Fundamental Physics of Radiology

W. J. Meredith, J. B. Massey

• Publisher: Butterworth-Heinemann
• Year: 2013
• Print ISBN:
  9780723607786
• eBook ISBN:
  9781483284354
• Edition: 3
• Topic: Medicine
• Subtopic: Diseases & Allergies

Fundamental Physics of Radiology, Third Edition provides a general introduction to the methods involving radioactive isotopes and ultrasonic radiations. This book provides the fundamental principles upon which the clinical uses of radioactive isotopes and ultrasonic radiation depend. Organized into four sections encompassing 45 chapters, this edition begins with an overview of the basic facts about matter and energy. This text then examines the technical details of some practical X-ray tubes. Other chapters consider the action of the X-rays on the screen to produce an emission of visible light photons in amount proportional to the incident X-ray intensity. This book discusses as well the fundamental aspects of the physical principles of radiotherapy, in which most attention is being given to gamma- and X-rays. The final chapter deals with the provision of adequate barriers and protective devices to guarantee the safety of the workers concerned. This book is a valuable resource for radiologists, physicists, and scientists.

CHAPTER I

MATTER AND ENERGY, RADIATION AND SPECTRA

Publisher Summary

This chapter discusses the importance of line spectra in radiology. Line spectra, as those of this type are usually called, are emitted by burning substances or, for example, by gases through which an electric discharge is passing. In direct contrast to the continuous spectrum, the wavelengths present in a line spectrum do not depend upon the temperature of the source, provided that this is high enough to cause light to be emitted at all, but entirely upon the chemical substance involved. The chapter discusses the basic facts about energy and matter. It also describes common properties and individual features of electromagnetic radiation. The smallest particle of an element is called an atom, while the smallest particle of a compound is a molecule. The measurement of the heat generated is often a very good method of assessing the amount of energy involved in a process.

THE universe in which we live is so vast, and the materials of which it is composed are so apparently infinite in their variety, that it is, at first, astounding to learn that it is all made up of various combinations of about 100 separate and distinct substances called elements. In fact, 95 per cent of the earth and its atmosphere is made up of no more than a dozen elements, whilst a mere four (hydrogen, oxygen, carbon, and nitrogen) make up about 95 per cent of the weight of the human body.

An element may be described as a single substance which cannot be made simpler by chemical methods. When one element joins with another, or with several others, the result is a chemical compound. For example, the elements sodium and chlorine combine together to form the compound sodium chloride, better known as common salt, whilst the gases hydrogen and oxygen can combine to form that most essential of compounds, water. A slightly more complex compound is limestone, which is made up of the elements calcium, carbon, and oxygen, whilst the tissues of our bodies, though very complex in structure, are compounds mainly composed of the four elements listed above, hydrogen, oxygen, carbon, and nitrogen.

Just as a chemical compound is made up of a number of different elements, so all material is made up of enormous numbers of extremely tiny particles. The smallest particle of an element is called an atom, whilst the smallest particle of a compound is a molecule. In common salt the molecule consists quite simply of one atom of sodium linked to one atom of chlorine, whilst in water two atoms of hydrogen are attached to one of oxygen. Limestone has a molecule of five atoms—one each of calcium and carbon and three of oxygen—whereas the molecule of a protein, one of the important materials of the body, may be made up of thousands or even millions of atoms.

A molecule may be split into its component atoms, but when this happens the chemical properties of the material will be changed. For example, when an electric current is passed through water the molecules may be split into the constituent hydrogen and oxygen, two gases with properties very different from those of the parent material.

It was, for a long time, believed that atoms were the smallest of particles and that they were indivisible. Now we know that they, too, in their turn are made up of even smaller, simpler particles. Just as a molecule can be separated into its constituent parts, so an atom can be separated into its components (though the process is much more difficult). As with the molecule such separation produces quite different materials and destroys the properties of the original. As will be discussed in greater detail later, an atom is like a miniature solar system having at its centre a nucleus wherein is concentrated practically all the weight of the atom, and which carries a positive electrical charge. This central ‘sun’ is surrounded by a number of very light and negatively charged particles called electrons.

