Surrounding the cells of the body there are the body fluid. These fluids are conductive solutions containing charged atoms known as ions. The principal ions are , and. The membrane of excitable cells nearly permits entry of potassium and chloride ions but effectively blocks the entry of. Amplifiers are used to process biopotentials are called bio-electric amplifiers but this designation applies to a large number of different types of amplifiers.
In general a transducer is defined as a device capable of converting one form of energy or signal to another for the purpose of measurement or control. In the man-instrument system, each transducer is used to produce an electrical signal that is an analog of the body phenomenon being measured. The transducer may measure temperature, pressure, flow or any other variables that can be found in the body but its output is always an electrical signal (V/A).
Example includes physiological variables such as body temperature, electrical activity of the heart, arterial blood pressure.
Types of Transducer:
- Active Transducer
- Passive Transducer
It can be utilized every known physical principle for converting non-electrical energy. However, not all principles are of practical importance in the design of actual transducers. Specially for biomedical applications. It is the characteristics that frequently but not always the same transduction principle used to convert from a non-electrical form of energy can also be used in the reverse direction to convert electrical energy into non-electrical forms. For example a magnetic loud speaker can also be used in the opposite directions as a microphone.
Some methods of Energy Conversion used in Active Transducers:
|Energy Form||Transduced Form||Device or Effect||Reversible|
|Mechanical||Electrical||Magnetic or Electrical Induction||Yes|
Passive Transducers utilize the principle of controlling a DC excitation voltage or an AC carrier signal. The actual transducer consists of a usually passive circuit element which changes its value as a function of the physical variable to be measured. The transducer is part of a circuit normally an arrangement similar to a Wheatstone Bridge, which is powered by an AC or DC excitation signal. The voltage at the output of the circuit refers the physical variable. There are only three passive circuit elements that can be utilized as passive transducers, resistors, capacitors and inductor.
Some Basic Transducers:
|Physical Variable||Type of Transducer|
|Force or Pressure||Piezoelectric, Strain gauge|
|Displacement||R , C , L, LVDT|
|Temperature||Thermocouple, Thermistors, RTD|
It is the device in which input is any form of energy but out is (V/A)
One kind of transducer that converts a physical parameter to an electrical output signal.
An electrode is used to pick up bio-signal from certain organs of the human body. It is special transducer that transforms an ionic current into an electronic current.
Electrodes are mainly two types:
à External Electrode/Body Surface Electrode: Are used on the surface of the body.
à Internal Electrode: Inserted into the body in the form of needles, wires, or implanted electronic circuits.
Types of Body Surface Electrode:
Ø Metal plate electrode.
Ø Suction electrode.
Ø Floating electrode.
Ø Flexible electrode.
Ø Dry electrode.
Types of Internal Electrode:
Ø Insulated needle electrode.
Ø Coaxial needle electrode.
Ø Bipolar Coaxial electrode.
Ø Fine wire electrode.
Ø Coiled fine-wire electrode.
Metal Plate Electrode:
One of the most frequently used forms of biopotential sensing electrodes is the metal-plate electrode. In its basic form, it consists of a metallic conductor in contact with the skin. An electrolyte get is used to establish and maintain the contact.
Fig: Body Surface electrodes: (a) Metal-plate electrode, (b) Metal-disk electrode,
(c) Foam pad electrode
A modification of the metal plate electrode that requires no straps or adhesive for holding it in place is the suction electrode. Such electrodes are frequently used in electrocardiography as the p0recordial (chest) leads because they can be placed at particular locations and used to take a recording. They consist of a hollow metallic cylindrical electrode that makes contact with the skin at the base. An appropriate terminal for the lead wire is attached to the metal cylinder, and a rubber suction bulb fits over its other base.
Electrolyte gel is placed over the contacting surface of the electrode, the bulb is squeezed and the electrode is then placed on the chest wall. The bulb is released and applies suction against the skin, holding the electrode assembly in place. This electrode can be used only for short periods of time; the suction and the pressure of the contact surface against the skin can cause irritation.
