Chapter 1 X-Ray to Radiography

 

 

   

 A discussion of the forms of radiation and the manner in which x-rays are produced should be of interest to the radiologic technologist. The inclusion of information about ionizing radiation in this manual will make it possible to better understand the x-ray machine in your practice or hospital. The greater your understanding of the x-ray machine, the higher will be the quality of the radiographs produced. More detailed descriptions of atomic structure, electromagnetic radiation, and the production and properties of x-rays are found in most radiation physics texts. The reader is referred to these books if additional information is needed. (Ball and Moore, 1980; Christensen et al., 1978; Hendee, 1970; Johns and Cunningham, 1977; Kodak 1980; Lamel et al., 1981; TerPogossian 1967; van der Plaats, 1980).

A brief discussion of the structure of matter will provide the reader with definitions and an understanding of matter than will make certain concepts easier to understand. 

1.1 Structure of Matter

Bohr's model of the atom places most of the mass of the atom in a

central core or nucleus that is much smaller than the atom. The diameter of the positively charged nucleus is estimated to be about 10-4 Angstrom units while the diameter of the atom is about 1 Angstrom (1A = lO-10m).

The nucleus consists of two types of particles. The positively charged particles are referred to as protons and have a charge equal in magnitude but opposite that of an electron. The number of protons is equal to the atomic number for the atom (Z) and defines the element. Neutrons are uncharged particles within the nucleus and are stable in that location. The sum of the number of protons (Z)plus the number of neutrons(NJ equals the mass number for the nucleus (A). (A = Z + N). Since particles of similar charge tend to repel each other, it is necessary that there be a nuclear force that exceeds the electrostatic repulsion forces so that the nucleus remains intact.

Electrons with a negative charge are within the atom and are bound to it by electrostatic forces of attraction that exist between particles of opposite charge. The electrons remain outside the nucleus because of their high velocity, while the high centripetal force exerted by the nucleus keeps the electrons from escaping. Atoms in their normal state are electrically neutral because the number of negatively charged electrons outside the nucleus equals the positive charge within the nucleus (Fig. 1-1).

Electrons are positioned in shells or energy levels around the nucleus. The shells are numbered K (inner shell), L (next outer shell), M (next outer shell), up to the Q 

Figure 1-1

   SCHEMATIC REPRESENTATION OF AN ATOM.

A model of an atom with the nucleus containing protons and neutrons as shown by the solid and hollow spheres in the center. The hollow spheres shown in the peripheral rings around the nucleus represent the electrons that rotate around the nucleus.

shell (Table l-l).The outermost shell of electrons is called the valence shell. If this shell is filled, the atom tends to be chemically unreactive. If there are a small number of electrons in the valence shell, the atom is said to have a positive valence and is chemically

reactive since it attempts to share this valence electron(s). If the valence shell is lacking but a few electrons to become filled, the atom is said to have a negative valence and is chemically reactive since it tries to attract an additional electron(s) to complete the shell . 

MAXIMUM NUMBER OF ELECTRONS THAT CAN OCCUPY EACH ELECTRON SHELL.

Table 1-1. 

An electron in an inner shell is attracted to the nucleus by a force greater than that which the nucleus exerts upon an electron further away. The energy required to remove an electron from the atom is termed the binding energy for that electron. Binding energies are negative because they represent amounts of energy which must be supplied to remove the electron. Binding energy for an electron in the K shell of an atom is always greater than for the L shell because the distance from the nucleus to the electron in the K shell is less than the distance to the electron in the L shell. Binding energy for electrons is greater as the atomic number of the atom increases because the nuclear charge is progressively greater. Any vacancy existing within an electron shell will be promptly filled by electrons cascading from lower energy levels further from the nucleus. During the shifting of electrons, any difference in binding energy between the original and final energy level of the electron is released as a photon of energy. This photon is considered an x-ray if its energy exceeds a level of 100 electron volts. An electron volt is the amount of energy that an electron gains as it is accelerated by a potential difference of 1 volt.

Problem: Calculate the energy required to displace an electron from the K shell to the L shell in tungsten.

Binding energy:

K shell = &endash;69,500 eV

L shell = &endash;11,000 eV

Answer:

Energy required is 69,500 &endash; 11,000 = 58,500 eV 

(This concept will be important when we discuss production of characteristic X-rays.)

Elements are conveniently described in the following manner, I.e.,

Co is the element symbol for cobalt

27 is the atomic number (number of electrons or protons) (Z)

58 is the mass number (number of protons and neutrons in the nucleus) (A)

+3 is the valence state and describes the number of electrons with in the valence shell and the chemical reactivity of the element.

Problem: If an atom has a nucleus with six protons and eight neutrons:

• a) What is the atomic number?
• b) What is the mass number?
• c) What is its electron number?
• d) What is the element which the atom represents?
• e) Is the atom stable?

Answer: a - 6, b -14, c- 6, d - carbon, e - no.

Certain terms are used in the description of the relationship between mass number (A) and atomic number (Z). An isotope is an element with the same atomic number but a different mass number. An isobar has the same mass number but a different atomic number. An isomer has the same mass and atomic number but different energy states. 

1.2 Types of Radiation

Radiation is the propagation of energy through space or matter. There are two basic types:

• l) corpuscular, and
• 2) electromagnetic. 

