Form of electromagnetic radiation….
Shorter Wavelength 0.01 to 10 nanometers
Higher Frequency 30 petahertz to 30 exahertz
High- Energy 120 eV to 120 keV
Great penetrating power.
Penetrate many materials opaque to light.
Travel with the speed of light ,in a straight line and can not
be focused by lens.
Photographic effect on silver halide crystal.
HEAT(99%) X-RAYS (<1%)
X-rays are produced
by energy conversion
when a fast moving
electron is suddenly
decelerated by a
Made of Pyrex glass that enclose a vacuum
containing two electrodes (This is a diode
Earliest X-Ray tube were cold cathode gas
KV & mAs are difficult to control
Presence of gas produces blackening of tube
thus shortening its life.
Variation in the no. & reduced speed of
Positive ion attracted by cathode cause spoiling
of cathode structure.
Modern X-Ray Tube is based on Coolidge tube.
Electrons produced at the cathode can be
accelerated by high potential difference toward
The purpose of vacuum in the modern X-Ray
tube is to allow the number and speed of the
electrons to be controlled independently.
Electrons are produced by heated tungsten
Accelerated across the tube(by potential diff
between electrodes)→gain kinetic energy
Hit the tungsten target
X-rays are produced by energy conversion
Source of e
A spiral of tungsten wire
0.2mm thick→ coils 1 cm long , 2mm dia
-for larger exposures
Modern x-ray may be supplied with single
more commonly a double filament.
Mounted side by side or one above the other
with one being larger than the other.
Only one filament is used for any given
When filament heated it become vaporize and
shorten the life of x-ray tube. so it should never
be heated for longer periods.
Tungsten that is vaporized acquire a bronze
colored sunburn on inner surface of glass tube.
Many modern x-ray circuits contain an
automatic filament boosting circuit.
High melting point of 33700C
Capable of stable electron emission at high
Higher mechanical strength and longer life.
Easily drawn out in to a fine filament.
Little tendency to vaporize.
When filament is heated its atom absorb
thermal energy and some of electrons in the
metal acquire enough energy to allow them to
move a small distance from the surface of
metal. The process of their escape known as
EDISON EFFECT :
The electron cloud surrounding the filament
produced by thermionic emission termed as Edison
SPACE CHARGE :
Collection of negatively charged electrons in the
immediate vicinity of filament is called as the space
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.
Made of nickel which surround the filament .
Its electrical force cause the electron stream to
converge on to the target anode in the required
size and shape.
Prevent spread out of electron stream and
bombardment of an unacceptably large area on
Negative bias voltage is applied to focus the electron
REQUIREMENT FOR AN IDEAL ANODE -:
High conversion efficiency.
High melting point .
High specific heat and thermal conductivity .
Low vapor pressure even at high temperature.
Suitable mechanical properties.
Tungsten with an atomic no of 74 and melting
point of 3370 degree , meets most of these
Target for e
2mm thick tungsten embedded in a large mass
High melting point 3370
High atomic number 74
Reasonable conductor of heat
Good conductor of Heat
A single exposure may raise the temp to 1000 c
Anodes are of two types-:
1) Stationary anode.
2) Rotating anode.
Target is small plate of tungsten ,2 or 3 mm thick
embedded in a larger mass of copper which has very
high heat storage capacity and conductivity for
efficient dissipation of heat.
Actual size of target is larger then the area
bombarded by the electron stream.
This is necessary because of relatively low melting
point of copper (10700C.).
If target is smaller ,the heat produced would melt the
The purpose of rotating anode is to spread heat
over a larger area of anode.
Ability of the x-ray tube to achieve high x-ray
output is limited by the heat generated at the
The rotating principle is used to produce x-ray
tube capable of withstanding the heat
generated by large exposure.
Anode consist of large disc of tungsten which
rotate at a speed of about 3600 revolution per
min when an exposure is made.
The electron will bombard a constantly
changing area of target and this will lead to
spread of heat over a larger area of anode.
Anode stem is made of molybdenum has a high
melting point [26000C] and is poor heat conductor.
Mb stem provide a partial heat barrier between
tungsten disc and bearing of anode assembly.
Length of mb stem is as short as possible to
decrease the inertia of tungsten disc.
Inertia of anode is reduced by decrease the weight
of anode itself.
Introduced by Philips medical system.
Has a metal casing .
Ceramic is used as insulator.
Al oxide commonly used ceramic insulator.
Metal part of tube is earth .
Because of this grounding & ceramic insulator there
is reduction in the effect of radiation.
Advantage of using metal as enclosure:-
Less off focus radiation.
Longer tube life with high tube current.
Higher tube loading.
