Any medico biological research is connected with receiving and registration of the relevant information. Despite a variety of the devices and methods used for this purpose it is possible to specify their general schemes and the principles of action. The questions considered in the head partially belong to cybernetics.
- Main body.
Block diagram of removal, transfer and registration of medicobiological information.
To receive and record information on a state and parameters of medicobiological system, it is necessary to have the whole set of devices.
The primary element of this set – sensor means measuring device called detachably – certainly contacts or interacts with the system and the remaining elements are generally apart from biomedical systems, in some cases, part of the measuring system can be allocated even over long distances from the object of measurement.
This system is a common and represents all possible real systems used in medical diagnostics and research. In medical electronics devices, sensor or directly outputs an electrical signal, or a signal such changes under the influence of the biological system. Thus, the pickup device converts the information medico-biological and physiological signal content in an electronic device. In medical electronics are two kinds of device removal: the electrodes and sensors. The final element of the measurement circuit is a measuring instrument that displays and logs information about a biological system in a form accessible to direct perception of the observer.
In many cases, between the unit pickup and measuring instruments there are elements that increase the initial signal and transmits it to a distance.
Classification of devices of removal.
Device removal provides medical information signals associated with various phenomena occurring in the living body-removal devices transitional link between the test organism and subsequent signal amplification devices, their display, registration, communication channel, handling, etc.
Subsequently basically be considered pickup device, an output signal which is an electric signal. We can say, then, that the removal device has a sort of translator information “physiological language” of the body in the “language of the electronic device which is indispensable to all subsequent stages of the conversion of the initial information.
The problem of removal of medical information is the first that inevitably occurs when contact with a doctor and an engineer at the intersection of medicine and technology. It is difficult to give priority in its decision a doctor or an engineer. For the successful design and implementation of device removal requires a deep knowledge of physiology, anatomy, physics, chemistry, features and operation of various electronic equipment. In order to obtain reliable results when measuring and recording, objectively reflect the essence of the processes occurring in the body, to make the right conclusion in the diagnosis, the doctor must be clear principles and methodology for the application of information retrieval devices, the advantages and disadvantages of different designs.
All medical parameters to be measured and recording can be divided into two groups: measured directly; measured indirectly.
The first group includes, for example, the movements associated with heart rate, body temperature, bioelectric potentials. Removing these values by means of the devices that are directly perceived by the test organism changes of various parameters: the mechanical movement, temperature, electric potential.
The second group of options are those that by themselves can not be measured, or the holding of such measurement is difficult, but the changes that lead to changes in other indicators more suitable for measuring. In other words, a doctor interested in certain aspects of life of the organism is estimated indirectly by measuring some auxiliary variables. For example, the electrical resistance of a certain area of the body may be interested in the physician as a particular characteristic of blood filling of the site; varying the degree of light absorption by tissue may be associated with changes in the volume of the organ or body part. Obviously, for the parameters measured indirectly, it is required to pay particular attention to the initial establishment of relationships between parameters of interest to the doctor, and actually measured by indirect indicators.
Regardless of the specific technical implementations removal devices can present a number of general requirements. They should ensure that: obtaining sustainable informative signal; minimum distortion of the desired signal; maximum noise immunity; ease of placement in the desired measurement location; no side – irritation or other – effects on the body; serializable (without changing characteristics) and reusable.
The whole set of different devices removal of medical information it is advisable to be divided into two large groups electrodes and transducers (converters).
Electrodes – is a special form of the conductors, by which part of the circuit composed of wire connected to another portion of the chain non-metallic conductivity type (e.g., with a particular part of the body, the body skin surface, and etc.). The electrodes are most often used for the removal of the electrical signal, actually existing in the test organism. They merely serve as a contact in a circuit performing retraction electrical signal with a certain degree of loss depends on the quality of the contact between the electrode and the part of the body with which it contacts. Depending on the type of tissue (or organ), bioelectrical activity that explores distinguished: electrocardiography (ECG) – a study of the electrical activity of the heart; electroencephalography (EEG) -Studies brain bioelectrical activity, electromyography (EMG) – an analysis of the electrical activity of muscles; electrooculography (EOG) – Research potential changes caused by the movement of the eyeball; electrogastrography (EGG) – the analysis of the variation of the electrical signals caused by the motor activity of the stomach..
