X-ray Tube Structure
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Direct answer
A rotating-anode X-ray tube is a vacuum diode: a heated thoriated-tungsten filament in a focusing cup (cathode) emits electrons that a high voltage accelerates onto a spinning tungsten-rhenium disc (anode), where X-rays and, overwhelmingly, heat are produced. The line focus principle bevels the target face 7 to 20 degrees so a long actual focal track projects as a small effective focal spot: effective spot = actual length × sin(target angle). Radiation intensity also falls towards the anode side — the heel effect — because photons emitted along the anode face are partly absorbed within the target itself. Everything else in the housing — oil bath, expansion bellows, lead lining, stator — exists to manage heat, insulation and leakage.
What you must remember
- Focal spot sizes are standardised: 0.3 mm fine for magnification and mammography work, 1.0-1.2 mm broad for general radiography; 0.1 mm is used in magnification mammography.
- Line focus principle: effective focal spot = actual focal spot × sin of the anode angle; a 12-degree target turns roughly a 5 mm track into about a 1 mm projected spot.
- Smaller target angle gives a smaller effective spot but a narrower usable field and a worse heel effect — the trade the tube designer makes.
- Heel effect: intensity is up to roughly 40 per cent lower at the anode end of the field; position the thicker body part (hip, thoracic spine) towards the cathode.
- The rotor, driven by external stator coils, spins at about 3,000 rpm on 50 Hz mains and up to about 10,000 rpm in high-speed mode, spreading heat along a longer track; bearings are silver-lubricated because oil would vaporise.
- Housing leakage must stay below 1 mGy in one hour at 1 metre with the shutter closed — the figure behind lead-lined housing design.
- Tube current (mA) is the flow across the vacuum; filament current (amperes) merely heats the filament — confusing the two fails many a viva.
Where students slip
Three confusions recur. First, mA versus filament current: tube current is electrons crossing the vacuum and is read in milliamperes; filament current heats the coil and is read in amperes — the mA selector actually varies filament temperature. Second, actual versus effective focal spot: a "1 mm tube" has a real electron track several millimetres long; sin of the bevel shrinks it, which is the entire point of the line focus principle. Third, heel effect direction: intensity is greater on the cathode side. A candidate who places the patient's thick hip towards the anode has just manufactured an underexposed half-film — examiners often hide exactly this in a positioning question.
Frequently asked questions
What is the line focus principle?
Angling the anode target face 7-20 degrees makes a long actual focal track project as a small effective focal spot (actual × sin of the angle), combining heat-loading capacity with geometric sharpness.
What is the heel effect and how is it used?
Intensity of the beam falls towards the anode side because photons leaving the target's depth are self-absorbed; the thicker patient part is therefore positioned towards the cathode.
Why does the anode rotate?
Rotation by the stator-driven rotor spreads the electron beam over a circular track rather than a fixed point, multiplying the heat area and permitting far higher exposures than a stationary anode.
What do tube rating charts tell you?
The maximum single-exposure time permissible at a given kVp and mA; operating above the curve risks anode melting or pitting and premature tube failure.
What is the permissible leakage radiation from a tube housing?
Less than 1 mGy (100 mrad) in one hour at 1 metre from the source, measured with the collimator fully closed at the maximum rating conditions.