Transcript Care of the X

02
X-ray Tube
2
Rotating Anode Tube
Rotating anode tube is
similar in many respects to
the stationary anode X-ray
tube, plus a number of
additional features (Figure).
3
Figure: Rotating anode tube and casing
4
Rotating anode X-ray tube
produce higher intensities of
X-ray beams than the stationary
anode tube. This is due to 2
factors:
The heat deposited on the
anode is spread over a much
larger area.
The cooling characteristics of
the rotating anode are
superior.
5
Figure: Rotating anode tube
6
Construction of the rotating
anode X-ray tube. Cathode has
the same basic construction
except that it is off-set from the
central axis of the tube in order
that electrons emitted from it
strike the
bevelled
surface of
the anode.
7
Anode Assembly
Anode is made of a tungsten
or molybdenum disk with an
accurately bevelled edge.
8
 The anode has a central hole
through which it is connected
to a beryllium anode stem and
hence to the
rotor of the
induction
motor
(Figure).
9
The diagrammatic representation
of the anode and rotor assembly
in figure below illustrates the
arrangement.
Figure:
X-ray tube insert
and stator
10
Insert tiub sinar-x dan stator
11
The stator is composed of 2
windings set into a circular
iron core (figures below).
12
Figure: Stator core and windings
13
Figure: Photograph of stator core
14
Rotor, stem and anode disk are
accurately balanced so that no
wobbling occurs when the whole
assembly rotates.
15
In the more modern rotating
anode tubes, the anode is
made of a solid block of
molybdenum
with a thin
coating of
an alloy of
tungsten and
rhenium on
the surface.
16
Molybdenum has double the
specific heat capacity of
tungsten and so produces an
anode of much
higher heat
storage capacity
in terms of Heat
units.
17
Rhenium has an atomic
number of 75 (tungsten, 74)
and so has a good conversion
of electron energy to X-rays. It
also has the advantage of
slowing down the ageing
process on the anode surface
caused by the inevitable pitting
of the surface during
exposures.
18
Induction Motor
Induction motor is used to drive
the rotor. The motor work on the
principles of electromagnetic
induction, particularly Lenz’s law.
The principle of this type of
motor is depicted in Figure
below.
19
Figure below shows more
efficient system, where more
magnets are used and a higher
flux linkage is obtained.
The copper drum
(the rotor) follows
the direction of
rotation of the
magnets.
20
The magnetic field from the
solenoids penetrates the glass
envelope of the X-ray tube and
interacts with the copper rotor on
the anode assembly (Figures
below).
21
Electron Focusing
During conduction, the anode of
the X-ray tube is positively
charged and the cathode is
negatively charged (Figure).
22
The electron space charge
emitted by the heated filament
is thus repelled from the
cathode and attracted towards
the anode, since electrons are
themselves negatively
charged.
23
In the case of a rotating
anode tube, the heat
caused by conversion of
their energy is spread over
a much larger area, the
disc’s focal track. For a
stationary anode, electrons
bombard a small,
rectangular area.
24
Figure: A conventional rotating anode disc.
(a) Face view; (b) In profile.
25
If both the anode and cathode were
flat plates, the electric field consists
of parallel lines starting from the
anode and finishing on the cathode
(Figure).
Figure: Focusing of the
electrons in an X-ray tubes.
The concave focusing cup
directs the electrons from
thermionic emitter F toward the central
axis, so that they strike the anode over
a small area.
26
The plume of electrons from the
thermionic emitter F, being
charged negatively, travel against
the direction of the electric field,
striking
the anode over a
width, W (W>F). The
situation  produces
an unacceptably large
focal area on the anode.
27
In the case of focussing-cup
cathode, the thermionic electrons
from F now experience a force
which is always towards the
central axis (as well as towards
the anode) owing to the shape of
the electric lines of
force produced by
the focusing cup
(Figure).
28
The plume of electrons is 
spread over a much smaller
width, w, on the anode (w<F)
and the electron beam is said to
have been focused. A line-focus
is produced.
Figure: The focusing cup directs
the electrons from thermionic
emitter F toward the central
axis, so that they strike the
anode over a small area.
29
Anode Heel Effect
Figure below shows crazing or
roughening of the target surface.
Such crazing results in a loss of
radiation output from the tube.
Photons of radiation being
absorbed to a greater extent as
they must now penetrate a
greater thickness of metal.
30
Roughening of the target
surface, due to cumulative
effects of heat stress,
creates pitting, the formation
of small crevices into which
electrons from the filament
may enter.
31
X-ray photons so produced
have a greater thickness of
anode substance to
penetrate before they
emerge from its surface.
This reduces the beam
intensity, compared with the
output from a new, smooth
target surface (Figure).
32
Figure: Effects of target surface pitting
33
Figure
(a) Effect of
anode
crazing.
(b) Photographs
of crazed
anodes.
34
Care of the X-ray tube
An X-ray tube is an
expensive and vulnerable
piece of equipment. If
damaged, it is unlikely to be
repairable.
35
Tube damage may be either
physical or thermal. Physical
damage is avoided simply by
ensuring that the tube is never
allowed to collide with any other
object, and that all movement is
reasonably smooth and steady.
Thermal damage is minimized
by:
36
Keeping the rate of heat
production within the safe
limits, determined by the tube
manufacturer.
Ensuring, before making each
exposure, that the anode can
safely accept the extra heat
which the exposure will bring.
37
38
X-Ray Tube Rating
X-ray Tube Rating
Definition
The rating of an X-ray unit is
that combination of exposure
settings which the unit can
just withstand without
incurring unacceptable
damage.
39
Any exposure gives rise to some
‘damage’ to the X-ray tube, since
the anode becomes slightly more
pitted and the filament becomes
slightly thinner. However, in this
context unacceptable damage
means that amount of damage
which will seriously impair the
performance of the unit for further
exposures or even to make the unit
completely inoperative.
40
Single Exposure Factors
The exposure factors which
are under the control of the
operator are:
Selectable Factors
Non-selectable factors
41
Selectable Factors
kVp, mA, exposure
time/mAs, focal spot
size, single / multiple
exposures, radiography
/ screening.
42
Non-selectable factors
Stationary Anode Tube
Rectification, thermal
capacity of anode/shield,
efficiency of heat loss from
anode/shield, anode angle,
filtration, rating of hightension cables and
transformers.
Rotating Anode Tube
As above, plus: anode
diameter, anode rotation
speed.
43
Rating Chart
Stationary Anode Tubes
Rating chart shows the
effect of varying the
quantities in the selectable
factors list, i.e. those which
may be altered.
44
For any particular X-ray
unit, the quantities in the
non-selectable factors list
is unalterable, and a rating
chart is used which is
applicable only to that unit.
45