FROM THE DEPARTMENT OF DIAGNOSTIC RADIOLOGY III (DIRECTOR: C.-G. HELANDER), SAHLGRENSKA SJUKHUSET, S-411 32 GOTHENBURG, SWEDEN.
FILTRATION IN SOFT TISSUE RADIOGRAPHY E. DEICHGRABER and S. REICHMANN In mammary radiography GROS (1967) used the K-emission from a molybdenum anode in combination with a molybdenum filter. This filter to some degree absorbes photons at energy levels above the K-emission lines, at 17 to 19 keY (BEARDEN 1968/69) more effectively than those just below the K-absorption edge. There has been some difference of opinion on the feasibility of the technique introduced by GROS (GAJEWSKI & MIKA 1968, MIKA & REISS 1968, JAEGER 1969, KYSER 1972, HACH 1972). It appears that the technique is applicable only for limited tissue thickness, the upper limit being five to six em. When this is exceeded, the filtering effect of the tissue counteracts the combined effect of the molybdenum anode and the molybdenum filter. The image is in fact made up of photons within the continuous spectrum above the K-emission lines. For this reason compression is strongly advised in radiography of the breast. Radiographic techniques similar to that of breast radiography have been successfully applied to periarticular tissues in different regions (FISCHER 1973 a, b, FISCHER & BRAUN 1973, REICHMANN et coll. 1974, DEICHGRABER & OLSSON, to be published). The limit of the tissue thickness pertaining to the technique of GROS should be valid to periarticular tissues as well. However, even in examination of such thin objects as finger joints this technique was found to be unsuitable, as too high an image contrast is obtained (REICHMANN et coll.). If a radiation quality were employed, corresponding Submitted for publication 28 May 1974.
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to a tube potential of 26 kV from a tungsten anode, filtering 0.5 mm AI, not only the soft tissues were well depicted but also the bone structure. In radiography of the shoulder joint the technique of GROS was completely unsuccessful due to the long exposure times. Therefore, it is to be recommended to use a tungsten target operating at low potentials for all types of radiography of periarticular tissues. The filtering in connection with a tungsten anode tube is incompletely known. KYSER (1972) presented the radiation spectrum of a combination of a tungsten anode tube with a molybdenum filter, which contained large amounts of energy below the K-absorption edge of molybdenum at 20 keY. However, he did not analyse their circumstances any further. Later an attempt was made to adapt the molybdenum anode to radiography of thick tissue volumes by combining the anode with other filtering materials, such as aluminium and silver (JOTTEN et coll. 1973). With the latter filters the continuous spectrum of the molybdenum anode was brought out and the K-emission lines were to some extent filtered away. GAJEWSKI & HEILMANN (1971) compared the image contrast obtained with a tungsten target to that obtained with one of molybdenum under identical operating conditions. A molybdenum filter (0.03 mm) was used in both cases, the tube window being made of beryllium. A glass filter (0.5 mm AI) was also combined with the tungsten target. Clinical radiography on non-screen film was performed but no spectrometry. Their results are partly at variance with the principles of GROS, insofar as the tungsten target was claimed to yield higher contrast under certain conditions. Thus the value of K-edge filtering in the examination of joints cannot be completely assessed from previous reports. An investigation of the K-edge filtering of two types of filter, molybdenum and tin, was therefore undertaken. A thin object was used, since it was considered that if the filtering effect had little or no value in such a case, it would be even more useless with thicker objects. Furthermore, Al-filtration in this type of radiography has also been considered. Materials and Methods The experiments were carried out by means of an ordinary diagnostic roentgen tube with a tungsten-rhenium target (Siemens Bi 125.30.50.MaR). The glass window of the tube had a filtering effect equivalent to 0.5 mm AI. It was operated by means of a 6-valve generator, according to the principles described by REICHMANN et coll. (1974). The tube potential and tube current were recorded at the high-voltage side of the generator. The leak current of the system was found to be completely insignificant. The tube potential was recorded by means of an oscilloscope and seemed to oscillate at an amplitude of ± 1 kV. In the following the maximal potential values are given. In the main experiments the tube was operated with five types of filters, one of these without extra filtration. In the other four molybdenum (0.06 mm), tin (0.11 mm), copper (0.06 mm), and aluminium (1.75 mm) were used as extra filters. The thicknesses were chosen so as to give approximately the same incident dose rate at 29 kV. The filters were placed next to the tube.