In principle, the above list of methods of research of bioelectric activity of various organs can be expanded.
In some cases, the electrodes can not be used for removing electric potentials actually available in the living body, and for applying to the body a certain external electric impact.
Such a situation occurs, for example, elektropletizmografii (rheography), by which examines fluctuations in the intensity of the blood flow in an organ or blood vessel. Alternating voltage sufficiently high frequency (30-250 kHz) is applied via electrodes to the body site under investigation. By measuring the variation of the total electrical resistance (impedance) tissue, including active and reactive component, you can judge the nature of the changes of blood filling the tissues.
To the electrodes, as an element removal of biomedical information, typically unique requirements: they must be fixed and quickly removed, have a low cost, high stability of electrical parameters, elasticity at a sufficient mechanical strength, not to give the artifacts and noise, non-irritating.
Sensor- (converter health information) – information retrieval device, which responds to its impact on the sensitive element of the measured value and the ongoing transformation of this impact in a form suitable for subsequent amplification, recording, processing, etc.
The type and design of the sensor depends on the type of conversion necessary, te. Determined by the specific non-electrical physical representation of the input signal and the output electrical signal, and also depend on the operating conditions of the sensor.
Nonelectric input values of sensors can be mechanical quantity (the linear and angular displacement, velocity, acceleration, pressure, frequency), physical (temperature, light, humidity), chemical (concentration of the substance composition) directly physiological (blood content of the tissue).
As output electrical quantities current, tension, full resistance (impedance), frequency (or a phase) alternating current or pulse signals usually serve.
Sensors of medicobiological information can be divided into two groups: biooperated and power.
The biooperated sensors change the characteristics directly under the influence of the medico-biological information arriving from object of measurement. In turn the biooperated sensors are subdivided into active (generating) and passive (parametrical).
The active sensor measured value is directly converted into an electrical signal, i.e. under the effect of the measured active sensors themselves generate a signal corresponding to the amplitude or frequency. These sensors are piezoelectric, inductive transducers, thermocouples.
Passive sensors under the influence of the input variable change their electrical parameters: resistance, capacitance or inductance. Unlike active (generating) passive sensors (parametric) sensors to obtain a corresponding output voltage or current in an electrical circuit includes an external power source. These sensors include capacitive, inductive, resistive probes.
Energy sensors, unlike the radio-controlled actively affect the organs and tissues. They create in the test body called an unmodulated energy flow with strictly defined, constant performance over time. Measured parameter influences the characteristics of the flow, it modulates the proportional change of the parameter. Energy Information converters require additional energy source for influencing the object and establishing the energy flow is not modulated. Because such probes can be noted for example, photoelectric and ultrasonic.
Each sensor is characterized by certain metrological parameters. The most important are: Sensitivity – the minimum change removes the parameters that can be consistently detected by this converter; dynamic range – the range of input values, the measurement of which is carried out without noticeable distortion of the maximum limit value to the minimum, or the limited sensitivity threshold level of interference; error – the maximum difference between the received and the nominal output values; reaction time – a minimum period of time during which the installation of the output value at a level corresponding to / changes in the level of the input variable.
Below sensors of temperature, parameters of system of breath, cardiovascular system and some other will be considered. The similar statement of material though does not correspond to the classification given here, it is represented preferable to relief of understanding of the considered problem. It is explained by the division of an organism into separate anatomic functional systems approved in classical physiology and clinical medicine: the blood circulatory system, system of breath, digestion etc. according to which it is convenient to divide devices of a removal of information on their state. As for electrodes, the description of these devices of a removal of information does not need division on separate anatomic systems of an organism.
Bioelectric signals that characterize the functional activity of various systems and organs are essentially low-current and occupy the low and infra-low frequencies.
In connection with this, the most important overall requirement for different electrodes, the requirement is a minimum loss of useful signal, especially in the transition resistance of the electrode – skin, which should strive to do the least.
