Comparison of CsI:Tl and Gd₂ O₂ S:Tb indirect flat panel detector x-ray imaging performance in front- and back-irradiation geometries

Purpose: The detective quantum efficiency (DQE) of indirect flat panel detectors (I-FPDs) is limited at higher x-ray energies (e.g., 100-140 kVp) by low absorption in their scintillating x-ray conversion layer. While increasing the thickness of the scintillator can improve its x-ray absorption efficiency, this approach is potentially limited by reduced spatial resolution and increased noise due to depth dependence in the scintillator's response to x rays. One strategy proposed to mitigate these deleterious effects is to irradiate the scintillator through the pixel sensor in a "back-irradiation" geometry. This work directly evaluates the impact of irradiation geometry on the inherent imaging performance of I-FPDs composed with columnar CsI:Tl and powder Gd₂ O₂ S:Tb (GOS) scintillators.

Methods: A "bidirectional" FPD was constructed which allows scintillator samples to be interchangeably coupled with the detector's active matrix to compose an I-FPD. Radio-translucent windows in the detector's housing permit imaging in both "front-irradiation" (FI) and "back-irradiation" (BI) geometries. This test device was used to evaluate the impact of irradiation geometry on the x-ray sensitivity, modulation transfer function (MTF), noise power spectrum (NPS), and DQE of four I-FPDs composed using columnar CsI:Tl scintillators of varying thickness (600-1000 µm) and optical backing, and a Fast Back GOS screen. All experiments used an RQA9 x-ray beam.

Results: Each I-FPD's x-ray sensitivity, MTF, and DQE was greater or equal in BI geometry than in FI. The I-FPD composed with CsI:Tl (1 mm) and an optically absorptive backing had the largest variation in sensitivity (17%) between FI and BI geometries. The detector composed with GOS had the largest improvement in limiting resolution (31%). Irradiation geometry had little impact on MTF(f) and DQE(f) measurements near zero frequency, however, the difference between FI and BI measurements generally increased with spatial frequency. The CsI:Tl scintillator with optically absorptive backing (1 mm) in BI geometry had the highest spatial resolution and DQE over all frequencies.

Conclusions: Back irradiation may improve the inherent x-ray imaging performance of I-FPDs composed with CsI:Tl and GOS scintillators. This approach can be leveraged to improve tradeoffs between detector dose efficiency, spatial resolution and noise for higher energy x-ray imaging.