as) United States
`a2) Patent Application Publication (0) Pub. No.: US 2008/0081972 Al
`(43) Pub. Date:
`Apr. 3, 2008
`Debreczeny
`
`US 20080081972A1
`
`(54) SYMMETRIC LED ARRAY FOR PULSE
`OXIMETRY
`
`(75)
`
`Inventor:
`
`Martin P. Debreczeny, Danville,
`CA (US)
`
`Correspondence Address:
`Covidien
`
`IP Counsel - Respiratory & Monitoring Solutions
`60 Middletown Avenue
`
`North Haven, CT 06473
`
`(73) Assignee:
`
`Nellcor Puritan Bennett
`Incorporated
`
`(21) Appl. No.:
`
`11/541,287
`
`(22)
`
`Filed:
`
`Sep. 29, 2006
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`AGIB 5/00
`
`(2006.01)
`
`(52) USGL scscssscssssccccenccsceccensse OOOS23: 600/310
`
`(57)
`
`ABSTRACT
`
`There is provided a sensor for pulse oximeter systems. The
`sensor comprisesafirst source of electromagnetic radiation
`configured to operate at a first wavelength, a second source
`of electromagnetic radiation configured to operate at a
`second wavelength, and a third source of electromagnetic
`radiation configured to operate at a third wavelength. The
`emission spectra ofthe first and third sources of electro-
`magnetic radiation overlap at
`their half power level or
`greater and correspond to a center wavelength in the range
`of 650 to 670 nm.
`
`pevceereoeneketl
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`
`
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`Detector/Decoder
`
`001
`
`Apple Inc.
`APL1008
`U.S. Patent No. 8,923,941
`
`Apple Inc.
`APL1008
`U.S. Patent No. 8,923,941
`
`001
`
`

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`US 2008/0081972 Al
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`Patent Application Publication
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`Apr. 3,2008 Sheet 2 of 4
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`US 2008/0081972 Al
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`Patent Application Publication
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`Apr. 3, 2008 Sheet 3 of 4
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`US 2008/0081972 Al
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`US 2008/0081972 Al
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`Apr. 3, 2008
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`SYMMETRIC LED ARRAY FOR PULSE
`OXIMETRY
`
`BACKGROUND
`
`1. Field of Invention
`[0001]
`[0002] The present invention relates generally to medical
`devices and, more particularly, to sensors used for sensing
`physiological parameters ofa patient.
`[0003]
`2. Description of the Related Art
`[0004] This section is intended to introduce the reader to
`various aspects ofart that may be related to various aspects
`ofthe present invention, whichare described and/or claimed
`below. This discussionis believed to be helpful in providing
`the reader with background information to facilitate a better
`understanding of the various aspects of the present inven-
`tion. Accordingly, it should be understood that these state-
`ments are to be read in this light, and not as admissions of
`prior art.
`In the field of medicine, doctors often desire to
`[0005]
`certain physiological
`characteristics of
`their
`monitor
`patients. Accordingly, a wide variety of devices have been
`developed for monitoring many such characteristics of a
`patient. Such devices provide doctors and other healthcare
`personnel with the information they need to provide the best
`possible healthcare for their patients. As a result, such
`monitoring devices have become an indispensable part of
`modern medicine.
`
`[0006] One technique for monitoring certain physiological
`characteristics of a patient is commonly referred to as pulse
`oximetry, and the devices built based upon pulse oximetry
`techniques are commonly referred to as pulse oximeters.
`Pulse oximetry may be used to measure various blood flow
`characteristics, such as the blood oxygen saturation of
`hemoglobinin arterial blood, the volumeofindividual blood
`pulsations supplying the tissue, and/or the rate of blood
`pulsations corresponding to each heart beat of a patient. In
`fact, the “pulse” in pulse oximetry refers to the time varying
`amount ofarterial blood in the tissue during each cardiac
`cycle.
`Pulse oximeters typically utilize a non-invasive
`[0007]
`sensor that transmits or reflects electromagnetic radiation,
`such as light, through a patient’s tissue and that photoelec-
`trically detects the absorption and scattering of the trans-
`mitted or reflected light in such tissue. One or more of the
`above physiological characteristics may then be calculated
`based upon the amountof light absorbed and scattered. More
`specifically, the light passed throughor reflected from the
`tissue is typically selected to be of one or more wavelengths
`that may be absorbed and scattered by the blood in an
`amount correlative to the amount of blood constituent
`presentin thetissue. The measured amount of light absorbed
`and scattered may then be used to estimate the amount of
`blood constituent in the tissue using various algorithms.
