Patent Publication Number: US-8986207-B2

Title: Systems and methods for providing sensor arrays for detecting physiological characteristics

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/260,743, filed Nov. 12, 2009, which is hereby incorporated by reference in its entirety. 
    
    
     SUMMARY 
     The present disclosure is related to signal processing systems and methods, and more particularly, to systems and methods for detecting one or more physiological characteristics of a subject using a sensor array. 
     In an embodiment, a sensor array may include a plurality of sensor elements capable of detecting physiological characteristics of a subject. The sensor elements in the array may be of any suitable type and may detect any number of physiological characteristics. Groups of sensor elements in the array may detect the same or different physiological characteristics. Each individual sensor element may detect a single physiological characteristic, or individual sensor elements in the array may detect more than one physiological characteristic. Sensor elements in the array may communicate with other sensor elements in the array or with a monitoring device connected to the sensor array. 
     In an embodiment, measurements received from a sensor array may be analyzed to select a suitable measurement type to obtain using the sensor array. The selection may include comparing characteristics of signals received from the sensor array to a predefined set of metrics and thresholds. The selection may also be made based on the locations of different sensor arrays or different sensor elements in sensor arrays. The selection may be made based on the correlation between signals received from the sensor array and models for available measurement types. A set of available measurement types may be predefined, and the suitable measurement type may be selected from the set of available measurement types. 
     In an embodiment, combinations of sensor elements may be evaluated for selection of a best combination or combinations of sensor elements to use for determining one or more physiological parameters. The combinations may include sensor elements from one or more sensor arrays. The measurement type obtained from the combinations of sensors may be a predefined measurement type. The measurement types obtained from the combinations of sensor elements may also be determined after the best combinations of sensor elements are selected. All possible combinations of available sensor elements may be evaluated before a final best combination of sensor elements is selected. Alternatively, combinations of available sensor elements may be evaluated only until a first suitable combination of sensor elements is found, and that suitable combination of sensor elements may be used for monitoring. 
     In an embodiment, a sensor array may be coupled to a monitoring device that obtains measurements from the sensor array and performs selection of combinations of sensor elements and suitable measurement types used for monitoring. The monitoring device may be coupled to other sensor units in addition to a sensor array and may be coupled to more than one sensor array. The monitoring device may test combinations of sensor elements from just one sensor array to which it is attached, or may test combinations of sensor elements from any number of sensor arrays to which it is attached. The monitoring device may be coupled to a multi-parameter monitor that may display information associated with the physiological parameters being determined, the various measurement types being obtained, and/or the combinations of sensor elements being used to obtain measurements. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The above and other features of the present disclosure, its nature and various advantages will be more apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings in which: 
         FIG. 1(   a ) depicts a block diagram of a sensor array according to an illustrative arrangement; 
         FIG. 1(   b ) shows a patient monitoring system according to an illustrative arrangement; 
         FIG. 1(   c ) is a block diagram of the patient monitoring system of  FIG. 1(   b ) coupled to a patient according to an illustrative arrangement; 
         FIG. 2  is a flowchart depicting an illustrative process for using a sensor array to take at least one physiological characteristic measurement according to an illustrative arrangement; and 
         FIG. 3  is a flowchart depicting an illustrative process for determining which physiological measurements can be detected using a sensor array according to an illustrative arrangement. 
     
    
    
     DETAILED DESCRIPTION 
     Monitoring the physiological state of a subject, for example, by determining, estimating, and/or tracking one or more physiological parameters of the subject, may be of interest in a wide variety of medical and non-medical applications. Knowledge of a subject&#39;s physiological characteristics (e.g., through a determination of one or more physiological parameters such as blood pressure, oxygen saturation, and presence of specific heart conditions) can provide short-term and long-term benefits to the subject, such as early detection and/or warning of potentially harmful conditions, diagnosis and treatment of illnesses, and/or guidance for preventative medicine. 
     One type of device that can be used to monitor the physiological state of a subject is an oximeter. An oximeter is a medical device that may determine, for example, the oxygen saturation of blood. An oximeter may include a light sensor that is placed at a site on a patient, typically a fingertip, toe, forehead or earlobe, or, in the case of a neonate, across a foot. The oximeter may pass light using a light source through blood perfused tissue and photoelectrically sense the absorption of light in the tissue. For example, the oximeter may measure the intensity of light that is received at the light sensor as a function of time. A signal representing light intensity versus time or a mathematical manipulation of this signal (e.g., a scaled version thereof, a log taken thereof, a scaled version of a log taken thereof, etc.) may be referred to as the photoplethysmograph (PPG) signal. In addition, the term “PPG signal,” as used herein, may also refer to an absorption signal (i.e., representing the amount of light absorbed by the tissue) or any suitable mathematical manipulation thereof. The light intensity or the amount of light absorbed may then be used to calculate the amount of the blood constituent (e.g., oxyhemoglobin) being measured and other physiological parameters such as the pulse rate and when each individual pulse occurs. 
     The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of light passed through the tissue varies in accordance with the changing amount of blood constituent in the tissue and the related light absorption. Red and infrared (IR) wavelengths may be used because it has been observed that highly oxygenated blood will absorb relatively less red light and more infrared light than blood with a lower oxygen saturation. 
     It will be understood that, as used herein, the term “light” may refer to energy produced by radiative sources and may include one or more of ultrasound, radio, microwave, millimeter wave, infrared, visible, ultraviolet, gamma ray or X-ray electromagnetic radiation. As used herein, light may also include any wavelength within the radio, microwave, infrared, visible, ultraviolet, or X-ray spectra, and that any suitable wavelength of electromagnetic radiation may be appropriate for use with the present techniques. 
     When the measured blood parameter is the oxygen saturation of hemoglobin, a convenient starting point assumes a saturation calculation based on Lambert-Beer&#39;s law. The following notation will be used herein:
 
 I (λ, t )= I   o (λ)exp(−( sβ   o (λ)+(1 −s )β r (λ)) l ( t ))  (1)
 
where:
 
λ=wavelength;
 
t=time;
 
I=intensity of light detected;
 
I o =intensity of light transmitted;
 
s=oxygen saturation;
 
β o , β r =empirically derived absorption coefficients; and
 
l(t)=a combination of concentration and path length from emitter to detector as a function of time.
 
