Previous: TRANSCUTANEOUS ELECTRICAL NEURAL STIMULATOR (TENS)

TRANSDUCER

Typical Number in Hospital: 100

Cost Bands: 2,3

References: 3,9

This is a device for the conversion of one form of energy to another. In medical work the best known examples are temperature, pressure, ion concentration, force and displacement transducers. The term could be applied to any energy-converting device such as a motor or light bulb but it is usually reserved for an electrically operated measuring device or actuator.

1. Pressure transducers. The electrical versions of these usually work by allowing the pressure to bend a diaphragm in the transducer and measure the bending by resistance strain gauges arranged into a bridge circuit which may be attached to the diaphragm (as in the case of most semiconductor gauges) or they may be remote from the diaphragm (unbonded) as in the case of wire or metallic film gauges. Such devices are used extensively for recording of blood pressure (via intra-arterial or intravenous cannulae), bladder pressure, etc.

2. Temperature transducers. The thermoelectric potential between two dissimilar metals (thermocouple), or the change in resistance due to temperature (thermistor) may be used to measure and record temperatures.

3. Displacement transducers. Distance moved, or position, may be measured by a potentiometer (e.g. as used to determine the position and orientation of some ultrasonic B-scan transducers) or the ratio of inductive coupling between two coils may be varied by the movement of a third coil or ferrite core (LVDT). Displacement transducers may also be used as velocity or acceleration transducers with suitable electronics.

4. Ultrasonic transducers. Sound or ultrasound may be produced by an inductive device such as the loudspeaker or magnetostrictive transducer, or be a piezoelectric element which expands or contracts according to the applied voltage. The advantage of the latter for medical applications is that they can work at high frequencies ( e.g. 5 MHz) and are reciprocal in that they can convert acoustic energy into electrical signals as well as the reverse.

An excellent list of medical transducer performance requirements is given on pages 7 and 8 of reference 9.