Radiation Physics
Screen/Film System Speed: Its Dependence on X-Ray Energy 1 Carl J. Vyborny, Ph.D.,2 Charles E. Metz, Ph.D., Kunio Doi, Ph.D., and Kurt Rossmann, Ph.D. 3 Dependence of the speed of various screen/film systems on x-ray energy was studied using the nearly monoenergetic x rays emitted by a filtered fluorescent source. The results show that response depends on screen phosphor composition and thickness. Barium and rare earth screens having K absorption energies lower than that of calcium tungstate are relatively more sensitive to x rays in the 40-70-keV region. INDEX TERM:
Radiography, apparatus and equipment
Radiology 125:811-816, December 1977
of incident x-ray energy absorbed by a radiographic intensifying screen is strongly dependent on the energy of the incident photons, with the nature of the dependence determined primarily by the elemental composition of the phosphor. The speeds of radiographic screen/film systems containing different phosphor materials therefore vary with x-ray energy in different ways: for example, the speed of a calcium tungstate system changes with photon energy in quite a different manner from that of a rare earth system. The relative speeds of different screen/film systems have almost always been measured by exposing them to the broad x-ray spectra emitted by a conventional x-ray tube (1, 2); however, this means that the response is averaged over the wide range of photon energies in the incident spectrum and does not clarify the exact relationship between the incident energy and the speed of the system. Calculations of the amount of energy attenuated or absorbed by x-ray phosphors can be used to estimate this dependence, but their usefulness is limited by the complex nature of the conversion of absorbed energy to film density, and the validity of these calculations in making precise predictions of system response has not been verified by experimental results. We have undertaken measurements to determine the relative speeds of various screen/film systems directly as a function of incident x-ray energy. A strong source of fluorescent x rays was used to study the relative number of photons needed to give a photographic density of 1.00 at different energies in a variety of screen/film systems. The results provide a detailed description of the energydependent response of systems using screens of widely different compositions.
T
TABLE
HE FRACTION
I:
SCREEN/FILM SYSTEMS
Screen
Screen Composition
Par Speed Hi-Plus Lightning-Plus X-Omatic Regular Trimax Alpha 4 Lanex Regular Lightning-Plus Trimax Alpha 4
CaW04 CaW04 CaW0 4 (0.75 Ba, 0.25 Sr)S04 (La202S) + 3(Gd20 2S) (La202S) + 3.6( Gd20 2S) CaW0 4 (La202S) + 3(Gd202S)
Film RP/S RP/S RP/S RP/S RP/S RP/S RP
XD
Film Spectral Sensitivity (Range) Blue-UV light Blue-UV light Blue-UV light Blue-UV light Blue-UV light Blue-UV light Blue-UV light Green-UV light
phosphor thicknesses of these screens are roughly 1:2:3, which allowed us to investigate the effect of phosphor thickness as well as composition. The other three screens-Kodak X-Omatic Regular, Kodak Lanex Regular, and 3M Trimax Alpha 4-were selected to provide a wide range of x-ray absorption properties. X-Omatic Regular screens contain barium strontium sulfate, whereas Lanex Regular and Alpha 4 are composed of some lanthanum oxysulfide and a much larger amount of gadolinium oxysulfide. The x-ray absorption properties of radiographic screens are strongly influenced by the K absorption edge energies of their high-atomic-number phosphor elements; these energies are 69 keV for tungsten, 37 keV for barium, and 50 keV for gadolinium. Kodak RP/S film was used with all six screens in order to provide a basis for comparison. This is a blue-sensitive film which is commonly used with calcium tungstate screens but is also moderately sensitive to the light emitted by the other phosphors. It is also a relatively fast film and was chosen so that the fluorescent fluence needed in the experiments could be generated within the loading capacity of the primary x-ray tube. In addition, two other films were selected, and each was used with one screen pair so that we could study the effect of the film on system sensitivity as a function of x-ray energy. One film, Kodak RP, was used with the Lightning-Plus screens; the other, 3M XD, was used with the Alpha 4 screens. Some of the
METHOD
Six different screen pairs were chosen for study. Three of them-Du Pont Par Speed, Hi-Plus, and LightningPlus-have calcium tungstate phosphors. The relative
1 From the Center for Radiologic Image Research, Department of Radiology, University of Chicago, and the Franklin McLean Memorial Research Institute (operated by the University of Chicago for the U.S. Energy Research and Development Administration), Chicago, III. Accepted for publication in July 1977. 2 Supported in part by training grant CA-05132 from the National Cancer Institute. 3 Deceased. sjh
811
812
CARL J. VYBORNY AND OTHERS
SCREEN- FILM CASSETTE (REMOVABLE)
I
w
FLUORESCENT
~FILTERI~TA~~ET J'
SODIUM IODIDE DETECTOR ASSEMBLY
Fig. 1.