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Two separate thin-walled basins containing distilled water and vegetable oil with equally high fluid levels were used as soft tissue substitute. The basins were placed alternatingly in the radiation beam, the transmitted radiation intensity being measured by means of an intensimeter, type Victoreen, model 440, serial 442. The spectral sensitivity of the ionization chamber was checked against a similar instrument (Labor Prof. Dr. Berthold, Modell TOL/D) with a constant sensitivity within the spectrum used. Control measurements displayed a standard error of ±0.087. In order to reduce the sensitivity, the entrance opening of the instrument was shielded by a lead plate with a hole of 5 mm diameter. For each tube potential and filter the intensities of transmitted radiation through oil and water, respectively, were measured and the quotient between them calculated. In some experiments the incident intensity was also measured, so that the absorbed radiation dose could be calculated (incident radiation dose minus transmitted radiation dose). The intensimeter measurements were supplemented by film dosimetry in certain cases. The radiation, having passed oil and water, exposed the film (Agfa-Gevaert Mamoray T 3) the exposures being made in a stepwise manner; the film was developed in a roll machine (DEICHGRABER et colI. 1974). By means of densitometry the relative mAs-values resulting in equal film density behind oil and water, respectively, could be assessed. In this way the same quotient could be calculated as was obtained from the intensimeter measurements. Differences in the spectral sensitivity of the two detectors could thus be recorded to a certain degree. Two experimental series were performed. In the first the effect of the K-edge absorption was tested. The phantom thickness of oil and water was 3 em and the potential was set at 24, 29, 34, and 39 kV. The intensity of the incident radiation was measured without phantom inserted into the beam. Transmitted radiation with strict collimation, eliminating all secondary radiation, was measured with the phantom in the beam. These measurements were repeated with uncollimated radiation, so as to imitate the conditions present in clinical radiography, where no secondary screening is used. By subtracting the transmitted intensity obtained with collimation from that obtained without collimation, a figure of the secondary radiation intensity in relation to the primary radiation could be obtained. The relative dose absorbed within the water layer of the phantom was calculated, water being considered equivalent to the major part of tissue in clinical examinations. In the same way the relative heating of the anode in the different types of exposure was calculated from the exposure data, a given dose transmitted by water being considered necessary for obtaining an image. In this way the contrast caused by the various tube potentials and filters could be analysed together with the absorbed dose and tube load. These experiments gave evidence that K-edge filters are not useful in clinical practice under the conditions given. Another series of experiments with increasing Al filtration was therefore performed. The measurements were based on previous clinical experiences. The soft tissue roentgen tube was mounted in a Lysholm skull table, the FFD being 70 em. Medium sensitivity industrial film (Mamoray T 3) and
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Table 1 Relative radiation dose in relation to contrast of the radiation relief and tube load. The contrast function implies the quotient between radiation intensities behind oil and water with a thickness of 3 em, respectively
Filter
Tube potential 24 kV
None Mo Sn Cu
Al
29 kV
34 kV
39 kV
ReI. dose
Intens. quot.
Tube ReI. load dose
Intens. quot.
Tube ReI. load dose
Intens. quot.
Tube ReI. load dose
Intens. quot.
Tube load
1.0 1.25 0.5 0.5 0.5
3.6 4.9 3.3 3.2 2.7
94 672 504 492 520
2.6 4.0 2.1 2.2 2.1
36 361 134 124 133
2.4 2.7 2.0 1.8 1.8
24 182 86 62 65
2.1 2.2 1.8 1.7 1.6
18 119 70 42 43
0.5 1.0 0.3 0.3 0.3
0.4 0.6 0.3 0.3 0.2
0.3 0.4 0.3 0.2 0.2
a potential of 25 kV for thin objects, such as finger joints, 32 to 35 kV for mediumsized objects, such as elbow joint, and 40 kV for thick objects, such as the shoulder joint, have previously been demonstrated to give the best results (DEICHGRABER et colI. 1974). Accordingly, a phantom consisting of 3 em oil and water, respectively, was tested at 25 kV. For phantom thicknesses of 5 and 7 em the tube potentials were 32.5 and 40 kV, respectively. The same measuring principle was utilized as in the first series, only collimated radiation being employed. The quotient between the transmitted intensities behind oil and water, respectively, was calculated as earlier. Beginning with tube filtration only (0.5 mm AI) AI-filters were added in a stepwise manner, the quotient being obtained for each step. When the quotient started to decline, it was assumed that the filtering was so effective as to influence the spectral distribution of the photons reaching the recording medium. The aim was to find the maximum AIfiltering which leaves the spectral distribution unaffected.
