Unit II GENERALIZED MEDICAL INSTRUMENT SYSTEM

Transcription

1 Unit II GENERALIZED MEDICAL INSTRUMENT SYSTEM The primary purpose of any medical instrumentation system is to measure or determine the purpose of some physical quantity that assists the medical personnel to make better diagnosis and start treatment. Majority of the systems used are electrical or electronic in nature. The major difference between this system of medical instrumentation and conventional instrumentation systems is that the source of the signals is living tissue or energy applied to living tissue. Measurand The physical quantity or the condition that the system measure is called the measurand.the source is a human body that generates bio-potential signals. The accessibility of the measurand is important because it may be internal (blood pressure), it may be on the body surface (electrocardiogram potential), it may emanate from the body (infrared radiation), or it may be derived from a tissue sample (such as blood or a biopsy) that is removed from the body. Most medically important measurands can be grouped in the following categories: bio potential, pressure, flow, dimensions (imaging), displacement (velocity, acceleration, and force), impedance, temperature, and chemical concentrations. The measurand may be localized to a specific organ or anatomical structure. Sensor Transducer is defined as a device that converts one form of energy to another. A sensor converts a physical measurand to an electric output. The sensor should respond only to the form of energy present in the measurand. The sensor should interface with the living system. Many sensors have a primary sensing element such as a diaphragm, which converts pressure to displacement. A variable-conversion element, such as a strain gage, then converts displacement to an electric voltage. Sometimes the sensitivity of the sensor can be adjusted over a wide range by altering the primary sensing element. Many variable-conversion elements need external electric power to obtain a sensor output.

2 Signal Conditioning Usually the sensor output cannot be directly coupled to the display device. Simple signal conditioners may only amplify and filter the signal or merely match the impedance of the sensor to the display. Often sensor outputs are converted to digital form and then processed by specialized digital circuits or a microcomputer. For example, signal filtering may reduce undesirable sensor signals. It may also average repetitive signals to reduce noise, or it may convert information from the time domain to the frequency domain. Output Display The results of the measurement process must be displayed in a form that the human operator can perceive. The best form for the display may be numerical or graphical, discrete or continuous, permanent or temporary depending onthe particular measurand and how the operator will use the information. Although most displays rely on visual sense, some information (Doppler ultrasonic signals-example) is best perceived by other senses (auditory sense). Auxiliary Elements A calibration signal with the properties of the measurand should be applied to the sensor input or as early in the signal-processing chain as possible. Many forms of control and feedback may be required to elicit the measurand, to adjust the sensor and signal conditioner, and to direct the flow of output for display, storage, or transmission. Control and feedback may be automatic or manual. Data may be stored briefly to meet the requirements of signal conditioning or to enable the operator to examine data that precede alarm conditions. Alternatively, data may be stored before signal conditioning, so that different processing schemes can be utilized. Conventional principles of communications can often be used to transmit data to remote displays at nurses stations, medical centres, or medical dataprocessing facilities. In many measurement systems, some external stimulus is given to the patient and its effect is measured. A good example is the recording of evoked response in EEG measurement. Measurements can be classified as in vivo and in vitro. In vivo measurement is made on or within the living organism itself, such as measurement of bool pressure in heart chambers. On the hand, in vitro measurement is made outside the body, such as measurement of blood glucose level in a blood sample drawn from the patient. ELECTRODES Bioelectric events have to be picked up from the surface of the body before they can be calibrated or processed for further action. This is done by the electrodes. Electrodes transfer ionic conduction into electronic conduction. The characteristics of a bio potential electrode are normally non-linear and are a function of current density at their surface. The equivalent circuit is shown in the fig.

3 Instrument Body Equivalent circuit of bio potentialelectrode E hc half-cell potential, C d electrode capacitance, R d - leakage resistance, R s series electrolyte and skin resistance. The resistance R d and C d represent the impedance associated with the electrode-electrolyte interface. R s is the series resistance associated with interfacial effects and the resistance of the electrode materials themselves. The potential developed at the electrode-electrolyte interface or metal electrode interface is called Half-cell Potential.The electrode is a metal and the electrolyte is body fluids. The battery E hc represents the half-cell potential. It is measured with reference to hydrogen electrode placed in the electrolyte near that metallic electrode. The half-cell potential developed can be expressed by the Nernst equation: E hc RT C f 1 1 = ln nf C 2 f 2 Where R gas constant, T absolute temperature in Kelvin, F faraday constant, n valency of ion, C 1 and C 2 concentration of the selected ion on the 2 sides of the membrane and f 1 and f 2 activity co-efficient of the ion on the 2 sides of the membrane. The impedance of the equivalent circuit is given by Z R d = R s j 2π fc R d d Variation of electrode impedance with frequency

