110 CATALYZING INQUIRY
The discussion below focuses only on a narrow slice of the very general problem of biological imaging,
as a broader discussion would go beyond the scope of this report.
4.4.12.1 Image Rendering^146
Images have been central to the study of biological phenomena ever since the invention of the
microscope. Today, images can be obtained from many sources, including tomography, MRI, X-rays,
and ultrasound. In many instances, biologists are interested in the spatial and geometric properties of
components within a biological entity. These properties are often most easily understood when viewed
through an interactive visual representation that allows the user to view the entity from different angles
and perspectives. Moreover, a single analysis or visualization session is often not sufficient, and pro-
cessing across many image volumes is often required.
The requirement that a visual representation be interactive places enormous demands on the
computational speed of the imaging equipment in use. Today, the data produced by imaging equip-
ment are quickly outpacing the capabilities offered by the image processing and analysis software
currently available. For example, the GE EVS-RS9 CT scanner is able to generate image volumes with
resolutions in the 20-90 mm range, which results in a dataset size of multiple gigabytes. Datasets of
such size require software tools specifically designed for the imaging datasets of today and tomorrow
(see Figure 4.5) so that researchers can identify subtle features that can otherwise be missed or misrep-
resented. Also with increasing dataset resolution comes increasing dataset size, which translates di-
rectly to lengthening dataset transfer, processing, and visualization times.
New algorithms that take advantage of state-of-the-art hardware in both relatively inexpensive
workstations and multiprocessor supercomputers must be developed and moved into easy-to-access
software systems for the clinician and researcher. An example is ray-tracing, a method commonly used
in computer graphics that supports highly efficient implementations on multiple processors for interac-
tive visualization. The resulting volume rendition permits direct inspection of internal structures,
without a precomputed segmentation or surface extraction step, through the use of multidimensional
transfer functions. As seen in the visualizations in Figure 4.6, the resolution of the CT scan allows
subtleties such as the definition of the cochlea, the modiolus, the implanted electrode array, and the lead
wires that connect the array to a head-mounted connector. The co-linear alignment of the path of the
cochlear nerve with the location of the electrode shanks and tips is the necessary visual confirmation of
the correct surgical placement of the electrode array.
In both of the studies described in Figure 4.5 and Figure 4.6, determination of three-dimensional
structure and configuration played a central role in biological inquiry. Volume visualization created
detailed renderings of changes in bone morphology due to a Pax3 mutation in mice, and it provided
visual confirmation of the precise location of an electrode array implanted in the feline skull. The
scientific utility of volume visualization will benefit from further improvements in its interactivity and
flexibility, as well as simultaneous advances in high-resolution image acquisition and the development
of volumetric image-processing techniques for better feature extraction and enhancement.
4.4.12.2 Image Segmentation^147
An important problem in automated image analysis is image segmentation. Digital images are
recorded as a set of pixels in a two- or three-dimensional array. Images that represent natural scenes
usually contain different objects, so that, for example, a picture of a park may depict people, trees, and
(^146) Section 4.4.12.1 is based on material provided by Chris Johnson, University of Utah.
(^147) Section 4.4.11.2 is adapted from and includes excerpts from National Research Council, Mathematics and Physics of Emerging
Biomedical Imaging, National Academy Press, Washington, DC, 1996.