The magnitude of transient resistance depends on the type of the metal material of the electrode properties of the skin, its contact area with the electrode and of conducting medium between them.
Contact resistance between the clean, dry skin and the electrode measured in hundreds of kilos. To reduce it between the skin and the electrode is usually laid gauze soaked in saline. This contact resistance is reduced to tens of kilos. In recent years, often using special conductive electrode paste, which gives a better result than simple electrolytes.
Contact resistance is also reduced with the increase in the area of the electrode-skin contact. However, the size of the area is determined by the required degree of localization of the source removes the biological signal. The larger the area of the electrode, the less information shall be removable signal localization of the lesion of the central nervous system with EEG, or changes in the electrical activity of the myocardium in the registration area of thoracic ECG leads. With increasing electrode area decreases noise immunity of the recording system. For example, in sports medicine ECG signal recording biopotentials difficult working muscles. To reduce these disturbances, tend ECG electrodes disposed in a region beneath the skin where there is no bone and muscle. It is a condition of placement in this case defines the electrode area. It is known that when a metal electrode is immersed in an electrolyte solution between the metal and there is some difference in potential (electrode potential). When current flows in the target this potential changes due to galvanic electrode polarization. The potential polarization dependent on many factors (the nature of the electrode, the electrolyte composition, temperature and so on.), And typically varies with time in a fairly wide range. By its size it can reach values greater than the value of the useful action potential. All this may eventually lead to the fact that the electrode polarization effect substantially distorts the shape of the detected signal, and in some cases make it impossible to check. Therefore, the polarization of the electrode is extremely undesirable phenomenon. It can be reduced by selection of the electrode material and electrode paste composition.
There are many types of metal electrodes. As a material used for manufacturing electrodes of gold, platinum, silver, palladium, stainless steel, alloys with iridium and other metals, alloys and chemical compounds, and researchers are often used not only different but also differently treated metals. Therefore, the subject of ongoing debate is the question of the influence of metal and a method of processing the nature of the results. Without going into details of this discussion, we note that by using different electrodes can be set to:
- a) the difference in contact resistance, given by the different types of electrodes; while essential here is the value of the input resistance (impedance) of the amplifying device;
- b) the difference in noise levels at the interface “electrode – tissue” and in terms of artifacts (noise) due to the movement of the electrode relative to the surrounding electrolyte, or due to the difference of corrosion current;
- c) the particular action of the metal electrodes to the tissue at the site of eavesdropping and the difference in reaction of the metal to change the content of chemicals in the surrounding tissues (due to redox reactions involving substances in tissues);
- e) the difference in the intensity of the polarization potentials.
In experiments with EEG recording, processed by computer, it is shown that the error register associated with the choice of metal and electrode processing method, does not exceed 10%, but can significantly increase when the amplifier input impedance below 107 ohms. Thus the best effect is obtained by using electrodes made from silver chloride. Such electrodes are prepared either by electrolytic chlorination silver or mixtures of silver powder pressing and chloride.
Designs and characteristics of electrodes depend in many respects on the purposes of their application. To destination electrodes can be divided into four groups:
- for single use (in the offices of functional diagnostics, etc.);
- for long, continuous monitoring bioelectric signals (in a reanimation, intensive care);
- for dynamic observation (in terms of physical activity in the wards of rehabilitation, sports medicine);
- for emergency use (in terms of emergency care, ambulance).
For short-term relief of thoracic ECG leads use a so-called electrode-sucker. This electrode is provided with a rubber bulb which enables simple and reliably fasten an electrode in the actual desired location thorax. However, such an electrode can not be used for continuous registration due to lack of integrity, loss of suction force of a rubber balloon and possible bleeding into the skin and subcutaneous tissue.
Needle electrodes until recently used only during surgery, but is now increasingly used for ECG and EEG systems continuous monitoring of critically ill injectable electrodes of thin platinum wire are easy to use and does not irritate the skin. However, their use is associated with the violation of the skin and the possibility of infection in the channel electrode, although in practice the infection is very rare.
