278 PHILlPS TECHNICAL REVIEW VOLUME 30

Infra-red thermography equipment for medical applications

M. Jatteau

The work on "passive" infra-red television, originallydirected towards military applications, has led to thedevelopment of cameras with opto-mechanical scan-ning. Once instruments of this type were available, thepossibilities of -e-e- and problems with - this imagingtechnique for medical and industrial use could beexamined. For example, a LEP camera with spiralscanning, operating in the spectral region of 4 fLm[1],

gave interesting images ofthe thermal radiation patternsof the human body [2] and allowed us to study meth-ods by which the information gathered could be usedto the best advantage.

The interest shown by the medical profession in thefirst thermography experiments encouraged us to con-sider the design of an equipment more suited to thistype of application, and as a start we wished to investi-gate the experimental conditions which might be en-countered in practice. For this purpose a thermographyequipment was built at LEP to a design which had anumber of features that enabled doctors to undertakea wide range of investigations. The instrument has ahigh thermal resolution, it can be used for plotting iso-therms (to an accuracy better than 0.05 "C) and it canalso operate in different spectral ranges (around 4 fLmand 10 fLm).

We shall describe this equipment and report on theexperiments performed with it by the Radiology Serviceof the Hospices Civils de Strasbourg, which is speciallyequipped for thermographic examinations.

Discussion of basic design requirements

Whatever the method used, the infra-red televisionimage is formed by transposing the wavelength of theradiation coming from the different elementary sur-faces of a scene into the visible spectrum. Despite acertain analogy with ordinary television the situationis different, for the eye has no way at all of checkingthe image against the natural scene. The infra-redimage can be used mainly in two different ways. In thefust, the image should allow a detail in the scene to beidentified or localized or just detected; to achieve this,the energy of the radiation originating from this detail

M. Jatteau, Ingénieur C.N.A.M., is with Laboratoires d'Electro-nique et dePhysique Appliquée, Limeil-Brévannes (Val-de-Marne},France.

and received by the infra-red television unit need onlybe different from the energy radiated by its surround-ings. The detection may then possibly be active [3].

In the other - and this is the case in thermography -there is an additional requirement: the luminancevariations in the image must represent the differencesin surface temperatures. This latter condition deter-mines the direction in which the equipment designshould proceed; in this case we must distinguish in thetotal radiation received from each element of the scenethat part which is not due to reflection but which theelement emits on its own: only this part is directlydependent on its temperature [4]. The detection musttherefore be passive.

E:

t

Fig. 1. Variation of the skin emissivity factor e as a function ofthe wavelength (post-mortem sample) [51.

The relationship between the energy of the emittedradiation and the surface temperature, i.e. the emissiv-ity B). of the surfaces studied must also be taken intoaccount and this can lead to the determination of apreferred spectral operating range. In the case ofmedical applications, the surface studied is the skin andaccording to certain authors [5][6] the healthy skinbehaves like a grey body (B). = a constant close tounity) beyond 6 fLm(fig. 1)., As far as we know, the "instant-view infra-red tele-vision" equipment in medical use up to the presentoperates in the spectral range around 4 fLm(indiumantimonide radiation detector). We wanted our equip-

1969, No. 8/9/10 MEDICAL INFRA-RED THERMOGRAPHY 279

ment to be able to operate at different wavelengths toallow comparison of the infra-red images obtained ineach case.The design requirements of the equipment were also

governed by the technical objectives we had in mind:on the one hand, the equipment must permit us todetermine the possibilities of thermography for differ-ent clinical applications and, on the other hand, itmust allow us to define the characteristics required formedical observations.In order to detect temperature differences of a few

hundredths of a degree Celsius near ambient tem-perature, the measurements of the radiated power mustbe carried out with a relative precision of better thanone thousandth. In practice this corresponds to thedetection of very low energy differences (a few nano-watts).The consequences of these conditions are as follows:

a) The detection must be carried out in the far infra-red. This conclusion is reached by considering therequired resolution in space and time, the optical per-formance that is physically attainable, and the charac-teristics of presently available radiation detectors inthis spectral range.b) It is preferable to use point detectors. This avoidsthe disadvantages resulting from local non-uniformityof photosensitive targets or linear arrays of infra-reddetectors.c) The resolutions in space and time are necessarilylimited since, for a given scanning system and a givendetector, the value ofthe minimum detectable tempera-ture difference is closely related to the product of theframe rate and the number of elements resolved in animage.d) When a point detector is used, scanning ofthe sceneis required and this should be achieved by opto-mechanical means.e) It is preferable to place the mechanical deflectionsystems in front of any optical focusing system. Thisavoids aperture modulation ofthe infra-red beam inten-sity, which can be detrimental to the required tempera-ture discrimination if the modulation depth is large. .The direct display of the infra-red images _isa very

useful possibility which we wanted to exploit. Displayis obtained on the screen of a cathode-ray tube. Forthis purpose the frame rate must be at least equal to1Hz (because ofthe limited time constants ofthe long-persistenee phosphors); quantum detectors rather thanbolometers, for example, must be used to provide thefast response necessary for this frame rate and also toprovide the spatial resolution required for medicalapplications, which is at least equal to 104 elementsper frame.The disproportion between the number ofluminance

steps which can be detected by an infra-red camera andthe number ofluminance steps observable on a cathode-ray tube screen [7] suggests that the image should berestored for successive temperature ranges.The amplitude non-linearity in the conversion of the

infra-red radiation to visible light and the selectivity ofthe quantum detectors imply that the temperaturemeasurements should be performed by comparisonwith calibrated radiation sources [8]. A very interestingfeature of the cathode-ray tube display is that suchcomparisons can be made directlyon the screen.These last two points led us to develop a device for

plotting isothermal zones, like the contour lines of ageographical map [9].

Description of the thermography ~quipment

We shall now describe in some detail the equipmentmade at LEP, stressing the points touched upon in thepreceding section.

The camera

1) Suitable quantum detectors.The principal quantum detectors which are available

in the far infra-red are summarized in Table J. Thefourth column givesthe specific detectivity D*(500 OK),which expresses the r.m.s. signal-to-noise voltage ratioobtained per watt of radiant power per unit area ofdetector and for a bandwidth of 1 Hz, when measure-ments are made from a black body at 500 "K. The lastcolumn permits a comparison of the detectors with re-spect to their own contribution to the thermal resolutionof a given camera in the region of T = 300 "K. Thesignificanee of the quantity listed here can be explained

[1) J. Cornillault, Télévision infrarouge dans le domaine spectralsitué au voisinage de 4 microns, Acta Electronica 7, 319-360,1963.

[2) J. Cayzac, Problèmes posés par la détection des faibles diffé-rences de température en télévision infrarouge, Acta Elec-tronica 9, 7-18, 1965.

[3) Active detection is defined by the fact that the objects can bediscerned, as in natural vision, by the radiation which theyreflect or transmit and which originates from one or severalsources illuminating the scene.

[4) G. Bruhat, Cours de physique générale. Thermodynamique,5th edition, revised by A. Kastler, publ. Masson, Paris 1962,pp. 591-677.

[5) R. Elam, D. W. Goodwin and K. Lloyd Williams, Opticalproperties of the human epidermis, Nature 198, 1001-1002,1963.

[6) E. Hendier, J. D. Hardy and D. Murgatroyd, Skin heatingand temperature sensation produced by infra-red and micro-wave irradiation, in: Temperature, its measurement andcontrol in science and industry, Vol. 3, Part 3, editor J. D.Hardy, Reinheld Publ. Co., New York 1963, pp. 211-230.

[7) F. Kerkhof and W. Werner, Television, Philips TechnicalLibrary, Eindhoven 1952.

[8) G.-A. Boutry and M. Jatteau, Sur la mesure des températuresde surface par télévision dans l'infrarouge, C.R. Acad. Sci.Paris 266 B, 214-217, 1968 (No. 4).

[9) M. Jatteau, Dispositif pour tracer les isothermes dans unappareil d'exploration optique au moyen de rayons infra-rouges, French Patent No. 122249, 1967. .

