An apparatus and method for directly measuring the depth-dependent gain and spatial resolution of turbid scintillators.

PURPOSE: Turbid (powder or columnar-structured) scintillators are widely used in indirect flat panel detectors (I-FPDs) for scientific, industrial, and medical radiography. Light diffusion and absorption within these scintillators is expected to cause depth-dependent variations in their x ray conversion gain and spatial blur. These variations degrade the detective quantum efficiency of I-FPDs at all spatial frequencies. Despite their importance, there are currently no established methods for directly measuring scintillator depth effects. This work develops the instrumentation and methods to achieve this capability. METHODS: An ultra-high-sensitivity camera was assembled for imaging single x ray interactions in two commercial Gd2 O2 S:Tb (GOS) screens (Lanex Regular and Fast Back, Eastman Kodak Company). X ray interactions were localized to known depths in the screens using a slit beam of parallel synchrotron radiation (32 keV), with beam width (~20 µm) much narrower than the screen thickness. Depth-localized x ray interaction images were acquired in 30 µm depth-intervals, and analyzed to measure each scintillator's depth-dependent average gain g ¯ ( z ) and modulation transfer function MTF(z,f). These measurements were used to calculate each screen's expected MTF(f) in an energy-integrating detector (e.g., I-FPD). Calculations were compared to presampling MTF measurements made by coupling each screen to a high-resolution CMOS image sensor (48 µm pixel) and using the slanted-edge method. RESULTS: Both g ¯ ( z ) and MTF(z,f) continuously increased as interactions occurred closer to each screen's sensor-coupled surface. The Regular yielded 1351 ± 66 and 2117 ± 54 photons per absorbed x ray (42-66 keV-1 ) in interactions occurring furthest from and nearest to the image sensor, while the Fast Back yielded 833 ± 22 and 1910 ± 39 photons (26-60 keV-1 ). At f = 1 mm-1 , MTF(z,f) varied between 0.63 and 0.78 in the Regular and 0.30-0.76 in the Fast Back. Calculations of presampling MTF(f) using g ¯ ( z ) and MTF(z,f) showed excellent agreement with slanted-edge measurements. CONCLUSIONS: The developed instrument and method enable direct measurements of the depth-dependent gain and spatial resolution of turbid scintillators. This knowledge can be used to predict, understand, and potentially improve I-FPD imaging performance.