reducing motion artifacts in photoplethysmograms by using light … · 2012-01-20 · figure 5:...
TRANSCRIPT
/ 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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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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−10
0
10
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30
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: [email protected]