Method and apparatus for ultrasonic detection of inclusions in a flowing fluid

Abstract

Inclusions in a flowing fluid, such as bubbles in a flow of blood, are detected by transmitting a continuous ultrasonic signal normally across the flow and detecting changes in the amplitude of the received ultrasonic signal, these changes being caused by changes in the acoustic impedance of the fluid due to inclusions.

Description

BACKGROUND OF THE INVENTION

The medical use of an extra-corporeal circulation system through which part or all of a patient's blood passes is now comparatively common. Such systems are used during certain surgical operations, for example heart and transplant operations, and also for example where a patient's blood is passed through an artificial kidney machine. In all such cases it is of paramount importance that blood returning to the patient's body should contain few or preferably no gaseous bubbles, as bubbles in the blood stream are apparently the cause of otherwise unexplained emboli, which can result in fatal brain haemorrhages.

For this reason, extra-corporeal circulation systems normally include a de-bubbler. It has also been proposed that the blood be checked for bubbles subsequent to the de-bubbler and before being returned to the body, and for this purpose there have been proposed ultrasonic devices for the detection of bubbles in a flow of blood. Those previously proposed detection devices have however had a number of disadvantageous features. In particular, they have involved the provision of a special chamber through which the blood flows and across which an ultrasonic beam is passed to detect any bubbles. The coupling of this special chamber into an extra-corporeal circulation system adds complications and in particular makes sterilisation more difficult, with attendant added risk to the patient. Moreover, the previously proposed detection devices have lacked sensitivity and discrimination in the detection of bubbles.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an improved method and apparatus for detecting inclusions in a flowing fluid.

Another object of the present invention is to provide an ultrasonic method and apparatus for detecting bubbles in a flow of blood.

Another object of the present invention is to provide an ultrasonic method and apparatus for detecting bubbles in a flow of blood which can be applied to the flow of blood in an existing path, the blood not having to be passed through a different or additional path for the purpose of the detection.

According to the present invention there is provided a method of detecting inclusions in a flowing fluid, the inclusions being of different acoustic impedance from the fluid, comprising positioning an ultrasonic transmitter and an ultrasonic receiver externally of a path within which the fluid is flowing such that when the transmitter transmits an ultrasonic signal into said fluid in said path a part of said signal is received from said fluid by the receiver, and detecting changes in the amplitude of said received signal due to inclusions in the fluid.

According to the present invention there is also provided apparatus for detecting inclusions in a fluid flowing in a path not forming part of the apparatus, the inclusions being of different acoustic impedance from the fluid, comprising an ultrasonic transmitter, an ultrasonic receiver, means to position the transmitter and receiver relative to said path such that when the transmitter transmits an ultrasonic signal into fluid flowing in said path a part of said signal is received from the fluid by the receiver, and means to detect changes in the amplitude of said received signal due to inclusions in the fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of an ultrasonic detector head,

FIG. 2 is a diagrammatic cross-section of the head of FIG. 1,

FIG. 3 is a schematic diagram of an ultrasonic apparatus for detecting gaseous bubbles in a flow of blood and including the head of FIG. 1,

FIG. 4 is a perspective view of an alternative form of ultrasonic detection head,

FIG. 5 is a diagrammatic cross-section of part of the head of FIG. 4, and

FIG. 6 is a diagrammatic cross-section of an alternative form of ultrasonic detector.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will first be specifically described in the form of apparatus for the ultrasonic detection of gaseous bubbles in a flow of blood, but thereafter alternative forms and uses of the invention will be briefly mentioned. It will be assumed that the apparatus is to be used in association with an extra-corporeal circulation system and that at the point in the system where the blood is to be checked for bubbles it is flowing in a plastics tube. The flow will normally be a pulsating one, either due to the action of the patient's heart or of a pump forming part of the system.

Referring to FIG. 1, the apparatus comprises an ultrasonic detector head made in two parts 1 and 2 which are separable but have mating plane surfaces 3 and 4 which in use of the head are in contact with one another. To ensure alignment the part 1 has two pegs 5 projecting from the surface 3 for co-operation with correspondingly positioned blind holes 6 in the part 2. Bolts (not shown) then pass through apertures (not shown) in the part 2 and thread into holes (not shown) in the part 1 to hold the parts 1 and 2 together. Alternatively the parts 1 and 2 may be hinged together.