     In pulse oximetry, by comparing the intensities of two wavelengths at different points in the pulse cycle, it is possible to estimate the blood oxygen saturation of hemoglobin in arterial blood. One common type of oximeter is a pulse oximeter, which may indirectly measure the oxygen saturation of a patient&#39;s blood (as opposed to measuring oxygen saturation directly by analyzing a blood sample taken from the patient) and changes in blood volume in the skin. Ancillary to the blood oxygen saturation measurement, pulse oximeters may also be used to measure the pulse rate of the patient. Pulse oximeters typically measure and display various blood flow characteristics including, but not limited to, the oxygen saturation of hemoglobin in arterial blood. 
     For example, using a pulse oximeter, saturation may be calculated by solving for the “ratio of ratios” as follows. 
     1. First, the natural logarithm of (l) is taken (“log” will be used to represent the natural logarithm) for IR and Red
 
log  I =log  I   o −( sβ   o +(1 −s )β r ) l   (2)
 
2. (2) is then differentiated with respect to time
 
                         ⅆ   log     ⁢           ⁢   I       ⅆ   t       =       -     (       s   ⁢           ⁢     β   o       +       (     1   -   s     )     ⁢     β   r         )       ⁢       ⅆ   l       ⅆ   t                 (   3   )               
3. Red (3) is divided by IR (3)
 
                         ⅆ   log     ⁢           ⁢       I   ⁡     (     λ   R     )       /     ⅆ   t             ⅆ   log     ⁢           ⁢       I   ⁡     (     λ   IR     )       /     ⅆ   t           =         s   ⁢           ⁢       β   o     ⁡     (     λ   R     )         +       (     1   -   s     )     ⁢       β   r     ⁡     (     λ   R     )               s   ⁢           ⁢       β   o     ⁡     (     λ   IR     )         +       (     1   -   s     )     ⁢       β   r     ⁡     (     λ   IR     )                     (   4   )               
4. Solving for s
 
     
       
         
           
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     Using log A-log B=log A/B, 
     
       
         
           
             
               
                 
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     So, (4) can be rewritten as 
                             ⅆ   log     ⁢           ⁢     I   ⁡     (     λ   R     )           ⅆ   t             ⅆ   log     ⁢           ⁢     I   ⁡     (     λ   IR     )           ⅆ   t         ≃       log   ⁡     (       I   ⁡     (       t   1     ,     λ   R       )         I   ⁡     (       t   2     ,     λ   R       )         )         log   ⁡     (       I   ⁡     (       t   1     ,     λ   IR       )         I   ⁡     (       t   2     ,     λ   IR       )         )           =   R           (   5   )               
where R represents the “ratio of ratios.” Solving (4) for s using (5) gives
 
     
       
         
           
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     From (5), R can be calculated using two points (e.g., PPG maximum and minimum), or a family of points. One method using a family of points uses a modified version of (5). Using the relationship 
                         ⅆ   log     ⁢           ⁢   I       ⅆ   t       =         ⅆ   I       ⅆ   t       I             (   6   )               
now (5) becomes
 
                                 ⅆ   log     ⁢           ⁢     I   ⁡     (     λ   R     )           ⅆ   t             ⅆ   log     ⁢           ⁢     I   ⁡     (     λ   IR     )           ⅆ   t         ≃       ⁢           I   ⁡     (       t   2     ,     λ   R       )       -     I   ⁡     (       t   1     ,     λ   R       )           I   ⁡     (       t   1     ,     λ   R       )               I   ⁡     (       t   2     ,     λ   IR       )       -     I   ⁡     (       t   1     ,     λ   IR       )           I   ⁡     (       t   1     ,     λ   IR       )                       =       ⁢         [       I   ⁡     (       t   2     ,     λ   R       )       -     I   ⁡     (       t   1     ,     λ   R       )         ]     ⁢     I   ⁡     (       t   1     ,     λ   IR       )             [       I   ⁡     (       t   2     ,     λ   IR       )       -     I   ⁡     (       t   1     ,     λ   IR       )         ]     ⁢     I   ⁡     (       t   1     ,     λ   R       )                       =       ⁢   R                 (   7   )               
which defines a cluster of points whose slope of y versus x will give R where
 
 x ( t )=[ I ( t   2 ,λ IR )− I ( t   1 ,λ IR )] I ( t   1 λ R )
 
 y ( t )=[ I ( t   2 ,λ R )− I ( t   1 ,λ R )] I ( t   1 ,λ IR )  (8)
 
 y ( t )= Rx ( t )
 
Once R is determined or estimated, for example, using the techniques described above, the blood oxygen saturation can be determined or estimated using any suitable technique for relating a blood oxygen saturation value to R. For example, blood oxygen saturation can be determined from empirical data that may be indexed by values of R, and/or it may be determined from curve fitting and/or other interpolative techniques.
 
     The foregoing is merely illustrative and any suitable processing techniques may be used to calculate pulse oximetry values. For example, Fourier transforms and continuous wavelet transforms may be used to process the PPG signals and derive blood oxygen saturation. 
     In regional oximetry, by comparing the intensities of two wavelengths of light, it is possible to estimate the blood oxygen saturation of hemoglobin in a region of a body. Whereas pulse oximetry measures blood oxygen based on changes in the volume of blood due to pulsing tissue (e.g., arteries), regional oximetry may examine blood oxygen saturation within the venous, arterial, and capillary systems within a region of a patient. A regional oximeter is another common type of oximeter, which may be used to calculate an oxygen saturation of a patient&#39;s blood in a non-invasive manner. For example, a regional oximeter may include a sensor to be placed on a patient&#39;s forehead and may be used to calculate the oxygen saturation of a patient&#39;s blood within the venous, arterial and capillary systems of a region underlying the patient&#39;s forehead (e.g., in the cerebral cortex). The sensor may include two emitters and two detectors: one detector that is relatively “close” to the two emitters and another detector that is relatively “far” from the two emitters. 
     For example, if I A  represents the intensity of the received/detected light associated with the “close” detector, 
                   I   A     ⁡     (     λ   ,   t     )           I   O     ⁡     (   λ   )         ,         
may be derived using Lambert-Beer&#39;s law, described above. Similarly, if I B  represents the intensity of the received/detected light associated with the “far” detector,
 
                   I   B     ⁡     (     λ   ,   t     )           I   O     ⁡     (   λ   )         ,         
may be derived using Lambert-Beer&#39;s law, described above. Light intensity of multiple wavelengths may be received at both the “close” and the “far” detectors. For example, if two wavelength were used, the two wavelengths may be contrasted at each location and the resulting signals may be contrasted to arrive at a regional saturation value that pertains to additional tissue through which the light received at the “far” detector passed (tissue in addition to the tissue through which the light received by the “close” detector passed, e.g., the brain tissue), when it was transmitted through a region of a patient (e.g., a patient&#39;s cranium). Other methods to calculate regional blood oxygen saturation are well known in the art.
 
