The cathode

The cathode is the negative electrode of the X-ray tube.

It serves as the source of electrons which are accelerated across the tube towards the anode to produce X-rays.

Structure of the cathode

The cathode assembly consists of two primary components:

Filament:

• A tightly coiled tungsten wire heated by an electric current.
• Tungsten is used because of its high melting point (≈ 3,370 °C) and low vapour pressure, which prevent rapid evaporation.
• Heating the filament causes thermionic emission.
• In thermionic emission, electrons gain enough energy to escape from the tungsten surface and form an electron cloud (space charge) on the surface of the filament.
• Most diagnostic tubes have two filaments (small and large). The smaller the filament, the smaller the focal spot size and the higher the spatial resolution (at the expense of heat capacity at the anode). For more on this see the line focus principle later on.

Focusing cup:

• A shallow, negatively charged metallic cup that surrounds the filament.
• Repels and focuses the negatively charged electrons into a narrow beam directed at the anode focal spot.
• Maintains beam precision and controls focal spot size, which directly affects image resolution.

Electron emission and control

Thermionic emission forms the electron source, while two electrical settings on the control console govern the beam:

• Tube current (mA):
  • Controls the number of electrons emitted per second.
  • Higher mA → more electrons → more X-ray photons → increased beam intensity (quantity).
  • Beam quantity ∝ mA
• Tube voltage (kV):
  • Determines the potential difference between cathode and anode.
  • Controls the kinetic energy of electrons which translates to the energy (penetrating power) of the resulting X-rays (which we will look at in the production of X-rays section). This is called beam quality.
  • Tube voltage also influences beam quantity. The higher kinetic energy of the electrons means more efficient X-ray production (more energy converted to X-rays rather than heat).
  • Beam quantity ∝ (kVp)²
  • In practice this increase in beam quantity is accounted for by a reduction in mA.

Therefore, generally speaking mA controls beam quantity/intensity and kVp controls beam quality/energy.

Space charge effect

At low kV or high filament currents, the electron cloud can become dense enough to repel further electrons, limiting electron emission. This space charge effect causes the tube current to deviate from a linear relationship with filament current. Modern generator circuits incorporate space charge compensation to maintain consistent output.

Key takeaways and exam tips

• The cathode = negative electrode; produces electrons via thermionic emission.
• Tungsten is used for its high melting point and low vapour pressure.
• Focusing cup shapes and directs the electron beam to the anode.
• mA controls beam quantity; kV controls beam quality and quantity (15% rule).
• Small focal spot → better detail, large focal spot → higher heat capacity.
• Space charge effect can limit electron flow; modern systems correct for it automatically.
• Common exam trap: mixing up the roles of mA (quantity) and kV (quality).

Up next

Next, we’ll examine the anode, the positive electrode of the X-ray tube, where electrons are rapidly decelerated to produce both X-rays and heat.