Abstract:
Axial stress or similar properties in a stressed tendon or ligament are measured by mechanical excitation of a shear wave in the tendon or ligament measured using ultrasonic displacement techniques at least two different longitudinal positions to derive a shear wave propagation speed. This shear wave propagation speed may be equated to an axial stress on the tissue using a model. Rapidly repeated measurements allow dynamic axial stress measurements to be obtained for clinical study.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with government support under AR056201 and EB015410 awarded by the National institutes of Health. The government has certain rights in the invention. 
     
    
     CROSS REFERENCE TO RELATED APPLICATION 
     Background of the Invention 
       [0002]    The present invention relates to an ultrasound device for measuring tissue properties and in particular to an ultrasound system that measures the stress borne by tendons, ligaments, and muscle in vivo. 
         [0003]    The ability to measure the stress that ligaments, tendons or muscle experience in vivo has considerable value in medical research and rehabilitative medicine. Prior techniques used in research settings are highly invasive and include insertion of a “buckle transducer” in series with the tissue or the threading of a fiber optic sensor through the tissue and detecting changes in light transmission associated with tension. Current noninvasive techniques are indirect, and require assumptions about relative loadings between different tissues to estimate internal tissue stress from external force measurements. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides a noninvasive technique for characterizing longitudinal stress in ligaments, tendons, and muscles under load by measuring the propagation speed of a shear wave induced by an external mechanical stimulator. Although the measurement of shear wave speed is generally known in the field of ultrasound elastography, the present inventors have established that the stress dependency of shear wave speed can provide a reliable measure of tissue stress in the range of tensions associated with in vivo tendons, ligaments, and muscles during use. In addition, the present inventors have developed an apparatus capable of providing measurements with sufficient temporal and spatial resolution to track the propagation of a shear wave between two or more locations within the mechanically loaded tendon, ligament, or muscle, important to measuring the high wave speeds possible in these tissues. 
         [0005]    Specifically, in one embodiment, the invention provides a device for in vivo dynamic measurement properties of connective tissue extending along a longitudinal axis, the device having a stimulator probe including an electromechanical actuator adapted to apply a superficial, periodic, mechanical transverse stimulation through a patient&#39;s skin to the tissue to generate a shear wave that travels longitudinally along the tissue when the stimulator probe is placed against the skin proximate to the connective tissue. A first motion sensor detects transverse motion of the tissue along a first transverse axis at a first longitudinal displacement from the stimulator probe when the first motion sensors are placed against the skin proximate to the tissue, and a second motion sensor detects transverse motion of the tissue along a second transverse axis at a second longitudinal displacement from the stimulator probe when the second motion sensors are placed against the skin proximate to the tissue. A processing circuit operates to: (a) receive a signal from the first motion sensor to provide a first transverse motion signal indicating the first transverse movement of the tissue at the first transverse axis over time and (b) receive a signal from the second motion sensor to provide a second transverse motion signal indicating the second transverse movement of the tissue at the second transverse axis over time. The processing circuit then (c) compares the first transverse motion signal to the second transverse motion signal to determine a travel time of passage of the shear wave between the first and second longitudinal axes and (d) output a value functionally related to the travel time. 
         [0006]    It is thus a feature of at least one embodiment of the invention to provide a device that can perform non-invasive, accurate, high-frequency shear wave speed measurements in tissues that are primarily tension-bearing, such as ligaments, tendons, and muscles. 
         [0007]    The repetition speed of the transverse mechanical stimulation between the first and second transverse axes may be selected to provide measurement of a range of shear wave speeds in excess of 20 meters per second. 
         [0008]    It is thus a feature of at least one embodiment of the invention to provide a device that can provide shear wave speed measurement in connective tissue under tension. 
