After thermionic emission at the cathode, electrons are accelerated across the vacuum toward the anode by the applied high voltage (kVp). These electrons gain kinetic energy proportional to the applied voltage.

When they collide with the anode target, two principal interactions occur:

• Bremsstrahlung radiation, produced when electrons are decelerated by the nuclear electric field

• Characteristic radiation, produced when inner-shell electrons are ejected and replaced

Only about 1% of the electron kinetic energy is converted into X-rays. The remaining ~99% is converted into heat.

Because of this large heat load, the anode must:

• Withstand extremely high temperatures

• Dissipate heat efficiently

• Maintain structural integrity

Most diagnostic X-ray tubes use a rotating anode. By rotating the target during exposure, the heat generated at the focal spot is distributed over a larger surface area, increasing heat capacity and allowing higher tube currents.

The target material is typically tungsten because it has:

• A high atomic number (improves X-ray production efficiency)

• A high melting point

• Good thermal conductivity

The angle of the anode target also influences the effective focal spot size through the line focus principle.