xray production.pptx

RADIOTHERAPY

1. Dr. ATUL VERMA PRODUCTION OF X-RAYS

2. Sir Wilhelm Conrad Roentgen German physicist Discovered X Rays in November 1895 ‘X’ in X rays stands for ‘Unknown’

3. PRODUCTION OF X-RAY X-rays were discovered by Roentgen in November,1985 whilst he was experimenting with the passage of electricity through a glass at low pressure.

4. PRODUCTION OF X-RAYS X rays are produced by energy conversion when a fast moving stream of electrons is suddenly decelerated in the “target” anode of x-ray tube. X ray tube ,made of Pyrex glass, is a diode tube i.e. contains two electrodes ,enclosed in vacuum. Electrons are produced at cathode by the process of thermionic emission and accelerated towards anode by applying a high potential difference between the two. Accelerated electrons hit the tungsten target resulting in production of x rays

5. During their passage from filament to target, each electron will have acquired energy ( E ) equal to the product of its charge ( e ) and the voltage difference between the two electrodes ( V ) E= eV

6. COMPONENTS OF AN X-RAY TUBE Glass enclosure Cathode Anode Tube housing

7. PURPOSE OF VACUUM IN GLASS TUBE Prevents collision of electrons with gas molecules Prevents production of secondary electrons The electrons suffer no impediment and therefore suffer no loss of energy

8. CATHODE The negative electrode. Consists of three elements : The filament ( source of electrons) The connecting wires ( supply voltage and amperage) A metallic focusing cup(negatively charged, converges electrons towards anode) The filament is made of a tungsten wire ,0.2 mm in diameter, coiled to form a vertical spiral.

9. thermionic emission When current flows through fine tungsten wire, it becomes heated and its atoms absorb thermal energy. Some electrons acquire enough energy to allow them to move from the surface of the metal This emission of electrons resulting from the absorption of thermal energy is referred to as thermionic emission.

10. WHY TUNGSTEN Better ductility. High melting point 3370 C Little tendency to vaporize at working temperatures of tube, this adds to better life expectancy. Other elements that may be used are Molybdenum and Rhenium.

11. ANODE The positive electrode of x ray tube Is of two types Stationary anode Rotating anode Made of tungsten. For mammography, Molybdenum anode is used

12. WHY TUNGSTEN 1) High atomic number (74) which makes it more efficient for the production of x-rays. 2) High melting point (3370 C). 3) Excellent heat absorbing and heat dissipating properties. 4) Does not vaporize easily.

13. HEEL EFFECT X-RAYS produced at various depth in the target vary in amount of attenuation, greater attenuation for those coming from depth Intensity of X-RAY beam decreases from cathode to anode direction

14. PHYSICS OF X-RAY PRODUCTION Produced by two different mechanism and give rise to: - BREMSSTRAHLUNG X-RAY - CHARACTERISTIC X-RAY

15. GENERAL RADIATION OR BREMSSTRAHLUNG “Breaking Radiation” Result from collision of high speed electron over a nucleus Electron with its electromagnetic radiation when passes in the vicinity of nucleus, it suffers a sudden deflection (coulomb forces of attraction) and acceleration Part or all of the energy of the electron is dissipated from it and propagates in space as electromagnetic radiation More than 99% of such radiation is lost as heat and less than 1% contributes to x-rays.

16. If an incoming free electron gets close to the nucleus of a target atom, the strong electric field of the nucleus will attract the electron, thus changing direction and speed of the electron. The Electron looses energy which will be emitted as an X-ray photon. The energy of this photon will depend on the degree of interaction between nucleus and electron Several subsequent interactions between one and the same electron and different nuclei are possible. X-rays originating from this process are called bremsstrahlung. Bemsstrahlung is a German word directly describing the process: "Strahlung" means "radiation", and "Bremse" means "brake”

17. CHARACTERISTIC RADIATION An electron with kinetic energy interacts with the atom of the target by ejecting an orbital electron Vacancy is filled by an outer electron with emission of electromagnetic radiation ( i.e x-ray) The Energy of the x-ray so produced is always constant for a given set of energy transition for a given element, i.e. characteristic for a particular energy transition.

