Cutting Planes
You can select directly the cutting plane if you are using a mouse. Use the scroll wheel to navigate the different slices. By selecting a point outside the cutting planes you enable zooming in and out. Rotate using your fingers or the mouse.
A cut plane is exactly as the name implies. A plane that cuts through, in this case, a volume. It is quite often used in Medical Visualization. You can look examples of applications using e.g. the MITK framework or with 3D Slicer and Osiris. The most common planes to use are the axial (horizontal), coronal (frontal) and the sagittal (longitudinal) plane. These planes are axis aligned as you can observe in the viewer above.
Volumes in medical imaging are typically described using a regular grid. Each cell in the grid can be addressed by index (i, j) in two dimensions or (i, j, k) in three dimensions, and each vertex has coordinates ( i ⋅ dx , j ⋅ dy ) in 2D or ( i ⋅ dx , j ⋅ dy , k ⋅ dz ) in 3D for some real numbers dx, dy, and dz representing the grid spacing. The spacing is given in the DICOM dictionary usually associated with the volume.
The axis-aligned plane can use this information to create an image i.e. the plane inside the volume can be subdivided into a grid, where each grid-cell takes a point describes a coordinate inside the volume. The values at each location is then (tri-linearly) interpolated and set in a texture to be displayed. Rather simple all around.
The volume comes from the publicly available Designed Database of MR Brain Images of Healthy Volunteers . I'll go into detail of the image modalities in a different post.