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).

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 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 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.

International System of Radiological Protection, based on the evaluation of the large body of scientific studies available to equate risk to received dose levels. The system’s health objectives are to manage and control exposures to ionizing radiation so that deterministic effects are prevented, and the risks of stochastic effects are reduced to the extent that is reasonably achievable.

The ICRP’s recommendations flow down to national and regional regulators, which have the opportunity to incorporate them into their own law. In most countries a national regulatory authority works towards ensuring a secure radiation environment in society by setting dose limitation requirements that are generally based on the recommendations of the ICRP.

The ICRP uses the following overall principles for all controllable exposure situations.

Justification: No unnecessary use of radiation is permitted, which means that the advantages must outweigh the disadvantages.

Limitation: Each individual must be protected against risks that are far too large through individual radiation dose limits.

Optimization: Radiation doses should all be kept as low as reasonably achievable (ALARA). This means that it is not enough to remain below the radiation dose limits, but that radiation doses are as low as reasonably achievable, which often means much lower than the permitted limit.

There are no limits to the medical exposure of pa-

tients. This is to be construed as meaning that when the examination under consideration is reasonably indicated, there is no dose limit that is not to be exceeded.

1.4 Overview of Doses in Radiological

Examinations

In the last two decades there has been a significant increase in the absolute number of tests using ionizing radiation. As a result, its current use in medicine represents the largest artificial source of ionizing radiation for the population in advanced countries.

For comparison of the doses received in different types of examinations, it is useful to use the effective dose. The effective dose is calculated from the doses absorbed by individual organs and is expressed as a single number. Its unit is the sievert (Sv). It enables an estimate of the risk of stochastic effects (the occurrence of fatal tumours and genetic changes) that can be compared to other risks. For children under 15 years of age, the effective dose of the same examination, i.e. the risk of stochastic effects from radiation exposure, is two to three times higher than for adults.

Table 1.1 lists the values of the effective dose and the risk of stochastic effects in some examinations. In general, the effective doses of standard radiological examinations differ significantly (0.01–10 mSv), but in most cases they are considerably lower than the effective dose in CT examinations (approximately 2–20 mSv). The average effective doses of interventional procedures using ionizing radiation are usually

Table 1.1 Typical effective doses in adults in connection with some medical procedures and their risks.

Diagnostic procedure (source) Typical effective dose Risk of developing fatal tumour*

average background radiation

average of 3 mSv/year tooth radiograph (intraoral) 0.005 mSv 1: 4,000,000 limb and joint radiographs (except hip) < 0.01 mSv < 1 : 2,000,000 chest radiograph (PA projection) 0.02 mSv 1 : 1,000,000 chest radiograph (PA and lateral projections) 0.1 mSv 1 : 200,000 skull radiograph 0.1 mSv 1 : 200,000 abdominal radiograph 0.7 mSv 1 : 30,000 lumbar spine radiograph 1.5 mSv 1 : 15,000 intravenous urography 3 mSv 1 : 7,000 barium meal 6–8 mSv 1 : 3,500–2,500 CT of head 2 mSv 1 : 10,000 CT of chest or abdomen 7–8 mSv 1 : 3,000–2,500 angiography of head or neck 5 mSv 1 : 4,000 diagnostic coronarography 7 mSv** 1 : 3,000 coronarography + PTA + stent introduction 15 mSv** 1 : 1,600 TIPS introduction 70 mSv** 1 : 300 bone scintigraphy 6 mSv 1 : 3,500 whole body PET/CT 15 mSv 1 : 1,500

* Fatal tumour = a tumour as a result of which the patient dies. ** Values may vary significantly, depending on the experience of the physician performing the examination and the difficulty of the examination (for example, for TIPS introduction, they vary within the range of 20–180 mSv).

in the range of 5–70 mSv, while in nuclear medicine they are in the range of 0.3–20 mSv. These doses can be compared to the dose we receive from the natural background, which is 3 mSv per year on average (in the range of 1.5–7.5 mSv per year). For a comparison of the risks posed by ionizing radiation with some other common risks, see Table 1.2.

Exposure to ionizing radiation during pregnancy.

Foetal exposure to ionizing radiation may lead to prenatal death, retardation of intrauterine growth, mental retardation, organ malformations, and the development of malignant tumours in a new-born child. The risk of these effects depends on the gestational age at the time of exposure and the size of the dose. In radiological examinations which do not directly irradiate the foetus the dose for the foetus is minimal and the risk of these changes is unlikely. In abdominal and pelvic examinations, some complications may occur. However, if the procedure is necessary and cannot be replaced by another method (e.g. ultrasonography or magnetic resonance), the risk is acceptable. For example, after a radiograph of the abdomen or lumbar spine, the risk of a tumour developing in the child increases from 0.067% to 0.084% and the increase in other risks is even lower. In examinations with a higher dose in the area of the abdomen, of course, the risk of developing the above complications increases.

Table 1.2 Risk of death resulting from common influences/ activities.

Cause Risk of death

the risk of carcinoma from food additives 1 : 1,000,000 pregnancy for mother 1 : 170,000 general anaesthesia 1 : 50,000 travel by jet plane (> 1,000 miles per year) 1 : 30,000 occupational injury 1 : 2,000 traffic accident 1 : 500 life in a big city (pollution) 1 : 160 smoking 10 cigarettes a day 1 : 5

1.5 Request for Radiological Examination

When requesting a radiological examination, the indicating physician has to fill in a form with two kinds of data: 1) Basic personal data of the patient (to correctly identify the patient), 2) Clinical data – the type and region of examination required, and also the patient’s anamnesis, significant clinical and laboratory findings, and the results of other examinations. For the radiologist, the request form is a primary source of patient data. The most important data is the clinical diagnosis (in relation to the examination required) and the question which is to be answered by the examination. For an examination in which the administration of a contrast medium is anticipated, an allergic anamnesis must also be given on the form.

1.6 Radiological Report

The report is a part of the examination. Generally, it consists of three parts: 1) the name of the examination and the technique applied, 2) the description of the examination (“objective” description of what is visible in images), and 3) the impression, where the information from the description of the examination with the clinical data is combined. In shorter descriptions, some or all of the parts are aggregated. In the description, we use terms that will be discussed with each method. The description should be sufficiently detailed to provide a good idea of the image even if we do not have the images available. The radiologist’s ability to draw a correct impression from the examination is to a large extent conditioned by the accuracy, completeness, and quality of the anamnestic and clinical data on the request form.