     PPG sensors may also be affixed to a subject and allow for the determination of the subject&#39;s blood pressure, for example, using continuous non-invasive blood pressure (CNIBP) techniques. For example, some continuous non-invasive blood pressure monitoring techniques have been developed that involve the use of two probes or sensors positioned at two different locations on a subject&#39;s body. The elapsed time, T, between the arrival of corresponding points of a pulse signal at the two locations may then be determined using the two probes or sensors. The estimated blood pressure, p, may then be related to the elapsed time, T, by
 
 p=a+b ·ln( T )  (9)
 
where a and b are constants that are dependent upon the nature of the subject and the signal detecting devices. Other blood pressure equations using elapsed time may also be used.
 
     Such a continuous and non-invasive blood pressure monitoring technique is described in Chen et al. U.S. Pat. No. 6,599,251, which is hereby incorporated by reference herein in its entirety. The technique described by Chen et al. may use two sensors (e.g., ultrasound or photoelectric pulse wave sensors) positioned at any two locations on a subject&#39;s body where pulse signals are readily detected. For example, sensors may be positioned on an earlobe and a finger, an earlobe and a toe, or a finger and a toe of a patient&#39;s body. In some approaches, a single sensor or probe location may be used to determine blood pressure, as described in U.S. patent publication No. 2009/0326386, published Dec. 31, 2009, which is hereby incorporated by reference herein in its entirety. 
     Similar sensors or probes may also be used to determine respiration rate and other respiratory properties (e.g., respiratory effort). For example, as described in more detail in U.S. Patent App. Pub. No. 2006/0258921, which is incorporated by reference herein in its entirety, the act of breathing may cause a breathing band to become present in a scalogram derived from a continuous wavelet transform of a PPG signal. This breathing band may occur at or about the scale having a characteristic frequency that corresponds to the breathing frequency. Furthermore, the features within this band (e.g., the energy, amplitude, phase, or modulation) or the features within other bands of the scalogram may result from changes in breathing rate (or breathing effort) and therefore may be correlated with various respiratory parameters of a patient. 
     As is well known in the art, sensors may also be used to determine other physiological characteristics of a subject. For example, an electrical physiological signal (EPS) sensor may be used to determine such signals as electroencephalographic (EEG) signals, electrocardiography (ECG or EKG) signals, electromyography (EMG) signals, or any other electrical physiological signal. Sensors may also be used to determine a subject&#39;s body temperature, a pulse transit times (PTT), or both. In some embodiments, PTT may be determined by using plethysmograph data in conjunction with ECG data. For example, PTT may be determined by comparing an ECG onset point with a PPG arrival point. An ECG signal may be processed in order to detect the QRS complex and to detect the R wave peak. The plethysmograph signal may be processed to detect the pulse timing. The PTT may then be calculated as the time between the R wave peak and the corresponding pulse peak. Other suitable techniques for calculating PTT are well know in the art and may also be used. Any of the aforementioned physiological characteristics may be determined using the sensor array of the present disclosure. 
       FIG. 1(   a ) shows illustrative sensor array  100 . Sensor array  100  may include a plurality of sensor elements  102 , which may be of the same or different types. Sensor array  100  may also include processing circuitry (e.g., one or more microprocessors), memory (e.g., RAM, ROM, and hybrid types of memory), storage (e.g., one or more hard drives, tape drives, or optical drives), and a communications interface (e.g., a wireless network controller, serial port, or infrared port). Sensor array  100  may be configured to be placed over a local area of a subject&#39;s body. Sensor array  100  may include a plurality of sensors elements  102  connected by flexible sheet  101 . Flexible sheet  101  may include any mesh, netting, fabric, plastic, other suitable polymer, or any combination of the foregoing. In some embodiments, flexible sheet  101  may be configured to be worn by a patient, for example, as an article of clothing. For example, flexible sheet  101  may be incorporated into a glove, hat, armband, headband, wristband, back strap, chest strap, or any other suitable article. One or more of sensors elements  102  of sensor array  100  may be, for example, an electrode, a complementary metal oxide semiconductor (CMOS) sensor, a charged coupled device (CCD) sensor, or a combination CMOS/CCD sensor. The CCD sensor may comprise a photoactive region and a transmission region for receiving and transmitting data whereas the CMOS sensor may be made up of an integrated circuit having an array of pixel sensors. Each pixel may have a photodetector and an active amplifier. One or more of sensor elements  102  may also be a flexible probe as described in U.S. patent application Ser. No. 12/508,180, filed Jul. 23, 2009, which is hereby incorporated by reference herein in its entirety. In an embodiment, sensor elements  102  in sensor array  100  may be configured to emit a single wavelength of light. For example, a first sensor element may emit only a RED light while a second may emit only an IR light. In an embodiment, sensor elements  102  in sensor array  100  may be configured to emit more than one wavelength (e.g., RED and IR light). 
     Each sensor element  102  of sensor array  100  may include one or more of an emitter portion and a detector portion. The detector portion may be configured to detect the intensity of light at particular wavelengths, such as the RED and IR wavelengths. Alternatively, one or more sensor elements  102  in sensor array  100  may be configured to detect an intensity of a single wavelength. In operation, light may enter a detector portion after passing through the subject&#39;s tissue. The detector portion may convert the intensity of the received light into an electrical signal. The light intensity may be directly related to the absorbance and/or reflectance of light in the subject&#39;s tissue. That is, when more light at a certain wavelength is absorbed or reflected by the tissue, less light of that wavelength may be received from the tissue by the detector portion. After converting the received light to an electrical signal, the detector portion may send the signal to a monitor (not shown), where physiological characteristics or parameters may be calculated based on the absorption of the RED and IR wavelengths in the subject&#39;s tissue. In an embodiment, some or all of the calculations may be performed by sensor array  100  itself, and the result of the calculations may be passed to a monitor. Each sensor element  102  in sensor array  100  (or a combination of sensor elements) may take the form of a local oxygen saturation sensor, a regional oxygen saturation sensor, a respiration rate sensor, a respiratory effort sensor, a blood pressure sensor, a temperature sensor, an EPS sensor, any other type of physiological measurement sensor, or any combination of the foregoing types of sensors. 
