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Ch.6 X-Ray Tube

QuestionAnswer
What conditions must exist for x-rays to be produced? A source of electrons, an appropriate target material, high voltage, and a vacuum.
What is the function of the cathode? Produce a thermionic cloud, conduct the high voltage to the gap between cathode and anode, and focus the electron stream as it heads for the anode.
Is the cathode on the negative or positive side of the x-ray tube? Negative
What does the cathode assembly consist of? Filament(s), focusing cup, and associated wiring.
What is a filament? A small coil of thoriated tungsten wire.
Where is the filament located? In the cathode assembly within the focusing cup.
What does the cathode assembly consist of? Filament(s), focusing cup, and associated wiring.
What is a filament? A small coil of thoriated tungsten wire.
Where is the filament located? In the cathode assembly within the focusing cup.
What are the three materials of choice for a filament? Tungsten (top choice), rhenium, and molybdenum.
Why is tungsten a desired material? High melting point and not easily vaporized.
What is the function of the filament? Provide sufficient resistance to the flow of electrons so that the heat produced will cause thermionic emission to occur.
The average x-ray has dual filaments, meaning a large filament and a small filament. What is the purpose of the small and the large filaments? Small filaments are used for better detail. Large filaments are used to dissipate the heat faster.
What happens when vaporization occurs? Produces particles that deposit on other surfaces and reduce the vacuum within the tube.
Where are x-ray photons produced? Anode target
What causes the tube gassy? Reduction of the vacuum caused by the small percentage of electrons that are permanently vaporized from the filament.
What causes tubes to have a mirrored image appearance? Vaporized tungsten from the filament and the anode.
What can cause a filament to break? Vaporization and rough handling of older tubes.
What happens when the x-ray machine is first turned on? A mild current is sent to the filament and remains in this preheated mode until immediately prior to an exposure.
What is the life of an average diagnostic x-ray tube filament? 6-9 hours (10,000-20,000 exposures)
What is the primary cause of premature tube failure? Radiographer's habit of holding the rotor switch prior to making exposures. Every second the rotor switch is is depressed, life is removed from the filament.
What was William D. Coolidge's contributions to radiography? Hot filament, focusing cup, the imbedded anode target, and various cooling devices (including the unique water-cooled tube).
What happens when the addition of a positive or negative potential difference is at the focusing cup? Causes the cup to attract or repel the thermionic cloud.
What are the three functions of the anode assembly? Target surface for high voltage electrons from the filament becoming the source of the x-ray photons, conducts the high voltage from x-ray photons, conducts high voltage from the cathode back to the x-ray generator circuitry, and primary thermal conductor
What is the purpose of the focusing cup? To narrow the thermionic cloud as it is driven toward the anode. Focuses the electrons toward one another in a convergence pattern.
Because electrons possess negative charges, do they travel in straight lines or diverge? Diverge
Describe the space charge effect? A phenomenon that as more and more electrons build up in the area of the filament, their negative charges oppose the emission of additional electrons, This also limits x-ray tubes to maximum mA ranges of 1,000-1,200.
What is the saturation current? Another filament phenomenon that affects the efficiency of the x-ray tube. As kVp increases, a greater percentage of the thermionically emitted electrons are driven toward the anode.
What does the anode assembly consist of and what is it's function? Anode, stator, and rotor and serves as the path for the high voltage flow during exposure.
What are the two types of anode? Stationary and rotating.
Which type of anode does radiography utilize because of their greater efficiency? Rotating anodes.
Why is it important to use an anode warm-up procedure? Failure to follow the warm-up procedure can cause the entire anode to crack if the molybdenum absorbs heat too rapidly and exceeds its expansion capability.
What is the focal track? The circular path that will be impacted by the electron beam.
What do the terms target, focus, focal point, and focal spot refer to? Area of the focal track that is impacted by the electron beam at one time.
What is the actual focal spot? The physical area of the focal track that is impacted.
What is the effective focal spot? Area of the focal spot that is projected out of the tube toward the object being radiographed.
What is the line-focus principle? Used to reduce the effective area of the focal spot. This permits the best resolution of detail while permitting as large an actual area as possible to increase thermal conductivity.
What happens when the focal spot is less than 45* degrees? The effective focal spot is smaller than the actual focal spot.
What is the most common diagnostic radiography target angle? 12* degree
What is the effective focal-spot controlled by? The size of the actual focal spot (which is controlled by the length of the filament) and the anode target angle.
What is the anode heel effect? Because of the geometry of an angle anode target, the radiation intensity is great on the cathode side. For this reason the cathode end of the x-ray tube should be position toward the denser (thicker) part of the body.
What is the stator and where is it located? Is the induction-motor electromagnets that turn the anode. This is the only part of the cathode or anode assemblies that is located outside of the vacuum of the envelope.
Created by: cccxray2013
 

 



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