X-RAY TUBE
LEAD SHIELD
4__----
-25cm .... -5 0cm 240cm' - - - _
Schematic diagram of the experimental arrangement (viewed from above).
properties of the various systems are listed in TABLE I; the ratios of high-Z-phosphor elements in the screens were determined from attenuation measurements (3). The x-ray source, which has been described in detail elsewhere (4), consisted of a diagnostic x-ray tube and a set of fluorescent targets of different elemental compositions (Fig. 1). The x rays emitted by the target were filtered to preferentially remove the K{j portion of the spectrum. Figure 2 shows the fluorescent spectrum from a dysprosium target before and after K,B filtration by a samarium filter; these spectra, which were measured with an intrinsic germanium detector, illustrate the highly monoenergetic nature of the x rays used. Target/filter combinations are listed in TABLE II together with the fractional contributions of target Ka and K; + K{j x rays to the total photon fluence rates of the individual beams. The screen/film system being studied was placed in a small vacuum cassette 50 cm from the target and exposed for one second with the largely monoenergetic fluence from the target/filter arrangement. This exposure was designed to yield a film density near 1.00 by means of adjustments of the primary tube current, or, within the kVp range for which the Ka x-ray purity of the exposing beam was nearly maximum (4), the primary tube kllovoltaqe. The cassette was then removed and the exposure repeated with a thin-window sodium iodide crystal recording the x-ray fluence. This process was repeated at least three times for each target and screen/film system with slightly different primary tube settings. After one system had been studied at all fluorescent energies, the experiment was repeated for the next system using the same arrangement. Exposed films were developed by hand for six minutes in TABLE II: Fluorescent Target Material SrO Mo Ag Te La Pr Sm Dy Er Vb W PI Au TI Pb
Filter Material
Nb Pd Sn BaO BaO Ce
Sm Dy Dy
HfH2 W W Pt Pt
December 1977
o z
Dy TARGET
:::>
NO FILTRATION
w
-J LL..
Z
o ~ o I a.. w
>
~
«
-J
w
a::
PHOTON ENERGY w u z w
Dy TARGET
.35mm Sm FILTER
:::>
...J
LL..
Z
o ~ o
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a.. w
>
~
« ...J
w a::
PHOTON ENERGY Fig. 2. Fluorescent x-ray spectrum from a dysprosium target before (above) and after (below) filtration for preferential removal of Kfj x rays. The spectra were measured with an intrinsic germanium detector and are normalized to the same Ka peak height.
Kodak KRX fluid. The developer tank was located in a water bath with temperature automatically maintained at 0 21 C (70 0 F) ± 0.05 0 , thereby providing reproducible development condltlons.
SPECTRAL PURITIES OBTAINED WITH THE FILTERED TARGET ARRANGEMENT Filter Thickness (mg/cm2)
Average Ka Energy (keV)
Ka Purity
Ka + K{j Purity
45 60 175 180 180 240 270 300 300 350 480 480 270 270
14.1 17.4 22.1 27.3 33.2 35.8 39.8 45.6 48.6 51.8 58.7 66.0 67.9 71.8 73.9
0.93 0.95 0.94 0.93 0.93 0.95 0.96 0.96 0.95 0.89 0.91 0.90 0.85 0.84
0.94 0.94 0.97 0.94 0.95 0.95 0.97 0.98 0.98 0.97 0.94 0.93 0.92 0.90 0.89
5
---1---
5
--f-- x -
Hi - Plus - RP/S
Radiation Physics
Omotic Regulor-RP/S
4
4
w
lJJ
u z w 3
u
....J lJ....