Results Measurements, series 1. The intensity of the secondary radiation varied slightly with tube potential; at the two lower it was 10 to 15 per cent of the primary radiation instensity, and 15 to 20 per cent at the higher potential. No correlation existed between secondary radiation intensity and the spectral composition of the transmitted radiation as manifested in the intensity quotient of the primary beam recordings. Thus it may be concluded that the different types of filtering should not possibly influence the amount of fogging radiation. Any differences in the recordings obtained with different filters should thus be expected to be related only to the transmitted primary beam. Table 1 gives three types of data obtained from the measurements of the collimated radiation: the relative absorbed dose, the intensity quotient obtained from the recordings with oil and water, respectively, and the tube load expressed in relative heat
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Table 2 Influence of the spectral sensitivity of silver on the intensity quotient (cf. Table 1). The intensimeter figures display the contrast.function of the radiation relief, the film figures indicate how these functions are modified when recorded on non-screen film
Filter
Tube potential 29 kV
24 kV
None Mo Al
Intensimeter
Film
Intensimeter
Film
3.6 4.9 2.7
4.4 4.6 2.5
2.6 4.0 2.1
2.1 2.7 1.6
units. Even though the degree of filtering varied relatively good agreement between the intensity quotient and the absorbed dose existed; a rising quotient caused the dose to increase to the same degree, regardless of how the rising quotient was obtained. The action of the K-edge could be demonstrated with the two filters with the K-edge absorption within the spectral range used (Mo and Sn). The influence of the molybdenum filter was at its maximum at a tube potential of 29 kV, whereas the same occurred at 34 to 39 kV for Sn. The change of the quotient was less marked for the tin filter. This may reflect a lesser filtering capacity of this filter than was obtained with the Mo-filter. The tube load varied within wide limits even in cases with identical intensity quotients. Thus concerning the effect on the radiation, it appears appropriate to choose the alternative giving the desired intensity quotient with the smallest tube load possible. This would result in shorter exposure times and less motion unsharpness. From this point of view no additional tube filtration proved a better alternative than any of the filters tested. The intensity quotients presented in Table 1 may, however, be misleading in one important respect. As long as the recording is made on film, the best recording medium appears to be medium sensitive industrial film without intensifying screens (DEICHGRABER et coll. 1974, 1975). Here the image is made up by the direct influence of photons on the silver bromide crystals. Silver has its K-absorption edge at 25.5 keY (BEARDEN 1968/69). Thus the film should be expected to be more sensitive above this energy level than below it, the sensitivity here changing markedly and stepwise. This assumption has been confirmed by analysing the spectral sensitivity of non-screen film (CORNEY 1966). For this reason the intensity quotient was measured for three filtering alternatives (no filter, Mo, AI) at 24 and 29 kV, film being used as detector instead of the intensimeter. These measurements did not appear to give the same precision as the intensimeter measurements, due to the fact that excessively high doses had to be used if strict collimation of the radiation was to be maintained. The results are presented in Table 2, where the intensity quotient obtained with film is compared with the one obtained with the intensimeter. At 24 kV no evident difference
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Table 3 Effect of increasing Al-filtration on the intensity quotient (cf. Table J). Pht. ~ phantom thickness
mmAI
0.000 0.125 0.250 0.500 0.750 1.000 1.750
Tube potential 25 kV Pht. 3 em
32.5 kV Pht. 5 em
40 kV Pht. 7 em
3.16 3.16 3.35 3.26 3.02 2.75
3.02 2.80 2.80 2.65 2.65
2.95 2.90 2.86 2.64 2.60 2.75 2.70
was seen between the two types of recording. At 29 kV the film quotient was lower in all three cases, indicating that the film was relatively more sensitive to photons of the highest energy levels. The most evident decrease occurred for the molybdenum filter, so that no great difference remained between the quotients of unfiltered rays and rays having passed the molybdenum filter. Measurements, series 2. It appears from Table 3 that at 25 kV the optimum filtration (0.75 mm AI) could be readily assessed. At the other two tube potentials the optimum was less well-defined. It was considered, however, that at 32.5 kV 0.75 mm Al should be recommended, 1.75 mm Al being optimal at 40 kV. The inherent filtration of the tube (0.5 mm AI) should be added to these values. The tube load and the dose are affected by these filtrations. The tube load was increased about 100 per cent in all three cases but only a moderate lowering of the dose occurred. In the three cases the dose reduction as compared with the case without extra filtration was 34, 33 and 44 per cent, respectively, the last value corresponding to 40 kV. Discussion In the present experiments a tube with a glass window (filtering effect, 0.5 mm AI) was used. The unspecific filtration of the window to some degree counteracted the specific effects of the K-edge filters. However, the window filtration is not only a matter of window material; it also increases with use, owing to precipitations of vaporized tungsten atoms originating from the cathode (HACH 1972). In soft tissue radiography of joints high dose rates-which imply high risks of such tungsten precipitations-are mostly unavoidable. A beryllium window might result in more marked K-edge filtration (GAJEWSKI & HEILMANN). The ageing of such tubes would be fairly rapid, however. Furthermore, few departments can afford to have a special apparatus for soft tissue radiography of joints. The same tube should be useful in
