Local incompressible registration for liver ablation surgery assessment

Purpose: In liver microwave ablation (MWA) surgery, the ablation area covers the tumor to generate tissue necrosis and treat the cancer. As the liver deforms during the operation, deviation between the target area determined during preoperative planning and the resultant ablation area is inevitable. Therefore, an accurate assessment of tumor coverage is crucial for treatment. Through registration between the pre- and postoperative livers, the ablation area is warped on the preoperative liver for the computation of tumor coverage. However, large deformations between the pre- and postoperative livers are caused by multiple factors, and these diverse deformations make registration a challenging task. The purpose of this paper was to develop an automatic method that can accurately register post- to preoperative livers. Methods: In the proposed method, nonrigid deformations caused by respiratory movement and edema are separately considered and estimated by the local incompressible model in the registration of livers. The pre- and postoperative livers are first aligned by a rigid registration based on a convex hull. In the nonrigid registrations, local incompressible constraints are then set on the liver and the ablation area to estimate the deformations caused by respiratory movement and edema, respectively. The concatenation of the rigid and nonrigid deformations is used to warp the ablation area on the preoperative liver. Results: The proposed method was evaluated using clinical CT datasets from 20 patients. The Dice similarity coefficient (DSC) between the preoperative and warped postoperative livers is 94.35%, the mean surface distance (MSD) between the livers is 1.65 mm, the mean Hausdorff distance (HDD) between the livers is 3.36 mm, and the mean corresponding distance (MCD) between the corresponding landmarks is 1.70 mm. Compared with five other state-of-the-art methods, the proposed method achieves automatic ablation assessment with highly accurate registration. Conclusions: The proposed method achieves a high accuracy for registering the livers. The sizes and positions of the ablation area and tumor are accurately compared for the assessment of ablation surgery.