Abstract:
A configurable system for obtaining a measurement of activity producing biopotentials in a subject, for example EEG or EMG biopotentials. The system includes a three electrode array positionable on the head of the patient to detect signals generated in the head of the subject. The array is connected to a monitor that includes a switch arrangement that is selectively configurable to direct the incoming signals received by the electrode array to specified inputs of a differential amplifier that creates signals that are displayed on the monitor. The switch arrangement is configurable to measure the activity of the subject in a conventional 1-channel measurement mode. The switch arrangement can also be configured to simulate a 2-channel measurement mode by alternating the configuration of the switch arrangement in a pre-determined manner.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for measuring the biopotential signals produced in a subject, and more specifically to an apparatus and method that is configurable to provide either a 1-channel operating mode or a mode resembling 2-channel operation. 
     BACKGROUND OF THE INVENTION 
     Electroencephalography (EEG) is a well established method for assessing the brain function by picking up the weak biosignals generated in the brain with electrodes on the skull surface. To obtain the biosignals, multiple electrodes are placed on the scalp of a patient in accordance with a recognized protocol. EEG has been in wide use for decades in basic research of the neural system of brain as well as clinically in diagnosis of various neurophysiological disorders. 
     The EEG signals received by the electrodes from the scalp are amplified by amplifiers which may be of the differential type to minimize electrical interference. Each amplifier has three inputs: 1) a positive signal input; 2) a negative signal input; and 3) a ground input. Consequently, even the most rudimentary 1-channel EEG measurement procedure requires the use of three electrodes. Applying electrodes to the scalp takes time and skill, requires skin preparation, e.g., removal of hair, and is especially difficult in a thick hair environment. 
     One of the special applications for EEG which has received much attention to during the 1990&#39;s is use of a processed EEG signal for objective quantification of the amount of brain activity for the purpose of determining the level of consciousness of a patient. In its simplest form, this usage of EEG allows for the automatic detection of the alertness of an individual, i.e. if he or she is awake or asleep. This has become a significant issue, both scientifically and commercially, in the context of measuring the depth of unconsciousness induced by anesthesia during surgery. Modern anesthesia practices use a sophisticated balancing technique with a combination of drugs for maintaining adequate hypnosis, analgesia, muscle relaxation, and/or suppression of the autonomic nervous system and blockage of the neuromuscular junction. The need for a reliable system for the monitoring of the adequacy of the anesthesia is based on both safety and economical concerns. An anesthesia dose which is too light can, in the worst case, can cause the patient to wake up in the middle of the operation and create a highly traumatic experience both for the patient and for the personnel administering the anesthesia. At the opposite extreme, the administration of too much anesthesia generates increased costs due to the excessive use of anesthesia drugs and the time needed to administer the drugs. Over dosage of anesthesia drugs also affects the quality and length of the postoperative period immediately after the operation and the time required for any long term post-operative care. 
     A significant main advancement in making the EEG-based measurement of the depth of unconsciousness induced by anesthesia an easy-to-use, routine procedure was a finding based on Positron Emission Tomography (PET) that determined that the effects of the anesthetic drugs on the brain are global in nature. This means that for many applications it is enough to measure the forebrain or frontal cortex EEG from the forehead of the subject. The forehead is both an easy to access and is a hairless location on the subject. Electrodes placed with an appropriate spacing between electrodes on the forehead can pick up an adequate signal originating from the anterior cortex in the brain. This discovery, together with development of a special algorithm, namely, the Bispectral Index (BIS), an electrode design requiring no skin preparation, as disclosed in U.S. Pat. No. 5,305,746, incorporated herein by reference, and a convenient integrated electrode array, as disclosed in U.S. Pat. No. 6,032,064, also incorporated herein by reference, have contributed to a viable commercial product manufactured and sold by Aspect Medical of Natick, Mass. capable of obtaining a measurement of the state or activity of the brain during delivery of anesthesia using an EEG system. 