....J lJ....
~3
::::>
::::> Z
Z
o ~ o ~
813
SCREEN/FILM SYSTEM SPEED
Vol. 125
f2 o
~2
2
w
lJJ
~
>
«
....J lJJ
....J W
Bo Kedge
W Kedge
20
30
40
50
60
70
1 L . - _ - L - _ - - l . . . _ - - - I - . l . . . - - _ - - J - . _ - - - - ' -_ _....I...-_---'
30
20
10
80
Fig. 3. Relative x-ray fluence needed to give a density of 1.00 in the Hi-Plus-RP IS system as a function of x-ray energy.
A given measurement provided the relative number of x-ray photons needed to produce the density measured on the film. These results were then converted to the relative x-ray fluence needed to give a density of 1.00 in the screen/film system using the H & D curve. In order to obtain the relative Ka fluence required for this density, it was necessary to make small corrections based on the known spectral impurities in the fluorescent beams (5). The final result for a particular system at a given K; energy was taken to be the average of the experimental trials. Since every fluorescent exposure was of equal duration, adjustments due to failure of the reciprocity law were not required.
40
50
60
80
70
PHOTON ENERGY (keV)
PHOTON ENERGY (keV)
Fig. 4. Relative x-ray fluence needed to give a density of 1.00 in the X-Omatic Regular-RP/S system as a function of x-ray energy.
5
--~--- Lonex
h
Regulor-RP/S
\ \
4
\
\
lJJ
\
U Z
\
\
4
~ 3
\
....J lJ....
\
Z
o ~ o
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IAI
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~
/'
II lI I
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l~........
~
~
__ IS"-
-~-------
....J
w
a::
RESULTS
Gd Kedge 1'--~----'~-~--"--....L..----'-----'
The relative x-ray fluence needed to give a density of 1.00 in the Hi-Plus-RP/S system as a function of x-ray energy is shown in Figure 3. The data points correspond to the average Ka energies at which the measurements described in the last section were made; the curve was fitted to these points by eye. Plotted in this manner, the results reflect the inverse of the speed or sensitivity of the system as a function of x-ray energy, since it takes a larger fluence to give a density of 1.00 in slower systems. The graph shows a large increase in required fluence for the Hi-Plus system at low x-ray energies, and this was seen in the other systems as well. At lower x-ray energies, nearly all of the photons incident upon a screen/film system are absorbed; thus it takes more x rays to deposit the same amount of energy in the screens. The fact that the required fluence also increases at higher energies in the Hi-Plus system is due to the incomplete absorption of incident x rays by the screens. At the tungsten K-edge energy level, however, there is a large drop in required fluence, i.e., a large increase in system speed, resulting from
10
20
30
40
50
60
70
80
PHOTON ENERGY (keV) Fig. 5. Relative x-ray fluence needed to give a density of 1.00 in the Lanex Regular-RP/S system as a function of x-ray energy.
greater photoelectric absorption by the tungsten atoms in the screen. Figure 4 shows the results obtained for the X-Omatic Regular system; the fluence scale is again relative and is normalized separately from those of the other systems. The relationship between energy and speed is very different from that of the Hi-Plus system: this was to be expected, as the X-Omatic system uses barium as the principal phosphor element, giving these screens a large absorption edge just above 37 keV. The X-Omatic system is thus relatively more sensitive to x rays in the 37-69-keV range, which (for example) comprise the bulk of the higher-energy components in an 80-kVp diagnostic spectrum. The Lanex Regular system (Fig. 5) contains the third type
814
CARL
J.
