Abstract:
A system for near infrared spectroscopy includes a controller that automates selection of light intensities for one or more light sources. The system may stepwise increase or decrease a current driving a light source until a signal received at a light detector is within a desired range. The system may maintain closed loop control over the intensity of a light source after the intensity has been set. The closed loop control may be based on a signal from a second light detector that senses light from the light source. Current/intensity settings may be established for each of multiple light detectors. In response to selection of a light detector, the corresponding current may be delivered to drive the light source.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. patent application Ser. No. 60/915402 filed on 1 May 2007 and entitled LIGHT INTENSITY CONTROL FOR NEAR INFRARED SPECTROSCOPY. For purposes of the United States of America, this application claims the benefit under 35 U.S.C. §119 of U.S. patent application Ser. No. 60/915402 filed on 1 May 2007 and entitled LIGHT INTENSITY CONTROL FOR NEAR INFRARED SPECTROSCOPY which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to near infrared spectroscopy (NIRS). Embodiments provide apparatus and methods for measuring the concentrations of compounds (typically biological compounds) in the tissues of living subjects using NIRS. 
       BACKGROUND 
       [0003]    Near Infrared Spectroscopy (“NIRS”) is a technique which involves emitting near infrared (“NIR”) light and receiving the NIR light after it has passed through a tissue or other medium of interest. NIRS can be applied to study and monitor biochemical compounds in the body. Emitted NIR light penetrates skin and other tissues and some of it is absorbed by biochemical compounds which have an absorption spectrum in the NIR region. NIR light which is not absorbed is scattered. Each biochemical compound has a different absorption spectrum. It is possible to estimate the concentration of biochemical compounds in the tissues by measuring characteristics of NIR light that has been detected after it has passed through the tissues. 
         [0004]    A typical NIRS apparatus emits light of a number of wavelengths (typically two or more wavelengths) and detects light after it has passed through tissues of a living subject. Since light detectors are only sensitive within a given range, it is necessary that the intensity of the light emitted be sufficient to be detected by the light detector. It is also necessary that the intensity of the light not be so great that it saturates the detector. 
         [0005]    There is a need for cost-effective, simple to operate apparatus for performing NIRS on living subjects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The appended drawings illustrate non-limiting example embodiments of the invention. 
           [0007]      FIG. 1  is a block diagram illustrating a NIRS apparatus according to an embodiment of the invention. 
           [0008]      FIG. 2  is a flowchart illustrating a method for operating NIRS apparatus according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]      FIG. 1  shows apparatus  10  for practicing NIRS. Apparatus  10  has several light emitters  12  (individually labeled  12 A,  12 B and  12 C). The number of light emitters may depend upon the intended application. Typically there will be two or three or four light emitters. Other numbers of light emitters are also possible. Some embodiments may provide five or more light emitters. 
         [0010]    In some embodiments, light emitters  12  comprise solid state lasers (such as laser diodes) or high intensity light emitting diodes, or other light emitters that emit light having an appropriate wavelength. Light from light emitters  12  is carried by an optical fibre  14  or other light conduit to a probe or patch  18  which can be placed against the skin of a subject. Patch  18  also has disposed upon it one or more light detectors  20 . 
         [0011]    In operation, light is emitted by light sources  12 . Each light emitter  12  emits light having a spectral character different from that of other light emitters  12 . For example, each light emitter  12  may emit light within a narrow wavelength band that is different from that of other light emitters  12 . Apparatus  10  can be made sensitive to changes in concentration of different compounds or other materials that interact differentially with light of different wavelengths. The wavelengths emitted by light emitters  12  are typically in the near-infrared portion of the spectrum (although the apparatus and methods described herein are not limited to any specific wavelengths or wavelength regions). 
         [0012]    The light is carried by optical fibre  14  to patch  18  where it enters the tissues of a patient. Within the tissues of the patient the light is backscattered. Backscattered light is picked up by light detector(s)  20 . Measuring the amount of backscattered light detected at detector  20  at various wavelengths permits changes in concentration of various biological compounds (and/or other materials present in the subjects&#39; tissues) to be monitored. 
         [0013]    It is desirable that the light output by each light emitter  12  have an intensity such that the backscattered (or transmitted) light emitted from that light emitter  12  and subsequently detected at detector  20  has an intensity in a portion of the range of detector  20  such that light detector  20  can detect changes in intensity of the backscattered (or transmitted) light and is not saturated. 
