VolView tutorial

VolView home page

VolView 2.0 is available in free and professional versions, the professional version having more features available.  It is a sophisticated volume rendering program built on top of the VTK platform.  This tutorial shows how to use VolView to view CT data as if it is a "virtual radiograph".

Screenshots at various points along the track are included and menu commands are in courier font.

Opening the volume file and making it manageable

To open a stack of DICOM images, one simply needs to have them all in the same folder and open one of them from the "open file" menu.  The program does the rest by seeking out all other images with the same spacing and making one big conglomerate file.

File>Open



Since volume rendering often eats up a lot of memory, usually the first thing I do is to crop out excess space that is not relevant - usually anything without a bone.

View>Cropping>Enable Crop



Applying the crop filter shrinks the file size and makes the file more manageable.

View>Filters>Filter>Utility>Crop
            >Apply Filter



If you have a system which is running on modest processor speeds or memory sizes, applying a shrinking filter reduces the number of voxels that need to be rendered and also can help speed things up.  Speed can also be increased by setting interpolation to "nearest" rather than "linear" in the Appearance menu and reducing the amount of Z-axis subsampling.  The quality of the final image is degraded slightly with all of these steps, but the gains in speed make them attractive.

View>Filters>Filter>Utility>Shrink Filter(VTK)
            >Apply Filter



Before going on to fiddling with the appearance settings, I usually save a file at this stage to make reloading a lot quicker down the track.

File>Save Volume



Setting the appearance


The appearance menu has everything that we need to change the appearance of the picture in the volume rendering window.  As we want to look at the CT data like a radiograph, do not enable shading.  "Interactive apply" is nice, but will be irritatingly slow on anything other than a top end system.
 Setting the "Gradient Opacity Mapping" slider is not needed here.  This sets opacity according to how large a density gradient next to the voxel is.

Set the "Scalar Color Setting" to white.  This sets all the voxels to white regardless of density value.  If you want, you can play around with the menu and get some pretty results.

View>Appearance>Scalar Color Mapping



Setting the "Scalar Opacity Mapping" is where all the fine adjustment to the volume is done.  Changing the location of point 2 along the x-axis defines the cut off between what is visible and what is not.

View>Appearance>Scalar Opacity Mapping



The easiest thing is to move it until the soft tissues just dissappear - this is often just after the hump on the histogram which denotes the soft tissues.

View>Appearance>Scalar Opacity Mapping



Next thing is to set the opacity mapping curve to the right of point 2.  Easiest is to place point 3 somewhere along the right hand side of the graph, but you can play with the settings as you like depending on how you want things to look.  If looking at a CT with metalware involved, usually a bit more tweaking of the opacity mapping graph is needed to make the picture nice.

View>Appearance>Scalar Opacity Mapping





An unadvertised feature of VolView 2.0 is the ability to run it in stereo.  You must have a graphics card capable of quad-buffered stereo or else things will not work.  Run volview -3 from the command line and enjoy.  Thanks to Lisa Avila from Kitware for the tip.

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Ezekiel Tan
Mar  2004
v.1.0