`[0008] Certain events can create error in these measure-
`ments. For example, pulse oximetry measurements may be
`sensitive to movementofthe sensor relative to the patient’s
`tissue, and various types of motion may causeartifacts that
`may obscure the blood constituent signal. Specifically,
`motion artifacts may be caused by moving a sensor in
`relation to the tissue, increasing or decreasing the physical
`distance between emitters and detectors in a sensor, chang-
`ing the angles of incidents and interfaces probed by the light,
`
`directing the optical path throughdifferent amounts or types
`of tissue, and by expanding, compressing, or otherwise
`altering tissue near a sensor.
`[0009]
`Pulse oximetry may utilize light sources that emit
`in at least two different or spectral regions, one that emits in
`the red region (typically about 660 nm) and onein the near
`infrared region (typically about 890-940 nm). Typically,
`LEDs are used as light sources and are held in close
`proximity, i.e., optically coupled, to a tissue location being
`probed. In the context of pulse oximetry, optical coupling
`refers to a relationship between the sensor and the patient,
`permitting the sensor to transmit light into the patient's
`blood profused tissue and permitting a portion of the light to
`return to the sensorafter passing through or reflecting from
`within the tissue. The quality of the optical coupling ofthe
`emitters and detectors is related to the amount oflight that
`actually enters the patient’s tissue and the portionofthelight
`received by the sensor that passes through the patient's
`blood profused tissue. As described earlier, motion and/or
`the application of excessive pressure can havethe effect of
`changing the relative optical coupling efficiency of the light
`sources and the detector. Even when two LEDs are mounted
`side by side, motion induced changes in optical efficiency
`have resulted in distortions of the photoplethysmographs
`produced by the two LEDs. The result of poor coupling,
`therefore, is a decrease in the accuracy ofthe sensor.
`[0010] Homogenizing the light sources using optical cou-
`pling devices is one way of mitigating the effect of motion-
`induced changes in optical efliciency on the accuracy of a
`pulse oximeter. Such techniques, however, generally require
`careful optical alignment, tend to be expensive, or reduce the
`optical coupling efficiency intothetissue.
`[0011]
`Sensor-to-sensor spectral variation of light sources
`used for oximeter sensors mayalsoaffect a pulse oximeter’s
`accuracy. Because hemoglobin (HbO, and HHb) spectra
`vary more rapidly as a function of wavelength at approxi-
`mately 660 nm than at approximately 940 nm, the precise
`spectral content of the 660 nm light source is more critical.
`Current manufacturing processes used to produce 660 nm
`LEDs result
`in a wide distribution of spectral content,
`potentially necessitating modification of the calibration
`model according to actual spectral content of the 660 nm
`source, thus adding cost to the system. Alternatively, choos-
`ing only LEDs that emit
`in a narrow wavelength range
`would result in lowproduction yields and higher sensorcost.
`Thus, costs are incurred either by limiting the range of
`wavelengths to reduce the need for calibration, or by allow-
`ing for a wider spectral content and inserting calibration
`models.
`
`SUMMARY
`
`[0012] Certain aspects commensurate in scope with the
`originally claimed inventionare set forth below. It should be
`understood that these aspects are presented merely to pro-
`vide the reader with a brief summary ofcertain forms the
`invention might take and, these aspects are not intended to
`limit the scope of the invention. Indeed, the invention may
`encompass a variety of aspects that may not be set forth
`below.
`In accordance with one aspectof the present inven-
`[0013]
`tion a sensor for pulse oximeter systems is provided. The
`sensor comprisesa first source of electromagnetic radiation
`configured to operate at a first wavelength, a second source
`of electromagnetic radiation configured to operate at a
`
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`US 2008/0081972 Al
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`second wavelength and a third source of electromagnetic
`radiation configured to operate at a third wavelength. The
`first and third sources of electromagnetic radiation overlap at
`their half power level or greater and correspond to a center
`wavelength in the range of 650 to 670 nm. A photodetector
`is configured to receive electromagnetic radiation from
`blood-perfusedtissue irradiated by the first, second and third
`sources of electromagnetic radiation.
