Medical images are typically generated as 2D projection images or sequences, as in radiography, or as stacks of 2D image slices, as in tomographic imaging. To use them for diagnostic or interventional purposes, the image data can be visualized as such, but they can also be shown as resliced images or as three-dimensional (3D) images. This chapter discusses the clinically relevant visualization methods.
Medical images are used not only for diagnostic purposes, but also often serve as the basis for a therapeutic or surgical intervention during which the instruments are guided by and navigate through the image content. Images can be obtained prior to and during surgery. Preoperative images, such as CT, MRI, and PET, can be used for accurate planning and can be acquired with the available diagnostic imaging modalities. However, the planning has to be accurately applied to the patient in the operating room. This requires a method to register geometrically the preoperative images and planning data with the surgical instruments. A computer can assist in both this planning and the registration, a process known as computer-assisted intervention.
To plan or simulate an intervention, preoperative images are imported in a 3Dgraphics computer workstation and manipulated as real 3Dvolumes. Planning is surgery-specific and typically consists of defining linear or curved trajectories to access a lesion, to position an implant, to simulate ablations and resections, or to reposition resected tissue.
Stereotactic brain surgery played a pioneering role in the development of computer-assisted interventions. It is based on the principle that a predefined area in the brain can be approached by a surgical instrument, such as an electrode or a biopsy needle, through a small burr hole in the skull. In order to realize this, a stereotactic frame (see Figure 8.1) with inherent coordinate system was developed and fixed to the brain. Images, planned trajectories, and instruments are all defined in this coordinate space. The first frame was built in 1908 by Horsley, a physician, and Clarke, an engineer. This instrument allowed them to reach a predefined area in the brain of a monkey (Macacus rhesus) with an electrode guided through a small burr hole in the skull. To define the target location, a brain atlas was used. It consisted of a collection of topographic maps with sketches of sagittal and frontal intersections of a standard Macacus rhesus brain.