Suppression of Motion Artifacts in Optical Action Potential Records by
Independent Component Analysis
Oto Janoušek1
, Jana Kolářová1
, Marina Ronzhina1
, Marie Nováková2
, Sridhar Krishnan3
1
Department of Biomedical Engineering, Brno University of Technology, Brno, Czech Republic
2
Department of Physiology, Faculty of Medicine, Masaryk University, Brno, Czech Republic
3
Department of Electrical and Computer Engineering, Faculty of Engineering, Architecture, and
Science, Ryerson University, Toronto, Canada
Abstract
Optical signals reflect electrical changes in the heart;
however, the presence of motion artifact (MA)
complicates the evaluation. Possibility of MA suppression
by independent component analysis (ICA) method is
presented in this article with an analysis of ICA efficiency
and its limitations.
Suppression of MA by ICA method was compared with
results obtained by state-of-the-art signal processing
method, the ratio method. Based on this comparison, the
ICA was found as highly precise and useful method for
motion artifact removal. ICA seems to be a promising
tool for analysis of optical signals recorded from the
heart surface.
1. Introduction
Electrical activity of the heart can be recorded
conventionally by the electrode technique or, newly, by
the optical method [1]. Optical method is based on
voltage sensitive dyes (VSD) utilization. VSD binds itself
on the cell membrane and changes it’s emission light
according to action potential (AP) voltage. Although
optical method has some significant advantages, it also
has certain limitations in practice. Most important
limitation is the presence of motion artifact (MA). MA
interferes with the AP recordings.
MA origin can be described as follows: emission light
collected by the photo-detector is modulated not only
with transmembrane potential, but also with the
movement of heart wall. The distance between photodetector
and light emitting cells of heart wall alters during
heart contraction. Increasing distance produces decreasing
intensity of the collected light. Moreover contraction of
the heart produces deformation of heart wall.
Deformation increases the total number of cells in the
photo-detector field-of-view. It also changes total light
intensity. These unwanted distortions of recorded optical
AP are commonly termed as MA.
MA may significantly misrepresent useful information
in AP records. There are three general approaches for its
suppression in the literature: (a) chemical agents causing
electro-mechanical uncoupling [2] (b) slight pressing of
the heart wall against glass plate [3] and (c) offline signal
processing techniques [4].
Signal processing techniques are superior to the other
approaches, because they do not affect physiology of the
heart. This article describes novel approach for
suppression of MA in AP records, based on independent
component analysis (ICA) [5]. Efficiency of ICA method
assessed in this article is compared with state-of-the-art
method, the ratio method.
2. Method
2.1. Experimental preparation
All measurements in this study were performed on rats.
Experiments followed the guidelines for animal treatment
approved by local authorities and conformed to the EU
law. After premedication with benzodiazepines (Apaurin,
2mg, i.m., Krka, Slovenia), the rat was deeply
anaesthetized by mixture of ketamin (60mg/kg of body
mass, Narkamon, Spofa, Czech Republic) and xylazin
(2mg/kg of b.m., Rometar, Spofa, Czech Republic), and
the chest was opened. Then the heart was excised with a
sufficiently long segment of ascending aorta. The aorta
was then cannulated, the heart was mounted on the
modified Langendorff apparatus [6] and placed in
thermostat-controlled Plexiglas bath (37°C) filled with
Krebs-Henseleit (K-H) solution of following composition
(in mM): NaCl 118, NaHCO3 24, KCl 4.2, KH2PO4 1.2,
MgCl2 1.2, glucose 5.5, Taurine 10, and CaCl2 1.2. The
perfusate was oxygenated with 95% O2 and 5% CO2. The
ISSN 0276-6574 Computing in Cardiology 2012; 39:641-644.641
heart was perfused in the mode of constant perfusion
pressure (85mmHg) and stabilized for 30 minutes. No
chemical agents were used for contraction suppression.
2.2. Optical AP recording
The isolated heart was placed in the bath with K-H
solution. Three-dimensional electrograms were recorded
for heart function monitoring. The optical method was
used for AP recording. Both electrograms and AP were
recorded by touch-less methods. Isolated heart was
beating freely without any mechanical restriction. For
optical method, the isolated heart was slowly stained [13]
with VSD di-4-ANEPPS, commonly used in cardiac
studies [7]. It was diluted in K-H solution and
retrogradely applied into coronary arteries. Fifteen
minutes long staining procedure was followed by the
washout, which removed redundant non-coupled VSD
molecules from the cardiac tissue. Finally, the heart was
kept without intervention for 30 minutes long period
during which AP was recorded.
VSD emits the fluorescence light after excitation [3].
