X Ray Production

PHYSICS OF MEDICAL IMAGING
1
X-ray Production, X-ray Tubes and Generators
MUHAMMED ANEES.K
Resident Medical Physicist

5/6/2015 Footer Text 2
The study and use of ionizing radiation in medicine started with three important
discoveries
 X rays by Wilhelm Roentgen in 1895.
 Natural radioactivity by Henri Becquerel in 1896.
 Radium-226 by Pierre and Marie Curie in 1898.
INTRODUCTION

Roentgen discovered x rays in 1895 while experimenting
with a Crookes “cold cathode” tube.
• Crookes tube is a sealed glass cylinder with two embedded
electrodes operated with rarefied gas.
• The potential difference between the two electrodes produces
discharge in the rarefied gas causing ionization of gas molecules.
• Electrons (cathode rays) are accelerated toward the positive
electrode producing x rays upon striking it.
A bit of History
Photograph of Roentgen’s apparatus

5/6/2015 4
William Roentgen discovered X-rays in 1895 &
determined they had the following properties
1. Travel in straight lines
2. Are exponentially absorbed in matter with the
exponent proportional to the mass of the
absorbing material
3. Darken photographic plates
4. Make shadows of absorbing material on
photosensitive paper
• Roentgen was awarded the Nobel Prize in 1901
• Debate over the wave vs. particle nature of X-
rays led the development of relativity and
quantum mechanics
A bit of History

The first x-ray photograph:
Roentgen’s wife Bertha’s hand

X-rays are one of the
main diagnostical tools
in medicine since its
discovery by
Wilhelm Roentgen in
1895
Current estimates show
that there are approximately
650 medical and dental X-ray
examinations per
1000 patients per year

“ Internal structures
of the body could be
made visible without
the necessity of surgery “

 To heat the filament
so that electrons are given off
 To connect the X-ray tube to a source of high voltage
Electrons are accelerated & convert their KE into X-rays
when impinge on the target of the X- ray tube
To produce X-rays

X-rays characteristics
• Highly penetrating, invisible rays
• Electrically neutral
• Travel in straight lines.
• Travel with the speed of light in vaccum:
300, 000 km/sec or 186, 400 miles/sec.
• Ionize matter by removing orbital electrons
• Induce fluorescense in some substances.
• Fluorescent screen glow after being stricken with photons.
• Can't be focused by lenses nor by collimators.

X-ray Imaging System
PRINCIPAL PARTS
 Operating Console
 High-voltage generator
 X-ray tube
PRIMARY FUNCTION
The system is designed to provide a large number of e- at cathode
with high kinetic energy focused to a small target at anode.

X-ray Tube Construction
Radiographic Equipment

 X-ray tube
 Main voltage supply
 High tension source
 Kilo-voltage control
 Filament circuit and milli-amperage control
 Timer
Features of X-ray equipment

Main supply Filament CKT including mA control
kV control kV indication
High Tension source
Exposure timing and switching
mA indication
X- Ray Tube
Block Diagram of X-ray equipment

X-ray Tube

How “X-rays” are created ?
 Power is sent to x-ray tube via cables
 mA is sent to filament on cathode side.
 Filament heats up – electrons are produced
 Positive voltage (kVp) is applied to anode
 Negative electrons are attracted across the tube to the positive anode.
 Electrons slow down and finally come to rest
 Electron beam is focused from the cathode to the anode target by the
focusing cup

Fundamentals of X ray Production
Bombardment of a thick target with energetic electrons
Electrons undergo a complex sequence of collisions & scattering proces
Which results in the production of bremsstrahlung and characteristic
radiation
 Bremsstrahlung Radiation
 Characteristic X-rays

18
X-ray Production
When Highly Energetic Electrons Interact with Matter &
Convert their Kinetic Energy into X-rays
X - rays

When the electrons from the cathode are accelerated at high voltage to the anode:
• 99% of the energy is dissipated as heat
(anode materials are selected to withstand the high temperatures they are able to withstand)
• 1% is given off as x-rays

 BREMSSTRAHLUNG RADIATION
 CHARACTERISTIC RADIATION
X-ray Production
Conversion of Electron Kinetic Energy into X-rays

