The purpose of the cathode is to provide a source of electrons and direct these electrons toward the anode (Fig. 2-2). The cathode consists of a coiled wire filament that emits electrons when heated. The filament in most x-ray tubes measures approximately 0.2 cm in diameter and 1 cm in length. It is mounted on rigid wires that support it and carry the electrical current that is used to heat the filament. The filament of the cathode is similar to the filament of a light bulb (Fig. 2-3). When a filament is heated, electrons are held less tightly by the nucleus of the atoms of the metal. In other words, the electrons become excited. When the energy level exceeds the binding energy, a cloud of electrons is formed and made available to travel to the anode.

Figure 2-2 Flow of electrons from the cathode to the anode.

Figure 2-3 A, Cathode filament construction showing a small (fine) and large (coarse) filament within the focusing cup. B, Light bulb containing a filament similar to the filament within the focusing cup of an x-ray tube.

The filament is constructed of tungsten because of its high melting point (3370°C) and high atomic number. The atomic number is the number of protons in the nucleus of an atom. This number is matched by an equal number of electrons traveling around the nucleus. A high atomic number is proportionate to the potential electron availability. A metal of this type is also necessary because of the great amount of heat produced at the filament. Some x-ray tubes, usually those used in small portable and mobile units, have a single filament. Most modern tubes have two filaments mounted side by side. One is smaller than the other, and each has a different capacity for heat and electron emission.

The filament is located in a concave cup called the focusing cup. The focusing cup is made of molybdenum because it has a high melting point and is a poor conductor of heat. As a result of the shape and electrical charge of the focusing cup, the electrons are confined and directed toward the anode side of the tube.

The filament is heated by a low-energy circuit. The amount of energy in the circuit is referred to as milliamperage (mA). As the milliamperage is applied and the filament is heated, electrons are released from their atomic orbits. The quantity of electrons produced depends on the heat of the filament. Because of its negative electrical charge, the electron cloud is attracted to the anode side of the tube. The electron stream must be accelerated to create an impact great enough to produce x-rays. Acceleration of the electrons is controlled by the kilovoltage applied between the anode and the cathode. Milliamperage and kilovoltage are discussed in more detail in Chapter 4.

Anode

The basic construction of the anode consists of a beveled target placed on a cylindric base. The target is composed of tungsten, which can withstand and dissipate high temperatures. The base of the target usually is made of copper. Copper acts as a conductor of heat and draws the heat away from the tungsten target. Temperatures in excess of 1000°C occur during x-ray production. If the heat were not removed efficiently, the metal on the target would melt, and the tube would be useless. Approximately 99% of the energy released at the impact of the electrons, in diagnostic radiography is in the form of heat. Only 1% is in the form of x-rays.

Other methods of cooling the x-ray tube include surrounding the glass tube with oil within the metal housing. The oil transfers the heat away from the anode. For tubes designed for heavy-duty radiography, the oil in the tube housing often is circulated through a heat exchanger.

In specialized radiography, targets other than tungsten are used. One such material, molybdenum, is used for mammography in a human application of radiography.

Types of Anodes.

The construction of the anode varies greatly. This variance is the main factor that differentiates one x-ray tube from another. The difference in anode type is associated with the maximum level of heat dissipation possible. The two main types are the stationary anode and the rotating anode.

STATIONARY ANODE.

Stationary, or “fixed,” anodes are found in dental and small portable radiography units. These units have a relatively small capacity for x-ray production (Fig. 2-4). As shown in Figure 2-5, the tungsten target area of the stationary anode is embedded on a cylinder of copper, with the face of the target angled down toward the window. The angle may range from 15 to 23 degrees, altering the “focal spot” size. The focal spot is the small area of the target with which the electrons collide. The focal spot is discussed in detail later.

Figure 2-4 Portable x-ray unit.

Figure 2-5 Stationary anode construction.

The primary limitation of the stationary anode is its inability to withstand large amounts of heat. Repeated bombardment by electrons and subsequent heat production can damage the target. Damage commonly seen from this repeated bombardment is a pitting of the target surface. Once a target has been damaged in such a way, the x-rays produced from that area scatter in undesirable directions (Fig. 2-6). Radiographs produced by an x-ray tube with a pitted target area appear lighter than expected.

Figure 2-6 Pitted anode target area showing scatter radiation resulting from the uneven target surface.

With the rapid development of increasingly powerful generators, temperature requirements far exceeded the capabilities of the stationary anode. This limitation prompted a search for a more efficient target area and resulted in the development of the rotating anode.

ROTATING ANODE.

The rotating anode is disk shaped and rotates on an axis through the center of the tube (Fig. 2-7). The disk is approximately 3 inches in diameter with a beveled edge. It is composed of tungsten or some similar alloy that can withstand high temperatures. The spindle on which the anode is mounted usually is made of molybdenum. Molybdenum dissipates the heat produced on electron impact. This heat reduction is necessary to reduce the heat flow to the rotor and bearing mechanism that spins the anode.