Christensen’s Physics of Diagnostic Radiology

fourth edition



Christensen's

Physics of
Diagnostic Radiology

THOMAS S. CURRY Ill, M.D.

Professor of Radiology, University of Texas
Southwestern Medical Center at Dallas
and Parkland Memorial Hospital

JAMES E. DOWDEY, Ph.D.

Associate Professor of Radiology (Physics),
Southwestern Medical Center at Dallas
and Parkland Memorial Hospital

ROBERT C. MURRY, Jr., Ph.D.

Associate Professor of Radiology (Physics)
Southwestern Medical Center at Dallas
and Parkland Memorial Hospital

4th Edition

Williams & Wilkins

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Library of Congress Cataloging-in-Publication Data

Curry, Thomas S., 1935-

Christensen's physics of diagnostic radiology.-4th ed. I

Thomas S. Curry III, James E. Dowdey, Robert C. Murry, Jr.

p. em.

Rev. ed. of: Christensen's introduction to the physics of

diagnostic radiology. 3rd ed. I Thomas S. Curry III, James E.

Dowdey, Robert C. Murry, Jr. 1984.

Includes bibliographical references.

ISBN 0-8121-1310-1

l. Diagnosis, Radioscopic . 2. Medical physics. I. Dowdey, James E .

II. Murry, Robert C . III. Christensen, Edward E., 1929-

Introduction to the physics of diagnostic radiology. IV. Title.

V. Title: Physics of diagnostic radiology.

[DNLM: I. Physics. 2. Radiography. 3. Radiology. 4. Technology,

Radiologic. WN 110 C976c]

RC78.C87 1990

616.07'57--<lc20

DNLM/DLC

for Library of Congress 90-5586

CIP

I st Edition, 1972
Reprinted 1973, 1975, 1976, 1977

2nd Edition, 1978
Reprinted 1979, 1981, 1982, 1983

3rd Edition, 1984
Reprinted 1985, 1987

4th Edition, 1990

Copyright © 1990 by Lea & Febiger. Copyright under the International Copyright Union.
All Rights Reserved. This book is protected by copyright. No part of it may be reproduced
in any manner or by any means, without written permission from the publisher.

PRINTED IN THE UNITED STATES OF AMERICA
Print Number: 12 11 10 9 8 7

Dedicated to: ---
MISS WINN
VAUDA
BARBARA



PREFACE

Why a new edition of a physics book? the amazing technology available, and let
Physics does not change. But our under­ the physicist worry about the technical de­
standing and use of physics continues to tails.
develop.
In our quest to include information that
The competent radiologist must know should be known, and reduce the amount
something about the physics of diagnostic that need not be known, we have in this
imaging. The tough question is exactly edition eliminated the material on copying
what physics should the radiologist know radiographs and subtraction techniques.
to function competently in the environ­ We have reduced the amount of informa­
ment in which he finds himself. As we sit tion on cinefluorography and non-image­
in our little room in Dallas and ponder this intensified fluoroscopy, and consolidated
question we realize that we will fail to in­ fluoroscopic imaging and recording into a
clude information that should be known, single chapter. Several areas, such as atten­
and undoubtedly we will include some uation and grids and beam restrictors, re­
things that need not be known. It is obvious main essentially unchanged. In other areas
that one individual is unlikely to become we have tried to update the material to in­
and remain an expert in physics and tech­ dicate new technology. These areas include
nology, plus diagnostic imaging. There is x-ray generators (solid-state devices), xe­
just too much to learn in each field, and rography (liquid toner), CT scanners (fast­
learned information becomes obsolete too imaging technology) and ultrasound (color
fast. We applaud the developing trend that Doppler). Obviously, the needed addition
pairs a radiologist and a radiologic physicist was in MRI, and a new chapter was added
as a team that provides the maximum pa­ to expand this topic.
tient benefit from diagnostic imaging stud­
ies. Thus, our goal is not to turn radiolo­ We certainly appreciate the positive re­
gists into physicists, but rather to allow sponse to this text, and hope the current
radiologists to understand and appreciate edition will continue to fill the needs of
radiology residents and student technolo­
gists.

Dallas, Texas Thomas S. Curry, III
James E. Dowdey
Robert C. Murry

vii



ACKNOWLEDGMENTS

Another edition is about wrapped up, selves from the demands and invasions of
causing us once again to reflect on those other academic and clinical responsibilities
who contributed so much time and talent. for several months. The understanding
and support of our friends and colleagues
Our friends at E.I. duPont de Nemours was absolutely spectacular, and this at­
and Company, Eastman Kodak Company, tempt to say thinks will fall far short of the
and Philips Medical Systems continued to genuine affection we hold for each of these
offer support. splendid individuals. We can name only a
few, and chose to single out those who were
Marty Burgin once again prepared all most directly inconvenienced because of
the new illustrations, and introduced us to our absence. In the physics department,
the wonders of computer graphics. Marty graduate students Tom Lane, Tim Black­
did much more than draw the pictures. She burn, and Jerome Gonzales provided re­
spent time helping us improve and refine search effort and critical review of manu­
many of the illustrations, especially those scripts. Our friend, John Moore, continued
dealing with electronics and MRI. We rec­ to come up with ideas and suggestions. Our
ognize and appreciate her considerable tal­ senior residents in diagnostic radiology
ent and kindness. must be the most understanding residents
in the cosmos. The seven who functioned
An amazing stroke of good fortune in an exemplary manner during the seven
guided a remarkable individual to our pho­ months they were being given little atten­
tography department. Only a few weeks tion from their assigned staff radiologist
after she joined our department, Claudia were Drs. Lori Watumull,Jim Fleckenstein,
Wylie had assumed responsibility for al­ Tom Fletcher, Christine Page, Diane
most all our photographic support. Clau­ Twickler, Mark Girson, and Brian Brue­
dia took great pictures, but her effort did ning. All our residents are wonderful peo­
not stop there. She organized the entire ple. Our colleagues on the teaching staff
book, chapter by chapter, and served as continued with their enthusiastic support
liaison between the authors and artist as and encouragement. Drs. Helen Redman
additions, alterations, and corrections re­ and George Miller absorbed most of the
quired attention. The enormous amount of increased clinical work caused by the fre­
time and dedication Claudia provided was quent unavailability of T.S.C. Drs. Geor­
one of the delightful surprises one hopes giana Gibson and Bill Erdman helped with
for but seldom finds. Thank you, Claudia. clinical MRI information and illustrations,
as did Drs. Hortono Setiawan and Rebecca
This textbook is made possible by the Harrell in CT and Ultrasound. We must
entire Department of Radiology, Univer­ mention those faculty members who really
sity of Texas Southwestern Medical Center helped with encouragement when things
at Dallas. Everything in this edition has didn't go smoothly. These include Drs.
been prepared by a physicist and a radi­
ologist sitting at a long table to compose
and revise the manuscript together. It was
necessary for the authors to distance them-

