Indirect Active Matrix Flat-panel Imagers Research Articles

Improved temporal performance and optical quantum efficiency of avalanche amorphous selenium for low dose medical imaging.

Active matrix flat panel imagers (AMFPIs) with thin-film transistor arrays experience image quality degradation by electronic noise in low-dose radiography and fluoroscopy. One potential solution is to overcome electronic noise using avalanche gain in an amorphous selenium (a-Se) (HARP) photoconductor in indirect AMFPI. In this work, we aim to improve temporal performance of HARP using a novel composite hole blocking layer (HBL) structure and increase optical quantum efficiency (OQE) to CsI:Tl scintillators by tellurium (Te) doping. Two different HARP structures were fabricated: Composite HBL samples and Te-doped samples. Dark current and optical sensitivity measurements were performed on the composite HBL samples to evaluate avalanche gain and temporal performance. The OQE and temporal performance of the Te-doped samples were characterized by optical sensitivity measurements. A charge transport model was used to investigate the hole mobility and lifetime of the Te-doped samples in combination with time-of-flight measurements. The composite HBL has excellent temporal performance, with ghosting below 3% at 10mR equivalent exposure. Furthermore, the composite HBL samples have dark current and achieved an avalanche gain of 16. Te-doped samples increased OQE from 0.018 to 0.43 for 532nm light. The addition of Te resulted in 2.1% first-frame lag, attributed to hole trapping within the layer. The composite HBL and Te-doping can be utilized to improve upon the limitations of previously developed indirect HARP imagers, showing excellent temporal performance and increased OQE, respectively.

Back-irradiated and dual-screen sandwich detector configurations for radiography.

Recent advances in thin film transistor array technology have enabled the possibility of "back-irradiated" (BI) indirect active-matrix flat-panel imagers (AMFPIs), in which x-rays first expose the scintillator through the optical sensor, and "dual-screen" AMFPIs, in which two scintillating screens are sandwiched around a bidirectional active matrix. We developed a theoretical treatment of such detectors. The theory is used to investigate possible imaging performance improvements over conventional "front-irradiation" (FI) AMFPIs, where the active matrix is opposite the x-ray entrance surface. Simple expressions for the modulation transfer function, normalized noise power spectrum, Swank factor ( ), Lubberts function , and spatial frequency-dependent detective quantum efficiency are derived and used to compute these quantities for a variety of FI, BI, and dual-screen detector configurations. is used as the figure of merit for optimizing and comparing the performance of the various configurations. Large performance improvements over FI single-screen systems are found possible with BI. Further improvements are found possible with dual-screen configurations. The ratio of the thicknesses of the two screens that optimizes DQE is generally asymmetric, with the thinner screen facing the incident flux. The optimum ratio depends on the x-ray attenuation length in the screen.

Development of solid-state avalanche amorphous selenium for medical imaging.

Active matrix flat panel imagers (AMFPI) have limited performance in low dose applications due to the electronic noise of the thin film transistor (TFT) array. A uniform layer of avalanche amorphous selenium (a-Se) called high gain avalanche rushing photoconductor (HARP) allows for signal amplification prior to readout from the TFT array, largely eliminating the effects of the electronic noise. The authors report preliminary avalanche gain measurements from the first HARP structure developed for direct deposition onto a TFT array. The HARP structure is fabricated on a glass substrate in the form of p-i-n, i.e., the electron blocking layer (p) followed by an intrinsic (i) a-Se layer and finally the hole blocking layer (n). All deposition procedures are scalable to large area detectors. Integrated charge is measured from pulsed optical excitation incident on the top electrode (as would in an indirect AMFPI) under continuous high voltage bias. Avalanche gain measurements were obtained from samples fabricated simultaneously at different locations in the evaporator to evaluate performance uniformity across large area. An avalanche gain of up to 80 was obtained, which showed field dependence consistent with previous measurements from n-i-p HARP structures established for vacuum tubes. Measurements from multiple samples demonstrate the spatial uniformity of performance using large area deposition methods. Finally, the results were highly reproducible during the time course of the entire study. We present promising avalanche gain measurement results from a novel HARP structure that can be deposited onto a TFT array. This is a crucial step toward the practical feasibility of AMFPI with avalanche gain, enabling quantum noise limited performance down to a single x-ray photon per pixel.

