Xray Tube Flashcards
Conditions necessary for X-ray production
- Source of electrons
- High voltage
- Target material
- Vacuum
X-ray tube construction
Cathode
Anode
Glass envelope
Protective housing
The cathode contains:
Filament
Focusing cup
The anode contains:
Target Stator Rotor Glass envelope Protective housing
What is the filament made out of and why?
Thoriated tungsten
For: low vaporization so tube won’t get gassy
High melting point so it won’t burn out the bulb
And high thermionic emission
Longer tube life
What is the significance of a dual focus filament?
Small filament for detail, large filament for heat
When does thermionic emission occur
After the filament has reached incandescence, glows orange or red
What does a biased focusing cup do?
Charges more negative than normal to narrow electron stream for detail. Ex: mammo tube
Grid controlled tube
Has 3 charged electrodes:
Anode
Cathode
Grid/focusing cup
Focusing cup charges negative which closes off electron flow then charges positive to allow exposure
PURPOSE: allow precise rapid exposures. Pulse
What is a stationary anode used for?
Dental unit.
Made out of tungsten rhenium allow embedded in copper
Low heat dissipation
What is a rotating anode made out of?
Tungsten rhenium on molybdenum disk.
Has a larger target area for higher heat dissipation
What is the target made out of and why?
Tungsten bc:
Higher atomic number (74) gives high efficiency x Ray production and high energy X-rays
High melting point allows high tube current without bubbling or pitting
High heat conducting ability to dissipate heat
Rhenium provides elasticity
Stator
Only part outside of the glass envelope
Electromagnets
Rotor
Copper cylinder/cuff attached by molybdenum stem
Inside the rotor is silver plated ball bearings which serve as a lubricant between cuff and core
How fast does a common anode spin?
3200-3600 rpm
How fast does a high speed anode spin?
10,000-12,000 rpm
Used in cath lab and Fluoro
What is the glass envelope?
Purpose: maintain the vacuum
Contains cathode and anode
Window is where primary beam exits
What can the envelope be made out of and which is better?
Heat resistant Pyrex
Can be metal: metal is better for:
Less likely to fail, high output of X-ray tubes, less off focus radiation
What does the protective housing do?
Provides mechanical support for tube, cushions tube from rough handling, cools the tube, isolated high voltage, controls leakage(which may not exceed 1 mGya/hr or 100 mR/ hr at 1 meter)
What is off focus radiation?
Radiation not produced at the focal spot but in the tube
If too much leaks, can cause ghosting of structures
What happens when tungsten is vaporized?
Gassy tube
Deposit on envelope cause increased filtration of the primary beam and can cause high voltage arcing
Breaking of the filament from overuse of the rotor
Space charge limited region
Too low of kV ranges not all electrons are driven to the target.
Below 70 KV space charge compensator comes in and adjusts filament amps to make up for electrons that didn’t get pushed over.
At about 70 kV, all electrons are pushed over called saturation current
What is the focusing cup made out of?
Molybdenum or nickel
Line focus principle
Want small effective focal spot for detail and large actual focal spot for heat dissipation
Smaller angle=smaller effective focal spot
What makes an anode heel effect more or pronounced?
Small effective focal spot size
Short SID
Large cassette size
What can cause tube failure in the anode?
Melting, putting, cracking of anode
Tungsten vaporization
Damage to rotor bearings (most common cause of tube failure)
What causes tube failure in cathode end?
Filament vaporization
High voltage arcing
Production of X-rays includes
- Low volts/high amps to warm filament
- Apply rotor to give proper thermionic emission
- Push exposure button-high volts/low amps to drive electrons to targets that the speed of light.
- X-ray/ heat production
