Volume rendering reconstruction (VR) is
a 3D semitransparent representation of the imaged structure. It has become the
favored 3D imaging technique with applications in every type of examination
performed with CT.
the advantage of VR than other 3D techniques
is all voxels (volume pixel) contribute
to the image. VR allows The images to display multiple tissues and show their
relationships between them
other 3D methods, VR displays are built by takes the entire volume of data,
calculate the contributions of each voxel (volume pixel) and manipulating data
along a line from the viewer’s eye through the data set, and displays the
resulting composite for each pixel of the display. However, VR techniques sum
the contributions of each voxel along the line. Each voxel is assigned an opacity
value based on its Hounsfield units. This opacity value determines the degree
to which it will contribute, along with other voxels along the same line, to
the final image. The process is repeated for the voxels along each line, with
each line producing one voxel in the VR image.
3D techniques, with VR no information
is ignored or discarded; every voxel contributes to the final image.
Examples of VR
images of the heart (A), the left atrium and pulmonary veins (B), and the
coronary artery tree (C). Images courtesy of the University of Michigan.
Volume rendering always accurately depicts 3D relationships,
especially on arterial phase–dominant images that show both arterial and venous
structures. Volume rendering not only allows display of the vascular anatomy
but also provides definition of soft tissue, muscle, and bone, which may
contribute to a more comprehensive understanding of pathologic processes.
The pixels in the final VR image can be assigned a
color, brightness, and degree of opacity. For example, normal soft tissue can
be assigned high transparency, contrasted vessels slight opaqueness, and bone strong
In many cases color is used, with the color intensity
varied to generate depth information for a traditional 3D impression. VR allows
the user a high degree of interactivity. The user can easily change the look of
the VR by changing variables such as the color scale, applied lighting, opacity
values, and window settings
The image can be rotated and viewed from any angle. By
varying opacity and window width and level functions, anatomy can be displayed
or made invisible. This allows the user to quickly classify structures based on
their attenuation. For example, adjusting the window settings can often remove
the soft tissue from the VR display so that the contrast-enhanced vascular
structures can be seen, without the need for time-consuming data set editing
CT angiography of the pulmonary vasculature.
Coronal MIP images (a, b) and
volume-rendered images (c, d) based on a
volume data set obtained with 64-section multi–detector row CT show an amazing
level of detail. Color mapping helps increase the 3D effect in d. The MIP images show a bit more vessel detail at the
periphery, produced with less operator interaction than was detail on the
In addition, volume rendering enables a
color display, which often improves the visualization of complex anatomy and 3D
relationships (Fig D).
The form of VR is Endolominal imaging that is
specifically designed to look inside the lumen of a structure. also called
virtual endoscopy, virtual bronchoscopy, and virtual colonoscopy.
internal vessels or organs are seen as if a virtual
endoscope is penetrating the body and viewing the organ from a virtual viewpoint.
The surface view with the possibility of making a virtual journey along a path
in a vessel or specific organ such as the colon, small intestine or the
The technique aims to simulate the view of an
endoscopist, hence it is commonly referred to as virtual endoscopy. Endoluminal
imaging visualizes a structure as if it were hollow and the viewer were inside
Because manually tracking such a path is time-consuming,
some software programs automatically calculate a centerline path through the
air or contrast-containing structure. Users may also change their virtual field
of view. That is they may look forward, backward, or to the sides and get
closer to the luminal wall or further away. Software can also correlate findings
on the 3D endoluminal imaging with the 2D cross-sectional source images to
allow better characterization and localization of abnormalities
Volume rendering is a flexible, accurate 3D imaging
technique that can help the radiologist more effectively interpret the large
volumes of data generated by modern CT scanners. To obtain accurate results
Volume rendering is widely used as part of CTA and
MRA, and in various applications such as cardiac imaging, orthopedic
applications and others.
Reconstructed 3-D data offers several
1. It enhances viewing of pathology
2. It equips
radiologists to deal with the large data sets that are available with the new
multi-detector CT scanners and to more easily compare current and previous
3. It improves service to referring physicians, since
selected 3-D images can be attached to the radiology report. These images
illustrate the diagnosis and may even be shown to patients while discussing the
condition and recommended treatment.