1975, British Journal of Radiology, 48, 858-859
Technical note Evaluation of a new screen/film combination By A. L Evans, B.Sc, Ph.D., M. Davison, M.A., D.Phil, J. McLellan, M.S.R., and W. B. James, F.R.C.P. (Glasgow), F.R.C.R. Department of Radiology, Southern General Hospital, Glasgow, and Department of Clinical Physics and BioEngineering, West of Scotland Health Boards, Glasgow (Received December, 1974)
ABSTRACT
The Trimax system offers film/screen combinations which reduce the X-ray exposure by factors of about 4. This is achieved without deterioration in image quality.
Buchanan, Finkelstein and Wickershein (1972) have described the characteristics of intensifying screens incorporating the rare earth phosphors terbium-activated lanthanum oxysulphide and terbium-activated gadolinium oxysulphide. They showed that exposure reductions of two or greater could be achieved even with the prototype screens. Similar screens have recently become commercially available (Types a4 and a8) together with suitable films (Types XD and XM)* sensitized to the green light emitted by the screens. This communication compares the physical characteristics of the new film/screen combinations with those in current use and gives an indication of their clinical usefulness.
simulate variations in patient absorption we have investigated the change in contrast with kVp using an aluminium step wedge of step depth 2-5 mm. Table II shows the difference in density per step. Finally, Fig. 1 shows the contrast transfer function (Bouwers, 1965) for some of the film/screen combinations at 80 kVp. This was calculated using
TABLE I Film-screen combination R Film Fast Tungstate R Film HR screens XDa4 XMa8
Characteristic curves were obtained by exposing the two extreme film/screen combinations to X-radiation from a tungsten anode tube at a focus to film distance of 40 in. The beam was filtered by 1-5 mm of copper to simulate the quality of radiation leaving the patient. Films were processed by Ilford Rapid R low temperature chemistry. For ease of comparison, the exposure for an optical density of 1-0 and the gamma of the film/screen combination have been summarized in Table I for 60 kVp, 80 kVp and 100 kVp radiation. All factors have been calculated relative to the film and screen combination used in this department, i.e., 3M Type R Polyester Film and Ilford fast tungstate screens. The manufacturers claim that when using the rare earth screens the contrast increases as the kVp increases. It has been suggested that this increase in film contrast with kVp might contribute to the use of higher kVp examinations with a consequent shortening of exposure time. In order to
R Film Fast Tungstate R Film HR screens XDa4 XMa8
1
60
12-7 0-8 0-2
R Film Fast Tungstate R Film HR screens XDa4 XMa8
PHYSICAL PARAMETERS
*Trimax system: 3M.
kVp
Relative exposure for optical Gamma density = 1
1
80
1 1-3 0-9 1
10-3 0-8 0-2
1 1-4 1
1 8-8
1 0-8
0-7 0-25
1-2 11
100
TABLE II VARIATION OF CONTRAST WITH KVP FOR ONE STEP IN AN ALUMINIUM STEP WEDGE
kVp
Fast Tungstate TypeR a4XD a8XM
60
80
100
0-35 0-45 0-40
0-20 0-30 0-25
015
0-25 0-20
Contrast is here denned as the ratio of the difference to the sum of the transmitted intensities for one step. 858
OCTOBER 1975
Technical note
y^
10
by a factor of about 4 although factors of between 3 and 5 were found for different batches of film. The gamma and latitude of all the screen/film combinations are broadly similar. The values of gamma do not confirm the manufacturers' claim of increasing contrast with kVp and the results given in Table II suggest that any change in gamma can be negligible compared with the concomitant decrease in subject contrast. The XD a4 combination gave a higher contrast than the Fast Tungstate-Type R combination at all kVp settings. We therefore conclude that a higher kVp may be used whilst maintaining the contrast given by Fast Tungsten screens. All the Trimax combinations have a resolution comparable to the Type R-Fast Tungstate combination, the slower XDa4 combination being marginally better than the XMa8. No Trimax combination has a resolution comparable to the high resolution screens.
film
HR ve A
st
- ^ .
