Dental Radiology E-Book

Andreas Fuhrmann

• Herausgeber: Thieme
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 275

Veröffentlichungsjahr: 2015

Beschreibung

<p><em>Dental Radiology</em> is an excellent guide book for both dental students and practicing dentists. Students will benefit from practical guidance on how and when to use the various imaging methods presented as well as key information on fundamental concepts. For experienced dentists, the book is a valuable guide for image analysis, interpretation of radiologic findings, and diagnosis of pathological changes. Modern imaging methods, the fundamentals of X-ray physics, examination strategies, and information on radiation protection and quality assurance are included.</p><p>Key Features:</p><UL><LI>Covers the entire range of dental imaging techniques, from intraoral radiography to panoramic radiography and cone beam computed tomography</li><LI>Presents information in a concise, easy to understand manner, enabling readers to quickly put imaging techniques into practice</li><LI>Includes almost 300 high-quality radiographs and color illustrations </LI></UL><p>This book serves as an excellent study guide for dental students as well as a practical radiological reference for experienced dentists.</p>

Leseprobe

Dental Radiology

Andreas Fuhrmann, DDSForensic OdontologistDepartment of Legal MedicineCenter for DiagnosticsUniversity Medical Center Hamburg-EppendorfHamburg, Germany

304 illustrations

ThiemeStuttgart · New York · Delhi · Rio de Janeiro

Library of Congress Cataloging-in-Publication Data

Fuhrmann, Andreas, author.  [Zahnärztliche Radiologie. English]  Dental radiology/Andreas Fuhrmann.       p. ; cm.  Includes bibliographical references and index.  ISBN 978-3-13-200421-4 (alk. paper) –   ISBN 978-3-13-200431-3 (e-book)  I. Title.   [DNLM: 1. Diagnostic Imaging. 2. Radiography, Dental—methods.   WN 230]  RK309  617.6’07572—dc23

                                                    2014047560

This book is an authorized translation of the 1st German edition published and copyrighted 2013 by Georg Thieme Verlag, Stuttgart. Title of the German edition: Zahnärztliche Radiologie

Translator: Suzyon O’Neal Wandrey, Berlin, Germany

Illustrators: Angelika Branner, Hohenpeissenberg, Germany; Joachim Hormann, Stuttgart, Germany

Eugen Roth poem translated and reproduced with kind permission of Carl Hanser Verlag, Munich, Germany© 2015 by Georg Thieme Verlag KG

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ISBN 978-3-13200-421-4

Also available as an e-book:eISBN 978-3-13200-431-3

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To Chrissi and Kati

A master advised his devoteesTo only believe in what one sees.And yet – this we should reconceive,Some only see what they believe.