     In an embodiment, sensor elements  102  or sensor array  100  may be connected to and draw its power from a monitor (not shown). In another embodiment, sensor elements  102  or sensor array  100  may be wirelessly connected to the monitor and include its own battery or similar power supply (not shown). The monitor may be configured to calculate physiological parameters based at least in part on data received from the sensor elements  102  in sensor array  100 . 
     Although in the example of  FIG. 1(   a ), twenty-six sensor elements are included in sensor array  100 , sensor array  100  may include any number of sensor elements. The sensor elements may be arranged in a grid layout, a rectangular layout, or a layout of any suitable geometry. For example, sensor array  100  may be incorporated with flexible sheet  101  and take a circular, square, rectangular, hexagonal, irregular, or free-form shape. Each sensor element  102  may be separated from the next sensor element  102  in the same row by width  108  and separated from the next sensor element  102  in the same column by height  110 . Width  108  and height  110  may be the same distance or different distances. In addition, width  108  and height  110  may remain constant throughout sensor array  100  or may vary from one portion of the array to the next. Other suitable geometries and layouts for sensor array  100  and flexible sheet  101  may also be used. 
     One or more sensors may be grouped or combined into a sensor element region, such as sensor element regions  104  and  106 . Sensor element regions may include any number of sensors of the same or different type used for determining the same or different physiological parameters. For example, some locations on a subject&#39;s body may be better suited to detect blood pressure, while other locations on a subject&#39;s body may be better suited to detect oxygen saturation. In an embodiment, sensor elements in a region corresponding to a location on a subject&#39;s body that is more suited for detecting a particular type of physiological parameter (e.g., blood pressure, pulse rate, or oxygen saturation) may include sensor elements operative to detect that type of parameter. In an embodiment, a single sensor element  102  may be capable of being used to determine more than one physiological parameter. For example, as described below in connection with  FIGS. 2 and 3 , sensor element  102  may detect one or more physiological signals and may be used to determine whatever physiological parameter or parameters that have the greatest signal quality or signal strength at that particular location. In this way, one or more of sensor elements  102  may be generic sensors capable of being used to determine multiple physiological parameters of different types. 
     Each sensor element  102  in sensor array  100  may communicate with other sensor elements via the monitor. Individual sensor elements may communicate directly or may communicate indirectly, for example, through a monitor connected to sensor array  100 . In some embodiments, sensor elements may communicate with the monitor wirelessly (e.g., using RFID or any other wireless network protocol). In an embodiment, sensor elements may also communicate with other sensor elements in the same sensor element region in order to coordinate measurement types, exchange measurement data, and determine the optimal site and/or best sensor element combination for a measurement as described in more detail below. 
       FIG. 1(   b ) is a perspective view of an embodiment of a patient monitoring system  10 . System  10  may include sensor array  12  and monitor  14 . Sensor array  12  may correspond to sensor array  100  ( FIG. 1(   a )). Sensor array  12  may include a plurality of sensor elements  16 , which may be of the same or different types. Each sensor element  16  of sensor array  12  (or a combination of sensor elements) may take the form of a local oxygen saturation sensor, a regional oxygen saturation sensor, a respiration rate sensor, a respiratory effort sensor, a blood pressure sensor, a temperature sensor, an EPS sensor, any other type of physiological measurement sensor, or any combination of the foregoing types of sensor. Sensor elements  16  of sensor array  12  may include one or more emitters for emitting light at one or more wavelengths into a patient&#39;s tissue. Sensor elements  16  of sensor array  12  may also include one or more detectors for detecting the light produced by one or more emitters that emanates from the patient&#39;s tissue after passing through the tissue. Any suitable physical configuration of emitters and detectors may be used. According to an embodiment, emitters and detectors of sensor array  12  may be located on opposite sides of a digit such as a finger or toe, in which case the light that is emanating from the tissue has passed completely through the digit. In an embodiment, emitters and detectors of sensor array  12  may be arranged so that light from the emitters penetrates the tissue and is reflected by the tissue into the detectors, for example, to obtain oximetry data from a patient&#39;s forehead. 
     In an embodiment, system  10  may include one or more additional sensor units. Sensor units used in addition to sensor array  12  may include single-parameter or multi-parameter sensors or probes, such as the physiological sensors discussed above with respect to sensor elements  16 . Sensor units used in addition to sensor array  12  may also include one or more sensor arrays, such as sensor array  13 , which may correspond to sensor array  100  ( FIG. 1(   a )). Sensor array  13  may take the form of any of the embodiments described herein with reference to sensor array  12 . Sensor array  13  may be the same type of sensor array as sensor array  12 , or sensor array  13  may be of a different sensor array type than sensor array  12 . Sensor arrays  12  and  13  may be capable of being positioned at two different locations on a subject&#39;s body; for example, sensor array  12  may be positioned on a patient&#39;s forehead, while sensor array  13  may be positioned at a patient&#39;s fingertip. In an embodiment, oxygen saturation measurements may be taken from both sensor array  12  positioned on a patient&#39;s forehead and sensor array  13  positioned at a patient&#39;s fingertip. The measurements obtained may be analyzed by monitor  14  to determine which sensor array location produces more reliable measurements based on signal integrity, confidence values, or any other suitable criteria. One of sensor array  12  and sensor array  13  may then be selected for continuous patient monitoring. In an embodiment, a combination of sensor elements from both sensor array  12  and sensor array  13  may be chosen for patient monitoring. The selection of sensor array elements for a certain measurement type will be described in more detail with respect to  FIG. 3 . One or more signals from one or more sensor elements and/or sensor arrays may be used in the measurement techniques described herein. 
     Sensor arrays  12  and  13  may each detect any signals that carry information about a patient&#39;s physiological state, such as an electrocardiograph signal, arterial line measurements, or the pulsatile force exerted on the walls of an artery using, for example, oscillometric methods with a piezoelectric transducer. It will be understood that any type of sensor element, including any type of physiological sensor element, may be used in one or more of sensor arrays  12  and  13  in accordance with the systems and techniques disclosed herein. It is understood that any number of sensor arrays measuring any number of physiological signals may be used to assess patient status in accordance with the techniques described herein. 