Going back from minute particles to the forms which matter usually takes in practice, we find that there are three, namely solids, liquids, and gases. Any material can exist in each of these forms, depending on the temperature. For example, water is normally gaseous at above 100° C, solid below 0° C., and liquid in between. The fundamental difference between the three states is merely the closeness of the constituent atoms or molecules. In a solid these particles are packed very close together, have generally fixed positions, and may, as in a crystal, be arranged in very orderly patterns. In a gas the particles are widely separated and move about quite freely, at random. Liquids have their particles closer together than gases but still have almost the same random movement of particles.

From what has been briefly said it would appear that all matter is made up of mass and electricity. However, there is a third factor of equal importance, and that is energy, a concept with which everyone is familiar and yet which is not easy to define. Though we now know that energy can be converted into mass, and vice versa (and we shall study some examples of this later), for the present it is convenient to keep the two factors apart and to discuss their properties separately.

Life may be regarded as one manifestation of the interaction of matter and energy, whilst radiology is intimately concerned with the relationship of matter with one particular form of energy—radiation. Therefore before embarking upon a detailed study of radiology it is necessary to be familiar with the basic facts about energy and matter.

ENERGY

Energy, in simple terms, can be described as the ability to do work, or as that which is being expended when work is being done. If we carry a case upstairs, work is done; if we compress a spring, work is done; if we pull apart a negatively and a positively charged particle against the electrical attraction that exists between them, work is done. In each of these cases a force is involved (the force of gravity, the resistance to compression, or an electrical force respectively), and they have been quoted to illustrate the fact that work usually involves overcoming a force. The amount of work done is proportional to the force and to the distance over which the force has been overcome. It is on this basis that the unit of work, or energy, the erg, is defined. Though it is sufficient for our purpose to know that the erg is the unit, the definition of the erg, together with definitions of other fundamental units, is given in an appendix at the end of the book.

Like matter, energy has many forms, the most important of which are probably:

1. Kinetic Energy.—This is the energy possessed by virtue of movement. If a piece of material of mass m moves with a velocity v, its kinetic energy is

.

2. Potential Energy.—This is the energy that a body has because of its position, for example, a weight on a high shelf, or a compressed spring.

3. Heat Energy.—Heat is the energy of the movement of atoms and molecules of any material. The level of this heat energy is indicated by the temperature. The higher the temperature, the greater is the movement of the particles and hence their energy.

4. Electrical Energy.—The energy associated with electricity is measured by multiplying the electrical charge being moved by the electrical force (the potential difference or voltage) against which it has been moved. In practical terms this is the product of the current (amperes), the pressure (volts), and the time (hours, etc.). The so-called ‘unit’ which is paid for in the electricity bill is a kilowatt-hour (1 kilowatt is 1000 watts, and watts are amperes × volts).

In radiology the electrical charge involved is often that of the electron (e), and a very convenient unit of energy is the electron-volt (eV.) or more frequently the kilo-electron-volt (keV.), which is the energy involved when an electron passes through a potential difference of 1000 volts.

