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RAD121
Chapter 6 Study Questions
| Question | Answer |
|---|---|
| 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. |
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queenxraynay
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