Radiography (General)

X-ray production

X-rays are the result of the bombardment of a metal target by electrons moving at high speed. This is done in a tube where a small area of the wall of this tube is not covered by lead. It is through this opening that the beam of X-rays useful for the formation of the image comes out.

Radiography: image formation.

Chest x-ray

Take the case of a chest x-ray in a person free of any pathology. The patient is placed between the x-ray tube and a cassette. The amount of X-rays arriving at the cassette will depend on the tissue being passed through. The X-ray beam will undergo a significant attenuation when it crosses bony structures (white color on the radiography), intermediate when it crosses muscular structures (more grayish color on the radiography), weak when it crosses lungs because alveoli are filled with air (black color on the X-ray). As the X-ray source and the cassette are fixed, the image obtained is a superposition of all the thoracic structures (bones, lungs, muscles). This overlap can mask lesions. The gray scale and the size of the lesions are also contributing factors to a possible defect in the perception of lesions.

Save the image. Standard radiography - Digital radiography.

A radiological image can be:

• fixed using a standard silver-based film. This process requires development hardware and a darkroom. As soon as the x-ray has been lost, there is no longer any way to get a second copy;

• recorded on a computer support when it is digitized because the information contained in the radiological image is now conveyed in the form of figures. Manipulation of these figures makes it possible to observe the region studied in different forms (visualization only of mediastinal tissues, bone, pulmonary parenchyma, etc.). These digital images can be archived on hard disks, optical disks, CDs, DVDs, etc. They can be transmitted to other centers by means of computer lines, or by telephone via a modem. Digital radiology is the basis of applications such as PACS (image archiving) or teleradiology.

This digitization of the image can be done in two ways:

• via a standard radiology installation and the use of phosphor plates. These phosphor screens are read by means of special equipment which reproduces the image in the form of figures;

• via an installation comprising special detectors. These detectors directly transform the “quantity of X-rays” received into figures. Digital acquisition is said to be “direct”.

A few words about the history of radiography

1895 Discovery of X-rays by Röntgen

X-rays were produced by the Crookes tube and the images were recorded on photographic plates. The first clinical or forensic applications appear in the following year.

1910 Following numerous deaths of doctors, the danger of X-rays has just been understood and the first radiation protection measures are put in place.

1913 Coolidge tube. The first X-ray tubes contained gas. The electrons produced by the cathode interacted with these gas molecules and the energy of the X-rays produced was disparate. This new tube introduces a new design and especially the vacuum inside this tube.

1915 Potter / Bucky table. Suppresses scattered radiation and thus improves image quality.

1918 Introduction of films by Eastman

1921 Conventional tomography by Ziedes de Plantes. It is a technique that allows you to visualize only one section of the body. If overlays of the different structures are avoided, the image obtained uses only the natural contrast of the fabrics.

1950 Image intensifier. Allows less irradiation when using fluoroscopy.