FROM THE DEPARTMENT OF DIAGNOSTIC RADIOLOGY III (DIRECTOR: ASSOC. PROF. C.-G. HELANDER), SAHLGRENSKA SJUKHUSET, S-411 32 GOTHENBURG, AND DEPARTMENT OF PHYSICS, DIVISION IV A (DIRECTOR: PROF. G. BROGREN), CHALMERS UNIVERSITY OF TECHNOLOGY, S-40220 GOTHENBURG, SWEDEN.

INTENSIFYING SCREENS IN SOFT TISSUE RADIOGRAPHY E. DEICHGRABER, S. REICHMANN and K.-G. STRID Low voltage radiation is nowadays more and more being employed in soft tissue radiography. The most important application is mammary radiography but the same technique is becoming increasingly common in examinations of the joints of the extremities. FISCHER (1973 a, b) and FISCHER & BRAUN (1973) used a tube with a molybdenum target and non-screen film in examinations of joints of fingers and toes, wrist, elbow and the tendon of Achilles. A tube with a tungsten-rhenium target and low inherent filtration (0.5 mm AI) was used by REICHMANN et colI. (1974) and DEICHGRABER & OLSSON (to be published) in examinations of finger joints (26 kV) and shoulder joints (40 kV) respectively. The radiation emitted at low tube potential involves comparatively large radiation doses to the patient. A recording medium of as high sensitivity as possible is thus desirable. The influence of different non-screen films on the image quality in mammary radiography was analysed by DEICHGRABER et colI. (1974). In relation to the radiation dose required a superior image was obtained on industrial film of medium sensitivity. Films of higher sensitivity gave rise to a disturbing quantum mottle, which definitely outweighed the gain in exposure reduction. A recording medium with high capacity Submitted for publication 5 March 1974.

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Table :1 Relative mAs values giving of three black-and-white films optimal density kV

Film

Density

ReI. exposure

26

Mamoray T 3 Curix RP I Red Seal

2.0 1.3 1.3

1.0 0.6 0.4

40

Mamoray T 3 Curix RP I Red Seal

2.0 1.3 1.3

1.0 0.6 0.3

to absorb photons would increase the sensitivity without also increasing the quantum mottle. This would be achieved by using intensifying screens, provided that they would not cause undue loss in image quality. Phantomtests Two phantoms were made of square plastic bottles and contained vegetable oil with layer thickness of 3 and 7 em respectively as measured in the beam direction. Within the oil there were five nylon wires varying from 0.25 to 1.0 mm in diameter. The bottles also contained small pieces of chalk embedded in an epoxy plastic. A metal disc was placed on the upper side of each bottle, the disc being practically impenetrable to the radiation used. The wires and the pieces of chalk were projected free of the discs. Any blackening of the film occurring within the area of the disc image would reflect the degree of secondary radiation and other fog factors. The thinner of the phantoms was used in combination with a tube potential of 26 kY, the thicker one with 40 kY. F our pairs of intensifying screens were tested. It was considered of importance that the unsharpness resulting from light scattering within the phosphorus layer should be kept at a minimum, thus two high-definition screens were chosen, viz. Kodak Xomatic Fine and Siemens Rubin Super. Kodak X-omatic screens have a rather regular crystalline texture, as compared with several other types (REICHMANN & HELANDER 1974 a). For this reason also Kodak X-omatic Regular was included in the tests although this screen presumably would give a higher degree of unsharpness due to its thicker phosphorus coating. The fourth screen selected was Ilford Fluorazure since it had been found to yield a much higher light output (higher radiation-to-light conversion factor) when exposed to soft radiation, than calcium tungstate screens (LEVY & WEST 1933).The screens were placed in a vacuum cassette and tested together with four films: a conventional roentgen film intended for 90 s processing time, Agf'a-Gevaert Curix RP 1; another black-and-white film for ordinary clinical use, intended for a somewhat longer processing time, Ilford Red Seal; a film that previously had been shown to have a low grain mottle (REICHMANN & HELANDER 1974 b), Agfa-Gevaert

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E. DEICHGRABER, S. REICHMANN AND K.-G. STRID

Table 2 The figures indicate the exposure reduction obtained with intensifying screens compared to exposure without screens on the same film (in per cent)

kV

Film

Density

Screen

X-omatic X-omatic Rubin Regular Fine Super

26

Mamoray Mamoray Mamoray Mamoray Mamoray Curix Curix Curix Curix Curix Curix Red Seal Red Seal Red Seal Red Seal Red Seal

1.0 2.0 1.0 2.0 3.0 1.0 2.0 1.0 1.25 2.0 2.3 1.0 2.0 1.0 2.0 2.5

double double back back back double double back back back back double double back back back