In practice, the electrode is filled with electrolyte gel and then attached to the skin surface by means of a double-sided adhesive-tap ring. The electrode element can be a disk made of metal such as silver, and often it is coated with AgCl. Another frequently encountered form of the floating electrode uses a sintered Ag-AgCl pellet instead of a metal disk. These electrodes are found to be quite stable and are suitable for many usages.
We know that one source of motion artifact in biopotential electrodes is the double layer of charge at the electrode-interface. To reduce this artifact floating electrodes are used, which offer a suitable technique and stabilize the interface mechanically.
The principle feature of this electrode is that the actual electrode element or metal disk is recessed in a cavity so that is does not come in contact with the skin itself. Instead, the element is surrounded by electrolyte gel in the cavity. The cavity does not move with respect to the metal disk, so it does not produce any mechanical movement of the double layer of charge.
Solid electrodes described so far cannot conform to the change in body-surface topography, which can result in additional motion artifact. To avoid such problems, flexible electrodes have been developed. A carbon-filled silicone rubber compound in the form of a thin strip or disk is used as the active element of an electrode. a pin connector is pushed into the lead connector hole, and the electrode is used in the same way as a similar type of metal-plate electrode. flexible electrodes are especially important for monitoring premature infants. Electrodes for detecting the ECG and respiration by the impedance technique are attached to the chest of premature infants, who usually weigh less than 2500 g. Conventional electrodes are not appropriate.
All the surface electrodes described so far require an electrolyte gel to establish and maintain contact between the electrode and the skin. Recent advances in solid-state electronic technology have made it possible to record surface biopotentials from electrodes that can be applied directly to the skin without an intermediate layer of electrolyte gel. The significant feature of these electrodes is a self-contained, very-high-input impedance amplifier.
Microelectrode (Internal Electrode):
Microelectrodes are electrodes with tips sufficiently small to penetrate a single cell in order to obtain readings from within the cell. The tip must be small enough to permit penetration without damaging cell. This action is usually complicated by the difficulty of accurately positioning an electrode with respect to a cell. This is used to measure bioelectric potential near or within a single cell. Several types of microelectrodes exist.
Body Surface Electrodes or Skin Surface Electrodes/External Electrodes:
This is to obtain bio-electric potentials from the surface of the body skin are found in many sizes and forms. Although any type of surface electrode can be used to sense ECG, EEG or EMG potentials. The larger the electrodes are usually associated with ECG where as smaller electrodes are used in EEG and EMG measurements.
Needle Electrode/Internal Electrode:
These are used to penetrate the skin to record EEG potentials from a local region of the brain or EMG potentials from a specific group of muscles. These needle electrodes are not inserted into the brain; they merely penetrate the skin without puncturing it. Generally they are simply inserted through a small section of the skin just beneath the surface and parallel to it.
Fig: Needle Electrode
Blood Gas Electrode:
Blood Gas Electrodes are used to measure physiological chemical parameter; partial pressure of and present in the blood. The partial pressure of a dissolved gas is the contribution of that gas to the total pressure of all dissolved gases in the blood. The partial pressure of a gas is proportional to the quantity of that gas in the blood.
It is used to measure the acid/base balance of a body fluid or blood. Perhaps the most important indicator of chemical balance in the body is the of the blood and other body fluids.
Most body fluids are slightly basic.
Blood Pressure Measurement:
Fig: Basic Anatomy of the Heart
1. Right Atrium,
2. Left Atrium,
3. Right Ventricular,
4. Left Ventricular.
SA node – Sino Artrial node,
AV node – Atrio Ventricular node
Blood pressure is a measurement of force applied to the walls of the arteries as the heart pumps blood through the body. The pressure is determined by the force and the amount of blood pumped and the size and flexibility of the arteries.
Blood pressure is continually changing depending on activity, temperature, diet, emotional estate, posture, physical state and medication.
