5.2 The X-ray Tube & X-ray Production

Inside the X-ray Tube

The x-ray tube is a vacuum tube with two electrodes housed in a glass or metal envelope. It converts electrical energy into an x-ray beam. Radiographers must know the tube's parts, how x-rays are produced, and how to protect the tube from heat damage.

The Cathode (Negative Electrode)

The cathode is the source of electrons. Its centerpiece is the filament, a small coil of thoriated tungsten. When the filament circuit heats the coil, electrons 'boil off' its surface in a process called thermionic emission, forming an electron cloud called the space charge. Surrounding the filament is the focusing cup, a negatively charged nickel or molybdenum recess that repels the electrons into a narrow, tight stream aimed at the anode. Most diagnostic tubes have dual filaments - a small filament for a small focal spot (better detail) and a large filament for a large focal spot (higher heat capacity). Selecting the small or large focal spot at the console energizes the matching filament.

The Anode (Positive Electrode)

The anode is the target the electrons strike. It is made of tungsten (often a tungsten-rhenium alloy) because tungsten's high atomic number (74) makes x-ray production efficient, and its high melting point (about 3,400 C) withstands intense heat. Two designs exist: the stationary anode (used in dental and some portable units) and the rotating anode (standard for general radiography). A rotating anode spins at 3,000-10,000 rpm so the electron beam strikes a constantly changing surface, spreading heat over a large focal track and allowing higher exposure factors.

The Line-Focus Principle and Focal Spot

The angled face of the anode is the target angle, usually 7-17 degrees. The line-focus principle exploits this angle: the actual focal spot (the area actually bombarded by electrons) is larger than the effective focal spot (the beam's projection toward the patient). A steeper anode angle produces a smaller effective focal spot, which improves spatial resolution while keeping enough actual area to dissipate heat. A smaller effective focal spot reduces geometric blur.

The Anode Heel Effect

The anode heel effect describes the uneven intensity of the beam along the cathode-anode axis: beam intensity is greater on the cathode side and weaker on the anode side because x-rays produced deeper in the angled target are absorbed by the 'heel' of the anode itself. Radiographers use this by placing the thicker/denser body part under the cathode (for example, the thicker abdomen or the shoulders in an AP thoracic spine) so more photons reach it, producing more uniform density.

How X-rays Are Produced

When high-speed electrons from the cathode slam into the anode, two interactions create x-rays.

Bremsstrahlung (Brems) Radiation

Bremsstrahlung means 'braking radiation.' An incoming electron passes near a tungsten nucleus, is decelerated and deflected by the positive nuclear charge, and loses energy as an x-ray photon. Because electrons brake by varying amounts, brems photons form a continuous spectrum of energies up to the peak kilovoltage. Bremsstrahlung is the dominant source of x-rays in the diagnostic range and produces the bulk of the beam at all kVp values.

Characteristic Radiation

Characteristic radiation occurs when an incoming electron ejects an inner-shell (usually K-shell) electron from a tungsten atom. An outer-shell electron drops in to fill the vacancy and releases a photon whose energy equals the difference in binding energies - a discrete peak characteristic of tungsten. K-characteristic x-rays of tungsten average about 69 keV, and they only appear when the tube is operated above about 69.5 kVp (the electron energy must exceed the K-shell binding energy). Below that threshold, the beam is essentially all bremsstrahlung.

Tube Heat and Heat Units

X-ray production is extremely inefficient: less than 1% of the electrons' energy becomes x-rays; more than 99% becomes heat. Managing heat protects the tube. Heat Units (HU) quantify heat load:

HU = kVp x mA x s x generator factor

The generator factor is 1.0 for single-phase, 1.35 for three-phase six-pulse, and 1.41 for three-phase twelve-pulse or high-frequency generators. Worked example: a high-frequency generator exposure at 80 kVp, 200 mA, and 0.1 s gives HU = 80 x 200 x 0.1 x 1.41 = 2,256 HU. A single-phase exposure at 70 kVp, 300 mA, and 0.2 s gives 70 x 300 x 0.2 x 1.0 = 4,200 HU. Radiographers compare calculated HU to the tube's anode heat-capacity chart and allow cooling between repeated high-technique exposures (for example, a scoliosis series or angiographic runs) to avoid target cracking or pitting.

Generators and Rectification

The generator determines how efficiently voltage is delivered to the tube and therefore affects both beam quality and heat load. Rectification converts alternating current into the direct current the tube requires, ensuring electrons always flow cathode to anode. Single-phase generators produce a pulsating waveform with 100% voltage ripple, wasting many low-energy photons; three-phase and high-frequency generators produce a nearly constant potential with low ripple (under about 4% for high-frequency), giving a higher average photon energy and more x-rays per mAs. That is why the same technique on a high-frequency unit produces a more penetrating beam - and greater heat units (factor 1.41) - than on a single-phase unit (factor 1.0). Tube failure most often results from repeated heat overload that pits or cracks the anode focal track, so respecting the anode cooling chart is both an economic and a safety habit.