280 PHILIPS TECHNICAL REVIEW VOLUME 30

as follows. The minimum temperature difference thatcan be detected between two black bodies near a tem-perature Tby a camera using opto-mechanical scanningand a point detector is proportional to the product:

{J

The first factor 0: characterizes the camera: A is thesensitive area of the detector, Q the aperture angle of

(1)

the camera with detector D, rotating prism Pandrocking. mirror F. The relatively slow discontinuousmotion of F is achieved by means of a cam C and onlyinvolves equipment which is relatively light and ofsmall dimensions.This system led to the use of a relatively large line-

deflection prism, but this enabled us to place the drivemotor inside the prism. The engineering ofthis element,achieved by mechanical milling of optical quality [10],

is all the more remarkable in view of its size. The incli-nation of the "line" scanning prism indicated in fig. 2is necessary to ensure that the angular vertical fieldscanned is divided into two equal parts by the horizontal

Table J. Data for the principal available detectors. The materials are used as photoconductorswhen not stated otherwise. The thermistor is listed solely for the sake of comparison sinceits time constant (~ 1 ms) is unsuitable for the applications envisaged. Maximum values ofD* for a field of view greater than 120° are given; D* may be increased for certain detectorsby limiting the field of view by means of a cooled diaphragm (see P. W. Kruse, L. D.McGlauchlin and R.B. McQuistan, Elements ofinfra-red technology; generation, transmissionand detection, WHey, New York 1963).

Limits of "effective"Operating Specific detec- spectral band Factor fJ in (I)

Detector temperature tivity D*(500 OK) for T = 300 oKin "K in m W-l HZI/2 Al A2 in m oK HZ-l/2

in !Lm in !Lm

InSb (photo-voltaic cell) 77 13x 107 3.85 5.45 1.48 x 10-8

InSbGe(Au) { 77 3 X 107 5 8.6 2.98 x 10-8

65 7x 107 5 8.6 1.27x 10-8Cd(Hg)Te 77 3 x 107 8.24 13 1.57 x 10-8Ge(Hg) ;;;;30 s x 107 8.1 14 0.6 x 10-8Ge(Cu) 4.2 9x 107 7.8 23 0.4 x 10-8

Thermistor I ambient I 1.5xl07 0 I 00 I 3.4 X 10-8

the diaphragm-detector system, IJ! the bandwidth ofthe amplifier and 1'1 the transmission factor of theinternal optical path of the camera. The second factor {J,whose value for T = 300 "K is given in the Table,characterizes the spectral sensitivity range. Here 1'2 isthe transmission factor in the space between black bodyand camera, Ds* the "effective" specific detectivity(defined from D*(500 OK) taking account of the effec-tive spectral response of the detector for a 300 "K blackbody) and dL/d). the monochromatic luminance of theblack body.2) Opto-mechanical scanning.

The scanning of the scene in two orthogonal direc-tions is achieved, on one hand, by means of a rock-ing mirror (period 0.5 s) and, on the other hand, bymeans of an octogonal prism with plane reflectingsurfaces rotating at constant speed (1500 rev/min). Therapid line scanning (the image being swept past thestationary point detector) is thus obtained from a uni-form rotary motion. Fig. 2 is a schematic diagram of

plane. With this arrangement there is no occultation ofthe field of view by any:fixedor mobile mechanical partand, therefore, no spurious modulation ofthe incomingradiation. The scanning distortion connected with thisinclination is acceptable. '3) Optical focusing system.The choice of the optical focusing system is partly

determined by the following considerations.The lo,wtemperatures required for the operation of

the infra-red detectors for medical cameras are generallyobtained by means of liquefied gases. Indeed, for theseapplications present cooling systems, however simple,are still too cumbersome (Joule-Thomson expansion [11]

requires high-pressure compressed gas of high purity,and Stirling-cycle cryogenerators [12] are not entirelyfree of noise and vibration, while both systems requirea certain amount of maintenance). Thus, the detectorhas to be placed in a cryostat. The dimensions ofdetector cryostats of sufficient capacity to ensureseveral hours of independent operation are generally

1969, No. 8{9{IO MEDICAL INFRA-RED THERMOGRAPHY 281

incompatible with the use of centred mirror systemswith wide apertures, unless high light losses are ac-cepted. Preference has therefore been given to an "offaxis" telescope comprising a parabolic mirror (0 infig. 2) which, by sliding along its optical axis, permitsfocusing at different distances; the cryostat Cr of thedetector, a Mullard Ge(Hg) cell, is in a fixed verticalposition.

respectively on the outside and inside of a ring R whichis mechanically linked to the prism and provided withslits.The rocking motion of the frame scanning mirror

is followed by a high-precision potentiometer Pot witha plastic track.A complete view of the camera arrangement is given

in fig. 3.

