reducing motion artifacts in photoplethysmograms by using light … · 2012-01-20 · figure 5:...

/ Biomedical Sensor Systems Group Philips Research / Signal Processing Systems Group, Electrical Engineering Department TU/e

Biomedica Life Science Summit, Eindhoven, the Netherlands, 7-8 April 2011

Introduction

Due to the ageing society prevention and lower-levelcare follow-up will become increasingly important. Pa-tient monitoring will consequently shift to ambulatorysettings. Successful application of pulse oximetry (Fig.1)in ambulatory settings requires an improved motionrobustness (Fig. 2). Therefore it has been investi-gated whether motion artifacts in photoplethysmograms(PPGs) can be reduced by using light source displace-ment as an artifact reference.

Figure 1: Pulse oximetry finger clipused to measure a patient’s heartrate and blood oxygenation.

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−2000

−1500

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0

500

1000

Bending Finger

Time [s]

Am

plitu

de [a

.u.]

PPGs Measured by a Finger Clip

Red PPGInfrared PPG

Figure 2: Motion distortedPPGs measured by a finger clip.

Measuring light source displacement

Light source displacement is measured by a laser diodeconfigured as an interferometer (Fig. 3). The laser’s mon-itor diode measures Doppler signals as a result of motion(Fig. 4). By interpreting the Doppler signals as the Carte-sian coordinates (xn(t), yn(t)) of a vector rotating at theDoppler frequency, displacement can be determined via:

∆Lsmi(t) =λ0

4π cos(θ)unwrap

[arctan

(yn(t)

xn(t)

)]. (1)

Motion

Milk flowInsert

Moving insert:

mimic blood pulse

Flexible

membrane

Delrin skin

phantom

Flow

channel

Monitor diode

Laser cavity

Doppler shifted

backscatter

Ball lens

Figure 3: Measuring displacementvia self-mixing interferometry.

Figure 4: Doppler signals in thespectrogram of the monitor signal.

Mimicking motion distorted PPGs

A moving laser diode illuminates a skin perfusion phan-tom to obtain motion distorted PPGs (Fig. 5):

• A shaker translates the laser diode to model thebasic effects of sensor deformation.

• The skin perfusion phantom is composed of a Del-rin skin phantom under which a pulsatile milk vol-ume models pulsatile blood volume.

Shaker

Laser Pen:855nm @ 0.45mW

Ball Lens

PPGPhotodiode

Pulsatile Flowfrom Roller

Pump

FlowCell

Delrin Skin Phantom

Monitor DiodeSignal

Laser DistanceTriangulation

Sensor

LinearStage

Figure 5: Experimental setup in which motion distorted PPGs are ob-tained by illuminating a skin perfusion phantom by a moving laser.

PPG motion artifact reduction

Laser displacement is used as a reference for the motionartifacts in a correlation canceler (Fig. 6), which reducesthe motion artifacts in the PPG significantly (Fig. 7).

vPD[k] = ppg[k] + ma[k]

∆lma[k]

eo[k]+

-h0∣∣∣∣∣

∣∣∣∣∣˜

∆Lsmi[k]∣∣∣∣∣

∣∣∣∣∣−1

˜∆Lsmi[k]

0.3 HzHPF

0.3 HzHPF

∆Lsmi[k]

vPD[k]

Normalization

Figure 6: NLMS algorithm thatuses the laser displacement as amotion artifact reference.

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Time [s]

v~P

D[k

], e

o[k

] [m

V]

NLMS Result

v~PD

[k] = ppg[k] + ma[k]

v~PD

[k] = ppg[k]

eo[k]

Figure 7: Artifact reductionachieved by the NLMS algorithm.

Conclusion

• Light source displacement can be measured accu-rately using self-mixing interferometry and can beused to reduce motion artifacts in the setup.

• Measurements in real-life situations have to showthe full potential of this method.

Valorization

• Robustness to artifacts is a key marketing issue forpulse oximetry. Expansion to ambulatory monitor-ing makes it even more important.

• The dedicated experimental platform may serve asa basis for a calibration tool for pulse oximeters.

Reducing motion artifacts in photoplethysmogramsby using light source displacement as an artifactreference: phantom studyRalph Wijshoff, Jeroen Veen, Alexander van der Lee, Lars Mulder, Marco Stijnen,Sjoerd van Tuijl and Ronald Aarts contact: R.W.C.G.R.Wijshoff@tue.nl