50 48 25 26

Mamoray Mamoray Mamoray Mamoray Curix Curix Curix Red Seal Red Seal Red Seal Red Seal

2.0 3.5 1.0 2.0

double double back back double back back double double back back

19

63

100

31 30

77 67 6 15 14 10 8 15 16

83 71 6 14 11 16 9 15 14

40

1.5

1.0 1.5

1.0 2.0 1.0 2.0

111 100

200 167

71

77

63

63

12 9 13

18 13 9

16

11

15

23 20 22 20

Fluorazure

14

10 6 2.9 4.1 3.5 8 5 8 5

13 17

10

1.2

2.2

1.5

1.3

2.7 2.9 2.2 3.6 3.3

6 19 16 0.5 1.0 1.6 0.9 2.0 1.2

Medichrome; finally, an industrial film of medium sensitivity which, previously had been shown to yield the best results when used without screens (DEICHGRABER et coll. 1974), Agfa-Gevaert Mamoray T 3 (previous name: Mamoray 2). This last film is not intended for exposure by light but none the less proved useful in combination with intensifying screens. The influence of exposures on the density levels of three types of black-and-white films was determined by sensitometry, phantoms not being used. The optimum rendering of weak signals on ordinary black-and-white films is expected to be obtained at the density D = 1.3, therefore the exposure values giving that density at 26 kV and 40 kV were measured and compared with those giving the density of D = 2.0 for industrial film. The results are given in Table 1. Table 2 presents the ex-

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a

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b

Fig. 1. Nylon wire (diameter 1 mm) in 3 ern of vegetable oil. Tube potential 26 kV. a) Non-screen film, IIford Red Seal, 100 mAs. b) Same film with double Kodak X-omatic Fine screens, 13 mAs. c) Same film with double Rubin Super screens, 13 mAs. The signal-to-noise ratio is most favourable in (b), despite a lower exposure than in (a).

c

posure reduction obtained for the three types of film when used in combination with the various screens in relation to the same films exposed without screens. The term 'intensification factor' has been avoided. The reason is that the exposure reductions listed in Table 2 indicate that these vary considerably with the level of density. It is evident that the exposure reductions were moderate when industrial film was used and that the high-definition screens gave the lowest reduction; the Fluorazure screen gave a high light output. The screen-film combinations were then tested on the phantoms. The Fluorazure screen was omitted, however, since sensitometry had revealed it to produce an unduly high mottle level. The remaining screens (Kodak X-omatic Fine and Regular, Siemens Rubin Super) were combined with the black-and-white films (Mamoray T 3, Curix RP 1, Red Seal) and with the Medichrome film. The target of the tube was 0.6 mm x 0.6 mm and the FFD 70 em. The quality of the image recorded on industrial film without intensifying screens was definitely superior to all the roentgenograms taken with the other films, whether exposed with or without screens. The most evident difference concerned the mottle level, which was lowest in the non-screen industrial film. For the remaining conventional films an interesting observation was made. In films exposed with double Kodak X-omatic Fine screens the signal-to-noise ratio was more favourable than in

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E. DEICHGRABER, S. REICHMANN AND K.-G. STRID

a

b

Fig. 2. Microphotographs from a roentgenogram of a thin metal wire on industrial roentgen film exposed with back screen. The image of the tubeside emulsion of the roentgen film (a) is much sharper than that of the back-side emulsion (b), where the screen has contributed to the exposure. (Magnification x 250.)

films exposed without screens, although the total exposure dose was lower when the screens were used (Fig. 1). With double screens the level of secondary radiation was lowered, owing to the filtering effect of the front screen (LAURELL 1932, LINDBLOM 1934, OOSTERKAMP 1946). This reduction contributed to a better signal-to-noise ratio. Thus it cannot with certainty be maintained that the screen roentgenogram, in this case, was produced with the aid of a larger number of photons than the non-screen one. Whatever the mechanism, the screens improved the signal-to-noise ratio and lowered the patient dose. When industrial film was used, on the other hand, none of the screens improved the image quality. In a comparison between the three conventional roentgen films (Curix, Red Seal, Medichrome), no significant difference appeared in roentgenograms taken under equal conditions. Rubin Super proved to produce a comparatively high mottle level (Fig. 1 c). This was most evident when double screens were used. Likewise, X-omatic Regular caused a rise in mottle, although a less disturbing one, possibly because this screen produced a higher degree of unsharpness than the other screens. An image quality comparable to the reference recording medium (non-screen industrial film) was thus obtained only with Kodak-X-omatic Fine and industrial film in combination both at 26 kV and 40 kV. It appears from Table 2, however, that the exposure reduction made possible with this combination is a modest one. In soft tissue radiography it is suggested only to employ back screens (PRICE & BUTLER 1970). The image is then to a large extent produced by photons absorbed directly in the silver emulsion of the film. It might be expected that the image would appear more distinct than when double screens are employed. In an additional test series thin metal threads were exposed on industrial film at 26 kV and 40 kV potentials. The same three screens were used as back screens as well as in pairs. Films were also exposed without screens. The films were analysed under the microscope at low

