05 chap 03 production of x-rays


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05 chap 03 production of x-rays

  1. 1. Chapter 3 Production of X-Rays X-rays were discovered by Roentgen in 1895 while studying the cathode rays (streams of electrons) in a tube. This new radiation could penetrate opaque substance, produce flourescence, blacken a photographic plate, and ionize gas. It is a form of electromagnetic radiation.Wilhelm Conrad Roentgen (1845-1923) the radiograph of Mrs. Roentgens hand 1
  2. 2. 3.1 The X-Ray Tube 2
  3. 3. 3.1 The X-Ray Tube (cont’d) The main components of an x-ray tube are the cathode and the anode, sealed opposite to each other in a highly evacuated vacuum tube. The cathode is a tungsten filament which when heated emits electrons. The anode consists of a copper rod and a piece of tungsten target (for producing the x-rays). The x-ray beam emerges through a thin glass window in the tube envelope. 3
  4. 4. 3.1 The X-Ray Tube (the anode) Tungsten is used as target because of its high Z (74) and high melting point (3370°C). Copper is used for its high heat conduction, heat is removed by oil (which also serves as an insulator to the tube housing from high voltage applied to the tube), water or air from outside the tube. Sometimes, a rotating anode is used to reduce the temperature of the target at any one spot. Anode hood is used to shield unwanted stray radiation. 4
  5. 5. 3.1 The X-Ray Tube (the anode, cont’d) 5
  6. 6. 3.1 The X-Ray Tube (the anode, cont’d)The focal spot is the target area from which the x-rays areemitted. It should be as small as possible for producing sharpradiographic images. For therapy tubes, relatively larger focalspots are acceptable since the image quality is not a primaryconcern.The apparent size of the focal spot is related to the apparentside α = A sinθ.In diagnostic radiology, the angle is small (6° -17°), producingapparent focal spot sizes from 0.1×0.1 to 2.0×2.0 mm. Fortherapy tubes, the angle is larger (30°), producing areas from5×5 to 7×7 mm. 6
  7. 7. Variation of X-Ray Intensity and Maximum X-ray Beam SizeBecause of the Anode Heel Effect 7
  8. 8. 3.1 The X-Ray Tube (the cathode)The cathode consists of a filament (to emit electrons), acircuit (to provide filament current), and a negativelycharged focusing cup (to direct electrons towards the anode).The size of the focal spot depends on the filament size. On adiagnostic tube, there are usually 2 filaments to provide“dual-focus”. 8
  9. 9. 3.2 Basic X-Ray Circuit 9
  10. 10. 3.2 Basic X-Ray Circuit (cont’d)The circuit can be divided into two parts:The high voltage (~100 kV) circuit to provide the acceleratingpotential for the electrons. The low voltage (~10V) circuit tosupply heating current to the filament.The filament current can be adjusted to control the x-rayoutput (intensity).The high-voltage can be adjusted by an autotransformer(rheostat, a variable resistor).The alternating voltage is characterized by itspeak voltage = 2 × line voltage 10
  11. 11. 3.2 Basic X-Ray Circuit (cont’d)For example, a 220V line voltage, after being stepped up byan x-ray transformer of turn ratio 500:1, the resultant peakvoltage applied to the x-ray tube is: 220 2 × 500 = 155,564 V = 155.6 kV The anode is positive relative to the cathode only through half of the cycle, during which the current flows through. During the other half, the voltage is reversed and the current cannot flow through. A machine operated this way is called self-rectified. 11
  12. 12. 3.3 Voltage Rectification (cont’d) 12
  13. 13. 3.3 Voltage Rectification (cont’d) electron flow A(-) ABCDEFGH A(+) HGCDEFBA 13
  14. 14. 3.3 Voltage Rectification (cont’d) Rectifier, valve or solid state (semiconductor), is used to prevent the current to flow in the wrong direction. Half-wave rectification allows the current to flow in the right direction (only half of the cycle). Full-wave rectification allows the current to flow throughout the cycle in the right direction. 14
  15. 15. 3.4 Physics of X-Ray ProductionThere are two mechanisms by which x-rays are produced:2. Bremsstrahlung: incoming electron interacts with the nucleus, giving out photons.3. Characteristic x-rays: incoming electron knocks out one of the atomic electrons, creating an hole in the shell; characteristic x-ray emitted as a result of the hole being refilled by an outer-shell electron. 15
  16. 16. 3.4 Physics of X-Ray Production (Bremsstrahlung)When an electron passes near a nucleus, itspath is deflected and loses energy asbremsstrahlung photons.0 < hν < E (electron energy) 16
  17. 17. 3.4 Physics of X-Ray Production (Bremsstrahlung, cont’d)For high-energy electrons (MeV), bremsstrahlung photonsemitted mostly in forward direction, for low energy electrons(100 keV), they are more isotropic (equal in all directions). 17
  18. 18. 3.4 Physics of X-Ray Production (Bremsstrahlung, cont’d) The probability of bremsstrahlung production ∝ Z2. The efficiency = 9 × 10-10 ZV For example, with tungsten target (Z=74), the efficiency for electrons accelerated through 100 kV is : 9 × 10-10 × 74 × 105 = 0.67% The rest of the energy (>99%) dissipated as heat. 18
  19. 19. 3.4 Physics of X-Ray Production (characteristic x-rays) The incoming electron, with energy E0, knocks out an electron, leaving the atom ionized. 19
  20. 20. 3.4 Physics of X-Ray Production (characteristic x-rays,cont’d)When an outer shell electron falls to an inner shell to fillthe vacancy, a characteristic x-ray is produced, with theenergy equal to the difference of the binding energies ofthe 2 shells involved. For example, if an L-shell electronfills a vacancy on the K-shell, the characteristic x-rayhas an energy = EK – EL. This energy is discrete, andcharacteristic to a particular element.Recall that the energy spectrum of bremsstrahlungphotons is continuous. 20
  21. 21. 3.5 X-Ray Energy Spectra 21
  22. 22. 3.5 X-Ray Energy Spectra (cont’d) 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. The quality of the x-ray beam is specified by a quantity half-value layer. 22
  23. 23. 3.6 Operating Characteristics tube filament current current tube voltage 23
  24. 24. 3.6 Operating Characteristics (cont’d)The relationship between x-ray output and filament current,tube current, and tube voltage.The x-ray output is measured by the quantity, exposure,defined as the amount of ionization produced per mass of air.The x-ray output is very sensitive to the filament current, thusit is important to keep it stable for constant output.The x-ray output is linearly proportional to the tube current,and varies approximately as squares of the tube voltage(kilovoltage). x − ray output ∝ I V 2 tube tube 24