Abstract:
A system for determining if signals present at bioelectric sensors derive from an intended source or from different, localized sources or artifacts includes a first sensor placed to detect the electric potential of interest and generate a first electric signal possibly representative of the electric potential of interest and a second sensor placed near the first sensor and preferably a relatively large distance away from the source. The second sensor detects the electrical potential of interest and generates a second electrical signal which also possibly represents the electrical potential of interest. An electronic circuit determines whether a difference between the electrical signals exceeds a certain threshold, thus indicating that either one or both of the signals is a measure of an artifact and not the electric potential of interest.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application represents a National Stage application of pending PCT/US2007/009748 filed Apr. 23, 2007 entitled “System for Measuring Electric Signals”, and further claims the benefit of U.S. Provisional Patent Application Ser. No. 60/794,275 filed Apr. 21, 2006 entitled “ECG Monitoring System”. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Phase II SBIR Contract No. W31P4Q-04-C-R293 awarded by DARPA. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention pertains to the art of measuring electric signals and, more particularly, to a system for measuring bioelectric signals that determines whether detected signals derive from a source of interest or result from an artifact or other undesired interference in acquired data. 
         [0005]    2. Discussion of the Prior Art 
         [0006]    It is widely known that electric fields are developed in free space from many different sources. For example, organs in the human body, including the heart and brain, produce electric fields. For a variety of reasons, it is often desirable to measure these electric fields, such as in performing an electrocardiogram (ECG). Actually, the measurement of bioelectric signals can provide critical information about the physiological status and health of an individual, and is widely used in monitoring, evaluating, diagnosing and caring for patients, as well as providing feedback for athletic training. Common methods of measuring electric potentials associated with a human employ gel-coated electrodes that must be secured directly to the skin of a subject. In addition, in recent years a number of alternate electrode technologies have been developed. While the alternate electrode techniques enable more convenient and comfortable measurement configurations, they are often prone to measurement artifacts. 
         [0007]    More specifically, resistive electrodes have been predominantly employed in connection with measuring electric potentials produced by animals and human beings. As the resistive electrodes must directly touch the skin, preparation of the skin to achieve an adequate resistive connection is required. Such resistive electrodes are the standard for current medical diagnostics and monitoring, but the need for skin preparation and contact rule out expanding their uses. Although attempts have been made to construct new types of resistive electrodes, such as making an electrically conductive fabric, providing a miniature grid of micro-needles that penetrate the skin, and developing chest belt configurations for heart related measurements or elasticized nets with resistive sensors making contact via a conductive fluid for head-related measurements, these alternative forms do not overcome the fundamental limitation of needing to contact the skin directly. This limitation leads to an additional concern regarding the inability to maintain the necessary electrical contact based on differing physical attributes of the patient, e.g. amount of surface hair, skin properties, etc. 
         [0008]    Another type of sensor that can be used in measuring biopotentials is a capacitive sensor. Early capacitive sensors required a high mutual capacitance to the body, thereby requiring the sensor to touch the skin of the patient. The electrodes associated with these types of sensors were strongly affected by lift-off from the skin, particularly since the capacitive sensors were not used with conducting gels. As a result, capacitive sensors were not found to provide any meaningful benefits and were not generally adopted over resistive sensors. However, advances in electronic amplifiers and new circuit techniques have made possible a new class of capacitive sensor that can measure electrical potentials when coupling to a source on the order of 1 pF or less. Examples of low noise electric field sensors can be found in U.S. Pat. Nos. 6,686,800 and 7,088,175, each of which is incorporated herein by reference. This capability makes possible the measurement of bioelectric signals with electrodes that do not need a high capacitance to the subject, thereby enabling the electrodes to be used without being in intimate contact with the subject. 
         [0009]    Substantial body motion during exercise or daily life can produce artifacts in any bioelectric measurement system, but these effects become more pronounced with many alternate electrode technologies. These artifacts are caused by mechanisms that are local to the skin and sensor, such as static electric potentials, electromyographic signals and piezoelectric artifacts. In contrast, the signal produced by the heart, brain or other organ originates within the body at a much greater distance from the sensor. Hence, there exists a need to determine when the data reflects a distant source, or results from local sources. Some such methods attempt to confirm that the data have the general structure expected. However, these may reject valid signals having non-standard features, and may accept artifact data that happen to conform to the expected model. Therefore, it is desirable to determine if data are valid independently of the specific data themselves. 