ix

X ACKNOWLEDGMENTS

Robert Parkey, George Curry, Peter An­ the manuscript. She accepted this chal­
tich, Geral Dietz, Robert Epstein, Mary lenge, and has shown great skill and ded­
Gaulden, William Kilman, Michael Lan­ ication as work has progressed. Equally im­
day, and Jack Reynolds, who has suffered portant, she has been friendly and patient
through four editions with us. A special as we return page after page marked up
thanks goes to Geoffrey Clarke, Ph.D., who with changes after we had promised that
spent many hours guiding us through the the final version was done. Eula has been
complexities of MRI. a genuine pleasure to work with.

Our search for help with word process­ As always, the final thanks must go to
ing provided a second delightful surprise. our wives and families for continuing to
Eula Stephens had been with us only a few support us and take care of us.
days when we approached her about typing

TSC
JED
RCM

CONTENTS

Radiation... . . .. .. . .. . . .. .. . .. . .. . . . . . . .. . .. . . .. . . .. .. . .. . . . . .. . .. . . . . .. . . . . . . . .. .. .. .. 1

2 Production of X Rays................................................................ 10

3 X-Ray Generators.................................................................... 36

4 Basic Interactions between X Rays and Matter.. . . .. . . . . .. . . .. . .. . . . .. . . . . . .. .. . . . 61

5 Attenuation .. . . .. . . .. . .. . . . . . . . . . .. .. . . . . . . . . . . . .. . . . .. .. . . .. . . . . . .. . . . . . . . . . .. . . . .. .. 70

6 Filters.................................................................................. 87

7 X-Ray Beam Restrictors .. . . . . . . . . . . . . . . . .. . . . . .. . .. . . .. . . . . . . . . . . . . . . . . . .. . .. . . . . . . . 93

8 Grids .................................................................................. 99

9 Luminescent Screens................................................................. 118

10 Physical Characteristics of X-Ray Film and Film Processing ..................... 137

11 Photographic Characteristics of X-Ray Film....................................... 148

12 Fluoroscopic Imaging................................................................ 165

13 Viewing and Recording The Fluoroscopic Image................................. 175

14 The Radiographic Image.. . . . . . . . . . . . .. . . .. . . . . . .. .. . . . .. . . . . . . . . . .. . . .. .. . .. . . . . . . . 196

15 Geometry of the Radiographic Image.............................................. 219

16 Body Section Radiography.......................................................... 242

17 Stereoscopy . .. .. .. . . .. . .. .. . .. .. . .. . .. . . . .. . . . . . . .. . .. . . .. . . . . . . . .. . .. . .. .. . . . . . . . . . . . 257

18 Xeroradiography..................................................................... 266

19 Computed Tomography............................................................. 289

20 Ultrasound............................................................................ 323

21 Protection.. . . . . .. .. .. . .. . .. .. . .. . .. .. . .. . . . . . .. . .. . .. . . .. . .. . . . . . . . . . . .. . .. .. . . . .. . . . . 372

22 Digital Radiography.................................................................. 392

23 Nuclear Magnetic Resonance ....................................................... 432

24 Magnetic Resonance Imaging....................................................... 470

Index.................................................................................. 505

xi

CHAPTER Radiation

1

Wilhelm Conrad Roentgen, a German ers. Then he held his hand between the
physicist, discovered x rays on November tube and the screen and, to his surprise,
8, 1895. Several fortunate coincidences set the outline of his skeleton appeared on the
the stage for the discovery. Roentgen was screen. By December 28, 1895, he had
investigating the behavior of cathode rays thoroughly investigated the properties of
(electrons) in high energy cathode ray the rays and had prepared a manuscript
tubes, which consisted of a glass envelope describing his experiments. In recognition
from which as much air as possible had of his outstanding contribution to science,
been evacuated. A short platinum elec­ Wilhelm Conrad Roentgen was awarded
trode was fitted into each end and when a the first Nobel Prize for Physics in 1901.
high-voltage discharge was passed through
this tube, ionization of the remaining gas ELECTROMAGNETIC SPECTRUM
produced a faint light. Roentgen had en­
closed his cathode ray tube in black card­ X rays belong to a group of radiations
board to prevent this light from escaping called electromagnetic radiation. Electro­
to block any effect the light might have on magnetic radiation is the transport of en­
experiments he was conducting. He then ergy through space as a combination of
darkened his laboratory room to be sure electric and magnetic fields (hence the
there were no light leaks in the cardboard name electromagnetic). Familiar members
cover. On passing a high-voltage discharge of the family of electromagnetic radiation
through the tube, he noticed a faint light include radio waves, radiant heat, visible
glowing on a work bench about 3 ft away. light, and gamma radiation.
He discovered that the source of the light
was the fluorescence of a small piece of Electromagnetic (EM) radiation is pro­
paper coated with barium platinocyanide. duced by a charge (usually a charged par­
Because electrons could not escape the ticle) being accelerated. The converse is
glass envelope of the tube to produce flu­ also true; that is, a charge being accelerated
orescence, and because the cardboard per­ will emit EM radiation. Right here at the
mitted no light to escape from the tube, he beginning we run into our first problem.
concluded that some unknown type of ray Physics, our beloved exact science, presents
was produced when the tube was ener­ a contradiction . The problem is just this:
gized. We can imagine his excitement as he in our discussion of atomic structure (see
investigated the mysterious new ray. He be­ Chap. 2) we will discuss electrons (charged
gan placing objects between the tube and particles) revolving around the nucleus in
the fluorescent screen: a book, a block of circular orbits while maintaining a precise
wood, and a sheet of aluminum. The energy. (This is a result of the Bohr theory
brightness of the fluorescence differed of the hydrogen atom.) This picture of
with each, indicating that the ray penc�­ atomic structure violates the converse state­
trated some objects more easily than oth- ment above. First, if an electron moves in
a circular orbit, it must have centripetal ac­
celeration (an acceleration toward the cen-