Further development of such electrodes are flexible screw electrodes (subcutaneous electrodes). Actually, an electrode made of high-grade steel, is located in the injection cannula analogous mandrill. After the puncture cannula is removed, leaving the electrode under the skin. The protruding above the skin of the electrode used to connect the discharge wires. The electrodes are characterized by high stability ECG recording within a week or more in the absence of any signs of tissue irritation. Proposed spray-electrodes. Electrode system consists of three components: a special conductive emulsion that penetrates deep into the skin, the silver discharge contact with the cable and covering plate. When a multi-day continuous monitoring of ECG electrodes showed high noise immunity, good elasticity due to their flat shape. A disadvantage of such electrodes is the high cost because inability to reuse.
There are many designs of electrodes for direct fetal ECG through the open cervix.
It electrodes for fetal by Junge, a special needle electrodes by Suling, electrode terminals by Noi and a modified version by Klock. The most common electrode terminals that are attached to the presenting part of the fetus. The second point of abstraction are vaginal or cervical secretions mother. Actually, an electrode made from a mixture of silver and silver chloride, can be used repeatedly, it is easily sterilized. Fixing the electrode is connected with some difficulties, easily surmountable after a short workout.
The EEG electrodes are used, the contact area with the skin which does not exceed 1.1 – 1.5 cm2. This is due to the need for more accurate localization of the foci of the bioelectrical signal source. Actually, the electrode is a cylinder or a ball of a small diameter, which is wrapped with sponge rubber soaked in saline. In another construction of the electrode as a conductive medium is applied electrode paste. The electrode is formed of a cylindrical cup. For ease of manipulation when applying a discharge wire attached to the electrode via an elastic flat plug which is inserted in a circular groove of the electrode.
Biopotential electrodes for removal muscles are flat circular metal plate whose surface is coated with a thin layer of gold or silver chloride. The diameter of these electrodes does not exceed 10-15 mm.
All of these variations are intended to drain electrodes of bioelectric signals from the skin surface and require its pretreatment. This procedure, even with a small number of electrodes is time consuming, especially tangible in emergency conditions. In such circumstances, as electrodes are often used conventional injection needles and special electrode complexes. Structurally, these complexes are made in the form of an equilateral triangular plate of insulating material with a side of 15-20 cm, in the corners of which are located cone-shaped electrodes 1-2 cm in height. The plate is applied properly in the region of the heart and provides a reasonably good and reliable contact. Sometimes a set of electrode is designed as a base cardioscope miniature self-powered.
A simple and economical is a multi-point electrode – modified flat oval electrode to the “bottom surface of which are attached several small needles. The height of the needle is approximately equal to the thickness of the upper layers of the epithelium. Electrodes of this type have a small contact resistance. Imposing such an electrode accompanied by irritation of the skin, which leads to a significant increase in blood flow in the skin, which in turn contributes to further reduce the transition resistance. Multipoint electrodes are widely used in conditions of emergency. For long-term observations of these electrodes are hardly used because of the instability of the transition resistance.
For EEG recording in the experiment and in the clinic can be used three main types of needle electrodes: unipolar, bipolar and coaxial. Comparing the waveform recorded by these three electrode types showed that bipolar needle provides a waveform which is the time derivative of the signal allocated using unipolar needle and substantially reduces the noise level. The signal recorded by means of coaxial needles, identical to the signal reserved unipolar needle, but the noise level at the same time much less.
In recent years, EMG is growing interest as a means of obtaining information on the behavior of the operator in the man – machine. In this case it must be good electrical contact of the electrode with the tissue for a long time. Conventional needle or surface electrodes are not suitable for this purpose, mainly because they have a high impedance, especially at low frequencies. Increasing the area to reduce the impedance causes the electrode records the total activity of adjacent groups of muscles.
Developed subcutaneous electrode of glassy carbon, devoid described drawbacks. The material has high conductivity and is chemically more inert than other known forms of carbon. The properties of the material resembles a conductive ceramic. Structurally, the electrode is in the form of a coil. Its height is 5 mm, a diameter of 4 mm cheeks, the cylinder axis – 2.5 mm.
