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Imaging Ch.7

The X-Ray Tube

QuestionAnswer
Protective Housing -Houses the x-ray tube. -Isolates the high voltages. -Provides a means to cool the tube. -Controls leakage and scatter radiation. -Mechanical support.
Isolates High Voltage Protects the radiographer from electrical shock.
Tube Cooling -Contains oil between the x-ray tube and the metal housing. -Conducts heat away from the x-ray tube.
Isotropic emission X-rays are emitted in all directions.
Window -The area in the tube housing which permits x-rays to pass through (useful beam). -Helps to focus x-rays in one direction.
The x-rays emitted through the window are called the _________________? Useful beam.
Leakage radiation -Any radiation leaving the tube housing from outside the window -Must be kept to less than 100mR/hr at 1m.
Mechanical Support -Provides a support for the x-ray tube. -Protects the tube from damage due to rough handling.
Glass or Metal Envelope -Inside of the Protective Housing. -Made of Pyrex glass or metal. -Pyrex glass can withstand tremendous heat. -Surrounds the anode and cathode. -Is considered a vacuum tube. -Does not allow a gas build up within the envelope.
Metal enclosure tube Maintain a constant electric potential between the electrons of the tube current and the enclosure.
Cathode The negative side of the x-ray tube.
The cathode composed of? Filament -Coli of wire used to boil off electrons. -Focusing cup. -Focuses the electron cloud so that X-rays are emitted more evenly.
Filaments are usually made of __________________? Thoriated tungsten.
Filament Current -A warm up current which flows through the filament and prepares it for the jolt of electricity necessary for x-ray production. -Produces a space charge (Electron cloud surrounding the filament).
Anode The positive side of the x-ray.
Functions of the anode include? -Electrical conductor. -Mechanical support for the target. -Good thermal radiator.
Target The area of the anode struck by the electrons from the cathode.
Atomic number Must be high so that it results in high efficiency in x-ray production.
Thermal dissipater Must be able to conduct heat away from the target.
High melting point Must be able to withstand high temperatures.
2 Types of Anodes 1. Stationary. 2. Rotating.
Stationary Anode -Used in dental and some portable units. -Does not rotate. -All electrons hit the same area on the anode. -Are not adequate for high technique procedures. -Composed of tungsten embedded in copper.
Rotating Anode -The anode rotates making the target area larger so that it does not wear out as quickly. -Allows for higher tube currents and exposure times. -Uses an induction motor to turn the anode.
Focal spot The area of the target (anode) from which x-rays are emitted.
Actual focal spot size The area on the anode target that is exposed to electrons from the tube current.
Effective focal spot Area projected onto the patient and the image receptor.
Line Focus Principle By angling the target, the effective focal spot size is made much smaller that the actual area of electron interaction.
Target Design: -Designed with an angle so that the effective focal spot is smaller than the actual focal spot. -Some have 2 angles and are called biangle targets that produce 2 focal spots for the tube
Heel Effect Due to the construction of the target, the x-ray intensity on the cathode side of the tube is greater than on the anode side.
The ________ the focal spot the greater the heel effect? Smaller.
Off Focus Radiation Undesirable radiation caused from electrons interacting with the target and bouncing in an unwanted direction causing scatter.
Causes of tube failure: -Excessive heat. -Maximal radiographic techniques. -Tube arch.
Radiation The filament heats up and radiates electrons to the anode.
Conduction The oil surrounding the tube conducts heat away from the tube / anode conducts heat away from the tube.
Convection Uese a fan to move heat away from the tube.
Open filament The filament wire becomes thinner and eventually breaks.
Radiographic rating chart Gives a range of safe techniques for a specific tube.
Anode cooling chart Specifies a thermal capacity of the anode and its heat dissipation characteristics.
Housing cooling chart Gives the characteristics of the tube housing cooling abilities.
Thermal energy is measured in? Heat units (HUs) or joules (J).
Single phases HU = kVp x mA x s.
Three phases and high frequency HU = kVp x mA x S x 1.4.
Protective Housing Guards against excessive radiation exposure and electric shock.
X-ray tubes Are designed with a glass or a metal enclosure.
The ____________ is the negative side of the x-ray tube; it has two primary parts, a filament and a focusing cup. Cathnode.
Tungsten Vaporization with deposition on the inside of the glass enclosure is the most common cause of tube failure.
The ___________ current is adjusted by controlling the filament current. X-ray tube.
_____________ at low kVp and high mA can be space change limited. Thermionic emissions .
The ___________ is the positive side of the x-ray tube; it conducts electricity and radiates heat and x-rays from the target. Anode.
__________ tube currents and ________ exposure times are possible with the rotating anode. Higher; shorter.
Focal Spot Is the actual x-ray source.
The ________________ is powered by an electromagnetic induction motor. Rotating anode.
The _________ the anode angle, the ________ the heel effect. Smaller; lager.
The _____________________ results in an effective focal spot size much less than the actual focal spot size. Line-focus principle.
The __________________ results in smaller effective focal spot and less radiation intensity on the anode side of the x-ray beam. Heel effect.
Excessive heat Results in reduced x-ray tube life.
Maximum radiographic techniques should never be applied to a __________________. Cold anode.
The most frequent cause of abrupt tube failure is electron arcing from the filament to the enclosure because of ________________. Vaporized tungsten.
Most x-ray imaging systems have a _________________ control that does not allow an unsafe exposure. Microprocessor.
Ceiling support 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 .
Floor-to-ceiling 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.
C-arm The image receptor is attached to the other end of the c-arm from the x-ray tube.
Isotropically With equal intensity in all directions.
Created by: sassyrad
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