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What I Forgot Over My Summer Vacation the Discovery The What I Forgot Over My Summer Vacation A Review of Oral Radiology I Wilhelm Conrad Roentgen 1845 - 1923 Steven R. Singer, DDS [email protected] The Discovery The Discovery • Roentgen discovered x-rays on November 8th, 1895 • Using a cathode ray tube and a piece of cardboard painted with platinocyanide, he was able to see a fluorescent glow in his darkened laboratory Roentgen’s Laboratory Source: http://compepid.tuskegee.edu/syllabi/clinical/small/radiology/chapter2.html The first radiograph – Mrs. Roentgen’s hand. 1 Enter Dentistry Roentgen’s Apparatus This radiograph was made on February 1st, 1896 by Dr. Walter Konig of Germany. Image from the Radiology Centennial Collection HITTORF-CROOKES' TUBES Image from the Cathode Ray Tube Site The Dangers Although many had noted difficulties associated with "X-ray burns," it was not until the death of Clarence Dally (1865- 1904), Edison's longtime assistant in X-ray manufacture and testing, that observers finally agreed that the magic rays could kill as well as cure. Image from the Radiology Centennial Collection Image from the Radiology Centennial Collection Forces • Electrostatic forces maintain electrons in the shells ( e- and protons) Radiation Physics • Centrifugal forces of revolving (orbiting) electrons balance electrostatic forces • Electron binding energy (ionization energy) – amount of energy required to remove an e- from its shell, specific for each shell 2 Forces Ionization • Energy required to remove e¯ from a given shell must exceed the electrostatic force of • An electrically neutral atom loses an electron attraction between e¯ and the nucleus • Atom becomes positively • Electrons in the K shell have the greatest charged ion binding energy due to the proximity to the • Electron becomes a free, nucleus negatively charged ion • Occurs by heating, • Binding energy of e¯ in successive shells collisions with high decreases energy x-rays or particles • Usually, outer shell electrons are lost ION Radiation Particulate Radiation • Transmission of energy through space and • Atomic nuclei or subatomic particles matter – Move at high speed • Occurs in two forms – Include – Particulate radiation • α particles – Electromagnetic (EM) radiation • β particles • Cathode rays Cathode Rays Linear Energy Transfer (LET) • High speed electrons • Capacity of particulate radiation to ionize • Man-made atoms depends on mass, velocity, and – X-ray tubes charge of the particle – Television sets (CRTs , not flat panel) • Rate of loss energy from particle as it – Computer screens (CRTs, not flat panel) moves along its track through matter is the – Cathode ray tubes Linear Energy Transfer 3 Linear Energy Transfer (LET) Electromagnetic Energy • Greater physical size and charge and • Movement of energy through space or matter as a combination of electric and magnetic fields lower velocity, the greater the LET • Includes radio waves, radiant heat, visible light, – α particles transfer more energy in a given and γ radiation path than β particles and are therefore more • Generated when the velocity (speed) of an damaging per unit dose electrically charged particle is altered – Both penetrate a relatively small distance into • Ionizing or non-ionizing tissue • Enough energy will knock an orbital electron out of its shell – ionizing radiation Dual Nature of Energy Wave Theory • Wave Theory – energy moves forward • Waves consist of electric and magnetic fields similar to a wave in water and travels at oriented in planes at 90º to each other the speed of light in a vacuum • Electric and magnetic waves oscillate perpendicular to the direction of the motion • Quantum Theory – energy moves as finite bundles or packets of energy (quanta of photons), with each photon moving at the speed of light and containing a specific amount of energy – an electron volt (eV) 4 X-ray Machine • Distinguish between – X-ray - an invisible beam of energy – X-ray receptor – Radiograph Definitions Definitions •An x-ray is an invisible beam or photon of • Image receptor is the material on which energy. the latent image is created. This may be a film, fluoroscope, or digital receptor. The receptor is processed, and a radiograph is made. Image courtesy of University of Pennsylvania Intraoral Image Receptors A radiograph is the resultant image. A Size 2 intraoral film B Size 2 PSP plate C Gendex #2 CCD sensor D Schick #2 CMOS sensor 5 Radiograph Power Supply and Control Panel X-ray Tube Head Support Arm Aiming Tube Cone Head X-ray Tube Head X-ray Machine Support arm • X-ray tube is positioned within the tube head with some components of the power Tube supply head • Tube may be recessed within the tube head to improve the radiographic image quality Aiming device 6 X-ray Head X-ray Tube Coolidge