DECEMBER 1952

AN EXPERIMENTAL X-RAY APPARATUS WITH MIDGET X-RAY TUBE

by P. J. M. BOTDEN, B. COMBÉE and J. HOUTMAN.621.386.1 :615.849: 620.179

The small, mass-produced and inexpensive high-tension. generator of a projection televisionreceiver has been found to be a most suitable voltage supply unit for a very small X-ra.y tube.The present artiele shows how this fact was made use of when the "KT" apporatus was developed.The X-ray tube of this apparatus is not only remarkable for its small size, bul also for a direc-tional effect in the emitted radiation, favourable to therapeutic application, which effect is notobserved in normal X-ray tubes.

Introduction

The X-ray tube which we will discuss in thepresent paper is probably the smallest ever designed.It is 45 mm long and 14 mm in diameter, includingthe earthed jacket. A power of 2.5 watts can bedissipated continuously at the anode, the maximumtube voltage is 25 kV, and the maximum tubecurrent 200 [LA.To complete this brief description,fig· 1 shows a photograph of the tube (which has

therapeutic applications of X-rays it can be mostuseful to the physician to have an X-ray source athis disposal, of such small dimensions that it caneasily be held and positioned by two fingers, andwhich can be inserted into small cavities of thebody if necessary. Sometimes radiographs of all sortsof objects have to be made for research or teachingpurposes, where a very soft radiation is required, an

Fig. 1. Midget X-ray tube (KT tube) with cable and plug. A match is shown by the sideof the X-ray tube (to the right) for comparison.

not yet been brought into regular production), withthe cable connecting it to the high-tension source.The larger tubular element at the left end of thecable is not the X-ray tube, but the plug.A few remarks will show that the design of this

midget X-ray tube (and the accessory high-tensionsource) was not merely inspired by an ambition todesign tbe smallest X-ray tube ofthe world. In certain

extremely small power is sufficient and a high pricefor the apparatus is undesirable. This defines ingeneral terms the possible application of the abovecharacterised "KT" tube and "KT" apparatus (fromthe Dutch for "smallest therapy", viz. "kleinstetherapie"). We shall discuss below the specialfeatures of this X-ray source, and those it lacks;it is convenient to compare it with the CT tube

165

166 PHILlPS TECHNICAL REVIEW VOL. 14, No. 6

(CT from Contact Therapy), described previoulsyin this review, which, of all X-ray tubes is mostsimilar to the tube under discussion 1)2).

We shall conclude the present papel: with a fewfurther observations on the possibilities of applica-tion.

Construction óf the X-ray tube

In fig. 2 a schematic sectional view of th~ tubeis, given. The simplicity and compactness of designshown by the figure are chiefly due to the fact thatthe X-rays leave the tube via the anode. For this

Fig. 2. Schematic cross-section of the KT tube. A anode can,Be beryllium plate, Au gold layer, G filament, M metalcylinder, R effective X-ray beam.

purpose the anode consists of a very thin layer ofgold deposited on a vacuum-tight plate of beryl-lium. Due to the very high atomic number of gold(79) a high efficiency of X-ray production is obtain-ed from the electrons slowed down in it; owing tothe very low atomic number of beryllium (4,) thiswill transmit the soft radiation excited at 25 kVwith negligible attenuation. The gold layer has to bevery thin in order to restrict the proper absorption ofthe excited X-radiation to a minimum; on the otherhand it should not be too thin, since all electronspassing through the gold layer are naturally lost forthe generation of X-rays.

The "inherent filter'? of the X-ray tube, i.e. thefilter which the X-rays, produced by the tube design-ed in the manner given above, must pass before theirpractical application, is equivalent to about 1.5 mmberyllium. The importance of this very low inherentfilter will be discussed presently.

