Physics of X-rays
Particles of Energy
When an electron is submitted to an
electrical field, it is accelerated and gets
a speed (an energy) proportional to the
applied voltage.
This energy is expressed in "eV," or
electron volt. When this electron
reaches the positive electrode, it has a
kinetic energy of 1000 eV or 1 KeV.
When speaking of particles, the unit of energy is the
"electron volt". This unit of measure is used for
Electromagnetic Radiation
X-ray is in the short wavelength range of the electromagnetic spectrum.
X-rays are of the same nature as radio waves and visible
light, but at a wavelength far beyond ultraviolet, and are
therefore not visible to human eyes. Wavelength
(angstroms) and frequency (hertz) are not convenient units
of measure for x-ray. For the x-ray spectrum, we use
Electro-magnetic Energy Spectrum
electron volts to measure the energy of photons.
X-rays and Interaction With Matter
X-rays that originate from the source, before they hit the body, are referred to as
incident beams.
When photons interact with materials, they can be:
1



Absorbed: they disappear, transferring their energy to the material
Scattered: they are deviated, with or without loss of energy
Transmitted: no interaction
Matter is composed of atoms, which are composed of elementary particles:
protons, neutrons and electrons. A relatively large space exists between the
electrons and the other particles. A single photon can pass through this cloud of
particles without touching any of them.
A hit, or a succession of hits, will deviate the photons, and may block them. The
more dense the material, the more dense the cloud of particles, and the more
these interactions can occur.
Absorption, Transmission and Scatter
Absorption increases with the density of the material. The material is what we
want to assess, but the absorption may cause biological damage.
Transmission is what we can see photographically, and the transmitted part of
the beam is used to assess the density of the various parts of the body. Highdensity tissue (bones) result in low transmission. Transmitted x-rays are also
used to assess the amount of radiation absorbed.
Scatter is undesirable. It interferes with the measurement of the transmitted
radiation and reduces the quality of the image. Scattered radiation is filtered out
whenever possible.
In x-ray imaging, we measure the transmitted part of the beam to assess the
density of the various parts of the body. Scattered radiation is filtered using a
grid. Grids are described in the Radiographic Components Module.
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Producing an Image
The x-ray image displays the part of the x-ray beam not absorbed by the tissues.
The lower the absorption, the darker the film. The radiograph displays differences
in tissue density.
The image to the right is the result on radiographic film. A radiographic film, once
developed, is like the negative film of a photo camera:


Low-density areas, like the lungs, do not block the beam as much as
high-density areas
The high level of transmitted radiation darkens the film
High-density bones stop the beam, and the film remains white
Scintillation and Photo-electric Effect
Some materials have specific reactions when they
receive photons. These materials are used in various
types of intensifying screens and x-ray detectors.
Screens will be discussed in a later section.
Scintillation effect (Fluorescent Screens)
A high-energy photon is converted into many lower
energy photons (visible light).
Photo-electric effect (Digital Image Detectors)
The photon energy is transferred to many lower energy
electrons, which are ejected from the material.
Ionized atom: positive ion
Ionization Effect
At the high energy levels of radiography, photons
are able to ionize matter: adding or removing an
electron to/from the atom.
At rest, the atoms of a material are electrically
3
neutral: as many electrons (negative) as protons in
the nucleus (positive). A high-energy photon, when
hitting an electron, may hit and eject an electron
from the atom and leave it in an unstable state
with a positive total charge.
This process, ionization, offers a way to detect
and
measure the x-rays quantity and strength. But it is
also responsible for potential biological damage.
Units of Measure
This topic explores the units used to measure
radiation. Using the ionization effect, an
Ionization Chamber measures the amount of
radiation passing through it.
By applying a voltage between two electrodes:



Electrons are removed from the atoms of
the gas by a photon, and are attracted by
the positive electrode.
Other electrons coming from the negative
electrode replace these electrons.
A measurable current results,
proportional to the number of incoming
photons.
To see an animation of the ionization chamber,
click on see animation.
Biological Damage
The probability of occurrence of cancer
increases with the absorbed dose of x-rays,
but severity of the effect is not dose-dependent.
Ionized atoms behave differently in living cells.
Ionization may cause damage to the DNA
chains, leading to cell malfunction (cancer) or
death.
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Even low doses involved in diagnostic imaging
present a risk. The higher the dose absorbed, the
higher the risk to develop a cancer or induce
genetic damage. Radiographic imaging presents
a low patient risk. However, the benefits expected
for the patient from an x-ray exam must always
be balanced against this risk.
Radiation Units: Output Intensity
Roentgen (R)
The measurement unit Röentgen is used to
express the intensity of a beam at a given point
in space. There is a direct relation between mAs
and R for any given equipment. mAs is tube
current (mA) multiplied by time in seconds (s).