         [0014]    The intensity of the detected light depends on a number of factors which may include:
       the intensity of light emitted by a light emitter  12 ;   the length of the path taken by light from the emitter  12  through the subject&#39;s tissues to detector  20 ;   the sensitivity of detector  20  to light from the light emitter  12 ;   the nature of the tissues through which the light propagates; and,   the efficiency of any optical paths or devices which transmit light from light emitter  12  to the subject and from the subject to light detector  20 .       
 
         [0020]    Apparatus  10  comprises a controller  22  which adjusts the output of each light emitter  12  in such a manner that the backscattered light detected at light detector  20  is within this desired range of operation, preferably somewhere near the center of the range of light intensities to which light detector  20  is sensitive. Since light detector  20  may have a sensitivity that is wavelength dependent, the desired intensity may be different for each light emitter  12 . 
         [0021]    Controller  22  controls current sources  24 A,  24 B and  24 C (shown as individually-controllable outputs of a power supply  25  in the illustrated embodiment) which regulate the current supplied to each of light emitters  12 A,  12 B and  12 C respectively. 
         [0022]    Controller  22  may comprise:
       a programmable controller, such as a digital signal processor, micro-processor, or the like;   logic circuits provided by a field programmable gate array (FPGA), a set of discreet logic circuits, an application specific integrated circuit (ASIC) or the like;   a combination thereof.       
 
         [0026]    Controller  22  has a calibration mode wherein it adjusts the outputs of light emitters  12  (i.e. the intensity of emitted light) in response to measurements of the light detected at light detector  20 . The outputs of light emitters  12  may be varied by varying the electrical current driving each light emitter  12  in some embodiments. Controller  22  varies the light output of each light emitter  12  (for example by adjusting the driving current) until the light detected at light detector  20  is in a suitable portion of the range of light detector  20 . 
         [0027]    If controller  22  completes its calibration sequence without being able to set the current driving a light source to a value which will result in the light source having a desired intensity then controller  22  may signal an alarm condition, for example by displaying a trouble light, a trouble message on a user interface, signaling an audible alarm, or the like. 
         [0028]    In typical embodiments, during operation, each light emitter  12  is pulsed. For example, each light emitter may be operated to emit a pulse of light a few milliseconds or microseconds long. Light emitters  12  may be operated such that only one light emitter  12  is operating at any given time. This permits the variation in the amount of backscattered light at the wavelength of each light emitter  12  to be independently determined. In alternative embodiments, two or more light emitters  12  may be operated simultaneously, but in different combinations at different times, to permit variations in the amount of light backscattered at each of a plurality of wavelengths to be determined. 
         [0029]    In the calibration mode, each light emitter  12  may be pulsed at a current level set by the corresponding current supply  24  under control of controller  22 . Controller  22  can determine from the intensity of light detected by light detector  20  at the instant the light emitter  12  is pulsed, whether or not a signal can be detected that corresponds to light being backscattered from light emitter  12  and also, whether or not that backscattered light has an intensity suitable to cause the detected signal to have a level within the desired range. If the backscattered light is too bright then controller  22  may reduce the current driving the light emitter  12  until the backscattered light has an intensity within a desired range. If the backscattered light is too dim, then controller  22  may increase the driving current of the light emitter  12  until the backscattered light is within the desired range. Adjustments to the current driving each light source  12  may be made in a step wise manner during calibration. 
         [0030]    In some embodiments, the size of the steps is varied, depending upon how different the light intensity detected at detector  20  is from the desirable light intensity. If the intensity of light detected at light detector  20  is very much greater or less than the desired light intensity then the current driving the light emitter  12  may be varied in relatively large steps. If the intensity detected at light detector  20  is not optimum but is fairly close to the optimum light intensity then the current driving the corresponding light emitter  12  may be adjusted in smaller steps. 
         [0031]    After each light emitter  12  has been adjusted so that backscattered light can be detected successfully at light detector  20  within a desired part of the range of light detector  20  then the current supplied to each of the light emitters  12  may be controlled to keep the current for each light emitter  12  (and therefore the intensity of light emitted by each light emitter  12 ) at the optimum value. 
         [0032]      FIG. 2  shows a method  40  according to an example embodiment of the invention. Method  40  may be implemented in a data processor or other programmable advice by providing instructions which are executed by the programable device to cause it to execute method  40 . 
