Every radiologic technologist must be able to recognizes the cause,types,mechanism and remedy for any faults in x ray tube.

1. FAULTS OF X-RAY TUBE &
ITS CARE
A PRESENTATION BY:
SANDESH GHARTI
MAGAR
B.Sc. MIT 12th BATCH
ROLL NO:132
IOM
1

2. Introduction
 X-ray tubes are a proven, cost effective way to
produce X radiation useful in medical imaging ,
inspections & scientific fields.
 For over 100 years , X-ray tubes have made advances
owing to new applications , materials , processing
equipment and design.
 Today two types of tubes are dominating i.e.
Stationary anode tube & Rotating anode tube.
2

3.  In X-ray production, less than 1% of the energy is used
to produce x-ray while the remaining 99% is
transformed into heat. This factor limits the useful life of
the x-ray tube.
3

4. GROSS ASPECT
 With use, X-ray tubes are found to undergo changes
which can be linked to a process of ageing & they
may also develop a fault as a result of misuse which is
mechanical or electrical.
 Many scientific disciplines are required & must be
controlled to produce a quality image & to make the
tube life longer. These includes:
4

5. 1.Thermodynamics
2.Material science
3.Heat transfer
4.Vacuum technology
5.High voltage
6.Electronics accessories
7.Atomic/Radiation disciplines
8.Manufacturing processes & many other
technologies
5

6. The deteriorating processes & the fault can be
develop due to following reasons:
 Normal ageing.
 Deficiencies in Manufacturing.
 Application Mismatch.
 Improper Drive by the power supply
6

7. 7
Normal Ageing
 X-ray tubes have a limited life because the
characteristics & material used begin a gradual
degradation & are consumed so that performance
gradually decreases.
 Ageing in tube can be raised due to faults
developed in any part of the x-ray tube as
mentioned below:

8.  Most tubes are manufactured with glass as the
vacuum wall vessel but the glass also perform the
task of insulating the tube electrode(cathode, anode
and ground) from leakage current and arc over.
8Faults in glass envelope

9.  With time and depending on used exposure
factors, tungsten metal from anode and filament
begins to evaporate on to the glass surfaces
causing eventual arc over and tube failure and
reduction in the insulating property
9

10. Precautions:
Various methods are used to mitigate the effects
of the evaporation such as :
 Sand blasting the glass(which increases the insulating
path)
 Using a hooded anode on stationary anode(a hood
reduces target evaporation on to the glass)
 Metal centered vacuum walls(which reduces filament
evaporation on to the glass in rotating anode)
 Use of ceramics.
10

11. NOTE:
These techniques do not eliminate metal evaporation
but greatly reduces its deposition and at the same time,
these techniques can produce other undesirable effects.
For e.g. : sand blasting glass can lead to glass particle
release which causes arc over.
11

12. Faults in anode
 Due to excessive heating of target, the original smooth
surface of target track takes an appearance of paved
pathway known as crazy paving.
 The roughening will reduces the radiation output and
will affect sharpness of outline in radiographic imaging.
 Due to this it may become necessary to increase the
exposure value above the normal.
 As a result, heavy radiographic exposure made on a
cold target can cause the anode disc to split radially.
 The wide cracking can cause imbalance of the anode
and increase noise from ball bearing.
12

13.  An overheated anode disc results in the change in
anode angle making the smallest FOV for large sized
film.
 This may also cause greater geometric blur in parts of
images.
Precautions:
Micro cracking in anode disc can be reduced
by using:
 Lowest necessary power.
 Largest possible focal spot.
 Longer exposure at reduced power rather than shorter
exposure at higher power.
13

14. Faults in rotor and its bearing
 Rotating anode tube consists of rotor which is
constructed from more than one metal i.e.; anode
stem of Mo & rotor’s body of Cu.
 Since the rotor and stem becomes hot, the thermal
expansion coefficient of these two metals are
important and different between them can leads to
mechanical changes after much use.
 High temperature and high speed will reduce
bearing life the most.
14