4 At low frequencies the impedance is dominated by the series combination of R s and R d, whereas at higher frequencies C d bypasses the effect of R d so that the impedance is now close to R s. Thus, by measuring the impedance of an electrode at high and low frequencies, it is possible to determine the component values for the equivalent circuit for that electrode. When a metal electrode comes in contact with an electrolyte (body fluid), the electrodes discharge ions into the solution. Ions combine with the electrode and produce a charge gradient. The spatial arrangement for the gradient is called Electrical Double Layer. Perfectly Polarizable Electrodes These are electrodes in which no actual charge crosses the electrode-electrolyte interface when a current is applied. The current across theinterface is a displacement current and the electrode behaves like acapacitor. Example: Ag/AgCl Electrode Perfectly Non-Polarizable Electrode These are electrodes where current passes freely across the electrode-electrolyte interface, requiring no energy to make the transition. Theseelectrodes see no overpotentials. Example: Platinum electrode Stable gradient exist at the interface and if not, there may be variations causing noise voltages called Artifact. Generally conductivity of the skin is directly proportional to the moisture content in the skin. Dry outer skin of the body is highly non-conductive. The skin has to be washed and rubbed to remove some outer cells to establish electrical contact. The skin area has to coated with an electrically conductive paste called Electrode Paste. The paste reduces the contact impedance and artifacts. Saline solution is best suited as an electrode paste. Purpose of Electrode Paste:It lowers the impedance between theelectrode and the skin and also helps to stick the electrode to the skin. It reduces artifacts. Electrode Material The body fluids, electrode paste, and the metal / electrode can form a battery causing ions to accumulate on the electrodes. Hence polarization occurs and causes some adverse effects. In order to reduce this, the electrode material must be inert to body chemicals. By coating a piece of pure silver with silver chloride, the silver-silver chloride electrode is developed. The half-cell potential for this electrode is 2.5mV and effectively minimises the artifacts. It also reduces the low frequency electrode-electrolyte impedance. Hence silver-silver chloride electrodes are most commonly used in medical instrumentation. PRACTICAL ELECTRODES FOR BIOMEDICAL MEASUREMENTS Many different forms of electrodes have been developed for different types of biomedical measurements. Three types are normally used in practice to measure bioelectric events.

5 1. Surface Electrodes: Used to measure ECG, EEG and EMG on the surface of the skin. 2. Depth and Needle Electrodes: Used to penetrate the skin to record EEG from a local region of the brain or EMG potentials from a specific group of muscles. 3. Micro Electrodes: Used to measure bioelectric potentials near a single cell. SURFACE ELECTRODES A small device that is attached to the skin to measure or cause electrical activity in the tissue under it is called Surface Electrode. Surface electrodes may be used to look for problems with muscles and nerves. Large area surface electrodes are used to sense ECG potentials and smaller ones for EEG and EMG potentials. Commonly used body surface electrodes are Metal plate electrodes, Suction electrodes, Floating electrodes,flexible electrodes, Adhesive tape electrodes and Multipoint Electrodes. i) Metal Plate Electrodes: These electrodes can be placed on the body surface for recording bioelectric signals. The basic metal plate electrode consists of a metallic conductor in contact with the skin with a thin layer of an electrolyte gel between the metal and the skin to establish this contact. A metal plate electrode is shown in the fig. Metals commonly used for this type of electrode include German silver (a nickel-silver alloy), silver, gold, and platinum. Electrode jelly is rubbed on the selected site of the body and the electrode is held in place at the skin site by a rubber strap or belt. Early electrodes were composed of metals such as tin, steel, aluminium etc. which are good conductors of electricity for therapeutic stimulation. The electrode was usually contained within the rubber casing with only one surface exposed to the patient. The interface between the metal electrode and skin was accomplished through a sponge or felt pad moistened with tap water. This reduces the skin electrode impedance, as the tap water is a good conductor of electricity. Distilled water is not preferred as it is devoid of free ions.they can be usedfor short-term diagnostic recording such as taking a clinical electrocardiogram or long-term chronicrecording such as occurs in cardiac monitoring.

