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RADT 334: X-ray Tube

Radiographic Physics Review Covering X-ray Tube

QuestionAnswer
What are the three main methods of x-ray tube support? Ceiling Support, Floor-to-Ceiling Support, C-Arm Support System
What is contained in the metal or glass enclosure of the x-ray tube? The main components of an x-ray tube
What is a cathode? It is the negative side of the x-ray tube. It has two parts: a filament and a focusing cup.
How are x-rays released from the protective housing? They are emitted from the special section of the x-ray tube called the window.
What is a filament? A coil of wire that is 1-2 cm long. It emits electrons when heated. Electrons are boiled off when current through the filament is sufficiently high.
What is a focusing cup? A negatively charged metal shroud that holds the filament.
What is an anode? It is the positive side of x-ray tube. It conducts electricity and radiates heat. It also contains the target.
What is the target? The area of the anode that is struck by the electrons from the cathode.
What is the induction motor? It is electromagnetic. It powers the rotating anode.
What are the typical events in a glass enclosure's life? Tungsten will vaporize and coat the inside of the glass enclosure. This alters the electrical properties which results in tube failure.
What are the typical events in a metal enclosure's life? It maintains a constant electric potential between the electrons of the tube current and enclosure. This results in longer x-ray tube life.
What is the choice of material for the filament? Thoriated Tungsten
Why is this material used in the filament? It provides for higher thermionic emission. With a melting point of 3410 degrees Celsius, it is not likely to burn out like a filament. It does not vaporize easily. The addition of 1-2% thorium enhances the efficiency of thermionic emission.
What are the properties of a small focal spot? It ranges from 0.1-1.0 mm. It is associated with the small filament.
What are the properties of a large focal spot? It ranges from 0.3-2.0 mm. It is associated with the large filament. If the mA is 400 or more, then the large focal spot must be used because of the heat capacity.
Define space-charge effect It is the phenomenon of the space charge that makes it difficult for subsequent electrons to be emitted by the filament because of electrostatic repulsion.
What are the two types of anodes? Stationary and Rotating
Which imaging application would a stationary anode be used for? It is used in imaging where high tube current and power are not required such as dental and portable imaging.
Which imaging application would a rotating anode be used for? It would be used in imaging that is capable of producing high-intensity x-ray beams in a short time such as general-purpose tubes.
What are some of the properties that support why Tungsten is the choice of material used for the anode target in general radiography? It has a high atomic number that results in high-efficiency x-rays. It has a high thermal conductivity which results in dissipating the heat produced. It has a high melting point of 3400 degrees Celsius. It gives added mechanical strength.
What is the line-focus principal? It makes the effective focal spot size much smaller than the actual focal spot size by angling the target.
How does the line-focus principal affect radiographic images? It improves spatial resolution and heat capacity.
What is the anode heel-effect? The intensity of x-rays that are emitted through the heel of the target is reduced because they have a longer path through the target and in which causes increased absorption.
How does the anode heel-effect affect radiographic images? It provides a more uniform radiation exposure of the image receptor. It results in smaller effective focal spot and less radiation intensity on the anode side of the x-ray beam.
What are some ways that tube failure can be prevented? Warming up the anode by using a low technique operation. Holding the rotor button down on the controls for appropriate time.
What are three causes of tube failure? Damage to bearings in rotor that cause warping, cracked or pitted anode, Tungsten coating the envelope that causes arching.
When using an X-ray tube rating chart, how do you know when the exposure will be safe? The combination of time and kVp is below the line.
When using an X-ray tube rating chart, how do you know when the exposure will be unsafe? The combination of time and kVp is above the line.
In an Anode cooling chart, what is the formula for the single phase generator? HU = kVp X mA X seconds = 0.7 J
In an Anode cooling chart, what is the formula for the three phase/high frequency generator? HU = 1.4 X kVp X mA X seconds = 1 J
Created by: pmbraswell
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