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RADR 1313 - Chapter 2, Part 1 - Notes

by: Michelle Notetaker

RADR 1313 - Chapter 2, Part 1 - Notes RADR 1313

Marketplace > Houston Community College System > Radiography > RADR 1313 > RADR 1313 Chapter 2 Part 1 Notes
Michelle Notetaker
Houston Community College System

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These notes are typed for Chapter 2, Part 1 from the book "Radiographic Imaging and Exposure" Fifth Edition! Some parts of these notes are from my professor's Powerpoint, so you may or may not see ...
Principles of Radiographic Imaging
Catherine Nobles
Class Notes
Radiography, X-ray, imaging, exposure
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This 5 page Class Notes was uploaded by Michelle Notetaker on Friday September 9, 2016. The Class Notes belongs to RADR 1313 at Houston Community College System taught by Catherine Nobles in Fall 2016. Since its upload, it has received 19 views. For similar materials see Principles of Radiographic Imaging in Radiography at Houston Community College System.

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Date Created: 09/09/16
RADR1303:CHAPTER2,PART1 X-RAY PRODUCTION – PG 16 -20 X-RAY TUBE CONSTRUCTION Ü Cathode : Filament and focusing cup Ü Anode : Target, stator, and rotor CATHODE Ü A negatively charged electrode Ü Electrode: Is used to conduct electricity Ü Comprisesa filament and a focusing cup Ü Figure 2 -1 on the right: Shows a double- filament cathode surrounded by a focusing cup Ü Filament: A coiled tungsten wire,which isthe source of electrons during x-ray production o Usually 2 – large and small o Onlyoneenergizedat a time Ü Focusing cup: Madeofnickelandnearlysurroundsthefilament o Negativelycharged o Purpose: Focuses electrons ANODE Ü A positively charged electrode composed ofmolybdenum,copper, tungsten, and graphite o Materialsusedthermal& electricalconductive properties Ü Consistsofa target and in rotating anode tubes, stator, rotor o Target § The part of the anode that is struck by the focused stream of electrons coming from the cathode § Stops the electrons and creates the opportunity forth e production ofx-rays § Either rotating or stationary § 3,000 to 10,000 revolutions perminute § 90% tungsten,10% rhenium § Tungsten and rhenium layer is embedded in a base of molybdenumandgraphite o Why is tungsten used as the coating for the anode? § Highatomic number – 74 § Highmeltingpoint o Stator: An electricmotorthatturnsthe motoratvery high speed o Rotor § Madeofcopper § Rigidlyconnected tothetargetthrough theanodestem (madeofmolybdenum),causing thetargetto rapidly rotate during x-ray production § Connected to target § Containshigh-strength bearings o Focal spot: The physical area of the target that is exposed to electrons during x -ray production o Why are rotating anodes more efficient? Becausetheycan withstand high heatloadsbecausetherotation causes a greaterphysicalarea,orfocaltrack,to be exposed to electrons o The Glass Envelope § Pyrexglasswith lead § Surrounds internalparts § Allairisremoved during manufacturing to create a vacuum § Containsa window § Three functions of the Glass Envelope • Allowsairto be evacuated • Providesinsulation • Dissipatesheat o Metal Envelopes § Commonly used today § Haveimprovedelectricalproperties § Extends tube life § Reduces“off-focus” radiation o Off -focus radiation: Occurswhenprojectileelectronsarereflectedandx -rays are produced from outside the focal spot o Lead Housing § Protective device – Stops leakage radiation § Supports forthe x-ray tube § Containsopeningsforelectricalcables § Containsa portorwindow o Leakage radiation: Referstoanyx-rays, other than the primary beam,thatescape the tube housing o Chain of events in x -ray production § Electrons are released from atoms at the filament (Releasing the electrons) • Filament is the source of electrons • Filament circuit is energized • Filament gets hot • Incandescence occurs • Electrons are “boiled off” or freed – this is called thermionic emission • Space charge occurs • Focusing cup holds electrons § Production ofhigh speed electrons • Focusing gets a strong negative charge • Anode getsa strong positive charge • Electrons are repelled by the focusing cup • Electrons leave the filament in a group • Positive anode pullsthe electrons • Combination ofpush/pullcreateshigh speed • Electrons have a high kinetic energy (kinetic – energy ofmovement) § Focusing electrons • Electrons hit the anode at the target • Size ofarea hit – Createslarge orsmallfocalspot • Isotropic emission occurs – X-ray fly-offin a sphericalpattern (like sparklers) • Lead housing / Glass envelope / Anode disc absorb unnecessary radiation § Stopping electrons • Electrons slam into the anode • Lose their kinetic energy • Lost energy is transformed TARGET INTERACTIONS – PG 20 -23 2 TYPES O F INTERACTIONS OCCURRING X-RAY TUBE Ü Bremsstrahlung Ü