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CIHR Strategic Training Program in Vascular Research
VASCPROG 560
Vascular Imaging Techniques
Module 2
X-ray Imaging
CIHR Strategic Training Program
In Vascular Research
Navigation through this Module
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When you have finished the module, please go to the Module 2 Quiz
under the Quizzes icon on the Course Home Page.
Credits
Information in this module was based on
lecture notes given by Robarts Scientists as
part of the coursework in the Department of
Medical Biophysics and the Biomedical
Engineering Program at the University of
Western Ontario, and from the Radiology
Residents’ Course entitled “Physics of
Diagnostic Imaging” given in partnership with
the Department of Diagnostic Radiology and
Nuclear Medicine, London Health Sciences
Centre, Imaging Research Laboratories,
Robarts Research Institute, St. Joseph’s
Health Centre, and The Lawson Research
Institute, all in London, Ontario.
Some of the figures in the module were
used with permission from “Figures from
the Essential Physics of Medical Imaging,
Second Edition. Jerrold T. Bushberg, J.
Anthony Seibert, Edwin M. Leidholdt Jr. &
John M. Boone. These are denoted by the
publisher’s mark, ©2003 Lippincott,
Williams & Wilkins.
Other material is based on information from
the following source:
Jackie Williams also wrote some of the
content and organized the module in its
present format for WebCT.
W. Huda & R. Slone. Review of Radiologic
Physics, 2nd edition. Lippincott Williams &
Wilkins. 2003. Material used with permission.
I wish to thank Dr. Ian Cunningham, who
provided much of the content for this
module and made many suggestions as to
its final form.
Objectives
At the end of this module, students should
have a good understanding of the
following:
Background Concepts
 Atoms
 Electricity
 Waves
 Electromagnetic radiation and xray photons
X-Ray Production
 Physics
 Equipment
X-Ray Interactions
 Attenuation, absorption, scatter,
interaction coefficients
X-Ray Imaging Equipment
 Film-screen radiography
 Fluoroscopy
 Digital radiography
Radiation Risks and Safety
 Biological effects of radiation
exposure
• X-ray tubes
 Radiation units: Exposure, absorbed
dose, dose equivalent
• High-voltage generators
 Radiation limits and regulations
• Collimators and x-ray beam
alignment
 Radiation protection
Introduction
X-ray imaging started with the discovery of x
rays by Prof. Wilhelm Conrad Roentgen in
1895 and remains the mainstay of modern
medical-imaging facilities around the world.
Today, medical radiography continues to
become safer, faster and more effective with
the development of digital detectors and fast
three-dimensional
computed
tomography
systems.
This module starts with a brief history of how
Dr. Roentgen first discovered x rays, continues
with a quick recap of terms used in electricity,
followed by a very basic overview of x-ray
production and interactions within the body,
and of how analogue and digital x-ray images
are formed. and ends with a brief description of
radiation safety. The module does not include
information on x-ray dose regimes, or certain
types of body imaging such as mammography,
or orthopedics, which are not related to
vascular imaging.
The description of how x rays (and all the
other imaging modalities) are used to
image the vasculature have been grouped
together in Module 8 – Angiography
Techniques.
We have tried to keep the number of
mathematical formulae in the module to a
minimum. Any formulae that are included
are just to show the relationships between
objects and how amounts are calculated. It
is not necessary for the purposes of the
course to memorize any of the formulae.
N.B. The correct convention for the use of
a hyphen when writing about x rays is:
It is x rays when used as a noun, but
It is x-ray when used as an adjective, e.g.
x-ray imaging, x-ray film etc.
Then and Now
Thought to be the first medical radiograph, the image on the left was obtained by Prof. Roentgen of his wife’s hand
(The hand of Mrs. Wilhelm Roentgen: the first X-ray image, 1895 In Otto Glasser, “Wilhelm Conrad Roentgen
and the early history of the Roentgen rays”. London, 1933. National Library of Medicine). Image quality is poor,
showing little about the patient except that she was married! The image on the right is a modern surface-rendered
image of a human hand obtained on a prototype high-resolution three-dimensional x-ray computed tomography
system.
The Discovery of X Rays
This short history is adapted from:
X Rays and Their Discovery (used with
permission).
Dr. Wilhelm Roentgen's discovery of x rays
in late 1895 in Wurtzburg, Germany,
serves as a classic example of an
accidental invention.
Roentgen was carrying out experiments
with a Crookes tube, which was a glass
chamber, with as much of the air removed
as was possible at the time, having two
pieces of enclosed metal in each end
connected across a battery. As Roentgen
applied large voltages to the device to
study the behaviour of electrons emitted
from the metal, he noticed that a piece of
phosphorus material, situated elsewhere in
the room, was glowing!
