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What is the primary support structure for the x-ray tube, which allows the greatest ease of movement and range of position?
ceiling support system
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What covers the x-ray tube and provides the following 3 functions?
protective housing
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What are the 3 funcitons of protective housing?
- reduces leakage radiation to less than 1 mGya/hr at 1 m
- provides mechanical support, thereby protecting the tube from damage
- serves as a way to conduct heat away from the x-ray tube target
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What surrounds the cathode (-) and the anode (+), which are the electrodes of the vacuum tube?
glass or metal enclosure
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What contains the tungsten filament, which is the source electrons?
cathode
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What anode is the tungsten-rhenium disc, which serves as a target for electrons accelerated from the cathode?
rotating anode
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What results from angled targets?
line-focus principle
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What is the variation in x-ray intensity across the x-ray beam that results from absorption of x-rays in the heel of the target?
heel effect
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Safe operation of the x-ray tube is the responsibilty of who?
radiographers
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Tube failure can be prevented. There are 3 caused of tube failure:
- A single excessive exposure causes pitting or cracking of the anode.
- Long exposure time causes excessive heating of the anode, resulting in damge to the bearing in the rotor assembly. bearing damage causes warping & rotational friction of the anode
- Even with normal use, vaporization of the filament causes tungsten to coat the glass or metal enclosure; this eventually causes arcing
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List the 3 methods used to support x-ray tubes and briefly describe each.
- Floor, wall, pedestal, or ceiling mounted, and C-arm or L-arm positioned.
- (answer taken directly from website)
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Where in an x-ray imaging system is thoriated tungsten used?
In cathodes to provide more electrons. In anodes for physical strength and stability.
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What is saturation current?
When all available electrons are projected from the cathode to the anode.
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Why are arcing and tube failure no longer major problems in modern x-ray tube design?
Heavy filaments and high-capacity anodes.
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Explain the phenomenon of thermionic emission.
Pass an electric current to heat a conductor and cause outer-shell electrons to be released from the conductor.
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What addition to the filament material prolongs tube life?
Broken filament, thorium added to filament wire.
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What is the reason for the filament to be embedded in the focusing cup?
In order to electrostatically shape the projectile electron beam.
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Why are x-ray tubes manufactured with two focal spots?
Small for better spatial resolution, large for higher radiation intensity.
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Is the anode or the cathode the negative side of the x-ray tube?
The cathode is the negative side.
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List and describe the two types of anodes.
Fixed and rotating.
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What are the three functions the anode serves in and x-ray tube?
X-ray tube target, electrical conductor, mechanical support, and thermal radiator.
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How do atomic number, thermal conductivity, and melting point affect the selection of anode target material?
- High atomic number = efficient x-ray production
- thermal conductivity = heat dissipation,
- high melting point = greater heat capacity
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How does the anode rotate inside a glass enclosure with no mechanical connections to the outside?
Induction motor (stator has electrical connections on the outside of the tube; the rotor inside the tube needs no connections).
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Define the heel effect and describe how it can be used advantageously.
Higher x-ray intensity on cathode side, which should be positioned to thicker anatomy.
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Explain the three causes of x-ray tube failure.
Cracked or pitted anode, induction motor failure, and open filament.
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What happens when an x-ray tube is space charge limited?
X-ray tube current is limited by the cloud of electrons surrounding the cathode. An increase in kVp will remove the space charge.
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What is a detent position?
X-ray tube locking-in at center and at a given SID
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Graph showing the cooling rate of an x-ray tube housing
Housing cooling chart
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Radiation emitted through the x-ray tube housing (other than the primary beam)
Leakage radiation
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Thermal Energy Unit to express the heat capacity (1HU = 1AVs = 1Ws =1J)
Heat unit (HU)
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Shroud inside the x-ray tube surrounding the cathode to concentrate the electrons on the target focal spot
Focusing cup
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3400 rpm or 10,000 rpm
Anode rotation speed
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Tungsten alloyed with thorium
Thoriated tungsten
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Cathode to anode electron flow
X-ray tube current
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X-ray tube capable of high-speed switching. Voltage applied to the focusing cup is the switch
Grid-controlled x-ray tube
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Method of heat transfer by a moving fluid medium (liquid or gas)
Convection
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Electron cloud in the vicinity of the filament
Space charge
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