the field of ultrasonic tissue characterization. Some
acoustic parameters, e.g., speed of sound and attenuation, have been successfully used for tissue characterization. One promising physical parameter for quantitative measurement is compressibility or compliance. The amount of compressibility or
compliance within tissues changes within regions of varying density. Diseased tissue, such as tumors, may be harder or softer than normal tissue, and thus have a different amount of compressibility.
Tissue compressibility is an important parameter which is used to detect the presence of diffuse or localized disease. Measuring changes in compressibility becomes important in the analysis of tissue for pathological conditions. Many tumors are
firmer than the surrounding normal tissue, and many diffuse diseases result in firmer or more tender pathology. Examples can be found in diffuse liver disease, prostate cancer, uterine fibroids, muscle conditioning or disease, and many other conditions.
Traditionally, physicians routinely palpate various regions of a patient's body to get an impression of tissue firmness or tissue softness. This technique is a form of remotely trying to sense what is going on in terms of tissue compliance. For
example, in a liver, if the compliance in an area is sensed to be different from compliance in the surrounding area, the physician concludes from the tactile sensations in his fingers that something is wrong with the patient. The physician's fingers are
used to perform a qualitative measurement.
The ability to quantitatively measure the compressibility or compliance of tissue in localized regions would help with (1) objective quantification of commonly used clinical signs, (2) localizing these measures, (3) making the measurements deep
in tissue with simple equipment, (4) constructing images of the compressibility or compliance parameter in vivo, which may be used alone or in conjunction with ordinary sonograms.
One technique has attempted to quantitatively measure the elasticity and compressibility of tissues by correlating patterns obtained in ultrasonic measurements of tissue movement in vivo. The method applies Fourier analysis to a clinical study
of patterns of tissue movement, specifically in the liver. The technique uses Fourier analysis to enable objective differentiation of different tissue types in pathologies on the basis of numerical features of the time-course of the correlation
coefficient between pairs of A-scans recorded with a particular time separation. Tissue oscillations resulting from periodic stimulus by waves resulting from ventricular contraction and pressure pulses in the descending aorta are measured to derive
patterns of movement. Fourier series transformation is used to analyze the data to quantitate the kinetic behavior of the tissue in vivo. See. Tristam et al.. "Application of Fourier Analysis to Clinical Study of Patterns of Tissue Movement,"
Ultrasound in Med. & Biol., Vol. 14, No. 8, (1988) 695-707.
In another approach, patterns of tissue movement are correlated in vivo. This technique basically studies details of the patterns of movement in tissues in response to a normal physiological dynamic stimulus such as cardiac motion. A method is
given for quantifying tissue movement in vivo from the computation of a correlation coefficient between pairs of A-scans with appropriate time separation. Tristam et al., "Ultrasonic Study of in vivo Kinetic Characteristics of Human Tissues," Ultrasound
in Med. & Biol., Vol. 12, No. 12 (1986) 927-937.
The waveforms of liver dynamics caused by aortic pulsation and vessel diameter variations are analyzed in still another method, involving a signal processing technique for analyzing radio frequency M-mode signals. The technique uses patterns of
movement in response to arterial pulsation to determine tissue characteristics. The technique measures displacement, velocity, and strain as a function of time in small deformations in tissue due to arterial pulsation. Wilson and Robinson, "Ultrasonic
Measurement of Small Displacements and Deformations of Tissue," Ultrasonic Imaging, Volume 4, (1982) 71-82.
Yet another method processes echoes in order to measure tissue motion in vivo. The motion patterns observed in vivo are correlated to arterial pressure pulse. Dickinson and Hill, "Measurement of Soft Tissue Motion Using Correlation Between
A-Scans," Ultrasound in Med. & Biol., Vol. 8, No. 3, (1982) 263-271.
All of the above techniques focus upon the dynamic motions of tissue in vivo. These methods are limited due to the complexity of tissue motion, and the behavior of the stimuli employed in those methods.
SUMMARY OF THE INVENTION
The present invention provides a pulse-echo system that has particular application in estimating compressibility in a target body. The target body may be any animal or human tissue, or any organic or inorganic substance that is compressible or
compliant. The term "animal tissue" includes "human tissue". An ultrasonic source is used to interrogate the target body. The detection of echo sequences may be at the ultrasonic source. The invention allows for accurate, localized determination and
imaging of an important parameter, compressibility, which has been used qualitatively in medicine for a very long time.