Eugen Roth

Contents

Preface

Acknowledgments

1 History and Development of Dental Radiography

2 Radiation Physics

2.1 Types of Radiation

2.2 Direct and Indirect Ionization

2.3 Corpuscular and Photon Radiation

2.3.1 Corpuscular Radiation

2.3.2 Photon Radiation

2.4 Interactions between Radiation and Matter

2.5 Fundamental Physical Processes Involved in the Transfer of Photon Energy to Matter

2.5.1 Excitation

2.5.2 Ionization

2.6 Interactions between X-rays and Matter

2.6.1 Absorption—Photoelectric Effect

2.6.2 Scattered Radiation—Compton Effect

2.7 Radioactivity

2.8 Production of X-rays

2.8.1 Dental X-ray Equipment

2.8.2 Additional Equipment Needed for Dose Limitation and Improvement of X-ray Image Quality

3 Dose Terms and Dose Units Used for Ionizing Radiation

4 The Biology of Radiation Effects

4.1 Fundamentals

4.2 Direct and Indirect Effects of Radiation

4.3 Effects of Ionizing Radiation on DNA

4.4 Repair Mechanisms for the Restoration of DNA

4.5 Biological Effects of Radiation Damage

5 Radiation Pathology

5.1 Natural Radiation Exposure

5.1.1 Cosmic Radiation

5.1.2 Terrestrial Radionuclides

5.2 Artificial Radiation Exposure

5.3 Stochastic and Deterministic Effects of Ionizing Radiation

5.3.1 Stochastic Effects

5.3.2 Deterministic Effects

6 Image Formation and Image Processing

6.1 Fundamentals

6.1.1 Summation Effect

6.1.2 Tangential Effect

6.2 Image Receptor-Independent Factors Influencing Image Formation

6.2.1 Object Contrast

6.2.2 Current Intensity and Exposure Time

6.2.3 Inverse Square Law

6.2.4 High Voltage

6.2.5 Scattered Radiation

6.3 Radiographic Film and Intensifying Screen-Dependent Factors that Influence Image Formation

6.3.1 Screenless Films

6.3.2 Radiographic Film with Intensifying Screens

6.4 Processing of Radiographic Films

7 Digital Dental Radiography

7.1 Sensors

7.1.1 Spatial Resolution

7.2 Storage Phosphor Imaging Plates

7.3 Advantages of Digital Radiography

8 Radiation Protection and Quality Assurance in Dental Radiology

8.1 History of Radiation Protection

8.1.1 Structure of the International Commission on Radiological Protection

8.1.2 Tasks and Content of the Various Activities of the International Commission on Radiological Protection

8.2 Responsibility for Radiation Protection

8.2.1 Supervisory Duty of the Government

8.2.2 Administration and Management of Safety

8.3 Need and Justification

8.4 Optimization of Radiation Protection

8.4.1 Limitation and Monitoring of Individual Dose Limits

8.4.2 Prevention of Accidents and Protection against Existing or Unregulated Radiation Risks

8.5 Implementation of Recommendations by the International Commission on Radiological Protection

8.6 Quality Assurance in Dental Radiology

8.6.1 Standards

8.7 Procedures to Ensure Compliance with Basic Principles of Radiation Protection

9 Practical Dental Radiography

9.1 Intraoral Radiography

9.1.1 Quality Criteria for Intraoral Radiography

9.1.2 Principles of Projection Geometry

9.1.3 Paralleling Technique

9.1.4 Bisecting-angle Technique

9.1.5 Right-angle Technique

9.1.6 Bitewing Radiography

9.1.7 Radiographic Measurement Techniques

9.1.8 Occlusal Radiography

9.2 Conventional Tomography

9.3 Panoramic Tomography

9.3.1 Panoramic Radiography with a Slit Collimator

9.3.2 Panoramic Radiography with an Intraoral Source

9.3.3 Rotational Panoramic Radiography

9.4 Cone Beam Computed Tomography

9.4.1 Technique and Image Formation in Cone Beam Computed Tomography

9.4.2 Limitations of Computed Tomography and Cone Beam Computed Tomography

9.4.3 Volume Size

9.4.4 Clinical Indications for Cone Beam Computed Tomography

10 Anatomy and Topography of the Facial Skeleton

10.1 The Teeth and Tooth-supporting Structures

10.2 The Mandible

10.3 The Maxilla

10.4 Panoramic Radiographic Anatomy

10.4.1 The Mandible

10.4.2 The Maxilla and Midface

11 Radiographic Findings and Diagnosis

11.1 Systematic Image Analysis and Interpretation

11.1.1 Film and Monitor Viewing Conditions

11.1.2 Steps from Findings to Diagnosis

11.2 Assessment and Diagnosis of the Most Common Pathological Changes

11.2.1 Carious Lesions

11.2.2 Horizontal Bone Loss with Vertical Bone Defects

11.2.3 Apical Periodontitis

11.2.4 Cystic Lesions

11.2.5 Malignant Lesions

11.2.6 Bone Diseases of the Jaw

11.2.7 Sialolithiasis

11.2.8 Tooth Fractures

References

Preface

Great progress has been made in dental radiology in the last 30 years. This is particularly true for technical developments, but also for education and training in dental radiology at universities.

