Production of x rays Components of X-ray Cathode kVp , mA , mAs . Line focus principle Heel effect anode Stationary anode x ray tube Rotating anode x-ray tube Grid controlled x-ray tube Saturation voltage Metal ceramic x – ray tube Processes of x- ray generation intensity of the x-ray beams Effect of kVp on x- ray beam Effect of tube current on x- ray beam learn with Me...........MK if you notice any mistake comment please ......

1. M K . Sadham hussain Bsc . MIT (2nd year )
PRODUCTION OF X -RAYS

2. Index
1. Production of x rays
2. Components of X-ray
3. Cathode
4. kVp , mA , mAs .
5. Line focus principle
6. Heel effect
7. anode
8. Stationary anode x ray tube
9. Rotating anode x-ray tube
10.Grid controlled x-ray tube
11.Saturation voltage
12.Metal ceramic x – ray tube
13.Processes of x- ray generation
14. intensity of the x-ray beams
15.Effect of kVp on x- ray beam
16.Effect of tube current on x- ray beam

3. Production Of X- Rays
X rays are produced by energy conversion when a fast
moving stream of electrons is suddenly decelerated in the
target anode of an X –ray tube .
Components of X - ray tube
Housing - Visible part of tube
Glass Enclosure (insert) - Vacuum ,Electrodes(anode, cathode)
the x-ray tube made of pyrex glass .
Tube Housing
X ray tube is always mounted inside a lead lined protective housing that is
designed to
-Prevent excessive radiation exposure
-Prevent electric shock to the patient and operator.
• Contains oil in which the tube is bathed.

5. Glass Enclosure
The x- ray tube is made of pyrex glass (silica and boron
trioxide ) that encloses a vacuum containing two
electrodes .
Pyrex glass have same coefficient of linear expansion.
(the coefficient of linear expansion is the change in length of specimen one unit
long when its Temperature changed by one degree .)
Features
• It can withstand heat and mechanical stress.
• It is an electrical insulator capable to withstand high voltage.
• It should be capable of being sealed to the electrodes with a
vacuum-tight, heat proof seal.

6. Glass enclosure
• It is necessary to seal the two electrodes of the x ray tube in vacuum
because if gas were present inside the tube the electrons that were
being accelerated toward the anode target would collide with the gas
molecules, lose energy and cause secondary electrons to be ejected from
the gas molecules.
• So the presence of secondary electrons would result in variation in the
number and in the reduced speed of electrons impinging on the target.
• So the purpose of vacuum in the modern x ray tube is to allow the
number and speed of accelerated electrons to be controlled
independently.

7. Cathode
It is the negative electrode of the X-ray tube.
It consists of
• Filament –source of electron.
• Connecting wires which supply the voltage (about 10 V) and current (3 to 5 A) .
• Metallic focusing cup.(Ni)
Filament
FILAMENT: Cathode filament made of thin tungsten wire which is a source of
electrons. It works on the phenomenon of Thermionic emission
Length :1.0 cm Diameters of spiral : 0.2 cm
Thermionic emission
The emission of electrons resulting from the absorption of thermal
energy .

8. Tungsten is used as filament material because of following Reasons:
• It has high melting point (3370C).
• Less tendency to vaporize
• High tensile strength means it can be drawn into wires.
• High thermal conductivity and specific heat.
• Appropriate Threshold or Work function
Focusing Cup .
It is the device surrounding the cathode filament in an X-ray tube.
This is actually a third electrode in the tube, called a Wehnelt electrode.
It is usually made up of Ni because of:
a) Light wt.
b) Poor thermal conductivity
c) High melting point.

9. Space charge
Electrons emitted from the tungsten filament form a small cloud in the
immediate vicinity of the filament.
This collection of negatively charged electrons forms what is called the
space charge.
Space charge effect
The tendency of the space charge to limit the emission of more
electrons from the filament is called the space charge effect.
Tube current (mA)
The x-ray tube current, measured in milliamperes (1 mA = 0.001 A),
The number of electrons flowing per second from the filament to the
target.
It is important to understand where these electrons come from, and
to remember that the number of electrons determines x-ray tube
current.

