In this article, an amorphous selenium (a-Se) flat-panel detector based on a high-gain, compact active pixel sensor (APS) architecture is proposed for digital radiography. The individual in-pixel APS circuits on the array consist of two thin-film transistors with read, reset, and DC bias signals supplied by two adjacent gate (row) lines by sequential row scanning. High-gain (>10), low dark current (∼1 fA), and 90% pixel area can be achieved for the x-ray detector at a 50 μm pixel pitch. A digital readout system is integrated with the 160 × 160 APS panel with 14-bit grayscale resolution and 160 μs integration time per scanning channel for x-ray image readout. The a-Se x-ray photoconductor is characterized by material structure and photocurrent response under x-ray irradiation. The fully integrated detector is demonstrated using x-ray phantom images.