Basic X-ray Properties PDF

# **X-rays: Discovery and Basic Properties** * **Discovery:** X-rays were discovered on November 8, 1895 by Wilhelm Conrad Roentgen, a German physicist. * **Initial Use:** X-rays were immediately used for medical purposes and many sophisticated applications were soon discovered. * For example...

# **X-rays: Discovery and Basic Properties** * **Discovery:** X-rays were discovered on November 8, 1895 by Wilhelm Conrad Roentgen, a German physicist. * **Initial Use:** X-rays were immediately used for medical purposes and many sophisticated applications were soon discovered. * For example, angiography was described in 1896, only one year after the discovery of x-rays. * Roentgen was awarded the first Nobel Prize for Physics in 1901 for his discovery. * **Importance:** X-ray imaging is still one of the most important and widely used diagnostic tests in people and animals despite advances in ultrasound, computed tomography, and magnetic resonance imaging. ## ***Figure 1.1: Discovery of X-rays*** * **Image A:** Wilhelm Conrad Roentgen accidentally discovered x-rays while working with a cathode ray tube. * He noted that applying high voltage caused fluorescence of a photographic plate. * Roentgen deduced that an unknown invisible radiation must be produced by the cathode ray tube. * **Image B:** The first known radiograph was made of Roentgen's wife's hand. ## **Basic Properties of X-rays** * X-rays and gamma rays are part of the spectrum of electromagnetic (EM) radiation. * The only distinction between them is their source. * X-rays are produced by electron interactions outside the nucleus. * Gamma rays are released from inside the nucleus of unstable atoms having excess energy. * It is incorrectly believed that gamma rays are more energetic than x-rays. * The energy of a gamma ray depends on the amount of energy released by the unstable atom. * The energy of an x-ray depends on the energy of the electron interacting with the atom. * Radiopharmaceuticals used in nuclear imaging generally emit gamma rays with similar energy to x-rays used for diagnostic imaging. ## **Familiar Types of EM Radiation** * Familiar types of EM radiation other than x-rays and gamma rays include: * Radio waves * Radar * Microwaves * Visible light ***Table 1.1: Wavelength and Origin of Common Types of Electromagnetic Radiation*** | Type of Electromagnetic Radiation | Wavelength | Origin | | ---------------------------------- | ---------- | -------- | | Radio Waves | 1 mm - 100 Km | Oscillating electrons in antennae | | Microwaves | 1 mm - 1 m | Oscillating electrons in antennae | | Infrared | 780 nm - 1 mm | Electron energy level transition | | Visible Light | 380 nm - 700 nm | Electron energy level transition | | Ultraviolet | 100 nm - 400 nm | Electron energy level transition | | X-rays | 0.1 nm - 10 nm | Bremsstrahlung, Characteristic | | Gamma Rays | 1 x 10^6 nm | Nuclear decay | * *From Bushberg JT, Seibert JA, Leidholdt EM, Jr, et al. The Essential Physics of Medical Imaging. 2nd ed. Lippincott Williams & Wilkins; 2012.* ## **Electromagnetic Radiation** * EM radiation is a combination of electric and magnetic fields that travel together, oscillating in orthogonal planes in sine-wave fashion. * **Figure 1.2:** This shows how electric and magnetic fields form sine waves. * **Sine waves** are characterized by two related parameters: frequency and wavelength. * **The velocity of EM radiation is constant:** the speed of light. * This results in an inverse relationship between frequency and velocity according to the following formula: * $V = fλ$ * where: * **V** = speed of light (m/s) * **f** = frequency in Hz * **λ** = wavelength in m ## **Properties of Electromagnetic Radiation** * **Figure 1.2:** All forms of electromagnetic radiation are characterized by oscillating electric and magnetic fields moving in panes at right angles to each other. * Any form of electromagnetic radiation is described by the wavelength, which is the distance between crests, and the frequency, which is the number of crests per unit time. * Frequency and wavelength determine the specific characteristics of that form of radiation. * **The velocity of EM radiation is the same:** the speed of light. * **The product of wavelength and frequency equals velocity:** $c = fλ$. * Therefore, as frequency increases, wavelength must decrease and vice versa. ## **The Photon Concept** * Properties of x-rays and gamma rays are given in Box 1.1. * Some properties of EM radiation cannot be explained adequately by the theories of wave propagation illustrated in Figure 1.2. * **Therefore, the photon concept was developed to explain the apparent particulate behavior of x-rays and gamma rays.** * A photon can be considered as a *discrete bundle of EM radiation* as opposed to a wave. * This makes it easier to understand how x-rays create an image or cause radiation damage. * **The terms x-ray and photon are used interchangeably in this book.** ***Box 1.1: Properties of X-Rays and Gamma Rays*** * Have no charge * Have no mass * Travel at the speed of light * Are invisible * Cannot be felt * Travel in a straight line * Cannot be deflected by magnetic fields * Penetrate all matter to some degree * Cause certain substances to fluoresce * Can expose photographic emulsions * Can ionize atoms ## **The Energy of EM Radiation** * **The energy of EM radiation is described according to the following formula:** * **Energy = Planck's constant x (speed of light / wavelength)** * **Planck's constant** is a proportionality constant between the energy of a photon and its wavelength. * **The speed of light is also a constant.** * **Therefore, the energy of EM radiation is inversely proportional to wavelength.** * **The biologic effects of EM radiation are a function of the energy.** * **The unit of energy for EM radiation is the electron volt (eV).** * One electron volt is the energy gained by one electron as it is accelerated through a potential difference of 1V. * This is a very small amount of energy. However, x-rays with energy of only 15 eV can produce ionization of atoms. * **Ionization occurs when an electron is ejected from the atom by an x-ray.** This creates an ion pair consisting of the negatively charged electron and the positively charged atom. * **Figure 1.3:** This shows the process of ionization and electron ejection. * When x-rays strike a person, they can result in ionizations of DNA leading to: * Mutations * Abortion or fetal abnormalities * Susceptibility to disease * Shortened lifespan * Carcinogenesis * Cataracts. * **Minimizing exposure of personnel working in a radiation environment is critical.** Radiation also causes ionizations in patients undergoing medical imaging procedures. * The risk of radiation injury from an imaging procedure is offset by the diagnostic value of the procedure. * Radiation workers are subject to repeated potential low-level exposure and the increased risk for damage. * **It is important to note that radiation damage to DNA can be amplified biologically.** DNA controls cellular processes that extend into subsequent generations of daughter cells. * Although only 15 eV of energy is required for ionization of biologic molecules, the energy of x-rays used for medical imaging is much greater. * Therefore, each x-ray photon can lead to multiple ionizations in tissue. ## **Biologic Injury** * **The relative risk of biologic injury from x-rays or gamma rays is greater than from other types of EM radiation.** * The wavelength of visible light is 10,000 times longer than the wavelength of x-rays. * The wavelength of radio waves is even longer, as shown in Table 1.1 * **Therefore, the energy of light waves and radio waves is many orders of magnitude lower than the energy of x-rays.** * **This means that light and radio waves do not produce tissue ionization or DNA damage.** * **Other forms of EM radiation, such as microwaves, can lead to biologic damage such as tissue heating but do not lead to molecular ionization.**