IMAGE QUALITY EVALUATION OF YVO 4 :Eu PHOSPHOR SCREENS FOR USE IN X-RAY MEDICAL IMAGING DETECTORS I. KANDARAKIS, 1 D. CAVOURAS, 1 * E. KANELLOPOULOS, 1 C. D. NOMICOS 2 and G. S. PANAYIOTAKIS 3 1 Department of Medical Instrumentation Technology, 2 Department of Electronics, Technological Educational Institution of Athens, Agiou Spyridonos Street, Aigaleo 122 10, Athens, Greece and 3 Department of Medical Physics, Medical School, University of Patras, 265 00 Patras, Greece (Received 8 December 1997; revised 3 March 1998; accepted 8 March 1998) AbstractÐYVO 4 :Eu is a phosphor material emitting red light (621 nm) and providing excellent spectral compatibility with red sensitive optical detectors. However, it has never been used in medical X-ray imaging. In this study the imaging performance of six laboratory prepared YVO 4 :Eu test screens with coating weights from 30 to 140 mg/cm 2 was examined by evaluating the modulation transfer function (MTF), the X-ray luminescence eciency (XLE), and the detective quantum eciency (DQE). DQE was evaluated by employing a method based on XLE and emission spectra measurements. The 30 mg/ cm 2 screen had a signi®cantly higher MTF than thicker screens while its zero frequency DQE peak value was 0.27 at 30 kVp. Results obtained indicate that YVO 4 :Eu may be of value for use in X-ray imaging detectors. # 1998 Elsevier Science Ltd. All rights reserved 1. INTRODUCTION Europium activated yttrium vanadate (YVO 4 :Eu 3+ ) is a phosphor material which, when irradiated, emits red light at 621 nm due to the presence of Eu 3+ ion activator. In a previous study (Panayiotakis et al., 1996) data on light output and spectral compatibility of YVO 4 :Eu 3+ with various light photon detectors have been reported. In that study, YVO 4 :Eu 3+ emission spectrum was found to have excellent spectral matching with ®lms sensitive to red light. Also, the sensitivity of the silicon photodiode employed in digital imaging detectors (Gurvich, 1995; Yae and Rowlands, 1997) matched the YVO 4 :Eu 3+ spectrum better than the spectra of commonly used phosphors (Kandarakis et al., 1997a; Panayiotakis et al., 1996). To our knowledge, YVO 4 :Eu 3+ has never been used in X- ray imaging screens or in other types of radiation detectors for medical imaging. In the present study, the imaging performance of the laboratory prepared YVO 4 :Eu 3+ screens was experimentally examined to evaluate the suitability of this material to be used in conventional or digital X-ray imaging. The following parameters were evaluated: 1. The modulation transfer function (MTF) that describes the diagnostic signal modulation, or high contrast, as a function of spatial frequency and determines the resolution limit of an imaging system. 2. The X-ray luminescence eciency (XLE) that gives the light energy ¯ux emitted by the phos- phor per unit of incident X-ray energy ¯ux aecting image brightness and quantum noise. 3. The detective quantum eciency (DQE) that expresses the image information content (Shaw, 1963; Shaw and Van Metter, 1984) as described by the signal-to-noise ratio (SNR) transfer from the input to the output of an ima- ging system: DQE SNRout=SNRin 2 : 1 DQE evaluation was based (Kandarakis et al., 1997b) on the determination of MTF, XLE, quantum detection eciency (QDE) and the energies of the incident X-ray photons and emitted light photons. 2. MATERIALS AND METHODS The phosphor screens used in our experiments had coating weights of approx. 30, 60, 80, 100, 120 and 140 mg/cm 2 , thus covering the range often used in X-ray imaging. All screens were prepared in our Radiation Measurements Vol. 29, No. 5, pp. 481±486, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1350-4487/98 $19.00 + 0.00 PII: S1350-4487(98)00058-4 *Author to whom correspondence should be addressed: Prof. D Cavouras, 37±39 Esperidon Street, Kallithea 17671, Athens, Greece. Fax: (+301) 5910 975; e-mail: cavouras@hol.gr, cavouras@medisp.teiath.gr 481