Patent Publication Number: US-2006016963-A1

Title: Medical sensors

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims benefit from U.S. Provisional Patent Application No. 60/577,494, filed Jun. 7, 2004, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to medical sensors generally.  
     BACKGROUND OF THE INVENTION  
      Medical light sensors are known. Some of them utilize the transmission of light through a body tissue having blood vessels to non-invasively measure important health indicators, such as heart rate, blood pressure and blood oxygen level. Another example of the use of such sensors is “light printing”, in which returned light measurements are used for personal identification, such as for identifying a unique handprint.  
      An exemplary, prior art, medical sensor  10  using light measurement technology is shown in  FIG. 1 , to which reference is now made. Light  14  is transmitted from LEDs  20  into skin tissue  24  in which blood vessels  12  are located. Light of varying wavelengths such as red light (e.g. 660 nm), as indicated by arrow  16 , and infrared light (e.g. 940 nm), as indicated by arrow  18  are used. The blood cells in blood vessels  12  absorb the light differently depending on the concentration of hemoglobin in the blood. The measurement of light transmitted through the tissue or reflected from the tissue can indicate the oxygen level at a certain time and continuous measurements over a period of time can yield the heart rate, blood pressure and average oxygen levels in the blood. Other medical measurements such as blood alcohol levels, glucose levels and body fat, can also be measured using this technology.  
      Reference is now made to  FIG. 2  which illustrates medical sensor  10  in more detail. As can be seen, medical sensor  10  comprises a multiplicity of photo detectors  32  which perform the measurements, connected in series to a current to voltage transformer  34 , an analog to digital converter  36  and a storage unit  38 . Unfortunately, photo detectors  32  are expensive and space consuming. The device requires separate components  34 ,  36  and  38 , typically connected externally to photo detectors  32 . Moreover, the device consumes power continuously while measurements are being taken. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:  
       FIG. 1  is a schematic illustration of a prior art medical sensor;  
       FIG. 2  is a partial block diagram illustration of the sensor of  FIG. 1 ;  
       FIG. 3  is a block diagram illustration of a novel, photo detecting medical sensor, constructed and operative in accordance with the present invention;  
       FIG. 4  is a schematic illustration of a sensor array, forming part of the sensor of  FIG. 3 ;  
       FIG. 5  is a circuit diagram illustration of a pixel of the sensor array of  FIG. 4 ;  
       FIG. 6A  is a schematic illustration of a plurality of photo detecting diodes such as can be used for the pixel of  FIG. 5 , each tuned to a different frequency of light; and  
       FIG. 6B  is a schematic illustration of an IR photo detecting diode such as can be used for the pixel of  FIG. 5 . 
    