The beryllium plate is soldered in a metal bushwhich functions as tube wall for this end of the tube("anode can"). This metal anode assembly is earthedand the cathode is connected to high voltage. Thecathode comprises a filament, consisting of a tung-

1) H. A. G. Hazeu, J. M. Ledeboer and J. H. van derTuuk, An X-ray apparatus for contact therapy, PhilipsTechn. Rev. 8, 8-15, 1946.

2) B. Combée and P. J. M. Bo td envSpecial X-ray tubes,Philips Techn. Rev. 13, 71-80, 1951 (No. 3).

sten coil of a few turns, and a small metal cylinderenclosing the filament and its supporting poles. Thiscylinder has a focusing eirec1;. on the electronsemitted by the filament. Moreover, it simplifies theproblems of the insulation between the cathode andthe closely adjacent earthed jacket: without thecylinder the electric field strengths at the thin lead-in poles would be much higher than they now areon the cylinder., The filament heating 'power can amount to about1 watt at the highest (1.4 V, 0.6 A). The coilled fila-ment will then give a saturation emissi~n of 200 p.A.

The focus and the radiation obtained

A description of an X-ray tube is n,ot completewithout some remarks regarding the configurationand the loading of the focal spot. In this case it ismost appropriate to begin with some general obser-vations, which will also explain the choice of theco~structional principle adopted.

Soft radiation is suitable for the therapeuticirradiation of very thin layers of tissue on or at thesurface of the body. The underlying healthy tissuereceives only relatively small doses in this case andis consequently very little affected (small depth pene-tration). Owing to the low voltage and the verysmall inherent filter, the radiation of the tube isextremely soft; the half value thickness of the radia-tion is only 0.035 mm of aluminium or 0.5 mm oftissue at a tube voltage of 25 kV, and at 10 kV it iseven 0.02 mm of Al or 0.3 mm of tissue. In thisrespect the radiation is similar to that of the pre-sent CT tube with a window of mica and beryllium(see the article referred to in note 2)).A second means of obtaining a very small depth

penetration is to place the focus of the X-ray tubeat a very short distance from the skin. This wasextensively commented on in the article referredto 1). This same article further stated that the small-est focus-skin distance, and therefore the smallestrelative depth doses can be realised if the anodeitself serves as exit window for the X-rays. Thisdesign principle also leads to a relatively simpletube construction. It is generally considered to bea drawback to this principle of design that therays passing obliquely through the' anode areattenuated more than those passing through per-pendicularly, resulting in a strong decrease of thedosage rate from the centre to the edges of theirradiated area. This makes it difficult to apply anexact dosage of radiation. For this reason a differentdesign was chosen for the CT tube, as may be seenin the article mentioned 1).

DECEMBER 1952 X·RAY APPARATUS WITH MIDGET X·RAY TUBE 167

We have, nevertheless, already observed that 'thedesign with anode - functioning as window waschosen for the KT tube. This does not imply, how-ever, that the objection referred to above wasignored; surprisingly enough this objection doesnot hold at all in this case! If the radiation intensityof the KT tube is measured in various directions 3),a diagram as given infig. 3 results. The intensity by

Fig. 3. Relative dosage rate of the KT tube, measured indifferent directions from the axis, at a distance of 150 mmfrom the focus, for two different tube voltages. In the axialdirection (0°) the dosage rate for both curves is made equalto 100. The asymmetry may be due to a slight inhomogeneityof the gold layer. (These measurements and those of fig. 4,weremade by Dr. W. J. Oos~erkamp and J. Propcr.)

no means decreases with increasing deviation fromthe axial direction, but even increases initially; atan angle of 20° to the axis it is at its maximum, andthe subsequent decrease is still rather gradual up toangles of about 50°. There is thus no question of astrong decrease in the dosage rate over the irradiatedarea from the centre to the edges. On the contracy,a very uniform dosage distribution is found for nottoo large, flat areas, even more Miform than with thespherical radiation diagram of normal Xvray tubes,since in our case the larger distance from the focalspot to the off-axis parts of the field is neutralisedby the initial increase in intensity shown in fig. 3.The phenomenon illustrated by fig. 3 has been

k~own for a long time, and can be explained by aconsideration ofthe mechanism whereby Xvraye areproduced in the anode. It is not observed in normalXvray tubes because of the rather th i ck anode thesetubes always contain. In the present case onlythe verythin foil of gold functions as an anode, and as a mat-ter of fact K ulenkampff was able to" demonstratethe directional effect as early as in 1928 by the useof specially designed tubes with a very thin anode 4).