The development of electronics, the widespread use of field-effect transistors in the input stage bioamplifier made it possible to receive huge quantities of input impedance of the amplifier, reaching up to hundreds of billions of ohms (hundreds GOhm). Such values of the input resistors allow virtually eliminate signal loss on the transition skin – dry electrode and create a capacitive electrode. A capacitive electrode is a metal plate coated with a special technology of a thin dielectric layer (varnish or metal oxide film). The metal plate is a plate of the capacitor, and the surface of the skin – the other. Bioelectrical signal changes the potential difference between the capacitor plates changes proportionally to its magnitude. This potential difference between the electrodes applied to the input of the amplifier, which amplifies the signal to a desired value. To reduce the influence of external noise similar circuits are usually built directly into the electrode body. Application capacitance electrode does not require any special pre-treatment of the skin.
Body temperature sensors.
Distinguish or core temperature of the body core and the temperature of the skin surface of the human body. The surface temperature depends on the ambient conditions: air temperature and humidity, clothes of hair, blood supply to the skin, etc. The core temperature of the body is a more reliable indicator of the thermoregulatory system of the body. In medical practice, make measurements of both values. To assess the core temperature measured by the temperature in the individual muscles and organs, rectal temperature, the temperature in the mouth, armpit, groin and umbilical fossa. When measuring the surface temperature are important for the diagnosis symmetrical temperature field, which allow to evaluate the intensity of the blood supply to any part or segment of the human body. For measurement of body temperature sensors are used as wire and semiconducting thermistors and thermocouples. The basis of wire and resistor semiconductor sensors put their properties change resistance with temperature. The change in resistance is estimated by the temperature coefficient of resistance (TCR). TCS can be either positive or negative. PTC suggests that an increase in temperature, the sensor resistance increases. When NTC is the ratio will be reversed. These sensors can be classified as passive (parametric) biocontrolled sensors. The value of TCS characterizes the sensitivity of the sensor. High TCS have semiconductor thermistors.
The basis of thermoelectric sensors (thermocouples) works on the principle of thermoelectric generators. The essence of it is the following: if on the one hand to connect the ends of two metallic plates made of different metals, such as iron and constantan, and heating the junction, between the free ends of the plates can register the appearance of an electromotive force, which is called thermoelectric power. The magnitude of this temperature dependent thermoelectric junction plates, and hence it can be used to judge the temperature. Such sensors are active (generator) biocontrolled sensors.
Among the characteristics that determine the quality of the temperature sensors must be distinguished:
- a) The linearity of the resistance R of the temperature.
High linearity different wire gauges and thermometers;
- b) the reaction time
- c) dimensional stability with time
Sensors of parameters of system of breath.
An objective study of the respiratory physician interested in quantitative characteristics of breathing (respiratory rate, tidal volume іi exhaled air, and others.) And qualitative characteristics of the process of external and tissue respiration (containing gases in inhaled and exhaled air, the partial pressures of oxygen and carbon dioxide in the blood the percentage of blood oxygen saturation and so on. d.). Respiratory rate – one of the most important parameters that characterize the functional activity of the respiratory system. Every act of breathing movements of the chest appears (the change in the length of its circumference) and the formation of oppositely directed air flow on inhalation and exhalation, having different temperatures. Fixation points recurrent chest movements can most easily be accomplished using three types of passive sensors biocontrolled.
Sensors of parameters of cardiovascular system.
For evaluation of the cardiovascular system using characteristics such as heart rate, systolic and diastolic pressure, tones and noises of the heart, tissue impedance, high performance circulation.
To register the frequency of peripheral pulses were extended piezoelectric transducers that use the piezoelectric effect. The piezoelectric effect is the appearance of electric charges of different signs on opposite surfaces of certain crystalline materials when mechanical deformations (tensile, compression, bending, etc.)