The introduction of digital image receptors can be viewed as a milestone in the technological development of dental radiology. The introduction of cone beam computed tomography gave dentists the first fully fledged technology for three-dimensional imaging of the oromaxillofacial region.

In parallel, radiation protection legislation has been thoroughly revised. Radiation protection and image quality are two closely intertwined pillars of the X-ray ordinance. Over the years, the International Commission on Radiological Protection has introduced increasingly concrete and detailed provisions. The “as low as reasonably achievable” (ALARA) principle, which states that the patient should only be exposed to as much radiation as reasonably necessary, must also be applied in full to dental radiology.

The key to high-quality radiographic diagnosis is comprehensive training during dental school; this serves to ensure that, in later practice, individuals will only be exposed to X-rays if really necessary. Dentists also learn in dental school how to perform practical radiation protection procedures and how to obtain radiographs of good diagnostic image quality. Moreover, continuing education and training during the course of professional practice is essential. Only those professionals who have up-to-date radiological knowledge and skills can perform X-ray procedures responsibly.

This book describes the essentials of dental radiography without which it would be impossible to understand the concepts of image quality and radiation protection. Likewise, it explains the fundamentals of X-ray physics and provides practical information and instructions for the different techniques used in dental radiography. The book is designed as a step-by-step guide to help students learn dental radiography. At the same time, it was conceived as a reference to help dentists in private practice and hospitals achieve optimal results in dental X-ray diagnostics in-office or in the university teaching setting.

A good knowledge of the techniques of intraoral radiography and dental panoramic radiography is crucial to obtaining good radiographic image quality. The technique of cone beam computed tomography is becoming more and more established. An exact knowledge of X-ray image formation and of the diverse possibilities for using the enormous amounts of data collected is necessary for optimal use of this wide-ranging technique.

At the end of the X-ray examination, the task at hand is interpretation of the findings, which plays an increasingly important role in dental radiology. Even panoramic radiographs can provide a large number of findings. Images from cone beam computed tomography yield far more diagnostic information. Therefore, many practical aids are given for the optimal interpretation of results.

In order to keep radiation exposure as low as possible, a justifying indication must be established before beginning any type of X-ray examination. A precise knowledge of the many diagnostic possibilities is also absolutely necessary, in order to respect the ALARA principle in dental radiology. Correct practical performance of the examination results in high diagnostic image quality. If all steps are performed and executed according to the latest standards of science and technology, then optimal diagnosis is assured.

Andreas Fuhrmann, DDS

Acknowledgments

First and foremost, I would like to thank Professor Dr. Friedrich Anton Pasler, who asked and encouraged me to write a textbook of dentomaxillofacial radiology. I give him great credit for this because, by doing so, he virtually designated his successor. I consider it a great honor that he chose me.

This book is the product of many years of practical and theoretical experience gained while teaching at the Department of Radiology of the Center for Dental and Oral Medicine, University Medical Center Hamburg-Eppendorf, Germany. However, it is not possible to write such a book without support. First, I would like to thank my wife Anne for her patience and understanding. Likewise, I am very much indebted to my staff for a wide range of support.

I would also like to express my gratitude to Dr. Christian Urbanowicz and Dr. Daria Wojciukiewicz from Thieme Verlag, who supervised the original German edition with large amounts of understanding and practical assistance, and to Angelika-Marie Findgott from Thieme Publishers Stuttgart, without whom there would have been no English edition. The English version has again shown that dental radiology is a very special and challenging field of radiology that demands a high level of expertise and a deep understanding of the complex issues involved. Suzyon O’Neal Wandrey, who provided the translation of the German edition, and Dr. Martina Habeck, who managed the editorial stages of the project, did an outstanding job in creating this English version and making many valuable suggestions for improvement. Finally, I would like to extend a note of thanks to Ruth Gutberlet, who translated the poem by Eugen Roth with wit and skill.