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ordinary work as well. This would probably lead to a rapid increase in window filtration, even if beryllium is used as window material. The tube of the present investigation has been moderately used. K-edge absorption filters in combination with tungsten target tubes do not seem to have any place in soft tissue examinations. It might have been hoped that the use of such filters would result in a narrowing of the spectral range in the incident radiation, this in turn leading to low patient doses with maintenance of high image contrast. The present results have clearly demonstrated that even if very heavy K-edge filtration is used in combination with thin objects, this effect cannot be obtained to any useful degree. The same image and dose characteristics may be obtained without any filtering if the tube potential is lowered. The K-edge filtration seems to give an unreasonably high tube load. Another effect that might be anticipated from the use of K-edge filtration would be a reduction of the exposure time. At low tube potentials the maximum mA-Ioad is rapidly reduced. This is not due to overheating of the anode but to the limited heating capacity of the cathode, so that the emission of electrons is impaired at low potentials. It might be argued that the introduction of a K-edge filter would allow the tube potential to be raised without contrast being adversely affected. This, in turn, would lead to a higher dose rate, resulting from increase of tube potentials as well as of tube current. However, the influence of the K-edge filters on the spectral distribution seems not to be sufficient to allow a shortening of the exposure time. On the contrary, considerable lengthenings seem to be unavoidable. The only K-absorption edge of significance in this context appears to be that of silver at 25.5 keY. It has been emphasized previously (DEICHGRABER et colI. 1974) that even though non-screen industrial film seems to be the most appropriate photographic recording material in mammary radiography with the molybdenum technique, it should be stressed that this recording medium unfortunately has such a low absorbing capacity in the spectral range in question. The introduction of the technique of GROS (1967), utilizing a molybdenum target and a molybdenum filter, has caused a subsiding interest in tungsten target tubes in this voltage range. This may be reasonable as regards mammary radiography but it is unjustified in soft tissue radiography of joints. It appears that with this type of tube a great deal of valuable information may be gained otherwise unobtainable (REICHMANN et colI., DEICHGRABER & OLSSON). The tungsten tubes for soft tissue radiography offered today by the manufacturers are merely standard tubes with extra low inherent filtration. Since these tubes only give a limited dose rate at low potentials, even the moderate increase in tube load induced by optimal Al-filtration seems to be unrecommendable. The filtration does not lead to drastic reductions, when expressed in relative figures. However, the total dose levels in this type of radiography are high, which implies that the filtering may give valuable reductions when expressed in absolute figures. It seems highly desirable that efforts be made to construct tubes with cathodes more suited to soft tissue radiography. The two focal spots of a tube
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could have equal size, the different cathodes being adapted to soft tissue radiography and ordinary radiography, respectively. Acknowledgement This investigation was supported by a grant from the Swedish Society of Medical Radiology.
SUMMARY The influence of K-edge filters in combination with tungsten target tubes has been analysed in the low voltage range. It appears that these filters do not offer any advantages with this target material. AI-filtration, applicable to different types of soft tissue radiography of joints is discussed.
ZUSAMMENFASSUNG Der Einfluss von K-Kanten Filtern in Kombination mit Tungsten Target Rohren wurde im Nieder-Volt Bereich untersucht. Es ist offen bar, dass diese Filter keinerlei Vorteil bei diesem Target Material bieten. AI-Filtrierung, anwendbar fiir verschiedene Typen der Weichgewebe Rontgenuntersuchung der Gelenke, wird diskutiert.
RESUME Les auteurs ont etudie I'influence de filtres a discontinuite d'absorption K avec des tubes Ie domaine des bas voltages. II resulte de cette etude que ces filtres ne presentent pas d'avantages quand ils sont utilises avec les anodes en tungstene, Les auteurs etudient la filtration par l'aluminium appliquee a differents types de radiographies des tissues mous periarticulaires.
a anode de tungstene dans
REFERENCES BEARDEN J. A.: X-ray wavelengths. In: The handbook of chemistry and physics. 49th ed, Edited by R. C. Weast. The chemical Rubber Co. Cleveland, Ohio 1968-69. CORNEY G. M.: X-rays and Gamma rays. In: The theory of the photographic process, p. 188. Edited by T. H. James. The Macmillan Compo New York and London 1966. DEICHGRABER E. and OLSSON B.: Soft tissue radiography in painful shoulder. To be published in Acta radiol. Diagnosis. - 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 Marnmographiegeraten. Electromedica 3 (1973), 90. GAJEWSKI H. und HEILMANN H.-P.: Experimentelle Untersuchungen zur optimalen Aufnahmetechnik bei der Mammographie. Fortschr. Rontgenstr, 115 (1971), 248.
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und MIKA N.: Optimierung von Bildkontrast und Bildscharfe bei der WeichstrahlMammographie. Deutscher Rontgenkongress 1968, S. 115. GRaS CH. E.: Methodologie. Symposium europeen de radiologie mammaire. J. Radiol. Electrol. 48 (1967), 638. HACH G.: Betrachtungen zur optirnalen Mammographie-Technik. Fortschr. Rontgenstr, 117 (1972), 208. JAEGER H.: Physikalisch-technische Grundlagen zur rontgenphotographischen Abbildung von Weichteilen. Rontgen-Bl. 23 (1969), 323. JOTTEN G., KYSER K. and OOSTERKAMP W. J.: X-ray source for mammography. Lecture held at the International Congress of Radiology, Madrid 1973. KYSER K.: Rontgenspektrometrische und rontgendosimetrische Untersuchungen der Strahlenqualitaten flir Weichstrahlaufnahmen. Fortschr. Rontgenstr, 116 (1972), 818. MIKA N. und REISS K. H.: Optimierung der Rontgenbelichtungstechnik mit Hilfe der Halbleiterspektrometrie. Rontgenpraxis 21 (1968), 164. REICHMANN S., DEICHGRABER E., STRID K.-G., HEYMAN F. and STRAND T.: Soft tissue radiography of finger joints. Acta radio!. Diagnosis 15 (1974), 439.