Puzzled
by
the
glow,
Roentgen
investigated the mysterious emissions over
the following weeks.
He tried to block the emissions by covering the
tube first by a piece of cardboard and then by a
piece of wood only to find that the phosphorus
still glowed. He also noticed that when he held
his hand between the tube and the phosphorus,
the light given off seemed to present an image
of his hand. Roentgen recorded such images on
pieces of film thus obtaining the first x-ray
images of human anatomy.
Roentgen’s Discovery of X-rays
What Roentgen had discovered was a form
of invisible, high-energy electromagnetic
radiation that could penetrate cardboard and
human tissues. When a potential difference
(thousands of volts) is applied between the
electrodes (pieces of metal) in the Crookes
tube, electrons are released from the
negative electrode. The electrons from this
negative electrode (cathode) are attracted
towards the second, positive electrode
(anode) with such force that they acquire
great speed and energy. When an electron
bombards the positively-charged electrode, it
is decelerated abruptly in the vicinity of a
heavy nucleus and its energy is converted to
x
rays
(more
specifically,
called
Bremsstrahlung radiation, meaning “braking
radiation” in German).
Both light and x rays are part of the same
physical
phenomenon
we
know
as
electromagnetic radiation.
X rays can simply be thought of as high energy
(i.e. very short wavelength, near 1nm, or high
frequency) light.
It is because of their high energy that x rays
can penetrate through many objects. However
there are differences in penetration through
different materials due to the differences in the
material densities and atomic numbers. For
example, x rays can penetrate through
fat/muscle easier than through bone. This
forms the basis for imaging with x rays.
Roentgen's first experiments took place in
November 1895 and communications to other
scientists were delivered by January of 1896.
His work was immediately duplicated and
confirmed by a number of other scientists. The
ability of x rays to shrink tumours was also
discovered and in addition to the acquisition of
diagnostic medical images, x rays began to be
used for radiation therapy (or radiation
oncology) of cancer.
X-Ray Imaging Today
Today, advanced digital computers and new x-ray detector technologies allow us
to see details of the human body never thought possible only a few years ago.
Until 30 years ago, x-ray imaging technology remained substantially unchanged
from the time of Roentgen. Film was still the principle image receptor, and there
was no opportunity to digitally store, process, enhance or retrieve the images.
The advent of the digital computer, however, changed that forever. Digital
subtraction angiography, where the computer is an integral part of the imaging
chain, has been used for over 30 years. X-ray film is being replaced by
completely digital image receptors, and hospitals are abandoning the use of film
and becoming completely film-less.
Examples of modern x-ray systems and typical vascular images are displayed on
the following page.
Modern X-Ray Systems
Typical angiographic x-ray imaging system
Arteriograms
Typical modern portable x-ray imaging system
Cerebral arteriogram
Advantages and Disadvantages of X-Ray Imaging
Advantages
X-ray equipment is readily available.
It is a reasonably inexpensive imaging
modality.
Disadvantages
Perhaps the biggest drawback is the
exposure to harmful radiation to
patients and personnel.
X-ray systems are simple to maintain.
There is little contrast between different
soft tissues, making it less useful as a
diagnostic tool for some pathology.
There are many experienced and trained
personnel available, as x-ray technology
has been in hospitals and clinics for quite
a long time.
X ray generates images that are
shadowgrams, or projection images,
which have no depth information.
Structure of Matter & Radiation : Atoms
Atoms are the basic foundation of everything
that exists. An atom is the smallest unit of
matter that is recognizable as a chemical
element, although they may also be broken up
into smaller parts. Most of the mass of an atom
is due to the atomic nucleus. The nucleus
consists of protons which have a positive
electrical charge and neutrons which have no
charge. Like charges repel, while dissimilar
charges attract. Therefore protons repel each
other because they each have a positive
charge. Neutrons, with no charge, allow some
separation between the protons, which
decreases the electrostatic repulsion and
makes the nucleus stable. For this reason
neutrons are necessary for two or more
protons to be bound into a nucleus.
Atoms also have electrons, which have a
negative charge. Each electron carries one unit
of negative charge and has a very small mass
compared to that of a neutron or proton. They
exist outside the nucleus, but surround it. An
atom with no net electrical charge has the
same number of electrons as protons (the
positive and negative charges cancel each
other out).
The atomic number of an atom is the
number of protons it contains which, if
the atom is neutrally charged, equals
the number of electrons. This is
important because the number of
electrons is related to the chemical
properties of the atom.
The atomic mass of an atom is the
total mass of the electrons, protons, and
neutrons in the atom.