Chapter 1 History and Development of Dental Radiography

1 History and Development of Dental Radiography

While conducting experiments in his laboratory at the Physics Institute of the University of Würzburg, Germany, on November 8, 1895, physicist Wilhelm Conrad Röntgen observed that “a paper screen washed with barium–platinocyanide lights up brilliantly and fluoresces equally well, whether the treated side or the other be turned toward the discharge tube. The fluorescence was observable two meters away from the apparatus.” (Fig. 1.1). These are the words that Röntgen used to describe the discovery he made while experimenting with cathode ray tubes. After repeating the experiments several times, he felt certain that he had indeed discovered “a new kind of rays.”

The first preliminary photographic evidence of this discovery probably showed his own hand, but the first “roentgenogram” on reliable record was that of his wife’s hand, taken on December 22, 1895.

Röntgen published the details of his discovery and findings in three communications:

• Preliminary communication dated December 28, 1895: On a New Kind of Rays (Fig. 1.2)

• Second communication dated March 9, 1896: On a New Kind of Rays

• Third communication dated March 10, 1897: Further Observations on the Properties of X-rays.

Röntgen himself referred to this new type of radiation as “X-rays.” At the meeting of German Physico-Medical Society on January 23, 1896, where he presented his discovery, it was decided to call them “Roentgen rays” instead. The name “X-rays” is still used in English-speaking countries today. Important dates in the history and development of dental radiography are described below.

• 1896: The first X-rays of the teeth were probably taken in late January and early February 1896. The first dental radiographs on record are those of Friedrich Otto Walkhoff, a German dentist who lay motionless on the floor for 25 minutes to achieve blur-free images during the long exposure time needed. Other dental X-rays from this early period were taken by German physicist Walter König. In the following years, the introduction of X-ray tubes with a platinum plate anode cut the exposure time in half.

Fig. 1.1 Fluorescence of intensifying screens of three different thicknesses.

• 1896: C. Edmund Kells described the first film-holder for dental radiography, which was made of highly permeable aluminum. Kells is thus credited as being the father of the paralleling technique. However, the glass plates used as image receptors at that time were generally held in the patient’s mouth by the dentist.

• 1897: The first double-coated celluloid films were made in 1897 but were not used in routine practice until 1923.

• 1897: Edison and Gehler developed intensifying screens that were coated with calcium tungstate.

• 1904–1907: The bisecting-angle technique of intraoral radiography was described by two independent researchers, Price (1904) and Cieszynski (1907) (Fig. 1.3).

• 1904–1925: The first dental X-ray machine was developed in 1904. However, the angular tube required for this machine was not developed until 1919 by Garretson, and was first installed in dental X-ray equipment in 1925. These technical advances made it possible to capture different angles of incidence of the beam with much better precision.

• 1921: French inventor André Bocage, the founder of conventional tomography, applied for a patent for a functional tomographic unit.

• 1921: Owing to the lack of suitable film-holders for the paralleling technique, Collins Le Master proposed the use of “a roll of absorbent cotton cemented or otherwise secured to the film holder,” to align the plane of the film perpendicular to the axis of the teeth.

• 1925: Howard R. Raper developed the bitewing examination technique for optimal detection and diagnosis of approximal caries.

Fig. 1.2 Detail of the title page of the Proceedings of the Meeting of the Würzburg Physico-Medical Society, Germany, in 1895.

Fig. 1.3a, b Image distortion in the root region associated with use of the bisecting-angle technique. (From: Pasler FA, Visser H. Zahnmedizinische Radiologie. 2nd ed. Stuttgart: Thieme; 2000. Farbatlanten der Zahnmedizin; Band 5.)

a Bisecting-angle technique: Positioning of the film packet.

b Zygomatic bone projecting to the root region.

Fig. 1.4 X-ray sphere introduced by Siemens.

• 1928: The first dental X-ray units with high-voltage protection were placed on the market.

