After loading the images from the PACS system, a volume of interest
for surface extraction should be defined within the data set. This can
be intuitively achieved by zooming and panning into the desired
section of the original images and by deselecting those slices, which
should not be involved in the 3D display process. If no volume of
interest is defined, the whole image sequence is used. The surface display is
initialized by selecting the corresponding display mode. As no interpolation
is used during projection to the
intermediate image plane, the voxels should be equal-sized in all
dimensions to avoid blurring due to stronger stretching in one
direction during the warp step. If necessary, to ensure equally sized voxels
and interactivity, the volume of interest is resampled to obtain a
new volume with a size of at most 
voxels. If a more accurate
view of some part of the volume is required, another volume of
interest can be constructed at this position and displayed within
another view of the workbench.
To improve the efficiency of working with the data, all frequently used interactions can be carried out by clicking into specific portions of the image. For supporting novice users, graphical tool tips appear if the mouse is moved over such a sensitive ``hot region''. Figure 4.18 shows, for example, the looking-glass symbol for the zooming region. Clicking on it and dragging the mouse zooms into and out of the data set. To avoid to distract expert users, the display of the hot-region symbols can also be disabled. A large region at the center of the image can be clicked to rotate the object, regions at the top of the image allow to specify the threshold and opacity for the surfaces. As the extraction of a new surface is the only non-interactive operation on the scene (see section 4.3.3 for details) the threshold is displayed numerically until the interaction is finished, the corresponding surface appears shortly after finishing the interaction. As mostly CT-data, which has a well-defined tissue-to-value correspondence, is visualized using surfaces, providing feedback only after the interaction has been finished poses no big problem. In addition, the surface threshold can be exactly specified using a numerical entry field. Currently, the parameters for two simultaneous surfaces can be defined using hot-regions within the image, which is sufficient for most applications.
Clipping planes can be defined and moved using six hot-regions at the bottom of the image. Two clipping planes perpendicular to each axis are defined, one for clipping data above, and one for clipping data below a specific coordinate value.
In addition, the windowing mechanism, which is familiar to radiologists,
can be used to enhance contrast and emphasize features of the original
volume data which is displayed on the clipping
planes. 
-movements of the mouse are used to define and modify the
window 
which maps all data values below 
to black, all
values above 
to white, and the values between and
to a uniform ramp of gray values.