Even if you zoom into image 4/9 and adjust the contrast, it's difficult to see - and impossible to measure - how much air might be retained in which regions. Fortunately, thanks to a tool called thresholding, we can specify and isolate a range of values that depicts the air left in Anne's lungs. The air left in the lungs after exhalation is known as dead air.

You should be on image 4/9, Anne's upper lung slice. If the polygon you made in Question #4 has disappeared, re-trace the outline of the lungs. Now, click the Threshold button. A new window appears with Minimum and Maximum slider bars. Set the Minimum to 249 and the Maximum to 255.

This thresholding range will select air within Anne's lungs and outside her body. If you choose a higher minimum, you will exclude some of the air that you want to measure. The range of pixels you chose will turn bright red, as shown below:
The solid red above Anne's lungs is the air in the room - which you don't want to measure. Instead, because you selected the lung tissue, you can direct WebImage to measure the area of only those pixels that represent air caught within Anne's upper lung slice.