The Xray Tube Part 1 Flashcards
Component of Crookes tube
Cathode
Aluminum cross and anode
Fluorescent image
Dark shadow
What tube did Roentgen used?
Crookes-Hittorf tube
No shielding =
X-rays emitted in all directions
A major advancement in tube design
Coolidge hot cathode tube
Two major hazards in early radiography
Excessive radiation exposure
Electric shock
Component in X-ray imaging system rarely seen by RT
X-ray tube
Contained in a protective housing
X-ray tube
Modern X-ray tube, Coolidge tube are
Vacuum tube
Size of X-ray tube
30-50 cm long
20 cm diameter
Two electrodes are called
Diode
Self rectification highest limits
30mA at 90kV
Overstepping the bounds can cause
Temperature rise in the anode target (thermionic emission)
External structures of X-ray tube
Support system
Protective housing
Glass or Metal enclosure
Internal structures
Anode
Cathode
Three methods of support system
Ceiling support system
Floor to ceiling support system
C-arm support system
Most frequently used support system
Ceiling support system
Have two perpendicular sets of ceiling mounted railings
Ceiling support system
Ceiling support system allows
Longitudinal and transverse travel
Floor to ceiling has
Single column with rollers at each end
Ceiling mounted and very flexible X-ray tube positioning
C-arm support system
Protective housing is made of
Lead lined metal
Generated isotropically and in all directions
X-ray photons
Limit the beam to window
Housing
Functions of housing
Controls leakage and scatter radiation
Isolates high voltages
Means to cool the tube
Any photons that escape from the housing
Leakage radiation
Other functions of housing
Mechanical support
Protection from damage
Uses of oil
More insulation
Thermal cushion
Housing is designed with
High voltage receptacles (avoid electric shock)
Don’ts in handling the tube
Never hold the tube during exposure
Never use the cables as handles
Glass enclosure is made of
Pyrex glass
Maintains vacuum inside the tube
Enclosure
Is 5cm2 with thin section glass
Window
Functions of vacuum
More efficient production of X-ray
Longer tube life
With the presence of air
Electron flow is reduced
Fewer X-rays produce
More heat generated
Incorporated with metal
Metal enclosure
Maintain constant electric potential
Metal enclosure
High capacity X-ray tubes uses
Metal enclosures
Negative side of the tube
Cathode
Coil of wire
Filament
Size of the filament
2mm diameter
1 to 2 cm long
Emits electrons when it is heated
Filament
Filament is made of
Thoriated tungsten
Provide higher thermionic emission
Tungsten
Melting point of tungsten
3410 C
1% to 2% thorium
Enhances efficiency of thermionic emission
Prolong tube life
Two filaments
Have greater variety of exposure
Dual focus filament
Rotating anode tube have
2 filaments
Lower mA station
300 mA or less
Most common cause of tube failure
Tungsten vaporization
A shallow depression
Focusing cup
House the filament
Focusing cup
Metal shroud that surrounds the filament
Focusing cup
Charge of Focusing cup
Negative
Effectiveness of Focusing cup
Size and shape
Charge
Filament size and shape
Position of Filament in the focusing cup
Adjusted by controlling the filament current
Tube current
Emission of electrons from a heated surface
Thermionic emission
Thermionic emission is also known as
Thermal electron emission
Process by which charge carriers move over a surface by induction of heat
Thermionic Emission
Formation of electron cloud near the filament
Space charge
Electrons build up in the area of the filament
Space charge effect
Electrons are difficult to emit by the filament because of
Electrostatic repulsion
Space charge limited
Thermionic emission at low kVp at high mA
As kVp increases, greater percentage of emitted electrons are driven toward the anode
Saturation current
The anode assembly
Anode
Stator
Rotor
Positive side of the tube
Anode
Functions of anode
Electrical conductor
Mechanical support
Thermal dissipator
Anode materials
Copper
Molybdenum
Graphite
It conducts electricity, radiates heat and contain the target
Anode
Types of anode
Stationary
Rotating
Used in dental imaging system
Portable imaging machines
Stationary anode X-ray tube
Use in general purpose X-ray tube
Rotating anode X-ray tube
Capable of producing high intensity X-ray beams in a short time
Rotating anode X-ray tube
Allows beam to interact with much larger target area
Rotating anode X-ray tube
Heat is not confined to a small area
Rotating anode X-ray tube
Does not move when exposure occurs
Stationary
Rotates during an exposure
Rotating anode
Made of graphite
Base
Made of tungsten-rhenium alloy
Target
Made of molybdenum
Parts holding the target
Area of the anode struck by the electrons
Target
Entire rotating disk is the target
Rotating anode
The target material in stationary anode
Tungsten alloy embedded in the copper anode
Target material of rotating anode
Tungsten with rhenium
Uses of molybdenum and graphite under the tungsten target
Anode lighter
Easier to rotate
Tungsten as material of choice
Atomic number (74) Thermal conductivity High melting point
A measure of the ability of the substance to conduct heat
Thermal conductivity
A form of extremely localized corrosion that leads to the creation of small holes in the metal
Putting corrosion
Actual target area for stationary tube
4mm2
Total target area for rotating anode
1760mm2
Rotating anodes revolve at
3600rpm
High capacity tube rotates at
10,000rpm
Shaft between anode and the rotor
Anode stem
Stem is made of
Molybdenum (poor heat conductor)
According to Crookes cathode rays travel in
Straight lines
