| Popis: |
Initial results from an examination of the performance of small-area, high-spatial-resolution, active matrix, flat-panel imager (AMFPI) prototypes, under conditions relevant to mammographic imaging, are reported. These prototypes are based on two 512x512 pixel designs: (a) a 75 μm pitch indirect-detection array incorporating a continuous photodiode surface operated with a 34 mg/cm 2 Gd 2 O 2 S:Tb phosphor screen and with a 150 μm thick structured CsI(Tl) scintillator on a fiber-optic plate (which was designed to provide high spatial resolution at the expense of light output); and (b) two 100 pm pitch direct-detection arrays with ∼47 pm and ∼100 pm thick PbI 2 photoconductive coatings. These arrays were specifically developed to examine the performance of AMFPI technology in high-resolution applications. Results from empirical studies of fundamental pixel signal performance are presented and serve to verify the correct functioning of both prototype designs. From such empirical studies, a variety of parameters were extracted for use in a theoretical assessment of the potential imaging performance of the prototypes. In this assessment, calculations of DQE (based on the cascaded systems formalism) were performed as a function of exposure, additive noise level, and spatial frequency to examine the sensitivity of the prototype designs to these parameters. For the indirect-detection prototype, DQE performance with the phosphor screen (up to ∼48%) is greater, under some conditions, than that of high-performance mammographic film-screen systems. However, in the case of the CsI(Tl) converter, better overall performance would probably be attained by trading off spatial resolution for higher light output. For the direct-detection prototype, the DQE performance of the array with the ∼47 pm thick PbI 2 coating is predicted to be as high as ∼75%. The calculations also indicate that, due to the relatively modest x-ray sensitivities thus far observed for this material, the performance will degrade at low mammographic exposures and at additive noise levels in excess of several thousand electrons. This predicted performance is encouraging although questions remain concerning the effects of the comparative low sensitivity and high degree of charge trapping and release exhibited by the PbI 2 prototypes. |
