section of physical medicine medical applications of microwave

Section of Physical Medicine

[May 10, 1950]

SAMUEL HYDE MEMORIAL LECTURENUMBER 9

Medical Applications of Microwave Diathermy:Laboratory and Clinical Studies 1

By FRANK H. KRUSEN, M.D., F.A.C.P.Professor of Physical Medicine, Mayo Foundation. Head of Section on Physical Medicine,

Mayo Clinic, Rochester, Minnesota, U.S.A.

IT is a great honour to have the privilege of delivering the Samuel Hyde Memorial Lectureand to follow, in honouring the memory of Samuel Hyde, such distinguished physicians asR. Fortescue Fox, William Gordon, H. Whitbridge Davies, Sir Henry Cohen and Philip S.Hench. Dr. Hyde was born at Stalybridge 101 years ago, was privately educated andafterwards entered King's College, London, in 1872. In 1875 he received the certificate ofhonour in obstetric medicine and in 1877 qualified as a member of the Royal College ofSurgeons of England. As a student, he took an active part in mission work and wasattached to St. Giles's Medical Mission. He was to retain the missionary spirit throughouthis life. In 1877 he settled at Buxton and devoted the rest of his all-too-short life to pioneeringefforts for the advancement of that branch of physical medicine known as "balneology andclimatology". He became editor of the Journal of British and Foreign Health Resorts, hefounded the British Balneological and Climatological Society, which was incorporated withthe Royal Society of Medicine in 1900, and later was merged with this Section of PhysicalMedicine. He was also editor of the Journal of the Balneological and Climatological Society.Dr. Samuel Hyde died in 1900 after a prolonged illness and was buried in Burbage in hisfiftieth year. As S. W. Foss has written: "There are pioneer souls that blaze their pathswhere highways never ran,. . ."Hyde was truly such a man, whose missionary zeal aided tremendously in the early

development of one of the most important branches of physical medicine.Sound development of the relatively new specialty of physical medicine will depend on

parallel expansion of practice, teaching and research. Research concerning the medicalapplications of physical agents should start in the laboratory. This should be followed byclinical research. Then, and then only, therapeutic applications should be begun andcarried on with continuing controlled research until such time as the clinical uses of theagent in question are well established. The studies I am about to describe have beenconducted with such chronologic sequence in mind.

DEVELOPMENT OF THE VARIOUS TYPEs OF DIATHERMYFor many years electrical currents or radiations of very high frequency have played

important roles in medicine and surgery for heating of tissues. Starting with the work ofD'Arsonval who demonstrated in 1890 that high-frequency electrical currents of 10,000cycles per second produce no muscular contraction but only heating when they are passedthrough the human body, physicians have been employing increasingly higher frequenciesfor heating of living tissues. By 1900, high-frequency currents of 1,000,000 to 3,000,000cycles per second (long-wave diathermy) were in use, and by 1935 electrical currents of stillhigher frequency, 10,000,000 cycles per second at a wavelength of 30 metres (short-wavediathermy), were being employed [1]. Soon thereafter, application was being made ofradiations at frequencies of 100,000,000 cycles per second and at a wavelength of 3 metres(ultra-short-wave diathermy) and writers were beginning to run out of diminutives.

'Studies done in conjunction with Dr. Julia F. Herrick, Division of Experimental Medicine,Mayo Foundation; Dr. Khalil G. Wakim, Section on Physiology, Mayo Clinic, and Dr. Gordon M.Martin, Section on Physical Medicine, Mayo Clinic; with the aid of the following Fellows of theMayo Foundation: Dr. Ursula M. Leden, Fellow in Physical Medicine; Dr. Ralph E. Worden,Fellow in Physical Medicine; Dr. Jerome W. Gersten, Fellow in Physical Medicine; Dr. James W.Rae, Jr., Fellow in Physical Medicine; Dr. Joseph P. Engel, Fellow in Physical Medicine; Dr. LouisDaily, Jr., Fellow in Ophthalmology, and Dr. R. Quentin Royer, Fellow in Dental Surgery.

25 641

642 Proceedings of the Royal Society of Medicine 26

THE DEVELOPMENT OF MICROWAVE DIATHERMYNow I am reporting on the use in medicine of radiations at a frequency of 3,000,000,000

cycles (3,000 megacycles) per second and at a wavelength of 10 cm. This procedure hasbeen called "microwave diathermy". The word "microwave" is a popular term designatinga certain range of waves in the radio-frequency spectrum. This range includes frequenciesfrom approximately 1,000 megacycles per second to 30,000 megacycles per second orhigher. When expressed in wavelengths, this region ofthe spectrum is from 30 cm. to1 cm.

Microwaves can be produced by means of specially constructed tubes such as the magnetronand the klystron. The magnetron was invented by Albert W. Hull of General ElectricResearch Laboratories in 1920 and the klystron was developed by D. L. Webster of StanfordUniversity in 1938.

Now, the scribes who set down the "Book of Genesis" could report everything in thethird person, because they were not participants in the work being reported. Your lectureris not so deluded as to consider in the same category the work reported there and thatwhich is to be reported here. That is not the point. Whatf am thinking of is that the oldscribes, being outsiders so to speak, were spared the use of the objectionable first personalpronoun which, for the sake of the reader who has not access to certain correspondence,I must use in recounting the genesis of the work on microwave diathermy. This pronounwill disappear after a few paragraphs.

In 1937 I became interested in microwaves. In that year Williams [2] reported thatelectromagnetic radiations with wavelengths of a few centimetres could be focused, andSouthworth [3] pointed out that such radiation could be directed along tubes. Theseobservations led Hemingway and Stenstrom [4] to conclude that these properties, whichwould permit the direction of a beam to a desired region, might make such radiationparticularly valuable in medicine. At that time 1 [5] agreed with Hemingway and Stenstromthat we should study the medical application of radiations having wavelengths of less thana metre.

In 1937 also, I entered into correspondence with Lee De Forest, the inventor of thetriode valve, concerning electromagnetic radiations of a wavelength of 3 cm. I told himthat if such radiation could be obtained in sufficient intensity, "it would seem that thetherapeutic possibilities might be great". In 1938I advised F. D. Jewett of the BellTelephone Laboratories, who was developing the magnetron tube, that "if it ever becomespossible to increase the wattage of such tubes, they should prove suitable for therapeuticpurposes". He informed me that a power of only 2 or 3 watts could be generated by thetubes then available and I replied that "for therapeutic purposes it is necessary to haveconsiderably higher wattage".

By July 1938 Dr. Albert W. Hull advised me: "I can promise you at least a hundred wattsat any wavelength down to 20 centimetres." In March 1939I learned of the developmentof the klystron tube at Stanford University and D. L. Webster of the Department of Physicswrote to me: "1 am convinced that therapy is one of the important lines to which we must startapplying the klystron very soon." I was also advised that "between 10 and 40 centimetresthe klystron has outputs of several hundred watts". It seemed that at last a tube of largeenough wattage to provide radiation of sufficient power for medical use had been found.The idea of using radiations of a wavelength of a few centimetres in treatment was, of

course, not an original one. At about the same time (1938 and 1939) that I was investigatingthe possibilities, Hollmann [6, 7] was discussing the possible application of radiations ofa wavelength of 25 cm. for treatment and predicted that these waves could be focused soas to cause heating of the deep tissues; without excessive heating of the skin. However, atthat time, equipment which could generate such high-frequency waves at sufficient wattagefor suitable biologic investigation was not available to Hollmann.