Piezoelectric properties are natural materials (quartz and tourmaline) and synthetic materials (Rochelle salt, potassium dihydrogen phosphate, barium titanate, lead zirconate titanate, etc.). Quantitatively piezoelectric estimated piezomodulus d, shall be proportional to the size of the emerging relationship between the charge Q and the applied force P: Q = dP. Furthermore, piezoelectric characterized mechanical strength dependence pezomodu- A temperature and humidity environment. As a rule, artificial piezoelectrics are piezoelectric, many times larger than piezoelectric quartz, but have a much lower mechanical strength, the greater dependence of the parameters of temperature and humidity environment.
Sensors working on the basis of piezoelectric effect, are among the active (generator) biocontrolled sensors.
Errors of devices of removal of information.
Errors devices removal of biomedical information – one of the links in the overall chain of measurement errors, depending on a number of technical and specific reasons. This makes it difficult to compare the results in the process of diagnosis and treatment.
There are uncertainties associated with:
- a) change of physical parameters of environment;
- b) specific features of an organism (anthropometrical data, options of structure and structure of fabrics, distinction экзоэндо gene psychophysiological reactions);
- c) inexact performance of process of measurement of physical parameters of an organism (fastening, orientation and coordination of the sensor with object, processing of skin, etc.);
- d) imperfection of measuring system: the distortions brought by the device of a sjem, the measuring device (a measurement method error), a measuring path — errors of the amplifier, the registrar or the indicator, information processing, etc.
Now it isn’t possible to give the exact characteristic of separate components of a full error of measurement of physiological parameters.
At an assessment of quality of various sensors and their comparison it is necessary to consider their main properties, various sources of emergence of errors of sensors:
owing to inexact reproducibility of function of the converter. Characteristics of converters of this look in an ideal have to be completely identical that is dictated by the requirement of possible interchangeability of sensors, owing to inconstancy of function of transformation in time. Aging of materials of which sensors are manufactured, leads change of parameters of sensitive elements owing to course of processes of corrosion, wear of mobile parts, etc. to indirect change of a type of the equation of transformation, the group of errors in total defining a so-called main error of sensors. Depending on nature of emergence of separate components of this main error distinguish errors technological, temperature, arising owing to action of harmful forces, from incomplete coincidence of function of transformation at increase and decrease of output size (from a transformation function hysteresis), from an elastic after-effect, etc.
Technological errors are caused, for example, by inaccuracy of keeping of the geometrical sizes of details, dispersion of parameters of initial materials. Also shkalovy errors caused by inaccuracy of control and adjustment of devices belong to such errors. Temperature errors are connected with instability of physical parameters of sensors at change of ambient temperature. Errors from harmful forces are caused by action on a sensitive element and mobile system of the sensor of friction forces, from an imbalance of mobile system, forces of an electromagnetic and electrostatic attraction. Errors from a hysteresis and an elastic after-effect are caused by powers of internal friction in material of sensitive and reformative elements, owing to the return impact of the sensor on the measured size. Converters can influence nature of course of the measured processes, distorting them and giving finally the wrong information on these or those measured parameters, the dynamic errors connected with a converter lag effect.
Errors of sensors can be to a certain extent considered if calibration operation is provided in the measuring device and a technique of measurement. The correct calibration of sensors and the subsequent links of a measuring path — the most important condition of reproducibility and comparability of results. Process of calibration demands a choice of its optimum technique and standardization as in principle the most various ways of its carrying out are possible. So, for example, at calibration of air microphones for a phonocardiography the following options can be used: calibration on a piston microphone, on a condenser microphone, on probe l on an electrodynamic microphone. However, experiments show that the most suitable for calibration of air microphones in the range of frequencies of 50 — 1000 Hz is the first method.
Many medical and physiological researches demand measurement of low-frequency signals of small amplitude. Thus in many cases extremely slow drift of entrance tension caused by the movements of a body and changes of characteristics of a living tissue (drift of the zero line) is observed.
Further improvement of measuring systems depends in many respects on standardization of schemes, a design of devices and methods of measurements. Very much attention is paid to questions of standardization of medical measuring systems now. We don’t stop on this problem as means and methods of its decision are beyond the far present chapter and demand not only and not so much in-depth scientific studies, but also broad organizational actions.
Қажетті материалды таппадың ба? Онда KazMedic авторларына тапсырыс бер