Abstract:
A system ( 100 ) for assessing a subject at risk of a soft tissue abnormality such as deep vein thrombosis (DVT), comprises a motion sensor ( 108 ) which is adapted to be fixed, in use, to a muscle ( 106 ) of the subject. The motion sensor includes a transmitter ( 206 ) configured to transmit a signal ( 400 ) representing motion of the sensor. A receiver ( 118 ) is configured to receive the signal from the transmitter of the motion sensor. A signal processor ( 112 ) is coupled to the receiver and configured to analyse first and second data sets received via the receiver from the motion sensor. The first and second data sets represent respective first and second oscillatory mechanical responses of first and second muscles of the subject resulting from mechanical stimuli. The analysis comprises determining first and second parameter sets characterising the first and second oscillatory mechanical responses, and comparing the first parameter set with the second parameter set to assess a possible presence of DVT in the subject.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to systems, methods and apparatus for assisting healthcare workers in the screening and detection of abnormalities and pathological states in soft tissue, and has particular application at least, but not confined to, the screening and detection of deep vein thrombosis (DVT) of the lower limbs. 
       BACKGROUND OF THE INVENTION 
       [0002]    Deep vein thrombosis (DVT) is a significant complication in all surgical and medical wards, as well as in other aspects of community life. It is a condition that has significant implications for pulmonary embolism, and possible death. There are also dangers of continuing morbidity in the legs, and also in the lungs, from the presence of venous thromboembolism. 
         [0003]    Currently, there is no recognised clinical assessment for DVT that has an accuracy greater than 60 percent, and many patients with DVT have no overt clinical findings. 
         [0004]    In cases of suspected DVT, it is necessary to undertake specific investigations for confirmation. These investigations are expensive, and include techniques such as Doppler ultrasonography which requires significant equipment and expertise. Tests for pulmonary embolism may also be necessary, involving methods such as computed tomography (CT) or chest scanning using a radioactive marker. Blood tests may also be performed. As will be appreciated, a number of these tests are invasive and/or uncomfortable for the patient. Many of these tests cannot be performed at the bedside. 
         [0005]    A particular concern is that many patients develop DVT with minimal or no significant changes or symptoms (e.g. pain and swelling), and as a result many patients are sent home early following surgery without any assessment of the possibility of DVT. Deaths from complications of venous thromboembolism have been known to result in such cases. 
         [0006]    There is, accordingly, a pressing need for a reproducible, objective investigation that can be performed by a trained technician, a trained clinician, or other healthcare worker. Ideally, assessment of the possible presence of DVT should be able to be conducted simply, with high reliability, at relatively low cost, and without the requirement for invasive or uncomfortable procedures. The present invention seeks to address these requirements. 
       SUMMARY OF THE INVENTION 
       [0007]    In one aspect, the present invention provides a system for assessing a subject at risk of a soft tissue abnormality such as deep vein thrombosis (DVT), comprising: 
         [0008]    a motion sensor, adapted to be fixed, in use, to a muscle of the subject, the motion sensor including a transmitter configured to transmit a signal representing motion of the sensor; 
         [0009]    a receiver, configured to receive the signal from the transmitter of the motion sensor; and 
         [0010]    a signal processor, coupled to the receiver, and configured to analyse first and second data sets received via the receiver from the motion sensor, the first and second data sets representing respective first and second oscillatory mechanical responses of first and second muscles of the subject resulting from mechanical stimuli, wherein the analysis comprises determining first and second parameter sets characterising the first and second oscillatory mechanical responses, and comparing the first parameter set with the second parameter set to assess a possible presence of DVT in the subject. 
         [0011]    For convenience of description, where the term ‘muscle’ is employed in this specification (including in the appended claims), this should be understood to encompass a single muscle, or one or more muscles comprising a muscle group. 
