ORAL ~R"ADIOLOGYPrinciples and Interpretation

STUART c. WHITE, DDS, PhDProfessor, Section of Oral and Maxillofacial Radiology

School of DentistryUniversity of California, Los Angeles

Los Angeles, CaliforniaMICHAEL I. PHAROAH, DDS, MSc, FRCD(C)

Professor, Department of Radiology

Faculty of DentistryUniversity of TorontoToronto, Ontario

Canada

FIFTH EDITIONSelected illustratians by Danald O'Cannor

with 1325 illustratians

~ ..A MosbyAn Affiliate of Elsevier

~ 'Y,1 MosbyAn Affiliate of Elsevier

11830 Westline Industrial DriveSt. Louis, Missouri 63146

ORAL RADIOLOGY: PRINCIPLES AND INTERPRETATIONCopyright @ 2004, Mosby, Inc. All rights reserved.

ISBN 0-323-02001-

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panoramic and skull radiographs for evidence of abnor-malities. In recent years there has been a major moveto digital imaging in dentistry. We added a new chapter,Digital Imaging, that explains the various types of digitalimaging available in dentistry, the underlying conceptsof how the different methods work, and their compar-ative strengths and weaknesses in clinical usage. Justwithin the last two years cone-beam tomography hasbecome available in dentistry. The chapter on special-ized radiographic techniques now explains how thistechnology works and how it is best used in dentistry.The chapter on dental caries has also been expandedand updated. The chapters on radiographic manifesta-tions of disease in the orofacial region have beenupdated to include the latest information on etiologyand diagnosis. The chapter on orofacial implants hasbeen expanded and updated to keep abreast of thisrapidly changing field. We improved the imagesthroughout the book and added examples of advancedimaging where appropriate.

Each new edition of this textbook provides the oppor-tunity to include recent progress in the rapidly chang-ing field of diagnostic imaging. We note with particularsatisfaction the recent recognition by the AmericanDental Association of Oral and Maxillofacial Radiologyas their ninth specialty. The ADA's definition of oral andmaxillofacial radiology is: the specialty of dentistry anddiscipline of radiology concerned with the productionand interpretation of images and data produced by allmodalities of radiant energy that are used for the diag-nosis and management of diseases, disorders, and con-ditions of the oral and maxillofacial region. It is thecontinuing goal of our text to present the underlyingscience of diagnostic imaging as well as describe thecore principles of image production and interpretationfor the dental student.

In this edition the chapters on panoramic imagingand extraoral radiographic examinations have beenextensively rewritten and expanded beyond radio-graphic technique and anatomy to provide new empha-sis on radiographic interpretation. These chapters nowlead the reader through systematic approaches to eval-uate the complex anatomic relationships displayed on

Stuart C. White

Michael J. Pharoah

ix

Ack owledqrnents

We have drawn upon the special talents of many of ourcolleagues as authors ofchapters, some for the first timeand others for return visits. We thank all for sharingtheir knowledge and skills. In particular we welcomethe first timers: Dr. Alan G. Lurie-Panoramic Imaging,Drs. Sotirios Tetradis and Mel L. Kantor-ExtraoralRadiographic Examinations, Drs. John B. Ludlow andAndre Mol-Digital Imaging, Dr. Ann Wenzel-DentalCaries, Dr. Ernest W.N. Lam-Paranasal Sinuses, Dr.Laurie C. Carter-Soft Tissue Calcification and Ossifica­tion, and Dr. Carol Anne Murdoch-Kinch-Developmen­tal Disturbances of the Face and Jaws. We also wish toacknowledge the insightful thoughts of Mr. Charlie

Brayer and Ms. Ruth K. Arbuckle related to x-ray filmand intensifying screens. The immense knowledge andexperience of all these individuals adds immeasurablyto this text. We are most grateful for the skillful and gen­erous support from the staff at Elsevier for their energyand creativity in the presentation of the content. Andfinally, we thank our students whose sharp eyes andminds constantly discover new ways for us to improveth is book;

Stuart C. WhiteMichael J. Pharoah

XI

Kathryn A. Atchison, DDS, MPHProfessor and Associate Dean for

Research and Knowledge ManagementUniversity of California, School of DentistryLos Angeles, California

Linda Lee, DDS, MSc, Dipl ABOP, FRCD(C)Assistant ProfessorDepartment of Biological and Diagnostic SciencesFaculty of Dentistry, University of Toronto;Staff Dentist, Department of Dental OncologyPrincess Margaret Hospital, University Health NetworkToronto, OntarioCanada

Byron W. Benson, DDS, MSProfessor, Department of Diagnostic SciencesBaylor College of Dentistry, Texas A&M University SystemDallas, Texas John B. ludlow, DDS, MS

Associate ProfessorDepartment of Diagnostic Sciences and General

DentistryUniversity of North Carolina, School of DentistryChapel Hill, North Carolina

Sharon L. Brooks, DDS, MS

ProfessorDepartment of Oral Medicine/Pathology/Oncology;Associate Professor, Department of RadiologyUniversity of Michigan School of MedicineAnn Arbor, Michigan Alan G. Lurie, DDS, PhD

Professor and Head, Oral and Maxillofacial Radiology;Professor, Diagnostic Imaging and TherapeuticsDivision of Oral and Maxillofacial RadiologyUniversity of Connecticut, School of Dental MedicineFarmington, Connecticut

Laurie C. Carter, DDS, MA, PhDAssociate Professor arid DirectorOral and Maxillofacial RadiologyDepartment of Oral PathologyVirginia Commonwealth University, School of DentistryRichmond, Virginia Andre Mol, DDS, MS, PhD

Assistant ProfessorDepartment of Diagnostic Sciences and General

DentistryUniversity of North CarolinaChapel Hill, North Carolina

Neil L. Frederiksen, DDS, PhDProfessor and Director, Oral and Maxillofacial RadiologyDepartment of Diagnostic SciencesBaylor College of DentistryTexas A&M University System Health Science CenterDallas, Texas

Mel L. Kantor, DDS, MPHProfessorDivision of Oral and Maxillofacial RadiologyDepartment of Diagnostic SciencesUMDNJ New Jersey Dental SchoolNewark, New Jersey

Carol Anne Murdoch-Kinch, DDS, PhDClinical Associate ProfessorDepartment of Oral Medicine, Pathology, and

OncologyUniversity of Michigan, School of Dentistry;

Attending PhysicianOral and Maxillofacial Surgery and Hospital DentistryUniversity of Michigan Health SystemAnn Arbor, Michigan

Ernest W.N. Lam, DMD, PhD, FRCD(C)Associate Professor and ChairDivision of Oral and Maxillofacial Radiology;Adjunct Associate Professor of OncologyDepartment of Dentistry, University of AlbertaEdmonton, AlbertaCanada

C. Grace Petrikowski, DDS, MSc, Dip. Oral Rad,

FRCD(C)Associate Professor, Department of RadiologyFaculty of Dentistry, University of TorontoToronto, OntarioCanada

v

.

vi CONTRIBUTORS

Ann Wenzel, PhD, DrOdontProfessor and HeadDepartment of Oral Radiology, Royal Dental CollegeFaculty of Health Sciences, University of AarhusAarhus, Denmark

Axel Ruprecht; DDS, MScD, FRCD(C)Professor and Director, Oral and Maxillofacial RadiologyDepartment of Oral Pathology, Radiology, and MedicineCollege of Dentistry;Professor, Department of RadiologyCollege of Medicine, University of IowaIowa City, Iowa

Vivek Shetty, DDS, Dr Med DentAssociate ProfessorDepartment of Oral and Maxillofacial SurgeryUniversity of California, School of DeptistryLos Angeles, California

Robert E. Wood, DDS, PhD, FRCD(C)Associate ProfessorDepartment of Oral RadiologyFaculty of Dentistry, University of Toronto;Active Staff, Department of Dental OncologyPrincess Margaret Hospital, University Health NetworkToronto, OntarioCanada

Sotirios Tetradis, DDS, rhOAssociate ProfessorDepartment of Oral and Maxillofacial RadiologyUniversity of California, School of DentistryLos Angeles, California

,

Conte nts

~ ... " 1 ITHE PHYSIC !> OF IONIZING RADIATION

I Radiallon I'tly~cs, 3In coIIoboro'itvlwill>Ai8UT G, RI(HARD~

PA RT II

BIOLOGIC EffECTS OF RADIATION2 RadIation Biology, 25

PA n II I

RADIATION SAfETY AND PROTfCTION3 Healt ll Physics. 47

NEIL L, ERWUIKSE N

P,UT IV _

IMAGING PRI NCIPLES AND TECHNIQUES4 X-Ray film, In tenllfying Sue"n•• and Grid . _71

