Advanced Biosignal Processing ||

Chapter 1Biosignals: Acquisition and General Properties

Amine Naıt-Ali and Patrick Karasinski

Abstract The aim of this chapter is to provide the reader with some basic andgeneral information related to the most common biosignals, in particular biopoten-tials, encountered in clinical routines. For this purpose, the chapter will be dividedinto two main sections. In Sect. 1.1, we will consider the basis of bipotential record-ing (i.e., electrodes, artifact rejection and safety). In the second section, some gen-eral properties of a set of biosignals, will be introduced briefly. This will concernessentially, ECG, EEG, EPs and EMG. This restriction is required to ensure anappropriate coherency over the subsequent chapters which will deal primarily withthese signals.

1.1 Biopotential Recording

As mentioned previously in the introduction to this book, biosignals are intensivelyemployed in various biomedical engineering applications. From unicellular actionpotential to polysomnogram, they concern both research and clinical routines. Sincethis book deals specifically with biopotentials (i.e. bioelectrical signals), a specialfocus on their acquisition is provided in this section.

As is the case in any common instrumentation system, biopotential recordingschemes include an observed process, a sensor and an amplifier. In our case, theobserved process is recorded from a human body which requires particular precau-tions to be taken into account.

Consequently, the following three most important aspects will be underlined inthis section:

1. The sensor: electrode description and its modeling will be given in Sect. 1.1.1.2. The power supply artifact: this point will be discussed in Sect. 1.1.2, in which

we provide a description of some common schemes,3. Safety: constraints and solutions are presented in Sect. 1.1.3.

A. Naıt-Ali (B)Universite Paris 12, Laboratoire, Image, Signaux et Systemes Intelligents, LiSSi, EA. 3956. 61,avenue du General de Gaulle, 94010, Creteil, Francee-mail: [email protected]

A. Naıt-Ali (ed.), Advanced Biosignal Processing,DOI 10.1007/978-3-540-89506-0 1, C© Springer-Verlag Berlin Heidelberg 2009

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2 A. Naıt-Ali and P. Karasinski

1.1.1 Biopotentials Recording Electrodes

Generally speaking, to ensure an appropriate interface between living tissue anda conductor, specific sensors are required to transform ionic concentrations toelectronic conductions. This sensor is, actually, an electrode in which a chemicalreaction produces this transformation.

The biopotentials are produced from cell activity that changes the ionic con-centration in intra and extra cellular environment. In electrical devices, electronactivity produces tensions and currents. Both of them are electrical phenomenonbut charge carriers are different. A current in an electronic circuit results from anelectron movement and from the displacement ions in living tissue. As mentionedabove, electrodes ensure the transformation from ionic conduction to electronic con-duction through a chemical reaction. Biopotential electrodes are regarded as secondkind electrodes (i.e. they are composed of metal, saturated salt from the same metaland an electrolyte made with common ion). For example, the most common elec-trode uses silver, silver chloride (Ag/AgCl) and a potassium or sodium chlorideelectrolyte. These electrodes present good stability because their potentials dependon common ion concentration and temperature. Moreover, electrodes are sensitiveto movement. In a steady state, electrochemical reactions create a double layerof charges that might be a source of artifacts during any possible movement (e.g.patient movement). To reduce the effect of these artifacts, one has to ensure that theelectrodes are properly fixed.

1.1.1.1 Equivalent Circuit

The electrode interface can be modeled by an equivalent electrical circuit, shownin (Fig. 1.1a). Here, E1 is the electrode potential. The charge double layer acts asa capacitor denoted by Ce, whereas Re stands for the conduction. It is worth not-ing that a more complex model has been proposed in the literature to take the skinstructure into account [9]. The corresponding electrical circuit is given in Fig. 1.1b.The paste-epidermis junction produces another DC voltage source, denoted E2.Epidermis can be modeled by an RC network.

Generally speaking, any equivalent circuit can be reduced to a DC voltage, placedin serial with an equivalent impedance.

1.1.1.2 Other Electrodes

Even if Ag/AgCl electrodes are commonly used in clinical routines, one can evokeother existing types, namely:

• glass micropipet electrodes for unicellular recording,• needle electrodes,• micro array electrodes,• dry electrodes.

1 Biosignals: Acquisition and General Properties 3

E1

Re Ce

a) Electrode

Ce

Rp

E2

Cs

E1

Rm

Electrode

Paste

Epidermis

Dermis

b) electrode skin interface

Re

Rs

Fig. 1.1 (a) Electrodeequivalent circuit,(b) electrode skin interfaceequivalent circuit

Of course, this is a non exhaustive list. For more details, the reader can refer, forinstance, to [9].

