Production of x rays & generators

Production of X Rays
Rakesh C A

PRODUCTION OF X-RAYS
• X-rays are produced by energy
conversion when fast moving
stream of electrons is suddenly
decelerated in the target anode of
an x-ray tube.

X-ray tube is made
of Pyrex glass that
encloses a vacuum
containing two
electrodes - anode
and cathode.

X-ray tubes are 2 types
1.Stationary anode x-ray tube
2. Rotatory anode x-ray tube

X-RAY TUBE STRUCTURE
Cathode
Anode
Filters
Tube housing
Tube envelope
Forms of tube used by Roentgen
in 1895–1896 for the production
of X rays.

CATHODE
• Negative terminal of the x-ray tube.
• Contains 3 elements.
1. Filament: source of electrons for the x-ray tube.
2. Connecting wires: supply voltage (10V) and
amperage (3-5 A) that heat the filament.
3. Metallic focusing cup: Made up of nickel and it
surrounds the filament.

FILAMENT
• Made of Tungsten wire.
• Diameter is about 0.2 mm
• Coiled to form vertical spiral of 0.2
cm in diameter and 1 cm or less in
length.
• Filament is source of electrons.

ADVANTAGES OF TUNGSTEN AS CATHODE
• High melting point (3370o C).
• Little tendency to vaporize.
• Ductility & Stability
• Malleability and strength
• Long life expectancy.
Disadvantages:-
Not an efficient electron emitting material.

TUBE CURRENT
• Is the number of electrons flowing from cathode
to anode per second.
• Measured in milli amperes(mA)
• The tube current is unidirectional - from cathode
to anode.

THERMIONIC EMISSION
•When current flows though
filament, it becomes heated up,
thermal energy is taken up by
electrons and they move a
small distance from the
surface.
•Emission of electrons
resulting from absorption of
thermal energy is k/a
thermionic emission.
EDISON EFFECT
•The electron cloud
surrounding the filament,
which is produced by
thermionic emission.

SPACE CHARGE
• Electrons emitted from
the tungsten filament
form a small cloud in
front of the filament.
This collection of
negatively charged
electrons forms space
charge.
SPACE CHARGE
EFFECT
• Tendency of space
charge to limit emission
of other electrons from
filament.

EQUILIBRIUM STATE
• As the electrons leave the
filament, it acquires a positive
charge attracting some electrons
back to itself.
• Number of electrons returning to
filament is equal to number of
electrons being emitted.
• As a result, space charge
remains constant with actual
number depending on filament
temperature.

METALLIC FOCUSSING CUP
• Made of nickel.
• Prevents bombardment of unacceptably large
target area
• The specially designed cup cause the electron
stream to converge to the target area on the
anode
• It is maintained at same negative terminal as
that of filament.
• In grid controlled tubes the cup (-1.5 kV) acts
as a switch

MODERN DAY X-RAY TUBES
• Contain single filament/ double filaments/
sometimes 3 filaments
DOUBLE FILAMENT ARRANGEMENT:-
• They are placed side by side or one above the
other.
• One filament is large, other one is small.
• Only one filament is used for any fixed x ray
exposure. Larger filament is used for longer
exposure.

DOUBLE FILAMENTS IN
FOCUSING CUP
Filament for large
focal spot
Filament for
small focal spot

HIGHLY SPECIALISED X-RAY TUBES
• Tube with 3 filaments
• Stereoscopic angiographic tube
In this tube, 2 focal spots are widely separated
producing stereoscopic film pair when 2 films are
exposed.
Used in angiography.

AUTOMATIC FILAMENT BOOSTING
CURCUIT
• When x-ray is turned on and no exposure is
made, (as in fluoroscopy) stand by current
heats the filament at low current (5mA).
• When exposures are needed, automatic circuit
will raise filament current to required value
and lower it to stand by after exposure.

ANODE
• The positive terminal of tube
STATIONARY ANODE ROTATING ANODE

STATIONARY ANODE
– Made of tungsten
– 2-3 mm thick.
– Embedded in large mass of copper
– Triangular/ rectangular shape
– Anode angle = 15-200
Adv of using tungsten:-
1. High melting point
2. High atomic number
3. Good absorption & dissipation of heat
Tungsten should be bonded with copper of anode to
further facilitate heat dissipation

ROTATING ANODE X-RAY TUBE
• Consists of
1. Rotating anode
2. Anode stem
3. Stator of induction coil
4. Rotor of induction coil
5. Ball bearings
6. Safety circuit

ROTATING ANODE
• Made of tungsten or alloy of tungsten with
Rhenium.
• Has beveled edge
• Angle of bevel is 6 to 20 degrees
• Speed of rotation is 3000rpm practically
The purpose of rotating anode is to spread the
heat produced during an exposure over a large
area of the anode.