Isotopes are atoms that have the same
atomic number (number of protons) but
have a different atomic mass, because
they have more neutrons than protons.
For instance most carbon has six
protons and six neutrons and thus
atomic mass of approximately 12. Some
carbon atoms have an atomic mass of
13 so thus have six protons and seven
neutrons. This can make the nucleus of
the atom unstable, resulting in nuclear
decay.
Properties of Elemental Particles
Particle
Mass
Charge
(in Atomic
mass units)
Properties
Proton (P)
1.007
+1
Nucleus of hydrogen
Neutron (N)
1.009
0
Hard to stop, hard to detect
Electron (e,e-,-)
0.0005
-1
Many in nature, easy to detect
Positron (e+, +)
0.0005
+1
Positively charged electrons (antimatter particle to electron)
Don’t last long, combine with e-
Photon (h)
Gamma-ray ()
0
0
Bundle of energy, never at rest
Neutrino ( °U°)
Less than
1/2000 of e-
0
Extra-Nuclear Structure
Arrangements
of
electrons in their
orbits surrounding
the nuclei of atoms
of various material.
Background: Electricity
Although you will have learned about
electricity in high school, this is just a
reminder of the key terms and
underlying concepts, so that you can
relate them to this module.
Electricity is a type of activity arising
from the existence of charge. The basic
unit of charge is that on a proton (called
a positive charge) or electron (called a
negative charge). Protons and electrons
are also referred to as charged particles.
Two particles with the same charges,
either both positive or both negative,
repel or drive away each other, while two
particles with unlike charges are
attracted.
Electrons are much lighter than protons
and circulate around the outside of the
nucleus of an atom.
Materials such as copper, whose outermost electrons are loosely held, are
conductors of electricity because they
allow an electric current to flow easily.
Electrons in materials like clay and
rubber are too tightly bound to their
atoms to conduct electricity. They are
insulators.
The quantity of charge is measured in
coulombs (C). One coulomb is the
charge on 6.24 x 1018 electrons (or the
same number of protons).
Continued
Background: Electricity
When the number of electrons increases
at one end of a conductor, such as a
copper wire, their negative charges repel
one another, forcing the foremost free
electrons towards the end with fewer
electrons. This is an electric current,
which is a measure of the number of
electrons moving along the wire. It is
measured in coulombs/sec or amperes
(A), 1 C/s = 1A.
Electrons flow easily along good
conductors, but sometimes they are
slowed down by bumping into atoms in
the wire. This braking effect is called the
conductor’s resistance. The longer a wire
the more resistance it has, but the thicker
the wire, the less resistance it has.
Terms
Electron (e): A small negative charge of
electricity.
Voltage (V): The electrical potential that
causes a current to flow in a closed
circuit. Voltage is measured in volts (or
millivolts mV or kilovolts kV) with a
voltmeter or multi-meter.
Current (I): The flow of electrons that
results when a voltage is applied to a
circuit. This unit is measured in amps (or
milliamps, or megamps) with an
ammeter or multimeter.
Electrons flowing through a resistance
result in a change in the potential energy
carried by electrons, measured in terms
of voltage (V).
Continued
Background: Electricity
Resistance (R): Resistance is the
impedance to current flow. All electrical
equipment has an inherent resistance measured in ohms (or milliohms or
megohms) with an ohmmeter or multimeter. Flowing current causes a
resistive object to heat up.
Watts: Energy is measured in joules (J).
A joule of electrical energy can move
from place to place along the wires.
When you transport one joule of energy
every second, the flow-rate of energy is
1 J/s, and "one joule per second" is one
watt.
Direct current (DC): Direct current is an
electrical current that flows through a
circuit in the same direction at all times
with constant strength. Batteries produce
direct current going from the positive to
the negative pole in a constant stream.
Alternating current (AC): This is the
current that comes from power stations
and is a periodic current that reverses at
regularly occurring intervals of time (60
times per second in North America) and
has alternately positive and negative
values. Transformers can “transform”
AC into high voltages before it gets sent
over long distances. At high voltages
less energy is lost as heat along the
way.
Continued
Background: Electricity
Ohm’s Law
Named after the German physicist George Simon Ohm, Ohm’s Law relates voltage, current,
and resistance in the following ways:
(a).
I V
R
(b).
V  IR
(c).
R V I
Where: I = current, V = Voltage, R = Resistance
For example, if a battery supplies 24 volts to a circuit consisting of a lamp with 112 ohms of
resistance, the current that flows is:
I = 24 ÷ 112 = 0.21 A, or 210 mA
If you are still fuzzy on some of these concepts and would like to know more, there is an
excellent website that explains them very clearly at:
HOW ARE WATTS, OHMS, AMPS & VOLTS RELATED?
