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X-Ray Tube Construction PowerPoint Presentation

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Published on : Aug 02, 2021 | Views : 4080 | Downloads : 14

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Slide 1-

Resident Physics Lectures (1st Year) Christensen, Chapter 2A X-Ray Tube Construction George David Associate Professor Department of Radiology Medical College of Georgia
Slide 2-

X-Ray Tube Converts Energy FROM electrical energy To Heat > 99% of incident energy X-Rays < 1% of incident energy Good! Our desired product *
Slide 3-

Heat and X-Ray Tubes Bad! Ultimately destroys tubes Limits single exposure beam intensity Limits ability to make repeated exposures over time *
Slide 4-

X-Ray Tube Components Housing Visible part of tube Glass Enclosure (insert) Vacuum Electrodes Cathode Filament Anode Target *
Slide 5-

Tube Housing Shields against leakage radiation Electrical insulation *
Slide 6-

Tube Housing Filled with Oil Helps cool tube Electrical insulation Bellows on end of tube allows oil to expand when hot. Insert
Slide 7-

Inside the Glass Insert Filament Similar to light bulb Glows when heated Target Large (usually) tungsten block
Slide 8-

X-Ray Tube Principle Filament heated Electrons released (“boiled” off) Thermionic emission *
Slide 9-

X-Ray Tube Principle Positive (high) voltage applied to anode relative to filament electrons accelerate toward anode target Gain kinetic energy electrons strike target electrons’ kinetic energy converted to heat x-rays *
Slide 10-

Electron Role Electron carries energy as kinetic energy Higher energy electron moves faster Electrons controlled by electric fields Accelerated Steered
Slide 11-

Requirements to Produce X-Rays Filament Voltage High Voltage + filament anode filament voltage source high voltage source
Slide 12-

Cathode (filament) Coil of tungsten wire similar to light bulb filament Cathode is source of electrons filament heated by electric current
Slide 13-

X-Ray Production(cont.) X-Rays produced by 2 distinct processes Characteristic radiation Bremsstrahlung
Slide 14-

Characteristic Radiation Interaction of high speed incident electron with orbital electron of target #1: Electron from filament removes inner-shell orbital electron from atom #2: electrons from higher energy shells cascade down to fill vacancies #3: characteristic x-ray emitted - - - + + ~ ~ + ~ K L - - #1 #2 #3 Electron from Filament
Slide 15-

Characteristic Radiation Consists only of discrete x-ray energies Energies correspond to difference between electron shells of target atom Specific energies characteristic of target material
Slide 16-

Bremsstrahlung interaction of moving electron from filament nucleus of target atoms + nucleus causes moving electron to change speed / direction Electron slows down (loses kinetic energy) Energy lost in form of Bremsstrahlung x-ray - Electron from Filament
Slide 17-

Bremsstrahlung (cont.) Bremsstrahlung means braking radiation Moving electrons have many Bremsstrahlung interaction small amount of energy lost with each interaction
Slide 18-

Bremsstrahlung (cont.) Random energy loss of moving electron Depends on distance from nucleus charge (Z) of nucleus Bremsstrahlung Energy Spectrum 0 - peak kilovoltage (kVp) applied to x-ray tube most Bremsstrahlung photons have low energy Don’t escape tube easily filtered by tube or filters outside tube Energy #
Slide 19-

Output Beam Spectrum Output photon beam consists of Characteristic Radiation characteristic of target material several discrete energies Bremsstrahlung continuous range of energies 0 - kVp setting most photons have low energy Spectrum depicts fraction of beam at each energy Energy #
Slide 20-

Tube Current (mA) rate of electron flow from filament to target Electrons / second Measured in milliamperes (mA) Not the same as filament current +
Slide 21-

Exposure Parameters kVp High voltage applied to x-ray tube mA Rate of electron flow from cathode to anode during exposure Time Duration of x-ray exposure mAs Product of mA & time
Slide 22-

Beam Intensity Product of # photons in beam Photon energy spectrum Units Roentgens (R) per unit time Measure of ionization rate of air Depends on kVp mA target material filtration
Slide 23-

Intensity & Technique Beam intensity proportional to mA Beam intensity ~ proportional to kVp2 + filament voltage source high voltage source
Slide 24-

Focal Spot Portion of anode struck by electron stream Focal spot size affects & limits resolution
Slide 25-

Focal Spots Most tubes have 2 filaments & thus 2 focal spots only one used at a time small focus improves resolution large focus improves heat ratings Electron beam strikes larger portion of target
Slide 26-

Focal Spot Size & Resolution The larger the focal spot the more it will blur a tiny place on the patient.
Slide 27-

Larger Focal Spot = Better Heat Ratings Electron beam applies huge amount of heat to target
Slide 28-

Larger Focal Spot = Better Heat Ratings The larger the area the electron beam hits, the more intense the beam can be without melting the target
Slide 29-

Target Angle Angle between target & perpendicular to tube axis Typically 7 – 15 degrees
Slide 30-

Line Focus Principle Actual (true) focal spot as seen from filament Apparent (effective, projected) focal spot as seen from tube port or patient Target angled 7-12o + Actual FS Apparent FS Patient
Slide 31-

Target Angle Small optimizes heat ratings limits field coverage + Large Target Angle (Small Actual Focal Spot) + Small Target Angle (Large Actual Focal Spot) Large poorer heat ratings better field coverage Same Apparent Focal Spot Size
Slide 32-

Heel Effect Intensity of x-ray beam significantly reduced on anode side beam goes through more target material exiting anode on anode side anode side x cathode side - - -
Slide 33-

Anodes Stationary Rotating Target is annular track spreads heat over large area of anode speeds 3600, 9600 rpm Faster = better heat ratings
Slide 34-

Rotating Anode Advantages better heat ratings Disadvantages More complex ($) Rotor drive circuitry motor windings in housing bearings in insert
Slide 35-

Rotating Anode Larger diameter Better heat ratings Heavier $$$ Materials usually tungsten