S Prolectlo n Geometry. 86

6 Pro cessing X·Ray FUm. 94

7 Radiograph ic Qua lity Auurano:e and Infection(ontrol, 110

8 Intraora l Radiograph ic Examinations, 121

9 Normal Rad iog raphic Anatomy, 166

10 Panora mic Imagi ng. 191ALAN c. LIIRIE

11 ht.aaral Radi09.a ph ic Examlna tlo n . , 210SOll.'OS TETllADI' • loiI El L. U.NTOR

12 Dig it al Imag i!l9 . 125101'11'1 ft_I UDl.OW . ...HDRf MOl

13 SpE'(la lil ed R..diographi< Ye<hniqu... . 245NEIL L, fRE DER'Kl fN

14 Guid e line. for Pr"luiblng De ntalRad iog rap hs, It>'>SMMlON L.- BROOKS . K...'IiBYN .... ""CHISON

P...RT vRADIOGRAPH IC INTERPRETATION OF PATHOLOGY15 Princip les of Radiogra phic Int er p ret at io n, 281

16 o.,n!a l Ca rie., 297.o.NNWE NlU

17 Periodontal Diseases, 314

18 DenIa l Anomalies, 330

19 Inflammatory l esions of the Jaws, ]66LIND'" l U

20 Cyst . of Ihe Jaws, 384

21 8en\gn Tumors of the Jaws, 410

22 Malig nant Di,eases of the Jaw" 458ROBEIIT E. WOOD

23 Diseases of Bone Manlfe,ted ;n the Jaw" 485

24 Syllemlc Di,ea ,es Man ilested In IheJaws, 516

25 Diagnosllc Imag ing 01 theTemporomand ibu la r Jo int, 5]8C. GAACE P,Tltl KOWSKI

26 Para na sal Sinuses, 576"' xu RUPM CHT • EBNtsl W.N tAM

27 Soft Tillue Ca lcilicalion and Os,ificat ion, 597tAUH IE C, CMlH H

28 Trauma to Teet h and fadal Struct ures, 61S

29 Development a l Di,turbance. of Ihe face andJaws, 6 39UdtOl J\NNE MURDOCH_KINCH

] 0 Salivary Gland Radiology, 658n OON W BENSON

31 Orolacla l Implant " 677VIV, K SHlTn • BUON W. BE NSON

PA R T . 0 N E"

The Physics .of IonizingRadiation

,

ORAL RADIOLOGYPrinciples and Interpretation

PA R T . 0 N E"

The Physics .of IonizingRadiation

In couodorotion with

ALBERT G. RICHARDS

•ICSPhylatlona

(HAPTER

Composition of MatterMatter is anything that occupies space and has inertia.It has mass and can exert force or be acted on by aforce. Matter occurs in three states-solid, liquid, andgas-and may be divided into elements and com­pounds. Atoms, the fundamental units of elements,cannot be subdivided by ordinary chemical methodsbut may be broken down into smaller (subatomic) par­ticles by special high-energy techniques. More than 100subatomic particles have been described*; the so-calledfundamental particles (electrons, protons, and neu­trons) are of greatest interest in radiology because thegeneration, emission, and absorption of radiationoccur at the subatomic level.

ATOMIC STRUCTURE

Because the atom cannot be directly observed, variousmodels are used to describe its structure, each of whichis capable of explaining observable actions. The phe­nomena associated with radiology employ the quantummechanical model proposed by Niels Bohr in 1913.Bohr conceived the atom as a miniature solar system, atthe center ofwhich is the nucleus, analogous to the sun.Electrons revolve around this nucleus at high speeds,analogous to the planets orbiting the sun. In all atomsexcept hydrogen, the nucleus consists of two primarysubatomic particles: protons and neutrons. A singleproton constitutes the nucleus of the hydrogen atom.

*The Particle Adventure: the fundamentals of matter and force:http://www.particleadventure.org.

Electrons orbit the nucleus of all atoms. All electronsare alike, as are all protons and neutrons.

Figure 1-1, A, illustrates Bohr's model, using a styl­ized rendering of three atoms. The paths of the elec­trons are drawn as sharply defined orbits to facilitategraphic representation of the generation of x rays andtheir interaction with matter. In reality the orbit shouldbe represented by broad parameters defining a spacein which the electron is most likely to be found. Theorbits, or shells, lie at defined distances from thenucleus and are identified by a letter (Fig. 1-1, B).The innermost shell is the K shell, and the next in orderare the L, M, N, 0, P, and Q shells. The shells also havenumbers for identification: 1 for the K shell, 2 for theL shell, and so on. These are the principal quantumnumbers, represented by the letter n. No known atomhas more than seven shells. Only two electrons mayoccupy the K shell, with increasingly larger numbers ofelectrons occupying the outer shells. The maximalnumber of electrons in a given shell is 2(n2

) , where nis the principal quantum number.

Electrons, protons, and neutrons have unique char­acteristics. The electron carries an electrical charge of-1, the proton a charge of +1, and the neutron nocharge at all. The mass of an electron at rest is about9.1 X 1O-28g. In contrast, the mass of a proton is 1.67 X

1O-24g, which is 1836 times the mass of an electron. Themass of a neutron is 1.68 X 10-24 g, making it 1852 timesheavier than an electron and slightly heavier than aproton. Most of the mass of an atom consists of protonsand neutrons concentrated in the nucleus. The nucleuscontributes only a small fraction (about 1/100,000) of thetotal size of an atom; most of the size of an atom con­sists of the cloud of orbiting electrons.

3

4 PART I THF PHYSICS OF IONI71NCo RA[)IATION

~ ~ )

Shell (( "j j~ .... . ,'* Lshell

K shell "K shell

BFIG. 1-1 A, Atomic structures of hydrogen, helium, andlithium showing orbiting electrons surrounding neutronsand protons in the nucleus. B, Atom showing the structureand identification of electron shells around the nucleus.

The number of protons contained in the nucleusdetermines the positive charge. Because any atom in itsground state is electrically neutral, the total number ofprotons and electrons it carries must be the same. Thenumber of protons in the nucleus also determines theidentity of an element. This is its atomic number (Z).Consequently, each of the more than 100 elements hasa definitive atomic number, a corresponding numberof orbital electrons, and unique chemical and physicalproperties. Nearly the entire mass of the atom consistsof the protons and neutrons in the nucleus. The totalnumber of protons and neutrons in the nucleus of anatom is its atomic mass (A).

The electrostatic attraction between a positivelycharged nucleus and its negatively charged electronsbalances the centrifugal force of the rapidly revolving

Nature of Radiation

IONIZATION

When the number of orbiting electrons in an atomis equal to the number of protons in its nucleus, theatom is electrically neutral. If an electrically neutralatom loses an electron, it becomes a positive ion and thefree electron is a negative ion. This process of formingan ion pair is termed ionization. Electrons can be lostfrom an atom by heating or by interactions (collisions)with high-energy x rays or particles such as protons.Such ionization requires sufficient energy to overcomethe electrostatic force binding the electrons to thenucleus. The electrons in the inner shells (K, L, and M)are so tightly bound to the nucleus that only x rays,gamma rays, and high-energy particles can removethem. In contrast, the electrons in the outer shells havesuch low binding energies that they can be easily dis­placed by photons of lower energy (e.g., ultraviolet orvisible light).

Particulate radiation consists of atomic nuclei or sub­atomic particles moving at high velocity. Alpha parti-

PARTICULATE RADIATION

Radiation is the transmission of energy through spaceand matter. It may occur in two forms: particulate andelectromagnetic.

electrons and maintains them in their orbits. Conse­quently, the amount of energy required to remove anelectron from a given shell must exceed the electrostaticforce of attraction between it and the nucleus. This iscalled the electron binding energy of the electron (or ion­ization energy) and is specific for each shell of eachelement. Electrons in the K shell ofa given element havethe greatest binding energy because they are closest tothe nucleus. The binding energy ofthe electrons in eachsuccessive shell decreases. For an electron to move froma specific orbit to another orbit farther from thenucleus, energy must be supplied in an amount equal tothe difference in binding energies between the twoorbits. In contrast, in moving an electron from an outerorbit to one closer to the nucleus, energy is lost andgiven up in the form of electromagnetic radiation (see"Characteristic Radiation," p. 13). The K-shell electronsor any other electrons of large (high-Z) atoms havegreater binding energies than those in comparableshells of smaller (low-Z) atoms. This is because largeatoms have more protons and thus bind the orbital elec­trons more tightly to the nucleus than do small atoms.

Lithium atom3 Electrons3 Protons3 Neutrons

--0--.