Special attention is given to, dry electrodes which are capacitive effect based.These electrodes are well suited for both long-duration and ambulatory recording.Associated with wireless transmission systems, they could represent the biopotentialrecording standard of the future.

1.1.2 Power Supply Artifact Rejection

The power supply artifact is not specific to biopotential measurements. This problemconcerns, rather, any instrumentation systems. Even if one can protect a strain gaugeor any industrial sensor by a shielded box; building a Faraday cage around a patient,in an operating room is impossible.

If the power supply artifact rejection is something new for you, you can carry outa very simple experiment using an oscilloscope, as follows: connect a coaxial cableat the input and take the hot point between two fingers. You will see on the screena 50 or 60 Hz sinusoidal noisy signal. The magnitude depends on your electricalinstallation. It can reach a few volts. Now, touch the grounding point with the sameor the other hand; the signal will decrease. Finally, put down the coaxial cable andincrease the oscilloscope gain to its maximum value; you will notice that the signalis still present. This “strange” signal is the power supply artifact.

From this experiment, one can wonder about this signal’s sources which makeit still present with two, one or even without any measurement point. Actually, thephysical phenomenon at the signal origin is a capacitive coupling created by dis-placement currents, described in the following section.

1.1.2.1 Displacement Currents

Displacement currents are described in electromagnetism theory, in particular, inMaxwell′s fourth equations:


c2∇ × B = jε0

+ �E�t

This equation establishes the equivalence between the conduction current densityand the temporal electrical field variation. Both of them can produce a magneticfield. The temporal electrical field variation term is useful for dielectric medium. Itexplains the reason for which an alternative current seems to go through a capacitordespite the presence of an insulator [5]. By analogy to the conduction current, thisterm is called displacement current.

The most important drawback when recording bio-potentials is that every con-ductor separated by an insulator forms a capacitor, including the human body. Actu-ally, a human is more of a conductor than an insulator. For this reason capacitors areformed by power supply wires, including ground wires. In this context, the oscillo-scope experiment can be modeled by the scheme shown in Fig. 1.2.

The capacitors represent a capacitive coupling between power supply wires andthe patient. The Cp value is approximately in the range 2 to 3 pF and 200 to 300pF for Cn according to the following references: [3, 10, 11]. Thus, the patient islocated at the center of the capacitor voltage divider which explains the presenceof “our low amplitude sinusoidal signal” on the oscilloscope whatever the measure-ment performed on the human body surface might be. Consequently, amplifyingbiopotentials using a standard amplifier is inappropriate. An instrumentation ampli-fier offers an initial solution since it amplifies the electrical tension between twoinputs In+ and In- (Fig. 1.3).

Power Line ( 50 or 60 Hz)

Cn

Cp

Fig. 1.2 Oscilloscopeexperiment


Ref

In −

In +

Vs

+

−

Fig. 1.3 First solution:Instrumentation amplifier

1.1.2.2 Common Mode Rejection

In theory, an amplifier provides at its output a voltage Vs, expressed by: Vs = Gd(VIn+ - VIn-) where Gd is the differential gain. In this case, the power supply artifactwill be canceled through subtraction. When two inputs are used, one should takeinto account the common mode voltage.

On the other hand, things are different in practice. The output voltage Vs will beexpressed by: Vs = Gd (VIn+ - VIn-) + Gc (VIn+ + VIn-)/2, where Gc is the commonmode gain. In this case, the power supply artifact generated by displacement currentsis still present. In addition, the power supply artifact is never the same at In+ andIn-. Hence, it appears as well in the differential mode.

1.1.2.3 Magnetic Induction

Excepting the displacement current, another physical phenomenon such as the mag-netic induction can produce power supply artifacts. Patient leads as well as theamplifier form a loop in which temporal magnetic field variations induce a voltage(Faraday law). A magnetic field is produced by transformers and any surroundingelectrical motors. In order to reduce magnetic field effects, one avoid being in close


proximity to magnetic sources. Moreover, one has to minimize the surface loop. Insuch situations, shielding is useless.

The solution illustrated in Fig. 1.3 is not appropriate in terms of safety. In suchsituations, the patient is directly connected to the ground.

1.1.2.4 The Right Leg Driver

Power supply artifact effects have been modeled through the equation described byHuhta and Webster [2]. This famous equation contains five terms, namely, the mag-netic induction, the displacement current in leads, the displacement currents in thehuman body, the common mode gain and finally the effect of electrode impedances,unsteadiness.

