Resident Physics Lectures Christensen Chapter 2 A XRay

Resident Physics Lectures • Christensen, Chapter 2 A 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 m. R / 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. Oil Insert Vacuum

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 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 (m. A) • rate of electron flow from filament to target ¨ Electrons / second • Measured in milliamperes (m. A) + • 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 (m. A) to anode as kilovoltage increases Saturation Voltage k. Vp

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 +