The anode

The anode is the positive electrode of the X-ray tube.

It receives accelerated electrons from the cathode and converts their kinetic energy into X-rays and heat. The anode design determines the efficiency, resolution, and heat-handling capacity of the X-ray tube.

Structure of the anode

There are two main anode designs used in diagnostic X-ray tubes:

Type	Description	Typical Use
Stationary anode	A fixed metal target (usually tungsten) embedded in a copper block.	Dental and portable units.
Rotating anode	A tungsten–rhenium target mounted on a rotating disk. Rotation spreads the electron impact over a larger surface area.	All modern diagnostic systems (radiography, fluoroscopy, CT).

Anode target material

Tungsten is almost universally used as the target material because it has:

• High atomic number (Z = 74): increases efficiency of X-ray production.
• High melting point (≈ 3,370 °C): tolerates large heat loads.
• Low vapour pressure: minimises evaporation at high temperatures.
• Good mechanical strength.

Some specialised tubes use different materials:

• Molybdenum (Z = 42) or rhodium (Z = 45): used in mammography to produce lower-energy characteristic radiation (17–23 keV range).

X-ray production at the anode

When electrons strike the anode target, their kinetic energy is converted into:

• Heat (≈ 99%).
• X-rays (≈ 1%).

Two types of X-rays are produced:

1. Bremsstrahlung radiation: continuous spectrum from deceleration of electrons near the nucleus.
2. Characteristic radiation: discrete energies from electron shell transitions within the target material.

We will examine these processes in detail in the X-ray Production section.

Line focus principle

The line focus principle allows for a small effective focal spot (sharp image) while maintaining a large actual area of electron impact (heat dissipation).

• The anode target is angled (typically 6–20°) relative to the electron beam.
• The effective focal spot = actual focal spot × sin(θ).
• Smaller anode angle → smaller effective focal spot → improved spatial resolution.
• However, very small angles reduce field coverage at larger image receptor sizes.

Anode heel effect

Because of the angled target, X-rays emitted towards the anode side pass through a greater thickness of target material than those towards the cathode side, resulting in:

• Higher intensity on the cathode side.
• Lower intensity (and lower energy) on the anode side.

This is known as the anode heel effect. It occurs because X-rays are attenuated as they travel through the anode itself. It can be exploited clinically by positioning the thicker body part under the cathode side of the beam.

Main factors influencing heel effect:

• Smaller anode angle → greater heel effect.
• Shorter source–image/detector distance → greater heel effect.
• Larger field size → greater heel effect.

Heat generation and dissipation

Approximately 99% of electron kinetic energy is converted into heat at the anode.
Effective heat management is critical to prevent tube damage.

Mechanisms of heat control:

• Rotation: spreads heat over a larger surface area.
• Molybdenum stem: poor thermal conductor, limits heat conduction to the rotor.
• Oil bath: dissipate heat from the anode to the tube housing.
• The anode itself acts as a heat reservoir, storing energy between exposures. Tungsten-rhenium alloy provides high melting point and good mechanical strength.
• The stored heat is gradually dissipated through conduction and radiation during idle periods.
  • The anode surface emits infrared radiation that transfers heat to the surrounding tube envelope and oil.
  • Conductive heat transfer moves heat away from the anode through the shaft and bearings to the rotor assembly.

Tube rating charts are used to specify safe exposure combinations of mA, kV, and exposure time to avoid overheating.

How can we apply the above clinically?

• The rotating anode enables high tube loading for CT and fluoroscopy.
• The line focus principle improves image resolution while maintaining heat tolerance.
• The heel effect can be used to optimise patient positioning.
• Tube rating charts ensure safe operation and extend tube lifespan.

Key takeaways and exam tips:

• The anode = positive electrode; converts electron energy into heat and X-rays.
• Tungsten is the primary target material (high Z, high melting point).
• Rotation spreads heat → higher exposure capacity.
• Line focus principle: effective focal spot = actual × sin(θ).
• Heel effect: beam intensity higher on cathode side.
• 99% of energy → heat, 1% → X-rays.
• Common exam questions:
  • “What factors influence the heel effect?” → remember angle, SID, and field size.
  • “How is heat dealt with at the anode?” → primary cooling path: radiation → conduction → oil → air. Also rotation, tungsten rhenium alloy etc.
  • “Why is tungsten used as a target material?” → high atomic number, high melting point, low vapour pressure, mechanically strong.

Up next:

Next, we’ll look at Tube Housing and Cooling. How the anode and cathode assembly are enclosed, insulated, and shielded to ensure efficient operation and radiation safety.