     The &#39;064 patent teaches a disposable EEG electrode array. One array has three electrodes for 1-channel measurement. A different array has four electrodes for 2-channel measurements. The 2-channel set-up is symmetrical in configuration and separately collects the signals between the mid-forehead and left and right mastoidal points, respectively. The 2-channel measurement configuration is used to determine the differences in the EEG signal in situations in which the right and left frontal hemispheres might be expected to produce different EEG signals. This can be caused, for example, by ischemia or burst suppression, i.e., EEG signals in discontinuous bursts, in either of the sides of the head, as well as artifacts in the EEG signals due to movement of the eyes of the subject or poor contact in one of the electrodes. 
     However, if it is desired to switch from 1-channel to 2-channel EEG measurements, with these prior art sensors it is necessary to remove the three electrode, 1-channel sensor and replace it with a four electrode, 2-channel sensor, and vice versa. This requires significant time and effort on the part of the technician taking the measurements as the first sensor must be removed before the second sensor can be positioned on the individual, and because the positioning of the second sensor must be precise in order to obtain an accurate measurement of the neurological activity of the subject. 
     It would, therefore, be desirable to develop a neurological activity sensor system which is capable of operation in both a 1-channel and 2-channel manner to obtain EEG measurements of the neurological activity of the subject. The sensor system should have as simple a construction as possible to minimize the amount of time and effort necessary to properly position the electrodes of the sensor on the subject prior to obtaining the measurements. 
     While the foregoing has discussed the use of EEG signals, it may also be desirable to obtain electromyographic (EMG) signals arising from the forehead of the subject. Should an anesthetized patient approach a state of consciousness, the frontalismuscle in the forehead of the subject may contract from a pain sensation or for other reasons. When sensed by appropriately placed electrodes, this muscle activity can provide an early indication that the subject is emerging from anesthesia. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a low cost sensor system of simple construction having an electrode array with three basic EEG electrodes capable of performing measurements of neurological activity in different portions of the brain, such as the overall frontal cortex of the brain or the left or right hemispheres of the forebrain. 
     A further object of the invention is to provide a sensor system capable of obtaining EMG signals from the head of a subject. 
     It is another object of the invention to provide a sensor system and method of operating same which can be configured to selectively operate in a conventional 1-channel mode or in a manner to approximate a 2-channel measurement. 
     It is still a further object of the invention to provide a sensor system wherein the electrode array is manufactured to be disposable. 
     The invention employs an electrode array of three electrodes. The sensor system uses a switching arrangement connected to the electrode array to route signals from each of three electrodes forming the array in a manner that allows measurement of the biopotential difference between any pair of the three electrodes of the system while using the remaining electrode in each case as a ground electrode. To this end, a signal from each of the three electrodes can be selected by the switching arrangement for use as a positive input signal, a negative input signal or a ground signal to a signal processing unit, such as a differential amplifier to obtain a biopotential difference used to measure the neurological or muscular activity of the subject. 
     The switching arrangement can route the signals from the electrodes to form a 1-channel measurement mode to monitor the neurological activity of either the left or right hemisphere of the forebrain or overall frontal cortex of the brain. The switching arrangement can also route signals from selected pairs of electrodes to the differential amplifier in a pre-determined, alternating fashion to provide an essentially 2-channel measurement of neurological activity. EMG signal data is obtained in an analogous manner. 
     The sensor system and method of the present invention have significant advantages compared to a fixed 1-channel set-up. First of all, the system allows for the optimization of the signal quality regarding the signal-to-noise ratio in the signals of the electrode array. The system can automatically choose to start a measurement using the electrode on the frontal hemisphere that is receiving the strongest signal and/or the least amount of noise by sampling the signals and noise levels generated by each frontal hemisphere and received by each electrode prior to starting any measurement. Secondly, by switching 1-channel measurements using selected pairs of electrode signals back and forth in a predetermined sequence, this system can also work as a surrogate for a true 2-channel measurement system. The system can also be configured to monitor the status of the electrodes, and detect the origin of any interference or signal artifacts and for the diagnosis of any physiological changes that generate lateral asymmetry in the frontal cortex neural activity, such as changes in blood flow in one of the carotid arteries. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following drawings illustrate the best mode currently contemplated of practicing the present invention. 
