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`US 2008008 I 9?2Al
`
`(19; United States
`(12) Patent Application Publication (10) Pub. No.: US 2003/0081972 A1
`(43) Pub. Date:
`Apr. 3, 2008
`Deb reczeny
`
`(54)
`
`SY.\/IME'I'RIC LED ARRAY FOR PULSE
`OXIMETRY
`
`(75)
`
`Inventor:
`
`Martin P. Debreezeny. lJar1ville_.
`(TA (Us)
`
`Correspondence Address:
`(Iovidien
`
`IP Counsel - Respiratory & Monitoring Solutions
`60 Middletown Avenue
`
`North Haven, (TT 06473
`
`(73)
`
`Assigttee:
`
`.\'e||eor Puritan Bennett
`Incorporated
`
`(21)
`
`(22)
`
`Appl.
`
`Filed:
`
`No.:
`
`lI!54l,287
`
`Sep. 29, 2006
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`A613 5/00
`
`(52) U.S. (TI.
`
`(2006.01)
`
`6llt};‘323;6()[)f3l{)
`
`(57)
`
`ABSTR;-\(.'T
`
`There is provided a sensor for pulse oximeter systems. The
`sensor conlprises it first source of electromagnetic radiation
`configured to operate at 21 first wavelength, :1 second source
`of electromagnetic radiation configured to operate at a
`second wavelengtli. and 3 third sounze 0|‘ electrutnagnetic
`radiation eonligured to operate at at third wavelength. 'I‘l1e
`emission speetra of the first amd third sources of electro-
`ntaguelic radiation overlap at
`their hall‘ power level or
`greater and correspond to :1 center wavelength in ll1e range
`of 650 to 6?0 um.
`
`
`
`__ I 20
`Pulse Oximeter
`
`24
`
`
`
`32
`
`5
`
`14
`
`Patient
`
`001
`
`Apple Inc.
`APL1008
`
`U.S. Patent No. 8,923,941
`
`Apple Inc.
`APL1008
`U.S. Patent No. 8,923,941
`
`001
`
`

`
`Patent Application Publication
`
`Apr. 3, 2008 Sheet 1 of 4
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`US 2008)'0081972 A1
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`Patent Appiication Publication
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`Apr. 3, 2008 Sheet 2 of 4
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`US 2008)'0081972 A1
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`52
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`L
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`64
`
` IRLED
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`65
`
`66
`
`FIG. 2
`
`lo
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`
`
`
`
`Wavelength (nm)
`
`
`
`FIG. 5
`
`O03
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`003
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`Patent Appiication Publication
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`Apr. 3, 2008 Sheet 3 of 4
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`US 2008)'0081972 A1
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`
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`EmissionIntensityTr
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`3- c
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`>=5'2
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`Wavelength T
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`FIG. 4-
`
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`FIG. 5
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`US 2008f008 l9’?2 A1
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`Apr. 3, 2008
`
`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 of a patient.
`[0003]
`2. Description ofthe Related Art
`[0004] This sectiort is intended to introduce I.l1e reader to
`various aspects ofart that may be related to various aspects
`ofthe present invention. which are described andfor claimed
`below. This discussion is believed to be helpful in providing
`the reader with background information to facilitate a better
`understanding o t‘ 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.
`l11 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 t.l1e inforrnation 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 oximcters.
`Pulse oximetry may be used to measure various blood flow
`characteristics, such as the blood oxygen saturation of
`hemoglobin in arterial blood. the volume ofi ndividual blood
`pulsations supplying the tissue. andfor the rate of blood
`pulsations corresponding to each heart beat ofa patient. In
`fact, the “pulse” in pulse oximetry refers to the time varying
`amount of arterial 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 patienl’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 amount of light absorbed and scattered. More
`specifically, the light passed through or 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
`present in the tissue. 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. l"or example. pulse oximetry measurements may be
`sensitive to movement of the sensor relative to the patient‘s
`tissue, and various types of motion may cause artifacts 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 iii a sensor. chang-
`ing the angles ofincidents and interfaces probed by the light,
`
`directing the optical path through different amounts or types
`of tissue. and by expanding. compressing. or otherwise
`altering tissue near a sensor.
`[0009]
`Pulse oximeu-y 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 one iii the 11ear
`infrared region (typically about 890-940 nm). Typically.
