Radiography and X-rays

How does it work?

Until a few years ago, radiography exploited the properties of X-rays to impress an X-ray film and this made it possible to transform the information content in possession of a radiogenic beam emerging from a body region into a diagnostic image.

When a radiographic film is exposed to X-rays it impresses itself and contains a "latent image, which is then transformed into a real image with procedures superimposable to those of any photographic film. If a radiopaque body is interposed between the X-ray source and the film," the radiations are totally absorbed by the body and do not reach the film, which is not exposed at that point. Therefore, the image of the body appears on the film in negative, ie white, exactly the opposite of what was seen for radioscopy.

Similarly, if a complex structure is interposed between the X-ray source and the film (such as the chest of a man for example), the high atomic number and thick formations (bones, mediastinum), which almost completely retain the radiation, appear clear on the film; those that hold them only partially (muscles, vessels, etc.) appear gray; those that are almost completely crossed (lungs) are dark. The whole of these components, light, gray and dark, constitutes the radiographic image and the exposed film is called radiogram or radiography.

So X-ray radiology exploits the fact that tissues with different densities and different atomic numbers Z absorb radiation in different ways:

High Z and densities: there is the maximum absorption, for which the fabrics almost completely retain the radiations resulting white on the film. The bones and the mediastinum have these characteristics;
Intermediate Z and densities: fabrics appear gray on the film, with a very varied scale. Muscles and vessels have these characteristics;
Low Z and density: the absorption of X-rays is minimal, so the image we get is black. The lungs (air) have these characteristics.

Radiation dose

In order to perform an X-ray examination, the overall quantity of X-rays arriving on the fluorescent screen, or on the film, must be sufficient.

Depending on the thickness and texture of the body to be examined, the incident beam must have appropriate intensity and penetration (energy). To vary these quantities, the operator acts, through the control table, on the combination of three factors: electric potential applied to the tube, current intensity of the tube, exposure time.

For example, if the patient is very large and muscular, it is necessary to use more penetrating radiation, with a shorter wavelength; if the organ to be studied has involuntary movements (heart, stomach), it is necessary to minimize the time of exposure.

If, on the other hand, the object is very stationary (bone), the exposure time can be relatively long and the intensity of the beam can be increased. The resulting image is sharper and richer in detail.
The current potential of the means of calculation allow to digitize, with sufficient resolution, the radiological images, thus allowing both their storage in memory (archive) and their processing (digital radiography). It consists in dividing the image into many surface elements (pixels), to which to assign - in binary code - the shade of gray value. The finer the subdivision of the image, the greater its resolution, therefore the greater the number of pixels to be digitized and stored.

Typically, a high definition image consists of at least one million pixels. Since digitization corresponds to one byte (binary word) for each pixel, such an image therefore occupies 1 megabyte (1MB) of memory.

The digitized images can allow the reconstruction and correction of geometric structures (elimination of deformations or artifacts), or the modification of shades of gray, to highlight even small differences between similar soft tissues. As soon as they are obtained, they are immediately visible on the monitor of a predisposed console. By means of digital radiography it is therefore possible to obtain more information from the radiographic images than the direct visual observation of the radiographic film allows. Furthermore, digitization allows less pollution (caused by the disposal of the exposed radiographic films) and economic savings (now all exist of a "radiographic investigation are released to the patient in the form of CD-Rom).

What are the rules for obtaining an optimal radiographic image?

for the radiological investigation to be more precise, the object to be X-rayed must be placed as close as possible to the X-ray film. If the object is far away, its image is enlarged and blurred;
to minimize the magnification and distortion of the image, the x-ray tube must be placed far from the object. teleradiography (This is particularly used in the examination of the chest.) Other times it may be useful, on the contrary, to place the tube very close or even in contact with the object. In this case we speak of plesioradiography;
in radiological investigations the expressions position and projection are often used. There position it is the attitude assumed by the patient during the examination. It can be erect, seated, lying down (supine or prone), on the side, etc. There projection refers to the path of radiation in the body. It is usually indicated with two adjectives: the first expresses the point of entry of the radiations into the body, the second the exit point. For example, postero-anterior projection means that the radiations penetrate the body from the posterior surface and emerge from the anterior one. The same projection can be performed by placing the patient in various positions. For example, the examination of the thorax is performed in the postero-anterior projection with the patient in an upright position; however, if the patient has a fractured foot (for an accident for example), the same projection can be performed in the seated projection and, if he is in very serious conditions, also in the horizontal position;
if the object to be X-rayed is mobile, it may be useful to take images in more or less rapid succession. In this case we speak of serioradiography. For example, the duodenum, due to its movements (peristalsis), constantly changes shape and attitudes; the execution of serial shots (at different times at regular intervals), called seriograms, allows to analyze the anatomical formation in the various subsequent attitudes. If the organ is equipped with very fast movements (heart, vessels), it is necessary to take radiograms at rapid cadence (rapid serigraphy) or even film shooting (obtained by means of a particular film camera applied to the image intensifier).