     In the drawings: 
     FIG. 1 is a perspective view of the sensor system for measuring biopotentials constructed according to the present invention and connected to a subject; 
     FIG. 2 is a plan view of the electrode array of the system of FIG. 1; and 
     FIG. 3 is a schematic view of the circuitry used in the system of FIG. 1 to direct input signals from the electrodes to signal processing unit inputs for measurement of signal differences between different selected pairs. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference now to the drawings in which like reference numerals designate like parts throughout the disclosure, the sensor measurement system of the present invention is indicated generally at  10  in FIG.  1 . The system  10  includes an electrode array  12  connected to a monitor  14  by a cable  16 . The array  12  transmits neurological activity signals received from the forehead  18  of the patient to the monitor  14  which carries out signal processing and numerically or graphically displays EEG or EMG data. The data may also be stored for future use. 
     As best shown in FIGS. 1 and 2, the electrode array  12  includes a central body  20  and a pair of side bodies  22  and  24  connected to the central body  20  by a pair of flexible arms  26 . The central body  20 , side bodies  22  and  24  and arms  26  are each formed of a flexible, resilient material which enables the arms  26  to flex with respect to the central body  20 . This allows the array  12  to conform to the shape of the subject&#39;s head  18  and to have the side bodies  22  and  24  positioned at the optional sites on the head  18  to detect activity producing biopotentials. The positioning of the array is shown generally in FIG.  1 . The preferred material used in the construction of the electrode array  12  is a thermoplastic material, which also allows the electrode array  12  to be formed as a single unit, if desired, as shown in FIG.  1 . 
     Each of the central body  20  and side bodies  22  and  24  includes an electrode  28 ,  30  and  32 , respectively, disposed on one side of the electrode array  12 . Each electrode  28 ,  30  and  32  is connected to a conductor  29 ,  31  and  33 , respectively, that transmits biopotential signals received by the electrodes  28 ,  30  and  32  from the forehead  18 . The electrodes and conductors are formed of a conductive material suitable for receiving and transmitting biopotentials, such as metallic foils or wires, vapor deposited or printed metallic layers, or the like. The electrodes  28 ,  30  and  32  and associated conductors  29 ,  31  and  33  are preferably formed on one side of the flexible, resilient material of array  12 . However, the electrodes and conductors may also be formed separately from the array  12  and individually placed on the array  12  in a necessary configuration and location. 
     The conductors  31  and  33  extend from each of the electrodes along the arms  26  and are connected, along with conductor  29 , to a connector  34  disposed on the central body  20 . The connector  34  is used to connect the cable  16  to the electrode array  12  and is formed as one half of a conventional electrical connection, such as a male or female plug portion. Preferably, the connector  34  is formed as a female plug portion including an aperture (not shown) for the reception of a male plug portion (not shown) located on the end of the cable  16  extending away from monitor  14 . The aperture exposes the end of each of the conductors  29 ,  31  and  33  leading from the electrodes  28 ,  30  and  32 , respectively, such that the plug can contact the conductors and receive a biopotential signal transmitted by the conductors  29 ,  31  and  33  from the electrodes  28 ,  30  and  32 , respectively, for transmission along the cable  16  to the monitor  14 . 
     The array  12  also includes adhesive material  40  disposed on each of the central body  20  and side bodies  22  and  24 , around the electrodes  28 ,  30  and  32 . The material  40  functions to secure the array  12  and each electrode  28 ,  30  and  32  against the skin of the forehead  18  of the subject so that biopotential signals from the forehead  18  can be picked up by the electrodes  28 ,  30  and  32 . The material  40  also prevents the movement of the array  12  and electrodes  28 ,  30  and  32  with respect to the forehead  18  to insure the electrodes remain in optimal locations on the forehead  18  for picking up the desired signals from the brain or head. The overall construction of the array  12  enables the array  12  to be disposed of in its entirety after use for measuring biopotential signals from the forehead  18  of a subject. 
     Referring now to FIGS. 1 and 3, the monitor  14  receives the signals picked up from the subject&#39;s head  18  by the electrodes  28 ,  30  and  32  via the cable  16 . The cable  16  includes three input signal leads  42 ,  44  and  46  which extend along the cable  16  and each correspond to and connect with one of the conductors  29 ,  31  or  33  in the connector  34  via the male plug portion. At the end of cable  16 , opposite the male plug portion, each lead  42 ,  44  and  46  is connected into a set of nodes  48 ,  50  and  52 , respectively. The nodes  48 ,  50  and  52  form part of a switching arrangement which includes three switches  56 ,  58  and  60 . Each switch  56 ,  58  and  60  is associated with one set of nodes  48 ,  50  and  52 , respectively, such that each switch can selectively contact each of the three nodes in each set. The switches are shown schematically in the drawing for illustrative purposes and may comprise solid state switching elements or other suitable components. 
