An Evaluation of Screen-Film Speed
Radiation Physics
Characteristics 1
J. Reynolds, M.D., J. Skucas, M.D., and J. Gorski, B.S. The relative speeds of six commercially available screen-film combinations were evaluated. There is a nonlinear relationship throughout the kVp range between the calcium tungstate, barium strontium sulfate, and rare earth screens. The speed of the last two types falls off considerably at lower kVp levels. INDEX TERMS:
Radiography, apparatus and equipment • Radiography, technique
Radiology 118:711-713, March 1976
•
• RESOLUTION of small, low-contrast objects is critical in many areas of radiology. Contrast can be improved by lowering the kVp, enhancing the resolution; Unfortunately, the resultant exposure time is thus prolonged, and consequent motion unsharpness degrades the image. Screens composed of different phosphor materials exhibit different speed characteristics throughout the diagnostic kVp range, and we have attempted a systematic evaluation of the speed characteristics of several screen-film combinations.
T
HE
FILM - - - -
~.;.......;,~--------r ~ o~
METHOD
Rather than study a theoretical nonscatter situation, we have attempted to duplicate actual clinical conditions. The screen-film combinations tested are listed in TABLE I, the equipment in TABLE II. A water-filled Plexiglas tank 25 cm wide, 50 cm long, and 25 cm deep was used as the scattering medium. Exposures were made using the spot-film device and geometry shown in Figure 1. Field size was limited to 12 X 12 cm. Cassettes containing the screen-film combination under evaluation were then exposed at 10-kV intervals from 50 to 120 kVp. The tube current was kept constant; only the exposure time was varied to obtain the required optical density. Films at slightly different exposure times were obtained at each kVp step. Optical densities were measured with the Macbeth QuantaLog densitometer, and graphs of exposure time versus optical density were plotted for each kVp step. Exposure times required to yield an optical density of 1 were then interpolated. All of the data were obtained in one continuous run. The developer temperatures were monitored constantly; Table I:
Fig. 1. System geometry. All exposures were made using a spot-film technique.
and for each system studied, the films came from the same package and the same cassette and the same set of screens were used at each kVp step. Initially there was considerable variation in the results due to poor control of generator kVp. The kVp on the Table II:
Screen
Film Kodak RP Kodak RP Kodak X-omatic G Kodak RP 3-M Type XD 3-M Type XD
Cassette Halsey Halsey Kodak Kodak Halsey Halsey
Equipment Used
General Electric MSI-850 generator 3-PHASE, 12-PULSE Maxiray 100 tube 11 0 target High-speed rotor Nominal focal spot: 1.0 mm Tube current: 400 mA station Filtration 3-mm aluminum equivalent Grid-8: 1 linear Focused at 80 cm 4 Line jmm 0 Film processing-90-sec. processor at 95 F
Screen-Film Combinations Evaluated
Du Pont Par Du Pont Hi-Plus X-omatic Regular X-omatic Regular 3-M Alpha-4 3-M Alpha-8
u
NU
Wafer Wafer X-omatic X-omatic Wafer Wafer
1 From the Department of Radiology, University of Rochester School of Medicine and Dentistry, Rochester, N. Y. Accepted for publication 55 in August 1975.
711
712
J.
REYNOLDS AND OTHERS
MSI-850 generator is controlled through a servo motordriven powerstat, with both the fluoroscopic and the filming kVps derived from it. When changing from the fluoroscopic to the spot-film mode, the powerstat must change from the preset fluoroscopic kVp to the desired radiographic kVp within a certain time interval (0.7 sec. on these units). If the powerstat does not reach its preselected value within this interval, it stops, and the radiographic kVp will not be the desired value. The sensitivity of the powerstat drive mechanism is also dampened in order to prevent voltage overshoot. Initially it was found that accuracy was reproducible only to ±6 kVp for each reading.2 Since accurate kVp is crucial in an evaluation of the speed-kVp characteristics, a highvoltage bleeder was connected to the circuit and pre-
March 1976
screen with X-omatic type G film is slightly faster than the Par-RP system above 65 kVp; below this kVp, its speed falls off sharply. Replacing the type G film with RP film essentially doubles the relative speed. However, since the change in speed is determined primarily by the screen and not by the film characteristics, the same speed variation is also present in the X-omatic RegularRP film combination. The X-omatic Regular-RP and the X-omatic Regular-G curves are thus parallel to each other throughout the kVp range studied.