How to measure Blood Pressure using a Sphygmomanometer:
The non-invasive ausculatory method is one of the most common ways of measuring a patient’s blood pressure. The subjects sit down and rest their arm on a table so the brachial artery is level with the heart. This is important when monitoring blood pressure as pressure is proportional to height ().
Usually when the doctor measures the patient’s blood pressure, he will pump the air into the cuff and use the stethoscope to listen to the sounds of the blood in the artery of the patient’s arm. At the start, the air is pumped to be above the systolic value. At this point, the doctor will hear nothing through the oscilloscope. After the pressure is released gradually, at some point, the doctor will begin to hear the sound (KorotKoff sounds) of the heart beats. At this point, the pressure in the cuff corresponds to the Systolic pressure. After the pressure decreases further, the doctor will continue hearing the sound (with different characteristics). And at some point, the sounds will begin to disappear. At this point, the pressure in the cuff corresponds to the Diastolic pressure.
To perform a measurement, we use a method called ‘oscillometric’. The air will be pumped into the cuff to be around 20 mmHg above average systolic pressure (about 120 mmHg for an average). After that the air will be slowly released from the cuff causing the pressure in the cuff to decrease. As the cuff is slowly deflated, we will be measuring the tiny oscillation in the air pressure of the arm cuff. The systolic pressure will be the pressure at which the pulsation starts to occur. We will use the MCU to detect the point at which this oscillation happens and then record the pressure in the cuff. Then the pressure in the cuff will decrease further. The diastolic pressure will be taken at the point in which the oscillation starts to disappear.
Electro Cardio Gram (ECG):
The ECG is a graphic recording or display of the time variant voltage produced the myocardium during the Cardiac Cycle.
How to perform ECG:
Ø Ten sensors are attached to the arms and chest around the heart area.
Ø These sensors listen the heart beat and make a wave pattern on graphic paper.
Ø Electrical impulse associated with heart contraction and relaxation are recorded.
Ø From the patern on ECG tracing the doctor can check how the heart is doing.
Fig: ECG curve
P – Wave:
It is the record of the movement of electrical activity through the upper heart chambers (right and left atrium) during contraction.
QRS – Complex:
It is the record of the movement of electrical activity through the lower heart chambers (right and left ventricles) during the contraction.
T – Wave:
This corresponds the period when the lower heart chambers (right and left ventricles) are relaxing electrically and preparing for the next muscle contraction.
Electro Encephalogram (EEG):
It is a representation (writing on paper or display on CRT) of the electrical activity of the brain. The technique involves the following:
Bio-potential pick up – Carnial or cerebral surface transducer electrodes.
EEG signal conditioning – Transducer output amplification and filtering.
EEG signal recording – Signal displayed on graphic recorder on CRT.
EEG signal analysis – Visual or computer interpretation of resulting EEG.
Why EEG is done?
1. Help detect and localize cerebral brain lesions.
2. Aid in studying epilepsy (recurrent transient attacks of disturbed brain function with irregular sensory and motion activity such as convolution)
3. Assist in studying sleep pattern.
4. Assist in diagnosis mental disorders..
Electro Myogram (EMG):
The bio-electric potentials associated with muscle activity constitute the electro my gram, abbreviated as EMG. The potentials may be measure at the surface of the body near muscle of interest or directly from the muscle by penetrate may be the skin needle electrodes. Since most EMG measurements are intended to obtain on indication of the amount of activity of a given muscles or group of muscles rather than of an individual muscle fiber. The pattern is usually a summation of the individual action potentials from the fibers constituting the muscle or muscles being measured through surface needle and fine wire electrodes.
Fig: EMG curve
Ultrasound waves are sound waves associated with frequencies above the audible range and generally extended upward from 20 KHz.
Ultrasound has become increasingly important in medicine and has taken its place along with X-Ray and nuclear medicine as a diagnostic tool.
Its main attraction as an image modality lies in its non-invasive character and ability to distinguish interfaces between soft tissues.