WE‐C‐I‐611‐03: Flat Panels Vs. IIs: A Critical Comparison

Over the past decade there has been rapid development of real‐time active matrix flat‐panel imagers (AMFPI) to replace the x‐ray image intensifiers (XRII) in fluoroscopy and angiography. The major advantages of AMFPI compared to the conventional XRII are: (1) compact and light weight; (2) free of distortion; (3) use of lower‐z material for detector window, which increases the quantum absorption efficiency of the detector; (4) free of veiling glare. Depending on the materials used for x‐ray detection, AMFPI are divided into two main categories: direct and indirect detection. Direct detection AMFPI employ a uniform layer of x‐ray sensitive photoconductor, e.g., amorphous selenium (a‐Se) to directly convert incident x‐rays to charge, which is subsequently read out by a two dimensional array of amorphous silicon (a‐Si) thin film transistors (TFTs). In indirect AMFPI, a phosphor such as structured cesium iodide (CsI) is used to convert x‐ray energy to optical photons, which are then converted to charge by integrated photodiodes at each pixel of the TFT array. The principle of operation, and advantages and disadvantages of both direct and indirect AMFPI will be described. The imaging performance of both approaches, and their comparison with the XRII will be presented. The evaluation of imaging performance includes: (1) the spatial frequency (f) dependent detective quantum efficiency (DQE); (2) DQE(f) as a function of x‐ray exposure; (3) temporal performance, i.e. lag and image persistence; and (4) veiling glare. Despite their differences in detector structure, the direct and indirect AMFPI have comparable imaging performance. The DQE(f) of AMFPI compares favorably to XRII except at the lowest exposure encountered in fluoroscopy (< 5 nGy), where the electronic noise of AMFPI degrades the DQE. To improve the DQE at low dose is the focus of many recent developments of both direct and indirect AMFPI. For direct AMFPI, photoconductors of higher z and x‐ray to charge conversion gain, e.g. lead iodide (PbI2) and mercuric iodide (HgI2), have been developed. The x‐ray to charge conversion gain for these new photoconductors is seven times higher than that of a‐Se. For indirect AMFPI, a thin layer of a‐Se avalanche photoconductor is being investigated as a replacement for a‐Si photodiodes. Under electric field of > 80 V per micron, avalanche multiplication occurs in a‐Se, which can amplify the signal in low dose applications. Sophisticated pixel structures (by incorporating more than three TFTs at each pixel) are also being proposed, which provide signal amplification at each pixel to reduce the electronic noise. This approach can be applied to both the direct and indirect AMFPI. The principle of operation, feasibility and advantages of each new AMFPI concept will be discussed.Educational Objectives:1. To understand the principle of operation and imaging performance of different AMFPI, and their advantages and disadvantages compared to XRII.2. To understand the limitation of existing AMFPI in low dose performance, and learn about the current development and advancement in AMFPI design to overcome this limitation.

Investigation into the physical characteristics of active matrix flat panel imagers for radiotherapy.

The effect of physical characteristics on active matrix flat panel imagers (AMFPIs) at megavoltage energies is studied. The detective quantum efficiency (DQE) of both direct and indirect AMFPIs is modeled using a modified cascade analysis combined with Monte Carlo simulations. It is found that for a given thickness of the sensitive layer less than about 1 mm, there should be no significant difference between the detection techniques, but for larger mass thicknesses (> or = 1 mm) there should be an advantage to using direct detection if such thick layers can be manufactured. The effect of the front plate on both direct and indirect techniques is also explored in terms of both the DQE and scatter rejection. It is found that for small sensitive layer thicknesses (< or =0.3 mm) a front plate thickness of about 1 mm Cu is optimal, whereas for larger mass thicknesses about 0.4 mm Cu should lead to better image quality.