         [0009]    The processing circuitry may operate to repeat the determining of the first and second transverse motion signals over multiple time periods with the probe providing intermittent pulsatile mechanical stimulation only between the time periods. 
         [0010]    It is thus a feature of at least one embodiment of the invention to provide time-of-flight measurements of broadband pulsatile shear waves in contrast to narrowband wavelength measuring techniques sometimes used with conventional imaging apparatus. 
         [0011]    The simulation probe may provide intermittent mechanical stimulation between time periods at a frequency of greater than  20  times per second. 
         [0012]    It is thus a feature of at least one embodiment of the invention to permit real-time dynamic analysis of tissue stress, for example, in motion studies of patients performing functional tasks such as walking, running, stair climbing, reaching, lifting weights or the like. 
         [0013]    The device may include a set of longitudinally spaced arrays of motion sensors, and the processing circuit may select the first and second motion sensor from among the longitudinally spaced array of motion sensors to vary longitudinal positions of the first and second transverse axes with respect to the tissue between measurements to provide the value functionally related to travel time through different longitudinal segments of the tissue. These values may be output as a function of longitudinal position. 
         [0014]    It is thus a feature of at least one embodiment of the invention to permit mapping of changes in stress-related properties to different longitudinal positions within a tissue. 
         [0015]    Alternatively, a mechanical actuator may be used for moving the first and second motion sensor longitudinally to vary the position of the first and second transverse axes between measurements to provide a value functionally related to travel time through different longitudinal segments of the tissue. 
         [0016]    It is thus a feature of at least one embodiment of the invention to provide a simple and low-cost system that can work with as few as a single pair of motion sensors. 
         [0017]    The processing circuit may also compare the signals from the first and second motion sensors to analyze at least one of amplitude change and frequency change and to output a second value functionally related to at least one of amplitude change and frequency change. 
         [0018]    It is thus a feature of at least one embodiment of the invention to permit the analysis of changes in shear wave amplitude and frequency content as it passes through the tissue which may reveal other tissue characteristics related to viscoelasticity. 
         [0019]    The comparison of the first received ultrasound and the second received ultrasound may determine a change in position of tissue at two different longitudinal depths to provide separate first and second transverse motion signals for each depth and output value functionally related to shear wave delay time for each depth. 
         [0020]    It is thus a feature of at least one embodiment of the invention to permit the analysis of depth-related stress variation, for example, in studying portions of tendons that originate in different muscles. 
         [0021]    The output value may be selected from the group consisting of shear wave delay, shear wave speed, elastic modulus, tissue viscosity, and longitudinal stress. 
         [0022]    It is thus a feature of at least one embodiment of the invention to provide a useful range of output dimensions all related to shear wave speed. 
         [0023]    In one embodiment, the first motion sensor may be a first ultrasonic transducer element detecting first ultrasonic data measuring first received ultrasound at a set of different times along the first transverse axis at the first longitudinal displacement from the stimulator probe when the first ultrasonic receiver is placed against the skin proximate to the connective tissue. In addition the second motion sensor may be a second ultrasonic transducer element detecting second ultrasonic radiofrequency data measuring a second received ultrasound at a set of different times along the second transverse axis at the second longitudinal displacement from the stimulator probe when the second ultrasonic receiver is placed against the skin proximate to the connective tissue. In this case, the processing circuit may receive the first and second received ultrasound to: (i) compare the first received ultrasound at different times over a time period to determine a change in position of the tissue between the different times to provide the first transverse motion signal indicating first transverse movement of the tissue at the first transverse axis over time; and (ii) compare the second received ultrasound at different times over the time period to determine a change in position of the tissue between the different times to provide the second transverse motion signal indicating second transverse movement of the tissue at the second transverse axis over time. 