Fig. 2. Schematic diagram showing the principle of the experimental LEP camera for thermo-graphy, The line scanning prism P (eight plane reflecting faces), made to rotate by motor MI,and the frame scanning mirror F, given a rocking motion by cam C and motor M2,sweep an image of the scene Se past the detector D, which is housed in a cryostat Cr. Theinfra-red image is focused on to D by the parabolic mirror O. The infra-red image conveyedby the detector signal d is displayed on the screen of a cathode-ray tube, whose time-base issynchronized by signals I and f from the optical reading head H with rotating slit ring Rand from the potentiometer Pot.

Since the opto-mechanical scanning is performedbefore the infra-red beam is focused, the system alwaysoperates on the optical axis of the telescope whoseparameters are determined in such a way that thedimensions of the circle of confusion remain belowthose of the sensitive area of the detector even at theminimum focusing distance (0.8 m).4) Synchronizing the scanning signals.

The synchronization of the line scanning signalsapplied to the image-display tube is provided by pulsessupplied from an optical reader head H consisting of acollimated light source and a photoelectric cell placed

Image-display device

The image-display device mainly consists of anoscilloscope of classical type (fig. 4) equipped with acathode-ray tube with a large fiat screen (diameter180 mm). A mobile trackhas been designed to carry the

[10] A. Charles-Georges and A. Salmon, Usinage de cavités depompage pour lasers "solides ", Acta Electronica 10, 315-330,1966.

[11] H. T. Hayward, Joule-Thomson liquefier, Cryogenics Symp.,Frankfort 1966, pp. 9-12.

[12] A. Daniels and F. K. du Pré, Closedcyclecryogenicrefriger-ators as integrated cold sources for infra-red detectors,AppI. Optics 5, 1457-1460, 1966.

282 PHILIPS TECHNlCAL REVJEW VOLUME 30

scanning signal generator and amplification circuits,the video amplifier, the d.c. power supplies and amonitoring oscilloscope. The isothermal-plotting de-vice described in the following paragraph is locatedalmost completely inside the camera.

J

T + LIT) undergo a high, perfectly linear amplification(ofthe order of 107 times). The average level ST + 1- LIScan be freely chosen between the extreme levels. Topreserve the accuracy of the isothermal zones which, attheir limit, occupy the entire field being scanned, very

Fig. 3. Camera equipped with a Ge(Hg) detector cooled to the temperature of liquid heliumin a double-walled cryostat.

Isothermal-zone plotting device

The principle of the isothermal-plotting device de-veloped at LEP is as follows [9]. The signal suppliedby the detector is progressively amplified, then ampli-tude-limited throughout the electronic chain (fig. 5) sothat only the amplitude variations ranging betweenlevels ST and Sa: + LIS (corresponding to Tand

low frequency signals must be amplified without dis-tortion. The upper limit has been chosen equal to themaximum frequency used in television broadcasting.For the device connected to the camera the values ob-tained are fmin = 0.18 Hz (which led us to use an elec-tronie chain with direct coupling) andfmax = 38 kHz.

The amplitude LIS is finally selected at the output S

1969, No. 8/9/10 MEDICAL INFRA-RED THERMOGRAPHY 283

Fig. 4. Overall view of the experirnental LEP t herrnography equipment. The camera ismounted on a mobile tripod with cradle head for easy manoeuvering without sacrificingstability. The image-display device, mounted on a mobile trolley, is shown on the left.

IR~~

Fig. 5. Block diagram of the isothermal-zone plotting device.

