Ghosting caused by bulk charge trapping in direct conversion flat-panel detectors using amorphous selenium

• Published in: Medical physics (Lancaster) 2005-02, Vol.32 (2), p.488-500
• Main Authors: Zhao, Wei, DeCrescenzo, G., Kasap, Safa O., Rowlands, J. A.
• Format: Article
• Language: English
• Subjects: amorphous selenium; amorphous semiconductors; Carrier generation; Charge carriers; charge measurement; Computer Simulation; diagnostic radiography; digital radiography; direct conversion; Electrodes; Electron hole recombination; electron traps; Electrons; Equipment Design; Equipment Failure Analysis; fluoroscopy; ghosting; image sensors; lag; Langevin recombination; Medical imaging; memory effect; Models, Theoretical; photoconducting materials; Radiation Dosage; Radiographic Image Enhancement - instrumentation; Radiographic Image Enhancement - methods; Radiometry - instrumentation; Radiometry - methods; selenium; Selenium - chemistry; Selenium - radiation effects; Static Electricity; Transducers; X-Ray Intensifying Screens; X‐ and γ‐ray sources, mirrors, gratings, and detectors; X‐ray detection; X‐ray detectors; X‐ray effects; X‐ray imaging; X‐ray optics
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.1843353

Direct flat-panel detectors using amorphous selenium (a-Se) x-ray photoconductors are gaining wide-spread clinical use. The goal of our investigation is to understand the physical mechanisms responsible for ghosting, i.e., x-ray induced change in sensitivity that results in image persistence, so that the knowledge can be used to consistently minimize ghosting artifacts in a-Se flat-panel detectors. In this paper we will discuss the effect on x-ray sensitivity of charge trapping in a-Se, which is the dominant source for ghosting in a-Se flat-panel detectors. Our approach is to correlate ghosting in electroded a-Se detectors with the trapped charge concentration measured by the “time-of-flight” (TOF) method. All measurements were performed as a function of radiation exposure X of up to ∼ 20 R at electric field strengths of E Se = 5 and 10 V ∕ μ m . The results showed that the x-ray sensitivity decreased as a function of X and the amount of ghosting decreased with increasing E Se . The shape of the TOF curves changed as a result of irradiation in a manner indicating trapped electrons in the bulk of a-Se. The density of trapped electrons n t increases as a function of X . A method was developed to determine the values of n t in the bulk of a-Se from the TOF measurements, and to predict the corresponding change in x-ray sensitivity. Our results showed that a recombination coefficient consistent with that predicted by Langevin produced good agreement between calculated and measured x-ray sensitivity changes. Thus it can be concluded that the trapping of electrons in the bulk of a-Se and their subsequent recombination with x-ray generated free holes is the dominant mechanism for ghosting in a-Se.