Transcript File
Resident Physics Lectures • Christensen, Chapter 2A X-Ray Tube Construction George David Associate Professor Department of Radiology Medical College of Georgia * X-Ray Tube Components • Housing Visible part of tube • Glass Enclosure (insert) Vacuum Electrodes » Cathode • Filament » Anode • Target X-Ray Tube * • Converts Energy FROM » electrical energy To » Heat • > 99% of incident energy • Bad! Ultimately destroys tubes » X-Rays • < 1% of incident energy • Good! Our desired product * Tube Housing • Shields against leakage radiation lead lined leakage limit » 100 mR / hour when tube operated at maximum continuous current for its maximum rated kilovoltage Tube Housing (cont.) • Shields against high voltage electrically grounded high voltage cable receptacles (wells) • housing filled with oil cools electrical insulation » all air removed bellows Vacuum » on end of tube » allows oil to expand when hot. Oil Insert Inside the Glass Insert • Filament Similar to light bulb Glows when heated • Target Large (usually) tungsten block target filament * X-Ray Tube Principle • Filament heated electrons gain energy electrons freed (“boiled” off) Thermionic emission - 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 Requirements to Produce X-Rays • Filament Voltage • High Voltage anode + high voltage source filament filament voltage source Cathode (filament) • Coil of tungsten wire similar to light bulb filament • Tungsten advantages high melting point little tendency to vaporize long life expectancy • Tungsten disadvantages not as efficient at emitting electrons as some other materials Cathode (filament) • Cathode is source of electrons • filament heated by electric current ~ 10 volts ~ 3-5 amps • filament current is not tube current Tube Current (mA) • rate of electron flow from filament to target Electrons / second • Measured in milliamperes (mA) + • Limited by filament emission (temperature / filament current) space charge (see next slide) Space Charge * • Electrons leave filament filament becomes positive » Negative electrons stay close • Electron cloud surrounds filament • Cloud repels new electrons from filament • Limits electron flow from cathode to anode + - Kilovoltage & Space Charge • raising kilovoltage gradually overcomes space charge + Higher fraction of electrons make it + + ++ - • At high enough kilovoltage saturation results All electrons liberated by filament reach target • Raising kilovoltage further has no effect on # electrons reaching anode Tube Current (mA) to anode as kilovoltage increases Saturation Voltage kVp Saturation Voltage + + + ++ - • kilovoltage at which a further increase does not increase tube current 100% of electrons already going to target • Tube current said to be emission limited tube current can only be increased by increasing filament temperature Focal Spot • portion of anode struck by electron stream • Focal spot sizes affects and limits resolution + Focusing Cup • negatively charged • focuses electron stream to target overcomes tendency of + electrons to spread because of mutual repulsion Focusing Cup Focal Spots • Most tubes have 2 filaments & thus 2 focal spots • only one used at a time • small focus improved resolution • large focus improved heat ratings Electron beam strikes larger portion of target Filament (cont.) • Large Filament normally left on at low “standby” current boosted before exposure (prep or first trigger) • With time tungsten from hot filament vaporizes on glass insert thins the filament filters the x-ray beam increases possibility of arcing » electrons attracted to glass instead of target + Cross Section of X-Ray Tube Dunlee Web Site: http://www.dunlee.com/new_tube_anatomy.html Cross Section of X-Ray Tube Dunlee Web Site: http://www.dunlee.com/new_target.html Line Focus Principle • Focal spot steeply slanted 7-15 degrees typical • Focal spot looks small from patient’s perspective + Actual FS Apparent FS Imaging size • Looks large from filament better heat capacity Patient Line Focus Principle • Actual (true) focal spot + as seen from filament Actual FS • Apparent (effective, projected) focal spot as seen from tube port or patient Apparent FS Patient Target Angle Angle between target & perpendicular to tube axis Typically 7 – 15 degrees + Target Angle, Q Line Focus (cont.) + Actual FS Apparent FS Target Angle, Q Apparent FS = Actual FS X sin Q Target Angle (cont.) • Large • Small – poorer heat ratings – better field coverage optimizes heat ratings Large Target Angle (Small Actual Focal Spot) Small Target Angle (Large Actual Focal Spot) + limits field coverage + Heel Effect • Intensity of x-ray beam significantly reduced on anode side • beam goes through more target material exiting the anode x anode side - - cathode side Anodes • Stationary • Rotating Target is annular track spreads heat over large area of anode speeds » 3600, 9600 rpm » Faster = much better heat ratings Rotating Anode • Advantages better heat ratings • Disadvantages More complex ($) » Rotor drive circuitry » motor windings in housing » bearings in insert Rotating Anode • Larger diameter Better heat ratings heavier » requires more support $$$ • Materials usually tungsten » high melting point » good x-ray production molybdenum (and now Rhodium) for mammography » low energy characteristic radiation Grid-controlled tubes • Grid used to switch tube on/off grid is third electrode relatively small voltage controls current flow from cathode to anode » Negative grid voltage repels electrons from filament » Grid much closer to filament than target • Applications grid speedy switching required » cine +