3) For these measurements see: W. J. Oosterkamp andJ. Proper, Acta Radiologica 37, 33·43, 195~ (No. I).Therein is given a full description of the method wherebysuch soft and heterogeneous X·rays may be measured.

4) H. Kulenkampff, Ann. Physik IV, 57, 597, 1928. Formore recent investigations into the phenomenon see:G. Sesemann, Ann. Physik V, 40, 66, 1941; O. Blunck,Ann. Physik. VI, 9, 373, 1951.

The electrons striking the anode with great velocity areslowed down on penetrating into the metal. According toclassical theory, each electron will emit radiation, the con-tinuous Xsradiation or "Bremsstrahlung", According to thistheory one can also expect the radiation to have zero intensityin the direction of movement of the electron, as also in thereverse direction, whereas the radiation will be at its maximumat a certain angle to this direction - depending on the electronvelocity. In a thick anode a penetrating electron mayrepeatedly change its direction by small amounts, due todeflection by atomic nuclei, with very small loss of energyeach time, b'efore it passes a nucleus so closely that it isstrongly decelerated and emits the Xvradiation observed. Thecontribution to the radiation, of this electron, will in thatcase possess the directional dependence as mentioned above,but with its last direction of movement as axis. Since theseaxes, for all the electrons, will have a random distributionof direction, the total radiation can reveal little or nothing ofthe described directional effect.

On the other hand, if the anode is very thin, the electronswill already have passed the entire layer after relatively fewencounters with atom nuclei. The emission of each contributionto the observed radiation can therefore only be preceded byfew deflections; consequently the "axes" for all contributionsdeviate but slightly from the axis of the Xsrny tube, and thetotal radiation will consequently· also show the directionaleffect with but little spread.If the anode functions as exit window, the opportunity for

the natural directional effect of the radiation to be observedbecomes still smaller in the case of a rather thick anode, sincethe earlier mentioned absorption effect (intensity decrease withincreasing angle), especially important in the case of thickanodes, is superimposed on this directional effect. Fig. 3 clearlyshows the increasing importance of this effect, even with thevery thin anode of the KT tube, 'as the tube voltage is decrea'sed(softer radiation).

Owing to the constructional principle adopted, itis possible with the KT tube to reduce the focus-skin distance to 1 mm if so desired, viz. by pres·sing the anode into the surface to be irradiated.Enormous dosage rates can be realised with only avery small power, if the distance between focus andobject is so small. With the above-mentioned maxi-mum permissible anode dissipation of 2.5 Wand avoltage of 25 kV, the KT tube will give 17000 rönt-gen units per minute at a distance of 1 cm from thefocus; at 10 kV and 2W dissipation (the tube currentis 200 (LA maximum), it will still give 7000 r/min!These are measured values (fig. 4); a separatelydetermined correction factor was applied for theabsorption of the soft radiation in the window ofthe dosimeter (see the article referred to in note 3)).At a focus distance of 1 mm even much larger dosagerates, ofsome hundred thousands r/min, are obtained.Naturally, if the focus distance is not very small, e.g.20 mm, which can also he reached with the CT tube,the KT tube is, as far as dosagerates are concerned, byfar the inferior of the CTtube, which was designed fora maximum power of 100Wand voltages up to 50kV.

