X-ray tube components and tube failure/ tube charts
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| Explain the protective housing of the X-ray tube | lead lined and surrounds the tube to prevent leakage radiation.
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| 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
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| Metal/Glass Envelope has two diodes, name them.. | Cathode (filament)
Anode (target)
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| 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.
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| Gassy Tube | filled with air molecules because the tube vacuum has deteriorated
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| Internal structure of the X-ray tube | Filmanet and Focusing Cup
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| filament | coil of wire approx. 2 mm in diameter. 1-2 cm long
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| focusing cup | metal cup that the filament is embedded in.
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| filament circuit | when tube is on a low current passes through filament, preparing it for an exposure.
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| 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.
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| Dual Focus Tube | come with two focal spots: small and large
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| 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.
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| Benefits of large focal spot | used with technical factors that require a lot more xray photons. Less detail, less patient dose.
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| Anode | positive side of the X-ray tubes
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| 2 types of anodes | rotating and stationary
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| 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.
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| 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.
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| Functions of the anode | electrical conductor, provides mechanical support for the target, good thermal conductor.
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| Anode materials | copper, graphite, molybdenum
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| Target materials | tungsten
(molybdenum or rhodium for mammography)
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| Target material characteristics | has high atomic number,
good thermal conductor
has high melting point
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| line focus principle | the effective focal spot size is smaller than the actual focal spot
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| actual focal spot | area on the target that's being bombarded with electrons. It is controlled by the length of the filament
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| effective focal spot | the focal spot that is projected down towards the patient. the actual source of radiation
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| Anode heel effect | xray intensity on the cathode side is higher than the intensity on the anode side.
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| Extra-focal radiation | electrons bounce off of the focal tract and enter and interact with an area outside the focal tract to produce xrays
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| 3 ways heat can be dissipated at the anode | radiation, conduction, convection
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| Causes of tube failure | melting or pitting of the anode
long exposure timers
filament vaporization
cracked anode
gassy tube
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| 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
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| 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.
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| Filament Vaporization | causes tungsten to vaporize and deposit itself on the inside of the envelope.
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| Cracked Anode | single, large exposure to a cold anode causes it to rapidly heat and crack
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| Gassy tube | tube vacuum deteriorates allowing air molecules in causing the electrons from the cathode to interact with air molecules and decrease tube output..
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