         [0024]    It is thus a feature of at least one embodiment of the invention to implement the motion sensors through ultrasonic transducers providing a variety of benefits including the ability to distinguish among motion at different tissue depths. 
         [0025]    The processing circuit may further receive the first and second ultrasound to provide a B-mode image of the tissue. 
         [0026]    It is thus a feature of at least one embodiment of the invention to permit the display of a reference image of the tissue useful for identifying properties within specific regions of the tissue anatomy. 
         [0027]    In one embodiment, the first and second motion sensors may be first and second skin contacting accelerometers, respectively, measuring acceleration along the first and second transverse axes, respectively. 
         [0028]    It is thus a feature of at least one embodiment of the invention to provide a low-cost sensor system that may be used for measuring the properties described above. 
         [0029]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a side elevational view of a human ankle and foot showing an example sensor/stimulator unit constructed according to one embodiment of the present invention as applied proximally to a patient&#39;s Achilles tendon to induce a shear wave in the tendon through the skin to enable the measurement of shear wave speed; 
           [0031]      FIG. 2  is a flowchart of a program executed on an electronic computer associated with the sensor/stimulator unit for calculation of shear wave velocity; 
           [0032]      FIG. 3  is a simplified and expanded view of a tendon and sensor/stimulator unit of  FIG. 1  showing the simplified waveforms obtained by the unit and their processing according to the program of  FIG. 2 ; 
           [0033]      FIG. 4  is a side elevational view in phantom of an alternative embodiment of the sensor/stimulator unit of  FIG. 1  providing for mechanical scanning of transducer elements instead of multiple transducer elements; 
           [0034]      FIG. 5  is a block diagram showing additional processing of the shear wave data obtained by the present invention for other tissue characterizations; 
           [0035]      FIG. 6  is an example screen display that may be provided by the present invention for generalized display of properties of tendon-like tissue; 
           [0036]      FIG. 7  is an alternative embodiment of the sensor stimulator of  FIG. 1  providing for simultaneous multidimensional B-mode imaging; 
           [0037]      FIG. 8  is an alternative display to that shown in  FIG. 6  providing dynamic measurement of applied stress to tissue during use; 
           [0038]      FIG. 9  is an alternative display to that shown in  FIGS. 6 and 7  showing tracking of stresses in different layers of the tissue possible with the present invention; 
           [0039]      FIG. 10  is a figure similar to that of  FIG. 1  showing an alternate embodiment of the invention which deduces shear wave travel using skin-mounted accelerometers. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0040]    Referring now to  FIG. 1 , a sensor/stimulator unit  10 , constructed according to one embodiment of the present invention, may provide a housing  11  having a front face  12  sized and shaped to place against skin  14  of a patient proximate to a tendon or ligament (henceforth tissue  18 ) extending along a longitudinal axis  20 . The housing  11  may be held against the skin by means of an elastic cuff  13  or the like. For example, the housing  11  may be applied with its front face  12  against the rear of a patient&#39;s ankle adjacent to and vertically aligned with the Achilles tendon. 
         [0041]    Exposed at a lower edge of the front face  12  of the housing  11  is a stimulator probe  22  attached to piezoelectric element  24 . The stimulator probe  22  is shaped to press against the skin overlying the Achilles tendon to conduct force from the piezoelectric element  24  through the skin to apply a periodic transverse stimulation pressure  26  to the tissue  18  inducing a longitudinally traveling shear wave pulse  28  traveling vertically upward therefrom. As is understood in the art, piezoelectric materials are those which change dimension under the influence of the electric field and thus can be used to provide mechanical motion under electrical control. 
         [0042]    Also exposed at the front face  12  of the housing  11  are multiple ultrasonic transducers  30  arrayed generally in a vertically extending longitudinal line. In a simplest case, two spaced apart ultrasonic transducers  30   a  and  30   b  may be positioned at predetermined locations in longitudinal separation from the stimulator probe  22  as discussed. These ultrasonic transducers  30  may emit ultrasonic waves  33  and measure returning echoes  35  in the manner of conventional ultrasound imaging transducers. 
