Chapter 6 Study Questions
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| 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
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| What are the basic parts of a cathode assembly? | filament/s, focusing cup, associated wiring
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| What is the purpose of the focusing cup? | This narrows the stream as it goes to the anode.
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| 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
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| 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
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| 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.
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| How does the anode heel affect radiation intensity? | Due to the geometry of this, the radiation intensity is greater on the cathode side.
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| What is the advantage of a high-speed rotor? | Faster anode rotation promotes better heat dissipation.
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| Why is it necessary for a vacuum to exist within the envelope? | This permits electrons to flow without encountering the gas atoms of air.
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| Define leakage radiation. | Any photon that escapes from the housing except at the port.
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| 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.
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| Actual Focal Spot | The physical area of the focal track that is impacted
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| 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.
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| Anode Assembly | Consists of the anode, stator, and rotor
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| Anode Cooling Chart | Permits the calculation of the time necessary for the anode to cool enough for additional exposures to be taken.
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| 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
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| 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.
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| Cathode Assembly | Consists of the filament, focusing cup, and associated wiring.
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| Dual Focus | A two-filament arrangement within the x-ray tube.
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| Effective Focal Spot | The area of the focal spot that is projected out of the tube toward the object being radiographed.
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| Envelope | The entire cathode and anode assembly is housed within this glass or metal structure
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| Extrafocal Radiation | Photons that were not produced at the focal spot; also called off-focus radiation
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| Filament | A small coil of thin thoriated tungsten wire
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| Focal Point | The portion of the anode where the high-voltage electron stream will impact; also: target, focus, focal spot, actual focal spot
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| Focal Spot | The portion of the anode where the high-voltage electron stream will impact; also: target, focus, focal point, actual focal spot
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| 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
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| Focus | The portion of the anode where the high-voltage electron stream will impact; also: target, focal point, focal spot, actual focal spot
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| Focusing Cup | A shallow depression in the cathode assembly designed to house the filament
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| Fractional Focal Spot | A very small focal spot; usually a fraction of a millimeter in size
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| Grid-Biased Tube | A type of tube that quickly regulates the flow of electrons, producing x-ray photons; also: grid-controlled, grid-pulsed
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| Grid-Controlled Tube | A type of tube that quickly regulates the flow of electrons, producing x-ray photons; also: grid-biased, grid-pulsed
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| Grid-Pulsed Tube | A type of tube that quickly regulates the flow of electrons, producing x-ray photons; also: grid-controlled, grid-biased
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| Housing Cooling Chart | Permits the calculation of the time necessary for the housing to cool enough for additional exposures to be made
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| Leakage Radiation | Any photons that escape from the housing except at the port
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| Line-Focus Principle | Used to reduce the effective area of the focal spot
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| 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
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| Protective Housing | The entire cathode and anode assembly is housed within this metal structure
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| 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
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| Rotating Anode | An anode assembly that turns during exposure
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| Rotor | A hollow copper cylinder or cuff that is attached to the anode disk by a molybdenum shaft
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| 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
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| 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
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| 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
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| Stationary Anode | An anode assembly that is immobile
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| Stator | Mutual-induction-motor electromagnets that turn the anode
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| Target | The portion of the anode where the high-voltage electron stream will impact; also: target, focus, focal spot, actual focal spot
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| 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
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| Window | A structure where the primary x-ray beams exit the envelope, which allows for less absorption of scatter radiation
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| Wiring | The parts within the cathode assembly that connect the elements of the assembly together
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| What is the function of the cathode? | to produce a thermionic cloud
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| What is the alloy called that is used in the filament? | Thoriated Tungston
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| What materials, other than tungston, could be used to make the filament? | Rhenium and molybdenum
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| How would a Rhenium Tungston alloy be used? | As anode target material.
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| What is the space charge effect? | During thermionic emission, electrons build at the filament and oppose production of more electrons.
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| What is the purpose of the focal cup? | It narrows the electron stream as it goes to the anode.
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| What is saturation current? | Once the current reaches a certain kVp, increasing kVp will not produce more electrons.
