Fig. 1. Illustration of model-based brain shift compensation. The patient's head and intraopera-tive images (e.g., iUS, iSV, and iFI) are continuously monitored using a common tracking sys-tem. The resulting sparse displacement data are assimilated into a biomechanical model to pro-duce whole-brain deformation from which an updated MR volume is generated for subsequent neuronavigation.

Fig. 2. Typical brain deformation field estimated by the inversion data assimilation scheme us-ing the brain-skull contact boundary conditions that allow the brain parenchyma to move towards or away from the skull. Thick yellow lines: cross-section of cortical surface acquired from co-registered iSV; Solid/dashed white lines: cross-sections of model-updated brain surface; Thin dashed white lines: skull inner-surface.

Fig. 3. Displacement map generated by applying a transformation matrix on the triangulated tu-mor boundary surface nodes.

Fig. 4. Overlays of 3D iUS before (red) and after (green) dural opening using transforms obtained from the optical tracking (a) or rigid body re-registration (b). Feature alignment was significantly improved after B-Spline nonrigid registration (c), suggesting the effectiveness of the registration technique in capturing feature displacement. The resulting parenchymal feature displacement vectors are shown in (d).

Fig. 5. Illustration of stereovision surface reconstruction: rectified left (a) and right (b) camera images after dural opening, which limit the search for correspondence points along a horizontal line. The reconstructed 3D cortical surface is shown in pMR image space (c). Cross-sections of a representative pMR axial image with respect to the reconstructed cortical surfaces (tumor cavity wall not included) are shown for four distinct surgical stages: (0) before dural opening; (1) after dural opening; (2) right before the start of tumor resection; and (3) after tumor resection.

Fig. 6. Representative axial and coronal uMR images after dural opening (left), in the beginning (~10% into resection; second column), middle (~50% into resection; third column), and at the end of resection (right), overlaid with corresponding cross-sections generated from reconstructed iSV surfaces (yellow line). Axial images are also overlaid with iUS (green). The alignments be-tween iUS, iSV, and uMR with parenchymal features indicate consistency between model esti-mates and intraoperative measurements.