FT
A - .A A~ ' , A
A A -A
0-5
0
FIG. 1. Contrast transfer at 80 kVp. (Upper) Errors of measurement for the XDa4 combination. (Lower) A comparison of the contrast transfer for four screen/film combinations. The contrast transfer for XD film used as non-screen film is also shown; the relative contrast is maintained at unity to five line pairs per mm.
the image of a Siemens line pairs resolution test object exposed in contact with the cassette. The densities of the images were roughly the same at a density of 1. A micro-densitometer scan was performed across the line pairs and the contrast, here defined as the ratio of the difference to the sum of the transmitted intensities of the light and dark bars, was subsequently calculated for each line pair frequency. The contrast was normalized to the calculated contrast at a frequency of 0-1 line pairs per mm.
CONCLUSION
The Trimax system offers the possibility of reducing exposures for the majority of diagnostic X-ray examinations by factors of about 4 without a reduction in the quality of the image. In clinical practice the lower exposures can be achieved by the use of shorter exposure times, or with a smaller focal spot, leading to an improvement in picture quality as movement or geometric unsharpness is reduced. Because of the lowered radiation dose this system should be applied particularly to abdominal and obstetric examinations. The lower exposures obtained by the use of lower milliampere values could permit the use of smaller and cheaper mobile machines for ward and theatre examinations. REFERENCES BUCHANAN, R. A., FINKELSTEIN, S. I., and WICKERSHEIN,
DISCUSSION
Table I shows that the a8 XM combination is significantly faster than the combination in current use in this department. Exposures could be reduced
859
K. A., 1972. X-ray exposure reduction using rare-earth oxysulphide intensifying screens. Radiology, 105,185—190. BOUWERS, A., 1965. The practical value of contrast transfer in radiology. In Diagnostic Radiologic Instrumentation, eds. R. D. Moseley and J. H. Rust (Springfield, Illinois)
Technical note Evaluation of a new screen/film combination By A. L Evans, B.Sc, Ph.D., M. Davison, M.A., D.Phil, J. McLellan, M.S.R., and W. B. James, F.R.C.P. (Glasgow), F.R.C.R. Department of Radiology, Southern General Hospital, Glasgow, and Department of Clinical Physics and BioEngineering, West of Scotland Health Boards, Glasgow (Received December, 1974)
ABSTRACT
The Trimax system offers film/screen combinations which reduce the X-ray exposure by factors of about 4. This is achieved without deterioration in image quality.
Buchanan, Finkelstein and Wickershein (1972) have described the characteristics of intensifying screens incorporating the rare earth phosphors terbium-activated lanthanum oxysulphide and terbium-activated gadolinium oxysulphide. They showed that exposure reductions of two or greater could be achieved even with the prototype screens. Similar screens have recently become commercially available (Types a4 and a8) together with suitable films (Types XD and XM)* sensitized to the green light emitted by the screens. This communication compares the physical characteristics of the new film/screen combinations with those in current use and gives an indication of their clinical usefulness.
simulate variations in patient absorption we have investigated the change in contrast with kVp using an aluminium step wedge of step depth 2-5 mm. Table II shows the difference in density per step. Finally, Fig. 1 shows the contrast transfer function (Bouwers, 1965) for some of the film/screen combinations at 80 kVp. This was calculated using
TABLE I Film-screen combination R Film Fast Tungstate R Film HR screens XDa4 XMa8
Characteristic curves were obtained by exposing the two extreme film/screen combinations to X-radiation from a tungsten anode tube at a focus to film distance of 40 in. The beam was filtered by 1-5 mm of copper to simulate the quality of radiation leaving the patient. Films were processed by Ilford Rapid R low temperature chemistry. For ease of comparison, the exposure for an optical density of 1-0 and the gamma of the film/screen combination have been summarized in Table I for 60 kVp, 80 kVp and 100 kVp radiation. All factors have been calculated relative to the film and screen combination used in this department, i.e., 3M Type R Polyester Film and Ilford fast tungstate screens. The manufacturers claim that when using the rare earth screens the contrast increases as the kVp increases. It has been suggested that this increase in film contrast with kVp might contribute to the use of higher kVp examinations with a consequent shortening of exposure time. In order to
R Film Fast Tungstate R Film HR screens XDa4 XMa8
1
60
12-7 0-8 0-2
R Film Fast Tungstate R Film HR screens XDa4 XMa8
PHYSICAL PARAMETERS
*Trimax system: 3M.