Background: Waves
X rays are a form of electromagnetic (EM) radiation.
Radiation is the transport of energy through space.
Electromagnetic radiation, including both visible light and x rays, can behave and interact with
matter in a wavelike manner, demonstrating wave properties such as reflection, refraction,
diffraction, and polarization.
Waves have certain properties. Wavelength  is the distance between two adjacent wave
crests.
Amplitude is the height of the wave from the mean.
f
Frequency (f) is the number of wave oscillations per unit of time, expressed in cycles per
second, or hertz (Hz).
Propagation
Electric field
maximum
(+ve)
Amplitude
Electric field
minimum (-ve)
1 cycle
Period (time)
(T)
Wavelength
()
Continued
Background: Waves


(lambda) is the wavelength, the distance between two identical points on a wave.
time
unit of time (orange box)
Frequency( f ) is the number of cycles per unit of time
(usually seconds).1Hertz (Hz) = 1 cycle per second.
Continued
Background: Waves
 v f
Wavelength (  ) is proportional to the velocity (v).
Wavelength is inversely proportional to the frequency (f).
In the case of Electromagnetic (EM) radiation, the equation becomes:
c f
where c = 3x108 m/s is the speed of light
For example, AM radio wave has a large wavelength (~200-600 m) and
a relatively low frequency (~500-1500 kHz).
Background: Electromagnetic Radiation
EM radiation propagates through space at the speed of light (3 x 108 m/s), in the form of
oscillating electric and magnetic fields.
While electromagnetic radiation can behave and interact with matter in a wave-like manner,
it can also behave in a particle-like manner where the particles are called photons. This is
called the “wave-particle duality” of EM radiation.
X rays are usually described by their “energy”, rather than their “frequency” or “wavelength”.
The energy of a photon is proportional to the frequency of the wave, as described in the next
panel.
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1meter
10-11 m (0.01 nm)
“hard x-rays”
10-9 m (1 nm)
“soft x-rays”
Electromagnetic spectrum from Gamma rays (short wavelength)
to radio waves (long wavelength). 1 nm = 10-9 m.
108
Background: X-Ray Photons
Photons may behave as waves or particles (photons), but they have no mass.
An x ray’s wavelength may be measured in angstroms (Å), where 1 Å is 10-10 m.

Photons are referred to as x rays if they are produced by electron interactions, and
as gamma rays if produced by nuclear processes. While they are identical, gamma
rays often have higher energies than x rays.
Each photon’s energy is given by the product of the photon frequency  hertz) and
Planck's constant (h = 6.626 x 10-34 joule seconds). Therefore, E = h.
Each photon’s energy (E) is inversely proportional to its wavelength )

E = h
Photon energy = (Planck’s constant) x (frequency of EM radiation)
Continued
Background: X-Ray Photons
The main difference between an x-ray
photon and a visible light photon lies in the
energy of each photon.
An x-ray photon has approximately 5,000
times the energy of an light photon. This
allows the x-ray photon to penetrate
materials
more
readily
than
light
photons. For example, if a person holds
his or her hand to a conventional light
source they would possibly notice a small
amount of light passing through the flesh of
the hand, but an x ray would pass more
freely. This ability of x rays to penetrate
flesh and other materials is what allows
them to be such a useful diagnostic tool in
medical imaging.
When EM radiation has enough energy
to remove outer electrons from an atom,
it is able to ionize that atom and is then
referred to as ionizing radiation. X
rays and gamma rays are ionizing
radiations.
When an electron is removed from a
neutral atom, it leaves behind a positive
ion sometimes called a free radical.
Free radicals can be produced from the
ionization of water molecules in the
body, and these free radicals are
responsible for the biological risk
associated with x-ray exposures.
X-Ray Production: Physics
X-rays are easily produced with low-cost and
sometimes portable equipment, which is the
major reason for the widespread use of x-ray
imaging. This next section describes the
physics and equipment required for the
production of x-ray radiation.
In medical radiography, the x-ray tube is
typically placed a metre or more from the
patient. As the emitted x rays pass through the
patient, they are absorbed or scattered by the
body’s tissues and recorded by a detector on
the opposite side. The most common detector
is silver halide film (regular photographic film
with extra silver), which gets darker where it
interacts with transmitted photons. X-rays are
deflected and absorbed to different degrees by
the different tissues in the patients body. The
amount of absorption depends on the tissue
composition.
For example, dense bone absorbs many more
x rays than soft tissues such as muscle, fat
and blood.
The amount of deflection depends on the
density of electrons in the tissues.