 If an incoming free electron gets close to the nucleus of a target atom
 The strong electric field of the nucleus will attract the electron
 Changing direction & speed of the electron
 Electron looses energy which will be emitted as an X-ray photon
 Energy of this photon depend on the interaction between nucleus and
electron
 X-rays originating from this process are called bremsstrahlung.
Bremsstrahlung Radiation

A large voltage is applied between two electrodes in an evacuated envelope
The cathode is negatively charged and is the source of electrons
the anode is positively charged and is the target of electrons
These electrons travel from cathode to anode & they were accelerated
The electrical potential difference between these electrodes attain kinetic energy
Emits Electromagnetic radiation, The name given as bremsstrahlung

 A part or all of its energy is dissociated from it
 An electron may have one or more bremsstrahlung interactions
 Bremsstrahlung photon may have any energy up to the initial energy of the
electron
Bremsstrahlung Spectrum

Bremsstrahlung Radiation
Bremsstrahlung is a German word
"Strahlung" means “radiation”
"Bremse" means “brake”

The high energy electron can also cause an electron close to the nucleus in a
metal atom to be knocked out from its place.
This vacancy is filled by an electron further out from the nucleus.
The well defined difference in binding energy,
characteristic of the material, is emitted as a monoenergetic photon.
When detected this X-ray photon gives rise to a characteristic X-ray line in the
energy spectrum.
Characteristic X-rays

Characteristic X-rays
• Characteristic radiation are emitted at discrete energies
• hv = Ek-El
• Ek and El are the electron binding energies of the K shell and the L shell

 The bremsstrahlung spectrum originates in the x-ray target.
 The characteristic line spectra originate in the target and
in any attenuators placed into the x-ray beam.
A typical spectrum of a clinical x-ray beam consists of:
• Continuous bremsstrahlung spectrum
• Line spectra characteristic of the target material and superimposed onto the
continuous bremsstrahlung spectrum

X-ray spectra are composed of:
1. Continuous bremsstrahlung spectra
2. In most cases, discrete spectra peaks known as characteristic x-rays.
keV

E max
Bremsstrahlung xrays form a
continuous energy spectra. The
frequency distribution is continuous
and shows that the Bremsstrahlung
process produces more low energy
that higher energy x-rays. The average
energy is approximately 1/3 of the
Emax.

The Emax or the maximum energy of
the x-rays measured as (keV) is equal
to
voltage applied to the Xray tube
(kilovolt peak or kVp).
E max
For example:
An applied voltage of 70 kVp produces an x-ray spectra with Emax of
70 KeV and average energy of about 23 keV.
70KeV
23keV

Filtration
It is the removal of x-rays as the beam passes through a layer of material
Added FiltrationInherent filtration of the tube
Attenuation of the x-ray beam

Includes
the thickness (l to 2 mm) metal insert at the x-ray tube port.
Glass (Si02) and aluminum effectively attenuate all x-rays in
the spectrum below approximately 15 keV:
Inherent filtration
Dedicated mammography tubes,
require beryllium (Z = 4) to improve the transmission of low-energy x-rays

Added Filtration
 Added filters attenuate the low-energy x-rays in the spectrum
 That’s slow-energy x-rays are absorbed by the filters instead of the patient,
 The radiation dose is reduced.
 More uniform x-ray exposure to the detector
Other common filter materials copper and plastic (e.g., acry
Aluminum (Al) is the most common ‘Added Filter material ‘"Bow-tie“
filters are used in CT to reduce
dose to the periphery of the patient

Adjust the size and shape of the x-ray
field emerging from the tube port
Collimators
Adjustable parallel-opposed lead
shutters define the x-ray field
Collimation of the x-ray field is identified by the collimator's shadows

X-RAY GENERATOR FUNCTION

Electrical power available to a hospital up to about 480 V
20,000 V to 150,000 V needed for x-ray production
Transformers are principal components of x-ray generators
they convert low voltage into high voltage through a process called
Electromagnetic Induction

Induction of an electrical
current in a wire conductor coil
by a moving magnetic field.
The direction of the current is
dependent on the direction of
the magnetic field motion.
Creation of a magnetic field by
the current in a conducting coil.
The polarity and magnetic field
strength are directly dependent
on the amplitude and direction of
the current

x-ray generator circuit
Rectifier circuit (AC DC )
Rectifier circuits divert the flow of electrons in the high-voltage circuit
Direct current is established from the cathode to the anode in the x-ray
tube
Despite the alternating current and voltage produced by the
transformer