Helium atom2 Electrons2 Neutrons2 Protons

Hydrogen atom1 Electron1 Proton

A

FIG, 1 -2 A vibrating negatively charged particle gener­ates electro magnetic radia tion, Oscillations of the part icle aretraced on a strip recorder; they are equal in frequency to theelectromag netic waves produced,

sufficient energy is associated with the radiation toremove orbital electrons fro m atoms in the irradiatedmatter, the radiation is ionizing.

Some of the properties of electromagnetic radiationare best expressed by wave theory, whereas others aremost successfully described by quantum theory. Thewave theory of electromagnetic radiation maintainsthat radiation is propagated in the form of waves, notunlike the waves resulting from a disturbance in water.Such waves consist of electric and magnetic fields ori­ented in planes at right angles to one another that oscil­late perpendicular to the direction of motion (Fig. 1-4).All electromagnetic waves travel at the velocity of light(3.0 x 108m per sec; the velocity of light is representedby the letter c) in a vacuum. Waves of all kinds exhibitthe properties of wavelength (A) and frequency (v).Wavelength and frequency of electromagnetic radia­tion are related as follows :

A x v = c = 3 X 108 meters/second

where A is in meters and V is in cycles per second(hertz). Wave theory is more useful for consideringradiation in bulk when millions of quanta are beingexamined, as in experiments dealing with refraction,reflection, diffraction, interference, and polarization.

Quantum theory considers electromagnetic radia­tion as small bundles of energy called photons. Eachphoton travels at the speed of light and contains a

CHAPTER 1 RADIATION PHYSICS

des, beta particles, and cathode rays are examples ofparticulate radiation. Alpha particles are helium nucleiconsisting of two protons andtwo neutrons. They resultfrom the radioactive decay of many elements. Becauseof their double charge and heavy mass, alpha particlesdensely ionize matter through which they pass. Accord-.ingly, they quickly give up their energy and penetrateonly a few microns of body tissue. (An ordinary sheetof paper absorbs them.) After stopping, alpha particlesacquiring two electrons and become neutral heliumatoms.

Beta particles are electrons emitted by radioactivenuclei. High-speed beta particles are able to penetratematter to a greater depth than alpha particles, to amaximum of 1.5 em in tissue. This deeper penetrationoccurs because beta particles are smaller and lighterand carry a single negative charge; therefore they havea much lower probability of interacting with matterthan do alpha particles. Accordingly, they ionize mattermuch less densely than alpha particles. Beta particlesare used in radiation therapy for treatment of skinlesions. Cathode rays are also high-speed electrons butare produced by manufactured devices (e.g. , x-raytubes) .

The capacity of particulate radiation to ionize atomsdepends on its mass, velocity, and charge. The rate ofloss of energy from a particle as it moves along its trackthrough matter (tissue) is its linear energy transfer (LET) .A particle loses kinetic energy each time it ionizes adja­cent matter; the greater its physical size and charge andthe lower its velocity, the greater is its LET. For example,alpha particles, with their high charge and low velocity,lose kinetic energy rapidly and have short path lengths(are densely ionizing); thus they have a high LET. Betaparticles are much less densely ionizing because of theirlighter mass and lower charge and thus have a lowerLET. They penetrate through tissue more readily thando alpha particles. .

ELECTROMAGNETIC RADIATION

Electromagnetic radiation is the movement of energythrough space as a combination of electric and mag­netic fields (Fig. 1-2). It is generated when the velocityof an electrically charged particle is altered. Gammarays, x rays, ultraviolet rays, visible light, infrared radia­tion (heat), microwaves, and radio waves are all exam­ples of electromagnetic radiation (Fig. 1-3). Gammarays are photons that originate in the nuclei of radioac­tive atoms. They typically have greater energy than xrays. X rays, in contrast, are produced extranuclearlyfrom the interaction of electrons with nuclei in x-raymachines. The types of radiation in this spectrum areionizing or nonionizing, depending on their energy. If

\ I \ I \

5

6 PART I THE PHYSICS OF IONIZING RADIATION

n ' ,... 1

Wavelength (nm)

MR imaging Visible light X-ray imaging

10,'3 ; i~1 01 ' 1109 107 105 103 110 l .r.9~}'t< !/ l 0-3 ,

:1:[,: : ::' , ,'~' ,,O'-r--r-I

,~10-10 10~ 10~ ~ · ' 10-4 10-2 102 104 106 1010

Photon energy (eV)

Radio Microwave Infrared Ultraviolet X-rays Gamma rays

FIG. 1-3 Electromagnetic spectrum showing the relationship among wavelength,photon energy, and physical properties of various portions of the spectrum. Photons withshorter wavelengths have higher energy. Photons used in dental radiography have awavelength of 0.1 to 0.001 nm.

Magnetic field

:. :;;

J

'f(ff~1

FIG. 1-4 The electric and magnetic fields associated with a photon.

specific amount of energy. The unit of photonenergy is the electron volt (eV) (Fig. 1-5). The rela­tionship between wavelength and photon energy is asfollows:

E = h x ciA,

where E is energy in kiloelectron volts (keV) , h isPlanck's constant (6.626 x 10-34 joule-seconds, or 4.3 x1O-18keV) , cis the velocity oflight, and A, is wavelength innanometers. This expression may be simplified as follows:

E = 1.24/A,

The quantum theory of radiation has been success­ful in correlating experimental data on the interaction

of radiation with atoms, the photoelectric effect, andthe production of x rays.

Typically, high-energy photons such as x rays andgamma rays are characterized by their energy, whereaslower-energy photons (ultraviolet through radio waves)are characterized by their wavelength.

The X-Ray MachineThe heart of an x-ray machine is the x-ray tube and itspower supply. The x-ray tube is positioned within thetube head, along with some components of the powersupply (Fig. 1-6). Often the tube is recessed within the

CHAPTER 1 RADIATION PHYSICS 7

Tube-

~ mL , ~

- r\.fl.~~~~;i~~:~t~~~~

1-Voltstoragebattery

t!~~:~~}j.~~~Ji~*~

FIG. 1-5 An electron volt is the amount of energyacquired by one electron accelerating through a potentialdifference of 1 volt (1.602 x 10-19 joules).

/

Electronicfocusing

cup

FIG. 1-7

+

Usefulx-raybeam

X-ray tube with the major components labeled.

B

FIG. 1-8 Dental x-ray machine circuitry with the majorcomponents labeled. A, Filament step-down transformer;B, filament current control (mA switch); C, autotransformer;D, kVp selector dial (switch); E, high-voltage transformer;F, x-ray timer (switch); G, tube voltage indicator (volt-meter);H, tube current indicator (ammeter); I, x-ray tube.

A::;1h ql :n';;lD ,

I If;:lm 1h··r ~ .... ~ "

f rHaiJ~1JJ ~b'(") t3 ~:r1,t}tl ~

E~----..,

C~

11~.11] H

-""-'7:':':"::'--'-~fHL- F '." .;

AC powersupply

Aimingcylinder

X-ray tube

Yoke

Oil

FIG. 1-6 Tube head (including the recessed x-ray tube),components of the power supply, and the oil that conductsheat away from the x-ray tube.

tube head to improve the quality of the radiographicimage (see Chapter 5). The tube head is supported byan arm that is usually mounted on a wall. A controlpanel allows the operator to adjust the time of exposureand often the energy and exposure rate of the x-raybeam.

strike the target in the anode, they produce x rays.For the x-ray tube to function, a power supply is nec­essary to (1) heat the filament to generate electrons,and (2) establish a high-voltage potential betweenthe anode and cathode to accelerate the electrons(Fig. 1-8).

X-RAY TUBE

All dental and medical x-ray tubes are called Coolidgetubes because they follow the original design of W. C.Coolidge introduced in 1913. The basic apparatus forgenerating x rays, the x-ray tube, is composed of acathode and an anode (Fig. 1-7). The cathode servesas the source of electrons that flow to the anode.The cathode and anode lie within an evacuated glassenvelope or tube. When electrons from the cathode

CathodeThe cathode (see Fig. 1-7) in an x-ray tube consists ofa filament and a focusing cup. The filament is the sourceof electrons within the x-ray tube. It is a coil of tungstenwire about 2 mm in diameter and 1em or less in length.It is mounted on two stiffwires that support it and carrythe electric current. These two mounting wires leadthrough the glass envelope and connect to both thehigh- and low-voltage electrical sources. The filament isheated to incandescence by the flow of current from

DADT I T1·H pI-lV~lr~ n~ InNI7INf: DAniAn"",

BFIG. 1-9 A, Focusing cup (arrow) containing a filament in the cathode of the tube froma dental x-ray machine. B, Focal spot area (arrows) on the target of the tube. The sizeandshape of the focal area approximate those of the focusina CUD.

the low-voltage source and emits electrons at a rate pro­portional to the temperature of the filament.

