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RAD121

Chapter 6 Study Questions

QuestionAnswer
What four conditions must exist for x-ray production to occur? --1.source of electrons--2.appropriate target material--3.high voltage--4.a vacuum
What are the basic parts of a cathode assembly? filament/s, focusing cup, associated wiring
What is the purpose of the focusing cup? This narrows the stream as it goes to the anode.
Explain the space charge effect. During thermionic emission the electrons build in the area of the filament and their negative charges oppose the production of more electrons
Why is tungsten the best metal for the x-ray source? --1.It has a high melting point--2.is difficult to vaporize--3.high atomic number enhances diagnostic x-ray photos--4.heat-conducting ability
Explain the line-focus principle. When there is a decrease in the anode angle, there will be a decrease in the effective focal spot. When there is an increase in the anode angle, there will be an increase in the effective focal spot.
How does the anode heel affect radiation intensity? Due to the geometry of this, the radiation intensity is greater on the cathode side.
What is the advantage of a high-speed rotor? Faster anode rotation promotes better heat dissipation.
Why is it necessary for a vacuum to exist within the envelope? This permits electrons to flow without encountering the gas atoms of air.
Define leakage radiation. Any photon that escapes from the housing except at the port.
How is off-focus radiation produced? Photons strike another object in the tube (cathode assembly, vaporized metal on a glass envelope, off-target sites on the anode, etc.).This is composed of photons that were not produced at the target.
Actual Focal Spot The physical area of the focal track that is impacted
Anode The positive side of the x-ray tube; serves as a target surface for high-voltage electrons from the filament, conducts the high voltage from the cathode back into the x-ray generator circuitry, and serves as the primary thermal conductor.
Anode Assembly Consists of the anode, stator, and rotor
Anode Cooling Chart Permits the calculation of the time necessary for the anode to cool enough for additional exposures to be taken.
Anode Heel Effect Due to the geometry of the angled anode target, the radiation intensity is greater(20% more) on the cathode side; 25% fewer photons on anode side
Cathode The negative side of the x-ray tube; produces a thermionic cloud, conducts the high voltage to the gap between the cathode and anode, and focuses the electron stream as it heads for the anode.
Cathode Assembly Consists of the filament, focusing cup, and associated wiring.
Dual Focus A two-filament arrangement within the x-ray tube.
Effective Focal Spot The area of the focal spot that is projected out of the tube toward the object being radiographed.
Envelope The entire cathode and anode assembly is housed within this glass or metal structure
Extrafocal Radiation Photons that were not produced at the focal spot; also called off-focus radiation
Filament A small coil of thin thoriated tungsten wire
Focal Point The portion of the anode where the high-voltage electron stream will impact; also: target, focus, focal spot, actual focal spot
Focal Spot The portion of the anode where the high-voltage electron stream will impact; also: target, focus, focal point, actual focal spot
Focal Track The portion of the anode where the high-voltage electron stream will impact; when discussing a rotating anode this describes the circular path that will be impacted by the electron beam
Focus The portion of the anode where the high-voltage electron stream will impact; also: target, focal point, focal spot, actual focal spot
Focusing Cup A shallow depression in the cathode assembly designed to house the filament
Fractional Focal Spot A very small focal spot; usually a fraction of a millimeter in size
Grid-Biased Tube A type of tube that quickly regulates the flow of electrons, producing x-ray photons; also: grid-controlled, grid-pulsed
Grid-Controlled Tube A type of tube that quickly regulates the flow of electrons, producing x-ray photons; also: grid-biased, grid-pulsed
Grid-Pulsed Tube A type of tube that quickly regulates the flow of electrons, producing x-ray photons; also: grid-controlled, grid-biased
Housing Cooling Chart Permits the calculation of the time necessary for the housing to cool enough for additional exposures to be made
Leakage Radiation Any photons that escape from the housing except at the port
Line-Focus Principle Used to reduce the effective area of the focal spot
Off-Focus Radiation Photons created somewhere in the tube other than the focal spot; can cause ghosting; not a result of scatter from patient; exit through tube window
Protective Housing The entire cathode and anode assembly is housed within this metal structure
Radiographic Tube Rating Chart A guide regarding the most common technical factor combinations that can be used without overloading the tube; also: tube rating chart
Rotating Anode An anode assembly that turns during exposure
Rotor A hollow copper cylinder or cuff that is attached to the anode disk by a molybdenum shaft
Saturation Current Filament phenomenon; as kVp increases, a greater percentage of the thermionically emitted electrons are driven toward the anode and is reached when there are no further thermionic electrons to be driven toward the anode; effects efficiency
Space Charge Effect As more and more electrons build up in the area of the filament, their negative charges begin to oppose the emission of additional electrons
Space Charge Cloud The ejection of electrons from the surface of the wire due to increased heat, causing an electron cloud; also: thermionic emission, electron cloud
Stationary Anode An anode assembly that is immobile
Stator Mutual-induction-motor electromagnets that turn the anode
Target The portion of the anode where the high-voltage electron stream will impact; also: target, focus, focal spot, actual focal spot
Tube Rating Chart Provides a guide regarding the maximum technical factor combinations for each filament of each tube that can be used without overloading the tube plotted in mA, kV, and time
Window A structure where the primary x-ray beams exit the envelope, which allows for less absorption of scatter radiation
Wiring The parts within the cathode assembly that connect the elements of the assembly together
What is the function of the cathode? to produce a thermionic cloud