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FILTRATION IN SOFT TISSUE RADIOGRAPHY E. DEICHGRABER and S. REICHMANN In mammary radiography GROS (1967) used the K-emission from a molybdenum anode in combination with a molybdenum filter. This filter to some degree absorbes photons at energy levels above the K-emission lines, at 17 to 19 keY (BEARDEN 1968/69) more effectively than those just below the K-absorption edge. There has been some difference of opinion on the feasibility of the technique introduced by GROS (GAJEWSKI & MIKA 1968, MIKA & REISS 1968, JAEGER 1969, KYSER 1972, HACH 1972). It appears that the technique is applicable only for limited tissue thickness, the upper limit being five to six em. When this is exceeded, the filtering effect of the tissue counteracts the combined effect of the molybdenum anode and the molybdenum filter. The image is in fact made up of photons within the continuous spectrum above the K-emission lines. For this reason compression is strongly advised in radiography of the breast. Radiographic techniques similar to that of breast radiography have been successfully applied to periarticular tissues in different regions (FISCHER 1973 a, b, FISCHER & BRAUN 1973, REICHMANN et coll. 1974, DEICHGRABER & OLSSON, to be published). The limit of the tissue thickness pertaining to the technique of GROS should be valid to periarticular tissues as well. However, even in examination of such thin objects as finger joints this technique was found to be unsuitable, as too high an image contrast is obtained (REICHMANN et coll.). If a radiation quality were employed, corresponding Submitted for publication 28 May 1974.
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to a tube potential of 26 kV from a tungsten anode, filtering 0.5 mm AI, not only the soft tissues were well depicted but also the bone structure. In radiography of the shoulder joint the technique of GROS was completely unsuccessful due to the long exposure times. Therefore, it is to be recommended to use a tungsten target operating at low potentials for all types of radiography of periarticular tissues. The filtering in connection with a tungsten anode tube is incompletely known. KYSER (1972) presented the radiation spectrum of a combination of a tungsten anode tube with a molybdenum filter, which contained large amounts of energy below the K-absorption edge of molybdenum at 20 keY. However, he did not analyse their circumstances any further. Later an attempt was made to adapt the molybdenum anode to radiography of thick tissue volumes by combining the anode with other filtering materials, such as aluminium and silver (JOTTEN et coll. 1973). With the latter filters the continuous spectrum of the molybdenum anode was brought out and the K-emission lines were to some extent filtered away. GAJEWSKI & HEILMANN (1971) compared the image contrast obtained with a tungsten target to that obtained with one of molybdenum under identical operating conditions. A molybdenum filter (0.03 mm) was used in both cases, the tube window being made of beryllium. A glass filter (0.5 mm AI) was also combined with the tungsten target. Clinical radiography on non-screen film was performed but no spectrometry. Their results are partly at variance with the principles of GROS, insofar as the tungsten target was claimed to yield higher contrast under certain conditions. Thus the value of K-edge filtering in the examination of joints cannot be completely assessed from previous reports. An investigation of the K-edge filtering of two types of filter, molybdenum and tin, was therefore undertaken. A thin object was used, since it was considered that if the filtering effect had little or no value in such a case, it would be even more useless with thicker objects. Furthermore, Al-filtration in this type of radiography has also been considered. Materials and Methods The experiments were carried out by means of an ordinary diagnostic roentgen tube with a tungsten-rhenium target (Siemens Bi 125.30.50.MaR). The glass window of the tube had a filtering effect equivalent to 0.5 mm AI. It was operated by means of a 6-valve generator, according to the principles described by REICHMANN et coll. (1974). The tube potential and tube current were recorded at the high-voltage side of the generator. The leak current of the system was found to be completely insignificant. The tube potential was recorded by means of an oscilloscope and seemed to oscillate at an amplitude of ± 1 kV. In the following the maximal potential values are given. In the main experiments the tube was operated with five types of filters, one of these without extra filtration. In the other four molybdenum (0.06 mm), tin (0.11 mm), copper (0.06 mm), and aluminium (1.75 mm) were used as extra filters. The thicknesses were chosen so as to give approximately the same incident dose rate at 29 kV. The filters were placed next to the tube.
