50 μm pixel size a-Se mammography imager with high DQE and increased temperature resistance

The imager presented in this paper has a special blocking structure that ensures very low dark current of less than 1 pA/mm² even with a 20 V/μm electric field. Hence the electric field can be increased from the generally applied 10 V/μm to 20V/μm, this reduces the energy required to produce an electron hole (e-h) pair from 60 eV to about 36 eV at the given (19.3 keV mean) mammo energy. Furthermore, with special doping and manufacturing processes this a-Se layer is very stable in the 0-70 C° temperature range as demonstrated by Ogusu et al. [1]. A new 5 cm × 5 cm size TFT array was developed with 50 μm pixel size, specifically for testing the resolution of photoconductor based imagers. The first new imager of this type had a 200 μm thick a-Se layer evaporated onto the array. Its MTF, NPS, and DQE values were evaluated using 28kVp Mo anode x-ray source with a 0.03mm thick Mo and an additional 2 mm thick Al filters. The MTF value is about 40% and 50% in x-and y directions at the Nyquist frequency of 10 lp/mm. The low frequency DQE at 20 V/μm electrical field is ~70% at 151 μGy dose and drops only about 10% when going down to 23 μGy. This new array also has excellent lag properties. The measured first frame image lag at 20 V/μm is less than 1%. Such low lag provides opportunities to use this material not only for mammography but also for breast tomosynthesis applications. Breast phantom images demonstrate that even the smallest 0.13 mm calcifications are clearly visible with this high-resolution imager.