--.--,...,.- --ALPHA 8 ./'
./
/
3
/ /
1.0
/
/
-- -
-_/'"-7...c--- __ -------
2--
0.8
PLUS
HI
/
X-RP
,,/
c ,,/
0.6
",,"
w w 0.4
...... ... ClI
/ALPHA
/
~
/ 4
/
/
/
--- ---
I.O+-------::.L---:~=--_=_=::---------
0.9
"'0.2
0.8 /" "
" ""
"
"
PAR-RP
" 0.7 80
60
tOO
120
KYP
I
60
80
100
120
KVP
Fig. 2. KVp versus speed of the Par screen-RP film combination expressed on a nonlogarithmic basis. Since the same rnA was used for all exposures, speed can be expressed simply by the exposure time needed to produce an optical density of 1.0 on the film.
Fig. 3. Speed comparison of screen-film systems evaluated. For comparison purposes, the Par-RP system speed is taken as 1.0 and all other system speeds are expressed relative to it. TABLE I indicates the screen-film combinations shown here.
cise kVp readings were then adjusted for each exposure. The secondary waveforms from the bleeder were displayed on an oscilloscope, allowing both kVp and exposure time to be monitored constantly.
DISCUSSION
RESULTS
A typical curve for the Par-speed screen-RP film combination is shown in Figure 2. Since absolute values for speed are of little practical use, a relative value of 1 was adopted for the Par-RP combination. Other systems were then compared to the Par-RP system (Fig. 3). The Kodak X-omatic Regular screens, containing europium-activated barium strontium sulfate (1), show a considerable decrease in speed at the lower kVp levels compared to calcium tungstate. The X-omatic Regular 2 This voltage variation has been discussed with the manufacturer, who states that steps have been taken to correct it.
Demonstration of the decreased efficiency of barium lead sulfate screens at the lower kVp levels is not new; in 1955, Mattson evaluated the Kodak 80 screens consisting of barium lead sulfate and found them to be most efficient at 110-120 kVp (2). Their efficiency relative to calcium tungstate decreased at both higher and lower kVp values. Ovitt et at. stated that there is little difference in screen-film speed variation in the 60-105 kVp range (3). However, they used step-wedge exposures without any added scatter and expressed the speed differences in kV rather than exposure time (a 10-kV difference in speed at 60 kVp is much greater than a 10-kV difference at 105 kVp). The Hi-Plus screen-RP film speed response was essentially flat. This is as predicted, since the Hi-Plus
Vol. 118
713
SCREEN-FILM SPEED CHARACTERISTICS
screen also contains calcium tungstate as the phosphor. It is of interest that in the 70-90 kVp range, the X-omatic Regular-RP system is only slightly slower than the Hi-Plus-RP system. The two rare earth screens we studied both employed the same phosphors, with the Alpha-4 screen containing an additive to limit the lateral diffusion of light. As expected, the speed curves of the Alpha-4 and Alpha-8 screens are essentially parallel. The marked fall-off at the lower kVp levels is surprising; in fact, the Alpha-4-XD system exhibited a speed less than that of the Par-RP system below 50 kVp. The Alpha-8-XD system exhibited its predicted 4X speed advantage only at approximately 80 kVp and higher; at the lower kVp ranges it dropped considerably. This speed comparison was obtained using an approximation of a typical clinical setting: the relative speeds were measured only with a scatter medium. Measurements without a scatter medium would have been considerably different. Since scattered x-ray photons have a slightly lower energy level than that of nonscattered photons, scatter tends to lower the average keV of the system slightly. Magnification also plays .a role: with no magnification, even wide-angle scatter radiations can impinge upon the screen, leading to a lowering of the average keV; with magnification of 2-4X, scattered radiations over more than a very small angle fall outside the field of view of the screen, so that the average radiation energy reaching the screen is higher. For the present study, we elected to simulate typical rnaqniflcatlons and amounts of scatter encountered in a clinical setting using a relatively well collimated beam. We believe these results are more meaningful than those obtained without scatter and magnification. The front cover of the Kodak X-omatic cassette is made of an aluminum compound, the Halsey Wafer a magnesium compound. Simultaneous exposure of both types of screenless front covers showed that the alumi-
