1. The X-Ray TubeThe X-Ray Tube

2.  X-ray tube is a component of x-rayX-ray tube is a component of x-ray
imaging system, it is contained in aimaging system, it is contained in a
protective housing and therefore isprotective housing and therefore is
inaccessibleinaccessible..

3. The eternal structure ofThe eternal structure of
xray tube consists of 3xray tube consists of 3
parts.parts.
 The support structureThe support structure
 The protective housingThe protective housing
 Glass or metal enclosureGlass or metal enclosure

4.  The xray tube and housing assemblyThe xray tube and housing assembly
are quite heavy therefore they requireare quite heavy therefore they require
a support mechanism so thea support mechanism so the
radiologist technologist can positionradiologist technologist can position
them.them.

5. Three main methods ofThree main methods of
xray tube suppotxray tube suppot
 Ceiling support systemCeiling support system
 Floor to ceiling support systemFloor to ceiling support system
 C-arm support systemC-arm support system

6. X-RAY TUBEX-RAY TUBE
 MADE OF THIN PYREX GLASS ORMADE OF THIN PYREX GLASS OR
METAL ENCLOSURE TOMETAL ENCLOSURE TO
WITHSTAND HIGH HEAT LOAD ANDWITHSTAND HIGH HEAT LOAD AND
MINIMIZE X-RAY ABSORPTONMINIMIZE X-RAY ABSORPTON
 IS GAS EVAUCUATEDIS GAS EVAUCUATED
 so electrons won’t collide with the airso electrons won’t collide with the air
molecules in the tubemolecules in the tube

7. THE X-RAYTHE X-RAY TUBETUBE
 The X-Ray tubeThe X-Ray tube
is theis the singlesingle
mostmost importantimportant
component ofcomponent of
the radiographicthe radiographic
system. It is thesystem. It is the
part thatpart that
produces the X-produces the X-
raysrays

8. Protective housingProtective housing
Made of lead & steelMade of lead & steel
 When x-rays are produced, they areWhen x-rays are produced, they are
emittedemitted isotropicallyisotropically
– Equal intensity in all directions
 We only use x-rays emitted through
the window or port
– Called the useful or primary beam

9.  Protective housing reduces theProtective housing reduces the
intensity of leakage radiation to lessintensity of leakage radiation to less
than 100mR/hr at 1m.than 100mR/hr at 1m.

12. Protective housingProtective housing
 X-rays that escape through theX-rays that escape through the
protective housing areprotective housing are leakageleakage
radiationradiation
 Provides mechanical support for theProvides mechanical support for the
tube and protects from rough handlingtube and protects from rough handling

13. Protective housingProtective housing
 Some tubes contain oil that serves asSome tubes contain oil that serves as
an insulator against electric shock andan insulator against electric shock and
as a thermal cushionas a thermal cushion
– Dissipate heatDissipate heat
 Some protective housing has coolingSome protective housing has cooling
fan to air-cool the tube and oilfan to air-cool the tube and oil

14. Internal componentsInternal components
CathodeCathode
 The negative side of the tube and hasThe negative side of the tube and has
two primary partstwo primary parts
– A filament and focusing cupA filament and focusing cup
 Filament = a coil of wire about 2mm inFilament = a coil of wire about 2mm in
diameter and 1 or 2 cm long.diameter and 1 or 2 cm long.

15. CathodeCathode
 FilamentFilament
– Dual-filamentDual-filament
 Focusing cupFocusing cup
– Negatively chargedNegatively charged

17. TungstenTungsten
 Filaments are usually made ofFilaments are usually made of
tungstentungsten
 Tungsten providesTungsten provides higher thermionichigher thermionic
emission than other metalsemission than other metals
 Tungsten has a very high melting pointTungsten has a very high melting point

18. FilamentFilament
 When current (mA) is applied to theWhen current (mA) is applied to the
coil of wire electron are ejectedcoil of wire electron are ejected
 The outer-shell electrons of theThe outer-shell electrons of the
filament atom are “boiled off”.filament atom are “boiled off”.
– This is known as thermionicThis is known as thermionic
emissionemission

