X-ray tube housing and cooling

The X-ray tube housing encloses the anode–cathode assembly, provides electrical insulation, prevents leakage radiation, and removes the large quantities of heat generated during exposure.

Structure of the tube housing

The tube housing consists of several key layers and components:

1. Glass or metal envelope
   • Maintains a vacuum to allow electrons to travel unimpeded from cathode to anode.
   • Prevents oxidation and contamination of internal components.
   • Metal envelopes (common in modern tubes) improve heat conduction and mechanical durability.
2. Oil bath
   • The evacuated tube is completely immersed in insulating oil that provides:
     • Electrical insulation between the tube and housing.
     • Thermal conduction, transferring heat away from the anode and envelope.
   • Some systems circulate the oil through pumps or radiator fins to improve cooling efficiency.
3. Protective metal housing
   • Heavy metal casing lined with lead shielding.
   • Limits leakage radiation, to < 1 mGy h⁻¹ at 1 m (International Electrotechnical Commission – IEC standard), by absorbing X-ray photons.
   • Provides structural support and mounting points for collimators, cables, and filtration systems.
4. Window
   • A thin section of the envelope or housing through which the X-ray beam exits.
   • Made of beryllium or thin glass to minimise attenuation.

Cooling mechanisms

Heat removal follows a multi-stage process:

Stages	Mechanism	Description
1. Anode rotation	Spreads heat over larger target area (focal track)	Prevents focal pitting and melting
2. Radiation	Infrared emission from hot anode surface to oil	Major path of heat transfer during exposure
3. Conduction	Through anode stem, rotor, and envelope into oil	Continuous thermal flow between exposures
4. Oil circulation	Natural convection or pumped flow	Carries heat to outer housing
5. Air cooling (fan)	Forced convection across housing fins	Dissipates heat to room air
6. Oil-to-air or oil-to-water heat exchanger	High-duty systems (CT, fluoroscopy)	Removes heat continuously for prolonged operation

Heat unit monitoring

To prevent tube damage, generator control systems continuously monitor heat units (HU):

HU = kVp x mA x time x generator constant

• Generator constants: 1.0 (single-phase), 1.35 (three-phase 12-pulse), 1.4 (high-frequency). We will discuss generators next.
• Tube rating charts specify safe combinations of kV, mA, and time.
• Cooling charts show the heat dissipation curve; operators must allow sufficient cool-down intervals between exposures.

Key takeaways and exam tips:

• The housing provides vacuum containment, insulation, cooling, and shielding.
• Leakage radiation < 1 mGy h⁻¹ at 1 m (IEC limit).
• Oil bath = electrical insulator + heat conductor.
• Beryllium window minimises attenuation of the useful beam.
• Cooling sequence: rotation → radiation → conduction → oil → air.
• Heat unit formula and function of tube rating charts are frequent exam topics.
• Failure to observe cooling intervals shortens tube life dramatically.

Up next:

Next, we’ll examine the generator system. How low-voltage alternating electrical current is converted into a stable direct high-voltage supply across the tube.