2 RADIATION

ter of the circular path); therefore, it accelerating charge. Any accelerating
should emit EM radiation. The loss of en­ charge not bound to an atom (including the
ergy would require the electron to change nucleus) will emit EM radiation.
its orbit and energy. As a matter of fact,
there was some heated discussion when Some time should be spent here discuss­
Bohr first introduced his theory that elec­ ing the production and structure of EM
trons could not possibly be in circular orbits radiation. To do that, we will start with a
because, being accelerated, they would single small charged ball. (Because we rec­
emit energy and spiral into the nucleus. ognize the electron as a charged particle,
Shifty-eyed physicists easily get around this we can put a charge on a ball by adding or
argument by saying that electrons, after all, taking away electrons. If we add electrons,
are standing waves about the nucleus and we will charge the ball negatively. Subtrac­
therefore do not represent accelerating tion of electrons results in a positively
charges. It is fair to say, however, that out­ charged ball.) We can only determine if we
side the atom a charge being accelerated are successful in placing a charge on the
will emit EM radiation. The energy that ball by observing its interactions with the
charged particles obtain in circular particle world around it. Coulomb studied the
accelerators is limited by the energy loss to forces that exist between two charged balls,
EM radiation as the particles move about and today we call the forces between
the accelerator. A cyclotron is a good ex­ charged objects "Coulomb forces." The
ample of an accelerator type that is EM­ force between two small balls having
radiation-limited . charges q1 and q2 is

But we haven't finished. In the same F = kq,q2
atomic structure discussion, we allow elec­ r2
tronic transitions from one energy state to
F = force (a vector)
another with emission of EM radiation en­ k = a constant whose value depends
ergy, but with no mention of acceleration.
In fact, we really think of instantaneous on the system of units
transitions across regions in which elec­ ..q, and q2 = charge on balls

trons cannot possibly exist. This concept of r = distance between balls.
energy level transitions seems to contradict
the first statement that EM radiation is pro­ This force is always along the line joining
duced by an accelerated charge. the two balls. (More accurately, it is the line
joining the centers of the two charge dis­
There are a couple of observations that tributions. If we use "point" charges we
must be made. First, the world of quantum don't have to worry about the charge dis­
physics (represented here by Bohr's the­ tribution.) If q1 and q2 are like charges
ory) does not always behave as we, living (same sign; positive and positive or nega­
in a somewhat larger world, might expect. tive and negative), the force is a repelling
Because x rays are produced in the quan­ force. If q1 and q2 are unlike charges, the
tum physics world, we must understand force is attractive. Gravitational force also
some of the laws governing that world. Sec­ has this mathematical form, but it is always
ond, in this book we have endeavored to attractive.
be as physically accurate as possible while
emphasizing those points that we feel cli­ We nearly always introduce the electric
nicians should understand. field (E) to describe the possible interactive
forces. We define the electric field for a
Perhaps we should then rephrase the charge distribution (q1) as the force that q1
statement: EM radiation, except for that would exert on a positive unit charge (q2
produced in energy level transitions (in­
cluding nuclear transitions), is produced by equals 1). Note that E has a unique value

and direction at each point surrounding a

charge. Figure 1-1 shows the E field sur-

RADIATION 3

Figure 1-2 Electric and magnetic fields sur­
rounding a positive charge moving with con­
stant velocity

Figure 1-1 Electric field surrounding a posi­ produces a "kink" in the electric field lines
tive charge at rest that moves outward from the charge with
a finite (but large) velocity. We can think of
rounding a point positive charge. Note that this kink as being the EM radiation. We
E is radially directed and falls off (de­ don't know any better way to describe this
creases in size) as llr2• The electric field for rather complex concept.
more complicated charge distributions
could be more or even less complicated. EM radiation is made up of an electric
(The electric field for an infinite plane uni­ field and a magnetic field that mutually
formly charged is constant everywhere.)
This electric field is sometimes called the support each other. Figure 1-3 shows the
static (electric is implied) field because the
charge is at rest. E and H fields and the direction of prop­
agation of the EM radiation. Note that E
If the charge moves with a constant ve­ and H are perpendicular, that they reverse
locity, we not only see an electric field (E) together, and that both E and H are per­
moving with the charge, but also a mag­ pendicular to the direction of propagation.
netic field (H) surrounding the line (path) The concept to be visualized is that E and
along which the charge is moving. (A more H interact to build each other up to some
detailed discussion of magnetic fields will value., then collapse together and build
each other up in the opposite direction.
be found in Chapter 23.) Figure 1-2 shows Energy is transmitted through space by the
EM radiation.
the electric field radially directed and the
circular magnetic field (H). Here E and H The radiation depicted in Figure 1-3
(both vectors) are perpendicular at any one
point. was produced by a charge oscillating back
and forth. Suppose we consider a radio
The next thing to do is to let the charge
accelerate. Here we have conceptual prob­ c
lems. At constant velocity, the electric field
moves along with the charge. But, with ac­ z
celeration, the charge moves to a new lo­
cation before the outer regions of the elec­ Figure 1-3 Representation of electromag­
tric field realize that the charge is at a netic radiation
different place than it should be. The elec­
tric field lags behind the charge, as does
the magnetic field. The lagging behind

