•
Prospects for X-Ray Exposure Reduction Using Rare Earth Intensifying Screens 1
•
Radiation Physics
Robert F. Wagner, Ph.D., and Kenneth E. Weaver, M.S. 2 From measurements of screen-film sensitometry, Wiener spectra, and x-ray spectra, the speed, mottle, and quantum efficiency of several rare earth screen-film systems have been characterized. These systems offer exposure reductions of from two- to tenfold over conventional calcium tungstate systems. At the slower end, the noise (mottle) and resolution are comparable to what is available with conventional systems. Moreover, films designed to be used in such systems can be made with significantly lower silver content. As the speed advantage is increased from two- to tenfold, however, the mottle also increases. INDEX TERMS: Diagnostic Radiology. Films. Radiations, exposure in diagnostic procedures • Radiography, apparatus and equipment Radiology 118: 183-188, January 1976
• Table I: Relative Exposure Factors* for ScreenjFilm Combinations Used in This Study-Hardened Beam (A smaller number represents less exposure)
years ago, a new generation of intensifying screens was developed utilizing rare-earth phosphors and promising exposure reductions of two- to twentyfold (1, 2). During this time, several of these screen-film systems have become commercially available and offer exposure reductions of two- to tenfold. From the rate at which new systems are being announced, it would appear that the radiologist will soon be deluged with a much broader selection of system speeds. One of the problems with new systems concerns "quantum mottle." This is the noise, blotchiness, or random clustering of film grains caused by the fact that screen-film radiography is quantum limited-that is to say, the effect of trying to form a picture with a relatively small number of photons is apparent visually, and can be quantified in the laboratory (3-10). This effect is of considerable importance since it can mask diagnostic information in certain radiographic procedures (11, 12). This paper describes the results of a pilot study directed toward this and related questions conducted in the Medical Physics Section of the Division of Electronic Products at the Bureau of Radiological Health. The screen-film systems studied represent the first samples received by the Bureau in response to its announcement of a program to study the new systems and their possibilities for reducing patient exposure in medical radiography (13, 14). The results of this study allow some conclusions to be drawn as to the possibilities for exposure reduction in general, and, in particular, with reference to currently acceptable levels of noise or quantum mottle. EVERAL
S
System
Relative Exposure Factor
PARjRP-54 TF-2jRP-54 DetailjRP-54 Alpha 8jXM Alpha 4jXM Alpha 8jXD Alpha 4jXD La-Exp'ljRP-54 Alpha 4jRP-54
1.00 0.57 3.97 0.10 0.16 0.26 0.42 0.52 0.74
* (1.0 net 0.0.); 80 kVp, 3.2 mm AI filtration; 19 mm AI added absorber.
Table II: Relative Exposure Factors* for ScreenjFilm Combinations Used in This Study-Soft Beam System
Relative Exposure Factor
PARjRP-54 TF-2jRP-54 OetailjRP-54 Alpha 8jXM Alpha 4jXM Alpha 8/XO Alpha 4jXO La-Exp'ljRP-54 Alpha 4/RP-54
1.00 0.59 3.30 0.14 0.25 0.37 0.63 0.66 1.10
* (1.0 net 0.0.); 80 kVp, 3.2 mm AI filtration; no added absorber. um tungstate screens-DuPont Par, Radelin TF-2, and DuPont Detail-all with Kodak RP film. These are familiar systems that can be used as points of reference for the newer ones. The rare earth screen-film systems are the 3M Trimax systems: Alpha 4 and Alpha 8 screens, with XM and XD film; experimental lanthanum screens used with Kodak RP film; and a system utilizing Alpha 4 screens and Kodak RP film.
SPEED AND NOISE MEASUREMENTS
In this study, we began with three conventional calci-
1 From the Medical Physics Section, Ionizing Radiation Branch, Division of Electronic Products, Bureau of Radiological Health, Food and Drug Administration, Rockville, Md. 20852. Accepted for publication in August 1975. The mention of commercial products herein is not to be construed as either an actual or implied endorsement of such products by the Department of Health, Education and Welfare. 2 Present address: Department of Radiology, George Washington University, Washington, D. C. 20038. shan
183
ROBERT F. WAGNER AND KENNETH E. WEAVER
184
Table III:
0.4
PAR
/
(......../mm)-I
0.2 /
0.1
OETAIL
RP-54
0.04 0.4
1.0 SPATIAL
2.0
4.0
10.
Scanning Parameters for Wiener Spectra
5 X 824,u effective scanning aperture (25X magnification; 0.1 N.A.) 1 polnt/LOu: 10,000 points per scan (repeated 4 times; quadruplets averaged) Smoothing in pairs Local mean calculated over 10mm neighborhood Autocovariance calculated up to 256 lags "Hanning" window Bandwidth > 0.45 cycles/rum (FWHM) Finite Fourier transform Correct for smoothing and aperture degradation (The above is repeated 10 times and results averaged, then repeated for another sample.)