     In an embodiment, sensor arrays  12  and  13  may be communicatively coupled to monitor  14  via a cable  24 . Sensor arrays  12  and  13  may draw power from monitor  14  through cable  24 . In an embodiment, a wireless transmission device (not shown) or the like may be used instead of or in addition to cable  24 , and sensor arrays  12  and  13  may include their own battery or similar power supply (not shown). Monitor  14  may be configured to determine physiological parameters (e.g., heart rate, blood pressure, blood oxygen saturation) based at least in part on data received from one or more sensor units, such as sensor arrays  12  and  13 , relating to light emission and detection. In an embodiment, the sensor unit itself may determine the physiological parameters, and the parameters may be passed to monitor  14 . Further, monitor  14  may include a display  20  configured to display the physiological parameters or other information about the system. In the embodiment shown, monitor  14  may also include a speaker  22  to provide an audible sound that may be used for various other embodiments, such as for example, sounding an audible alarm if a patient&#39;s physiological parameters are not within predefined normal ranges. 
     In the illustrated embodiment, system  10  may also include a multi-parameter patient monitor  26 . The multi-parameter patient monitor  26  may include a cathode ray tube display, a flat panel display (as shown) such as a liquid crystal display (LCD) or a plasma display, or may include any other type of monitor now known or later developed. Multi-parameter patient monitor  26  may be configured to determine physiological parameters and to provide a display  28  for information from monitor  14  and from other medical monitoring devices or systems (not shown). For example, multi-parameter patient monitor  26  may be configured to display an estimate of a patient&#39;s blood oxygen saturation generated by monitor  14  (referred to as an “SpO 2 ” measurement), pulse rate information from monitor  14 , and blood pressure measurement from monitor  14  on display  28 . In an embodiment, multi-parameter patient monitor  26  may display information associated with different types of measurements received from sensor arrays  12  and  13 . In an embodiment, the information may include information used to choose a certain measurement type to be obtained by sensor arrays  12  and  13 . In an embodiment, multi-parameter patient monitor  26  may display information identifying the combination of sensor elements being used to obtain certain measurements. The combinations and measurement types may be chosen, for example, by the processes described below with respect to  FIGS. 2 and 3 . Monitor  26  may include a speaker  30  that may be used to sound audible alarms based on data received from monitor  14 . 
     Monitor  14  may be communicatively coupled to multi-parameter patient monitor  26  via a cable  32  or  34  that is connected to a sensor input port or a digital communications port, respectively, and/or may communicate wirelessly (not shown). In addition, monitor  14  and/or multi-parameter patient monitor  26  may be coupled to a network to enable the sharing of information with servers or other workstations (not shown). Monitor  14  may be powered by a battery or by a conventional power source such as a wall outlet. 
     Calibration device  80 , which may be powered by monitor  14  via a cable  82 , a battery, or by a conventional power source such as a wall outlet, may include any suitable signal calibration device. Calibration device  80  may be communicatively coupled to monitor  14  via cable  82 , and/or may communicate wirelessly (not shown). In some embodiments, calibration device  80  is completely integrated within monitor  14 . In some embodiments, calibration device  80  may include a manual input device (not shown) used by an operator to manually input reference signal measurements obtained from some other source (e.g., an external invasive or non-invasive physiological measurement system). 
     Calibration device  80  may also access reference signal measurements stored in memory (e.g., RAM, ROM, or a storage device). For example, in some embodiments, calibration device  80  may access reference blood pressure measurements from a relational database stored within calibration device  80 , monitor  14 , or multi-parameter patient monitor  26 . The reference blood pressure measurements generated or accessed by calibration device  80  may be updated in real-time, resulting in a continuous source of reference blood pressure measurements for use in continuous or periodic calibration. Alternatively, reference blood pressure measurements generated or accessed by calibration device  80  may be updated periodically, and calibration may be performed on the same periodic cycle. Reference blood pressure measurement may be generated when recalibration is triggered. 
       FIG. 1(   c ) is a block diagram of a patient monitoring system, such as patient monitoring system  10  of  FIG. 1(   b ), which may be coupled to a patient  40  in accordance with an embodiment. Certain illustrative components of sensor array  12  and monitor  14  ( FIG. 1(   b )) are shown in  FIG. 1(   c ). Sensor arrays  12  and  13  may include similar functionality, and it will be understood that any of the concepts, components, features, and operations discussed in connection with sensor array  12  may be applied to sensor array  13  as well (e.g., emitter portion  17  and detector portion  18  of sensor array  12  may be similar to emitter and detector portions of sensor array  13 ). It will be noted that patient monitoring system  10  may include one or more additional sensor arrays or probes, which may take the form of any of the embodiments described herein with reference to sensor arrays  12  and  13  ( FIG. 1(   b )), or any other suitable sensor units. For the purpose of illustration and not limitation, one emitter portion and one detector portion are shown in sensor array  12 , but any number or type of sensor elements may be included in sensor array  12 . Additional sensor elements in sensor array  12  may include additional emitters and detectors, blood pressure sensors, temperature sensors, EPS sensors, any other type of physiological measurement sensor, or any combination of the foregoing types of sensors. In an embodiment, multiple sensor elements (distributed in one or more sensor arrays) may be located at multiple different body sites on a patient. 
     The sensor elements in sensor array  12  may include an emitter portion  17 , a detector portion  18 , and an encoder  42 . Emitter portion  17  and detector portion  18  may each include one or more sensor elements  16  ( FIG. 1(   b )). In the embodiment shown, emitter portion  17  may be configured to emit at least two wavelengths of light (e.g., RED and IR) into a patient&#39;s tissue  40 . Hence, emitter portion  17  may include a RED light emitting light source such as RED light emitting diode (LED)  44  and an IR light emitting light source such as IR LED  46  for emitting light into the patient&#39;s tissue  40  at the wavelengths used to calculate the patient&#39;s physiological parameters. In one embodiment, the RED wavelength may be between about 600 nm and about 700 nm, and the IR wavelength may be between about 800 nm and about 1000 nm. Individual sensor elements  16  ( FIG. 1(   b )) within emitter portion  17  may each be configured to emit a single wavelength. For example, a first sensor element emits only a RED light while a second sensor element only emits an IR light. In another example, the wavelengths of light used are selected based on the specific location of the sensor. 