4 RADIATION

transmission tower. The purpose of the which makes the electrons oscillate back
tower is to transmit radio signals that, as and forth in the antenna at the frequency
you might have guessed, are EM radiation. of the radio wave . Consequently, the an­
The transmission is accomplished by ac­ tenna detects the EM radiation by the elec­
celerating charge up and down a conductor trons moving under the influence of the E
in the tower. As the charge is accelerated field in the EM radiation. In this type of
up the tower, we might get one section of detection, the EM radiation looks and be­
the EM radiation shown in Figure 1-3. As haves as a wave. As the frequency of the
the charge moves down the tower, the fol­ EM radiation increases, however, a point is
lowing section of the EM radiation is pro­ reached in the frequency range (not a sin­
duced. When the charge changes direction, gle precise frequency) at which the electron
so do the E and H fields. Thus, a radio can no longer follow the electric field. At
wave can be produced by forcing charge to frequencies in this range and higher, the
move up and down the tower by applying electrons interact with the EM radiation as
an alternating voltage to the tower. The if the EM radiation were an energy bundle,
frequency of the radio wave is just how rather than made up of waves. Later, we
many times per second the charge changes will see that coherent scattering of x rays
direction. Of course, we tune our radios at is by an interaction of the wave type, while
home to a given frequency to find a par­ photoelectric absorption is an interaction
ticular station. The information we obtain of the energy bundle (called a photon)
from the radio, perhaps music, is super­ type . To stay away from exotic physics, it
imposed on the EM radiation generated by is necessary to discuss EM radiation as if it
the tower. How that is done is another story. were comprised of both particles (photons).
and waves . Let us hasten to add that EM
We would be in excellent shape if EM radiation will behave as only one of the two,
radiation didn't interact with the particles and that this behavior is always the same
of our old world. Obviously, without such for a given interaction (or experimental
interaction we wouldn't be here to wonder measurement).
about EM radiation anyway. Life-giving en­
ergy from the sun gets here by EM radiac To finish the radio detection, we note
tion. one problem: there are a number of radio
stations broadcasting at the same time. We
Maybe it would be instructive to see how must pick one frequency (the station we
radio waves interact with your radio. Just want to listen to) and discard the rest. This
a little while ago we talked about EM ra­ selection is done by a "tank" circuit on the
diation in the form of radio waves being end of the antenna. We tune the tank cir­
emitted from a radio tower. (We didn't say cuit (by the station selection knob) to keep
there that the energy emitted was emitted only one frequency. What we hear on the
outward from the tower in something of a radio is the second part of the other story
doughnut-shaped pattern. The maximum introduced in the paragraph on radio
intensity is emitted perpendicularly to the transmission.
tower, while there is no intensity directly
above the tower. That pattern is best for In the first chapters in this text, we will
us, because most of us don't listen to a radio be interested in how the E part ofthe EM
while directly above the tower.) What we radiation reacts with electrons. Later,
need to detect the radio wave is some an­ when discussing nuclear magnetic reso­
tenna (an electrical conductor) in the radio nance, we will consider reactions with the
radiation pattern. When the radio wave H (magnetic) part ofthe field that are pro­
(the E and H fields) passes the antenna, the duced by oscillating electrons.
E-field part of the EM radio wave exerts a
force on the electrons in the antenna, The interactions of different kinds of
EM radiation are difficult to understand.

RADIATION 5

Some are explained only if they are as­ Because all types of electromagnetic ra­
sumed to be particles, while others are ex­ diation have the same velocity, the fre­
plained only by theories of wave propa­ quency of the radiation must be inversely
proportional to its wavelength. All types of
gation. It is necessary to discuss radiation in the electromagnetic spectrum
electromagnetic radiation as if it were differ basically only in wavelength. The
composed of both particles and waves. wavelength of a radio wave may be 5 miles
long, while a typical x ray is only 1 billionth
Wave Concept of Electromagnetic of an inch. The wavelength of diagnostic
Radiation x rays is extremely short, and it is usually

Electromagnetic radiation is propagated expressed in angstrom units (A) or nano­
through space in the form of waves. They
may be compared to waves traveling down meters (nm). An angstrom is 10-10 m, while
a stretched rope when one end is moved a nanometer is 10-9m. Therefore, one nm
up and down in a rhythmic motion. While
the waves with which we are familiar must is equal to 10 A. Or, if you prefer, one A
be propagated in a medium (such as the
example of the rope, waves traveling in wa­ is equal to 0.1 nm. You may wish to refresh
ter, or sound waves traveling in air), elec­ your memory of various prefixes by refer­
tromagnetic waves need no such medium; ring to Table 1-1. The wavelength of most
that is, they can be propagated through a
vacuum. Waves of all types have an asso­ diagnostic x rays is between 1 and 0. 1 A.
ciated wavelength and frequency. The dis­
tance between two successive crests, or The wavelength of an electromagnetic
troughs, is the wavelength of the wave, and wave determines how it interacts with mat­
is given the symbol A (the Greek letter ter. For example, an electromagnetic wave

lambda, the initial for length). The number 7000 A (700 nm) long can be seen by the

of waves passing a particular point in a unit human exe as red light, and a wavelength
of time is called the frequency, and is given
the symbol v (the Greek letter nu, the initial of 4000 A is seen as blue light. The fre­
for number). If each wave has a length A,
and v waves pass a given point in unit time, quency of blue light may be calculated by
the velocity of the wave is given by
knowing its wavelength (4000 A = 4 X 10-7
V =A X v
m):
For example, if the wavelength is 4 ft and
the frequency is 60 waves/min, then c
c = Av or v = -
V = 4 ft x 60/min
V = 240 ft/min A
3 x 10• m/sec
EM radiation always travels at the same ve­ v=
locity in a vacuum. This velocity is 186,000
4 x 10-7m
miles per second (3 X 108 meters per sec­ v = 7.5 x 1014/sec

ond), which is usually referred to as the Blue light, with a wavelength of 4000 A,
velocity of light and given the symbol c.
Therefore, we may express the relation­ has a frequency of 7.5 x 1014 vibrations
ship between velocity, wavelength, and fre­ per second. Similarly calculated, the fre­
quency as
quency of an x ray of wavelength 0.1 A is
c = AV 3 X 1019vibrations per second.