Just when 1 had finally found tubes that gave promise of being of sufficient wattage,suddenly all such tubes became mysteriously unavailable and it was not until the secret ofradar was finally revealed that it became evident that the supply of such tubes had beendesignated for military use and that the tubes were being employed only for this secretwartime development. As soon as the war was over, I found in January 1946 just whatI had been looking for. On a visit to the Massachusetts Institute of Technology, I saw alarge experimental model of a microwave generator which contained a water-cooled multi-cavity magnetron with a maximal output of 400 watts and a frequency of 3,000 megacyclesper second. Shortly thereafter, in June 1946, 1 obtained one of these devices' and, with theaid of Drs. Herrick, Wakim and Martin, began our studies on microwave diathermy. We'We are indebted to the Raytheon Manufacturing Company of Waltham, Massachusetts, for

providing the early models of the microwave apparatus employed in our studies.

27 Section of Physical Medicine 643

also obtained a small, portable experimental model (Fig. 1) of a microwave generator,containing an air-cooled multicavity magnetron with a maximal output of 125 watts, whichoperated either on a frequency of 3,000 megacycles per second or a frequency of 2,450megacycles per second, depending on the magnetron employed.

FIG. 1.-Small portable experimental model of microwave generator containing an air-cooledmulticavity magnetron with maximal output of 125 watts at a frequency of 3,000 megacycles persecond. (Weight 34 pounds [15-4 kg.], dimensions 15 by IOj by 9j in. [38 by 27 by 23 cm.]) Twohemispherical directors of different sizes are shown.

FIG. 2 (Inset to FIG. 1).-The three types of microwave directors which were employed in thesestudies. (a) The director of type A is a hemispheric type 4 in. (10-16 cm.) diameter; (b) the directorof type B is a hemispheric type, 6 in. (15-24 cm.) in diameter; (c) the director of type C is a rectangularor "coiner reflector" type.

The multicavity air-cooled magnetron tube was developed through the genius of aresearch group at the UJniversity of Birmingham and was of tremendous importance inperfecting radar which, it has been said, "won the Battle of Britain". Prior to 1940, suitablegenerators of microwaves for radar did not exist. In September 1940, a British technicalmission headed by Sir Henry Tizard brought the multicavity magnetron tube to the UJnitedStates and in a short time American manufacturers had produced this multicavity magnetrontube for use in microwave radar. The output of these tubes could be as high as 1,000,000watts and the microwaves which they develop have optical properties so that they can bereflected, refracted or diffracted. They can be focused by a suitable type of metal lens to assharp a beam as a searchlight. Microwaves can be selectively absorbed by certain mediabut whether or not the various biologic media will absorb microwaves selectively has notas yet been determined.

FIRST STUDIES ON THE MEDICAL APPLICATIONS OF MICROWAVE DIATHERMYDr. Julia F. Herrick, after years of experience with the Signal Corps of the United States

Army in studies of radar, began our work with experimental studies, in vitro, on microwavediathermy. Then Dr. Ursula Leden, Dr. Herrick, Dr. Khalil G. Wakim and I [8] beganstudies on living animals. These studies continued, and I wish to say that most of theactual work was done by my associates rather than by me.

-------- ---- ---


Up to the time when this work began, the only reports of the effect of microwaves on theliving organism were of several studies [9-11] conducted by members of the armed forces.of the United States in order to answer a problem of morale. They were meant to dispelfears of possible ill-effects of exposure to radar on personnel connected with radar work.These reports were concerned only with exposure to radar pulses-that is, very brief, rapidbursts of energy-and not with exposure to continuous microwave energy such as we haddecided should be employed in medicine.The equipment which we used for our first experiments on living animals produced

continuous microwave energy at a frequency of 3,000 megacycles per second, correspondingto a wavelength of 10 cm. Much of our work was done with the small, portable unit con-taining an air-cooled magnetron tube. The energy was transported by a coaxial cable fromthe generator to the director. From this hemispherical director, the energy was radiatedon to the bodily surface that was under treatment.As our work had developed, we had used dead tissue for investigation of directors, of

which we had employed three types (Fig. 2): the director of type A was a hemispherictype, 4 in. (I016 cm.) in diameter; the director of type B was a hemispheric type, 6 in.(15-24 cm.) in diameter, and the director of type C was a corner reflector. When a director oftype A was employed a circular pattern (Fig. 3A) was produced, in which the peak, or 100%,area of heating was in a circle toward the periphery and not at the centre of the pattern.When a director of type B was used, the same pattern (Fig. 3B) was obtained but it covereda larger area. The energy distribution pattern for a director of type C (Fig. 3c) was verydifferent. Maximal heating was at the centre of an elliptical pattern. All of these patternswere determined, as was said, in dead tissue and do not indicate the equalizing effect of thecirculation which obtains in living tissue.

L........II j ~Director AA. -Surface heating pattern in dead

tissue for director A at a distance of 1 in.(2-54 cm.) from the skin.

21

c. -Surface heating pattern in deadtissue for director C at a distance of 2 in.(5 -08 cm.) from the skin.

B.- Surface heating pattern in dead tissue for director Bat a distance of 2 in. (5 08_cm.) from the skin.

FIG. 3.


For our early studies on living tissue, only a director of type A was employed. Weconfined our studies to the area where heating was nearly maximal, as determined by theheating pattern. Trained dogs without anmsthesia were employed in the majority of thepainless experiments. When surgical procedures were necessary, anesthesia was producedby pentobarbital sodium. After considerable preliminary experimentation, it was finallydetermined that an output of 75 millianmperes, corresponding to 65 watts, and a spacing of5 cm. from director to bodily surface for a period of twenty minutes were both effective andsafe. Our observations in this regard were later confirmed by Salisbury, Clark and Hines [12]who concluded that "the area of cross section of a typical 10 centimetre wave guide is about28 sq. centimetres. Accordingly one must have a total power of about 90 watts to reachthe danger level".Measurement of local temperature of tissues was accomplished by inserting needle

thermocouples into the subcutaneous tissue and muscle at various depths and cutaneoustemperatures were taken with contact thermocouples. The thermocouples were removedduring the actual application of the high-frequency radiation. The exact spots where theneedle thermocouples were to be inserted were marked and the needles were inserted to thedesired depths that were marked on the shafts. The first readings following application ofradiation were taken one minute after discontinuation of the heating. Temperatures ofmuscle were recorded at two depths (1 9 cm. and 3 cm.). We were soon convinced thatmicrowaves could heat tissues satisfactorily.With the technique which we employed, there was a rise in cutaneous temperature of

approximately 30 to 50 C. and approximately the same rises occurred in subcutaneoustissue and muscle to a depth of 3 cm. (Fig. 4). However, the increase in cutaneous and

43-

4Z79 scle _ 2

i5 0 X ~ ~ 141uIs XII ,XI.41 -----------

40 -e in utnus tissue39 subcutaneoust

34

36 r_____ II I I I I

0 8 Z6 30 40 50 60 70Control Time in minutes

FIG. 4.-Increases in temperature of skin, subcutaneous tissue and muscle, after exposure tomicrowaves, of the thigh of an aniesthetized animal on which a surgical procedu're had beenperformed.

subcutaneous temperatures was somewhat greater than the increase in temperature ofmuscle. Superficial muscle, it may be said, rose in temperature more than deep muscleand also cooled more rapidly.

Approximately the same results were obtained when the director was placed at a distanceof 5 cm. from the surface and exposure was of 75 milliamperes or 65 watts, as when thedirector was placed at a distance of 2 cm. from the surface and exposure was of 45 milli-amperes or 32 5 watts. In all cases under both conditions, superficial tissues cooled morerapidly than deeper tissues. In intact animals, cooling was always complete and rapid;temperature returned to control value within twenty-five to thirty minutes. In anasthetizedanimals on which surgical procedures were performed, cooling was slower and temperatureusually did not return to control values but formed a new base line 10 C. or more abovethe original control value.For measurement of flow of blood a modification of the Dumke and Schmidt [13] bubble

flow meter was employed. It consists of a coil of glass tubing which is introduced betweenthe ends of a sectioned blood vessel. A bubble is introduced by means of a syringe; thisbubble is pushed through the meter by the flow of blood and is then caught in the trap toprevent air embolism. The volume of the meter is calibrated, so that by measuring thetime necessary for the bubble to flow from marker to marker, the rate of flow of bloodcan be calculated.