VVBORNV AND OTHERS
December 1977
,
5
---1---
5
Par - RP/S
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---1--2
\
\
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0
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Screen/Film System Speed: Its Dependence on X-Ray Energy 1 Carl J. Vyborny, Ph.D.,2 Charles E. Metz, Ph.D., Kunio Doi, Ph.D., and Kurt Rossmann, Ph.D. 3 Dependence of the speed of various screen/film systems on x-ray energy was studied using the nearly monoenergetic x rays emitted by a filtered fluorescent source. The results show that response depends on screen phosphor composition and thickness. Barium and rare earth screens having K absorption energies lower than that of calcium tungstate are relatively more sensitive to x rays in the 40-70-keV region. INDEX TERM:
Radiography, apparatus and equipment
Radiology 125:811-816, December 1977
of incident x-ray energy absorbed by a radiographic intensifying screen is strongly dependent on the energy of the incident photons, with the nature of the dependence determined primarily by the elemental composition of the phosphor. The speeds of radiographic screen/film systems containing different phosphor materials therefore vary with x-ray energy in different ways: for example, the speed of a calcium tungstate system changes with photon energy in quite a different manner from that of a rare earth system. The relative speeds of different screen/film systems have almost always been measured by exposing them to the broad x-ray spectra emitted by a conventional x-ray tube (1, 2); however, this means that the response is averaged over the wide range of photon energies in the incident spectrum and does not clarify the exact relationship between the incident energy and the speed of the system. Calculations of the amount of energy attenuated or absorbed by x-ray phosphors can be used to estimate this dependence, but their usefulness is limited by the complex nature of the conversion of absorbed energy to film density, and the validity of these calculations in making precise predictions of system response has not been verified by experimental results. We have undertaken measurements to determine the relative speeds of various screen/film systems directly as a function of incident x-ray energy. A strong source of fluorescent x rays was used to study the relative number of photons needed to give a photographic density of 1.00 at different energies in a variety of screen/film systems. The results provide a detailed description of the energydependent response of systems using screens of widely different compositions.
T
TABLE
HE FRACTION
I:
SCREEN/FILM SYSTEMS
Screen
Screen Composition
Par Speed Hi-Plus Lightning-Plus X-Omatic Regular Trimax Alpha 4 Lanex Regular Lightning-Plus Trimax Alpha 4
CaW04 CaW04 CaW0 4 (0.75 Ba, 0.25 Sr)S04 (La202S) + 3(Gd20 2S) (La202S) + 3.6( Gd20 2S) CaW0 4 (La202S) + 3(Gd202S)
Film RP/S RP/S RP/S RP/S RP/S RP/S RP
XD
Film Spectral Sensitivity (Range) Blue-UV light Blue-UV light Blue-UV light Blue-UV light Blue-UV light Blue-UV light Blue-UV light Green-UV light
phosphor thicknesses of these screens are roughly 1:2:3, which allowed us to investigate the effect of phosphor thickness as well as composition. The other three screens-Kodak X-Omatic Regular, Kodak Lanex Regular, and 3M Trimax Alpha 4-were selected to provide a wide range of x-ray absorption properties. X-Omatic Regular screens contain barium strontium sulfate, whereas Lanex Regular and Alpha 4 are composed of some lanthanum oxysulfide and a much larger amount of gadolinium oxysulfide. The x-ray absorption properties of radiographic screens are strongly influenced by the K absorption edge energies of their high-atomic-number phosphor elements; these energies are 69 keV for tungsten, 37 keV for barium, and 50 keV for gadolinium. Kodak RP/S film was used with all six screens in order to provide a basis for comparison. This is a blue-sensitive film which is commonly used with calcium tungstate screens but is also moderately sensitive to the light emitted by the other phosphors. It is also a relatively fast film and was chosen so that the fluorescent fluence needed in the experiments could be generated within the loading capacity of the primary x-ray tube. In addition, two other films were selected, and each was used with one screen pair so that we could study the effect of the film on system sensitivity as a function of x-ray energy. One film, Kodak RP, was used with the Lightning-Plus screens; the other, 3M XD, was used with the Alpha 4 screens. Some of the
METHOD
Six different screen pairs were chosen for study. Three of them-Du Pont Par Speed, Hi-Plus, and LightningPlus-have calcium tungstate phosphors. The relative
1 From the Center for Radiologic Image Research, Department of Radiology, University of Chicago, and the Franklin McLean Memorial Research Institute (operated by the University of Chicago for the U.S. Energy Research and Development Administration), Chicago, III. Accepted for publication in July 1977. 2 Supported in part by training grant CA-05132 from the National Cancer Institute. 3 Deceased. sjh
811
812
CARL J. VYBORNY AND OTHERS
SCREEN- FILM CASSETTE (REMOVABLE)
I
w
FLUORESCENT
~FILTERI~TA~~ET J'
SODIUM IODIDE DETECTOR ASSEMBLY
Fig. 1.