The filament lies in a focusing cup (Fig. 1-9, A; see alsoFig. 1-7), a negatively charged concave reflector madeof molybdenum. The focusing cup electrostaticallyfocuses the electrons emitted by the incandescent fila­ment into a narrow beam directed at a small rectangu­lar area on the anode called the focal spot (Fig. 1-9, B;see also Fig. 1-7).·The electrons move in this directionbecause they are repelled by the negatively chargedcathode and attracted to the positively charged anode.The x-ray tube is evacuated to prevent collision of themoving electrons with gas molecules, which wouldsignificantly reduce their speed. This also preventsoxidation ann "hnr-nrmr" of rhe filamerir .

AnodeThe anode consists of a tungste.Q.target embedded in acopper stem (see Fig. 1-7). The purpose of the targetinan x-ray tube is to convert the kinetic energy of the elec­trons generated from the filament into x-ray photons.This is an inefficient process with more than 99 % of theelectron kinetic energy converted to heat. The target ismade of tungsten, a material that has several charac­teristics of an ideal target material. It has a high atomicnumber (74), high melting point, high thermal con­ductivity, and low vapor pressure at the working tem­peratures ofan x-ray tube. A target made ofa high atomicnumber material is best because it is most efficient inproducing x rays. Because heat is generated at theanode, the requirement for a target with a high meltingpoint is clear. Tungsten also has high thermal conductivity,thus dissipating heat into the copper stem. Finally, the

low vapor pressure of tungsten at high temperatures alsohelps maintain the vacuum in the tube at high operat­ing temperatures.

The tungsten target is typically embedded in a largeblock of copper to dissipate heat. Copper, a goodthermalconductor, dissipates heat from the tungsten, thusreducing the risk of the target melting. In addition,insulating oil between the glass envelope and thehousing of the tube head carries heat away from thecopper stem. This type of anode is a stationary anode.

The focal spot is the area on the target to which thefocusing cup directs the electrons from the filament.The sharpness of the radiographic image increases asthe size of the focal spot-the radiation source­decreases (see Chapter 5). The heat generated per unittarget area, however, becomes greater as the focal spotdecreases in size. To take advantage of a small focal spotwhile distributing the electrons over a larger area of thetarget, the target is placed at an angle to the electronbeam (Fig. 1-10). The projection of the focal spot per­pendicular to the electron beam (the effectivefocal spot)is smaller than the actual size of the focal spot. Typi­cally, the target is inclined about 20 degrees to thecentral ray of the x-ray beam. This causes the effectivefocal spot to be almost 1.x 1mm, as opposed to theactual focal spot, which is about 1 x 3mm. The effect isa small apparent source of x rays and an increase insharpness of the image (see Fig. 5-2) with a larger actualfocal spot for heat dissipation.

Another method of dissipating the heat from a smallfocal spot is to use a rotating anode. In this case the tung­sten target is in the form of a beveled disk that rotateswhen the tube is in operation (FiR. 1-11) . Asa result, the

CHAPTER 1 RADIATION PHYSICS 9

1 mm

FIG. 1-10 The angle of the target to the central ray ofthe x-ray beam has a strong influence on the apparent sizeof the focal spot. The projected effective focal spot is muchsmaller than the actual focal spot size.

Target

Anode (+)

Effective focal _ ------r__spot size

Focusing cupand filament

Cathode (-)

-----~:~I===~~... ....t i

. .

ray ~\!... 3mm" ...1 mm..... !

Actual focalspot size

Glass lube

/ /

nII

\../

Electron stream

---X-ray beam

FIG. 1-11 X-ray tube with a rotating anode, which allowsheat at the focal spot to spread out over a large surface area.

electrons strike successive areas of th e target, wideningthe focal spot by an amount corresponding to the cir­cumference of the beveled disk and distributing theheat over this expanded area. As a consequence, smallfocal spots can be used with tube currents of 100 to 500milliamperes (rnA), 10 to 50 times that possible with sta­tionary targets. The target and rotor (armature) of themotor lie within the x-ray tube, and the stator coils (whichdrive the rotor at about 3000 revolutions per minute )lie outside the tube. Such rotating anodes are not usedin in traora l dental x-ray machines but may be usedin tomographic or cephalometric units and in medicalx-ray machines requiring higher radiation output.

" I

POWER SUPPLY

A brief review of some aspects of an electric circuit maybe useful in understanding the power supply in an x­ray machine. An electric current is the movement ofelectrons in a conductor, for example, a wire. The rateof the current flow-the number of electrons movingpast a point in a second-is measured in amperes. Itdepends on two factors: the pressure, or voltage, of thecurrent, measured in volts , and the resistance of theconductor to the flow of electricity, measured in ohms.Ohm's law relates these units:

v == I x R

where Vis the electric potential in volts, lis the currentflow in amperes, and R is the resistance of the conduc­tor in ohms. Such an electric circuit is often comparedto a simple water supply system in which the rate ofwater flow through a pipe (amperes) depends both on

the water pressure (volts) and the pipe resistance ordiameter (ohms).

The primary functions of the power supply of anx-ray machine are to (1) provide a low-voltage currentto heat the x-ray tube filament by use of a step-downtransformer and (2) generate a high potential differ­ence between the anode and cath ode by use of a high­voltage transformer. These transformers and th e x-raytube lie within an electrically grounded metal housingcalled the head of the x-ray machine. An electrical insu­lating material, usually oil, surrounds the transformers.

Tube CurrentThe filament step-down transformer (see Fig. 1-8, A)reduces the voltage of the incoming alternating current(AC) to about 10 volts . Its operation is regulated by thefilament current control (rnA switch) (see Fig. 1-8, B),which adjusts the resistance and thus the current flowthrough the low-voltage circuit, including the filament.This in turn regulates the temperature of the filamentand thus the number of electrons emitted. The tubecurrentis the flow of electrons through the tube, that is,from the filament to the anode and then back to thefilament through the wiring of the power supply. ThernA setting on the filament current control refers tothe tube current, which is measured by the ammeter(see Fig. 1-8, H) .

Tube VoltageA high voltage is required between the anode andcathode to generate x rays. An autotransformer (see

10 PART I THE PHYSICS OF IONIZING RADIATION

Fig. 1-8, C) converts the primary voltage from the inputsource into the secondary voltage. The secondaryvoltage regulated by the kilovolts peak (kVp) selectordial (see Fig. 1-8, D). The kVp dial selects a voltage fromdifferent levels on the autotransformer and applies itacross the primary winding of the high-voltag~ trans­former. The kVp dial therefore controls the voltagebetween the anode and cathode of the x-ray tube. Thehigh-voltage transformer (see Fig. 1-8, E ) provides thehigh voltage required by the x-ray tube to accelerateth e electrons from th e cathode tothe anode and gen­erate x rays. It accomplishes this by boosting the peakvoltage of the incoming line current to as high as 60 to

100kV, thus boosting the peak energy of the electronspassing through the tube to as high as 60 to 100keV.The kVp selector dial setting thus determines the peakkilovoltage across th e tube (see Fig. 1-8, /) .

Because the line current is AC (60 cycles persecond) , the polarity of the x-ray tube alternates at thesame frequency (Fig. 1-12, A) . When the polarity of thevoltage applied across the tube causes the target anodeto be positive and the filament to be negative, the elec­trons around the filament accelerate toward the posi­tive target and curren t flows through th e tube (Fig.1-12, B). Because the line voltage is variable, the voltagepotential between the an ode and cathode vari es. As the

1/60 sec 3/120 secI .I II I

U

/

.,'- Inverse voltage

(reverse bias)

~ +110 ...---.......> «I /ui E(!) '~ Cii

~ <IlE<Il~

:..J 00a OW> «I C

w all!!z 0)-

:J .!!lg

-110

A1/120 sec

II

+70> e.>: «I -,ui E(!) '5. Q)

~~ <Il F~

<Il,a eJ> <..> 1

«I IW all0 0)'a .!!lz g-c

B

1/120 sec

>- <t: 10I-U5 Ez Ew

~I-~ :;

o?:c al.ca: ::l

X I- II

- One impulse -III

Time- IIIII

1/60 secC

FIG. 1-12 A, A 60-cycle AC line voltage at a primary transformer. B, Voltage at theanode varies up to the kVp setting (70 in th is case). C, The intensity of radiation producedat the anode increases as the anode voltage increases. (Modified from Johns HE, Cun­ningham JR: The physics of radiology, ed 3, Springfield, III, 1969, Charles C Thomas.)