Bisecting the surface 3 longitudinally is a channel 7 of rectangular cross-section, the longer dimension of the cross-section being disposed parallel to the surface 3. In use of the apparatus a plastic tube 8 (see FIG. 2) forming part of an extra-corporeal circulation system, and in which is flowing the blood to be investigated for bubbles, can be laid in the channel 7 and then confined therein by bringing the parts 1 and 2 together. The dimensions of the channel 7 are selected such that when so confined the cross-section of the tube 8 is resiliently deformed from the normal circular shape to the rectangular shape of the channel 7, but without restriction of the internal cross-sectional area of the tube 8, which, if it occurred, would restrict the blood flow and might even promote the formation of bubbles. The ends 9 of the channel 7 and the corresponding portions of the surface 2 are tapered or otherwise suitably shaped to ease the transition in the shape of the cross-section of the tube 8 from circular to rectangular and vice versa.

It is particularly to be noted that in use of the apparatus the blood is confined to the path defined by the tube 8 forming part of the extra-corporeal circulation system, and does not need to pass through any special chamber or other flow path additional to that circulation system. There are therefore no additional sterilisation problems.

The parts 1 and 2 comprise rectangular metal blocks 10 and 11 respectively, and rectangular metal caps 12 and 13 respectively removably secured thereto by bolts (not shown). Passing through the blocks 10 and 11 normal to the surfaces 2 and 3 are respective apertures 14 and 15, which house an ultrasonic transmitter and an ultrasonic receiver respectively.

Referring also to FIG. 2, the transmitter and receiver are in substance the same, so only the transmitter will be described in detail. The transmitter is formed by a piezoelectric crystal 16 in the shape of a thin rectangular block mounted with its general plane normal to the length of the channel 7, and hence normal to the flow of blood in the tube 8, that is parallel to the plane of the paper in FIG. 2. The crystal 16 is retained in place by supports 17 made of transparent plastics material which are themselves held in place by small metal plates 18 retained by screws 19. The supports 17 are initially thin rectangular blocks but are bent to a slight "U-shaped" cross-section when they are pressed by the plates 18 against the two smallest faces of the crystal 16.

The effect of the supports 17 pressed against opposite ends of the crystal 16 is to pre-stress and hence damp the crystal 16, so that when in use an oscillatory electric signal is supplied to the crystal 16 it does not vibrate at its natural resonant frequency, but at some other frequency determined by the extent of the damping.

The plastics material of the supports 17 is preferably the same as that of the tube 8, which may be polyvinyl chloride, and the surfaces 20 of the supports 17 which in use are to bear against the opposite sides of the tube 8 are, in the absence of the tube 8, just above the bottom surface of the channel 7 and the surface 4 respectively. In this way firm pressure is ensured, and acoustic mis-match minimized. Moreover, because of the pre-stress on the crystal 16 fluctuations in the pressure on the crystal 16 due to pressure pulsations in the blood flow will have little effect on the operation of the transmitter or on the output signal supplied by the receiver.

The input oscillatory electric signal in the crystal 16 of the transmitter is supplied over leads (not shown) which pass into the cap 15 and are secured to electric terminals therein which in turn are connected to the crystal 16. In a similar way an output oscillatory electric signal is derived over leads (not shown) from the receiver.

A further feature of the mounting of the crystal 16 of the transmitter and receiver in the supports 17 of plastics material, is that it minimises the direct transmission of ultrasonic energy from the transmitter to the receiver via the material of the head itself.

The shape of the crystal 16 of the transmitter is such that when energized it emits an ultrasonic signal in the form of a thin and substantially parallel beam, the general plane of the beam coinciding with the general plane of the crystal 16. The beam passes normally through the supports 17 and the tube 8 in the longitudinal mode in the direction indicated by the arrows 21, and in so doing substantially fills the cross-sectional area of the tube 8. On emerging from the tube 8 it impinges on the crystal 16 of the receiver which thereupon supplies the output oscillatory electric signal.

Reference will now be made to FIG. 3 which shows the electronic circuitry which with the head described above forms the complete apparatus. In FIG. 3 the head is indicated by the transmitter 25 and the receiver 26 disposed one on each side of the tube 8. The transmitter 25 is energised by an oscillatory 27 which supplies a continuous wave signal the frequency of which may lie in the range 25 to 400 kilohertz, a frequency of 65 kilohertz having been found particularly suitable in one embodiment of the apparatus.

The output signal derived from the receiver 26 is supplied via an alternating current amplifier 28 to a demodulator 29 the output of which is connected to a band-pass filter 30 and back via an automatic gain control circuit 31 to the amplifier 28. The output of the filter 30 is connected via a level control circuit 32 to a trigger circuit 33, and also to a magnetic tape recorder 34, preferably of the cassette type.

The output of the trigger circuit 33 is connected to an audible alarm 35 such as a buzzer, and also to a counter 36 which may also be connected to the alarm 35. A clock 37 which generates a clock pulse or other suitable timing signal is connected to the counter 36 and to the recorder 34. Read-out devices 38 and 39 are connected to the counter 36 and the clock 37 respectively.