Reducing artifact magnitude under a given threshold can be achieved with a spe-cific reference device. The idea consists in suppressing the common mode voltageVc using a -Vc voltage as reference. In others words, the patient is situated in afeedback loop whose aim is the common mode suppression [10]. A basic scheme isillustrated in Fig. 1.4.

The amplifier A1 provides a differential voltage Vs and a common voltage Vcbetween R1 and R2. A2 amplifies Vc with a negative gain. Hence, –GVc is appliedto the human body through R3 which acts as a current limiter. The third ampli-fier, denoted A3, allows an operating mode without using any offset voltage. More-over, one has to point out that all DC voltages provided by electrode skin interfaces(Fig. 1.1b) are not particularly the same and can produce undesirable offset voltage.In this scheme, R4 and C2 determine the low cutoff frequency.

Nowadays, this circuit has become somewhat classic. It is commonly used as anexample in numerous amplifier design datasheets and application notes.

Generally speaking, the right leg driver circuit can be used for all biopotentialrecordings (e.g., EEG, EMG, EOG, etc.). It is also called the active reference circuit.

Instrumentation Amplifier

Vs

Vc

R1

R2

A1

A2R3

A3

R4

C2

R5

R6

Isolation Amplifier

Isolation barrier

Fig. 1.4 Driven right leg circuit


On the other hand, it is also important to underline the fact that the configurationof the two electrodes is also used in some specific applications such as in telemetryor ambulatory recording. Consequently, it is still subject to modeling and designimprovement. For instance, the reader can refer to: [8, 11, 7, 1].

1.1.3 Safety

Safety objectives deal with unexpected incidents. Misused and defective devices aretwo risky aspects of biopotentials recording. Maybe one cannot cite the entire list ofpossible scenarios, but it seems obvious that one has to avoid any sensitive, or evenlethal electrical stimulation.

1.1.3.1 Current Limitation

All the leads connected to the patient are potentially dangerous. For example, inthe right led driver (Fig. 1.4) there are three contacts with the patient. The resis-tor R3 limits the current supplied by the voltage source A2. The inputs In+ andIn- shouldn’t supply any current except in case of A1 dysfunction, In+ or In- canbecame a DC voltage source. Therefore, R4 and R3 operate as a current limiter aswell and are useless for amplification.

1.1.3.2 Galvanic Isolation

Grounding is another important default source. If several devices are connected tothe patient, the ground loop can generate sensitive stimulation. Obviously, a moredangerous case occurs when the patient is accidentally in contact with the powersupply line. An electrocution is unavoidable if the current finds, through the patient,a pathway to the ground!

Safety grounding requires an efficient galvanic isolation (i.e. elimination of allelectrical links between electronic devices inside the patient side and the powersupply system). Electronic manufacturers propose isolation amplifiers that provideisolated DC supply, isolated ground and an analog channel through the isolation bar-rier. Galvanic isolation justifies the two different grounds symbols used in Fig. 1.4.Isolated ground (or floating ground) inside the patient is totally disconnected fromthe power supply ground. There is no pathway whatsoever for the power linecurrent.

1.1.4 To Conclude this Section. . .

In this field, technological progress has tended to take the form of system miniatur-izations; low consumption battery powered systems; hybrid designs (by includingdigital systems), secured transmission and so on.

What about tomorrow? Certainly, this trend will persist and one can imagine asingle chip device that integrates electrodes, amplifiers, codecs, digital data process-ing codes, as well as a wireless transmission system.


In the next section, the reader will find some basic biopotential properties thatnaturally cannot be exhaustive due to the numerous different cases and variousapplications.

1.2 General Properties of Some Common Biosignals

As explained earlier, the purpose of this section is to present some basic generalitiesrelated to the most common biosignals used in clinical routines (i.e., ECG, EEG, EPand EMG).

1.2.1 The Electrocardiogram (ECG)

The ECG is an electrical signal generated by the heart’s muscular activity. It isusually recorded by a set of surface electrodes placed on the thorax. The number ofchannels depends on the application. For instance, it could be 1, 2, 3, 6, 12 or evenmore, in some cases such as in mapping protocols (e.g. 64 or 256 channels).

Generally speaking, the ECG provides a useful tool for monitoring a patient,basically when the purpose consists in detecting irregular heart rhythms or prevent-ing myocardial infarctions.