c = velocity of light (m/sec) The complete spectrum of electromag-
A = wavelength (m)
v = frequency (per sec). Table 1-1. Prefixes

FACTOR PREFIX SYMBOL

109 giga G
1 os mega M
103 K
10-1 kilo d
10-3 deci m
10-6 milli
10-• micro j.L
10-12 nano n
pico p

6 RADIATION

netic radiation covers a wide range of wave­ called Planck's constant. (Planck's constant
lengths and frequencies. The various parts in SI units is 6.62 X 10-34 joules seconds
of the spectrum are named according to
the manner in which the type of radiation U·s]). Planck's constant is normally given
is generated or detected. Some members
of the group, listed in order of decreasing in the SI units. We made the conversion to
wavelength, are keV·sec to make the calculation of the en­
ergy of x-ray photons have the units of kilo­
Radio, television, 3 x 10s to 1 em electron volts.) The mathematical expres­
radar: 0.01 to 0.00008 em sion is written as follows:

Infrared radiation: (8ooo A) E = hv

Visible light: 7500 (0.000075 em) to E = photon energy

Ultraviolet radiation: 3900 A h = Planck's constant
Soft x rays: 3900 to 20 A v = frequency.
Diagnostic x rays: 100 to 1 A
Therapeutic x ray and 1 to 0.1 A The ability to visualize the dual charac­
teristics of electromagnetic radiation pre­
gamma rays: 0.1 to 1Q-4 A sents a true challenge. But we must un­
avoidably reach the conclusion that EM
There is considerable overlap in the wave­ radiation sometimes behaves as a wave and
lengths of the various members of the elec­ other times as a particle. The particle con­
tromagnetic spectrum; the numbers listed cept is used to describe the interactions be­
are rough guides. It is again stressed that tween radiation and matter. Because we
the great differences in properties of these will be concerned principally with inter­
different types of radiation are attributable actions, such as the photoelectric effect and
to their differences in wav�length (or fre­ Compton scatter, we will use the photon
quency). (or quantum) concept in this text.

The wave concept of electromagnetic ra­ The unit used to measure the energy of
diation explains why it may be reflected, photons is the electron volt (eV). An elec­
refracted, diffracted, and polarized. There tron volt is the amount of energy that an
are some phenomena, however, that can­ electron gains as it is accelerated by a po­
not be explained by the wave concept. tential difference of 1 V. Because the elec­
tron volt is a small unit, x-ray energies are
Particle Concept of Electromagnetic usually measured in terms of the kilo­
Radiation electron volt (keV), which is 1000 electron
volts. We will usually discuss x rays in terms
Short electromagnetic waves, such as of their energy rather than their wave­
x rays, may react with matter as if they were lengths, but the two are related as follows:
particles rather than waves. These particles
are actually discrete bundles of energy, and c
each of these bundles of energy is called a c = ll.v or v = -

quantum, or photon. Photons travel at the ll.

speed of light. The amount of energy car­ and
ried by each quantum, or photon, depends
E = hv
on the frequency (v) of the radiation. If the
�Substituting for v,
frequency (number of vibrations per sec­
ond) is doubled, the energy of the photon he
is doubled. The actual amount of energy E =­
of the photon may be calculated by mul­
tiplying its frequency by a constant. The ll.
constant has been determined experimen­
tally to be 4.13 X 10-18 ke V·sec, and is The product of the velocity of light (c) and
Planck's constant (h) is 12.4 when the unit

RADIATION 7

of energy is keV and the wavelength is in All others are derived from these nine
angstroms. The final equation showing the units, although some are given special
relationship between energy and wave­ names. Table 1-3 gives the seven funda­
length is mental quantities to which the seven base
SI units refer. The MKS units for these
= 12.4 seven quantities are identical to the SI
E units. For comparison, Table 1-3 also lists
those common cgs (centimeter-gram-sec­
A ond) units that have been used elsewhere
in this book. A blank entry does not mean
E = energy (in keV) that there is no cgs equivalent, but that we
A = wavelength (in A). have not used those units elsewhere and do
not wish to add complications. Note that
Table 1-2 shows the relationship between the two supplementary units at the bottom
energy and wavelength for various pho­ of Table 1-3 (radians and steradians)
tons. merely formalize the use of radians for an­
gular measurements. There are 21T radians
If a photon has 15 eV or more of energy in 360° (i.e., in a complete circle), so one
it is capable of ionizing atoms and mole­ radian is about 57.3°; there are 41T stera­
cules, and it is called "ionizing radiation." dians in a sphere.
An atom is ionized when it loses an elec­
tron. Gamma rays, x rays, and some ultra­ Units for any physically known quantity
violet rays are all types of ionizing radia­ can be derived from these nine SI units.
tion. Table 1-4 lists some quantities mentioned
in this book, and all but one also have a
UNITS name in SI units. Again, those common
units used elsewhere are also listed. Of
While writing this text we encountered course, each unit in the table can be ex­
a problem regarding the units used to pressed in SI base units only, and a column
measure various quantities. Whenever pos­ is included to show this. Sometimes ex­
sible we have tried to use SI units. Some­ pressing one derived SI unit in terms of
times this resulted in very large or very other derived SI units reveals some un­
small numbers, and often in such cases we derlying principles of physics. For instance,
used cgs system units. A brief description we note that the SI unit of power is the
of units will help you to follow the various watt, which in base SI units equals 1
units used in this text. m2kg. The meaning of the base SI units

The SI system (Systeme Internationale s3
d'Unites) is a modernized metric system might be immediately apparent to a phys­
based on the MKS (meter-kilogram-sec­ icist, but ]Is Uoules per second) is a little
ond) system. The SI system was originally easier to comprehend. This is just energy
defined and given official status by the per unit time, or power. Electrical power
Eleventh General Conference on Weights would be even harder to understand in
and Measures in 1960. (A complete listing base SI units, but V · A (volts times amps)
of SI units can be found in the National is our comfortable definition.
Bureau of Standards Special Publication
330, 1977 edition.) There are only seven The SI unit of radionuclide activity is the
base units and two supplementary units. becquerel (Becquerel has finally made the
big time, after discovering radioactivity in
Table 1-2. Correlation Between Wavelength 1896, the same year that Roentgen discov­
and Energy ered x rays). The fact that one Bq is the