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| mA controls what? | The # of electrons available.
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| kVp controls what? | The acceleration of the electrons from the cathode to the annode.
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| Where are grid-biased tubes found? | capacitor discharge units and angiography units
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| Recorded detail are impacted by what? | Filament length and width.
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| What is filament length? | 7-15 mm
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| What is filament width? | 1-2 mm
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| What is filament thickness? | .1-.2
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| 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.
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| 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
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| The envelope of x-ray tubes can be made from what? | metal or glass
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| What are 2 causes of tube failure? | --1.tube arcing--2.filament breakage
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| Vaporized tungston deposits on the inner surface of the glass envelope can cause what? | high-voltage arcing, and decreased efficiency.
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| What is the focusing cup made from? | nickel
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| The focusing cup has what electrical charge? | negative
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| What 3 functions does the anode have? | --1.provides a target surface--2.conducts high voltage--3.primary thermal conductor
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| Where are high speed incident electrons from the filament suddenly stopped? | the anode
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| 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.
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| What are the 2 types of anodes? | --1.stationary--2.rotating
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| Stationary anodes are limited to what? | Low-power functions, such as those of dental units
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| Why does diagnostic radiography utilize rotating anodes? | because of their greater efficiency
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| isotropically | in all directions
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| Biased focusing cup | causes exiting electron beam to be focused into a narrower stream
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| What are stationary anodes composed of? | rhenium-alloyed tungston imbedded in a 45-degree angled copper end of a copper rod.
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| 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.
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| Tungsten's atomic number | 74
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| What benefit does rhenium provide? | greater elasticity when the focal track expands rapidly due to intense heat
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| Molybdenum's atomic number | 42
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| 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.
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| Pitting of the focal track is caused by what? | vaporization of focal track material; reduces efficiency
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| What is the glass envelope window made of in mammography tubes? | berylium
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| What is the advantage of a graphite backed anode? | You can double the heat-loading capacity without increasing bearing wear.
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| What is the purpose of an anode warm-up procedure? | raises heat to near operating range to prevent cracking; helps maintain strong vacuum.
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| What is different about a stress-relieved anode? | Does not require an elaborate tube warm-up procedure.
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| What is the point source of x-ray photons in addition to being the point that OID and SID are measured from? | the target
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| Which anodes have a greater heat-loading capacity? | high-speed anodes
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| What is the effective focal spot controlled by? | --1.actual focal spot--2.target angle
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| What is the actual focal spot controlled by? | --1.filament size--2.angle of anode
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| 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.
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| Most common diagnostic x-ray angle is what? | 12-degrees
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| What is the target angle range for x-ray tubes? | 7-17 degrees
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| What are stators? | Self-induction motor magnets that turn the anode.
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| Where are stators? | Outside the envelope.
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| 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.
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| 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.
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| Harmonics | Name of the tone a high-speed rotating cuff makes at 5000-7000 rpm capable of shattering the glass envelope.
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| What is a common cause of tube failure? | Bad bearings because of long use at high temperatures.
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| What is the envelope made of? | pyrex or metal
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| Why is metal becoming a more common choice for envelopes? | they eliminate the problem of tungston erosion
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| What is the primary function of the envelope? | To maintain the vacuum.
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| Where is the window on the envelope? | Where the primary beam exits.
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| 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.
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| What must x-rays be a part of to be considered useful? | the primary beam
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| What is the protective housing made of? | lead-lined cast steel
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| What is the rule for leakage radiation limit? | LEAKAGE RADIATION MUST BE LIMITED TO 100mR/hr @ 1 METER!!!
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| What does the dielectric oil do? | --1.Insulates--2.Promotes cooling
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| What additional devices might help cool the housing/oil? | --1.fan--2.heat exchanger
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| What are the three most impertant factors set by the radiographer? | kVp, mA, and time
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| What are the 3 ways heat is dissapated? | --1.Radiation(99%)--2.Conduction--3.Convection
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| Excessive heat results in what? | reduced x-ray tube life
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| 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.
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