`l_,EDs are used as light sources and are held in close
`proximity. i.e.. optically coupled. to a tissue location being
`probed. ln the context of pulse oximetry, optical coupling
`refers to a relationship between the sensor and the patient,
`permitting the sensor to transniit light into the patient‘s
`blood profused tissue and permitting a portion of the light to
`return to the sensor after passing through or reflecting from
`within the tissue. The quality of the optical coupling of the
`emitters and detectors is related to the amount of light that
`actually enters the patient’ s tissue and the portion of the light
`received by the sensor that passes through the patienr’s
`blood profilsed tissue. As described earlier. motion andfor
`the application of excessive pressure can have the ellect of
`changing the relative optical coupling efliciency of the light
`sources and the detector. Even when two I.EDs are mounted
`side by side. motion induced changes in optical efliciency
`have resulted in distortions of the photoplethysniographs
`produced by the two l.lil)s. The result of poor coupling.
`lllerefore. is a decrease in the accuracy of the sensor.
`[0010]
`Ilomogenizing the light sources using optical cou-
`pling devices is one way of mitigating the eflect of motion-
`induced changes in optical efiiciency on the accuracy of a
`pulse oximeter. Such techniques. however. generally require
`careful optical alignment. tend to be expensive, or reduce the
`optical coupling elliciency into the tissue.
`[0011]
`Sensor-to-sensor spectral variation of light sources
`used for oximeter sensors may also atlect a pulse oxi1neter’s
`accuracy. Because hemoglobin (HbO2 and HHb) spectra
`vary more rapidly as a function of wavelength at approxi-
`mately 660 mu 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
`l.lEDs result
`in a wide dislributiort of spectral content.
`potentially necessitating niodification 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 low production yields and higher sensor cost.
`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 invention are set forth below. lt should be
`understood that these aspects are presented merely to pro-
`vide the reader with a briel‘ summary ol‘ certain forms the
`invention might take zuid, 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 aspect of the present inven-
`[0013]
`tion a sensor for pulse oximeter systems is provided. The
`sensor comprises a first source of electromagnetic radiation
`configured to operate at a first wavelength. a second source
`of electromagnetic radiation configured to operate at a
`
`006
`
`006
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`US 2008f008 l9’?2 A1
`
`Apr. 3, 2008
`
`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
`waveleltgth in tl1e range of 650 to 670 nm. A photodetector
`is configured to receive electromagnetic radiation from
`blood-perfused tissue 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 electrornagnetic 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
`[00l5]
`present invention a sensor comprising a first light emitting
`diode configured to emit
`radiation having a maximum
`intensity corresponding to wavelengths in a red region of the
`electromagnetic spectrum. The sensor also comprises a
`second LEI) gonfigurcd to operate in the near-itifrarcd
`region of the electromagnetic spectrturt and a third LEI)
`configured to operate in the red region of the electromag-
`netic spectrum. The third LED has a maximum intensity at
`a wavelength greater than 650 um and greater than the
`wavelength at which the first LED has a maximum. The first
`LED and third LED are spectrally symmetrical with respect
`to a center wavelength in the range 650 to 670 mu.
`
`BRIILI7 l)[ES(‘RIPTl()N O1’ Tllli 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 em exemplary embodiment of the
`present invention:
`[0019]
`FIG. 3 illustrates an emission intensity plot of an
`emitter in accordance with an embodiment of the 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 coniiguration for
`LEDs of 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 exetnplary embodi-
`ment of the present invention.
`
`DETAILED DESCRIPTION OF SPECIFIC
`l-_":Ml30l'JIMENTS
`
`[0023] One or more specific embodiments of the present
`invention will be described below. In an elfort 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 development of any such
`actual
`implementation_. as in any engineering or design
`project. numerous implementation-specific decisions mttst
`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
`development effort 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 tnotion or spectral
`variation of light 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. I. 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 electnnnagnetic radiation from the blood perfused
`tissue of a patient 14. The electromagnetic radiation origi-
`nates from emitters 16. A photoelectric current is generated
`when the 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 of the
`electromagnetic spoctrtun. As will be explained in greater
`detail below. the emitters 16 may be oriented to provide
`spatial symmetry about an axis. Additionally, the emitters 16
`may be spectrally sytrunetrical 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 by the tissue. the sensor 11 may also be configured
`to provide calibration data to the pulse oximeter 20 via an
`encoder 21. Pulse oximetry algorithms typically use coetli-
`cients indicative ofcertain parameters ofa particular system.
`The particular set of coellicients 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 Cl.ililEtl't?l1l sets of coelli-
`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
`coefiieicnts 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 1] to compute various physiological param-
`eters. The pulse oximeter 20 may also include a random
`access memory (RAM) 26 tor 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
`
`O07
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`US 2008f008l972 A1
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`Apr. 3, 2008
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`to ligl1t drive circuitry 32 and a switch 34. The light drive
`circuitry 32 controls light emissions ofthe 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 (AID) 40.
`belhre arriving at the microprocessor 24.
