The ITK (Insight Segmentation and Registration Toolkit) provides various image processing algorithms including multi-dimensional image enhancement, segmentation, and registration. The toolkit is an open-source software development framework based on object-oriented C++ scheme. Since the ITK supports cross platforms and covers the Apache 2.0 license, it allows the users to modify and distribute the developed source codes freely and easily. For these reasons, the ITK is widely used for processing and analyzing medical images. It also supports GPU acceleration for advanced users.