CHAPTER 1 RADIATION PHYSICS

tube voltage is increased, the speed of the electronstoward the anode increases. When the electrons strikethe focal spot of the target, some of their energy con­verts to x-ray photons. X rays are produced at the targetwith greatest efficiency when the voltage applied acrossthe tube is high. Therefore the intensity of x-ray pulsestends to be sharply peaked at the center of each cycle(Fig. 1-12, C). During the following half (or negativehalf) of the cycle, the polarity of the AC reverses, andthe filament becomes positive and the target negative(see Fig. 1-12, B). At these times the electrons stay inthe vicinity of the filament and do not flow across thegap between the two elements of the tube. This half ofthe cycle is called inverse voltageor reverse bias (see Fig.1-12, B). No x rays are generated during this half of thevoltage cycle (see Fig. 1-12, C). Therefore when an x­ray tube is powered with 6O-cycle AC, 60 pulses ofx raysare generated each second, each having a duration of1/120 second. This type of power supply circuitry, inwhich the alternating high voltage is applied directlyacross the x-ray tube, limits x-ray production to half theAC cycle and is called self-rectified or half-wave rectified.Almost all conventional dental x-ray machines areself-rectified.

A tube energized with a self-rectifying power supplymust not be operated for extended periods. With over­use the target may get so hot that it emits electrons, andduring the negative half cycle, the inverse voltage maydrive electrons from the target to the filament, causingthe filament to overheat and melt. The glass envelopealso may be damaged if the electrons are driven in thewrong direction by the reverse bias on the tube.

Some dental x-ray manufacturers produce machinesthat replace the conventional 6O-cycle AC high-voltagecurrent of the x-ray tube with a high-frequency powersupply. This effect is an essentially constant potentialbetween the anode and cathode. The result is that themean energy of the x-ray beam produced by these x-raymachines is higher than that from a conventional half­wave rectified machine operated at the same voltage..This is because the number of lower-energy (non­diagnostic) x rays is reduced. These photons are pro­duced as the voltage across the x-ray tube rises fromzero to its peak and then decreases back again to zeroduring the voltage cycle in the half-wave rectified mach­ine. For a given voltage setting and radiographic density,the images resulting from these constant-potentialmachines have a longer contrast scale and lower patientdose compared with conventional x-ray machines.

TIMER

A timer is built into the high-voltage circuit to controlthe duration of the x-ray exposure (see Fig. 1-8, F). The

11

timer controls the length of time that high voltage isapplied to the tube and therefore the time duringwhich tube current flows and x rays are produced.Before the high voltage is applied across the tube,however, the filament must be brought to operatingtemperature to ensure an adequate rate of electronemission. Subjecting the filament to continuousheating at normal operating current is not practicalbecause maintaining the filament at a high temperaturefor a long period shortens its life. Failure of the fila­ment is a common source of malfunction of x-ray tubes.To minimize filament burnout, the timing circuit firstsends a current through the filament for about half asecond to bring it to the proper operating temperature.After the filament is heated, the timer then appliespower to the high-voltage circuit. In some circuitdesigns, a continuous low-level current passing throughthe filament maintains it at a safe low temperature. Inthis case the delay to preheat the filament before eachexposure is even shorter. Accordingly, the machineshould be left on continuously during working hours.

Some x-ray machine timers are calibrated in frac­tions and whole numbers of seconds. The time inter­vals on other timers are expressed as number ofimpulses per exposure (e.g., 3, 6, 9,15). Such numbersrepresent the number of impulses of radiation emittedduring the exposure; thus the number of impulsesdivided by 60 (the frequency of the power source) givesthe exposure time in seconds. Therefore a setting of30 impulses means that there will be 30 impulses ofradiation and is equivalent to a half-second exposure.

TUBE RATING AND DUTY CYCLE

Each x-ray machine comes with tube rating specifica­tions that describe the maximal exposure time the tubecan be energized without risk of damage to the targetfrom overheating. These specifications describe ingraph form the maximal safe intervals (seconds) thatthe tube can be used for a range of voltages (kVp) andfilament current (rnA) values. These tube ratings gen­erally do not impose any restrictions on tube use fordaily intraoral radiography. If a dental x-ray unit is tobe used for extraoral exposures, however, the tube­rating chart should be mounted by the machine for easyreference.

Duty cycle relates to the frequency with which succes­sive exposures can be made. The heat buildup at theanode is measured in heat units defined by the follow­ing equation: heat units (HU) =kVp x rnA x seconds.The heat storage capacity for anodes of dental diag­nostic tubes is approximately 20kHU. Because of heatgenerated at the anode, the interval between successiveexposures must be long enough for its dissipation. This

12 PART I THE PHYSICS OF IONIZING RADIATION

characteristic is a function of the size of the anode andthe method used to cool it. The cooling characteristicsof anodes are described by the maximal number of heatunits it can store without damage and the heat dissipa­tion rate, which can be determined from the coolingcurves provided by the manufacturer of each tube,

Production of X RaysElectrons traveling from the filament to the targetconvert some of their kinetic energy into x-ray photonsby the formation of bremsstrahlung and characteristicradiation.

Pho on ofmaxima l enorgy

/

High-energyrt/A~electron ..,

Direct hit

/

~

BFIG. 1-13 Bremsstrahlung radiation is produced by thedirect hit of electrons on the nucleus in the target (A) or bythe passage of electrons near the nucleus, which results inelectrons being deflected and decelerated (B).

~Photon of

lower energy(bremsstrahlung

radiation)

Near miss

Altered path

~of deflecteds--- deceleratedelectron

-,

~j

/

A

1. The continuously varying voltage difference be­tween the target and filament, which is characteristicof half-wave rectification, causes the electrons strik­ing the target to have varying levels of kinetic energy.

2. The bombarding electrons pass at varying distancesaround tungsten nuclei and are thus deflected tovarying extents. As a result, they give up varying 'amo­unts ofenergy in the form ofbremsstrahlung photons.

3. Most electrons participate in many bremsstrahlunginteractions in the target before losing all theirkinetic energy. As a consequence, an electron carriesdiffering amounts of energy at the time of eachinteraction with a tungsten nucleus that results inthe generation of an x-ray photon.

High-energyelectron ......~t--~~....,,;:::::::::-j~-

BREMSSTRAHLUNGBremsstrahlung interactions, the primary source of x­ray photons from an x-ray tube, are produced by thesudden stopping or slowing of high-speed electrons atthe target. (Bremsstrahlung means "braking radiation"in German.) When electrons from the filament strikethe tungsten target, x-ray photons are created if theelectrons hit a target nucleus directly or if their pathtakes them close to a nucleus. If a high-speed electrondirectly hits the nucleus of a target atom, all its kineticenergy is transformed into a single x-ray photon (Fig.1-13, A). The energy of the resultant photon (in keY)is numerically equal to the energy of the electron. Thisin turn is equal to the kilovoltage applied across thex-ray tube at the instant of its passage.

Most high-speed electrons, however, have near orwide misses with atomic nuclei (Fig. 1-13, B). In theseinteractions, a negatively charged high-speed electronis attracted toward the positisely charged nuclei andloses some of its velocity. This deceleration causes theelectron to lose some kinetic energy, which is given offin the form of many new photons. The closer the high­speed electron approaches the nuclei, the greater is theelectrostatic attraction on the electron, the brakingeffect, and the energy of the resulting bremsstrahlungphotons.

Bremsstrahlung interactions generate x-ray photonswith a continuous spectrum of energy. The energy ofan x-ray beam may be described by identifying the peakoperating voltage (in kVp). A dental x-ray machineoperating at a peak voltage of 70,000 volts (70 kVp) , forexample, applies a fluctuating voltage of as much as70 kVp across the tube. This tube therefore producesx-ray photons with energies ranging to a maximum of70,OOOeV (70keV) . Fig. 1-14 demonstrates the contin­uous spectrum of photon energies produced by anx-ray machine operating at 100 kVp. The reasons forthis continuous spectrum are as follows:

CHAPTER 1 RADIATION PHYSICS 13

(/Jc

~s:c. / Bremsstrahlung radiation'0Qi.c§ CharacteristicC -----'/ radiation.~10Ciia:

hence are characteristic of the target atoms. Charac­teristic radiation is only a minor source of radiationfrom an x-ray tube.

Factors Controllingthe X-Ray BeamThe x-ray beam emitted from an x-ray tube may be mod­ified by altering the beam exposure length (timer),exposure rate (rnA), beam energy (kVp and filtration) ,beam shape (collimation), and target-patient distance.

10 20 30 40 50 60 70 80 90 100

Photon energy (keV)

FIG. 1-14 Spectrum of photons emitted from an x-raybeam generated at 100 kVp. The vast preponderanceof radiation is bremsstrahlung, with a minor addition ofcharacteristic radiation .

