Evaluating scintillators used in radiation detectors of medical imaging systems by the effective fidelity index method

Objective: The performance of medical X-ray image receptors depends: (1) on the scintillator light emission efficiency; and (2) on the compatibility o f the scintillator light spectrum with the spectral sensitivity of the ligh t detector (film, photocathode, or photodiode), employed in conjunction wit h the scintillator. In this study, a scintillator performance measure, the effective fidelity index (EFI), is defined as function of both the scintill ator light emission efficiency and spectral compatibility Materials and Met hod: CsI:Na, Gd2O2S:Tb and La2O2S:Tb scintillators were employed in the for m of phosphor screens prepared in our laboratory with various coating thick nesses. The screens were irradiated with X-rays employing tube voltages ran ging between 50-120 kVp. Results: The EFI performance of CsI:Na was found t o increase with screen coating thickness and it was best when combined with the orthochromatic film or the ES/20 photocathode. Gd2O2S:Tb showed peak E FI performance at 70 mg/cm(2) coating thickness and it was well combined wi th the light detectors considered. Conclusion: In accordance with our resul ts, CsI:Na may be employed in radiography when adequately protected against humidity. Gd2O2S:Tb suitability for conventional imaging was verified and it was found that it may be useful in all types of digital imaging. La2O2S: Tb could also be used in digital detectors of imaging applications demandin g medium X-ray tube voltages. (C) 1999 Elsevier Science Ireland Ltd. All ri ghts reserved.