X-RAY TUBE
a. All modern x-ray tubes are known as Coolidge tubes (Figure 2-1). In this
tube, electrons are supplied by an electrically heated filament. The electrons are
accelerated by a high electric field to the anode or target. When an electron strikes the
target, part of its energy is degraded to heat (~99%) and the remainder goes toward
producing x-rays. As the voltage (potential) is increased, the minimum wavelength
radiated decreases, thus producing higher energy x-rays. Very few electrons give up
their total energy in a single encounter; therefore, many photons of energy lower than
that expected will be produced.
b. The current, which heats the filament, is sometimes referred to as the tube
current. As the current is increased, the number of electrons produced is increased.
Increasing the number of electrons increases the number of x-rays produced.
Therefore, we can say that an increase in the tube current increases the quantity of xrays. Tube current is expressed in terms of milliamperes (mA).
c. In expressing x-ray energies, it is customary to state the peak kilo-voltage
(kVp) used. Increasing the potential, or kVp, increases the acceleration and the energy
of the electrons. This results in the production of higher energy x-rays. If electrons are
accelerated across a potential of 100 kVp, we can produce x-rays having a maximum
energy of 100 kilo electron volts (keV). So we can say that increasing the kVp will increase the quality or hardness of the x-ray. Quality or hardness refers to the
penetrating ability of x-rays.
d. The target used in these tubes is usually tungsten or tungsten-molybdenum.
This material is used because of certain properties that are desirable for use in the xray
tube. For instance, with all the heat produced when the electrons strike the target, it
is desirable to have a material with a high melting point. Tungsten's melting point is
3,370° C. One other desirable property of tungsten is the fact that it produces a usable
characteristic x-ray, which will be discussed later.
e. X-rays are emitted in a broad energy spectrum, ranging from an amount of
energy equivalent to the maximum energy of the accelerated electron down to the
minimum energy x-ray, which can penetrate the window of the x-ray tube. This
spectrum is composed of two components--continuous and characteristic x-rays.
Figure 2-1. X-ray tube.
f. Continuous x-rays exhibit a range of energies because not all of the electrons
striking the target lose all their energy. This continuous emission is known as
bremsstrahlung, or braking radiation, from the German language. Electromagnetic
theory holds that a moving electric charge will radiate energy whenever it is
accelerated. The same is true in cases where a charged particle is decelerated.
g. Characteristic x-rays appear as sharp peaks if superimposed over the
continuous spectrum (Figure 2-2). The wavelengths of these x-ray emissions are
unique characteristics of the element used as the target material. In the x-ray tube, the
accelerated electron occasionally ejects one of the orbital electrons from a shell of one
of the atoms in the target. This loss of a negative charge gives the atom a net positive
charge and thus attracts an electron from an outer shell or a free electron to fill the vacated space. The abrupt change in velocity when the attracted electron reaches its
final position results in the emission of an x-ray possessing an energy characteristic of
the target material.
Figure 2-2. X-ray spectrum.
h. The use of x-rays for radiography calls for hard x-rays--that is, x-rays with
energy high enough to penetrate the subject and expose the film. The x-ray beam also
contains a range of low energy or soft x-rays. These x-rays do not contribute anything
to radiography, but they can add to the patient's dose; therefore, we must remove these
soft x-rays from the beam. We do this by using a technique known as filtration. By
inserting a dense material into the beam, we can remove soft x-rays, thus leaving a
beam containing only hard x-rays. X-ray units come with the inherent filtration of their
components; however, we can provide additional filtration as may be required.
Materials such as aluminum or copper are used for additional filtration.
i. Due to the possibility of scatter radiation in x-ray facilities, it is important to
limit the size and shape of the x-ray beam. This is the concept of collimation. Of equal
importance here is that only the area of interest in the patient should be exposed to the
beam. Collimation is an aid in limiting the patient's dose from x-rays.
j. In considering filtration, we use the concept of half value layers. A half-value
layer (HVL) is that thickness of a specified material which, when placed in the path of a
given beam of radiation, reduces the exposure rate by one-half. Materials used are
lead or aluminum and copper equivalents of lead. This concept is also used when
considering the shielding required for x-ray facilities.
For Informational Purposes Only - Based On US Army Radiation Safety Training