Physics of Radiation PDF Lecture

Physics of Radiation Dr. Mustafa Waseem B.D.S., M.Sc.,  Introduction Radiology is the science that deals with diagnosis, therapeutic and researches application of high energy radiation. Dental radiography is a process of image production for a...

Physics of Radiation Dr. Mustafa Waseem B.D.S., M.Sc.,  Introduction Radiology is the science that deals with diagnosis, therapeutic and researches application of high energy radiation. Dental radiography is a process of image production for an object inside the oral cavity or outside through the use of x-ray radiation. Radiologic examination is an integral component of the diagnostic procedure.  why Dentists often make radiographic images? To obtain additional information beyond that available from a clinical examination or their patient's history. ❖ Information from these images is combined with the clinical examination and history to make a diagnosis and formulate an appropriate treatment plan  Nature of Radiation Radiation is the transmission of energy through space and matter. It may occur in two forms: (1) electromagnetic (2) particulate  X – Ray was discovered by (Roentgen) in 1895,it travels in a form of pure energy and the basic unit is x – ray photon or (quantum).  X – Ray photons travel with a wave motion called (sine – wave) and the distance between the crests of these waves called (wave – length) which measured by a unit (A˚) Angstrom. The X – ray photons wave length used in diagnostic radiography is ranged between 0.1 – 0.5 A˚ ❖ The amount of energy contained in each photon called (photon energy) which depend on 1. Wave length 2. Frequency of x – ray The high frequency of X – ray the shorter wave length photons this shorter wave length photon has more energy than a low frequency long wave length type of X – ray photons.  Comparison between x – ray and light 1. Both belong to the same electro – magnetic radiation family. 2. Both travel in straight lines at the same speed which is 186,000 miles per seconds. 3. Both affected the photographic films and made them black. 4. Both not affected by magnetic fields 5. X-ray and light cast the shadows of the objects in the same manner 6. X-ray has the ability to penetrate objects that the light cannot pass through 7. X-ray has the ability to ionize atoms 8. X-ray has the ability to produce light (blue light) when it hits some objects and this phenomena called (fluorescence). 9. X-ray is invisible Components of X-ray machine and generation of X-ray The primary components of an x-ray machine are the x ray tube and its power supply, positioned within the tube head.  A control panel allows the operator to adjust the duration of the exposure, and often the energy and exposure rate of the xray beam. Often, the tube is recessed within the tube head to increase the source- to-object distance and minimize distortion. X–ray tube  X–ray tube is composed of a cathode and an anode situated within an evacuated glass envelope or tube.The glass of the tube is leaded to prevent (the generated X – ray) from escaping in all directions. While the window is of unleaded glass so that X – ray exist out through this window.  The cathode consists of a filament and a focusing cup. The filament is the source of electrons within the x-ray tube. It is a coil of tungsten wire approximately 2 mm in diameter and 1 cm or less in length, and typically contains approximately 1% thorium, which greatly increases the release of electrons from the heated wire. The filament lies in a focusing cup. The electrons emitted by the filament into a narrow beam directed at a small rectangular area on the anode called the focal spot.  The anode in an x-ray tube consists of a tungsten target embedded in a copper stem. ❖ The purpose of the target in an x-ray tube is to convert the kinetic energy of the colliding electrons into x-ray photons. The tungsten target is typically embedded in a large block of copper which functions as a thermal conductor to remove heat from the tungsten, reducing the risk of the target melting. The conversion of the kinetic energy of the electrons into x-ray photons is an inefficient process, with more than 99% of the electron kinetic energy converted to heat. To produce x-rays, electrons stream from the filament in the cathode to the target in the anode, where the energy from some of the electrons is converted into x-rays.  Selection of target material ❖characteristics of an ideal target material, including the following: 1. High atomic number, allows for efficient x-ray production. 2. High melting point, to withstand heat produced during x-ray production. 3. High thermal conductivity, to dissipate the heat produced away from the target. 4. Low vapor pressure at the working temperatures of an x-ray tube, to help maintain vacuum in the tube at high operating temperatures  Collimation Is a process used to control the size and shape of X – ray beam. In diagnostic radiography its essential to get the diameter of circular X – ray beam at patients skin surface is not great than 2.75 inches, while for Rectangular X – ray beam the dimensions at the skin should be approximately 1½ × 2 inches.  Filtration X – ray used in dentistry must be able to penetrate dental hard tissues (teeth and bone). The longer wave length X – ray (soft X – ray) are not useful in diagnostic radiology thus removal of these long wave length photons from the beam by passing the beam through a filter made from Aluminum which absorb most of long wave length photons (soft X – ray), the resulting X – ray beam will consist mainly of X – ray photons with short wave length, high energy photons and high penetrating power that’s why they named (hard X – ray beam).  Types of filtration: 1. Inherent filtration: done by filter built-in to the X – ray machine by manufacturer (as glass wall, the insulating oil and the metal housing of the tube). The inherent filtration tends to increase with age because some of tungsten metal of both target and filament is vaporized and deposited on the inside of the tube window. 2. Added filtration : done by using aluminum sheet as extra filter. *[total filtration = inherent filtration + added filtration]  Types of radiation 1. Central ray: is X– ray photons that traveling in very center of the cone of radiation (radiation beam), and it’s commonly used to fix and locate the position of X – ray beam. 2. Bremsstrahlung radiation: radiation produced when projectile electron is slowed by the electric field of target atom nucleus. 3. Characteristic radiation: radiation produced when an outer shell electron fills an inner shell void (empty orbital). 4. Primary radiation: Radiation emerging from the X – ray machine in form of collimated useful X – ray beam 5. Secondary radiation: Radiation result from interaction of primary beam with matter 6. Leakage radiation: x-ray that escape through the protective housing and result in unnecessary exposure of the patient and radiologic technologist and have no value in diagnostic radiology.  Definition of terms used in X – ray interaction:- ✓ Scattering: - change in direction of photon with or without a loss of energy. Absorption: - deposition of energy i.e. removal of energy from the beam. Attenuation: - reduction in the intensity of X – ray beam caused by absorption and scattering attenuation = absorption + scattering. Ionization: - removal of an electron from neutral atom.