Radiographic Physics Week 4 RTEC A Objectives General Science Review Atomic Structure Atomic Interactions in the Tube • A few days after Roentgen's initial public announcement of his discoveries, a doctor in America took X-ray photographs of a person with gunshot wounds in his hands • Why can you see the bullet fragments? Atomic Models • The theory of how radiation is created is based on the BOHR model of the atom. • Electrons orbit around a nucleus ATOM Structure of the Nucleus • central core of the atom • contains p+ & n0 (& other particles) • identifies the atom as a specific element • held together by fundamental force ("strong") Binding Energy Binding Energy K L M Shells Law of Electrostatics • Repulsion/Attraction – like charges repel, unlike charges attract • The inverse square law – the force between two charges is directly proportional to the product of their magnitudes - The more energy they have the more attracted/repelled they will be How “X-rays” are created SEE: MAN MADE RADIATION (PG.93) TO PRODUCE X-RAYS YOU NEED: • A SOUCE OF ELECTONS • A FORCE TO MOVE THEM QUICKLY • SOMETHING TO STOP THEM SUDDENLY How Are X-rays Made? • X-rays are produced when electrons strike a metal target. • The electrons are liberated from the heated filament and accelerated by a high voltage towards the metal target. • The X-rays are produced when the electrons collide with the atoms and nuclei of the metal target. The X-ray Photons that exit the tube are many energies – “polyenergetic or heterogenous beam” How “X-rays” are created • Power is sent to x-ray tube via cables • mA (milliamperage) is sent to filament on cathode side. • Filament heats up – electrons “boil off” • Negative charge How “X-rays” are created • Positive voltage (kVp) is applied to ANODE • Negative electrons = attracted across the tube to the positive ANODE. • Electrons “slam into” anode – suddenly stopped. • X-RAY PHOTONS ARE CREATED Kinetic energy • Energy of motion • The electrons KINETIC energy is converted to electromagnetic or PHOTON energy kVp & mAs • they are electromagnetic waves of shorter wavelength and higher energy than normal light. But the debates over the nature of the rays – waves or particles? – • Photons can be described both as waves and particles. Electromagnetic Energy Spectrum • continuous range of energy – spectrum indicates that the distribution of energies exist in an uninterrupted band rather than at specified levels • released by accelerating charged particles – moves through space or matter as oscillating magnetic & electric fields • needs no carrier medium but can have one • can penetrate or interact with matter ELECTROMAGNETIC RADIATION • maybe described as wavelike fluctuations of electric and magnetic fields. • There are many kinds of electromagnetic radiation; • EMS illustrates that visible light, microwaves, and radio waves are within the electromagnetic spectrum. All the electromagnetic radiations have the same velocity, ie, 186,000 miles per second; however, they differ greatly in wavelength. The Electromagnetic Spectrum • X-rays have wavelengths much shorter than visible light, but longer than high energy gamma rays. The Electromagnetic Spectrum energy (eV) (EMS) f requency (Hz) wavelength (m) 25 10 10 10 -15 10 gamma 10 5 1020 xray -10 10 ultraviolet 10 0 1015 -5 light 10 infrared 10 -5 1010 microwave 10 0 TV/FM radio radio 10 -10 105 long waves 10 5 10 -15 100 10 10 10 -20 10 -5 10 15 General Characteristics of EMS X-ray photons: • Have no mass or physical form • travel at speed of light (c) in a vacuum (or air) – c = 3 x 108 m/s or 186,400 miles per sec • travel in a linear path (until interaction occurs) • dual nature: wave vs. particle • unaffected by – electric or magnetic fields – gravity Wavelength The shorter the wavelength – the higher the frequency • Wavelength refers to the distance between two consecutive wave crests • Frequency refers to the number of cycles per second (cps); its unit of measure is the hertz (Hz), which is equal to 1 cps. • Frequency and wavelength are closely associated with the relative energy of electromagnetic radiations. More energetic radiations have shorter wavelengths and higher frequency. The relationship among frequency, wavelength, and energy is illustrated in the electromagnetic spectrum (Fig 1). X-rays can ionize atoms… What is Ionization? Some radiations are called ionizing because they have the energetic potential to break apart electrically neutral atoms, resulting in the production of negative and positive ions. When an electron is added or removed from the atom- it is ionized