111111
`
`1111111111111111111111111111111111111111111111111111111111111111111111111111
`US 20080081972A 1
`
`(19) United States
`( 12) Patent Application Publication
`Debreczeny
`
`{10) Pub. No.: US 2008/0081972 Al
`Apr. 3, 2008
`(43) Pub. Date:
`
`Publication C lassification
`
`(5 1)
`
`Int. C t.
`A61B 5100
`(52) U.S. C l .
`........................................ 600/323; 600/3 10
`
`(2006.01)
`
`(57)
`
`ABSTRACT
`
`There is provided a sensor fo r pulse oximeter systems. The
`sensor comprises a firs t source of electromagnetic radiation
`configured to operate at a first wavelength, a second source
`of electromagnetic radiation configured to operate at a
`second wavelength, an d a third source or e lectromagnetic
`radiation configured to operate at a thi rd wavelength. The
`emission spectra of the first and third sources of electro(cid:173)
`magnetic radiation overlap at their hal f power level or
`greater and correspond to a center wavelength in the range
`of 650 to 670 nm.
`
`(54) SYMMETRIC LED ARRAY FOR PULSE
`OXIMETRY
`
`(75)
`
`Inventor:
`
`M artin P. Debreczeny, Dan ville,
`CA (US)
`
`Correspondence Address:
`Covidien
`IP Counsel - Respiratory & Mon itoring Solutions
`60 M iddletown Avenue
`North Elaven, C T 06473
`
`(73) Assignee:
`
`Nellcor P ur itan Ben nett
`Incor por ated
`
`(21) AppL No.:
`
`111541,287
`
`(22) Filed:
`
`Sep . 29, 2006
`
`10 ---.._.
`
`r·····;··----- -----------(-~Q ________________________________________ ____________________ _____ _____________________________ ,
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`Patent Application Publication
`
`Apr. 3, 2008 Sheet 2 of 4
`
`US 2008/0081972 Al
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`~60
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`Patent Application Publication Apr. 3, 2008 Sheet 3 of 4
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`US 2008/0081972 Al
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`

`

`Patent Application Publication
`
`Apr. 3, 2008 Sheet 4 of 4
`
`US 2008/0081972 Al
`
`70
`
`70
`
`FIG. 6
`
`005
`
`

`

`US 2008/0081972 Al
`
`Apr. 3, 2008
`
`1
`
`SYMMETRIC LED ARRAY FOR PULSE
`OXIMETRY
`
`BACKGROUND
`
`I. 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 of the Related Art
`(0004)
`·n1is section is intended to introduce the reader to
`various aspects of art that may be related to various aspects
`of the present invention, which are described and/or claimed
`below. This discussion is believed to be helpful in providing
`the reader with background information to facilitate a better
`understanding of the various aspects of the present inven(cid:173)
`tion. Accordingly, it should be ll!ldersrood that these state(cid:173)
`ments are to be read in this light. and not as admissions of
`prior art.
`In the field of medicine, doctors often desire to
`10005)
`monitor certain physiological characteristics of their
`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 infonnation 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.
`10006] 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 conunonly referred to as pulse oximeters.
`Pulse oximetry may be used to measure various blood flow
`characteristics, such as the blood oxygen saturation of
`hemoglobin in arterial blood. the volume of individual blood
`pulsations supplying the tissue, and/or the ra te of blood
`pulsations corresponding to each heart beat of a patient. In
