Pinnacle 3D Profile Tool
The profile tool is shown in this screenshot. It is the icon with axes and a green squiggle on the third row and left most.
CT-PROFILETOOL.jpg
After clicking the icon, the box will disappear and you can click and hold the right button while you draw a line across the image. When you release the mouse button, a graph will appear representing the Hounsfield numbers across the line you drew. The Hounsfield number ranges from 0 (air) up to about 4000 (metal). Because the graph size is dynamically determined, differences of 50-100 can be hard to see, but if you look at later examples (PAROTID), you can see differences of much less. The graph size has been deliberately stretched to match the length of the Profile line drawn, and it is obvious to see that there are three spikes up and three depressions down to base line. The three depressions represent AIR as the line starts in the air to the right of the patient and ends in air to the left of the patient. The central portion is air in the aerodigestive tract. The three spikes are the bones of the mandible. The right mandible is predominantly solid though you can see a vague flicker at the top. This is much more clear in the left mandible where the parallel cortices are plainly visible.
CT-PROFILE.jpg
The profile across the aerodigestive tract in a bit more details shows that the division between the air and soft tissue is reasonably sharp. As a mattter of routine use, I use the value 800 to define what is air V tissue. This corresponds well to what is seen on lung windows. A value of closer to 1000 provides a better match when using soft tissue windows. This possible variability is a feature when describing all boundaries whether air, fat, parotid, muscle, bone or tooth.
CT-PROFILE-AIR.jpg
On bone window, the presence of the teeth within the mandible is better appreciated. What is not appreciated is the fact that teeth have a much higher CT number than mandible, meaning that it is possible to autocontour teeth quite easily. The mandibular bone rarely rises above 200, while the teeth are well separated at 2500.
CT-PROFILE-TEETH.jpg
The difference between parotid, muscle and fat is visible on high contrast settings (soft-tissue/brain). Since this can be seen, it is possible to define CT number ranges which relate to each tissue. The boundary of ~950 separates fat from parotid, and the boundary ~1030 separates fat from muscle. The image shows that fat has a CT range between 850 and 900. The muscle definition in this image is slurred, but the plateau at 1050 is consistent with muscle. The choice of numbers varies between individuals depending in the obesity of the patient (fat is better!), and the fattiness of the parotid (fat is better!).
CT-PROFILE-PAROTID.jpg
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