, _f The present invention relates to sensors, and is particularly concerned with sensors for use in the calibration of the acoustic % output of medical ultrasound equipment used for diagnosis or therapy.

Ultrasound has many medical uses, including scanning to 5 determine various internal body conditions such as the size and condition of a foetus in a womb, and for heat treatment, for example in the treatment of cancer. Ultrasound does however have a heating effect when it passes through the body and this effect can be significant, particularly when, as in many medical applications, a 10 focussed beam is used.

It is therefore important that acoustic wave apparatus be carefully calibrated in order to insure that operators of the apparatus should have a good knowledge of the effect that ultrasound will be having on a patient. In one calibration method a thermocouple 15 is positioned in a gel having the characteristics of tissue, and an ultrasound beam from the equipment under test is focussed on the thermocouple. There are two problems with this system. Firstly the thermoelectric effect is small and only becomes significant when the beam is focussed on the thermocouple. This makes it very difficult to 20 align the thermocouple and the beam. Secondly conventional thin wire thermocouples present a significant obstacle to ultrasound beams, giving rise to relative movement between the wires and the surrounding medium causing viscous heating effects which distort the test results. There is therefore a requirement for a thermocouple which allows 25 for easier alignment and which gives rise to.minimal perturbation of the ultrasound beam.

According to the present invention a thermal and acoustic sensor includes a first electrode of a first material and a second electrode of a second material, the electrodes meeting to form one junction of a 30 thermocouple, the junction being separated from a tip of a third ι electrode by a thickness of a layer of piezoelectric material.

The third electrode might be of any convenient material which could be one of the first or second materials.

The piezoelectric layer might be a plastic material such as, for example, polyvinylidene fluoride or a copolymer of difluoroethylene with trifluoroethylene or tetrafluoroethylene. These materials can be obtained in the form of thin films, a film typically having a thickness of 2 microns.

Suitable materials are silver or gold for the first material and nickel for the second material. They might conveniently be positioned on surfaces of the piezoelectric material in paint form by painting or spraying, using vapour deposition, or otherwise.

In a preferred form of. the invention for use in the calibration of acoustic wave equipment a device consists of a thin piezoelectric film having thin gold and nickel electrodes forming a junction of a thermocouple on a first surface thereof and a thin gold electrode with its tip underlying the junction on a second surface thereof. When the electrodes are connected to circuits to form respectively a thermocouple and a pressure sensor and the device is positioned in a test medium through which is passed a focussed beam from an acoustic generator the output of the thermocouple will be negligible if it lies outside the focussed beam whilst the pressure sensor, whilst giving maximum output ^* when lying at the focal point of the beam, will give some output whenever the beam impinges on the piezoelectric film.

One embodiment of the invention, and one use of the invention, will now be described, by way of example only with reference to the accompanying diagrammatic drawings, of which;

Figure 1 is a plan view of a thermal and acoustic sensor according to the invention,

Figure 2 is an elevation, in section along line II-II of Figure 1, of a detail of the sensor, and

Figure 3 s an elevation, in section, of a sensor being used to assist in the focussing of an acoustic wave generator.

A sensor 10 (Figure 1) consists of a thin layer 11 which might be a 5 micron thick film of piezoelectric material such as, for example, polyvinylidene fluoride, on a first surface 20 (see Figure 2) of which are positioned a first electrode 12 of a first material such

as, for example, gold and a second electrode 13 of a second material such as, for example, nickel. The electrodes are joined together at a junction 1 to form one junction of a thermocouple. The electrodes f( 12, 13. terminate at terminals 16, 17 respectively.

5 On a second surface 21 of the thin layer 11 is positioned a third electrode 18, of any suitable material which might, for example, be the same as that of one of the first two electrodes 12, 13. The third electrode 18 has a tip 19 lying such that the same normal to the layer 11 passes through both the junction 15 and the tip 1 (see 10 particularly Figure 2). The electrode 18 terminates at a terminal 23- The electrodes 12, 13, 18 can be positioned on the layer 11 by any convenient means, such as, for example, by painting or spraying, by vacuum deposition, or otherwise and should be a thin as possible, though in practice they will usually have to be thicker than 0.3 15 microns.

In use the terminals 16, 17 are used to connect the first and second electrodes 12, 13 into a first circuit, shown generally at 30 in Figure 1, such that the system acts as a thermocouple sensitive to the temperature at the junction 15. The circuit 30 includes 20 temperature indicating means 31- The terminal 23 of the third electrode and one of the terminals 16, 17, shown in Figure 1 as terminal 16 of the first electrode 12, are connected to a second circuit, shown generally at 40 in Figure 1, which includes a pressure indicator 4l such that pressure variations affecting the film in the 25 region of the junctions 19. 15 can, as a result of the piezoelectric effect, be measured.

In a typical use of the sensor, for the calibration of an acoustic wave generator to be used as a diagnostic ultrasound scanner, a sensor 10 is embedded in a bulk 0 (Figure 3) of gel having 30 characteristics simulating those of tissue. An acoustic wave generator 51 is positioned such that an ultrasonic beam 52 issuing

# therefrom has a focal point 53 falling as near as can be estimated to the junction 15• In general, and as illustrated in Figure 3. the

♦ focal point will fall neither on the junction 15 nor on the layer 11, 35 and as a result the temperature indicator 30 will show no change as a result of switching on the beam 5 . However, provided that the beam

2 impinges on the layer 11 there will be a response from the pressur indicator l. The nearer the focal point 53 is to the layer 11 the greater will be the response of the pressure indicator l, allowing the beam 2 to be focussed to position the focal point 53 on the laye 11, and the nearer the focal point 53 to the disposition of junction 15 and tip 19 the greater the response of the pressure indicator 4l, allowing the position of the focal point 53 on the layer 11 to be moved until it lies on this disposition, and hence on the junction 15, at which stage the temperature indicator 31 will respond and calibration can start.

It will, of course be realised that whilst the sensor herein described was developed for .the calibration of acoustic generators for use in medical ultrasound processes its uses are by no means limited to this function. Other possible uses are, for example, in environmental testing and for burglar alarms. Multisensor arrangements are possible, enabling temperature and pressure fields to be mapped simultaneously. Such multisensor arrangements could, for example, be incorporated into systems containing several different materials so as to mimic the more complex systems, such as the interface between soft tissue and bone, within the body.

It will also be realised that other embodiments of the invention are possible. For example, using known miniaturisation techniques it should be possible to make sensors small enough to be used in probes for insertion into living tissue, or into regions in which ultrasound treatment is being carried out.