15.  With operation, the lubricant (which is usually silver
or lead metal) wear off of the ball and race surfaces
leaving steel to steel contact which leads to binding
or jamming.
15

16. Precautions:
Deteriorating process in bearing can be
reduced by:
 Maintaining normal speed of rotation.
 Avoiding exposure on cold cathode.
 Lowering the heat subjection to bearing.
 Using effective lubricating materials
16

17. Faults in the stator windings
 If the faults develop in the stator windings, there is
no power supply to make the anode rotate.
 As the anode remains stationary due to no power
supply, the load that should have been applied to
the rotating anode will unknowingly be applied to
a stationary one.
 This is very likely to overload & overheat the anode
& can result in cracking of anode disc.
 The tube can be protected against this type of
accident by arranging the circuit which prevent the
exposure from taking place when the stator is
without its power supply.
17

18. Faults in the filament
 Failure of the filament to heat when its circuit is energized
may be due to:
i. Break in the filament itself.
ii. Faults in the circuit which supplies to
power it.
 Since the filament is heated for every exposure & the heat
vaporize the tungsten from it, the filament becomes older
& thinner nearly to break which seems like fracture of
filament.
18

19. Fig: Flowchart of filament Failure
19
No development of images on the film
No production of x-rays
No reading on mA meter
No thermionic emission
Failure of the filament to heat

20. Precautions:
Filament life can be extended, during the
use of the tube if the filament is energized only for the
shortest periods.
20

21. Faults in the vacuum
 With long use, the x-ray tube may become gassy due
to which the vacuum is spoilt.
 As a result, the milliamperes becomes erratic & runs
away so that it reaches the high values..
 A gassy tube will go from bad to worse if it continues
to be used & the best course of action is to stop
using until the tube replacement can be obtained.
21

22. Deficiencies in Manufacturing
1. Immediatefailure
 No matter how hard a manufacturers tries, not all
tubes are made exactly same.
 Small difference exist, but the manufacturer needs to
make sure that such differences don’t impact the
tube operations.
 Immediate failure consists of:
22

23. i) Weed out by Test:
 After a tube is produced & processed, it is subjected to
different testing's as mentioned below:
- Battery test(to make sure it meets the
performance standard established for that
model).
- High voltage stability test(to removes
gases & particles.
- And others pertinent characteristics
are tested & measured.
23

24. - Cathode emission test.
- Filament volt ampere
characteristics
- Focal spot size test.
- Thermal loading.
- Noise, vibration( for rotor & stator)
 Tubes that don’t meets the specifications are
rejected & analyzed. So that corrections can be
made immediately
24

25. ii)Hold Period:
 Sometimes, despite the satisfactory testing if
tubes are held for 2-4 weeks, they don’t perform
satisfactorily esp. under high voltage conditions.
 The change in performance is usually caused by
tiny vacuum leaks which don’t allow high voltage
performance.
 Such degradation of performance is rare but can
be improved immediately.
25

26. iii)Improper Materials:
 Modern materials like:
- Oxygen free copper,
- Controlled expansion cobalt alloys,
- Rhenium fused Tungsten,
- High hot strength alloys,
- Vacuum grade graphite,
- High temperature grade,
- Ceramics & technical glasses,
have vastly improved tube performance.
26

27.  Despite these efforts, a high level of quality
assurance is necessary to guarantee these
material quality.
 Otherwise, due to improper materials, x-ray
tube can felt defects & faults.
27

28. iv)Process failure:
 New processes such as:
- Vacuum re-melted metals,
- Turbo-molecular vacuum pumps,
- High temperature hydrogen gas firing,
-High temperature vacuum processing,
have improved x-ray tube performance.
 Automation has helped to insure more consistent product.
 However if these processes/equipment utilized become
faulty or the control is lost, a well-tuned process can easily
fail & marginal or reject tubes can result.
28

29. 2. Latent failure
 Latent or unpredictable failures which occurs in
time are often unforeseen & sometimes may
not be attributable to a known cause.
 Latent failure consists of:
29

