X-Ray Production PDF

X-Ray Production Overview of XR Production What are the components required to produce XR tungsten filament (cathode) tungsten target (anode) vacuum chamber (glass enclosure/envelope)...

X-Ray Production Overview of XR Production What are the components required to produce XR tungsten filament (cathode) tungsten target (anode) vacuum chamber (glass enclosure/envelope) focusing cup What is thermionic emission emission of electrons from a heated metal (cathode) as the temperature increases, the surface electrons gain energy → overcoming binding energy of e- → allows them to move a short distance off the surface → emission explain briefly the process of how XR is made (refer to thermionic emission, electron acceleration and Bremsstrahlung) X-Ray Production 1  1. current applied to Tungsten filament → heating of filament (T >2000) → electrons in filament becomes very energetic → released electrons via thermionic emissions 2. electrons are accelerated towards the positive anode, via 2 processes Apply high voltage between filament and target (anode), causing a potential difference This can be changed to control the electron energy (e.g., 50-150 kVp for most applications, 20-40 kVp for mammo) 3. when energetic electrons strike the Tungsten target → lose their kinetic energy via 3 different methods (excitation, ionization, radiative losses → Bremsstrahlung), some of which results in production of XR 4. excitation produces heat (99%), ionization and radiative losses produce XR (1%) Model Answer: 1. Filament current applied through tungsten filament at cathode. 2. Heats up filament to produce enough energy to overcome binding energy of electrons (thermionic emission). X-Ray Production 2  3. Electrons released from filament. 4. Tube voltage is applied across the x-ray tube to achieve acceleration of e-. This can be changed to control the electron energy 5. Electrons, therefore, are accelerated towards positively charged anode, which transmit energy (kinetic energy) 6. The electrons strike the anode and the energy is released via interactions with the atoms in target - 3 interactions: excitation, ionisation and Bremsstrahlung 7. The target emits radiation (predominantly heat > 99% and a few XRs < 1%) 8. These x-ray photons leave the x-ray tube through the window as an x-ray beam travelling towards the patient. 9. They pass through the patient to the detector to produce the x-ray image Component of XR tube 1. Glass enclosure What is the purpose of the glass enclosure/envelope maintain a vacuum, which allow the amount and speed fo electrons to be controlled independently How does the glass enclosure/envelope achieve their function? Without vacuum, electrons that are emitted from the tungsten cathode would collide with gas particles on their way to reach the tungsten anode this produces secondary electrons, which will be of variable reduced speeds when striking target → wide variation in the energy of XR produced When would a metal tube insert be used instead of a glass tube? When greater heat loading is required e.g. interventional, cardiovascular What is the tube shielding material? 2-3mm lead sheet Legal limit of leakage is 1 mGy/hr @1m from anode X-Ray Production 3  2. Cathode What are the filaments in the XR machine made of? why? Tungsten, sometimes Tungsten + Rhenium (10%) due to high melting point (3422 C) high atomic number → good thermionic emitter What is the temperature that Tungsten must be heated to for XR production? 2200C What is the space charge effect at the filament? filament = thin coiled wire of tungsten current placed through wire → heats up the wire → thermionic emission of electrons this creates an imbalance in the tungsten atoms at the cathode where there are uneven number of negative charge (electrons) and positive charge (protons of the tungsten nucleus) → net positive charge which exert some pull on the emitted electrons → electron cloud surrounding the filament → reaches an equilibrium point → further electron emitted into the cloud will → electron in the cloud leaves for the anode this limits emission of electrons What is mA and what dose it measure? miliampere ampere = unit of measurement to describe the current/movement of electrons → measures the QUANTITY of electrons travelling from cathode to anode relationship is direct: 100 mA has twice the electrons of 50 mA current measures quantity (total number) of XR produced Focusing cup - what is it? what is the function? how? what is it usually made up of? X-Ray Production 4  shallow depression on the surface of cathode, usually made up of nickel help concentrate electron beam towards the focal spot of anode cup has negative charge and the shape helps converge electrons into the focal spot (target). Otherwise, as electrons are negatively charged, their electrostatic repulsion will cause them to spread out as they reach the anode → only some hitting the target (basically focusing the beam to hit the target) 3. Generator What is function of generator generator applies a voltage across the tube to create a negative to positive gradient across the tube and accelerate e- across towards anode 4. Anode Anode - what are the types? stationary and rotating Ideal characteristics of anode High melting point High atomic number Z (beam intensity is directly proportional to Z) High conductivity Low vapour pressure Mechanically stable How does a fixed/stationary anode work? What are the applications W target embedded in copper electrode Heat removed by conduction via copper Used in mobile fluoroscopy or dental (low current/low mAs) studies - up to 8mA However, inadequate for higher tube current studeis (e.g., general XR, cardio, CT, fluoro) X-Ray Production 5  Function of a rotational anode? Why is it needed? what is the structure? used in most radiography as electrons strike the target, they lose kinetic energy via 3 main methods (excitation, ionisation and radiative losses i.e., Bremsstrahlung) Excitation (99%) produces heat. This can melt the tungsten anode and is the major limitation of XR production. therefore, a rotational anode can have