Animals were anxsthetized and heparinized and the meter was placed in the femoralvein. Changes in flow of blood caused by heating of the thigh were observed before, duringand after exposure to microwave diathermy. In nine successful experiments, flow of bloodafter exposure to microwaves increased an average of 109%. Significant increase in bloodflow (Fig. 5) usually occurred toward the end of the exposure to microwaves.

- Ilcrowaves 75 N.A. 5 cm.

40

(.J39 1 s leg 38k-

3b Subcutaneous tissue-35 -. -- -- --

33-5031

k40ts

0 10 20 30 40 50 boContro1 Time in minutes

FIG. 5.-Increase in flow of blood in the femoral vein during and after heating by microwaves,of the thigh of an antsthetized animal; also, increase in temperature of skin, subcutaneous tissueand muscle after the exposure.

The heat produced locally in the tissues by exposure to microwaves causes an increaseof venous return and vasodilatation effected through local reflex mechanisms and directmetabolic effects. At this point in our studies, we concluded that microwave diathermyprovided a means of heating tissues by which we could obtain accurate localization bydirection of a beam of energy toward any surface of the body. We concluded, also, thatabsorption of such radio-energy was greater than that at longer wavelengths.At the frequency which we were using, "the absorption of radio-energy in water at

1000 F. is in the order of 7,000 times the absorption at 27 megacycles now commonly usedfor short-wave diathermy" [8]. With better absorption, more efficient heating can beexpected. Although we recognized the dangers of excessive dosage of microwaves, we con-cluded that, with proper technique, they could be employed effectively for heating of livingtissues. The wide variety of patterns, and the possibility of placing the director in anyposition, provided great flexibility in therapeutic application. Freedom from encumberingpads, cables or towelling, commonly used with short-wave diathermy, was possible; more-over, the fact that the radiation could be beamed and localized in the manner of a spotlightfacilitated application. Finally, these studies indicated a desirable relationship betweencutaneous and internal temperatures which would permit adequate internal heating up toa depth of 3 cm. without undue heating of the cutaneous surface.

STUDIES OF THE EFFECT OF MICROWAVE DIATHERMY ON ISCHAEMIC TIssuEsAbout this time, the American Federal Communications Commission assigned the

frequency of 2,450 megacycles per second (a wavelength of about 12 cm.) for use in physicalmedicine. Since this particular wavelength was close to the one we already had beenstudying, we continued our studies at the assigned wavelength. Dr. Ralph E. Worden,Fellow in Physical Medicine of the Mayo Foundation, continued the studies under oursupervision. He attempted to determine the effect of microwaves on the temperature oftissue of which the blood supply was normal, as compared with their effect on the tempera-ture of ischaemic tissue. Also, he sought to determine, if possible, an optimal period ofradiation for heating of tissue [14].A hemispherical director of type A was employed in these experiments and dogs weighing

12 kg. or more were used exclusively. When the effects on tissues with reduced blood supplywere studied, the dogs were anesthetized with pentobarbital sodium given intravenously and,through an abdominal incision, an adjustable clamp was inserted around the aorta distal tothe origin of the mesenteric artery. The incision was closed but the handle of the clamp, con-


taining the adjustment screw, was left extending.outside the abdomen. Thus it was possibleto produce ischemia of the tissues by clamping the aorta before exposure to microwaves.

Cutaneous temperatures were recorded by means

4, of a thermistor [15, 16] (a thermally sensitive10 cm. _ resister with a negative temperature coefficient)., < 9i'\\3i Temperatures of subcutaneous tissue and muscle

were recorded with needle thermocouples. AllJ/ "' -- " \\'\1 observations of temperature were continuously

\j..registered galvanometrically on moving photo-/ ,tt cm t 1l2 ', graphic film. The hair over the thigh of the dog

7+ \ \s/w- as clipped and an outline of the heating pattern/'a\/ ! /f1 was drawn on the thigh with gentian violet (Fig. 6).se.'50% The area of 100% concentration of energy, as

previously patterned, was outlined and in that area

-Sw / the temperatures of the skin, subcutaneous tissue,\,,/ and superficial and deep muscle were recorded

before and after exposure to microwaves. Tem-Area of perature of muscle was measured at a depth of 15

lS°entratYon and 3 cm. In all these studies, the output of thegenerator was maintained at 30 watts and the

FIG. 6. -The approximate site of distance of the director from the skin was 2 5 cm.exposure to microwaves and the heat- The aortic clamp was tightened when ischemia wasing pattern produced by a hemispheric desired. After a control period, thermistor anddirector of type A at a distance of 2-5 thermocouples were removed and the region was

cm. from the skin. exposed to microwaves. Then the thermistor and

thermocouples were replaced and the temperatureswere recorded, beginning within thirty seconds after irradiation had been discontinued.During the first five or ten minutes of exposure, temperature of ischemic tissues did not

differ appreciably from the temperature of tissues with intact circulation (Fig. 7). There

Temperature rises in thigh of dogafter microwave radiation with and without ischemia

0 Subcutaneous tissue8¢ 3X [Superficial Muscle

*Deep muscle

Control Ischemia Control Ischemia Control Ischemia Control Ischemia5 10 15 zo

Periods of radiation - minutesFIG. 7.-Experiments were based on exposure for various periods of time. The control is the average

increase of temperature after exposure when the circulation was intact.

was no evidence of burning. However, after exposures of fifteen or twenty minutes, thetemperature of ischemic tissues rose slightly higher than that of tissues with intact circula-tion. Moreover, in the presence of ischaemia 4 of the 5 animals exposed for fifteen minuteswere burned, as were all 6 of those exposed for twenty minutes. In no instance did burningoccur when the circulation was intact. Furthermore, in no instance did burning occur whenthe circulation was intact and irradiation of the same animal at the same site was repeatedfor six consecutive periods of twenty minutes each.

The highest temperature obtained when the circulation was intact was 44.6' C. and therewas no evidence of burning. After ischemic tissues had been exposed to microwaves fortwenty minutes, the highest temperature was only 42.30 C., yet there was gross evidence ofburning. The height of the temperature alone is not an indicator as to whether or not tissueswill be burned. Duration of exposure and the presence or absence of the protective

648 Proceedings of the Royal Socidy of Medicine 32

mechanism for dissipation of heat provided by the circulating blood are also factors.In all cases in which burns occurred, the first evidence of burning appeared over the

femur where it crossed the zone of 100% concentration of energy in the irradiation patternof the director (Fig. 8). The greater the prominence of the femur the more severe was theburn. Apparently there is reflection from the bone, with concentration of heat in the over-lying tissues. Therefore, microwave diathermy should be employed with caution in areaswhere there are bony prominences.

Left1thigh

100 % energy)concentration -

50Deep

49 musle leb

Superficia_48 musclemuscle

%46 .- /

tj45 -N kin4

-In bleb- BlebSubcutaneoust4 __tissue

43

4 0 I I ' ! I ISuperI ficial42 muscle

41 -Subcutaneoustissue

40-0 1 2 3 4 5 6 7 6 9 10 11 12 13

Ilinutes after radiation

FIG. 8.-Temperature variations after microwave irradiation when blebs are present. The rightupper corner shows the relationship of the heating pattern to the femur. In all cases in whichburning occurred, the first evidence of burning was over the femur and at a site in which it crossedthe area of 100% concentration of energy.

