Summary/Review Questions Chapt 6 Test 11/6 Rad Science/CRB
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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
What covers the x-ray tube and provides the following 3 functions?
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
What surrounds the cathode (-) and the anode (+), which are the electrodes of the vacuum tube?
glass or metal enclosure
What contains the tungsten filament, which is the source electrons?
What anode is the tungsten-rhenium disc, which serves as a target for electrons accelerated from the cathode?
What results from angled targets?
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?
Safe operation of the x-ray tube is the responsibilty of who?
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
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)
Where in an x-ray imaging system is thoriated tungsten used?
In cathodes to provide more electrons. In anodes for physical strength and stability.
What is saturation current?
When all available electrons are projected from the cathode to the anode.
Why are arcing and tube failure no longer major problems in modern x-ray tube design?
Heavy filaments and high-capacity anodes.
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.
What addition to the filament material prolongs tube life?
Broken filament, thorium added to filament wire.
What is the reason for the filament to be embedded in the focusing cup?
In order to electrostatically shape the projectile electron beam.
Why are x-ray tubes manufactured with two focal spots?
Small for better spatial resolution, large for higher radiation intensity.
Is the anode or the cathode the negative side of the x-ray tube?
The cathode is the negative side.
List and describe the two types of anodes.
Fixed and rotating.
What are the three functions the anode serves in and x-ray tube?
X-ray tube target, electrical conductor, mechanical support, and thermal radiator.
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
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).
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.
Explain the three causes of x-ray tube failure.
Cracked or pitted anode, induction motor failure, and open filament.
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.
What is a detent position?
X-ray tube locking-in at center and at a given SID
Graph showing the cooling rate of an x-ray tube housing
Housing cooling chart
Radiation emitted through the x-ray tube housing (other than the primary beam)
Thermal Energy Unit to express the heat capacity (1HU = 1AVs = 1Ws =1J)
Heat unit (HU)
Shroud inside the x-ray tube surrounding the cathode to concentrate the electrons on the target focal spot
3400 rpm or 10,000 rpm
Anode rotation speed
Tungsten alloyed with thorium
Cathode to anode electron flow
X-ray tube current
X-ray tube capable of high-speed switching. Voltage applied to the focusing cup is the switch
Grid-controlled x-ray tube
Method of heat transfer by a moving fluid medium (liquid or gas)
Electron cloud in the vicinity of the filament
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