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magnification. In the front emulsion layer an extremely well defined image appeared when a back screen alone had been employed, whereas the silver layer in contact with the back screen presented a much more blurred image of the same thread (Fig. 2). The loss of definition was considerable even with the finest screen. Thus the introduction of one intensifying (back) screen seems likely to cause such a high loss of definition that addition of a front screen only slightly increases the total unsharpness. At close inspection the images of the threads did not vary significantly in definition whether exposed with a back screen alone or with screen pairs. In summary: All conventional roentgen films tested gave rise to a lower image quality than did non-screen industrial film. Kodak X-omatic Fine screen improved the image quality when conventional films were employed. However, when combined with industrial film this screen caused a slight decrease in quality, mostly due to increased loss of detail. Rubin Super introduced a high mottle level. X-omatic Regular caused more loss of definition than X-omatic Fine. The mottle level was also probably raised. In combination with industrial film, however, X-omatic Regular appeared most suitable for further investigation. X-omatic Fine and Rubin Super were excluded since they reduced the exposure dose only modestly. Rubin Super, moreover, caused increased mottle. Therefore it was considered reasonable to supplement the experiments with clinical trials to determine if the reduction in quality introduced by the X-omatic Regular screens has any diagnostic significance. Clinical experiences

The soft tissues of the shoulder joint were chosen for the clinical tests since these have proved somewhat difficult to demonstrate in non-screen industrial film of medium sensitivity, due to rather long exposure times needed. Clinical experience with Mamoray T 3 has shown the optimum tube potential to be 40 kV (DEICHGRABER & OLSSON). The inherent filtering of the tube was equivalent to 0.5 mm AI. The FFD was 70 cm. Patients admitted for soft tissue examination of the shoulder were examined according to this technique in a.p. projection and served as a reference material. For each shoulder an additional film was exposed for comparison, with as near as possible identical centering. Cassettes with double Kodak X-omatic Regular screens were used. As this procedure was found to increase the image contrast due to a lowered level of secondary radiation, the tube potential was increased to between 40 and 50 kV. Eight patients were examined, Medichrome film being employed. The thin fat tissue layer located on the lateral aspect of the shoulder joint beneath the deltoid muscle (Fig. 3 a) was demonstrated in the reference films but was more or less indistinguishable in Medichrome films. The image quality of these films was thus so poor that they were considered useless. Next Mamoray T 3 films were used in 35 examinations with 50 to 60 kV being used. At 60 kV the contrast obtained in films exposed with screens was approximately identical to that obtained in the reference material. Well-developed fat tissue layers in the shoulder region were clearly visible

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E. DEICHGRABER, S. REICHMANN AND K.-G. STRID

a

b

Fig. 3. Shoulder joint without radiographic evidence of pathology. a) Nonscreen industrial film, the fat tissue layer appears distinct. b) Same film with intensifying screen. Only part of the layer visible to the point indicated by the arrow.

even on films exposed with screens. The unsharpness was more marked, however, which was most clearly evident in the bone structure. Neither did this blur allow extremely thin fat tissue layers to be discerned, thus simulating a true disappearance, as occurring in conditions with a regional inflammatory edema (Fig. 3 b). Consequently the use of screens produced an image quality, which might lead to false diagnoses. This deterioration was not considered to be outweighed by the reduction of the exposure. Discussion

The purpose was to determine whether intensifying screens provide for increased absorption of photons as compared with non-screen film, so that the exposure may be reduced without undue loss in image quality. The assumption that this should be possible proved to be true for films having a low silver content, i.e, films intended for conventional clinical diagnostic work. Such films combined with one type of screens produced a definite improvement of image quality compared to exposure without screens, at the same time allowing a significant dose reduction. Thus the basic idea proved to be correct. However, the same principle could not be successfully applied to that type of film which ought to be used in most cases of soft tissue radiography, viz. industrial film of medium sensitivity with a high silver content. This film used without screens gave an image of extraordinary quality but with screens the image quality deteriorated because of loss of definition. The low voltage radiation employed in soft tissue radiography is easily absorbed. Thus, at this energy level, even a thin phosphorus layer of an intensifying screen should be able to absorb a considerable number of photons. In principle, it should