168 PHILIPS TECHNICAL REVIEW VOL. 14.,No. 6

In normal Xsray therapy tubes the focus is madeas small as possible in order to obtain a sharpboundaryto the fieldto be irradiated (small penumbrawidth of diagram edges); in the KT tube, on the

20000r/min

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Fig. 4·.Absorption characteristic of the radiation of the KTtube. The dosage rate obtained at 1 cm distance from the focuswhen the radiation is weakened by an aluminium filter ofvarying thickness is plotted for four values of the tube voltage.The minimum filter, the inherent filter of the tube, is equivalentto about 1.5 mm beryllium or 0.03 mm aluminium. For thiscase the graph gives a value of 17 000 r/min at 25 kV and3600 r/min at 10 kV. (The figures along the horizontal axisare not correct: a constant value of 0.02 is to be added toall-of them.) All curves were measured at a tube currentof 100 [LA; the 25 kV curve, therefore, corresponds to aload of 2.5 W, the 10 kV eurve to 1 \V. The measurementswere made at a distance of 2.5 cm from the focus and recal-culated to a distance of 1 cm by use of the inverse square law:a correction was made for the smaller air absorption at 1 cmdistance as against 2.5 cm distance from the focus.

contrary, the electrode configuration was so designedas to spread the focus over the entire anode surface(diameter 6.5 mm). Owing to the rather large focaldimensions the loading of2.5 Wis permissible and theabove-mentioned very large dosage rates are obtain-ed. It is true that part of the gain with regard to thedepth penetration, obtained by the selected principle

Fig. 5. (a) If the focus (fl) is small, the dosage rate decreasesas the inverse square of the distance. The dose thus rapidlydecreases from A to e.g.A'. (b) If the focus is large, the dosagerate decreases more slowly than as the inverse square of thedistance. The decrease is intensified, however, by the extraabsorption of oblique rays in the object 0: point A' profitsless from the largeness of the focus than point A, since therays' originating from f2 have to travel' a' longer distance inthè: tissue than those odginäiing lióm fl' .

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of construction, is sacrificed if the focus is so large,for if the focus is not small in proportion to thefocus-skin distance, the dosage will, for geometricalreasons, not follow the inverse square law, but willdecrease much more slowly with increasing distance.Thus, the dosage rate at 1 mm distance will notbé 100 times larger but, according to our computa-tion, only 25 times as large as at 1 cm distance.Still, one can see from these figures that the distanceeffect is still considerable. Moreover, it !s enhancedby the effect of the absorption of the rays in thetissue: points at some distance below the skinsurface profit less by the largeness of the focus,therefore, than do points on the skin surface (seefig. 5). The resulting decrease of the dosage withdepth is shown infig. 6. It will be seen, consequently,that only a thin layer 'of skin of a few tenths ofone millimetre is effectively irradiated.

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Fig. 6. Dosage rate of the KT tube computed for variousdepths in the irradiated body, at a focus-skin distance of 1mm,for two different tube voltages. The dosage rate on the skinwas set equal to 100 for both curves. The broken-line curvesrepresent the dosage rate which would be obtained with apoint focus.

It will he seen from this, that the lack of sharpness of thefield boundary, which was mentioned above as the current ob-jection against a broad focus, will not be of much significaneein this case. For this lack of sharpness will only be appreciablein planes at some distance behind the diaphragm, in the caseunder discussion - the diaphragm being represented by theanode cap placed on the skin - at a certain depth below theskin where the dose is already relatively small.If the anode is placed on or near to the skin of the patient,

the focus is of nearly the same area as the irradiated skin sur-face..Naturally this is conducive to a stillfurther equalisation ofthe dosage distribution over the irradiated surface (see above).

