Ch. 5/Ch. 7 X-Ray Tube Flashcards
What are the four conditions necessary for the production of X-ray photons?
- Need a source of electrons (filament)
- Appropriate target material (anode disk made of Tungsten)
- High voltage - generate power
- Vacuum tube - X-ray tube
What are the two main sides of an X-ray tube?
- Cathode side
- Anode side
What is the purpose of the glass or metal envelope in an X-ray tube?
To encase both electrodes and maintain a vacuum
The envelope protects the internal components and ensures the proper functioning of the X-ray tube.
What is thermionic emission in the context of X-ray production?
The release of electrons from the filament due to heat
This process is initiated by heating the filament, allowing electrons to escape and form a thermionic cloud.
What happens when high voltage is applied to the thermionic cloud?
Electrons are accelerated towards the anode target
The kinetic energy of these electrons results in the production of X-rays and heat upon colliding with the anode.
What is the primary function of the cathode assembly in an X-ray tube?
To produce the thermionic cloud and focus the stream of electrons to the anode
The cathode assembly includes the filament, focusing cup, and associated wiring.
Fill in the blank: The anode disk in an X-ray tube is typically made of _______.
Tungsten
Tungsten is used due to its high atomic number and melting point, making it ideal for X-ray production.
True or False: The X-ray tube is sometimes referred to as a diode.
True
The term ‘diode’ refers to the presence of two electrodes in the tube system.
What materials are typically used for the housing of an X-ray tube?
Metal protective housing, lead lined
The lead lining is essential for protecting against leaking radiation.
What are the parts of the cathode assembly in an X-ray tube?
- Filament
- Focusing Cup
- Associated wiring
Each part plays a role in the generation and focusing of electrons towards the anode.
What is a filament in radiography?
A small coil of wire made of Thoriated Tungsten, typically 0.1-0.2 mm thick, 1-2 mm wide, and 7-15 mm long.
What is the purpose of using small filaments?
To achieve more image detail for extremities using low mA (25-200 mA).
What is the purpose of using large filaments?
To achieve less image detail for the axial skeleton using higher mA (225-1400 mA).
What is the function of a filament?
It is coiled to create enough resistance in the electrical current to cause a ‘back up’ and then a release of electrons, known as ‘Thermionic Emission’.
Why is tungsten used for filaments?
Tungsten is chosen for its high melting point and difficulty to vaporize.
What problems can arise from vaporized tungsten?
- Not all electrons reach the anode; some float around or turn to gas.
- When tungsten cools and hardens, it deposits on the glass envelope, causing the ‘Minoring effect’.
What is arcing in relation to filaments?
Arcing occurs when current jumps to tungsten on the window, making a snapping noise and potentially puncturing the tube.
What is the melting point of tungsten?
Tungsten’s melting point is 3310°C.
What is thoriated tungsten?
It is tungsten with 1-2% thorium added to increase the efficiency of thermionic emission and extend the life of the tube.
What are other metal options for filaments?
Rhenium (melting point 3170°C) and Molybdenum (melting point 2620°C).
What happens when 10% of the filament’s diameter has vaporized?
There is a risk for breakage due to the filament becoming very brittle and thin.
What causes filament breakage?
- Too cold of a filament.
- Excessive vaporization leading to brittleness.
What is the effect of added unwanted filtration?
It absorbs some of the primary beam protons at the window, reducing the intensity of the primary beam.
What causes tube failure in radiography?
Issues such as arcing, added unwanted filtration, and filament breakage can all lead to tube failure.
What should be done with the exposure button during an X-ray procedure?
It should be depressed in one motion, except for pediatric patients or difficult patients.
What is the role of the stator in an X-ray tube?
It activates the rotor to start spinning the anode.
What happens after the thermionic cloud is formed?
The high voltage circuit is activated to send the thermionic cloud over to the anode, producing X-rays.
What is the average tube use for an X-ray machine?
10,000-20,000 exposures or 8-9 hours of ‘beam on’ time.
What is the Focusing Cup made of?
Nickel.
What is the main function of the Focusing Cup?
To narrow the thermionic cloud as it travels over to the anode.