Tubes Coolidge Tubes • Basic design introduced in 1913 • Composed of – Anode • Requires power supply to: – Cathode – source of electrons – Heat the filament to generate electrons – Evacuated glass tube – Establish a high-voltage potential between the anode and the cathode to accelerate the • Electrons from cathode strike anode, electrons producing x-ray photons Cathode X-ray tube, circa • Filament (source of electrons) 1926 Cathode – Coil of Tungsten wire 1 cm long and 0.2 cm in diameter – Mounted on 2 stiff wires for support and carrying electric current X-ray beam – Wires connect to high and low voltage electric sources – Incandescence of the wire causes the release Anode of electrons (boiling off of electrons) 7 Cathode Cathode Features • Nickel or molybdenum focusing cup – • Focusing cup – directs the electrons produced by the Focusing negatively charged filament toward the focal spot on the Cup concave reflector anode (charge repels • Electrons travel from cathode to anode by electrons) repellent forces on the negatively charged cathode and attractive forces on the positively charged anode Anode Rotating Anode • Tungsten target embedded in copper stem • Converts the kinetic energy of electrons from the filament into photon energy (x- rays) • Inefficient process – 99% of energy is lost as heat Diagram from White & Pharoah 3rd Edition Tungsten Target Modern Dental X-ray Tube • Embedded in a large block of copper (a good thermal conductor) – Dissipates heat from the tungsten – reduces risk of target melting • Insulating oil surrounds the x-ray tube • Stationary anode 8 Focal Spot Coolidge X-ray Tube circa 1930 • Area on target where focusing cup directs electrons from the filament • Sharpness of radiographic image increases as the size of the focal spot decreases Source: http://antiquescientifica.com/archive47.htm Focal Spot Focal Spot • Heat generated increases as the size of the focal spot decreases • Target is angled to decrease effective focal spot size while maximizing distribution of electrons over a large target • Projection of focal spot is 90º to the electron beam (effective focal spot is smaller than the actual focal spot) Diagram from White & Pharoah 3rd Edition Focal Spot Smaller focal spot = sharper images 9 Production of X-rays X-ray Production • Current heats the filament of the cathode, releasing electrons – Thermionic Emission • Higher temperatures produce more free electrons (electron cloud) • Electrons accelerate from the cathode to the tungsten target on the anode • X-rays are produced as the electrons decelerate or stop Electric Circuits Power Supply • Electric current is the movement of • Heats the filament using low-voltage electrons through a conductor current to generate • Rate of flow (electrons/second past a set electrons • Establishes a high- point) is measured in amperes voltage potential • Depends on voltage and resistance of the between the anode and the cathode to conductor to flow, measured in ohms accelerate the • Ohm’s law: V = IR, V=potential, I=current, electrons R=resistance Tube Current (mA) Power Supply (Electric Circuits) • Determines the number of electrons produced • Tube current (mA) • The higher mA, the greater the number of • Tube voltage (kVp) electrons generated, and consequently, the • Together, mA and more x rays produced kVp determine the • Tube current is the flow of electrons from the intensity of the x ray filament to the anode, then back to the filament beam, along with the target material and through wiring of the power supply the filtration used 10 Tube Current (mA) Tube Voltage (kVp) • Filament step-down transformer reduces • Requires high voltage between the anode and voltage of the alternating current to ~ 10V the cathode to generate x rays • Autotransformer converts the primary voltage • Controlled by the mA switch, adjusting from the input source to the secondary voltage resistance and current through the low- • High voltage transformer provides the high voltage circuit voltage required by the x-ray tube to accelerate • Regulates temperature of the filament and electrons from the cathode to the anode number of electrons emitted Tube Voltage (kVp) Tube Voltage (kVp) • Peak voltage of the incoming line current is boosted to 60 -100 kV • kVp determines the energy of the • Boosts peak energy of the electrons to 60 -100 electrons that generate the photons from keV the target • Line voltage is variable (alternating current), so • kVp determines the maximum energy intensity is greatest at peak of each positive portion of the cycle (quality) of the x rays produced • No x-rays are generated during the negative • For tungsten targets, at least 70 kVp must portion be used to produce the K-characteristic radiation Half-Wave Rectification Rectification • AKA self-rectification • Limits x-ray
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