As a Radiologic Technologist I can answer this.
Anything "black" on a CT Scan is extremely low density, like air in the gut or in the lungs.
Anything "pure white" is extremely dense....like bone, gallstones or some type of "contrast material" which often is used to outline structures such as the urinary system (kidneys down to the bladder), the gallbladder, the entire circulatory system, including the heart, or the entire GI system (mouth to anus).
Shades of grey correspond to tissue which is more or less dense.
When a child swallows an object, we can see its location via x-ray.
When a stupid drug smuggler swallows containers of drugs, they also easily show up on x-ray.
Posted 788 days ago