Using either the slider bar or the < and > arrow keys, move to the second image in the stack, which shows a chest X-ray, one method that doctors use to examine lungs. X-rays do a good job showing abnormalities such as tumors, but not the damage from emphysema.

Instead, doctors rely on another imaging technique, Computed Tomography, which reveals much more information. Unlike an X-ray machine that looks at the entire chest, a CT scanner only depicts individual slices of an organ. The white lines on the X-ray image show the location of the digital slices used in this activity.

Soon you'll be examining those very slices to measure the amount of air remaining after the patients have exhaled.