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X-ray Tube
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X-rays
X-rays are generated from
the interaction of accelerated
e ’s & a target metal
(tungsten).
Patient is placed between Xray tube and silver halide film.
X-rays passed through the
body are absorbed in direct
proportion to tissue density.
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X-rays penetrating the body strike
the silver halide film and turn it dark,
the more x-rays that penetrate, the
darker the area inscribed on the film.
Bones & metal absorb or reflect
X-rays are inscribed area on film is
“lighter” or “more white”.
Soft tissues allow more X-rays to
penetrate are inscribed area on film
is “darker”.
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Visualizing tissues of similar
density can be enhanced using
“contrast agents”.
Contrast agents: dense fluids
containing elements of high
atomic number (barium, iodine).
Contrast agents absorbs more
photons than the surrounding
tissue or cavity appears lighter.
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These contrast agents can be
injected, swallowed, or given by
enema.
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electron beam generator
tungsten target metal
resultant X-ray beam
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Diagnostic Medical X-Ray Unit
Diagnostic
(portable)
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Dental (Diagnostic) X-Ray Unit
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Diagnostic Medical X-Ray Unit
Diagnostic x-ray tube in a
mobile x-ray unit
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X-Ray Tube Components
Diagnostic Medical X-Ray Unit
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Tube Components
X-ray tube is one of
the components of a
basic radiographic unit.
Other components are:
 Operating Console
 X-ray tube
 Automatic exposure
control
 Exposure control
 Beam limiting device
Diagnostic x-ray tube in a
mobile x-ray unit
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X-ray tube
comprise of
3 components:
Cathode
structure
Anode structure
Glass envelope
Diagnostic x-ray tube in a
mobile x-ray unit
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Functions of X-Ray Tube
 An x-ray tube converts its input of
electrical energy into an output of X-ray
energy.
 In the case of low power equipment, an
x-ray tube also acts as a rectifier.
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Production of X-Rays
Requires 3 main components:
–Electron source
–Method of accelerating electron
–Method of
stopping
(braking)
electron
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X-Ray Tube
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Types of X-Ray Tube
Types of X-Ray Tube
2 types of x-ray tube:
Stationary Anode
X-Ray Tube
Rotating Anode
X-Ray Tube
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Stationary Anode X-ray Tube
Low electric power.
Relative simplicity of design and
construction and therefore low cost.
Suitable for the production of
X-rays at low or medium intensities.
Used for applications such as
dental radiography and mobile work
where no sophisticated procedures
such as rapid sequential imaging
are required.
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Rotating Anode X-ray Tube
 Higher X-ray intensities and
electrical power are provide by
the anode tube since it has
more efficient anode cooling.
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Stationary Anode Tube
Stationary anode tube: insert and housing
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Construction of
Stationary Anode Tube
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Glass envelope
Glass envelope is used to
enclose the vacuum within
the x-ray tube.
The envelope is joined to
the copper anode at one
end and the nickel cathode
support at the other reentrant seal.
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The glass must be a good
electrical insulator, or a
substantial current will flow
through it when a potential
difference is applied between
the anode and cathode.
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Cathode
Cathode consists of the
following components:
 Filament
 Focusing cup
 Supporting wires
 Cathode support
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Filament(s)
The source of electron is a
filament, heated by an
electric current. The current
increases the vibration of
atoms within the filament so
much that it emits heat and
light.
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So much heat energy is
acquired by the atoms that
some of their electrons
(outer) can break free and
temporarily leave the
filament’s surface. This
phenomenon is known as
thermionic emission.
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The rate at which electrons
are emitted rises with the
filament’s temperature.
Thermionic emission:
emission of electrons from the
surface of a metal after heated
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The filament is made of thin
coiled tungsten wire for the
following reasons:
Tungsten is a good
thermionic emitter.
Tungsten has a low vapour
pressure, i.e. it does not
vaporise easily. It therefore
lasts a long time.
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Tungsten is rugged and able
to be drawn into the thin wire
required.
Easy to be shaped
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An x-ray tube may have 2
filaments of different sizes
placed side-by-side. This is
known as a dual-focus tube.
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The 2 filaments produce 2
different sizes of electron foci
on the anode, one for
general use and the other
(smaller) for fine-focus
applications requiring less
geometric unsharpness.
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Most modern tubes have
twin foci, e.g. 1.2 mm and
2.0 mm, 0.6 mm and 1.2
mm, 0.3 mm and 2.0 mm,
etc.
