GB2124376A - Ultrasonic radiometers - Google Patents

Abstract

An ultrasonic radiometer comprises a fluid container (2) in which a target assembly (14) is completely submerged in a fluid (4). The target assembly (14) is held submerged in the fluid by three tethers or flexible lines (24) each secured at one end to the base of the container. In this way the target assembly (14) is suspended so that it is neutrally buoyant. An ultrasonic radiation source (28) is directed at a conical surface (22a) of the target assembly. The downward radiation force on the target assembly is counteracted by means of a magnetic force applied to the assembly by a coil (10) which is disposed around a magnet (20) which depends from the target (22a). The magnitude of the current which is supplied to the coil (10) to restore the target assembly (14) to its null position provides an output indication of the magnitude of the ultrasonic energy to which the target is subjected. <IMAGE>

Description

SPECIFICATION Radiometers The present invention relates to devices for measuring radiation (hereinafter referred to as radiometers).

Ultrasonic energy needs to be measured for a variety of reasons. Furthermore, exposure of personnel to ultrasonic energy is often required to be monitored, for example, hospital patients are subject to ultrasonic radiation through diagnostic or therapeutic instruments.

One previously proposed device for measuring ultrasonic energy is described in a paper by J. A.

Rooney in "Ultrasound in Med a Biol" Vol. 1 pp 13-1 6 1 973. This device includes a target, submerged in water, for intercepting a beam of ultrasonic energy. When the beam impinges upon the target, the target experiences a force which is proportional to the ultrasonic power. The force, which is approximately 67 mg per watt of ultrasonic power can then be measured using a chemical balance coupled to the target by lines, extending down into the water and attached to the target. The problem which arises with this device is that the range of power required to be measured can vary from a few watts of ultrasonic power (such as is produced by therapeutic instruments) to a few milli-watts (such as is produced by diagnostic instruments).To accommodate such a wide range of power, the chemical balance needs to be highly sensitive.

However, such a highly-sensitive device is susceptible to ambient vibration and errors due to the effects of surface tension on the supporting lines at the water/air interface.

Another device is described by Farmery and Whittingham in "Ultrasound in Med a Biol" Vol 3 pp 373-379 1 978. This device uses a galvanometer movement with a target suspended in liquid paraffin for rotation about a vertical axis.

Whilst this device overcomes the surface tension problems of that previously discussed by disposing the entire device in the fluid, it is very subject to external vibrations. This has to some extent been overcome by the use of a highly damping fluid. However liquid paraffin has strongly frequency dependent absorption so that results for ultrasound sources of different frequency would not be directly comparable.

According to the invention there is provided an ultrasonic radiometer comprising a target and suspension means therefor entirely disposed within a mass of fluid such that the target is in a state of neutral buoyancy, said suspension means comprising a plurality of tethers arranged so as to render the target insensitive to external vibration whilst allowing deflection downwardly in response to a radiation force, means for directing a radiation source at the target, means for applying a counter-free to maintain the target in a predetermined position, and means for providing an output representative of the magnitude of the counter-force and thereby the magnitude of the radiant energy to which the target is subjected.

Such an ultrasound radiometer is extremely sensitive and has been found to be capable of prototype form of making measurements to better than 10 yW. The radiometer is moreover relatively immune to vibration.

Preferably the fluid is water which has a substantially frequency independent absorption of ultrasonic radiation.

An ultrasonic radiometer embodying the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a side elevation of a radiometer; Figures 2 to 5 are side elevations to a reduced scale of the radiometer of Figure 1 showing various modified tethering systems; Figure 6 is a circuit diagram of the radiometer of any of the modifications; and Figure 7 is a schematic side elevation showing a modified container design for a radiometer.

As shown in Figure 1 a container 2 holds a fluid 4, preferably water. A support platform 6 is mounted on the base of the container 2.

Mounted centrally on the platform 6 is a former 8 carrying a coil 10. Also mounted on the platform 6 and equiangularly spaced about the vertical axis of the former 8 are three posts 12.