Tissues with high electron densities cause
more x-ray scattering than those of lower
density. Thus, since fewer photons reach the
x-ray film after encountering bone or metal
rather than tissue, the x-ray film will look
brighter for bone or metal. The darker areas
on the film indicate high transmission
intensities while brighter areas show places
of low exposure.
So, to recap, the production of x-rays requires
a number of different components:
The Generator: This provides high voltage
power to the x-ray tube.
The X-ray Tube: Converts the electric power
from the generator into x-ray photons.
The Detector: Captures, the radiographic
images. In the case of film, the detector
acts as the storage and display medium
as well. Modern electronic detectors
depend on computer storage and
display.
X-Ray Production: Physics
X rays are generated when negatively
charged electrons in motion are
deflected by positively charged atomic
nuclei. The deflection of the electrons
involves loss of kinetic energy (velocity),
which is emitted as electromagnetic
radiation (x rays) in the form of photons.
X-ray machines produce x rays by the
bombardment of high-energy electrons
on a metal (generally tungsten) target. In
medical imaging, this process takes
place in a heated cathode x-ray tube
(diagram on next page).
This heated cathode tube is the source
of the electrons which are accelerated by
applying a voltage of between 20 and
150 kilo electron volts (keV) between
the cathode and the tungsten anode. The
voltage is provided by a special
generator.
Diagnostic x rays can be generated by two
separate processes, one of which is called
bremsstrahlung (from a German word
meaning “braking”), and characteristic xray production (caused by electron
behaviour following ionization). Most x
rays produced in the x-ray tube are by the
bremsstrahlung process, whereby incident
electrons are slowed down (braked) by
their interaction with nuclear electric fields.
As x rays are transmitted through
materials, they may be absorbed or
scattered so that their intensity decreases
exponentially. The amount of absorption is
related to the attenuation coefficient of the
material and the energy of the x rays.
Dense materials absorb more x rays than
less dense materials, which is why bone
appears as the most dominant feature on
x-ray films. This is described in more detail
under x-ray Interactions later in this
module.
X-Ray Production: The High Voltage Generator
The utility of an x-ray tube depends on one
critical factor: a stable, high-power, high
voltage
electrical
supply.
Recent
developments in electrical design have
greatly increased the available power, while
decreasing the size and heat production of
modern x-ray generators. In North America,
utility companies provide an electric power
supply of 120volts (V) alternating current
(AC) that oscillates at a frequency of 60
cycles/sec (60Hz). X-ray systems need
higher supply voltages (around 440V). A
generator increases the voltage to that
required by the x-ray tube, and rectifies the
waveform from AC to direct current (DC).
The different elements of the generator are:
Power Circuit - An x-ray generator must
supply between 40 and 125kV between
the cathode and the anode to generate a
diagnostically useful x-ray spectrum. This
circuit must also provide sufficient current
between 2 and 1200 mA. The power
circuit is the primary purpose of the
generator.
kV Control - The generator must supply not
only voltage and current, but also control
the voltage supply so that exposures are
made with appropriate parameters.
Filament Circuit - The filament circuit
provides the current needed to heat the
filament (for thermionic emission) and the
current needed for x-ray production.
Stator Control - The generator provides the
high frequency waveform that rotates the
anode within the vacuum housing of the
x-ray tube. This circuit is critical, since the
tube will be destroyed if exposure takes
place with a stationary anode.
Continued
X-Ray Production: The High Voltage Generator
Microprocessor - All modern generators
are microprocessor-controlled for increased
stability and reliability. The microprocessor
also interfaces with other subsystems, such
as digital cameras, patient positioning
equipment, and gantry motion control.
Exposure Control - Most new generator
systems provide the capacity to monitor
and adjust x-ray exposure automatically,
guaranteeing correct exposure at all times.
This capacity is called automatic brightness
control.
TRANSFORMERS
The generator produces high voltages that can
be up in the 100,000V range, although the
general electrical supply only provides a few
hundred volts. There are different transformers
inside the generator that change the input
voltage to high and low voltages. Transformers
have two wire coils wrapped around a common
iron core. One is the primary coil. Current in the
primary coil produces a current in the secondary
coil
by
magnetic
induction.
Step-up
transformers increase the voltage and stepdown transformers decrease it.
RECTIFICATION
AC power supplies the electric current that flows
alternately in each directions. Rectification
within the generator changes the AC voltage to
DC voltage across the x-ray tube by the use of
diodes, which only permit current flow in one
direction. The use of high frequency generators
and various rectification approaches has
increased the efficiency and reduced the core
size in virtually all modern x-ray systems.
X-Ray Production: X-Ray Tube
Production of X-rays in a Vacuum Tube
Lateral view of the cathode and anode of a stationary anode x-ray tube.
