Enhanced a-Se X-ray imager for mammography

a-Se is a well-known material for X-ray imaging. It is a direct x-ray conversion photoconductor-detector material with less image blurring than scintillator-photodiode based imagers, which provides excellent resolution. The newly improved imager has a special blocking structure that ensures very low dark current. This allows the electric field to be increased from the standard 10 V/μm up to 20 V/μm. The higher field reduced the energy required to produce an e-h pair from 50 eV/e-h to about 30 eV/e-h increasing the sensitivity by 60%. Special doping processes and blocking layers also improved the a-Se's thermal stability to extend over the whole 0-70 C° temperature range. The high resolution is evident from the Modulation Transfer Function (MTF) value of >40% at the Nyquist Frequency of 10-lp/mm. The Detective Quantum Efficiency (DQE) was determined at different dose rates and as a function of spatial frequency. The higher electrical field increased the signal level at each given dose. The result was a 70% DQE value at low spatial frequencies for a high dose of 157 μGy. However, the low frequency DQE only dropped by about 10% to 60% at a low dose of 23 μGy. When the electric field at low dose was decreased to 10 V/μm the low frequency DQE fell to less than 40%. This new imager shows very low image lag. This is important for high frame rate breast tomosynthesis imaging applications. Ghosting properties of the imager were also tested. At 20 V/μm the ghosting was less than 1/4th of that measured at 10 V/μm for an array, with relatively higher image lag. However, ghosting was practically 0 for an array having relatively low image lag, a further major advantage. Finally, breast phantom images resolved even the smallest calcifications, embedded in the phantom.