Thickness Of Phosphor Screen Research Articles

SU‐FF‐I‐74: A Modulation Transfer Function Comparison of Dual‐Screen CR Systems

Purpose: The goal of this study is to measure MTF of two dual‐screen CR systems, and to explain the correlation between their system resolution and the physical properties of phosphor screens. Method and Materials: Modulation transfer function (MTF) is commonly used method to characterize the performance of an imaging system. The well‐established edge method is used to quantify the MTF of digital radiographic systems. We use a sharply polished edge slanted with respect to sampling detector grid, which allows measurement of oversampled edge function profiles. The focus of this study is on the comparison of the presampled MTFs obtained for different thicknesses of phosphor screens and pixel sizes. MTFs were measured with both diagnostic and 6 MV x‐rays. The storage phosphors used in this study were two commercially available CR plates: Kodak EC‐L fast and Kodak EC‐L regular cassettes. Results: The scan times per pixel for both high (0.171 mm) and low (0.342 mm) resolution are much shorter than 558 μs of the dominant luminescence lifetime for , used in this study. Consequently, the spatial resolution of KODAK 2000RT CR system is lower in the scan direction than in the moving direction due to the afterglow effect. A comparison of the MTFs with Kodak EC‐L fast and regular cassettes indicates that the spatial resolution of dual‐screen CR systems does not depend on the total thickness of phosphor screens but is determined by the thickness of the thicker phosphor screen in the dual‐screen CR system. Conclusion: MTFs in laser scan direction roll off faster than in phosphor transport direction with KODAK 2000RT CR system. The resolution is related to the thickness of individual phosphors layers.

Experimental investigation of the optical signal, gain, signal-to-noise ratio and information content characteristics of X-ray phosphor screens

In this study, the quality of medical images produced by X-ray phosphor screens is described by a model based on the light generation and emission properties of the phosphor material. Combined detector gain (CDG), modulation transfer function (MTF), detective quantum efficiency (DQE) and information capacity (IC) of the screens were expressed and evaluated as functions of emitted optical fluence, incident X-ray fluence and emitted optical spectrum. Phosphor screens with coating thickness ranging from 21 mg/cm2 to 137 mg/cm2 were prepared in the laboratory and were irradiated by X-rays with X-ray voltages from 50 to 140 kVp. Experimental data were obtained in both transmission and reflection modes of measurement (light emission from both screen sides). Results showed that most image quality parameters (CDG, MTF, DQE) depend strongly on phosphor screen thickness. CDG and DQE also depend on X-ray tube voltage. However, the total quantity of information (IC), which depends on both the incident X-ray fluence and phosphor material type, was not found to be significantly affected by phosphor thickness.

Vacuum ultraviolet and x-ray luminescence efficiencies of Y3Al5O12:Ce phosphor screens

Phosphor screens are used in many applications to convert incident radiation to visible wavelengths that can be readily measured by modern detectors. For many such systems it is necessary to tailor the phosphor screen thickness to the wavelength range of interest in order to maximize the efficiency. Here, we describe the experiments made to determine the luminescent efficiency of thin Y3Al5O12:Ce (P-46) phosphor screens (1–22 mg cm−2) in the vacuum ultraviolet and x-ray ranges. We use current and previous measurements to obtain fitting parameters for a granular unidimensional radiation transfer model developed. In the model, we take into account the effects of scattering and absorption of the luminescent light emitted and we discuss the effects of grain size and shape on the model. Finally, we find that a screen thickness of ∼1.7 mg cm−2 is optimum for incident radiation between 1.2 and 124 nm but this thickness becomes wavelength dependent at shorter wavelengths (0.025–1.2 nm).

Monte Carlo studies of metal/phosphor screen in therapeutic X-ray imaging

In order to provide the optimal design parameters, we have investigated the characteristics of a combination of metal plate and phosphor screen, a typical X-ray sensor in electronic portal imaging system, by using Monte Carlo simulation. Detection efficiency and spatial resolution of the X-ray sensor were estimated based on the bremsstrahlung spectrum of the incident X-ray and for various thicknesses of metal plate and phosphor screen. The detection efficiency was calculated from the total absorbed energy and the spatial resolution was defined from the spatial distribution of the absorbed energy. Simulation results showed a trade-off relationship between the detection efficiency and the spatial resolution. It was also found that the detection efficiency and the spatial resolution were mainly determined by thickness of metal plate and phosphor screen, respectively. The simulation will be useful when determining the optimal thickness of phosphor screen as well as of metal plate for the sensor design of therapeutic X-ray imaging.

A vacuum ultraviolet study of rare-earth phosphor screens with application as diagnostics for hot plasmas (abstract)

In a previous work [Zurro et al., Rev. Sci. Instrum. 68, 680 (1997)], we outlined designs for several detector systems, based on rare-earth phosphor powders, with application as broadband VUV plasma radiation monitors in the TJ-II flexible heliac device. In addition to their immunity to microwave power, to ground loops, and to any kind of EMI, the phosphor powders considered as suitable candidates can withstand the high temperatures of the TJ-II vacuum chamber (150 °C), while possessing good sensitivity across the x-ray to UV range and fast response times. Among those selected was Y3Al5O15:Ce (P-46), which emits at 530 nm and when examined, it was found to be particularly suitable for our application due to its very fast response time (∼0.1 μs). We report on absolute efficiency measurements performed between 40 and 200 nm using a calibrated VUV source, and made on thin P-46 phosphor screens, having thicknesses between 1 and 20 mg/cm2. We combine these results with measurements made at visible wavelengths and in the x ray [A. Baciero et al., in Diagnostics for Experimental Thermonuclear Fusion Reactors 2, edited by P. E. Stott et al. (Plenum, New York, 1998)] to obtain fitting parameters for models used to predict phosphor response efficiencies in transmission and reflection modes. These models can be used to tailor the phosphor screen thickness to the radiation range of interest while reducing background noise. Such screens are currently used on the TJ-II.