         [0043]    Each of the stimulator probe  22  and the ultrasonic transducers  30  communicate with a local signal processing circuit  32  which may provide signals to the stimulator probe  22  and the ultrasonic transducers  30  from electronic computer  34  and may sample and digitize the data from the ultrasonic transducers  30  for transmission to the electronic computer  34 . Generally, signals from each of the ultrasonic transducers  30   a  and  30   b  provide a time series of ultrasound RE data. 
         [0044]    The electronic computer  34  may include one or more processors  36  communicating with a memory  38  holding a program  43  as will be described below. In addition, the electronic computer  34  may communicate with the signal processing circuit  32  to send data to the stimulator probe  22  and ultrasonic transducers  30  and to collect data from the ultrasonic transducers  30  that may also be stored in memory  38  for processing. As will be described below, the electronic computer  34  may further change the particular ultrasonic transducers  30  selected as ultrasonic transducers  30   a  and  30   b  or may control mechanical elements to scan the transducers  30   a  and  30   b  in an alternative embodiment also discussed below. 
         [0045]    The electronic computer  34  communicates with a graphics display  39  of a type known in the art as well as human input controls  40  such as a keyboard, mouse, touchscreen or the like, allowing a human operator to input data and control the acquisition of data using the present device. 
         [0046]    Referring now also to  FIGS. 2 and 3  as well, the sensor/stimulator unit  10  operating with the electronic computer  34  may make repeated measurements of shear wave propagation speed, for example, at a repetition frequency of greater than 20 hertz and at rates from 50 to 100 hertz. At the beginning of each periodic measurement, stimulation pressure  26  is applied to the tissue  18  as indicated by process block  42 , for example, providing an impulse signal to the piezoelectric element  24  (for example, a short square wave pulse or sine pulse) and then quieting the piezoelectric element  24  for the remainder of the measurement. It will be understood that this pulse provides a range of frequencies of stimulation as a result of its short duration. Typically the duty cycle of the transducer operating in this fashion will be less than ten percent, meaning that the piezoelectric element  24  remains quiet without motion for most of the time during operation of the sensor/stimulator unit  10 . 
         [0047]    The transverse stimulation pressure  26  passes through a gel or ultrasound conducting pad layer  44  then through the skin  46  and into the superficial tendon  48  (gastrocenemius) and then through the deep tendon  50  (soleus) and then into other muscle and tissue  52 . 
         [0048]    The result of the transverse stimulation pressure  26  is to produce a shear wave pulse  28  in the superficial tendon  48  (and 28′ in the deep tendon  50 ) traveling upwardly and longitudinally (along the Y-axis as shown) along the tissue  18 . It will be appreciated that some shear waves will also pass up the other layers including the gel layer  44 , skin layer  46 , and muscle  52  such as may be distinguished from the pulses  28   a  by intensity, speed, or depth. 
         [0049]    The shear wave pulses  28  arrive first at ultrasonic transducer  30   a  longitudinally displaced from the stimulator probe  22  and positioned to direct ultrasound waves  33  transversely along a first transverse axis  31   a  (also indicated as the X-axis) through each of the layers  44 ,  46 ,  48 ,  50 , and  52  and receive return echo signals  35  at various points along that path, for example, from tissue interfaces and other elements providing “speckle”. The result is a series of time domain raw ultrasound radiofrequency signals  60  (shown distinguished by index i and each providing a “frame” of data) having time domain defined by portions corresponding to each of the layers  44 ,  46 ,  48 ,  50  and  52  and more generally providing an echo signal amplitude as a function of time reference to a corresponding timing of the emission of the ultrasound wave  33 . 
         [0050]    The shear waves  28  next arrive at the second ultrasonic transducer  30   b  displaced longitudinally along axis  20  to a second transverse axis  31   b  further beyond the ultrasonic transducer  30   a  of first transverse axis  31   a.  Typically the first transverse axis  31   a  and second transverse axis  31   b  will have longitudinal separation of 1 to 2 centimeters. Ultrasonic transducers  30   a  and  30   b  may be single element transducers or multielement transducers. 
         [0051]    Like ultrasonic transducer  30   a,  ultrasonic transducer  30   b  are positioned to direct ultrasound wave  33  transversely through each of the layers  44 ,  46 ,  48 ,  50 , and  52  and received return echo signals  35  in return providing raw ultrasound signal  60 ′. The acquisition of this set of echo signals  60   i  and  60 ′ i  is indicated by process block  66  and may be stored in memory  38  for later processing. 
