Real time flat panel detectors based on amorphous selenium (a-Se) have demonstrated to be the most advanced technology for direct conversion X-ray imaging in various medical applications. In continuation of real time detector development, ANRAD Corporation introduce in this paper a large size 14 inches X 14 inches active area detector built with an amorphous selenium (a-Se) converter coated on a TFT array. This new detector is a scaled up version of the 9 inches X 9 inches presented last year based on a TFT array with 150 um x 150 um pixel and a 1000 mm thick a-Se PIN structure operated at 10V/um. DQE(f=0) measurements were performed in low dose range and demonstrated to be in agreement with a linear model including 2500e of electronic noise. It is also shown that the spatial resolution (MTF) could be controlled by selenium coating process and can almost reach the theoretical limit defined by the pixel pitch. Finally, the first 14 inches X 14 inches chest image is presented.
As amorphous selenium based flat panel detectors gain more interest for direct, real-time x-ray imaging, we report in this paper the performance of such a detector by ANRAD Corporation. This new detector is based on a 1536 X 1536 array of amorphous silicon TFT pixels coupled with a 1000 micrometers selenium converter biased at 10 V/micrometers . Each 150 micrometers X 150 micrometers pixel is made of a thin film transistor, a storage capacitor and a collecting electrode having a geometrical fill factor of 77% and an effective fill factor of nearby 100%.
The use of selenium alloys for direct conversion fluoroscopy flat-panel detectors has been underestimated. The purpose of this paper is to demonstrate the salient features of a selenium-based detector designed for R&F applications. The detector has an active area of 30 cm X 27 cm and comprises 2048 X 1792 pixels at 150 micrometer pitch. The geometric fill factor is 66%, but experimental evidence supports the fact that internal electric field bending leads to an effective fill factor approaching unity. The detector is designed to support full resolution images at 15 frames/second, and 896 X 1024 resolution at 30 frames/second. The detector is coated with a simple coplanar 'p-i-n' selenium diode structure which has a dark current less than 100 pA/cm2. The thickness of this structure is 1000 micrometer to absorb 77% of a NEMA standard fluoroscopy beam. Measurements show we have obtained an x-ray sensitivity of 4400pC/mR/cm2, which translates to 1212 collected charges per absorbed x-ray. Resolution was measured to be near the theoretically predicted values, with a modulation of 63% at the Nyquist limit of 3.33 lp/mm. Phantom images were obtained at a frame rate of 15 frames per second, and negligible lag was observed in this image sequence.
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.