• 1931: Hofrath and Broadbent introduced cephalometric radiography into orthodontics.

• 1933: The first X-ray systems housing the X-ray tube and generator in a single unit were developed. The Siemens X-ray sphere is the most famous representative of this group of dental X-ray machines (Fig. 1.4). The so-called single-tank construction method was thus introduced into dental radiography (Fig. 1.5).

• 1933: Numata developed a panoramic radiography method capable of imaging the entire upper or lower dental arch using a slit-beam technique (Fig. 1.6).

• 1946–1952: Paatero independently developed a similar system in 1946. Unlike Numata, Paatero continued to develop his invention. Between 1949 and 1952, he introduced an X-ray machine for panoramic tomography or pantomography (Fig. 1.7).

Fig. 1.5 Modern technology has greatly reduced the size of the dental X-ray head. A large older model is shown on the left, and a significantly scaled-down modern dental X-ray head (Heliodent 70) is shown on the right. The space required for the transformer is highlighted in blue.

• 1956: This year marked the introduction of the Panorex X-ray machine, the first double eccentric panoramic unit.

• 1959–1961: In this period, there was continued development proceeding from the pantomograph to the orthopantomogram.

• 1961: Magnification was introduced to panoramic radiography by Ott and Blackman (Fig. 1.8 and Fig. 1.9).

• 1974: Further development of dental panoramic tomography started in 1974.

• 1982: The Zonarc panoramic X-ray machine (Fig. 1.10), which had many new scanning programs for the head and neck region, was introduced (Fig. 1.11).

Fig. 1.6 Schematic diagram of a panoramic radiograph (based on Numata’s method of slit-beam intraoral panoramic radiography).

Fig. 1.7 Panoramic radiograph of a patient by Paatero.

Fig. 1.8 Status-X panoramic X-ray apparatus by Siemens.

Fig. 1.9 This magnified panoramic radiograph taken with the Status-X shows a residual root in the right maxillary sinus of the upper jaw.

Fig. 1.10 Zonarc panoramic radiography device for supine patients.

Fig. 1.11 Zonarc panoramic radiography device for midfacial imaging.

Fig. 1.12 NewTom cone beam computed tomography system for supine patients.

Fig. 1.13 Galileos cone beam computed tomography system.

• 1985: The Scanora, a combined system for panoramic radiography and conventional spiral tomography, was introduced in 1985.

• 1987: This year marked the beginning of digital radiography in dental, oral, and maxillofacial surgery.

• 1995: Digital technology for panoramic radiography was introduced.

• 1995: “Transverse panoramic tomography” was the buzzword in 1995.

• 1998: Digital cone beam computed tomography (CBCT) was introduced as a tomographic method designed specifically for imaging of bony structures in the head region (Fig. 1.12).

• 2007: The Galileos digital CBCT system for standing and seated patients (Fig. 1.13) was launched on the market.

• 2010: A combined panoramic radiography and CBCT system was introduced.

Chapter 2 Radiation Physics

2.1 Types of Radiation

2.2 Direct and Indirect Ionization

2.3 Corpuscular and Photon Radiation

2.4 Interactions between Radiation and Matter

2.5 Fundamental Physical Processes Involved in the Transfer of Photon Energy to Matter

2.6 Interactions between X-rays and Matter

2.7 Radioactivity

2.8 Production of X-rays

2 Radiation Physics

2.1 Types of Radiation

All life on our planet is exposed to different types of naturally occurring radiation. This radiation generally occurs in the form of electromagnetic waves that differ only by wavelength. Some radiation can be seen or felt if it occurs at a few specific wavelengths but, in most cases, radiation cannot be detected by our sense organs. This applies in particular to rays with very short wavelengths beyond the ultraviolet end of the light spectrum.

Radiation is defined as the emission and propagation of energy.

When radiation strikes an object, the energy generated in the radiation field triggers interactions in the object. The body can withstand a good deal of radiation but biological damage starts when it is exposed to short-wavelength radiation.