30. i)Process optimization:
 Many process are used on tubes and their parts have
evolved over many years & through practical experiences.
Such as:
- Outgassing,
- Vacuum pumping,
- Seasoning.
 Unless there is very clear contrary evidence, manufacturers
are reluctant to change a process for the fear of unknown
consequences.
 So it is difficult to find a suitable compromise & once a
process works it is often best to leave it alone.
30

31. ii)Marginal or Poorly understood
processes:
 Some failures are caused by effects that are not well
known. Such as:
- Why does dielectric oil sometimes become dark
& have foreign materials, yet the tube operate ok?
- While the other system exhibit arcing, yet why
the tube & cooling oil and surrounding look & test ok?
31

32. Application mismatch
 In early mammography, a standard diagnostic tube was
used to produce mammograms
 So the resulting diagnosis was poor & radiation burns
often resulted.
 Over several years, it was learned that molybdenum
radiation, at voltage of about 25-30 kvp with very small
focal spot is best to produce the quality image of breast
on mammograms.
 Application mismatch can be described as below:
32

33. i) Low kv/High mA emission:
 A common mismatch can occur when a tube designed for
high voltage use is used at lower voltages.
 Due to this reason, the filament has to be run at high
emission current to overcome the limited emission.
 In a particular rotating anode tube, operated at 125 kvp &
300 mA when decreased to 50 kvp & 300 mA, the
filament must be operated at 16% more power to
overcome the lower tube voltage which can shortens the
tube life.
33

34. ii)Temperature:
 A basic rule for x-ray tubes is that temperature is
always consider as the biggest enemy.
 The more power applied, the shorter the tube life.
 However without adequate power, there may not be
enough x-radiation intensity to get the job done.
 Filament evaporation causing unwanted metallic
deposition will eventually lead to insulator arc over.
34

35. Improper drive by the power supply
 In an x-ray source, the power supply provides all the
necessary power to operate the tube including the
filament, rotor, stator, accelerations,
 Thus, the power supply is an integral part of the x-ray
tube.
 Power supply can be described as below:
35

36. i)Supply Impedance:
 Most critical characteristic of the power supply is its
impedance.
 For stationary anode tube with high impedance, means
that it contains a lot of resistance.
 So, in case of damage to tube and sensitive electronics,
damage is minimized by maintaning good control of
voltage through impedance.
 A rotating anode tube operates under much high power
condition, sometimes over 100 KW or almost 1000 times a
stationary anode.
 Here, the supply cannot have high impedance other wise
it would not support the required power.
36

37. ii)High frequency:
 The glass-to-metal seals in a tube are made from kovar
or similar alloy consisting of iron, nickel & cobalt alloy
of which are highly magnetic.
 Under high frequency, the magnetic materials are
subjected to magnetic hysteresis loss, eddy currents
loss & the skin effect which sap the energy from the
current flow.
 Currently, frequencies of up to 40 kHz are employed.
 For the cathode & anode, high frequency, high voltage
supplies are employed, but these are rectified to DC.
37

38. iv)Rotation speed:
 For rotating tubes, bearing life as well as filament
evaporation is a major consideration for tube life.
 Power supply to the rotor & stator determines the
rotation speed.
 So, the power supply should be maintained very wisely.
38

39. v)Filament Boost:
 A current limiting or controlling device is place in series
between the constant current source & filament.
 The amount of current controls the temperature of
filament.
 One of the problem with a constant current source, is that
it brings the filament up to the selected operating
temperature gradually.
39

40.  To bring the filament up to temperature more quickly ,
a current boost is applied when the current to the
filament is 1st turned on.
 That is , instead of supplying the normal operating
current to the filament, a higher current is provided for
a pre selected short duration.
40

41. Fig: Chart ShowingFilament Boosting
41

42. Care of the X-ray Tube
42

43. Introduction
 With proper care, the life of the tube can be extended
with normal use.
 The following precautions should be applied for safe
use of x ray tube and its care:
 MINIMIZE THE FILAMENT BOOST TIME
Current applied for too long will shorten filament
life and will lead to unstable operation.
43

