The purpose of this study was to address image lag in target tracking and its influence on the accuracy of target tracking. Fluoroscopic images were obtained using a direct type of dynamic flat-panel detector (FPD) . Image lag properties, modulation transfer function (MTF) , and profile curves were measured. Digital phantoms of metal sphere with and without blurred edge due to image lag were simulated using the results, respectively, and then superimposed on breathing chest radiographs of a patient. The moving target with and without image lag was traced using a template-matching technique. In the results, the image lag for the first frame after X-ray cutoff was 2.0% and decreased to less than 0.1% in the fifth frame. In the measurement of profile curves on the edges of tungsten material plates, the effect of image lag was seen as blurred edges of the plate. The blurred edges of the plate were also indicated as reduction of MTF. However, the target could be traced without tracking error. The results indicated that there was no effect of image lag on target tracking in usual breathing speed in a radiotherapy situation.