         [0012]    Embodiments of the invention take advantage of the observation that, upon clinical examination, in certain pathological states there is a change in the calf muscles that may be identified in a response to a percussive stimulus applied to the tissues. In DVT, it has been observed that the normal mobility of the calf is reduced. It has thus been observed that tapping the calf, which under normal circumstances has considerable mobility like the bounce of jelly, results in a more ‘dough-like’ response in the presence of DVT. It has been found, however, that skill and experience are required in order to master the use of this technique for the detection of DVT. It is reasonably anticipated that similar effects will also occur in comparable pathological states of other soft tissues. 
         [0013]    Embodiments of the invention further take advantage of the fact that disorders such as DVT are normally present in only one comparable muscle of the subject. Thus, for example, if a DVT is present in the right calf muscle, the response of this muscle to a mechanical stimulus should be different from the response of the subject&#39;s left calf muscle, in which DVT is unlikely to be present. 
         [0014]    Embodiments of the present invention therefore advantageously employ a consistent, reproducible, objective analysis technique, comprising determining and comparing suitable parameter sets of corresponding pairs of muscles of the subject, in order to detect significant differences indicative of the possible presence of DVT or other abnormality. 
         [0015]    Accordingly, in embodiments of the invention the first and second muscles are corresponding muscles of respective left and right limbs of the subject. Particularly, the first and second muscles are left and right calf muscles of the subject. 
         [0016]    Embodiments of the invention decompose the oscillatory mechanical responses of the first and second muscles, such that the first and second parameter sets each comprise at least one frequency parameter and at least one corresponding damping parameter characterising the oscillatory mechanical response. Typically, the first and second parameter sets each comprise frequency parameters and corresponding damping parameters of two or more oscillatory components of the mechanical responses of the first and second muscles of the subject. 
         [0017]    According to embodiments of the invention, a possible presence of DVT in the subject is assessed based on a difference between the frequency parameters and/or the damping parameters of the first and second parameter sets. For example, a lower frequency response and/or more rapid damping in one of the first and second muscles, as compared with the other, is indicative of the possible presence of DVT in the subject. The frequency response may typically be a resonant frequency, while damping may be measured as a damping factor. 
         [0018]    For the purpose of comparing the first parameter set with the second parameter set, the signal processor may be configured to present information relating to the first and second parameter sets on a display, for evaluation and further diagnostic consideration, by an operator such as a trained technician, trained clinician, or other healthcare worker. 
         [0019]    In other embodiments, further statistical analysis of the first and second parameter sets, obtained over multiple responses of the first and second muscles of the subject to multiple mechanical stimuli, may be performed in order to assess the statistical and/or clinical significance of differences between the first and second parameter sets, and to provide a specific indication, such as a likelihood or probability of the presence of an abnormality such as DVT in the subject. 
         [0020]    According to embodiments of the invention, the motion sensor comprises an accelerometer. Advantageously, the accelerometer comprises a multi-axis accelerometer, such as a two-axis or three-axis accelerometer. 
         [0021]    In some embodiments, a communications channel between the transmitter and the receiver comprises a wired connection. 
         [0022]    In other embodiments, a communications channel between the transmitter and the receiver comprises a wireless connection. Advantageously, the use of a wireless connection between the motion sensor and the signal processor avoids the possibility that the presence of physical connections may influence the mechanical response of the first and second muscles resulting from the applied mechanical stimuli. 
         [0023]    In various embodiments, the signal communicated from the transmitter to the receiver may be an analog signal, which may be sampled and digitised at the receiver, or may be a digital signal, having been sampled and digitised at the motion sensor. 
         [0024]    In another aspect, the invention provides an apparatus for assessing a subject at risk of DVT, comprising: 
         [0025]    a receiver, configured to receive a signal from a transmitter associated with a motion sensor, wherein the motion sensor is adapted to be fixed, in use, to a muscle of the subject, and is configured to transmit a signal representing motion of the sensor; and 
         [0026]    a signal processor, configured to analyse first and second data sets received via the receiver from the motion sensor, the first and second data sets representing respective first and second oscillatory mechanical responses of first and second muscles of the subject resulting from mechanical stimuli, wherein the analysis comprises determining first and second parameter sets characterising the first and second oscillatory mechanical responses, and comparing the first parameter set with the second parameter set to assess a possible presence of DVT in the subject. 