X-RAY TUBE
LEAD SHIELD
4__----
-25cm .... -5 0cm 240cm' - - - _
Schematic diagram of the experimental arrangement (viewed from above).
properties of the various systems are listed in TABLE I; the ratios of high-Z-phosphor elements in the screens were determined from attenuation measurements (3). The x-ray source, which has been described in detail elsewhere (4), consisted of a diagnostic x-ray tube and a set of fluorescent targets of different elemental compositions (Fig. 1). The x rays emitted by the target were filtered to preferentially remove the K{j portion of the spectrum. Figure 2 shows the fluorescent spectrum from a dysprosium target before and after K,B filtration by a samarium filter; these spectra, which were measured with an intrinsic germanium detector, illustrate the highly monoenergetic nature of the x rays used. Target/filter combinations are listed in TABLE II together with the fractional contributions of target Ka and K; + K{j x rays to the total photon fluence rates of the individual beams. The screen/film system being studied was placed in a small vacuum cassette 50 cm from the target and exposed for one second with the largely monoenergetic fluence from the target/filter arrangement. This exposure was designed to yield a film density near 1.00 by means of adjustments of the primary tube current, or, within the kVp range for which the Ka x-ray purity of the exposing beam was nearly maximum (4), the primary tube kllovoltaqe. The cassette was then removed and the exposure repeated with a thin-window sodium iodide crystal recording the x-ray fluence. This process was repeated at least three times for each target and screen/film system with slightly different primary tube settings. After one system had been studied at all fluorescent energies, the experiment was repeated for the next system using the same arrangement. Exposed films were developed by hand for six minutes in TABLE II: Fluorescent Target Material SrO Mo Ag Te La Pr Sm Dy Er Vb W PI Au TI Pb
Filter Material
Nb Pd Sn BaO BaO Ce
Sm Dy Dy
HfH2 W W Pt Pt
December 1977
o z
Dy TARGET
:::>
NO FILTRATION
w
-J LL..
Z
o ~ o I a.. w
>
~
«
-J
w
a::
PHOTON ENERGY w u z w
Dy TARGET
.35mm Sm FILTER
:::>
...J
LL..
Z
o ~ o
J:
a.. w
>
~
« ...J
w a::
PHOTON ENERGY Fig. 2. Fluorescent x-ray spectrum from a dysprosium target before (above) and after (below) filtration for preferential removal of Kfj x rays. The spectra were measured with an intrinsic germanium detector and are normalized to the same Ka peak height.
Kodak KRX fluid. The developer tank was located in a water bath with temperature automatically maintained at 0 21 C (70 0 F) ± 0.05 0 , thereby providing reproducible development condltlons.
SPECTRAL PURITIES OBTAINED WITH THE FILTERED TARGET ARRANGEMENT Filter Thickness (mg/cm2)
Average Ka Energy (keV)
Ka Purity
Ka + K{j Purity
45 60 175 180 180 240 270 300 300 350 480 480 270 270
14.1 17.4 22.1 27.3 33.2 35.8 39.8 45.6 48.6 51.8 58.7 66.0 67.9 71.8 73.9
0.93 0.95 0.94 0.93 0.93 0.95 0.96 0.96 0.95 0.89 0.91 0.90 0.85 0.84
0.94 0.94 0.97 0.94 0.95 0.95 0.97 0.98 0.98 0.97 0.94 0.93 0.92 0.90 0.89
5
---1---
5
--f-- x -
Hi - Plus - RP/S
Radiation Physics
Omotic Regulor-RP/S
4
4
w
lJJ
u z w 3
u
....J lJ....
