Methods for determining the relative spatial change in subsurface resistivities across frequencies in tissue

Abstract

Non-invasive devices and systems to determine tissue wetness/hydration based on relative changes in subsurface resistivities in tissue below an electrode array applied to a human body across different frequencies. For example, these a sensor including arrays of current-injecting and voltage-sensing electrodes may be placed on a subject's back to determine lung wetness. Systems and methods for determining tissue water content, systems and methods for determining lung wetness, sensors for determining relative changes in subsurface resistivities across frequencies and systems and methods to determine which arrays of electrodes in a sensor to use to determine relative changes in subsurface resistivities across frequencies are all described.

1 .- 12 . (canceled) 
     
     
         13 . A method of non-invasively determining lung wetness, the method comprising:
 determining subsurface resistivities measured at different frequencies for an active region of a subject's back;   analyzing the subsurface resistivities for at least part of the region to determine an indicator of lung wetness; and,   providing an output of the indicator allowing a determination of lung wetness to be made.   
     
     
         14 . The method of  claim 13 , further comprising analyzing subsurface resistivities by determining relative spatial changes in resistivies measured at different frequencies for each of a plurality of spatial locations. 
     
     
         15 . The method of  claim 14 , wherein changes in resistivies for a spatial location are calculated using at least one of: 
       
         
           
             
               
                 
                   ρ 
                   H 
                 
                 
                   ρ 
                   L 
                 
               
               , 
               
                 
 
               
                
               
                 100 
                 * 
                 
                   
                     ρ 
                     H 
                   
                   
                     ρ 
                     L 
                   
                 
               
               , 
               
                   
               
                
               
                 100 
                 * 
                 
                   
                     
                       ρ 
                       L 
                     
                     - 
                     
                       ρ 
                       H 
                     
                   
                   
                     ρ 
                     L 
                   
                 
               
               , 
               or 
             
           
         
         
           
             
               100 
                
               
                 
                   ( 
                   
                     
                       ρ 
                       L 
                     
                     - 
                     
                       ρ 
                       H 
                     
                   
                   ) 
                 
                 / 
                 
                   ρ 
                   H 
                 
               
             
           
         
         where: ρL is a resistivity measured at a low frequency
 ρL is a resistivity measured at a high frequency 
 
       
     
     
         16 . The method of  claim 14 , wherein changes in resistivies for a spatial location are based on:
 differences in resistivities measured at different frequencies; and,   relative percentage differences in resistivities measured at different frequencies.   
     
     
         17 . The method of  claim 14 , wherein at least some of the different spatial locations are at different depths. 
     
     
         18 . The method of  claim 13 , further comprising determining the indicator using differences in resistivies measured at different frequencies for different depths. 
     
     
         19 . The method of  claim 13 , further comprising determining a slope of relative spatial changes in subsurface resistivities measured at different frequencies extending from a more superficial region of the subject's body to a deeper region of the subject's body. 
     
     
         20 . The method of  claim 19 , further comprising determining the indicator at least in part depending on whether when the slope is at least one of:
 above a predetermined value;   above a threshold;   positive.   
     
     
         21 . The method of  claim 13 , further comprising:
 determining an average of differences in subsurface resistivities measured at different frequencies from a central subsurface region; and,   determining the indicator at least in part using the average.   
     
     
         22 . The method of  claim 21 , further comprising:
 comparing the average to the threshold value; and,   determining the indicator at least in part based on results of the comparison.   
     
     
         23 . The method of  claim 13 , further comprising:
 determining a slope of relative spatial changes in subsurface resistivities measured at different frequencies extending from a more superficial region of the subject's body to a deeper region of the subject's body;   determining an average of differences in subsurface resistivities measured at different frequencies from a central subsurface region; and,   determining the indicator based on whether the slope is positive and the average is above a threshold.   
     
     
         24 . The method of  claim 13 , the indicator comprising a map indicating differences of resistivities measured at different frequencies for different depths within the active region. 
     
     
         25 . The method of  claim 13 , further comprising measuring the subsurface resistivities using a sensor comprising a plurality of spaced apart electrodes. 
     
     
         26 . The method of  claim 25 , further comprising placing the sensor on the subject's back so that the active region at least one of:
 extends cranially to caudally along the subject's back, lateral to the subject's spine; and, is between the scapula and spine.   
     
     
         27 . The method of  claim 26 , further comprising arranging the sensor in a proximal to distal active region on the subject's back so that the proximal to distal axis of the sensor extends cranially to caudally along the subject's back, and wherein the active region of the sensor is positioned lateral to the subject's spine. 
     
     
         28 . The method of  claim 26 , further comprising:
 applying currents at a plurality of frequencies from the sensor on a region of a subject's back;   detecting voltages in the active region using the sensor.   
     
     
         29 . The method of  claim 26 , further comprising:
 applying current to a first pair of the spaced apart electrodes; and,   detecting voltages using a second pair of spaced apart electrodes.   
     
     
         30 . The method of  claim 29 , further comprising selecting a resistivity array including the first and second pairs of electrodes to control a spatial location in which resistivities are measured. 
     
     
         31 . The method of  claim 30 , further comprising selecting a resistivity array by calculating a score based on a value for at least one of:
 signal error;   depth of the spatial location; and,   resistivity array location.