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therefore be possible to make intensifying screens adapted for soft tissue radiography. Inhomogeneous crystalline texture of intensifying screens may introduce mottle into the image. The phosphorus layer, therefore, must meet very high demands for homogeneity. The lower the un sharpness caused by the screen, the higher these demands must be, because mottle may be rendered invisible by unsharpness (GooDENOUGH et coll. 1972). The unsharpness introduced by screens varies inversely with the thickness of the phosphorus layer. A thinner coating of the screens chosen for this investigation to reduce unsharpness to an acceptable degree decreased the light output so much that the reduction of the exposure became too small. The present results indicate that the only screen-film combination which could be considered acceptable yielded an exposure reduction of about 60 per cent of that used for nonscreen industrial film. Against this dose reduction must be weighed the special maintenance necessary for these screens. In this type of diagnostic procedure the screens must be cleaned with utmost care. Even the finest dust particles will be clearly reproduced in the films and may be confused with calcific deposits in the soft tissues. The inherent conflict between light output and image unsharpness would indicate that available intensifying screens should not be used for soft tissue radiography. This conclusion is probably true as regards the fluorescent substances of the screens used. At low tube potential the light output of calcium tungstate and barium lead sulfate rapidly decreases (MATTSSON 1955). The modification of zinc sulfide used in the Fluorazure screen, on the other hand, gives a relatively high output when exposed to soft radiation. Screens for soft tissue examinations should be based upon a fluorescent substance with a favourable light output for soft radiation. Technical improvements should thus aim at the production of extremely thin phosphorus layers with a high light output. The unsharpness of the image introduced by intensifying screens is to a considerable extent caused by light scattering within the phosphorus layer. The optical properties of this layer determine the degree of unsharpness. The factors most often discussed are the thickness of the layer, the grain size, and the presence or absence of a reflecting coating at the boundary between the phosphorus layer and the screen base (HARTMANN 1931, KLUG 1937, SCHOBER & KLETT 1954, PIWONKA et coll. 1965). Two additional factors deserve consideration. The X-omatic screens were examined under the microscope at low magnification in reflected and transmitted light and the Rubin Super screen in reflected light only. Other screens (CaWo Universal, CaWo Fin, Du Pont Par Speed, Siemens Saphir) were examined in the same way. In reflected light refraction or reflection phenomena or both were observed at the boundary between the binding substance and the crystals (Fig. 4 a). The conclusion was that they had different refractive indices. Thus, all the screens were optically inhomogeneous, which should enhance light scattering. This applied particularly to screens with more than one layer of crystals. Furthermore, the binding material invariably was opaque. Crystals at the surface of the layer were more easily observed in trans-

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E. DEICHGRABER, S. REICHMANN AND K.-G. STRID

a

Fig. 4. Microphotographs of intensifying screens in a) reflected light (Siemens Rubin Super) and b) transmitted light (CaWo Fin). Superficial crystals in (a) appear black with a halo of light that indicates differences in refractive index between crystals and binder. In the bright areas of (b) no superficial crystals were present. No sharp image of crystals of the deeper layers could be obtained, owing to the opacity of the binder. (Magnification x 115.) b

mitted light than those situated in deeper layers, even if they were not covered by overlying crystals (Fig. 4 b). This means that the opacity of the binder also should enhance light scattering. Further progress as regards intensifying screens suitable for soft tissue radiography should imply that fluorescent substances with a high light output for soft radiation be used. The zinc sulfide of the Fluorazure screen (LEVY & WEST 1933) might be considered as well as the rare-earth oxysulfides described by BUCHANAN et coll, (1972). The fluorescent substance should be dispersed into a thin, preferably monocrystalline, layer of densely packed, minute and regular crystals. The binder should be absolutely clear and possess the same refractive index as the crystals. Colour substances may be added to the binder as usual, provided the binder not being opacified. Another

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possibility bears on the principle applied in the production of intensifying screens of cesium iodide in image intensifiers (STEVELS et colI. 1973). Precipitation of a suitable fluorescent agent as needle crystals, all oriented in the beam direction, should be favourable for constructing a screen for soft tissue radiography, once problems of a technical nature have been solved. The choice of the film type is of great importance. The industrial film of medium sensitivity appears to be the most suitable one as a starting point for further investigation. However, it should have to be modified so that it can be used in combination with screens prepared as suggested. It is evident that further research for promotion of a refined technique of soft tissue radiography should include simultaneous attention to the qualities of films and intensifying screens.

