Most of us have found ourselves using our hands to throw shadows on the wall. It’s pretty tempting to do it in the cinema, if you’re sitting under the projector during the credits. It works because the light cannot get through your hands and so casts a shadow.
X-rays are a form of light – they are very energetic and can pass through your soft tissues really easily. They cannot get through bone in the same way. So if you stand in front of an X-ray gun, then you will cast a shadow on the wall behind. Of course, we can’t see X-rays and so we have to use a special type of film to capture the shadow. The first X-ray pictures were little more than shadow puppets, with only the bones really visible (below left). As you will see if you read about the other imaging techniques, all human organs are slightly different. Modern X-ray machines coupled to powerful computers can do slightly better. They use softer X-rays to try and pick up on the differences between organs. As you can see on the right – it almost works. The skin is pretty much see through and you can make out muscles, organs and lungs.
One solution to the lack of contrast between organs is to inject the patient with iodine. Iodine atoms are difficult for X-rays to pass through and so the blood vessels throw darker shadows. Because organs have lots of blood vessels, they tend to throw darker shadows than the fat tissue. The result can be a rather clearer picture than before. Using an additional agent to enhance contrast is common to all imaging techniques. The agent is different every time, but the aim is the same – to make the thing you are interested in stand out.
X-ray scans and CAT scans are really just the same thing. A CAT scan uses a computer processor to construct the image instead of using film.
Find out about the other imaging techniques.