WAVELENGTH (A) ENERGY (keV)

0.0005 24,800
0.08 155
0.1 124
1.24 10

8 RADIATION

Table 1-3. Sl Base and Supplementary Units

QUANTITY Sl Sl FAMILIAR cgs
UNIT SYMBOL cgs SYMBOL
NAME UNIT
m em
Sl base units: meter kg centimeter g
Length kilogram s gram s
Mass second- A second
Time ampere K
Electric current kelvin mol
Temperature mole cd
Amount of substance candela
Luminous intensity rad
sr
Sl supplementary units: radian
Plane angle steradian
Solid angle

Table 1-4. Sl Derived Units with Special Names

QUANTITY Sl UNIT Sl EXPRESSED IN EXPRESSED IN MORE
NAME SYMBQL Sl BASE UNITS OTHER Sl UNITS FAMILIAR
Frequency
Force hertz Hz s UNIT
Energy m·kg
Power newton N
Charge 52
joule J m2kg N·m erg (cgs)
J
watt w S2 -or V·A
coulomb c m2kg s

53

A·s

Radioactivity becquerel Bq s J curie
Absorbed dose gray Gy kg rad
m2 c roentgen
Electric potential volt v 52 kg
F w gauss
Capacitance farad Wb A·s
T -
Magnetic flux weber --
tesla A
Magnetic flux density kg c
(magnetic induction) v
m2·kg
53· A V·s

A2s• Wb
m2kg m2

m2kg
52·A

�

52· A

RADIATION 9

decay of one nucleus per second may be a ingly complex world should uniformly use
little inconvenient, but even simplicity has something like SI units. A sixteenth-cen­
its price. The SI unit of radiation absorbed tury English peasant may not have needed
dose is the gray, which again refers to the to convert a speed of furlongs per fortnight
quantity of ionizing radiation energy ab­ into inches per second, and we should not
sorbed per unit of mass. There is no special have to either.
SI unit corresponding to the familiar ra­
diation exposure unit of the roentgen. The SUMMARY
roentgen was originally defined as the
charge produced in a given mass of air, and Wilhelm Conrad Roentgen discovered
comparable SI units are C/kg (C = cou­ x rays on November 8, 1895. X rays are
lomb). members of a group of radiations known
as electromagnetic radiations, of which
A good table is needed to convert the light is the best-known member. They have
older units and United States units (based a dual nature, behaving in some circum­
on the British engineering system) to SI stances as waves and under different con­
units. A foot is about 0.305 m, a joule is 10 ditions as particles. Therefore, two con­
million ergs, and so forth. We will un­ cepts have been postulated to explain their
doubtedly continue to use a combination characteristics. A single particle of radia­
of units from different systems for some tion is called a photon, and we will discuss
time. For instance, we purchase electric x rays in terms of photons.
power in kilowatts (SI units), potatoes in
pounds (British engineering unit), and REFERENCE
heat energy in BTUs (British thermal
units), all different systems. An increas- l. Glasser, 0.: Wilhelm Conrad Roentgen and the
Early History of the Roentgen Rays. Springfield,
IL, Charles C Thomas, 1934.

CHAPTER Production of X Rays

2

DIAGNOSTIC X-RAY TUBES present inside the tube, the electrons that
were being accelerated toward the anode
X rays are produced by energy conver­ (target) would collide with the gas mole­
sion when a fast-moving stream of elec­ cules, lose energy, and cause secondary
trons is suddenly decelerated in the "tar­ electrons to be ejected from the gas mol­
get" anode of an x-ray tube. The x-ray tube ecules. By this process (ionization), addi­
is made of Pyrex glass that encloses a vac­ tional electrons would be available for ac­
uum containing two electrodes (this is a di­ celeration toward the anode. Obviously,
ode tube). The electrodes are designed so this production of secondary electrons
that electrons produced at the cathode could not be satisfactorily controlled. Their
(negative electrode or filament) can be ac­ presence would result in variation in the
celerated by a high potential difference to­ number and, more strikingly, in the re­
ward the anode (positive or target elec� duced speed of the electrons impinging on
trode). The basic elements of an x-ray tube the target. This would cause a wide varia­
are shown in Figure 2-1, a diagram of a tion in tube current and in the energy of
stationary anode x-ray tube. Electrons are the x rays produced. Actually, this princi­
produced by the heated tungsten filament ple was used in the design of the early so­
and accelerated across the tube to hit the called "gas" x-ray tubes, which contained
tungsten target, where x rays are pro­ small amounts of gas to serve as a source
duced. This section will describe the design of secondary electrons. The purpose of the
of the x-ray tube and will review the way vacuum in the modern x-ray tube is to al­
in which x rays are produced. low the number and speed of the acceler­
ated electrons to be controlled independ­
Glass Enclosure ently. The shape and size of these x-ray

It is necessary to seal the two electrodes
of the x-ray tube in .a vacuum. If gas were

TUNGSTEN
TARGET

COPPER ELECTRONS
A NODE HEATED TUNGSTEN FILAMENT

+

Figure 2-1 The major components of a stationary anode x-ray tube
10

PRODUCTION OF X RAYS 11

tubes are specifically designed to prevent form a vertical spiral about 0.2 em in di­
electric discharge between the electrodes. ameter and 1 em or less in length. When

