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X-RAY TUBESBY AUGUSTIN PCS
CONTENTSIntroductionTypesConstruction of each tubesFeatures of each tubeLine focus principle & anode heel effectTube ratingTube failureCare of tubeConclusion
A tube is air evacuated device.In which the x-rays are produced.Characteristics-:
i) produce electrons ii) attract electroniii) physical protective casingiv) radiation protectionPurpose:
provide x-rays from near source.
Requirement:Ability to control quality and quantity.X-ray emission from desired portmovement Basic principle:High voltage across anode and cathodeelectrons emittedaccelerated toward anode Interact and produce x-ray.
According to the cathode
Two typesCold cathode tube(Crookes tube)Hot cathode tubes (all other types)According to the anode
Three typesStationary anode x-ray tubeRotating anode x-ray tube
( conventional radiography tubes, mammography tube, grid control x-ray tube)Rotating envelope x-ray tube
Crookes TubeCrookes 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 TubeConsists :Tube EnvelopeAnodeCathodeAnti Cathode
Tube EnvelopeConsists of a partially evacuated glass bulb .Material sodium or cerium Air pressure of 10-6 to 5x10-8 .
AnodeMaterial- Platinum(At no-78, mp-1768oc)
(At no -13, mp- 660oc)Anti cathodeMaterial - Copper plate
OperationField 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.
DisadvantageUnreliableAir absorbed by wall, reduces pressure.Overheating due to heavy use.Intensity and energy of x-ray cannot be controlled independently.
COOLIDGE TUBE OR STATIONARY ANODE TUBE
COOLIDGE TUBECrookes tubes improved by William Coolidge in 1913.Called Coolidge tube or hot cathode tube.Characteristic features-
i) high vacuum ii) heated filamentIii) thermionic emission
Construction Tube envelopeAnode assemblyCathode assembly
Tube EnvelopeSpherical tempered glass tube with two cylindrical arms- cathode and anode arm.Made of borosilicate ( Pyrex) Internal pressure 10-8 mmHgHold both electrodes
Anode assemblyConsists TargetMaterial tungsten embedded in copper.Thickness- 2-3 mm, Size 1.8 to 2.2 mmShape Circular, square or rectangularAnode angle 6-20oTypical angle 16.5oHigh atomic no. W- 74High melting point W- 3370ocEvaporating point -5600oc
Cathode assemblyFocusing cupFilament
Focusing cupMade of molybdenum Nickel coated (anticorrosive layer)Streamline the thermionic cloud.
FilamentMade of tungstenHigh melting point-3370ocHigh evaporation point- 5600ocHighly ductile and malleable
COOLIDGE TUBE OperationCathode filament heated, emits electron.Emitted electron accelerated towards positively charged anode.X-ray produced- bremsstrahlung and characteristic .
COOLIDGE TUBEAdvantagesStability.Intensity and energy of x-ray can be controlled independently.
ROTATING ANODE X-RAY TUBE
ROTATING ANODE X-RAY TUBEThe rotating anode tube is an improvement of the Coolidge tube.
Characteristic feature-i) anode rotates ii) tungsten rhenium alloy
ROTATING ANODE X-RAY TUBEConstruction
Glass envelope Anode assemblyCathode assembly
ROTATING ANODE X-RAY TUBEGlass envelop Borosilicate glassSupports electrodesShape - cylindrical Size - 30 50 cm length
20 cm broadThinner window size 5x5 cm
thickness 0.2mmHighly evacuatedInternal pressure- 10-8 mmHgCeramic metal (adv prevent off focus radiation)
ROTATING ANODE X-RAY TUBEAnode assembly Target trackMaterial- 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 12oRpm- 9000Shape beveledSize -10/12/15 cm (diameter)
material molybdenum thin and longrestrict conduction of heatRotor
material copper and iron high thermal capacity and conductivityrotation of anode 9000 rpm
ROTATING ANODE X-RAY TUBE
Made of metal ballsLubrication silver powder( adv liquid gallium lubricated fluid dynamic bearing)Stator induction Motor
Material electromagnet Supplied by AC power ROTATING ANODE X-RAY TUBE
Cathode AssemblyFocusing cup
Same as in Coolidge's tube.Effectiveness of focusing cup depends upon Shape , size , applied charge and length of filament ROTATING ANODE X-RAY TUBE
FilamentDual filamentThe size varies from 2.2mm to 7mm in length in small filamentThe size varies from 7mm to 15mm in length in large filament99 % tungsten and 1% thorium (prevent breakage, prevent evaporation)
ROTATING ANODE X-RAY TUBE
ROTATING ANODE XRAY TUBEADVANTAGESProvide greater area for bombardment Better heat dissipation than stationary anode tube
MAMOGRAPHY X-RAY TUBEX-ray tube dedicated for mammography.Characteristic features
I)Low kv Ii) Characteristic radiation Iii) High spatial resolution
MAMOGRAPHY X-RAY TUBECONSTRUCTION
GLASS ENVELOPE ANODE ASSEMBLYCATHODE ASSEMBLY
Glass envelope Borosilicate glass/ ceramic metalWindow- berrylium (z=4, mp=1287oc)
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 TUBECathode assembly
consistsFocusing Cup - MoFilament W for MO trackRh for Rh track
(Added filter Mo)
GRID CONTROLLED XRAY TUBESUsed in fluoroscopy unitsPurpose- reduced pt doseInterval 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 TUBEMost advanced technology till date.Idea of rotating envelope 1917
Characteristic featuresElectron beam is shaped and controlled by magnetic field.Cooling surface of anode in direct contact with cooling fluidAnd a rotating envelope
ROTATING ENVELOPE TUBEIt consists of four systemTube envelope systemElectron emission systemMagnetic deflection systemCooling system
ROTATING ENVELOPE TUBETube envelope system
Material non magnetic stainless steelDirectly Attached to anode diskAnnular/Circular Window (Thickness 0.2 mm) Max rotational speed 150Hz
ROTATING ENVELOPE TUBEAnode assemblyConsists
Target track- 90% tungsten & 10% RheniumAnode Disk Tungsten , Zirconium, molybdenum body(TZM Alloy)BP- 4612, MP- 2600
ROTATING ENVELOPE TUBEElectron emission system
Cathode AssemblyConsists Focusing cup Flat EmitterShape of emitter circular Material tungsten Thickness -100m Diameter - 5m Meander like path for filament heating .
RETMagnetic deflection system
Three coilsR- coils - Deflect the beam radial direction onto focal spot. (flying focal spot in z- direction,)Q-coils focus the beam to determine the sizePhi- coils deflection of flying focal spot in tangential direction
ROTATING ENVELOPE TUBEMicrocontroller
controls individual coil currents (real time )Electronically adjusted focal spotBest image quality.
ROTATING ENVELOPE TUBECooling 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
AdvantagesBetter heat dissipation.Various size multiple focal spotLonger 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 principles
i)Incr anode angle incr effective focal spot size ( decr intensity of x-ray beam, poor image quality)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 CHARTCapacity of focal spot supplied by manufacture.
Shows maximum power delivered for a given exposure time without over loading.
Tube failureAnode pitting.Rapid heat increase at anode.Filament vaporization.
CARE OF X-RAY TUBEWarm up the anode following manufactures recommendationMinimize 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
ConclusionMastery 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 di