NZ218636A - Microwave thermography aerial - Google Patents

Description

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Public- - Dst.*; . 2z.N0V.J98? P.O. ",o. ., .1. 5P.?r. _2_M6-. Initials NEW ZEALAND PATENTS ACT, 1953 No.: Date: Divided out of No. 206,774 6 January 1984 COMPLETE SPECIFICATION MICROWAVE THERMOGRAPHIC APPARATUS >/We. THE UNIVERSITY COURT OF THE UNIVERSITY OF GLASGOW. The University, Glasgow G12 8Q(]> Scotland, a British body Corporate, U hereby declare the invention for which X/ we pray that a patent may be granted to jp&/\is, and the method by which it is to be performed, ,.y'^ particularly described in and by the following statement:- ©\ -v * \ 4 - 1 - -9 0CTJ987^1 (followed by page la) \ /?• # 2 18636 1 a - This invention relates to apparatus for use in microvave thermography.

Microwave thermography is a known technique for use in diagnosis of bio-medical maladies, which functions by detecting a temperature variation in an affected subcutaneous part of the body with respect to the normal body temperature.

Infra-red thermography, which is also known, is only effective in determining body surface temperatures but waves of longer wavelength than infra-red have the power to penetrate substantial thicknesses of body tissue and it is those waves, referred to simply as microwaves, which are emitted by a deep lying part of a body and reach the surface with sufficient power to the detected. Apparatus used for microwave thermography is thus a form of radio receiver sensitive to waves in the range of frequencies capable of penetrating appreciable thickness, for example several cm, of body tissue. This frequency range is roughly 1-10 GHz. The actual waves produce a "noise signal" the intensity of which is observed and interpreted to give an indication of the temperature of the object from which the wave originated.

A known form of microwave thermography apparatus incorporates a form of comparator receiver known as the Dicke type comprising a reference load which is maintained at a controlled and measured temperature (in practice a temperature close to normal body temperature) generating a reference noise signal corresponding with the controlled temperature, a switching device which feeds the reference noise signal alternately with the noise signal emanating from a sensing aerial placed in proximity to the object the temperature of which is to be investigated to a receiver v^hich combines the signals to produce an output signal of the form G (Tg - TQ) where G represents the receiver gain, T^ represents the signal m lO 2 186 3 generated by the reference load and T represents the b signal from the aerial. The receiver output signal is then added to a signal directly derived from in order to provide a measure of alone.

Since for the frequencies of interest the gain G cannot be highly stabilized in the known apparatus whereas can be accurately measured, is normally chosen and set to be as close as possible to the temperature Tc so that measurement error coming from the unstabilized gain factor of the receiver is made as small as possible.

Any loss of signal in circuits or components at or preceding the input to the receiver degrades the noise figure (or equivalent effective noise temperature) of the receiver. These equivalent parameters (noise figure and effective noise temperature) are a most important measure of the performance of the receiver for this application. Degradation of this parameter causes a proportionate degradation of the source temperature measuring resolution of the apparatus and/or an increase in the measurement time required to achieve a given source temperature measuring resolution. It is therefore important to minimize circuit or component signal losses in the essential circuits and components preceding or at the input of the receiver viz. the aerial, the switch and associated conductors. For the known Dicke receiver the signal switch and its associated signal circuits are significant sources of signal loss.

In practical terms the known Dicke receiver configuration described previously suffers from three disadvantages: 1) It requires a two-throw switch. Two-throw electro-mechanical switches capable of operating at microwave signal frequencies and having signal path losses acceptably low for^the application considered 2 18636 here can be constructed but the rate of switching is severely limited since contact bounce can cause problems and such a switch has a limited operating life at the high switching frequency required to avoid 5 or reduce receiver amplifier and detector flicker noise. Solid-state microwave switches, which normally use semi-conductor diodes for the switched elements, can have a very high switching rate and a very long operating life but a two-throw form of solid 10 state switch has signal losses which are high for the application considered here. 2) When the switch connects the receiver to the aerial, a thermal noise signal travels from the receiver to the aerial arising from the effective noise temperature of the receiver. At the known forms of aerial there exists a significant signal reflection coefficient for the aerial/source interface which reflects back into the receiver a portion of'the receiver noise signal and thus causes the measurement 20 of source temperature to be in error by an amount proportional to this reflected signal. 3) The existence of a significant reflection coefficient as described in 2) above, which is equivalent to an impedance mis-match, causes the gain of the 2 5 receiver to change from the gain obtained when the input of the receiver is connected by the switch to the matched impedance reference load. The change of gain, occurring as it does at the switching frequency, causes a spurious output signal from the receiver and 30 hence an error in the measurement of the source temperature.