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Electron Sources (e-)
e- produced via thermionic emission
Electric current is supplied through the
filament
Frequently, 2
filaments control
the size of the area
of interactions of
e- with the target
and therefore the
image quality
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Focusing cup
Focusing cups are usually made
of either nickel, molybdenum or
stainless steel. Cup shape metal
Used to focus electrons towards
target material. These materials
possess high melting point and
is a relatively poor thermionic
emitter.
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Anode
Anode
Anode is constructed of the
2 materials copper and
tungsten, known as a
compound anode.
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Anode consists of:
The block and stem is
made of copper.
The target or an inset of a
thin (~ 2-3 mm) tungsten
plate on the inclined face of
the copper block. Electrons
from the filament are
focused on the target.
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 1% of energy is converted to
X-rays, plus heat.
The anode rotates to increase
heat load
capacity.
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e- accelerates to positively
charged anode
Requires high potential
difference to produce x-rays
Diagnostic energy range usually
30 kV – 150 kV
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Target material
The target material is
made of tungsten for the
following reasons:
Tungsten has a high
atomic number Z of 74.
Intensity of an x-ray
beam is proportional to Z.
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Tungsten has a low vapour
pressure, so that it does not
readily vaporise at its normal
working temperature.
Tungsten has a high melting
point (3387°C), so that it can
withstand the heat generated
during an x-ray exposure
without melting.
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Tungsten has a relatively
good thermal conductivity (½
that of Cu), thus enabling a
rapid transfer of heat from
the small focal area to the
anode block by conduction.
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Tungsten is a suitable
material for machining into
the shape and size required.
Focal spot: the area within
the target material where
electrons hit the target to
produce x-ray
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The design of the cathode
assembly is such that a socalled line focus is produced
on the anode. In fact, the
electrons from the filament
are focused onto a thin
rectangle rather than a line.
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The face of an anode is set
an angle with respect to the
axis of the X-ray beam by an
amount known as the anode
angle.
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Effective focus, a
is given by:
a = r sin 
where, r = true length of the
line focus
and
 = anode angle
The effective focus is
smaller than the true focus,
a < r.
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Advantages of using a
line focus
 Size of effective focus is smaller
than the real focus (figure).
The filament
may thus be
relatively long
without giving
rise to poor
geometric unsharpness.
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Thermionic emission from the
filament is proportional to its
surface area so that a long
filament is able to provide the
high values of mA required in
many
exposures.
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 The area over which the heat
is deposited is the area of the
real focus. The temperature
rise experienced by a large
area is less than that
experienced by a small area
for the same amount of heat,
since more atoms are able to
take part in the heat transfer
process.
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The line-focus principle enables
the heat to be spread over an
area about 3 times larger than the
effective focus, so enabling a
compromise between the
requirements of minimising the
temperature rise in the target
(requiring a large focal area) and
minimising the size of the X-ray
source (requiring a small focal
area).
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Anode Heel Effect
Anode Heel Effect
Anode heel effect is due to the
higher absorption of those
X-rays which pass through the
greater thickness of target.
This effect produced
irregularities in the surface of
the target as a result of
prolonged use of the tube.
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The formation of such irregularities
in the target is known as pitting
and is caused by vaporisation of
the tungsten from the target
(Figure).
Figure:
Effect of anode
grazing
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The anode heel effect
produces a reduction in
X-ray intensity for those
X-rays which are emitted
from the anode at neargrazing angles to the face
of the target.
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Figure shows anode heel
variation of X-ray intensity.
Intensity IF > IC > ID > IE . Beyond
D, e.g. at E, there is no significant
radiation, due to absorption within
the anode.
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The anode heel effect may
have no practical
significance, particularly if
field size is small. When the
whole width of the beam is
used, however, its intensity
variation may cause a
detectable contrast in image
density, from one end to the
other.
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The high temperatures
reached by the anodes of
such tubes result in thermal
stresses within the anode
which may cause distortion
of the target angle with a
consequent reduction in
radiation output, a restriction
in field size and in extreme
cases cracking of the anode.
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The use of rhenium tungsten
alloy as a target material
helps to minimize (pitting)
crazing of the focal area and
the use of high speed, large
diameter, compound anodes
permits increased loading of
the focal area. (faster heat
transfer).
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Crazing is reduced by using
a rhenium tungsten alloy
(10% rhenium, 90%
tungsten) as the target
material. Radiation output is
therefore maintained at a
higher level giving the X-ray
tube a longer useful life.
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