In this paper, we report measurements from a prototype 1024 X 1024 selenium-based flat panel detector suited for interventional digital mammography applications. This detector is based on an amorphous silicon TFT array, with a pixel pitch of 85 micrometer and a fill factor of 70%. A 200 micrometer layer of amorphous selenium is used to directly convert the incident x-rays into electrical charges. The detector electronics, TFT array, and selenium converter structure are designed to operate at a frame rate of 10 images per second. Experimentally, this detector yields an x-ray sensitivity of nearly 290 electrons/absorbed x-ray nearly 100% absorption of x-rays at a beam energy of 18 keV, a high spatial resolution (limited only by the pixel pitch up to the Nyquist limit), and quantum-noise limited operation down to the lowest exposures currently investigated. Images from the ACR phantom and contrast detail phantom reveal all embedded targets in the phantoms, which indicates the potential of this technology for digital mammography.