         [0027]    In a further aspect, the invention provides an analysis method for assessing a subject at risk of DVT, the method comprising the steps of: 
         [0028]    receiving a first data set representing an oscillatory mechanical response of a first muscle of the subject resulting from a first applied mechanical stimulus; 
         [0029]    receiving a second data set representing an oscillatory mechanical response of a second muscle of the subject resulting from a second applied mechanical stimulus; 
         [0030]    analysing the first and second data sets to determine corresponding first and second parameter sets characterising the first and second oscillatory mechanical response respectively; and 
         [0031]    comparing the first parameter set with the second parameter set to assess a possible presence of DVT in the subject. 
         [0032]    In a still further aspect, the invention provides a method of assessing a subject at risk of DVT, comprising the steps of: 
         [0033]    applying a mechanical stimulus to a first muscle of the subject, resulting in an oscillatory mechanical response of the first muscle; 
         [0034]    acquiring a first data set representing the oscillatory mechanical response of the first muscle, and analysing the first data set to obtain a first parameter set characterising the oscillatory mechanical response; 
         [0035]    applying a mechanical stimulus to a second muscle of the subject, resulting in an oscillatory mechanical response of the second muscle; 
         [0036]    acquiring a second data set representing the oscillatory mechanical response of the second muscle and analysing the second data set to obtain a second parameter set characterising the oscillatory mechanical response; and 
         [0037]    comparing the first parameter set and the second parameter set to assess possible presence of DVT in the subject. 
         [0038]    Further features, benefits and advantages of embodiments of the invention will be apparent from the following description, which is provided by way of example only, and should not be considered as limiting of the scope of the invention as defined in any of the preceding statements, or in the claims appended hereto. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    Embodiments of the invention will now be described with reference to the accompanying drawings, in which like reference numerals indicate like features, and wherein: 
           [0040]      FIG. 1  is a schematic diagram of a system embodying the invention; 
           [0041]      FIG. 2  is a schematic diagram of a motion sensor suitable for use in the system of  FIG. 1 ; 
           [0042]      FIG. 3  is a diagram illustrating position and support of a leg of a subject arranged for testing by the system of  FIG. 1 ; 
           [0043]      FIG. 4  is a graph of a motion sensor data set representing an exemplary mechanical response obtained according to an embodiment of the invention; 
           [0044]      FIG. 5  is a graph illustrating decomposition of a data set into three decaying sinusoidal components according to an embodiment of the invention; 
           [0045]      FIG. 6  is a chart illustrating an illustrative statistical analysis of frequency parameters obtained in accordance with an embodiment of the invention; and 
           [0046]      FIG. 7  is a chart illustrating an illustrative statistical analysis of damping parameters obtained in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0047]      FIG. 1  is a schematic diagram of a system  100  embodying the invention. The system  100  captures data sets obtained by applying mechanical stimuli to muscles of a subject. Shown schematically in  FIG. 1  is a cross-section through the lower leg  102  of a subject, comprising bones  104  (i.e. fibula and tibia), muscles  106  (i.e. peroneals, tibialis posterior, gastrocnemius, and/or soleus muscles). Collectively, the various muscles making up the back portion of the lower leg are called ‘the calf’. 
         [0048]    In accordance with the exemplary system  100 , one or more motion sensors  108  are fixed to the exterior of the limb, e.g. at the rear and/or side of the calf. In some embodiments of the invention, a single motion sensor, such as a multi-axis accelerometer, may be sufficient. It may be advantageous to provide multiple motion sensors to enable data from each to be combined and/or selection of signals from a sensor providing the strongest or cleanest signal. 
         [0049]    The system  100  further includes signal processing apparatus  110 , which is configured to receive and analyse information captured by the motion sensor, or sensors,  108 . As shown in  FIG. 1 , the signal processor  110  comprises a combination of hardware and software, in which the hardware and/or the software are configured to embody various features of the invention. The signal processing apparatus  110  may comprise a standard hardware configuration, such as a personal computer, a smartphone or other portable device, or any other suitable computing and communications platform. Alternatively, the signal processing apparatus  110  may comprise custom or semi-custom hardware, including programmable components such as microprocessors, or programmable-logic devices such as a field programmable gate array (FPGA). Where programmable-logic devices are used, these may be configured to include functional blocks implementing features of discrete devices, such as microprocessors, memory devices, and custom analog and/or digital components. As will be appreciated by electronics system designers and engineers, various hardware and/or software implementation options are available, falling within the scope of the present invention. 