Diagnostic Ultrasound is applied for obtaining image of almost the entire range of internal organs in the abdomen. These include the kidney, lever, pancreas, bladder, the foetus during pregnancy etc.
= Density of medium
= Velocity of Ultrasound
= Wave length
What is Impedance () ?
The acoustic impedance () of material is a measure of its opposition to the propagation of sound waves. It is sometimes characterized as a measure of the efficiency with which the signal propagates in the material. The unit of acoustic impedance is Rayl.
The greater the difference in acoustic impedance, the greater the amount of reflected energy.
How to make Ultrasound?
Basic Transducer Structure:
The matching layers may be made from glass or epoxy. A baking material such as epoxy, loads the back side of the element. The face plate protects the transducer assembly and also may act as an acoustic lens. Face plates are often made from silicon or polyurethane.
How Ultrasonic Imaging done?
Instruments must include an electrical signal source capable of driving the transmitter, which consists of a piezoelectric crystal. The same crystal can be used for receiving echoes or a second crystal may used. After amplification, the received information is displayed in one of several display modes.
Ø Thermal radiation
Ø Radioactive radiation
Various Rays and Particles:
Origin of X – Ray:
X – Rays are actually electro-magnetic waves as are light and radio waves. The main difference is a matter of frequency or wave length. X- Rays are high energy waves that pass through the body and indicate relative tissue density as a photo sensitive plate. Essentially bones are dense and pass less X-rays than soft tissues such as blood vessels, organs or muscles. The X-ray that does not pass through (transmitted) is absorbed and stored within the body in accumulating doses.
X – Machines:
X –Ray machine produces high energy electromagnetic wave that penetrate the body during medical procedure. These machines serve diagnostic and therapeutic purpose.
Fig: X – Ray Machine
Target Anode: Positive electrode contains target (W). Anode target may stationary or rotary.
Cathode: Negative cathode consists of a thin wire called filament.
Control of X-Ray:
à Filament heat control (mA) for exposure strength.
à Kilovolt control (kV) for penetration depth and contrast.
à Timing device for time exposure length.
Production of X-Ray:
The tube commonly used is called vacuum tube. It consists of an evacuated glass tube containing a cathode and the target (anode). The cathode is a tungsten filament and can be heated by the current supplied by a step down transformer. The tungsten filament emits electrons i.e. thermions which are accelerated to the target (anode) by maintaining a difference of potential between the cathode and the target. Is necessary to control current in order to heat the filament which ultimately controls the emission of electrons. Most of the energy is heat (99%) and the rest energy is X-Rays. Emission of X-Ray is controlled by varying the high V, I and the time of exposure.
Properties of X – Rays:
Ø They are electromagnetic radiations of very small wave length of the order of cm and are photons of high energy.
Ø They travel with the velocity of light ()
Ø They affect a photographic plate.
Ø They are not detected by magnetic or electric field and therefore they do not possess any charge.
Ø Similar to light X-rays due to the energy, liberate photo-electrons from some metals when incident on them.
Ø They ionize the gas through which they pass.
Ø They are highly penetrating and can pass through many solids, liquids, gases but X-ray can’t penetrate through heavy metals and bones. Therefore X-rays cast the shadow of this objects when placed in their path.
This property has been used in detecting the feature of bones and flows in precious casting
Use of X-Rays:
Ø Radio therapy.
Ø Detection Depts.
Unit for measuring Radioactivity:
Ø Curie (Ci) (Conventional unit)
Ø Amount of radioactivity in 1 gm of radium.
Bq – International unit
Bq = 1 dps.
Ø Roentgen (R): Unit of radiation exposure or amount of X-ray radiation that will produce.
Ø Radiation Absorbed Dose (RAD) in tissue: 1 RAD is the radiation dose that will result in energy absorption.
1 RAD = 1 R
Ø REM – for X-ray radiation. It is the measurement of absorbed dose.
Ø Sievert (Sv): It is the international unit for measurement of absorbed dose.