Acknowledgement This investigation was supported by a grant from the Swedish Society of Medical Radiology.

SUMMARY Certain screen-film combinations were tested in the low voltage range (26 to 40 k V, tungsten target) and compared with an optimal non-screen industrial film. The dose reduction was of no consequence as compared with the loss in image quality. Some physical properties of the screens giving rise to the failure are discussed.

ZUSAMMENFASSUNG Gewisse Schirm-Film Kombinationen wurden im Niedervoltbereich (26-40 kY, Tungsten Target) gepriift und mit einem optimalen nicht-Schirm industriellen Film verglichen. Die Yerminderung in der Dosis war bedeutungslos verglichen mit dem Verlust in der Bildqualitat, Einige physikalische Eigenschaften der Schirme, die zu dieser Verschlechterung fiihren, werden diskutiert,

RESUME Les auteurs ont etudie certaines combinaisons d'ecrans et de films dans le domaine des bas voltages (26-40 kV, anode en tungstene) et Ies ont comparees avec un film industriel sans ecran optimal. La reduction de dose est sans interet quand on la compare avec la perte de qualite de l'image, Les auteurs examinent certaines proprietes physiques des ecrans qui sont la cause de cet echec,

REFERENCES BUCHANAN R. A., FINKELSTEIN S. 1. and WICKERSTEIN K. A.: X-ray exposure reduction using rare-earth oxysulfide intensifying screens. Radiology 5 (1972), 185. DEICHGRABER E. and OLSSON B.: Soft tissue radiography in painful shoulder. To be published in Acta radiol. Diagnosis.

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REICHMANN S. and BUREN M.: Film quality in mammary radiography. Acta radiol. Diagnosis 15 (1974), 93. FISCHER E.: Der Nachweis einer chronischen Insertionstendopathie am Epicondylus humeri durch Weichstrahlaufnahmen. Fortschr, Rontgenstr. 119 (1973), 358. - Die progressive Sklerodermie der Finger im Weichstrahlbild. Fortschr. Rontgenstr. 119 (1973), 372. - und BRAUN J.: Neue diagnostische Moglichkeiten an den Extremitaten durch Weichstrahlaufnahmen mit Mammographiegeraten, Electromedica 3 (1973),90. GOODENOUGH D. J., ROSSMANN K. and LUSTED L. B.: Radiographic application of signal detection theory. Radiology 105 (1972), 199. HARTMANN J. H.: Verstarkerfolien, ihre Beurteilung und Eigenschaften. Fortschr. Rontgenstr. 49 (1931), 758. KLUG H.: Vergleichende Untersuchung der gebrauchlichen Durchleuchtungsschirme und Verstarkerfolien. Fortschr. Rontgenstr. 55 (1937), 19l. LAURELL H.: Dunne Bleifolien als Sekundarblenden, Acta radiol. 13 (1932), 193. LEVY L. and WEST D.: A new and much more rapid intensifying screen allowing great alterations in radiographic technique. Brit. J. Radiol. 6 (1933), 85. LINDBLOM K.: Secondary screening by means of filtering. Acta radiol. 15 (1934), 620. MATTSSON 0.: Practical photographic problems in radiography. Acta radiol. (1955) Supp!. No. 120. OOSTERKAMP W. J.: Eliminating scattered radiation in medical X-ray photographs. Philips Techn. Rev. 8 (1946), 97. PIWONKA R" VOIGT G. und PETRI E. C.: Der Einfluss der wirklichen Korngrosse des Calciumwolframat-Leuchtstoffes auf die Detailwiedergabe von Rontgen-Verstarkerfolien. Rontgen-Bl. 18 (1965), 79. PRICE J. L. and BUTLER P. D.: The reduction of radiation and exposure time in mammography. Brit. J. Radiol. 43 (1970), 251. REICHMANN S. and HELANDER c.-G. (a): Homogeneity of intensifying screens. Acta radio!. Diagnosis 15 (1974), 449. - - (b) High-voltage radiography. Theory and clinical application. Acta radiol. Diagnosis 15 (1974), 56l. STEVELS A. L. N., DE PAUW A. D. M. and DAAMS J. L. C.: Luminescent screen with 'pile structure'. Medicamundi 18 (1973), 149. SCHOBER H. und KLETT c.: Untersuchungen tiber die Zeichenscharfe von Verstarkerfolien. Rontgen-Bl, 7 (1954), 214.

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Intensifying screens in soft tissue radiography.

Certain screen-film combinations were tested in the low voltage range (26 to 40 kV, tungsten target) and compared with an optimal non-screen industria...
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