The connecting wires must be sealed into current flows through this fine tungsten
the glass wall of the x-ray tube. During op­ wire, it becomes heated. When a metal is
eration of the x-ray tube, both the glass and heated its atoms absorb thermal energy,
the connecting wires are heated to high and some of the electrons in the metal ac­
temperatures. Because of differences in quire enough energy to allow them to move
their coefficients of expansion, most metals a small distance from the surface of the
expand more than glass when heated. This metal (normally, electrons can move within
difference in expansion would cause the a metal, but cannot escape from it). Their
glass-metal seal to break and would destroy escape is referred to as the process of
the vacuum in the tube if special precau­
tions were not taken. Because of this prob­ thermionic emission, which may be de­
lem, special alloys, having approximately
the same coefficients of linear expansion as fined as the emission of electrons resulting
Pyrex glass, are generally used in x-ray from the absorption of thermal energy.
tubes. The electron cloud surrounding the fila­
ment, produced by thermionic emission,
Cathode
has been termed the "Edison effect." A
The negative terminal of the x-ray tube
is called the cathode. In referring to an x­ pure tungsten filament must be heated to

ray tube, the terms cathode and filament a temperature of at least 2200° Cto emit a

may be used interchangeably, a statement useful number of electrons (thermions).
that is not true for other types of diode Tungsten is not as efficient an emitting ma­
terial as other materials (such as alloys of
tubes. In addition to the filament, which is tungsten) used in some electron tubes. It
the source of electrons for the x-ray tube, is chosen for use in x-ray tubes, however,
because it can be drawn into a thin wire
the cathode has two other elements. These that is quite strong, has a high melting
are the connecting wires, which supply
point (3370° C), and has little tendency to
both the voltage (average about 10 V) and
the amperage (average about 3 to 5 A) that vaporize; thus, such a filament has a rea­
sonably long life expectancy.
heat the filament, and a metallic focusing
cup. The number (quantity) of x rays pro­ Electrons emitted from the tungsten fil­
duced depends entirely on the number of ament form a small cloud in the immediate
electrons that flow from the filament to the vicinity of the filament. This collection of
negatively charged electrons forms what is
target (anode) of the tube. The x-ray tube
current, measured in milliamperes (1 rnA called the space charge. This cloud of neg­
= 0.001 A), refers to the number of elec­
trons flowing per second from the fila­ ative charges tends to prevent other elec­
ment to the target. It is important to un­ trons from being emitted from the filament
until they have acquired sufficient thermal
derstand where these electrons come from, energy to overcome the force caused by the
and to remember that the number of elec­ space charge. The tendency of the space
trons determines x-ray tube current. For charge to limit the emission of more elec­
example, in a given unit of time, a tube
trons from the filament is called the space
current of 200 rnA is produced by twice as charge effect. When electrons leave the fil­
many electrons as a current of 100 rnA, and
200 rnA produces twice as many x rays as ament the loss of negative charges causes
100 rnA. the filament to acquire a positive charge.
The filament then attracts some emitted
The filament is made of tungsten wire, electrons back to itself. When a filament is
heated to its emission temperature, a state
about 0.2 mm in diameter, that is coiled to of equilibrium is quickly reached. In equi­
librium the number of electrons returning

12 PRODUCTION OF X RAYS

to the filament is equal to the number of FOCUSING
CUP
electrons being emitted. As a result, the
number of electrons in the space charge Figure 2-2 A double filament contained in a
remains constant, with the actual number focusing cup
depending on filament temperature.
served by looking into the beam exit port
The high currents that can be produced of an x-ray tube housing (do not forget to
by the use of thermionic emission are pos­ remove the filter).
sible because large numbers of electrons
can be accelerated from the cathode (neg­ Two additional filament arrangements
ative electrode) to the anode (positive elec­ may be seen in highly specialized x-ray
trode) of the x-ray tube. The number of tubes. A tube with three filaments (triple­
electrons involved is enormous. The unit focus) is available. Another special appli­
cation is a stereoscopic angiographic tube.
of electric current is the ampere, which In this tube the two focal spots are widely
may be defined as the rate of "flow" when separated (about a 4-cm separation). When
l coulomb of electricity flows through a two films are exposed, using a different
conductor in l sec. The coulomb is the focal spot for each film, a stereoscopic film
equivalent of the amount of electric charge pair is produced. This tube is useful in an­
giography when rapid exposure of multi­
carried by 6.25 X 1018 electrons. There­ ple stereoscopic film pairs is desired. In­
fore, an x-ray tube current of l 00 rnA (0.1
A) may be considered as the "flow" of 6.25 tervals as short as 0.1 sec between
x 1017 electrons from the cathode to the
anode in l sec. Electron current across an exposures can be obtained with stereo­
x-ray tube is in one direction only (always scopic tubes.

cathode to anode). Because of the forces of Vaporization of the filament when it is
mutual repulsion and the large number of heated acts to shorten the life of an x-ray
electrons, this electron stream would tend tube, because the filament will break if it
to spread itself out and result in bombard­ becomes too thin. The filament should
ment of an unacceptably large area on the never be heated for longer periods than
anode of the x-ray tube. This is prevented necessary. Many modern x-ray circuits con­
tain an automatic filament-boosting circuit.
by a structure called the cathode focusing When the x-ray circuit is turned on, but no
cup, which surrounds the filament (Figs. exposure is being made, a "standby" cur­
2-2 and 2-4). When the x-ray tube is con­ rent heats the filament to a value corre­
sponding to low current, commonly about
ducting, the focusing cup is maintained at
the same negative potential as the filament. 5 rnA. This amount of filament heating is
The focusing cup is designed so that its
electrical forces cause the electron stream all that is required for fluoroscopy. When
to converge onto the target anode in the exposures requiring larger tube currents
required size and shape. The focusing cup are desired, an automatic filament-boost­
is usually made of nickel. Modern x-ray ing circuit will raise the filament current
tubes may be supplied with a single or, from the standby value to the required
more commonly, a double filament. Each value before the exposure is made, and
filament consists of a spiral of wire, and
they are mounted side by side or one above
the other, with one being longer than the

other (Fig. 2-2). It is important to under­

stand that only one filament is used for any
given x-ray exposure; the larger filament
is generally used for larger exposures. The
heated filament glows and can be easily ob-

PRODUCTION OF X RAYS 13

lower it to the standby value immediately _c;;; i:�i�i�20°(t)(- )
after the exposure. CATHODE
ANODE
Tungsten that is vaporized from the fil­ :I
ament (and occasionally from the anode) is
deposited as an extremely thin coating on II
the inner surface of the glass wall of the x­ II
ray tube. It produces a color that becomes 1I
deeper as the tube ages. Aging tubes ac­ II
quire a bronze-colored "sunburn." This
tungsten coat has two effects: it tends to �
APPARENT
FOCAL SPOT