         [0052]    The signals  60  and  60 ′ may be acquired at an extremely high rate based on the simple transducer structure of the present invention providing in excess of at least 8000 frames per second and typically in excess of 14,000 frames per second for each transducer  30   a  and  30   b.    
         [0053]    It will be appreciated that for a given elapsed time after generation of the emitted ultrasound wave  33  associated with each signal  60 , the signal  60  will indicate echoes returned from different depths within this tissue  18  unique to different of layers  44 ,  46 ,  48 ,  50 , and  52  along transverse axis  31   a.  Accordingly, a time window  68  may be applied to each signal  60  being a predetermined fixed time delay after the excitation signal  29  of the emitted ultrasound wave  33  to isolate signal portions relevant to particular layers. For example, a time window  68   a  may provide for corresponding depth signal portion  70  among the different signals  60  relevant to the layer  48  and time window  68   b  may provide for signal portion  70  (not shown) among the different signals  60  relevant to layer  50 . 
         [0054]    Similar windows  68   a  and  68   b  may be applied to the signals  60 ′ to generate depth signal portion  70  for corresponding layers positioned along axis  31   b.    
         [0055]    At process blocks  72 , signal portions  70  for each window  68   a  and  68   b  for each of signal  60  and  60 ′ are correlated (i.e., between sequentially acquired signals  60  for corresponding windows  68  of corresponding depths, and independently between sequentially acquired signals  60 ′ for corresponding windows  68  of corresponding depths) to determine separately the relative transverse motion of the tissue  18  along the transverse axes  31   a  and  31   b.  This transverse motion will be determined from the timeshift necessary for maximum correlation times the approximate sound speed of ultrasound transversely through the tissue  18 . It will be appreciated that precision with respect to knowing the transverse displacement is not required and that the instrument may be used for revealing relative changes as well as providing absolute quantitative measurements. 
         [0056]    Successively calculated displacements for successive signals  60  yield transverse motion signals  76  being a set of displacements over time indicating the transverse motion of the particular tissue element of layer  48  or  50  at axis  31   a.  Similarly, successively calculated displacements for successive signals  60  yield transverse motion signals  76 ′ being a set of displacements over time indicating the transverse motion of the particular tissue element of layer  48  or  50  at axis  31   b.    
         [0057]    The motion signals  76  and  76 ′ describe the evolution of transverse tissue deformation caused by the propagation of shear wave pulse  28  as it propagates along the tissue  18 . Accordingly, two motion signals  76  and  76 ′ may be then compared, as indicated by process block  78 , to determine a Δt value being equal to time it took the shear wave pulse  28  to travel between the axes  31   a  and  31   b.  It will be understood that this Δt value is inversely proportional to the shear wave speed and that shear wave speed may be determined simply by knowing the longitudinal separation between axes  31   a  and axis  31   b.    
         [0058]    The processes of process block  66 ,  72  and  78  makeup process block  80  shown in  FIG. 2 . 
         [0059]    The speed of propagation of a shear wave is typically much lower than the speed of propagation of a compression wave through the tissue  18 , for example, with compression waves traveling at 1800 to 2000 meters per second and shear waves traveling from approximately 10 to 100 meters per second depending on the stress applied to the tendon. Accordingly, it will be appreciated that high spatial and temporal accuracy is necessary to resolve shear wave speed differences over the short distance of the separation of the transducers  30  at axes  31   a  and  31   b.  Shear wave speed is substantially more sensitive to tissue stress (above speeds of over approximately 15 meters per second) than measurement of compression wave speed. 
         [0060]    This measurement of shear wave propagation speed may be repeated by looping back to process block  42  to apply a new stimulation pulse between measurements of process blocks  66 ,  72  and  78  indicated generally by process block  82 . The shift at process block  82  is optional and the repeated measurements may be made at the same location for example under dynamic loading. 
         [0061]    During or subsequent to the process of process blocks  42 ,  80 , and  82 , stress measurements may be determined from the shear wave speeds as indicated by process block  83 . In one embodiment, stress on the tissue  18  may be derived according to the following equation modeling the tissue  18  as a Timoshenko beam as follows: 
         [0000]    
       