         [0010]    Therefore, there exists a need in the art for a system that can determine when the data taken by a bioelectric measurement system reflect a distant source, or result from local artifacts. There also exists a need for less intrusive electrode technology, together with the capability to tolerate artifacts at the input, and allow for measurement configurations which were previously not practical. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention is directed to a system for discerning the validity of bioelectric data and adds to a basic measurement system one or more additional sensors that measure related signals. Based on known relations between the sensors and a primary source of the bioelectric signals, and on known response characteristics of the sensors, a relation of the expected signals produced by the source on the multiple sensors can be predicted. If the observed signals do not show the expected relation, then it is concluded that a significant part of the measured signal from at least one of the sensors compared must derive from a cause other than the primary source, and therefore is due to sources local to the sensor. For example, if two sensors are very close to each other and comparatively far from the signal source, the signals they measure from that source should be similar. Any difference in the two signals must therefore come from some local source. If the difference is significant, then the data measured during the period in which the difference appears are suspect. 
         [0012]    The invention generally includes a sensor system for measuring an electric potential of interest generated by a source such as the heart or brain of the human body. A first sensor is placed at a first measurement location to detect the electric potential of interest and generate a first electric signal possibly representative of the electric potential of interest. A second sensor is placed at a second measurement location near the first sensor and preferably a relatively large distance away from the source. The second sensor detects the electrical potential of interest and generates a second electrical signal which also possibly represents the electrical potential of interest. Optionally, an adjuster or circuitry is provided for altering the first and second electrical signals to compensate for changes caused by placement of the sensors or the electrical characteristics of the sensors themselves. For example, the adjuster could adjust the gain or amplification of the signal produced by each sensor. In another embodiment, the adjuster involves filtering the data from one sensor to account for known changes in the source signal between first and second measurement locations should a sensor be moved from the second measurement location to the first measurement location. 
         [0013]    A comparator compares the first and second signals to produce a comparison result representing a measurement of that difference. An electronic circuit determines whether the comparison result exceeds a certain threshold level, thus indicating that either one or both of the signals is a measure of an artifact and not the electric potential of interest. 
         [0014]    The threshold level may either be static or dynamic. In one embodiment, the comparator compares the magnitude of the signals. In another embodiment, it checks for a time offset of the signals produced by each of the two sensors. 
         [0015]    The sensor system may incorporate various different types of devices. In one preferred embodiment, the sensor system is incorporated into an audio generating device. In another embodiment, the sensor system is enclosed and the sensors themselves are in the arms or shoulders of a garment. In yet another embodiment, the sensor system is independent of the garment and attached thereto through some type of connection mechanism and, as such, is removably attached to the garment. 
         [0016]    In use, the sensor system measures electrical signals by using the sensors, then adjusts the signals to compensate for placement and/or electrical characteristics of the signals. The two signals are then compared to see whether or not the differences in the signals reach a threshold to determine whether or not the signals are caused by a local artifact or the distant source. 
         [0017]    In yet another embodiment, one or more additional sensors are used in the system. For example, a third sensor may be used and signals generated by the third sensor analyzed and compared with each of the first two sensors as described above in regards to the first and second sensors. The resulting signal from each additional pair of sensors considered valid may be averaged in with the signal from the existing sensors to obtain a more accurate final signal. 