Radiation Physics
num cover absorbed approximately 12 % more radiation. Thus the X-omatic systems evaluated here are at a slightly greater speed disadvantage. However, it was elected to use the cassettes as shown in TABLE I. It is believed that this is more realistic; most radiologists probably would mount the Kodak X-omatic screens in the Kodak X-omatic cassettes; few would consider mounting the 3-M rare earth screens in the X-omatic cassettes. The present evaluation thus covers the ·cassette-screen-film system in a scatter medium simulating actual use rather than an isolated theoretical but unrealistic setting. CONCLUSION
Six different commercially available film-screen combinations were evaluated with respect to speed at various kVp levels. The efficiency of the rare earth screens and screens composed of barium strontium sulfate decreases at lower kVp ranges when compared to calcium tungstate screens. This decrease can become critical when subject contrast is being enhanced by the use of a lower kVp. J. Skucas, M.D. Department of Radiology University of Rochester School of Medicine and Dentistry Rochester. N. Y. 14642
REFERENCES 1. Buchanan RA: Trends in x-ray intensifying screens and image intensifiers. Presented to the Institute for Graphic Communication Conference on New Developments in Medical X-Ray Systems and Techniques, Ipswich, Mass., 17-19 Jun 1973 2. Mattsson 0: Practical photographic problems in radiography with special reference to high-voltage technique. Acta Radiol (Suppl
120): 1-206, 1955 3. Ovitt TW, Moore R, Amplatz K: The evaluation of high-speed screen-film combinations in angiography. Radiology 114:449-452, Feb 1975
Radiation Physics
Characteristics 1
J. Reynolds, M.D., J. Skucas, M.D., and J. Gorski, B.S. The relative speeds of six commercially available screen-film combinations were evaluated. There is a nonlinear relationship throughout the kVp range between the calcium tungstate, barium strontium sulfate, and rare earth screens. The speed of the last two types falls off considerably at lower kVp levels. INDEX TERMS:
Radiography, apparatus and equipment • Radiography, technique
Radiology 118:711-713, March 1976
•
• RESOLUTION of small, low-contrast objects is critical in many areas of radiology. Contrast can be improved by lowering the kVp, enhancing the resolution; Unfortunately, the resultant exposure time is thus prolonged, and consequent motion unsharpness degrades the image. Screens composed of different phosphor materials exhibit different speed characteristics throughout the diagnostic kVp range, and we have attempted a systematic evaluation of the speed characteristics of several screen-film combinations.
T
HE
FILM - - - -
~.;.......;,~--------r ~ o~
METHOD
Rather than study a theoretical nonscatter situation, we have attempted to duplicate actual clinical conditions. The screen-film combinations tested are listed in TABLE I, the equipment in TABLE II. A water-filled Plexiglas tank 25 cm wide, 50 cm long, and 25 cm deep was used as the scattering medium. Exposures were made using the spot-film device and geometry shown in Figure 1. Field size was limited to 12 X 12 cm. Cassettes containing the screen-film combination under evaluation were then exposed at 10-kV intervals from 50 to 120 kVp. The tube current was kept constant; only the exposure time was varied to obtain the required optical density. Films at slightly different exposure times were obtained at each kVp step. Optical densities were measured with the Macbeth QuantaLog densitometer, and graphs of exposure time versus optical density were plotted for each kVp step. Exposure times required to yield an optical density of 1 were then interpolated. All of the data were obtained in one continuous run. The developer temperatures were monitored constantly; Table I:
Fig. 1. System geometry. All exposures were made using a spot-film technique.