....J lJ....
~3
::::>
::::> Z
Z
o ~ o ~
813
SCREEN/FILM SYSTEM SPEED
Vol. 125
f2 o
~2
2
w
lJJ
~
>
«
....J lJJ
....J W
Bo Kedge
W Kedge
20
30
40
50
60
70
1 L . - _ - L - _ - - l . . . _ - - - I - . l . . . - - _ - - J - . _ - - - - ' -_ _....I...-_---'
30
20
10
80
Fig. 3. Relative x-ray fluence needed to give a density of 1.00 in the Hi-Plus-RP IS system as a function of x-ray energy.
A given measurement provided the relative number of x-ray photons needed to produce the density measured on the film. These results were then converted to the relative x-ray fluence needed to give a density of 1.00 in the screen/film system using the H & D curve. In order to obtain the relative Ka fluence required for this density, it was necessary to make small corrections based on the known spectral impurities in the fluorescent beams (5). The final result for a particular system at a given K; energy was taken to be the average of the experimental trials. Since every fluorescent exposure was of equal duration, adjustments due to failure of the reciprocity law were not required.
40
50
60
80
70
PHOTON ENERGY (keV)
PHOTON ENERGY (keV)
Fig. 4. Relative x-ray fluence needed to give a density of 1.00 in the X-Omatic Regular-RP/S system as a function of x-ray energy.
5
--~--- Lonex
h
Regulor-RP/S
\ \
4
\
\
lJJ
\
U Z
\
\
4
~ 3
\
....J lJ....
\
Z
o ~ o
\
\
\
~\
I:
a. 2
Iff
IAI
" ''6__ ,6f l~···/
~
/'
II lI I
lJJ
> «
l~........
~
~
__ IS"-
-~-------
....J
w
a::
RESULTS
Gd Kedge 1'--~----'~-~--"--....L..----'-----'
The relative x-ray fluence needed to give a density of 1.00 in the Hi-Plus-RP/S system as a function of x-ray energy is shown in Figure 3. The data points correspond to the average Ka energies at which the measurements described in the last section were made; the curve was fitted to these points by eye. Plotted in this manner, the results reflect the inverse of the speed or sensitivity of the system as a function of x-ray energy, since it takes a larger fluence to give a density of 1.00 in slower systems. The graph shows a large increase in required fluence for the Hi-Plus system at low x-ray energies, and this was seen in the other systems as well. At lower x-ray energies, nearly all of the photons incident upon a screen/film system are absorbed; thus it takes more x rays to deposit the same amount of energy in the screens. The fact that the required fluence also increases at higher energies in the Hi-Plus system is due to the incomplete absorption of incident x rays by the screens. At the tungsten K-edge energy level, however, there is a large drop in required fluence, i.e., a large increase in system speed, resulting from
10
20
30
40
50
60
70
80
PHOTON ENERGY (keV) Fig. 5. Relative x-ray fluence needed to give a density of 1.00 in the Lanex Regular-RP/S system as a function of x-ray energy.
greater photoelectric absorption by the tungsten atoms in the screen. Figure 4 shows the results obtained for the X-Omatic Regular system; the fluence scale is again relative and is normalized separately from those of the other systems. The relationship between energy and speed is very different from that of the Hi-Plus system: this was to be expected, as the X-Omatic system uses barium as the principal phosphor element, giving these screens a large absorption edge just above 37 keV. The X-Omatic system is thus relatively more sensitive to x rays in the 37-69-keV range, which (for example) comprise the bulk of the higher-energy components in an 80-kVp diagnostic spectrum. The Lanex Regular system (Fig. 5) contains the third type
814
CARL
J.
VVBORNV AND OTHERS
December 1977
,
5
---1---
5
Par - RP/S
4\
---1--2
\
\
4 ;I'~
w
/
~
/
,6
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