18. The high energy electron can also cause an electron close to the nucleus in a metal atom to be knocked out from its place. This vacancy is filled by an electron further out from the nucleus. The well defined difference in binding energy, characteristic of the material, is emitted as a monoenergetic photon. When detected this X-ray photon gives rise to a characteristic X-ray line in the energy spectrum .

19. 23 When an outer shell electron falls to an inner shell to fill the vacancy, a characteristic x-ray is produced, with the energy equal to the difference of the binding energies of the 2 shells involved. For example, if an L-shell electron fills a vacancy on the K-shell, the characteristic x-ray has an energy = E K – E L . This energy is discrete, and characteristic to a particular element. Recall that the energy spectrum of bremsstrahlung photons is continuous.

20. X-RAY ENERGY SPECTRA X-ray photon produced by the x-ray machine are heterogeneous in energy Spectrum shows a continuous distribution of energy (bremsstrahlung) superimposed by discrete energies (characteristic)

21. FACTORS AFFECTING X-RAY PRODUCTION The output of an X-RAY tube is often describe by the terms: QUALITY : the penetrability of an X-RAY beam QUANTITY : the number of photons comprising the beam EFFICIENCY : the ratio of output energy as x-rays to input energy deposited by electrons

22. The factors affecting those characteristic of x-rays are: Anode target material Tube voltage Tube current Beam filtration

23. ANODE TARGET MATERIAL Incident electrons are more likely to have radiative interactions in higher Z material The energies of characteristic x-rays produced in the target depend on the target material The efficiency of bremsstrahlung radiation production is roughly proportional to atomic number Thus the target material affects the quantity and efficiency of bremsstrahlung photons and the quality of the characteristic radiation I ∝ Z

24. TUBE VOLTAGE (Kv) determines the maximum energy in the bremsstrahlung spectrum and affects the quality of the output spectrum also affects the efficiency of bremsstrahlung x-ray I ∝ kV²

25. TUBE CURRENT(mA) proportional to the number of electrons flowing from the cathode to the anode. So it will depend on the tube current. Greater the current, greater the number of electron therefore more radiation produced. I ∝ mA

26. BEAM FILTRATION modifies the quantity and quality of the x-ray beam by absorbing the low energy photons in the spectrum (this process is also k/a beam hardening) reduces the number of photons (quantity) and increases the average energy, also increasing the quality

27. Properties of x-rays X-rays travel in straight lines. X-rays cannot be deflected by electric field or magnetic field. No medium of propagation required X-rays have high penetrating power. Photographic film is affected by X-rays. Ionization of a gas results when an X-ray beam is passed through it.

28. 35 If no filtration, the theoretical energy spectrum is a straight line. However, due to inherent and additional filtration, the low-energy portion of the spectrum is reduced, making the beam ‘ harder ’, that is, more penetrating. The maximum energy on the spectrum is the energy of the incoming electron, which equals to the value of the applied voltage peak (e.g. 100 keV electrons from 100 kVp). The average energy is approx. 1/3 of the maximum energy .

29. Linear accelerator. Linac...

30. Introduction: Linear accelerator  is a type of  particle accelerator  that greatly increases the  kinetic energy  of charged  subatomic particles  or  ions  by subjecting the  charged particles  to a series of  oscillating   electric potentials  along a  linear beamline . This method of particle acceleration was invented by   Leó Szilárd . It was patented in 1928 by  Rolf Widerøe , who also built the first operational device.

31. The major components of medical Linear Accelerator Power Supply Modulator Magnetron Or Klystron Electron Gun Wave Guide system Accelerator Tube Bending Magnet Treatment Head (Straight Beam) Treatment Head (Bent Beam)

32. Medical Linear accelerator Treatment Head (Straight Beam) Power Supply Modulator Electron Gun Magnetron Or Klystron Wave Guide system Treatment Head (Bent Beam) Bending Magnet Accelerator Tube A block diagram of typical medical Linear Accelerator

33. A power supply Provides DC power to the modulator Modulator the magnetron or klystron Deliver the pulses to the electron gun Pulsed microwaves the accelerator tube or structure via a waveguide systems. electrons

34. Magnetron controls the power and frequency of radio frequency waves. Radio frequency waves are posted to wave guide by magnetron This is synchronised with injection of electrons into wave guide by electron gun These electrons are accelerated in vacuum created accelerated tube which is made of copper

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