©2003 Lippincott, Williams & Wilkins.
Continued
X-Ray Production: X-Ray Tube
Cathode: The cathode excites electrons
to the point where they become free from
their parent atom and then join the
electron beam.
The cathode is a
negative electrode that propels the
electron beam towards the positive
electrode (the anode).
Anode: The anode attracts the free
electrons and accelerates them through
the electromagnetic field that exists
between the anode and cathode. This
increases the velocity of the electrons,
building potential energy. The higher the
kilo electron volts (keV) applied to the
anode, the greater the speed at which the
electrons are propelled through the gap
between the cathode and anode.
The electrons then hit the tungsten target
(this target can also be molybdenum,
palladium, silver or other material),
causing the release of the potential
energy built up by the acceleration of the
electrons in the electron beam. Most of
this energy is converted to heat and is
radiated by the copper portions of the
anode. The remainder is refracted off the
target in the form of high energy photons,
which forms the x-ray beam.
Glass
envelope:
The
above
components are sealed into a glass
envelope. This allows for gases and
other impurities to be pumped out of the
tube, creating the vacuum necessary for
proper performance. The x-ray creation
process must occur in a vacuum so as
not to disrupt the electron beam, and also
to allow for proper filament performance
and durability.
X-Ray Production: Collimators and X-ray Beam Alignment
The x-ray beam is aligned using
collimators, which restrict the x-ray
beam to the region of clinical interest.
This reduces the radiation dose to the
patient. Proper collimation can also
improve the contrast of the image by
reducing the amount of scattered
radiation.
In x-ray imaging systems, the collimator
is mounted onto the x-ray tube (see
diagram on following page). It is used to
define the dimensions of the beam
which is to be incident on the subject
and the detector.
For convenience, and to reduce x-ray
exposure to the patient, the alignment is
accomplished using a light and mirror
system inside the collimator assembly. By
carefully controlling the positioning of the
light source and the mirror, the light field
can be made to give an exact display of the
x-ray field. If the mirror is out of adjustment,
the light field will not match the x-ray field,
causing a mis-exposure of the patient.
Thus, it is necessary to perform system
calibrations on a regular basis to ensure
that the two fields match.
A collimator is made of a highly
absorbing material such as lead, which
permits x or gamma rays to only travel
within a pre-defined cone or pyramid.
Continued
X-Ray Production: X-ray Beam Alignment
© 2003 Lippincott,
Williams &
Wilkins.
X-Ray Interactions: Transmission and Attenuation
What happens to an x ray when it
encounters the object to be imaged?
It either:
Passes right through the object
Is Absorbed by the object, or
Is Scattered by the object
X-ray attenuation is the decrease in
the number of photons in an X-ray
beam due to interactions with the
atoms of a material substance.
Attenuation is due primarily to two
processes, absorption and scattering.
In both processes, the x-ray photon
interacts with the atoms of the
material. In scattering, the x-ray photon
continues with a change in direction
with or without a loss in energy.
In absorption, the energy of the x-ray
photon is completely transferred to the
atoms of the material.
In photoelectric (PE) absorption, all of
the x-ray photon energy is used to eject
one of the inner shell electrons from an
atom. For more information on this please
go to:
Photoelectric Effect
In Compton scattering, some of the xray energy is used to eject an electron
from an atom, and the x-ray is scattered
with a reduced energy. For more
information on this please go to:
Compton Scattering
In coherent or elastic scattering, the xray is scattered with no energy transferred
to the material.
In all of these interactions, the x-ray
photon energy is removed from the
primary x-ray beam, and the process
contributes to the beam attenuation.
Continued
X-Ray Interactions: Transmission and Attenuation
(Beer’s Law)
X-ray attenuation follows
the natural exponential law
expressed by the equation
on the left.That is to say, if a
thickness of material “a”
reduces the intensity of the
beam by 1/2, then a
thickness of 2a will reduce
the beam to ¼ of its original
intensity.
Continued
X-Ray Interactions: Attenuation Coefficient
5
Attenuation
Coefficient
Bone
Muscle
Fat
1
0.1
10
50
100
150
500
Photon Energy (keV)
Attenuation coefficient tells how well a material blocks x rays. The Attenuation
Coefficient depends on the type of material being imaged. For example, the density
influences the coefficient (bone has a high density compared to soft tissues), and the
chemical composition. Lead effectively blocks x rays completely.
X-Ray Imaging Equipment: Film-Screen Systems
Currently, most diagnostic radiographic
systems use a phosphor screen to convert the
x rays to light, rather than expose the file
using x-rays directly. A phosphor screen emits
light in response to x-rays absorption. The
resulting optical image is conventionally used
to expose a photographic film as shown
below. This method is referred to as filmscreen radiography and has been used since
the discovery of x rays 100 years ago.