6 Demerits of metal electrodes: 1. Metal plates may not be flexible enough to maintain adequate contact with certain body parts. 2. Some may not be of comfort to the patients. 3. Size of the electrode makes it difficult to use for smaller treatment areas. ii) Suction Electrodes: These electrodes are made of a foil of themetal so as to be flexible, and sometimes they are produced in the form of a suction electrode. It is more practical and suited to attach to the skin to make a measurement and then move it to anotherpoint to repeat the measurement. Mainly used to record ECG, EEG and for underlying soft tissues as it is Compact in size and only the rim is in contact with the skin. Suction-cup electrode It can be placed on the skin very quickly and easily by a mere pinch of the fingers. No adhesives or tapes are necessary to hold it in place or to insure contact with the skin. It can be placed on the exact reference point marked on the skin. If suction is lost, the electrode can quickly be re-lubricated (Cardeez electrode jelly) and replaced exactly on the same spot. Suction can be maintained from one to several hours, depending upon the size of the suctioncup and its electrode, and the amount of electrode jelly used. Because of the slight suction created, continuous contact between the electrode and the skin is insured. Shaving of body hair is not necessary unless the subject is heavily endowed with hair. Demerit: Prolonged use of the electrode on flexor surfaces of the body such as the chest or abdomen may cause uneasiness to the patient. iii) Adhesive tape Electrodes: There is possibility of the electrode paste to squeeze out against the skin due to the pressure of the surface electrode. Hence some kind of adhesive may be required. An adhesive tape electrode can be used instead. It consists of a light weight metal screen backed by a pad for electrode paste. The adhesive backing holds the electrode in place and retards evaporation of the electrolyte present in the paste.

7 iv) Multipoint Electrodes: These electrodes are commonly used for Bio-telemetry, patient monitoring systems, ECG measurements and achieveseveral active, fine skin contacts.low resistance contact with the patient can be achieved under any ambience conditions. It requires no preparation with jelly or paste. Noise voltage potentials can be minimized as the electrodeskin movement can be reduced. v) Floating Electrodes: This electrode practically eliminates movement artefact by avoiding any direct contact of the metal with the subject. Electrode paste or jelly is the only contacting medium which serves as the electrolytic bridge. The actual metal disk is recessed in a cavity. Movement artifacts can be reduced it is even possible to record muscle potentialsfrom moving athletes or during doing some physical task without significant artifact. This electrode is also called Liquid Junction electrode. The whole assembly is fixed to the skin with a double sided adhesive tape ring. The electrode is made of silver and coated with silver chloride. DEPTH ELECTRODES Depth electrodes are thin, wire like plastic tubes with metal contact points spread out along their length. These electrodes are placed directly into the brain, but they do not require that a large opening be made in the skull. Depth electrodes are inserted through burr holes drilled in the skull. The patient is usually awake while the electrodes are being placed, but may be sleeping. Depth electrodes are used to record electrical activity directly from superficial layers of brain. Made of alloy wires bonded to a stainless steel wire coated with varnish and offers less impedance with the skin. They may be to 0.25mm in diameter offering acute measurements to measure oxygen tension. NEEDLE ELECTRODES The basic needle electrode consists of a solid needle, usually made of stainless steel, with a sharp point. An insulating material coats the shank of the needle up to a millimetre or two of the tip so that the very tip of the needle remains exposed. When this structure is placed in tissue such as skeletal muscle, electrical signals can be picked up by the exposed tip. The electrode can also be shielded by the metal of the needle and can be used to pick up much localized signals in tissue. Fine wires can also be introduced into tissue using a hypodermic needle, which is then withdrawn. This wire can remain in tissue for acute or chronic measurements.

8 Normally these are used to record nerve action potential in Electro-neurography, EMG and animal research. In EMG measurements these electrodes create an interface beneath the surface of the skin and have lower impedance and less movement artifacts compared to surface electrodes. MICRO ELECTRODES A microelectrode is an electrode of very small size, used in electrophysiology for recording of neural signals or electrical stimulation of nervous tissue and in studying the electric behaviour of cells. These microelectrodes are made from inert metals with high Young modulus such as tungsten, stainless steel, platinum and iridium oxide and coated with glass or polymer insulator with exposed conductive tips. These are also called as intracellular electrodes. There will not be any damage to the cells during insertion. It will be inside the cell whereas a reference electrode is placed outside the cell. Depending on the cell size (50 microns), the tip diameter of these electrodes is set between 0.5 to 5 microns. Micro electrodes are of two types metallic and non-metallic. The non-metallic is called Micropipette. i) Metal Microelectrode These electrodes are formed by a process called Electro pointing. It is the process of electrolytically etching the tip of a fine tungsten or stainless steel wire to a fine point. The micro tip is coated with an insulating material. The electrode is shown in the figure. Chloriding and developing by the photographic developer solution to the tip reduces the impedance. The position of the electrode is shown in the figure below.