         [0050]    The exemplary signal processor  110 , as illustrated in  FIG. 1 , comprises a microprocessor component or functional block  112 . The microprocessor  112  is interfaced to, or otherwise operably associated with, a non-volatile memory/storage device  114 . The non-volatile storage  114  may be a hard disk drive (e.g. if the signal processor  110  is implemented using a personal computer), or may include a solid-state non-volatile memory, such as read-only memory (ROM), flash memory, or the like. The microprocessor  112  is also interfaced to volatile storage  116 , such as random access memory (RAM) which contains program instructions and/or transient data relating to the operation of the signal processor  110 . 
         [0051]    In a conventional configuration, the non-volatile storage device  114  maintains program and data content relevant to the normal operation of the signal processing apparatus  110 . For example, if the apparatus  110  is implemented using a personal computer, smartphone, or the like, the storage device  114  may contain operating system programs and data, as well as other executable application software necessary to the intended functions of the signal processing apparatus  110 . The storage device  114  may also contain program instructions which, when executed by the microprocessor  112 , instruct the apparatus  110  to perform operations in accordance with an embodiment of the present invention, for assessing a subject at risk of DVT. In operation, instructions and data held in non-volatile storage  114  may be transferred to volatile memory  116  as required. 
         [0052]    The microprocessor  112  is also operably associated with a communications interface  118  in a conventional manner. The communications interface  118  enables communication between the signal processor  110  and the one or more motion sensors  108 . Communications between the signal processing apparatus  110  and each motion sensor  108  may be via a wired connection, such as a Universal Serial Bus (USB) connection, or may be via a wireless connection, such as a Bluetooth, Bluetooth Low Energy (BLE), Wi-Fi, or other wireless communications channel. In the exemplary embodiment of the system  100 , communication is conducted via a wireless BLE channel. 
         [0053]    In use, the volatile storage  116  includes a corresponding body  120  of program instructions configured to perform processing and operations embodying features of the present invention, as described in greater detail below, particularly with reference to  FIGS. 4 to 7 . 
         [0054]    The signal processing apparatus  110  further includes a display  122  interfaced with the microprocessor  112 , enabling information to be communicated to an operator of the apparatus  110 . As shown in  FIG. 1 , the display  122  is integrated with the apparatus  110 , and may be, for example, a touchscreen display of a smartphone or similar device. Alternatively, the display  122  may be incorporated into a custom- or semi-custom-designed signal processing apparatus  110 . In still other embodiments, the display  122  may be the display of a personal computer, a tablet device, a notebook, or other similar conventional computing platform. 
         [0055]      FIG. 2  is a schematic diagram of a motion sensor suitable for use in the system of  FIG. 1 . The exemplary motion sensor  108  comprises an accelerometer  202 , a local processor  204 , and a communications interface  206 . 
         [0056]    Various commercially available components may be used in the design and implementation of the motion sensor  108 . For example, the motion sensor  202  may be the part number KXTJ9 Tri-axis Accelerometer, available from Kionix Incorporated of Ithaca, N.Y. The local processor  204  may comprise part number CC2541, Bluetooth Low Energy and Proprietary System-on-Chip, available from Texas Instruments Incorporated of Dallas Tex. The CC2541 part comprises all of the components necessary to communicate with the accelerometer  202 , and to implement a wireless BLE communications interface for transmitting accelerometer data to the signal processor  110 . The CC2541 part requires only an external antenna  206  for transmitting and receiving BLE signals at 2.4 GHz. 
