Text 11. Thermocouple

These work by the Seebeck effect, which is a combination of the Peltier effect by which a small voltage exists at the junction of two unlike metals, and a second effect credited to Lord Kelvin which produces a small voltage along a conductor in a temperature gradient. Both effects are proportional to the temperatures involved. The total voltage produced in a circuit including a number of thermocouples is zero as there is no temperature difference around the loop. Thus two thermocouples are normally employed. One is maintained at a reference temperature (e.g. the freezing point of water) and the other acts as the thermometer.

The sensitivities of common thermocouples range from 6.5 to 80 [u]V/[d]C with accuracies from 0.25% to 1%. Several thermocouples can be arranged in series to form a thermopile to increase the sensitivity. The advantages of thermocouples are their fast response (down to 1 ms), small size (down to 12 [u]m diameter), ease of fabrication and long-term stability. Their disadvantages are small output voltage, low sensitivity, and need for reference temperature. Small thermocouples can be inserted into catheters and hypodermic needles. Some clinical electronic thermometers employ thermocouples.

Text 12. Ultrasonic Transducer

Ultrasonic waves can be created by the magnetostrictive effect by which the shape of a metal rod is modified by passing an alternating electric current through a coil wound around it. If high frequencies are required, a piezoelectric transducer is more effective.

Some natural materials (e.g. quartz) and some synthetic materials exhibit the piezoelectric effect, by which electric charges appear on their surfaces when they are deformed. Some materials also exhibit the reverse effect by which electric charges applied across the element (crystal) cause it to deform. Thus it can act as a microphone or as a sound generator.

In medical apparatus the transducer element is usually a thin disc or plate of lead zirconate titanate (PZT) of which each face is silvered to form an electrode. It can be made to vibrate at the frequency of an oscillator (as in doppler instruments and physiotherapy apparatus) or if excited by a short electric pulse (as in A- and B-scanners) it will ring at its resonant frequency. The duration of the ringing must be limited to provide a short pulse of sound, and this is achieved by attaching sound-absorbing material to the back of the element. The front face is applied to the skin through focusing and impedance matching layers and a coupling oil or jelly.

Doppler instruments commonly make use of separate crystals for transmit and receive, which are angled together so that reception of echoes is most effective at a chosen depth. Pulse- echo equipment usually employs the same crystal for both functions with some form of focusing lens.

Multi-element transducers are now common in real-time B- scanners. The piezoelectric element is divided into many parallel strips. In real-time linear array scanners there may be 64 parallel elements stretching 10 cm or so, and these are used in groups so that the active area of the crystal is electronically moved along, simulating movement of a square transducer. In electronically steered array scanners each element is driven at a different time so that the ultrasound beam may be made to point in (or receive from) any direction. Focusing of the transmitted wave, and dynamic focusing during the receiving phase can be achieved in a similar way.