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Two separate thin-walled basins containing distilled water and vegetable oil with equally high fluid levels were used as soft tissue substitute. The basins were placed alternatingly in the radiation beam, the transmitted radiation intensity being measured by means of an intensimeter, type Victoreen, model 440, serial 442. The spectral sensitivity of the ionization chamber was checked against a similar instrument (Labor Prof. Dr. Berthold, Modell TOL/D) with a constant sensitivity within the spectrum used. Control measurements displayed a standard error of ±0.087. In order to reduce the sensitivity, the entrance opening of the instrument was shielded by a lead plate with a hole of 5 mm diameter. For each tube potential and filter the intensities of transmitted radiation through oil and water, respectively, were measured and the quotient between them calculated. In some experiments the incident intensity was also measured, so that the absorbed radiation dose could be calculated (incident radiation dose minus transmitted radiation dose). The intensimeter measurements were supplemented by film dosimetry in certain cases. The radiation, having passed oil and water, exposed the film (Agfa-Gevaert Mamoray T 3) the exposures being made in a stepwise manner; the film was developed in a roll machine (DEICHGRABER et colI. 1974). By means of densitometry the relative mAs-values resulting in equal film density behind oil and water, respectively, could be assessed. In this way the same quotient could be calculated as was obtained from the intensimeter measurements. Differences in the spectral sensitivity of the two detectors could thus be recorded to a certain degree. Two experimental series were performed. In the first the effect of the K-edge absorption was tested. The phantom thickness of oil and water was 3 em and the potential was set at 24, 29, 34, and 39 kV. The intensity of the incident radiation was measured without phantom inserted into the beam. Transmitted radiation with strict collimation, eliminating all secondary radiation, was measured with the phantom in the beam. These measurements were repeated with uncollimated radiation, so as to imitate the conditions present in clinical radiography, where no secondary screening is used. By subtracting the transmitted intensity obtained with collimation from that obtained without collimation, a figure of the secondary radiation intensity in relation to the primary radiation could be obtained. The relative dose absorbed within the water layer of the phantom was calculated, water being considered equivalent to the major part of tissue in clinical examinations. In the same way the relative heating of the anode in the different types of exposure was calculated from the exposure data, a given dose transmitted by water being considered necessary for obtaining an image. In this way the contrast caused by the various tube potentials and filters could be analysed together with the absorbed dose and tube load. These experiments gave evidence that K-edge filters are not useful in clinical practice under the conditions given. Another series of experiments with increasing Al filtration was therefore performed. The measurements were based on previous clinical experiences. The soft tissue roentgen tube was mounted in a Lysholm skull table, the FFD being 70 em. Medium sensitivity industrial film (Mamoray T 3) and
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Table 1 Relative radiation dose in relation to contrast of the radiation relief and tube load. The contrast function implies the quotient between radiation intensities behind oil and water with a thickness of 3 em, respectively
Filter
Tube potential 24 kV
None Mo Sn Cu
Al
29 kV
34 kV
39 kV
ReI. dose
Intens. quot.
Tube ReI. load dose
Intens. quot.
Tube ReI. load dose
Intens. quot.
Tube ReI. load dose
Intens. quot.
Tube load
1.0 1.25 0.5 0.5 0.5
3.6 4.9 3.3 3.2 2.7
94 672 504 492 520
2.6 4.0 2.1 2.2 2.1
36 361 134 124 133
2.4 2.7 2.0 1.8 1.8
24 182 86 62 65
2.1 2.2 1.8 1.7 1.6
18 119 70 42 43
0.5 1.0 0.3 0.3 0.3
0.4 0.6 0.3 0.3 0.2
0.3 0.4 0.3 0.2 0.2
a potential of 25 kV for thin objects, such as finger joints, 32 to 35 kV for mediumsized objects, such as elbow joint, and 40 kV for thick objects, such as the shoulder joint, have previously been demonstrated to give the best results (DEICHGRABER et colI. 1974). Accordingly, a phantom consisting of 3 em oil and water, respectively, was tested at 25 kV. For phantom thicknesses of 5 and 7 em the tube potentials were 32.5 and 40 kV, respectively. The same measuring principle was utilized as in the first series, only collimated radiation being employed. The quotient between the transmitted intensities behind oil and water, respectively, was calculated as earlier. Beginning with tube filtration only (0.5 mm AI) AI-filters were added in a stepwise manner, the quotient being obtained for each step. When the quotient started to decline, it was assumed that the filtering was so effective as to influence the spectral distribution of the photons reaching the recording medium. The aim was to find the maximum AIfiltering which leaves the spectral distribution unaffected.