ANODE STEM
• Made up of molybdenum
• It has high melting point
but poor conductor of heat
- It protects ball bearings
from un-desirable heat
• The length of the
molybdenum should be as
short as possible
(length ÞinertiaÞ load
on the bearings)

STATOR OF INDUCTION COIL
– Provides magnetic field necessary for
induction of current.
ROTOR OF INDUCTION COIL
– The magnetic field provided by stator
induces current in copper rotor
– This current provides power for rotation
of anode assembly

BEARINGS
• Increases life of the tube.
• Lubricant used is silver.
• Silver is suitable in vacuum
– Ball bearings

SAFETY CIRCUIT
• There is short delay (0.5 – 1 sec) between
application of force and full rotation of anode
due to inertia.
• Safety circuit prevents un-necessary exposure
during this delay.

HALF LIFE OF ROTATING X-RAY TUBE
• Depends on roughing
and pitting of surface of
anode exposed to
electron beam.
• Prevented by using
alloy of 90% tungsten
and 10% rhenium

Why increased speed of
rotation?
" speed of rotation Þ ability of anode to withstand heat
MODIFICATIONS TO INCREASE SPEED OF ANODE
1.Decrease anode-stem length (¯ inertia)
2.Use of two sets of ball bearings.
3.Decrease weight of anode (¯ inertia)
- compound anode disc
- molybdenum or graphite

Compound Anode
Compound anodes contain combination of rhenium, molybdenum, and graphite being
application based

GRID CONTROL X-RAY TUBE
• It contains its own switch, which allows it to be turned
on and off rapidly (as in cineflurography)
• Third electrode - focusing cup – is used to control the
flow of electrons from filament to target.
• Focusing cup is negative to filament. (in conventional,
it is connected to the filament)
• Voltage(-1.5 kV) applied between focusing cup and
filament acts as switch

Emission limited
Or
Temperature limited
Using resistors

FOCAL SPOT
• Area of tungsten target i.e. bombarded by electrons
from the cathode.
• Large focal spot – greater heat loading
v/s
• Small focal spot – better resolution

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Anode is inclined so it forms an angle with the plane perpendicular to
electron beam. The angle is anode angle.
Effective or apparent focal spot is smaller than actual focal spot d/t
anode angle.

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Normal anode angle is 60 –200
220000
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• Apparent focal spot µ sine (anode angle)
• For FFD 40 inch, anode angle is usually no smaller
than 150 (heel effect)

HEEL EFFECT
• Intensity of x-rays depends on the angle at which
the x-rays are emitted from the focal spot.
• The intensity of beam towards anode side is less
than that towards cathode side.
• Intensity of the beam towards the anode side of
the tube is less because of absorption of some of
the x-ray photons by the target itself.

Production of x rays & generators

CLINICAL IMPORTANCE
1. Used for obtaining balanced densities in
radiographs of body parts of different
thickness, i.e. thicker parts towards cathode
2. When FFD is increased, heel effect is
reduced.
3. For smaller films, less heel effect.

USEFUL RADIATION –
PROJECTED TOWARD THE PATIENT

TUBE HOUSING
• Consists of metal case made up of Aluminum alloy
lined on the inside by a layer of lead which protects
and supports the glass x-ray tube insert.
– Tube housing provides an efficient radiation barrier where
in the x-rays produced in the x-ray tube are attenuated in
all the directions except at the tube port.
– Provides shielding for the high voltages required to
produce x rays
• The tube housing is packed with industrial grade oil
to provide electrical and thermal insulation.

GLASS ENCLOSURE
• Is a sealed evacuated tube made up of borosilicate
into which are mounted the anode and cathode.
• Accelerated electrons ® collide with gas molecules
® secondary electrons (less speed) ®wide variation
in tube current and energy of x ray produced.
• The purpose of the vacuum in the modern x-ray tube
is to allow the number and speed of accelerated
electrons to be controlled independently.
• The shape and size of these x-ray tubes are specially
designed to prevent electric discharge between
electrodes.

Disadvantages of glass enclosure:
• On long term use, tungsten vaporizes and
form thin coat on inner surface of glass
wall of x-ray tube.
• It gives bronze colored sunburn to x-ray
tube.