`[0014]
`In accordance with another aspect of the present
`invention there is provided a sensor comprising a first source
`of electromagnetic radiation configured to operate at a first
`wavelength, a second source of electromagnetic radiation
`configured to operate at a second wavelength and a third
`source of electromagnetic radiation configured to operate at
`a third wavelength. A photodetector is configured to receive
`electromagnetic radiation from the blood-perfused tissue,
`and the first and third sources of electromagnetic radiation
`are symmetrically disposed spatially relative to the photo-
`detector.
`
`In accordance with yet another aspect of the
`[0015]
`present invention a sensor comprising a first light emitting
`diode configured to emit
`radiation having a maximum
`intensity corresponding to wavelengths in a red regionof the
`electromagnetic spectrum. The sensor also comprises a
`second LED configured to operate in the near-infrared
`region of the electromagnetic spectrum and a third LED
`configured to operate in the red region ofthe electromag-
`netic spectrum. The third LED has a maximumintensity at
`a wavelength greater than 650 nm and greater than the
`wavelengthat which the first LED has a maximum. Thefirst
`LED and third LEDare spectrally symmetrical with respect
`to a center wavelength in the range 650 to 670 nm.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0016] Advantages of the invention may become apparent
`upon reading the following detailed description and upon
`reference to the drawings, in which:
`[0017]
`FIG.
`1
`illustrates a block diagram of a pulse
`oximeter system in accordance with an exemplary embodi-
`ment of the present invention;
`[0018]
`FIG. 2 illustrates spatial symmetry of the light
`sources in accordance with an exemplary embodimentof the
`present invention:
`[0019]
`FIG. 3 illustrates an emission intensity plot of an
`emitter in accordance with an embodiment ofthe present
`invention;
`[0020]
`FIG. 4 illustrates the emission intensity plots of
`two emitters spectrally symmetrical relative to a central
`wavelength in accordance with embodiments of the present
`invention:
`[0021]
`FIG. 5 illustrates an electrical configuration for
`LEDsof a pulse oximeter in accordance with an exemplary
`embodiment of the present invention; and
`[0022]
`FIG. 6 illustrates a cross-sectional view ofa pulse
`oximeter sensor in accordance with an exemplary embodi-
`ment ofthe present invention.
`
`DETAILED DESCRIPTION OF SPECIFIC
`EMBODIMENTS
`
`[0023] One or more specific embodiments of the present
`invention will be described below. In aneffort to provide a
`concise description of these embodiments, not all features of
`an actual implementation are described in the specification.
`
`It should be appreciated that in the developmentofany such
`actual
`implementation, as in any engineering or design
`project, numerous implementation-specific decisions must
`be made to achieve developer’s specific goals, such as
`compliance with system-related and business-related con-
`straints, which may vary from one implementation to
`another. Moreover,
`it should be appreciated that such a
`developmenteffort might be complex and time consuming,
`but would nevertheless be a routine undertaking of design,
`fabrication, and manufacture for those of ordinary skill
`having the benefit of this disclosure.
`[0024]
`In accordance with aspects of the present inven-
`tion, techniques are disclosed for reducing the susceptibility
`of pulse oximeters to error caused by motion or spectral
`variation oflight sources. Additionally, techniques are dis-
`closed that allow for the operation of pulse oximetry systems
`with a broad spectral content and, potentially, without cali-
`bration.
`
`[0025] Turning to FIG. 1, a block diagram of a pulse
`oximeter system in accordance with an exemplary embodi-
`ment of the present invention is illustrated and generally
`designated by the reference numeral 10. The pulse oximeter
`system 10 includes a sensor 11 having a detector 12 which
`receives electromagnetic radiation from the blood perfused
`tissue of a patient 14. The electromagnetic radiationorigi-
`nates from emitters 16. A photoelectric current is generated
`whenthe electromagnetic radiation scattered and absorbed
`by the tissue arrives at the detector 12. The current signal
`produced by the detector 12 is amplified by an amplifier 18
`prior to being sent to the pulse oximeter 20.
`[0026] The emitters 16 may be one or more LEDs con-
`figured to emit in the red and near infrared regions ofthe
`electromagnetic spectrum. As will be explained in greater
`detail below, the emitters 16 may be oriented to provide
`spatial symmetryabout an axis. Additionally, the emitters 16
`may be spectrally symmetrical about a central wavelength to
`eliminate the use of a spectrum calibration model.