CHARACTERISTIC RADIATION

Characteristic radiation occurs when an electron fromthe filament displaces an electron from a shell ofa tung­sten target atom, thereby ionizing the atom. When thishappens, a higher energy electron in an outer shell ofthe tungsten atom is quickly attracted to the void in thedeficient inner shell (Fig. 1-15). When the outer-shellelectron replaces the displaced electron, a photon isemitted with an energy equivalent to the difference inthe two orbital binding energies. Characteristic radia­tion from the K shell occurs only above 70 kVp with atungsten target and occurs as discrete increments com­pared with br$:ll1sstrahlung radiation (see Fig. 1-14).The energies of characteristic photons are a function ofthe energy levels of various electron orbital levels and

EXPOSURE TIME

Figure 1-16 portrays the changes in the x-ray spectrumthat result when the exposure time is increased whilethe tube current (rnA) and voltage (kVp) remain con­stant. When the exposure time is doubled, the numberof photons generated at all energies in the x-ray emis­sion spectrum is doubled, but the range of photonenergies is unchanged. Therefore changing the timesimply controls the quantity of the exposure, thenumber of photons generated.

TUBE CURRENT (rnA)

Figure 1-17 illustrates the changes in the spectrum ofphotons that result from increasing tube current (rnA)while maintaining constant tube voltage (kVp) andexposure time. As the rnA setting is increased, morepower is applied to the filament, which heats up andreleases more electrons that collide with the target toproduce radiation. Therefore the quantity of radiationproduced by an x-ray tube (i.e., the number of photonsthat reach the patient and film) is directly proportional

Higher-energy-Ievelelectron

Characteristicradiation(photon)

Vr ,. '.1

.' Vacancy '<,,,/" ,~~

~

Recoilelectron I

@ABC 0 'FIG. 1-15 Characteristic radiation. A, An incident electron in an inner orbit ejects a pho­toelectron, creating a vacancy. B, This vacancy is filled by an electron from an outer orbit.C, A photon is emitted with energy equal to the difference in energy levels between thetwo orbits. 0, Electronsfrom various orbits may be involved, giving rise to other photons.The energies of the photons thus created are characteristic of the target atom.

High-energyelectron

14 PART I THE PHYSICS OF IONIZING RADIATION

o 10 ~ ~ ~ W ~ roPhoton energy (keV)

FIG. 1-16 Spectrum of photon energies showing that asexposure time increases (kVp and tube voltage held con­stant), so does the total number of photons. The meanenergy and maximal energy of the beams are unchanged.

o 10 20 30 40 50 60 70 80 90 100

Photon energy (keV)

FIG. 1-18 Spectrum of photon energies showing that asthe kVp is increased (tube current and exposure time heldconstant), there is a corresponding increase in the meanenergy of the beam, the total number of photons emitted,and the maximal energy of the photons.

/100 kVP

/ /90kVP

80kVP

CIlC~ 100 ·s:c,

'0 75

~~ 50c

~~ 25QiII:

1-second exposure

/ 2-second exposure

. ,

50

100

:g 100

~s:c-oIii.c

~ 50cQ)

>'iiiQiII:

o 10 ~ ~ ~ ~ ~ roPhoton energy (keV)

FIG. 1-17 Spectrum of photon energies showing that astube current (mA) increases (kVp and exposure time heldconstant), so does the total number of photons. The meanenergy and maximal energy of the beams are unchanged.Compare with Fig. 1-16.

to the tube current (rnA) and the time the tube is oper­ated. The quantity of radiation produced is expressedas the product of time and tube current. The quantityof radiation remains constant regardless ofvariations inrnA and time as long as their product remains constant.For instance.. a machine operating at lOrnA for 1second (lOmAs) produces the same quantity of radia­tion when operated at 20 rnA for 0.5 second (lOmAs),although in practice some dental x-ray machines fallslightly short of this ideal constancy.

TUBE VOLTAGE (kVp)

Figure 1-18 shows the influence of changing tubevoltage (kVp) on the spectrum of photon energies in

an x-ray beam. Increasing the kVp increases the poten­tial difference between the cathode and anode, thusincreasing the energy of each electron when it strikesthe target. This results in an increased efficiency of con­version of electron energy into x-ray photons, and thusan increase in (1) the number of photons generated,(2) their mean energy, and (3) their maximal energy.The increased number of photons produced per unittime by use of higher kVp results from the greater effi­ciency in the production of bremsstrahlung photonsthat occurs when increased numbers of higher-energyelectrons interact with the target.

The ability of x-ray photons to penetrate matterdepends on their energy. High-energy x-ray photonshave a greater probability of penetrating matter,whereas relatively low-energy photons have a greaterprobability ofbeing absorbed. Therefore the higher thekVp and mean energy of the x-ray beam, the greater thepenetrability of the beam through matter. A useful wayto characterize the penetrating quality of an x-ray beam(its energy) is by its half-value layer (HVL). The HVL isthe thickness of an absorber, such as aluminum,required to reduce by one half the number of x-rayphotons passing through it. As the average energy of anx-ray beam increases, so does its HVL. The term beamquality refers to the mean energy of an x-ray beam.

FILTRATION

Although an x-ray beam consists of a spectrum of x-rayphotons of different energies, only photons with suffi­cient energy to penetrate through anatomic structuresand reach the image receptor (usually film) are usefulfor diagnostic radiology. Those that are of low energy(long wavelength) contribute to patient exposure (and

o 10 20 30 40 50 60 70

Photon energy (keV)

FIG . 1-19 Filtering an x-ray beam with aluminum prefer­entially removes low-energy photons, thereby reducing thebeam intensity and increasing its mean energy.

risk) but do not have enough energy to reach the film.Consequently, to reduce patient dose, the less-pene­trating photons should be removed. This can be accom­plished, in part, by placing an aluminum filter in thepath of the beam. Fig. 1-19 illustrates how the additionof an aluminum filter alters the energy distribution ofthe unfiltered beam. The aluminum preferentiallyremoves many of the lower-energy photons with lessereffect on the higher-energy photons that are able topenetrate to the film. .

In determinations of the amount of filtrationrequired for a particular x-ray machine, kVp and inher­ent filtration of the tube and its housing must be con­sidered. Inherenrfiltration consists of the materials thatx-ray photons encounter as they travel from the focalspot on the target to form the usable beam outside thetube enclosure. These materials include the glass wall

CHAPTER 1 RADIATION PHYSICS

CIlc:-§ 100.J::.C-

o~E~ 50

~~Qia:

/ Nonfiltered beam

/ Filtered beam(AI filter)

15

of the x-ray tube, the insulating oil that surrounds manydental tubes, and the barrier material that prevents theoil from escaping through the x-ray port. The inherentfiltration of most x-ray machines ranges from the equiv­alent of 0.5 to 2 mm of aluminum. Total filtration is thesum of the inherent filtration plus any added externalfiltration supplied in the form of aluminum disks placedover the port in the head of the x-ray machine. Gov­ernmental regulations require the total filtration in thepath of a dental x-ray beam to be equal to the equiva­lent of 1.5mm of aluminum to 70kVp, and 2.5mm ofaluminum for all higher voltages (see Chapter 3).

COLLIMATION

A collimator is a metallic barrier with an aperture in themiddle used to reduce the size of the x-ray beam (Fig.1-20) and therefore the volume of irradiated tissuewithin the patient. Round and rectangular collimatorsare most frequently used in dentistry. Dental x-raybeams are usually collimated to a circle 23

/ 4inches

(7 em) in diameter. A round collimator (see Fig. 1-20,A) is a thick plate of radiopaque material (usually lead)with a circular opening centered over the port in the x­ray head through which the x-ray beam emerges. Typi­cally, round collimators are built into open-endedaiming cylinders. Rectangular collimators (see Fig. 1-20,B) further limit the size of the beam to just larger thanthe x-ray film. It is important to reduce the beam to thesize of the film to reduce further unnecessary patientexposure. Some types of film-holding instruments alsoprovide rectangular collimation of the x-ray beam (seeChapters 3 arid 8) .

Use of collimation also improves image quality.When an x-ray beam is directed at a patient, the tissues

AB

FIG . 1-20 Collimationof an x-ray beam (dotted area) is achieved by restricting its usetulsize. A, Circularcollimator. 8, Rectangular collimator restricts area of exposure to just largerthan the detector size.

16 PART I THE PHYSICS OF IONIZING RADIATION

, .... .

,"~~~'-~~~--~;;~~::~:~_.-:=------------

FIG. 1-21 The intensity of an x-ray beam is inversely pro­portional to the square of the distance between the sourceand the point of measure.

absorb about 90 % of th e x-ray photons and 10% of thephotons .pass through the patient and reach the film.Many of the absorbed photons generate scattered radi­ation within the exposed tissues by a process calledCompton scattering (see below). These scattered photonstravel in all directions, and some reach the film anddegrade image quality. Collimating the beam to reducethe exposure area and thus the number of scatteredphotons reaching the film can minimize the detrimen­tal effect of scattered radiation on the images.

