The working principles of a diagnostic xray tube. The same applies for other imaging modalities.

1. DIAGNOSTIC X-RAY TUBES
Dr.Sherin Evelin Kenny
Radiodiagnosis resident
Dept.of radiology,AHRI

2. X rays are produced as a result of
bombardment of tungsten target (anode)
by free electrons from an electrically
heated cathode or filament in an
evacuated glass tube.

4. CATHODE
• Negative terminal
• Produce electrons by ‘Thermionic principle’.
Consist of:
Filament
Metallic focussing cup

5. Electric current applied
Cathode gets heated up to high
temp.
Electrons generated by
thermionic emission
Acceleration of electrons
towards anode with high
voltage application

6. a. Filament
 Tungsten wire- 0.2 mm diameter
 Coiled into a vertical spiral –
0.2 cm diameter
1 cm length-Increases SA
 THERMIONIC EMISSION-Process of emission
of electrons resulting from absorption of
thermal energy, when a material is heated at
high temperature(2200°C for tungsten.)
 EDISON EFFECT- Electron cloud surrounding
cathode ,produced by thermionic emission

7.  Space charge
The collection of negatively charged electrons in
the vicinity of filament.
Only electrons acquiring adequate thermal
energy can escape.
The tendancy of space charge to limit emission of
more electrons from the filament-
Space charge effect
 2 filaments used in modern Xray tubes – One
longer

8. Why tungsten?
Ductile
High melting point 3370 0C
Little tendency to vapourise

9. b.Focussing Cup
 To focus/converge the electron beam
 Nickel

10.  The number of X-rays produced = The number
of electrons hitting the target (anode)
 X-ray tube current measured in
milliAmpere(mA)

11. ANODE
 Positive electrode
 Converts KE of
electrons to x ray
photons(1%) and
heat(99%)
 2 types-
Stationary and
Rotating

12. a. Stationary Anode
 Uses- Portable Xray and Dental imaging
 2-3mm thick small plate of tungsten embedded
in copper bars is used at 15-200
Copper-for better heat dissipation.

13. b.Rotating Anode
 A disc of Tungsten rotating at
3000-3600 rpm
 A bevelled edge of 15-200
 Rotation spreads the heat
produced during exposure
over a large area of the
anode
 Powered by an induction
motor

14. • Life of anode disc is limited by roughening
and pitting of the surface due to thermal
• stress
•
 90% Tungsten & 10% Rhenium-
 Resistant to roughening
 Good thermal capacity
 Molybdenum and Rhodium-used in
mammographic xray tubes.

15. slits/grooves in anode
disc.
Allows expansion without
producing expansile
mechanical stress.

16. Line focus principle
 Focal spot- area of the tungsten target in the
anode where electrons strike.
Heat is distributed over the focal spot
uniformly.

17. A larger focal spot reduces the thermal damage to
the target anode.
However, a smaller focal spot is needed for
radiographic detail and accuracy
Line focus principle provides detail of a small focal
spot while allowing large amount of heat
dissipation with a slant of 6-20° in the anode
target.

18.  With this angle the apparent focal spot
becomes much smaller than the actual focal
spot (sine function of the angle)
 Angle of inclination – 60 to 200

20. Anode heel effect
Byproduct of line focus principle.

21. The intensity of the x-ray beam that leaves the x-ray
tube is not uniform throughout all portions 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.”
The intensity of the beam toward the anode side of
the tube is less than that which angles toward the
cathode.
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22. Other pre requisites
Vaccum glass enclosure
 Pyrex glass
 Vacuum-Removes obstruction(air molecules like
N2) to electrons.
The ejected electrons can lose energy and/or get
scattered by colliding with molecules in the air.
Coolant oil
Electrical insulation
Heat dissipation

23. Lead encasement
Lead has larger nucleus and higher density electron cloud.
Lead absorbs radiation and thus protects from electrons and
photons scattered by air/glass (scattered radiation).
Filters
Absorb low energy photons.
If not filtered it will get absorbed in soft tissue and add to
the dose of x-ray without any useful contribution.
A thin sheet of aluminium is used as filter.