`[0027]
`In addition to providing a signal corresponding to
`the amount of electromagnetic radiation scattered and
`absorbed bythe tissue, the sensor 11 mayalso be configured
`to provide calibration data to the pulse oximeter 20 via an
`encoder 21. Pulse oximetry algorithmstypically use coefli-
`cients indicative ofcertain parameters ofa particular system.
`The particular set of coefficients chosen for a particular set
`of wavelength spectra is determined by the value indicated
`by encoder 21 corresponding to a particular light source in
`a particular sensor. In one configuration, multiple resistor
`values may be assigned to select different sets of coefli-
`cients. In this instance, the encoder 21 may include one or
`a plurality of resistor values and a detector 22 located in the
`pulse oximeter 20 reads the resistor values and selects
`coeflicients from a stored table. Alternatively, the encoder 21
`may be a memory that either stores the wavelength infor-
`mation or the coeflicients. Thus, the encoder 21 and the
`decoder 22 allow the pulse oximeter 20 to be calibrated
`according to the particular wavelengths of the emitters 16.
`[0028]
`In an exemplary embodiment, the pulse oximeter
`20 includes a microprocessor 24 that processes data received
`from the sensor 11 to compute various physiological param-
`eters. The pulse oximeter 20 may also include a random
`access memory (RAM) 26 for storing data and an output
`display 28 for displaying computed parameters. A time
`processing unit (TPU) 30 may be provided to control the
`timing of the pulse oximeter 20. The TPU may be coupled
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`to light drive circuitry 32 and a switch 34. The light drive
`circuitry 32 controls light emissions of the emitters 16, and
`the switch 34 receives and controls the gating-in of the
`amplified signals from the detector 12. The received signals
`are passed from the switch 34 through a second amplifier 36,
`a filter 38, and an analog-to-digital converter (A/D) 40,
`before arriving at the microprocessor 24.
`[0029] Uponreceiving the signals, the microprocessor 24
`calculates the oxygen saturation and other physiological
`parameters. In calculating the physiological parameters, the
`microprocessor 24 uses algorithms stored on a read-only
`memory (ROM) 44 and data stored in the RAM 26. As
`discussed above, the algorithms typically use coefficients
`which correspond to the wavelengths of light used and
`calibrate the pulse oximeter 20 to the particular wavelengths
`being used.
`Implementation of spectral symmetry tech-
`niques may, however, eliminate such calibration.
`[0030] The display 28 outputs the physiological param-
`eters, such as oxygen saturation, calculated by the pulse
`oximeter 20. The block diagram of the pulse oximeter
`system 10 is exemplary, and it should be understood that
`various alternative configurations may be possible. For
`example, there may be multiple parallel paths of separate
`amplifiers, filters, and A/D converters for multiple light
`wavelengths or spectra received, Additionally, implementa-
`tion of spectral symmetry techniques may obviate the
`encoder 21 and decoder 22.
`
`Spatial symmetry of the emitters 16 may provide a
`[0031]
`level of immunity against motion induced artifacts. FIG. 2
`diagrammatically illustrates the emitter portion of a sensor
`having emitter LEDs 64, 65 and 66 symmetrically oriented
`relative to an axis 62 in accordance with an exemplary
`embodiment of the present invention. The first LED 64 is
`positioned on one side of the center LED 65, while the
`second LED 66 ts positioned on the otherside ofthe center
`LED 65. The axis 62 runs through the center ofthe center
`LED 65 and may representthe long axis of a patient’s finger
`to which the LEDs may couple. The center LED 65 typically
`emits radiation in the infrared (IR) or near infrared (NMR)
`range, while the LEDs 64 and 66 have similar spectral
`outputs in the red range of approximately 600 to 800 nm to
`help ensure that any coupling issues that may occur due to
`movement oftissue relative to one LED may be compen-
`sated for by the other LED. For example, if the finger moves
`away from the LED 64, resulting in poor coupling with LED
`64, the coupling ofthe finger with LED 66 maystill exhibit
`good coupling or even improved coupling due to the move-
`ment.
`
`[0032] As discussed above, pulse oximeters typically
`employ light sources that operate in the near infrared (NIR)/
`infrared (IR) and the red range of the electromagnetic
`spectrum. The different wavelengths oflight generate dif-
`ferent levels of current in the photodetector 12. As the red
`range produces a lower photocurrent in the photodetector
`12, LEDsthat emit in this range may beselected as the LEDs
`64 and 66. Because the signal from the two LEDs 64 and 66
`is additive, the signal-to-noise ratio of the sensor may be
`increased, thus, providing better readings.