Used to express output (exposure) of an
x-ray device.
Quantity of ionization (electrical
charge) produced in a unit mass of air.
Inverse Square Law
The intensity of an x-ray beam is inversely
proportional to the square of the distance from
the x-ray source.
In the example to the right, the distance was
doubled (x2) from the tube in the previous
screen. Invert the two, square it, and it indicates
that there is one quarter the intensity, at twice
the distance. One quarter the intensity results in
5mR of exposure.
Absorbed dose is dangerous and cumulative.
Distance is your friend.
Radiation Units: Absorbed Dose
5
In your job as a service engineer, a measurement device (film badge) is used to
determine your exposure to x-ray. But you must wear it to have it work! The
amount of absorbed radiation is measured in RADs or REMs.
RAD (Radiation Absorbed Dose)


Quantity of energy absorbed, per kilogram, in a unit mass of material
Used to express the dose received by an object
REM (Radiation Equivalent Man)




Absorbed dose equivalent, regarding biological effects
Biological effects depend on the type of ionizing radiation
For x-rays, 1 REM = 1 RAD
Applies only to body tissues, used to express the dose received by patients
and radiation workers
Surface Dose
Due to attenuation (absorption and scatter)
inside the body, the maximum absorbed
dose is measured at the skin level.
You will hear the tems surface dose, skin
dose, patient dose or entrance dose.
These terms all refer to the absorbed dose
measured at the skin level.
Radiation Unit Standards
Two standards exist for the measurement of radiation: European SI units
(System International) and the old, conventional standard (RAD/REM). Below
is a table showing the conversion.
SI units
Conventional
Absorbed dose
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1 Gray (Gy) = 1
J/Kg
1 Sievert (Sv)
=
100 RAD or 1cGy (centigray) = 1 Rad
=
100 REM or 1mSv (millisievert) = 100 mRem
Exposure
Exposure is also expressed in Grays. It represents the amount of radiation
necessary to deliver an absorbed dose of 1 Gray.
1 Gray (exposure) =
115 Röentgen
Examples of Typical Exposure
Chest exam
Skull exam
Abdomen exam
Average natural radiation (per
year)
Intra-Venous Urography (IVU)
CT abdomen exam
2 mREM
7 mREM
100 mREM
0.02 mSv
0.07 mSv
1 mSv
200 mREM
2 mSv
250 mREM
1000 mREM
2.5 mSv
10 mSv
We are all exposed to ionizing radiation from sun, cosmic rays and some natural
elements existing on earth (e.g. radon). One chest film is 100 times less than the
natural radiation received per year. To see a list of radiation amounts and its
impact on the body, click on body effects.
Review
X-ray is electromagnetic radiation, with a spectrum far beyond ultraviolet light,
with higher energy, making it able to penetrate matter.
An x-ray image reflects the difference in transmission through the different
tissues and bone.
X-ray is an ionizing radiation:


The ionizing property of x-ray is used to detect and measure x-ray
beams.
It is also responsible for biological damage.
The output of an x-ray tube is measured in Röentgen (conventional) or in Gray
(SI units).
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The absorbed dose is expressed in RAD or REM (conventional) or in Gray or
Sievert (SI units)
Simplified X-ray Tube
This topic describes the basic theory of x-ray tube
operation, and the impact of changes in x-ray tube
variables. A tube has several major characteristics:





A current (typically 3-5 amps) passes through
the filament and heats it. ·
The heated filament of the cathode produces a
cloud of free electrons. The temperature of the
filament determines the quantity of free
electrons.
These electrons are attracted to and accelerated
toward the target anode by the electrical field
(kV).
The flow of electrons is an electrical current
(mA).
The electrons, upon hitting the target (anode),
are converted to heat (99%) and x-rays (1%).
To see an animation of the similarities between
electrodes and x-ray tubes, click on see animation.
X-ray Variation with kV
Variation in the kV of a tube has two major effects on
the x-ray spectrum:


