X-RAY TUBES
BY AUGUSTIN PCS
CONTENTS
• Introduction• Types• Construction of each tubes• Features of each tube• Line focus principle & anode heel effect• Tube rating• Tube failure• Care of tube• Conclusion
INTRODUCTION
• A tube is air evacuated device.• In which the x-rays are produced.• Characteristics-:
i) produce electrons ii) attract electroniii) physical protective casingiv) radiation protection
• Purpose:provide x-rays from near source.
INTRODUCTION
Requirement:• Ability to control quality and quantity.• X-ray emission from desired port• movement
Basic principle:• High voltage across anode and cathode• electrons emitted• accelerated toward anode • Interact and produce x-ray.
TYPES
• According to the cathodeTwo types
• Cold cathode tube(Crookes tube)• Hot cathode tubes (all other types)
• According to the anodeThree types
• Stationary anode x-ray tube• Rotating anode x-ray tube( conventional radiography tubes, mammography tube, grid
control x-ray tube)• Rotating envelope x-ray tube
Crookes Tube
Crookes Tube
• Crookes tube• Historically , x-rays were discovered
radiating from experimental discharge tubes, called Crookes tube.
• Invented by William Crookes and others.• Commonly known as cold tubes.• Used till 1920.
Crookes Tube
Consists :• Tube Envelope• Anode• Cathode• Anti Cathode
Tube Envelope• Consists of a partially evacuated glass bulb .• Material – sodium or cerium• Air pressure of 10-6 to 5x10-8 .Anode• Material- Platinum• (At no-78, mp-1768oc)Cathode• Material- Aluminium(At no -13, mp- 660oc)Anti cathode• Material - Copper plate
CROOKES TUBE
Operation• Field emission or cold emission.• DC voltage (100kv) across cathode and anode.• Gas atoms ionized , created positive ion.• Positive ion attracted to cathode and create more
electron.• High speed electron strike anode.
CROOKES TUBE
Disadvantage• Unreliable• Air absorbed by wall, reduces pressure.• Overheating due to heavy use.• Intensity and energy of x-ray cannot be
controlled independently.
CROOKES TUBE
COOLIDGE TUBE OR
STATIONARY ANODE TUBE
COOLIDGE TUBE
• Crookes tubes improved by William Coolidge in 1913.
• Called Coolidge tube or hot cathode tube.• Characteristic features-
i) high vacuum ii) heated filamentIii) thermionic emission
COOLIDGE TUBE
Construction • Tube envelope• Anode assembly• Cathode assembly
COOLIDGE TUBE
Tube Envelope• Spherical tempered glass tube with two cylindrical arms-
cathode and anode arm.• Made of borosilicate ( Pyrex) • Internal pressure 10-8 mmHg• Hold both electrodes
Anode assemblyConsists
Target• Material –tungsten embedded in
copper.• Thickness- 2-3 mm, Size – 1.8 to 2.2 mm• Shape – Circular, square or rectangular• Anode angle – 6-20o
• Typical angle – 16.5o
• High atomic no. – W- 74• High melting point – W- 3370oc• Evaporating point - 5600oc
COOLIDGE TUBE
Cathode assembly• Focusing cup• Filament
COOLIDGE TUBE
Focusing cup• Made of molybdenum • Nickel coated (anticorrosive layer)• Streamline the thermionic cloud.
Filament• Made of tungsten• High melting point-3370oc• High evaporation point- 5600oc• Highly ductile and malleable
COOLIDGE TUBE
COOLIDGE TUBE
Operation• Cathode filament heated, emits electron.• Emitted electron accelerated towards
positively charged anode.• X-ray produced- bremsstrahlung and
characteristic .
COOLIDGE TUBE
Advantages• Stability.• Intensity and energy of x-ray can be
controlled independently.
ROTATING ANODE X-RAY TUBE
ROTATING ANODE X-RAY TUBE
• The rotating anode tube is an improvement of the Coolidge tube.
Characteristic feature-i) anode rotates
ii) tungsten – rhenium alloy
ROTATING ANODE X-RAY TUBE
Construction
• Glass envelope • Anode assembly• Cathode assembly
ROTATING ANODE X-RAY TUBE
Glass envelop • Borosilicate glass• Supports electrodes• Shape - cylindrical • Size - 30 – 50 cm length
20 cm broad• Thinner window – size 5x5 cm
thickness 0.2mm• Highly evacuated• Internal pressure- 10-8 mmHg• Ceramic metal (adv prevent off
focus radiation)
ROTATING ANODE X-RAY TUBEAnode assembly Target track• Material- 90% tungsten and 10%
rhenium(high thermal capacity, less roughening)
Anode diskBase – molybdenum ( advanced graphite alloy, light wt and heat dissipation)Anode angle- 7-17o ,typical angle – 12o
Rpm- 9000
Shape – beveled
Size -10/12/15 cm (diameter)
• Anode stemmaterial – molybdenum
thin and longrestrict conduction of heat
• Rotor material – copper and iron high thermal capacity and conductivityrotation of anode 9000 rpm
Anode stem
ROTATING ANODE X-RAY TUBE
• Bearing• Made of metal balls• Lubrication – silver
powder• ( adv – liquid gallium
lubricated fluid dynamic bearing)
• Stator induction Motor • Material –
electromagnet• Supplied by AC power
ROTATING ANODE X-RAY TUBE
• Cathode AssemblyCathode Assembly• Focusing cupFocusing cup
• Same as in Coolidge's tube.Same as in Coolidge's tube.• Effectiveness of focusing Effectiveness of focusing
cup depends upon cup depends upon • Shape , size , applied charge Shape , size , applied charge
and length of filament and length of filament
ROTATING ANODE X-RAY TUBE
• FilamentFilament• Dual filamentDual filament• The size varies from 2.2mm to The size varies from 2.2mm to
7mm in length in small filament7mm in length in small filament• The size varies from 7mm to The size varies from 7mm to
15mm in length in large 15mm in length in large filamentfilament
• 99 % tungsten and 1% thorium 99 % tungsten and 1% thorium
• (prevent breakage, prevent (prevent breakage, prevent evaporation)evaporation)
ROTATING ANODE X-RAY TUBE
ROTATING ANODE XRAY TUBE
ADVANTAGES
• Provide greater area for bombardment
• Better heat dissipation than stationary
anode tube
MAMOGRAPHY X-RAY TUBE
• X-ray tube dedicated for mammography.