kVp
Relative exposure for optical Gamma density = 1
1
80
1 1-3 0-9 1
10-3 0-8 0-2
1 1-4 1
1 8-8
1 0-8
0-7 0-25
1-2 11
100
TABLE II VARIATION OF CONTRAST WITH KVP FOR ONE STEP IN AN ALUMINIUM STEP WEDGE
kVp
Fast Tungstate TypeR a4XD a8XM
60
80
100
0-35 0-45 0-40
0-20 0-30 0-25
015
0-25 0-20
Contrast is here denned as the ratio of the difference to the sum of the transmitted intensities for one step. 858
OCTOBER 1975
Technical note
y^
10
by a factor of about 4 although factors of between 3 and 5 were found for different batches of film. The gamma and latitude of all the screen/film combinations are broadly similar. The values of gamma do not confirm the manufacturers' claim of increasing contrast with kVp and the results given in Table II suggest that any change in gamma can be negligible compared with the concomitant decrease in subject contrast. The XD a4 combination gave a higher contrast than the Fast Tungstate-Type R combination at all kVp settings. We therefore conclude that a higher kVp may be used whilst maintaining the contrast given by Fast Tungsten screens. All the Trimax combinations have a resolution comparable to the Type R-Fast Tungstate combination, the slower XDa4 combination being marginally better than the XMa8. No Trimax combination has a resolution comparable to the high resolution screens.
film
HR ve A
st
- ^ .
FT
A - .A A~ ' , A
A A -A
0-5
0
FIG. 1. Contrast transfer at 80 kVp. (Upper) Errors of measurement for the XDa4 combination. (Lower) A comparison of the contrast transfer for four screen/film combinations. The contrast transfer for XD film used as non-screen film is also shown; the relative contrast is maintained at unity to five line pairs per mm.
the image of a Siemens line pairs resolution test object exposed in contact with the cassette. The densities of the images were roughly the same at a density of 1. A micro-densitometer scan was performed across the line pairs and the contrast, here defined as the ratio of the difference to the sum of the transmitted intensities of the light and dark bars, was subsequently calculated for each line pair frequency. The contrast was normalized to the calculated contrast at a frequency of 0-1 line pairs per mm.
CONCLUSION
The Trimax system offers the possibility of reducing exposures for the majority of diagnostic X-ray examinations by factors of about 4 without a reduction in the quality of the image. In clinical practice the lower exposures can be achieved by the use of shorter exposure times, or with a smaller focal spot, leading to an improvement in picture quality as movement or geometric unsharpness is reduced. Because of the lowered radiation dose this system should be applied particularly to abdominal and obstetric examinations. The lower exposures obtained by the use of lower milliampere values could permit the use of smaller and cheaper mobile machines for ward and theatre examinations. REFERENCES BUCHANAN, R. A., FINKELSTEIN, S. I., and WICKERSHEIN,
DISCUSSION
Table I shows that the a8 XM combination is significantly faster than the combination in current use in this department. Exposures could be reduced
859
K. A., 1972. X-ray exposure reduction using rare-earth oxysulphide intensifying screens. Radiology, 105,185—190. BOUWERS, A., 1965. The practical value of contrast transfer in radiology. In Diagnostic Radiologic Instrumentation, eds. R. D. Moseley and J. H. Rust (Springfield, Illinois)