The operation of the apparatus will now be described.

With the transmitter 25 energized the ultrasonic beam which passes the tube 8 and impinges on the receiver 26. If there is a smooth flow of blood without bubbles in the tube 8 then the signal supplied by the receiver 26 is similar to the input signal and in particular is of constant amplitude. If however a bubble passes across the ultrasonic beam this has the effect of temporarily interposing a different acoustic impedance in the beam path, so there is a transient dip in the amplitude of the output signal. After amplification this dip is detected by the demodulator 29 as a pulse. The pass bond of the filter 30 is selected to pass only signals of frequency corresponding to the transient resulting from a bubble passing across the thin ultrasonic beam. In one particular instance these transients had frequencies of about 400 kilohertz and upwards.

The automatic gain control circuit 31 operates to maintain the amplitude of the amplified output signal substantially constant for the purpose of minimising relatively long-term fluctuations due for example to pulsations originating from the pump. The response time of the automatic gain control circuit 31 is however long relative to transients caused by bubbles.

Pulses passing the filter 30 are recorded by the recorder 34 and also trigger the trigger circuit 33 if of an amplitude exceeding that set by the level control circuit 32, which may of course be incorporated in the trigger circuit 33. Pulses supplied by the trigger circuit 33 pass to the alarm 35 and to the counter 36.

The precise form and operation of the counting, read-out and recording arrangements are not of the essence of the invention and can be adapted by well known techniques to give a required output or outputs in a required form. For example, where the presence of bubbles is particularly critical the alarm 35 may be operated on the occurrence of each bubble, whilst in a less critical situation it may be controlled to operate only on the occurrence of a predetermined cumulative total number of bubbles or only if a predetermined number of bubbles per unit time is exceeded.

Similarly the read-out devices 38 and 39 may indicate the cumulative total number of bubbles and the elapsed time since counting began, or may indicate a moving average of the number of bubbles per unit time.

The timing signal supplied by the clock 37 to the recorder 34 is recorded to provide a time scale against which the pulses passing the filter 30 are recorded. In addition provision may be made for a voice track to enable a voice/time/bubble record of say a surgical operation to be made for subsequent analysis.

The apparatus is sufficiently sensitive to discriminate small bubbles which are close together, and to permit the size of individual bubbles to be determined by analysis of the pulses passing the filter 30.

Referring now to FIGS. 4 and 5, these show an alternative and simpler form of ultrasonic transducer head usable in certain applications. The head comprises a body 40 of slightly resilient plastics material of generally C-shape. The aperture 41 of the body 40 is dimensioned and shaped to receive a plastics tube 42 through which passes a flow of blood, as in the embodiment described above. The resilience of the body 40 enables it to be deformed to permit lateral insertion of the tube 42. Preferably the tube 42 is deformed to rectangular cross-section as described above.

Embedded in respective limbs of the body 40 are an ultrasonic transmitter and receiver formed by piezoelectric crystals 43 and 44. The material of the body 40 bears on the surfaces of the crystals 43 and 44 nearest to and further from the tube 42 so as to provide the pre-stressing described above. Air gaps 45 are interposed between the other surfaces of the crystals 43 and 44 and the material of the body 40, or such air gaps may be omitted.

Apart from the points specifically mentioned, the general form and operation of the apparatus incorporating a head as described with reference to FIG. 4 and FIG. 5 is similar to that of the apparatus described with reference to FIGS. 1 to 3.

In some cases it may be necessary to investigate for the presence of bubbles within a patient's body, for example during decompression of divers or in the case of bends. For this purpose an ultrasonic detector head as shown in FIG. 6 to which reference is now made may be used. This head comprises a separate ultrasonic transmitter 50 and receiver 51 mounted on a suitable strap 52 which can be placed tightly around a patient's limb. The transmitter 50 and receiver 51 are both generally as described above and each comprises a damped piezoelectric crystal. Preferably both the transmitter 50 and the receiver 51 are mounted so that their direction of emission and reception of ultrasonic energy is controllable, for example by angular movement of the transmitter 50 and receiver 51 relative to the strap 52.

In use of such a head the transmitter 50 is positioned so that an ultrasonic beam is directed towards an artery 53 to be investigated. If the blood flow is normal there will be a small amount of ultrasonic energy reflected to the receiver 51, but if a bubble passes through the transmitted beam there will be a transient increase in the level of the reflected ultrasonic energy, which can be detected. It is necessary for the transmitter 50 and receiver 51 to be accurately directed towards the artery 53, and focussing of the ultrasonic energy may be improved by providing each with acoustic lenses.