The compressibility of a material is normally defined as the inverse of the bulk modulus of the material. Thus,
v=a change in volume;
V=the original volume;
F=force applied to the volume;
a=area across which the force is applied.
In the present instance, it may be generally assumed in determining relative compressibilities within a material that the terms "F" and "a" will remain constant along an axis of compression, and that the terms "l" and "L" may be employed in place
of v and V, where
l=a change in the length of a segment of interest along an axis of compression, and
L=the original length of the segment.
Thus, the compressibility of any given segment or layer within a material relative to another segment or layer may be estimated from the relationship K.sub.1 =K.sub.2 (l.sub.1 /L.sub.1)/(l.sub.2 /L.sub.2), where
K.sub.1 =compressibility of a first segment or layer;
l.sub.1 =change in length of the first segment or layer along an axis of compression in response to a given force;
L.sub.1 =original length of the first segment;
l.sub.2 =corresponding change in length of a second segment or layer;
L.sub.2 =original length of the second segment or layer; and
K.sub.2 =compressibility of the second segment or layer.
In those instances where absolute value of compressibility of a segment or layer is desired, such a value may be estimated from the relationship
F=a change in compressive force, and
a=the area of application--typically, the cross-sectional area of a transducer forced against a material which includes the segment or layer of interest.
In the present invention, the velocities of sound in different segments or layers may be employed, together with time measurements, to calculate distances within the segments or layers. The ultrasonic signals also provide a precise measuring
tool. The velocities of sound may be determined using the apparatus and procedures disclosed in the application entitled "Transaxial Compression Technique for Sound Velocity Estimation," Ser. No. 7/438,695, filing date Nov. 17, 1989.
The invention contemplates sonically coupling an ultrasonic source to a target body; energizing the ultrasonic source to emit a first ultrasonic signal or pulse of ultrasonic energy from the source along an axis into the target body; detecting
from a region within the target body a first echo sequence including a plurality of echo segments resulting from the first transmitted signal; displacing the target body along the axis while maintaining coupling between the ultrasonic source and the
target body; energizing the ultrasonic source to emit a second ultrasonic signal along the axis into the target body; and detecting from the region within the target body a second echo sequence including a plurality of echo segments resulting from the
second transmitted signal; measuring the differential displacement of the echo segments. A plurality of first ultrasonic signals or pulses of ultrasonic energy may be emitted and a plurality of first echo sequences detected before displacing the target
body. Then a plurality of second signals and pulses are emitted along a plurality of parallel paths and a plurality of second echo sequences are detected.
In one embodiment, a transducer is the ultrasonic source and is sonically coupled to direct an ultrasonic signal or pulse of ultrasonic energy into the tissue along a radiation axis such that movement of the transducer along the axis effects a
change in compression of the tissue.
In a preferred embodiment of the present invention, the ultrasonic source is a transducer sonically coupled to a tissue of interest. A first pulse of ultrasonic energy is emitted along a path into the target body and the arrival of a first echo
sequence (A-line) including one or more echo segments is detected from regions within the tissue along the path resulting from the first pulse of ultrasonic energy. Thereafter, compression is changed within the tissue along the path. The compression
change may be accomplished by transaxially moving the transducer along the path to compress or displace a proximal region of the tissue. A second pulse is emitted, and the arrival of a second echo sequence including one or more echo segments common to
the first echo sequence is detected in response to the second pulse. The differential displacements of at least one echo segments is measured. The echo sequences detected are from common regions within the tissue.
A comparison of the first and second echo sequences or waveforms with intervening compression reveals a generally decreasing displacement of tissue structures with depth. In a homogeneous medium, the rate of decrease will tend to be asymptotic.
Of particular interest is the differential displacement per unit length--i.e., strain. In a homogeneous compressible medium, the strain will tend to be constant along the axis of displacement. In a non-homogeneous medium, the strain varies along the
axis of displacement.
The strain of a tissue may be calculated using the arrival times of first and second echo sequences from proximal and distal features in a target body--i.e., tissue--using the following equation: ##EQU1## t.sub.1A =arrival time of a first echo
sequence from a proximal feature; t.sub.1B =arrival time of a first echo sequence from a distal feature;
t.sub.2A =arrival time of a second echo sequence from a proximal feature; and
t.sub.2B =arrival time of a second echo sequence from a distal feature.