What is the alloy called that is used in the filament? Thoriated Tungston
What materials, other than tungston, could be used to make the filament? Rhenium and molybdenum
How would a Rhenium Tungston alloy be used? As anode target material.
What is the space charge effect? During thermionic emission, electrons build at the filament and oppose production of more electrons.
What is the purpose of the focal cup? It narrows the electron stream as it goes to the anode.
What is saturation current? Once the current reaches a certain kVp, increasing kVp will not produce more electrons.
mA controls what? The # of electrons available.
kVp controls what? The acceleration of the electrons from the cathode to the annode.
Where are grid-biased tubes found? capacitor discharge units and angiography units
Recorded detail are impacted by what? Filament length and width.
What is filament length? 7-15 mm
What is filament width? 1-2 mm
What is filament thickness? .1-.2
What is the purpose of the filament? To provide sufficient resistance to the flow of electrons so that the heat produced will cause thermionic emission to occur.
What is the purpose of the cathode? to produce a thermionic cloud, conduct high voltage to the gap from the cathode to the anode, and focus the electron stream as it heads for the anode
The envelope of x-ray tubes can be made from what? metal or glass
What are 2 causes of tube failure? --1.tube arcing--2.filament breakage
Vaporized tungston deposits on the inner surface of the glass envelope can cause what? high-voltage arcing, and decreased efficiency.
What is the focusing cup made from? nickel
The focusing cup has what electrical charge? negative
What 3 functions does the anode have? --1.provides a target surface--2.conducts high voltage--3.primary thermal conductor
Where are high speed incident electrons from the filament suddenly stopped? the anode
What is the purpose of grid-biased tubes? Gives precise control of the thermionic cloud. Changes charge of focusing cup from negative to positive to stop electrons from flowing.
What are the 2 types of anodes? --1.stationary--2.rotating
Stationary anodes are limited to what? Low-power functions, such as those of dental units
Why does diagnostic radiography utilize rotating anodes? because of their greater efficiency
isotropically in all directions
Biased focusing cup causes exiting electron beam to be focused into a narrower stream
What are stationary anodes composed of? rhenium-alloyed tungston imbedded in a 45-degree angled copper end of a copper rod.
What are rotating anode disks composed of? Molybdenum with the target focal track material being rhenium-alloyed tungsten backed by a thicker layer of molybdenum or graphite.
Tungsten's atomic number 74
What benefit does rhenium provide? greater elasticity when the focal track expands rapidly due to intense heat
Molybdenum's atomic number 42
Why is molybdenum used as the primary target material for mammography x-ray tubes? It's ability to emit a more uniform range of lower energy photons which permit better soft tissue imaging.
Pitting of the focal track is caused by what? vaporization of focal track material; reduces efficiency
What is the glass envelope window made of in mammography tubes? berylium
What is the advantage of a graphite backed anode? You can double the heat-loading capacity without increasing bearing wear.
What is the purpose of an anode warm-up procedure? raises heat to near operating range to prevent cracking; helps maintain strong vacuum.
What is different about a stress-relieved anode? Does not require an elaborate tube warm-up procedure.
What is the point source of x-ray photons in addition to being the point that OID and SID are measured from? the target
Which anodes have a greater heat-loading capacity? high-speed anodes
What is the effective focal spot controlled by? --1.actual focal spot--2.target angle
What is the actual focal spot controlled by? --1.filament size--2.angle of anode
Rules of the Line Focus Principle --1.As the actual focal-spot size increases, the effective focal spot size increases--2.When the target angle is less than 45-degrees, the effective focal spot is smaller than the actual focal spot.
Most common diagnostic x-ray angle is what? 12-degrees
What is the target angle range for x-ray tubes? 7-17 degrees
What are stators? Self-induction motor magnets that turn the anode.
Where are stators? Outside the envelope.
Of what is the rotor composed and how is it attached? the rotor is a copper cuff attached to the anode disk by a molybdenum shaft.
Why do the ball bearings between the rotor cuff and the anode shaft use silver plating as their lubrication? Liquids produce gas at high temperatures and reduce the vacuum in the tube.
Harmonics Name of the tone a high-speed rotating cuff makes at 5000-7000 rpm capable of shattering the glass envelope.
What is a common cause of tube failure? Bad bearings because of long use at high temperatures.
What is the envelope made of? pyrex or metal
Why is metal becoming a more common choice for envelopes? they eliminate the problem of tungston erosion
What is the primary function of the envelope? To maintain the vacuum.
Where is the window on the envelope? Where the primary beam exits.
What is purpose of the protective housing? --1.Controls leakage and scatter radiation--2.Isolates high voltages--3.Provides a means to cool the tube--4.Provides cushioning from rough handling by operators.
What must x-rays be a part of to be considered useful? the primary beam
What is the protective housing made of? lead-lined cast steel
What is the rule for leakage radiation limit? LEAKAGE RADIATION MUST BE LIMITED TO 100mR/hr @ 1 METER!!!
What does the dielectric oil do? --1.Insulates--2.Promotes cooling
What additional devices might help cool the housing/oil? --1.fan--2.heat exchanger
What are the three most impertant factors set by the radiographer? kVp, mA, and time
What are the 3 ways heat is dissapated? --1.Radiation(99%)--2.Conduction--3.Convection
Excessive heat results in what? reduced x-ray tube life
What are the 7 recommendations for extending tube life? --1.Anode warm-up procedure--2.Depress rotor switch in one motion--3.lower mA stations--4.lower-speed rotor--5.Repeated exposures near tube loading limits should not be made--6.Rotate tube housing gently--7.Don't use the tube if rotor bearings are loud.
Created by: queenxraynay on 2008-07-18



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