A typical ECG beat mainly has 5 different waves (P, Q, R, S and T), as shown inFig. 1.5. These waves are defined as follows:

– P wave: this corresponds to atrial depolarisation. Its amplitude is usually lowerthan 300 �V and its duration is less than 0.120 s. Furthermore, its frequency mayvary between 10 and 15 Hz,

– QRS complex: This is produced after the depolarisation process in the right andleft ventricles. Its duration usually varies from 0.070 s to 0.110 s and its amplitude

Fig. 1.5 Normal heart beats showing some essential indicators, generally measured by cliniciansfor the purpose of diagnosis


is around 3 mV. It should also be pointed out that the QRS complex is often usedas a reference for automatic heart beat detection algorithms,

– T wave: this low frequency wave corresponds to the ventricular polarisation;– ST segment: this corresponds to the time period during which the ventricles

remain in a depolarised state,– RR interval: this interval may be used as an indicator for some arrhythmias,– PQ and QT intervals: they are also used as essential indicators for diagnostic

purposes.

As it is well known, heart rhythm varies according to a person’s health (fatigue,effort, emotion, stress, disease etc.). For instance, in the case of cardiac arrhyth-mias, one can emphasize the following types: ventricular, atrial, junctional, atrio-ventricular and so on [6]. Special cases, including advanced processing techniqueswill be presented in Chaps. 2, 3, 4 and 5.

1.2.2 The Electroencephalogram (EEG)

The EEG is a physiological signal related to the brain’s electrical activity. Its varia-tion depends on numerous parameters and situations such as whether the patient ishealthy pathological, awake, asleep, calm and so on. This signal is recorded usingelectrodes placed on the scalp. The number of electrodes depends on the applica-tion. Generally speaking, the EEG may be used to detect potential brain dysfunc-tions, such as those causing sleep disorders. It may also be used to detect epilepsiesknown as “paroxysmal” identified by peaks of electrical discharges in the brain.

A considerable amount of the EEG energy signal is located in low frequencies(i.e., between 0 and 30 Hz). This energy is mainly due to five rhythms, namely, �, �,�, � and �. They are briefly described as follows:

1. δ rhythm: consists of frequencies below 4 Hz; it characterizes cerebral anomaliesor can be considered as a normal rhythm when recorded in younger patients,

2. θ rhythm: having a frequency around 5 Hz, it often appears amongst children oryoung adults,

3. α rhythm: generated usually when the patient closes his eyes, its frequency islocated around 10 Hz,

4. β rhythm: frequencies around 20 Hz may appear during a period of concentra-tion or during a phase of high mental activity,

5. γ rhythm: its frequency is usually above 30 Hz; it may appear during intensemental activity, including perception.

Above 100 Hz, one can note that the EEG energy spectrum varies according to a1/f function, where f stands for the frequency. When recording the EEG signal peaksand waves may appear at random epochs (e.g. with cases of epilepsy). Moreover, itis important to note that other biosignals may interfere with the EEG signal during


(a)

(b)

(c)

Fig. 1.6 Recorded EEG signals: (a) From a healthy patient (eyes open) (b) from a healthy patient(eyes closed) – (c) from an epileptic patient

the acquisition phase (e.g. ECG or EMG). The amplitude of the EEG signals variesfrom a few micro volts up to about 100 �V. As mentioned above, the number ofelectrodes required for the acquisition depends on the application. For instance, insome applications, a standard such as 10–20 system may be used.

For the purpose of illustration, some EEG signals recorded from a healthy patient(eyes open, eyes closed) as well as for an epileptic patient are presented in Fig. 1.6.Additionally, the reader can refer to Chaps. 6, 7 and 8 for advanced processedtechniques.

1.2.3 Evoked Potentials (EP)

When a sensory system is stimulated, the corresponding produced response is called“Evoked Potential” (EP). Nervous fibres generate synchronized low-amplitude


action potentials, also called spikes. The sum of these action potentials providesan EP that should be extracted from the EEG, considered here as noise. Generally,EPs are used to diagnose various anomalies such as auditory or visual pathways (seealso Chaps. 9, 10, 11 and 14).

There are three major categories of evoked potentials:

1. Somatosensory evoked potentials (SEP): these are obtained through musclestimulations,

2. Visual evoked potentials (VEP): for which a source of light is used as astimulus,

3. Auditory Evoked Potentials (AEP): they are generated by stimulating the audi-tory system with acoustical stimuli. In Fig. 1.7, we represent a simulated signalshowing a Brainstem Auditory Evoked Potentials (BAEP), thalamic sub-corticalpotentials and late potentials (cortical origin).