20. Focusing cupFocusing cup
 The filament is embedded in a metalThe filament is embedded in a metal
cup that has a negative chargecup that has a negative charge
 Boiled off e- tend to spread out due toBoiled off e- tend to spread out due to
electrostatic repulsion. The focusingelectrostatic repulsion. The focusing
cup confines the e- cloud to a smallcup confines the e- cloud to a small
areaarea

23. Filament CurrentFilament Current
 When the x-ray imaging system is firstWhen the x-ray imaging system is first
turned on, a low current passesturned on, a low current passes
through the filament to warm it andthrough the filament to warm it and
prepare it for the thermal joltprepare it for the thermal jolt
necessary for x-ray productionnecessary for x-ray production
 The current is not enough to energizeThe current is not enough to energize
the tube, just warm the wire of thethe tube, just warm the wire of the
filamentfilament

24. Space-charge effectSpace-charge effect
 The cloud of e- = space chargeThe cloud of e- = space charge
 As the space charge becomes moreAs the space charge becomes more
negative by the boiling off of morenegative by the boiling off of more
electrons it makes it difficult for moreelectrons it makes it difficult for more
e- to be emittede- to be emitted
– Electrostatic repulsionElectrostatic repulsion
– Space-charge effectSpace-charge effect
– Space-charge limiting at low kVp & high mASpace-charge limiting at low kVp & high mA

26. Dual-focus tubesDual-focus tubes
 Most diagnostic tubes have two focalMost diagnostic tubes have two focal
spots; large & smallspots; large & small
 Large is used when large body partsLarge is used when large body parts
are imagedare imaged
 Small is used when better spatialSmall is used when better spatial
resolution is desired – better detailresolution is desired – better detail
 Filament sizeFilament size

28. Dual-focus tubesDual-focus tubes

29. AnodeAnode
 Anode is the positive side of the x-rayAnode is the positive side of the x-ray
tubetube
 The anode conducts electricity,The anode conducts electricity,
radiates heat and contains the targetradiates heat and contains the target
 Two types of anodesTwo types of anodes
– Stationary & RotatingStationary & Rotating

30. Stationary AnodeStationary Anode
 Used for dental x-rays, some portableUsed for dental x-rays, some portable
imagingimaging
 Used when high tube current andUsed when high tube current and
power are not required because theypower are not required because they
are not capable of producing high-are not capable of producing high-
intensity x-ray beams in a short timeintensity x-ray beams in a short time

32. Anode FunctionAnode Function
 An electrical conductorAn electrical conductor – the anode– the anode
receives electrons emitted by thereceives electrons emitted by the
cathode and conducts them throughcathode and conducts them through
the tube to the connecting cables andthe tube to the connecting cables and
back to the high-voltage generator.back to the high-voltage generator.

33. Anode FunctionAnode Function
 Mechanical support for the targetMechanical support for the target
 Dissipates heatDissipates heat
– 99% of the kinetic energy from the e- is99% of the kinetic energy from the e- is
converted into heat; 1% is converted intoconverted into heat; 1% is converted into
x-raysx-rays
– Copper, molybdenum and graphite areCopper, molybdenum and graphite are
common anode materialcommon anode material

34. A layered anodeA layered anode
increases heat capacityincreases heat capacity

35. TargetTarget
 Is the area of the anode struck by theIs the area of the anode struck by the
e-from the cathodee-from the cathode
 Tungsten is the material of choice forTungsten is the material of choice for
the target in general radiographythe target in general radiography

39. Rotating AnodeRotating Anode
 Is powered by an induction motorIs powered by an induction motor
 The stator is on the outside of theThe stator is on the outside of the
glass, consist of a series ofglass, consist of a series of
electromagnetselectromagnets
 The rotor is a shaft made of bars ofThe rotor is a shaft made of bars of
copper and soft iron built into onecopper and soft iron built into one
massmass