1!ECEMBER 1952 X-RAY APPARATUS WITH MIDGET X-RAY TUBE

The above-mentioned limit of 2.5 watts to theloading of the anode is set by the temperature bothofthe focus and the jacket. We need not distinguishbetween continuous and short-duration loading inthe case of the KT tube. The heat capacity of thegold foil is so small that the maximum focus temper-ature is always reached, even in the briefest periodof loading likely to be used. The temperature ofthe outer' face of the anode rises in proportion tothe time of loading, in accordance with the curve

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Fig. 7. Temperature time curve for the outer surface of theanode of the KT tube in free air under load. The tube wasloaded to 2.5 W from t = 0 to t = 120 sec, the tube thenbeing switched off. (If the anode is placed on the skin of ti epatient, the temperature will rise a little more slowly, owingto the thermal conductivity of the body.)

represented infig. 7, which shows that on prolongedloading temperatures not much higher than 90 oeare reached. This temperature is, of course, too highfor direct contact with the skin of the patient, sothat irradiation in this manner is limited to 10 or 20seconds - which will, however, in general be amplysufficient for the application of the necessary dose!For irradiation at a certain, albeit small, distance

(in which case e.g. a small localizer is fitted to thetube) the anode temperature is not at all important.At other places, the jacket, which, if the tube isfully loaded, has to dissipate 3.5 watts in total(2.5 W from the anode, 1 W from the cathode),becomes hardly warmer than the hand, and it cantherefore be held by the fingers without discomfort,or allowed to touch the skin or the mucous mem-branes if tissue parts in body cavities have to beirradiated.

Jacket and cable

The position of the X-ray tube in the jacket intowhich it is built, is shown infig. 8. The tube is con-nected to the high-tension source by means of avery flexible cable 1.5 metre long, with "Podur"(polyvinyl chloride) insulation and a metal braiding.The metal jacket is bonded to this braiding at oneend and fixed at the other end to the anode can.Thus the anode is earthed via the braiding.A second protective "Podur" sleeve is drawn over

the metal braiding and makes a liquid-tight seal

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with the metal jacket. The X-ray tube can thereforebe easily disinfected, which is necessary for applica-tion in body cavities. The total diameter of this veryflexible cable, built up as described, is only 8 mm.

The thin cable stands up very well to the highvoltage to which it is subjected. The X-ray tube isoperated by D.e. voltages up to 25 kV; it has beenshown that the free cable (i.e. without X-ray tube)does not break down, even if subjected to a directvoltage of 150 kV at room temperature. When sub-jected to alternating current, which is always muchmore dangerous to cables, the test showed that thecable could stand a voltage of 60 kV peak value forthirty minutes without breaking down.

The high-tension generator

In developing the KT apparatus we have beenable to avail ourselves of an earlier development inthe field of television .. A high-tension generatorhad been developed in Eindhoven for supplying thecathode-ray tube used in projection television 5),possessing exactly the properties we needed for theKT tube supply. This generator produces a wellsmoothed direct voltage of 25 kV at a load of 150 [LA,its characteristic being such that the short-circuitcurrent is not much greater than the normal workingcurrent, and the output capacitance is only small, sothat the energy which will be discharged through thecathode-ray tube in the event of a fault (breakdown)is not excessively large. The last two properties, all'important to the avoidance of overloading of thefluorescent screen, are also essential to our purpose;it is obvious that all overloading of the very smallKT tube, however short, should be avoided.We have consequently been able to copy, in

broad outline, the design of the high-tension genera-tor referred to, which has already been described inthis review. In the present article, therefore, we shallgive only a résumé of the principle and mention thedeviations from the earlier construction which werefound to be necessary.

Fig. 8. Position of the X-ray tube (KT) in the jacket, andsectional view of the cable. PI "Podur" insulation, Af".metalbraiding, P2 outer "Podur" sleeve.

5) Projection-television: receiver, Ill: G. J. Siezen andF. Kerkhof, The 25 kV anode voltage supply unit, Philipstechno RIlV. 10, 125-134, 1948.