Since the observable difference in temperature between normal and diseased tissue can be and often is less than 1 Celsius degree the need for high accuracy in the 35 temperature resolution of the' apparatus will be understood.

* New Zealand Patent Specification 206,774 claims apparatus for use in microwave thermography, said apparatus comprising a signal-receiving aerial sensitive to microwave radiation emanating from a body having a mean body temperature, a reference load capable of being maintained at a chosen temperature substantially equal to said mean body temperature and of emitting a thermal noise signal at microwave frequency corresponding to that temperature, a receiver having an input impedance matched to the impedance of said reference load and incorporating means for comparing the thermal noise signals emanating from said aerial and from said reference load, and a three-port non-reciprocal circulator interconnecting said aerial, reference load and receiver, said circulator having respective ports taken in circulation order connected to the noise-signal output of said reference load, to the aerial via a single throw on-off microwave switch, and to the receiver input.

According to the present invention there is provided apparatus for use in microwave thermography, said apparatus comprising a signal-receiving aerial sensitive to microwave radiation emanating from a portion of a living-tissue body having a mean tissue impedance, said aerial comprising a tubular metallic waveguide having a radiation receiving end of circular'cross-section dimensioned to receive radiation modes up to and including said tubular metallic waveguide having a signal output end incorporating a coaxial cable connection , a broad band-waveguide to coaxial-line mode transformer connected between the receiving end and the output end of said tubular metallic waveguide and arranged to deliver only radiation of the TE^ mode to the coaxial cable connection , and the interior of the tubular metallic -waveguide being filled with low-loss dielectric material providing the aerial with a planar radiation-receiving-end face and a receiving-end impedance close to the mean tissue impedance of said body.

The present invention will now be exemplified with reference to ccompanying drawings and description, wherein the drawings: -<Nvr ^ Fig. 1 diagramatically illustrates a known form of 9 OCT 19873 * 2 1863 apparatus for use in microwave thermography; Fig. 2 diagramatically illustrates a form of apparatus for use in microwave thermography in accordance with the present invention; Fig. 3 is a diagram illustrating the effects of signal reflection at the body surface/aerial interface for the Fig. 1 apparatus; Fig. 4 illustrates the signal paths taken in the apparatus of Fig. 2 firstly with the switch in its open position and secondly with the switch in its closed position; Fig. 5 is a block diagram of a complete microwave thermography system incorporating the Fig. 2 apparatus; Figs. 6, 7 and 8 are sectional views through different forms of aerial; Fig. 9 is a sectional end view of the Fig. 6 aerial; and Fig. lO illustrates a further form of aerial.

In the drawings 1 denotes a reference resistive load thermally controlled to operate at a selected reference temperature and which provides a corresponding microwave frequency noise signal output; 2 denotes an aerial the purpose of which is to receive microwave radiation from the body under investigation, said radiation emanating from a source of temperature T ; and 3 denotes O a receiver arranged to amplify signals applied to it and indicate in intelligible form the chosen characteristics of the signals so received. In the known apparatus illustrated in Fig. 1, 4 denotes a two-way switch arranged to oscillate between its two positions in one of which the load 1 is connected to the receiver 3 and in the other of which the aerial 2 is put into connection with the receiver 3.

In operation of the know device of Fig. 1 the aerial 2 is placed in contact with body and the switch 4 2 18636 caused alternately to connect the aerial 2 and the reference load 1 to the receiver 3. When the aerial 2 is connected to the receiver the signal Tg from the aerial 2 is passed to the amplifier then when the switch 5 breaks the connection to the aerial it connects the reference load to the receiver and the reference signal Tq is fed to the receiver so that the receiver issues an amplified difference signal at the switching frequency the level of amplification depending upon G, the gain of 10 the receiver.