Radiation in the infrared range is perceived as heat radiation that is not uncomfortable. Ultraviolet radiation, however, starts to induce damage associated with chemical reactions in the skin that can produce sunburn.

Radiation at even shorter wavelengths is able to knock electrons out of an atom. This process is called ionization and such radiation is referred to as ionizing radiation.

Note

There are two basic types of radiation: ionizing radiation and nonionizing radiation.

2.2 Direct and Indirect Ionization

Radiation is further characterized by the manner in which its ionizing effects occur.

In the case of directly ionizing radiation, the energy from electrically charged particles is transferred directly to the irradiated structures. Alpha particles, beta particles, and protons are types of directly ionizing radiation.

In indirectly ionizing radiation, interactions with the irradiated matter result in the generation of electrically charged particles which, in turn, transfer their energy to surrounding structures. X-rays and gamma rays are forms of indirectly ionizing radiation.

2.3 Corpuscular and Photon Radiation

Ionizing radiation can be divided into two types, according to its physical consistency: corpuscular radiation and photon radiation.

The fact that nuclei may be stable or unstable is one reason why these two types of radiation occur. Stable nuclei are an important part of our everyday lives and are generally harmless. Unstable nuclei, on the other hand, emit ionizing radiation and it is important to protect the body from this type of radiation as well as possible.

2.3.1 Corpuscular Radiation

Alpha and beta radiation are common types of corpuscular radiation. Corpuscular radiation is directly ionizing radiation generated by the spontaneous decay of unstable atomic nuclei. This process is known as radioactivity.

Alpha particles consist of helium nuclei, while beta particles are high-energy free electrons.

Alpha and beta rays consist of particles with rest mass; therefore, they have a relatively short range. Typically, alpha radiation has a range of only a few centimeters in air. Its range in tissue is less than 1 mm because the attenuation of radiation passing through tissue is very large. The range of beta radiation depends on the energy of the radiation and varies from a few meters in air to only a few millimeters in tissue.

2.3.2 Photon Radiation

X-rays and gamma rays are forms of electromagnetic radiation.

Fig. 2.1 Electromagnetic waves. B: magnetic flux density; E: electric field strength. (Adapted from: Zabel H. Kurzlehrbuch Physik. Stuttgart: Thieme; 2011.)

Fig. 2.2 The electromagnetic spectrum. (From: Zabel H. Kurzlehrbuch Physik. Stuttgart: Thieme; 2011.)

Electromagnetic waves are made from electric and magnetic fields. Electric fields and magnetic fields are closely related. Electromagnetic waves are transverse waves because the electric field waves are perpendicular to the magnetic flux density (Fig. 2.1).

Unlike corpuscular radiation, photon radiation has no rest mass. Therefore, it travels at the speed of light in air as well as in a vacuum.

The entire electromagnetic spectrum consists of photon and electromagnetic radiation wavelengths, ranging from very long-wavelength radio waves to very short-wavelength gamma rays (Fig. 2.2).

2.4 Interactions between Radiation and Matter

The main physical processes involved in energy transfer are the excitation and ionization of atoms exposed to radiation.

Knowledge of the structure of atoms is therefore needed to understand the processes associated with ionization.

From the work of Ernest Rutherford (1911) and Niels Bohr (1913), it is known that an atom consists of a positively charged nucleus surrounded by a shell of negatively charged electrons that travel in circular orbits around the nucleus.

The number of electrons corresponds to the atomic number or nuclear charge of the atom. The atomic number is equivalent to the number of protons in the nucleus. The characteristic position of an element on the periodic table is determined by its atomic number. Atoms normally have an equal number of electrons and protons, so they are electrically neutral.