1.0
Prospects for X-Ray Exposure Reduction Using Rare Earth Intensifying Screens 1
•
Radiation Physics
Robert F. Wagner, Ph.D., and Kenneth E. Weaver, M.S. 2 From measurements of screen-film sensitometry, Wiener spectra, and x-ray spectra, the speed, mottle, and quantum efficiency of several rare earth screen-film systems have been characterized. These systems offer exposure reductions of from two- to tenfold over conventional calcium tungstate systems. At the slower end, the noise (mottle) and resolution are comparable to what is available with conventional systems. Moreover, films designed to be used in such systems can be made with significantly lower silver content. As the speed advantage is increased from two- to tenfold, however, the mottle also increases. INDEX TERMS: Diagnostic Radiology. Films. Radiations, exposure in diagnostic procedures • Radiography, apparatus and equipment Radiology 118: 183-188, January 1976
• Table I: Relative Exposure Factors* for ScreenjFilm Combinations Used in This Study-Hardened Beam (A smaller number represents less exposure)
years ago, a new generation of intensifying screens was developed utilizing rare-earth phosphors and promising exposure reductions of two- to twentyfold (1, 2). During this time, several of these screen-film systems have become commercially available and offer exposure reductions of two- to tenfold. From the rate at which new systems are being announced, it would appear that the radiologist will soon be deluged with a much broader selection of system speeds. One of the problems with new systems concerns "quantum mottle." This is the noise, blotchiness, or random clustering of film grains caused by the fact that screen-film radiography is quantum limited-that is to say, the effect of trying to form a picture with a relatively small number of photons is apparent visually, and can be quantified in the laboratory (3-10). This effect is of considerable importance since it can mask diagnostic information in certain radiographic procedures (11, 12). This paper describes the results of a pilot study directed toward this and related questions conducted in the Medical Physics Section of the Division of Electronic Products at the Bureau of Radiological Health. The screen-film systems studied represent the first samples received by the Bureau in response to its announcement of a program to study the new systems and their possibilities for reducing patient exposure in medical radiography (13, 14). The results of this study allow some conclusions to be drawn as to the possibilities for exposure reduction in general, and, in particular, with reference to currently acceptable levels of noise or quantum mottle. EVERAL
S
System
Relative Exposure Factor
PARjRP-54 TF-2jRP-54 DetailjRP-54 Alpha 8jXM Alpha 4jXM Alpha 8jXD Alpha 4jXD La-Exp'ljRP-54 Alpha 4jRP-54
1.00 0.57 3.97 0.10 0.16 0.26 0.42 0.52 0.74
* (1.0 net 0.0.); 80 kVp, 3.2 mm AI filtration; 19 mm AI added absorber.
Table II: Relative Exposure Factors* for ScreenjFilm Combinations Used in This Study-Soft Beam System
Relative Exposure Factor
PARjRP-54 TF-2jRP-54 OetailjRP-54 Alpha 8jXM Alpha 4jXM Alpha 8/XO Alpha 4jXO La-Exp'ljRP-54 Alpha 4/RP-54
1.00 0.59 3.30 0.14 0.25 0.37 0.63 0.66 1.10
* (1.0 net 0.0.); 80 kVp, 3.2 mm AI filtration; no added absorber. um tungstate screens-DuPont Par, Radelin TF-2, and DuPont Detail-all with Kodak RP film. These are familiar systems that can be used as points of reference for the newer ones. The rare earth screen-film systems are the 3M Trimax systems: Alpha 4 and Alpha 8 screens, with XM and XD film; experimental lanthanum screens used with Kodak RP film; and a system utilizing Alpha 4 screens and Kodak RP film.
SPEED AND NOISE MEASUREMENTS
In this study, we began with three conventional calci-
1 From the Medical Physics Section, Ionizing Radiation Branch, Division of Electronic Products, Bureau of Radiological Health, Food and Drug Administration, Rockville, Md. 20852. Accepted for publication in August 1975. The mention of commercial products herein is not to be construed as either an actual or implied endorsement of such products by the Department of Health, Education and Welfare. 2 Present address: Department of Radiology, George Washington University, Washington, D. C. 20038. shan
183
ROBERT F. WAGNER AND KENNETH E. WEAVER
184
Table III:
0.4
PAR
/
(......../mm)-I
0.2 /
0.1
OETAIL
RP-54
0.04 0.4
1.0 SPATIAL
2.0
4.0
10.
Scanning Parameters for Wiener Spectra
5 X 824,u effective scanning aperture (25X magnification; 0.1 N.A.) 1 polnt/LOu: 10,000 points per scan (repeated 4 times; quadruplets averaged) Smoothing in pairs Local mean calculated over 10mm neighborhood Autocovariance calculated up to 256 lags "Hanning" window Bandwidth > 0.45 cycles/rum (FWHM) Finite Fourier transform Correct for smoothing and aperture degradation (The above is repeated 10 times and results averaged, then repeated for another sample.)
1.0
