Biomedical Engineering Reference
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discontinuous. Artificial loss of signal is usually easy to identify if caused by
the in-plane flow. It is due to the fact that the vessel geometry can be appre-
ciated above and below the in-plane segment. On several occasions, vessels
may become so intertwined that vascular loops are formed. Presaturation pulse
obliterates the signal intensity from inferior vessel taking course. Similarly, re-
versed flow in the internal or external carotid arteries will be undetectable. The
invisibility is because of the superior presaturation pulse. For better results,
the maximum intensity pixel (MIP) approach was suggested as described in the
following section.
3.2.1.4 2D-TOF Imaging of Venous Anatomy
Images of cortical veins have been generated with the use of 2D TOF imaging.
When superimposed on axial MR images, a map of the cortical veins can be
produced to relate intraparenchymal pathology with cortical venous anatomy.
This approach has been successful for preoperative localization of lesions prior
to surgery. For this, a presaturation pulse is applied to eliminate arterial signal
using slice thickness 2.0- 2.9 mm. The method allows the relatively fast coverage
of a large anatomical region without compromising in-flow enhancement. The
trade-off in this approach is the decrease in image-resolution.
3.2.1.5 MIP Projection Ray Tracing Technique
The MIP projection ray tracing technique is used for viewing 2D TOF angiograms
after data acquisition and reconstruction (see Fig. 3.10). As a result, multiple pro-
jection images are generated from volume of stacked axial slices' image data.
When volume data is projected onto a two-dimensional plane, each pixel in
the projection image depends on the pixels along each line, or ray, through the
volume of data. Several procedures can be used to determine pixel intensity
in the projection image. Using MIP, the projection pixel is assigned the maxi-
mum pixel intensity found along a ray traversing the imaging volume. Additional
parallel rays are passed through the volume until a complete projection image
of the vessels is obtained. Once all pixel intensities for a projection have been
calculated, the process may be repeated for any other projection angles (see
Fig. 3.11). Typically, 18 projection angiograms are obtained at 10 increments.
The collections of projected images may be viewed as a cine loop to give the
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