X-Ray Tube Physics: Design, Operation and Faults

What was Rontgen working with, describe and explain?

Crookes tube: a partially evacuated glass tube with a cold cathode

What came from ionising the residual gas and what was output dependent on?

Electrons came from ionising the residual gas, so output depended on gas pressure

How were early gas tubes?

unstable , unpredictable, hazardous, tempermental

What tube do we still use today and what is it?

Coolidge: a heated tungsten filament in a high vacuum

What factors of the tube can be controlled independently?

tube current and tube voltage

What was created in 1929 and why?

the rotating anode, to spread the heat

What is every modern tube from mobile to CT scanners based on?

Coolidge tube.

What is used in modern high performance tubes?

Glass envelope, or metal and ceramic

What stops the hot filament oxidising and lets electrons cross tha gap?

The vacuum lets electrons cross the gap without collisions and stops the hot filament form oxidising

How is the cathode assembled?

Two thoriated tungsten filaments, broad and fine, sharing a common lead

What is tube current?

the flow of electrons across to the anode.

What does heating the tungsten filament do to outer electrons?

Heating the tungsten filament gives its outer electron enough energy to escape the metal surface

Why is thorium added in tungsten?

Thorium is added to the tungsten because it lowers the energy needed for electrons to leave

What do the freed electrons do?

The freed electrons form a cloud around the filament (the space charge) and wait there until the kV is applied

What does emission do as it rises steeply with a filament temperature?

Emission rises steeply with a filament temperature

- a small amount change in filament current gives a large charge in the number of electrons available

What is the target layer on a molybdenum or graphite disc?

Tungsten-rhenium target layer on a molybdenum or graphite disc

What is the focal track and the line focus principle?

the line focus principle gives a large actual focal spot for heat and a small effective spot for resolution

What does the stem, rotor and bearing inside the vacuum do?

the stator windings sit outside the envelope and drive the rotor through the glass

Does tungsten have a high melting point?

yes

What are electrons hitting in the anode?

What is the chemical symbol for tungsten?

the electrons are hitting the tungsten target and colliding with the focal spot.

Why is the housing lead lined?

Lead lined, radiation leakage is limited

What does transformer oil in the housing provide?

Transformer oil provides electrical insulation and carries heat away from the insert

What do expansion bellows do?

Expansion bellows take up the oil as it warms

What are the cable sockets for?

For the high tension supply

What does the exit port have?

inherent filtration

What does the rotating anode do?

Spreads the load around a track instead of one spot

What does the anode mass and diameter do?

More material to soak up heat before it matters

What does oil do?

Insulates, and carries heat from insert to housing

What does cooling rate do?

How fast the tube recovers between exposures

Explain what can go wrong in the filament?

Tungsten slowly evaporates: the filament thins, resistance rises, and it eventually fails.

- The deposits on the glass adds filtration and gives arcing a path

Explain what can go wrong in the vacuum?

A slow leak or outgassing makes a "gassy" tube: unstable current and arcing.

Explain what can go wrong in the anode?

Thermal shock can crack a cold disc, hence warm-up.

Repeated overheating pits and roughens the focal track, cutting output.

A stalled rotor plus and exposure melts the target.

Explain what can go wrong in the housing and generator?

Oil leaks, bellows failure, arcing at high tension cable socket if the grease is neglected, rectifier or transformer faults.

Explain what can go wrong in bearings?

Run hot, in vacuum, on solid lubricant, Wear gives a noisy or slow rotor, then a failure to reach speed.

The most common reason a tube is replaced.

List the things that can protect the tube?

- Warm-up procedure

- Tube rating and anode cooling charts, now enforced automatically by the generator

- Heat interlocks

- QA of the output

- half value layer

- focal spot size

List the things that radiographers should notice?

- A rotor that sounds different

- Warm-up prompts and heat interlocks

- Exposure that terminate early or refuse to start

- Factors creeping up over months for the same examination

- Oil on or under the housing

Falling tube output at routine QA

Who sets the mAs and kV?

radiographer

What does the generator's computer choose from the tube rating data?

chooses the mA and time combination (and often the focal spot)

What does rectification do?

Rectification, because the anode must never act as a cathode once it is hot

Where does the mA meter sit and why?

The mA meter sits at the earthed centre tap of the secondary, so it can read safely at the console

What does the timer control?

The timer controls exposure duration and termination

What does the stator circuit drive?

What is the standard rpm?

What is a fast rpm?

The stator circuit drives an induction motor: around 3000 rpm standard, 9000 to 10000 rpm high speed

What does prep on the exposure switch do?

he rotor coming up to speed and the filament being boosted

Where is all of the above packaged?

high frequency generator on modern units including mobiles

Why is mamography an outlier?

the tissue contrast it needs is at low energy.

What do tube faults show up as?

show up at the console as noise, interlocks, early terminations and creeping factors