         [0057]    A more complete off-the-shelf motion sensor  108  may be obtained from corporations such as Texas Instruments, in the form of components such as the CC2541 SensorTag reference design, which is a ‘development kit’ comprising the CC2541 System-on-Chip, the KXTJ9 Accelerometer, along with additional sensors for detecting and recording movement, magnetic fields, humidity, pressure and temperature. The SensorTag reference design is configured for communications with smartphones and other mobile devices executing either the android or iOS operating systems. It is supported by software development tools, and the ability to download software/firmware updates directly from a smartphone or other device. Accordingly, although the SensorTag reference design incorporates sensors that may not be required in a basic implementation of the invention, it provides a convenient development platform for motion sensors  108  embodying the invention. 
         [0058]      FIG. 3  is a schematic diagram illustrating exemplary position and support of a leg  300  of a subject, arranged for testing using the system  100  of  FIG. 1 . Supports  302 ,  304  are provided for the upper leg and foot, respectively, and one or more motion sensors  108  are fixed, e.g. via a strap, tape or other suitable fixing mechanism, to the lower leg  102 . The arrangement of supports  302 ,  304  is provided by way of example only, and other arrangements are possible, e.g. the provision of upper leg support  302  may be optional. In this position, the subject may relax his or her calf muscle, such that a trained technician, clinician, or other healthcare worker may apply a mechanical stimulus, such as a percussive impulse (represented by arrow  124  in  FIG. 1 ) resulting in an oscillatory mechanical response of the subject&#39;s calf muscles. The oscillatory mechanical response causes corresponding voltages to be generated along the tri-axis detectors within the accelerometer  202 . In the case of the KXTJ9 accelerometer part, the voltages are digitised on-chip, at a selectable resolution of 8, 12 or 14 bits, and transferred to the local processor  204  via an inter-integrated circuit (I 2 C) standard communications interface. The local processor  204  is programmed to collect, format and transmit the data set corresponding with the digitally sampled accelerometer readings via the transmitter  206  to the receiver  118  of the signal processor  110 . In an exemplary embodiment, the BLE communications channel supports a sampling rate of about 800 samples per second, using a 12-bit resolution. Increasing the bandwidth of the communications channel, and/or improving the efficiency of transmission (e.g. by compressing the transmitted data) may enable higher resolution data acquisition, such as one kilosamples/s at up to 14 bits per sample. 
         [0059]    As noted above, in the exemplary embodiment a tri-axis accelerometer is employed. This provides motion information, in the form of voltage levels representing acceleration, along three orthogonal axes. Embodiments of the invention may utilise acquired data from a single axis (e.g. the axis providing the strongest signal), or may combine data from multiple axes by computing a resultant magnitude of the acceleration vector. Computation of the magnitude of the acceleration vector may be implemented by software executing on the local processor  204 , or on the signal processing apparatus  110 . Advantageously, computing the resultant acceleration vector magnitude at the local processor  204  reduces the volume of information that must be transmitted via the BLE communications link from the motion sensor  108  to the signal processor  110 . 
         [0060]      FIG. 4  shows a graph  400  of a motion sensor data set representing an exemplary oscillatory mechanical response obtained using the arrangement and apparatus described above with reference to  FIGS. 1 to 3 . Time is represented on the horizontal axis  402 , while the accelerometer voltage level corresponding with the resultant acceleration magnitude is represented on the vertical axis  404 . As can be seen from the resulting acquired data trace  406 , a mechanical percussive impulse is applied after approximately 0.17 seconds, resulting in a generally oscillatory motion of the subject&#39;s calf, apparent in the portion  408  of the trace  406 . This motion substantially settles over a period of approximately 0.8 seconds. A low level of additive noise is also visible, particularly on the steady state portions of the trace  406 . The goal of analysis of the oscillatory mechanical response of the subject&#39;s calf can therefore be defined as extracting a relevant parameter set, useful for assessing the possible presence of DVT, from the acquired data, in the presence of additive noise. 