Results of sun burning of x-ray tube
1. It filters x-ray beam and gradually changes its
quality.
2. It increases probability of arcing between glass
and electrodes at higher kVp which may
puncture the tube.
SOLUTION Þ Metallic Enclosures

CERAMIC/ METALLIC X-RAY TUBES
• Has metal encasing
and 3 ceramic
insulators – high
voltage cables and
anode stem
• Anodes up to 2000 g
can be supported
(conventional tubes –
700g)

CERAMIC INSULATORS
• Made of aluminum oxide. They
insulate high voltage parts of x-ray
tube from metal envelope.
• The use of insulators allows a more
compact tube design.

ADVANTAGES OF METALLIC ENCLOSERS
• Less off focus radiation.
• Longer tube life with high tube currents.
• Higher tube loading.
• Adequate electrical safety
• Compact size

OFF FOCUS RADIATION
It is due to electron
back scatter from
anode interacting
with metal other than
the focal track and
striking anode a
second time to
produce X-rays.
Decreased by
1.Placing collimator
2.Lead diaphragm as
close to X-ray tube as
possible.
3.Using a metal
enclosure – attracts off
focus radiation to the
grounded metal tube.
USEFUL
OFF-FOCUS

COOLING MECHANISM OF X-RAY TUBE
• Almost all energy put into x-ray tube is converted
into heat and <1% is converted into x-rays.
The process of heat
dissipation:
Conduction: Through solid
parts of anode.
Convection: Through oil
surrounding the tube.
Radiation : Occurring
through the vacuum of the tube
which passes off the heat to glass
envelope or from metallic
housing through air into the
atmospheric air.

There are three basic requirements for X-Ray
production in an X-Ray tube:
1. A source of electrons (cathode)
2. A target to stop the electrons (anode)
3. A method of accelerating the electrons
from the source to the target (PD
maintained across the tube)
1% of the incident energy is converted to X-Rays
the remainder is converted into heat

Produced by 2 different processes
GENERAL RADIATION
CHARACTERISTIC
RADIATION
• Reaction of electron with
nucleus of tungsten atoms,
producing x-rays.
• Collision between high speed
electrons and electrons in the
shells of target tungsten atoms.

GENERAL RADIATION
When electron passes near the nucleus of tungsten atom, the +ve charge of the nucleus acts on the –ve
charge of the electron.The electron is attracted towards the nucleus and deflected from its original
direction.The electron will lose some energy and slows down when its direction changes.The kinetic energy
lost by the electron directly in the form of a photon of radiation called general radiation /
bremsstrahlung.

CHARACTERISTIC RADIATION
Results when electrons bombarding the target eject electrons from the inner orbits of the target atoms.
Removal of an electron from a tungsten atom causes the atom to have an excess positive charge, and the
atom thus becomes a positive ion

In the process of returning to its normal state the ionized atom of tungsten may
get rid of excess energy in two ways.

An additional electron (auger electron) expelled by the atom and carry off the
excess energy - does not produce x-rays.

An alternative way to get rid of excess energy is for the atom to emit
radiation that has wave lengths with in the x-ray range.

BRAKE RADIATION
CHARACTERISTIC
RADIATION
• Only about 5 % of
electrons participate
• These 5% produce
most of the X-Rays
• There is no
ionization
• No characteristic
radiation below 70
kV
• In the 70 to 150 kv
range contribution is
10-28 %
• Ionization of target
atom

Intensity of X ray beams
Intensity of an x ray beam
=
Number of photons x Energy of each photon
•Varies with:
– Kilovoltage (proportional to kVp2)
– X ray tube current
– Target material
• Atomic number – quantity(number) of bremsstrahlung
– quality(energy) of characteristic radiation
– Filtration

FILTRATION
• The spectrum of x-ray beam includes a wide range of
energies – low to high.
• For diagnostic purposes very low intensity x-rays are
not useful & increase the patient dose and decrease
the quality of the image.
• Hence the x-ray beam reaching the patient is filtered
so as to make it most useful for image formation
without increase in patient dose.

Filtration of x-ray beam – 2 types
Inherent Filtration:
Provided by the tube
envelope, tube housing and
the tube port.
Added Filtration:
Provided by the use of
Aluminum and
Copper across the
beam path.

First X ray made in public.
• Hand of the famed
anatomist, Albert von
Kölliker, made during
Roentgen's initial
lecture before the
Würzburg Physical
Medical Society on
January 23, 1896.

