Innovations in the detector technology have been driving the field of medical x-ray imaging towards higher efficiency and broader clinical applications. To inspire additional innovations, this imaging symposium brings three detector experts from both academia and industry to highlight recent innovations in x-ray detectors.
Dr. Zhao will talk about both direct and indirect active matrix flat panel imagers (AMFPIs) with avalanche gain. AMFPIs have been widely used in x-ray imaging applications since the beginning of this century. While AMFPIs offer many advantages, e.g. high geometric accuracy and fast readout, their performance at low dose has been hindered by the electronic noise. This presentation will review the factors affecting the image quality of both direct and indirect AMFPIs, and introduce several AMFPI detector concepts involving amorphous selenium with avalanche gain. The impact of radiation geometry will also be discussed, and how it may be used to increase the detective quantum efficiency (DQE) of AMFPIs at all spatial frequencies will be presented.
Dr. Antonuk will describe how to improve the signal-to-noise performance of x-ray detectors through polycrystalline mercuric iodide converters and advanced polycrystalline silicon pixel circuits. The imaging performance of large area x-ray detectors based on AMFPI array backplanes employing amorphous silicon thin-film transistors (TFTs) is constrained by a relatively high level of electronic additive noise compared to imaging signal. This results in degraded performance under conditions of low exposure per image frame or at high spatial frequencies. In this presentation, two methods of addressing this constraint will be described. The very high sensitivity of polycrystalline mercuric iodide makes it an interesting candidate for the replacement of conventional a-Se x-ray converters. Alternatively, the effect of electronic additive noise can be significantly reduced through the introduction of a pixel amplification circuit in an active pixel architecture or through the introduction of single-photon-counting pixel circuits - both based on polycrystalline silicon TFTs.
Mr. Ullberg will discuss the benefits and basic design considerations of photon counting detectors, which offer some unique features for x-ray imaging. If designed correctly, photon counting detectors have no readout noise and no dark counts. This is an important feature in for example low dose CT imaging where the total dose is distributed over a large number of projections from different angles. In addition to this, it is also possible to incorporate pulse height discrimination of each photon event, thus enabling the recording of images from multiple energy intervals in a single exposure. Mr. Ullberg will show that the position and energy resolution in a detector are degraded by physical effects such as charge sharing between pixels and escape of fluorescent photons. In order to limit the effects of these events, charge sharing corrections can be incorporated in the design of the detector. Such corrections are however impacting other parameters of the detector such as maximum count rate and power dissipation. Therefore, the final design is a tradeoff between different parameter optimizations.
(1) Understand how the degradation in imaging performance of AMFPIs at low dose levels can be addressed through charge gain in amorphous selenium, which can be used either as an x-ray photoconductor in direct AMFPIs or as an optical sensor in indirect AMFPIs.
(2) Understand how signal-to-noise limitations of large-area, digital x-ray detectors based on AMFPI array backplanes can be addressed through innovations involving a high sensitivity polycrystalline mercuric iodide converter or more sophisticated pixel circuits based on poly-Si thin-film-transistors.
(3) Understand the benefits of photon counting in x-ray detectors and the performance tradeoffs from different pixel sizes.
Funding Support, Disclosures, and Conflict of Interest: W. Zhao acknowledges funding support from the NIH (R01EB026267 and R21 EB019526-01A1). She also acknowledges collaboration with Analogic Corp. and Hamamatsu Photonics, and a research grant from Siemens Healthcare. Dr. Antonuk's research is partially supported by NIH grant R01EB022028. C. Ullberg is an employee of Direct Conversion AB.