Blisters occurred where the femur crossed the area of greatest intensity of radiation, whenthe aorta was clamped and the tissues ischtamic. After the aorta had been released and thetemperatures had returned to control levels, the same area again was exposed to micro-waves for fifteen minutes and temperatures again were recorded at the original sites. Itwas known that fluids are heated to high levels by microwaves; therefore, after threeminutes the needles were quickly withdrawn from the subcutaneous tissue and superficialmuscle and inserted into a bleb. The temperature in the bleb was much higher than thatof the surrounding tissues. After three more minutes, the needles were removed from thebleb and replaced in their original positions, whereupon the usual cooling curve wasresumed. This gave evidence of the high absorption of microwaves in regions of localizedaccumulation of tissue fluids, and suggested that regions containing fluid such as cedemafluid, regions of effusion, abscesses, the eye, the gall-bladder or the urinary bladder, mightbe overheated by microwaves unless the dosage was carefully modified. However, it issuggested that, in proper dosage and when indicated, microwaves will be extremely effectivein heating such regions.Another interesting physiologic phenomenon was observed (Fig. 9). The cooling of

muscle when circulation was intact was moderately rapid and resulted in a smooth curve,while the rate of cooling of ischeemic muscle was slow, even though it had been heated toa higher temperature to begin with. On release of the aorta, however, cooling of thepreviously ischiemic muscle occurred rapidly and within two minutes the rate of coolingfollowed that of normal tissues.The skin and subcutaneous tissues, with circulation intact, were heated very rapidly and

reached high values after five or ten minutes of exposure. The temperature of superficialand deep muscle rose more slowly and all four layers reached a peak in twenty minutes.It was interesting to note that, after thirty minutes of exposure, all of the temperaturesdropped below the peak achieved at twenty minutes. It appears that factors which favourcooling become more effective after twenty minutes of exposure. Increasing circulation,as shown previously, is largely responsible for this cooling effect.


Temperature gradients at various levels may vary rather markedly at different periodsof exposure. In some instances the muscle layers were the hottest and in others the sub-cutaneous layer was the hottest. Further studies may reveal that by varying not only theduration of exposure but the output of the generator, and the distance of the director fromthe skin, different gradients may be obtained.