44.  WARM UP THE ANODE FOLLOWING
THE MANUFACTURER’S
RECOMMENDATION
In order to prevent tube from the thermal
shock, preheat the tube anode by performing
the following procedure. This procedure should
be performed if the system has not been
energized for 2 hours or longer.
44

45.  USE LOW TUBE CURRENT
The high filament current required to produce high tube
current(mA) will cause evaporation of the tungsten from
the filament and then , it will be deposited on to the
glass envelope.
45

46.  LIMIT OPERATION TO 80%OF
MAXIMUM SINGLE EXPOSURE
RATING
 Although higher power levels are both possible and
permitted this reduction will help assure long focal
track life.
 Also, it will minimize the reduction in radiation
output associated with a roughened focal track.
46

47.  DO NOT ROTATE THE TUBE HOUSING
RAPIDLY FROM ONE POSITION TO
ANOTHER
 Sudden movement will produce tension or pressure
to the x-ray tube.
 The gyroscopic effect may crack or otherwise
damage the rotor.
47

48. FOLLOWRating Charts
 Failure of the x ray tube can be prevented with the use
of tube rating chart.
 It is essential that radiologic technologist be able to
understand and read these charts even though many
of these charts are now digitally stored.
 There are 3 types of rating charts:
 The radiographic rating chart
 The anode cooling chart
 The housing cooling chart
48

49. RADIOGRAPHIC RATING
CHART
 The radiographic rating chart is most important
because it conveys which radiographic techniques are
safe and which techniques are unsafe for x-ray tube
operation.
 The x axis and y axis shows scales of the two
radiographic parameter-time and kVp.
 For a given mA ,any combination of kVp and time that
lies below the mA curve is safe and above is unsafe
49

50.  If an unsafe exposure was made ,the tube might fail
abruptly but a x ray imaging system have a built in
safety features that would not allow an exposure to
be made.
 There are different chart for each filament (large or
small focal spot) ,the speed of anode rotation
(3400rpm or 10,000rpm) , the target angle , and the
voltage rectification.
 An appropriate radiographic rating chart is
supplied with each replacement of x ray tube and
can be different from that of original tube.
50

51. Fig: RADIOGRAPHIC RATING
CHART
51

52. THE ANODE COOLING CHART
 The thermal capacity of an anode and its heat
dissipation characteristics are contained in a rating
chart known as anode cooling rating chart.
 Unlike the radiographic rating chart , the anode cooling
chart does not depend on the size of filament or the
rotation speed.
 The rate of cooling is rapid at first and gradually slows
as the anode cools.
 In addition to determining the maximum heat capacity
of the anode , it is used to determine the time required
for complete cooling after any level of heat input.
52

53. Fig: Anode Cooling Chart
53

54. HOUSING COOLING CHART
 Ability of entire tube housing to withstand heat is
represented by housing cooling rating chart which has
chart similar to anode rating chart and used in same way.
 X ray tube housing generally have a maximum heat
capacity lies in the range of 1 to 1.5 million HU.
 Complete cooling of tube housing after maximum heat
capacity requires for 1 to 2 hours.
 About twice the amount of time is required without fan
powered air circulation.
54

55. Fig: House cooling chart
55

56. Summary:
• Tube fault can occur in any part of the tube like glass
envelope , anode , filament, rotor , stator winding ,
vacuum etc.
• Excessive heat results in reduced x ray tube life.
• Maximum radiographic techniques should never be
applied to a cold anode.
• Tube rating charts printed by manufacturers of x ray
tubes aid the radiologic technologists in using
acceptable exposure levels to maximize the x ray
tube life.
56

57. REFERENCE:
 Radiologic science for
technologist by STEWART
CARLYLE BUSHONG
 the essential physics of
medical imaging by JERROLD T.
BUSHBERG
 Chesney’s equipment for
students radiographers by P.H.
CARTER
57

58. 58
Thank you