• Characteristic featuresI) Low kv
Ii) Characteristic radiation Iii) High spatial resolution
MAMOGRAPHY X-RAY TUBE
• CONSTRUCTION
• GLASS ENVELOPE • ANODE ASSEMBLY• CATHODE ASSEMBLY
Glass envelope• Borosilicate glass/ ceramic metal• Window- berrylium (z=4, mp=1287oc)Anode • Material-
molybdenum – (z=42, mp=2623oc) rhodium - (z=45)
• Anode angle (0 or 16)/ tube tilt(24 or 06)
MAMOGRAPHY X-RAY TUBE
MAMOGRAPHY X-RAY TUBE
• Cathode assemblyconsists
Focusing Cup - MoFilament
• W for MO track• Rh for Rh track(Added filter – Mo)
GRID CONTROLLED XRAY TUBES
• Used in fluoroscopy units• Purpose- reduced pt dose• Interval between exposure.• Focusing cup functions as a switch.
• Focusing cup act as a third electrode.• Highly negatively charged -1.5 kV
ROTATING ENVELOPE TUBE
ROTATING ENVELOPE TUBE
• Most advanced technology till date.• Idea of rotating envelope – 1917Characteristic features
• Electron beam is shaped and controlled by magnetic field.
• Cooling surface of anode in direct contact with cooling fluid
• And a rotating envelope
ROTATING ENVELOPE TUBE
It consists of four system• Tube envelope system• Electron emission system• Magnetic deflection system• Cooling system
ROTATING ENVELOPE TUBE
• Tube envelope systemi. Material – non magnetic stainless steelii. Directly Attached to anode diskiii. Annular/Circular Window (Thickness – 0.2
mm) iv. Max rotational speed – 150Hz
ROTATING ENVELOPE TUBE
• Anode assembly• Consists
Target track- 90% tungsten & 10% Rhenium
Anode Disk – Tungsten , Zirconium, molybdenum body(TZM Alloy)
BP- 4612, MP- 2600
ROTATING ENVELOPE TUBE
• Electron emission systemCathode AssemblyConsists
Focusing cup Flat Emitter
Shape of emitter – circular Material – tungsten Thickness -100µm Diameter - 5µm
Meander like path – for filament heating .
RET
• Magnetic deflection systemThree coilsR- coils - Deflect the beam radial
direction onto focal spot. (flying focal spot in z- direction,)
Q-coils – focus the beam to determine the size
Phi- coils – deflection of flying focal spot in tangential direction
ROTATING ENVELOPE TUBE
• Microcontroller controls individual coil
currents (real time )Electronically adjusted focal
spotBest image quality.
ROTATING ENVELOPE TUBE
• Cooling system (convective)Anode disk comes in direct contact with
cooling oil.Oil rotation is turbine flow Flow of oil - during exposure is 25ltr/min
- during pump running 8 ltr/minOil – Mineral oil
• Advantages• Better heat dissipation.• Various size multiple focal spot• Longer tube life.• can be used in high kV and High mA
technique for prolonged duration ( ie. High mAs)
ROTATING ENVELOPE TUBE
Line focus principle• Introduced in 1918• Relation between anode angle and focal
spot • Three principlesi) Incr anode angle – incr effective focal spot
size ( decr intensity of x-ray beam, poor image quality)
ii) decr anode angle – decr effective focal spot size (incr intensity of x-ray beam, better image quality)
iii)without change in anode angle increasing the bombarding area incr actual focal spot ,effective and provide better heat dissipation.
Anode heel effect
• Varying intensity of x-ray at anode side is lesser than cathode side.
• Beveled anode absorbs some x-ray photon
TUBE RATING CHART
• Capacity of focal spot supplied by manufacture.
• Shows maximum power delivered for a given exposure time without over loading.
Tube failure
• Anode pitting.• Rapid heat increase at
anode.• Filament vaporization.
CARE OF X-RAY TUBE• Warm up the anode following manufactures
recommendation• Minimize filament boost (prep) time.• Use lower tube current (mA).• Follow rating chart.• Do not exceed anode thermal capacity or heat
dissipation rate of the target.• Do not make high mA exposure on cold target.• Do not rotate the tube housing rapidly from one
position to another
Conclusion
Mastery is not about perfection, but a process ,a journey.
• So lets be sure there is going to be more advanced technology and there will be x-ray tubes with better heat dissipation and lesser patient dose.