Apart from the points specifically mentioned, the general form and operation of apparatus incorporating a head as described with reference to FIG. 6 is similar to that of the apparatus described with reference to FIG. 1 to FIG. 3.

Various other modifications can of course be made without departing from the invention as defined by the appended claims. For example other forms of ultrasonic transducer, such as magnetostrictive devices may be used in place of piezoelectric crystals for the ultrasonic transmitter and receiver.

Although described above only with reference to the detection of bubbles in blood, the invention can be adapted for use in many other circumstances where it is required to detect inclusions in a flowing fluid, the inclusions being of different acoustic impedance from the fluid. The flowing fluid may be liquid or gaseous, and the inclusions may be gaseous or liquid bubbles or solid particles.

Specific examples of other uses include the detection of bubbles in photographic solutions; checking outgassing operations; detecting bubbles in liquid sodium loops in fast nuclear reactors; detecting bubbles in superheated water in heat exchangers; and detecting bubbles in fuel lines.

Claims

I claim:

1. Apparatus for detecting inclusions in a fluid flowing within a flexible tube of substantially circular cross-section and not forming part of said apparatus, said inclusions being of different acoustic impedance from said fluid, said apparatus comprising:

an ultrasonic detector head having defined therein an aperture of substantially rectangular cross-section, said aperture being dimensioned to receive said tube and to deform said tube to adopt a substantially rectangular cross-section;

a pair of flexible elongate supports mounted to respective ones of a pair of opposite surfaces of said aperture, said supports being mounted parallel to and opposite one another and each extending substantially entirely across said surface to which it is mounted, and each support being above said surface to which it is mounted, whereby, when said tube is received in and deformed by said aperture, each of said supports is urged against substantially the whole of a respective one of a pair of opposite sides of said substantially rectangular cross-section of said tube;

an ultrasonic transmitter mounted to said ultrasonic detector head and operatively coupled to one of said supports to transmit an ultrasonic signal, via said one support and the wall of the tube, into and across said fluid flowing within the tube, said signal being transmitted through the flow in the form of a planar beam substantially transverse to the direction of flow and traversing substantially the whole of the cross-section of the flow;

means for energizing said transmitter to cause it to continuously transmit said signal;

an ultrasonic receiver mounted in said ultrasonic detector head and operatively coupled to the other of said supports to receive said signal; and

means coupled to said receiver to detect a change in amplitude of the signal received by the receiver caused by an inclusion passing through said planar beam.

2. Apparatus according to claim 1 wherein said ultrasonic transmitter and said ultrasonic receiver comprise respective ultrasonic transducers each including a transducer element.

3. Apparatus according to claim 2 wherein said transducer elements are piezoelectric crystals.

4. Apparatus according to claim 2 wherein said transducer elements are magnetostrictive devices.

5. Apparatus according to claim 2 wherein said means for energizing said ultrasonic transmitter comprises oscillator means connected to energize said transmitter with a continuous wave electric signal.

6. Apparatus according to claim 5 wherein said electric signal has a frequency in the range 25 to 400 kilohertz.

7. Apparatus according to claim 5 wherein said electric signal has a frequency of 65 kilohertz.

8. Apparatus according to claim 1 wherein said head is a generally C-shaped body formed of resilient material and said aperture is the central aperture within said body.

9. Apparatus according to claim 1 wherein said ultrasonic detector head comprises a metallic body comprising two parts movable with respect to one another to allow said tube to be received thereby, said aperture being elongate and respectively defined by a pair of mating surfaces of said two parts.

10. Apparatus according to claim 1 wherein said supports are of a plastic material.

11. A method of detecting inclusions in a fluid flowing within a flexible tube of substantially circular cross-section, said inclusions being of different acoustic impedance from said fluid, said method comprising the steps of:

deforming at least part of said tube so that it adopts a substantially rectangular cross-section;

positioning a pair of flexible elongate supports so that they are parallel to and opposite one another and so that they are each urged against a respective one of a pair of opposite sides of said substantially rectangular cross-section of said tube;

energizing an ultrasonic transmitter operatively coupled to one of said supports to transmit an ultrasonic signal, via said one support and the wall of the tube, into and across said fluid flowing within the tube, said signal being transmitted through the flow in the form of a planar beam substantially transverse to the direction of flow and traversing substantially the whole of the cross-section of the flow; and

detecting a change of the signal received by an ultrasonic receiver operatively coupled to the other of said supports caused by an inclusion passing through said planar beam.

12. A method according to claim 11 wherein the flowing fluid is blood and the inclusions are bubbles.