43

anithsehemia , inAorta released o80 Ilccae

fiften mnuteofirraiatin wih mcrowves.cutaeou tise afterotn weve n hrt iue

~~~~~~~~~~~~~~~~~directorC

41 so~~~~~~~~~~~~~70

40 Olicr-owavedirector A

The rate of colig o ishxmc tssu wa ofheaing30 Vae s n hrrt wavent~~ ~ ~ ~ diathermy

1'z~~~~~~~~~~~0

0o Z 4 t 8 10 14lor1mts eZ 0 10 00 30Olinute,s after radiation Duration of heating in minutes

FIG. 9.-Comparison of cooling curves, with FIG. 10.-Relationship between the increase inand without ischuemia, in deep muscle after the temperature of deep muscles and that of sub-fifteen minutes of irradiation with microwaves, cutaneous tissues after ten, twenty and thirty minutesT'he rate of cooling of ischlemic tissue was of heating. Values shown represent ratio of rise inslower than that of normal tissue even though deep muscles to rise in subcutaneous tissues X 100.the rise in temperature was greater. Releasing All increases in temperature of deep muscles werethe aorta resulted in rapid cooling of ischmmic less than in those in subcutaneous tissue. A valuetissues. of 100 would indicate a rise equal to that in sub-

cutaneous tissue.

COMPARISON OF THE HEATING EFFECTS OF SHORT-WAVE ANDMICROWAVE DIATHERMY

Dr. James W. Rae, Jr., another Fellow in Physical Medicine, contributed a supervisedcomparative study ofthe temperatures produced by microwave and short-wave diathermy [17].A portable microwave generator supplying continuous microwave energy at a frequency of2,450 megacycles per second and at a wavelength of about 12 cm. was employed for theseexperiments. Four short-wave diathermy machines, all products of reputable firms and inwide clinical use, were tested. The wavelengths of these four machines varied from 6 to 25 m.Three needle thermocouples were inserted into the tissue, one into the subcutaneous layer,one into the muscle at a depth of 1.5 cm. and one into the muscle at a depth of 3 cm.Cutaneous temperatures were recorded with a thermistor. Temperatures were then recordedon moving photographic film. Four intact and well-trained dogs were used in this study.Basal conditions were approached by permitting the dog to lie quietly on the table forthirty minutes or longer before measurements were taken. The thigh was chosen as thesite of application and was kept free of hair by frequent clipping.One hundred and forty-four experiments were performed with the four short-wave

diathermy machines and two microwave directors (types A and C). Twenty-four experimentswere carried out with each type of apparatus. These twenty-four experiments consisted ofsix experiments on each of 4 dogs, and the six experiments consisted of two at each of threeperiods of radiation; that is, ten, twenty and thirty minutes. The averages of the studieson the heating effect of short-wave diathermy indicate that, before treatment, the temperaturegradient in the various layers increases from without inward, the skin being the coolest andthe deep muscle the warmest. With short-wave diathermy, the temperature of all fourlayers increases after ten minutes, after twenty minutes and after thirty minutes but thedeeper muscle is heated less and the gradient is reversed. With microwave director A,comparable rises in temperature occur but peak temperatures are reached in twenty minutes.With a microwave director of type C, the highest temperatures are reached in ten minutesand higher temperatures are reached at all depths in the tissue levels than are reached withshort-wave diathermy or with a director of type A.The relationship between temperatures of deep muscle and subcutaneous tissue after

microwave diathermy was different from that after short-wave diathermy (Fig. 10). Withshort-wave diathermy, the rise in temperature of deep muscle, in comparison to the rise intemperature of subcutaneous tissue, was less than was the comparative rise when microwaveswere employed.


DEVELOPMENT OF AN IMPEDANCE MATCHING TRANSFORMERTO INCREASE HEATING OF DEEPER TISSUES

We were aware of the fact that because of high reflection at the surface of the skin, theheating efficiency of the electro-magnetic waves we were using is low. To make the transferof energy from air to tissues more efficient, an impedance matching device or "transformer"was designed by Herrick and her associates [18]. This improved considerably the heatingof subcutaneous tissues, muscle and bone. It was concluded that the device would increasethe transfer of thermal energy to the tissues and might also make it possible to heatselectively small portions of tissue. Impedance matchers made of the dielectric mycalex,or of mycalex and polystyrene, were tested and proved satisfactory for this purpose.69 observations were made on 9 normal men [19]. For impedance matching, a cylindricalblock of mycalex with a dielectric constant of 8-0, a diameter of 5-08 cm. and a thickness of10-32 mm. was placed on the skin in the microwave field. In one series of observations,after irradiation for one minute at a plate current of 100 milliamperes, there was an averagerise of cutaneous temperature of 1.080 C. at the point not covered by the mycalex cylinderand, at the point covered by the mycalex cylinder, there was an average rise of cutaneoustemperature of 2-350 C. In another group, there was an average rise in temperature ofmuscle at a point not covered by mycalex of 0.700 C. and at the point covered by mycalexthe average rise in temperature of muscle was 2-21' C.A few experiments also were performed in order to determine the change in temperature

of the mycalex cylinder when it was exposed to the microwave field. When the cylinderwas placed at a distance of 4 cm. from the director, the maximal recorded rise in temperature,either on the surface or in the substance of the cylinder, was 0.10 C. after one minute ofirradiation at a plate current of 100 milliamperes.

It was apparent that significantly higher temperatures are recorded in the region coveredby a dielectric than in uncovered regions. The dielectric constant of the impedance matcherwe used was 8-0. This constant may not be the ideal one and m~aterial with different dielectricconstants may result in even higher transmission of energy from air to the tissues. Weconcluded that impedance matching by placing of an appropriate dielectric between airand tissue can be used to decrease the amount of microwave energy reflected by tissue.Further, we concluded that by choice of certain combinations of current intensity andduration of irradiation, some regions might be heated satisfactorily while adjacent regionsreceived relatively little heat.

STUDIES OF DIELECTRIC CONSTANTS OF VARIOUS TISSUES ATVARIOUS MICROWAVE FREQUENCIES

Having proved that microwave diathermy was effective in heating certain localizedportions of the body, we wished to explain if possible the experimentally observed tempera-ture distribution produced in the tissues by microwaves. Therefore, Herrick and herassociates [20] studied the dielectric constant and loss of freshly excised samples of varioustissues. The objective was to increase our understanding of the heating of these tissues.The amount of heat developed in animal tissues from microwave diathermy is dependent onthe dielectric properties of the tissues. If these properties vary with the frequency, it isdesirable to measure them at the actual frequencies we are using. The dielectric propertiesof certain body fluids were also studied. These studies aided in the designing of animpedance matcher for increasing the transfer of microwave power into a particular tissue,such as muscle.The equipment employed for measuring the dielectric constant and loss was a special

microwave dielectrometer which was designed by Dr. D. J. Jelatis. Accurate determinationsof the dielectric constant and loss revealed varying figures for different types of tissue(Table I). It is hoped that these determinations will be helpful to others in future studies.

TABLE I.-DIELECTRIC CONSTANT AND Loss FACTOR AS DETERMINED* WITH THE MICROWAVEDIELECTROMETER DESIGNED BY D. J. JELATIS

Dielectric Loss factorMaterial constant K Tan 8

Muscle .. .. .. .. 48 0-29

Fat:Homogeneous .. 3.9 0-17More fibrous .. .. 7-2 0 19

Bone .. .. .. .. 7-5 013Tendon.. .. .... 32 0-46Marrow:Red .. .. .. .. 71 0.20Yellow .. .. .. 4-2 0-20*Determinations made on different tissues of the horse.


The dielectric properties of tissue important in microwave diathermy also were studied atvarious frequencies (Table II). There were considerable differences in the results obtainedat 1,000, 3,000 and 8,600 megacycles.

TABLE II.-DIELECTRIC PROPERTIES OF VARIOUS TISSUES IMPORTANT IN MICROWAVEDIATHERMY

Frequency and electrical data1,OCO Mc. 3,000 Mc. 8,600 Mc.per sec. per sec. per sec.

Material Temp. 'C. K Tan 8 K Tan 8 K Tan 8Muscle:Horse .. .. 38 52 0 49 48 0-29 43 0-42Dog .. .. 38 52 0 44 37 0 30 40 0-45

Fat of horse:Homogeneous 38 3-8 0 33 3 9 0 17 3 9 0 22

More fibrous .. 38 7.5 0.36 7-2 019Fat of dog 38 5-3 0-29 3.7 0-17 3.5 0-16Bone of horse .. Room 8-0 0 11- 7.5 0 13 8-0 0-17Tendon ofhorse. . Room 39 0 37 32 0-46Marrow of horse:Red . . .. Room 7-8 0.23 7.1 0-20 7.2 0 51Yellow.. .. Room 4-3 0 18 4-2 0-20 4-3 0.24

Skin of dog .. Room 35 0-31 34 0-28 28 0 49

EFFECT OF MICROWAVE DIATHERMY ON BONE, BONE-MARROW ANDADJACENT TISSUES

