A hybrid biomechanical intensity based deformable image registration of lung 4DCT

Deformable Image Registration (DIR) has been extensively studied over the past two decades due to its essential role in many image-guided interventions. Morfeus is a DIR algorithm that works based on finite element biomechanical modeling. However, Morfeus does not utilize the entire image contrast and features which could potentially lead to a more accurate registration result. A hybrid biomechanical intensity-based method is proposed to investigate this potential benefit. Inhale and exhale 4DCT lung images of 26 patients were initially registered using Morfeus by modeling contact surface between the lungs and the chest cavity. The resulting deformations using Morfeus were refined using a B-spline intensity-based algorithm (Drop, Munich, Germany). Important parameters in Drop including grid spacing, number of pyramids, and regularization coefficient were optimized on 10 randomly-chosen patients (out of 26). The remaining parameters were selected empirically. Target Registration Error (TRE) was calculated by measuring the Euclidean distance of common anatomical points on both images before and after registration. For each patient a minimum of 30 points/lung were used. The Hybrid method resulted in mean±SD (90th%) TRE of 1.5±1.4 (2.8) mm compared to 3.1±2.0 (5.6) using Morfeus and 2.6±2.6 (6.2) using Drop alone.