         [0018]    Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of preferred embodiments when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0019]      FIG. 1  schematically illustrates a system for measuring electric signals according to a preferred embodiment of the invention; 
           [0020]      FIG. 2  is a graph depicting two electrocardiograms measured with closely spaced sensors; 
           [0021]      FIG. 3  is a graph depicting the electrocardiogram of  FIG. 2  and also showing the magnitude of the difference between the signals; 
           [0022]      FIG. 4  is a graph depicting the electrocardiogram of  FIG. 2  and also showing regions of traces identified as an artifact; 
           [0023]      FIG. 5  is a graph depicting the electrocardiogram of  FIG. 4  and also showing regions of traces identified as an artifact having been removed; 
           [0024]      FIG. 6  schematically illustrates the system for measuring electrical systems incorporated with the sleeves of a garment; 
           [0025]      FIG. 7  schematically illustrates the system for measuring electrical systems incorporated with the shoulders of a garment; and 
           [0026]      FIG. 8  schematically illustrates the system for measuring electric signals incorporated with an audio device. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    With initial reference to  FIG. 1 , a sensor system  10 , constructed in accordance with the present invention, is arranged to measure signals from a bioelectric source  12  within a body  13  of an individual  15 , such as a medical patient, animal, test subject or the like. Bioelectric source  12  creates an electric potential of interest  16  which is depicted as a cardiac signal, but could also be generated by other muscle, nerve or brain action. A first sensor  17  is placed at a first measurement location  18  so as to detect electric potential of interest  16  and generate a first electrical signal  19  possibly representative of electric potential of interest  16 . Similarly a second sensor  20  placed at a second measurement location  21  near first sensor  17  so as to detect electrical potential of interest  16  and generate a second electrical signal  22  possibly representative of electric potential of interest  16 . First and second sensors  17  and  20  are positioned near to each other and far enough from source  12  that signal produced by source  12  will be similar at each sensor  17 ,  20 . 
         [0028]    As indicated in  FIG. 1 , a plurality of sensors, including first and second sensors  17  and  20 , could be supported by a common housing or carrier  24 . This arrangement provides for convenient placement and control in the positioning of the sensors relative to each other. In addition, the use of common housing/carrier  24  assures an optimized relationship between sensors  17  and  20 , while maintaining sufficient electrical and mechanical isolation to avoid coupling unwanted signals into the sensors. 
         [0029]    The first and second signals  19 ,  22  from first and second sensors  17 ,  20  may be passed through first and second adjusters  26 ,  27  for altering first and second electrical signals  19 ,  22  respectively to compensate for changes in first and second electrical signals  19 ,  22  caused by placement of first and second sensors  17 ,  20  and electrical characteristics of first and second sensors  17 ,  20  to create first and second altered electrical signals  28 ,  29 . For example, adjuster  26  preferably adjusts a gain or a time offset of electrical signal  19  to match a gain or time offset of electrical signal  22 . Since amplifiers and signal adjusters are well known, the details of such electrical devices will not be discussed here. The use of first and second adjusters  26 ,  27  is completely optional and when adjusters  26 ,  27  are not used the unaltered first and second electrical signals  19 ,  22  are processed further as set forth below. 
         [0030]    Referring now to  FIG. 2 , a chart  30  is shown as an example of first and second altered electrical signals  28 ,  29  which in this case are two ECG traces derived from first and second altered electrical signals  28 ,  29  obtained from first and second sensors  17 ,  20 . The resulting altered signals  28 ,  29  are compared by a comparator  31  to produce a comparison result  32  which can be seen in  FIG. 3  depicting a chart  33  showing the magnitude of the difference or comparison result  32  between the traces. 
         [0031]    A threshold generator  35  generates a threshold  36  which can be seen in a chart  37  depicted in  FIG. 4 . Chart  37  indicates that threshold  36  identifies artifact regions or locally produced artifacts  38 . This is done when the magnitude of comparison result  32  is compared with threshold  36  produced by threshold generator  35 . Depending on implementation, threshold  36  is a static value, or is set dynamically, possibly incorporating duration information. If comparison result  32  exceeds threshold  36 , an interpretation system, i.e. an electronic circuit  40  suggests that first and second electrical signals  19 ,  22  represent locally produced artifacts  38 . In other words, if result  32  exceeds threshold  36 , interpretation system  40  will indicate that the data is suspect. Otherwise, interpretation system  40  will expect that the data is reliable. While in this example the magnitude of the comparison is used in another embodiment, time offset data of the first and second electrical signals  19 ,  22  are used to detect locally produced artifacts. 
         [0032]    The first and second altered signals  28 ,  29  can optionally be added, averaged or otherwise combined to produce a result with a better signal-to-noise ratio than either individually, with the resultant signal being used by interpretation system  40 . System  10  from sensors  17 ,  20  onward is implemented in analog electronics, or signals  19 ,  22  can be digitized at any point and the further processes performed digitally. 
