Radiography - Physical principles
Radiography is based on the use of X-rays. These rays were discovered at the end of the 19th century. They are a form of electromagnetic wave, much like the microwaves used in portable cell phones, or the light that we perceive with our eyes. The sole difference is their frequency: that of x-rays is higher, and thanks to this matter can be penetrated to a greater or lesser extent.
Thus, the tissues that make up our organs absorb more or less x-rays and with a simple photosensitive film placed behind the patient a photographic image can be obtained.
This image is the projection in one plane of each of the organs through which x-rays have passed… One sort of loses the notion of depth.
A dense structure like bone absorbs a high percentage of the incident X-ray beam. For this reason, they appear as clear areas on the photograph.
By contrast, low density structures like the lungs filled with air absorb a small percentage of the X-ray beam. They thus appear as black on the image.
Between these two extremes, note that muscles appear as dark gray and fat as light gray.
Radiography rests on the principle of differential absorption of x-rays by tissues. The results obtained are photographs, in shades of gray, that the physician will be able to interpret.
The apparatus is made up of an X-ray emitting tube, an examination table and a receiver that contains the radiographic film.
Let’s take the example of a chest X-ray.
The patient is standing, chest against the examination table, which is in a vertical position.
The x-ray tube is placed behind the patient. It projects a marker of light that enables the correct positioning of the emitter.
The patient is asked to breathe in deeply and, for about 1 second, he is x-rayed in order to make a recording on the radiographic film.
The film is then removed and developed in the same manner as is a traditional roll of photographic film. But note that, more and more, current technology tends to replace traditional radiosensitive film with digital recording plates.
With the digital format we can put together a bank of images that can be consulted or transmitted to a colleague. It also makes it possible to touch up the photo, to zoom in or to emphasize contrasts for example.
The radiologist observes and interprets the photographs
- either using a light box
- or on a control screen if digital images are involved
The information provided by the x-rays is purely morphological.
In this photo, you can easily find:
the lungs – the left lung being smaller because of the presence of the heart. You can make out the ribs, the clavicles and even the bronchi.
You can also locate the dome of the diaphragm, which marks the boundary between the thoracic and abdominal cavities.
In this photo there are opacities in both lungs. An opacity is a radio opaque area, which appears brighter than its surroundings. This indicates pulmonary metastases.
By contrast, a clear area corresponds to a radio transparent zone that is darker than its surroundings. This is the case in this radiograph of a lower limb, which reveals a fracture.
Contrast medium is sometimes administered to enhance the contrast of some structures that wouldn’t be seen without. This is the case for the esophagus which gets mixed up with the other organs.
For that, patients are required to drink a mixture of barium sulphate and water. Because it blocks the passage of X-rays, barium filled organs stand out better on the final images and the photos show, successively, the esophagus ... then the stomach ... then the duodenum.
This examination is called an Upper Gastro-Intestinal Tract Radiography, or “Upper GI” for short.
The injection of a contrast medium based on iodine, into the bloodstream makes it possible to see very fine structures like blood vessels.
Thus, to treat an aneurysm in a cerebral artery – that is a local distension of an artery in the brain -- an incision is made in the groin, at the femoral artery, and then a very fine tube called a catheter is introduced, which passes successively from the femoral artery to the abdominal aorta, then into the thoracic aorta so as to go up through the carotid artery to the brain.
That is when we inject the contrast medium.
The progression of the catheter, which is itself radio-opaque is followed on a control screen.
This catheter is used to treat the aneurysm. This is called interventional radiography. In this sequence you can see how the aneurysm is recapped with a titanium wire which is wound around itself.
X-rays present a danger that cannot be underestimated.This radiation can produce changes in DNA and so damage cells. But everything here is a question of dosage.
As the effects are cumulative even if the dosages employed are very low, the number of radiographic examinations must be limited and they should not be prescribed for pregnant women.
The personnel, who are themselves exposed to a chronic risk, are of course protected. At the precise moment of the irradiation, they are behind a leaded screen that blocks the radiation.
If his or her presence beside the patient is necessary, then the medical professional wears leaded gloves and a leaded apron.
Furthermore, the iodine based contrast media can provoke allergic reactions in some patients.