The fact that the electrons revolve around the nucleus in different paths is important for various reactions involving photons. The orbits or shells are represented, from the nucleus outwards, by the letters K, L, M, N, O, P, and Q. Each shell can accommodate a fixed number of electrons. The shell capacity ranges from two electrons in the K shell to 18 electrons in the M shell. The number of electrons that can be accommodated in a shell increases with the atomic number of an element. The higher the atomic number of an element, the larger the number of electrons that it can accommodate. This, in turn, has an effect on the absorption behavior of the element.

Note

The more electrons an element has, the more photons it can absorb. This fact plays an important role in the recognition of different tissues and in diagnostics.

2.5 Fundamental Physical Processes Involved in the Transfer of Photon Energy to Matter

When incoming photons strike matter, two physical processes may occur: excitation or ionization.

2.5.1 Excitation

The addition of energy to an atom from an external source can cause electrons to be displaced from an inner shell of the atom to an outer shell. This very brief state is called excitation. In the process of excitation, no electron is ejected from the atom and no ionization occurs. Since the shells of an atom must always be full, the vacancies created in now incomplete inner shells must, in turn, be filled by electrons from outer shells. In the process, the electrons that fall from the outer shells to fill the vacancies in the inner shells emit energy in the form of electromagnetic photon radiation. The vacancies left in the outer shell resulting from this electron movement must, in turn, be filled by other free electrons.

If these electron shell-to-shell transitions occur in the region of the innermost shell, then enough energy is generated for the production of X-rays. Since the wavelength of these X-rays is dependent on the specific type of anode material, this kind of radiation is referred to as characteristic X-rays. Conversely, electron shell-to-shell transitions in the region of the middle shells result in the production of ultraviolet light, and those in the region of the outermost shell result in the production of low-level energy perceived as visible light.

Note

Electron shell-to-shell transitions in the region of the innermost shell of an atom result in the release of energy in the form of electromagnetic radiation at wavelengths in the range of X-rays.

2.5.2 Ionization

Ionization occurs when the energy of an incident photon is completely absorbed and transferred to an electron. The amount of energy transferred in the process is so great that it overcomes the binding energy of the electron, knocking the electron out of its shell and ejecting it from the atom. Such an atom is said to be ionized. The ejected electron can, in turn, ionize other atoms.

2.6 Interactions between X-rays and Matter

When incident X-rays collide with matter, they pass through the matter but are attenuated in the process. In physical terms, the energy of the photons is either absorbed or propagates further as scattered radiation. The amount of attenuation is determined by the thickness, density, and atomic number of the irradiated material.

Note

As X-rays pass through the body, the X-ray beam is attenuated, or weakened as a result of the absorption of photons and the production of scattered radiation.

2.6.1 Absorption—Photoelectric Effect

The absorption of radiation takes place mainly within the inner shells of an atom. Since X-ray beams consists of photons, X-ray absorption is also referred to as the photoelectric effect (Fig. 2.3). A fraction of the energy of the incoming X-ray photon is used to eject an electron from its inner shell, and the emitted photon receives the remaining energy as kinetic energy. Subsequently, the ejected electron induces more photoelectric effects in other atoms.

As in excitation, the vacancy left in the inner shell of an atom due to the ejection of electrons is then filled by electrons dropping down from outer shells. The energy released in the process is emitted from the atom in the form of characteristic X-radiation.

The photoelectric effect plays a crucial role in diagnostic radiography, where voltages of up to 100 kilovolts (kV) are applied. The lower the kilovoltage level, the greater the photoelectric absorption, and the higher the atomic number of the irradiated material, the greater the photoelectric effect. This is why bony structures are best visualized at lower kilovoltage levels of around 50 kV. From levels of around 60 kV, the photoelectric effect decreases sharply with increasing kilovoltage, with a proportional increase in scattered radiation. The size of the two fractions is approximately equal at 60 kV.

Fig. 2.3 Photoelectric effect. (From: Schwenzer N, Ehrenfeldt M, eds. Chirurgische Grundlagen. 4th ed. Stuttgart: Thieme; 2008.)

