Cathode

The cathode of an x-ray tube is a negatively charged electrode. It comprises a filament and a focusing cup. Figure 2-1 shows a double-filament cathode surrounded by a focusing cup. The filament is a coiled tungsten wire that is the source of electrons during x-ray production.

Most x-ray tubes are referred to as dual-focus tubes because they have two filaments: a large filament and a small filament. Only one filament is energized at any one time during x-ray production. If the radiographer selects a large focal spot when setting the control panel, the large filament is energized. If a small focal spot is chosen, the small filament is energized. The focusing cup is made of nickel and nearly surrounds the filament. It is open at one end to allow electrons to flow freely across the tube from cathode to anode. It has a negative charge, which keeps the cloud of electrons emitted from the filament from spreading apart. Its purpose is to focus the stream of electrons.

Anode

The anode of an x-ray tube is a positively charged electrode composed of molybdenum, copper, tungsten, and graphite. These materials are used for their thermal and electrical conductive properties. The anode consists of a target and, in rotating anode tubes, a stator and rotor. The target is a metal that abruptly decelerates and stops electrons in the tube current, allowing the production of x-rays. The target can be either rotating or stationary. Tubes with rotating targets are more common than tubes with stationary ones. Rotating anodes are manufactured to rotate at a set speed ranging from 3000 to 10,000 revolutions per minute (RPM). Figure 2-2 shows how a rotating anode and stationary anode differ in appearance.

The target of rotating anode tubes is made of a tungsten and rhenium alloy. This layer, or track, is embedded in a base of molybdenum and graphite (Figure 2-3). Tungsten generally makes up 90% of the composition of the rotating target, with rhenium making up the other 10%. The face of the anode is angled to help the x-ray photons exit the tube. Rotating targets generally have a target angle ranging from 5 to 20 degrees. Tungsten is used in both rotating and stationary targets because it has a high atomic number of 74 for efficient x-ray production and a high melting point of 3400° C (6152° F). Most of the energy produced by an x-ray tube is heat, so melting of the target can sometimes become a problem, especially with high exposures.

In order to turn the anode during x-ray production, a rotating anode tube requires a stator and rotor (Figure 2-4). The stator is an electric motor that turns the rotor at very high speed. The rotor (made of copper) is rigidly connected to the target through the anode stem (made of molybdenum), causing the target to rotate rapidly during x-ray production. High-strength ball bearings in the rotor allow it to rotate smoothly at high speeds.

During x-ray production, most of the energy produced at the anode is heat, with x-ray energy being a very small percentage. Heat can pose a problem if allowed to build up, so it is transferred to the envelope and then to the insulating oil surrounding the tube. Many tube assemblies also have a fan that blows air over the tube to help dissipate heat.

Rotating anodes can withstand high heat loads. The ability to withstand high heat loads relates to the actual focal spot, which is the physical area of the target that is bombarded by electrons during x-ray production. With stationary targets, the focal spot is a fixed area on the surface of the target. With rotating targets, this area is represented by a focal track. Figure 2-5 shows the stationary anode’s focal spot and the rotating anode with its focal track. The size of the focal spot is not altered with a rotating anode, but the actual physical area of the target bombarded by electrons is constantly changing, causing a greater area—a focal track—to be exposed to electrons. Because of the larger area of the target being bombarded during an exposure, the rotating anode is able to withstand higher heat loads produced by greater exposure factors. Rotating anode x-ray tubes are used in all applications in radiography, whereas stationary anode tubes are limited to studies of small anatomic structures such as the teeth.

X-ray Tube Housing

The components necessary for x-ray production are housed in a glass or metal envelope. Figure 2-6 shows the appearance of a glass x-ray tube. Metal envelopes are more commonly used because of their improved electrical properties.

A disadvantage of a glass envelope x-ray tube is that tungsten evaporated from the filament during exposure can deposit on the inside of the glass, especially in the middle portion of the envelope. This evaporation could affect the flow of electrons and cause the tube to fail. Replacing all of this section of glass with metal prevents these problems and extends the tube life. An additional advantage of a metal envelope is the reduction of off-focus radiation. Off-focus radiation occurs when projectile electrons are reflected and x-rays are produced from outside the focal spot. The metal tube envelope can collect these electrons and conduct them away from the anode.

The envelope allows air to be evacuated completely from the x-ray tube, which allows the efficient flow of electrons from cathode to anode. The envelope serves two additional functions: It provides some insulation from electrical shock that may occur because the cathode and anode contain electrical charges, and it dissipates heat in the tube by conducting it to the insulating oil that surrounds the envelope. The purpose of insulating oil is to provide more insulation from electrical shock and to help dissipate heat away from the tube. All of these components are surrounded by metal tube housing except for a port, or window, which allows the primary beam to exit the tube. It is the metal tube housing that the radiographer sees and handles when moving the x-ray tube. The tube housing is lined with lead to provide additional shielding from leakage radiation. Leakage radiation refers to any x-rays, other than the primary beam, that escape the tube housing. The tube housing is required to allow no more than 100 mR/hr of leakage radiation to escape when measured at 1 m from the source while the tube operates at maximum output. Electrical current is supplied to the x-ray tube by means of two high-voltage cables that enter the top of the tube assembly.

Bremsstrahlung Interactions

Bremsstrahlung is a German word meaning “braking” or “slowing down radiation.” Bremsstrahlung interactions occur when a projectile electron completely avoids the orbital electrons of the tungsten atom and travels very close to its nucleus. The very strong electrostatic force of the nucleus causes the electron suddenly to “slow down.” As the electron loses energy, it suddenly changes its direction, and the energy loss then reappears as an x-ray photon (Figure 2-7).

In the diagnostic energy range, most x-ray interactions are bremsstrahlung. The diagnostic energy range is 30 to 150 kVp. At less than 70 kVp (with a tungsten target), 100% of the x-ray beam consists of bremsstrahlung interactions. At greater than 70 kVp, approximately 85% of the beam consists of bremsstrahlung interactions.

Characteristic Interactions

Characteristic interactions are produced when a projectile electron interacts with an electron from the inner shell (K-shell) of the tungsten atom. The electron must have enough energy to eject the K-shell electron from its orbit. K-shell electrons in tungsten have the strongest binding energy at 69.5 keV. For a projectile electron to remove this orbital electron, it must possess energy equal to or greater than 69.5 keV. When the K-shell electron is ejected from its orbit, an outer-shell electron drops into the open position and creates an energy difference. The energy difference is emitted as an x-ray photon (Figure 2-8). Electrons from the L-, M-, O-, and P-shells of the tungsten atom are also ejected from their orbits. However, the photons created from these interactions have very low energy and, depending on filtration, may not even reach the patient. K-shell characteristic x-rays have an average energy of approximately 69 keV; therefore, they contribute significantly to the useful x-ray beam. At less than 70 kVp (with a tungsten target), no characteristic x-rays are present in the beam. At greater than 70 kVp, approximately 15% of the beam consists of characteristic x-rays. X-rays produced through these interactions are termed characteristic x-rays because their energies are characteristic of the tungsten target element.

To summarize, when bremsstrahlung and characteristic interactions are compared, most x-ray interactions produced in diagnostic radiology result from bremsstrahlung. There is no difference between a bremsstrahlung x-ray and a characteristic x-ray at the same energy level; they are simply produced by different processes.