and for each system studied, the films came from the same package and the same cassette and the same set of screens were used at each kVp step. Initially there was considerable variation in the results due to poor control of generator kVp. The kVp on the Table II:
Screen
Film Kodak RP Kodak RP Kodak X-omatic G Kodak RP 3-M Type XD 3-M Type XD
Cassette Halsey Halsey Kodak Kodak Halsey Halsey
Equipment Used
General Electric MSI-850 generator 3-PHASE, 12-PULSE Maxiray 100 tube 11 0 target High-speed rotor Nominal focal spot: 1.0 mm Tube current: 400 mA station Filtration 3-mm aluminum equivalent Grid-8: 1 linear Focused at 80 cm 4 Line jmm 0 Film processing-90-sec. processor at 95 F
Screen-Film Combinations Evaluated
Du Pont Par Du Pont Hi-Plus X-omatic Regular X-omatic Regular 3-M Alpha-4 3-M Alpha-8
u
NU
Wafer Wafer X-omatic X-omatic Wafer Wafer
1 From the Department of Radiology, University of Rochester School of Medicine and Dentistry, Rochester, N. Y. Accepted for publication 55 in August 1975.
711
712
J.
REYNOLDS AND OTHERS
MSI-850 generator is controlled through a servo motordriven powerstat, with both the fluoroscopic and the filming kVps derived from it. When changing from the fluoroscopic to the spot-film mode, the powerstat must change from the preset fluoroscopic kVp to the desired radiographic kVp within a certain time interval (0.7 sec. on these units). If the powerstat does not reach its preselected value within this interval, it stops, and the radiographic kVp will not be the desired value. The sensitivity of the powerstat drive mechanism is also dampened in order to prevent voltage overshoot. Initially it was found that accuracy was reproducible only to ±6 kVp for each reading.2 Since accurate kVp is crucial in an evaluation of the speed-kVp characteristics, a highvoltage bleeder was connected to the circuit and pre-
March 1976
screen with X-omatic type G film is slightly faster than the Par-RP system above 65 kVp; below this kVp, its speed falls off sharply. Replacing the type G film with RP film essentially doubles the relative speed. However, since the change in speed is determined primarily by the screen and not by the film characteristics, the same speed variation is also present in the X-omatic RegularRP film combination. The X-omatic Regular-RP and the X-omatic Regular-G curves are thus parallel to each other throughout the kVp range studied.
--.--,...,.- --ALPHA 8 ./'
./
/
3
/ /
1.0
/
/
-- -
-_/'"-7...c--- __ -------
2--
0.8
PLUS
HI
/
X-RP
,,/
c ,,/
0.6
",,"
w w 0.4
...... ... ClI
/ALPHA
/
~
/ 4
/
/
/
--- ---
I.O+-------::.L---:~=--_=_=::---------
0.9
"'0.2
0.8 /" "
" ""
"
"
PAR-RP
" 0.7 80
60
tOO
120
KYP
I
60
80
100
120
KVP
Fig. 2. KVp versus speed of the Par screen-RP film combination expressed on a nonlogarithmic basis. Since the same rnA was used for all exposures, speed can be expressed simply by the exposure time needed to produce an optical density of 1.0 on the film.
Fig. 3. Speed comparison of screen-film systems evaluated. For comparison purposes, the Par-RP system speed is taken as 1.0 and all other system speeds are expressed relative to it. TABLE I indicates the screen-film combinations shown here.
cise kVp readings were then adjusted for each exposure. The secondary waveforms from the bleeder were displayed on an oscilloscope, allowing both kVp and exposure time to be monitored constantly.
DISCUSSION
RESULTS
A typical curve for the Par-speed screen-RP film combination is shown in Figure 2. Since absolute values for speed are of little practical use, a relative value of 1 was adopted for the Par-RP combination. Other systems were then compared to the Par-RP system (Fig. 3). The Kodak X-omatic Regular screens, containing europium-activated barium strontium sulfate (1), show a considerable decrease in speed at the lower kVp levels compared to calcium tungstate. The X-omatic Regular 2 This voltage variation has been discussed with the manufacturer, who states that steps have been taken to correct it.