E Ae

of the electronic chain, whose gain can be adjusted bymeans of an attenuator Att. Each amplifier Az ... Anis protected from saturation effects by a limiterLI ... Ln-l and the gain of the preamplifier Al isdetermined such that it will not be saturated by themaximum signal corresponding to the extreme tem-peratures chosen (0-60 "C), In order to obtai n animage which represents only the isothermal zones,every signal whose level reaches at least the upperthreshold of selection is fed back to the black level byan inversion signal originating from the selectioncircuit S-E and amplifier Ae. The output amplifier AF

applies the resulting signal to the grid of the cathode-ray tube.

The reference level maintained throughout the ampli-fier chain corresponds to the optimum bias of thedetector. (This is obtained either by means of a corn-pensating circuit when photoconductors are used ordirectly in the case of photovoltaic detectors.) Thedifferent levels are scanned by continuously "shifting"the signals supplied by the detector D along the entirescale. This is done by means of circuit Sh in fig. 5which varies the resistance of the bias circuit or thebias voltage.

284 PHILIPS TECHNICAL REVIEW VOLUME 30

Technical characteristics

The main technical characteristics of the LEP equip-ment are listed in Table IJ. The isothermal-plottingdevice offers the facilities described below and illus-trated by some results obtained in the spectral rangearound 10 !Lm.a) Accurate examination ofthe content ofthe infra-redimages: . The isothermal zones can be plotted atany temperature between 0 °C and 60°C, regardless

mally contrasted image (fig.Bb).e) Rapid discrimination of regions higher in tempera-ture than the isotherm displayed. This possibility isuseful when, during the examination, the isothermdelimits several dark zones each of which may bewarmer or colder (fig. 9).f) Possibility of differential and absolute temperaturemea~urements by comparison with reference sources.With a Ge(Hg) detector the maximum precision of the

Table n. Technical characteristics of the LEP thermography equipment.

Repetition rateDefinition

Angular field scannedAngular resolutionFocusing distance

2 frames per second100 lines per frame135 picture elements per lineabout 420 x 560 miIIiradians (24 x 32 degrees)4 miIIiradiansfrom 0.8 m to infinity

IOperation at 10 {lm

(detector Ge(Hg) at 4 OK)

< 0.05 °C

o to 60°C

Operation at 4 {lm(detector InSb at 77 OK)

from 0.05 °C to 60°C

Thermal resolution around3000KWidth of isothermal zonesplottedTemperature range of iso-thermal-zone plotting

from O. I °C to 60°C

o to 60°C

of the extent of these zones (although of course limitedto the element resolved or the scanned field) to amaximum precision of 0.1 °C using a 4 !Lmdetectorand better than 0.05 °C using a 10 !Lmdetector. Thisallows a detailed examination ofthe images to be made:differences in radiated energy equal to t~e minimumdetectable difference are clearly brought up; precisegeometrical location of the thermal details is obtained(fig. 6a, b, c) ..b) Thermal mapping by superimposing isotherms ob-tained at different levels on ope photographic film oron the screen of a storage tube (fig. 7).c) Continuous variation from 0:05 °C to 60°C of thetemperature range corresponding to the range ofluminances presented by the picture. This variationallows the accuracy to which the isotherms are plottedto be freely chosen (fig. 6a, c). Moreover, the contrastof the .displayed images can be improved by adjustingthe width of the amplitude range selected as a f~nctionofthe extreme temperatures ofthe scene, having regardto the properties of the phosphor of the cathode-raytube screen (or the photographic film). This is demon-strated by fig. Ba, b.d) Superimposition of the isothermal zones on a nor-

comparisons made directlyon the image correspondsto a temperature difference between two "black"sources of less than 0.05 oe.Conclusions derived from medical experiments using theequipment

Since the end of 1967 the LEP equipment has beenused experimentally in France at the Central RadiologyService (Director: Professor Gros) of the HospicesCivils de Strasbourg where a team of doctors andphysicists are studying the fundamental problems ofmedical thermography. Several thousand examinationshave been performed by this team with a Barnes ther-mograph in various clinical services [131 and varioustypes of equipment have been tried out. Everyexamina-tion is performed in a thermography room kept at aconstant temperature (21 ± 1 0c) [141, approximately15 minutes after the patient has been prepared.An examination made with the LEP equipment con-

sists in photographing an image with the best possiblecontrast and then plotting a series of four to six iso-therms. These plots are made for the temperature levelsconsidered to be of interest during a previous analysismade by direct observation of the screen of the display