different areas exposed to electron stream over time → overcomes heating structure: tungsten discs which rotates fast (several thousands RPMs, at 50 Hz), stem of Molybdenum (poor conductor of heat to prevent heat transfer to mechanism that spins the disc), rotation rotor (blackened, to ease heat transfer), silver lubricated bearings between stem and rotor (allow very fast rotation at low resistances) What other way can the anode prevent overheat? angling the anode, to about 6 to 20 degrees this increases the SA for heat dispersion the angle decrease focal spot size → increase the spatial resolution. What is the focal spot (actual vs effective) and what is this significance? what affects the size of the focal spot? actual focal spot = where electrons land in the anode effective focal spot = where xr land on patient. Defines the amount of blur. Smaller effective focal spot = greater spatial resolution i.e., we want small effective focal spot focal spot size is affected by the anode angle What are the focal spot values used in mammography, general XR and portable XR? mammography: 0.3 and 0.1mm general XR: 0.6mm and 1.2mm X-Ray Production 6  portable XR device: stationary anode (does not rotate) → limit their tube rating What is the line focus principle? how is this used in radiography? relationship between actual focal spot (area where e- strikes the anode) and effective focal spot size (focal spot as viewed from the image plane a.k.a area where XR strikes the patient) good image spatial resolution requires a small focal spot. Heat dissipation requires a large focal spot ⇒ this conflict is solved by slanting the anode face (introducing an angle) Line focus principle describes the reduction in effective focal spot size compared to actual focal spot size due to anode angle allows larger heat loading while minimise size of focal spot (less blur → improved spatial resolution) What is the optimal angle for anode? why? 12-15 15 - larger angle = larger effective focal spot - less spatial resolution of the image X-Ray Production 7  6-15 is good balance, allowing larger heat loading while minimising size of focal spot What is the issue associated with angling of the anode? Negative consequence of an angled anode → variation of beam intensity across the XR field. This is known as the anode-heel effect. steeper (smaller) angle = greater heel effect. this is sometimes referred to as "heel cut-off" Why does the anode-heel effect occur? what 4 factors affects the anode-heel effect? what is their relationship 1. cathode side = stronger side Production of XR does not only occur on the surface of the anode, but throughout the material XR produced deep within the material must travel increased distance to travel out of the material. This process causes the XR photons to be absorbed by the anode heel → less energy (more attenuation) of the XR beam However, this distribution is not even → spectrum of intensity in the exiting beam (differential XR attenuation). XRs formed closer to the cathode have the smallest travelling distance → less attenuated → more intense beam XRs formed closer to the anode have higher travelling distance to exit → more attenuated → less intense beam → decreased XR intensity in the anode side of the beam X-Ray Production 8  2. Smaller angle = worsening (greater) heel effect (angle = 1/heel effect) smaller angle → greater exit distance → greater loss of XR photon in the anode side of beam → increased difference in intensity between anode side and cathode side of XR beam → greater heel effect smaller angle also → increased loss of XR photon → greater loss of energy. This is referred to as the heel cut-off effect (anode cut-off) X-Ray Production 9  3. larger focus to film distance = less heel effect (FFD = 1/heel effect) focus to film distance (FFD) = distance between tube and film the central beam is the most uniform → if there are more central beam vs peripheral beam, there will be a smaller variation in intensity larger distance → captures more central beam → less difference in intensity across XR beam → less heel effect X-Ray Production 10  4. smaller film = less heel effect (if same FFD) (film size = heel effect) same principle as above - smaller film → larger percentage of central beam → less heel effect How is heel effect reduced? Larger focus-image receptor distance X-Ray Production 11  Smaller field size (for equal focus-image receptor distances) Larger anode angle What are the some applications of the anode-heel effect (3) 1. mammography: thicker breast tissue tends to be closer to chest wall → position chest wall closer to cathode (chest wall closer to cathode - CCC) stronger XR beams on the chest wall side (to penetrate thicker tissue) 2. CXR in CXR, the beam orientation should be vertical with cathode down → stronger XR (cathode side) for upper abdomen, weaker (anode side) beam for lung 3. AP thoracic spine XR soft tissues tend to be increased over the lower thoracic spine → cathode side on the lower spine (cathode down) → more penetration anode = upper thoracic spine, cathode = lower thoracic spine Where is the cathode side for AP: thoracic spine, femur (adults vs paeds?), tibia/fibula, humerus and forearm XR? thoracic spine: cathode side on abdomen (belly denser than lungs) femur: cathode on head of femur (upper thigh is thicker) for paeds, however - cathode towards knee to avoid gonad exposure tibia/fibula: cathode on knee (upper calf is thicker) humerus: cathode on shoulder (upper arm is thicker) forearm: cathode on elbow (upper forearm is thicker) 5. Filtration (see more in Filtration, Grids and Collimation) What are the filters of XR machines? X-Ray Production 12  blocking of soft XR that are not usual for imaging but still add dose (entrance skin dose) inherent filter: normal component of the XR tube that block soft XR (e.g., windows, envelope) added filter = extra metal used to achieve a safe level of total filtration total filtration = inherent + added filter XR Tube filtration: what is the minimum filtration required for general, dental and mammography? general: 2.5mm Al (1.5mm Al permanent) Dental (