10. mAs ( milli ampere second )
Amount of x-ray produced in x-ray tube .
mAs = tube current *scan time per second
Current passed time .the exposure time
mAs has no effect on energy of x- ray beam they only effect the intensity
of beam increasing the mAs increases the beam intensity and also increases
the contrast
kVp (kilo voltage peak ) (tube potential)
This is the maximum or peak electron energy as they flow through the
x-ray tube
During an x-ray exposure , the cathode is negatively charged and anode is
positively charged . The difference in charge is called the kilovoltage of
the tube potential .
Increasing the kVp the increases the beam intensity (like increasing mAs),
but
Increasing the kVp also increases the beam energy

11. This is only prime exposure variable that can change the energy of the
x-ray beam .
Increasing kVp increases the patient dose and receptor exposure but
decreases the image contrast .
Focal Spots
The focal spot is the area of the tungsten target (anode) that
is bombarded by electrons from the cathode. Most of the
energy of the electrons is converted into heat, with less than
1% being converted into x rays.
Most tubes have 2 filaments & thus 2 focal spots, only one used at a time
Small focus – improved resolution
Large focus – improved heat ratings ( Electron beam strikes larger portion
of target thus spreading heat produced to a large area).

13. Line focus principle
A small focal spot is required for producing good radiographic detail but it
may also lead to overheating of target.
Whereas large focal spot allows greater heat loading but doesn't
produce sharp image.
This problem was solved in 1918 by the development of line focus
principle .
The line focus principle helps resolve this issue by starting that an
angulation of the anode surface will result in an apparent decrease in the
focal spot size .
The apparent focal spot (projected focal spot ) size can be determined the
sine of the angle of the anode surface .
Apparent focal spot size = real focal spot size * sin anode angle
The angle varies as per tube design with a range value of 6 degrees to about
20 degrees .

16. Line focus principle
• the principle that viewing a sloped surface at an angle
reduces its apparent size . In an x-ray tube the angling of
the anode results in the effective focal spot being smaller
than the actual focal spot .
• The electron stream bombards the target, the surface of
which is inclined so that it forms an angle with the plane
perpendicular to the incident beam.
The anode angle differs according to individual tube design and may vary
from 6 to 20°. Because of this angulation, when the slanted surface of the
focal spot is viewed from the direction in which x-rays emerge from the x-
ray tube, the surface is foreshortened and appears small. It is evident,
therefore, that the side of the effective, or apparent, focal spot is
considerably smaller than that of the actual focal spot.
 The unfortunate bi-product of the line focus principle is the
Anode heel effect .

17. heel effect
Heel effect – the intensity of the x-ray beam that leaves the x-
ray tube is not uniform throughout all portion of the beam .
The intensity of the beam depends on the angle at which the x-
rays are emitted from the focal spot. This variation is termed the
"heel effect."
anode heel effect
• Varying intensity of x-ray at anode side is lesser than cathode side.
• Beveled anode absorbs some x ray photona
• anode angle (from 7to 20degree) induces a variation of the x- ray
output in the plane comprising the anode – cathode axis
• The magnitude of influence of the heel effect on the image depends on
the factors such as :
1. Anode angle 2. size of film
3. Focus to film distance

19. Three clinically important aspects of the heel effect are
1) The intensity of film exposure on the anode side of the X ray tube is
significantly less than that on the cathode side of the tube.
2) the heel effect is less noticeable when larger focus film distances are
used.
3) For equal target film distances , the heel effect will be less for smaller
films.
• the heel effect is less noticeable when larger focus-film distances are
used. Third, for equal target film distances, the heel effect will be less
for smaller films.
• This is because the intensity of X ray beam nearest the central ray is more
uniform than that toward the periphery of the beam

20. Anode
Anodes (positive electrodes) of x-ray tubes are of two types, stationary or
rotating.
Stationary Anode : The anode of a stationary anode x-ray tube consists of a
small plate of tungsten, 2- or 3-mm thick, that is embedded in a large mass of copper. The
tungsten plate is square or rectangular in shape, with each dimension usually being greater
than 1cm. The anode angle is usually 15 to 20°.
We use TUNGSTEN as a target material because of following reasons:
• High atomic no.(74)
• High melting point (3370 °C)
• High thermal conductivity.
• It doesn’t vaporize easily.
The rather small tungsten target must be bonded to the much larger copper
portion of the anode to facilitate heat dissipation. In spite of its good thermal
characteristics, tungsten cannot withstand the heat of repeated exposures. Copper
is a better conductor of heat than tungsten, so the massive copper anode acts to
increase the total thermal capacity of the anode and to speed its rate of cooling

21. If the tungsten target were not sufficiently large to allow for some cooling around
the edges of the focal spot, the heat produced would melt the copper in the
immediate vicinity of the target.
All the metals expand when heated, but they expand at different rates. The bonding
between the tungsten target and the copper anode provides technical problems
because tungsten and copper have different coefficients of expansion. If the bond
between the tungsten and the copper were not satisfactorily produced, the tungsten
target would tend to peel away from the copper anode.
Advantage of stationary anode x- ray tube
• Compact unit
• Less costly
Application
• Dental x-ray sets , small portable and mobile x- ray units with
limited out put .

