Determination of the detective quantum efficiency of a prototype, megavoltage indirect detection, active matrix flat-panel imager

• Published in: Medical physics (Lancaster) 2001-12, Vol.28 (12), p.2538-2550
• Main Authors: El-Mohri, Youcef, Jee, Kyung-Wook, Antonuk, Larry E., Maolinbay, Manat, Zhao, Qihua
• Format: Article
• Language: English
• Subjects: active matrix flat‐panel imager; Amorphous semiconductors; amorphous silicon; Biophysical Phenomena; Biophysics; Computers; detective quantum efficiency; diagnostic radiography; digital simulation; Dosimetry; electronic portal imager; Equipment Design; Field effect devices; image sensors; Imaging detectors and sensors; Medical image noise; Medical imaging; Medical X‐ray imaging; Modulation transfer functions; Monte Carlo Method; Monte Carlo methods; Phosphors; photodiodes; Photodiodes; phototransistors; photoresistors; Quantum Theory; radiation therapy; sensitivity; Sensitivity and Specificity; thin film transistors; Verification; X‐ and γ‐ray sources, mirrors, gratings, and detectors; X‐ray detection
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.1413516

After years of aggressive development, active matrix flat-panel imagers (AMFPIs) have recently become commercially available for radiotherapy imaging. In this paper we report on a comprehensive evaluation of the signal and noise performance of a large-area prototype AMFPI specifically developed for this application. The imager is based on an array of 512×512 pixels incorporating amorphous silicon photodiodes and thin-film transistors offering a 26×26 cm2 active area at a pixel pitch of 508 μm. This indirect detection array was coupled to various x-ray converters consisting of a commercial phosphor screen (Lanex Fast B, Lanex Regular, or Lanex Fine) and a 1 mm thick copper plate. Performance of the imager in terms of measured sensitivity, modulation transfer function (MTF), noise power spectra (NPS), and detective quantum efficiency (DQE) is reported at beam energies of 6 and 15 MV and at doses of 1 and 2 monitor units (MU). In addition, calculations of system performance (NPS, DQE) based on cascaded-system formalism were reported and compared to empirical results. In these calculations, the Swank factor and spatial energy distributions of secondary electrons within the converter were modeled by means of EGS4 Monte Carlo simulations. Measured MTFs of the system show a weak dependence on screen type (i.e., thickness), which is partially due to the spreading of secondary radiation. Measured DQE was found to be independent of dose for the Fast B screen, implying that the imager is input-quantum-limited at 1 MU, even at an extended source-to-detector distance of 200 cm. The maximum DQE obtained is around 1%—a limit imposed by the low detection efficiency of the converter. For thinner phosphor screens, the DQE is lower due to their lower detection efficiencies. Finally, for the Fast B screen, good agreement between calculated and measured DQE was observed.