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RADT 334 X-ray Tube
Radiographic Physics Review Covering X-ray Tube
| Question | Answer |
|---|---|
| What are the external components of the x-ray tube? | support system, protective housing, and the enclosure |
| What are the different types of support systems? | ceiling support system, floor-to-ceiling support system, and C-arm support system |
| What is the ceiling support system? | Consists of two perpendicular sets of ceiling-mounted rails, and a telescoping column attaches the x-ray tube housing to the rails |
| What is the floor-to-ceiling support system? | Has a single column with rollers at each end, one attached to a ceiling-mounted rail and the other attached to a floor-mounted rail |
| What is the C-arm support system? | Shaped like a C and is ceiling mounted, located in interventional radiology suites, and has an image receptor attached to the other end of the C-arm from the x-ray tube |
| What does the protective housing do? | guards against excessive radiation exposure and electric shock, controls leakage and scatter radiation, isolates the high voltages and provides a means to cool the tube |
| The enclosure can be made of what two materials? | glass or metal |
| What does the enclosure of the x-ray tube do? | it maintains a vacuum inside the tube, which allows for more efficient x-ray production and a longer tube life |
| Why is the glass of the enclosure made of Pyrex? | to enable it to withstand the heat generated |
| How was the enclosure of the tube improved? | made of metal rather than glass to maintain a constant electric potential between the electrons of the tube current and the enclosure; have a longer life and are less likely to fail |
| What are the internal components of the x-ray tube? | the cathode and the anode |
| What is the cathode? | the negative side of the x-ray tube; it has two primary parts, a filament and a focusing cup |
| What is the filament of the cathode? | a coil of wire that emits electrons when heated through thermionic emission |
| What material is the filament made of? | thoriated tungsten, which provides for higher thermionic emission than other metals and does not vaporize easily |
| What is the focusing cup? | where the filament is embedded, negatively charged to keep electron beam to a small area of the anode |
| Why are there two focal spots in the cathode? | the small focal spot is used when better spatial resolution is required (bone work), and the large focal spot is used when large body parts are imaged and when higher techniques are used that will generate high heat |
| What is the space charge effect? | the cloud of electrons makes it hard for subsequent electrons to be emitted because of electrostatic repulsion |
| What is the anode and what types of anodes are there? | the positive side of the x-ray tube that conducts electricity and radiates heat and contains the target; stationary and rotating |
| When are stationary anodes used? | dental imaging x-ray systems, some portable imaging systems, and other special-purpose units in which high tube current and power are not required |
| When are rotating anodes used? | with general purpose x-ray tubes because they must be capable of producing high-intensity x-ray beams in a short time |
| What is the target of the anode and what is it made of in stationary anode tubes? | the area of the anode struck by the electrons from the cathode; it is made of a tungsten alloy (usually with rhenium) |
| Why is tungsten the material of choice for the target for general radiography? | it has a high atomic number for higher efficiency x-ray production, it has a thermal conductivity for dissipating the heat produced, and it has a high melting point and can stand up under high tube current without pitting or bubbling |
| What is the actual x-ray source? | the focal spot; it is where x-rays are emitted |
| What is the line-focus principle? | an angled target was designed that allows for a larger area for heating while maintaining a small focal spot for better spatial resolution; this design is called the line-focus principle |
| What is the difference between the effective focal spot and the actual focal spot? | the effective focal spot is the area projected onto the patient and image receptor, and the actual focal spot is where the electrons interact with the target |
| What does the line-focus principle result in? | an effective focal spot size much less than the actual focal spot size, with improved 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 therefore increased absorption |
| The smaller the anode angle, the ___________. | larger the heel effect |
| What types of exams can the anode heel effect be used in advantage? | foot, chest, and abdomen |
| The anode heel effect results in what? | smaller effective focal spot and less radiation intensity on the anode side of the x-ray beam |
| Which way is the cathode positioned in regards to the anode heel effect? | cathode is positioned to the thicker side of the anatomy and results in more uniform radiographic density |
| List three causes of tube failure. | using maximum techniques before warming up the anode, maintaining the anode at elevated temperatures for prolonged periods of time, and electron arcing from the filament to the enclosure because of vaporized tungsten (most frequent cause) |
| What is the radiographic rating chart? | it conveys which radiographic techniques are unsafe for x-ray tube operation; for a given mA, any combination of kVp and time that lies below the mA curve is safe, and any combination of kVp and time that lies above the mA curve is unsafe |
| What is the anode cooling chart? | contains the thermal capacity of an anode, and its heat dissipation characteristics; used to determine the length of time required for complete cooling after any level of heat input |
| What is the housing cooling chart? | reports the cooling time for the x-ray tube after a number of exposures |
Created by:
mschnell
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