What sort of things are difficult to image in x rays?

Low-density materials: X-rays are more easily absorbed by dense materials, so low-density materials like air, fat, and water can be difficult to image.

Objects that are too thick: X-rays can only penetrate a certain amount of material, so objects that are too thick can be difficult to image.

Objects that contain a lot of metal: Metal objects can scatter or absorb x-rays, making it difficult to see what's behind them.

Objects that are moving: Moving objects can be difficult to image because they can blur or distort the x-ray image.

Some medical devices: Certain medical devices, such as pacemakers and stents, can interfere with x-ray imaging.