To avoid back-propagation, the placement of a diode of correct orientation in
the
high-voltage circuit allows electron flow during only .one half of the AC cycle
If an alternating voltage were applied directly to the x-ray tube, Electron back-
propagation could occur and the cathode is positive with respect to the
anode.
If the anode is very hot, electrons can be released by thermionic emission, and
such electron bombardment could rapidly destroy the filament of the x-ray tube.

Single-Phase X-Ray Generator
x-ray generator uses a single-phase input line voltage source (e.g 220 V at 50 A)
Produces either a single-pulse or a two-pulse DC waveform depending on the high-
voltage rectifier circuits.
bridge rectifier routes the electron flow so that the electrons always emerge
from the cathode and arrive at the anode.
The x-ray tube current for a specific filament current is nonlinear below 40 kV due to
Space Charge Effect

Single-phase x-ray generator
Single-Phase full-
wave rectified
circuit shows the basic
components and their
locations in the primary or
secondary side of the
generator.

Three-Phase X-Ray Generator
x-ray generators use a three-phase AC line source
Three voltage sources,
each with a single-phase AC waveform oriented one-third of a cycle (120 degrees)
apart from the other two (i.e., at 0, 120, and 240 degrees).
high-powered triode, tetrode, or pentode circuits on the secondary side of the circuit
control the x-ray exposure timing.
Theses witches turn the beam on and off during any phase of the applied voltage
within
extremely short times

beam on / off
during any phase
of the applied
voltage within
Extremely
short times
(millisecond or
better accuracy)
v
e

Timing The X-ray Exposure In Radiography
Electronic Timers
Digital timers have largely replaced electronic timers
Digital timer circuits have extremely high reproducibility and microsecond accuracy.
The timer activates and terminates a switch on either the primary or the secondary
side of the x-ray tube circuit
Precision and accuracy of the x-ray exposure time depends on the type of exposure
switch employed in the x-ray system X-ray systems
Use a countdown timer to terminate the exposure in timer or exposure switch failure

Phototimers
Phototimers are often used instead of manual exposure time settings in radiograph
Measuring the actual amount of radiation incident on the image receptor
Terminating x-ray production when the proper amount is obtained.
It helps provide a consistent exposure to the image receptor by compensating for
thickness and other variations in attenuation in a particular patient and from patient to
patient

X-ray Tube: Cathode
 A filament : small coil of thin thoriated tungsten wire
2 mm dia. & 1 to 2 cm long.
(A trace of thorium in the tungsten wire increases the efficiency of the electron
emission and prolongs the life of filament)
other materials : Molybdenum , Rhenium
 The emission of electron from the filament based on thermionic emission- the
metal is heated to sufficient temperature (about 2500 degree Celsius) to enable
the free electrons to leave the metal surface. The number of electrons emitted
depends upon the temperature. The higher the temperature, the greater is the
emission of electrons.

 The amount of thermion emission increases rapidly as the emitter temperature is
raised.
 The emission is markedly affected by temperature changes. Doubling the
temperature of an emitter may increase electron emission by more than 107 times.
 Small changes in the work function of the emitter can produce enormous effects
on emission.
 Low work function, high melting point, High mechanical strength , high emission
Efficiency
Cathode

 Tungsten: Φ = 4.52 eV
 High melting point (3400 0C)
 Thoriated tungsten (1-2 %): Φ = 2.63 eV (Th - 3.4 eV)
 Thermionic emission – (1700 0C)
 Can be drawn into very thin wire, Greater mechanical strength, withstands positive
ion bombardment, Low tendency to vapourise , Long structural & emission life.
Disadvantage:
 High operating temperature (>2500 0K)
 High work function
 Low emission efficiency.
Commonly used Thermionic Emitters

 The filament is embedded in a metal shroud called the focusing cup.
 All electrons accelerated from cathode to anode are electrically negative, the
beam tends to spread out due to electrostatic repulsion, and some electrons can
even miss the anode completely.
 The focusing cup is negatively charged so that it condenses the electron beam to
a small area of the anode.
 There can be two filaments with focusing cups, providing
different focal spots on the anode.
Focusing Cup