Fig. 1.7 Simulated Auditory Evoked Potentials. BAEP (waves: I- II, II, III, IV and V); Thalamicand sub-cortical potentials (waves: N0, O0, Na, Pa, and Nb); Late potentials (waves: P1, N1, P2and N2)

1.2.4 The Electromyogram (EMG)

EMG is a recording of potential variations due to voluntary or involuntary muscleactivities. The artefact’s amplitude (about 5 �V) resulting from muscular contrac-tion is superior to that of the EEG and the time period varies between 10 and 20 ms.This signal can be used to detect some specific abnormalities related to the electricalactivity of a muscle. For instance, this can be related to certain diseases including:

• muscular dystrophy,• amyotrophic lateral sclerosis,• peripheral neuropathies,• disc herniation.


5 10 15 20 25 30–2

–1.5

–1

–0.5

0

0.5

1

Time (s)

Nor

mal

ized

am

plitu

de

(b)(a)

Fig. 1.8 Arm EMG acquisition, (a) electrods position, (b) corresponding recorded signal for aperiodic “open-close” hand movement

• From Fig. 1.8(a,b), we show respectively an example of an arm muscle EMG andits corresponding recorded signal due to a periodic “open/close” hand movement.(See also Chaps. 12 and 13).

1.3 Some Comments. . .

Generally speaking, an efficient biomedical engineering system requires a particu-lar optimization of its various components which might include both software andhardware. For this purpose, implementing advanced signal processing algorithms insuch a system becomes interesting, mainly if the following aspects are taken intoaccount:

1. Acquisition system: its performance, its power consumption and its size, areregarded as essential technical features,

2. Processing algorithms: their efficiency is clearly important, but what about theircomplexity? Can the processing be achieved in real-time?

3. Processing system: which platform is best suited for a given algorithm? One witha mono-processor or a multi-processor? Should the algorithm be implemented ina mobile system? In such cases, what about power consumption?

4. Transmission system: does the application require a real-time transmission ofdata? Which protocol should be used? Does one have enough bandwidth? Shouldwe compress data [4]? Is there any protection against channel errors?

5. Data security: since we deal with medical signals, how should one protectthe data? Is any watermarking or encryption required? What about the locallegislation?

In addition, another non-technical aspect should be taken into account. It essen-tially concerns the “development cost”. This important financial considerationshould never be overlooked!


1.4 Conclusion

We have tried throughout this first chapter to provide the reader with the basicsof biopotential acquisition systems as well as some common general properties ofECG, EEG, EPs and EMG signals. As one will notice, no specific cases have beenevoked. This can be explained by the fact that some of them will be subject toadvanced analysis and study in subsequent chapters.

Finally, we advise the reader to pay special attention to the references proposedby the authors at the end of each chapter.

References

1. Dobrev D, Neycheva T and Mudrov N (2005) Simple two-electrode biosignal amplifier Med.Biol. Comput. 43:725–730

2. Huhta J C and Webster J G (1973) 60 Hz Interference in Electrocardiography. IEEE Trans.Biomed. Eng. 20:91–101

3. Metting-Van-Rijn A C, Peper A A et al. (1990) High-quality recording of bioelectric events.Part 1 Interference reduction theory and practice. Med. Biol. Comput. 28:389–397

4. Naıt-Ali A and Cavaro-Menard C (2008) Compression of biomedical images and signals.ISTE-WILEY

5. Feynman R P, Leigthon R B et al. (1964) The Feynman lectures on physics. Addison-Wesley,Boston, MA

6. Sornmo L and Laguna P (2005) Bioelectrical signal processing in cardiac and neurologicalapplications, Elsevier Academic Press, New York

7. Spinelli E M and Mayosky M A (2005) Two-electrode biopotential measurements: power lineinterference analysis. IEEE Trans. Biomed. Eng. 52:1436–1442

8. Thakor N and Webster J G (1980) Ground free ECG recording with two electrodes. IEEETrans. Biomed. Eng. 20:699–704

9. Webster J G (1998) Medical instrumentation Application and Design, Third Ed.10. Winter B B and Webster J G (1983) Driven-right-Leg Circuit Design. IEEE Trans. Biomed.

Eng. 30:62–6611. Wood D E, Ewins D J et al. (1995) Comparative analysis of power line interference between

two or three electrode biopotentials amplifiers. Med. Biol. Comput. 43:63–68

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