The detect menu present the tissue classification based on pixel gray value that compared by Hounsfields Unit in table. So you can see the organ classification of the image that yow want to know. Otherwises, the grayscale image that are you entered must be have the Window and Level control value, to get the HU value inside it.
You develop it better than i have expected, good job. But i think you should fix the display for window and level and also the description of the image. because i can see it clearly.