40. ElectromagneticElectromagnetic
inductioninduction
 As current is applied to the statorAs current is applied to the stator
sequentially so the magnetic fieldsequentially so the magnetic field
rotates on the axis of the statorrotates on the axis of the stator
 This magnetic field interacts with theThis magnetic field interacts with the
metal (ferromagnetic rotor) causing itmetal (ferromagnetic rotor) causing it
to rotate in unison with the magneticto rotate in unison with the magnetic
field of the statorfield of the stator

42. Focal spotFocal spot
 The area of the anode’s target whereThe area of the anode’s target where
x-rays are emittedx-rays are emitted
 The smaller the focal spot the betterThe smaller the focal spot the better
the resolution of the resultant imagethe resolution of the resultant image

43. Focal spotFocal spot
 Unfortunately, as the size of the focalUnfortunately, as the size of the focal
spot decreases, the heat of the targetspot decreases, the heat of the target
is concentrated into a smaller areais concentrated into a smaller area
 This is the limiting factor to focal spotThis is the limiting factor to focal spot
sizesize

44. Line-focus principleLine-focus principle
 By angling the target, the effectiveBy angling the target, the effective
area of the target is much smaller thanarea of the target is much smaller than
the actual area of electron interactionthe actual area of electron interaction

45. Line-focus principleLine-focus principle
 EffectiveEffective
FocalFocal
SpotSpot

46. Target angleTarget angle
 The smaller the target angle theThe smaller the target angle the
smaller the effective focal spotsmaller the effective focal spot
 Angles from 5 degrees to 15 degreesAngles from 5 degrees to 15 degrees
 Biangular targets are available thatBiangular targets are available that
produce two focal spot sizesproduce two focal spot sizes

47. Biangular targetsBiangular targets

48. The second factor of effectiveThe second factor of effective
focal spot is the incoming sizefocal spot is the incoming size
of e- streamof e- stream

49. Focal spot size of the cathodeFocal spot size of the cathode

50. Anode Heel EffectAnode Heel Effect
 Because of the use of line-focusBecause of the use of line-focus
principle the consequence is that theprinciple the consequence is that the
radiation intensity on the cathode sideradiation intensity on the cathode side
of the x-ray field is higher than that onof the x-ray field is higher than that on
the anode sidethe anode side

51. Heel EffectHeel Effect
 Because the e- on the anode sideBecause the e- on the anode side
must travel further than the e- that aremust travel further than the e- that are
close to the cathode side of the target,close to the cathode side of the target,
the anode side x-rays have slightlythe anode side x-rays have slightly
lower energy than the cathode side x-lower energy than the cathode side x-
raysrays

52. Anode Heel AffectAnode Heel Affect
 The smallerThe smaller
the anodethe anode
angle, theangle, the
larger thelarger the
heel affectheel affect

53. Extrafocal RadiationExtrafocal Radiation
 X-ray tubes are designed so that theX-ray tubes are designed so that the
projectile e- interacts with the target.projectile e- interacts with the target.
However, some of the e- bounce offHowever, some of the e- bounce off
the target and land on other areasthe target and land on other areas
 This caused x-rays to be produced outThis caused x-rays to be produced out
side the focal spotside the focal spot

54. Extrafocal RadiationExtrafocal Radiation
 These rays can also be called off-These rays can also be called off-
focus radiationfocus radiation
 Extrafocal radiation is undesirableExtrafocal radiation is undesirable
because it extends the size of thebecause it extends the size of the
focal spot, increases patient skin dosefocal spot, increases patient skin dose
& reduces image contrast& reduces image contrast

55. Off-focus radiationOff-focus radiation

56. Fixed diaphragm in theFixed diaphragm in the
tube housingtube housing

57.  Another solution is the metalAnother solution is the metal
enclosure x ray tube so that reflectedenclosure x ray tube so that reflected
electrons are extracted and conductedelectrons are extracted and conducted
awayaway

58. ThanksThanks