As kV increases, photon energy increases
kV has a significant effect on the relative
amplitude of characteristic radiation
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High-energy (60-120 kV) x-rays penetrate better than
low energy, and are used for dense or thick parts of
the body. Low-energy (20-40 kV) x-rays are used for
soft tissue.
Low-Energy Filtration
Low-energy x-rays are harmful to the patient and not
useful for imaging. They are filtered out by passing
the beam through a sheet of material (normally
aluminum for a tungsten anode).
Any material in the x-ray beam path absorbs more of
the low-energy radiation (harmful for the patient)
than the high-energy radiation.
Film and screenOverview
The Film and Intensifying Screens section gives you information about how xrays are converted to viewable images. Today, the most common way to record
a radiographic image is to use film. Tomorrow, it may be through the use of a
digital detector and softcopy read from a review station. Until film is replaced, it is
important to understand how an image is created, and the variables that affect
how an image is recorded.
After completing this Section, you should be able to:




Explain how x-ray images are transferred to film
Identify the purpose of fluorescent intensifying screens and describe how
the screen converts x-rays to light
Describe the composition of film
Describe the characteristics of film and the effects of film processing
This is the last section of the X-ray Principles module.
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Introduction: X-rays to Light
The most common radiographic recording device is film. This topic describes how images are
recorded, and some of the variables affecting image recording. Film is sensitive to light, therefore
requires a light-proof container (or cassette). X-rays have little effect on film.
Film is not a good medium for recording x-rays directly;
only 2-3% of x-rays are absorbed by the film.
What then, is the best way to create an image on the film?
One way to produce an acceptable x-ray image on film is to convert x-rays to light. Since film is
more sensitive to
light than x-rays, fewer x-rays are required to produce
a visible image.
Fluorescent Intensifying Screens
This topic describes the conversion of x-rays to light, and introduces concepts and
components important to evaluating and servicing x-ray equipment.
Fluorescent Intensifying Screens were developed to reduce patient exposure to xrays. Because film is more sensitive to light than x-rays, converting x-rays to light
permits less x-ray dosage to create an acceptable film image. Intensifying screens use
the scintillation effect to convert x-rays to visible light.
Fluorescent Intensifying Screen Because the
screen intensifies the x-ray beam to make the
10
film image, it is often just referred to as an Intensifying Screen.
Scintillation effect:


A high-energy photon (x-ray) is converted into many low-energy photons (visible
light)
Phosphor, Cesium Iodide, and Selenium are good scintillation materials
Screen Location
Intensifying screens are mounted in a padded cassette
that allows the film to be placed between the screens. A
soft padding placed between the cassette and the
screen assures a tight seal. It also allows for proper filmto-screen contact.
Note that the screen is curved. This insures the proper
contact between the screen and film, and squeezes air
out of the cassette when closed. If the screen is not in
contact with the film, image detail will be lost.
It is important that intensifying screens match film
sensitivity to the same light frequency.
Screen Construction
Intensifying Screens have the following layers (from top
to bottom):




External protective coating
Bonded phosphor crystals
A thin layer of reflecting material
Plastic base support

Calcium Tungstate emits blue light photons.
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The phosphor layer can be made
from a variety of materials
depending on the desired
effect:

Rare Earth emits green light photons.
Light Imaging
X-ray film images are produced primarily by the
visible light from the intensifying screen.
To see an animation of x-ray photons converted
to blue light.
Intensifying Screen Efficiency
Intensifying screen efficiency is measured in two ways:

Intrinsic Efficiency: The ability of the fluorescent material to convert x-ray
photons to light photons.
o Calcium Tungstate (blue light) about 5%
o Rare Earth (green light) about 20%
For example, a 51keV x-ray photon striking a Calcium Tungstate screen
will emit blue light photons with an energy of 3eV. If the screen is 100%
efficient, one x-ray photon will produce 17,000 blue light photons
(51,000/3=17,000). Since Calcium Tungstate is 5% efficient, only 850
photons are produced.

Screen Efficiency: The ability of the light created by the screen to reach
the film. Typically, only about 50% of light photons produced reach the
film.
The next screen shows the effect of increasing x-ray energy on the output of light
photons.
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kV and the Intensifying Screen
As the x-ray tube kV increases, the amount of light from
the screen increases. This concept will be used when
we discuss Automatic Exposure Control (AEC) in the
Components Module.
To see an animation of the effect of increased kV on blue
light output.
This graphic shows that if the x-ray keV is increased (by
increasing kV ), more light is produced. For example,
increasing keV from 51 to 90, light photon output
increases from 17,000 to 30,000.
Screen Speed
The "speed" of an intensifying screen is determined
by the thickness of the fluorescent phosphor layer.
Fast Screen
Has a relatively thick phosphor layer with:




