184
Level 2
Board Level
anterior and posterior wall of the vessel, underestimation from
angle is not possible.
In comparison, for area measurement tracings of objects that are
not uniform in cross-section such as heart chambers and diseased
vessels, there are two different sources of error: location and angle.
Clearly, even if the image plane is perpendicular to the structure,
the cross-sectional area measured is dependent on location. For
this example, there is a larger cross-sectional area in the middle of
the object than at either end. The other error source is related to
angle. As in the previous example, off-axis beams tend to increase
the cross-sectional area.
Fig. 62
3D spiral CT of an abdominal aortic aneurysm
(possible measurement errors)
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Scenario 3:
Fig. 63
Longitudinal plane and area underestimation
Along the longitudinal axis of the nonuniformcross-sectional object,
there are again two distinct sources of error: location and angle. If
the imaging plane does not intersect this object in the center, then
the area measurement will naturally be smaller. In this case, if the
angle is incorrect,the area measurement will be smaller than reality.
In the cardiac world,when this occurs when viewing a chamber, the
image is said to be “foreshortened.”
Calculated From the Radius
Just as with tracing, angle effects can be a significant source of error
in area measurements calculated from a radius. Recall that area is
a two-dimensional measurement whereas the radius is only a one-
dimensional parameter. As discussed in Appendix A: Mathematics,
the area is proportional to the radius squared, or:
Area r
∝
2
.
Therefore,any error in the radiusmeasurement is squared to produce
the area calculation. This source of error is depicted in
Figure 64
.
Fig. 64
Errors in radius measurement are squared for area
KEY CONCEPT
18. Video Display and Monitors
Most newer ultrasound systems are nowusing Liquid Crystal Display
(LCD) monitors. Before the proliferation of LCD technology, most
monitors were driven by cathode ray tube (CRT) technology. The
older technology of CRT monitors went through a few iterations
such that later models were created with higher definition and faster
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