The arrival times of the echo segments from a common point detected in response to a first and second pulse of ultrasonic energy are compared. The common points may be found in features occurring within the echo signal. The time shifting of the
two echo segments is used to determine compressibility.
Thus, if no change in arrival time has occurred with an intervening compressive force, it follows that a target body has not been compressed along the travel path leading to the source of the echo segments. On the other hand, if the arrival time
of the second echo segment is smaller than the arrival time of the first echo segment, it is clear that compression has occurred and that the target body is compressible. Moreover, the difference in arrival times, taken together with other available
data, makes it possible to quantify the compressibility of the target body.
In another embodiment of the invention, body segments which extend along the transmission path of the ultrasonic pulses are selected within a target body and separate first and second echo segments detected from within each body segment. Thus, a
series of first and second echo segments is detected for the body segments selected for interrogation. Preferably, the echo segments are detected from the proximal and distal ends of body segments relative to the ultrasonic source. Measurement of the
time shifts of echo segments in the first and second echo sequences which correspond to the proximal and distal ends of each body segment are then made. By studying the time shifts, it becomes possible to determine whether changes in compressibility
occur along the ultrasonic beam within the target body.
A preferred embodiment of the invention involves (1) sonically coupling a material with a known Young's Modulus and speed of sound to the surface of the target body; (2) emitting a first pulse of ultrasonic energy along a path through the
material into the target body; (3) detecting a first echo sequence including a plurality of echo segments, from within the target body resulting from the first pulse; (4) forcing the material against the target body sufficiently to displace the target
body while maintaining acoustic coupling between the material and the target body; (5) emitting a second pulse of ultrasonic energy along the path through the material into the target body; and (6) detecting a second echo sequence including a plurality
of echo segments common to the first echo sequence, resulting from the second pulse. The presence of the material with a known Young's modulus and speed of sound makes it possible to determine the Young's modulus of the target body. If the target body,
itself, has multiple layers, it also becomes possible to determine the Young's moduli of the individual layers. The application of Young's modulus to these matters is explained later in this description.
The present invention takes advantage of the acoustical properties of physically compressible or displaceable materials. These materials--for example, animal or human tissues--often contain a large number of acoustic "scatterers". The
scatterers, being small compared to the wavelength of the sound frequencies involved, tend to reflect incident sound energy in all directions. For example, in homogeneous tissue regions, scatterers may comprise a collection of nearly identical
reticulated cells. The combined reflections from each scatterer create a background echo signal called speckle. A particular arrangement of scatterers will shift in response to axial forces from the transducer, changing the time an echo is received
from the arrangement. The echoes received from the various arrangements of scatterers form an echo sequence. A selected echo segment or wavelet of the reflected RF signal corresponds to a particular echo source within the tissue along the beam axis of
the transducer. Time shifts in the echo segment or wavelet are examined to measure compressibilities of tissue regions. It is important that the shape of the echo segment or wavelet not change significantly, due to compression, such that identification
of the wavelet is not possible, and that the signals not be decorrelated beyond an acceptable range. The time shift can be determined by analyzing the data in a computer or by a visual examination, but the analysis will generally be easier with a
computer.
Studying an internal region of the human body is accomplished by sonically coupling an ultrasonic transducer to the body so as to emit an ultrasonic signal along an axis into the region, and such that movement of the transducer along the axis
relative to the region will change the compression of the body between the transducer and the region; energizing the transducer to emit a first signal along the axis into the body and the region; detecting the arrival at the transducer of a plurality of
spaced echo segments resulting from the first signal and coming from the region; moving the transducer along the axis relative to the region sufficient to change the compression of the body between the transducer and the region while maintaining said
sonic coupling; energizing the transducer to emit a second signal along the axis into the body and said region; detecting the arrival at the transducer of each echo segment resulting from the second signal; and determining the strains produced in
segments of the region between the pairs of echo segments.
