Technical note: Preprocessing of portal images to improve image quality of VMAT‐CT

Abstract

BackgroundThe concept of volumetric modulated arc therapy‐computed tomography (VMAT‐CT) was proposed more than a decade ago. However, its application has been very limited mainly due to the poor image quality. More specifically, the blurred areas in electronic portal imaging device (EPID) images collected during VMAT heavily degrade the image quality of VMAT‐CT.PurposeThe goal of this study was to propose systematic methods to preprocess EPID images and improve the image quality of VMAT‐CT.MethodsOnline region‐based active contour method was introduced to binarize portal images. Multi‐leaf collimator (MLC) motion modeling was developed to remove the MLC motion blur. Outlier filtering was then applied to replace the remaining artifacts with plausible data. To assess the impact of these preprocessing methods on the image quality of VMAT‐CT, 44 clinical VMAT plans for several treatment sites (lung, esophagus, and head & neck) were delivered to a Rando phantom, and several real‐patient cases were also acquired. VMAT‐CT reconstruction was attempted for all the cases, and image quality was evaluated.ResultsAll three preprocessing methods could effectively remove the blurred edges of EPID images. The combined preprocessing methods not only saved VMAT‐CT from distortions and artifacts, but also increased the percentage of VMAT plans that can be reconstructed.ConclusionsThe systematic preprocessing of portal images improves the image quality of VMAT‐CT significantly, and facilitates the application of VMAT‐CT as an effective image guidance tool.

IMAGE QUALITY IMPACT OF DIFFERENT PHOSPHOR ACTIVATOR MATERIALS IN Gd 2O2S BASED EPID SYSTEMS

“…In Figure 7, the experimental (aMTF) is demonstrated. The difference between the theoretical MTF and the experimental aMTF are due to the effect of pixel sampling, the effect of scatter radiation and any software algorithms that are imposed in the image [6], [11]. The theoretically calculated MTF of Gd2O2S:Tb demonstrates higher values than the experimental one, for the available spatial frequencies 0 up to 8 lp/cm.…”