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 Glass Enclosure (insert) * Housing Visible part of tube Glass Enclosure (insert) Vacuum Electrodes Cathode Filament Anode Target

X-Ray Tube Converts Energy FROM To Heat X-Rays electrical energy * 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 * 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 on end of tube allows oil to expand when hot. Vacuum Oil Insert

Inside the Glass Insert Filament Similar to light bulb Glows when heated Target Large (usually) tungsten block filament target

X-Ray Tube Principle Filament heated electrons gain energy * 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 filament filament voltage source + high voltage source

Cathode (filament) Coil of tungsten wire Tungsten advantages 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 * 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 to anode as kilovoltage increases 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) 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 Imaging size Looks large from filament better heat capacity + Actual FS Apparent FS Patient

Line Focus Principle Actual (true) focal spot as seen from filament Apparent (effective, projected) focal spot as seen from tube port or patient + Actual FS Apparent FS Patient

Target Angle Angle between target & perpendicular to tube axis Typically 7 – 15 degrees + Target Angle, Q

Line Focus (cont.) Apparent FS = Actual FS X sin Q + Actual FS Target Angle, Q Apparent FS = Actual FS X sin Q

Target Angle (cont.) Large Small Same apparent focal spot! + + poorer heat ratings better field coverage Small optimizes heat ratings limits field coverage Large Target Angle (Small Actual Focal Spot) Small Target Angle (Large Actual Focal Spot) + + Same apparent focal spot!

Heel Effect Intensity of x-ray beam significantly reduced on anode side beam goes through more target material exiting the anode x - - - cathode side anode 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 Disadvantages better heat ratings More complex ($) Rotor drive circuitry motor windings in housing bearings in insert

Rotating Anode Larger diameter Materials 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 speedy switching required cine يصد grid +