         [0033]    In the embodiment above, first and second sensors  17  and  20  are at positions  18  and  21  in proximity to each other. It should be noted that positions  18  and  21  could effectively be the same position if, for instance, sensors  17  and  20  contacted body  13  at a plurality of discrete locations and those locations for sensor  17  were interleaved with those of sensor  20  such that the average of the contact locations for each of the two sensors was essentially the same location. 
         [0034]    In the embodiment above, first and second sensors  17  and  20  are of the same kind, with differing positions, i.e., as stated above, first sensor  17  is located at first measurement location  18  and second sensor  20  is located at second measurement location  21 . Since the difference in position is small compared with the location of source  12 , the signal from source  12  will present similarly to each of sensors  17 ,  20 . However, other signal sources from within sensors  17 ,  20  themselves, the interface between sensors  17 ,  20  and individual  15 , within individual  15  in proximity to one or the other sensor  17 ,  20 , or other causes will not present similarly to each sensor  17 ,  20 , and will therefore create a signal difference which can be identified by system  10 . Note that system  10  in each case is used to determine whether or not an observed signal originates from intended signal source  12 , or more locally to sensors  17 ,  20 . 
         [0035]    Alternatively, sensors  17  and  20  are co-located or closely located, yet receive signals in differing ways. For instance, they might be capacitive sensors with differing stand-offs from the skin. As such, they would respond similarly to distant intended signal source  12 , but very differently to signals or artifacts  38  generated at or near the skin. The processing chain is the same, except that the signal-scaling elements, i.e., adjusters  26  and  27 , would model the differences in sensor response between sensors  17  and  20 . 
         [0036]    Additionally, system  10  can be implemented with more than two sensors, such as including an additional sensor (not shown), also in proximity to sensors  17  and  20 . Signals can then be compared in pairs through the same process described above. Just as signals deriving from sensors  17  and  20  are compared to infer whether either of them contains signals that are locally generated, so can signals deriving from sensor  17  and the additional sensor be compared, and likewise signals deriving from sensor  20  and the additional sensor can be compared. If more than three sensors are used, more pairwise combinations can be established. If the number of sensors in proximity is N, the number of pairwise comparisons possible will be [N*(N−1)/2]. The signals from any pair(s) of sensors for which the comparison suggests that the signals are not locally generated can then be analyzed individually, or combined by averaging or other techniques. 
         [0037]    If the location or characteristics of the locally generated signal are of interest, they can be determined by this invention. For instance, in the example above, if the comparisons between sensor  17  and the additional sensor, and between sensor  20  and the additional sensor both suggest the presence of a locally-generated signal, yet the comparison between sensors  17  and  20  suggests no locally-generated signal, then the locally-generated signal will be concluded to have been measured by the additional sensor. 
         [0038]    Additionally, since useful bioelectric signals are generally formed by a difference between sensors that see differing presentations of bioelectrical source  12 , additional instances of the invention can be implemented using another set of sensors not in proximity to those of system  10 . For example, as shown in  FIG. 1 , an additional system (not separately labeled), similar to system  10 , includes a third sensor  43  placed at a measurement location  44  so as to detect electric potential of interest  16  and generate a third electrical signal possibly representative of electric potential of interest  16 . A fourth sensor  45  placed at a fourth measurement location  46  near third sensor  43  so as to detect electrical potential of interest  16  and generate a fourth electrical signal possibly representative of electric potential of interest  16 . 
         [0039]    A series of electronic devices  50  located downstream of third and fourth sensors  43 ,  45  are represented as a box. Electronic devices  50  optionally include third and fourth adjusters and also a second comparator  54  for comparing third and fourth altered electrical signals to produce a second comparison result, as well as a second electronic circuit for determining if the second comparison result suggests that the third and fourth electrical signals represent electric potential of interest  16  or locally produced artifacts. In such a system, data can be inferred to reflect intended source  12  if any pair of sensors that agree sufficiently well. More sensors improve the probability that such a pair will exist. Multiple such pairs can be considered individually, or averaged together to form a composite signal as represented by a line going from electronic devices  50  to interpretive system  40 . Interpretive system  40  can compare signals which the additional system infers are not locally generated with signals which system  10  infers are not locally generated to obtain a view of intended source  12  from differing perspectives. In either case, interpretive system  40  can remove locally produced artifacts  38  from electrical signals  28 ,  29  as shown in  FIG. 5  on chart  60  at signal portions  65 . 