5. Chemical Energy.—Energy can be ‘locked up’ in chemical compounds and released under certain circumstances. For example, the detonation of an explosive institutes chemical changes and much energy is released. Or less violen...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface to The Third Edition
• Preface to The First Edition
• CHAPTER I : MATTER AND ENERGY, RADIATION AND SPECTRA
• CHAPTER II : ATOMS AND NUCLEI
• CHAPTER III : RADIOACTIVITY
• CHAPTER IV : RADIOACTIVITY—MATERIALS
• CHAPTER V : THE PRODUCTION OF X-RAYS
• CHAPTER VI : THE INTERACTION OF X- AND GAMMA RAYS WITH MATTER—I
• CHAPTER VII : THE INTERACTION OF X- AND GAMMA RAYS WITH MATTER—II
• CHAPTER VIII : THE EFFECTS OF X-RAYS
• CHAPTER IX : THE MEASUREMENT OF X-RAY QUANTITY
• CHAPTER X : THE ROENTGEN AND ITS MEASUREMENT
• CHAPTER XI : THE GEIGER-MÜLLER AND SCINTILLATION COUNTERS AND THE THERMOLUMINESCENCE DOSEMETER
• CHAPTER XII : ABSORBED DOSE AND THE RAD
• CHAPTER XIII : FILTERS AND FILTRATION
• CHAPTER XIV : THE PHYSICAL BASIS OF DIAGNOSTIC RADIOLOGY
• CHAPTER XV : THE X-RAY FILM AND ITS PROCESSING
• CHAPTER XVI : THE PROPERTIES OF THE X-RAY FILM
• CHAPTER XVII : INTENSIFYING AND FLUORESCENT SCREENS AND XERORADIOGRAPHY
• CHAPTER XVIII : GEOMETRIC FACTORS WHICH INFLUENCE THE RADIOGRAPHIC IMAGE
• CHAPTER XIX : THE EFFECT OF X-RAY ABSORPTION ON THE RADIOGRAPHIC IMAGE
• CHAPTER XX : THE EFFECTS AND CONTROL OF SCATTERED RADIATION
• CHAPTER XXI : THE RADIOGRAPHIC EXPOSURE
• CHAPTER XXII : THE DIAGNOSTIC X-RAY TUBE AND SHIELD
• CHAPTER XXIII : THE ELECTRICAL CIRCUITS OF THE X-RAY UNIT
• CHAPTER XXIV : THE RATING OF THE X-RAY TUBE
• CHAPTER XXV : FLUOROSCOPY
• CHAPTER XXVI : TOMOGRAPHY
• CHAPTER XXVII : ULTRASONICS IN CLINICAL MEDICINE
• CHAPTER XXVIII : RADIOACTIVE ISOTOPES IN CLINICAL MEDICINE
• CHAPTER XXIX : THE PHYSICAL PRINCIPLES OF RADIOTHERAPY
• CHAPTER XXX : TELETHERAPY DOSAGE DATA: GENERAL CONSIDERATIONS
• CHAPTER XXXI : TELETHERAPY DOSAGE DATA FOR CLINICAL USE
• CHAPTER XXXII : OUTPUT MEASUREMENTS AND THE USE OF ISODOSE CHARTS
• CHAPTER XXXIII : PATIENT DOSAGE
• CHAPTER XXXIV : BEAM MODIFICATION
• CHAPTER XXXV : COLLIMATORS AND ‘BEAM-DIRECTION’ DEVICES
• CHAPTER XXXVI : THE TREATMENT PRESCRIPTION
• CHAPTER XXXVII : SOME SPECIAL TECHNIQUES
• CHAPTER XXXVIII : TELETHERAPY SOURCES
• CHAPTER XXXIX : ACCEPTANCE TESTS AND CALIBRATION
• CHAPTER XL : GAMMA-RAY SOURCES FOR PLESIOTHERAPY
• CHAPTER XLI : PLESIOTHERAPY DOSAGE CALCULATIONS
• CHAPTER XLII : PARTICLE RADIATIONS IN RADIOTHERAPY
• CHAPTER XLIII : GENERAL PRINCIPLES AND MATERIALS
• CHAPTER XLIV : DEPARTMENTAL PROTECTION
• CHAPTER XLV : PROTECTION INSTRUMENTS AND PERSONNEL MONITORING
• APPENDIX I: S.I. UNITS
• APPENDIX II: THE EXPONENTIAL LAW OF RADIOACTIVE DECAY, OR OF PHOTON BEAM ATTENUATION
• APPENDIX III: MODULATION TRANSFER FUNCTION AND ASSOCIATED CONCEPTS
• INDEX