22. Rotating anode x- ray tube
• Large disc of tungsten/ alloy of tungsten
• Beveled edge
• Carbon coating
• Stem
• Rotor
• Stator coils
• Lubricants

24. Rotating anode x- ray tube
 Limitations of stationary anode tube were overcome by rotating anode
tube which were introduced in 1936.
 The ability of the X ray tube to achieve high X ray output is limited by
the heat generated at the anode.
 The rotating anode principle is used to produce X ray tubes capable of
withstanding the heat generated by large exposures.
 The anode of the rotating anode tube consists of a large disc of
tungsten which rotates at about 3000 rpms.
 The usual speed of anode rotation using 60-cycle current varies between
3000 and 3600 rpm
 The tungsten disc has a beveled edge. The angle of the bevel may vary
from 6 to 20 degree (same principle as line focus principle).

25.  Purpose Of Rotating Anode is to spread the heat produced during an
exposure over a large area of anode
 If the target is made to rotate at a speed of 3600 rpm, however , the
electrons will bombard a constantly changing area of the target.
 The power to effect rotation is provided by a magnetic field produced
by stator coils that surround the neck of the x ray tube outside the
envelope.
 The magnetic field produced by the stator coils induces a current in the
copper rotor of the induction motor and this induced current provides
the power to rotate the anode assembly.
 Heat generated in a solid tungsten disc is dissipated by radiating through
the vacuum to the wall of the tube and then into the surrounding oil and
tube housing.

26.  In the rotating anode tube, absorption of heat by the anode assembly is
undesirable because heat absorbed by the bearings of the anode
assembly would cause them to expand and bind.( but in stationary anode heat
is dissipated by absorption and conductivity is provided by the massive copper anode.)
 Because of this problem the stem which connects the tungsten target to
the remainder of the anode assembly is made of molybdenum.
 Mo has high melting point (2600deg C) and is a poor conductor of
heat.
 Thus the Mo stem provides a partial heat barrier between the tungsten
disc and the bearings of the anode assembly.
 If the speed of the rotation is increased , the ability of the anode to
withstand heat will become greater.

27. Three modifications to increase velocity:
1.Length of the anode stem to be made short
2. Bearings to be placed as far apart as possible
3. Inertia of the anode is reduced by decreasing
the weight of the anode.
HALF-LIFE
• Depends on roughing and pitting of surface of anode exposed to
electron beam.
• Prevented by using alloy of 90% tungsten and 10% rhenium.

28. GRID CONTROLLED X-RAY TUBES
o A grid controlled x-ray tube contains its own switch , which
allows the x -ray tube to be turned on and off rapidly , as is
required with cinefluorography.
o A third electrode is used in the grid controlled tube to control
the flow of electrons from the filament to the target. This
third electrode is the focusing cup that surrounds the
filament.
SATURATION VOLTAGE
If the potential applied across the tube is insufficient to cause almost
all electrons to be pulled away from the filament the instant they are
emitted , a residual space charge will exist about the filament.

29. 40 kVp defines the location of the saturation point of
this x-ray tube. below 40 kVp ,the current flowing in the
tube is limited by the space charge effect

30. Metal/Ceramic X-Ray Tubes
 A high-performance x-ray tube introduced by Philips Medical Systems
 The trade name - ceramic Super Rolatix tube
 This tube has a metal casing instead of the usual glass envelope, and has
three ceramic insulators
 Two insulators provide insulation for the two (positive and negative)
high-voltage cables, and one supports the anode stem.
We shall explore the advantages of such construction.

31.  that the anode rotates on an axle which has bearings at each end to
provide greater stability and reduce stress on the shaft.
 This additional support allows use of a more massive anode, with anode
weights of 2000 g possible. Anodes in conventional x-ray tubes are
generally limited to no more than 700 g.
 Ceramic insulators are used to insulate the high voltage parts of the x-
ray tube from the metal tube envelope.
 Aluminum oxide is a commonly used ceramic insulator
 Ceramic insulators have long been used to attach high-voltage
transmission lines to overhead supporting towers.
 The three ceramic insulators are labeled 4 (high-voltage connectors) and
8 (anode shaft)
 The use of ceramic insulators allows a more compact tube design.