Increased image brightness
Decreased image resolution
Reduced image blurring (caused by patient
movement)
Shorter exposures and reduced patient dose
Slow Screen
Has a relatively thin phosphor layer with:
 Decreased image brightness
 Increased image resolution
 Longer exposures and increased patient dose
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A slow screen is used when detailed
imaging is needed (e.g., for the fine
bones of a hand.)
Radiographic components
Introduction
The Radiographic Components module provides training on the purpose and operation
of all major components of a radiographic system. This training provides fundamental
knowledge for service personnel working with x-ray systems.
At the end of this module, you should be able to:
 Identify the major components and characteristics of an x-ray tube
 Describe the purpose and different designs of generators
 Describe the functions of the bucky assembly and the collimator
 Identify the different patient positioners
 Describe tomography and how it differs from conventional x-ray
Start by selecting the first section tab, Overview. Then, select the first topic.
Proceed through each section and topic. Then, take the test.
Overview
The X-ray Tube section gives you information about the design and operation of
an x-ray tube. The tube is the single most important component of an x-ray
system. As a service engineer, you will be required to install, troubleshoot, and
verify the proper operation of x-ray tubes. The content in this section is essential
introductory and background material to perform those functions.
After completing this section, you should be able to:





Identify basic x-ray tube components and describe their functions
Describe focal spot characteristics and effects
Explain techniques for tube cooling
Interpret a tube rating chart
Describe the difference between continuous and pulsed tube operating
modes
In the next section, you will learn about x-ray generators and how they power and
control x-ray tube operation.
14
X-ray Tube Components
Introduction
The tube is the key component in an x-ray system.
To understand the production of x-rays in an x-ray
tube, let's look first at an overview of the x-ray
system.
An x-ray system consists of six primary
components:





X-ray Tube and Hanger
X-ray Generator
Collimator
Chest Stand
Table
Major Components
The x-ray tube is composed of the tube casing
(housing) and its insert.
The tube insert consists of the:



Frame
Cathode
Anode
To see the major components of the tube
insert,
Tube Housing
The x-ray tube insert is mounted inside a
protective housing. The housing is made of
aluminum and lined with lead. The housing:



Controls scatter and leakage radiation
Isolates high voltage
Cools the tube
Frame
The outer part of the insert (sometimes called the enclosure or frame) can be
made of glass, steel, or copper. The enclosure:
15



Provides a structural base for the cathode and anode
Provides high-voltage insulation
Allows a vacuum to be maintained
Cathode
The cathode provides the electron stream
for x-ray production. The cathode consists
of:

Filaments, which are coils of
tungsten wire that, when heated,
provide the electron stream for x-ray
production
A focusing cup, which surrounds
the filament and concentrates, or
focuses, the electron beam

Dual Focus Cup
Some focusing cups have two filaments.


The large filament is used for higher power levels
needed for dense anatomy. (High mA, high kV, or
both)
The small filament is used for lower dosage levels,
which provides better resolution.
Focus and focal spot will be discussed in the next topic,
Focal Spot Characteristics.
Anode
X-rays are produced by energy conversion
when electrons strike the anode. The anode
assembly is electrically positive and consists
of 3 major components:
16