An x-ray image is essentially a shadow image
of the body. Different tissues attenuate the xray beam differently and thus the number of x
rays exiting from a patient depends on the
tissues in the path of the beam.
Typical x-ray
“screen-film”
imaging
configuration
Bone absorbs x rays well and so attenuates
the beam. Thus the areas falling in the
shadow of the bone appear light or
underexposed on an x-ray film image
because relatively few x rays exit the patient
and little light is produced in the phosphor
screen. Conversely, areas falling under the
shadow of soft tissue (e.g. fat/muscle) appear
dark on an x-ray image because a large
number of x rays exit the patient and a lot of
light is given off from the phosphor screen to
overexpose the film.
X-Ray Imaging Equipment: Digital Radiography
The limitations in the practicality of the filmscreen methods are fueling the development of
alternative x-ray imaging techniques. Filmscreen radiography is an analog method, but in
recent years there has been considerable
research effort in finding digital alternatives to
analog radiography.
Digital x-ray imaging refers to methods in
which the image information is represented as
a matrix of numbers whose value corresponds
to the x-ray transmission. An example of digital
image would be a computer scanned version of
a film image.
In general, a digital detector would absorb x
rays and produce an electric signal, either
directly or indirectly via multiple stages, as the
output. The electrical signal can then be
assigned numerical (digital) values according
to its amplitude, and these numbers can be
stored in a 2-dimensional array to be displayed
as an image on the computer screen.
Once in digital format, images can be
stored and transferred as data files. Digital
imaging also allows improved image
quality via independent optimization of the
detector and the display components of
the imaging method. In addition, digital
images can be displayed on a computer
monitor, their appearance can be altered
via image processing, and computer
software can be use to aid disease
diagnosis.
Digital radiographic systems have already
been introduced in clinical situations.
These are indirect methods in which the
number of x rays incident on a detector is
converted to electronic signal in a
multistage
process.
As
previously
mentioned, a film image can be digitized
but in this case the image quality can
ultimately be only as good as the original
film image.
X-ray Imaging Equipment: Fluoroscopy
Fluoroscopy refers to real-time imaging
carried out to observe motion within the
body. Procedures where fluoroscopy is used
include the barium swallow and the barium
enema carried out to study the intestine.
Fluoroscopic imaging of the blood vessels,
angiography, involves a contrast agent
injected into the vessel to increase its x-ray
absorption and thus its contrast in the x-ray
image. Angioplasty is an example of an
interventional technique carried out with
fluoroscopic guidance. In angioplasty, a
catheter is guided within a blood vessel to a
site of occlusion (e.g. atherosclerotic plaque).
Then a small balloon on the catheter is
inflated to break apart the occlusion thus
restoring normal blood flow through the
diseased vessel.
.
For human eye to perceive continuous
motion, a series of still images has to be
presented to the eye at a quick
succession. For example, TV monitors
show images at a rate of about 30 per
second. Thus fluoroscopic x-ray imaging
involves the acquisition of a large number
of x-rays per second over a total time
which can last into tens of minutes.
Fluoroscopy thus differs greatly from
radiography (e.g. mammography and
chest imaging) in that the patients are
exposed with relatively long x-ray
exposures. Thus to keep the total x-ray
dose low, each of the fluoroscopic images
is made with relatively low exposure. The
small number of x rays used to build up
each image places fundamental limitations
on image quality and also very stringent
requirements on the fluoroscopic x-ray
imaging system.
Continued
X-Ray Imaging Equipment: Fluoroscopy
Because of the low number of x rays
used for each image, fluoroscopic x-ray
detectors must be very sensitive.
Fluoroscopy is currently performed with
the help of a device called the Image
Intensifier (II). In an II, the incident x
rays first impinge upon a phosphor
screen which gives off light. This light is
then absorbed in a photocathode which
gives off electrons. The electrons are
accelerated in a vacuum with the help
of high potentials and acquire great
speed and energy. The electrons are
focused, with the help of magnets, onto
a small, secondary phosphor where
light is given off. This process yields an
amplified light signal at the secondary
phosphor. The secondary phosphor
light image is captured with a video
camera and the video signal is
presented on a monitor.
X-rays
X-ray Image Intensifier
Measuring X-Ray Radiation
Absorbed dose is a physical quantity which represents the energy imparted by radiation onto an
absorbing material. It is expressed in:
Rad (traditional Unit)
Gray (Gy) - SI Unit 1 Gy = 1 joule per kilogram; 1 Gy = 100 rads
Dose Equivalent (DE) may be regarded as an expression of dose in terms of its biological effect. DE
takes account of the fact that, for a given absorbed dose, such as 1 Gray, a radiation of one type and/or
energy may give rise to a greater biological effect than a radiation of another type and/or energy.
