Liver CBCT Reconstruction by Prior-Knowledge Guided Motion Modeling and Biomechanical Modeling

Abstract

We developed a new liver CBCT reconstruction technique, which solves a deformation vector field (DVF) to deform a prior high-quality liver CT/CBCT to the new CBCT. The DVF also enables fast on-board liver tumor tracking, dose warping, and adaptive radiation therapy. The new liver CBCT reconstruction technique is based on the foundation of “2D-3D deformation,” which solves the DVF by matching 2D projections simulated from the deformed prior image to 2D on-board CBCT projections. The accuracy of 2D-3D deformation is limited by the low-contrast liver tumors, which lack sufficient intensity variations from liver parenchyma in 2D projections. The new technique applies two strategies to enhance the accuracy of 2D-3D deformation: 1) prior-knowledge guided motion modeling; and 2) finite-element-analysis based biomechanical modeling. To improve the DVF accuracy at the liver boundary, especially for the inferior liver boundary, the liver is contoured on a prior liver 4D-CT and the contoured liver volume at each phase is then density-overridden to enhance the boundary contrasts. Deformable registration is then performed between the 4D-CT phase images to study the liver boundary motion pattern to extract a motion model. Based on the patient-specific liver motion model, an initial DVF is generated and input into 2D-3D deformation for refinement. Based on the refined DVF, biomechanical modeling is further applied to improve the intra-liver DVF accuracy. Biomechanical modeling uses the DVF at the liver boundary to drive finite element analysis, which fine-tunes the DVFs within the liver volume, including those of the tumor. The fine-tuned DVF is fed back into 2D-3D deformation to form an iterative loop until convergence. The efficacy of the reconstruction algorithm was tested on 3 liver cancer patients. Each patient had a prior CT and a new CT. On-board cone-beam projections were simulated from the new CT for reconstruction. The low-contrast liver tumors were contoured on the prior and new CT images. The new tumor contour (“ground-truth”) was compared against those propagated from the prior tumor contour by the solved DVFs. Their DICE coefficients were calculated to evaluate the reconstruction accuracy. Compared with the other techniques, the new technique generated CBCTs with the highest accuracy (Table 1) for the liver tumor.Abstract 206; Table 1The DICE coefficient between the deformed tumor volume and the “ground-truth” tumor volumePatient No.No. of projections for reconstruction2D-3D deformation2D-3D deformation w/ biomechanical modelingThe new technique150.6020.8510.924100.6220.8750.926200.6490.8920.927250.6580.8350.885100.6710.8690.889200.6710.8820.897350.4110.5630.772100.4110.5980.780200.4110.6610.802 Open table in a new tab The new technique substantially improved the accuracy of the reconstructed liver tumor volume. It can also reduce 3D/4D-CBCT imaging dose by acquiring fewer projections for reconstruction.