X-Ray tube X-Ray production Anode heel effect

1. X-RAY TUBE & PRODUCTION
OF X-RAYS.
Presented by- Tajdar Amber
Radiology 1st Year Resident

2.  Major Components Of X-ray Tube
X-RAY TUBE DESIGN
Cathode- Negatively charged
Anode- Positively charged
Glass envelope- Supports the anode and
cathode structure
X-rays are produced by energy conversion when a fast moving
stream of electrons from the cathode is suddenly decelerated in the
target anode of an x-ray tube.

3. Cathode Assembly consist of
 Filament
 Focusing cup
Filament
 Tungsten wire
 Embedded in a focusing cup which is negatively charged
 Powered by 10-15volts at 4-6 amperes
 Thermionic emission
 Source of electrons
 About 0.2mm in diameter, coiled form a vertical spiral about
0.2cm in diameter and 1cm or less in length
Current
 Filament current- Flow of electrons through the filament in order to raise its
temperature and release electrons.
 X-Ray Tube current- the number of electrons flowing per second from the
filament to the target.(measured in milliamperes)

4. Advantages of Tungsten filament
o High melting point(3370°C)
o Good thermionic emitter
o Rugged and can be twirled into thin wire
o Less tendency to vaporize
o Stability and strength
o Long life expectancy
o Trace amount of thorium with tungsten is found to be
efficient for electron emission and prolong the life of
filament

5. Thermionic emission
When current flows through the filament, the heating cause
the release of electrons from the surface of the tungsten
metal wire. The electrons are liberated at a rate
proportional to the filament current.
Therefore, Electrons emitted resulting from the absorption
of thermal energy is known as thermionic emission.
The electron cloud surrounding the filament, produced by
thermionic emission has been termed “The Edison Effect”.

6. Space charge effect
Electrons emitted from the tungsten filament form a cloud in
vicinity of the filament. This collection of these negatively
charged electrons is called the space charge.
The tendency of space charge is to limit emission of further
electrons from the filament is known as Space charge effect.
Equilibrium state
when a filament is heated to its emission temperature, a
state of equilibrium is quickly reached
 The number of electrons returning to the filament is equal to the
number of electrons being emitted
 Hence, space charge will remain constant depending on the filament
temperature.

7. X-Ray tubes may be supplied with single or, more commonly, a double filament.
Single filament
Double filament
 Broad focus, larger filament
Higher current, lower resolution, larger area of exposure
 Fine focus, smaller filament
lower current, higher resolution, small area of exposure
Triple focus(three filament)- used for Angiography
Single/Double/Triple Filament Modern X-Ray tube.

8. Focusing Cup
Focusing cup is concave
Made of Molybdenum nickel alloy
High melting point
Poor thermionic emitter
Converge the electrons to the focal spot on
target.
Supports the filament

9. Anode
Positively charged
Attracts the electrons when the exposure button is pressed
Made of
Tungsten in all radiographic tubes including digital mammography
 Higher X-Ray production efficiency due to higher atomic number
 Improved heat absorption capacity and rapid dissipation of the heat away from the target area
 High melting point
 Wider dynamic range of digital detectors relaxing the requirement of X-Ray beam
Molybdenum and Rhodium anode targets are used in conventional
mammography
 Characteristic X-rays near 20 keV, desired for breast imaging

10. Two types- Stationary or Rotating
Stationary anode
 Tungsten(Anode target) is fixed within a large mass of
copper block
 Low output machines
 Eg Dental, Mobile radiography
Disadvantage(or problem with stationary anode)
In spite of tungsten good thermal characteristics, it can’t withstand the heat of
repeated exposure.
Copper base is used to increase the total thermal capacity of the anode and speed
its rate of cooling but copper and tungsten have different coefficients of expansions,
hence, the tungsten target would tend to peel away from the copper anode

11. Rotating anode X-Ray tube
The purpose of the rotating anode is to spread the heat produced during an
exposure over a large area of the anode, and made it capable of
withstanding the heat generated by large exposures
Tungsten anode target is rotating
High end machine which requires producing high intensity X-rays in short
time
Eg- general purpose X-Ray machines such as DR, Angiography.

