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Physics Chapter 6
X-ray tube components and tube failure/ tube charts
Question | Answer |
---|---|
Explain the protective housing of the X-ray tube | lead lined and surrounds the tube to prevent leakage radiation. |
What does the xray protective housing protect from? | protects against electrical shock. Provides mechanical support Contains oil that serves as an electrical insulator and thermal cushion |
Metal/Glass Envelope has two diodes, name them.. | Cathode (filament) Anode (target) |
What is the purpose of the metal/glass envelope? | to maintain a vacuum. The production of xrays would be less efficient without it because the electrons would interact with the particles in the air. |
Gassy Tube | filled with air molecules because the tube vacuum has deteriorated |
Internal structure of the X-ray tube | Filmanet and Focusing Cup |
filament | coil of wire approx. 2 mm in diameter. 1-2 cm long |
focusing cup | metal cup that the filament is embedded in. |
filament circuit | when tube is on a low current passes through filament, preparing it for an exposure. |
Space Charge Effect | cloud of electrons around the filament that becomes so large that it makes it difficult for additional electrons to be boiled off. |
Dual Focus Tube | come with two focal spots: small and large |
Benefits of small focal spot | used with mA stations less than 300. Provides better spatial resolution. Don t use all the time because of tube burn out. Better detail, more patient dose. |
Benefits of large focal spot | used with technical factors that require a lot more xray photons. Less detail, less patient dose. |
Anode | positive side of the X-ray tubes |
2 types of anodes | rotating and stationary |
rotating anode | used for general purpose X-ray tubes because of the high intensity of the xrays that are produced. Moving focal spot, less detail. |
Stationary anode | used in dental. Very high tube current and power aren't required. Very limited output can heat up and burn out if used for diagnostic xray. |
Functions of the anode | electrical conductor, provides mechanical support for the target, good thermal conductor. |
Anode materials | copper, graphite, molybdenum |
Target materials | tungsten (molybdenum or rhodium for mammography) |
Target material characteristics | has high atomic number, good thermal conductor has high melting point |
line focus principle | the effective focal spot size is smaller than the actual focal spot |
actual focal spot | area on the target that's being bombarded with electrons. It is controlled by the length of the filament |
effective focal spot | the focal spot that is projected down towards the patient. the actual source of radiation |
Anode heel effect | xray intensity on the cathode side is higher than the intensity on the anode side. |
Extra-focal radiation | electrons bounce off of the focal tract and enter and interact with an area outside the focal tract to produce xrays |
3 ways heat can be dissipated at the anode | radiation, conduction, convection |
Causes of tube failure | melting or pitting of the anode long exposure timers filament vaporization cracked anode gassy tube |
Melting or pitting of the anode | temp of the anode is excessive due to exposure exceeding the tube rating chart. Excessive heat can produce many small pitts or melts on the surfaace of the focal tract |
Long exposure timers | results from maintaining the anode at elevated temperatures for a prolonged exposure time of 1-3 seconds. Anode will seize up and quit rotating. |
Filament Vaporization | causes tungsten to vaporize and deposit itself on the inside of the envelope. |
Cracked Anode | single, large exposure to a cold anode causes it to rapidly heat and crack |
Gassy tube | tube vacuum deteriorates allowing air molecules in causing the electrons from the cathode to interact with air molecules and decrease tube output.. |