A Novel Active Pixel Sensor With On-Pixel Analog- to-Digital Converter for Mammography C. D. Arvanitis, S. E. Bohndiek, G. Segneri, C. Venanzi, G. Royle, A. Clark, J. Crooks, R. Halsall, M. Key- Charriere, S. Martin, M. Prydderch, R. Turchetta and R. Speller Abstract–Two acitve pixel sensor (APS) architectures have been evaluated for mammography. Firstly, a 525 x 525 APS array of 25 x 25 µm 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 µm 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 ± 2 e- 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 x 64 array, is almost free of column FPN, reads at rates more than 3700 f/sec and exhibits 44±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. I. INTRODUCTION MOS sensors with on-pixel analogue buffer amplifier [1], low read related noise [2], and fast image acquisition through massive parallel read out are being developed for scientific applications. As technology downscales both increased sensitivity in the blue part of the spectrum [3] and higher fill factor [4] are expected. Large area CMOS monolithic active pixel sensors (APS) with zero dead area can be produced due to advances in stitched technology, making these a viable alternative to flat panel imagers. Planar and advanced digital mammography [5] could benefit from the on-pixel characteristics of CMOS APS. Low Manuscript received November 13, 2006. This work is supported by the RC- UK Basic Technology Multidimensional Integrated Intelligent Imaging (MI- 3) programme (GR/S85733/01). C. Arvanitis, S. Bohndiek, G. Segner, C. Venanzi, G. Royle and. R. Speller are with the department of Medical Physics and Bioengineering at University College London, London, UK (e-mail: akostas@medphys.ucl.ac.uk). A. Clark, J. Crooks, R. Halsall, M. Key-Charriere, S. Martin, M. Prydderch and R. Turchetta are with the Rutherford Appleton Laboratories Chilton, Didcot, Oxfordshire, OX11 0QX, UK noise, high fill factor and good spectral matching with CsI:Tl phosphors will improve the “image quality” in digital mammography. In contrast enhanced mammography higher signal-to-noise ratio at regions with lower contrast medium concentration of the breast could be achieved. Fast image acquisition is essential for tomosynthesis and mammotomography to minimize patient motion artifacts. II. METHODS AND MATERIALS Two monolithic APS have been employed in this study; a standard 3T APS (Fig. 1. a) and a novel APS that offers high pixel level integration (Fig. 1. b) utilizing the recent advances in standard CMOS technology. Description of the two pixel architectures is given below. The performance of the two sensors in terms of read noise, full well capacity and signal-to- noise ratio has been evaluated optically through the photon transfer curve (PTC) technique. The two sensors were optically coupled via a fiber optic plate to structured CsI:Tl phosphors, and their x-ray imaging performance has been evaluated by measuring their energy dependent modulation transfer function (MTF), noise power spectrum (NPS) and detection quantum efficiency (DQE). Fig 1. Schematic diagram of the two different pixel architectures, a) the 3T APS with the nwell photodiode, the reset, the source follower (TX), and the column select transistor b) the OPIC with the read out transistors, on-pixel ADC, comparator, and “hit flag”. A. APS Pixels The 3T APS is a 525 x 525 array based on almost 100% fill factor technology with 25 x 25 µm pixel pitch and on-chip column parallel 10-bit analog-to-digital converters (ADC) that reads at 10 frames/sec [6]. Each pixel has 4 n-well photodiodes (PD) as sensing elements, reset, source follower (TX), and row select transistors (Fig. 1. a) offering the highest possible fill factor. The novel APS test structure with on-pixel ADC will be referred to in this paper as On Pixel Intelligent CMOS (OPIC). It allows high-speed read-out (>3700 frames/sec) and snap shot digital imaging. This structure comprises a 72 x 64 pixel array with 30 x 30 µm pixel pitch. Each pixel has an n-well C