In Fig. 3 the line 8 represents the body surface/ aerial interface at which an impedence mismatch reflection coefficient ^ occurs. The source temperature viewed by the aerial is Tg and the receiver 3 has an input effective 15 noise temperature of T . When switch 4 connects the aerial to the receiver the noise signal originating from the receiver input follows the path 7 to interface 8 there to be partly reflected along path 7B as a^signal of magnitude^/) TR. The partly transmitted receiver-noise 20 ' signal which follows the path 7A into the body is assumed to be absorbed and has no further effect. The signal from source Tg initially follows the path 9, and is partly reflected from the interface 8 along the path 9B with subsequent absorption in the body, and partly 25 transmitted along the path 9A into the receiver with magnitude of signal .(1-^)^. The resultant receiver signal is then Tg + />(TR - Tg) and measurement error of source temperature T resulting from the impedence mis- O match reflective coefficient can only be zero when T is equal to T which is not a practical possibility since T is not readily controllable. However this measure-R ment error can be minimised by minimising reflection coefficient /° and for this purpose it is necessary to design aerial 2 to have an impedance which is as close 35 as possible to the mean body-tissue impedance as will 2 18636 be explained. Of course, when switch 4 connects the reference load 1 to the receiver 3, because the load impedance is matched there is an absence of reflection of the receiver input noise signal and the signal delivered 5 to the receiver is simply TQ.

As has been explained previously the known Dicke-type receiver (see Fig. 1) suffers from the three particular disadvantages of (1) limited switching frequency and short operating life or relatively high signal loss, (2) 10 receiver effective input temperature reflected error signals, and (3) error signal-production by modulation of the receiver gain at the switching frequency resulting from aerial impedance mis-match. The arrangement of the components at the input of the receiver as shown in 15 Fig. 2 is intended largely to overcome the above stated disadvantages.

In Fig. 2, 5 denotes a 3-port microwave circulator of the non-reciprocal type, for example a ferrite circulator, the ports being connected in order firstly 20 to the output of the reference load 1, secondly to the aerial 2 by way of a single-throw on-off microwave switch 6, and thirdly to the receiver 3. The circulator 5 and the single-throw switch 6 are the essential component parts which provide the advantages of 25 Dicke comparator receiver type operation while largely overcoming the above stated disadvantages. In Fig. 2 the reference load is always connected into the circulator 5 and the receiver 3, and the switch 6 has the function only of connecting and disconnecting the 30 aerial 2 to and from the circulator 5. Fig. 4 shows the paths taken by the signals from the reference load, from the receiver input, and from the aerial. The signal paths through the non-reciprocal circulator itself are (with reference to Fig, 4) Ato B, B to C, and 35 C to A. The practical construction of such a circulator 2 1 3 6 3 is such that the signal loss along these paths is acceptably small for the application considered here and the isolation provided between the ports A, B and C in the reverse direction is such that signal transmission in 5 such reverse direction can. be neglected.

The single-throw switch 6 used in the arrangement of Fig. 2 has, in the practical form of a solid-state diode (not transistor] switch, a significantly lower signal loss than the tvo-throw switch of the known arrangement 10 of Fig. 1 which as previously explained is of great importance. Where the switch 6 is in the open position it presents an open circuLt or very large impedance mismatch to signals travelling towards it from either the aerial 2 or the circulator 5 and these signals will thus 15 experience complete or nearly complete reflection at the switch.

With the svitch 5 in the open position the signal path is, with reference to Fig. 4, from the reference load 1 at temperature TQ through the circulator 5 from 20 A to B, from the port B of the circulator 5 to the open-circuit of the switch 6 where it is reflected back to the port B of the circulator 5, through the circulator 5 from B to C and hence into the receiver 3. The noise signal resulting from the input effective noise 25 temperature (T ) passes through the circulator 5 from C to A to the reference load 1 which is impedance matched so that there is an absence of any reflected signal. The source signal (Tg) is totally reflected at switch 6 and therefore does not enter circulator 5. The output 30 signal from the receiver will for this case be GT^ where G is the gain of the receiver amplification stage.