    
      It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.  
     DETAILED DESCRIPTION OF THE INVENTION  
      In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.  
      Applicant has realized that VLSI (very large scale integration) CMOS (complementary metal-oxide semiconductors) image sensors may be implemented for medical sensors. Doing so may provide significant size and cost reduction benefits.  
      Such CMOS image sensors, integrated onto a wafer like any VLSI component, are generally cheaper and smaller than photo detectors  32  and are typically used in cameras for imaging. Unlike photo detectors  32 , which measure the amount of light of a specific wavelength impinging on a given area, cameras image a scene and indicate the shades and color of light across the scene. Typically, cameras have pixels of about 5 μm 2  each. For camera usage, a large number of pixels (e.g. 1 million) are used. Each pixel typically contains a photo sensitive diode which discharges a capacitor or capacitive element. The sensitivity of CMOS image sensors is typically 6 bits and the signal to noise ratio (SNR) is 40 db. However, current CMOS image sensor technology is not sufficient for medical photo detection which requires a sensitivity of at least 10 bits and an SNR of at least 70 db.  
      Applicant has realized that the CMOS image sensor may be changed for medical photo detection in a number of ways. The sensitivity to light of the CMOS image sensor may be increased by increasing the size of the component pixels. Alternatively or in addition, the output of multiple pixels may be averaged for a single measurement. The structure of the photo diode may be changed to more advantageously respond to incident light.  
      Reference is now made to  FIG. 3 , which illustrates the elements of an innovative medical sensor  50 , constructed and operative in accordance with a preferred embodiment of the present invention. Medical sensor  50  may comprise a multiplicity of photo sensors  52  (two are shown), an analog to digital converter  54 , an on-chip memory  56  and a CPU  58 . In alternative embodiments of the present invention, sensor  50  may also comprise a communication block  55  and/or an audio block  57 .  
      As described in more detail hereinbelow, photo sensors  52  may measure incident light and may generate a voltage signal in response thereto. Analog-to-digital converter  54 , such as are well known in the art, may convert the voltage signal to a digital signal which may be processed by CPU  58 . The results may be stored in on-chip memory  56 .  
      CPU  58  may check the results against ranges of acceptable measured values for each type of measurement (e.g. heart rate, oxygen level, etc.). When the measurements may be outside of this range, CPU  58  may issue an alarm sound and may send an alarm message.  
      Medical sensor  50  may be used for monitoring purposes, which may require continuous measurements. Several measurements may be required for a single reported measurement (e.g. measurement of oxygen level may last 20 seconds but may require 20 measurements for a reliable result.) CPU  58  may operate medical sensor  50  briefly during the measurement time (e.g. 1 msec) and may be shut off until the next measurement. This may reduce the amount of power utilized.  
      Communication block  55  may be any suitable communication device, such as one which may communicate via any standard communication technology, such as infrared, Bluetooth, Wlan etc., and may provide the results generated by CPU  58  to an external device, such as a monitor. Audio block  57  may provide such results in an audio manner. Alternatively, audio block  57  may generate status sounds to indicate the status of the device, such as on/off, an error, etc.  
      It will be appreciated that medical sensor  50  may be manufactured as a single chip, thereby significantly lowering the cost of such sensors. Medical sensor  50  may be operated with no-battery technologies, such as hand movement charging, solar batteries and/or thermoelectric devices utilizing body heat. Certain elements of such technologies, such as the charge booster and the control for the thermoelectric device, may be integrated inside the chip.  
      Reference is now made to  FIG. 4 , which details one photo sensor  52 . Photo sensor  52  may comprise a multiplicity of CMOS photo pixels  54 , arranged in an array, for light intensity measurements. In a preferred embodiment of the present invention, photo pixels  54  may have an at least  10  times larger area than pixels of current CMOS image sensors, thereby to respond to a greater amount of light with less noise. Thus, pixels  54  may be approximately 50-200 μm 2 .  
      To further reduce noise, the output of multiple CMOS sensor pixels  54  may be averaged together for a single measurement. For example, white noise may be reduced by 3 dB for every factor of two pixels combined together. In the embodiment of  FIG. 4 , the output of each column of the array may be connected together and averaged in an associated averager  56 . Averager  56  may be a separate unit or part of CPU  58 .  
      In accordance with a preferred embodiment of the present invention, each column or group of columns of photo sensor  54  may be tuned to a different frequency λ i  of light. In one embodiment, each column may be covered with a light filter attuned to the desired frequency λ i  of light. For example, for red, the light filter may pass light above of 600-660 nm.  
      For example, photo pixels  54  may be covered with a red, green or blue light filter. Red, blue and green light passing these filters may be measured directly. For any other color in the visible range, a combination of the measurements through the filters may be used.  
      Infrared light passes through the filters with no disturbance, and may be measured separately or simultaneously with red light. As a separate measurement, it may be measured using any filter or with no filter. Alternatively, the intensity of the infrared light may be measured by a non-infra-red (e.g. green) pixel together with measurement of an infra- red pixel. The infra-red intensity may be determined by CPU  54  by subtracting the green pixel measurement from the red pixel measurement. If desired, a calibration may be performed before the measurements to account for ambient light.  
      Reference is now made to  FIG. 5 , which details one photo pixel  54 . Pixel  54  may comprise a diode  60 , a reset transistor  62 , a charge transfer transistor  64  and a row select transistor  66 . Reset transistor  62  may be controlled by a RESET signal and may transfer a Vdd level charge to a node  68  when activated.  
      Diode  60  may also be connected to node  68  and may comprise an n-well  63 . Diode  60  may be separated from other diodes by p-wells  65 . Diode  60  may discharge the charge at node  68  in the presence of light, typically during an “integration period”. At the end of the integration period, the level of charge at node  68 , which may be correlated to the light intensity, may be measured.  
      Node  68  may control charge transfer transistor  64 , which may receive a Vdd supply level and be connected in series with row select transistor  66 . At the beginning of the integration period, the charge level at node  68  may be high, which may be sufficient to fully activate charge transfer transistor  64  to transfer most, if not all, of Vdd supply to the input of row select transistor  66 . However, as integration continues, diode  60  may utilize some of the charge at node  68 , thereby reducing the charge of node  68  and thus, slightly turning off charge transfer transistor  64 . The more light seen by diode  60 , the less the charge will be at node  68  and therefore, the less charge transferred to the input of row select transistor  66 .  
      When the ROW SELECT signal may be activated, typically at the end of the integration period, if at all, row select transistor  66  may transfer whatever charge there is to a column  70 . It will be appreciated that column  70  may combine the outputs of a multiplicity of pixels  54  (which may have an averaging effect) and may provide such outputs to its associated averager  56  which may divide the output by the number of pixels in the column. It will be appreciated that, instead of a column, the output of a row may be combined together.  
      Reference is now made to  FIGS. 6A and 6B , which illustrate two implementations of diode  60  for absorbing different wavelengths of light, one ( FIG. 6A ) for absorbing different wavelengths of visible light and one ( FIG. 6B ) for absorbing infrared (IR) light.  
      In  FIG. 6A , n-well  63  may be buried within a silicon substrate  67  at varying distances d i , d j  and d k  from a surface  69  of the silicon substrate. As shown in  FIG. 6A , n-well  63 - 1  may be buried at a distance of d 1  within silicon substrate  67  while n-well  63 - 2  may be buried at a distance d 2  and n-well  63 - 3  may be buried at a distance d 3 . Each n-well  63  may be sensitive to light of a different wavelength λ i , where the longer the wavelength, the deeper the light may penetrate, or: 
 
 d   i   f= ( e   λ     i   ) 
 
      For infrared light, which has a longer wavelength, of about 900 nm, n-well  63 -IR may be a deeper n-well (i.e. have a larger well depth T IR ) as shown in  FIG. 6B , than those for visible light. N-well  63 -IR may be implanted from a distance d IR  from surface  69  until a depth d IR +T IR , where d IR =0-2 μm and T IR =1-4 μm. This may result in increased sensitivity to the infrared range.  
      It will be appreciated that, in addition to the n-wells  63  at variable distances, sensitivity to different wavelengths of light may be increased by depositing filters  71  over the surfaces  69  where n-wells  63  may be implanted.  
      Returning to  FIG. 4 , different columns of photo sensor  52  may be sensitive to different wavelengths of light. This may implemented with filters  71 , with n-wells at variable distances or both, as desired. The light impinging on photo sensor  52  may be provided either by a single white LED, which may provide multiple wavelengths or several LEDs, of different colors, may be used.  
      While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.