Next, Dr. Joseph P. Engel [21], also a Fellow in Physical Medicine, investigated under ourguidance the effect of microwaves on bone, bone-marrow and adjacent tissues. Our purposewas to study the temperature relationships between bone and soft tissue following micro-wave diathermy and to investigate the effects of daily exposure to microwaves for prolongedperiods. Osborne and Frederick [22] produced with microwaves average temperatures of105.10 F. in the frontal sinuses of dogs, using a director of type C. This was an increaseof 100 F. more than the control temperatures.

Furthermore, studies on dead tissues have indicated that the temperature of bone couldbe increased with a frequency of 2,450 megacycles but not to the high levels produced insoft tissues, and it was postulated that the exceedingly low dielectric constant of animalbone-marrow would prevent absorption of any measurable amount of energy, so that littleor no increase in temperature would be obtained.

In our studies, microwave diathermy was applied to the legs of anesihetized or traineddogs and measurements of temperature were made before and after irradiation by means ofthermistors and thermocouples placed in the adjacent soft tissues and in the cortex andbone-marrow of the upper third of the tibia. When bone was exposed to microwaves forthirty minutes, in such a way that muscle lay between the bone and the source of radiation,the locations of the thermistors and thermocouples, and the direction of irradiation, wereas shown in Fig. 11A. UJnder these conditions, average increases of temperature were asfollows: subcutaneous tissues, 4.3 C.; overlying tibialis anterior muscle, 4. 6 C.; tibial cortex,3 8°C., and tibial marrow, 3 0° C. (Fig. 1 I B.)The rectal temperature is given only to show Microwave iryadihtion- Subcutaneous tissuehow the entire body presumably reacted. 75 ma current --Bone cortexreacted. ~~Director .'----

Spacino-Z inches from - Bone marrowdirector to skin. Overlyinj muscle

+ +43-- ~~~~~~~~~~~~~~~~~~(tibiaLliscinticus)~~Rectal

41

4

C_ Controlo 5 io 15 20 25 30 35 40 45 50 55 b o5 70O

Tliime in minutes

FIG. 11A. FIG. I1B.FIG. 11A.-Transverse section of the hind leg of the dog 1 5 cm. below the tibial tuberosity, showing

the position of each thermistor when temperatures were observed. Arrows indicate the directionof irradiation on the lateral aspect of the leg when muscle lay between the source of radiation andthe bone being studied; 11B.-Temperatures produced in the lateral aspect of the hind leg of theanaesthetized dog after varying periods of microwave diathermy. The cooling curve begins afterthirty minutes of exposure.


When subcutaneous bone was exposed to microwaves for thirty minutes, in such a waythat skin and subcutaneous tissue lay between the bone and the source of radiation, thearrangement of thermistors and thermocouples, and the direction of irradiation, were as isshown in Fig. 12A. UJnder these circumstances, the average increases of temperature wereas follows: subcutaneous tissues, 4-10C.; tibial cortex, 5.0° C.; tibial marrow, 3.40 C.;adjacent gastrocnemius muscle, 4.30 C. (Fig. 12B). No comment is necessary concerningthe temperature of tissue beneath bone or concerning the rectal temperature.When an impedance matching transformer was placed in the field of microwave radiation,

there was an increase in the temperature of tissues of approximately 300% over the values

Microwave irradiation- -- Sutcutaneous tissue75 ma. current Bone cortex

,,4_ _, Director "A" .Bone marrowSpacin§-2 inches from Adjacent muscle

.e L director to skin Tissue beneath boneRectal

0 5 lO iS 20 25 30 35 40 45 50 55 60 65 70Time in minutes

FIG. 12A. FIG. 12B.FIG. 12A.-Transverse section of the hind leg of the dog 15 cm. below the tibial tuberosity showing

the position of each thermistor when temperatures were observed. Arrows indicate direction ofirradiation on the medial aspect of the leg when skin and subcutaneous tissue lay between the sourceof radiation and the bone being studied; 12B.-Temperatures produced in the medial aspect of thehind leg of the anesthetized dog after varying periods of microwave diathermy. The cooling curvebegins after thirty minutes of exposure.

_Subcutanleous tissue @ Borne cortex`| OverlyinM muscle Bone mariow

Microwave gelleiator output-50 mlia. Microwave Venerator output -5o maiDirector 'A" DirecLor "A'5pacino-z in. fronm director to skirl Spacrn{-2 in. from director to stunDuration of exposuYe-l0 mniii. Dui'ation of exposuire-10iom.n

43

42Z

4-

4040

3939

387

367

Without transformer With transformecFIG. 13.-Effect of an impedance matching transformer on the temperature of bone and adjacent

tissues of the trained dog, when only skin and subcutaneous tissue lay between the bone and thesource of radiation.

obtained when no transformer was used (Fig. 13). When skin, subcutaneous tissue and alayer of muscle lay between the bone and the source of radiation, the increases in temperatureof bone cortex and bone-marrow were considerably less than the increases in temperatureof subcutaneous tissues and overlying muscle. When the microwaves were applied tobone, and only skin and subcutaneous tissue lay between the bone and the source ofradiation, significant increases of temperature were produced in bone cortex and bone-marrow without the transformer being in use. Marked rises of temperature occurred whenthe transformer was used.


Repeated daily exposure of the leg of the dog to microwaves with a director of type Aat a distance of 2 in. (5 -08 cm.) and a power output of 65 watts, for forty-five minutes duringa period of eight weeks, produced no significant changes in the hkmatocrit reading, con-centration of hkemoglobin or blood cell counts. However, repeated exposure to microwavescaused more rapid healing of the surgical wounds in the soft tissue than occurred in theabsence of such exposure.

Five dogs gave roentgenologic evidence that, under treatment with microwave diathermy,an artificially produced defect of bone filled in earlier than did a similar defect in the oppositeuntreated leg which served as a control.

EFFECTS OF MICROWAVE DIATHERMY ON THE EYENext we studied in the laboratory the effect of microwave diathermy on enucleated eyes

and on the eyes of intact animals. Daily, a Fellow in Ophthalmology of the Mayo Foundation,and his associates [23] determined the changes in the temperature of orbital tissues andaqueous and vitreous humours of the eye after exposure to microwaves. Different distancesof the director from the eye, exposures of various durations and power outputs of the generatorof various intensities were used. Ophthalmoscopic studies were made before and afterexposure; and after enucleation, histologic sections and microscopic examinations weremade.

It was known that microwaves would be likely to heat the eye to high temperatures andthat, in excessive doses, damage might be done. It was apparent also that the distanceof the director from the eye was an important factor in dosage.

Repeated exposures for thirty minutes every other day with a generator output of 94watts at a distance of 5 in. (12-70 cm.) produced no observable pathologic effects. Dailyexposures under the same conditions at a distance of 3 in. (7-62 cm.) produced no observablepathologic effects. However, one exposure for thirty minutes at a generator output of94 watts, and at a distance of 2 in. (5-08 cm.) produced cloudiness of the cornea twenty-fourhours after exposure, and the pupil was half dilated and unreactive. But the eye of anotherdog treated in exactly the same fashion once daily for ten days sustained no clinicallyobservable pathologic effects. Corneal clouding developed in the eye of a third dog sotreated for eight days and, six days after the last exposure, an anterior cortical rosettecataract was observed and a posterior cortical cataract developed later.

Corneal clouding developed in the eyes of two animals which were exposed to microwaveradiations with a generator output of 122 watts and at a distance of I in. (3 -81 cm.). In aneye of one of these animals, an anterior cortical cataract, and later a posterior corticalcataract, developed. Similar damage to the eye was reported by Salisbury and his asso-ciates [12] as a result of experiments performed on rabbits. They found that cataracts wereformed on exposure for ten minutes at a field intensity of about 3 watts per square centi-metre and at a wavelength of 12 cm. These cataracts developed three to ten days afterexposure.

It is evident that microwave diathermy should be applied with extreme caution in theregion of the eye. Otherwise cataracts, such as are represented in Fig. 14A and B, will beproduced. It is apparent, however, that the eye can be readily screened from microwaveradiations and we are planning to investigate suitable methods of screening it. Likewisewith proper use of an impedance matching transformer, regions near the eye could beheated, with minimal heating of the eye itself.

FIG. 14A. 1FIG. 14A.-Cataract produced in the eye of a dog following exposure to microwave diathermy.FIG. 14B.-Cataract produced in the eye of a rabbit following exposure to microwave diathermy.AUG.-PHYS. MED. 3

654 Proceedinrgs of the Royal Society of Medicine 38

EFFECT OF MICROWAVE DIATHERMY ON THE CIRCULATION OF BLOOD AND ONTEMPERATURE OF TISSUES OF NORMAL HUMAN BEINGS

