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1. Introduction Miroslav Heřman
Radiology is a clinical medical discipline based on imaging. Imaging methods are used in diagnostics, as well as for guiding therapeutic procedures. The objective of this radiology textbook is primarily to teach a student to correctly indicate individual imaging examinations and to know their common indications and contraindications, as well as to learn to assess and report of the common types of examinations needed in clinical disciplines. The path to these goals leads through an understanding of the basic principles of imaging methods.
1.1 Origin and Properties of X-rays X-rays are electromagnetic waves with a very short wavelength of 10–8–10–12 m. In radiodiagnostics, the wavelengths of 10–9–10–11 m are used. X-ray properties: an X-ray is invisible, it propagates linearly at the speed of light, and in a vacuum it decreases with the square of distance. It passes through an object in which it is partially absorbed and scattered, and the amount of an X-ray absorbed and scattered depends on the composition of the object (its average atomic number, density, and thickness) and the quality of the X-ray (its wavelength). In the object, the X-ray induces the ionization and excitation of atoms. X-rays produce a blackening of the photographic material (the so-called photochemical effect), the origin of visible light in luminophores (the so-called luminescent effect), and the excitation of some substances, which is used in digital radiography. The biological effects of ionizing radiation are significant. Generation of X-rays: in radiology, the source of X-rays is an X-ray tube (Fig. 1.1). X-rays are generated by the rapid braking of very fast-flying electrons in a mass with a high proton number (e.g. tungsten).
occur if the dose in a tissue or an organ exceeds a certain threshold. An example may be an acute radiation syndrome or local effects on the skin. In radiology, we encounter only stochastic effects, as we use low doses of radiation. The stochastic effects are delayed zero-threshold effects and each dose, even a very small one, corresponds to a certain probability of their occurrence. The most serious effects include the origin of malignant tumours and genetic changes. The aim of protection against ionizing radiation in radiology is to prevent the occurrence of deterministic effects and to limit the stochastic effects to an acceptable level.
1.3 Principles of Radiation Protection Radiation protection is defined by the International Atomic Energy Agency as the protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this. The reduction of the expected dose and the measurement of a human dose uptake are fundamental to radiation protection. The International Committee on Radiation Protection (ICRP) recommends, develops and maintains the
X-ray tube
housing
1.2 Biological Effects of X-rays The radiation absorbed in a human body has negative effects, which are conditioned primarily by the excitation and ionization of the mass atoms. At the cellular level, the most significant damage is that done to the DNA molecule. The dividing cells are those most sensitive to X-rays. That is why we particularly consider the indication of X-ray procedures in the pelvis and abdomen and all X-rays in children. The biological effects of ionizing radiation on the organism are divided into deterministic and stochastic. Deterministic effects are of the threshold type – the effect will only
A
C
W useful X-ray beam
Fig. 1.1 X-ray tube scheme. The X-ray tube is an evacuated glass tube stored in a lead housing. The cathode (C) is heated to emit electrons which are greatly accelerated as a result of the high voltage between the cathode and the anode (A). Accelerated electrons collide with anode material and their kinetic energy changes to heat (99%) and X-rays (1%). A rotating anode is used (rotor – R) for better cooling. The useful radiation beam leaves the X-ray tube by the window (W) in the X-ray tube housing.
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