A target, which receives a stream of
electrons from the cathode
A rotor of an induction motor, which
holds the target and drives its rotation
at a rate of approximately 10,000 rpm.
This rotation dissipates the heat
generated during the production of xray
A bearing assembly, which supports
the rotor and target assembly, and
provides smooth rotation of the target
X-ray Generation
The process by which the x-ray tube produces xrays is very inefficient. Only one percent of the
energy produced is converted to x-rays; the rest is
dissipated as heat.
To see an animation and hear a general overview
of x-ray generation,.
To see a text-only explanation,
Focal Spot Characteristics
This topic describes how an electron beam from an
x-ray tube is controlled to produce the sharpest
possible x-ray image . For the service engineer, this
information is important to help resolve problems
with image quality.
Central Ray
The primary x-ray beam, leaving the tube target
surface at 90 degrees, is called the central ray. It
does not necessarily pass through the point of
maximum beam intensity.
The central ray is often used in radiographic
applications to align the x-ray system with the
anatomical area of interest.
17
Actual Focal Spot
The electron beam from the tube filament lands on the
angled target surface and creates the focal spot. This
area, bombarded by a stream of electrons, is not a
pinpoint source of energy, but a rectangular area called
the actual focal spot.
The actual focal spot is the area on the anode target
where electrons strike, and where x-rays and heat are
produced.
Effective Focal Spot
The effective focal spot, or effective focal area, is the
area from which the x-ray beam appears to come.
Viewed from the perspective of the central ray, the
effective focal area is reduced to a square. See the
graphic to the right. The x-ray tube's effective focal spot
is commonly called the focal spot.
Focal Spot Size and Shape
The size and shape of the actual focal spot are determined
by the:
· Length of the tube filament · Diameter of the filament wire coil
· Width and length of the focusing cup slot · Depth of the
focusing cup
The effective focal spot size is determined by viewing the
target at a 90 degree angle from the anode target. The x-ray
beam energy leaving the target surface is projected through the
tube's glass window , and then through a collimator positioned
to aim the beam at the anatomical area of interest. Collimators,
and other system components, will be discussed in later
sections.
As a service engineer, you may be required to measure the
focal spot size. The next screen describes how you will do this.
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Measuring Focal Spot Size
As a service engineer, you will use a star pattern to
determine focal spot size. This is used most frequently on
mammography and cardiovascular systems.
Star Pattern
The star test pattern is positioned about midway between
the focal spot and the film, parallel to the film, and in the
central ray of the beam. An exposure is made using kV and
mA comparable to that used clinically.
After developing the film, the service engineer then makes
some measurements from the x-ray image and puts those
measurements in a formula that will indicate focal spot size.
(See Job Card XPM-910, contained in Class C
documentation.)
Another tool for measuring effective focal spot size is a pin-hole camera. This
method, not commonly used today, produces an image of the focal spot on a
piece of x-ray film.
Anode Heel Effect
In an x-ray tube, x-ray energy is distributed
away from the target surface with an uneven
intensity. The distribution of x-ray beam
intensity is greatest on the side of the target
closest to the filament. This phenomenon is
called the anode heel effect.
This variation in beam intensity will cause a
detectable variation in film density. This is also
called "cut-off," since the image may be cut off
as the x-ray intensity decreases.
The x-ray technologist must carefully consider
the heel effect when positioning the patient for
an exposure. The thickest section of the patient
should be positioned on the cathode side of the
target.
19
The central beam is used as a dividing point of the
target surface. The side closest to the filament is called
the "cathode side." The side furthest from the filament is
called the "anode side."
Penumbra
Radiographic images are compiled from x-ray beams arriving at different angles.
A single point source of x-rays would create sharp images, similar to the images
of a slide projector or other optical system. But multiple beams striking a
rectangular area create multiple images, or shadows, of that focal spot area.
Unfortunately, unlike visible light, x-rays cannot be focused. Multiple beams of
energy from the focal spot create multiple shadows or images. These images,
around the outer edge of an object, are slightly out of focus. This out-of-focus
"shadow" around the outer edge is referred to as the penumbra. The penumbra
is visible as a lack of sharpness in the filmed x-ray image.
To distinguish it from other causes (movement blur, for example) the effect of
penumbra is termed geometric unsharpness.
Tube Heating and Cooling
This topic describes the effects of heat on an x-ray tube, and the process of
limiting and measuring those effects. As a service engineer, you need to be able
to evaluate the consequences of heat on the tube, and also understand tube
specifications and heat ratings.
Tube Heating
As discussed in the Principles Module of this training course, a small number of
the electrons that bombard the target anode have their energy transferred into xrays. The majority of the energy transfer results in heat.
Results of heat generation in a tube include:
20



Reduction of the filament wire diameter, as it is heated repeatedly over
time
Softening the rotor bearing surfaces, which can lead to premature
bearing failure and bearing noise
Etching on the anode target surface, resulting in an imperfect focal spot
Tube and Housing Cooling Charts
Both the tube anode and the tube housing have cooling
charts. These charts show the rate at which the tube (or
housing) loses heat.
The tube cooling curve (chart) is tracked by software so
that the maximum heat storage capacity of the anode is not
exceeded.
Older, non-computer-controlled systems, included electronic
circuitry to prevent exceeding the tube's maximum heat rating
for any given exposure. A transducer "heat sensor" was also
included in the tube unit's housing. Its purpose was to open
the x-ray exposure start circuit when the heat limit had been
reached.
Types of Cooling
In order to maintain the efficiency, and life, of an x-ray tube, it
is important to limit the temperature rise that occurs with xray production. There are four methods of cooling an x-ray
tube unit:



Natural air convection flowing across the tube unit
(low duty cycle applications)
Forced-air cooling fan attached to the tube unit
(radiography and R & F rooms)
Oil circulator pump with a heat exchanger attached
to a forced-air cooling fan (high-duty-cycle
21
Forced Air Cooled Tube
fluoroscopic rooms, vascular, angio, and CT systems)
. Oil circulator pump attached to a water-cooled heat
exchanger (highest duty cycle for vascular and angio
Pulsed Tube Operating Mode
Exposure Switching
There are two methods of switching tube current on and off to the xray tube:


Switching the primary circuit of the high-voltage
transformer causes the secondary anode and cathode
voltages to rise and fall between 1 to 12 times per second.
(Generators and transformers will be discussed in the next
section.)
Switching the beam current on and off using a bias power
supply in the secondary circuit.
Primary Switching
Primary switching typically uses solid-state semiconductors called
silicon-controlled rectifiers (SCR). Primary switching works well for
exposure rates of 1 to 12 exposures per second. Above that rate,
efficiency decreases.
Continuous Tube Operating Mode
Two methods are used to control the production of x-rays:


Continuous
Pulsed (discussed in the next topic )
In order to troubleshoot and effectively isolate x-ray system problems, you should have a
22
fundamental understanding of these two processes. Now, let's discuss the continuous operating
mode.
Prep Signal
In a Continuous system, the sequence of events leading to the production of x-rays begins with a
"prep signal" to the x-ray generator. The operator moves the hand switch to the "prep" position,
which is the first switch position (the first "click"). Generators will be covered in detail in the next
section.
Review (tube)







The tube is the key component in any x-ray system
The major components of the tube insert include:
o Frame
o Cathode
o Anode
The central ray is often used in radiographic applications when aligning the x-ray system wi
the anatomical area of interest
The actual focal spot is the area on the anode target where electrons strike, and where xrays and heat are produced
The anode heat storage capacity is expressed as Heat Units (HU)
An x-ray tube's kW rating is a specification of the maximum safe combinations of kV, mA an
exposure time which may be used without thermal damage to the tube
Continuous and grid-pulsed methods are used to control the production of x-rays
23
Generator
Purpose
The x-ray generator supplies DC power to the x-ray tube. The generator is comprised of several
major assemblies:




Power distribution unit (PDU)
Step-up transformer
Operator console
Exposure control circuitry
The main functions of the x-ray generator are:



Control kV applied to the x-ray tube, thereby controlling x-ray beam penetration
Control the amount of mA in the tube, thereby controlling the quantity of x-ray beams
Control exposure time and switching
24
kV Settings
Based on patient size and anatomy, kV selection ranges from 40 to 150 kV.
mA Settings
Depending on the x-rayed anatomy, mA selection ranges from 10 to 1000 mA.
Time Settings
Depending on patient anatomy, exposure time selection ranges from 1 ms to 10 seconds.
Using the automatic exposure control circuitry (AEC), time can also be controlled automatically.
AEC will be discussed in the next section.
Generator Control Panel
Types of Generators
Generators use different methods of creating high-voltage (kV). This topic will help you understand
how kV output affects the quality of radiation produced by the x-ray system.
The three main types of generators are:



Single phase
Three phase
High frequency
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Let's look at the kV waveforms produced by a single phase generator.
Single Phase
Generators use different methods of creating high-voltage output. Let's look at the simplest, and lea
expensive.
A single phase generator uses single phase, 50/60 Hz, AC input. This AC voltage, from the facilit
mains power, ranges from 220 to 240 volts. Feeding the sine wave input to a specific lead (tap) on
the step-up transformer provides the desired kV output.
The stepped up voltage, now kilovoltage, is full-wave rectified and fed to the anode and cathode
the tube. Notice how much the tube input voltage varies (ripples).
Three Phase
This type of generator uses three-phase power to produce a more uniform kV output, with
much less ripple than single phase. Three phase generators are classified as either 6 pulse or
12 pulse output.
Six Pulse
A three phase generator is supplied with three AC inputs. Note on the graphic that each input
is out of phase with the other inputs.
Each input is fed to a "wye" step-up transformer. The wye transformer steps up the three
AC inputs. The rectifier circuit rectifies and combines the three inputs.
The result is an input to the tube that has six peaks, or pulses. Note the significant reduction in
ripple (from 100% to 13%) compared with the single phase system.
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Three Phase
Twelve pulse
The 12 pulse kV method is an improvement over 6 pulse
rectification. A combination wye-delta step-up transformer is used to double the six pulse input to 1
pulses.
This method exhibits less ripple, and further reduces the patient exposure to harmful radiation.
High Frequency (HF)
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The most recent x-ray generator design is capable of producing a kV output
virtually free of ripple. Three phase AC power is converted to DC and applied to
an inverter/chopper circuit.
The inverter converts the DC voltage back to AC at frequencies of 20 kHz and
higher. These higher frequencies are fed into a step-up transformer and rectified
just like a single phase generator. The resulting DC signal is then filtered to
further reduce ripple, and then applied to the anode and cathode of the tube.
The graphic shows the kV output as a flat DC voltage. The radiation output is
uniform, and contains minimal harmful, low-energy, radiation. HF generators are
smaller and lighter than single and three phase units
Comparison of Generator Types


