The coronary arteries are three dimensional structures, but angiography produces two dimensional images. For this reason, multiple views are required to fully understand the image. A classic example of this is shown below:
Figure 1: The importance of two views
Figure 2-1: Severe stenosis with parallel alignment. Camera sees a small sliver of opacified vessel.
Now imagine a coronary artery with very severe stenosis, and a slit-like section of lumen. If the viewing angle is parallel to the axis of the slit, it will be obvious to the viewer that there is significant stenosis. This is shown below. The yellow represents cholesterol plaque, and the black slit represents radiopaque contrast.
Figure 2-2: Severe stenosis with perpendicular alignment. Camera sees contrast opacification nearly from wall to wall, disguising the severity of the stenosis.
On the other hand, if the viewing angle is perpendicular to the axis of the slit, it would be very difficult to discern the presence of stenosis.
In addition to the presence of eccentric stenoses which can be missed as described in the section above, angiographers must also consider the course of the vessel.
First, imagine viewing a vessel parallel to its course.
From this angle, the vessel will appear as a circle. The double headed arrow is the maximal diameter of contrast visible to the image intensifier. It would be very easy to overlook the severe stenosis.
Figure 3-2: When the vessel is parallel to the imaging plane, the entire length of the vessel is compressed into a single circle when projected onto a plane.
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Viewing a vessel in line with its path produces a foreshortened image, and can disguise the presence of stenoses.
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Now, imagine viewing a vessel perpendicular to its course.
Figure 3-1: When the vessel is perpendicular to the imaging plane, the stenoses will be well visualized.
From this angle, the vessel will be seen in its entirety from one end to the other. Any stenosis in the middle of the vessel will be seen easily.
For example, the double headed arrow in Figure 3-1 shows the maximal diameter of contrast visible to the image intensifier — not very much!
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When vessels are imaged in the ideal fashion (perpendicularly), this is colloquially referred to as “laying the vessel out.”
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Many novice angiographers fail to appreciate the “purpose” of a given angle. Just because a given vessel is opacified and within the field of view does not mean the view is well suited to evaluating the vessel of interest. In addition to foreshortening, another common reason for poor vessel visualization is overlap with another vessel.
Figure 4-1: LAO cranial view of the left sided vessels. The mid to distal LAD is well visualized, but many of the other vessels are overlapping with one another making them hard to tell apart.
Figure 4-2: Same as figure 4-1, but with annotated anatomy.
For example, consider Figures 4-1 and 4-2. In this view, the mid to distal LAD and most of the diagonal branch (D1) are fairly well visualized. By contrast, it is almost impossible to trace the course of the LCx and OM. They are very overlapped. When I added the annotations, I needed to repeatedly skip forward and back a few frames and compare other angles to figure out which vessel was which. I am still not fully certain I labeled them correctly! But that is not the point of this view. This view is really targeted at LAD and the diagonal bifurcations.