     It will be understood that, as used herein, the term “light” may refer to energy produced by radiative sources and may include one or more of ultrasound, radio, microwave, millimeter wave, infrared, visible, ultraviolet, gamma ray or X-ray electromagnetic radiation. As used herein, light may also include any wavelength within the radio, microwave, infrared, visible, ultraviolet, or X-ray spectra, and that any suitable wavelength of electromagnetic radiation may be appropriate for use with the present techniques. Detector portion  18  may be selected to be specifically sensitive to the chosen targeted energy spectrum of the emitter portion  17 . 
     In an embodiment, an individual sensor element may be configured to detect the intensity of light at both the RED and IR wavelengths. Alternatively, individual sensor elements in the array may each be configured to detect an intensity of a single wavelength. In operation, light may enter detector portion  18  after passing through the patient&#39;s tissue  40 . Detector portion  18  may convert the intensity of the received light into an electrical signal. The light intensity is directly related to the absorbance and/or reflectance of light in the tissue  40 . That is, when more light at a certain wavelength is absorbed or reflected, less light of that wavelength is received from the tissue by the detector portion  18 . After converting the received light to an electrical signal, detector portion  18  may send the signal to monitor  14 , where physiological parameters may be calculated based on the absorption of the RED and IR wavelengths in the patient&#39;s tissue  40 . Monitor  14  may also determine physiological parameters based on signals received from sensor elements of sensor array  12  that are not included in emitter portion  17  or detector portion  18 . Monitor  14  may determine any physiological parameter based on signals received from sensor elements, including oxygen saturation, blood pressure, pulse rate, respiratory rate, heart rate, any other suitable physiological characteristic, or any combination of suitable physiological characteristics. Each physiological parameter may be determined based on signals received from a single sensor element or a combination of sensor elements. 
     In an embodiment, encoder  42  may contain information about sensor array  12 , such as what type of sensor array it is (e.g., whether the sensor array is intended for placement on a forehead or digit), the different sensor elements contained in the sensor array, and/or the wavelengths of light emitted by emitter portion  17 . This information may be used by monitor  14  or calibration device  80  to select appropriate algorithms, lookup tables and/or calibration coefficients stored in monitor  14  or calibration device  80  for calculating the patient&#39;s physiological parameters. 
     Encoder  42  may contain information specific to patient  40 , such as, for example, the patient&#39;s age, weight, and diagnosis. This information about a patient&#39;s characteristics may allow monitor  14  to determine, for example, patient-specific threshold ranges in which the patient&#39;s physiological parameter measurements should fall and to enable or disable additional physiological parameter algorithms. This information may also be used to select and provide coefficients for equations from which, for example, oxygen saturation, pulse rate, blood pressure, and other measurements may be determined based at least in part on the signal or signals received at sensor array  12 . For example, some oximetry sensors rely on equations to relate an area under a pulse of a photoplethysmograph (PPG) signal to determine blood pressure. These equations may contain coefficients that depend upon a patient&#39;s physiological characteristics as stored in encoder  42 . The information stored in encoder  42  may be used in retrieving a set of coefficients from calibration device  80  to be used for calculations. Encoder  42  may, for instance, be a coded resistor which stores values corresponding to the type of sensor array  12  or the type of each sensor element in sensor array  12 , the wavelengths of light emitted by emitter portion  17 , and/or the patient&#39;s characteristics. In an embodiment, encoder  42  may include a memory on which one or more of the following information may be stored for communication to monitor  14 : the type of the sensor array  12 , the different types of sensor elements in sensor array  12 , the wavelengths of light emitted by emitter portion  17 , the particular wavelength each sensor element in sensor array  12  is monitoring, a signal threshold for each sensor element in sensor array  12 , any other suitable information, or any combination thereof. 
     In an embodiment, signals from detector portion  18  and encoder  42  may be transmitted to monitor  14 . In the embodiment shown, monitor  14  may include a general-purpose microprocessor  48  connected to an internal bus  50 . Microprocessor  48  may be adapted to execute software, which may include an operating system and one or more applications, as part of performing the functions described herein. Also connected to bus  50  may be a read-only memory (ROM)  52 , a random access memory (RAM)  54 , user inputs  56 , display  20 , and speaker  22 . 
     RAM  54  and ROM  52  are illustrated by way of example, and not limitation. Any suitable computer-readable media may be used in the system for data storage. Computer-readable media are capable of storing information that can be interpreted by Microprocessor  48 . This information may be data or may take the form of computer-executable instructions, such as software applications, that cause the microprocessor to perform certain functions and/or computer-implemented methods. Depending on the embodiment, such computer-readable media may include computer storage media and communication media. Computer storage media may include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules or other data. Computer storage media may include, but is not limited to, RAM, ROM, EPROM, EEPROM, flash memory or other solid state memory technology, CD-ROM, DVD, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by components of the system. 
     In the embodiment shown, a time processing unit (TPU)  58  may provide timing control signals to a light drive circuitry  60 , which may control when emitters in emitter portion  17  are illuminated and multiplexed timing for the RED LED  44  and the JR LED  46 . TPU  58  may also control the gating-in of signals from detector portion  18  through an amplifier  62  and a switching circuit  64 . These signals are sampled at the proper time, depending upon which light sources are illuminated. The received signals from detector portion  18  may be passed through an amplifier  66 , a low pass filter  68 , and an analog-to-digital converter  70 . The digital data may then be stored in a queued serial module (QSM)  72  (or buffer) for later downloading to RAM  54  as QSM  72  fills up. In one embodiment, there may be multiple separate parallel paths having amplifier  66 , filter  68 , and A/D converter  70  for multiple light wavelengths or spectra received. TPU  58  may also control the timing and operation of additional sensor elements in sensory array  12  that are not depicted in  FIG. 1(   c ). 