.
Cathode design determines the size of focal spot
Factors determining the size of focus
1. Size and shape of filament
2. Dimension of focusing cup
3. Depth at which the coil is kept in the slot
4. Electric field associated with focusing cup
Focal spot size determines amount of x-rays falling on image receptor and resolution
(definition) of image
Focusing Cup

Cathode and focusing cup

Cathode: filament + focusing cup
 Two electrical currents flow in an x-ray tube.
 The filament current : It is the flow of electrons through
the filament to raise its temperature and release
electrons.
 The electrical current: It is the flow of released
electrons from the filament to the anode across the x-
ray tube. This current, referred to as the tube current,
varies from a few to several hundred milliamperes
Cathode

Space charge
•Collection of negatively charged electrons in the vicinity of filament when no
voltage applied btw cathode and anode – space charge
•Number of electrons in space charge remain constant
•Tendency of space charge to limit the emission of more electrons
from the filament is called space charge effect
Filament current → filament temperature → rate of thermionic emission

Space Charge Effect
 The two currents (filament and tube current) are separate but interrelated. One
of the factors that relates them is the concept of “space charge.”
 At low tube voltages, electrons are released from the filament more rapidly than
they are accelerated toward the target.
 A cloud of electrons, termed the space charge, accumulates around the filament.
This cloud of negative charges opposes the release of additional electrons from
the filament until they have acquired sufficient thermal energy to overcome the
force caused by the space charge.
 At higher tube voltages, space charge cloud is overcome by applied potential
difference.

 At low filament currents, a saturation voltage is reached above which the current
through the x-ray tube does not vary with increasing voltage.
Influence of tube voltage and filament current upon tube current
 At the saturation voltage, tube current is limited by the rate at which electrons are
released from the filament.
 Above the saturation voltage, tube current can be increased only by raising the filament’s
temperature in order to increase the rate of electron emission. In this situation, the tube
current is said to be temperature or filament emission limited.
 To obtain high tube currents and x-ray energies useful for diagnosis, high filament
currents and voltages between 40 and 140 kV must be used.
 With high filament currents and lower tube voltages, the space charge limits the tube
current, and hence the x-ray tube is said to be space-charge limited.

Space charge cloud
Temperature limited
Space charge cloud
shield the electric
field for tube voltages
of 40kvp and less
( space charge limited )
above 40kvp space
charge cloud is
overcome by
voltage applied

Filament current Vs Tube current
• At low tube potential (40kVp and lower), tube current will be space charge limited.
• Space charge places upper limit on tube current (space charge compensation necessary
for change in tube current)
• At higher tube potential, tube current will be emission limited.
• Emission limited tube current cannot be increased by increase in tube potential.
• Emission limited tube current can be changed only by increasing the filament heating.

Filament current Vs Tube current

.
The anode is provided with positive potential in the x-ray tube
Function of Anode
 It serves as a target surface for the highly energetic electrons thereby becoming the
source of X-rays.
 Serves as the electrical Conductor. ie, receives electrons emitted by the cathode and
conducts them through the tube to the connecting cables and back to the high-
voltage section of the x-ray machine.
 Provides mechanical support for the target.
 Serves as the primary thermal conductor.

Anode: TARGET
TARGET: The area of electron bombardment is the place where the both
heat and X-rays are produced. SO it should be made of a metal that is
able to withstand high temperatures without melting and is efficient in
the production of X-rays.

Tungsten is chosen as efficient target material because it has
 High melting point (33600 C)
 High Z element (more bremsstrahlung yield)
 Fairly a good conductor of heat. So heat can be passed
reasonably quickly away from the small area where it is
produced and the rise in temperature at that area is prevented
from being too great.
 It does not vaporize easily. The presence of metal vapour
inside an
X-ray tube would spoil the vacuum which essential for its proper
operation
 It can be machined and made smooth. Smooth anode surface

Focal Spot
Focal spot-the area on the anode, which is bombarded by the electrons. The focal spot
becomes the source of the X-ray in an X-ray tube.
The electron beam produced from an helical filament in a rectangular slot, covers
rectangular area. That is focal spot is rectangular. The x-ray tube with rectangular focal
spot is described as LINE FOCUS TYPE
Focal spot size depends upon the following factors:
 The size and shape of the filament,
 The dimension of the focusing cup and the depth of filament in it
 Electric field associated with the focusing cup
 The spacing between cathode and anode .