Spectral shift of emitted light reflects transmembrane
potential of stained heart cells. Emitted light was
collected by photo-detector, transformed by optical to
voltage transducer to voltage signal, and saved to PC.
Excitation was produced by laser (λ = 488nm) and
transferred to the heart wall by optical probe (Avantes,
Netherlands). The optical probe passed through Plexiglas
bath wall and ended in proximity of the left ventricular
wall. Micromanipulator allowed to set-up the probe in a
position close to the heart wall but did not restrict
movement of the beating heart [8]. The same optical
probe was used for transfer of emitted light to the photodetector
(6 of 7 optical fibres in the multi-fibre probe
were used for excitation light, 1 was used for light
emission). The three-segment photodiode (RGB Color
Sensor S9032-2, Hamamatsu Photonics, Japan) was used
as a photo-detector. Light intensity in red, green and blue
parts of emission spectra was recorded by corresponding
segment of photodiode.
Signals with duration approximately 60 minutes were
recorded with sample frequency 2kHz. The digital signals
were stored on a hard disk for offline processing.
Afterwards, signals were offline pre-processed and
analyzed by ICA in Matlab R2007b (MathWorks, 2007).
2.3. Independent Component Analysis
Independent Component Analysis (ICA) [5] is a
method for separating of the multivariate statistical data
into its subcomponents. ICA can be also described as a
statistical and computational technique for revealing
hidden factors that underlie sets of random variables,
measurements, or signals [9]. Generative model of ICA
assumes that the mixed signals are the product of
instantaneous linear combination of the independent
sources. Such a model can be mathematically described
as [5]:
MiMiii sasasax ...2211  (1)
for all i = 1, ..., M, where x1, ..., xM are the observed
random variables, s1, ..., sM are source random variables,
and aij,i,j = 1, ... M are real coefficients. Source random
variables, si, must be statistically mutually independent.
Recorded optical signals in this study correspond to the
ICA generative model. Optical signal is a mixture of
optical action potential, motion artifact, and optical noise
recorded by three-segment photodiode as:
NatMAatB
NatMAatAPatG
NatMAatAPatR
BB
GGG
RRR



)()(
)()()(
)()()(
(2)
where R(t), G(t) and B(t) are recorded optical signals in
three different ranges of fluorescence wavelengths (red,
green, blue), AP(t) is AP, MA(t) represents MA and N is
the optical noise, which is time invariant. Coefficients aR,
aG, aB represent amplification of red, green and blue
segment of the photo-detector. Note that coefficient aR
and aG must be opposite because of spectral shift of VSD
during AP occurrence. AP was omitted in blue signal B(t)
because VSD emitted light only at red and green parts of
spectra.
AP can be extracted from optical signals by several
ICA algorithms; in this article, WASOBI algorithm [10]
was used.
2.4. Ratio method
Ratio method [11] was used for computing the AP
template. Ratio of light from red and green parts of
emission spectra was computed as follows:
NatMAatAPa
NatMAatAPa
tG
tR
tAP
GGG
RRR
ratio



)()(
)()(
)(
)(
)( (3)
where APratio(t) is a template of AP, R(t), G(t) are
recorded optical signals (red, green), AP(t) is an action
potential, MA(t) represent motion artifact and N is optical
noise. Coefficients aR, aG, represent amplification of red
and green segments of used photodiode. Ratio method is
probably the most utilized signal processing technique for
motion artifact suppression nowadays. Although this
method is not ideal, it still remains state-of-the-art method
for motion suppression. For this reason, it was chosen for
comparison with ICA.
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3. Results
3.1. Number of independent components
Number of independent components emerged from
scree plot of eigenvalues of matrix composed from red,
green and blue optical signals reveals (Figure 1). The line
breaks at position number = 2; it suggests that there are
two independent variables in the data set. This is in
accordance with theoretical assumptions that optical
signals are supposed to be mixture of two sources, AP
and MA.
Figure 1. Plot of eigenvalues
3.2. ICA limitation
ICA, belonging to blind source separation methods
group, performs source signals separation without a prior
knowledge of original sources character. There is
assumption of source signal character, however: source
signals must be mutually independent.
Independency has been evaluated by comparison of the
joint probability density function (pdf) of measured MA
and modelled AP (Luo-Ruby [12]) with product of its
marginal densities. Comparison of products of separate
AP’s and MA’s pdfs (Fig. 2, part B) and its joint pdf (Fig.
2, part A) reveals that MA and AP are not fully
statistically independent. Nevertheless dependency of
MA and AP is really negligible, and ICA gives
satisfactory results.
3.3. Efficiency of ICA
Efficiency of ICA was evaluated by comparison of AP
obtained by ICA, APICA(t), with AP obtained by ratio
method, APICA(t). Both results are almost identical, as can
be seen in Fig. 3, part A and part B. Correlation
coefficient between APratio(t) and APICA(t) is 0.995.