         [0040]    Using electrode technologies alternative to traditional wet electrodes, a more comfortable and less intrusive arrangement is employed to collect signals  19 ,  20  from subject  15 . When such arrangements produce high quality data, they are used for detailed medical analysis. However, if the data is of lower quality, relatively simple measurements such as heart rate are extracted. Either type of arrangement benefits greatly by sensor system  10 , as described above, for determining when data is reliable verses unreliable. 
         [0041]      FIG. 6  depicts another preferred embodiment in which a sensor system  220  is incorporated into a shirt or garment  221 . Sensors  229  that produce a vector across a heart zone are embedded in two armbands  230 ,  231 , which might be part of shirt  221 , or attached to shirt  221 . Attachable armbands  230 ,  231  preferably have a common ground. Multiple sensors  229  can be incorporated in each armband  230 ,  231 , with sensor system  10  being used to infer when sensors  229  are providing useable data. As in the previous description, sensor system  220  may use individual sensors or may incorporate a plurality of discrete sensors  229 . Electrical connections  232  between sensor armbands  230 ,  231  may optionally be incorporated into shirt  221 . Similarly, an electronics package  233  may be detachable from or integrated with the other components. 
         [0042]      FIG. 7  depicts another preferred embodiment in which a multiple sensor system  235  with electrodes or sensors is incorporated into conductive fabric in a shoulder  239  of shirt  221 . System  235  relies on very weak resistive and weak capacitive coupling to subject  15 . Capacitive sensors  240  are incorporated into shoulders  239  or arm bands  242  of shirt  221 , with the weight of shirt  221  holding sensors  240  to subject  15 . Sensor system  10  is used to infer which sensors  240  are providing useable data. These sensors  240  are preferably held in contact with or near body  13  merely by the weight of shirt  221  and the multiplicity of sensors  240 , in combination with sensor system  10 , allows the determination of which sensors  240  are providing valid signals at any given time. Again, there are electrical connections  241  incorporated into shirt  221 , and an integrated or detachable electronics package  233  is provided. Although  FIG. 7  depicts sensor package  233  at sleeve  242 , it can be placed wherever most convenient. 
         [0043]    Finally,  FIG. 8  depicts another preferred embodiment of the invention including a system  300  of sensors, which utilizes body attachments already commonly made with portable music players to form a MP3/CD/Radio ECG system to make a bioelectric measurement of cardiac and/or other bioelectric signals. This system  300  utilizes a hardware configuration often currently worn in an ear  319  and includes at least one earpiece  320 , along with audio electronics module  324  often worn on an arm  326  or hip  327 . 
         [0044]    Wires  329  contain strands which, in addition to the usual audio functions, provide power and signal connections to electronics module  324 . At this point, it should be noted that, although  FIG. 8  depicts two electronics modules  324 , only one would be implemented in a given system  300 . For this reason, one electronics module  324  on hip  327  is shown in phantom. In any case, electronics module  324  contains a sensor system of one or more individual sensors which provides a measurement vector across the heart, enabling a cardiac signal to be measured. In addition, electronics module  324  includes sensor system  10  discussed above. Ear piece  320  preferably contains a sensor or a sensor system that measures the bioelectric potential of interest  16  at ear  319 . Sensor system  300  is preferably a single sensor making a resistive, capacitive or hybrid (resistive/capacitive) connection to ear  319 , or has two or more such sensors in an implementation of sensor system  10  discussed above. Because there will likely be local measurement artifacts  38  resulting from the movement of subject  15 , multiple sensors may be at each location, and preferably system  10  is used to infer when the sensors are providing useable data. 
         [0045]    Although described with reference to various preferred embodiments of the invention, it should be readily understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof. In general, the invention is only intended to be limited by the scope of the following claims.