Once we had completed thorough studies of the effect of microwave diathermy onlaboratory animals, we turned our attention to its effect on normal human beings.Dr. J. W. Gersten, Fellow in Physical Medicine, collaborated with us [24] in determiningthe effect of various outputs of microwaves, and of different periods of exposure to micro-wave diathermy, on the peripheral circulation and on the tissue temperature in the exposedlimb of man. Fifty normal human volunteers were studied. For the temperature studies,thermocouples were placed on the skin, in the subcutaneous tissues and in the muscle ata depth of 1 5 cm. The studies on the flow of blood in the human subjects were done onthe exposed and on the contralateral extremity with a venous occlusion plethysmographand compensating spirometer recorder [25]. The output of the generator was either 60 or80 watts and the duration of exposure varied from one to thirty minutes. After controlmeasurements of flow of blood and temperature had been established, the forearm wasexposed to microwaves and the studies on flow of blood and temperature were repeatedafter the microwaves were turned off.

Significant increases in flow of blood and in temperature of the tissues were produced inthe extremity exposed to microwaves. Changes in bodily temperature, heart rate and bloodflow in the unexposed extremity were insignificant. The average rise in temperature ofmuscle (Fig. 15) was significantly greater than that of the subcutaneous tissues, while the

,)7t~~-----&,-riuscle9. - _ =_FIG.15.-Effects of ex-A 5 posure to microwaves (805 x , x x " watts) on temperature oft 4 _ x tissue of the treated ex-

5kin tremity. Readings of tem-23 - / Stibcutaneous perature were taken one

tissue minute after microwaves2-_ { had been turned off. Each

N g point is the average of tento twenty-six observations.

0 5 1 0 15 20 25 30

Duration of heating - minutes

average increase in the subcutaneous temperature was greater than that of the skin. Aftertwenty minutes of irradiation with an 80 watt output, the increase of temperature was6-75 C. in muscle, 5 8° C. in subcutaneous tissue and 4 7° C. in the skin. Although thetemperatures decreased after twenty minutes of exposure (Fig. 16), the flow of blood con-tinued to increase and reached its height after thirty minutes of exposure, at which time

70 Skin _60 L0 ubcutaitQous tissuei

50 - luscle

8 4>X40 -|Blood flow

3030ijL~~ to5 10 15 20 30

minutes minutes minutes minutes minutes

Durtction of heatingFIG. 16. Effects of exposure to microwaves (80 watts) on blood flow and on temperature of tissue

in the treated extremity. Readings of blood flow were taken five minutes after the microwaves hadbeen turned off. Readings of temperature were taken one minute after the microwaves had beenturned off. The height of each bar represents the average of nine to twenty-six observations.

Section of Physical Medicine 655

the average increase had reached 65 %. It was interesting to observe that after thirty minutesof heating, the decline in the temperature of the tissue from the peak attained at twentyminutes was directly proportional to the increase of flow of blood during the same period.

DELETERIOUS EFFECTS OF EXCESSIVE DOSES OF MICROWAVE DIATHERMYCertain deleterious effects of microwave diathermy have been noted. Wise, Castleman and

Watkins [26] showed that when heavy doses of microwaves were employed, doses sufficientto cause injury to soft tissues, shortening and deformity of bones, or partial or completeepiphyseal destruction, might occur. They concluded, however, that when used in clinicaldosage, there was no appreciable effect on growth of bone.

Richardson, Duane and Hines [27] confirmed the observations that microwaves, whenapplied to the eye at a power output of 100 watts and at a wavelength of 12 cm., for fifteenminutes, would produce cataractous lenticular opacities. Then Feucht, Richardson andHines [28] found that microwaves of a wavelength of 12 cm. caused a greater increase intemperature of tissues containing metallic implants than in temperature of control tissues.The increase in temperature was of sufficient magnitude to cause gross damage in the tissuecontiguous to the metal and between the metal plate and the surface of the tissues.

EARLY STUDIES CONCERNING TREATMENT WITH MICROWAVE DIATHERMYAfter facts had been established and techniques developed by laboratory studies on animals,

and by clinical research on normal human beings, we began clinical treatment of a selectedgroup of patients [29-31]. Over a period of approximately two years, 481 patients receiveda total of 4,807 treatments. The number of treatments per patient varied from one to 110.Duration of a single treatment was from twenty to thirty minutes. An output of 60 to100 watts was used and in most instances the dosages varied between 80 and 100 watts.The conditions treated with microwave diathermy were mostly lesions of the shoulder,particularly periarthritis, tendinitis with calcification or acute bursitis, since it was thoughtthat this group of conditions could be studied fairly readily. Of all patients treated 76-6 percent had lesions of the shoulder. Bursitis in other regions of the body, conditions followingodontectomy, fibrositis, osteo-arthritis and a few miscellaneous conditions made up theother 23.4 per cent.

Lesions of the shoulder.-Analysis of clinical improvement of the shoulder followingtreatment with microwave diathermy, massage and exercise was helpful in determining thegeneral clinical results. Periarthritis, subdeltoid bursitis and similar lesions of the shoulderfrequently cause limitation of motion at the shoulder joint. Measurement of range of motionin this joint provided an objective method of evaluation, which could be considered togetherwith the subjective findings reported by the patient. Certainly, statistically valid conclusionscannot be drawn, yet the observations are sufficiently extensive to allow some clinical impres-sions concerning the effect of this type of heating.

Final clinical findings were divided arbitrarily into four categories: no improvement,slight improvement, good improvement and excellent improvement. The shoulders of approx-imately 72 per cent of the patients who could be traced were placed in the category of eithergood or excellent improvement. Only slight improvement was obtained by 240% and nobenefit by 40%. Our clinical impression is that lesions of short duration, such as acute bur-sitis, are more amenable to the treatment described than are more chronic disabilities, suchas periarthritis of long standing or "frozen shoulder". The results obtained comparefavourably with those obtained with short-wave or long-wave diathermy.A few patients in this series, in addition to the customary supplementary treatments

mentioned, previously had received roentgen therapy, injections of procaine, needling of abursa or manipulation of the involved shoulder.Minor undesirable reactions were noted in examination of only 16 of the 481 patients,

an incidence of 3.-3 0. Fifteen patients noted increase in pain which could be attributed tothe microwave diathermy. The increase in pain was described variously as "drawing sen-sation", "throbbing", "cramping", "deep pain" or "dull ache". In several instances treat-ment could be continued simply by reducing the output of the machine or changing theposition of the director. The pain of one patient was aggravated equally as much by short-wave diathermy as by microwave diathermy. One patient received a first degree burn about2 cm. in diameter.

Sixteen patients were treated with both microwave and short-wave diathermy. In compar-ing the subjective reactions to the two types of diathermy, 9 patients reported that theypreferred microwave diathermy. The reasons generally expressed for this preference werethat there was less systemic heating and less perspiration, no weight or pressure from theapplicator, good relaxation and yet adequate local heating. Most of the 7 patients whopreferred short-wave diathermy stated that they felt that they obtained more heat over alarger area.


Our preliminary survey indicates that microwave diathermy has no striking effect on theabsorption of calcium in cases of tendinitis with calcification or in cases of bursitis withcalcification.

Following odontectomy.-Thirty-five patients with bilaterally impacted third molars forwhich removal was indicated were selected for the study [32]. In 19 of the cases, the extentof operation on the two sides was approximately the same. Maxillary and mandibular thirdmolars were removed on one side only at the first operation and on the other side at a secondoperation. As nearly as possible all factors which would alter the postoperative course ofswelling and trismus were kept constant. To determine the effect of microwave diathermy,only one side, following one operation, was exposed to microwaves. Treatment was givenfor thirty minutes daily on each of the first four days after operation on this one side.Microwave diathermy was not given following operation on the other side. Measurementsof swelling arid trismus were made postoperatively on both sides. The amount of swellingand also the amount of trismus were measured by means of calipers.During the second, third, fourth and fifth postoperative days (Fig. 17A) swelling was less

severe on the side that was exposed to microwaves than on the side not treated. Trismus wasless severe on the third, fourth and fifth postoperative days when the jaw was treated with

? J \\\\ _ ffi f 9 0X~~~~20

7 ao

% t~~~~~~~~~~~/4

: Untreadted

4 2 3 s 7 1 2 4 s7Treat4% 0~~~~~10 b.

_6FIG 1%7A FIG 17B

T%~~~4

Treated

0 4

i. 3 4 5 6 70

1 2. 3 4 5 6 7

D cixsfter odontectomUo D%sy cifteir odontectom~j

FIG. 17A. FIG. 17B.