`[0029] Upon receiving the signals. the microprocessor 24
`calculates the oxygen saturation and other physiological
`parameters. In calculating the physiological parameters. the
`rtlicroprocessor 24 uses algoritiuns stored on a read-only
`memory (ROM) 44 and data stored in the RAM 26. As
`discussed above. the algorithms typically use coeflicients
`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. sttch 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 AID 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 artilacts. 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 l.l1e present invention. The first I.l.iIJ 64 is
`positioned on one side of the center l..|..il) 65. while the
`second LED 66 is positioned on the other side of the center
`LED 65. The axis 62 runs through the center of the center
`LED 65 and may represent the long axis ofa patient‘s linger
`to which the LEDs may couple. The center LED 65 typically
`emits radiation in the infrared (IR) or near infrared [NMR]
`range, while the Llil)s 64 and 66 have similar spectral
`outputs in the red range of approximately 600 to 800 rim to
`help ensure that any coupling issues that may occur due to
`movement of tissue 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 of the finger with LED 66 may still exhibit
`good coupling or even improved coupling due to the move-
`lTlCl'll.
`
`[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 difl'erent wavelengths of light generate dif-
`ferent levels of current in the photodetector 12. As the red
`range produces a lower photocurrent in the photodetector
`12. I.I.iDs that emit it1 this range may be selected as the LE] )s
`64 and 66. Because the signal from the two L1.il)s 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 I.-l?.Ds 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 syntmetry
`of the Ll3ii)s 64 a11d 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 maxilntun at a center wavelength KC. The wave-
`lengths L54 and K56 of the l_l.il)s 64 and 66 are shown
`symmetrically disposed about the center wavelength ?t._,. To
`provide spectral symmetry. two emitters are used that have
`emission intensity maxima at wavelengths equidistant in
`nanometers from the central wavelength he and on opposite
`spectral sides of the central wavelength Ac. which may be
`selected to be 660 nm. for example. Specifically. the two
`wavelengths 164 and R66 are selected to have maxima at
`wavelengths that overlap at their half power level or greater
`at the center wavelength Kb. such that when sununed together
`they achieve a maximum intensity at the center wavelength
`1],. where the maximum intensity is greater than that of
`either LED 64 or LED 66 alone. For example. if the spectral
`bandwidth of the wavelengths 1:, A54 and R66 are the same,
`two wavelengths }»,_.,4 and 166 may be selected to correspond
`to the half power level or greater of the center wavelength
`kg. 111 other words. if the LED 64 has a maximum at 650 nm
`and the LED 66 has a maximum at 670 ntn and the
`respective signals have not decreased beyond their half
`power level (-3 dB) at 660 nm. then the additive maximum
`oflhc LEI) 64 and I..I.iI) 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
`[HbO3 and HI-lb] spectra vary more rapidly as a Function of
`wavelength at 660 nn1 than at 940 nm. Therefore. the precise
`spectral content of the red light source is more critical than
`that of the NIIUIR light source. Accurate predictions of
`oxygen saturation may be achieved by modification of the
`calibration model according to the spectral content of the
`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 LEDs 64 and 66 having maxima at wavelengths K54 and
`156, which are symmetrical about the center wavelength Ac.
`e.g.. approximately 660 nm. are illustrated. A maximum at
`the center wavelength.
`indicated by the dashed line 67.
`occurs due to the additive elfects of the LEDs 64 and 66
`
`emitting at the spectrally symmetrical wavelengths K54 and
`3.56 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 AC, they may combine to
`create a maximum at the center wavelength 1,. Thus. the
`technique of spectral synunetry may eliminate the wave-
`length specific calibration because the LEDs 64 and 66 are
`selected to be sununed to create a maximum at the center
`wavelength L, for which the pulse oximeter may already be
`programmed. It is inherent in this technique that the wave-
`length ntaximum. and not die 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
`maxiinuin at the center wavelength AC. a wider range of light
`source spectra may be used. For example the range of
`currently allowed wavelengths for the 660 nm LEDs is
`approximately 650 nm to 670 nm. According to the tech-
`niques presented herein. however, it may be possible to use
`
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`US 2008f008 l9’?2 A1
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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 um.
`such as 648 um, and second I.I§il) n1ay be selected to have
`an emission peak at a wavelength greater than 650 um. such
`as 6'r'2 run. As long as sig_nals from the first and second LEDs
`overlap at half power (-3 dB) or greater. at peak will be
`created by the overlap. Assuming that each LED has an
`equivalent spectral bandwidth. there will be a peak at 660
`um. Alternatively. the first LED can be selected to emit at
`640 D111 and the second IJEIJ can be selected to emit at 660
`nm. thus providing spectral symmetry at 650 nm. Again. as
`long as the signals emitted front the first and second LEDs
`overlap at ha] I‘ power or greater at 650 nm. there will be a
`peak at 650 nm. The use of LEDs producing maximas at
`wavelength other than 660 um. however, may require a
`calibration model to compensate for the lack ofabsorbance
`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
`bandwidth ofthe particular LEDs. the photosensitivity of the
`detector and limits on the spectrophotographic response of
`hemoglobin at wavelengths otherthan 660 mn. Specifically.
