-
Positive electrode of xray tube
Anode
-
Negative electrode of xray tube
Cathode
-
Process of boiling off electrons from the filament
Thermionic emission
-
Define space charge
Cloud of electrons around filament
-
Composition of filament
Thoriated tungsten
-
Two parts of cathode
Filament and focusing cup
-
Three functions of anode
- Electrical conductor
- Mechanical support
- Good thermal dissipater
-
Define line focus principle
Effective focal spot is smaller than actual focal spot
-
Area on target where electrons strike
Actual focal spot
-
Two types of anodes
Stationary and rotating
-
Composition of stationary anode
Tungsten rhenium alloy embedded in copper anode
-
Composition of rotating anode
Tungsten rhenium alloy disc with molybdenum and graphite
-
Three types of tube supports
- Ceiling
- Column supported by floor or ceiling
- C-arm
-
Part of glass enclosure where xrays are emitted
Tube window
-
Federal regulation for amount of leakage radiation
100 mR per hour at distance of 1 meter
-
How is heat dissipated from xray tube?
Oil cooled and air cooled
-
Area on patient or image receptor where electrons are directed
Effective focal spot
-
Three reasons tungsten is used for anode
- High atomic number
- High melting point
- Thermal conductivity
-
Type of motor used to rotate anode
Induction/inductor
-
Two parts of motor
Rotor and stator
-
Two speeds anode rotates
- Regular: 3,600 rpm
- Fast: 10,000 rpm
-
What happens when you press the rotor button (boost)?
- Anode rotates up to speed
- Filament heated to boil off electrons
-
What happens when you press the exposure button?
Electrons are accelerated across the tube and abruptly stopped to produce xrays
-
Angle of anode
5-15 dgrees
-
Define heel effect
Radiation intensity is greater on the cathode side of the xray field than on the anode side
-
Most common reason xray tube fails
Tungsten vaporization on the inside of glass enclosure leads to arching
-
Define target
Area of anode struck by electrons
-
Three methods to dissipate heat
- Conduction
- Convection
- Radiation
-
Term for emitted in all directions (360 degrees)
Isotropically
-
Three essential items needed to produce xrays
- 1. Hot filament for thermionic emission
- 2. kVp application to accelerate electrons across xray tube
- 3. Abrupt stoppage of electrons at target of anode
-
Purpose of glass envelope
Creates a vacuum
-
Why is a metal envelope preferred over glass (pyrex)?
- 1. Maintains constant electric potential between electrons and enclosure
- 2. Longer life
- 3. Less likely to fail
-
Advantage of rotating anode
Heat of anode spread across larger surface area (1000 times); increased heating capacity
-
Composition of stem anode? Why is that material used?
Molybdenum - poor heat conductor
-
List reasons anode fails in xray tube
- Surface melting and pitting
- Bearing and motor damage
- Tungsten vaporization
- Cracking of anode
-
List reasons cathode fails in xray tube
- Tungsten vaporization
- Open filaments (wire breaks from thinning due to use)
-
Purpose of radio graphic rating chart
Conveys which techniques are safe or unsafe for tube operation
-
Purpose of anode cooling curve
- Shows amount of heat units anode can hold
- Shows time required for heated anode to cool
- Shows how long you have to wait before you can take next exposure
-
Formula for calculating heat units
HU=mA x time x kVp Single phase
HU=mA x time x kVp x 1.4 Three phase/high frequency
-
Can one chart be transferred to all units
No, each machine has a rating chart specific to that unit
-
List the two main byproducts of xray production
-
How much heat is produced when electrons strike the target? How much xray?
99% heat, <1% xray
-
The efficiency of xray production increases with what?
Increasing kVp
-
What is emitted when an outer shell electron fills an inner shell void
Characteristic radiation
-
Why is characteristic radiation called that?
Characteristic of binding energy of target element
-
'Bremsstrahlung' means what?
Slowing down or braking
-
In the diagnostic range, most xrays produced are what type?
Bremsstrahlung
-
The type of radiation that is not emitted below 70 kVp
Characteristic
-
Define xray emission spectrum
Relative number of xrays emitted, plotted as a function of the energy of each xray
-
What type of xrays have a range of energies and form a continuous emission spectrum?
Bremsstrahlung
-
What type of xrays have fixed energies and form a discrete emission spectrum?
Characteristic
-
Function of filtration
Remove low energy or soft rays from xray beam that would only add to patient's skin dose
-
Total filtration in xray unit operating at 90 kVp
2.5 mm Al eq
-
Define HVL
Thickness of designated absorber required to reduce or decrease the intensity of the primary beam by 50% of one half of it's initial value
-
List types of compensating filters
-
Amount of added filtration in xray unit
2.0 mm Al eq
-
What makes up inherent filtration
Glass envelope, insulating oil
-
Advantage of three phase power generation
Voltage never drops to zero
-
Purpose of autotransformer
Steps up the voltage in the kVp circuit
-
Purpose of filament circuit
Provide low voltage and high amperage to heat the filament of the xray tube
-
Purpose of kVp circuit
Produce the kilo-voltage needed to penetrate the body part being xrayed
-
Two types of automatic exposure control
- Ionization chamber
- Photo timer
-
Two limitations for AEC
- Positioning is critical
- Minimum time exposure
-
Purpose of AEC
Machine decides when to terminate the exposure
-
Define back up timer
Setting place on timer to shut off machine in case the machine doesn't shut off before hand
-
Where is the ion chamber located for AEC? Photo timing?
Ion chamber - between patient and IR
Photo timing - below image receptor
|
|