INTERACTIONS X-ray production begins at the atomic level Energy (photons) are released when the electron collides with another electron, or passes close to the nucleus of the atom – the change in energy of the shells –produces photons X-ray PRODUCTION in the TUBE X-rays are produced by the conversion of e- radiation (photons) A large potential difference is applied across the two electrodes in an evacuated envelope Neg. charged electrode (cathode): source of e- Pos. charged electrode (anode): target of e- Cathode Anode 35 INTERACTIONS IN THE TUBE • BREMS (Bremsstrahlung) • CHARACTERISTIC • HEAT TUBE INTERACTIONS - e (electrons) are released from the cathode are accelerated towards the anode • Heat = 99% • X-ray = 1% – Bremsstrahlung (Brems) = 80% – Characteristic = 20% The CATHODE is the X-ray Tube Cathode e- source – a tungsten wire filament surrounded by a focusing cup Electricity sent though the wire at the Cathode Electricity heats the filament and releases e- via thermionic emission ELECTRONS ARE “BOILED OFF” & suspended in a space cloud. 38 When the ELECTRONS FROM THE CATHODE – INTERACT WITH THE ELECTRONS IN THE TUNGSTEN ANODE – X-RAY PHOTONS ARE CREATED e- (electrons) are released from the cathode are accelerated towards the anode X-ray Tube Anode Tungsten anode disk Focal track area Stator and rotor make up (spreads heat out the induction motor over larger area than Rotation speeds stationary anode Low: 3,000 – 3,600 configuration) rpm High: 9,000 – 10,000 rpm Molybdenum stem (poor heat conductor) connects rotor with anode to reduce heat transfer to rotor bearings • HIGH VOLTAGE TO ANODE – ATTRACTS – ELECTRONS FROM CATHODE • CURRENT TO STATOR CAUSES ROTATION OF ANODE Rotating Anode INTERACTIONS IN THE TUBE • HEAT = 99% X-RAYS = 1% • BREMS (BRESTRAULUNG) (80%) • CHARACTERISTIC (20%) Start with the incoming line… Tube or Patient ? Heat • Most kinetic energy of projectile e- is converted into heat – 99% • Projectile e- interact with the outer-shell e- of the target atoms but do not transfer enough energy to the outer-shell e- to ionize Heat INCOMING HEAT ELECTRONS FROM CATHODE - 8 p+ + 8e- = neutral atom pass by the ELECTRONS IN THE e TUNGSTEN TARGET – do not interact with e – causes them to VIBRATE – RELEASING e SMALL AMOUNTS OF HEAT Heat is an excitation rather than an ionization X-ray Photons – BREMS Creates a polychromatic spectrum – xrays of different energies 50 Energy (photons) are released when the e passes close to the nucleus, then changes direction BREMS RADIATION • Electron • Passes by nucleus • Changes direction • Energy released as a PHOTON • bremsstrahlung : “breaking” radiation Bremsstrahlung is a german word meaning slowed-down radiation Characteristic Radiation – 2 steps • Projectile e- with high enough energy to totally remove an inner-shell electron of the tungsten target • Characteristic x-rays are produced when outer-shell e- fills an inner-shell void • All tube interactions result in a loss of kinetic energy from the projectile e- CHARACTERISTIC (in tube) • Electron hits inner shell e in orbit – knocked out & creates a hole • Other E’s want to jump in • Energy released as PHOTONS • It is called characteristic because it is characteristic of the target element in the energy of the photon produced Characteristic Bremsstrahlung Radiation • Heat & Characteristic produces EM energy by e- interacting with tungsten atoms e- of the target material • Bremsstrahlung is produced by e- passing by closely with the nucleus of a target tungsten atom – the change in direction of the electon – releases a photon of energy REVIEW What type of Interaction? Where does it occur? X-RAY TUBE: STRAIGHT LINE IN = E FROM TUBE WAVY LINE OUT = PHOTONS Review Quiz Why you see what you see • The films or images have different levels of density – different shades of gray • X-rays show different features of the body in various shades of gray. • The gray is darkest in those areas that do not absorb X-rays well – and allow it to pass through • the images are lighter in dense areas (like bones) that absorb more of the X-rays. IMAGES • DENISITY = THE AMOUNT OF BLACKENING “DARKNESS” ON THE RADIOGRAPH • CONTRAST = THE DIFFERENCES BETWEEN THE BLACKS TO THE WHITES Density • mAs • mA = AMOUNT of electrons sent across the tube combined with TIME (S) = mAs • mAs controls DENSITY on radiograph primary function of mAs is DENSITY Contrast • Kilovolts to anode side – kVp • Kilovolts controls how fast the electrons are sent across the tube • kVp – controls CONTRAST on images Questions ?
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