X ray production

1. PRODUCTION OF X-RAYS
Dr. Khazi Mohammed Athar

2. X-Ray tube

3. KILOVOLTAGE,MILLIAMPERAGE
Two sources of electrical energy are required and are derived from AC
mains using transformer.
1) Filament heating voltage(10V), current (10A)
2) Accelerating voltage(typically 30-150kV) between anode and
cathode; this derives current of electrons(typically around 0.5-
1000mA).
The mA is controlled by adjusting filament voltage and current and
thus filament temperature.

4. 1) Cathode: Negative terminal of tube is cathode.
It has filament which is the source of electrons made up of
Tungsten wire (0.2mm in diameter)coiled in vertical spiral
filament(0.2cm in diameter and 1cm or less in length).
Connecting wires which supply heating filament voltage (about 10V) and
amperage(average about 3-5A) which heats the filament and metal
focussing cup.
When metal is heated, atoms absorbs thermal energy adequate in amount
to escape from its nuclear attraction by a process called Thermionic
emission.
Electron cloud surrounding filament produced by thermionic emission is
termed as Edison effect. Pure tungsten must be heated to atleast to 2200°c
to emit useful number of electrons.

5. Electrons emitted from tungsten filament form a small cloud in
immediate vicinity of filament. This collection is called Space charge.
The tendency of space charge to limit emission of more electrons from
filament is called space charge effect.
High currents that can be produced by thermionic emission are possible
because large numbers of electrons can be accelerated from cathode to
anode.
The number of x rays produced depends on number of electrons that flow
from filament to target anode
The tube current measured in mA refers to number of electrons flowing
per second from filament to target.

6. Tungsten is not as efficient as it’s alloys in emitting electrons but it can
be drawn into thin wire and quite strong, has high melting
point(3370°)and little tendency to vaporise.
2) Glass enclosure: Two electrodes are sealed in Pyrex glass in a
vacuum. Vacuum allows the number and speed of accelerating electrons
to be controlled independently.
Pyrex can withstand high temperatures it is used in x ray tubes
Connecting wires must be sealed into glass wall.
3) Focusing cup: is maintained at negative potential.
So that it’s electrical forces causes electron stream to converge onto
target in required size and shape.
It is usually made up of Nickel.

7. 4) Anode: Tungsten is chosen as target material because,
I) Has high atomic number
II) High melting point
III) Good material for absorption and rapid dissipation of heat.
Positive electrode can be stationary and rotating
Stationary: Small plate(square or rectangle, each side >1cm) of tungsten
embedded in large mass of copper. Anode angle is usually 15-20°.
Focal spot is area of anode bombarded by electrons from cathode. Most of
energy of electrons is lost in the form of heat, with less than 1% being
converted to x rays.
Line focus principle: Because of angulation of anode(6-20°) when focal spot
is viewed from direction in which x rays emerge, surface is foreshortened
and appears smaller.

8. Decrease in projected focal spot size is directly related to sine of angle
of anode.
Rotating: The purpose of rotating anode is to spread heat over large area
produced during exposure.
The rotating anode consists of large disc of tungsten or an alloy of
tungsten, which rotates at speed of 3600 rpm (theoretically)and disc has
beveled edge. Angle of bevel 6-20°
The power to effect rotation is provided by magnetic field produced by
stator coils that surround neck of x ray tube outside the envelope.

9. Magnetic field produced by coils induces current in copper rotor of
induction motor, this induced current provides power to rotate the
anode assembly.

10. Metallic lubricant like silver is used to reduce friction in bearings
Stem made up of Molybdenum is used to connect Tungsten target with
rest of anode assembly in order to avoid undesirable absorption of heat by
bearings of anode as Molybdenum is poor conductor of heat.
Length of Molybdenum is kept as short as possible to reduce load on
bearings.
Anode made up of alloy of Tungsten and Rhenium make it more
resistant to surface roughening.
GRID CONTROLLED X-RAY TUBE
A third electrode is used in focussing cup that surrounds that’s filament.
Negative bias voltage around 1500V is applied to cup relative to filament
to punch off flow of electrons.It act as on an off switch.

11. Intensity of x ray beam that leaves x ray tube is not uniform throughout
all portions of beam.
Intensity of beam toward
anode side is significantly
less than cathode side.
This factor can be used in
obtaining balanced densit
ies in radiographs of body
parts of different thickness
Thicker parts should be pl
aced towards cathode side

12. Tube shielding: Tube housing is lined with lead and serves to absorb
primary and secondary x rays that would otherwise produce needless
exposure to patient and personnel.
Off focus radiation: Produced when high speed electrons interact with
metal other than focal tract of anode usually from backscattering from
anode.It can partly controlled by collimators or lead diaphragm.

13. Characteristic Brehmsstrahlung

16. THANK YOU