Biomedical Engineering Reference
In-Depth Information
The presence of vascular stenosis may produce areas of increased flow ve-
locity within the stenosis segment. So, these regions (having relatively less flow)
further slow the flow in the poststenotic vortices. Turbulence is also frequently
encountered distal to the stenosis. These factors will produce intravoxel incoher-
ence, spin saturation in the slowly flowing eddies, and loss of signal intensity.
Large ulcerations are often difficult to image because of the slow and com-
plex flow within the location of ulceration. Use of the shortest TE and smallest
voxel size can substantially minimize signal loss. In spite of these measures,
some signal will be lost adjacent to the stenosis region. It is possible to over-
estimate the degree of stenosis when interpreting the carotid MR angiograms
(see Fig. 3.9). In-plane flow can also compromise the quality of 2D TOF an-
giograms. When a blood vessel runs parallel to the imaging plane, the blood will
experience multiple RF pulses. As a result, the blood will eventually become
saturated. The portion of the vessel coursing through the imaging plane may ex-
hibit little or no signal intensity. These vessels appear as artificially narrowed or
Figure 3.9:
Three-dimensional coronal inflow targeted MIP angiography of
carotid arteries is shown. Boxes in both rows represent isolated single carotid
arteries at 1.5 mm 32 partitions.