`fact, the "pulse" in pulse oximetry refers to the time varying
`amowll of arterial blood in the tissue during each cardiac
`cycle.
`10007] Pulse oximcters typically utilize a non-invasive
`sensor that transmits or reflects electromagnetic radiation,
`such as light, through a patient's tissue and that photoelec(cid:173)
`trically detects the absorption and scattering of tl1e trans(cid:173)
`mitted or reflected light in such tissue. One or more of the
`above physiological characteristics may then be calculated
`based upo n the amount oflight 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 algoritluns.
`10008] Certain events can create error in these measure(cid:173)
`ments. For 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 in a sensor, chang(cid:173)
`ing the angles of incidents 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 oximetry may utilize light sources that emit
`in at least two different or spectral regions, one that emirs in
`the red region (typically about 660 urn) and one in the near
`infrared region (typically about 890-940 run). 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,
`pem1itting the sensor to transmit light into the patient's
`blood profi.tsed tissue and permitting a portion of the light to
`return to the sensor afier 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 patient's
`blood profused tissue. As described earlier, motion and/or
`the appl ication of excessive pressure cnn have the effect of
`changing the relative optical coupling efficiency of the light
`sources and tl1e detector. Even when two LEDs are mounted
`side by side, motion induced changes in optical efficiency
`have resulted in distortions of the photoplethysmograpbs
`produced by the two LEDs. The result of poor coupling,
`therefore, is a decrease in the accuracy of the sensor.
`(0010] Homogenizing the light sources using optical cou(cid:173)
`pling devices is one way of mitigating tl1e effect of motion(cid:173)
`induced changes in optical efficiency on the accuracy of a
`pulse oximeter. Such techniques. however. generally require
`caref1ll optical alignment, tend to be expensive, or reduce the
`optical coupling efficiency into the tissue.
`(0011) Sensor-to-sensor spectral variation of light sources
`used fo r oximeter sensors may also affect a pulse oximeter's
`accuracy. Because hemoglobin (Hb02 and HHb) spectra
`vary more rapidly as a function of wavelength at approxi(cid:173)
`mately 660 run 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 tUJl
`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(cid:173)
`ing only LEOs 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 uced for calibration. or by allow(cid:173)
`ing for a wider spectral content and inserting cal ibration
`models.
`
`SUMMARY
`
`10012) Certain aspects commeusurate in scope with the
`originally claimed invention are set forth below. It should be
`understood that these aspects are presented merely to pro(cid:173)
`vide the reader with a brief summary of certain forms the
`invention might take and, these aspects are not intended to
`limit the scope of the invemion. Indeed, the invention may
`encompass a variety of aspects that may not be set forth
`below.
`10013)
`In accordance with one aspect of the present inven(cid:173)
`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
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`