Focal Spot
To produce radiographic images with sharp edges the size of the focal spot should be
small. Ie, smaller the focal spot size better the spatial resolution of the image.
But…
As the size of the focal spot decreases, the heating of the target is concentrated in a
small area and damages the target area of X-ray production.

Focal Spot: Line focus principle
The conflicting demands can be facilitated by
angling the target.
Sloping the anode face provides a larger area for
heating, while maintaining small effective
focal spot. The principle behind this design is
called Line-focus principle. Effective size
Actual size

Anode Angle & Effective Focal spot size
Effective focal length =Actual focal length x sin 

Angled Target: advantages & disadvantages
Advantages:
 Large actual focal size: more x-ray yield
: better heat dissipation
Small effective focal size: for image sharpness
Disadvantages:
Steeper anode angle restricts the field size
Heel effect

Anode angle and useful beam

Heel Effect
Reduction in the x-ray beam intensity toward the anode side of the x-ray field
X-rays are produced isotropically at depth in the anode structure.
therefore experience more attenuation than those directed toward the cathode side of the
field
Photons directed toward the anode side of the field transit a greater thickness of the anode

Heel Effect

Anode Configuration
Two types of anodes:
1. Stationary : The simplest type of x-ray tube has a stationary (i.e., fixed) anode. It consists
of a tungsten insert embedded in a copper block. The copper serves a dual role:
 It supports the tungsten target
 It removes heat efficiently from the tungsten target . Heat must be conducted away
quickly before it can melt the anode.
 Dental x-ray and portable x-ray machines (where high tube current and power are not
required) use fixed anode tube x-ray tubes.

Types of Anode
Rotating Anode:
For most of the diagnostic x-ray application, the rotating anode is used. The purpose of
the rotating anode is to spread the heat produced during an exposure over a large area
of the anode and consequently higher x-ray output capabilities.
 In rotating-anode tubes the entire rotating disc is the target.
 The rotating anode is a heavy disc mounted on a Molybdenum stem, which functions as
its support.
 Disc is made up of W-Rh alloy, or Tungsten followed by graphite or Molybdenum base.

Types Of Anodes

Rotating Anode
To compare the target areas of typical stationary-anode and rotating-anode
x-ray tubes with 1mm focal spot
(The area on the target that is struck by electrons)
 Actual area of the ST is 1 mm x 4 mm = 4 mm2.
 Diameter of Rotating anode = 7 cm, r of the target area is approximately 30
mm. So Total target area = 754 mm2.

Advantage of Rotating Anode
 Higher tube currents and shorter exposure times are possible with rotating anode.
 The rotating-anode x-ray tube allows the electron beam to interact with a much larger
target area, and therefore the heating of the anode is not confined to one small spot
as in a stationary-anode tube.
 Less geometric un-sharpness and movement un-sharpness in the image due to smaller
focal spot and shorter exposure timings.

•Rotated Anode x-ray tube
Conventional X-ray Tube

Conventional X-ray Tubes
•Rotated Anode x-ray tube

Target
The target is the area of the anode struck by the electrons from the cathode.
Tungsten is the material of choice for the target for three main reasons:
Atomic number-tungsten’s high atomic number, 74, results in higher-efficiency x-
ray production and in higher-energy x-rays.
Thermal conductivity-tungsten has a thermal conductivity nearly equal to that of
copper. It is therefore an efficient metal for dissipating the heat produced.
High melting point-any material, if heated sufficiently, will melt and become liquid.
Tungsten has a high melting point (3410 oC compared with 1083 oC for Cu) and
therefore can stand up under high tube current without pitting or bubbling
Further It can be machined and made smooth which avoids attenuation of a
fraction of x-ray intensity .

