한국외대 / 김동식*
Lag signals occur at images sequentially acquired from a flat-panel (FP) dynamic detector in fluoroscopic imaging due to charge trapping in photodiodes and incomplete readouts. This lag signal produces various lag artifacts and prevents analyzing detector performances because the measured noise power spectrum (NPS) values are reduced. In order to design dynamic detectors, which produce low lag artifacts, accurately evaluating the detector lag through its quantitative measurement is required. A lag correction factor can be used to both examine the detector lag and correct measured NPS. To measure the lag correction factor, the standard of IEC62220-1-3 suggests a temporal power spectral density under a constant potential generator for the x-rays. However, this approach is sensitive to disturbing noise and thus becomes a problem in obtaining accurate estimates especially at low doses. The Granfors-Aufrichtig (GA) method is appropriate for noisy environments with a synchronized pulse x-ray source. However, for the x-ray source of a constant potential generator, gate-line scanning to read out charges produces a nonuniform lag signal within each image frame and thus the conventional GA method yields wrong estimates. In this paper, we first analyze the GA method and show that the method is an asymptotically unbiased estimate. Based on the GA method, we then propose three algorithms considering the scanning process and exposure leak, in which line estimates along the gate line are exploited. We extensively conducted experiments for FP dynamic detectors and compared the results with conventional algorithms.
Dong Sik Kim, Eunae Lee