The primary purpose of medical instrumentation is to measure or determine the presence of some physical quantity that may some way assist the medical personnel to make better diagnosis and treatment. Accordingly, many types of instrumentation systems are presently used in hospitals and other medical facilities. The majority of the instruments are electrical or electronic systems, although mechanical systems such as ventilators or spirometers are also employed. Because of the predominantly large number of electronic systems used in medical practice, the concepts explained hereafter are mostly related to electronic medical instruments.
Certain characteristic features, which are common to most instrumentation systems, are also applicable to medical instrumentation systems. In the broadest sense, any medical instrument would comprise of the following four
basic functional components :
1) Measurand :
The physical quantity or condition that the instrumentation system measures. The source for the measurand is the human body which generates a variety of signals. The measurand may be on the surface of the body (electrocardiogram potential) or it may be blood pressure in the chambers of the heart.
2) Transducer / Sensor :
A transducer is a device that converts one form of energy to another. Because of the familiar advantages of electric and electronic methods of measurement, it is the usual practice to convert into electrical quantities all non-electrical phenomenon associated with the measurand with the help of a transducer. For example, a piezo-electric crystal converts the mechanical vibrations into an electrical signal and therefore, is a transducer. The primary function of the transducer is to provide a usable output in response to the measurand which may be specific physical quantity, property or condition. In practice, two or more transducers may be used simultaneously to make measurements of a number of physiological parameters. Another term "sensor" is also used in medical instrumentation systems. Basically, a sensor converts a physical measurand to an electrical signal. The sensor should be minimally invasive and interface with the living system with minimum extraction of energy.
3) Signal Conditioner :
Converts the output of the transducer into an electrical quantity suitable for operation of the display or recording system. Signal conditioners may vary in complexity from simple resistance network or impedance matching device to multi-stage amplifiers and other complex electronic circuitry. Signal conditioning usually include functions such as amplification, filtering (analog or digital), analog-to-digital and digital-to-analog conversion or signal transmission circuitry. They help in increasing the sensitivity of instruments by amplification of the original signal or its transduced form. The information obtained from a sensor / transducer is often in terms of current intensity, voltage level, frequency or signal phase relative to a standard. Voltage measurements are the easiest to make, as the signal from the transducer can be directly applied to an amplifier having high input impedance. However, most of the transducers produce signal in terms of current, which can be conveniently converted into voltage by using operational amplifiers with appropriate feedback.
4) Display System :
Provides a visible representation of the quantity as a displacement on a scale, or on the chart of a recorder, or on the screen of a cathode ray tube or in numerical form. Although, most of the displays are in the visual form, other forms of displays such as audible signals from alarm or foetal Doppler ultrasonic signals are also used. In addition of the above, the processed signal after signal conditioning may be passed on to :
• Alarm System, with upper and lower adjustable thresholds to indicate when the measurand goes beyond preset limits.
• Data Storage, to maintain the data for future reference. It may be a hard copy on a paper or on a magnetic or semiconductor memories.
• Data Transmission, using standard interface connections so that information obtained may be carried to other parts of an integrated system or to transmit it from one location to another.
In most of the medical instrumentation systems, some form of calibration is necessary at regular intervals during their operation. The calibration signal is usually applied to the sensor input or as early in the signal conditioning chain as possible.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]