         [0033]    In block  42 , an initial starting current is set for each light emitter  12 . This initial value may be approximately at the threshold current for operating each light emitter  12 . In block  44 , an appropriate target signal level is set and one of light emitters  12  is selected for initial adjustment. In block  45 , the selected light emitter is operated with a current at the initial value and the resulting signal received at light detector  20  is measured. 
         [0034]    In block  46 , a determination is made as to whether a signal detected is within the desired range. If there is a “yes” result in block  46  then a flag is set in block  47  to indicate that the selected light emitter  12  has been adjusted. Block  48  then determines if all light emitters  12  have been adjusted. In the event of a “no” result in block  48  then the next light emitter  12  is selected in block  49  and method  40  returns to block  45  where the next light emitter  12  is selected. 
         [0035]    In the event of a “yes” result in block  48 , all light emitters  12  have been adjusted and method  40  ends at block  99 . 
         [0036]    In the even of a “no” result in block  46 , block  50  determines whether the current selected for the current light emitter  12  has a value that is outside of an allowable current range. If so then, in block  51 , the current driving the selected light emitter  12  is brought back into the allowable range and in block  52  the target signal level is evaluated to determine whether it could be reduced. If there exists an allowable target signal level lower than the existing target signal level then, in block  54 , the target signal level is set to the lower value and control returns to block  44 . If there is no lower signal level allowed then in block  55 , a flag is set indicating that it was not possible to achieve suitable signal levels and method  40  terminates at block  99 . 
         [0037]    In the event of a “no” result at block  50 , then block  57  determines whether or not the detected signal is less than the maximum allowable detected signal. If yes, then the driving current for the current light emitter  12  is decreased in block  58 . If no, then the driving current for the current light emitter  12  is increased in block  59 . In block  60 , method  40  selects the next light emitter  12  and returns to block  45  for further processing. 
         [0038]    It can be appreciated that, in some embodiments at least, the methods and apparatus of this invention are advantageous because they automatically take into account differences in the sensitivity of light detector(s)  20  to different wavelengths of light. 
         [0039]    In some embodiments, after the desired intensity of each light emitter  12  has been determined, the light output of each light emitter  12  is controlled with reference to a signal from a separate light detector (not shown) that directly detects the light emitted from the light emitter  12  before that light passes through tissues of a subject. The light intensity of each light emitter  12  may be controlled in a closed-loop control. 
         [0040]    Separate calibrations may be provided for each of a number of different light detectors  20  located at different locations to detect light that has been backscattered by a section of tissue and/or has passed through the section of tissue. Controller  22  may be programmed or otherwise configured to apply different driving currents to light sources  12  depending upon which light detector  20  is being monitored. For example, there may be ten different light detectors  20  at different distances from or different positions relative to the point at which light is emitted into the subject from optical fibre  14 . A different set of driving current values (or other intensity-determining values) may be determined for each light detector  20 . Controller  22  may select a set of current values corresponding to a particular light detector  20  and then operate the light emitters  12  with those current values while monitoring the light detector  20  and then repeat the procedure for other light detectors  20 . 
         [0041]    While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. For example:
       Light detected at light detector  20  is not necessarily backscattered light. The light may be detected after passing through a section of tissue in a forward direction.   Light detector  20  is not necessarily mounted on patch  18 . Light detector  20  could be located remotely from patch  18 . Light detector  20  could be provided on a separate probe or patch from patch  18  or light could be carried to light detector  20  by an optical fiber or other optical conduit extending from patch  18  to light detector  20 .       
 
         [0044]    Certain implementations of the invention comprise computer processors which execute software instructions which cause the processors to perform a method of the invention. For example, one or more processors in a NIRS apparatus may implement the methods of the invention by executing software instructions in a program memory accessible to the processors. The invention may also be provided in the form of a program product. The program product may comprise any medium which carries a set of computer-readable signals comprising instructions which, when executed by a computer processor, cause the data processor to execute a method of the invention. Program products according to the invention may be in any of a wide variety of forms. The program product may comprise, for example, physical media such as magnetic data storage media including floppy diskettes, hard disk drives, optical data storage media including CD ROMs, DVDs, electronic data storage media including ROMs, flash RAM, or the like. The computer-readable signals on the program product may optionally be encoded, compressed or encrypted. 
         [0045]    Where a component (e.g. a software module, processor, assembly, device, circuit, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention.