Electromagnetic radiation may be produced in a variety of ways. One method is the acceleration or deceleration of electrons. For example, a radio transmitter is merely a source of high-frequency alternating current that causes electrons in an antenna wire to which it is connected to oscillate (accelerate and decelerate), thereby producing radio waves (photons) at the transmitter frequency. In an x-ray tube, electrons boiled off from a hot filament are accelerated toward a tungsten anode by a high voltage on the order of 100 kilovolts (kV). Just before hitting the anode, the electrons will have a kinetic energy in kiloelectron volts equal in magnitude to the kilovoltage (eg, if the voltage across the x-ray tube is 100 kV, the electron energy is 100 keV).When the electrons smash into the tungsten anode, most of them hit other electrons, and their energy is dissipated in the form of heat. In fact, the anode may become white-hot during an x-ray exposure, which is one reason for choosing an anode made of tungsten, with a very high melting point. The electrons penetrate the anode to a depth less than 0.1 mm.
A small fraction of the electrons, however, may have a close encounter with a tungsten nucleus, which, because of its large positive charge, exerts a large attractive force on the electron, giving the electron a hard jerk (acceleration) of sufficient magnitude to produce an x-ray photon. The energy of the x-ray photon, which is derived from the energy of the incident electron, depends on the magnitude of the acceleration imparted to the electron. The magnitude of the acceleration, in turn, depends on how closely the electron passes by the nucleus. If one imagines a target consisting of a series of concentric circles, such as a dart board, with the bull’s-eye centered on the nucleus, more electrons clearly will impinge at larger distances than in the bull’s-eye; hence, a variety of x-ray photon energies will be produced at a given tube voltage (kV) up to a maximum equal to the tube voltage (a hit in the bull’s-eye), where the electron gives up all its energy to the x-ray photon. Increasing the voltage will shift the x-ray photon spectrum to higher energies, and higher-energy photons are more penetrating. The radiation produced in this manner is called Bremsstrahlung (braking radiation) and represents only about 1% of the electron energy dumped into the anode by the electron beam; the other 99% goes into heat.
The electron beam is made to impinge on a small area on the anode of the order of 1 mm in diameter in order to approximate a point source of x-rays. Because a radiograph is a shadow picture, the smaller the focal spot, the sharper the image. By analogy, a shadow picture on the wall (such as a rabbit made with one’s hand) will be much sharper if a point source of light such as a candle is used rather than an extended light source such as a fluorescent tube. The penumbra (or unsharpness) of the shadow will depend not only on the source size, but also on the magnification, as can be illustrated by making a shadow of one’s hand on a piece of paper using a small light source such as a single light bulb. The closer you bring your hand to the paper (the smaller the magnification), the sharper the edges of the shadow. Similarly, magnification of the x-ray image produced by the point source is less, the closer the patient is to the film and the farther the source is from the film.
Lange – Basic Radiology