Xray Tube Part 1 Flashcards
Travel of the xray
Cathode
Aluminum cross and anode
fluorescent image
dark shadow
Demonstation by crookes, it travels in straight lines
Cathode rays
Dr. Roentgen used _____ to make the first xray image
Crookes-hittorf tube
It was a major advancement in tube design
Coolidge Hot Cathode Tube
Two major hazards plagued early radiography
Excessive radiation exposure
Electric shock
A component of the xray imaging system rarely seen by radiologic technologist
Xray tube
Xray tube is contained in
Protective housing
Modern xray tube is a
Vacuum tube
Xray tube contains 2 electrodes:
Anode
Cathode
Size of the xray tube
30-50 cm long
20 cm in diameter
Two principal parts of the xray tube:
Anode (rotating)
Cathode
Tube that has two electrodes
Diode
Self rectification highest limits are about
30 mA at 90 kVp
External structure of the xray tube:
Support structure
Protective housing
Glass or metal enclosure
Internal structure:
Anode and cathode
3 main methods of xray tube support
Ceiling support system
Floor to ceiling support system
C-arm support system
The most frequently used
Consist of two perpendicular sets of ceiling mounted rails
Ceiling support system
Has a single column w/ rollers at each end (attached to the ceiling and to the floor)
Floor to ceiling support system
Ceiling mounted and provide very flexible positioning
C-arm support system
Tube is housed in a _________protective housing
Lead line metal
Generated isotropically or in all directions
Xray photons
It limits the beam to window
Housing
Controls leakage , scattered radiation , isolates the high v and provides a mean to cool the tube
Housing
Photons that escape from the housing except at the port
Leakage radiation
Leakage radiation must not exceed
100 mR /hr at 1 meter
Provides mechanical support and protection
Housing
Provides more insulation and thermal cushion
Oil
To protects againts electrical shock , the housing incorporates
High voltage receptacles
Maintains the vacuum inside the tube
Glass enclosure
Glass enclosure is made of ______ to enable to withstand the heat generated
Pyrex
It is where useful beam is emitted
Window
Size of the window
5cm ^2
Allows more efficient production of xray and tube life
Vacuum
With the presence of air:
Electron flow is reduced
Few xrays
More heat
Maintain a constant electric potential between the electrons of current and enclosure
Metal enclosure
Negative side of the tube
Cathode
2 parts of the cathode
Filaments
Focusing cup
Emits electrons when heated
Filament
Filaments are made up of
Thoriated tungsten
Provides higher thermionic emission and melting point is 3410 c
Tungsten
Most rotating anode xray tubes have
Two filaments
The most common tube failure
Tungsten vaporization
Designed to house the filament
Metal shroud that surrounds the filament
Focusing cup
Has a negative charge to confine electron beam to small area of anode
Focusing cup
Effectiveness of focusing cup
Size and shape
Charge
Filament size and shape
Position of filament
Emission of electrons fr heated surface
Thermionic emission / thermal electron emission
The process which charged carriers move over a surface by the induction of heat
Thermionic emission
Formation of electron cloud near the filament
Space charge
More electrons build up the filament, negative charges begin to oppose the emission
Space charge effect
As kVp increases , great percentage of the thermionically emitted electrons are driven towards anode
Saturation current
Parts of the anode
Anode target
Stator
Rotor
Positive side of the tube
Anode
Functions of the anode
Electrical conductor
Mechanical support
Thermal dissipator
Anode materials
Copper
Molybdenum
Graphite
Conducts electricity and radiates heat and contains target
Anode
2 types of anode
Stationary
Rotating
Used in dental xrays , does not require high tube current
Stationary anode xray tube
General purpose xray tubes , capable to produce high tube current
Rotating anode xray tube
Does not move when an exposure occurs
Stationary anode
Rotates during exposure
Rotating anode
Made of graphite for solid base
Base
Made of tungsten - rhenium alloy
Target
Parts holding target is made of
Molybdenum
Area of the anode struck by electrins from cathode
Target
Target material on stationary
Tungsten alloy in copper anode
Target material on rotating
Tungsten w/ rhenium
High capacity xray tubes have ________ under the tungsten target making it lighter
Molybdenum or graphite layer
Why tungsten is the material of choice
Atomic number
Thermal conductivity
High melting point
Measure of ability of a substance to conduct heat
Thermal conductivity
Melting point of tungsten
3422 c / 6192 F
Melting point of copper
1083 c / 1981.4 F
Leads to creation of small holes in the metal
Pitting corrosion
Actual target area of stationary
4mm2
Target area of rotating
1,760 mm2
Shaft between the anode and the rotor
Anode stem
Stem is made up of _____ because of poor conductor of heat
Molybdenum