Obrázek1 1 •Biosignals and their processing •Thermometry Lectures on Medical Biophysics Dept. of Biophysics, Medical faculty, Masaryk University in Brno Obrázek1 2 What is a biosignal? ØDefinition: a biosignal is a human body variable that can be measured and monitored and that can provide information on the health status of the individual. In most cases it is an electric voltage. Ø ØExamples: –EKG (ECG): a V(t) biosignal which provides information on cardiac physiology / pathology –A US image: small voltage arising in elementary transducer by receiving reflection from tissue interface. –A CT tomogram: a m(x, y) biosignal for which the attenuation coefficient value is measured for each patient voxel at the position (x,y) in a slice of patient. –A 3-D MRI image: a SD (x,y,z) biosignal for which the hydrogen spin density (SD) is measured for each patient voxel at the position (x,y,z) in the patient each. SEAM%20Figure_4 ECG logo_mri Obrázek1 3 Types of Biosignals •ACTIVE (body generated) biosignals: the energy source for measurement derives from the patient himself (“internal source”) •Electrical active biosignals (known as BIOPOTENTIALS) e.g., EKG, EEG, EMG, ERG (electroretinogram) (ERG), EGG (electrogastrogram) etc •Non-electrical: e.g., temperature, blood pressure •PASSIVE (body modulated) biosignals: the energy source is from outside the patient (“external source) e.g., X-ray in CT •In this lecture we discuss active biosignals only Obrázek1 4 Origin of Biopotentials ØCells transport ions across their membrane leading to ion concentration differences and therefore charge differences - hence generating a voltage. ØMost cell groups in the tissues of the human body do not produce electric voltages synchronously, but more or less randomly. Thus most tissues have a resultant voltage of zero as the various random voltages cancel out. ØWhen many cells produce voltages synchronously the resultant voltage is high enough to be measurable e.g., EMG - muscle fibre contraction, most cells of the fibre perform the same electric activity synchronously and a measurable electric voltage appears. Obrázek1 5 Instruments for Measuring Active Biosignals ØBiopotentials: measuring device consists of: –Electrodes: enable an electrical conductive connection between the examined body part with the measuring system –Signal processor (amplifier, ADC, electrical filters to remove noise, and unwanted frequencies etc) –Recorder (also called read-out device, today usually a computer monitor or a chart recorder) ØNon-electric active biosignals: electrodes are replaced with appropriate sensors electrodes 12 Two types of disposable ECG electrodes Medical Temperature probes Medical temperature sensors Obrázek1 6 Monitoring Biosignals in an Intensive Care Unit gesu_02_img0124 If a new drug were as effective at saving lives as Peter Pronovost’s checklist, there would be a nationwide marketing campaign urging doctors to use it. nk_monitor Obrázek1 7 Electrodes for Biopotentials: contact Voltage Problems • •Problem: electrodes produce ‘contact voltages or contact potentials’ when put in contact with body! Polarisable electrodes produce variable contact voltage (via an electrochemical reaction) and hence are not suitable for accurate measurements. Non-polarisable electrodes produce a constant contact potential and hence are used when accurate measurements are required. Electrodes should be made of noble metals (metals which resist corrosion and oxidation). •Non-polarisable electrode: accurate measurements of biopotential. In practice, the silver-silver chloride (Ag-AgCl) electrode is most often used. •Polarisable: the contact voltage varies with movement of patient, humidity (sweating), chemical composition of ambient medium etc. ØConcentration polarisation: the concentration of ions changes around electrodes due to electrochemical processes. ØChemical polarisation, gases are liberated on the surface of the electrodes. Obrázek1 8 Non-polarisable Ag-AgCl electrode electrode Obrázek1 9 Electrodes for Biopotentials: Electrode Sizes ØMacro or Microelectrodes. Latter used for biosignals from individual cells. Small tip diameter (<0.5 m) and made of metal (polarisable) or glass (non-polarisable). The glass microelectrode is a capillary with an open end filled with an electrolyte of standard concentration. • ØSuperficial or needle