KEY TERMS Anode: A positively charged electrode. Atom: A basic part of matter that consists of a nucleus and a surrounding cloud of electrons. Atomic number: The number of protons in an atom's nucleus. Cathode: A negatively charged electrode. Electromagnetic radiation: A method of transporting energy through space, distinguished by wavelength, frequency, and energy. Electromagnetic spectrum: Electromagnetic radiation grouped according to wavelength and frequency. Electron: A negatively charged particle that travels around the nucleus. Excitation: A process in which an electron is moved to a higher energy level within the atom. Fluorescence: The ability of a substance to emit visible light. Frequency: The number of cycles of the wave that pass a stationary point in a second. Gamma rays: Electromagnetic radiation emitted from the nucleus of radioactive substances. Infrared rays: Electromagnetic radiation, beyond the red end of the visible spectrum, characterized by long wavelengths. Ionization: A process in which an outer electron is removed from the atom so that the atom is left positively charged. Neutron: A neutral particle located in the nucleus of an atom. Photons: A bundle of radiant energy (synonymous with quanta). Proton: A positively charged particle located in the nucleus of an atom. Quanta: A bundle of radiant energy (synonymous with photons). Radiant energy: Energy contained in light rays or any other form of radiation. Radiograph: A visible photographic record on film produced by x-rays passing through an object. Shell: An electron's orbital path and energy level. Ultraviolet rays: Electromagnetic radiation, beyond the violet end of the visible spectrum, that is characterized by short wavelengths. Vacuum: An area from which all air has been removed. Wavelength: The distance between two consecutive corresponding points on a wave. X-rays: A form of electromagnetic radiation similar to visible light but of a shorter wavelength. X-ray beam: A number of x-rays traveling together through space at a rapid speed.

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DEFINITION OF X-RAYS

Knowledge of the nature and behavior of x-rays is the first step in understanding the production of a radiograph. The veterinary radiographer does not need detailed knowledge of the underlying radiologic physics, but a basic understanding of certain principles and terms is necessary to produce quality radiographs. Many of the basic components will affect the image that is produced. This will improve the ability of the radiographer to produce a quality diagnostic image on the first attempt which reduces retakes and radiation exposure.

X-rays are defined as a form of electromagnetic radiation similar to visible light but of much shorter wavelength. Electromagnetic radiation is a method of transporting energy through space and is distinguished by its wavelength, frequency, and energy. Essentially, there are two characteristics of electromagnetic radiation: particles and waves.

We will first consider the wave. All radiant energy travels in a waveform along a straight path and is measured by its wavelength. In a series of waves the distance between two consecutive, corresponding points on a wave is called the wavelength (Fig. 1-1). Electromagnetic radiation that has a short wavelength has a high frequency. Electromagnetic radiation that has a long wavelength has a low frequency. Frequency is measured by the number of cycles of the wave that pass a stationary point per second (cycles per second). The higher the frequency, the more penetrating power the energy has through space and matter.

All forms of electromagnetic radiation are grouped according to their wavelength and frequency in what is called the electromagnetic spectrum . Examples of electromagnetic radiation are radio waves, television waves, radar, infrared rays, the visible spectrum of light, ultraviolet rays, x-rays, and gamma rays (Fig. 1-2).

Electromagnetic radiation behaves as a particle, as well as a wave. Atoms consist of small particles called protons, neutrons , and electrons. An atom has a nucleus with a surrounding cloud of electrons ( Fig. 1-3). The nucleus of an atom contains protons, which are positively charged, and neutrons, which are neutral. Electrons, which are negatively charged, travel

Figure 1-1 Wavelength motion showing two corresponding points on consecutive waves.

Figure 1-2 The electromagnetic spectrum .

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around the nucleus in specific orbits, which are called shells. X-rays are produced when charged particles (electrons) are slowed down or stopped by the atoms of a target area. This process occurs inside the x-ray tube to create an x-ray beam.

An x-ray beam is composed of bundles of energy that travel in a wave. These bundles of energy, or quanta, are referred to as photons. The photons have no mass or electrical charge. Photons consist of pure energy and are transported, or “carried,” by the wave.

Electromagnetic radiation can carry a wide range of energies. The energy of the radiation is proportional to the wavelength. The shorter the wavelength, the greater the energy. Therefore in radiography, x- Figure 1-3 Model of an atom . rays that have a shorter wavelength penetrate farther than rays that have longer wavelengths.