Demonstration of the decreased efficiency of barium lead sulfate screens at the lower kVp levels is not new; in 1955, Mattson evaluated the Kodak 80 screens consisting of barium lead sulfate and found them to be most efficient at 110-120 kVp (2). Their efficiency relative to calcium tungstate decreased at both higher and lower kVp values. Ovitt et at. stated that there is little difference in screen-film speed variation in the 60-105 kVp range (3). However, they used step-wedge exposures without any added scatter and expressed the speed differences in kV rather than exposure time (a 10-kV difference in speed at 60 kVp is much greater than a 10-kV difference at 105 kVp). The Hi-Plus screen-RP film speed response was essentially flat. This is as predicted, since the Hi-Plus
Vol. 118
713
SCREEN-FILM SPEED CHARACTERISTICS
screen also contains calcium tungstate as the phosphor. It is of interest that in the 70-90 kVp range, the X-omatic Regular-RP system is only slightly slower than the Hi-Plus-RP system. The two rare earth screens we studied both employed the same phosphors, with the Alpha-4 screen containing an additive to limit the lateral diffusion of light. As expected, the speed curves of the Alpha-4 and Alpha-8 screens are essentially parallel. The marked fall-off at the lower kVp levels is surprising; in fact, the Alpha-4-XD system exhibited a speed less than that of the Par-RP system below 50 kVp. The Alpha-8-XD system exhibited its predicted 4X speed advantage only at approximately 80 kVp and higher; at the lower kVp ranges it dropped considerably. This speed comparison was obtained using an approximation of a typical clinical setting: the relative speeds were measured only with a scatter medium. Measurements without a scatter medium would have been considerably different. Since scattered x-ray photons have a slightly lower energy level than that of nonscattered photons, scatter tends to lower the average keV of the system slightly. Magnification also plays .a role: with no magnification, even wide-angle scatter radiations can impinge upon the screen, leading to a lowering of the average keV; with magnification of 2-4X, scattered radiations over more than a very small angle fall outside the field of view of the screen, so that the average radiation energy reaching the screen is higher. For the present study, we elected to simulate typical rnaqniflcatlons and amounts of scatter encountered in a clinical setting using a relatively well collimated beam. We believe these results are more meaningful than those obtained without scatter and magnification. The front cover of the Kodak X-omatic cassette is made of an aluminum compound, the Halsey Wafer a magnesium compound. Simultaneous exposure of both types of screenless front covers showed that the alumi-
Radiation Physics
num cover absorbed approximately 12 % more radiation. Thus the X-omatic systems evaluated here are at a slightly greater speed disadvantage. However, it was elected to use the cassettes as shown in TABLE I. It is believed that this is more realistic; most radiologists probably would mount the Kodak X-omatic screens in the Kodak X-omatic cassettes; few would consider mounting the 3-M rare earth screens in the X-omatic cassettes. The present evaluation thus covers the ·cassette-screen-film system in a scatter medium simulating actual use rather than an isolated theoretical but unrealistic setting. CONCLUSION
Six different commercially available film-screen combinations were evaluated with respect to speed at various kVp levels. The efficiency of the rare earth screens and screens composed of barium strontium sulfate decreases at lower kVp ranges when compared to calcium tungstate screens. This decrease can become critical when subject contrast is being enhanced by the use of a lower kVp. J. Skucas, M.D. Department of Radiology University of Rochester School of Medicine and Dentistry Rochester. N. Y. 14642
REFERENCES 1. Buchanan RA: Trends in x-ray intensifying screens and image intensifiers. Presented to the Institute for Graphic Communication Conference on New Developments in Medical X-Ray Systems and Techniques, Ipswich, Mass., 17-19 Jun 1973 2. Mattsson 0: Practical photographic problems in radiography with special reference to high-voltage technique. Acta Radiol (Suppl
120): 1-206, 1955 3. Ovitt TW, Moore R, Amplatz K: The evaluation of high-speed screen-film combinations in angiography. Radiology 114:449-452, Feb 1975