         [0061]    According to exemplary embodiments of the invention, a starting assumption for the analysis, which may be performed using suitably configured software, firmware and/or hardware implemented on the signal processing apparatus  110 , is that the acquired data set  406  may be represented as a superposition of two or more exponentially decaying sinusoidal waveforms. On this assumption, a waveform such as the trace  406  may generally be represented in the following form: 
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         [0062]    In the above equation, N represents the number of exponentially decaying sinusoidal components to be used in fitting the acquired data set  406 . Experiments conducted by the inventors have indicated that N=3 is generally a suitable choice. Each component also has an amplitude A, a characteristic frequency ω, an associated phase Φ, and a damping coefficient γ. The function n(t) represents the additive noise. It should also be noted that, for the purposes of analysis, the signal is a discrete time sequence of samples of the motion sensor voltage. 
         [0063]    Various techniques are available for analysing signals such as that shown in the graph  400  of  FIG. 4 . For example, Fourier analysis could be employed, based upon the Fast Fourier transform (FFT). However, the FFT may suffer from limited frequency resolution, spectral leakage, picket fence effect, and scalloping loss. While these may be mitigated through choice of proper apodisation, averaging, zero padding, up-sampling, and other well-known approaches, the use of FFT-based methods for analysis of short, aperiodic, signals is likely to be suboptimal. 
         [0064]    An alternative technique would be to use a nonlinear regression technique, such as nonlinear least-squares fitting, in order to estimate the unknown parameters of the above equation. Such general techniques may be unstable or unreliable in the presence of additive noise, and may perform poorly when fitting large numbers of unknown parameters. Accordingly, it may be preferable to employ more-targeted techniques for fitting or estimating the parameters of a sum of exponentially decaying sinusoids in the presence of additive background noise. One such targeted technique is Prony&#39;s method, however it has been shown that better performance can be obtained using a Matrix Pencil Method (MPM) as described in Hua and Sarkar, ‘Matrix Pencil Method for estimating parameters of exponentially damped/undamped sinusoids in noise’,  IEEE Transactions on Acoustics, Speech and Signal Processing,  Volume 38, No. 5, May 1990. 
         [0065]    Using the MPM technique, a generally damped oscillatory response, such as the trace  406  shown in the graph  400 , may be represented as a superposition of a plurality of exponentially-damped sinusoids, in accordance with the above equation, as illustrated by the graph  500  shown in  FIG. 5 . Again, the horizontal axis  502  represents time, while the vertical axis  504  represents the equivalent acceleration amplitude in volts, relative to the steady state reading. The acquired data set has been decomposed into three oscillatory components, comprising a dominant component  506 , and two smaller components,  508 ,  510 . By design, the MPM technique minimises error due to noise, and thus any further exponentially decaying sinusoidal components would necessarily be of smaller magnitude relative to the three components  506 ,  508 ,  510  shown in the graph  500 . It is, accordingly, apparent that the inclusion of additional components in the analysis would be of minimal benefit. Indeed, increasing the number of parameters to be estimated may impact upon the stability and reliability of the analysis method. 
         [0066]    A key insight of the present inventors is that the mechanical response of a muscle is altered in the presence of DVT. While there is considerable variation between muscles of different subjects, the expected variation between corresponding muscles of pairs of limbs of a single subject is expected to be considerably smaller. Therefore, the possible presence of DVT in one muscle of a subject, such as the right calf muscle, may be assessed by comparison with the corresponding muscle of the other limb, e.g. the left calf muscle. As described below, with reference to  FIGS. 6 and 7 , a suitable comparison may be made using parameter sets derived from analysis, such as described above, of the mechanical response of each muscle. 
         [0067]    According to embodiments of the present invention a parameter set is derived from each data set using MPM analysis. The characteristic frequencies and damping coefficients (or, equivalently, damping factors) of the principal exponentially decaying sinusoidal component have been found to comprise useful parameter sets for the purpose of comparison. Highly statistically significant differences in these parameters have been identified in subjects experiencing DVT in one calf muscle. 
