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 her after 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:
Measurand Physical quantity or condition that the instrumentation system measures is called the Measurand. 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 or it may be blood pressure in the chambers of the heart.
Transducer or 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 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 a 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 instrmentation 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.
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 a 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.
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 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.
In many measurements in the medical field, some form of stimulus or energy is given to the patient and the effect it has on the patient is measured. The stimulus may be visual in the form of flash of light or audio tone or direct electrical stimulation of some part of the nervous system. A typical example is that of recording of the evoked response with EEG machine when visual/audible stimulus is given to the subject under test.
In some simations, it is required to have automatic control of the transducer, stimulus or signal conditioning part of the system. This is achieved by using a feedback loop in which part of the output from the signal conditioning or display device is fed back to the input stage. Control and feedback may be automatic or manual. Almost all measuring and recording equipment is now controlled by microprocessor as this makes it possible to design equipment that requires minimal user intervention, calibration and set up procedure.
Measurements on the human body can be made at several levels on the functional systems and sub-systems. For example; it is easiest to make measurements on the human body as a whole due to accessible environment. Examples of measurement made on the human body are recording of electrocardiogram and measurement of temperature . The next level of measurements can be made on the major functional systems of the body such as the cardiovascular system, the pulmonary system and so on. Many of the major systems communicate with each other as well as with external environment. Me functional systems can be further sub-divided into sub-systems and organs and still smaller units up to the cellular and molecular level. Measurements in the medical field are made at all these levels with specially designed instruments with appropriate degree of sophistication.
Measmements in the medical field can be classified into two types: in invivo and in invitro. In invivo measurement is made on or within the living organism itself, such as measurement of pressure in the chambers of the heart. On the other hand, in invitro measurement is performed outside the body. For example; the measurement of blood glucose level in a sample of blood drawn from the patient represent in invitro measurement.