PHYSICAL PROPERTIES OF X-RAY ELECTROMAGNETIC RADIATION The physical properties of x-ray electromagnetic radiation, listed as follows, have diagnostic, medical, and research applications:

1. Wavelength is variable and is related to the energy of the radiation. 2. Travel is in a straight line. Direction can be altered, but the new path is also a straight line. 3. Because of the extremely short wavelength, x-rays can penetrate materials that absorb or reflect visible light. They are gradually absorbed the farther they pass through an object. The amount of absorption depends on the atomic number, the physical density of the object, and the energy of the x-rays. 4. Certain substances have the property of fluorescence (i.e., they can emit visible light). Crystalline substances such as calcium tungstate or rare-earth phosphors fluoresce (emit light) within the visible spectrum after absorbing electromagnetic radiation of a shorter wavelength (i.e., x-rays). 5. X-rays produce an invisible image on photographic film that can be made visible by processing the film. 6. X-rays have the ability to excite or ionize the atoms and molecules of the substances including gases through which they pass. Excitation is a process in which an electron is moved to a higher energy level within the atom. Energy is required to initiate this change. Ionization is a process in which an outer electron is completely removed from the atom so that the atom is left positively charged. This process requires more energy than excitation. 7. X-rays can cause biologic changes in living tissue. A biologic change occurs either by direct action of excitation and ionization on important molecules in cells or indirectly as a result of chemical changes occurring near the cells. Affected cells may be damaged or killed.

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GENERATION OF X-RAYS

X-rays are generated when fast-moving electrons (small particles bearing a negative charge) collide with any matter. This is best achieved in an x-ray tube. The x-ray tube consists of two electrodes, a cathode and an anode, that have opposite electrical charges. The controls, generators, and transformers associated with a radiology machine control the amount of electricity that reach the x-ray tube.

As electrons have a negative charge at the cathode, they are attracted to the positive pole (anode) in the tube, and they collide with the positively charged target. This collision results in the production of x-radiation and a great amount of heat. Heat is the result of the interaction of the electrons and the atoms in the target. In fact, in diagnostic x-ray tubes, 99% of the energy from fast-moving electrons is converted into heat and 1% into x-ray energy.

DISCOVERY OF X-RAYS

On November 8, 1895, Wilhelm Conrad Roentgen discovered x-rays, an invaluable contribution to science. A professor of physics, Roentgen was the director of the new Physical Institute of the University of Würzburg, Germany. “Gas” tubes were being used at the time to conduct experiments with cathode rays. A vacuum was created in the tube by pumping out the air, and a current of electrons was passed through the tube. The tube consisted basically of a cathode (negative electrical charge) and an anode (positive electrical charge). The difference in electrical charge potential between the two electrodes caused the electrons to accelerate toward the tube end, where they interacted with the glass, producing x-rays. Figure 1-4 Roentgen viewing a radiograph of his Roentgen then wrapped the glass tube with dark wife's hand. paper, and during activation he saw a greenish illumination from a piece of cardboard across the room. The cardboard was painted with a fluorescent material called barium platinocyanide. This fluorescent material had been used previously to detect cathode rays. After further investigation, Roentgen presented a written report to the Society of Physics and Medical Sciences at the University of Würzburg on November 28, 1895. With his findings, he also submitted a radiograph of the hand of his wife, which he had produced with his own x-ray tube (Fig. 1-4).

By 1896, thousands of manuscripts and many books on x-rays had been published. X-rays were used immediately for medical and surgical diagnosis. And by as early as April 1896, changes in skin color caused by exposure to x-rays, similar to a sunburn, were reported. This discovery of skin color changes resulted in the use of x-rays for radiation therapy.

In recognition of Roentgen's discovery, he was awarded the Nobel Prize in 1901. This was the first Nobel Prize awarded in the field of physics.

Interestingly, a professor Goodspeed in Philadelphia had also made the discovery of x-rays in 1890, but he did not recognize their medical significance.

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KEY POINTS

• Energy travels in waves, the length of which is measurable. • X-rays with a shorter wavelength have a higher frequency and penetrate farther than rays having longer wavelengths. • X-radiation is a form of electromagnetic radiation produced when electrons moving with great speed collide with matter. • The ability of x-rays to excite and ionize molecules within cells can cause severe damage or death to those cells. • The first written report concerning x-rays and their use for medical and surgical diagnosis was made in 1895. The author and discoverer was Wilhelm Roentgen.

Lavin, Lisa. Radiography in Veterinary Technology, 4th Edition . W.B. Saunders Company, 072006.

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