     In an embodiment, microprocessor  48  may determine the patient&#39;s physiological parameters, such as SpO 2 , blood pressure, pulse rate, respiratory rate, heart rate, any other suitable physiological parameter, or any combination of suitable physiological parameters, using various algorithms and/or look-up tables based on the values of the received signals and/or data corresponding to the light received by detector portion  18  or signals received from other sensor elements of sensor array  12 . Signals corresponding to information about patient  40 , and particularly about the intensity of light emanating from a patient&#39;s tissue over time, may be transmitted from encoder  42  to a decoder  74 . These signals may include, for example, encoded information relating to patient characteristics. Decoder  74  may translate these signals to enable the microprocessor to determine the thresholds based at least in part on algorithms or look-up tables stored in ROM  52 . User inputs  56  may be used to enter information about the patient, such as age, weight, height, diagnosis, medications, treatments, and so forth. In an embodiment, display  20  may exhibit a list of values which may generally apply to the patient, such as, for example, age ranges or medication families, which the user may select using user inputs  56 . 
     In an embodiment, microprocessor  48  may analyze data received from sensor array  12  to determine a certain measurement type (e.g., a certain physiological parameter) to obtain from sensor array  12 . In an embodiment, microprocessor  48  may analyze data received from different combinations of sensor elements from one or more sensor arrays to identify an optimal sensor element or combination of sensor elements to obtain a certain measurement type (e.g., a certain physiological parameter). 
       FIG. 2  shows illustrative process  200  for using a sensor array to take at least one physiological measurement. Some locations on a subject&#39;s body may yield better measurements for certain measurement types than other locations on the subject&#39;s body. By using an array, such as sensor array  100  ( FIG. 1(   a )), a medical practitioner can apply the array to a general location on a subject&#39;s body (e.g., the forehead, wrist, arm, back, head, or hand), and an optimal combination of sensor elements in the array may be selected for use in determining a particular measurement. 
     At step  202 , desired monitoring information may be received. The desired monitoring information may include, for example, a test specification identifying variables for testing. For example, healthcare personnel may input a test specification into a monitoring device (e.g., monitor  14  ( FIG. 1(   b ))) connected to a sensor array (e.g., sensor array  100  ( FIG. 1(   a ))). The test specification may indicate which types of measurements to take (e.g., oxygen saturation, blood pressure, and/or pulse rate), the measurement frequency (for continuous measurement, for example, every five seconds), an absolute time a measurement is needed (for single measurements, for example, within 60 seconds), a desired signal integrity or confidence value, or any other suitable information. 
     At step  204 , the desired measurement types may be determined from the received desired monitoring information. For example, the monitoring information may indicate that measurements for local oxygen saturation, regional oxygen saturation, blood pressure, PTT, one or more EPS, respiration rate, respiratory effort, temperature, and pulse rate may be desired. At step  206 , sensor array  100  ( FIG. 1(   a )) or the monitor connected to sensor array  100  (e.g., monitor  14  ( FIG. 1(   b ))) may determine if one or more statistical models are available for the desired measurement type or types. If such models are available, they may be accessed at step  208 . For example, in an embodiment, a data modeling processor included with sensor array  100  ( FIG. 1(   a )) or the monitor connected to sensor array  100  (e.g., monitor  14  ( FIG. 1(   b ))) may include a linear or non-linear statistical data modeling module that identifies valid signal segments for a particular measurement type. The modeling processor (which may take the form of an artificial neural network (ANN) in some embodiments) may be trained to identify signal segments that are valid for use in determining physiological parameters. For example, in some embodiments, the data modeling processor may perform one or more regression analyses (e.g., using linear or nonlinear regression techniques) on the input data. Valid signal segments may then be identified and may include segments identified as not having artifact (or having less than some threshold level of artifact), segments that are not stale (e.g., segments collected within some user-defined freshness time threshold), segments with a suitable signal-to-noise ratio, segments with a suitable signal quality, segments with any other suitable characteristics, or segments with any combination of the foregoing characteristics. 
     The data modeling processor may operate directly on the detected signal itself (e.g., a PPG signal) or some transform of the detected signal (e.g., a continuous wavelet transform of a PPG signal). In some embodiments, the data modeling processor may also operate on a scalogram derived from the transformed signal, a wavelet ratio surface, the real part of the wavelet transform, the imaginary part of the wavelet transform, the modulus of the wavelet transform, the energy density of the wavelet transform, or any combination of the foregoing signals. For example, the data modeling processor may recognize the pulse band in a scalogram derived from a continuous wavelet transform of a PPG signal prior to corruption by artifact. The data modeling processor may then detect an unrecognizable (or low fidelity) pulse band during artifact corruption. 
     In an embodiment, the data modeling processor may learn signal characteristics associated with a particular physiological parameter to be determined using a supervised learning phase. In an embodiment, the data modeling processor may implement a self-organizing map (SOM) feature (e.g., using a Kohonen map) that is trained using an unsupervised learning phase. A reinforcement learning phase (e.g., one that discovers a policy that minimizes some long-term cost metric) may additionally or alternatively be employed. 
     The statistical models accessed at step  208  may additionally or alternatively include an indication of one or more metrics defining signals suitable for use in determining a particular type of measurement. For example, signal quality and signal strength may be two useful metrics. The statistical models may include threshold values for one or more of these metrics. Measurements may be indicated as valid only when threshold values for all available metrics are met. For example, to determine respiration rate, a breathing band may be identified in a scalogram of a detected signal. Depending on the location of the sensor (e.g., sensor element  102  of  FIG. 1(   a )) in the array (e.g., sensor array  100  of  FIG. 1(   a )), signals of varying qualities may be detected. For example, artifact or other types of noise may corrupt, distort, or make the breathing band difficult to identify in the scalogram. Thus, signal quality may be lower at some locations than at other locations. As another example, the pulse band may be identified in order to determine the subject&#39;s pulse rate. If a strong pulse band is detected by the data modeling processor, this signal may be associated with a high signal strength whereas signals with weak pulse bands may be associated with lower signal strengths. 
