Detective quantum efficiency of a CsI:Tl scintillator based scanning slot x-ray detector for digital mammography

An investigation was made of the key determinants of the detective quantum efficiency at zero spatial frequency (DQE(0)) of a CsI:TI scintillator based scanning slot x-ray detector for digital mammography. The slot x-ray detector was made of a prismatic type thallium activated CsI scintillator (150 mu m thick) optically coupled to CCDs by fiber optical image guides. Monte Carlo calculations were performed to generate the CsI:TI scintillator Swank factor on the basis of the energy deposition from pencil beam x-ray sources and light transmission within the CsI:TI scintillator. A theoretical expression for the detector DQE(0) was obtained which was used to investigate the detector imaging performance as a function of x-ray energy, x-ray exposure, CCD electronic noise level, and optical coupling efficiency of the fiber optic image guide. The Swank factor of the CsI:TI scintillator was close to unity at x-ray energies below Iodine K-edge (33.2 keV), but decreased to similar to 0.8 at higher x-ray energies up to 40 keV. DQE(0) of the slot x-ray detector was similar to 75% at 15 keV but decreased to similar to 40% at 30 keV. Optimum DQE(0) performance of the slot x-ray detector was generally obtained at a detector x-ray exposure level above similar to 5 to 10 mR and an electronic noise level below similar to 50 electrons rms. A drop in the optical coupling efficiency of the image guide from 1.0 to 0.3 reduced the detector DQE(0) from similar to 75% to similar to 55% in the mammography x-ray energy range. The key finding in this study is that the choice of the x-ray energy has a major impact on the DQE(0) of a CsI:TI scintillator based slot x-ray detector. Since the x-ray photon energy also affects x-ray tube loading, mean glandular dose and subject contrast, the choices of optimal x-ray spectra from current mammography x-ray tubes require further investigation.