Presentation on theme: "Physics of Radiation Therapy"— Presentation transcript:
1Physics of Radiation Therapy Lecture III: X-ray Production/Clinical Radiation Generators
2Production of X Rays The X-Ray Tube Components Glass tube – maintains vacuum necessary to minimize electron interactions outside of the target areaCathode – contains filament and focusing cupAnode – contains x-ray targetFigure 3.1 from Faiz Khan
3The X-Ray Tube The Cathode Tungsten filament (high melting point – 3370 ° C)Thermionic emission – electron production as a consequence of heatingFocusing cup – “directs” electrons to anodeDual filaments (diagnostic tubes) – necessary to balance small focal spots and larger tube currents
4The X-Ray Tube The Anode Tungsten target Heat dissipation High melting pointHigh Z (74) – preferred since bremsstrahlung production a Z2Heat dissipationCopper anode – heat conducted outside glass into oil / water / airRotating anode (diagnostic tubes) – larger dissipation areaAnode hood – copper and tungsten shields intercept stray electrons and x rays
50.1 – 2.0 mm in diagnostic tubes, The AnodeFocal-spot sizeLine focus principleapparent focal spot (a) is smaller then actual (A) due to target angle q :a = A sin qApparent focal spots0.1 – 2.0 mm in diagnostic tubes,5 – 7 mm in therapy tubesFigure 3.2 from Faiz Khan
6The AnodeHeel effectSince the x-rays are produced at various depths in the target, they suffer varying amounts of attenuation in the targetThere is greater attenuation for x-rays coming from greater depths than those from near the surface of the target.Consequently, the intensity of the x-ray beam decreases from the cathode to the anode direction of the beam.This variation across the x-ray beam is called the heel effect.
7Basic X-Ray Circuit Consists of two parts: High-voltage circuit – provides x-ray tube accelerating potentialFilament circuit – provides filament currentFigure 3.3 from Faiz Khan
8Basic X-Ray CircuitVoltages can be increased or decreased using “step-up” or “step-down” transformersWire coils wound around iron ringsCoils create a magnetic field within the ringCurrent and voltages (V) on opposite sides (primary and secondary) of the ring are proportional to the number of “turns” (N):VP / VS = NP / NSStepwise and continuous voltage adjustments can be made with autotransformers and rheostats
9Basic X-Ray Circuit Voltage Rectification Alternating current is used to energize x-ray tubesX-ray tubes are designed to operate at a polarity where the anode is positiveRectifiers permit current flow in only one directionRectifiers placed in series in HV circuit provide half-wave rectification
10Basic X-Ray CircuitRectifiers can be arranged in a “diamond” arrangement to provide full-wave rectification – conduction during both halves of alternating (AC) voltageFigure 3.4 from Faiz Khan
11X-Ray Production Bremsstrahlung (“braking” radiation) Electromagnetic radiation emitted when an electron losses energy as a consequence of coulomb interaction with the nucleus of an atomFigure 3.6 from Faiz Khan
12X-Ray Energy SpectrumThe bremsstrahlung photon’s energy is equal to the difference between electron’s incident and final energiesThis leads to an “energy spectrum” (the Kramer’s spectrum):IE = K Z (Em- E)
13X-Ray Energy Spectrum Characteristic X Rays Ka x rays – most “intense” transition to K-shell (most commonly by transitions from adjacent (L) shell), Kb x rays – second most intense, etc.For tungsten: Ka ≈ 60 keV, Kb ≈ 70 keVFigure 3.8 from Faiz Khan
14X-Ray Angular Distribution Angular distribution of x raysAngular distribution becomes more “forward peaked” as the electron energy increasesFigure 3.7 from Faiz Khan
15X-Ray Spectrum X-Ray Spectrum Composite of Kramer’s spectrum and characteristic x raysFiltration reduces lower-energy componentRule of thumb is average energy ≈ 1/3 maximum energyHalf Value Layer (HVL) is a common descriptorFigure 3.9 from Faiz Khan
16Clinical Radiation Generators Treatment UnitKilovoltage RangeHalf-Value LayerClinical UseGrenz Ray» 20 kVp≈ mm AlDermatological UseContact Therapy40-50 kVp≈ 1-2 mm AlFew mmSuperficial TherapykVp≈ 1-8 mm AlSkin LesionsOrthovoltagekVp≈ 1-4 mm CuDeep TherapySupervoltagekVp≈ 10 mm PbReplaced by Co-60
19The Linear Accelerator Components Microwave Power SectionDC Power SectionTreatment HeadAccelerator Guide
20The Linear Accelerator DC Power SectionProduces properly-shaped pulses of DC power; these pulses are shaped in the pulse-forming network of the modulator and delivered to the electron gun and microwave power section at the proper frequency through a high-voltage switching device (the thyratron).
21The Linear Accelerator Microwave Power Section – Provides microwave power amplification (utilizing either a magnetron or klystron) and transmits the amplified microwaves to the accelerator guide.Accelerator Guide – A cylindrical tube in which electrons, injected by the electron gun, are accelerated by the amplified microwaves. The accelerated electrons exit the waveguide and enter the treatment head.
22The Linear Accelerator Treatment HeadContains the beam shaping, steering, and control components of the linear accelerator. These components are: the bending magnet, x-ray target, electron scattering foils (most accelerators), x-ray flattening filter, dose monitoring chambers, and beam collimation system.
23Microwave PowerElectrons gain energy through continued exposure to an increasing electric fieldThe process is analogous to a “surfer riding a wave”Microwave cavities create that environment – they are machined to dimensions that resonate at microwave frequencies (S-Band accelerators – 3000 MHz)
24Microwave PowerIn a fashion similar to that by which electrons gain energy from microwaves, microwave power can be amplified by the deposition of electron kinetic energy if electrons (grouped in “buncher” cavities) arrive in “catcher” cavities at the cavities’ resonant frequenciesFigure 4.7 from Faiz Khan
25Electron Acceleration Electrons can be accelerated in microwave cavities that are arranged in a linear configuration such that the microwaves’ “phase velocity” is matched to the velocity of the electrons traveling through the guide(Note that energy gain will be proportional to waveguide length)Figure 1-11 from Hendee
26The Treatment HeadShielded Housing – lead shielding reduces unwanted radiationBending Magnet – provides electron energy selectionthe magnetic field intensity B is set such that electrons possessing the appropriate energy (momentum mev) are bent through the radius r that allows passage through the magnet’s exit port:mev µ B r
28The Treatment HeadX-Ray Target – transmission-type tungsten target in which electron produce bremsstrahlung radiation; inserted only during x-ray beam production, removed during electron-beam productionFlattening Filter – (photon beams) metal filter placed in the x-ray beam to compensate for the “forward peaked” photon distribution and produce a “flat” beam
29The Treatment HeadScattering Foils – (electron beams) thin metallic foils inserted in the electron beam to spread the beam and obtain a uniform electron fluenceMonitoring Chambers – transmission ionization chambers used to monitor dose rate, total integrated dose and beam symmetry.Collimation System – fixed and movable beam limiting devices, normally made of lead, used to shape and size the beam
30Cobalt-60 (Megavoltage unit) Co-60 is most commonly used because of its superior specific activity per gram, its greater beam intensity per curie, and higher average photon energy.A typical Co-60 source is 1 to 2 cm in diameter, and possesses an activity of about 3000 to 5000 Curies.Co-60The source activity is limited by the physical size of the source.The large size of the Co-60 source produces a significant geometric penumbra (will be discussed later on)Does not give appropriate dose profile for deep seated tumors (in thoracic area)Replaced by linear accelerators