Characteristic Ü Both producesx-ray photons BREMSSTRAHLUNG INTERACTION Ü Occurswhenprojectile electron avoids the orbitalelectrons ofthe tungsten atom Ü Electron travels close to the tungsten atom nucleus Ü Nucleus – Electrostatic force slows down the electron Ü Electron looses energy and changes direction Ü Energy loss reappears as an x -ray photon CHA RACTERISTIC INTERACTION Ü Occursprojectileelectroninteractswithanelectronfrom theinnerKshellof tungsten atom Ü Electron ejects a K-shell electron Ü Outer-shell electron drops into open position Ü Createsan energy difference Ü Energy difference is emitted as an x-ray photon TO SUMMARIZE THE INTERACTIONS : Ü Less than 70 kVp – 100% Bremsstrahlung interactions Ü Greater than 70 kVp – 15% Characteristic interactions,the restare Bremsstrahlung Ü The majority of x-ray interactions produced in diagnostic radiology are produced by Bremsstrahlung interactions Ü There is no difference between a Bremsstrahlung x -ray and a Characteristic x-ray at the same energy level; They each are just produced by differentprocesses X-RAY EMISSION SPECTRUM – PG 22 -23 Ü Widerangeand intensity of the energy of x-rays emitted Ü Spectrum (range ofenergies)changes with differentexposure and kVp settings Ü X-rays measured in –kiloelectrons- volts (keV) or 1000 electrons Ü Polyenergetichasmanyenergies o Lowest – 15-20 keV o Highest – kVp set Ü Produces energies ranging from 15 keV to 80 keV Ü Fewest number of x-rays occur at the extreme low and high end ofthe spectrum Ü Greatestnumberoccurbetween 30 and 40 keV foran 80kVp exposure technique X-RAY EXPOSURE – PG 23 -26 Ü Produced byusing 2 switches o Rotor,orPrep button o Exposure,or x-ray button Ü Rotor or Prep button o Mustbeactivatedbeforetheexposurebutton o Activatesthe filamentcurrent o Activatesthe statorwhich acceleratesthe anode attop speed o Causesan electricalcurrentto be induced acro ss the filament in the cathode Ü Filament Current o Activated by the prep or rotor button o This current heats the tungsten filament o Heatingofthefilamentcausesthermionicemission Ü Thermionic emission o The boiling off to electrons from the filament o Whenthe tungsten filament gains enough heat (therm), the outer-shell electrons (ions) of the filament atoms are boiled off,oremitted,from the filament o Space charge: A cloud ofelectronsthatisformed around the filament as they are boiled off during thermionic emission § Space charge effect: The tendency of the space charge not to allow more electrons to be boiled off of the filament Ü Exposure or X -ray button o Pushing thisbutton beginsthe x -ray exposure o Cathode becomeshighly negatively charged o Anode becomes highly positively charged o Electrons strike the anode o Kineticenergy ischanged to electromagneticenergy X-RAY QUALITY AND QUANTITY – PG 26 -31 RADIOGRAPHER MANIPULATES : Ü kVp,mA,Exposure time Ü Bymanipulating theseexposurefactors,theradiographer is able to alter both quantity and quality of the x-ray beam Ü What is meant by quantity of the -ray beam?Indicates the number of x-ray photons in the primary x-ray beam Ü What is meant by the quality of the-ray beam?Indicates the x-ray beams penetrating power kVp OR KILOVOLTAGE Ü Determinesthespeed theelectronsmovein thetubecurrent Ü é kVp ê speed of electrons o They are directly proportional Ü Why? A highervoltage results in a great repulsion of the electrons from the cathode and a greater attraction ofelectrons towards the anode Ü The speed that the electrons in the tube current move – determines the quality or energy ofthe x-rays o QUALITY = ENERGY Ü The quality or energyofthe x-rays produced determines the penofthe primary beam Ü Direct or P roportional Relationship o é kVp é QualityorEnergy é Penetration o ê kVp ê QualityorEnergy ê Penetration Ü kVp also effects quantity of-rays o An increase in kVp resultsin more x -rays being produced o Increase in kVp increases the efficiency of x-ray production Ü How? o 1% x-rays, 99% heat o Increasing kVp will increase the number of electrons boiled off during thermionic emission therefore a larger number ofx-rays are produced o HigherkVpequalshigherprotonsused o Example : 1% of100 = 1 and 1% of1000 = 10 MILLIAMPERAGE (mA) o Unitused tomeasuretubecurrent § Tube current:Measuresthenumberofelectronsflowingperunitoftimebetweencathodeandanode o Quality or QuantityQuantity o Directly proportional: o é mA é Quantity – electrons /x-ray o ê mA ê Quantity – electrons/x-ray o mA DOES NOT EFFECT QUALITY Ü Exposure Time o Determinesthelength oftimethatthex -ray tube produces x-rays o Expressed as:Seconds,milliseconds as a decimalorfraction o Quantity ofelectrons and x-rays is directly prto the exposure time o WhenmAismultipliedbyexposuretime,theresultisknownasmAs o Mathematically: mAxs=mAs o Quantity ofelectrons flowing from the cathode to the anode and x -rays produced is torectly proportional mAs


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