Europium-activated phosphors for use in X-ray detectors of medical imaging systems

Abstract

Three europium-activated phosphors Y2O2S:Eu, Y2O3:Eu, and YVO4:Eu emitting red light were studied to investigate their suitability for radiographic cassettes or digital imaging systems. Screens were prepared from phosphor powders with various coating thicknesses by sedimentation. To assess phosphor light producing efficiency in relation to patient dose, each screen was X-rayed using 40-120 kVp and the number of light photons emitted per X-ray incident was experimentally and theoretically evaluated. Additionally, the capability of the emitted light to sensitize films or to generate electrons in silicon photodiodes used in digital imaging systems was examined. Y2O2S:Eu screens were most efficient in light emission, and when combined with either red sensitive films or Si photodiodes, they were found superior to Y2O3:Eu or YVO4:Eu screens in film grain or electron signal generation. In many cases they were also found superior to terbium-activated phosphors. Provided that several problems related to industrial production (special dyes, reflective backing, crossover effects) are dealt with, those europium-activated screens could be employed in low-tube-voltage radiographic applications.

Experimental and theoretical assessment of the performance of Gd2O2S:Tb and La2O2S:Tb phosphors and Gd2O2S:Tb–La2O2S:Tb mixtures for X-ray imaging

“…where m is the mass attenuation coefficient of the phosphor for X-rays calculated using published data [27] and T is the screen-coating weight; h Q is the intrinsic X-ray to light conversion efficiency, giving the fraction of absorbed X-ray energy converted to light within the phosphor material; G L (a,s) is the light transmission efficiency, expressing the fraction of light produced that reaches the screen output [3,5,9,10,25,28,29], with a and s being the coefficients of light absorption and light scattering, and " E is the mean energy of X-rays determined by the relation:…”

“…Fig. 7 The DQE data for various 80 mg/cm 2 Gd 2 O 2 S±La 2 O 2 S mixtures and X-ray tube voltages Appendix Theoretical light emission efficiency Light emission efficiency was theoretically evaluated considering that it is the product of the following quantities [3,5,9,10,28,29]:…”

“…Grain size affects both emission efficiency and spatial resolution [5,11,16,18]; however, sizes in the range from 5 to 10 mm are generally accepted as a satisfactory compromise between emission efficiency and spatial resolution [5,18]. The phosphors were used in the form of layers (screens) prepared in our laboratory [9,10,19,20,21,22] with various coating weights ranging from approximately 25 to 180 mg/ cm 2 , which are often used in various types of X-ray imaging applications. Preparation was performed by sedimentation of the phosphor powder on fused silica substrates.…”

“…Data obtained by the electrometer were converted into light intensity (energy per unit of area and time) using manufacturer's photosensitivity data [9,10,19,20,21,22]. Secondly, the exposure rate X (mR s ±1 ), related to the incident X-ray beam, was determined by a PTW ionization chamber dosimeter (type no.…”

“…The light emission efficiency [1,2,3,4,5,6,7,8,9,10,16], which was evaluated by determining the number of emitted photons (NEP) of light per incident X-ray 2. The modulation transfer function (MTF), which describes the variation of image contrast with spatial frequency and expresses spatial resolution [5,10,11,12,13,14].…”