A target assembly 14 includes a base member 1 6 carrying a downwardly-depending member 18, for example of PERSPEX (Registered Trade Mark) which in turn carries a cylindrical magnet 20. The magnet 20 is arranged to engage the former 8 and the member 18 has a collar 1 8a which is arranged to support brass trimming weights (not shown).

The base member 16 carries a hollow, airfilled, target body 22 having a dished (preferably conical) roof 22a of thin metal. The target assembly 1 4 as a whole is buoyant but is held submerged in the water by three tethers or flexible lines 24 (broken lines) each secured at one end to the base member 1 6 and each secured at the other end to a corresponding one of the three pillars. The tethers may be of a synetheticresin, flexible, tape, Secured to each line 24 at a central portion thereof is a counterweight 26; the three counterweights 26 each being of the same magnitude.

A source 28 of ultrasonic sound projects into the water 4 to direct ultrasonic energy on to the dished roof 22a of the target body 22.

An optical device 4 (a microscope for example) is mounted on the wall of the container 2 to view an alignment mark 42 (shown in exaggerated size for the sake of clarity) on the magnet 20.

As shown in Figure 6, the coil 10 is connected in series with a meter 34, and a variable resistor 30 and the series combination is connected across a DC source 32.

In operation trimming weights are initially added to or removed from the collar 1 8a to bring the alignment mark 42 into precise alignment with the optical device 40. In this position the portions of each line 24 on opposite sides of the associated weight 26 should be at right angles to each other.

The sound source 28 is then energised. The force resulting from the bombardment of the roof 22a with sound waves will depress the assembly 14 and so the alignment mark 42 will move out of alignment with the optical device 40. At this point the coil 10 is energised. The current drawn by the coil will create an electromagnetic force which will act to repel the magnet 20. By varying the value of the resistor 30, the current flowing through the coil can be adjusted until alignment mark 42 is again in alignment with the optical device 40.

At this point the current indicated by the meter 34 is recorded and this will given an indication of the energy of the incident ultrasonic sound waves.

Alternatively any form of variable voltage power supply could be used to produce the required current adjustment.

It will be appreciated that the arrangement of the target assembly in a totally submerged state with neutral buoyancy both avoids surface tension effects and renders the radiometer highly sensitive. In particular, the neutral buoyancy provides the radiometer with a non-linear forcedisplacement characteristic; however, because the target assembly is always returned to its original position (a null method of operation) the radiometer has the same high sensitivity independently of the magnitude of the ultrasonic energy.

The arrangement of the lines 24 to extend in substantially horizontal and vertical directions is believed to provide the radiometer with its maximum sensitivity.

In the modified radiometer outlines in Figure 2, the target assembly is arranged to have a greater density than that of water, but the neutral buoyancy is maintained by replacing the weights 26 with floats 26a. The floats can, as shown in Figure 2, be totally submerged or instead be allowed to float on the surface of the water.

In the modified radiometer shown in Figure 3, the target assembly can have a greater or lesser density than the supporting fluid. The target assembly is then maintained in a state of neutral buoyancy by a combination of weights 26 and floats 26a.

In the modified radiometer of Figure 4 the supports 1 2a are or are fixed to the sides of the container 2. As previously the weights 26 are suspended from flexible ties 24. As with the arrangement of Figure 1 the tie portions are in horizontal and vertical directions to achieve good sensitivity.

The modified radiometer of Figure 5 is suitable for use only in the case where the target and magnet assembly has the same overall density as the fluid. In this situation it is possible to omit the weights or floats of the previous embodiments and thereby produce a similar structure. The ties 24a are elastic and allow extension when the target assembly is depressed by the force of the ultrasonic radiation. This suspension system is not as sensitive as those previously described but does give a sensitivity of the order of 5 microwatts for 0.1 mm deflection which may be adequate in applications where the advantages of structural simplicity are greatest as, for example, for a portable radiometer.

A further modification of the radiometer is shown in Figure 7. In this case the container 2a is provided with a narrower central extension 34 in which the magnet 20 is disposed and about which the former 8 and coil 10 are located.

Suitable suspension systems for this form of container are those shown in Figures 4 and 5.