electrodes. Superficial electrodes are metallic plates of different shape and size. Good electric contact is ensured by a conducting gel. Their shape is often dish-like (see the Ag-AgCl electrode in the previous slide). Needle electrodes are used for recording of biopotentials from a small area of tissue. Used mainly for muscle biopotentials or long-term recording of heart or brain potentials. Obrázek1 10 Bipolar and Unipolar Electrode Pairs •Bipolar electrode pair – both are placed in the electrically active region. • •Unipolar electrode pair, one electrode has a small area and is placed in the electrically active region. The second electrode (usually with a large area) is placed in an electrically inactive region (this electrode is called ‘indifferent’). •Exclusion: Wilson central terminal used in EKG. lead1 A bipolar ECG electrode pair – depiction of the 1st limb lead – see also next slide Obrázek1 11 Signal processing: Amplifier ØA high-fidelity (HiFi) amplifier is one which amplifies the biosignal without changing its shape (distortion) at different frequencies. The amplifiers of modern medical devices must fulfil this condition. ØGain (amount of amplification) of an amplifier in dB •gain = 20log(Uo/Ui) •o = output i = input • •The user deals only with a correct setting of different filters (to supress some artefacts). • • Obrázek1 12 ECG - electrocardiogram ØECG (EKG) is the strongest and most often measured active biopotential. ØIn Europe 3 electrodes are placed on extremities (2 on arms, 1 on left leg), 6 electrodes are placed on chest. The right leg is used for an electrode which partially removes interfering voltages. ØA pair of electrodes between which a voltage is measured, is called a lead. Every lead gives info on different parts of the heart. ØThe chest leads are formed by chest electrodes and the so-called Wilson central terminal (connected limb electrodes, in principle) Calibration 1mV voltage impulse Obrázek1 13 Einthoven triangle image006 Heart is modeled as a source of dipole electric field limbleads Placement of triagle vertices (electrodes) can be a problem! Or not? Obrázek1 14 Multiple electrodes placed on the surface of the body allow us to calculate voltage values throughout the torso (V (x,y,z) biosignal). Thus, we can localise problems with stimulus conduction throughout the myocardium. 2D and 3D Biopotential images (an example of another way how to record heart activity) Obrázek1 15 EEG Ø-waves: f = 8-13 Hz, amplitude (A) max 50 V. Body and mind at rest. Ø-waves: f = 15 - 30 Hz, A = 5 - 10 V. Healthy people at full vigilance. Øϑ-waves: f = 4 - 7 Hz, A > 50 V. Physiological in children, in adults pathological. Ø-waves: f = 1 - 4 Hz, A = 100 V. Occurs in deep sleep under normal circumstances. In vigilance pathological. •In EEG record, some other patterns of electric activity can appear, characteristic of different brain diseases e.g., spike-wave complexes in epilepsy. •Brain biopotentials can be both spontaneous and evoked. Evoked potentials can be caused by sensory stimuli (vision, audition) or by direct stimulation by e.g. magnetic fields. 1306 eeg2 Note: The frequencies and amplitudes of individual waves are rather different in various resources! They also depend on the age of the patient. Obrázek1 16 Colour Brain Mapping: V (x,y,z) biosignal (the colours represent intensity of electric activity of individual parts of the brain) http://www.mybrainmap.com.au/wp-content/uploads/2016/09/1.jpg Attention Deficit Hyperactivity Disorder (ADHD) Obrázek1 17 Anaesthesia: The EEG and the Bispectral Index (optional) ØThe Bispectral index monitor is a neurophysiological monitoring device which continually analyses a patient's electroencephalograms during general anaesthesia to assess the level of consciousness (too little anaesthetic and patient remembers, too much leading to brain damage). The essence of BIS is to take a complex signal (like the EEG), analyse it, and process the result into a single number which can be easily monitored. The BiS is the bottom trace. Obrázek1 18 Comments on BiS etc. (optional) The Bispectral Index is an example of a “descriptive indices”. These are not real physical quantities. They are calculated from many measured parameters and by searching knowledge databases which contain measurements of many different patients (of various