`if the LEDs only have a spectral bandwidth of twenty
`nanometers. the spectrally symmetrical I..EiI)s can only have
`peaks twenty nanometers or less apart (Le. ten nanometers
`from a desired center wavelength
`the
`[0037]
`I11 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 11ot necessarily be greater than
`tl1e 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 [..l-IDs 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 li'om halfpower level on the blue side of
`the signal from LED 64 to the half power level on the red
`side ofthe 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 andfor spatial symme-
`try. The two Ll:i[)s 64 and 66 are electrically configured to
`emit
`light coincidentally. whereas the center LED 65 is
`configured to emit light while the I..F.Ds 64 and 66 are oh‘.
`[0039] Turning to FIG. 6. a cross sectional view of a
`sensor in accordance with an exemplary embodiment of the
`present invention is illustrated and generally designated by
`the reference numeral 68. The cross sectional view of the
`sensor 68 shows a plurality of Ll-£l')s 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 movement of 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 which limits
`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 111otion-
`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 system that
`does not have spectral symmetry,
`it may be desirable to
`calibrate the pulse oximeter. When spectral symmetry tech-
`niques are implemented. the calibration may be unnecessary.
`as described above. In the event that spectral symmetry is
`implemented and the its is not 660 nm,
`it may still be
`desirable to calibrate according to the particular 1,.
`[004]]
`Several advantages are achieved by implementing
`the techniques described herein. For example, spatial sym-
`metry may provide anti-correlation olimotion-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
`l.l1e tissue couples with another l.l.il) operating at the same
`wavelength. Additionally. the sttrnnlod signal from ll1e sym-
`metrically disposod LIiDs may provide a stronger signal
`than a single LED to improve the signal-to-noise ratio for
`wavelengths which have a weaker photodetection effect.
`[0042] The implementation of spectral symmetry may also
`provide a stronger signal at wavelengths which have a
`weaker photo detection eliect. The combined emission
`strength of the two I.,l.iDs spectrally oriented about a central
`wavelength may provide a stronger signal for detection if
`each of LEDs have emission wavelengths which overlap
`above their hall’ power level. as described above with
`reference to FIG. 4. Furtherntore. the spectral sylurnetry
`allows the use of LEDs having a wider range of spectral
`content. Selecting light sources having maxima symmetri-
`cally disposed about a center wavelength in the range of 650
`to 670 um 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 when paired
`with an l..ED having a peak emission wavelength symmetri-
`cally disposed about the center wavelength. as long as the
`spectra of the LEDs overlap at the center wavelength at their
`respective hall" power levels or greater.
`[0043]
`lmplementation of spectral synunetry may also
`allow for calibration-free sensors. Assuming that the wave-
`length maximum and not the spectral width of the LEDs is
`the most important aspect of 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
`cneflicients 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 of light in all other spectral regions. In
`this way. the detected spectral band width would be prima-
`
`O09
`
`009
`
`

`
`US 2008t008 l9’?2 A1
`
`Apr. 3, 2008
`
`rily determined by the spectral width ofthe optical bandpass
`filter. This aspect of the invention would have the additional
`advantage of greatly limiting tl1e influence of ambietit 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
`ttiodifications and altemative fonns. specific embodiments
`have been shown by way of example in the drawings and
`have been described in detail herein. However, it should be
`ttnderstood that the invention is not intended to be limited to
`
`tech-
`the present
`|.l1e partictilar forms disclosed. Indeed,
`niques may not only be applied to nieasurements of pulse
`oximetry. but these techniques may also be utilized for the
`measurement andfor analysis of other blood or tissue con-
`stituents. Rather. the invention is to cover all modifications.
`equivalents. and altematives falling within the spirit and
`scope of the invention as defined by the following appended
`claims. It will be appreciattxi by those working in 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:
`:1 second source of electromagnetic radiation configured
`to operate at a second wavelength;
`a third source of electroniagnetic radiation configured to
`operate at a third wavelength. wherein the emission
`spectra of the first and third sources of electrotnagnetic
`radiation overlap at their half power level or greater and
`correspond to a center wavelength in the range of 650
`to 670 rim; and
`a photodetector configured to receive electromagnetic
`radiation from blood-perfused tissue irradiated by the
`first. second and third 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 sp