     If no statistical models are available at step  206  or after accessing the statistical models at step  208 , process  200  continues to determine if prior valid measurement data is available at step  210 . If prior data is available, then that data may be accessed at step  212 . The prior measurement data may represent previously-known good data and be used to extrapolate a statistical model at step  214 , using, for example, a linear or nonlinear regression analysis or other statistical pattern recognition or modeling techniques. After a model has been extrapolated at step  214  or if no prior measurement data is available at step  210 , a sensor array element combination may be determined for testing at step  216 . For example, a series of contiguous or non-contiguous sensor elements (e.g., sensor elements  102  of  FIG. 1(   a )) in a sensor region (e.g., sensor regions  104  or  106  of  FIG. 1(   a )) may represent one element combination. In an embodiment, all possible element combinations in sensor array  100  ( FIG. 1(   a )) may be tested for each measurement type to yield the preferred measurement, as described in more detail in connection with  FIG. 3 . In an embodiment, arbitrary combinations may be tested until a suitable measurement (e.g., meeting the modeling metric thresholds) is discovered. In an embodiment, combinations of sensor elements from more than one sensor array (e.g., sensor arrays  12  and  13  ( FIG. 1(   b )) may be tested as a single combination. 
     In some embodiments, location data is accessed at step  216  in order to determine sensor array element combinations for testing. For example, based on historical or empirical data, a location corresponding to a particular area of a subject&#39;s temple may be a more suitable location for determining blood pressure while a different area of the subject&#39;s temple may be more suitable for determining oxygen saturation. If location data is available about the current sensor array (e.g., the array&#39;s positioning on the subject&#39;s body), this data may be used to determine appropriate element combinations for testing (e.g., depending on the measurement type or types desired). 
     At step  218 , test measurements may be iteratively made at the sensor array element combinations selected at step  216 . For example, test measurements may be made serially, one after another, or multiple measurements may be made simultaneously using different combinations of sensor array elements. The measurements made at step  218  may be used for continuous patient monitoring for the desired physiological characteristic, or the measurements may also be used to select a measurement type to obtain from the selected sensor array combination, for example, by the process discussed below with respect to  FIG. 3 . 
       FIG. 3  is a flowchart depicting an illustrative process  300  for determining which physiological measurements can be detected using a sensor array. Sometimes, it is desirable to combine multiple sensors into a single sensor in order to minimize the number of sensors (and leads) connected to a subject. By combining multiple sensor elements into an array, such as sensor array  100  ( FIG. 1(   a )), measurements for multiple different measurement types may be taken with a single application of the array. In addition, the sensor array may be applied to different parts of a body from which different physiological measurements can be detected. 
     At step  302 , a sensor array element or element combination is selected. Sensor array elements may be selected from one or more sensor arrays (e.g., sensor arrays  12  and  13  ( FIG. 1(   b )) for a single combination. At step  304 , a test measurement may be taken using the selected sensor array element or element combination. For example, some or all of the sensor elements in sensor region  104  ( FIG. 1(   a )) of sensor array  100  ( FIG. 1(   a )) may be used to obtain a PPG signal. The test measurement may then be analyzed at step  306 . For example, sensor array  100  ( FIG. 1(   a )) or the monitor connected to sensor array  100  (e.g., monitor  14  ( FIG. 1(   b ))) may compare the test measurement to one or more models. The models may indicate a valid signal for use in determining a physiological characteristic or combination of physiological characteristics. 
     At step  308 , sensor array  100  ( FIG. 1(   a )) or the monitor connected to sensor array  100  (e.g., monitor  14  ( FIG. 1(   b ))) may determine if the test measurement is valid for any available measurement type. For example, in an embodiment, the available measurement types may include local oxygen saturation, regional oxygen saturation, blood pressure, temperature, one or more EPS, respiration rate, respiratory effort, and PTT. If, at step  308 , the test measurement is valid for one of these measurement types, then the sensor element or element combination used for taking the test measurement may be stored as a possible “best” sensor elements for that measurement type at step  310 . If the test measurement is not valid for any desired measurement type, then a new element or element combination may be tested. To be a “valid” measurement for any available measurement type, the test measurement may have to meet one or more metric thresholds, such as signal quality and signal strength. In some embodiments, a valid test measurement may be determined by correlating the test measurement with one or more models representative of available measurement types. If the correlation exceeds some threshold correlation, then the test measurement may be classified as a valid measurement for that measurement type. In an embodiment, both model correlation and metric thresholds are used to determine valid measurements. 
     At step  312 , sensor array  100  ( FIG. 1(   a )) or the monitor connected to sensor array  100  (e.g., monitor  14  ( FIG. 1(   b ))) may determine if there are more sensor elements, element combinations, or both to test. For example, all sensor elements and/or element combinations may be tested. As another example, a user may set a threshold signal integrity metric for a given measurement type. After the threshold signal integrity has been met by a valid test measurement, a test specification may dictate that no further test measurements be made for that measurement type. In addition, as described above, test specifications may indicate a desired measurement frequency or absolute time a measurement of a particular type is needed (e.g., in order to be passed to another process). Thus, new sensor elements or element combinations may be automatically tested until a new measurement is needed by some other process or until a user stops the measurement test cycle. At step  314 , the best sensor element or element combination may be outputted for each measurement type. For example, the best sensor element or element combination may be saved to memory, recorded to a storage device, logged to a file, displayed (e.g., on a monitoring system or display screen), or used for monitoring the corresponding measurement type. 
     The foregoing is merely illustrative of the principles of this disclosure and various modifications can be made by those skilled in the art without departing from the scope and spirit of the disclosure.