1969, No. 8/9/10 MEDICAL INFRA-RED THERMOGRAPHY 285

Fig. 6. Image of isothermal regions of the breasts. Isotherms areplotted with different temperature resolutions ilTand for differ-ent temperatures. a) iJ T < 0.05 "C at a certain temperature Tl.b) iJT < 0.05 "C at a temperature T2 < Tl. c) ilT "'" 0.2 "C atT3"", Tl.

[13) Colloque international de thermographie médicale, Stras-bourg J 966, J. Radiologie 48, 1-98, J 967.

[14) Ch. Gras, M. Gautherie and P. Bourjat, Conception d'unesalie de thermographie médicale, J. Radiologie 49,371-372,J 968 (No. 5).

Fig. 7. Thermal map of a leaf of Ficus elastica. Width of theisotherms less than 0.05 -c, temperature difference between iso-therms vs 0.1 °C, total temperature range ss J.2 oe. The temper-ature increases from the edge of the leaf to the circular regionat the left.

Fig. 8. Infra-red images of a face. a) Demonstration of luminancecontrasts in the hair. b) Demonstration of contrasts in thetemperature of the skin. The contour of the face and oneisotherm are superimposed on this image.

It would not be possible to observe the contrasts reproducedin (a) and (b) in a single picture.

.------------------------------- -

286 PHILlPS TECHNICAL REVIEW VOLUME 30

Fig. 9. I nfra-red images of a face as recorded by the isothermalplotter. a) Isotherms for T = 34 oe. b) Temperature regionsabove 34°C.

The outline of the face was recorded by the same equipmentbut at a different adjustment.

tube (thermoscopy). Figs. 10 and 11, which havekindly been provided by the Strasbourg RadiologyService, show the results of examinations made withthe LEP equipment operating in the 10 fLm region.

Usefulness and problems of medical thermography

Thermography is considered by medical ex-perts [13][15] to be an investigation technique cornple-mentary to the more conventional scientific methods(radiology, scintigraphy, histology, etc.). Tt is attractivenot only because of the originality of the type of infor-mation provided but also because it is harmless both tothe patient and the practitioner. The main special

problems to which it seems thermography can contrib-ute effectively are, in particular, mammography (thedetection and location of breast cancer), the study ofvascular diseases, rheumatology, placentography, laryn-gology, the study of the effects of drugs and vaccines,the study of the development of skin grafts, etc.

Nevertheless, even today it seems that the develop-ment of medical therrnography is linked up with thestudy of the fundamental problems posed by the exactinterpretation of the images from the physical andmedical points of view [16].

In particular, these problems relate to the determina-tion of differential and absolute temperatures from theinfra-red radiation, the knowledge of the thermo-stabi-lization mechanisms which result in a variation of skintemperatures with time, the understanding of the pro-cesses by which internal pathological phenomena can"project" themselves so as to be revealed by thermalanomalies, and the improvement of thermographicequipment for a better adaptation to medical applica-tions.

A number of detailed results

I) Thermography corroborates the radiological andclinical examinations in approximately 80 % of casesof breast cancer.2) The plotting of precise isotherms permits better loca-tion of pathological sites than on a normal image. Thisshould lead to a closer correlation between the thermo-graphic and the radiographic examinations.3) As a result of its high temperature resolution and thepossibilities offered by the isothermal-plotting device,the LEP equipment has enabled some 20% morepathological cases to be detected than when a thermo-graph using a bolometric detector was used. It appears,however, that the sensitivity ofthe equipment would besufficient even if the minimum detectable temperaturedifference is as large as 0.1 oe [17][18] and correspondsto the minimum width of the isotherms.4) After a short period of adaptation, the user is nottroubled by the low frame rate for direct observationofthe images. This direct observation permits the studyof dynamic thermal phenomena such as the natural andartificial cooling of the skin surface and the effect ofdrugs and vaccines.