`US 2008/0081972 Al
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`Apr. 3, 2008
`
`2
`
`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
`U1eir half power level or greater and correspond to a center
`wavelength in the range of 650 to 670 nm. A photodeteetor
`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 e lectromagnetic 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(cid:173)
`detector.
`In accordance with yet another aspect of the
`(0015)
`present invention a sensor comprising a first light emitting
`diode configured to emil radiation having a maximum
`intensity corresponding to wavelengths in a red region of the
`electromagnetic spectmm. The sensor also comprises a
`second LED configured to operate in the near-infrared
`region of the electromagnetic spectmm and a tl1ird LED
`configured to operate in the red region o f the e lectromag(cid:173)
`netic spectrum. The third LED has a maximum intensity at
`a wavelength greater than 650 nm 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 nm.
`
`BRI EF DESCRIPTION OF TH E DRAWINGS
`
`(0016] Advantages of the invention may become apparent
`upon reading the following detailed description and upon
`reference to the drawings, in wl1ich:
`(0017] FIG. 1 illustrates a block diagram of a pulse
`oximeter system in accordance with an exemplary embodi(cid:173)
`ment of the present invention;
`10018] FIG. 2 illustrates spatial symmetry of the light
`sources in accordance with 311 exemplary embodiment of the
`present invention:
`(00191 FIG. 3 illustrates an emission intensity plot of an
`emitter in accordance with an embodiment of the present
`invention;
`100201 FIG. 4 illustrates the emission intensity plots of
`two eminers spectrally symmetrical relative to a central
`wavelength in accordance witl1 embodiments of the present
`invention:
`(0021) FIG. 5 illustrates an electrical configuration for
`LEDs of a pulse oximeter in accordance with au exemplary
`embodiment of the present invention; and
`10022) FIG. 6 illustrates a cross-sectional view of a pulse
`oximeter sensor in accordance with an exemplary embodi(cid:173)
`ment of the present invention.
`
`DETAJLED DESClUPTION OF SPECIFIC
`EMBODIMENTS
`
`10023) One or more specific embodiments of the present
`invention will be described below. In an eflort to provide a
`concise description of these embodiments. not all features of
`an acn1al 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 must
`be made to achieve developer's specific goals, such as
`compliance with system-related 31ld business-related con(cid:173)
`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 manufacn1re for those of ordinary skill
`having the benefit of this disclosure.
`(0024]
`In accordance with aspects of the present inven(cid:173)
`tion. teclmiques are disclosed for reducing the susceptibility
`of pulse oximeters to error caused by motion or spectral
`variation of light sources. Additionally, techniques are dis(cid:173)
`closed that allow for the operation of pulse oximetry systems
`with a broad spectral content and, potentially, without cali(cid:173)
`bration.
`[0025] Turning to FIG. 1, a block diagram of a pulse
`oximeter system in accord311ce with an exemplary embodi(cid:173)
`ment of the present invention is illustrated and generally
`designated by the reference numeral10. The pulse oximeter
`system 10 includes a sensor 11 having a detector 12 which
`receives electromagnetic radiation from the blood perf11sed
`tissue of a patient 14. The electromagnetic radiation origi(cid:173)
`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 tl1e 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 LEOs con(cid:173)
`figured to emit in the red and near infrared regions of the
`electromagnetic spectnun. As will be expla ined in greater
`detail below, the emitters J 6 may be oriented to provide
`spatial symmelly about 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 amoun t of electromagnetic radiation scattered and
`absorbed by the tissue, the sensor llmay also be configured
`to provide calibration data to the pulse oximeter 20 via 311
`encoder 21. Pulse oximetry a.lgoritl1ms typically use coeffi(cid:173)
`cients indicative of certain parameters of a particular system.
`Tb.e particular set of coefficients chosen for a part icu Jar set
`of wavelength spectra is detemlined 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 coeffi(cid:173)
`cients. ln this instance, U1e e ncoder 21 may include one or
`a plurality of resistor values and a detector 22 located in the
`pulse oximeier 20 reads the resis tor values and selects
`coefficients from a stored table. Alternatively, the encoder 21
`may be a memory that either stores the wavelength infor(cid:173)
`mation or the coefficients. Thus, the encoder 21 and the
`decoder 22 allow the pulse oximeter 20 to be calibrated
`according to tl1e particular wavelengths of the emitters 16.
`(0028]
`In an exemplary embodiment, the pulse oximeter
`20 includes a microprocessor 24 that processes data received
`from tbe sensor 11 to compute various physiological param(cid:173)
`eters. Tbe 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 (TPV) 30 may be provided to control the
`timing of tl1e pulse oximeter 20. The TPU may be coupled
`
`007
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`