Produced when high speed electron interact
with metal surface other than the focal track of
Main source is electron backscatter from the
Metal enclosure decrease off focus radiation by
attracting off focus electron to the grounded
small fraction of e scatter from the target and
are accelerated back to the anode, outside focal
create a low-intensity x-ray
Increases patient exposure, blurring, and
Lead collimator placed near
the x-ray tube output port
(Metal enclosure at the same
electrical potential as anode)
Cause of tube failure
Extremely high voltage.
Anode melting .
Scarring of glass tube.
Tube become gassy.
Appropriate radiographic factor (ma,kvp and
Rating derived from tube rating chart should
never be exceeded.
Anode should never be run unnecessarily.
Preparation time should be kept as short as
Adequate cooling of the tube housing must be
Space charge production
Application of potential difference
Cathode focusing cup
Interaction of electron beam
with x-ray tube target
< 1% x-rays
Kilo Volt peak
( max) Potential difference btw electrodes
energy of e
Kilo electron Volt
Kinetic energy of e when acc by 1 Volt
(With kVp 100 the MAX energy expected will b 100 keV)
milli Ampere second (Tube current)
Current carried by flow of e in tube
Depends on no. of e/ filament current
Up to saturation voltage an increase in kvp
produce a significant increase in x-ray current.
Above this further increase in kvp produce
very little change tube current.
Below saturation voltage current flowing is
limited by space charge effect.
Above this it has no influence on tube current.
What is focal spot.,?
That part of anode impacted by
small focal spot
large focal spot
Larger the area over which heat is
developed, less danger of destroying the tube.
To have sharper anatomical picture ,the
apparent focal spot size should be of smaller
Surface of target anode is inclined so that it forms an
angle with the plane perpendicular to the incident
Anode angle may vary from 6 -20degree.
Size of apparent focal spot is smaller then that of
actual focal spot and is directly related to the sine of
the angle of anode.
Smaller the angle ,the apparent focal spot also
“a reduction in the x-ray beam intensity
toward the anode side of the x-ray field.”
Intensity of x-ray beam that leave the x- ray
tube is not uniform throughout all portion.
Depends on the angle at which x-ray emitted
from the focal spot.This variation is known as
Intensity of beam toward anode is less than
the steeper the anode, the more
noticeable the heel effect becomes.
less prominent with
distance (subtends a
The thicker portion of the anatomical part is
placed beneath the cathode end of the x-ray
S. Guilbaud, Education Director
The following anatomical parts may be imaged
using the anode heel effect:
Tibia & fibula
S. Guilbaud, Education Director
interacts with the
nucleus of target
interacts with an
orbital electron of
Involves two processes
e comes within the proximity of a nucleus
→ Columbic forces attract and decelerate the e,
→ Loss of kinetic energy and change in
An x-ray photon with energy equal to the
kinetic energy lost by the electron is produced
Distance between the bombarding electron and
the nucleus determines the energy lost ,
closer it gets to nucleus greater the energy loss.
Incident e with energy > binding energy
eject e from inner orbits
e from outer shell (L) loses energy and
The energy lost by this electron is
emitted as x-ray photon
Energy of x-ray photon depends on
element and shell not on energy of
incident e (CHARECTERISTIC)
In tungsten (K-72 L-11) k characteristic
radiation is ~59 keV
Results when the electrons bombarding the target eject
electrons from the inner orbits of the target atoms
The wavelength of the x-rays produced are characteristic
of the atom that has been ionized and is not changed by
the kVp used (Of course, the applied kilovoltage must
be high enough to excite the characteristic radiation)
The quantity of x rays generated is proportional to the
atomic number of the target material (Z), the square of
the kilovoltage [(kVp)2], and the milliamperes of x-ray
tube current (mA)
The quality (energy) of the x-rays generated depends
almost entirely on the x-ray tube potential (kVp)
Photon energy Vs.
The position of the spectrum
,on the x-axis the further to
the right it is the higher the
The area under the spectrum
, the greater the area the
greater the quantity.
Max photon energy
Max kinetic energy of e
Max potential diff betw electrodes
Number of e that flow (mAs)
Max energy - max kinetic energy
i.e.., kVp across electrodes
Min energy - filters
Typically peaking -at about 1/3 of the
projectile electron energy
Effect of added filtration
What is the function of filters.?
…..Removes the low energy
Tube current (mA):
increases the amplitude of the spectrum (area under the
Tube voltage (kV):
increases the kinetic energy , amplitude of the spectrum
and shifts it to the right.
will decrease the amplitude, more so on the left side than
changing to a better target material will increase
amplitude AND shift discrete spike to the right.
the continuous x-ray
at all kV settings
at least 70 kV. (k-shell
binding energy of
tungsten is 69.5 keV).
fixed, discrete, energies.