- Dr. Bipin Joseph

The X-Ray Tube Development Wilhelm Conrad

Roentgen used a Crookes-Hittorf tube to make the first x-ray image.

There was no shielding so x-rays were emitted in all directions.

The X-Ray Tube Development This is the variety of

tube designs available in 1948.

The X-Ray Tube Development

Two major hazards plagued early radiography.

Excessive radiation exposure Electric Shock

Moderns tubes are designed to overcome these problems The modern tube is based on the Coolidge

tube

Collimator

Manual controls

Transformer

X –ray tube

Newer X ray tube

The improvement

X-Ray Tube Components Housing

Visible part of tube Glass

Enclosure(insert) Vacuum Electrodes

Cathode Filament

Anode Target

*

X-Ray Tube

Converts Energy FROM

electrical energy

To Heat

> 99% of incident energy Bad! Ultimately destroys tubes

X-Rays < 1% of incident energy

*

Inside the Glass Insert

Filament Similar to light bulb Glows when heated

Target Large (usually) tungsten block

target filament

Requirements to Produce X-Rays

Filament Voltage High Voltage

+

filamentanode

filamentvoltagesource

highvoltagesource

X-Ray Tube Principle

Filament heated electrons gain energy electrons freed (“boiled” off) Thermionic emission

--

*

Space Charge Electrons leave filament

filament becomes positive Negative electrons stay close

Electron cloud surrounds filament Cloud repels new electrons from filament Limits electron flow from cathode to anode

+ ---

*

X-Ray Tube Principle

Positive (high) voltage applied to anode relative to filament electrons accelerate toward anode target

Gain kinetic energy electrons strike target

electrons’ kinetic energy converted to heat x-rays

+

*

Cathode

Fillament + Focussing cup + Connecting wires

Tungsten filament- 0.2mm diameter wire is coiled to spring of 0.2cm diameter and 1 cm length

Cathode (filament)

Coil of tungsten wire Tungsten advantages

high melting point 3370’C little tendency to vaporize long life expectancy

Tungsten disadvantages not as efficient at emitting

electrons as some other materials

Cathode (filament)

1-3% Thorium added for better emission

Cathode is source of electrons filament heated by electric current

~ 10 volts ~ 3-5 amps

filament current is not tube current

Filament (cont.)

Large Filament normally left on at low “standby” current boosted before exposure (prep or first trigger)

With time tungsten from hot filament vaporizes on glass insert- sunburn thins the filament filters the x-ray beam increases possibility

of arcing electrons attracted to

glass instead of target

+

Focusing Cup

Negatively charged Focuses electron

stream to target overcomes tendency of

electrons to spread because of mutual repulsion

+

FocusingCup

Focal Spot portion of anode struck by electron

stream Focal spot sizes affects and limits

resolution

+

Focal Spots

Most tubes have 2 filaments & thus 2 focal spots

only one used at a time small focus

improved resolution large focus

improved heat ratings Electron beam strikes larger portion of

target

Focal Spot Size & ResolutionThe larger the focal spot the more it will blur a tiny place on

the patient.

Focal Spot Size & Heat

The larger the area the electron beam hits, the more intense the beam

can be without melting the target

Tube Current (mA)

Rate of electron flow from filament to targetElectrons / second

Measured in milliamperesmilliamperes (mA)

Limited byfilament emission (temperature / filament current)space charge (see next slide)

+

Stationary Anode

Target AngleAngle between target &

perpendicular to tube axisTypically 6-20 degrees

+

Target Angle,

Target Angle Small

optimizes heat ratings limits field coverage

+

Large Target Angle(Small Actual Focal Spot)

+

Small Target Angle(Large Actual Focal Spot)

• Large– poorer heat ratings– better field coverage

Anode angle

Anode angle

Remember If anode angle is small Then focal spot is also small So good radiographic detial And small area of exposure

Anode

The Target

Tungsten-Rhenium is used as the target for the electron beam.

Rhenium is used to increase the surface properties to minimise the pitting and cracking of the target

The Rotating Anode

The rotating anode allows the electron beam to interact with a much larger apparent target area.

The heat is not confined to a small area.

Rotating anode

Rotating anode

Rotating Anode

Advantages Better heat ratings

Disadvantages More complex ($)

Rotor drive circuitry motor windings in housing bearings in insert

Rotating Anode

Larger diameter Better heat ratings heavier

requires more support costly

Materials usually tungsten

high melting point good x-ray production

molybdenum (and now Rhodium) for mammography (sometimes) low energy characteristic radiation

Molybdenum and graphite as insulator

The anode stem The anode stem is made from

molybdenum(2600’) It is made appropriately thin as to

minimize the heat conduction towards the rotor

Lubrication Not oil Not graphite

Silver is the best available in high vaccum, bearing wear is negligible

Safety circuit

Delay of 0.5-1second for anode to reach full speed

Breaking The Rotating Anode

When the anode is spinning at the correct speed, the exposure can be made.

After the exposure is completed, it slows by reversing the motor.

This is necessary to avoid excessive wear and tear of the bearings

Heel effect

Beam Intensity

Product of # photons in beam energy per photon

Units Roentgens (R) per unit time Measure of ionization rate of air

Depends on kVp mA target material filtration

Grid-controlled tubes

Grid used to switch tube on/offgrid is third electroderelatively small voltage

controls current flowfrom cathode to anodeNegative grid voltage repels electrons from filamentGrid much closer to filament than target

Applicationsspeedy switching

required+

grid

Tube Housing

Shields against leakage radiation lead lined leakage limit

100 mR / hour when tube operated at maximum continuous current for its maximum rated kilovoltage

*

Tube Housing (cont.)

Shields against high voltage

Housing filled with oilOil

Vacuum

Insert

Off focal radiation

Collimator

Aluminium and copper sheet to remove low energy X rays

Dissipation of heat from the target

Even with the anode rotating, some melting occurs.

The heat must be rapidly dissipated from the target.

The anode dissipates heat by radiating towards the glass envelop

Tube cooling

Newer X ray tube

Super Rolatix ceramic X ray tube

Super rolatix ceramic X ray tube

Summary

Earliest X ray tubes Basic principle Cathode Focusing cup Stationary anode Anode angle Rotating Anode

Anode stem Grid control Tube housing Collimator Tube cooling Super rolatix tube

Thank you

Wilhelm Conrad Roentgen