Eugene Atlas

This paper describes a novel area detector for direct conversion and readout of the x-ray energy that eliminates multiple conversions and coupling stages which degrade performance. The pixel array and readout electronics are fabricated on the same piece of silicon. The detector consists of a uniform layer (approximately 300 micrometers) of amorphous selenium alloy vapor-deposited on an electronic readout array fabricated using conventional complementary metal oxide semiconductor (CMOS). The CMOS array features 66 micrometer pixels in a 1024 X 832 array providing a 5.5 X 6.75 cm image area. Each pixel has active circuitry including signal amplification, pixel selection and reset, while peripheral circuitry on one end of the array provides shift registers, sample and hold and multiplexing. The CMOS readout array was fabricated at a standard facility on a 10-cm diameter silicon wafer using 2 micrometer CMOS process. Fifteen separate image sensors were assembled for evaluation in a 3 X 5 format to provide a 20 X 27 cm composite field of view. Missing data between sensors is recovered by acquiring three sub-exposures, between which the array is translated diagonally approximately 2 mm. Total exposure time for an average breast is less than one second. Conversion efficiency was found to be approximately 120 electrons per absorbed x-ray (19 keV average). Electronic readout noise was measured to be 2.4 ADU corresponding to approximately 500 electrons. Detective quantum efficiency was found to be 0.65 at low spatial frequency (0.25 lp/mm) and at 0.2 at high spatial frequency (8 lp/mm) for x-ray fluence ranging from 5 - 35 mR. Images of an ACR phantom show visualization of all of the fibers, specks and masses when displayed with a linear lookup table on a high-resolution monitor. These studies demonstrated that there is a slight but measurable image retention evident as 'ghost' images. The two most effective means to reduce this effect are flushing the sensors with infrared light or x-rays between exposures and reversing the applied voltage on the selenium layer. A number of improvements designed to increase sensitivity and reduce noise also have been identified and are being implemented. Sample images were acquired from four volunteer human subjects at exposure factors identical to their film-screen mammograms. The results suggest that the detector performance is suitable for further clinical investigation.

We describe the readout architecture and progress to date in the development of hybrid PIN diode arrays for use as vertex de,tectors in the sse environment The architecture supports a self-timed mechanism for time stamping hit pixels, storing their xy coordinates and later selectively reading out only those pixels containing interesting data along with their coordinates.The peripheral logic resolves ambiguous pixel ghost locations and controls pixel neighbor readout to achieve high spatial resolution.A test lot containing 64 X 32 pixel arrays has been processed and is currently being tested.Each pixel contains 23 transistors and six capacitors consuming an area of 50 J.U1l by 150 J.1ffi and dissipating about 20 J.LW of power.

A pixel cell has been designed which has been optimized for a data push architecture readout. It retains the features of preceding designs which allow time stamping, analog signal processing, XY address recording, ghost elimination and sparse data transmission. It eliminates a number of problems inherent in previous designs by use of sampled data techniques, destructive readout sampled data techniques, and current mode output drivers. This document examines the motivation for this new pixel and covers the theory of operation of the various blocks. A discussion of the tradeoffs affecting speed, power, device size, and radiation stability is included as well.