This type of container avoids the need to locate any electrical parts in the fluid and, moreover, simplifies changing of the coil as may, for example, be required when changing the range of power which the radiometer is being used to measure.

When the fluid used is water the radiometer is substantially frequency independent. When selecting a fluid other than water care must be taken to use a fluid which is substantially frequency independent.

If oil is used, which would render the radiometer frequency dependent, compensation must be provided when making measurements with different frequency ultrasonic beams.

While the radiometer described relies on manual adjustment to produce a required reading it will be appreciated that a servo system could be used instead to maintain automatically the target assembly in a predetermined position. Thus a transducer could be used to provide an electrical output representative of the position of the target assembly. The electrical output is then used to control a servo motor coupled to the variable tapping in the resistor 30 to vary the resistance of the resistor, in a sense to maintain the target assembly in its predetermined 'null' position.

Preferably a digital display is used to provide an indication of the ultrasonic energy measured by the radiometer.

A suitable form of transducer would be a sensing coil (not shown) disposed around magnet 20 and concentric with the coil 10.

It will be appreciated that various types of transducer would be suitable for measuring the amount and sense of the deviation of the target assembly from its 'null' position.

Where different sources of ultrasonic energy are used targets of different size and shape may be needed to be sure that all the radiated energy is collected. The target body 22 can thus be readily detached from the base member 1 6 and replaced by another target body, which has a shape specifically adapted to the shape of the beam which is under test.

The shape of the target body surface which is directed towards the radiation source is preferably such that at each point the surface is inclined to the direction of the incident radiation. Thus for a parallel beam a conical surface with its axis in the direction of the beam is appropriate. The surface design should be such that the net radiation force is as nearly as possible in the direction in which the target is free to move,-i.e. downwardly- whilst scattering the radiation away from the source to avoid interference effects.

The described radiometers have been found capable of measuring power of less than 10 micro watts.

Because of the manner in which the target assembly is supported by tethers it can be readily transported and is generally insensitive to ambient vibration.

It will be appreciated that instead of using electromagnetic force to maintain the target assembly in its predetermined position other forces can be used.

The radiometers described have been found to have a linear response up to the threshold at which cavitation occurs.

The radiometers described are generally econmical to manufacture.

It will be appreciated that while the described radiometers are used to measure ultrasonic radiation they can equally be used to measure other forms of radiation which impart a physical force when impinging upon a target.

Claims (10)

Claims

1. An ultrasonic radiometer comprising a target and suspension means therefor entirely disposed within a mass of fluid such that the target is in a state of neutral buoyancy, said suspension means comprising a plurality of tethers arranged so as to render the target insensitive to external vibration whilst allowing deflection downwardly in response to a radiation force, means for directing a radiation source at the target, means for applying a counterforce to maintain the target in a predetermined position, and means for providing an output representative of the magnitude of the counter-force and thereby the magnitude of the radiant energy to which the target is subjected.

2. A radiometer as claimed in claim 1, wherein the fluid is water.

3. A radiometer as claimed in claim 1 or 2, wherein the surface of the target is inclined at substantially all points to the direction of the incident radiation.

4. A radiometer as claimed in claim 3, wherein the target surface is conical with an axis in the direction of the incident beam.

5. A radiometer as claimed in any one of the preceding claims, wherein the means supplying the counter-force is a coil which when energised electromagnetically couples a magnetic member fixed rigidly to the target.

6. A radiometer as claimed in any one of the preceding laims, wherein neutral buoyancy of the target is achieved by the attachment of the tethers of weights and/or buoyant members in accordance with the relative density of the target and the fluid.

7. A radiometer as claimed in claim 6, wherein each tether has a point of flexure at the location where the weight or buoyant member is attached; the angle of the tether at the point of flexure being arranged to be approximately 900.

8. A radiometer as claimed in any one of the preceding claims, wherein there are three tethers equiangularly positioned around the target.

9. A radiometer as claimed in any one of the preceding claims, wherein the target is removably mounted to allow a target to be used which has a surface shape which is specifically adapted to receive radiation from the source which is being measured.

10. An ultrasonic radiometer substantially as herein described with reference to any one of the accompanying drawings.