Biomedical Engineering Reference
In-Depth Information
Rapid back-projection reconstruction
Although volume visualization is an area of continuing
progress, there is another need that is growing with new
types of imaging techniques: visualization of a 3D struc-
ture using a set of projection images, i.e., images acquired
from different orientations. Concerns of exposure to ra-
diation and the need for high speeds make projection
images a preferred mode of acquisition for some appli-
cations. Such requirements may be difficult to meet while
acquiring the images in 3D mode. In this case, if the
geometric nature of the object is known, a representative
shape of the object can be reconstructed using image
back-projection techniques. Although this principle is
similar to the tomographic reconstruction used in CT, lack
of sufficient accuracy and angular resolution may restrict
the use of such approaches. Back-projection refers to
a graphic technique in which a raster or scan line of an
image is smeared, i.e., painted parallel to the direction in
which the image is acquired, on an image reconstruction
buffer. By back-projecting corresponding scan lines from
each projection image into the reconstruction buffer,
transverse-images of the desired 3D image can be
obtained (
Fig. 6.6-10
)
[29]
. Recent advances in graphics
hardware, such as texture mapping, enable rapid re-
construction using the back-projection technique rather
than traditional generic CPU based approaches.
functions through simulation and modeling. These visu-
alization techniques belong to two classes of fifth gener-
ation systems.
6.6.4.1 Fifth generation systems I
(modeling and simulation)
Modeling represents a significant thrust in biomedical
research. It attempts to build analytical formulations that
describe physiological functions and involves such multi-
disciplinary fields as finite-element modeling and com-
putation fluid dynamics (
Figs. 6.6-11-6.6-14
). Visualiza-
tion needs in modeling have been minimal in the past,
primarily due to the complexity of the models. The focus
was on quantitative or analytical verification rather than
on visual validation thought to be optional. However, over
the years, the scope of models that could be built and the
extent to which they can be studied became limited when
all the necessary input information to define model pa-
rameters became unavailable. The new approach that was
needed to address such modeling problems emerged in
visualization, since it could enable researchers to qualita-
tively validate the model. Visualization techniques that
are employed in modeling packages are more suitable for
engineering applications than for biomedical applications,
which deal with large volumetric images. Visualization
techniques that are needed for biomedical modeling and
simulation applications should be compatible with finite-
element
[49-51]
and fluid dynamics techniques
[52]
.
This will enable researchers to build rapid prototypes of
themodels for quick visual validation and determine some
parameters of the model
[53]
. Also, this approach can
enable them to build the model using actual geometry
rather than the simplified representation the earlier
modeling techniques preferred.
6.6.4 Imitative visualization
The development of illustrative and investigative visual-
ization systems paved the way to the development of
visualization systems that focus on creating realism
through visualization. Such virtual realism could be ex-
hibited in visualizing the structures or in visualizing the
Figure 6.6-15 Virtual workbench. (Left) Schematic diagram illustrating the concept of reach-in work space. (Right) Prototype model of the
workbench. (Photos courtesy of Timothy Poston, Luis Serra.)