`[0033]
`In addition to the LEDs 64 and 66 being physically
`disposed in a symmetrical relationship, or in the alternative,
`the wavelengths of the LEDs 64 and 66 may be spectrally
`symmetrical with respect to one another. Spectral symmetry
`of the LEDs 64 and 66 may be implemented in combination
`with or independent from the spatial symmetry described
`
`above. FIG. 3 illustrates the emission intensity plot of an
`exemplary light emitter relative to the wavelength. Specifi-
`cally, FIG. 3 illustrates that the emitter exhibits an emission
`intensity maximumat a center wavelength i,. The wave-
`lengths 4.,, and A4,, of the LEDs 64 and 66 are shown
`symmetrically disposed about the center wavelength A... To
`provide spectral symmetry, two emitters are used that have
`emission intensity maxima at wavelengths equidistant in
`nanometers fromthe central wavelength 4. and on opposite
`spectral sides of the central wavelength ‘,., which may be
`selected to be 660 nm, for example. Specifically, the two
`wavelengths A,, and A, are selected to have maxima at
`wavelengths that overlap at their half powerlevel or greater
`at the center wavelength i... such that when summedtogether
`they achieve a maximumintensity at the center wavelength
`i... Where the maximumintensity is greater than that of
`either LED 64 or LED 66 alone. For example, if the spectral
`bandwidth of the wavelengths A, A,4 and A,, are the same,
`two wavelengths 0, and A,, may be selected to correspond
`to the half powerlevel or greater of the center wavelength
`d,.. In other words, if the LED 64 has a maximumat 650 nm
`and the LED 66 has a maximum at 670 nm andthe
`respective signals have not decreased beyond their half
`powerlevel (-3 dB) at 660 nm, thenthe additive maximum
`of the LED 64 and LED 66 will occur at 660 nm. Thus, a
`stronger signal at the center wavelength, such as 660 nm
`may be achieved through spectral symmetry techniques.
`[0034] Additionally,
`the use of spectral symmetry may
`eliminate the need for a calibration model. The hemoglobin
`(HbO, and HHb) spectra vary more rapidly as a function of
`wavelength at 660 nmthan at 940 nm. Therefore, the precise
`spectral content of the red light source is more critical than
`that of the NIR/IR light source. Accurate predictions of
`oxygen saturation may be achieved by modification of the
`calibration model according to the spectral content ofthe
`particular red light sources being used, as discussed above.
`Spectral symmetry techniques, however, may be used to
`obviate calibration.
`
`[0035] Referring to FIG. 4, the emission intensity of the
`two LEDs64 and 66 having maxima at wavelengths A, and
`Age: Which are symmetrical about the center wavelength i._,
`e.g., approximately 660 nm, are illustrated. A maximumat
`the center wavelength,
`indicated by the dashed line 67,
`occurs due to the additive effects of the LEDs 64 and 66
`emitting at the spectrally symmetrical wavelengths 4, and
`Age Which overlap at their half power level or greater. As
`discussed above, even though the LEDs 64 and 66 are not
`operating at the center wavelength A... they may combine to
`create a maximumat the center wavelength A... Thus, the
`technique of spectral symmetry may eliminate the wave-
`length specific calibration because the LEDs 64 and 66 are
`selected to be summed to create a maximumat the center
`wavelength 4. for which the pulse oximeter may already be
`programmed. It is inherent in this technique that the wave-
`length maximum, and not the spectral width of the light
`source, is used for the calibration of oximeter sensor light
`sources.