         
           
             
               
                 
                   v 
                   = 
                   
                     
                       ( 
                       
                         
                           
                             
                               k 
                               ′ 
                             
                              
                             μ 
                           
                           + 
                           σ 
                         
                         ρ 
                       
                       ) 
                     
                     
                       1 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0062]    where υ is shear wave longitudinal speed determined by knowledge of the separation of the axes  31  discussed above; 
         [0063]    k′ is a shear correction factor empirically determined for a particular tissue and geometry being studied; 
         [0064]    μ is the shear elastic modulus (that may be determined empirically for a particular tissue type); 
         [0065]    ρ is the effective tissue density (generally known for a particular tissue type and its surroundings); and 
         [0066]    σ is the axial stress on the tissue. 
         [0067]    The inventors have determined that axial stress dominates this equation (1) when even moderate stresses are applied to tendon tissue allowing the equation to be simplified to: 
         [0000]      σ≈ρv 2    (2)
 
         [0068]    Alternatively, a measurement may be made using equation (1) at zero axial stress to deduce a constant 
         [0000]    
       
         
           
             ( 
             
               
                 
                   k 
                   ′ 
                 
                  
                 μ 
               
               ρ 
             
             ) 
           
         
       
     
         [0000]    to be used in extracting axial stress. 
         [0069]    This value may then be output as indicated, for example, by process block  85  of  FIG. 4 . In addition to or alternatively various shear wave speed derived parameters may be output including but not limited to shear wave delay, shear elastic modulus and density. These latter two measures may be determined simply by making assumptions about shear wave speed (for example, by empirical measurement) and solving equation (1) for different variables. 
         [0070]    It will be appreciated that the present technique may provide not only quantitative axial stress but qualitative axial stress in cases where only qualitative indications of stresses are required, for example, as provided by the shear wave speed itself. Such measures may be useful for qualitative displays of tissue properties. 
         [0071]    Referring now to  FIGS. 1 and 2 , at the conclusion of each measurement of process block  80 , and prior to the repetition of process block  82 , the location of the ultrasonic transducers  30   a  and  30   b  may be shifted along the tissue  18  (with or without shifting the stimulator probe  22 ) to measure stress-related properties at different longitudinal portions of the tissue  18 . These properties may include changes in stress in the tissue or changes in the other properties described above. In cases where the stress along the tissue  18  may be assumed to be substantially constant, insight and other tissue properties such as elastic modulus may be better revealed. 
         [0072]    Referring to  FIG. 1 , this movement of the location of transducers  30   a  and  30   b  may be performed by simply selecting among different pairs of ultrasonic transducers  30  in a longitudinal array of ultrasonic transducers  30  only some of which are activated. Alternatively, as shown in  FIG. 4 , a pair of individual ultrasonic transducers  30   a  and  30   b  may be mounted on a movable carriage  84  translated by electrically controlled actuator  86  such as a stepper motor or the like driving a lead screw  88  to physically translate the ultrasonic transducers  30  along the axis  20 . This actuator  86  may be controlled by the computer  34  through the signal processing circuit  32 . 
         [0073]    Referring now to  FIG. 5 , the motion signals  76  and  76 ′ as described above may be further analyzed to determine a change in the shape of the shear wave pulse  28  as it progresses through the tissue such as may reveal a damping effect of the tissue, for example, caused by tissue viscosity or the like. Accordingly process block  78  may be supplemented to provide not only a Δt value indicating the propagation delay of the shear wave pulse  28  through the tissue  18  but also a decrease in amplitude or a change in spectral content represented by the shear wave pulse  28 , for example, produced by a Fourier transform of the motion signals  76  and  76 ′ or a similar measure such as power spectra difference. 
         [0074]    Referring to  FIG. 6 , repeated execution of process blocks  42 ,  80  and  83  (without necessarily shifting the position of the transducers  30 ) may be used to create a dynamic stress plot  100  showing a representation of the stress on the tissue  18  over time, for example, with a patient walking or performing other activity. This stress plot  100  may be displayed on the display  39  for analysis, for example, together with a video image  102  of the patient showing a frame of the patient activity such as walking coordinated with a cursor  104  moving over the stress plot  100  to a corresponding time. 
