The principle of X-ray imaging
General characteristics[edit | edit source]
X-Ray methods are practically the most important in diagnostic medicine. It's due to the fact that they are the most widely used and the cheapest. From the time of the discovery of X-rays by Wilhelm Conrad Röntgen in 1895, they have been continuously improved to today's combined use of the effects of X-rays and modern computer detection and imaging methods.
Their principle is the different absorption and scattering of X-rays in individual body tissues. X-rays are electromagnetic waves similar to light, but with a wavelength 100 000× times smaller. While visible light has a wavelength of 400-760 nm, X-rays have a wavelength of about 0.05 nm. X-rays are ionizing, harmful to health, unlike visible light. It is therefore necessary to protect oneself from X-rays (lead aprons, exposure reduction, dosimetry).
The nature of the origin of X-rays[edit | edit source]
X-rays are produced in an X-ray tube consisting of a positively charged anode and a negatively charged cathode. The cathode is usually made of tungsten and is heated by a DC electric current of 2-3 A and emits negatively charged electrons. A Direct current voltage of up to 150 000 V is applied between the cathode and the anode. A strong electric field is generated between the two electrodes, which causes the electrons to move at high speed and strike the anode, which brakes them and generates a large amount of heat (99%) from their kinetic energy and only about 1% of the X-rays. They exit through the opening of the lamp towards the patient. X-rays can be so-called characteristic (depending on the material of the anode) and braking radiation.
The basis for the characteristic X-ray radiation is that the electrons in the anode knock other electrons out of the atomic shells. Electrons from higher levels take their place and the energy difference is emitted as X-rays.
X-rays are either hard, with shorter wavelengths, well penetrating through tissues, or so-called soft, with longer wavelengths, less penetrating through tissues. The larger the anode current, the harder the X-rays are. It is also true that the greater the cathode current, the more intense the X-rays.
X-ray image formation[edit | edit source]
X-rays from the X-ray tube pass through the patient's body, striking the tissues. Photoelectrons are produced (photoeffect, Compton scattering and electron-positron pair formation) and these allow the tissue to be imaged. The X-ray photons pass through the wall of the lamp, then the low-energy photons are absorbed in the primary aperture (aluminium) and, after passing through the organ, are absorbed in the so-called Bucky aperture (thin lead strips), deposited just in front of the film. Tissues such as muscle and adipose tissue are very poorly depicted, whereas bone tissue as well as air bubbles, e.g. in the stomach, are shown with high contrast. Therefore, soft tissues such as the oesophagus, intestine, gall bladder are not visible on the X-ray. If we want to make them visible, we inject contrast agents into the body of the X-ray. These include, for example, baryte (so-called positive contrast) or iodine contrast agents, which we use when examining the thyroid gland. However, the contrast agent can also be air, or oxygen or helium. It is helium that is used in the imaging of the ventricles of the brain using negative contrast.
The X-ray image is a shadow image of a particular organ, with the black and white contrast of the image highly dependent on the graded absorption of X-rays by the tissues. This results in graded blackening of the X-ray film. In addition, the X-ray image is also dependent on the nature and thickness of the tissue and the hardness or softness of the X-rays. Finally, the image is noticeably affected by the photographic emulsion and the quality and processing of the film.
The annual X-ray dose is a maximum of 5 mSv for chronic - cancer effects and 50 mSv for acute - organ effects.
Links[edit | edit source]
References[edit | edit source]
SVOBODA, Milan. Základy techniky vyšetřování rentgenem. 2. edition. Prague. 1976. pp. 605.
BLAŽEK, Oskar. Obecná radiodiagnostika. 1. edition. Praha : Státní pedagogické nakladatelství, 1974. pp. 180.
