X-ray films are taken of different areas of the body. If the body part contains bone it will show up as white because bone is dense and absorbs X-rays more than muscle or fat. If the body part contains gas or air it will show up as black because X-rays easily pass through gas.
Different Tissue Types
When you look at this chest X-ray, notice that the lungs look black because they are filled with air and the X-rays can pass through the air. The spine, ribs and shoulders appear white because they are dense and stop the X-rays from passing through. The heart appears gray because it is more dense than air and lung tissue but less dense than bone.
The image on the far right is of the abdomen of a patient who was injected with radiographic contrast material. The contrast medium contains iodine and when injected the body gets rid of it by excreting it through the kidneys. If you look closely toward the top of the image you can see the patient's kidneys. You can also see contrast media running from the kidneys to the bladder located toward the bottom of the image.
You actually see only the inside of the kidney called the renal pelvis because that is where the contrast medium gets excreted. You do not see the body of the kidney and you do not see any other internal organs, such as the stomach, liver, pancreas, gall bladder or spleen because they are not dense enough to stop the X-rays from penetrating.
This is an X-ray of a patient with a knee replacement. As before you can see the bones, in this case of the femur, tibia and fibula but notice how much more dense or white the replacement parts are. They are made of metal which is more dense than bone and shows up on the X-ray as being even more white than bone. Notice also that you cannot see any muscle, tendons or ligaments.
This X-ray is a lateral view of a thigh bone or femur with the knee and knee cap on the right side of the image. You can see a small dark vertical line about two thirds of the distance from the left side of the image and the end of the femur; that line is a fracture of the patient's femur.