Generator
Ordinary Generator
Mechanical energy Electrical energy
X-ray generator
device that supplies electric power to the X-ray tube
•Electric supply Generator X-ray tube
•Electric supply : 220 V, 50Hz, 3 phase, AC power

Why does x ray tube require electrical energy ?
• To boil of electrons from cathode - filament circuit(10V)
•To accelerate from cathode to anode - high voltage circuit(40-150kVp)
•To regulate the length of exposure - timer circuit
cathode (+) anode
(-)

Generator
Control panel
- kVp
-mA
-Exposure time
Transformer assembly
-Grounded metal box
-Low voltage filament
transformer
-High voltage transformer
-Group of rectifiers
-Oil (Insulator)

•Filament circuit - 10 V
• High voltage circuit - 40-150 kVp

Transformer
•A transformer is a device that either increases or decreases the
voltage in a circuit
•When current flows in primary coil it creates a magnetic field in the
core, and this induces current in the secondary coil
•Current only flows in the secondary circuit when the magnetic field is
increasing or decreasing,(i.e. when there is some potential difference
between the two ends of primary coil), hence requirement for AC
current
v
Primary coil
Secondary coil
Switch

Laws of transformer
1. The voltage in the two circuits is proportional to the
number turns in the two coils
N P = VP
NS VS
• More turns in the secondary coil - step up transformer
• Fewer turns in the secondary coil - step down transformer
2. A transformer cannot create energy. An increase in
voltage must be accompanied by a corresponding decrease
in current
Vp Ip = Vs Is
• The output of Transformers is an alternating current(AC)

•Filament circuit - 10V
Step down transformer
• High voltage circuit - 40-150kVp
Step up transformer

Autotransformer
Electric supply Autotransformer Generator X-ray
tube
Principle of self induction
110v/55
230v/115
320v/160
230v/115
Turns of windings
Incoming power voltage
Autotransformer
Step down transformer(filament)
- 10 V
-Turns in secondary coil less
Step up transformer (cathode to anode)
- 40-150 kV
- Turns in secondary coil > 600

PROBLEM- Output of Transformers is alternating current(AC)
Alternating current
(-) (+)
X-ray tube

PROBLEM- Output of Transformers is alternating current(AC)
Alternating current
(+)
X-ray tube
(-)

Rectification
Electric supply
Autotransformer
Generator
AC
Rectifier
DC
X-ray tube
•Rectification is the process of changing the alternating
current into direct current
•A rectifier is the device that allows electrical current in
one direction but does not allow current to flow in the other
direction

High voltage rectifiers
• Two types
– vacuum – tube type (thermionic diode tubes)
– solid state rectifier
• Modern x-ray tubes use solid state rectifier
which are more reliable and have a longer
life.
• Selenium was the first material and silicon
is most widely used in modern days.
• The heart of a solid state rectifier is a semi
conductor.

Solid state semiconductor - Rectifier
Conduction band
Forbidden gap
Valency band

Conduction band
Forbidden gap
Valency band
Conductor

Conduction band
Forbidden gap
Valency band
Insulator (10 eV)

Conduction band
Forbidden gap
Valency band
Semi-conductor (1 eV)
Semiconductor acts like an insulator at low temperature
and acts like a conductor at room temperature

N-type semiconductors
• The material with five
valence electrons is
added as a impurity to
silicon lattice creating a
extra electron.
• Impurity- arsenic or
antimony.
(N – Negative)
• These are called
donors.

P-type semiconductors
• In this an impurity with
only three valance
electrons is added to
silicon crystal.
• This leaves behind a
Hole (P – Positive)
• Impurity- indium,
gallium and aluminum.
• These are called
acceptors.

P-N junction
• It is formed when N-type
and P-type crystals are
joined.
• This will create a
depletion layer which is
opposite in polarity to the
adjacent material.

• If voltage is applied across
current will flow or not flow
depending upon the polarity.
• Forward bias
When the negative pole of a
battery is connected to the N-type
and positive pole to the P-type.
Electrons will start flowing
from negative to positive
direction.
• Reverse bias
When negative pole is
connected to P-type and
positive to N-type.
Electrons will stop flowing.
•P-N diode conducts current in a forward direction only,
hence it meets the definition of a rectifier.

1/60 sec
1 cycle
Two rectifiers
connected in series
with X-ray tube
Half wave rectification
1 PULSE
Advantage:
Prevents damage to the tube in inverse cycle.
Disadvantage:
Half of the power will be wasted.

1/60 sec
1 cycle
Diode bridge
Full wave rectification
2 PULSE
Seen in Modern X ray tubes.
This is brought about by using 4rectifiers connected in
opposite circuits.
Advantage:
Utilizes full electrical potential
Prevents tube damage.