What is the thermionic cloud also referred to as?
Space charge.
What does the Space Charge Effect describe?
The phenomenon where more electrons build up in the area of the filament, opposing the emission of additional electrons.
What is the purpose of the anode assembly in an X-ray tube?
It serves as a target area for incident electrons from the cathode filament.
What are the three functions of the anode assembly?
- Serves as a target area for incident electrons
- Becomes the source of X-ray photons
- Serves as a primary thermal conductor.
What percentage of the kinetic energy of the incident electrons is converted into heat?
99%
Only 1% of the energy is converted into x-ray photons.
What are the two types of anodes used in X-ray equipment?
- Stationary
- Rotating
What is the primary material used in the construction of a stationary anode?
Rhenium-alloyed tungsten
It is embedded into a 45° angle end of a copper rod and does not turn during the exposure, making it only good for low volume x-ray equipment
What is the size range of rotating anodes?
5-13 cm in diameter
What material is the whole disk of a rotating anode made from?
Molybdenum
Why is the focal track/ target track (area) made of rhenium-alloyed tungsten?
To give strength and flexibility
What additional material can be used to support the rotating anode?
Graphite
Why is tungsten the material of choice for anodes?
- High atomic number (74)
- High melting point (1000-2000°C)
- Good heat conductor
What is the target area on the anode where the electron thermionic cloud could impact called?
Focal track
What are other terms that refer to the precise point where the electron impacts on the focal track?
- Target
- Focus
- Focal Point
- Focal Spot
Where are x-ray photons created?
At the focal spot
What is measured from the focal spot?
SID (Source to Image Distance)
What is the approximate distance from the anode to the bottom of the collimator?
About 12 inches
What is the effect of frequent ‘overloading’ of the tube?
Pitting caused by high exposure technique
‘Overloading’ refers to excessive radiation exposure settings which can damage the anode.
What can cause a cracked anode?
Not warming up the tube prior to use
Warming up the tube is essential to prevent thermal shock/ cracking to the anode.
What is the average KVP for the standard warm-up procedure?
65-75
What is the average mA used in the standard warm-up procedure?
200-300
What is the duration of the exposure during the standard warm-up procedure?
One 1-second exposure
Two 2-second exposures
What is the purpose of the rotor in an x-ray tube?
To rotate the anode for heat dissipation
What is the composition of the rotor in an x-ray tube?
Hollow copper cylinder attached to the anode with a molybdenum shaft
The materials are chosen for their thermal conductivity and strength.
What are the common RPM ranges for high-speed rotors?
common rotors are 3200-3600 RPM and high speed rotor are 10,000-12,000 RPM
Higher RPMs improve heat dissipation and efficiency of the x-ray tube.
What happens at the end of the exposure in relation to the rotor?
A reverse current runs through the rotor/stator to quickly slow it down
This process helps prevent damage to the rotor by allowing it to decelerate safely.
What is the function of the stator in an x-ray tube?
To turn the rotor using an induction motor
The stator is located outside the vacuum tube and is essential for initiating rotor movement.
What causes the rotor to not turn if the stator stops working?
Bad induction motor
If the stator fails, it can lead to overheating of the anode and potential tube failure.
What noise indicates bad ball bearings in the rotor?
Grinding noise during exposure
Flattened balls in the bearings can cause uneven rotation and overheating.
Fill in the blank: The stator is the only part located ______ of the vacuum tube.
outside
Its external placement allows for effective operation without compromising the vacuum integrity.
True or False: The rotor rotates at the same rate as the anode disk.
True
The rotor is mechanically connected to the anode disk, ensuring synchronized rotation.
What is the function of the envelope for the diode?
Encapsulates the diode (cathode and anode)
Made of heat-resistant glass (like Pyrex) or metal with a glass window
What is the purpose of the window area of the tube?
Made of thinner glass to absorb less X-ray photons
Allows for the exit of primary beam while filtering out unnecessary photons
What is the significance of the vacuum in the envelope?
Removes all air molecules to allow electrons to travel from cathode to anode
Achieved through a vacuum pump
What is the function of the protective housing?
