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RadT 334
Radiographic Physics Review Covering X-ray Tube
| Question | Answer |
|---|---|
| What is "guards against excessive radiation exposure and electric shock"? | Protective housing |
| What holds the x-ray tube? | Glass or metal enclosure |
| What does the metal enclosure do? | Maintains a constant electric potential between the electrons of the tube current and the enclosure. |
| What is the glass enclosure tube made of and what does it do? | It is made of Pyrex, which enables the tube to withstand large amounts of heat. |
| Positive side of the x-ray tube. | Anode |
| Negative side of the x-ray tube. | Cathode |
| Coil of wire that emits electrons when it is heated. | Filament |
| Metal shroud that holds the filament. | Focusing cup |
| Area of anode struck by the electrons from the cathode. | Target |
| Allows the electron beam to interact with much larger target areas. | Rotating anode |
| Turns the anode.It has two parts, the stator and the rotor. It works through electromagnetic induction. | Induction motor |
| Thin section of a glass envelope through which the useful beam emerges. | Window |
| What happens if gas enters into the glass or metal enclosures? | Electron flow from the cathode to the anode is reduced, x-ray production drops and heat is generated, which causes tube failure. |
| What happens when the glass enclosure ages? | Tungsten vaporizes and coats the inside of the enclosure. The electrical properties are altered and tube currents stray and interact with the glass enclosure, which results in arcing and tube failure. |
| How does the metal enclosure affect the longevity of the x-ray tube? | It maintains constant electric potential between electrons of the tube current and the enclosure, which causes the tube to have a longer life and be less likely to fail. |
| What are filaments made of? | Thoriated tungsten |
| Why is thoriated tungsten the choice material for the filaments? | Tungsten provides for higher thermionic emission. It has a high melting point and is least likely to burn out. It does not vaporize easily. When 1-2% of thorium is added to tungsten, efficiency of thermionic emission is increased and tube life increases. |
| What are the two focal spot sizes? | Small focal spot and large focal spot. |
| When is the small focal spot used? | When better spatial resolution is required |
| When is the large focal spot used? | When large body parts are imaged and when other techniques that produce high heat are required. |
| What is space charge? | A cloud of electrons, known as space charge, makes it difficult for subsequent electrons to be emitted by the filament because of electrostatic repulsion. |
| What are the two types of anodes? | Stationary and rotating |
| What is a stationary anode used for? | Dental x-ray imaging systems, some portable imaging systems, and other special-purpose units in which high tube current and power are not required. |
| What is the rotating anode used for? | There are for general purpose because they are capable of producing high-intensity x-ray beams in a short time. This allows for the electron beam to interact with a much larger target area. |
| What is the choice of material used for the anode target in general radiography? | Tungsten |
| What three properties make tungsten the choice material for the target anode? | Atomic number, thermal conductivity, and high melting point. |
| What is the line-focus principle? | It is a concept of maintaining a small focal spot while heating a large area. |
| What is the Anode Heel Effect? | The intensity of x-rays that are emitted through the heel of the target (anode) is reduced because they have a longer path through the target and therefore, increased absorption. |
| How is the Anode Heel Effect obtained? | It is obtained by placing the cathode over the thicker portion of the anatomy. |
| What is the advantage of the line-focus principle? | Increased spatial resolution and heat capacity. |
| What are three causes of tube failure? | Heat dissipation, maintaining the anode at elevated temperatures for prolonged periods of time and the filament. |
| Why is the filament considered in tube failure? | The high temperatures that are emitted from it cause slow vaporization of tungsten, which can plate the inside of the glass or metal enclosure. This can lead to arcing of the tube and tube failure. |
| What happens when elevated temperatures have been maintained for long periods of time? | Excessive heating of the bearings results in increased rotational friction and an imbalance of the rotor anode assembly. |
| What three ways is heat dissipated? | Radiation, conduction, convection |
| What is the radiographic rating chart used for? | It conveys radiographic techniques that are safe and unsafe for x-ray tube operation. |
| What is the anode cooling chart? | This chart contains the thermal capacity of an anode and its heat dissipation characteristics. This one does not depend on the filament size or the speed of rotation. |
| What is the housing cooling chart? | It is the same as the anode cooling chart. It contains the thermal capacity of an anode and its heat dissipation characteristics and does not depend on filament size or the speed of rotation. Complete cooling requires 1-2 hours. |
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