Table of Contents
X rays are part of the electromagnetic spectrum and are really a form of light. Like visible light, X-ray photons travel at the speed of light, and they can produce a latent image on film. Unlike visible light, X rays can penetrate most opaque matter, make some materials fluorescent, and produce ionization of some materials.
Bremsstrahlung radiation results when an electron passes near the nucleus of an atom. The close passage of the electron to the nucleus causes the electron to change its course thus losing much of its energy in the process. In the world of quantum particles, energy is always exchanged in discreet particles of light known as photons. The loss of energy by the electron as it is deflected by the heavy nuclei in the anode target produces a very high energy photon of light called an x-ray. The dental x-ray tube produces Bremsstrahlung radiation.
Electricity flows through a wire like water flows through a pipe. If the water is under low pressure, it will flow slowly. If it is under high pressure, the water flows faster and more water flows through the pipe each second. In Electricity, the “pressure” in the wire is measured in volts, or Kilovolts (thousands of volts), and the amount of electricity, called the current, flowing through the wire each second is measured in amperes or in milliamperes (a milliampere is one thousandth of an ampere).
Electricity under high voltage (great pressure) flows so fast that it can jump across gaps in the wire. Think of water under high pressure rushing out of a fireman’s hose. The nozzle on the fireman’s hose constricts the flow of water to make the water shoot out further. This constriction in the water line can be thought of as resistance to the flow of water. In electricity, wires come with various degrees of resistance depending on their composition and diameter.
When current flows through a high resistance wire, the wire will heat up, just like the filament in a light bulb.
When high voltage pushes current across a low resistance gap, the electrons that make up the current can jump the gap with great speed. Both of these principles are operating in an x-ray tube.
The x-ray machine takes energy from an electrical source (usually a 220V outlet) and converts it to two separate voltage streams.
One stream is a low voltage source which can be varied so that different amounts of current can flow through the high resistance wire that composes the filament in the cathode. The current that flows through the filament is measured in milliamperes (mA).
The second stream is very high voltage, measured in kilovolts (thousands of volts at peak voltage — kVp). This voltage is applied across the gap between the anode and the cathode.
How KVP and MA come together in an x-ray tube
The schematic of the x-ray tube is reproduced here for the reader’s convenience.
The low voltage stream (measured in milliampers–mA) flows through the high resistance wire in the heating element of the cathode pictured on the left side of the diagram above. The heating element resides inside a metal covering called a filament focusing cup. Whenever a piece of metal gets red hot, electrons become excited and tend to “boil” off the surface. The higher the temperature of the metal, the larger the number of electrons that boil off. As the electrons boil off the filament, it would normally become progressively more positively charged, and the negatively charged electrons would simply fall back into filament. But that’s not what happens in an x-ray tube. That’s where the high voltage stream comes in.
The high voltage stream is applied across the anode and cathode connections at either end of the tube. This applies thousands of volts (generally 70 thousand) across the gap between the filament on the cathode (negative) side, and the tungsten target on the anode (positive) side of the system. This voltage causes the negatively charged electrons that boil off the filament to be attracted to the positively charged anode, just like a magnet attracts iron filings. The gap is filled with a vacuum so nothing interferes with the flow of electrons across the gap (low resistance). This is a bit like applying thousands of pounds of pressure to the water in a pipe. Obviously, water under very high pressure would shoot out of the open end of the pipe like the stream of water through the fireman’s hose.
In the X-ray tube, the huge kVp across the gap supplies so much pressure to the stream of electrons generated by the hot filament that the electrons go speeding across the gap with tremendous velocity. The focusing cup is negatively charged and repels the negatively charged electrons so they are all pushed to the center of the cup and end up focused in a tight beam. The electrons hit the tungsten target so hard that they “explode” into a shower of high energy photons (see the explanation of bremsstrahlung radiation above) . These photons are the x-rays. X-ray photons are like the photons in visible light except that they contain so much energy that they can penetrate opaque objects. But even with their great energy, objects of varying density can block some of them, casting shadows on whatever screen is there to stop the photons that get through, just like regular light. In the case of medical x-rays, the “screen” is generally an x-ray film or a digital sensor.
The energy of the x-rays is controlled by the high voltage kVp. When the kVp is increased, the x-ray photons that are produced have a shorter wavelength, and thus have higher energy and pack more of a punch.
The number of x-ray photons produced (i.e. the intensity of the beam) depends on the number of electrons that boil off the filament. This depends on the temperature of the heating element which in turns depends on the current flowing through the filament. The amount of current flowing through the filament is controlled by the mA setting.
Kilovoltage and milliampere settings
In dental x-ray units, the kVp and mA are set by the manufacturer and are rarely changed by the end user. (Some machines allow such changes, but practitioners very rarely make changes from factory default settings.) The only variable that is normally adjusted by the operator is the time that the kVp is applied across the x-ray tube. Modern tubes use tenths and hundredths of a second as a standard measure of time, but older machines used “pulses”, each pulse being 1/60 of a second. Many of these machines are still in use, so if you see a unit with a dial labeled in whole numbers, it is probably measuring time in pulses. To get the actual time, multiply the number on the dial by 1/60 of a second. 6 pulses would be a tenth of a second.
The most common settings for dental x-ray units are 70 kVp (Kilovoltage peak) or 90 kVp. If the kVp were to be changed on a dental x-ray unit, a 15% increase in kilovolt age would double the density on the radiograph. In this case, an operator would have to cut the exposure time in half to keep the same density on the film. The kilovolt age is responsible for the quality of the x ray beam. Milliamperage (mA) is responsible for the quantity or number of rays produced. For dental use, the normal range for Milliamperage is between 7 and 15 mA.
According to federal guidelines, a chart with the settings for time (seconds or impulses), kVp, and mA for the techniques most commonly used must be posted near the control panel of each x-ray unit in the office.
An x-ray beam with the lowest possible kilovolt age should be used, but not less than 60 kVp. Filtration equivalent to 2.5 mm of aluminum should be used for 70 kVp or more. Those units operating below 70 kVp should have the equivalent of 1.5 mm of aluminum.
The photons produced by the x-ray tube come in a range of energies. It is desirable to limit the output of the x-ray tube to only the most energetic photons. Low energy photons are more easily absorbed by soft tissue and would generally not reach the film. On the other hand, they represent an increased absorbed dose of radiation to the patient. Since aluminum is transparent to high energy x-rays, but more opaque to low energy x-ray photons, the low energy photons are filtered out by placing a flat aluminum disk in the path of the radiation beam. Most modern machines are factory set to produce 70 kVp radiation, so most x-ray tubes come with a 2.5 mm thick aluminum filter.
The x-ray radiation wavelengths and penetration characteristics are controlled according to three variables:
1. Filament Temperature: The higher the temperature of the filament, the more electrons are released. We measure the level of current as milliamperes (mA). Increasing the mA increases the number of electrons emitted from the cathode. This, in turn, increases the number of x-rays produced.
2. Kilovolt age: The voltage between the negatively charged cathode and the positively charged anode is expressed in peak kilovolts (kVp). Increasing the kVp increases the speed of the electrons that strike the target. Higher kVp settings produce shorter wavelength (higher energy) x-rays. These have more penetrating power than longer wavelength x-rays.
3. Time: A timer on the x-ray tube controls the number of seconds that electrons are produced by the cathode. This also influences the number of x-rays produced.
Penetration and the x-ray image
When x-ray beams enter an object, they have a uniform distribution of high energy wavelengths. The x-rays are then absorbed to a greater or lesser degree depending on what tissue they encounter before they strike the x-ray film. In this way, the