Nader Safavian MSc, Presenter: Nothing to Disclose

Karim S Karim PhD, Abstract Co-Author: Consultant, ActivPixel Innovations, Inc

John A. Rowlands PhD, Abstract Co-Author: Research grant, General Electric Company Consultant, Analogic Corporation, Montreal, Canada Institutional research agreement, Koninklijke Philips Electronics NV

Passive pixel sensors (PPS), comprising an X-ray detector and a thin film transistor (TFT) readout switch, are the norm in large area direct detection amorphous silicon (a-Si) flat panel imagers. While compact, they are vulnerable to noise from the data line and external panel readout electronics. An alternative to PPS is the active pixel sensor (APS). Like new large area detector materials (e.g. PbO, HgI2, avalanche selenium), APS circuits improve signal-to-noise ratios (SNR) by performing on-pixel amplification. APS circuits can be implemented using existing a-Si TFT manufacturing technology by simply adding an additional TFT per pixel without affecting the fill factor. A novel two TFT APS circuit is presented and shown to exhibit high SNR and immunity to variations in the manufacturing process, TFT metastability and the ambient temperature gradients.

We investigated the performance in terms of Detective Quantum Efficiency (DQE) for APS circuits fabricated in both a-Si and polycrystalline TFT technologies integrated with an overlying amorphous selenium (a-Se) direct X-ray detector. Using in-house semiconductor manufacturing facilities, many prototype APS designs in a-Si technology with the pixel pitch ranging from 100 um to 250 um pixels were fabricated. We compared the APS results to state-of-the-art PPS panels for real-time fluoroscopy data. We measured the SNR, transient readout speed, and lifetime of the fabricated APS circuit using thermal and electrical stress.

DQE(0) for the APS circuit was computed to be 0.4 at 0.1 µR for digital fluoroscopy. Input referred electronic noise was verified to be 380 and 260 electrons for the a-Si and poly-Si APS circuits respectively. The APS circuits can be operated at up to 60 frames per second for a 1000 x 1000 pixel array. The APS output is stable with changing temperature (20–80oC) and electrical stress for 500 hours.

Using an overlying a-Se direct detector, the APS pixels improve the DQE by 18% at 0.1 µR compared to state-of-the-art PPS pixels. As such, they can enable a variety of electronic noise limited digital x-ray imaging modalities without resorting to newer detectors (e.g. PbO) or backplane readout electronics (e.g. CMOS).

If these findings are verified clinically, the higher SNR obtained with APS technology could be used to either obtain a higher quality image or optimize for lower dose.

Safavian, N, Karim, K, Rowlands, J, Active Pixels for Large Area Direct Detection Medical X-ray Imaging.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8015605.html