The purpose of this paper is to analyze the image quality of a selenium-based flat panel detector suited for digital interventional mammography. To characterize the image quality, the DQE was measured at various x-ray exposures. The results indicate that when the detector is quantum noise limited, the DQE is independent of the exposure. A measurement of the quantum detection efficiency of 90% indicates that an electrostatic field shaping effect within the selenium layer gives a greater collection efficiency than that predicted simply by the geometric fill factor of each pixel collection electrode. Measurements were also conducted to determine the relative strength of ghost images on the detector. An image of a high contrast object using an exposure of 183 mR was acquired, followed by a low exposure 6 mR flat field image. No visual indication of a ghost could be found in the latter image even after appropriate windowing and leveling of the image was performed. A subjective comparison of image quality between film/screen and the detector was conducted by acquiring images of the ACR phantom under various exposure conditions. The digital images were printed on film using optimally adjusted LUT's. The resulting images were randomly presented to 15 non-trained observers, who assessed a score for each image. The comparison results show that the image quality obtained with the digital detector is superior to the images acquired with film/screen.
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.