# X-ray Tube Structure > X-ray tube structure in Radiology Technology: cathode and rotating anode, line focus principle, heel effect, focal spots, tube rating and cooling charts. - Canonical URL: https://prepelephant.com/topics/allied/radiology-and-imaging-technology/xray-tube-structure - Exam / course: Allied Health · Subject: Radiology and Imaging Technology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "X-ray Tube Structure", PrepElephant, https://prepelephant.com/topics/allied/radiology-and-imaging-technology/xray-tube-structure ## 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. ## Choosing safely off the tube rating chart A tube rating chart looks intimidating until you read it as a simple question: at this kVp and this mA, how long can one exposure run? Suppose a single-phase unit must deliver 80 kVp at 200 mA for a heavy lateral lumbar spine on a large patient. Find the 200 mA line, move up to 80 kVp, and read across to the maximum time — say 2 seconds. At 3 seconds the point sits above the curve: the anode face can melt or crater, and repeated abuse pits the track, eventually shedding tungsten onto the glass ("target wandering" and tube failure). The remedy is not brute force: raise kVp slightly, use a larger focal spot, or split the exposure. Heat that does get in must get out. The anode cooling curve shows storage decaying over minutes to tens of minutes; the housing cooling curve is far slower — a housing holding several hundred thousand heat units may need an hour to shed them. In a busy barium or fluoroscopy list, cumulative housing heat, not single-exposure ratings, is what actually limits throughput. ## 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.