[15) Therrnography and its clinical applications, Ann. New YorkAcad. Sci. 121, art. I, 1-304, 1964.

u ei R. Genève, Introduction à la therrnopraphie médicale, ActaElectronica 12, 7-19, 1969, (No. I).

l17] Ch. Gros, M. Gautherie, P. Bourjat and C. Vrousos, Ther-mographie des affections marnrnaires, in: Medical ther-mography, Proc. Boerhaave Course Postgrad. Med. Educ.,Leiden 1968, pp. 68-81.

118) Ch. Gros, M. Gautherie, P. Bourjat and C. Vrousos, Lesapplications médicales de la thermographie infrarouge, ActaElectronica 12,63-119, 1969, (No. I).

1969, No. 8/9/10 MEDICAL INFRA-RED THERMOGRAPHY 287

+-Fig. 10. Infra-red images of fe-male thorax showing an encystedabscess in the left breast.At the top an image with normalcontrast.1-5: five recordings of isothermalzones with temperatures increas-ing from J to 5.

The isothermal zones aresuperimposed on the trace repre-senting the outline of the body.

"""*Fig. 11. Infra-red images offemale thorax with epithelioma,left breast ulcerated.At the top an image with normalcontrast.1-4: four recordings of isother-mal zones with temperatures in-creasing from 1 to 4.5: image of entire region withtemperatures above that repre-sented by the isotherms in 4.

The isothermal zones aresuperimposed on the trace repre-senring the outline of the body.

288 PHILIPS TECHNICAL REVIEW VOLUME 30

In some cases, the thermal contrasts can be improvedby artificial cooling of the body region studied; this isdue to the fact that regions of healthy skin have adifferent cooling rate from pathological areas [18].

5) With regard to the spectral range, the comparisonof images of normal contrast obtained simultaneouslyfrom one subject with LEP equipment operating in the10 !Lmrange (Ge(Hg) detector) and with the Barnesthermograph tends to confirm the remarks concern-ing the spectral emissivity of the skin (see p. 278).The infra-red radiant power emitted by healthy and.pathological areas as compared with that emitted bythe black body, when viewed at different angles of inci-dence, wavelengths, and temperatures, has still to beinvestigated.6) In view of the variations in skin temperature as afunction of time [18][19] the duration of observation ofthe body region concerned must be sufficiently short toensure that errors due to such variations are smallerthan the minimum temperature difference which can bedetected by the equipment. This duration depends par-ticularly on the body region concerned, the efficiencyof the artificial cooling or, if the cooling is natural, theadaptation time of the patient, and the ambient tem-perature (fig, 12). The recording of a certain numberof preselected isotherms must be done in a time whichis, at most, equal to this period of observation. Thislimits the duration of image formation. For example,for a patient-adaptation time reduced to five minutes

25

200~----3~O----~60b-----~90~--~~~O~m~m_t

Fig. 12. Experimental curves (solid lines) and theoretical curves(dashed lines) of the variation of the skin temperature ofvarious parts of the body as a function of the time t.These curvesdepend on the ambient temperature Ta. (Taken from Ch. Gros,M. Gautherie, P. Bourjat and C. Vrousos [18].) 1 forearm,T.= 23. 5°C. 2 abdomen, Ta = 22.1 °c. 3 hand, Ta =21.2 °c.4 foot, Ta = 21.8 °c. The precise initial temperatures are indi-cated for each case.

and an examination which consists of recording tenisotherms (precision 0.1 "C) the duration of imageformation should not exceed 4 seconds.7) It is convenient to record isotherms of different levelsin different colours on the same photograph for aneasier interpretation of the results.8) The dimensions of the element resolved on a breastmay be a few millimetres at a working distance of 1.5to 2 metres; this value is the result of a compromisebetween the ideal conditions of observation and thetechnical requirements.