169

170 PHILlPS TECHNICAL REVIEW VOL. 14, No. 6 "

An oscillator circuit, consisting of a coil Landits own capacitance Cp, is huilt into the anodecircuit of a pentode. The anode current is periodical-ly interrupted by a saw-tooth voltage applied tothe grid of the pentode. At each interruption adamped oscillation occurs in the circuit with apeak value:

.Vmax = imax iL/Cp;

imax is the anode current at the moment of inter-ruption. With suitable 'circuit constants, a voltagepeak of the order of 10 kV can he obtained and theinterruption can he given a rather high rate ofrepetition, e.g. of the order of a 1000 times a second. 'The intermittent high voltage III the oscillatorcircuit is converted by a cascade rectifier, composedof a number of valves and capacitors, into a directvoltage which is a multiple of Vmax- viz., in our case,25kV.The rather high repetition frequency mentioned

above is one of the essential items of the design.As a consequence of this high frequency the result-ing direct voltage can be well smoothed with arelatively small capacitance. This amounts to about2000 pF; the ripple in the voltage on the X-ray tube,at 200 !LAtube current, is onlyabout 100 V.The television generator supplies a positive volt-

age of 25 kV relative to earth. We needed anegative voltage for the KT tube; this can beobtained by merely reversing the valves in the cas-cade rectifier.For the television tube a fixed voltage of 25 kV

is required. In our case it was desired that the X-raytube should also he operable at lower voltages. Wehave obtained a continuously adjustable tube volt-age by making the anode voltage of the pentodevariable: this involves variation of the anodecurrent imax at the moment of interruption and,consequently, Vmax (see the formula given above).In the television generàtor the cathode filaments

of the cascade valves are fed from a windingcoupled to the coil (L) in the anode circuit, andconsequently supplying a fixed voltage. In our casethe voltage on the coil is variable, preventing theuse of this simplemethod; we had thus to provide forthe valves a separate filament transformer, with hightension insulation. We have so arranged it, however,that this transformer is switched on and off simul-taneously with the high tension. The service life ofthe valves is therefore not unduly shortened byuseless burning. The resulting slight delay in theexcitation of X-rays due to the filaments of thevalves first having to be heated and the capacitorsto be charged, after switching on the high tension,

was not considered objectionable in this case; itamounts to less than t second. The filament trans-former for the X-ray tube cathode is also switched onand off simultaneously with the above-mentionedfilament transformer, Consequently no heat is gen-'erated in the tube so long as it is not under hightension and the temperature of the jacket does notrise unduly from its initial value. By means of anadjustable resistor in series with' the last-mentionedfilament transformer, the current in the-Xvray tubecan be continuous,ly varied from 0 to 200 !LA.

Adjustments of tube current and tube voltage are not com-pletely independent of each other; moreover, no coupling hasbeen provided between the two adjustments to preclude thepossibility of the maximtim voltage (25 kV) or the maximumpower (2.5 W) being exceeded. Such refinements would havemade the apparatus too complicated and expensive, in view

25kV,'

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Fig. 9. Calibration curves for the voltage regulation of the. KT apparatus. The high tension on the KT tube is varied by

adjustment of the pentode anode voltage Va. The relation-ship between the two depends also, however, on the tubecurrent, which is separately controlled. - This calibrationchart can be made identical for all KT apparatus by means ofa variable resistor.

of its restricted purpose. The only consequences for the userare that he, should bear in mind the limit of 2.5 W whenselecting the values of the tube current and tube voltage andthat he should consult a set of calibration curves when pre-setting these values (see fig. 9). If the user intends to raisethe voltage whilst the apparatus is in use, he shouldadjustt he tube current to the new, smaller value beforehand.

The high tension generator was built into acabinet, shown infig. 10, together with the requisitepower supply apparatus, controls, switches, etc. Thecable, with the attached X-ray tube, is connectedto it by means of a plug, insulated for high tension,which is tightened by means of a coupling nut.This nut at the same time presses on a pin, whichfirst connects the braiding of the cable (and con-sequently the anode and the jacket of the X-raytube) with the earth of the apparatus and thencloses the anode circuit of the pentode. A smallcylindrical opening to hold the X-ray tube when not

DECEMBER 1952 X-RAY APPARATUS WITH MIDGET X-RAY TUBE

in use has been provided in the lid of the cabinet(in the photograph the tube is just being with-drawn).