SIZE

filter the x-ray beam, gradually changing Figure 2-3 The line focus principle
the quality of the beam, and the presence

of the metal on the glass increases the pos­

sibility of arcing between the glass and the focusing cup. The electron stream bom­

electrodes at higher peak kilovoltage (kVp) bards the target, the surface of which is

values, which may result in puncture of the inclined so that it forms an angle with the

tube. One of the reasons metal, as opposed plane perpendicular to the incident beam.

to glass, x-ray tube enclosures have been The anode angle differs according to in­

developed is to minimize the effect of dep­ dividual tube design and may vary from 6

osition of tungsten on the tube wall. We to 20°. Because of this angulation, when the

will discuss metal tubes later in this chapter. slanted surface of the focal spot is viewed

Line Focus Principle from the direction in which x-rays emerge
from the x-ray tube, the surface is fore­

The focal spot is the area of the tungsten shortened and appears small. It is evident,

target (anode) that is bombarded by elec­ therefore, that the side of the effective, or
trons from the cathode. Most of the energy apparent, focal spot is considerably smaller
of the electrons is converted into heat, than that of the actual focal spot. If the

with less than 1% being converted into x decrease in projected focal spot size is cal­

rays. Because the heat is uniformly distrib­ culated, it is found that the size of the pro­

uted over the focal spot, a large focal spot jected focal spot is directly related to the

allows the accumulation of larger amounts sine of the angle of the anode. Because sine

of heat before damage to the tungsten tar­ 20° = 0.342 and sine l6.SO = 0.284, an

get occurs. The melting point of tungsten anode angle of 16.5° will produce a smaller
is about 3370° C, but it is best to keep the focal spot size than an angle of 20°. Thus,
temperature below 3000° C. The problems as the angle of the anode is made smaller,

posed by the need for.a large focal spot to the apparent focal spot also becomes

allow greater heat loading, and the con­ smaller.

flicting need for a small focal area to pro­ Some newer 0.3-mm focal spot tubes

duce good radiographic detail, were re­ may use an anode angle of only 6°. Such

solved in 1918 with the development of the small angles permit the use of larger areas
line focus principle. The theory of line of the target for electron bombardment

focus is illustrated in Figure 2-3. The size (and heat dissipation), yet achieve a small

and shape of the focal spot are determined apparent focal spot size. For practical pur­

by the size and shape of the electron stream poses, however, there is a limit to which the

when it hits the anode. The size and shape anode angle can be decreased as dictated

of the electron stream are determined by by the heel effect (the point of anode cut­

the dimensions of the filament tungsten off). For general diagnostic radiography

wire coil, the construction of the focusing done at a 40-in. focus-film distance, the

cup, and the position of the filament in the anode angle is usually no smaller than 15°.

14 PRODUCTION OF X RAYS

Focal spot size is expressed in terms of �Tungsten Focusing
the apparent or projected focal spot; sizes Cup
of 0.3, 0.6, 1.0, and 1.2 mm are commonly Target
employed. ANODE �TRON CATHODE

Anode (+) --RT-s� (-)

Anodes (positive electrodes) of x-ray - c, _-_ -_-___
tubes are of two types, stationary or rotat­
ing. The stationary anode will be discussed Filament
first because many of its basic principles
also apply to the rotating anode. Figure 2-4 Lateral view of the cathode and
anode of a stationary anode x-ray tube
Stationary Anode. The anode of a sta­
tionary anode x-ray tube consists of a small high temperature is reached by any metal
in the immediate vicinity of the focal spot.
plate of tungsten, 2- or 3-mm thick, that is If the tungsten target were not sufficiently
large to allow for some cooling around the
embedded in a large mass of copper. The edges of the focal spot, the heat produced
tungsten plate is square or rectangular in would melt the copper in the immediate
shape, with each dimension usually being vicinity of the target.
greater than 1 em. The anode angle is usu­
ally 15 to 20°, as discussed above. All the metals expand when heated, but
they expand at different rates. The bond­
Tungsten is chosen as the target material ing between the tungsten target and the
for several reasons. It has a high atomic copper anode provides technical problems
number (74), which makes it more efficient because tungsten and copper have differ­
for the production of x rays. In addition, ent coefficients of expansion. If the bond
because of its high melting point, it is able between the tungsten and the copper were
to withstand the high temperature pro­ not satisfactorily produced, the tungsten
duced. Most metals melt between 300 and target would tend to peel away from the
1500° C, whereas tungsten melts at copper anode.
3370° C. Tungsten is a reasonably good
material for the absorption of heat and for Rotating Anode. With the development
the rapid dissipation of the heat away from of x-ray generators capable of delivering
the target area. large amounts of power, the limiting factor
in the output of an x-ray circuit became the
The rather small tungsten target must x-ray tube itself. The ability of the x-ray
be bonded to the much larger copper por­ tube to achieve high x-ray outputs is limited
tion of the anode to facilitate heat dissi­ by the heat generated at the anode. The
pation. In spite of its good thermal char­ rotating anode principle is used to produce
acteristics, tungsten cannot withstand the x-ray tubes capable of withstanding the
heat of repeated exposures. Copper is a heat generated by large exposures.
better conductor of heat than tungsten, so
the massive copper anode acts to increase The anode of a rotating anode tube con­
the total thermal capacity of the anode and sists of a large disc of tungsten, or an alloy
to speed its rate of cooling. of tungsten, which theoretically rotates at
a speed of about 3600 revolutions per min­
The actual size of the tungsten target is ute (rpm) when an exposure is being made.
considerably larger than the area bom­ In practice, the anode never reaches a
barded by the electron stream (Fig. 2-4). speed of 3600 rpm because of mechanical
This is necessary because of the relatively
low melting point of copper (1070° C). A
single x-ray exposure may raise the tem­
perature of the bombarded area of the
tungsten target by 1000° C or more. This