With the switch 6 in the closed position the receiver input effective noise temperature signal traverses circulator 5 from port C to port A but gives rise to no 35 reflection because reference load 1 is impedance matched. 2 186 However the signal from reference load 1 at temperature Tq now passes through the circulator 5 from A to B and through the switch.6 to the aerial 2 where it is partly reflected at interface 8 in a manner similar to that 5 explained with reference to Fig. 3 and therefore combines with the part transmitted signal emanating from the source at temperature Tg to be delivered from aerial 2 through the switch 6, into the circulator 5 from B to C / and hence to the receiver 3. If the reference load 10 temperature TQ is set at or close to the source temperature T , which is a practical possibility, the measurement error resulting from aerial impedance mismatch reflection is eliminated or minimized. The output signal from the amplifier will be GTg for this case. 15 The receiver output arising from the repeated switching of switch 6 is then the difference between the two cases described above, that is G(Tg - T^) , and it will be seen that because the temperature of the reference load is chosen to be at or close to the source 20 temperature Tg there is a minimum dependency upon gain G. Furthermore by virtue of using circulator 5 and matched load 1 the second of the disadvantages previously described of the known Dicke comparator receiver configuration is avoided. A still further improvement in 25 measurement efficiency can be achieved by design of aerial 2 in the manner which has been explained whereby the reflection coefficient j> is minimized.

By way of example Fig. 5 illustrates the Fig. 2 arrangement forming part of a complete microwave 30 thermography apparatus from which it will be observed that receiver 3 is of the known superheterodyne type incorporating local oscillator 3A set at 3.1 GHz, mixer 3B operating in the band 2.6 - 3.6 GHz, intermediate frequency amplifier/detector 3C operating over the 35 5 - 500 MHz band, low frequency amplifier 3D operating ? 1 863 at about 1 KHz and coherent detector 3E which is switched in common with switch 6 by means of a 1 KHz modulator 30. The output from receiver 3 is delivered to a processor 31 together with a signal directly obtained from reference 5 load 1 in order for the processor 31 to output a signal proportional only to Tg.

Turning now to the preferred design of aerial 2 it will be appreciated that the electromagnetic wave impedance of body tissues is normally between 60 and 150 10 ohms and the impedance of normal still air is in the region, of 377 ohms. If the microwave transmission had to pass through air there would be a large signal loss by reflection at the interface between the body surface and the air (in addition to attenuation losses). For 15 this reason the aerial is designed to present an impedance similar to the body tissue impedance and, in use, is pressed against the surface of the body so that the air interface is removed. The aerials to -be described are of circular cross-section dimensioned to 20 propagate fields of TE-q mode and less because the TE^ mode provides the lowest reflection coefficient at the signal-receiving end of the aerial and the best source positional resolution within the near field. The impedance of the aerial is dependent upon its dimensions 25 and upon a filling of low-loss dielectric material so that selection of the dielectric material enables the wave impedance of the aerial to be near the mean impedance of the body tissues. The aerials additionally are capable of operating over a wide bandwidth, of the order 30 of several hundred megahertz, and have low loss.

In Figs. 6, 1 and 8 the aerial 2 is formed by a metallic circular cross-section waveguide tube 10 filled with low-loss dielectric material 13 held in place by a seal 14 formed by a disc of epoxy resin inserted into 35 the mouth or signal-receiving end of the tube 10. At 2 1863 -lithe signal-output end the tube 10 has a metallic closure penetrated by connections 15,16 of a standard coaxial cable connector. Because tube 10 is arranged to support TE^ mode and the lower mode (TMq^) the aerial also includes 5 a mode transformer whereby only the TE^ mode signal is delivered into the coaxial cable connector. In the Figs. 6 and 7 embodiments this mode transformer is in the form of two radial fins 11, 12 within tube 10 disposed in line opposite one another but separated as shown in end view 10 in Fig. 9. In Fig. 6 the fins are peripherally contoured to the shape of an exponential curve so that the gap between the fins varies exponentially from a maximum at the signal-receiving end of the aerial to a minimum at the signal output end. In Fig. 7 the fins 11, 12 are 15 peripherally formed vith steps having the average effect of an exponential gap. Connection 15 is made to fin 11 and connection 16 is made to fin 12 which additionally incorporates a quarter-wave section 22 to present a high impedance at the junction with connection 16. In the 20 Fig. 8 embodiment the mode transformer comprises a rod 23 extending diametrically across tube 10 and connected to conductor 16, conductor 15 being connected directly to tube 10- The mode transformers of Figs. 6 and 7 are preferred to that of Fig. 8 because the fin contouring 25 provides a smooth transition for the electromagnetic fields between the waveguide tube 10 and the coaxial transmission line and additionally causes the aerial to operate over a relatively wide bandwidth.

