A Novel Active Pixel Sensor With On-Pixel Analog-to-Digital Converter for Mammography

Two active pixel sensor (APS) architectures have been evaluated for mammography. Firstly, a 525 times 525 APS array of 25 times 25 mum pixels with greater than 80% fill factor, on-pixel analogue buffer amplifier and column parallel 10-bit ADC has been optically coupled to a 115 mum thick CsI:Tl phosphor with columnar structure. Critical performance parameters such as photon transfer curves, MTF, and DQE have been measured. With a readout rate of 10 f/sec, the sensor has a read noise of 114 plusmn 2e- and suffers from read related column FPN. A DQE of 35 % close to zero frequency has been measured for this array. Secondly, a novel APS test structure with on-pixel ADC, i.e. On Pixel Intelligent CMOS (OPIC), has been designed to eliminate the above limitations. This sensor, a 72 times 64 array, is almost free of column FPN, reads at rates more than 3700 f/sec and exhibits 44 plusmn 5 e- read noise. Additionally, a "hit flag" can be set to select only pixels above a threshold for sparse readout. By adjusting the threshold, segmentation of different regions of the image can be performed. The time above threshold can also be recorded, offering a technique to eliminate over-exposed mammograms. Both sensors had 10% MTF at 9 cycles/mm when coated with the structured CsI:Tl. X-ray images displaying the sensors capabilities are also presented.