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RADT 334

Radiation Physics- Chapter 6

QuestionAnswer
Defend the choice of material used most frequently in the filament. Thoriated tungsten, provides for higher thermionic emission than other metals, melting point 3410C
List three causes of tube failure and how to prevent them. Excessive heat results in reduced x-ray tube life, maximum radiographic techniques should never be applied to a cold anode, and most frequent is electron arcing from the filament to the enclosure because of vaporized tungsten
External components of the x-ray tube Quite heavy so there has to be kind of support system, there is also a protective housing, and a glass or metal enclosure
Three kinds of support systems Floor-to-ceiling, ceiling, and c-arm
Internal components of the x-ray tube Anode and cathode
Anode Positive side of the x-ray tube and it conducts electricity and radiates heat and contains the target
Cathode Negative side of the x-ray tube
Two parts of the cathode Filament and focusing cup
Filament Coil of wire, 2mm in diameter and 1-2 cm long, electric current is conducted through the coil causing it to glow and emit a large quantity of heat
Focusing cup The filament is embedded in a metal shroud, effectiveness is determined by its size, shape, and the position of the filament
Protective housing Controls leakage and scatter radiation, isolates the high voltage, and provides a means to cool the tube
Glass or metal enclosure X-ray tube is a vacuum containing two electrodes (anode and cathode)
Improved metal enclosures Maintain a constant electric potential between electrons of the tube current and the enclosure
Electron cloud Space charge (when the electrons are in vicinity of the filament before acceleration to anode)
Space charge effect The cloud of electrons makes it hard for subsequent electrons to be emitted because of electrostatic repulsion
Saturation current Tube current rises with increasing voltage to a maximum value
Ceiling support system Most used, two perpendicular sets of ceiling-mounted rails, preferred in detent
Floor-to-ceiling support system Single column with rollers on each end, one attached to a ceiling-mounted rail and the other attached to a floor-mounted rail
C-arm support system Ceiling mounted and provide very flexible x-ray tube positioning
Small focal spot Used when better spatial resolution is required (bone work)
Large focal spot When large body parts are imaged and when higher techniques are used that will generate high heat
Three functions of an anode Electrical conductor, mechanical support for the target, and thermal dissipater
Whats is the anode made up of? Tungsten (high atomic number, high melting point), molybdenum, and graphite (lower mass density, high melting point)
Stationary anode Used in dental x-ray imaging systems, some portable imaging systems, and other special-purpose units in which high tube current and power are not required
Rotating anode Capable of producing high-intensity x-ray beams in a short time,
Line focus principle The focal spot is the area of the target from which x-rays are emitted; requires smaller focal spot to get better spatial resolution
Anode heel-effect Weaker intensity of x-ray beam on anode side, the smaller the anode angle, the larger the effect, used on feet, chest, and abdomen
X-ray tube rating Charts that guide the technologist in the use of x-ray tubes
Cooling chart Thermal capacity of an anode, and its heat dissipation characteristics are contained in a rating chart
Create a list of properties to support the choice of material mostly used for the anode target in general radiography Tungsten is the material used in the general radiography, used for 3 reasons: high atomic number, thermal conductivity, and high melting point
Created by: meechthebeech91