12. Rotating anode assembly consist of
o Anode disk
o Anode stem
o Rotator assembly
o High tension connection from
positive end of the tube circuit
Anode disk
 Tungsten(90%) alloy with rhenium(10%) to avoid cracking and has more resistant to
surface roughening and has a higher thermal capacity compare to pure tungsten’
 Tungsten layered with Molybdenum/Graphite; easier to rotate
(Molybdenum and graphite is lighter and has lesser specific gravity which help to
decrease disc inertia)
 Electron beam interacts with larger area on rotating target and hence heat is not
concentrated at one point; good thermal dissipation
 High speed rotation 3400rpm – 10000rpm

13. • The rotor is made of copper which is actually part of
a motor that is made to rotate by the device called
Stator
• The stator is a set of electric coils that produces a
very strong magnetic field outside of the glass
envelope that in turn “pulls along” the copper rotor
• The stator causes the rotor to rotate very fast just
like any electric motor. The stator is also called an
Induction motor
• The bearings are very sensitive and if the X-RAY tube
heats up abnormally high, they can be damaged and
can cause the rotor to rotate at less than 3200rpm. If
this occurs, the target surface can be damaged or
“pitted”.

14. Line Focus Principle
The focal spot is the area of the tungsten target(anode) that is
bombarded by electrons from the cathode.
Most of the energy of electron is converted into heat, with less
than 1% being converted into X-Rays.
therefore, we need a large focal spot to allow greater heat
loading, and the conflicting need for a small focal area to
produce good radiographic detail, were resolved in 1918 with
the development of Line Focus Principle.
Line Focus Principal
o Effective focal spot smaller than the actual focal spot
o Angling the target 5°-20°
o Decrease in target angle or anode angle also decreases the effective
focal spot
Actual focal
spot

15. Anode Heel Effect
The intensity of the X-Ray beam is greater towards the
cathode side than near the anode
Most of the energy is absorbed at the anode heel
This effect depends on the anode angle
o Steeper the angle more is the effect
o Source to detector distance(SID) should be increased to minimize the
effect
A B
Image A- Patient positioned with thinner region towards the cathode
Image B- Patient positioned with thicker region towards the cathode.

16. X-Ray photons that are not directed at the focal spot area.
It has no diagnostic value
Low intensity beam
Increases patient radiation dose
Reduces image contrast
Use of lead collimators(exit port of the X-Ray tube) can reduce
the off focus radiation.
Off Focus Radiation

17. Glass Envelope
Evacuated tube made of borosilicate (Pyrex) to withstand high heat
generated
Encompasses anode and cathode
Good electrical insulator
Long term usage leads to tube arcing (tungsten vaporizes and
forms a thin coating on the inner surface of the glass)
Improvement using metal/ceramic which maintains constant
electrical potential between the electrons and envelope thus
resulting in longer life

18. Tube Housing
Metal case made of aluminium alloy
Lead lined inside layer protects and supports
the glass tube insert
Oil bath- provide electrical and thermal
insulation
Tube housing provides efficient radiation
barrier, shielding for high voltage
Leakage radiation from tube housing
Limit- not exceeding 114mR/hr (1mGy/hr) at
1meter distance

19. Cooling Mechanism Of X-Ray Tube
Heat generated in the X-Ray tube is dissipated by radiation from the anode
Radiation- transfer of heat by emission of IR
Conduction- transfer of heat through solid parts of anode
Convection- transfer of heat through oil bath

20. Tube rating charts
 The maximum temperature that can be safely accepted by an X-
Ray tube is a function of the heat energy produced during the
exposure
o Tungsten can be safely raised to 3000 °C above this level
considerable vaporization of the tungsten target
occurs
 The Heat Unit (HU) is defined as the product of
current(mA) and kVp and time(sec) for single-phase power
supplies
 The safe limit within which an X-Ray tube can be operated
for a single exposure can be easily determined by the
Tube Rating Chart supplied with all X-Ray tube
 Every machine has its own unique chart
 The highest technique factors that are possible without
damaging the target.

21. Anode Heating and Cooling Curves
 This chart help to determine the length of time the
tube must be allowed to cool before additional
exposure can be made
Eg- if a rapid filming procedure required 20exposures,
the total anode heating will be 100,000 J. It could be
determined from the chart that it would require
approximately 6min for the tube to cool from 100,000 to
10,000 J
 Thermal capacity of anode and its heat dissipation
characteristics.