INVERSE SQUARE LAW

The intensity of an x-ray beam at a given point (numberof photons per cross-sectional area per unit exposuretime) depends on the distance of the measuring devicefrom the local spot. For a given beam the intensity isinversely proportional to the square of the distancefrom the source (Fig . 1-21) . The reason for this decreasein intensity is that the x-ray beam spreads out as it movesfrom the source. The relationship is as follows:

.!!. _ (D2)2

12 - (D1

) 2

where lis intensity and D is distance. Therefore if a doseof 1 gray (Gy) is measured at a distance of 2m, a doseof 4Gy will be found at 1 m , and 0.25Gy at 4m.

Therefore changing the distance between the x-raytube and patient has a marked effect on beam intensity.Such a change requires a corresponding modificationof the kVp or mAs if the exposure of the film is to bekept constant.

Interactions of X RaysWith MatterThe mtensity ot an x-ray beam IS reduced by interactionwith the matter it enco un ters. This attenuation results

FIG. 1-22 Photons in an x-ray beam interact with theobject primarily by Compton scattering (A), in which casethe scattered photon may strike the film and degrade theradiographic image by causing film fog, or photoelectricabsorption (8). Relatively few photons undergo coherentscattering within the object (e) or pass through the objectwithout interacting and expose the film (D).

from interactions of individual photons in the beamwith atoms in the absorber. The x-ray photons are eitherabsorbed or scattered out of the beam. In absorption,photons ionize absorber atoms and convert theirenergy into kinetic energy of the absorber electrons. Inscattering, photons are ejected out of the primary beamas a result of interactions with the orbital electrons ofabsorber atoms. In a dental x-ray beam there are threemeans of beam attenuation: (1) coherent scattering,(2) photoelectric absorption, and (3) Compton scatter­ing (Fig. 1-22) . In addition, about 9% of the primaryphotons pass through the patient without interaction(Table 1-1) .

COHERENT SCATTERING

Coherent scattering (also known as classical, elastic, orThompson scattering) may occur when a low-energy inci­dent photon (less tharr -Hlke'V) passes near an outerelectron of an atom (wh ich has a low binding energy).The incident photon interacts with the electron bycausing it to become momentarily excited at the samefrequency as the in coming photon (Fig. 1-23). The inci­dent photon ceases to exist. The excited electron thenreturns to the ground state and generates another x-rayphoton with the same frequency and energy as in theincident beam. Usually the secondary photon is emittedat an angle to the path of the incident photon. In effect,the direction of the incident x-ray photon is altered.This interaction accounts for only about 8% of thetotal number of interactions (per exposure) in a dentalexamination (see Table 1-1). Coherent scattering con­tributes very little to film fog because the total quantity

,CHAPTER 1 RADIATION PHYSICS

Scattered photon

FIG. 1-23 Coherent scattering resulting from the inter­action of a low-energy incident photon with an outerelectron, causing the outer electron to vibrate momen­tarily. After this, a scattered photon of the same energy isemitted at a different angle from the path of the incidentphoton.

TABLE 1-1 .

Fate of 2,000,000 Incident Photonsin Bitewing Projections

. PRIMARY SCATIERED

INTERAVFION PHOTONS PHOTONS* TOTALt

Coherentscattering 148,905 156,234 305,139

Photoelectricabsqrplion 536,208 522;082 1. 05 B,2~0

Compton scattering 1,131,878 1,098,720 2,230,598

Exit 183,009 758,701 941,710

TOTAL 2,000,000 2,535,737 4,535,737

From Gibbs SJ: Personal communication, 1986.*Scattered photons result from primary, Compton, and coherent interactions.'Note that the sum of the total number of photoelectric interactions andphotons that exit the patient equals the total number of incident photons.

of scattered photons is small and its energy level is toolow for much of it to reach the film.

PHOTOELECTRIC ABSORPTION

Photoelectric absorption is critical in diagnosticimaging. This process occurs when an incident photoncollides with a bound electron in an atom of the absorb-

17

ing medium. At this point the incident photon ceasesto exist. The electron is ejected from its shell andbecomes a recoil electron (photoelectron) (Fig. 1-24).The kinetic energy imparted to the recoil electron isequal to the energy of the incident photon minus thatused to overcome the binding energy of the electron.The absorbing atom is now ionized because it has lostan electron. In the case of atoms with low atomicnumbers (e.g., those in most biologic molecules), thebinding energy is small. As a result the recoil electronacquires most of the energy of the incident photon.Most photoelectric interactions occur in the K shellbecause the density of the electron cloud is greater inthis region and a higher probability of interactionexists. About 30% of photons absorbed from a dentalx-ray beam are absorbed by-the photoelectric process.

An atom that has participated in a photoelectricinteraction is ionized as a result of the loss of an elec­tron. This electron deficiency (usually in the K shell) isinstantly filled, usually by an lrshell electron, with therelease of characteristic radiation (see Fig. 1-15). What­ever the orbit of the replacement electron, the charac­teristic photons generated are of such low energy thatthey are absorbed within the patient and do not fog thefilm.

Recoil electrons ejected during photoelectricabsorption travel only short distances in the absorberbefore they give up their energy through secondary ion­izations. As a consequence, all the energy of incidentphotons that undergo photoelectric interaction isdeposited in the patient. Although this is beneficial inproducing high-quality radiographs, because no scat­tered radiation fogs the film, it is potentially deleteri­ous for patients because of increased radiationabsorption.

The frequency of photoelectric interaction variesdirectly with the third power of the atomic number of theabsorber. For example, because the effective atomicnumber ofcompact bone (Z = 13.8) is greater than thatof soft tissue (Z = 7.4), the probability that a photon willbe absorbed by 8. photoelectric interaction in bone isapproximately 6.5 times greater than in an equal thick­ness of soft tissue. This difference is readily seen ondental radiographs as a difference in optical density ofthe image. It is this difference in the absorption thatmakes the production of a radiographic image possible.

COMPTON SCATTERING

Compton scattering occurs when a photon interactswith an outer orbital electron (Fig. 1-25). About 62% ofthe photons that are absorbed from a dental x-ray beamare absorbed by this process. In this interaction theincident photon collides with an outer electron, which

18 PART I THE PHYSICS OF IONIZING RADIATION

Char ctenstic Highor-er .rgy vel

radiauci elec ron

(p oton) /\ •Photo Ira \ \

1

V cancy~

L

)

A 8 C 0FIG. 1- 4 Photo Icc ric abs rpt ion. A, Pho to I c trk nbs p tion DC ur wh n a ll incl­de nt pho ton gives up i1 11 its ene rgy to an inn r I ctron je led from the atom (a ph oto­electron ), 8, An let tron vacancy in th nner o rb it resul ts in i nization o the d am . C, Anelectron I om a higher pnl' rgy level filb the vacancv and emits characteri ic r dia tion ,D, All orbits ar subsequently lllled cornpletinc the energy exchange.

Scattered photon~ of lower energy

~

Hecoll electron

FIG. 1-25 Compton absorption occurs when an incident photon interacts with an outerelectron, producing a scattered photon of lower energy than the incident photon and arecoil electron ejected from the target atom.

receives xmenc energy and recoils from the point ofimpact. The path of the incident photon is deflectedby its interaction and is scattered from the site of thecollision. The energy of the scattered photon equalsthe energy of the incident photon minus the sum ofthe kinetic energy gained by the recoil electron and itsbinding energy. As with photoelectric absorption,

L.ompton scattenng results In the loss of an electronand ionization of the absorbing atom. Scatteredphotons continue on their new paths, causing furtherionizations. Similarly, the recoil electrons also give uptheir energy by ionizing other atoms.

The probability of a Compton interaction is directlyproportional to the electron. density of the absorber. The

CHAPTER 1 RADIATION PHYSICS 19

number of electrons in bone (5.55 x 1023/cc) is greaterthan in soft tissue (3.34 x 1023/ cc) ; therefore the prob­ability of Compton scattering is correspondinglygreater in bone than in tissue. In a dental x-ray beam,approximately 62% of the photons undergo Comptonscattering.

Scattered photons travel in all directions. The higherthe energy of the incident photon, however, the greaterthe probability that the angle of scatter of the second­ary photon will be small and its direction will beforward. Approximately 30% of the s.cattered photonsformed during a dental x-ray exposure (primarily fromCompton scattering) exit through the patient's head.This is advantageous to the patient because some of theenergy of the incident x-ray beam escapes the tissue, butit is disadvantageous because it causes nonspecific filmdarkening. Scattered photons darken the film whilecarrying no useful information because their pathsare altered.

