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7 Scout Based Attenuation Characterization
A patient’s size, shape and density define the attenuation characteristics seen by the X-rays. An
understanding of the desired image quality (noise and contrast) and the attenuation
characteristics of the region being scanned can be used to determine the required scan
technique factors (kVp and mA).
Automatic characterization of the patient’s attenuation can enable a CT system to aid the user in
determining the scan technique factors required to achieve the desired image quality. The scout
image is a measurement of the distribution of the attenuation for the patient. This distribution of
attenuation can be analyzed to give a metric of total attenuation (or patient size) and a metric of
patient shape.
One size metric that can be used is Water Equivalent Diameter, Dw. This is the diameter of a
uniform cylinder of water that gives the same total attenuation as the patient. Although Dw
assumes a circular water object, patients are rarely circular. Most anatomical regions, however,
can be approximated as ovals. Therefore, in addition to Dw, it is beneficial to have a shape
metric to indicate how non-circular the anatomy is. One shape metric is the Oval Ratio, OR. The
OR is the ratio of the major and minor diameters, with a value of 1 indicating a circle. Together,
these two metrics can be determined at every location over the selected scan range of the
patient to create a 3D characterization of the attenuation. This information can then be used by
different applications, such as SmartmA, Auto Gating and kV Assist.
The following are some examples of phantom images and their representative Dw and OR
characterizations:
Illustration 9: 35cm Polyethylene Phantom with Water Equivalent Diameter = 33.6cm (OR=1)
Illustration 10: 20x32cm Polyethylene Phantom shown with overlay indicating: Left - Water
Equivalent Diameter = 33.6cm (OR=1) and Right - Ellipse based on Dw and OR (OR=1.58)
Revolution CT User Manual
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Chapter 21 General Information 659