An empirical method for lag correction in cone-beam CT

Image lag degrades image quality in cone-beam CT (CBCT), resulting in contrast reduction, lack of CT number accuracy and uniformity, and skin-line artifacts. The magnitude of such degradation and the extent to which imaging performance can be improved by means of a lag correction method were investigated. Measurements were performed using a radiotherapy CBCT guidance system (Elekta Synergy XVI, Elekta Oncology Systems, Atlanta, GA), for which the imaging system is based upon a RID1640-AL1 flat-panel imager (Perkin Elmer, Wiesbaden, Germany). Image lag and its relationship to various parameters including signal magnitude, photon energy, and frame number were investigated, and an empirical lag correction method was developed to manage lag artifacts. The correction method was simply the subtraction from the current frame by previous frames weighted by the temporal response function. The CatPhan 500 phantom (The Phantom Laboratory, Salem, NY) within an irregularly shaped body annulus was used to demonstrate the magnitude of artifacts with and without lag correction. CBCT images after correction demonstrated improvement in skin-line reconstruction, CT number accuracy, image uniformity, and contrast-to-noise ratio. Lag artifacts can be reduced by means of algorithmic correction of the projection images. Lag correction is most important for all shapes of objects having contrast inserts. For circular/cylindrical objects, lag correction does not improve the skin-line artifact but can improve low contrast visibility adjacent to high contrast objects.