 Protective Housing
 The x-ray tube is always mounted inside a lead-lined protective housing designed to
control two serious hazards
 Excessive radiation exposure
 Electric shock
 When x-rays are produced, they are emitted isotropically i.e. with equal intensity in
all directions.
 Only those x-rays are used which are emitted through special section of x-ray tube,
called the window.
 The thin window serves to allow maximum emission of x-rays with minimum
absorption in the glass envelope.
Conventional X-ray Tube: Parts

 A properly designed protective housing reduces the level of leakage
radiation to less than 100 mR/hr at 1 m when operated at maximum
conditions.
 It also provides mechanical support for the x-ray tube and protects the tube
from damage caused by rough handling.
 The protective housing around some x-rays tubes contains oil that serves as
both an electrical insulator and a thermal cushion.
 Some protective housings have a cooling fan to air-cool the tube or the oil in
which the x-ray tube is immersed.
X-ray Tube

 The x-ray tube is a special kind of vacuum tube.: 20-35 cm long and 15 cm in diameter.
 The glass envelope is made of Pyrex glass to enable it to withstand the tremendous heat
generated, maintains a vacuum inside the tube.
 This vacuum allows for more efficient x-ray production and longer tube life.
 The tube is evacuated to pressure less than 10-7 mm Hg
 If tube becomes glassy, x-ray production will fall off and tube will fail.
 As glass envelope tube age, some tungsten vaporize and coats the inside of the glass
envelope. This alters the electric potential of the tube, allowing tube current to stray and
interact with the glass envelope; the result is arcing and tube failure.
Glass Envelope

 Because of this problem, a recent improvement in tube design incorporates metal
rather than glass as part or all of the envelope.
 Metal envelope tubes maintain a constant electric potential between the electron of
the tube current and the envelope.
 Therefore they have longer life and less likely to fail.

There are two primary parts; the cathode and the anode. Each of these is called an
electrode
Any tube with two electrodes is called a diode. An x-ray tube is a special type of
diode.
Conventional X-ray Tube

Those x-rays emitted through the window are called the useful beam.
Other x-rays that escape through the protective housing are leakage radiation –not
desirable.
X-ray Tube

Conventional X-ray Tubes

Factors affecting x-ray beam
Quality and Quantity

• The energy of the x-rays is determined by the voltage
applied.
• The amount of x-rays is determined by the current.

Factors affecting x-ray beam quality and quantity
• Anode material
• Voltage applied (kVp)
• Tube Current (mA)
• Filters used

Different anode materials will produce different
characteristic x-ray spectra and different amounts of
bremsstrahlung radiation.
1. Anode material

Note that increasing the applied voltage or kVp will increase the maximal energy,
the average energy and the intensity of the x-rays. Characteristic x rays do not
change with a change in kVp
40keV 75keV
2. Voltage (kVp)

100 mA
200 mA
75 keV
Increasing the current (ie mA) will not change energy of the beam only the intensity
(i.e. the amount) of x-rays. The quantity of x-rays is directly proportional to the tube
current.
3. Tube current (mA)

Power Ratings
The energy per unit time that can be supplied by the x-ray generator or received by the
x-ray tube during operation.
Power delivered by an electric circuit is equal to the product of the voltage and the current
Power = 100 kVp X A max for a O.I-second exposure
Maximum tube current (Amax) for 100 kVp and 0.1-second exposure t
The maximal tube current for high-powered generators can exceed I,OOOmA (I A) for short exposures

Heat Unit (HU)
simple way of expressing
the energy deposition on and dissipation from the anode of an x-ray tube
Energy (HU) = Peak Voltage (kVp) X Tube Current (mA) X Exposure time
For continuous x-ray production (fluoroscopy),
the HU/sec is defined as follows
Energy (HU) = kVp X mA

X-ray Exposure Rating charts
Operational limits of the x-ray tube for single and multiple exposures and the
permissible heat load of the anode and the tube housing.
The single-exposure chart contains the information to determine whether a
proposed
exposure is possible without causing tube damage.
Rating chart is specific to a particular x-ray tube and must not be used for other
tubes.
Charts show the limitations and allowable imaging techniques for safe
operation

A single-exposure rating chart
Multiple-exposure rating charts
Anode heat input and cooling chart
Housing Cooling Chart
X- ray exposure rating chart

Anode heat input and cooling chart

Housing Cooling Chart

Thank you
for your attention!!!

X Ray Production

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