Helps to control leakage radiation
Leakage radiation is considered secondary radiation, not scatter radiation
What is leakage radiation?
Radiation exiting the protective housing
The protective housing is lined with lead to minimize leakage
What is the maximum allowable leakage radiation?
1 mGya per 1 hour at 1 meter (3.3 ft)
(1 mGy subscript a)
## Footnote
Leakage tested using an ionization chamber
What is the purpose of the special dielectric oil?
Protects against electrical shock and acts as a thermal cushion
Located between the X-ray tube and protective housing
What are the additional functions of the protective housing?
Isolates high voltage wires, helps cool the tube, protects from rough handling
Essential for safety and performance of the X-ray system
What are tube rating charts used for?
Help the tech prevent thermal damage
Each X-ray tube has its own rating chart
What information does a tube rating chart provide?
Max technique that can be used with the tube
Includes plotting KVp, mA, and time on the chart
How many types of charts are there for tube rating and cooling?
Three types of charts
tube rating chart, anode cooling chart, housing cooling chart
What does the anode cooling chart calculate?
The time it takes for the anode to cool down before another exposure can be made.
What is the formula to calculate heat units (Hu)?
Hu = MA x time x KVp x rectification unit.
What is the rectification unit for single-phase equipment?
1Ø= 1.0
What is the rectification unit for three-phase equipment?
3Ø=1.35.
What is the rectification unit for high-frequency equipment?
1.40.
Calculate the heat units generated by an exposure of 80 KVp, 200 mA, and 0.2 seconds on a single-phase rectified unit.
3200 HU.
What does the housing cooling chart measure?
The time it takes the protective housing to cool down before additional exposures can be made.
Which cools first, the anode or the housing?
The anode.
List one recommendation for extending tube life.
- Warm up anode by manufacturer’s recommendations
- Do not hold rotor switch unnecessarily
- Use lower mA stations when possible
- Use a lower speed rotor when possible
- Do not make repeated exposures
- Do not rotate tube housing rapidly
- Do not use tube when you can hear loud bearings.
What does Actual Focal Spot (AFS) describe?
The physical area on the focal track that the electrons are impacting.
What is the Effective Focal Spot (EFS)?
The area of the focal spot that is being projected out of the tube towards the object (patient).
What does Spatial Resolution (SR) relate to?
It relates to the Effective Focal Spot (EFS).
Fill in the blank: The formula for calculating heat units is Hu = MA x time x KVp x _______.
[rectification unit].
What principle is used to reduce the effective focal spot for better spatial resolution?
Line Focus Principle
This principle helps manage heat production while maintaining effective focal spot size.
How does filament size affect the effective focal spot (EFS)?
Small filament = Smaller AFS = Smaller EFS; Large filament = Larger AFS = Larger EFS
AFS refers to the actual focal spot size.
What is the relationship between anode angle and effective focal spot?
As the anode angle increases, AFS and EFS increase
Most target angles range from 7-11°, with 10° being most common.
What is a disadvantage of having a very small anode angle?
Very small useful primary beam
This can affect imaging quality.
What effect does the line focus principle have on primary beam intensity?
Anode Heel Effect
It causes intensity variation from anode side to cathode side.
What causes the variation in primary beam intensity in the Anode Heel Effect?
Some of the primary beam gets absorbed by the heel of the anode
This leads to more photons on the cathode side and fewer on the anode side.
What is the maximum percentage variation of photons from the anode to the cathode side?
Up to 45% total variation
This can be significant in imaging.
What is the impact of the Anode Heel Effect on imaging the thorax or abdomen?
Can be used to advantage based on patient thickness and composition
Helps optimize image quality in these body types.
What is Off Focus Radiation?
X-rays produced outside the actual focal spot
These occur when electrons from the thermionic cloud stray outside the target area.
What percentage of the primary beam can Off Focus Radiation make up?
25-30%
These photons are typically low energy, increasing Entrance Skin Dose.
What should the tube housing and collimator shutters do regarding Off Focus Radiation?
Should absorb a majority of it
This minimizes unnecessary radiation exposure.