Fig.11 shows the cabinet with side walls removed.

We shall merely mention here three cases forwhich the apparatus has proved its usefulness:(1) the irradiation of warts and ha em an gi 0m a e(benign tumors of the blood vessels), more especial-

Fig. 10. The KT apparatus, ready for use. The tube current and the tube voltage areadjusted by means of the control knobs and meters. Note the connection of the "Podur"cable, with the midget KT tube at its end, to the apparatus. When not in use, this tubeis housed in an opening in the apparatus.

Methods of application

Physicians who have been working with the KTapparatus for a number of months will presentlyreport elsewhere on its use for therapeutic irradiation.

ly the so-called strawberry marks (fig. 12); in mostcases of this type the patients are very young children,also incubator children, and the relatively smallCT tube is still too large to be convenient in suchcases; (2) the irradiation of the cornea of the eye,

171

172 PHILlPS TECHNICAL REVIEW VOL. 14, No. 6

Fig. 11. The apparatus open. The black box on the left contains the high-tension coil andthe cascade rectifier. On the right to the rear the power unit supplying the pentode.Shown in the foreground are the unscrewed coupling nut and the plug (withdrawn), bymeans of which the cable is connected. To the right of this is a safety fuse shown unscrewed.

e.g. after transplantation (fig. 13), and (3) the ir-radiation of very superficially located small tumoursof the mouth and throat cavities. For each ofthese cases special smalllocalizers for the limitationof the field may be useful, and aluminium filters ofvarious thicknesses may be put in the localizers ifin a special case a somewhat harder radiation isrequired.The KT tube can also be usefully employed in

another field, viz. the testing of materials. In supportof this observation we may mention the fact (thoughthis can hardly serve to justify its own existence!)that the KT tube renders good service in the pro-duction of - KT tubes; the rolled beryllium platefor the anode is tested for the absence of impurities,and the thin layer of gold for homogeneity, in bothcases by making X-ray photographs with the KTapparatus. In general the very soft (and not undulyintense) radiation of the KT tube can he employedto good advantage for the examination of very lightmaterialor very thin layers.

Lastly, the KT tube lllay in our opllllOn beextremel y useful in the hi 0 log i c a I field. An

Fig. 12. Application of the KT tube for the irradiation of astrawberry mark. (This photograph was put at our disposalby Dr. G. J. van der Plaats, radio-therapeutist at thehospitalof St. Annadal. Maastricht.)

DECEMBER 1952 X-RAY APPARATUS WITH MIDGET X-RAY TUBE 173

Fig. 14. X-ray photograph of a shrub-leaf, taken with the KT tube. In this case the voltage was18 kV, the tube current 100 [LA,the distance from focus to film 30 cm and the exposure time 180 sec.

interesting example is the examination of the effectof X-rays on bacteria or other organisms present ina liquid solution. The KT tube provides a neatway of doing this; the tube is dipped in the solutionwhich is stirred until - according to the laws ofprobability - each volume element of the liquidhas been near the anode for a sufficiently long timeto absorb the required total dose. A more obviousapplication of the KT apparatus is the making of

Fig. 13. Application of the KT tube for after-irradiation ofa transplan ted cornea. The tube is fitted, in this case, wi tha very small cone-shaped localiser, in order to restrict thebeam of rays to the requisite small diameter. Since the depthpene tra tion ofthe radiation is extremely small, special measuresfor protection against secondary radiation are not necessary.(The photograph was put at our disposal by MI'. P. J. L. Schol-te, radio-therapeutist at the Municipal Hospital, the Hague.)