Source signal for both methods was the same.
Both signals APratio(t) and APICA(t) are shown at the
same scale in left part of Fig. 3. APs from both methods
are almost completely overlapping, indicating that ICA
results are the same as ratio method results. MA obtained
by ICA is also almost completely overlapping with blue
part of emitted light B(t), where only movement of the
heart contributed to recorded light intensity.
Figure 2. Joint probability density function of AP and MA
(part A) and product of it’s marginal densities (part B)
4. Conclusions
ICA can be used for MA suppression in simultaneously
recorded optical signals. Both requirements on ICA input
signals (independency, non-Gaussianity) are fulfilled.
ICA can reveal AP and MA with very good accuracy.
Results showed that ICA is comparable with ratio
method, which is state-of-the-art signal processing
method for moving artifact restriction.
Although both methods give comparable results, there
are some advantages and disadvantages of ICA, when
compared with ratio method. ICA is superior over ratio
method in case of different amplification of input sensors
– i.e. segments of photodiode in this article – because
ICA transforms scales of input data in a pre-processing
steps. The shape of AP is therefore independent on input
sensors amplification. On the other hand, ICA is more
computationally demanding, and for this reason it may be
more time-consuming and has more enormous memory
requirements than ratio method. It may limit usage of
ICA especially in on-line MA removal approaches.
1 1.5 2 2.5 3
0
5
10
x 10
10
Number of independent components
Eigenvalue
Scree Plot
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Figure 3. AP obtained by ratio method (part A), by ICA
(part B) and corresponding electrogram (part C)
Acknowledgment
This work was supported by the grant projects of the
Grant Agency GACR 102/09/H083 and GAP102/12/2034
and by the grant of Masaryk University
MUNI/A/0846/2011. Author greatly thanks to Professor
Sri Krishnan for his excellent help and useful advices, as
well as for opportunity to work with his research group at
Ryerson University, Canada. Author also thanks Canada
Research Chairs Program, and Nadání Josefa, Marie a
Zdeňky Hlávkových foundation for financial support of
research fellowship in Canada.
References
[1] Rosenbaum DS, Jalife J. Optical Mapping of Cardiac
Excitation and Arrhythmias, New York: Futura Publishing
Company, Inc. 2001.
[2] Danshi L, Nattel S. Pharmacological elimination of motion
artifacts during optical imaging of cardiac tissues: Is
blebbistatin the answer?. Heart Rhythm. 2007;4:1547-71.
[3] Loew LM. Potentiometric dyes: Imaging electrical activity
of cell membrane. Pure Appl. Chem. 1996;68:1405–9.
[4] Efimov IR, Nikolski VP, Salama G. Optical Imaging of
the Heart. Circ Res. 2004;94:21-33.
[5] Hyvärinen A, Karhunen J, Oja E. Independent Component
Analysis, New York, 2001,
[6] Nováková M, Moudr J, Bravený J. A modified perfusion
system for pharmacological studies in isolated hearts.
Analysis of Biomedical Signals and Images 2000;15:162-
4.
[7] Fialová K, Kolářová J, Provazník I, Nováková M.
Comparison of Voltage-Sensitive Dye di-4-ANEPPS
Effect in Isolated Hearts of Rat, Guinea Pig, and Rabbit.
Computing in Cardiology, 2010;37:565-8.
[8] Kolářová J, Fialová K, Janoušek O, Nováková M,
Provazník I. Experimental Methods for Simultaneous
Measurement of Action Potentials and Electrograms in
Isolated Heart, Physiol. Res., 2010;59(Suppl. 1):71-80.
[9] Semmlow J. Biosignal and Medical Image Processing,
Abingdon, 2008.
[10] Yeredov A. Blind separation of Gaussian sources via
second-order statistics with asymptotically optimal
weighting, IEEE signal processing letters 2000;7:197-200.
[11] Montana V, Farkas DL, Loew ML. Dual-wavelength
ratiometric fluorescence measurements of membrane
potential, Biochemistry, 1989; 28: 4536-9.
[12] Luo C, Rudy. Model of the Ventricular Cardiac Action
Potential - Depolarisation, Repolarisation and Their
Interaction, Circulation Research, 1991;68:1501-26.
[13] Nováková M, Nogová K, Bardoňová J, Provazník I.
Comparison of Two Procedures of Loading with VoltageSensitive
Dye Di-4-ANEPPS in Rabbit Isolated Heart,
Computers in Cardiology, 2008;1081-1084.
Address for correspondence.
Oto Janoušek
Kolejní 4, Brno, 60200, Czech Republic
janouseko@feec.vutbr.cz
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