xray tubes

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.