PULSED POTENTIAL
Fluctuating voltage from 0 to its Max.
Disadvantage:
- Loss of exposure time
- Low energy X-rays
-Increased patient dose
- Tube life
1/60 sec
1 cycle
2 PULSE

Three phase generators
•Three phase generator consists of three single phase currents
out of step with each other by 1200
00
1200 2400
•Supply power at near constant potential
•2000 mA, 150 KV, 1 msec

Three phase transformers
• Consists of three sets of primary and
secondary windings.
• Two types of configurations:
Delta
Wye (star)
• Generally
Primary – delta
Secondary – wye / delta

Six pulse six rectifier
Six solid state
rectifiers are used.
Six max. positive
voltages per cycle

Six pulse twelve rectifier.
Employs 12 rectifiers
Advantage:
allows a 150 kV
generator to allow a
wide range of voltage
(-75 to +75 kV)

Twelve pulse
Secondary winding
will have both delta
and wye connection
Advantage:
Delta will lag
the wye by 30degrees
which fills the ripple

Ripple factor
•The ripple factor is the variation in the voltage across the
x-ray tube expressed as the percentage of maximum value
Single phase 100%
Triple phase
Six pulse six rectifier 13.5%
Six pulse twelve rectifier 13.5%
Twelve pulse 3.5%

Advantages
• Produces X rays efficiently throughout the
exposure.
• No time is spent in bombarding the X ray tube with
low energy electrons.
• High tube ratings for extremely short exposure
times.
• Produces radiographs with short exposure times
and high repetition rates.

kVp
mA
AUTOTRANSFORMER
150 kVp to rect.
.
.
.
.
.
+11OV
.
STEP UP
STEP DOWN
TO X-RAY TUBE FILAMENT
-11OV
.
+75kVp
-75kVp

Transformer rating
•The rating of a transformer states the maximum safe output of its
secondary winding , expressed in kilowatts
kW = kV x mA
1000
•Transformer rating is determined when the generator is under
load ( 100 kVp)

Power storage generator
•Provides means of supplying power
for the tube independent of an
external power supply ( mobile
radiographic equipment)

Storage generators
Battery powered generator
Electric supply
Nickel-cadmium battery
DC chopper
Transformer
Rectifier
X-ray tube
•Supplies constant kVp and mA
Capacitor discharge generator
Electric supply
Step up transformer
Large capacitor
X-ray tube
•KV falls during exposure at
the rate of 1KV for each mA
Advantage:
Small and easy to move.
Limitations:
Must be charged prior to use.
Cannot be used for thicker
body parts – abdomen
Limitation:
Heavy and requires regular
battery maintenance.
Advantages:
Stores considerable energy
Independent of power supply

Medium frequency generator
• V = f n A
Electric supply (50 Hz)
Invertor (DC chopper)
Power (6500 Hz)
Transformer
13000 Hz constant potential
• Transformer is made more compact and small
• Constant nearly ripple free voltage to the X-ray tube

Falling load generators
•Produce X ray exposure in shortest possible time by
operating X ray tube at its maximum Kilowatt rating.
•Disadvantages:
Causes Focal spot blooming
Reduces Tube life
Expensive

Exposure switching
•A switch is a device that turns the high voltage to
the X-ray on and off
Primary switching
- Takes place in the primary coil
- All general purpose generator
- Easier and cheaper
- 1-2 millisecond
-Cannot produce repeated
exposure
Silicon controlled rectifiers
(thyrister)
Secondary switching
- Secondary circuit of high voltage
- Special purpose generator like in
angiography , cinefluoro
- Costly
- 0.5 ms (shorter exposures)
- Repeated exposure
Triode vacuum tube
Grid controlled X-ray tubes

Exposure timers
•To control the length of an X-ray exposure
Exposure timer
Electromechanical Automatic exposure control Pulse counting timer
Photomultiplier
Ionisation chamber
Solid state

VALVE RECTIFERS
X-RAY GENERATOR-THEN
STEP UP TRANSFORMER
STEP DOWN
TRANSFORMER

DUAL TUBE 800 mA HIGH FREQUENCY GENERATOR & CONTROL ASSEMBLY IN SINGLE CABINET
CAPACITORS
HT
TIMER & ANODE CONTROL
MICROPROCESSORS
X-RAY GENERATOR- NOW !

The first medical X ray taken
Drs. Edwin and Gilman Frost perform the
first American medical X-ray in Reed Hall,
Dartmouth College on February 3, 1896
The first American medical X-ray : 14-
year-old Eddie Murphy’s fractured
ulna.

Production of x rays & generators

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