     The outputs of switches  56 ,  58  and  60  are connected to the inputs of a signal processing unit, shown as differential amplifier  62  which amplifies the biopotential signals transmitted from the leads  42 ,  44  and  46 . For a signal processing unit comprising a differential amplifier, the output of switch  56  is connected to a positive signal input  64  of amplifier  62 , the output of switch  58  is connected to a negative signal input  66 , and the output of switch  60  is connected to a ground input  68  via ground  63 . The signals transmitted to the positive signal input  64  and negative signal input  66  are used to establish a signal difference that is amplified by the differential amplifier  62  to create an output signal in conductor  70  which is processed and used to drive a display  72  for the monitor  14 . 
     The monitor  14  also includes a plurality of buttons  74   a, b, c , and  d  disposed on monitor  14 . The buttons  74  are operably engaged with the switching arrangement and are used to control the configuration of the switches  56 ,  58  and  60  in order to alter the connections between the signal leads  42 ,  44 , and  46  and amplifier  62 . For EEG signals, this obtains various EEG measurements from the signals from the frontal cortex of the subject&#39;s forehead  18  or different sections thereof, which are displayed on the monitor  14 . 
     To operate system  10 , the cable  16  is connected to the electrode array  12  which is positioned on the subject&#39;s forehead  18  with each electrode  28 ,  30  and  32  in a desired location and secured to the patient&#39;s forehead by the adhesive material  40 . By operating one of the buttons  74   a, b,  or  c,  the user selects the configuration of the switches  56 ,  58  and  60  within the monitor  14 . The configuration of the switches determines how the biopotential signals obtained by the electrodes  28 ,  30  and  32  from the subject&#39;s forehead  18  will be utilized by differential amplifier  62 . For example, when the switches  56 ,  58  and  60  are in the configuration shown in FIG. 3, the signal from the electrode  30  is utilized as the positive signal input  64 , the signal from the electrode  28  is utilized as the negative signal input  66 , and the signal from the electrode  32  is utilized as the ground input  68 . For EEG signals, this would measure the biopotential signal existing in one of the hemispheres of the patient&#39;s forebrain, i.e. the right hemisphere shown in FIGS. 1 and 2. By operating a different button  74 , the configuration of the switches  56 ,  58  and  60  will change such that signals from different electrodes  28 ,  30  and  32  will be utilized as the positive signal input  64 , negative signal input  66  and ground input  68  for the amplifier  62  to measure the biopotential signal existing in the other forebrain hemisphere or in the overall frontal cortex of the brain. Thus by changing the configuration of the switches with buttons  74   a,    74   b,  or  74   c,  and hence the inputs to differential amplifier  62 , a user can determine the neurological activity in the right hemisphere of the forebrain, in the left hemisphere of the forebrain, or in the overall frontal cortex pursuant to an EEG measurement performed in the conventional 1-channel mode of the system  10 . 
     Further, monitor  14  can contain a control  76  such that when button  74   d  is operated, a computer program or other control element, is initiated to periodically alternate the configuration of the switches  56 ,  58  and  60  in a specified manner. This allows the monitor  14  and system  10  to alternately measure the neurological activity in each hemisphere of the forebrain to obtain a measurement similar to that of a 2-channel EEG measurement mode. Thus, the system  10  can be selectively operated in either a selected 1-channel or 2-channel surrogate measurement mode simply by operating the appropriate button  74  on the monitor  14  associated with the desired measurement mode. 
     Operation of system  10  to obtain EMG biopotential signals is carried out in a manner analogous to that described above in connection with obtaining EEG signals. 
     By sensing properties such as the signal strength and/or signal noise in conductors  42 ,  44 , and  46 , as by signal sensor  78  and connection  80 , control  76  can be used to provide signals of highest quality to differential amplifier  62 , thereby to improve the quality of the output signal in conductor  70 . Signal sensor  78  may also be used to provide and indication of the status of the electrodes of array  12 . 
     Various alternatives are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.