2.6.2 Scattered Radiation—Compton Effect

Besides the photoelectric effect, the second physical process that plays an important role in diagnostic radiology is the Compton effect, or Compton scattering. The Compton effect also involves the ejection of an electron from its atomic shell. However, unlike the photoelectric effect, outer-shell electrons are affected rather than inner-shell electrons. Because outer-shell electrons have relatively low binding energy, they can be more easily ejected from an atom. Therefore, only a fraction of the incoming photon energy is required to eject these electrons. The remaining energy is deflected from the atom, at an angle of 0 to 180 degrees, as a scattered photon. Therefore, the angle of scattering can be quite large (Fig. 2.4). In spite of the energy loss, the ejected electron and scattered radiation can still undergo further ionization interactions.

Since scattered X-rays can travel in all directions, they strike the image receptor (e. g., X-ray film) from arbitrary angles. Therefore, scattering results in a significant reduction of contrast.

The Compton effect is a major factor determining the energy level used in dental X-ray imaging. At 60 kV, photoelectric and Compton-effect interactions are approximately equal.

Classical scattering without energy loss and pair production does not play a role in diagnostic radiology. The reason for this is that classical scattering only occurs with very “soft” X-rays and pair production only occurs in the mega electron volt (MeV) range.

Fig. 2.4 Compton scattering. (From: Schwenzer N, Ehrenfeldt M, eds. Chirurgische Grundlagen. 4th ed. Stuttgart: Thieme; 2008.)

Note

The photoelectric effect has its greatest effect at low voltages of up to ~50 kV. X-ray images produced in this low-voltage range are characterized by high contrast. This high contrast is possible because the scattered radiation component is very small, resulting in very sharp visualization of bony structures. On the other hand, these so-called soft-rays are associated with a very large degree of absorption and thus disproportionately high radiation exposure.

This is why all dental X-ray machines that used to operate at 50 kV were prohibited. Today, radiation protection guidelines stipulate that the kilovoltage of dental X-ray machines must not be less than 60 kV. This operating kilovoltage achieves a good compromise between the quality of the dental X-ray image and radiation exposure.

2.7 Radioactivity

Atoms with stable nuclei have an equal number of protons and neutrons. If, however, the number of neutrons differs from the number of protons (that is, if the neutron to proton ratio is too low or too high), then the atom has an unstable nucleus. Unstable nuclei undergo radioactive decay at variable rates and are ultimately converted back into stable nuclei. Nuclei undergoing radioactive decay are termed radioactive nuclides (radioactive isotopes), or radionuclides (radioisotopes) for short. Radioactivity refers to the process of radioactive decay.

Radioactive radiation consists mainly of alpha particles (helium nuclei), beta particles (electrons), and gamma rays (very high-energy electromagnetic radiation).

It is categorized by source as terrestrial, cosmic, or artificial radiation. The different types of radiation have different mechanisms of action.

2.8 Production of X-rays

2.8.1 Dental X-ray Equipment

In dental X-ray equipment, the X-ray generator and X-ray tube are contained together in the same radiation-proof housing. This modern single-tank design allows the manufacture of small and easy-to-handle X-ray machines, which are very well suited for use in dental radiography (Fig. 2.5 and Fig. 2.6).

The X-rays are generated in a glass X-ray tube (Fig. 2.7). The glass X-ray tube contains all the essential technical components required for the generation of X-rays.

The X-ray tube and the generator producing the high and low voltage needed for the production of X-rays are located together in the radiation-proof housing. The radiation-proof X-ray tube housing is completely filled with oil, which acts as an insulator.

The radiation-proof housing of all X-ray machines must be inspected regularly to prevent leakage radiation. Oil leakage is a sure sign of defects in the radiation-proof housing. If oil leakage is detected, operation of the equipment must be stopped immediately.

Fig. 2.5 Dental X-ray tube with a short spacer cone (may no longer be used today).

X-ray Tube Design

X-rays are produced when high-speed electrons coming from the cathode are suddenly decelerated or stopped by the anode (Fig. 2.8).