FIG. 17A. The average degree of swelling of 19 patients as determined by subtracting the pre-operative from the post-operative measurement of the distance between lingual embrasure of themandibular first and second molars and the cutaneous surface of the cheek; 17B. The averageseverity of trismus of 19 patients as determined by subtracting the postoperative from the pre-operative measurement of the distance between the edges of the maxillary and mandibular centralincisors when the mouth was opened maximally.

microwaves following odontectomy than when microwave diathermy was not applied(Fig. 17B). Subjectively, postoperative pain and soreness were relieved, especially duringthe actual period of heating.

CONCLUSIONSWhile more clinical investigations must be done before the exact place of microwave

diathermy in clinical practice will be known and until such investigations can be made,microwave diathermy should be employed clinically with caution, nevertheless certainconclusions can be drawn from the studies conducted so far.

It may be concluded that, as have most forms of heat employed therapeutically, so micro-wave diathermy has certain shortcomings. These include the following: (1) Living tissuesmay be damaged by excessive doses of microwave diathermy. (2) Ischaemic tissues are especi-ally susceptible to burning by excessive doses of microwave diathermy. (3) Bodily regionscontaining large amounts of fluid may be heated excessively by microwave diathermy.(4) Tissues overlying bony prominences may be heated excessively by microwave diathermy.


(5) The eye may be damaged and cataracts may be formed by excessive amounts of micro-wave diathermy. For these reasons, it is concluded that when employed clinically microwavediathermy should be applied with particular caution over or near the following: the eye,ischemic tissues, regions in which there are effusions, collections of fluid or marked cedema,tissues containing metallic bodies, bony prominences, epiphyseal regions of growing bones orregions in which there are tendencies to haemorrhage.Microwave diathermy has a number of advantages over the methods of applying heat

locally to living tissues which have previously been employed therapeutically. From ourobservations it may be concluded that these embrace the following: (1) Microwave diathermyprovides a new, extremely flexible and effective means of heating living tissue which permitsaccurate localization by direction of a beam of energy toward any surface of the body.(2) Absorption of microwave energy is greater than the absorption of energy produced byhigh-frequency radiations of longer wavelength. (3) Greater rises in the temperature of muscleat a depth of 3 cm., as compared with rises in temperature of subcutaneous tissue, are pro-duced with microwave diathermy than with short-wave diathermy. (4) When microwavediathermy is employed for heating tissues, there is a desirable relationship between cutaneousand internal temperatures which permits adequate internal heating, at least to a depth of 3cm., without undue heating of the skin. (5) Proper application of microwave diathermy willproduce marked and readily controlled or modified increases in the temperature of subcu-taneous tissues, muscle, bone cortex and bone-marrow. (6) Microwave diathermy will notonly increase the temperature but also will produce marked increases in flow of blood intissues to which it is applied in experimental animals and in man. (7) Microwave diathermywill produce heating of local regions with almost no systemic heating. (8) Microwave dia-thermy can be employed clinically for effective heating of local regions of the body andit can be applied with maximal comfort to the patient.

Finally it may be concluded that the use of an impedance matching transformer duringapplications ofmicrowave diathermy will decrease reflection of radiation by the skin and resultin greatly increased transmission of energy to the deeper tissues. Thus, use of the impedancematching transformer provides a new and effective method for accurate localization of heatin deeper tissues.

It can be concluded that because of accuracy of localization, great flexibility of applicationand extreme effectiveness in production of heat at depth, microwave diathermy may wellbecome a valuable new therapeutic agent.

REFERENCES

1 KRUSEN, F. H. (1935) Short wave diathermy; preliminary report, J. Amer. med. Ass., 104,1237.

2 WILLIAMS, N. H. (1937) Production and absorption of electromagnetic waves from 3 cm. to6 mm. in length, J. appl. Physics, 8, 655.

3 SOUTHWORTH, G. C. (1937) New experimental methods applicable to ultra short waves, J. appl.Physics, 8, 660.

4 HEMINGWAY, ALLAN, and STENSTROM, K. W. (1939) Physical Characteristics of Short WaveDiathermy. In Handbook of Physical Therapy. Ed. 3, Chicago, American Medical Associa-tion, p. 214.

5 KRUSEN, F. H. (1941) Physical Medicine; the Employment of Physical Agents for Diagnosisand Therapy. Philadelphia, p. 397.

6 HOLLMANN, H. E. (1938) Das Problem der Behandlung biologischer Korper im Ultrakurzwellen-Strahlungsfeld. In Danzer, H., Hollmann, H. E., Rajewsky, B., Schaefer, H. and Schliephake.E.: Ultrakurzwellen in ihren medizinischbiologischen Anwendungen. Leipzig, Chap. 4, p. 232.

7 (1939) Zum Problem der Ultrakurswellenbehandlung durch Anstrahlung, Strahlen-therapie, 64, 691-702.

8 KRUSEN, F. H., HERRICK, J. F., LEDEN, URSULA, and WAKIM, K. G. (1947) Microkymatotherapy:Preliminary report of experimental studies of the heating effect of microwaves ("Radar")in living tissues, Proc. Mayo Clin-, 22, 209.

9 DAILY, L. E. (1943) A clinical study of the results of exposure of laboratory personnel to radarand high frequency radio, U.S. Nav. M. Bull., 41, 1052.

10 LIDMAN, B. I., and COHN, CLARENCE (1945) Effect of radar emanations on the hematopoieticsystem, Air Surgeon's Bull., 2, 448.

11 FOLLIS, R. H., Jr. (1946) Studies on the biological effect of high frequency radio waves (radar).Amer. J. Physiol., 147, 281.

12 SALISBURY, W. W., CLARK, H. W., and HINES, H. M. (1949) Exposure to microwaves, Electronics,22, 66.

658 Proceedinqs of the Royal Societ?, of Medicine 42

13 DUMKE, P. R., and SCHMIDT, C. F. (1943) Quantitative measurements of cerebral blood flow inthe macacque monkey, Amer. J. Physiol., 138, 421.

14 WORDEN, R. E., HERRICK, J. F., WAKIM, K. G., and KRUSEN, F. H. (1948) The heating effectsof microwaves with and without ischemia, Arch. Phys. Med., 29, 751.

15 BECKER, J. A., GREEN, C. B., and PEARSON, G. L. (1946) Properties and uses of thermistors-thermally sensitive resistors, Electrical Engineering, 65 (Transaction Section), 711.

16 DRUMMETER, L. F., Jr., and FASTIE, W. G. (1947) A simple resistance thermometer for blood-temperature measurements, Science, 105, 73.

17 RAE, J. W., Jr., HERRICK, J. F., WAKIM, K. G., and KRUSEN, F. H. (1949) A comparative studyof the temperatures produced by microwave and short wave diathermy, Arch. Phys. Med.,30, 199.

18 HERRICK, J. F., JELATIS, D. J., and LEE, G. M. Microwave Transformer Matching AnimalTissue to Free Space. Unpublished data.

19 GERSTEN, J. W., WAKIM, K. G., and KRUSEN, F. H. (1950) A method for decreasing reflectionof microwaves by tissue, Arch. Phys. Med., 31, 281.

20 HERRICK, J. F., JELATIS, D. J., and LEE, G. M. The Dielectric Properties of Tissues Importantin Microwave Diathermy. Unpublished data.

21 ENGEL, J. P., HERRICK, J. F., WAKIM, K. G., GRINDLAY, J. H., and KRUSEN, F. H. The Effectof Microwaves on Bone and Bone Marrow and on Adjacent Tissues. Arch. Phys. Med.(In press.)

22 OSBORNE, S. L., and FREDERICK, J. N. (1948) Microwave radiations; heating of human andanimal tissues by means of high frequency current with wavelength of twelve centimeters(the microtherm), J. Amer. med. Ass., 137, 1036.

23 DAILY, Louis, Jr., WAKIM, K. G., HERRICK, J. F., PARKHILL, E. M., and BENEDICT, W. L.(1950) The effects of microwave diathermy on the eye: An experimental study, Amer. J.Ophthal. (In press.)

24 GERSTEN, J. W., WAKIM, K. G., HERRICK, J. F., and KRUSEN, F. H. (1949) The effect of micro-wave diathermy on the peripheral circulation and on tissue temperature in man, Arch. Phys.Med., 30, 7.

25 BERRY, M. R., BALDES, E. J., ESSEX, H. E., and WAKIM, K. G. (1948) A compensatingplethysmokymograph for measuring blood flow in human extremities, J. Lab. Cliii. Med.,33, 101.

26 WISE, C. F., CASTLEMAN, BENJAMIN, and WATKINS, S. L. (1949) Effect of diathermy (short waveand microwave) on bone growth in the albino rat, J. Bone Jt. Surg., 31A, 487.

27 RICHARDSON, A. W., DUANE, T. D., and HrNEs, H. M. (1948) Experimental lenticular opacitiesproduced by microwave irradiations, Arch. Phys. Med., 29, 765.

28 FEUCHT, BARBARA L., RICHARDSON, A. W., and HINES, H. M. (1949) Effects of implantedmetals on tissue hyperthermia produced by microwaves, Arch. Phys. Med., 30, 164.

29 WAKIM, K. G., HERRICK, J. F., MARTIN, G. M., and KRUSEN, F. H. (1949) Therapeuticpossibilities of microwaves; experimental and clinical investigation, J. Amer. med. Ass., 139,989.

30 MARTIN, G. M., RAE, J. W., Jr., and KRUSEN, F. H. (1950) Medical possibilities of microwavediathermy, 5th. med. J., 43, 518.

31 RAE, J. W., Jr., MARTIN, G. M., TREANOR, W. J., and KRUSEN, F. H. (1950) Clinical experienceswith microwave diathermy, Proc. Mayo Clin., 25, 441.

32 ROYER, R. Q., WAKIM, K. G., LOVESTEDT, S. A., and KRUSEN, F. H. The Influence of MicrowaveDiathermy on the Swelling and Trismus Resulting from Odontectomy. Unpublished data.

section of physical medicine medical applications of microwave

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