What is the function of the cathode in an X-ray tube?
The cathode in an X-ray tube is responsible for producing and directing electrons toward the anode. It generates electrons through thermionic emission and focuses them into a narrow beam aimed at the focal spot on the anode.
The cathode produces electrons by thermionic emission and focuses them toward the anode, controlling tube current and beam intensity.
The cathode therefore controls the number of electrons available for X-ray production and directly influences tube current (mA) and beam intensity.
Understanding the physics
The cathode consists primarily of a tungsten filament housed within a focusing cup.
When an electrical current passes through the filament, it heats to a very high temperature. At sufficiently high temperatures, electrons gain enough energy to escape from the metal surface. This process is called thermionic emission.
The number of electrons released depends on the filament temperature, which is controlled by the filament current. Increasing filament current increases electron emission and therefore increases tube current (mA).
The focusing cup, typically made of molybdenum or nickel, carries a negative charge and electrostatically narrows the cloud of emitted electrons. This focusing ensures that electrons strike a small, well-defined area on the anode known as the focal spot.
The cathode does not determine electron energy, that is controlled by the kilovoltage (kVp) applied between cathode and anode. Instead, the cathode determines how many electrons are available to be accelerated.
Because X-ray photon production is proportional to the number of electrons striking the anode, the cathode directly influences X-ray beam intensity.
Where this matters clinically
Changes in filament current alter mA and therefore affect:
Beam intensity
Image receptor exposure
Radiation dose
Focal spot size, partly determined by cathode design, also influences spatial resolution. A smaller effective focal spot improves sharpness but increases heat concentration at the anode.
Understanding the cathode is essential for linking tube current to beam output and image quality.