The 100 kV setting in all three illustrations results in a different average kV
for each type of generator.
Because the average kV is lower for the single phase generator, it is
necessary to make an exposure twice as long (compared with the 12
pulse) to produce the same film density.
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
The shorter the exposure time, the less patient motion is a factor. In
addition, patient exposure to harmful low-energy radiation is reduced..
6 and 12 pulse generators are found on older systems. HF generators have been
replacing them since
1990. HF generators are used on all newer
systems.Battery powered models are found in all mobile units. Capacitive
discharge generators may be found in older installations, or military field
applications.
Battery powered
Battery powered generators are used primarily in mobile x-ray equipment. They
are high frequency generators using rechargeable batteries as a power source.
The batteries can be:


Nickel Cadmium
Lead-acid
Batteries not only provide cordless operation, but also considerably reduce
electrical installation requirements, since the electrical system is used only to
recharge the batteries.
The main characteristics of a battery powered generator are:




Cordless operation
Same performance as high frequency generators, but lower power output
A standard wall outlet is the only electrical requirement for recharging the
batteries
Used for mobile systems
Capacitor Discharge
Capacitor discharge generators work on the same principle as battery
powered generators, except the battery is replaced by a capacitor. Capacitors
have a much smaller capacity than batteries, and their voltage decreases
constantly while discharging.
Here are some characteristics of the capacitor discharge generator:
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




Smaller and less expensive than battery generators
No rapid succession exposures, because time is needed to recharge the
capacitor after an exposure
A wait time indicator tells when the system is ready for a new exposure
Can be operated from simple electrical outlets, ideal for mobile systems
Low power output capability: for example, 50 mAs @ 100 kV
BUCKY
Purpose
A bucky provides the housing for three assemblies:



Cassette tray
Grid
Automatic exposure control
A bucky can be in a horizontal (table) or vertical (chest stand) orientation.
Let's look first at the cassette tray, which holds and centers the film cassette.
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Cassette Tray
The cassette tray is a mechanical component of the x-ray system. Since
operators often have a difficult time with mechanics, it is important for you to
understand its operation and be able to troubleshoot basic mechanical problems.
Purpose
A cassette tray:
Holds the cassette

Centers the cassette within the tray
In order to follow the x-ray tube movements, the cassette tray is positioned in a
track below the tabletop.
Grid: Purpose
In 1913, Gustav Bucky introduced the radiographic grid, a device placed between
the patient and the cassette. The purpose of the grid is to:


Reduce the amount of scatter radiation reaching the film
Increase contrast in the x-ray image
Scatter radiation is a noise factor. It causes a fogging which impairs radiographic
quality, lessening image contrast. Scatter provides no useful information. Refer to
the X-ray Principles training module for more about scatter.
Grid Design
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A radiographic grid is composed of strips of lead, separated by aluminum, a
material that is relatively transparent to x-rays. The graphic shows how grid
removes scatter radiation.
Because the scatter radiation, represented by dotted lines, moves in various
directions, it is largely absorbed by the lead strips.
The transmitted x-rays pass untouched through the grid and reach the film.
Stationary and Moving Grids
The grid can be stationary, or it can move during the xray exposure.
Stationary Grid
Used in a stationary position, a grid may cast a pattern
of strips onto the x-ray image. They appear as a series
of fine, parallel, unexposed lines.
Moving Grid
To eliminate grid lines from an image, the grid can be
moved transversely during an exposure. Bucky grid
movement is usually a simple side-to-side oscillation.
Movement begins before the start of an exposure.
Moving the grid:
 Blurs the grid line images
 Removes scatter
 Has no effect on the primary beam transmission
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Grid Specifications
These grid characteristics impact x-ray quality image: amount of radiation
exposure to the patient, and ease and range of use of the grid for the
technologist. When troubleshooting image quality problems, you should
understand the significance of some grid specs.
Grid Ratio
The grid ratio is the relationship between the height of the lead strips to the
distance between them. In general, the higher the ratio of the grid:




The more scatter radiation is absorbed
The greater the contrast in the x-ray image
The higher the radiation exposure to the patient
The greater the necessity to accurately position the grid under the tube
Grid ratios in the 8-12 range are used most frequently.
Grid Frequency
Another important factor when considering grid specifications is the fineness, or
frequency, of the grid. This is the number of lead strips (or lines) per cm (or per
inch).
In general, a higher grid frequency results in:


