## 6 Brief Overview of the CAD Modeling Process for the Reconstruction of Cranial Defects  
In general, image data are captured from the bioimaging of CT scans for the design of a CAD model of skull defect reconstruction. The patients with skull defects will receive CT scans of the brain with slice thickness from the skull base to convexity, including the region of the skull defect. It is understood that a high-resolution CT scan represents a slice as thin as 1-2 mm. A 1.25 mm thick of CT slice has been shown for a 3-D titanium mesh reconstruction of a defective skull [26].  
Advances in hardware and software have increased the feasibility of accurate 3-D CAD reconstruction of skull skeletons [12; 13; 14]. The two-dimensional (2-D) images of brain CT scans can be edited with medical institution-specific or commercial software. As shown in Figure 2B,D,F, and Figure 3G,L, by using an image-editing interface approach to process the CT digital information, the contour of the skull defect can be manually reconstructed under the CT bone window image. The image-editing interface provides a region-growing function to reconstruct the flap and the original cranial skull [43], so that the external appearance and symmetry of the CAD model can therefore be verified. Specifically, the symmetry of the reconstructed 3-D artificial CAD flap will be simultaneously evaluated via axial, coronal, and sagittal views (as shown in Figure 2C,E,G and Figure 3H,J,K,M).  
Figure 1: Diagram of computer-assisted design/manufacturing (CAD/CAM) techniques and their potential application in perspective brain sciences.  
Figure 2: A 3-D CAD model for the skull defect reconstruction of case 1. (**A**) Initial computed tomography (CT) scan, revealing malignant in the left cerebral hemisphere with profound brain swelling, causing brainstem compromise. The patient received cranioplasty using autogenous bone graft. Resorption of autogenous bone has been shown with single arrows in (**D**). The 3-D CAD/CAM-fabricated implants will be made for surgery. Reconstructed contour of the skull defect as labeled in red, axial view in (**B**), coronal view in (**D**), and sagittal view in (**F**). Curvature of bony loss, double arrows in (**B**) can be compensated using newly reconstructed contour. Reconstructed 3-D models presented as the axial view in (**C**), coronal view in (**E**), and sagittal view in (**G**). The reconstructed artificial flap is labeled in green.  
Figure 3: A 3-D CAD model for the skull defect reconstruction of case 2. Axial (**A**,**B**), and coronal (**C**,**D**) CT images showing supra- and infratentorial acute epidural hematoma with mass effect. (**E**,**F**) The 3-D CAD reconstructions demonstrating skull defects in the demonstrated case underwent decompressive craniectomy and removal of hematoma. After having a full recovery of consciousness, the patient received cranioplasty using a 3-D CAD/CAM-fabricated titanium mesh. The reconstructed contour of the skull defect, as labeled in red, axial view in (**G**), coronal view in (**I**), and sagittal view in (**L**). The reconstructed artificial flap was labeled in green, as shown in the axial view (**H**), coronal view (**J**,**K**), and sagittal view (**M**). (**N**) CAD/CAM titanium implants for the reconstruction of skull defects. The patient, fixed with the Mayfield skull clamp, was placed in a prone position during the operation. The skull defect was located at the left of posterior fossa. Note that the titanium implant smoothly follows the contour to the cranial curvature of posterior fossa. R: right; L: left.