Results Measurements, series 1. The intensity of the secondary radiation varied slightly with tube potential; at the two lower it was 10 to 15 per cent of the primary radiation instensity, and 15 to 20 per cent at the higher potential. No correlation existed between secondary radiation intensity and the spectral composition of the transmitted radiation as manifested in the intensity quotient of the primary beam recordings. Thus it may be concluded that the different types of filtering should not possibly influence the amount of fogging radiation. Any differences in the recordings obtained with different filters should thus be expected to be related only to the transmitted primary beam. Table 1 gives three types of data obtained from the measurements of the collimated radiation: the relative absorbed dose, the intensity quotient obtained from the recordings with oil and water, respectively, and the tube load expressed in relative heat
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Table 2 Influence of the spectral sensitivity of silver on the intensity quotient (cf. Table 1). The intensimeter figures display the contrast.function of the radiation relief, the film figures indicate how these functions are modified when recorded on non-screen film
Filter
Tube potential 29 kV
24 kV
None Mo Al
Intensimeter
Film
Intensimeter
Film
3.6 4.9 2.7
4.4 4.6 2.5
2.6 4.0 2.1
2.1 2.7 1.6
units. Even though the degree of filtering varied relatively good agreement between the intensity quotient and the absorbed dose existed; a rising quotient caused the dose to increase to the same degree, regardless of how the rising quotient was obtained. The action of the K-edge could be demonstrated with the two filters with the K-edge absorption within the spectral range used (Mo and Sn). The influence of the molybdenum filter was at its maximum at a tube potential of 29 kV, whereas the same occurred at 34 to 39 kV for Sn. The change of the quotient was less marked for the tin filter. This may reflect a lesser filtering capacity of this filter than was obtained with the Mo-filter. The tube load varied within wide limits even in cases with identical intensity quotients. Thus concerning the effect on the radiation, it appears appropriate to choose the alternative giving the desired intensity quotient with the smallest tube load possible. This would result in shorter exposure times and less motion unsharpness. From this point of view no additional tube filtration proved a better alternative than any of the filters tested. The intensity quotients presented in Table 1 may, however, be misleading in one important respect. As long as the recording is made on film, the best recording medium appears to be medium sensitive industrial film without intensifying screens (DEICHGRABER et coll. 1974, 1975). Here the image is made up by the direct influence of photons on the silver bromide crystals. Silver has its K-absorption edge at 25.5 keY (BEARDEN 1968/69). Thus the film should be expected to be more sensitive above this energy level than below it, the sensitivity here changing markedly and stepwise. This assumption has been confirmed by analysing the spectral sensitivity of non-screen film (CORNEY 1966). For this reason the intensity quotient was measured for three filtering alternatives (no filter, Mo, AI) at 24 and 29 kV, film being used as detector instead of the intensimeter. These measurements did not appear to give the same precision as the intensimeter measurements, due to the fact that excessively high doses had to be used if strict collimation of the radiation was to be maintained. The results are presented in Table 2, where the intensity quotient obtained with film is compared with the one obtained with the intensimeter. At 24 kV no evident difference
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Table 3 Effect of increasing Al-filtration on the intensity quotient (cf. Table J). Pht. ~ phantom thickness
mmAI
0.000 0.125 0.250 0.500 0.750 1.000 1.750
Tube potential 25 kV Pht. 3 em
32.5 kV Pht. 5 em
40 kV Pht. 7 em
3.16 3.16 3.35 3.26 3.02 2.75
3.02 2.80 2.80 2.65 2.65
2.95 2.90 2.86 2.64 2.60 2.75 2.70
was seen between the two types of recording. At 29 kV the film quotient was lower in all three cases, indicating that the film was relatively more sensitive to photons of the highest energy levels. The most evident decrease occurred for the molybdenum filter, so that no great difference remained between the quotients of unfiltered rays and rays having passed the molybdenum filter. Measurements, series 2. It appears from Table 3 that at 25 kV the optimum filtration (0.75 mm AI) could be readily assessed. At the other two tube potentials the optimum was less well-defined. It was considered, however, that at 32.5 kV 0.75 mm Al should be recommended, 1.75 mm Al being optimal at 40 kV. The inherent filtration of the tube (0.5 mm AI) should be added to these values. The tube load and the dose are affected by these filtrations. The tube load was increased about 100 per cent in all three cases but only a moderate lowering of the dose occurred. In the three cases the dose reduction as compared with the case without extra filtration was 34, 33 and 44 per cent, respectively, the last value corresponding to 40 kV. Discussion In the present experiments a tube with a glass window (filtering effect, 0.5 mm AI) was used. The unspecific filtration of the window to some degree counteracted the specific effects of the K-edge filters. However, the window filtration is not only a matter of window material; it also increases with use, owing to precipitations of vaporized tungsten atoms originating from the cathode (HACH 1972). In soft tissue radiography of joints high dose rates-which imply high risks of such tungsten precipitations-are mostly unavoidable. A beryllium window might result in more marked K-edge filtration (GAJEWSKI & HEILMANN). The ageing of such tubes would be fairly rapid, however. Furthermore, few departments can afford to have a special apparatus for soft tissue radiography of joints. The same tube should be useful in
