Illustration 12: Large Thorax Phantom shown with overlay indicating: Left - Water Equivalent
Diameter = 33.8cm (OR=1) and Right - Ellipse based on Dw and OR (OR=1.14)
Since these metrics are dependent on the attenuation distribution, Dw and OR measurements
can vary across the entire anatomical region. In an anatomical area like the abdomen, which
contains little air pockets, the Dw is similar to the average physical diameter of the patient. But
for an area like the chest, which contains the lungs, Dw may be significantly smaller than the
average diameter.
An example of this behavior can be seen in the figure below of a scout torso phantom with the
measured Dw and OR at different locations.
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
Chapter 21 General Information 661
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