BEAM ATTENUATION

As an x-ray beam travels through matter, its intensityis reduced (attenuated). This results from loss of in­dividual photons, primarily through photoelectricabsorption and Compton scattering interactions. Thereduction of beam intensity is predictable because itdepends on physical characteristics of the beam andabsorber. A monochromatic beam of photons, a beamin which all the photons have the same energy, providesa good example. When only the primary (not scattered)photons are considered, a constant fraction of the beamis attenuated as the beam moves through each unitthickness of an absorber. Therefore 1.5 em ofwater mayreduce a beam intensity by 50%, the next 1.5cm byanother 50% (to 25% of the original intensity), and soon. This is an exponential pattern of absorption (Fig.1-26). The HVL described earlier in this chapter is ameasure of beam energy describing the amount of anabsorber that reduces the beam intensity by half; in the

DIFFERENTIAL ABSORPTION

Absorber thickness (em)

FIG. 1- 26 Exponential decay of intensity in a homoge­neous photon beam through the absorber, where the HVL is1.5 em of absorber. The curve for a heterogeneous x-raybeam does not drop qu ite as precipitously because of thepreferential removal of low-energy photons and theincreased mean energy of the resulting beam.

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SECONDARY ELECTRONS

In both photoelectric absorption and Compton scat­tering, electrons are ejected from their orbits in theabsorbing material after interaction with x-ray photons.These secondary electrons give up their energy in theabsorber by either of two processes: (1) collisionalinteraction with other electrons, resulting in ionization01- excita tio n of the affected atom, and (2) radiativeinteraction : . wind . produce bremsstrahlung radiation,resultin in the emission of low-energy x-ray photons.

condai )" electrons eventually dissipate all theircllcny

) ,. mostly as heat by collisional interactions, andcome to rest.

The importance of photoelectric absorption andCompton scattering in diagnostic radiography relates todifferences in the way photons are absorbed by variousanatomic structures. The number of photoelectric andCompton interactions is greater in hard tissues than insoft tissues. As a consequence, more photons in thebeam exit the patient after passing through soft tissuethan through hard tissue. Thus while the incident beam,the beam striking the patient, is spatially homogenous,the remnant beam, the beam that exits the patient, is spa­tially heterogeneous. This remnant beam strikes theimage receptor (film), resulting in greater exposure ofthe film behind soft tissue than behind hard tissues. Itis this differential exposure of the film that allows a radi­ograph to reveal the morphology of enamel, dentin,bone, and soft tissues.

,20

TABLE 1·2

Summary of Radiation Quantities and Units

PART I THE PHYSICS OF ION IZING RADIATION

QUANTITY SI UNIT

Exposure Air kerma (Gy)

Absor bed dose Gray (Gy)

Equivalent dose Sievert (Sv)

Effective dose Sievert (Sv)

Radioactivity Becquerel (Bq)

TRADITIONAL UNIT

Roentgen (R)

Rad

Rem

Curie (Ci)

CONVERSION

1 Gy = 100 rad1 rad = 0.01 Gy (1 cGy)

1 Gy - I 00 rad1 rad e 0 .0 1Gy (1 cGy)

1 Sv= 100rem1 rem = 0.01 Sv (1 cSv)

1 Bq = 2.7 X 10-11Ci1 Ci = 3.7 x 101DBq

Data from The NIST Reference on Constants, Units, and Uncertainty: http ://physics.nist.gov/cuu/Units/units.html.

preceding example, the HVL is 1.5 em. The absorptionof the beam depends primarily on the thi ckness andmass of the absorber and the energy of the beam.

The range of photon energies in an x-ray beam iswide. Low-energy photons are much more likely thanhigh-energy photons to be absorbed. As a consequencethe superficial layers of an absorber tend to remove th elow-energy photons and transmit the higher-energyphotons. Therefore as an x-ray beam passe s throughmatter, the intensity of the beam decreases, but themean energy of the resultant beam increases. In con­trast to the absorption of a monochromatic beam, an x­ray beam is absorbed less and less by each succeedingunit of absorber thickness. For example, the first 1.5 emof water might absorb about 40% of the photons in anx-ray beam with a mean energy of 50 kVp. The meanenergy of the remnant beam might increase 20% as aresult of the loss of lower-energy photons. The next1.5 ern of water removes only about 30% of the photonsas the average energy of the beam increases another10%. If the water test object is thick enough, the meanenergy of the remnant beam approaches the peakvoltage applied across the tube and absorption becomessimilar to that of a monochromatic beam.

The attenuation of a beam depends on both theenergy of the incident beam and the composition ofthe absorber. In general, as the energy of the beamincreases, so does the transmission of the beam throughthe absorber. When the energy of the incident photonis raised to the binding energy of the K-shell electronsof the absorber, however, the probability of photoelec­tric absorption increases sharply and the number oftransmitted photons is greatly decreased. This is calledK-edge absorption. (The probability that a photon willinteract with an orbital electron is greatest when the

energy of the photon equals the binding energy of theelectron; it decreases as the photon energy increases.)Photons with energy less than the binding energy of K­shell electrons interact photoelectrically only with elec­trons in the L shell and in shells even farther from thenucleus. Rare earth elements are sometimes used asfilters because their Kedges (50.2keV for gadolinium)greatly increase the absorption of high-energy photons.This is desirable because these high-energy photons arenot as likely to contribute to a radiographic image asmid-energy photons.

DosimetryDetermining the quantity of radiation exposure or doseis termed dosimetry. The term dose is used to describe theamount of energy absorbed per unit mass at a site ofinterest. Exposure is a measure of radiation based on itsability to produce ionization in air under standard con­ditions of temperature and pressure (STP) .

UNITS OF MEASUREMENT

Table 1-2 presents some of the more frequently usedunits for measuring quantities of radiation. In recentyears a move has occurred to use a modernized versionof the metric system called the Sf system (Systerne Inter­national d'Unites) " . This book uses SI units. The SIsystem uses base units including the kilogram (mass),the meter (length), the second (time) , the amphere

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*The NIST Reference on Constants, Units, and Uncertainty:http:/ /physics.nist. gov/ cuu/Units/units.httnl.

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CHAPTER 1 RADIATION PHYSICS 21

ExposureExposure is a measure of radiation quantity, the capac-

ity of radiation to ionize air. The 81 unit of exposure is

air kerma, an acronym for kinetic energy released in matter:

Kerma measures the kinetic energy transferred from

photons to electrons and is expressed in units of dose

(Gy), where 1 Gy equals 1 joule/kg. Kerma is the sum

of the initial kinetic energies of all the charged parti-

cles liberated by uncharged ionizing radiation (neu-

trons and photons) in a sample of matter, divided by

the mass of the sample. It has replaced the roentgen

(R), the traditional unit of radiation exposure

measured in air.

Effective DoseThe effective dose (E) is used to estimate the risk in

humans. It is the sum of the products of the equivalent

dose to each organ or tissue (HT) and the tissue weight-

ing factor (WT):

E = L WT X HT

The tissue weighting factors include gonads, 0.20;

red bone marrow, 0.12; esophagus, 0.05; thyroid, 0.05;

skin, 0.01; and bone surface, 0.01. The unit of effective

dose is the sievert (Sv). The use of this term is described

more fully in Chapter 3.Absorbed DoseAbsorbed dose is a measure of the energy absorbed by

any type of ionizing radiation per unit mass of any type

of matter. The SI unit is the gray (Gy), where 1 Gy

equals 1 joule/kg. The traditional unit of absorbed

dose is the rad (radiation absorbed dose), where 1 rad is

equivalent to 100 ergs/g of absorber. One gray equals

100 rads.

RadioactivityThe measurement of radioactivity (A) describes the

decay rate of a sample of radioactive material. The SI

unit is the becquerel (Bq); 1 Bq equals 1 disintegra-

tion/second. The traditional unit is the curie (Ci),

which corresponds to the activity of 1 g of radium

(3.7 x 1010 disintegrations/second). Accordingly, 1 mCi

equals 37 megaBq; and 1 Bq equals 2.7 X 10-11 Ci.Equivalent Dose

The equivalent dose (HT) is used to compare the bio-

logic effects of different types of radiation to a tissue or

organ. It is the sum of the products of the absorbed

dose (DT) averaged over a tissue or organ and the radi-

ation weighting factor (WR):

HT = L WR X DT

Equivalent dose is expressed as a sum to allow for the

-__0. o. that the tissue or organ is exposed to more

one type of radiation. The radiation weighting

is chosen for the type and energy of the radia-

involved. Thus high-LET radiations (which are

-radiations) have