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ordinary work as well. This would probably lead to a rapid increase in window filtration, even if beryllium is used as window material. The tube of the present investigation has been moderately used. K-edge absorption filters in combination with tungsten target tubes do not seem to have any place in soft tissue examinations. It might have been hoped that the use of such filters would result in a narrowing of the spectral range in the incident radiation, this in turn leading to low patient doses with maintenance of high image contrast. The present results have clearly demonstrated that even if very heavy K-edge filtration is used in combination with thin objects, this effect cannot be obtained to any useful degree. The same image and dose characteristics may be obtained without any filtering if the tube potential is lowered. The K-edge filtration seems to give an unreasonably high tube load. Another effect that might be anticipated from the use of K-edge filtration would be a reduction of the exposure time. At low tube potentials the maximum mA-Ioad is rapidly reduced. This is not due to overheating of the anode but to the limited heating capacity of the cathode, so that the emission of electrons is impaired at low potentials. It might be argued that the introduction of a K-edge filter would allow the tube potential to be raised without contrast being adversely affected. This, in turn, would lead to a higher dose rate, resulting from increase of tube potentials as well as of tube current. However, the influence of the K-edge filters on the spectral distribution seems not to be sufficient to allow a shortening of the exposure time. On the contrary, considerable lengthenings seem to be unavoidable. The only K-absorption edge of significance in this context appears to be that of silver at 25.5 keY. It has been emphasized previously (DEICHGRABER et colI. 1974) that even though non-screen industrial film seems to be the most appropriate photographic recording material in mammary radiography with the molybdenum technique, it should be stressed that this recording medium unfortunately has such a low absorbing capacity in the spectral range in question. The introduction of the technique of GROS (1967), utilizing a molybdenum target and a molybdenum filter, has caused a subsiding interest in tungsten target tubes in this voltage range. This may be reasonable as regards mammary radiography but it is unjustified in soft tissue radiography of joints. It appears that with this type of tube a great deal of valuable information may be gained otherwise unobtainable (REICHMANN et colI., DEICHGRABER & OLSSON). The tungsten tubes for soft tissue radiography offered today by the manufacturers are merely standard tubes with extra low inherent filtration. Since these tubes only give a limited dose rate at low potentials, even the moderate increase in tube load induced by optimal Al-filtration seems to be unrecommendable. The filtration does not lead to drastic reductions, when expressed in relative figures. However, the total dose levels in this type of radiography are high, which implies that the filtering may give valuable reductions when expressed in absolute figures. It seems highly desirable that efforts be made to construct tubes with cathodes more suited to soft tissue radiography. The two focal spots of a tube
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could have equal size, the different cathodes being adapted to soft tissue radiography and ordinary radiography, respectively. Acknowledgement This investigation was supported by a grant from the Swedish Society of Medical Radiology.
SUMMARY The influence of K-edge filters in combination with tungsten target tubes has been analysed in the low voltage range. It appears that these filters do not offer any advantages with this target material. AI-filtration, applicable to different types of soft tissue radiography of joints is discussed.
ZUSAMMENFASSUNG Der Einfluss von K-Kanten Filtern in Kombination mit Tungsten Target Rohren wurde im Nieder-Volt Bereich untersucht. Es ist offen bar, dass diese Filter keinerlei Vorteil bei diesem Target Material bieten. AI-Filtrierung, anwendbar fiir verschiedene Typen der Weichgewebe Rontgenuntersuchung der Gelenke, wird diskutiert.
RESUME Les auteurs ont etudie I'influence de filtres a discontinuite d'absorption K avec des tubes Ie domaine des bas voltages. II resulte de cette etude que ces filtres ne presentent pas d'avantages quand ils sont utilises avec les anodes en tungstene, Les auteurs etudient la filtration par l'aluminium appliquee a differents types de radiographies des tissues mous periarticulaires.
a anode de tungstene dans
REFERENCES BEARDEN J. A.: X-ray wavelengths. In: The handbook of chemistry and physics. 49th ed, Edited by R. C. Weast. The chemical Rubber Co. Cleveland, Ohio 1968-69. CORNEY G. M.: X-rays and Gamma rays. In: The theory of the photographic process, p. 188. Edited by T. H. James. The Macmillan Compo New York and London 1966. DEICHGRABER E. and OLSSON B.: Soft tissue radiography in painful shoulder. To be published in Acta radiol. Diagnosis. - 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 Marnmographiegeraten. Electromedica 3 (1973), 90. GAJEWSKI H. und HEILMANN H.-P.: Experimentelle Untersuchungen zur optimalen Aufnahmetechnik bei der Mammographie. Fortschr. Rontgenstr, 115 (1971), 248.
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und MIKA N.: Optimierung von Bildkontrast und Bildscharfe bei der WeichstrahlMammographie. Deutscher Rontgenkongress 1968, S. 115. GRaS CH. E.: Methodologie. Symposium europeen de radiologie mammaire. J. Radiol. Electrol. 48 (1967), 638. HACH G.: Betrachtungen zur optirnalen Mammographie-Technik. Fortschr. Rontgenstr, 117 (1972), 208. JAEGER H.: Physikalisch-technische Grundlagen zur rontgenphotographischen Abbildung von Weichteilen. Rontgen-Bl. 23 (1969), 323. JOTTEN G., KYSER K. and OOSTERKAMP W. J.: X-ray source for mammography. Lecture held at the International Congress of Radiology, Madrid 1973. KYSER K.: Rontgenspektrometrische und rontgendosimetrische Untersuchungen der Strahlenqualitaten flir Weichstrahlaufnahmen. Fortschr. Rontgenstr, 116 (1972), 818. MIKA N. und REISS K. H.: Optimierung der Rontgenbelichtungstechnik mit Hilfe der Halbleiterspektrometrie. Rontgenpraxis 21 (1968), 164. REICHMANN S., DEICHGRABER E., STRID K.-G., HEYMAN F. and STRAND T.: Soft tissue radiography of finger joints. Acta radio!. Diagnosis 15 (1974), 439.
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