For the purpose of eliminating unwanted external 30 surface currents on the tube 10 the aerial 2 additionally incorporates a quarter-wave choke 17 at its signal-receiving end. In the Fig. 6 embodiment this choke 17 is in the form of a single coaxial sleeve 18 externally secured to the pertaining end of tube 10 and forming a % coaxial annular air gap. In' the Fig. 8 embodiment the - -■ in^fiwa^iigfeiii Irtfrritiitni—r—" C 2 186 choke 17 is in the form of first and second coaxial sleeves 18, 19 each forming a coaxial annular air gap, the axial length of the gap between sleeves 18 and 19 being less than that of the gap between sleeve 18 and 5 tube 10.

For the purpose of maintaining aerial 2 at approximately body temperature thereby to eliminate the unwanted effect of heat flow from the body to the aerial I which would introduce measurement error, the aerial 2 10 additionally comprises a heater 20 which may be either in the form of a heating coil or a heating blanket disposed exteriorly of tube 10 and preferably thermostatically controlled for example by sensor 21 connected in a feedback loop with the drive to heater 20. Heater 15 20 and sensor 21 are shown Ln each of Figs. 6,7 and 8. .

The aerials 2 which have been described are intended to provide mode transformation over as short a distance as possible whilst providing the necessary bandwidth and at the same time minimising axial aerial length so that 20 the volume of dielectric material is minimised, the latter being important because the dielectric material is the main source of signal loss in the aerial. One size of aerial 2 which has been found to be suitable comprises tube lO being between 5 and 10 cm in length and 2.5 cm 25 in diameter with fins 11, 12 each made of brass or copper 6 mm thick, the dielectric material being EccoFlo Hi-K 12 which has a relative dielectric constant in the range 10-50 approximately.

In a still further embodiment, illustrated in Fig. 30 10, the aerial 2 comprises circular-section copper tube 10 at the signal-receiving end which is mandrel-shaped into a rectangular section waveguide at the signal-output end, the transition between circular and rectangular sections being of the order of two wavelengths or greater 35 in length and thereby providing mode transformation to the 2 18636 13 coaxial line connector connected into the rectangular section which itself only propagates the TEQl mode. The propagation requirements for both cross-sectional shapes are simultaneously met provided that X >0.853 D and Y < 0.718 D where D is the diameter of the circular section, X and Y being the rectangular section dimensions. As previously the tube lO is dielectric filled. 2 18636

Claims (2)

WHAT WE CLAIM IS

1. Apparatus for use in microwave thermography, said apparatus comprising a signal-receiving aerial sensitive to microwave radiation emanating from a portion of a living-tissue body having a mean tissue impedance, said aerial comprising a tubular metallic waveguide having a radiation receiving end of circular cross-section dimensioned to receive radiation modes up to and including TE^, said tubular metallic waveguide having a signal output end incorporating a coaxial cable connection , a broadband-waveguide to coaxial-line mode transformer connected between the receiving end and the output end of said tubular metallic waveguide and arranged to deliver only radiation of the TE^ mode to the coaxial cable connection , and the interior of the tubular metallic waveguide being filled with low-loss dielectric material providing the aerial with a planar radiation-receiving-end face and a receiving-end impedance close to the mean tissue impedance of said body.

2. Apparatus for use in microwave thermography according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings. <r... .W|.. o!L. hj*/ihelr authorised Agent*, A. J. PARK A SON.