9 Position of electrode The equivalent circuit at the electrode position is shown below. Two electrodes are required for measurement of biopotentials, one microelectrode and the other reference electrode. The total potential developed will be the sum of instantaneous potentials developed at the electrodes including the membrane potential. E A - metal electrode- electrolyte potential at the microelectrode tip. E B reference electrode electrolyte potential. E C membrane potential. R A resistance of the connecting wire, R S - resistance of the shaft of the microelectrode. R FA, R WA, C WA refer to the impedance of the microelectrode tip-intercellular fluid interface. R IN resistance of the intercellular fluid, R B resistance of the wire connected to the reference electrode.

10 R FB, R WB, C WB refer to the impedance of the reference electrode-extracellular fluid interface. R EX resistance of the extracellular fluid. CD distributed capacitance between the insulated shaft of the microelectrode and the extracellular fluid. The capacitance between thetip of the microelectrode and the intracellular fluid is negligible as the potential difference across it remain constant. The impedance of the microelectrode tip is inversely proportional to the area of the tip and frequency. When the electrode output is coupled with an amplifier, low frequency components will be attenuated, if the input impedance of the amplifier is not high. In that case, it will act as a high pass filter. ii) Micropipet or Non-metallic Electrode Most microelectrodes used for intracellular recording are glass micropipettes, with a tip diameter of < 1 micrometre, and a resistance of several megaohms. The micropipettes are filled with a solution that has a similar ionic composition to the intracellular fluid of the cell. A chlorided silver wire inserted in to the pipet connects the electrolyte electrically to the amplifier and signal processing circuit. The voltage measured by the electrode is compared to the voltage of a reference electrode, usually a silver chloride-coated silver wire in contact with the extracellular fluid around the cell. In general, the smaller the electrode tip, the higher its electrical resistance, so an electrode is a compromise between size (small enough to penetrate a single cell with minimum damage to the cell) and resistance. These electrodes suffer from high source impedances and fabrication difficulty.

11 AMPLIFIERS Bio-signals are recorded as potentials, voltages, and electrical field strengths generated by nerves and muscles. The measurements involve voltages at very low levels, typically ranging between 1 µv and 100 mv, with high source impedances and superimposed high level interference signals and noise. The signals need to be amplified to make them compatible with devices such as displays, recorders, or A/D converters for computerized equipment. Amplifiers adequate to measure these signals have to satisfy very specific requirements. They have to provide amplification selective to the physiological signal, reject superimposed noise and interference signals, and guarantee protection from damages through voltage and current surges for both patient and electronic equipment. Amplifiers featuring these specifications are known as bio-potential amplifiers. The quality of bio-signal acquisition in a medical instrument purely depends on medical preamplifier design.the design of physiological signal amplifier or biomedical pre-amplifier has to satisfy the following requirement. 1. The voltage gain should be greater than 100 db so that the signal be delivered properly to drive the recorder. 2. Amplifier should have low frequency response from DC to the required frequency of the particular bio-signal. 3. Gain and the response should be uniform throughout the bandwidth. 4. It must have large input impedance and low output impedance. 5. CMRR has to be large to eliminate 50 HZ interference from the mains.the amplifier should provide the best possible separation of signal and interferences. 6. The measured signal should not be distorted and should not suffer from drift. 7. The amplifier has to offer protection of the patient from any hazard of electrical shock 8. The amplifier itself has to be protected against damages that might result from high input voltagesas they occur during the application of defibrillators or electrosurgical instrumentation. PREAMPLFIERS For biophysical measurements, the amplifiers include i) ac / dc universal amplifier with neutralization, current injection, low leakage current, low dc drift etc. These are necessary to make intracellular or extracellular measurements in case of ECG, EEG, EMG etc. ii) an ECG amplifier with 12 lead selection and patient isolation iii) transducer amplifier suited for bridge measurements on strain gauges etc for blood pressure measurements and in other biophysical measurements like temperature etc.