`
`Furthermore, because the light sources are spec-
`[0036]
`trally symmetrical and their intensities may add to have a
`maximumat the center wavelengthA... a wider range of light
`source spectra may be used. For example the range of
`currently allowed wavelengths for the 660 nm LEDsis
`approximately 650 nm to 670 nm. According to the tech-
`niques presented herein, however, it may be possible to use
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`LEDs emitting outside the range of wavelengths between
`650 and 670 nm. For example, a first LED can be selected
`to have an emission peak at a wavelength less than 670 tim,
`such as 648 nm, and second LED may beselected to have
`an emission peak at a wavelength greater than 650 tum, such
`as 672 nm. As long as signals fromthefirst and second LEDs
`overlap at half power (-3 dB) or greater, a peak will be
`created by the overlap. Assuming that each LED has an
`equivalent spectral bandwidth, there will be a peak at 660
`nm. Alternatively, the first LED can be selected to emit at
`640 nm and the second LED canbeselected to emit at 660
`nm, thus providing spectral symmetry at 650 nm. Again, as
`long as the signals emitted fromthe first and second LEDs
`overlap at half power or greater at 650 nm, there will be a
`peak at 650 nm. The use of LEDs producing maximasat
`wavelength other than 660 nm, however, may require a
`calibration model to compensate for the lack of absorbance
`of hemoglobin at that particular wavelength. Additionally,
`the actual range of wavelengths that may be implemented
`may be limited by several factors,
`including the spectral
`bandwidthofthe particular LEDs, the photosensitivity ofthe
`detector and limits on the spectrophotographic response of
`hemoglobin at wavelengths other than 660 nm. Specifically,
`if the LEDs only have a spectral bandwidth of twenty
`nanometers, the spectrally symmetrical LEDs can only have
`peaks twenty nanometers orless apart (i.e. ten nanometers
`from a desired center wavelength
`the
`[0037]
`In an alternative exemplary embodiment,
`implementation of the spectral symmetry techniques may
`produce a peak having a broader spectral bandwidth, rather
`than increasing the magnitude of the signal at the center
`wavelength. Specifically, the peak generated by summing
`the emitted wavelengths may not necessarily be greater than
`the peak generated by the individual LEDs 64 and 66
`themselves. For example, the summed peak may have a
`magnitude approximately equivalent to the magnitude of
`peaks generated by the LEDs 64 and 66 alone. Accordingly,
`the intensity of the emissions across the spectra will be
`relatively flat between the wavelengths being used and at the
`center wavelength. The combined signal would provide a
`broader spectral bandwidth a the center wavelength, as the
`bandwidth extends fromhalf powerlevel onthe blue side of
`the signal from LED 64 to the half power level onthe red
`side of the LED 66.
`
`[0038] An exemplary schematic of the electrical configu-
`ration of the multiple LEDs is illustrated in FIG. 5. The
`configuration of the LEDs may be the same regardless of
`whether the emitters provide spectral and/or spatial symme-
`try. The two LEDs 64 and 66 are electrically configured to
`emit
`light coincidentally, whereas the center LED 65 is
`configured to emit light while the LEDs 64 and 66 are off.
`[0039] Turning to FIG. 6, a cross sectional view of a
`sensor in accordance with an exemplary embodimentofthe
`present inventionis illustrated and generally designated by
`the reference numeral 68. The cross sectional view of the
`sensor 68 showsa plurality of LEDs 64, 65 and 66 oriented
`about an axis 62. The center LED 65 and the detector 16 are
`bisected by the axis 62. The LEDs 64, 65, and 66 transmit
`electromagnetic radiation through a patient’s tissue 14
`which is detected by the detector 16. ‘The housing 70 of the
`pulse oximeter 68 may be designed to limit movementof the
`patient 14 relative to the LEDs 64, 65, 66 and the detector
`16, thus, reducing artifacts due to motion and poor coupling
`of the LEDs 64, 65, 66 and detector 16 to the patient.
`
`Specifically, the sensor 68 includes a curved shape about the
`patient’s tissue 14 which permits rocking movement accord-
`ing to the curved shape of the housing 70, but whichlimits
`other movement. Movement such as rocking along the
`curvature of the housing 70 may be anti-correlated by the
`spatial symmetry of the sensor 68, thus reducing motion-
`induced artifacts.
`
`[0040] As stated above, spatial symmetry techniques may
`be used in combination with or independent of the spectral
`symmetry technique. When implemented in a systemthat
`does not have spectral symmetry,
`it may be desirable to
`calibrate the pulse oximeter. Whenspectral symmetry tech-
`niques are implemented, the calibration may be unnecessary,
`as described above. In the event that spectral symmetry is
`implemented and the A,
`is not 660 nm,
`it may still be
`desirable to calibrate according to the particular A...