ethnic origins) with different health status. Complete algorithms of calculations and contents of knowledge databases are producer secrets. The doctor needs only get acquainted with meaning of the respective index and the values which it can have, but it is not necessary to know how it is calculated. It is usually enough to give some information about the patient for the computer to correctly search in the knowledge databases. It is almost always necessary to enter age, sex, race, body height and mass. There are sometimes questions about e.g. length of fingers or toes. Such “strange questions” are frequent in monitoring of the cardiovascular system. However, they can be important. When the answers are omitted, the software can use an incorrect statistical patient model and an incorrect index value will be displayed. Obrázek1 19 Artefacts •Definition: features of signals not arising from the target tissue •They arise from electromagnetic waves in the environment (e.g., 50Hz electricity supply, mobile phones), patient movement, patient sweating etc. Obrázek1 20 EKG Artefacts Tremor ACInterference 50Hz AC superimposed on the EKG Muscle tremors WBaseline Moving baseline from patient movement, dirty electrodes, loose electrodes http://mauvila.com/ECG/ecg_artifact.htm Obrázek1 Switched limb leads přehozené končet přehozené hrudní svody Switched chest leads Doctor must be able to recognise it!!!!!! Obrázek1 22 Some EEG artefacts Pulse wave artefact: movement of electrode arising from patient pulse under the electrode. EKG signal artefact: EKG signal also picked up by the EEG electrodes. Both easily recognized because they are periodic. http://www.brown.edu/Departments/Clinical_Neurosciences/louis/artefct.html Obrázek1 23 Temperature Measurement “If a part of the human body is warmer or even colder than the surrounding parts, it is necessary to look for the disease focus in this place” Hippocrates Obrázek1 24 Main purposes of temperature measurements Ømonitoring of ill patients Ømonitoring of physiological reactions Ømonitoring of hyperthermia treatment Important specifications of thermometers: Øaccuracy Øresponse time (determined by heat capacity of the sensor and its conductivity) Obrázek1 25 1. Point temperature measurement – measurement of temperature at individual points in the body Ø Contact •Dilation thermometers based on expansion (mercury and alcohol thermometers) • Digital thermometers based on thermistor sensors (resistance of the thermistor changes with temperature) •Digital thermometers based on thermocouple sensors (voltage produced varies with temperature) Ø Contactless (ear tympanic thermometer) Ø 2. Temperature distribution on the surface of the body (thermography) Ø Contact (use of sensors placed on skin) Ø Contactless – IR camera (other lecture) Types of Thermometry in diagnostics Obrázek1 26 Mercury-in-glass thermometer gives maximum temperature Its capillary is narrowed to avoid return of mercury into the reservoir. Disadvantage: long response time (long time necessary for a stable reading 3 - 5 min.) Medical high-speed thermometer: Alcohol filled – the capillary is not narrowed, the temperature must be read during the measurement, response time 1 min. Dilatation thermometers (i.e., based on expansion of some substance) Obrázek1 27 Digital Thermometers Obrázek1 28 B00008BFSZ Removable hygienic tip Tympanic (ear) thermometer geniuspic They are based on the measurement of infra-red radiation which is emitted from the ear drum. The temperature reading is obtained only 1 second after attachment of the sensor to the distal end of the acoustic meatus. Obrázek1 Physical principle of the temperature determination based on measurement of infrared radiation •Stefan-Boltzmann law – dependence of the so-called spectral density of a black body radiation on temperature 29 http://www.qtest.cz/bezdotykove-teplomery/img/princip-mereni/teorie_fig03.gif Wavelength Obrázek1 30 Digital Thermometers: Thermistor sensor based R – resistance temperature in Kelvin T Ro – resistance at temperature To B – constant Obrázek1 31 Digital thermometers: Thermocouple sensor based digital-thermocouple Digital thermocouple sensor nbtc_bar2_02 Thermovoltage U = a(t – t0) Obrázek1 Authors: Content collaboration and language revision: Vojtěch Mornstein, Jan Dvořák, Věra Maryšková Carmel J. Caruana, Ivo Hrazdira