Photon counting is an emerging detection technique that is promising for mammography tomosynthesis imagers. In photon counting systems, the value of each image pixel is equal to the number of photons that interact with the detector. In this research, we introduce the design and implementation of a low noise, photon counting pixel for digital mammography tomosynthesis in 0.18 mu m crystalline silicon complementary metal-oxide semiconductor technology. The design comprises of a low noise, charge-integrating amplifier, a low offset voltage comparator, a decision-making unit, a mode selector, and a pseudorandom counter. Theoretical calculations and simulation results of linearity, gain, and noise of the photon counting pixel are presented. (c) 2006 American Vacuum Society.