112 CATALYZING INQUIRY
benches. Similarly, a scanned image of a magazine page may contain text and graphics (e.g., a picture of
a park). Segmentation refers to the process by which an object (or characteristics of the object) in an
image is extracted from image data for purposes of visualization and measurement. (Extraction means
that the pixels associated with the object of interest are isolated.) In a biological context, a typical
problem in image segmentation might involve extracting different organs in a CT scan of the body.
Segmentation research involves the development of automatic, computer-executable rules that can
isolate enough of these pixels to produce an acceptably accurate segmentation. Segmentation is a central
problem of image analysis because segmentation must be accomplished before many other interesting
(a) (b) (c)FIGURE 4.6 Volume renderings of electrode array implanted in feline skull.
In this example, scanning produced a 131 MB 16-bit volume of 425 × 420 × 385 samples, with resolution of 21 ×
21 × 21 microns. Renderings of the volume were generated using a ray-tracing algorithm across multiple proces-
sors allowing interactive viewing of this relatively large dataset. The resolution of the scan allows definition of the
shanks and tips of the implanted electrode array. Volumetric image processing was used to isolate the electrode
array from the surrounding tissue, highlighting the structural relationship between the implant and the bone.
There are distinct CT values for air, soft tissue, bone, and the electrode array, enabling the use of a combination of
ray tracing and volume rendering to visualize the array in the context of the surrounding structures, specifically
the bone surface. The volume is rotated gradually upward in columns (a), (b), and (c), from seeing the side of the
cochlea exterior in (a), to looking down the path of the cochlear nerve in (c). From top to bottom, each row uses
different rendering styles: (1), summation projections of CT values (green) and gradients (magenta); (2), volume
renderings with translucent bone, showing the electrode leads in magenta.
SOURCE: Courtesy of Chris Johnson, University of Utah; see also http://www.sci.utah.edu/stories/2004/
spr_imaging.html.