`[0041]
`Several advantages are achieved by implementing
`the techniques described herein. For example, spatial sym-
`metry may provide anti-correlation of motion-induced arti-
`facts and increase the signal-to-noise ratio. Motion-induced
`artifacts are typically a result of changes in the coupling of
`the sensor with the patient’s tissue. The spatial symmetry
`anti-correlates the motion induced artifacts by providing two
`LEDs symmetrically disposed about an axis of movement
`such that as the patient’s tissue moves away from one LED
`the tissue couples with another LED operating at the same
`wavelength. Additionally, the summed signal from the sym-
`metrically disposed LEDs may provide a stronger signal
`than a single LED to improve the signal-to-noise ratio for
`wavelengths which have a weaker photodetectioneffect.
`[0042] The implementation of spectral symmetry may also
`provide a stronger signal at wavelengths which have a
`weaker photo detection effect. The combined emission
`strength of the two LEDsspectrally oriented about a central
`wavelength may provide a stronger signal for detection if
`each of LEDs have emission wavelengths which overlap
`above their half power level, as described above with
`reference to FIG. 4. Furthermore, the spectral symmetry
`allows the use of LEDs having a wider range ofspectral
`content. Selecting light sources having maxima symmetri-
`cally disposed about a center wavelengthin the range of 650
`to 670 nm allows for a summed signal with a maximum
`within the 650 to 670 nm range. Thus, LEDs emitting
`outside of the 650 to 670 nm range may be used whenpaired
`with an LED having a peak emission wavelength symmetri-
`cally disposed about the center wavelength, as long as the
`spectra of the LEDsoverlap at the center wavelength at their
`respective half power levels or greater.
`[0043]
`Implementation of spectral symmetry may also
`allow for calibration-[ree sensors. Assuming that the wave-
`length maximumand not the spectral width of the LEDs is
`the most important aspectof the calibration, a center wave-
`length can be selected about which LED pairings are spec-
`trally symmetrical. The pulse oximeter 20 can be set to
`operate according to a center wavelength, i.e. according to
`coefficients associated with the center wavelength, and no
`calibration is required. If variable spectral width due to the
`use of two LEDs is found to limit
`the accuracy of the
`measurement, an optical coating could be applied either to
`the light source or detector to limit the spectral width. For
`example, the detector could be coated with a material that
`passes light only with bands around 660 and 890 nm,but
`blocks the detection oflight in all other spectral regions. In
`this way, the detected spectral band width would be prima-
`
`009
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`009
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`US 2008/0081972 Al
`
`Apr. 3, 2008
`
`rily determined by the spectral width ofthe optical bandpass
`filter. This aspect of the invention would have the additional
`advantage ofgreatly limiting the influence of ambient light
`on the measured signal. Examples of suitable coatings
`include multilayer dielectric films and light-absorbing dyes.
`[0044] While the invention may be susceptible to various
`modifications and alternative forms, specific embodiments
`have been shown by way of example in the drawings and
`have been described in detail herein. However, it should be
`understood that the invention is not intendedto be limited to
`the particular forms disclosed. Indeed,
`the present
`tech-
`niques may not only be applied to measurements of pulse
`oximetry, but these techniques may also be utilized for the
`measurement and/or analysis of other blood or tissue con-
`stituents. Rather, the inventionis to cover all modifications,
`equivalents, and alternatives falling within the spirit and
`scopeof the inventionas defined by the following appended
`claims. It will be appreciated by those workingin the art that
`sensors fabricated using the presently disclosed and claimed
`techniques may be used in a wide variety of contexts.
`What is claimed is:
`1. A sensor for pulse oximeter systems comprising:
`a first source of electromagnetic radiation configured to
`operate at a first wavelength;
`a second source ofelectromagnetic radiation configured
`to operate at a second wavelength;
`a third source of electromagnetic radiation configured to
`operate at a third wavelength, wherein the emission
`spectra of thefirst and third sources ofelectromagnetic
`radiation overlap at their half power level or greater and
`correspond to a center wavelength in the range of 650
`to 670 nm; and
`a photodetector configured to receive electromagnetic
`radiation from blood-perfused tissue irradiated by the
`first, second andthird sources ofelectromagnetic radia-
`tion.
`2. The sensor of claim 1, wherein the first and third
`sources of electromagnetic radiation emit in the red region
`of the electromagnetic spectrum.
`3. The sensor of claim 1, wherein the first source of
`electromagnetic radiation has an emission maximum atless
`than 670 nmandthe third source has an emission maximum
`in greater than 650 nm, the emission maximumof the third
`source occurring at a wavelength greater than the emission
`maximumof the first source and whereinthefirst and third
`sources of electromagnetic radiation are spectrally equidis-
`tant from the center wav