The present invention is of particular interest in interrogating organic tissue, especially human and other animal tissue. Thus, as a transducer is pressed against such a material, scatterers in a region within the material are displaced from
one position to another. For elastic materials, release of the pressure enables the scatterers to return to their original position. A principal object of such interrogation is to use echo signals from the tissue in strain studies which may reveal the
presence of abnormalities. In general, when employing a transducer to transmit signals into a living body, care should be taken to coordinate the transducer signals with naturally occurring signals. Thus, in the human body, the transducer should
normally be activated at times which will minimize interference by signals such as aortic and vessel pulses.
This invention may be used in the detection of diseases such as breast cancer and prostate cancer to accurately detect and locate tumors at an early stage. Another advantage of the invention is the avoidance of ionizing radiation from x-rays.
It will be noted at this point that the invention is contemplated to have significant applications other than in medicine. One such application, for example, is in the quality grading of beef. The invention may be used to quantitate the
tenderness of beef before and after slaughter. This ability is economically important in determining when to slaughter cattle. Other applications would include, for example, interrogation of materials and products such as cheese or crude oil that are
physically displaceable by the movement of a transducer.
It will be noted that the transducers employed in the present invention need not be in direct contact with the materials to which they are applied. It is necessary, however, that transducers be sonically coupled to the materials in a manner such
that movement of the transducers will result in displacement of the materials. Sonic coupling methods and agents are well known in the art.
It will be also noted that a material may be displaced according to the invention either (a) by advancing a transducer against a compressible elastic material to increase compression, or (b) by retracting a transducer from a compressed position
within the material. Changing compression means compressing or decompressing the target body.
As noted above, it is not necessary that an echo from a discrete feature in a tissue or other compressible material be employed. It is sufficient that an identifiable echo segment be present in the echo signal resulting from a transmittal
signal. Even though the physical features within a material responsible for a selected echo segment may not be clearly known, the selected echo segment is an adequate reference for the purposes of the invention. Thus, the compression of a material and
signal travel times determined before and after such compression may be based upon comparison of time shifts in the echo segments. Similarly, the recovery of an elastic material from an initially compressed condition and the signal travel times before
and after such recovery or decompression may be based upon comparisons of time shifts in the echo segment.
The present invention may also be employed for estimating compressibility or compliance in targets having multiple layers. It will be noted that the terms "compressibility" and "compliance" in the present context have generally similar
connotations. In any event, the compressibility in each of the progressively deeper layers is estimated by employing the same techniques discussed above. According to the present invention, the compressibility may be estimated in each layer from only
two echo sequences along the axis of radiation. The echo sequence may be divided into echo segments corresponding to the layers. Thus, imaging of the compressibility parameter in a plane or volume of a target body may also be accomplished by
appropriate lateral translation of the transducers. Other objects and advantages of the invention will become readily apparent from the ensuing description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a shows an embodiment of the invention where one transducer is sonically coupled to a target body to interrogate a distal tissue region within the target body.
FIG. 1b shows a plot of an RF echo signal originating from the distal tissue region interrogated in FIG. 1a.
FIG. 2a shows the transducer of FIG. 1a imparting a small compression to a proximal region of the target body.
FIG. 2b shows a plot of the time shifted RF echo signal originating from the distal tissue region as interrogated in FIG. 2a.
FIG. 3 shows a one dimensional spring model of tissue before and after compression.
FIG. 4 shows a one dimensional spring model of tissue with a totally incompressible section before and after compression.
FIG. 5 shows the equipment set up for experiment 1.
FIG. 6 shows the equipment set up for experiment 2.
FIG. 7 shows an apparatus embodiment of the invention in which a transducer is coupled to a target body via a stand-off device containing an acoustic coupling fluid.
FIG. 8 is a block diagram depicting an apparatus embodiment of the invention controlled by a computer.
DETAILED DESCRIPTION
FIG. 1a shows the transducer 10 sonically coupled to a target body 15. An ultrasonic pulse 18 is shown propagating within beam 20 toward a echo source 25 on beam axis 12. As the pulse 18 propagates through the target 15, corresponding echoes
are generated and arrival times noted at the transducer aperture 11. The combination of all echoes generated from reflections within the beam 20 is the echo sequence or A-line corresponding to pulse 18. A radio frequency ("RF") signal plot of the
A-line acquired from pulse 18 is shown in FIG. 1b. The amplitude of the signal in millivolts is plotted against echo arrival times in microseconds (.mu.s). Later arrival times correspond to progressively deeper regions within the target body 15. An
echo segment or echo wavelet 30, within a chosen arrival time Window, is selected as a reference. The time window may be selected based on anatomical data from ultrasound imaging, or may be arbitrary, e.g., every x micro seconds. The echo segment or
wavelet 30 originates from the echo source 25.
FIG. 2a shows the transducer 10 being translated along axis 12 to impart a small compression (y.sub.1) to the tissue. Alternatively, as shown in FIG. 7, a transducer 80 may be associated with a stand-off device 85 which allows the transducer so
to be acoustically or sonically coupled to the target body 90 without being in direct contact with the target body. In this case the stand-off 85, and not the transducer, compresses the target.
After the transducer 10 Compresses the target, a second pulse 22 is emitted and the corresponding A-line segment is acquired from a desired depth within the tissue. FIG. 2b shows the RF plot of a time shifted A-line corresponding to pulse 22.
The echo segment or wavelet 32 associated with echo source 25 is also time shifted. The time shifted wavelet 32 is tracked within the selected time window using standard pattern matching techniques. The window selected must be such that the wavelet of
interest will not be shifted out of the window. This selection may involve the size of the window or the positioning of the window. The window selected should reveal both wavelets or echo segments. The arrival time of echo segment or wavelet 32 is
prior to that of echo segment or wavelet 30 above, since the distance between aperture 11 and feature 25 was shortened by the compression .DELTA.y.sub.1.
In a preferred embodiment of the invention, a transducer is positioned on or otherwise coupled to a target tissue and advanced axially toward the target to compress the target. Alternatively, the invention may be practiced by retracting a
transducer from a previously compressed position. Since the relatively large aperture size of the transducer precludes penetration of the tissue, small tissue displacements occur instead. A pulse is emitted prior to the displacement, and a first echo
sequence received in response to the pulse is recorded. Following displacement, a second pulse is emitted and a second echo sequence is recorded in response to transmission. Next, a comparison of the waveforms is made to reveal a decreasing
displacement of the tissue structures with depth. The decrease will generally be asymptotic in character.
In the foregoing embodiment, a single compression of a homogenous target body, and a repetitive sinusoidal waveform signal have been described. It will be apparent, however, that other conditions may be employed. Thus, multiple compressions,
other waveforms and other signal sources, such as array transducers, may be used. These signal sources, for example may be non-repetitive and may generate spike-like signals.
In tissue that is not homogeneous, the shifting of tissue in various segments will differ. For example, if a segment of tissue is less compressible than the overall tissue containing the segment, the tissue in the segment will compress or strain
less than if the segment of tissue were of the same compressibility as the tissue as a whole. Alternatively, when a segment is more compressible than the tissue as a whole, the segment will compress or strain more than if the segment were of the same
compressibility as other segments.
Referring to FIG. 3, a strain model is shown which illustrates how Young's Modulus may be employed in explaining the application of the present invention to compressible materials, notably human organs and tissue. Young's modulus is a basic
property of elastic materials and elastic materials can be characterized by their Young's moduli. Human tissue, accordingly, may be similarly described.
Briefly stated, Young's Modulus for any given material is the numerical ratio of the stress applied to the material to the resulting strain in the material. Thus, Y=F/(A)(S)=P/S, where Y is Young's Modulus for a given material; F is the total
force applied to the material; A is the area of application of the force; S is the strain; and P is pressure. It will be recognized from the relationship of these several factors that the Young's modulus of a material is a measure of the stiffness of
the material.
The model in FIG. 3 represents four segments A, B, C and D of a compressible body wherein each segment is uniformly compressible and equal in length when not compressed. Each segment in FIG. 3 is represented by a spring which is identical to the
other springs. The springs in the left hand array reflect the condition of the model at a state of no compression. The springs in the right hand array represent the condition of the model when a compressive force applied to the top most segment has
displaced the top of that segment by a distance 4.DELTA.y. It may be seen that point x has been displaced by a distance 4.DELTA.y to point x'. It may also be seen that this total displacement has been distributed equally across each one of the springs,
thereby causing each segment to shorten by the same amount. Thus, segment or spring A has been shortened by .DELTA.y from a to a'; segment or spring B has been shortened by .DELTA.y from b to b', etc. The net effect, however, has been to displace each
segment progressively more, going from segment or spring D to segment or spring A.
The total compression of the model is shown by the change in length 4.DELTA.y. The change in length of segment A is calculated as 4.DELTA.y-3.DELTA.y=.DELTA.y. The total compression of segments B-D is shown by the change in length 3.DELTA.y.
The change in length of segment B is calculated as 3.DELTA.y-2.DELTA.y=.DELTA.y. The total compression of segments C and D is shown by the change 2.DELTA.y. The change in length of segment C is calculated as 2.DELTA.y--.DELTA.y=.DELTA.y. Finally, the
total compression of segment D is shown by the change in length .DELTA.y from d to d'. The change in length of segment D is calculated as .DELTA.y-0=.DELTA.y. The change in length of each segment is equal to .DELTA.y. Each segment, as long as all of
the segments are equal in compressibility, compresses by the same net amount .DELTA.y.
The strain of each segment may be computed as .DELTA.y/1, where 1 is the initial (uncompressed) length of the segment. This strain value is in fact the quantity of most interest for purposes of display. Clearly in this case, the strain in this
one-dimensional system is constant for each segment, reflecting the fact that all springs are equal. The strain is, however, affected by the initial displacement.
On the other hand, if a segment in the above strain model were totally incompressible, the incompressible section would show no strain, but its presence would nevertheless affect the compression of the other sections. Referring to FIG. 4, for
example, one of the springs C in the model of FIG. 3 has been replaced by a totally stiff spring (so stiff, it can actually be replaced by a thin rod which is incompressible). Now one of the segments is incompressible. Using the same total compression
in FIG. 3 of 4.DELTA.y, the total compression of segments A-D is shown by change 4.DELTA.y in overall length. The change in length for Segment A is now calculated as 4.DELTA.y-2/3x 4.DELTA.y=4(12/3).DELTA.y=(4/3)(.DELTA.y).
The total compression of segments B-D is shown by the change (8/3)(.DELTA.y) in overall length. The change in length of segment B is now calculated as 8/3.DELTA.y-4/3.DELTA.y=(4/3)(.DELTA.y).
The total compression of segment C is zero. The change in length for segment C is calculated as (4/3)(.DELTA.y)-(4/3)(.DELTA.y)=0.
The total compression of segment D is shown by the change in length from d to d'. The change in length of segment D is calculated as (4/3)(.DELTA.y)-0=(4/3)(.DELTA.y).
Each of the segments A, B, and D is compressed equally, since they are represented by equal springs. However, the amount by which each one of these segments is compressed is larger than in the prior example, because the same displacement
4.DELTA.y is now divided over 3 springs, and not 4 as before. The segment C, represented by a incompressible rod shows no strain, but its presence affects the compression of the others. 0 In conclusion, as long as segments, represented by springs, have
the same Young's modulus, they will show equal strain which may be measured. The magnitude of this strain is dependent on the initial compression and on the number of equal segments. A segment of different Young's modulus can be discerned due to the
different strain effects it introduces. Its presence changes the strains of the surrounding segments. Thus, changes in strain within different segments of a tissue may be detected by using a spring model of the tissue.
As explained above, the presence of an abnormality or defect in an otherwise homogeneous tissue causes the baseline strain of the surrounding homogeneous segments to change, because of the requirement that the integral of all the strains along
the strain path (area under strain profile) be equal to the initial displacement. In other words, "normal" tissue strain is influenced by the size and Young's modulus of an abnormal segment. Thus, only relative measurements can be made using the strain
model alone. These measurements are useful, but absolute measurements are also desirable.
It becomes possible to determine compressibility within a tissue in absolute terms using a strain profile which includes the tissue together with a coupling medium with a known Young's modulus and speed of sound. Thus, a layer of a material
having a known Young's modulus and speed of sound may be interposed as a layer between a transducer and the tissue, and the method of the invention may then be applied to obtain a strain profile of the combined layers. The known layer may consist of
compressible or compliant material such as rubber, sponge, gels, etc. The material should be compressible and provide for an ultrasonic transmission path to the tissue. The material may be echogenio, but it is not necessary.
Using the method of the invention, sonic measurements are made before and after a force is applied to a transducer so as to compress the known layer and the unknown tissue. The resulting strain data are used to produce a strain profile. The
strain measurements may then be converted to Young's Modulus measu | | |