         [0075]    Referring now to  FIGS. 7, 8 and 9 , in one embodiment, the sensor/stimulator unit  10  may make use of an array  105  of ultrasonic transducers  30  having columns extending along the longitudinal axis  20 . Depth information is obtained using the known speed of ultrasound transmission from to determine the depth at which tissue structures generated the ultrasound echo. This depth information is obtained for each element, allowing for the acquisition of a two dimensional B-mode image  108 . This B-mode acquisition may occur before or after the processing of shear wave propagation speed measurement. Portions  111  of the B mode image may be shaded or colored to reflect tissue properties determined by the present mention, for example, by matching the data determined from process block  85  with a shifting of the transducers  30   a  and  30   b  corresponding to longitudinal location in the B-mode image. This shear wave speed-related data may also be displayed in addition to or alternatively along only a single dimension per plot line  113 , and quantitative information  114  may also be provided as derived above with respect to process block  82  of  FIGS. 2 and 5 . A key  115  may be provided to decode the shading to quantitative values or ranges. 
         [0076]    During the determination of shear wave speed, ultrasound is collected at a small number of (two or more individual ultrasonic transducers)  30  may be actuated to obtain high frame rate information. Alternatively, planar wave imaging may be used to achieve for high-frame rate data acquisition. 
         [0077]    Referring now to  FIG. 9 , the ability to rapidly measure axial stress in multiple tissue layers (for example, tissue layers  48  and  50 ) of  FIG. 3  allows simultaneous display of axial stress or a similar quantity for each of the different tissue layers. In this way depth-related differences in tissue properties may be determined, for example, showing a deviation between axial stress  110  for superficial tendon layers versus axial stress  112  for deeper tendon layers. 
         [0078]    Referring now to  FIG. 10 , in an alternative embodiment, the ultrasound transducers  30   a  and  30   b  may be replaced with skin-mounted accelerometers  116   a  and  116   b,  each providing an axis of sensitivity along respective axes  31  as described above. The accelerometers  116 , for example, may be microelectromechanical devices having low mass and high sensitivity, for example, the ADX L212 accelerometer from Analog Devices of Massachusetts having sensitivity of +/−2 g with the Z-axis aligned with axis  31 . 
         [0079]    A signal from the z-axis accelerometers  116   a  and  116   b  may provide motion signals  76  and  76 ′ as discussed above and these motion signals  76  and  76 ′ may be processed as described above, for example, with respect to  FIGS. 2, 3, and 5 , to provide the measurements also discussed above. 
         [0080]    In this respect, it will be understood that transducers  30  and accelerometers  116  both provide a function of motion sensors of transverse motion. Generally, the signals from the accelerometers  116  will indicate a second derivative of position of the tissue  18  and will measure a position that is a combined effect of shear waves moving through multiple layers of tissue  18  as opposed to the transducers  30  which may distinguish between motion at different layers and measure motion directly. This second derivative signal can be integrated twice to provide a position or may be used directly as if the acceleration signals were position signals, either of which will provide an indication of shear wave speed. 
         [0081]    In one embodiment, the accelerometers  116  may be precisely spaced in a supporting housing (not shown) including damping elements to prevent the communication of vibration through the housing between the accelerometers  116  or between the stimulator probe  22  and the accelerometers  116 . 
         [0082]    Close coupling of the accelerometers  116  to the skin of the patient may be provided a pressure sensitive adhesive (not shown) or by an elastic cuff  118  holding the accelerometers  116   a  and  116   b  in a fixed longitudinal separation and biased against the skin. The same cuff  118  may support the stimulator probe  22  operating as discussed above to be spaced from the lower accelerometer  116   b.  The material of the cuff  118  may be selected to provide very little coupling between the stimulator probe  22  and the accelerometers  116 , for example, by using a material that is relatively compliant and that has high damping measured in the longitudinal direction. 
         [0083]    Multiple accelerometers  116  may be arrayed longitudinally along axis  20  to provide for the ability to make shear wave measurements at different longitudinal locations along the tissue  18  as discussed above with respect to the ultrasound transducers  30 . Alternatively, two accelerometers  116   a  and  116   b  may be mounted as shown in  FIG. 4  in place of the ultrasound transducers  30   a  and  30   b  and moved mechanically along the surface of the skin, coupled to the mechanical carriage  84  by a motion damping material and by using a lubricating medium between the face of the accelerometers  116  and the skin to permit this motion. 
         [0084]    In other respects, this embodiment may make use of the components and techniques described above with respect to the ultrasonic transducer system. 
         [0085]    Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0086]    When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0087]    References to “a controller” and “a processor” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
         [0088]    “Diameter” as used herein should not be understood to require a cylindrical or circular element but to simply describe a diameter of a circumscribing cylinder closely conforming to the element. 
         [0089]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications are hereby incorporated herein by reference in their entireties.