IEM - Xray Tube

X-ray tube

A vacuum diode containing cathode and anode electrodes that produces diagnostic x-rays.

Cathode

Negative electrode of the x-ray tube; houses the filament and focusing cup and supplies electrons.

Anode

Positive electrode of the x-ray tube; receives electrons, conducts electricity, provides mechanical support, and dissipates heat.

Diode

Any electronic tube with two electrodes; an x-ray tube is a specialized high-vacuum diode.

Ceiling Support System

Most frequently used tube-support device; two perpendicular ceiling rails allow longitudinal and transverse movement of the tube.

Floor-to-Ceiling Support System

Support with a single column riding on floor and ceiling rails; variant is the floor-mounted system.

C-Arm Support System

C-shaped support used in interventional radiology suites; holds tube on one end, image receptor on the other.

Protective housing

Metal enclosure surrounding the x-ray tube that limits leakage radiation, provides mechanical protection, and contains oil for insulation and cooling.

Useful beam

X-rays that exit the tube through the designated window for imaging purposes.

Leakage radiation

X-rays that escape the protective housing in directions other than the window; add no diagnostic value and increase exposure.

Diala A oil

Insulating oil inside the protective housing that electrically insulates and thermally cushions the tube.

Glass enclosure

Made of Pyrex glass that maintains the vacuum and withstands intense heat inside many x-ray tubes.

Metal enclosure

Metal vacuum envelope that keeps a constant electric potential, reduces arcing, and lengthens tube life; standard in high-capacity tubes.

Tube window

5 cm² thin segment of the enclosure through which the useful x-ray beam exits.

Filament

Small thoriated-tungsten coil (2 mm in diameter and 1–2 cm long) that emits electrons when heated (thermionic emission).

Thermionic emission

“Boiling off” of electrons from a heated filament due to high filament current.

Thoriated tungsten

Tungsten alloyed with 1–2 % thorium; offers high melting point (3410 °C) and efficient electron emission, prolonging filament life.

Focusing cup

Negatively charged metal shroud surrounding the filament that electrostatically narrows the electron beam toward the anode.

Grid-controlled tube

X-ray tube whose negatively charged focusing cup (grid) acts as a rapid on/off exposure switch.

Space charge

Cloud of electrons that forms in front of the heated filament just before acceleration to the anode.

Space-charge effect

Limitation of additional electron emission by repulsion from the existing space charge; prominent at low kVp and high mA.

Saturation current

Condition where all available filament electrons are drawn to the anode; further kVp increase no longer raises tube current.

Emission-limited

Operating state when tube current is capped by electron availability (saturation current), not by space-charge effects.

mA station selector

Console control that chooses discrete filament currents (100 mA, 200 mA, etc.) thus selecting focal spot size.

Small focal spot

Actual target area 0.1–1 mm used with ≤ 300 mA for high spatial resolution imaging.

Large focal spot

Target area 0.3–2 mm used with ≥ 400 mA or high-heat techniques and large body parts.

Stationary anode

Fixed tungsten-in-copper target used in dental, portable, and low-power tubes.

Rotating anode

Spinning disc target that spreads heat over a larger area, enabling high-intensity, short-exposure imaging.

Target (focal track)

Area of the anode struck by projectile electrons; entire disc for rotating anodes, small insert for stationary anodes.

Induction motor

Electromagnetic device that spins the rotating anode without a mechanical shaft through the vacuum.

Stator

Series of external electromagnets around the tube neck that create a rotating magnetic field for the induction motor.

Rotor

Internal copper/iron cylinder attached to the anode disc; turns when energized by the stator’s magnetic field.

Anode stem

Narrow molybdenum shaft connecting rotor to anode disc; poor heat conductor to protect bearings.

Line-focus principle

Design that uses an angled anode surface to create a small effective focal spot while maintaining a larger actual spot for heat dissipation.

Biangular target

Anode with two different target angles, providing two distinct focal spot sizes on one disc.

Focal spot

Region on the anode from which x-rays emanate; its size affects image spatial resolution.

Heel effect

Variation in beam intensity caused by absorption within the angled anode, producing higher intensity on the cathode side.

Off-focus radiation

X-rays produced when electrons scatter to areas outside the focal spot, enlarging the field and reducing image contrast.

Heat dissipation

Removal of anode heat by radiation, conduction, and convection to avoid tube damage.

Radiation (heat transfer)

Emission of infrared radiation from the anode surface to its surroundings.

Conduction (heat transfer)

Movement of heat through the anode material to the stem and housing oil.

Convection (heat transfer)

Transfer of heat from circulating oil or air around the tube to external cooling surfaces or fins.

Radiographic rating chart

Graph that shows safe combinations of kVp, mA, and exposure time for a specific x-ray tube.

Anode cooling chart

Graph displaying anode heat storage capacity and required cooling time after exposures, expressed in heat units (HU).

Housing cooling chart

Chart indicating heat capacity and cooling characteristics of the tube housing/oil assembly.

Heat unit (HU)

Thermal energy measure; HU = kVp × mA × s (single-phase) or 1.4 × kVp × mA × s (three-phase/high-frequency).

Three-phase/high-frequency

Multiplier (1.4) applied when calculating heat units for non-single-phase generators due to greater heat production.

Tungsten

Primary target metal (Z = 74) chosen for high atomic number, high melting point (3400 °C), and good thermal conductivity.

Molybdenum

Low-density metal used in anode stems and as a backing layer to reduce rotational mass and improve heat storage.

Graphite

Lightweight carbon form sometimes layered under tungsten to decrease anode mass and enhance rotation speed.

X-ray tube failure (three causes)

Tungsten vaporization/deposition (most common), bearings damage from overheating, and filament burnout from excessive current.