Less visible grid lines on the x-ray image
Thinner grid line material
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A collimator is an x-ray beam restrictor. It is attached to the opening in the x-ray
tube housing. The collimator:




Controls the dimensions of the x-ray beam
Aligns the x-ray beam, the patient, and the bucky prior to exposure
Limits patient and technician exposure to radiation
Reduces scatter radiation
The next screen describes x-ray beam control.
Grid Cutoff
Grid cutoff is the result of poor alignment between the primary x-ray beam and
the lead strips of the grid. The resulting image is light in the area where the
cutoff occurs. The next several screens explore the different types of grid cutoff.
Parallel Grid Cutoff
As the graphic to the right demonstrates:


The cutoff of primary radiation results in progressively decreased density
toward the edges of the film
The lead strips in the grid make shadows proportionate to their distance
from the central beam
Positioning the tube too close to the grid may cause parallel grid cutoff. Let's look
at a comparable problem with a focused grid.
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Off-Center Cutoff
When the grid is not centered on the central beam of the tube, there is a uniform
loss of radiation over the entire surface of the grid.
The focal distance is correct, but the tube is mis-positioned relative to the
convergent line of the grid, as shown in the graphic. The results are:


All the lead strips are out of correct alignment with the primary x-ray
beam
The entire film shows a lighter density than it would with proper alignment
This is probably the most common kind of grid cutoff. The problem is usually
attributed to incorrect exposure settings. The loss of radiation due to off-center
cutoff can be minimized with low-ratio grids and long focal distance.
Operation of AEC
All three detector circuits output a signal, which is input to the AEC circuit. The AEC
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circuit includes the following stages:
AEC circuit
1. The integrator "adds" the output from the detector.
2. The comparator compares the integrator output to a voltage reference that equals a desired
film density.
3. When the sum of the input voltage equals the reference voltage, the comparator creates
4. an "exposure stop" signal, which is fed back to the generator.
The reference voltage compensates for variations in: kV, time, cassette size.
Examples of AEC
AEC circuits can be used as entrance or exit detectors. An entrance detector is located above
the x-ray film, and an exit detector is located below the film.
Ion Chamber (table and chest stand)
The ion chamber illustrated to the right is installed under the grid in the bucky assembly. The
operator selects one or more of three ionization chambers to detect radiation exposure. The
primary purpose of an ion chamber is to convert transmitted radiation to an electrical signal.
The technologist must position the anatomical area of interest over the active ion chamber in
order to produce the correct exposure. Next, we will discuss how the chamber is positioned
Ion Chamber
The ion chamber in the wall stand contains three sensing areas. The Illustration shows the
location of the three sensing areas:



Sensing area 2 is at the center of the x-ray beam.
Sensing areas 1 and 3 can be selected to cover an exposure of two symmetrical parts
of the body, such as the lungs or the kidneys.
Care should be taken to center the patient and the detector areas.
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PMT
A PMT (photomultiplier tube) is typically used in mammography. It is centered
underneath the breast.
When stimulated by x-ray, the fluorescent intensifying screen produces light. The
light is converted to an electrical signal by the photomultiplier tube. The electrical
signal is proportional to the light emission from the screen, which is proportional
to the x-ray film density.
The photomultiplier tube is connected to the AEC circuit, which terminates the
exposure when the proper film density has been reached.
Silicon Cell (Digital Detector)
The silicon cell converts x-ray to light, and light to an electrical signal. The signal
outputs to a microprocessor, which controls the AEC.
A digital detector includes the following detector components:



A scintillator that converts x-ray to light
An array of photocells that convert light to an electrical signal
An amplifier that converts the electrical signal to a digital output
Review



A cassette tray holds the cassette and centers the cassette within the
tray.
The x-ray tube unit must be centered accurately with the center of the
bucky.
A grid reduces the amount of scatter radiation reaching the x-ray film.
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



The most common method of eliminating grid lines from an image is to
move the grid during an exposure.
Two important grid specifications are:
o Grid ratio: the relationship between the height of the lead strips
and the distance between them
o Grid frequency: the number of lead strips (or lines) per cm (or per
inch)
Grid cutoff is the result of poor alignment between the primary x-ray
beam and the lead strips of the grid.
Two advantages of using AEC:
o Limited patient exposure to radiation
o Fewer operator mistakes, resulting in fewer re-takes of the x-ray
exam.
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