TU‐G‐141‐03: An On‐Board 4D‐CBCT Reconstruction Technique Using Limited‐Angle Projections Based On Motion Modeling and Free‐Form Deformation (MM‐FD)

Purpose: To develop an on‐board 4D‐CBCT(OB‐4D‐CBCT) reconstruction technique using prior information and limited‐angle projections for 4D inter/intra‐fractional target verificationMethods: The OB‐4D‐CBCT at each phase is considered as a deformation of planning 4D‐CT at one selected phase (prior image). The MM‐FD technique solves the deformation field maps(DFMs) with a two‐step approach: 1. MM: a principal component analysis‐based method is applied to the planning 4D‐CT to extract a motion model. Coarse estimation of DFMs are obtained by optimizing motion model parameters to meet data‐fidelity constraint for limited‐angle on‐board projections. 2. FD: the coarse DFMs are further fine‐tuned by free‐form deformation based on data‐fidelity constraint and deformation‐energy minimization, using constrained‐optimization algorithm ASD‐POCS. OB‐4D‐CBCT is then reconstructed by deforming prior volume based on final DFMs. The 4D Digital Extended‐cardiac‐torso(XCAT) Phantom was used to evaluate MM‐FD. A lung patient with 3‐cm diameter lesion was simulated to have various anatomical and respirational changes from 4D‐CT to OB‐4D‐CBCT including respiration amplitude change, lesion size change, lesion average‐position change, and phase shift between lesion and body respiratory cycle. The lesions were contoured in both the reconstructed and ‘ground‐truth’ OB‐4D‐CBCT for comparison. 3D volume percentage‐difference (VD) and center‐of‐mass shifts(COMS) were calculated for evaluation. The MM‐FD technique was compared with MM‐only and FD‐only techniques. Results: For all patient scenarios, the mean original VD/COMS between prior volume and true OB‐4D‐CBCT were 137.65%/15.5mm. Using orthogonal‐15‐degree scan angle, the mean VD/COMS between reconstructed and true OB‐4D‐CBCT for MM‐only, FD‐only and MM‐FD techniques were 64.67%/4.9mm, 98.80%/12.1mm and 20.90%/1.3mm, respectively. For orthogonal‐30‐degree scan angle, the corresponding results were 63.87%/4.9mm, 79.92%/9.9mm and 15.23%/0.5mm. For single‐view 30‐and 60‐degree projections, the mean VD/COMS for MM‐FD technique were 32.67%/2.9mm and 19.67%/1.1mm, respectively. Conclusion: The MM‐FD technique substantially improves the reconstruction accuracy for OB‐4D‐CBCT using limited‐angle projections. It can potentially improve the inter/intra‐fractional 4D‐localization accuracy for lung SBRT. Research partially supported by grant from Varian Medical Systems.