`US 2008/0081972 Al
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`Apr. 3, 2008
`
`3
`
`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 (AID) 40,
`before arriving at the microprocessor 24.
`(0029] Upon receiving the signals, the microprocessor 24
`calculates the oxygen saturation and other physiological
`parameters. ln 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 algoritluns typically use coeliicients
`which correspond to the wavelengths of light used and
`calibrate the pulse oximeter 20 to the particular wavelengths
`being used. Jmplementation of spectral symmetry tech(cid:173)
`niques may, however, eliminate such calibration.
`(0030] The display 28 outputs the physiological param(cid:173)
`eters. such as oxygen saturation. calculated by the pulse
`oximeter 20. The block diagram o f the pulse oximeter
`system 10 is exemplary. and it should be understood that
`various a lternative 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(cid:173)
`tion of spectral symm etry techniques may obviate the
`encoder 21 and decoder 22.
`[0031) Spatial symmetry of the emitters 16 may provide a
`level of immunity against motion induced artifacts. FlG. 2
`diagrammatically illustrates the emitter portion of a sensor
`having emitter LEOs 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, w hile the
`second LED 66 is positioned on tl1e other side of tlle center
`LED 65. The axis 62 runs through tlle center of the center
`LED 65 and may represent the long axis of a patient's finger
`to which tl1e LEOs may couple. The center LED 65 typically
`emits radiation in the infrared (IR) or near infrared (NMR)
`range, while the LEOs 64 and 66 have similar spectral
`outputs in the red range of approximately 600 to 800 nm to
`help ensure tl1at any coupling issues tl1at may occur due to
`movement of tissue relative to one LED may be compen(cid:173)
`sated for by tlle other LED. For example, if the finger moves
`away from tlle LED 64, resulting in poor coupling with LED
`64, tlle coupling of the finger with LED 66 may still exhibit
`good coupling or even improved coupling due to the move(cid:173)
`ment.
`(0032] As discussed above, pulse oximeters typically
`employ light sources that operate in tlle near infrared (N1R)/
`infrared (IR) and the red range of the electromagnetic
`spectrum. The diJTerent wavelengths of light generate dif(cid:173)
`ferent levels of cturent in the photodetector 12. As the red
`range produces a lower photocurrent in the photodetector
`12, LEOs that emit in tlus range may be selected as the LEOs
`64 and 66. Because the s ignal from the two LEOs 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 LEOs 64 and 66 being physically
`disposed in a symmetrical relationship, or in the alternative.
`the wavelengths of the LEOs 64 and 66 may be spectrally
`symmetrical with respect to one another. Spectral symmetry
`of tl1e LEOs 64 and 66 may be implemented in combination
`with or independent from the spatial symmetry described
`
`above. FlG. 3 illustrates the emission intensity plot of an
`exemplary light emitter relative to the wavelength. Specifi(cid:173)
`cally, FlG. 3 illustrates that the emitter exhibits an emission
`intensity maxirmun at a center wavelength t..,. The wave(cid:173)
`lengths /..54 and /..66 of the LEOs 64 and 66 are shown
`symmetrically disposed about the center wavelength "-c· To
`provide spectral synunetry. two emitters are used tl1at have
`emission intensity maxima at wavelengths equidistant in
`nanometers from the central wavelengd1 "-c and on opposite
`spectral sides of the central wavelength "-c, which may be
`selected to be 660 run, fo r example. Specifically, the two
`wavelengths /..54 and /..66 are selected to have maxima at
`wavelengths that overlap at their half power level or greater
`at the center wavelength "-c such that when summed together
`they achieve a maxinlllm intensity at the center wavelength
`"-c, where the maximum intensity is greater than that of
`eitller LED 64 or LED 66 alone. For example, if the spectral
`bandwidth of the wavelengths "-c• /..54 and /..66 are the same,
`two wavelengths /..54 and /..66 may be selected to correspond
`to the half power level or greater of the center wavelength
`"-c· ln o ther words, if the LED 64 has a maximum at 650 run
`and the LED 66 has a maximum at 670 nm and the
`respective signals have not decreased beyond tlleir half
`power level ( - 3 dB) at 660 run. then d1e additive maximtllll
`of the LED 64 and LED 66 will occur at 660 run. 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 tl1e need for a calibra tion model. The hemoglobin
`(I-Ib02 and HHb) spectra vary more rapidly as a function of
`wavelength at 660 mu than at 940 run. Therefore. the precise
`spectral content of the red light source is more critical than
`that of the NlR!lR 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 LEOs 64 and 66 having maxima at wavelengths /..54 and
`/..66 , wh.ich are symmetrical about the center wavelength "-c,
`e.g., approximately 660 nm. arc illustrated. A maximum at
`the center wavelength, indicated by the dashed line 67,
`occurs due ro the additive elTects of the LEOs 64 and 66
`emitting at the spectraily synunetrical wavelengths /..54 and
`/..66 which overlap at their half power level or greater. As
`discussed above, even though the LEOs 64 and 66 are not
`operating at the center wavelength "-c• they may combine to
`create a maximum at the center wavelength "-c· 1l1us, the
`technique of spectral symmetry may eliminate tlle wave(cid:173)
`lenglh specific calibration because the LEDs 64 and 66 a re
`selected to be summed to create a maximum at the center
`wavelength "-c for which the pulse oximeter may al.ready be
`programmed. It is inherent in this technique that the wave(cid:173)
`length maximum, and not the spectral width of the light
`source. is used for the calibration of oximeter sensor light
`sources.
`(0036] Furthermore. because the light sources are spec(cid:173)
`trally synunetrical and their intensities may add to have a
`maximum at the center wavelength "-c· a wider range of light
`source spectra may be used. For example tlle range of
`currently allowed wavelengths for the 660 nm LEOs is
`approximately 650 mn to 670 nm. According to the tech(cid:173)
`niques presented herein, however, it may be possible to use
`
`008
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`