33. Using metal as the x-ray tube enclosure (instead of glass) offers several
advantages, the three most important being:
1. Less off-focus radiation
2. Longer tube life with high tube currents
3. Higher tube loading
The metal envelope is grounded; this grounding plus use of ceramic
insulation allows adequate electrical safety despite the tube's small size.
Off-Focus Radiation
Off-focus radiation is produced by an x-ray tube when high-speed electrons
interact with metal surfaces other than the focal track of the anode (usually
other parts of the anode).
The main source of off-focus electrons is electron backscatter from the
anode. These scattered electrons may then strike the anode a second time
and produce x-rays from areas other than the anode focal track

34. Off-focus radiation may be partly controlled by placing the collimator, or a lead
diaphragm, as close to the x-ray tube as possible.
The metal enclosure decreases off-focus radiation by attracting off-focus electrons to the
grounded metal wall of the x-ray tube.
The metal enclosure thus decreases off-focus radiation by removing many off-focus
electrons.
Longer Tube Life
We have already mentioned the problem of deposition of tungsten (from
the anode) on the glass wall of x-ray tubes. This tungsten eventually builds
up enough to act as an electrode and cause arcing between the glass and
filament.
Tungsten deposition is of greatest concern when high tube currents (high
mA) are used. A metal enclosed x-ray tube has its metal enclosure
connected to ground, and deposition of tungsten will not alter this
grounding.
For this reason, the useful life of a metal-enclosed x-ray tube will be greater
than that of a glass tube, especially when used for high tube currents, such
as in angiography.

35. High Tube Loading
The more massive metal anode of this tube allows significantly
higher tube currents (more mAs) to be used because of the
larger heat storage capacity of the anode. This allows a higher
mAs setting for single exposures.
In addition, there is increased capacity for serial exposures
because of better cooling that results from more efficient
transfer of heat to the oil through the metal enclosure, as
compared to a glass enclosure (metal is a much better heat
conductor than glass).
X----------x----------x

36. Processes of X-Ray Generation
X ray s are generated by two different processes.
 general radiation, or bremsstrahlung .
 characteristic radiation.
General radiation or Bremsstrahlung
When an electron passes near the nucleus of a tungsten atom, the
positive charge of the nucleus acts on the negative charge of the
electron. The electron is attracted toward the nucleus and is thus
deflected from its original direction. The electron may lose energy
and be slowed down when its direction changes. The kinetic energy
lost by the electron is emitted directly in the form of a photon of
radiation. The radiation produced by this process is called general
radiation or bremsstrahlung (from the German for "braking radiation").

38. CHARACTERISTIC RADIATIONS
• These radiations result when the electrons bombarding the target eject electrons from
the inner orbits of the target atoms.
• X-rays produced are called characteristic because these are characteristic of the atom that
has been ionised .
• Characteristic photon energy is equal to the difference between binding energy of the
electron shells involved.(Ex. If l shell electron fills k shell vacancy then k-l =characteristic
photon energy).
• When outer shell electrons fill inner shell vacancies ,a characteristic cascade occurs . This
produces several x-ray photons of different energies.
• Any outer shell electron can fill an inner shell vacancy,the most likely it is the adjacent
shell.
• K shell emissions are highest in energy and are the only emissions useful to us.
• To get k characteristics we must use at least 70kv(k shell binding energy of tungsten is
69.5kev).

40. INTENSITY OF THE X-RAY BEAMS
• The intensity of the x-ray beam is defined as the number of
photons in the beam multiplied by the energy of each photon.
• The higher the atomic number of the target Atoms , the
greater will be the efficiency of the production of x-rays.
• Thus, for the continuous spectrum, the atomic number of
the target material partly determines the quantity of x rays
produced.
• The atomic number of the target material determines the
energy, or quality, of characteristic x rays produced.
For example, the K-shell characteristic x rays for tungsten (Z = 74) vary from 57 to 69
keV; those of tin (Z = 50) vary from 25 to 29 keV ; and those of lead (Z = 81) have
energies between 72 and 88 keV .

41. EFFECT OF kVp ONXRAY BEAM
THE kVp DETERMINES THE MAXIMUM ENERGY OF THE
XRAYS PRODUCED.
INTENSITY IS PROPORTIONAL TO (kVp)2

42. EFFECT OF TUBE CURRENT ON X-RAY BEAM
TH NUMBER OF ELECTRONS DEPEND DIRECTLY ON THE
TUBE CURRENT (mA) USED.THE GREATER THE CURRENT
,THE MORE ELECTRONS WILL BE PRODUCED AND
CONSEQUENTLY MORE X RAYS WILL BE PRODUCED.