X-ray Production & Spectra Lecture Notes PDF

X-ray production & X-ray spectra FUNDAMENTALS OF RADIATION AND RADIATION SAFETY MODULE TIMELINE 2024-25 Mock...

X-ray production & X-ray spectra FUNDAMENTALS OF RADIATION AND RADIATION SAFETY MODULE TIMELINE 2024-25 Mock Assessment Seminar Seminar Seminar 3 1 2 MCQ Exam Lecture Lecture Lecture Lecture 2 4 6 7 Lecture Lecture Lecture Lecture Lecture Lecture 1 3 5 8 9 10 Holidays SEPTEMBER OCTOBER NOVEMBER DECEMBER JANUARY Virtual 1 3 October 2024 Simulations Radiation safety - legislation & practice 2 2 1 3 On-Campus Simulations 2 TOPICS KEY: Skills for module Fundamentals of completion Radiation Radiation Safety Radiation Protection Assessment Skills & in Context Assessment Module Learning Objectives Define the electromagnetic spectrum and its application to diagnostic imaging. Describe the x-ray tube design and explain the x-ray production process  Appreciate how Gain familiarity with Understand the X- exposure the design of an X- ray production parameters affect ray tube process X-ray production Awareness of how Introduction to the exposure X-ray spectrum parameters affect the X-ray spectrum 6th November 2023 X-ray Production & X-ray Spectra 4 Song of the week! Rate the song ⓘ Start presenting to display the poll results on this slide. X-ray tubes Terminology refresh Tube voltage (kV) Tube current (mA) Exposure time (s) Voltage applied Value of electric Duration for which across the X-ray current formed by the electron tube flow of electrons current flows Accelerating across tube potential that drives electrons from cathode to anode Anatomy of an X-ray tube Parts of the X-ray Tube Anode Target Cathode Filament Held at Tungsten Held at Thin positive embedded negative tungsten potential in anode potential wire Usually Area of Where Heated to copper – anode that electrons release How do electrons get why? electron are electrons from cathode to anode? beam hits produced Parts of the X-ray tube Focussing cup Evacuated glass vessel Held at negative Minimises electron potential interactions with Focusses electron other atoms As X-ray tubes age the cloud Maximises X-ray vacuum worsens and tubes become ‘gassy’ production in target Maximises current Can allow surge of electron flow (repulsive flow – tube arcing force) Parts of the X-ray tube Shielded metal Stops the x-rays being irradiated out of housing the tube in all directions Surrounds the glass vessel Oil Used to remove heat from the tube and works as insulation from the high voltage Useful X-ray beam directed Usually made of Beryllium towards the patient Window The area where we allow X-rays to emerge from the tube Remaining X-ray photons contained within the tube Set of electric coils outside the glass vessel Stator – uses electromagnetic induction to cause the anode to rotate Filament Material Heating Thin Tungsten (W) wire – has high Thermionic emission Heated by filament resistance current ~3-6A Heating releases High atomic electrons number Z=74 Resulting electron current 5-10x less than filament current Usually two filaments of different sizes to allow two focal spot sizes Smaller filament has limited filament current, therefore limited electron flow Anode Made of Tungsten - very high melting temperature (3422 ºC) Doped with Rhenium – prevents cracking at high temperature Because anode is angled, effective focal spot size is smaller than area of Set at angle to electron beam to electron beam hitting anode (actual direct X-rays towards patient focal spot) Effect of focal spot size Anode heel effect Some of X-ray beam Effect is greatest at self-attenuated within bottom corner of the anode anode (anode heel) Smaller anode angle gives Reduced X-ray beam Will yield reduced smaller effect focal spot intensity reaching the image quality in this BUT larger heel effect detector area Therefore position This is a cathode of X-ray problem for X- tube over most dense area of patient ray imaging… Anode types Stationary Rotating Simplest tube Disc shaped anode Primarily found in Rotates during dental X-ray as exposure to spread exposures generate out heat generation little heat Used in all non-dental Cheaper to X-ray tubes manufacture Approx 99% of electron energy converted to heat, ~1% to X-rays X-ray generators / circuitry Requirements for our circuit X-ray tube High voltage across the X-ray tube (kV) Anode to be +ve, o ~ o Cathode to be -ve The problems… X-ray tube Mains supply is AC so Voltage is ~230V not o ~ o voltage varies +ve to at kV level -ve How do we increase How to we make the the voltage? voltage constant +ve? Range of circuit components Capacitor Diode Switch Take 5 mins to remind yourself (find out) what each of these does… Transformer Resistor What can we use to increase voltage? First problem solved! Voltage increased to desired level Use mutual induction within a step up transformer to increase voltage from mains supply to tens of kV What can we do to make voltage only +ve? Diode only allows current to “Self rectified” waveform. flow in one direction – known No –ve voltage but only ‘on’ as rectification for half the required time Use ‘rectifying bridge’ to solve this A Anode X-ray AC tube Voltage supply Cathode B This uses four diodes in a specific pattern to produce a full wave rectification spectrum. Whether A or B is at a positive potential the current flows the same way through the x-ray tube. How does this work? What is the effect of this on our waveform? Second problem solved? Voltage only +ve, but is it constant enough? What can we do to smooth out the waveform? Second problem solved? Voltage only +ve, but is it constant enough? The capacitor stores and then releases energy. This then reduces the variation (ripple) in the voltage. Further smoothing of the waveform Feed these into the Split the transformed circuit at three different (increased) waveform points (times). into three Creates a three phase supply. Pass these three phases through a diode rectifying bridge to remove –ve cycles Produces 3 peaks where there would have been 1. Gives more constant voltage than a single phase rectified beam. Works better than the capacitor. Effect on our waveform? Most modern X-ray generators do this slightly differently… We now have: High voltage waveform (tens of kV)! Approximately constant +ve voltage! Can therefore use this voltage across our X-ray tube! High frequency generator Most common generator used in modern X-ray circuits DC voltage chopped Mains AC is rectified rapidly by inverter, to This high frequency is and smoothed to give give high frequency (5 transformed to a DC voltage kHz – 50kHz) higher voltage alternating voltage Produces ‘constant’ Transformed voltage is voltage, negligible rectified and smoothed ripple. All X-rays generated at peak kV. High frequency kV waveform kV High frequency Low ripple Approx. constant voltage supply enables approx. constant peak X-ray energy 3 phase Time X-ray production / spectrum Preparatory (prep) stage When X-ray exposure switch is pressed half way Current applied Voltage applied to filament → to stator – thermionic causes anode to emission of rotate electrons Exposure stage When X-ray exposure switch is pressed fully Potential difference (kV) applied across tube Electrons accelerate to target, X-rays produced Methods of X-ray production Rapid deceleration of fast moving electrons Electron transition between inner shells Let’s make X-rays! Part 1… Bremsstrahlung (braking radiation) Electron (-) – nucleus (+) interaction electron No collision! Many When electron changes possible direction/speed energy paths is released in form of X-rays electron Electron can lose any X-ray amount of energy so X- rays of any energy generated Maximum energy loss = energy that incoming electron had (keV) – this is the max. X-ray energy produced  Bremsstrahlung spectrum Relative A continuous energy Spectrum with intensity spectrum no filtration Quantity of photons greatest at lowest energies Filtration removes lowest energies from the beam Energy (keV) Emission spectrum with filtration Let’s make X-rays! Part 2… Characteristic radiation X-ray energies well defined (monoenergetic) Characteristic of the anode material (not affected by e- energy) Energy released corresponds to binding energies of electron shells  Electron binding energies – incoming Characteristic energies electron must have more energy than this to eject the orbiting electron Tungsten (W), Z = 74 2 P -0.02 keV 12 O -0.07 keV 32 N -0.6 keV 18 M -2.8 keV 8 L -11.0 keV 2 K -69.5 keV For Tungsten: Kα = 59.3keV, Kβ = 67.6keV Effect on X-ray spectrum What is the purpose of oil in the X-ray tube? ⓘ Start presenting to display the poll results on this slide. Which components were used in the X-ray circuit? ⓘ Start presenting to display the poll results on this slide.  I will record an additional set of slides that describe how X-ray control parameters affect the X-ray spectrum, and add to Canvas… Summary