Corpuscular Radiation

Corpuscular radiation is the transport of energy contained in moving particles of matter that are usually submolecular in nature. The energy carried by the particle is dependent on the mass of the particle and its speed. Corpuscular radiation may be composed of particles that are electrically charged or are neutral. Typical corpuscular radiations are :

• 1) alpha particles (helium nuclei),
• 2) protons (hydrogen nuclei), or
• 3) electrons.

Moving electrons are called beta rays if they are emitted by radioactive nuclei or are called cathode rays if they are electrons that are accelerated by another method. The manner in which these particles interact with other particles of atomic or subatomic size depends on the:

• l) velocity of the particle,
• 2) mass of the particle, and
• 3) charge of the particle.

Most of the reactions are similar to "billiard ball" type collisions; that is, if the accelerated particle "hits" another particle, a portion of its energy is lost and the energy is transferred to the second particle. Gradually, through a series of these collisions, all of the energy of the particle is lost.

Corpuscular radiation is important in the understanding of x-rays and their production, since the "cathode rays produced within the x-ray tube is a form of corpuscular radiation. The interaction of this stream of electrons with the target in the x-ray tube produces the x-ray beam used both in diagnostic x-ray studies and in x-radiation therapy. 

Table 1-2

Where:

E = energy transported by a photon measured in keV (kilo electron volts = keV

lambda = wavelength (Ångstrom)

12.4 = conversion factor for converting the wavelength of an electromagnetic radiation into the energy carried by a photon (keV Ångstrom) 

or

Where:

E = energy transported by a photon measured in keV (kilo electron units = keV)

lambda = wavelength (nm)

1.24 = conversion factor for converting the wavelength of an electromagnetic radiation into the energy carried by a photon (keV mm) 

Electromagnetic Radiation

Electromagnetic radiation is another method of transporting energy through space. This movement of energy can most easily be explained by considering both the

• l) wave nature, and
• 2) particulate nature of the radiation.

Examples of electromagnetic radiation with increasing frequencies in cycles per second are:

• l) electrical waves,
• 2) radio waves,
• 3) television waves,
• 4) radar,
• 5) infrared rays,
• 6) visible spectrum,
• 7) ultraviolet rays,
• 8) x-rays, and
• 9) gamma rays (Fig. 1-2).

Electromagnetic radiations are characterized by a constant speed measured in a vacuum which is 3 x 1010 cm per second or the speed of visible light. The wavelength can vary and is a function of the energy. The frequency of the wave propagation is also variable and elated to the wavelength by the following formula:

where:

C = velocity of light(meters/sec)

lambda = wavelength (meters)

v (nu) = frequency (seconds)

The particulate nature of electromagnetic radiation is recognized in the fact that the radiation carries energy in the form of discrete bundles or "quanta." These bundles of energy are referred to as photons. They travel at the speed of light and the energy of the electromagnetic radiation can be seen to be proportional to the wavelength of the radiation in the following formula. The shorter the wavelength, the greater the energy transfer by the photon.

It can be seen from the formula (Table 1-2) that electromagnetic radiation can carry a wide range of energies. In diagnostic radiology, a beam of x-rays is referred to as being "hard" when the mean energy of the beam is high. The beam is referred to as being "soft" when the mean energy of the beam is low. The beam of x-rays produced by a x-ray tube consists of many different energies and is therefore referred to as heterochromatic or polychromatic. If a beam of electromagnetic radiation is of only one energy, the beam is called monochromatic.

Primary and secondary are another pair of terms used in the description of radiation. Primary radiation refers to the unaltered radiation that exists in the form and energy level that it possessed when formed. Secondary radiation refers to radiation with an altered, different energy level and/or a new direction resulting from an interaction between the primary radiation and any matter. These definitions will be used many times in discussion of the production, interaction and use of diagnostic x-rays. Physical properties of one form of electromagnetic radiation are listed in Table 1-3. 

1.3 Ionizing Radiation

Interaction of radiation with matter results in one of the following:

• l) elastic collision,
• 2) excitation, or
• 3) ionization.

It is possible for the radiation to interact with atoms or molecules so that all or only a part of the energy of the radiation is transferred to the atom or molecule. This transfer of energy may result in an elastic collision with the energy dissipated as heat.

Another possibility is that the atom or molecule can experience the interaction of excitation and return to a stable state by releasing the excess energy present in the form of long wavelength radiation that can be seen with the eye as visible light or in the form of heat. 'This type of energy transfer is seen when the x-ray beam strikes the intensifying screen used in diagnostic radiography.

When radiation carries enough energy to remove an electron from the atom or molecule, it is referred to as ionizing radiation and the process is called ionization. This can be a property of both corpuscular and electromagnetic radiation. Two ions are formed:

• l) one positively charged consisting of the nucleus and electrons minus the ejected electron, and
• 2) the negatively charged ejected electron.

Ionization is the basis for the biological effect of electromagnetic and corpuscular radiation. Ionization may occur whenever the primary or secondary x-ray beam strikes the patient or those assisting in positioning the patient. 

Table 1-3

PHYSICAL PROPERTIES OF ONE FORM OF ELECTROMAGNETIC RADIATIONS&endash;X-RAYS.

 References:

THESE WERE FOUND UNDER A LARGE OAK TREE.