12 Various amplifiers used are 1. Differential Amplifier: The main task of the differential amplifier is to reject the line frequency interference that is electrostatically or magnetically coupled into the subject. The desired bio potential appears as a voltage between the two input terminals of the differential amplifier and is referred to as the differential signal. The line frequency interference signal shows only very small differences in amplitude and phase between the two measuring electrodes, causing approximately the same potential at both inputs, and thus appears only between the inputs and ground and is called the common mode signal. Strong rejection of the common mode signal is one of the most important characteristics of a good bio potential amplifier. Most amplifiers that are used to measure bioelectric signals are of differential type. 2. DC Amplifier: Generally of negative feedback type and suited for medium gain applications. These suffer from low CMRR capability and are employed as pen drive amplifiers in direct writing recorders. 3. Chopper Amplifier: Preferred to convert low level frequency signal to high level and are available as mechanical and non-mechanical chopper amplifiers. These possess excellent common mode rejection capability and very low drift with high sensitivity. 4. Isolation Amplifier:A form of differential amplifier that allow measurement of small signals in the presence of a high common mode voltage by providing electrical isolation and an electrical safety barrier. These amplifiers are also used for amplifying low-level signals in multi-channel applications. They can also eliminate measurement errors caused by ground loops. Chopper Amplifier Chopper amplifiers are basically DC amplifiers that operating on low-frequency signals. These are extensively used in medical electronics to measure bio signals, as they are very weak and needs to be amplified with high gain. A very high gain DC amplifier can be used to support this with low drift and low offset. But the DC amplifiers exhibit a certain amount of drift where the output will continuously shift even in the absence of input is zero or if it is constant.a solution to the problem of drift is the use of Chopper Amplifier that employs a chopper circuit to break up the slowly varying input signal into an alternating signal with an amplitude proportional to the input direct current and with phase dependant on the polarity of the original signal. The AC voltage is then amplified by an AC amplifier and rectified to get back the amplified DC signal. This device serves to be a better device for signals of narrow

13 bandwidth and eliminates the drift. Stability and accuracy can also be achieved. The schematic of the chopper amplifier is shown in figure.. The chopping frequency is in the audible range and a BJT or FET switch can serve as a chopper. A LPF can be used to remove ac ripple of chopping frequency. Non-mechanical Chopper Amplifier: A photoconductor and a neon lamp driven by an oscillator can also serve the purpose of a chopper amplifier as shown in the figure. A photoconductor or photodiode can be used as non-mechanical choppers for modulation (dc to ac conversion) and demodulation (ac to dc back). A photoconductor exhibits a large resistance when dark and vice-versa when illuminated by light. The current through it varies and thus the device can act as switch by means of incident light. A neon lamp flashes light on the photoconductor at a frequency equal to the oscillator frequency, resulting in chopping the dc waveform. The same setup can be used for the demodulator side. For one half of the oscillator cycle, neon lamp1 will flash and for the other lamp2 will flash. The lamps could be collector loads in an astable multivibrator. At the input, there is a low level dc input. Whenever the light falls on the first photoconductor, its resistance reduces and input capacitor charges. Thus by alternate cycles, we have a square wave across the capacitor. This is

14 applied to AC amplifier and the amplified wave is obtained at the output. The two photoconductors recover the dc signal by their demodulating action and the output capacitor becomes charged to the peak of the output voltage. Then this dc output voltage is passed through a LPF to remove ripples and finally the amplified dc output is obtained. The transition time between the high and low resistance states of the photoconductors limit the chopping rate up to a few hundred Hz even though the oscillator can deliver high frequency driving voltage. Mechanical Chopper Amplifier A simple Mechanical Chopper Amplifier is shown in the figure. The chopper S 1, alternatively connects the input terminal C of the AC amplifier to terminal B connected to the ground. When connected to B, the amplifier input terminals are short circuited and the input voltage is zero. The input of the amplifier consists of an AC voltage varying from zero to the value of the input existing when the signal is connected (S 1 open). By this process, a steady DC is chopped into a train of pulses having a frequency equal to the rate of the chopper. After amplification, the chopped signal is rectified by the diode. The rectified signal is then filtered and the DC and slow varying signal components are recovered. Instead of rectifying the output of the amplifier with a diode, a second chopper contact operating synchronously with the first may be used as shown in the figure. The rapidity of the

15 movement of mechanical choppers is limited due to their inertia. They are available in 60 and 400 Hz units. They have finite lifetime.