Biomedical Signals & Systems

Agung W. Setiawan

Signal?

Biomedical Signal• Biomedical signal as some

– natural (endogenous) or manmade (exogenous), – continuous, – time-varying

record that carries information about the internal functioning of a biomedical system. • A signal can be

– a system input, or – a system output

as the result of one or more inputs, or carry information about a system state variable. • In physiological systems, a signal can be

– an electrical potential, – a force, a torque, – a length or pressure, or – a chemical concentration of ions or of molecules including hormones or cytokines. – in the form of nerve impulses that lead to the contraction of muscles or the release of

neurotransmitters or hormones. • In an optical system, a signal can vary with position (x, y, z) as well as with time, t, and

wavelength, λ.

Biomedical Signal

• Acquired by a sensor, a transducer, or an electrode, and is converted to voltage / current for processing and storage.

• Endogenous signals are continuous (analog). • Example

– ECG, – Blood velocity in an artery measured by Doppler ultrasound.

• Biomedical signals are invariably noisy because of– Interfering signals from the body, – Noise picked up from the environment, – Noise arising in electrodes and from signal conditioning

amplifiers.

Hormones as Biomedical Signal

• Hormones – A type of physiological signal. – Quantified by its concentration in a compartment,

• such as the blood or extracellular fluid.

• Usually used as control substances as part of a closed-loop physiological regulatory system.

• Example– The protein hormone insulin,

• secreted by the pancreatic beta cells, • acts on cells carrying insulin receptor molecules on their cell membranes to

increase the rate of diffusion of glucose from the blood or extracellular fluid into those cells.

• [Northrop, 2000].

Analog Biomedical Signal

• After initial acquisition and conditioning, analog signal may be converted to discrete form by ADC.

• In discrete form, a signal can be more easily– stored – processed numerically by

• discrete filtering or • other nonlinear discrete transforms.

• Discrete signals are very important because of – the ability of DSP algorithms to reveal their properties in

the time, frequency and joint time-frequency domains

Signals from physiological systems• Endogenous biomedical signals from physiological systems are acquired for

a number of reasons:– For purposes of diagnosis– For postsurgical intensive care monitoring– For neonatal monitoring– To guide therapy and for research

• Such signals include, – ECG, – EEG, – EMG, – nerve action potentials, – muscle force, – blood pressure, – temperature, – respiration,

• Signals can also be rates or frequencies derived from other signals; e.g., – heart rate and – respiratory rate.

– hemoglobin psO2,

– blood pCO2, – blood glucose concentration, – concentrations of various hormones & ions in body

fluids, – heart sounds, – breath sounds, etc.

Frequency of Biomedical Signal

• In general, the frequency of endogenous physiological signals range from nearly dc (1.2x10-5 Hz or 12 µHz, a period of 24 h) to several kHz.

• This apparent low frequency is offset in many cases by massively parallel and redundant signal pathways in the body (as in the case of motor neurons innervating muscles).

Nonstationary• Signals from physiological systems have another property nonstationary (NS). • NS The physical, biochemical and physiological processes that contribute to their

origins change in time. • Example: Arterial blood pressure (ABP).

– The ABP has a waveform with the almost-periodic rhythm of the heartbeat. • Many physiological factors affect the heart rate and the heart’s stroke volume; • The body’s vasomotor tone is under control by the autonomic nervous system.

– The time of day (diurnal rhythm), emotional state, blood concentration of hormones (epinephrine & norepinephrine), blood pH, exercise, respiratory rate, diet, drugs, blood volume and water intake all affect the ABP.

• Over a short interval of several minutes, ABP waveform is relatively invariant in shape and period short-term stationary (STS).

• In fact, many physiological signals can be treated as STS; others change so rapidly that the STS assumption is not valid. – For example, certain breath sounds which change from breath to breath should be treated

as NS.

Signals from man-made instruments

• Energy (photons, sound, radioactivity) is put into the body to measure physiological parameters and structures. – Doppler ultrasound, used to estimate blood velocity in

arteries and veins, (5 to 10 MHz). – The transducers, filters, amplifiers, mixers, etc., used in a

Doppler system must operate in the 5 to 10 MHz range.

• The blood velocity Doppler signal itself lies in the audio frequency range [Northrop, 2002].

Some ways to describe signals• There are many ways to characterize 1-D & 2-D signals,

– signals that vary as a function of time, or – spatial dimensions x and y.

• A signal can be described in terms of its – statistical amplitude properties, – its frequency properties and, – if non-stationary, – its time-frequency properties.

• The signal itself can be– a voltage (ECG record), – a chemical concentration (e.g., calcium ions in the blood), – a fluid pressure (e.g., blood pressure), – a sound pressure (e.g., the first heart sound), etc.

System• A set of interacting or interdependent components forming an

integrated whole or a set of elements and relationships which are different from relationships of the set or its elements to other elements or sets.

• Some systems share common characteristics, including:– A system has structure, it contains parts (or components) that are

directly or indirectly related to each other;– A system has behavior, it contains processes that transform inputs into

outputs (material, energy or data);– A system has interconnectivity: the parts and processes are connected

by structural and/or behavioral relationships.– A system's structure and behavior may be decomposed via subsystems

and sub-processes to elementary parts and process steps.• The term system may also refer to a set of rules that governs

structure and/or behavior.http://en.wikipedia.org/wiki/System

• The common feature of all systems is that each one is formed from a collection of interconnected, interacting, interdependent, dynamic elements.

• The elements can be physical entities such as – neurons, – mechanical components (springs, masses, dashpots), – electronic circuits (resistors, capacitors, op amps, etc.), or – abstract, causal relations such as those found in

economics.

• A system can be continuous (i.e., analog) or discrete (i.e., digital).

System

System & Signal Processing• An area of systems engineering, electrical engineering and applied mathematics

that deals with operations on or analysis of analog as well as digitized signals, representing time-varying or spatially varying physical quantities.

• Signals of interest can include sound, electromagnetic radiation, images, and sensor readings, for example biological measurements such as electrocardiograms, control system signals, telecommunication transmission signals, and many others.

• The goals of signal processing can roughly be divided into the following categories.– Signal acquisition and reconstruction, which involves measuring a physical signal,

storing it, and possibly later rebuilding the original signal or an approximation thereof.• For digital systems, this typically includes sampling and quantization.

– Quality improvement, such as noise reduction, image enhancement, and echo cancellation.

– Signal compression (Source coding), including audio compression, image compression, and video compression.

– Feature extraction, such as image understanding and speech recognition.

http://en.wikipedia.org/wiki/Signal_processing

Biology & Medicine System• In biology and medicine, many

systems can be identified. • It is important to realize that

none of these systems is isolated; all physiological systems are interconnected to some degree.

• Each system has identifiable components which interact.

• Each also has one or more inputs (excitations or commands) and one or more parameters which can be considered to be outputs (responses).

• Some of these inputs and outputs are observable signals, others are unobservable.