         [0068]    By way of example,  FIG. 6  is a chart  600  illustrating a preliminary statistical analysis of characteristic frequency parameters obtained in accordance with an embodiment of the invention. The horizontal axis  602  of the chart shows the characteristic frequency (in radians per second), while the vertical axis  604  represents the number of events (out of 10 repeated tests for the left leg, and nine for the right), falling within each identified characteristic frequency band. The result of this analysis is a histogram  606  for the left leg of the subject, to which a corresponding normal distribution curve  608  has been fitted, and a further histogram  610  for the right leg, to which a corresponding normal distribution curve  612  has been fitted. In this particular case, the subject is known, from other tests, to have DVT in the right calf muscle, but not in the left calf muscle. The reduction in the characteristic frequency resulting from the presence of the DVT is statistically significant, having a p-value of 0.01 (i.e. less than 0.05) when analysed using the statistical t-test. 
         [0069]      FIG. 7  shows a similar chart  700  resulting from a preliminary statistical analysis of damping factors. In this case, the horizontal axis  702  represents damping factor, while the vertical axis  704  once again shows the number of events within each identified damping factor range obtained from a sample of 10 repeated mechanical stimulus impacts. A histogram  706  representing damping factors for the left calf muscle has been fitted with a corresponding normal distribution curve  708 , while a histogram  710  corresponding with damping factors for the right calf muscle has been fitted with a normal distribution curve  712 . There is, once again, a clear difference between the left and right calf muscle response, and t-test analysis has established that the difference is statistically significant, having a p-value of 0.02. It is clear that a higher damping factor is observed in the muscle having DVT. 
         [0070]    In accordance with embodiments of the invention, the above analysis may be performed by the signal processor  110 , and graphs such as the response  400  shown in  FIG. 4 , and results of statistical analyses such as the charts  600 ,  700  shown in  FIG. 6 or 7 , rendered on the display  122 . An operator, i.e. a trained technician, clinician or other healthcare worker, will thus be able to assess the possible presence of DVT in the subject of the test by visual inspection of the displayed results. 
         [0071]    According to some embodiments of the invention, the statistical analysis may be used in further comparisons performed by the signal processor  110  in order to offer the operator a preliminary assessment of likelihood of the presence of DVT. For example, based upon the analysis of aggregate data using computational and statistical methods including, but not confined to, logistic regression, the results may be used to assess the differences in characteristic frequency and/or damping factor between a series of tests performed on left and right calf muscles of the subject. If the likelihood or probability of the differences is above a clinically and empirically determined threshold (or any other desired criterion, depending upon requirements) then the signal processor  110  may highlight to the operator the likelihood or probability that DVT is present, and identify the relevant muscle. 
         [0072]    Advantageously, therefore, embodiments of the invention provide a useful diagnostic tool that may be used by healthcare workers to identify patients having a high likelihood of DVT. Such patients may then be referred for further testing and diagnosis. Used as a preliminary test, embodiments of the invention may be able to eliminate the likelihood that DVT is present in individual subjects, thus avoiding the need for unnecessary further testing, which may be costly, invasive and inconvenient. 
         [0073]    While particular embodiments and variations of the invention have been described herein, further modifications and alternatives will be apparent to persons skilled in the relevant art. For example, while the system described with reference to  FIGS. 1 and 2  employs a motion sensor  108  which performs data acquisition and digital transmission, along with a signal processing apparatus  110  that receives the digital signals and performs further processing and analysis, other arrangements are also possible. For example, additional analysis and processing could be performed on-board the motion sensor unit  108 . Furthermore, a wired, rather than a wireless, connection may be provided between the motion sensor unit  108  and the signal processing apparatus  110 . Furthermore, analog signals may be obtained by the motion sensor, e.g. voltage representing acceleration or other motion parameters, and transmitted in analog form to the signal processing apparatus  110 , which would receive and digitise the analog signals. Various combinations and arrangements of the above alternatives are also possible, and fall within the scope of the present invention. 
         [0074]    In other variations, alternative forms of analysis may be employed to determine parameter sets characterising muscle response to mechanical stimuli, such as modal analysis. 
         [0075]    Accordingly, the particular embodiments described in detail should be understood as provided by way of example, for the purpose of teaching the general features of the invention, but should be understood as not limiting of the scope of the invention, which is defined in the following claims.