DE = Absorbed Dose x Quality Factor (Q)
- Q depends on the type of radiation.
- Q = 1 for gamma, x-ray and beta
- Q = 10 for alpha particles Helium nuclei (2 protons + 2 neutrons)
Q is used to compare the biological damage producing potential of various types of radiation, given equal
absorbed doses. The effectiveness of radiation in producing damage is related to the energy loss of the
radiation per unit path length. The term used to express this is linear energy transfer (LET). Generally, the
greater the LET in tissue, the more effective the radiation is in producing damage.
REM (Traditional Unit)
Sievert (Sv) - SI Unit; 1 Sv = 100 rem
Exposure is a quantity that expresses the ability of radiation to ionize air and thereby create electric
charges that can be collected and measured. Measured in:
- Roentgens (R). 1 R = 2.58 x 10-4 c/kg of air (Traditional unit)
- Coulomb/kilogram (SI Units)
Radiation Safety – Biological Effects
The biological effects of radiation depend
on the amount of energy absorbed by the
cells and where in the cell the energy is
absorbed. Biological effects are divided
into deterministic and stochastic effects.
Deterministic
effects
include
the
following:
erythema,
desquamation,
cataracts, decreased white blood count,
organ atrophy, fibrosis and sterility. The
onset of any of these somatic effects
depends on the absorbed dose, dose
rate and the extent of the body area
exposed. These effects have a dose
threshold, and the intensity of the effect
increases
with
increasing
dose.
Stochastic effects include cancer and
genetic risk.
The Intensity and energy of any x-ray
exposure are two major factors to
consider in radiation safety. Intensity
refers to the number of x-ray photons in
the x-ray beam, or the number of x-ray
photons entering or exiting the patient. It
should not be confused with penetration
ability. Factors controlling the x-ray beam
intensity are the mA (milliamp), kV
(kilovoltage) and pulse width (time). The
greater the number of electrons
accelerated through the X-ray tube,
and/or the greater the mA, the higher the
x-ray intensity. There is a linear
relationship between mA and intensity:
When mA is doubled, the number of xray photons produced are doubled,
assuming that kV and time are kept
constant. To increase film density while
maintaining contrast, mA is increased but
kV remains unchanged.
Continued
Radiation Safety – Biological Effects
The penetrating ability of the beam is
determined by the energy of the beam,
which is controlled by voltage applied
across the x-ray tube. The higher the
voltage the more energy the electrons
acquire and the more they can lose as
they travel toward the anode. When an
electron is accelerated though a
potential difference of 1 volt it acquires
the energy of 1 electron volt (1 eV).
When it is accelerated through a
potential difference of 100,000 volts
(100 kV), it acquires 100 keV of
energy, which it can lose as it slows
down and release as a 100-keV X-ray.
Since the relationship between kV and
intensity is exponential, a 15% increase
in kV is equivalent to doubling the mAs.
This "15% rule" is used to adjust
techniques to maintain desired film
density. While the number of photons
increase, the primary effects of
increasing kV are the increased
penetration of the beam with concurrent
reduction in energy absorbed and
increased scatter. This reduction in the
energy absorbed by bone and tissue
reduces the contrast between them and
results in lower image contrast.
Radiation Safety – Protection of Personnel
Radiation risks should be minimized by
utilizing techniques and procedures that
keep exposure to a level as low as
reasonably achievable (ALARA).
Patient dose is minimized through the
use of tightly collimated x-ray fields, such
that only the organ(s) of interest are
irradiated.
Personnel shielding, such as lead
aprons, thyroid shields and eye
protection is designed to effectively
attenuate scatter X-ray levels, not
primary beam exposures. As the X-rays
are scattered they undergo loss of
energy and penetration ability. A 0.5-mm
lead apron is approximately equivalent to
two half-value layers for the scatter
radiation associated with a 100-kV beam.
The half-value layer is the thickness of a
given material that reduces the intensity
of the radiation to 50%. The
effectiveness of attenuation decreases
with increasing kV.
The technique factors for the examination
(such as KV, MA, exposure time) are
optimized to ensure optimum image
quality without excessive dose. In
addition, the receptor material is
designed so that as many of the exit
photons as possible are captured and
contribute to the image. For example, if
only 50% of the photons are captured
and contribute to the image, the patient
entrance exposure (and hence the dose)
would need to be doubled for the same
image quality.