`US 2008/0081972 Al
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`Apr. 3, 2008
`
`4
`
`LEOs 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 ~un,
`such as 648 nm, and second LED may be selected to have
`an emission peak at a wavelength greater than 650 ~un, such
`as 672 nm. As long as signals from the first and second LEOs
`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
`1101. Alternatively, the first LED can be selected to emit at
`640 nm and the second LED can be selected to emit at 660
`mn, thus providing spectral symmetry at 650 nm. Again, as
`long as the signals emitted from the first and second LEOs
`overlap at half power or greater at 650 nm, there wiU be a
`peak at 650 nm. The use of LEOs producing maximas at
`wavelength other than 660 nm, however, may require a
`calibration model to compensate for the lack of absorbance
`of hemoglobin at that particular wavelength. Additio nally,
`the actual range of wavelengths that may be implemented
`may be limited by several factors, including the spectral
`bandwidth of the particular LEOs, the photosensitivity of the
`detector and limits on the specrrophotographic response of
`hemoglobin at wavelengths other th,:m 660 nm. Specifically,
`if the LEOs only have a spectral bandwidth of twenty
`nanometers, the spectrally synunetrical LEOs can only have
`peaks twenty nanometers or less apart (i.e. ten nanometers
`from a desired center wavelength
`In an alternative exemplary embodiment.
`the
`(0031)
`implementation of the spectral symmetry teclUliques may
`produce a peak having a broader spectral bandwidth, rather
`than increasing the magnimde of the signal at the center
`wavelength. Specifically, the peak generated by stunming
`the emilted wavelengths may not necessarily be greater than
`the peak generated by the individual LEOs 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 U1e center wavelcngU1, as the
`bandwidth extends from half power level on the blue side of
`the signal from LED 64 to the half power level on the red
`side of the LED 66.
`(0038] All exemplary schematic of the electrical coufigtl(cid:173)
`ration of the multiple LEDs is illustrated in FIG. 5. The
`config11ration of the LEOs may be the same regardless of
`whether the emitters provide spectral and/or spatial symme(cid:173)
`try. The two LEOs 64 and 66 are electrically configured to
`emit light coincidentally, whereas the center LED 65 is
`configured to emh ligl1t wllile the LEOs 64 and 66 are off.
`(0039) Tuming 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 ofLEDs 64, 65 and 66 oriented
`about an axis 62. The center LED 65 and the detector 16 are
`bisected by the axis 62. The LEOs 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 LEOs 64, 65, 66 and the detector
`16, thus. reducing artifacts due to mo tion ru1d poor coupling
`of the LEOs 64, 65, 66 and detector 16 to the patient.
`
`Specifically, tl1e sensor 68 inch1des a curved shape about the
`patient's tissue 14 which permits rocking movement accord(cid:173)
`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 motion(cid:173)
`induced artifacts.
`[0040] As stated above, spatial symmetry tecbJliques 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
`cal ibrate the pulse oximeter. When spectral symmetry tech(cid:173)
`niques are implemented, the calibration may be tUmeccssa.ry,
`as described above. In the event that spectral symmetry is
`implemented and the A.c is not 660 om, it may still be
`desirable to calibrate according to the particular f...c.
`(0041] Several advantages are achieved by implementing
`the techniques described herein. For example, spatial sym(cid:173)
`metry may provide anti-correlation of motion-induced arti(cid:173)
`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
`LEOs 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 sununed signal from the sym(cid:173)
`metrically disposed LEOs may provide a stronger signal
`than a single LED to improve the signal-to-noise ratio for
`wavelengths which have a weaker pbotodetection effect.
`[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 LEDs spectrally oriented abo ut 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. Furthennore, the spectral symmetry
`aUows the use of LEOs having a wider range of spectral
`content. Selecting light sources having maxima symmetri(cid:173)
`cally disposed about a center wavelength in the range of 650
`to 670 11111 allows for a summed signal with a maximum
`within tl1e 650 to 670 om range. Thus, LEOs emitting
`outside of the 650 to 670 tliD rru1ge may be used when paired
`with an LED having a peak emission wavelength synliDetri(cid:173)
`cally disposed about the center wavelength, as long as the
`spectra of the LEOs overlap at the center wavelength at their
`respective half power levels or greater.
`Implementation of spectral symmetry may also
`[0043)
`allow for calibration-free sensors. Assuming that the wave(cid:173)
`length maximum a11d not the spectral width of the LEOs is
`the most important aspect of the calibration, a center wave(cid:173)
`length can be selected about which LED pairings are spec(cid:173)
`trally symmetrical. The pulse oximeter 20 can be set to
`operate according to a center wavelength, i.e. according to
`coefficients associated with tl1e center wavelength, and no
`calibration is required. If variable spectral width d ue to the
`use of two LEOs 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 otlu:r spectral regions. In
`this way, tlte detected spectral band width would be prima-
`
`009
`
`

`

`US 2008/0081972 Al
`
`Apr. 3, 2008
`
`5
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`rily determined by the spectral width of the o