Not relevant to the main trust of the article but barium sulphate is not radioactive, it just efficiently absorbs X-rays. Radioactive markers are I believe most commonly used in PET scans, Wikipedia suggests flourine-18 as the common isotope used.
you are very correct - authors are doing a fascinating and smart parallel, but the analogy (contrast X-RAY imaging) is wrong. A radioactive element (F-18) marking a glucose molecule for tracking sugar metabolism in the human body with an imager (PET) is what they mean. This is one of many techniques in the field of nuclear medicine or molecular imaging.