Fig. IS. X-ray photographs of honey bees after feeding themsugar water mixed with barium sulphate as contrast medium.Photographs 5, 3 and 2: half an hour after feeding withrespectively 1%, 5% and 10% BaSÛ4; photographs 1, 4. and6 ditto an hour after feeding.

The intestines of the bee lie in its abdomen and consist chieflyof the pouch-shaped honey stomach, serving as temporarystorage place for the food taken, the stomach or middleintestine, i.e. the digestive organ proper, and the rectum. Thelight spots seen on the photographs in the abdomen of eachbee, are the honey stomach and the middle intestine. Theirdifferences in size are due to differences in appetite; the honeystomach of bee no. 6, for example, is so well filled that itsabdomen is considerably longer than that of the other bees.(The experienced beekeeper can see from the size of theabdomen if his bees collect much or little honey.) The organsof bee no. 2 are clearly shown in their normal position: themiddle intestine lies in a loop. None of the six photographsshows positively the contrast medium in the rectum.

By means of photographs such as these, the absorption ofthe food by and its passage through the intestines of thehoney bee could be studied. (The photographs were made incooperation with Drs. A. J. de Groot, Lab. for ComparativePhysiology, Utrecht, to whom we also owe this explauation.)

contact photographs of biologicalobjects, so verythin that this could be done nearly - but still notquite - with light rays. Fig. 14 gives as an examplean X-ray photograph, made with a KT apparatus,

174 PHILlPS TECHNICAL REVIEW VOL. H, ..No. 6

of a relatively thick, barely translucent leaf of ashrub, Fig: 15 is still more striking; it reproducesXvray photographs of bees, after imbibing a sugarsolution mixed with barium sulphate, the well-knownc'ontrast medium for radiological stomach examin-ation; the six photographs correspond to differentperiods of time after feeding.It may suffice to give these examples here,

Summary. The experimental X-ray tube described in thisarticle (KT tube fromthe Dutch for smallest therapy, "kleinstetherapie", tube) is 45 mm long and 14 mm in diameter, in-cluding the earthed metal jacket. The X-rays are excited in avery thin layer of gold, deposited on a beryllium plate, whichat the same time servesas exit windowfor the rays. The tubecan be loaded with a voltage of 25kV and a power -ofmaxi-mum 2.5 W. The half-value thickness of the radiation is verysmall, viz. 0.5 mm tissue at 25 kV and 0.3mm tissue at 10kV,owing to the small "inherent filter" of the tube (equivalentto 1.5mm beryllium).Very high dosagerates of some hundredsof thousands of röntgen units per minute can be obtained,

in spite of the very small power, and a rapid deercase of thedose in proportion to depth, owing to the fact that the focuscan be brought within 1mm distance of the area to bc irradiat-ed. The dose distribution across the irradiated area is veryuniform; due to the method of construction, the rays that passthe window obliquely are weakened' more than axial rays,but this effect is .more than compensated for by the naturaldirectional effect of the continuous X-radiation, which is aconsequence of its mechanism of origin, and is effective in theKT tube owing to the extreme thinness of the gold layer.

The X-ray tube is connected to the high-tension source bymeans of a flexible cable, 1.5 m long and 8 mm thick, with"Podur" insulation and earthed metal braiding. The high-tension generator is, in broad outline, identical with the well-known small 25 kV generator for projection television. A lowshort-circuit current and a relatively small output capacitanceare essential to avoid overloading of the very small tube inthe event of a fault (breakdown).

In contrast to the television generator, the generator of thcKTapparatusis continuously adjustable forvoltages up to 25kVand currents up to 200 (lA.The KT tube may be applied thera-peutically for the removal of haemangiomae, the -irradiationof the cornea after transplantation and the treatment ofsuperficial tumors in the mouth and throat cavities. In addi-tion, the tube can render good service in the testing of verylight metals or very thin layers and for X-ray photographsof all sorts of biologicalobjects; a series of radiographs ofhoney bees is, i.a., given as example.