Improvements in thermography equipment

The present conclusions of the medical profession,and the technical suggestions resulting from the devel-opment and practical use of the experimental equip-ment described, have enabled us to define the principalcharacteristics of a more suitable thermography equip-ment for medical applications.Returning now to the design requirements formu-

lated on the basis ofthe different objectives establishedby ourselves, particularly with respect to the method ofimage display on a 'cathode-ray tube screen, it ispossible to define the technical characteristics of a newequipment in which the angular resolution would beincreased with respect to the previous version (1.5milliradians) and the scanned field would be adaptedto the medical "scenes" (about 210x280 milliradians,i.e. 12X 16 degrees), as would the thermal resolution(0.1 "C). The 10 !Lmspectral range would be preferred(using Ge(Hg) point detectors or, in the near future,CdHgTe detectors cooled to the temperature of liquidnitrogen). The possibilities offered by the isothermal-plotting device would be retained and even increasedby semi-automatic recording, recording in colour, andthe superimposition of a normally contrasted picture.A different arrangement of the opto-mechanical

scanning systems (pyramidal "line" analysing prism)would permit the development of a more compactcamera than the one used in the experimental equip-ment.It is possible that the introduetion of picture tubes

employing an electro-optical effect and eliminating allflicker (such as TITUS, developed at LEP [20]) willsoon lead to the development of a new class of thermo-graphy equipment. Up to now the duration of a framehas been limited by the phosphor time constant, but

[lOl A. C. Burton, The pattern of response to cold in animals andthe evolution of homeothermy, in: Temperature, its measure-ment and control in science and industry, Vol. 3, Part 3,editor J. D. Hardy, Reinhold Publ. Co., New York 1963,pp. 363-371.

[20] G. Marie, Un nouveau dispositif de restitution d'imagesutilisant un effet électro-optique: le tube TITUS, PhilipsRes. Repts. 22, 110-132, 1967. See also this issue, page 292.

1969, No. 8/9/10 MEDICAL INFRA-RED THERMOGRA,PHY

these new tubes will remove this restrietion - the onlyrestrietion that will remain being due to the phenome-non studied. Among other things, this will permithigh-definition images to be obtained without reducingthe temperature resolving power.

Extending the applications of thermography

Thermography mayalso have numerous applicationsin the scientific and industrial fields. Many phenomenaare, in fact, closely related to thermal dynamics, andthe knowledge of surface temperatures can, forexample, permit the location of internal heat sites orthe conditions of heat exchange with the environment.The qualities required for the equipment will vary

according to each field of application. The followingare the most important characteristics: a high geo-metrical resolving power for the inspection of minia-turized or integrated electronic circuits, rapid analysisfor dynamic thermal phenomena on models for super-sonic wind-tunnels, a high sensitivity for the fields ofplant and animal biology, a high observation distancefor meteorology and aerial thermal mapping (whereproblems will be caused by the transmission of theatmosphere). Insofar as the range of temperatures isclose to the ambient, the fundamental problems of

thermography will, however, be similar to thosementioned above with regard to medical applications.

For this reason the equipment described should, inour opinion, be suitable in many cases for studying anddetermining the experimental conditions particular toone application or another and can thus contribute todefining the required technical characteristics of moresophisticated equipment.

Summary. LEP have developed a thermography equipment whosethermal resolution is better than 0.05 "C at 300 "K and whichcan operate in different spectral ranges (particularly 4 and 10microns). It includes a special device for plotting isotherms toan accuracy better than 0.05 °C. The author first outlines thechieftechnical objectives which were used to determine themainrequirements for the equipment design. He then describes thecamera, which is equipped with a single-cell quantum detector(InSb for 4 (.Lm, Ge(Hg) for 10 (.Lm) and an opto-rnechanicalscanning system. The electronic device which allows the iso-thermal zones of the surfaces examined to be viewed directlyonthe screen of the display tube is briefly outlined. The possibilitieswhich this method of representation opens up are discussed andillustrated with a few examples. The last part of the article isdevoted to applications of thermography, particularly in themedical field. The results of numerous examinations carried outby the Hospices CiviIs de Strasbourg with a LEP thermographyequipment allow a nlumber of conclusions to be drawn. On thebasis of this experience and the technical improvements that canstill be made in the present camera, it is possible to draw up aspecification for an improved version of the equipment.

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