Resident Physics Lectures 1 st Year Christensen Chapter

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

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

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

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

* Tube Housing �Shields against leakage radiation �Electrical insulation

Tube Housing Filled with Oil �Helps cool tube �Electrical insulation �Bellows on end of tube allows oil to expand when hot. 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 released (“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

Electron Role �Electron carries energy as kinetic energy �Higher energy electron moves faster �Electrons controlled by electric fields �Accelerated �Steered +

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 �Cathode is source of electrons �filament heated by electric current

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

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

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 L K ~ + + ~ - - - # Energy

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 L K Electron from Filament - ~ + + ~ - -

Bremsstrahlung (cont. ) �Bremsstrahlung means braking radiation �Moving electrons have many Bremsstrahlung interaction � small amount of energy lost with each interaction L K ~ + + ~ - - -

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

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

Tube Current (m. A) �rate of electron flow from filament to target �Electrons / second � Measured in milliamperes (m. A) �Not the same as filament current +

Exposure Parameters �k. Vp � High voltage applied to x- ray tube �m. A � Rate of electron flow from cathode to anode during exposure �Time � Duration of x-ray exposure �m. As � Product of m. A & time

Beam Intensity �Product of � # photons in beam � Photon energy spectrum �Units � Roentgens (R) per unit time � Measure of ionization rate of air �Depends on � k. Vp � m. A � target material � filtration

Intensity & Technique �Beam intensity proportional to m. A �Beam intensity ~ proportional to k. Vp 2 + high voltage source filament voltage source

Focal Spot �Portion of anode struck by electron stream �Focal spot size affects & limits resolution +

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

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

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

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

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

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

Target Angle • Large – 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 Size

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

�Stationary Anodes �Rotating � Target is annular track � spreads heat over large area of anode � speeds � � 3600, 9600 rpm Faster = 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 � $$$ �Materials � usually tungsten